ERRATA SHEET PAGE NO LINE NO SHOULD READ 78 11 One Of

ERRATA SHEET PAGE NO LINE NO SHOULD READ

78 11 one of which functions as an .... 141 28 the chlorides of iron, vanadium and copper ... 110 34 radiation polarised parallel to the plane of incidence is absorbed by surface species. Installation of a polariser ...... 151 21 a) ZDDPs at room temperature ... 202 18 3) ZDDP and parent phosphorodithioic acid ... 129 7 attributed to the symmetric and antisymmetric .. Imperial College London

-LUB-R-I CANT—ADD ITIVES a im n wi= a r LU6RKANT A$VlTi\/£S COwr/BNiNp- p H0SPHORU3

Philippa Mary Cann

A Thesis Submitted for the Degree of DOCTOR OF PHILOSOPHY of the University of London and also for the DIPLOMA OF IMPERIAL COLLEGE

June 1982

Lubrication Laboratory Department of Mechanical Engineering Imperial College London SW7 I

ABSTRACT

Organophosphorus additives have been used for many years as load-carrying and anti-oxidant agents for lubricating oils. This thesis describes an investigation into the load-carrying properties of a series of zinc dialkyldithiophosphates (ZDDP) and related compounds. A short literature review is presented which concludes that despite extensive research into ZDDPs the mechanism of their anti-wear action is still not fully understood. A basis of this study therefore was an investigation into the AW mechanism of ZDDPs. The problem was studied in two ways. Firstly, the friction-temperature properties of the additives were evaluated on a High Frequency Reciprocating test device. The second approach was to investigate chemical films formed on steel surfaces during immersion in additive solutions. Analysis of these films was by infrared reflection-absorption spectroscopy. This study concludes that the load-carrying properties ofZDDPs are not due simply to their thermal degradation products and that the anti-oxidation reactions of ZDDPs play an important role in determining L-C properties. Also ZDDPs operate by forming thick non-conducting (boundary) films especially at high temperatures, which reduce friction. II

ACKNOWLEDGEMENTS

I would like to. thank the following:

Professor A Cameron and Dr H Spikes for their encouragement and supervision throughout this project. Reg, Tony and Paul for their technical support and assistance. All members of the Lubrication Laboratory, both past and present, for their forbearance and help with the engineering aspects of this project. Jane for the excellent typing, and endless patience with this thesis. Cinderhill Pharmacy for the photographs. Rolfe and Nolan Computer Services PLC for the photocopying. Graham, my mother and Elf (Aquataine) Ltd for financial support and encouragement. Ill

CONTENTS

Title Page

Abstract i Acknowledgements II Contents III List of Figures and Tables VIII Nomenclature XIII

CHAPTER ONE GENERAL INTRODUCTION AND OUTLINE OF PROJECT

1.1 Introduct ion 1.2 Frict ion 1.3 Wear 1.4 Type and Role of Additives in Boundary Lubrication 1.5 Introduction to Project 1.6 Outline of Project

CHAPTER TWO GENERAL REVIEW OF ZINC PIALKYLDITHIO- PHOSPHATE LITERATURE 2.1 General Introduction 8 2.2 Solution Chemistry of ZDDPs 9 2.2 Thermal Decomposition of ZDDPs 9 2.2 Discussion 21 2.3 Anti-oxidant Action in Solution 24 2.3.1 Literature Review 24 2.3.2 Discussion 31 2.4 Relationship Between AW and AO Action 34 2.5 Nature of ZDDP Films on Metal Surfaces 34 2.6 Load-Carrying Properties of MDDPs 39 2.7 Mechanism of ZDDP L-C Action 42

CHAPTER THREE PREPARATION OF ORGANOPHOSPHORUS ADDITIVES 3.1 Introduction 46 3.2 Preparation of 0,0 Dialkyldithiophosphoric Acids 46 3.2.1 Experimental 46 3.2.2 Purification of Dithiophosphoric Acids 47 IV

3.2.3 Analysis of Dithiophosphoric Acids 48 3.3 Preparation of ZDDP Salts 48 3.3.1 Purification of Basic and Neutral Zinc Salts 50 3.3.2 Characterisation of ZDDPs 52 3.4 Analysis of Commercial Additives 52 3.4.1 Analysis of Commercial ZDDPs 53 3.5 Discussion 53

CHAPTER FOUR INFRARED SPECTRA AND MOLECULAR STRUCTURE OF ZINC DIALKYLDITHIQPHOSPHATES AND RELATED COMPOUNDS 4.1 Introduction 55 4.2 Infrared Spectroscopy 55 4.3 Literature 57 4.4 IR Spectra of Dialkyldithiophosphoric Acids 58 4.4.1 Experimental and Introduction to Results 58 4.4.2 -S-H Bond 58 4.4.3 -P-O-C Bond 60 4.4.4 P=S Thiophosphoryl Bond 62 4.4.5 P-S Bond 73 4.5 IR Spectra ZDDPs 73 4.5.1 Literature Survey Crystal and Molecular Structure -of MDDPs 7 3 4.5.2 IR Spectra ZDDPs - Experimental 81 4.5.3 Results and Discussion 81 4.5.3.1 P-O-C Bond 81 4.5.3.2 P-S Bond 91 4.5.3.3 Zn-S Bond 91 4.5.3.4 Discussion of ZDDP Results 95

4.6 IR Spectroscopic Analysis of Lubrizol 1395 96 V

CHAPTER FIVE INFRARED SPECTROSCOPIC ANALYSIS OF THIN FILMS ON METAL SURFACES 5.1 General Introduction 100 5.2 Analysis of Thin Films on Metal Surfaces 100 5.3 Infrared Reflection-Absorption Spectroscopy 102 5.3.1 Introduction 102 5.3.2 Applications of Infrared Reflection- Absorption Spectroscopy 103 5.3.3 Theory and Development of Infrared Reflection-Absorption Spectroscopy 105 5.3.4 Optimisation of Experimental Conditions 110 5.4 Experimental 113 5.4.2 Equipment - IR Spectrometer 113 5.4.3 Sample Optics 115 5.5 Preliminary Experiments 118 5.6 Experimental Procedure 120 5.6.1 Room Temperature Tests 120 5.6.2 High Temperature Tests 120 5.6.3 Scanning Procedure 122 5.6.4 Reference Spectra 122 5.7 Results 123 5.7.1 Di-ethylphosphorodithioic Acid 123 5.7.1.1 Room Temperature 124 5.7.1.2 100°-150°C 127 5.7.1.3 160°-200°C 133 5.7.2 Zinc Diethyldithiophosphate 134 5.7.2.1 Room Temperature Tests 134 5.7.2.2 100°-150°C Tests 137 5.7.2.3 160°-200°C Tests 137 5.7.3 Commercial Additive - Lubrizol 1395 142 5.7.3.1 Room Temperature Tests 142 5.7.3.2 100°-150°C Tests 144 5.7.3.3 160°-200°C Tests 144 5.8 General Discussion 148 5.9 Conclusions 151 VI

CHAPTER SIX LUBRICATION OF STEEL BY ZINC DIALKYL- DITHIOPHOSPHATE S 6.1 Introduction 153 6.2 Methods of Studying Load-Carrying Additives 153 6.3 Test Apparatus and Procedures 157 6.3.1 HFR Rig 157 6.3.2 Description of HFR Device 157 6.3.3 Test Procedure 160 6.3.4 Cleaning Procedure 161 6.3.5 Solvents, Additives, Metals Employed 161 6.4 Preliminary Results 164 6.4.2 Other Preliminary Tests 164 6.5 Main Results 168 6.5.1 Testing Programme 168 6.5.2 No Additives 195 6.5.3 Organophosphorus Additive Tests 195 6.5.3.1 Zinc Dialkyldithiophosphates 195 6.5.3.2 Dialkylphosphorodithioic Acids 201 6.6 Mixed Additive Tests 201 6.6.1 Introduction 201 6.6.2 Results 203 6.6.2.1 Di-ethyl Additives 203 6.6.2.2 Di-iso-propyl Additives 204 6.6.2.3 Organophosphorus/Preoxidised Hexadecane Additive Mixtures 205 6.6.2.4 ZDDP and Anti-oxidant 205 6.7 Discussion 206 6.7.1 Effect of Initial Concentration 213 6.7.2 Effect of Alkyl Chain Length 214 6.7.3 Presence of Acid Impurities 215 6.7.4 Addition of Polar Oxidised Species 215 6.7.5 General Discussion 216 6.7.6 Nature of the Boundary Film 220 6.7.7 Comparison of HFR Test Results with Previous Work 221 6.8 IR Spectroscopic Analysis of Bulk Oil Changes 223 6.9 General Conclusions 224 VII

CHAPTER SEVEN GENERAL CONCLUSIONS AND SUGGESTIONS FOR FUTURE WORK 7.1 Introduction 226 7.2 Chapter 4. IR Spectroscopy of ZDDPs 226 7.3 Chapter 5. IR Reflection-Absorption Studies of Surface Films 227 7.3.1 Conclusions 227 7.3.2 Future Work 2 28 7.4 Chapter 6. Lubrication of Steel by ZDDPs. 228 7.4.1 Conclusion 228 7.4.2 Future Work 229 7.5 Mechanism of ZDDP AW Action 229

APPENDIX 1 Analysis of Organophosphosphorus Additives by Thin Layer Chromatography 231

REFERENCES 234 VIII

LIST OF FIGURES

Page No

1.1 Typical Load-Carrying Additives 4 2.1 Metal Dialkyldithiophosphates 8 2.2 ZDDP Thermal Decomposition Mechanism from Ashford et al (23) 12 2.3 Mechanism of ZDDP Thermal Degradation from Luther et al (24) 13 2.4 Mechanism of ZDDP Thermal Degradation from Dickert and Rowe (27) 15 2.5 Mechanism of ZDDP Thermal Degradation from Brazier and Elliott (28) 17 2.6 Mechanism of ZDDP Thermal Degradation from Jones and Coy (30) 20 2.7 Reaction Mechanism of Hydrocarbon Auto- oxidation in the Liquid Phase (33) 24 2.8 Mechanisms of ZDDP Chain Breaking from Burns et al (35) 26 2.9 Reaction Profiles for the Decomposition of Cumene Hydroperoxide in the Presence of ZDDP from Burns et al (38) 28 2.10 Ionic Versus Free Radical Hydroperoxide Decomposition (40) 30 2.11 Summary of ZDDP Anti-oxidant Reaction Mechanisms 33 4.1 Infrared Spectrum of n-Hexadecane 56 4.2 Rotational Isomers of (RO) P(S)SH (68) 59 2 4.3 IR Spectrum of Di-ethylphosphorodithioic Acid 63 4.4 IR Spectrum of Di-n-propylphosphorodithioic Acid 64 4.5 IR Spectrum of Di-iso-propylphosphorodithioic Acid 65 4.6 IR Spectrum of Di-n-butylphosphorodithioic Acid 66 4.7 IR Spectrum of Di-n-hexylphosphorodithioic Acid 67 4.8 IR Spectrum of Di-n-octylphosphorodithioic Acid 68 4.9 Tetramethyldiphosphorus Disulphide (TMPDS) 70 4.10 Proposed Molecular Structures of ZDDPs 75 IX

4.11 Resonance Structures of MDDPs from Gallopoulos (66) 76 4.12 Equilibrium Chelate Structures of ZDDPs in Solution from Heilwell (81) 77 4.13 Molecular Structure of the Zinc Di-iso-propyldithiophosphate Dimer from Lawton and Kototailo (86 ) 79 4. 14 IR Spectrum of Zinc Di-ethyldithiophosphate 82 4. 15 IR Spectrum of Zinc Di-n-propyldithiophosphate 83 4. 16 IR Spectrum of Zinc Di-iso-propyldithiophosphate 84 4. 17 IR Spectrum of Zinc Di-n-butyldithiophosphate 85 4. 18 IR Spectrum of Zinc Di-iso-butyldithiophosphate 86 4. 19 IR Spectrum of Zinc Di-sec-butyldithiophosphate 87 4. 20 IR Spectrum of Zinc Di-n-hexyldtihiophosphate 88 4. 21 IR Spectrum of Zinc Di-n-octyldithiophosphate 89 4. 22 IR Spectrum of Lubrizol 1395 90 4. 23 IR Spectrum of ZDDPs in 400-200cm~1 region 94 4. 24 IR Spectrum for Di-ethyl and Di-n-octyl derivative in the 900-500cm_1 spectral region 97 5.1 Transmission and Reflection Infrared Sampling Methods 101 5.2 Model of a Thin Film on a Metal Surface 104 5.3 Orientation of Polarisation Before and After Reflection from a Metal Surface 104 5.4 Phase Shift for Light Reflected from a Metal Surface from Greenler (103) 107 5.5 Definition of Absorption Factor (A) 109 5.6 Absorption Factor as a Function of Angle of Incidence for Parallel Polarisation from Greenler (105) 111 5.7 Optimum Number of Reflections Shown as a Function of Incidence Angle from Greenler (105) 112 5.8 Reflectance Unit 116 5.9 Schematic Diagram of Sample Optics 117 5.10 Effect of Polarisation Angle on Absorption 118 5.11 Appearance of a Reflection-Absorption Band After Different Number of Reflections 121 5.12 IR Reflection Spectra of Steel Surfaces After Immersion in Di-ethylphosphorodithioic Acid Solutions at 20°C 124 X

5.13 IR Reflection Spectra of Steel Surfaces After Immersion in Di-ethylphosphorodithoic Acid Solutions at 20°C 125 5.14 IR Reflection Spectra from Steel Surfaces After Immersion in Di-ethylphosphorodithoic Acid Solutions at 100-150°C 323 5.15 IR Reflection Spectra from Steel Surfaces After Immersion in Di-ethylphosphorodithioic Acid Solution at 200°C 132 5.16 IR Reflection Spectra from a steel surface after immersionin Zinc Di-ethyldithiophosphate Solution at 80-100°C 135 5.17 IR Reflection Spectra from Steel Surfaces Immersed in Zinc Di-ethyldithiophosphate Solution at 200°C 138 5.18 IR Transmission Spectrum of the Bulk Decomposition Precipitate of Zinc Di-ethyl dithiophosphate in Hexadecane 139 5.19 IR Reflection Spectra from a Steel Surface After Immersion in Lubrizol 1395 Solution at 100-150°C 143 5.20 IR Spectra for Lubrizol 1395 Test Results at 200 °C 145 6.1 Schematic Diagram of the High Frequency Reciprocating Device 155 6.2 Overall View of the Test Apparatus 156 6.3 Oscilloscope Traces 158 6.4 Typical CRO Traces 159 6.5 Test Repeatability of HFR Friction Device 167 6.6 HFT Test Results for Pure Hexadecane 171 6.7 HFR Test Results for Preoxidised Hexadecane 171 6.8 Zinc Di-ethyldithiophosphate 172 6.9 Zinc Di-n-propyldithiophosphate 174 6.10 Zinc Di-iso-propyldithiophosphate 175 6.11 Zinc Di-n-butyldithiophosphate 177 6.12 Zinc Di-iso-butyldithiophosphate 179 6.13 Zinc Di-sec-butyldithiophosphate 131 6.14 Zinc Di-n-hexyldithiophosphate 132 6.15 Zinc Di-n-octyldithiophosphate 133 6.16 Lubrizol 1395 184 XI

6.17 Di-ethylphosphorodithioic Acid 186 6.18 Di-iso-propylphosphorodithioic Acid 187 6.19 Phosphorodithioic Acid Additive Mixtures 188 6.20 Zinc Di-ethyldithiophosphate Additive Mixtures 189 6.21 Zinc Di-iso-propyldithiophosphate Additive Mixtures 191 6.22 Preoxidised Hexadecane Additive Mixtures 193 6.23 o.l% w/w ZDDP and Antioxidant 194 6.24 Typical Coefficient of Friction/ECR Curves for 0.0015M Solution of a Pure ZDDP in Hexadecane 207 6.25 Diagrammatic CRO Traces Showing the Effect of Increased Stroke Length on Boundary Film Formation 219 Appendix 1 Analysis of ZDDP's by Thin Layer Chromatography 23 3 XII

LIST OF TABLES

3.1 Prepared Dialkyldithiophosphoric Acids 51 3.2 Prepared Zinc Dialkyldithiophosphates 51 3.3 Specified Composition of Lubrizol 1395 52 4.1 Summary of v(-SH) Band Positions 60 4.2 Summary of Literature P-O-C Band Assignment 62 4.3 Assignment of P-O-C Skeleton Vibration

Frequencies for (RO)2P(S)SH 72 4.4 Assignment of v(P=S) Band Positions for

(RO)2P(S)SH 72 4.5 Assignment of v(P-S) Band Positions for

(RO)2P(S)SH 74 4.6 Assignment of P-O-C Skeleton Vibration Frequencies for ZDDP 92 4.7 Assignment of ZDDP P-S/P=S Vibration Frequencies 93 4.8 Summary of Results of v(M-S) 95 4.9 Summary of Lubrizol 1395 IR Band Assignments 99 5.1 Summary of IR Peak Positions and Assignments for Figures 5.12 and 5.13 126 5.2 Summary of IR Peak Positions and Assignments for Figure 5.14 131 5.3 Summary of IR Peak Positions and Assignments for Figure 5.15 131 5.4 Summary of IR Peak Positions and Assignments for Figure 5.16 136 5.5 Summary of IR Peak Positions and Assignments for Figures 5.17 and 5.18 14° 5.6 Summary of IR Peak Positions and Assignments for Figure 5.20 147 6.1 Specified Composition of EN31 Steel 163 6.2 Summary of Organophosphorus Additive Tests 169 6.3 Summary of Mixed Additive Tests 170 6.4 Summary of Friction Results for 0.0015M ZDDP Solutions 208 6.5 Effect of Increased Additive Concentration 209 6.6 Summary of HFR Results for Mixed Additive Tests 211 XIII

NOMENCLATURE

MDDP Metal Dialkyldithiophosphate ZDDP Zinc Dialkyldithiophosphate DTP Dithiophosphoric L-C Load-carrying AW Antiwear EP Extreme Pressure AO Anti-oxidant v Wavenumber in cm 1 IR Infrared R-A Reflection-absorption R Reflectance d Film thickness n Refractive index A Wavelength a Absorption coefficient in cm 1 A Absorption factor K Absorption constant J_ Perpendicular || Parallel ECR Electrical Contact Resistance M Coefficient of friction CRO Cathode Ray Oscilloscope HFR High Frequency Reciprocating Hz Cycles per second ISL Initial Seizure Load WL Weld Load MHL Mean Hertz Load TLC Thin Layer Chromatography 1

CHAPTER 1 GENERAL INTRODUCTION AND OUTLINE OF PROJECT

1.1 Introduction

The phenomenon of friction and its reduction has been studied since the earliest times (1). Whenever one surface is brought into contact and then moved against another, friction and subsequently wear occurs. One explanation of friction is that all metal surfaces are microscopically rough so that when they slide together these asperities interlock and resist motion (2)« An alternative theory proposed by Bowden (6) is that friction is due to shearing of localised welds formed by surface asperities under high pressures. To reduce these effects a lubricant is introduced. Two general lubrication regimes are recognised depending on the conditions experienced, fluid film and boundary. Fluid film lubrication occurs under lightly loaded contacts and at high speeds, when the two surfaces are completely separated by a lubricant film. Low friction results from fluid shear rather than metal shear and negligible wear occurs, depending on the physical properties of the lubricant film. Under more severe conditions, of high loads and slow speeds, the bulk oil film collapses and the boundary region results. The concept of boundary lubrication was first established by Hardy and Doubleday in 1919 (3). It has been defined as the state when sliding surfaces are separated by such thin films of lubricant that the chemical and physical nature of the surface and the lubricant are of major importance (4).

1.2 Friction

Friction in the boundary region has been explained by interaction between molecules adsorbed on the two metal surfaces (3) (5). An alternative theory is that isolated 2 metal asperities weld when placed together under load. Friction was thought to arise from shearing of the metal junctions formed (6), although this mechanism has also been shown to contribute to the wear process (4). A boundary lubricant modifies the metal surfaces by forming a thin film of molecular dimensions whose shear strength is lower than that of the bulk metal. Friction and wear characteristics are determined by the physical and chemical properties of the surface film. The absence of which causes severe wear and high friction. Both oxygen and moisture play important roles in determining friction and wear in boundary lubrication (8) (7). Very clean or freshly exposed metal surfaces strongly adhere when brought together, due to their high surface energy. Oxide films and other surface contaminants reduce surface energy and prevent welding (9). Although the two lubrication regimes have been presented separately, in practice considerable overlap can occur. A third regime is very common, that of mixed lubrication. It occurs under moderately loaded contacts. Where a separating oil film is maintained although intermittent penetration by surface asperities takes place.

1.3 Wear

The problems of friction and wear are related, although the reduction of wear is perhaps of more commercial importance. The occurrence and prevention of wear of sliding metal surfaces has been extensively investigated in recent years Wear has been defined as the progressive loss of material from the operating surface of a body occurring as a result of relative motion at the surface (11). It is generally identified as mild or severe depending on the rate of material lost. The detrimental results of severe wear have been well documented; piston ring scuffing, pitting, bore polishing, etc. Some aspects of wear, however, can be beneficial, eg running-in. A number of wear mechanisms have been identified (15), 3 the most common being adhesion, corrosion and abrasion. Continuous wear of metal surfaces is due to a number of effects, material is removed in the lubricant without resulting in severe wear. Both adhesion due to localised welding of surface asperities and mechanical interlocking of asperities contribute to this form of wear Adhesion is also a primary cause of wear of clean metal surfaces, due to shearing of intermetallic junctions resulting in high friction and severe wear . Abrasion is due to the formation of hard oxide particles from the surface which are not carried away in the lubricant. It is probably the most serious form of wear. EP additives reduce wear by preventing junction growth and metallic contact, although excessive EP activity can cause corrosive wear due to formation of a brittle surface f i lm. The reduction of friction and wear of sliding surfaces by lubricant additives is well established. Organic compounds have been incorporated since the mid 19th century to enhance lubricant performance. Since the 1930's the use of additives has been widespread. Due to the complexity of present day lubricants a brief review of additive types studied in this project is given.

1.4 Type and Role of Additives in Boundary Lubrication

A fully formulated lubricant contains many different types of additives. They can be divided into two main categories; those that alter the physical characteristics of the oil, eg viscosity index improvers and pour point depressants, and those that are chemical in action. These include oxidation inhibitors, load carrying, detergents, anti-corrosive and oiliness additives. Oiliness additives are usually long chain polar molecules such as acids, esters, alcohols and amines. They operate at low temperatures by physical adsorption on the metal surfaces, forming a solid film (12). In this way metal- metal contact is reduced. At temperatures near the bulk melting point of the adsorbate melting of the solid film occurs disrupting 4

chemical structure (13). An increase in friction and wear is observed at this temperature. Load carrying additives reduce wear and friction in boundary lubricated contacts. They can be divided into anti-wear (AW) or extreme pressure (EP) agents depending on their action and operating temperature (48). AW additives operate under mixed lubrication conditions by reacting with surface asperities so preventing metal-metal cont act. As the load and/or temperature increases the separating oil film collapses and the number of metallic contacts increases. Temperature and wear of the metal surfaces increases dramatically as the molecular layer is insufficient to prevent asperity welding. The onset of severe wear is taken as the start of the EP region. EP additives are thought to work by reacting with freshly exposed metal, forming thin inorganic films which prevent welding of the surfaces . Most authors (48) agree that the temperature of the contact is the critical factor controlling the onset of the EP region. EP additives are designed to work at high temperatures and/or high loads. Their operation has been called a form of controlled corrosion (17), and careful matching of lubricant to application is required or practical problems can arise (14). Some additives can work in both EP and AW regions, zinc dialkyldithiophosphates (ZDDP), are generally regarded as AW additives that show moderate EP activity. Load carrying additives usually contain a sulphur, phosphorus, or chlorine atom, or a combination of these. Some examples of commonly used additives are given below:

AW ro or \ EP R = cresyl

0 Dibenzyl Disulphide

Tricresyl Phosphate

FIGURE 1.1 Typical Load Carrying Additives 5

ZDDP's combine roles as antioxidant and load carrying additives. Lubricant oxidation occurs readily at high temperatures encountered under boundary conditions. Oil oxidation products are acidic in nature and therefore possibly corrosive. They also lead to the formation of sludges and increased oil viscosity. Antioxidants work by decomposing reaction intermediates formed during the oxidation process or by deactivating metal catalysts which promote oxidation. A review of the antioxidant behaviour of ZDDP's is given in Chapter 2.

1.5 Introduction to Project

In the last twenty-five years the role of ZDDP's as lubricant additives has been extensively studied. Though considerable effort has been expended in establishing the mechanism of load carrying, research has tended to be fragmentery and results more dependent on the experimental method chosen than fundamental properties of ZDDP's. As a consequence the AW action of ZDDP's is still not fully understood. In recent years it has become increasingly important that this problem should be resolved. Development of new catalytic engine emission control systems has made the reduction of ZDDP concentration in motor oils desirable. Thermal decomposition products of ZDDP's have been found to act as catalyst poisons (14). At the same time reduced oil viscosities have been introduced to improve fuel efficiency. These are more dependent on ZDDP action, since lower oil viscosities mean reduced oil film thickness. Increased ZDDP efficiency is obviously desirable for both the above reasons. A better understanding of ZDDP action is also important in considering certain practical problems ' associated with their use. ZDDP containing oils have been found to be unsuitable for use with silver bearings (14). Chemical attack of lead-bronze bearing alloys and tin rich white metals at high temperatures has also been reported (14). 6

During a study of the literature it became increasingly obvious that the antiwear action of ZDDP's was still not fully understood. It was felt that an investigation into a fundamental aspect of the load carrying action of ZDDP's would still be profitable. This was the basis of the PhD project undertaken.

1.6 Outline of the Project

The role of ZDDP's is very complex especially in a fully formulated oilwhere additive interference can occur (16). Most investigators (57) (58 ) agree that minor impurities formed during ZDDP's synthesis play an important role in determining the load carrying capacity of ZDDP's. Even so many investigators have used impure ZDDP's (19), occasionally of unspecified structure (18). As a consequence a study of fundamental AW action of pure ZDDP's and the effect of major impurities likely to be found in a commercial ZDDP was proposed. A first priority was the synthesis and purification of a number of ZDDP's and related compounds. This is detailed in Chapter 3. The prepared compounds and a typical commercial ZDDP were characterised by a number of analytical techniques to establish purity and the identity of any major impurities. Previous investigations of ZDDP AW action have been concentrated in three general areas of research: a. Mechanism of thermal decomposition. b. Nature of surface film. c. Friction and wear testing.

These aspects are reviewed in full in Chapter 2. Two of them have been studied in this project.

1. Nature of the Surface Film

ZDDP's in hydrocarbon solution were reacted with metal surfaces. Any chemical films formed were characterised using 7 infra-red reflection-absorption spectroscopy. A technique which provides information on the identity, structure and density of surface films.

2. Friction Tests

For reasons detailed later, the ZDDP's were tested on the High Frequency Reciprocating rig rather than a standard friction device. Coefficient of friction and electrical contact resistance were monitored as functions of oil temperature, so that the formation of a boundary film and its effect on friction could be observed.

These two techniques were combined with analysis of the lubricants before and during testing, to provide information on the formation, and nature of the surface films; its effect on wear and friction and accordingly bulk oil changes. The thesis is presented in three parts:

1) Introduction and literature survey of ZDDP action. (Chapters 1 and 2). 2) Preparation and characterisation of additives. (Chapters 3 and 4). 3) Experimental techniques, results and general discussion. (Chapters 5, 6 and 7). 8

CHAPTER 2 GENERAL REVIEW OF ZINC DIALKYLDITHIOPHOSPHATE LITERATURE

2.1 General Introduction

Metal 0,0 dialkyl dithiophosphates (MDDPs) (Figure 2.1) have been used as antioxidants for lubricating oils for many years. Originally their primary function was as antioxidants, their anti-wear properties not at first being recognised.

RO S S OR \ / ^ / p p / \ / \ RO S M S OR

R = alkyl group M = metal atom

FIGURE 2.1 Metal Dialkyldithiophosphate

In the past twenty-five years there has been extensive investigation of both the AW and antioxidant properties of ZDDPs. Most research has been directed at understanding the mechanism of ZDDP action, so work has been concentrated on three general topics: a) Behaviour of ZDDPs in solution. b) Composition of surface films. c) Load carrying properties of ZDDPs.

In this review each of these areas will be discussed separately. 9

2.2 Solution Chemistry of ZDDPs

2.2.1 Thermal Decomposition of ZDDPs

The thermal degradation of ZDDPs has been extensively studied in recent years. Despite this the results are inconclusive and no final reaction mechanism has been agreed. Most workers (18) to (30) have found three main types of ZDDP degradation product:

i) olefins. ii) sulphur compounds - mercaptans, sulphides, disulphide, hydrogen sulphide, iii) polymeric residue.

Earlier workers (18) to (21) generally failed to identify some of the more reactive sulphur compounds, especially the disulphides. This has been attributed to the analytical techniques chosen and the operating conditions used (22). From the degradation products formed a wide range of contradictory mechanisms have been proposed. Most authors have preferred an ionic mechanism, though some have suggested radical or radical/ionic processes. A short review is presented below, outlining the most important contributions to the literature.

Feng (18) published one of the first papers detailing a mechanism of thermal decomposition. He identified two major volatile products, hydrogen sulphide and olefins,and also a solid decomposition residue. A two stage reaction mechanism was proposed to explain this result. The first step was formation of an olefin by an intramolecular $-elimination reaction. Conversion of thiono (P=S) to thiolo (P=0) then occurs. The second stage is liberation of hydrogen sulphide and formation of a thioanhydride. Formation of a polymer with structure (I) was also postulated. 10

0 0 II II -P-S-Zn-S-P-S- n OR OR

Feng failed to observe any disulphide products possibly due to their high boiling points. These were probably retained in the residue, giving an abnormally high sulphur content when analysed. Gallapoulos (19) in an investigation of MDDP mineral oil blends concluded that thermal stability is determined by: a) Nature of the metal atom. b) Nature of the base oil. c) Presence of basic barium sulphonate.

He also found that the thermal stabilities of the three MDDPs studied did not correlate with their antiscuff performance. In a discussion of Feng's mechanism, Gallopoulos concluded that thermal decomposition data of ZDDPs was not always consistent with general considerations of 8-elimination reactions. As both unspecified and impure salts and mineral oils were used only very general conclusions can be drawn on the relationship between thermal stability and antiscuff performance. Hanneman and Porter (20) studied the pyrolysis of a number of isomeric amyl alcohol ZDDPs. Volatile products were analysed by gas-liquid chromatography (GLC). No sulphur products were identified and isomeric olefins were found to be the major constituents. It was concluded that the results were consistent with an E^ unimolecular elimination reaction. Initial formation of the carbonium ion is followed by elimination of a proton from the B carbon atom. Rearrangement could then occur to 11 form a more stable carbonium ion, thus providing an explanation for the spread of olefin isomers identified. There was also evidence that the reaction was acid catalysed which would support the above mechanism. Early studies (20) (21)where analysis of volatile products was by gas-liquid chromotography generally failed to identify sulphur decomposition products especially the disulphides. This is probably due to the low column temperatures necessary for the resolution of olefin isomers (22). Ashford and co-workers (23) also studied volatile decomposition products using GLC. A sample of purified zinc di-(4-methyl pentyl-2) dithiophosphate was decomposed in glass between 130°C and 190°C. Less volatile products were solvent extracted from the residue, and i the evolved products analysed by GLC and Mass Spectroscopy. Products identified include olefins, thiols and sulphides (mono, di and tri). An overall reaction mechanism was postulated which involved the elimination of both carbonium ions and free radicals (Figure 2.2). The first stage is the thermal isomerisation of the thiono form (I) to the thiolo form (II). The induction period observed at decomposition temperature below 185°C is possibly due to this reaction, it ends when the concentration of acidic products is sufficient to initiate acid—catalysed decomposition of the zinc salt by route A. A carbonium ion mechanism accounts for this decomposition possibly by elimination of both alkyl groups. Further decomposition reactions of (II) are possible, route C involves homolytic scisson of the P-S band to produce RS*radicals. This accounts for the formation of di and trisulphides. Analysis of the residue indicated a complex structure possibly of the form III. Luther and co-workers (24) investigated the thermal decomposition of zinc di-n-butyldithiophosphate and zinc di-iso-butyldithiophosphate using thermogravimetric analysis. Different product ratios were obtained for each ZDDP. The n-butyl salt decomposed to form predominantly di-n-butyl sulphide, with smaller quantities of n-butyl RO O P — S — Zn — + R H H" H2S RO Zn — 0' m

RSH RH -+- RSH 1

o s v R S + 1 % /SR 2 P / \ / \ RO S Zn "0 S — zn — E

0 RSSR RO^ NS — Zn + RS < " SH

R RO^ ^S— Zn

Figure 2.2 ZDDP Thermal Decomposition Mechanism from Ashford et al(23) 13

mercaptan and little C^ olefins and hydrogen sulphide. The isobutyl salt formed isobutyl mercaptans and C^ olefins as the main products. The quantities of hydrogen sulphide and diisobutyl sulphide were much smaller. A radical mechanism was proposed on the basis of these results.

OR -Zn-S-P OR

0 S-R / -Zn-S-P \ 0-R

0 S-R / -Zn-S-P + • R \ 0

0 -Zn-S-P + • SR 0

r S R- + • S-R ^ 2

RS- + H' RH

RSH ^Olefin + H2S

FIGURE 2.3 Mechanism of ZDDP Thermal Degradation from Luther et al (24)

Ashford (23) considered the last reaction unlikely since it requires rather high temperatures (250°-450°C). In a later paper Luther and co-workers (25) (26) studied the thermal decomposition of zinc di-isopropyldithiophosphate in hydrocarbon oils. At temperatures below 173°C a reaction of first or second order was found to precede the main reaction which was also of zero order. A mechanism was proposed to explain the observed results, the initial reaction being the hydrolysis of the ZDDP to a phosphorodithioic acid and basic zinc dithiophosphate. 14

ZDDP decomposition is catalysed by thiophosphoric acid generated by a hydrolysis reaction (equation 1)

[(RO)2PS2]2 Zn + H20 [(RO)2PS2] ZnOH + (RO)gPS2H

(1)

A mechanism was proposed to explain the formation of observed products (alkenes, mercaptans and dialkyl sulphides) A polymer degradation product with the structure (I) was also suggested.

Zn Zn

S S I

RO-P-O-P-OR II II S 0

Dickert and Rowe (27) studied the thermal decomposition of a series of purified MDDP's to determine the effect of both the cation and nature of the alkyl group on the rate of thermal decomposition. Decomposition was in glass at 155°C, analysis of products was by mass spectroscopy, IR and/or vapour phase chromatography. Major decomposition products identified included olefins, mercaptans, hydrogen sulphide and a polymeric residue. (III). They concluded that the reaction rate is dependent on: a) size of the metal cation; rate of decomposition increases with decreasing metal cation size. b) structure of alkyl group; rate of decomposition increases with increasing number of hydrogens on the 3 carbon atom.

A mechanism involving an isomerisation followed by intramolecular elimination of an olefin was postulated to explain the observed results (Figure 2.4). The initial isomerisation (A) is considered to be responsible for the observed induction period and is thought to be acid catalysed. Formation of the olefin (R) is by RO S — Zn- RO^ ^ S—Zn- ro v ^s—Zn- 4- r P / \ RO RS 0 / \ s oh

RO .S —Zn- RO S—Zn- RO ^ S—Zn - H

RSH + | P+ p ^ p / \

0 RS 0 S 0"

n + i RO P— O P — OR 2 RSH R-S-R -f H9S II II s 0

FIGURE 2.4 Mechanism of ZDDP Thermal Decomposition from Dickert and Rowe (27) 16

an intramolecular (cis) elimination and not by a carbonium ion mechanism of earlier workers (20) (23). Formation of olefin isomers previously cited as evidence for a carbonium ion mechanism is considered to be due to olefin isomerisation catalysed by acids at the high temperature used.

Brazier and Elliot (28) in their study stressed the importance of the purity of the compounds used. They investigated the thermal degradation of both pure and commercial grade ZDDPs using thermogravimetric analysis. Their results indicated that ZDDPs broke down in two distinct stages:

a) At relatively low temperatures products being alkene and mercaptan. b) At a higher temperature, hydrogen sulphide, sulphide and disulphides formed.

A result which supports the order of product evolution found by Dickert and Rowe (27) to be:

1) Alkene 2) Mercaptan 3) Hydrogen sulphide.

A mechanism was outlined to explain observed results (Figure 2.5). The reaction was found to be autocatalytic. Thermal stability of the zinc salts was found to improve when dithiophosphoric acid impurities were removed. RO S

H P r'h / \ / \ RO S Zn RO s Zn

0 o SR RSH P—S—Zn P / \ o o s Zn

2 RSH RSR -+- H2S

FIGURE 2.5 Mechanism of ZDDP Thermal Decomposition from Brazier and Elliott (28 ) 18

The paper recognised that the formation of disulphides was not explained by the above mechanism. This was considered to be more complex involving the formation of both ionic and radical species (RS-). Coy and Jones (29) (30) have recently published a comprehensive study of ZDDP oil soluble degradation products using 'H and 31P nuclear magnetic resonance spectroscopy. In contrast to earlier workers who concentrated on sulphur degradation products, Coy and Jones by following the fate of phosphorus, identified several previously undetected reaction intermediates and decomposition products. Three ZDDP's were studied, di-n, iso and sec butyl salts all as 25% w/w solutions in mineral white oil. The rate of thermal degradation was found to depend on the structure of the alkyl group, concentration and temperature. A mechanism based on the HSAB (hard and soft acids and bases) (31) concept was proposed to explain the observed products and intermediates. The thiophosphoryl group is considered to be typical soft base and as such reacts preferentially with soft acids. Thus:

-P=S + R-X -P-S-R + X"

occurs in preference to:

-P=0 + R-X + -P-O-R + X~

The first stage in the decomposition can be summarised as a transfer of alkyl groups from oxygen to sulphur atoms (P-O-R to P-S-R). This isomerisation is a two stage process, involving attack by the incoming nucleophile on the a-carbon atom of the alkyl group. Earlier workers (27) have suggested that this process is acid catalysed. The relative thermal stability of branched and long chain ZDDPs observed by previous workers (27) is thus explained as due to steric inhibition of nucleophilic attack at the a-carbon atom. 19

H H H n butyl P-O-C-C-C-H nu (little steric hindrance) H H H

iso butyl P-0-CH2-C-H nu (considerable steric hindrance)

Rate of thermal decomposition n butyl > iso butyl. Increasing the number of hydrogen atoms on the 3 carbon atom facilitates nucleophile attack. The rate determining step in the decomposition was considered to occur early in the reaction sequence. After an initial induction period the reaction was observed to proceed autocatalytically even at room temperature. The above isomerisation could therefore be the rate determining step. Decomposition products identified include, S,S,S-trialkyl tetrathiophosphate (SPSR)^, disulphides, sulphides, mercaptans and an insoluble white precipitate. Elemental analysis of this precipate gave atomic ratios for Zn:P:S of 1:1.5:0.7 (Structure, Figure 2.6). Spedding and Watkins (32) have recently published a study of the thermal decomposition of ZDDPs both as bulk and in oil solution at 160°C and 200°C under nitrogen. A variety of analytical techniques were used to characterise the products, which were predominantly a mixture of alkyl sulphides and a zinc polyphosphate. A hydrolytic mechanism was proposed to explain the observed results. Removal of water from the system suppressed the decomposition. 20

Alkyl Group Transfer

r— 0 s" 0 sr \ / \ / lst Nu p P Alkyl Group

/ \ / ^ r—0 s r-0 s

0 sr " o y sr \ / / 2nd P Alkyl Group

/ \ r-0 s o sr

Formation of Products

0 0 II II rs-p — 9r + sr rs —p —o 4- r0s

Polymeric Pi,esidue

0 0 0 0 II . . II . . II ll o— p— 0 -+- 0 — p —-0 —p—o—p—0

sr sr 0 sr -t-

~sr

eventually forming a precipitate of type

0 0 0 . II II II 0 p 0 p — 0 p —sr I. I I "o/sr -0 sr

FIGURE 2.6 Mechanism of ZDDP Thermal Decomposition from Jones and Coy (30) 21

2.2.2 Discussion

It will be clear from the preceding survey that there is a wide disparity between authors both in results and in interpretation. This is hardly surprising since different authors have rarely studied the same ZDDPs and have usually employed different temperatures, purities, carrier oils and other experimental conditions. In view of the wide variation in results it is possible to draw only general conclusions.

a) Products

i) Most authors agree on the type of decomposition products formed:

Olefins (27) (23) (20) (18) Hydrogen sulphide (28) (24) (18) (27) Sulphides (24) (23) (32) (28) (29) Disulphides (29) (23) (32) (28) Mercaptans (28) (29) (27) (23) S,S,S Trialkyltetrathiophosphates (29) Polymeric residue (29) (18) (23) (32) (27)

Several sets of workers have failed to identify certain sulphur decomposition products. This may be due to their high boiling points which are commonly in excess of experimental operating temperatures, so that those sulphur compounds could be retained in the residue. This would also cause anomolous results in the elemental anlaysis of the residue.

ii) Composition of Polymer Residue

Most investigators report the formation of a white polymeric residue. No satisfactory chemical analysis of this product has been reported, possibly due to variable composition and experimental difficulties in isolating a sufficiently pure sample. In some work high sulphur content has led workers to 22

postulate a structure in which the Zn-S-P bonds are unbroken, but a more likely explanation is the presence of other low volatility sulphur containing products. The suggestion that this residue forms an insitu polymer film on metal surfaces is inconsistent with the results of surface analysis where atomic ratios for Zn:S:P are found to be relatively low in sulphur indicating a breakdown of the Zn-S-P bonds. The polymer structure suggested by Coy and Jones (29)' (30), and Watkins and Spedding (32) consists of a P-O-P backbone (below).

0 0 0 II II -P-0-P-0-P-

0- 0- SR n

Zn-S-P ratios for this type of polymer are in general agreement with the results of surface analysis (46). Infrared analysis by the author (5.7) of bulk precipitate formed during testing is in agreement with the above structure. b) Effects of Structure on Thermal Stability

i) Metal Cation

Thermal stabilities of MDDPs have been shown to increase as the size of the metal cation increases, in the order

Cu(II) < Ag(I) < Zn(II) < Cd(II) Pb(II (27)

increasing thermal stability

This may be due to increasing covalency of the M-S bond reducing the ease of formation of the

R0 S

P 23

species in solution. In Coy and Jones (30) mechanism it is the bimolecular nucleophilic attack of this anion that is possible the rate determining step.

ii) Alkyl Group

Thermal stability generally increases as alkyl chain length increases and also as the number of g hydrogens in the alkyl group decreases (27) (30).

a 3

P-0-CH2-CH2-CH2

It has been postulated that this is due to steric hindrance of nucleophilic attack at a carbon atom by long and branched chain alkyl groups (30). c) Mechanisms

Various reaction schemes have been proposed based on ionic (27) (28), free radical (24) or hydolytic mechanisms (26) (32). Several workers (73) (30) have postulated that the first stage is a two step isomerisation involving alkyl transfer from oxygen to sulphur atoms. The induction period observed by many workers is possibly due to this rearrangement which is also acid catalysed. From the evidence reviewed therefore no general principle can be drawn, though the thermal decomposition of a wide range of ZDDPs both pure and commercial have been investigated under a variety of experimental conditions. It is thought that at low temperatures an ionic mechanism involving double alkyl group migration predominates. This is initiated by hydrolysis of the neutral ZDDP to form a dialkylphosphorodithioic acid which catalyses the reaction (26). After concentration of the acidic species reaches a critical level the reaction is autocatalytic. The rate determining step occurs early in the reaction mechanism, possibly during the alkyl migration, and explains observed induction periods. At higher temperatures it is possible that an alternative mechanism may predominate, since two different mechanisms have been observed depending on the degradation temperature (26). 24

2.3 Mechanisms of Anti-oxidant Action

2.3.1 Literature Review

It is generally accepted that auto-oxidation of hydrocarbons in the liquid phase occurs by a free radical process which involves the formation of thehydroperoxide (ROOH) as a primary oxidation product. The important chain carrying species are the peroxy (RO*^) and hydrocarbon (R») radicals. The following mechanism can be assumed for the low temperature liquid phase oxidation of hydrocarbon (33).

Initiation rh - R • (1)

Propagation r- + ro - (2) °2 ^ 2

ro2. + RH + rooh + R (3) rooh + RO • + • oh (4)

Termination ro • + RO + roor + 0, (5) 2 2 Formation of ro* + RH + roh + r• (6)

Products •oh + RH h2o + r- (7) rooh + RH 2 roh (8) + rooh r1cor2 h20 (9)

r1cor2 acids (10)

FIGURE 2.7 Reaction Mechanism of Hydrocarbon Auto- Oxidation in the Liquid Phase

The breakdown of hydroperoxides (equation 4) during oxidation leads to the formation of new radicals and is responsible for the autocatalytic nature of the process. The main products are alcohols or ketones which are further oxidised to form acids (33). Two main types of anti-oxidant are recognised:

A. Those that work by breaking the propagating chain by reaction with peroxy radicals to form non-radical products (radical inhibitors). 25

B. Those that work by preventing the build up of hydroperoxide which can initiate further oxidation (peroxide decomposers).

ZDDPs are known to function by both the above mechanisms.

2.3.1.1 Mechanism of ZDDP Chain Breaking

The ability of ZDDPs to act as radical inhibitors was first demonstrated by Colcough and Cuneen (34) in 1964. They investigated the extent to which three dithioates prevented the oxidation of squalene. It was suggested that electron transfer from a sulphur atom to a peroxy radical was responsible for chain inhibition. Burns and co-workers (35) independently confirmed this result in a later paper in 1965. Correlation of chemical structure and anti-oxidant ability indicated that both sulphur and metal atoms were required in a molecule for it to exhibit chain-breaking properties. The only decomposition product identified was disulphide, although other workers (36) have identified solid polymeric residues. Two possible reaction mechanisms were suggested. These are shown in Figure 2.8. Electron transfer from a sulphur atom to a peroxy radical in mechanism I leads to the intermediate formation of a free dithiophosphate radical (A). This was considered unlikely as this species could also propagate a peroxy radical chain. An alternative mechanism (II), also proposed an electron transfer forming a stabilised intermediate (B). Attack of a second peroxy radical leads to dimerisation of dithiophosphate radicals before peroxy propagation can occur.

There is however little supporting evidence for either mechanism as few ZDDP-peroxy products have been identified. sv .or ro s ro a s ^ / ^ P \ ^ / + POr' r ROJ + / \ p ; . 4- zn—s or P /.V ro' S Zn /P RO \S4-z. RO

ro ^s s or

n ro. S / /P\ Disulphide

O-QR ro-o ro s —rs or I ro s co ctj

P ^ Disulphide X \ ro S 4Zn S+Zn +- Ror + Zn

FIGURE 2.8 Mechanism of ZDDP Chainbreaking from Burns et al (35) 27

2.3.1.2 Hydroperoxide Decomposition by ZDDPs

The ability of ZDDPs to act as catalytic peroxide decomposers was first recognised by Kennerly and Patterson (37), although it is only recently that detailed investigation into the mechanism of the process has been completed. Reaction with peroxides has been easier to study than that with peroxy radicals, due to the availability of stable peroxide reagents such as cumene hydroperoxide. Burns et al (38) studied the decomposition of cumene hydroperoxide in the presence of zinc di-iso-propyldithiophosphate at 70°C. They concluded that the reaction occurs in three stages.

a) An initial fast auto-inhibitive stage (Area I). b) A slow stage which they considered to be an induction period (II). c) A final fast stage (III) following approximately first order kinetics.

These reaction profiles are shown diagrammatically in Figure 2.9. Burns and his fellow workers found that the onset of reaction stage III identified as being responsible for the catalytic decomposition previously reported by Kennerly et al (37) (Figure 2.9) was determined by:

a) Concentration, high initial concentrations increased the induction period. An optimum concentration of _ 5 ^2.5 x 10 M gave no appreciable induction period (curves A, B and C). b) Presence of radical traps, such a Toponol O increased the induction period (curve D). c) Presence of oxygen. Long induction periods were observed for reactions carried out under nitrogen or in sealed vacuum flasks.

A number of non-phosophorus products were identified, relative yields of each being dependent on the initial concentration of ZDDP used. 28

In chlorobenzene at 70°C under oxygen

Time

Zinc di-isopropyl dithiophosphate A 2.5 x 10~5M B 2.5 x 10"4M C 2.5 x 10~3M D 2.5 x 10"5M + Topanol 0 0.5 x 10_2M

FIGURE 2.9 Reaction Profiles for the Decomposition of Cumene Hydroperoxide in the Presence of ZDDP from Burns et al (38) 29

Three organophosphorus products were isolated: a) Disulphide formed during the first reaction stage. b) White precipitate which formed during the first and second stage (B). c) A final product isolated as a white powder with an

empirical formula of ZnO.ZnSO^.6H20.

A mechanism was proposed for the first stage to explain the simultaneous formation of the disulphide and an alcohol. it was suggested that electron transfer occurs to form disulphide and an alcohol. No explanation is offered for the auto-inhibitive effect of this reaction, although it is suggested that other reactions also occur during this first stage. The onset of the third stage is controlled by the formation of an unspecified catalyst which is considered to be a product or reactive intermediate derived from the ZDDP. The effect of free radical inhibitors such as Toponol 0 indicates that this catalyst is a free radical or derived via a free radical intermediate. The addition of zinc oxide or the unidentified white powder (B) was found to completely inhibit the third stage. Ross and Impartio (39) in a later paper also suggested that free radical inhibitors prevented the formation of a Lewis acid type catalyst which is responsible for the hetrolytic decomposition of ROOH. They identified the insoluble product (B) as the basic ZDDP, the overall equation for the formation of which is given below.

[(RO)2P(S)S]2 Zn + ROOH + [(RO)2P(S)S]6 Zno + Basic ZDDP

[(RO)2P(S)S] 2 + ROH Disulphide Alcohol 30

Bridgewater and co-workers (40) have recently published a detailed study of the kinetics of ZDDP decomposition of hydroperoxide. They assumed that both ionic and free radical decomposition of the hydroperoxide is possible (Figure 2.10).

0 II

ROOH + ROH + R-C-R

0 RO- R-C-C- (a)

H+ I L roh 5 r-c=ch2 (B) (C)

FIGURE 2.10 Ionic Verses Free Radical Hydroperoxide Decomposition (40)

In this study the reaction profile was simplified by using a large excess of hydroperoxide at relatively high temperatures (^100°C). Several reaction products were identified, including acetophenone (A), 2-phenyl propanol (B) and 2-phenyl propene (C). It is assumed that the promoter is responsible for the rapid decomposition of hydroperoxide observed in the third stage. They investigated a number of possible compounds which could be derived from ZDDPs at the temperatures used. These include the corresponding basic ZDDP (I), disulphide (II) and dialkylphosphorodithioic acid (III).

[(r0)2ps2]6zn40 [(ro)2ps2i2

(I) (ii)

(ro)2ps2h

(III) 31

They concluded the following:

(1) Product (A) (Figure 2.10) is formed by a free radical mechanism that is independent of the promoter used. This is supported by examination of product yields of acetophenone obtained by Buns et al (38). (2) Products (B) and (C) (Figure 2.10) are formed from cumene hydroperoxide by ionic decomposition which takes place via a catonic chain reaction. (3) The catalyst formed from ZDDPs for this reaction is the dialkylphosphorodithioic acid.

A mechanism was therefore proposed for the ionic decomposition of hydroperoxide giving as products alcohols, olefins, water and hydrogen peroxide. The "promoter" a dialkylphosphorodithioic acid is thought to be formed by the hydrolysis of neutral ZDDP (26).

[(RO)2P(S)S]2 Zn + H20 - [(RO) P(S)S] ZnOH + (RO)2P(S)SH

The reaction is reported to be second order with respect to the hydroperoxide when it is promoted by neutral (I) and basic ZDDP (II) but first order when promoted by the disulphide (III). Bridgewater therefore proposed that the first stage in the decomposition of a ZDDP is the hydrolysis of neutral ZDDP to form the phosphorodithioic acid.

[(RO)2PS2] Zn + H20 * [(RO)2PS2] ZnOH + (RO)2PS2H

(1)

Formation of the acid removes water from the system and explains the second order kinetics. It is also applicable to the basic salts (II) because at high temperatures these are unstable and decompose to corresponding neutral ZDDP and zinc oxide. 32

The first order kinetics of disulphide promoted hydroperoxide decomposition is explained by equations 2 and 3.

[(RO)2PS2J2 2(RO)2PS2* (2)

disulphide

(RO)2PS2- + ROOH + (RO)2PS2H + RC>2 • (3)

dialkylphosphorodithioic acid

The acid (III) is then removed from the system by the oxidation reaction (4).

2(RO)2 PS2H + ROOH + [(RO)2PS2]2 + ROH + HgO (4)

A cyclic reaction scheme is established generating water and a steady state concentration of acid.

2.3.2 Discussion

Taking into account all these observations, the following tentative reaction scheme is proposed by the author, this is summarised below (Figure 2.11). The effect of increased initial ZDDP concentration on the rate of hydroperoxide decomposition (Figure 2.9) suggests -5 -4 that there is a critical concentration (2.5 x 10 - 2.5 x 10 m) above which accumulation of the catalyst which induces the third stage of the reaction, does not occur. A mechanism for the first stage has been suggested (38). Two possible products from this, ZnO and basic ZDDP have been found to inhibit the third stage. It is suggested therefore that a high initial ZDDP concentration causes auto-inhibition by accumulation of insoluble basic products which inhibit acid formation. (This would, incidentally ZDDP -boo' (RQ)2 Ps2 + KETONE + ZnO

H, 0

(R0)2P(S).SH -BQQb (RQ)2PS$J TQPONQL O ^ non radical products CO CO B ROOH

A: free radical decomposition

B: cationlc decomposition H20 + ROH

FIGURE 2.11 Summary of ZDDP Antioxidant Reaction Mechanisms 34

also imply that basic ZDDP ought to be avoided in commercial ZDDPs if effective anti-oxidation is desired).

2.4 Relationship between Antiwear and Anti-oxidant Reactions in Solution

Most studies of ZDDP reactions in hydrocarbon solution have considered antiwear and anti-oxidant effects seperately. Little is known of the relative importance of these reactions during lubrication. Coy and Jones (41) have recently published a study of the degradation of ZDDPs during testing in an Amsler wear test machine. They have concluded that degradation processes are mainly oxidative. The main products identified include disulphide and a basic zinc compound. Willermet and co-workers (59) in a series of papers have studied the anti-oxidant reactions of ZDDPs and their effect on wear rates. They concluded that the products derived from ZDDP anti-oxidant reactions do not show anti-wear activity. This is contrary to earlier findings (57) in which the disulphide a major product of antioxidant reaction has been shown to have AW/EP properties.

2.5 Composition of Surface Films

The idea that ZDDPs reduce wear by forming protective films on rubbing metal surfaces was first suggested over twenty years ago (42) (43). The identification of these films is of obvious importance in determining the mechanism of AW action, and has been extensively investigated, although little agreement has been reached. Attempts to correlate film formation with load carrying efficiency have also been made (44). The analysis of chemical films on roughened surfaces produced during wear tests, poses many experimental problems. Some workers therefore have studied films formed during static immersion tests. The relevance of these films to those formed under rubbing conditions is questionable, although some 35

workers (45) have concluded that broadly similar films are formed in both static and dynamic tests. Early workers (42) (44) used radio trace techniques to Yv study the distribution of sulphur and phosphorus in films formed by labelled ZDDPs. Loeser and co-workers (44) studied both rubbed films on cast iron cams and tappets and static films formed by immersion of metal specimens in test solutions at 80-150°C. No definite compounds were identified in the surface films, although zinc and phosphorus content increased more rapidly than sulphur with increased temperature and pressure. Localised chemical films occured in areas of high load and temperature, with a relatively high sulphur content. 32 Furey (42) in a study of a P labelled ZDDP demonstrated that a strongly adherent film is found on rubbing valve-train 32 metal surfaces. The greater P activity was concentrated on hihgly stressed areas of the valve-train mechanism. It was concluded that AW action involves formation of a phosphorus containing solid lubricant film by chemical reaction with the rubbing surfaces. Bird and Galvin (45) applied photo-electron spectroscopy to the study of both static and dynamic films formed by ZDDPs. They concluded that both ZDDPs (a commercial and a pure sample) gave broadly similar films in both tests. Differences were observed in the thickness of films formed in the immersion tests between the pure and commercial additives. This was attributed mainly to the differences in thermal stability of the two additives, rather than the presence of impurities in the commercial ZDDP, although the presence of acidic species in the commercial additive would be expected to decrease thermal stability. The composition of the films was variable indicating a polymeric structure containing zinc, phosphorus and some sulphur of the original additive. The presence of several compounds suggested by other workers, eg zinc phosphide, zinc phosphate, zinc sulphide and iron phosphate, was discounted. In a later paper Bird and co-workers (46) examined surface films found by n-butyl and s-butyl zinc salts during wear testing. A modified Amsler wear test machine was used 36

to prepare films of a large surface area. By holding the coefficeint of friction constant throughout the test (^0.07y) thick uniform films were produced. Analysis of the films was by X-ray photo-electron spectroscopy and electron probe micro-analysis. Two types of film were formed by both additives, a thiophosphate film and a ferrous sulphide film. High wear score marks were found to accompany the sulphide films. Element compositions for the two types of film are given below: Zn P S Thiophosphate film 1.0 : 1 : 0.2 Score mark 1.0 : 1 : 0.3

X-ray photo-electron spectroscopy was also used by Baldwin (47) to characterise wear surfaces. Only sulphur was identified, the conclusions were that the active AW species is a metallic sulphide. It was also suggested that polar additives decrease the AW efficiency of organosulphur additives (16). Debies and Johnson (53) applied scanning Auger microprobe analysis to determine spatial distribution of species formed in wear scars. Zinc di-iso-decyl dithiophosphate was tested as 5% additive by weight concentration in a pure mineral oil. Zinc, sulphur and phosphorus were identified in the Auger spectrum. Sulphur and zinc Auger images were found to coincide indicating a zinc/sulphur species possibly Zn-S. These areas were associated with least scoring. Phosphorus was not detected in these areas but was widely distributed with no evidence of segregation. A combination of analytical techniques was applied by Coy and Quinn (49) to the examination of surface films formed on EN31 steel during wear tests. Films formed under both EP and AW conditions were studied. In the AW region films with atomic ratios Zn:P:S of 1:5:1.4 were observed, a polymer type film similar to that proposed by Feng (50) was suggested. In the EP region a relatively high percentage of sulphur was found in the surface of the scars compared to the AW results. EP activity was therefore associated with the 37

formation of an iron sulphide film. A combination of technique was also applied by Francis and Ellison (51) who used X-ray fluorescence and infrared reflection-absorption spectroscopy to characterise surface films formed by ZDDPs on metal mirrors. Two types of film were examined, static films formed by immersion in hydrocarbon solution, and those formed by rubbing. Both basic and neutral zinc salts were studied. A number of components in the films were identified, these included zinc sulphate, zinc oxide and adsorbed neutral ZDDP. Under oxidising conditions a further component possibly thiophosphate was also observed. Sulphate formation was also facilitated by the presence of hydroperoxide, or at high temperatures where oxidation of the hydrocarbon would occur. It was therefore suggested that sulphate is produced by the oxidation of sulphur in the ZDDP molecule in the presence of peroxides. Analysis of films formed by rubbing indicated a further component characterised by a single sharp IR absorption band at 9.6 microns, tentatively identified as a metal phosphate. In a recent paper Georges et al (52) in their investigation of scuffing wear examined both static and rubbed films using Auger electron spectroscopy and secondary ion mass spectroscopy. Two types of adherent film were formed; a brown mutlilayer film, which was gradually covered by a blue film. The brown film was composed of adsorbed ZDDP and hydrocarbons on a thick layer composed of phosphorus, sulphur, oxygen and zinc, approximately 1000 monolayers thick. The composition of which is thought to be polymers of the type previously discussed (23)-(30) or mixed compounds such as sulphides, sulphates and phosphates. The blue film which forms on top of the brown film consists essentially of iron oxide. The wear rate was found to decrease significantly on formation of these interface films. Several authors (45) (49) (52) have suggested that ZDDPs form films similar in composition to the inorganic residue found during thermal decomposition. Feng et al (50) postulated that solid thick film could be deposited in situ on rubbing metal surfaces. Four mechanisms are proposed in 38

which the metal surface is not involved in the reaction, these are:

1) Polymerisation to form large molecules on the metal surface. 2) Chemical combination of two or more molecules to form a solid surface layer. 3) Isomerisation of a molecular structure on the surface to form a new structure with better anti-wear properties. 4) Decomposition of a molecule to form new products which can be deposited as a lubricant film.

Feng considered that ZDDPs found polymer films of the^ type

0 0 II II -P-S-Zn-S-P-S- I I n _ or or .

The atomic ratios Zn:P:S of a polymer of this type is 1:2:3, which is higher in sulphur than those found in practice. Coy and Jones (30 ) in their study of the thermal degradation of ZDDPs suggest a polymer with the structure

~ 0 0 0

-O-P-O-P-O-P-O-

0- sr sr

Zn:P:S ratios for this polymer were found to be 1:1.0:0.6 which is in more general agreement with AW surface films found by other workers (45) (46). Although this area of ZDDP research has been extensively investigated there is still no positive identification of any particular chemical species as providing AW properties. A number of general conclusions have however been drawn from the literature review.

1) The type of film formed by ZDDPs is not the same for all test conditions but is dependent on temperature, load and running time. 39

2) Films found in the antiwear region generally have a relatively high phosphorus content and possibly have a layered polymer structure. 3) Films associated with the EP region have a relatively high sulphur content usually identified as iron/zinc sulphide.

2.6 Load-Carrying Properties of Metal Dithiophosphates

Load-carrying additives can be divided into two types, anti-wear and extreme pressure.* ZDDPs are usually considered to be mainly AW additves with moderate EP activity (48). The load-carrying properties of MDDPs have been evaluated in both engine (54) and laboratory bench tests (55)-(58). Those most commonly used include four-ball, Timken, Falex and pin-on-disc devices. In studies of wear in an internal combustion engine Larson (54) suggests that ZDDPs reduce surface wear caused by rubbing of mating parts, such as occurs in valve train combination, oil pumps etc. Larson showed that a ZDDP derived form a mixture of x two 2° alcohols gave better AW results of two similar alkyl chains were attached to the same P atom, ie

R10 S S 0R2

// X p r * r2 / \ /\ R1 S-Zn-S 0R2

Anti-oxidant effectiveness was also demonstrated during Chevrolet L-4 and Lawson engine tests. These results supported the conclusion that anti-oxidant effectiveness was dependent on ease of thermal decomposition, and that primary alkyl ZDDPs are less effective in preventing oil oxidation than those having secondary alkyl groups. Correlation of antiscuff performance with thermal 40

stablility has been demonstrated by a number of workers (55) (57) in both engine and bench tests. Studies of ZDDP load-carrying performance have been published for a variety of laboratory test devices (55)—(57). Unfortunately little systematic investigation of pure ZDDPs has been published. In many cases the chemicals studied contain substantial amounts of other additives (eg, Barium sulphonate) or are unspecified. Rowe and Dickert (55) published one of the first studies to use pure metal dithiophosphates in their investigation of load-carrying performance on a pin-on-disc machine. They concluded that the AW effectiveness of a series of metal di-iso-propyl dithiophosphate increase in the order Ag(I) < Pb(II) < Cd(II) Zn(II) Cu(II) This, with the exception of silver is the order of decreasing thermal stability and decreasing ionic radius o-f theYmetal. Also that wear rates decreased as the number of hydrogens on the 6 carbon atom increases and therefore as the thermal stability decreases. Most tests have been unable to differentiate between 1° and 2° alkyl chains. Jayne and Elliott (56) used four ball and Timken OK load test in the evaluation of ZDDP antiscuff/action performance. From comparison of these results those from earlier work they concluded that:

1) ZDDPs prepared from secondary alcohols should be better than those derived from primary alcohols of the same carbon number. 2) Zinc dialkyl/diaryl and zinc diaryl phosphorodithioates should be worse than the dialkyl phosphorodithioates derived from both primary and secondary alcohols.

Allum and Forbes (57) studied both EP and AW properties of a series of MDDPs using the four ball machine. They concluded that the nature of the metal atom is a major factor in determining these properties. The order of decreasing EP activity was found to be

BI (III) _> Ag(I), Pb(II) > Sb (III), Sn(II) _> Cd(II)

Fe(III) Ni(II) > Zn(II) 41

The order of decreasing AW activity is

Cd(II), Zn(II) Ni(II) _> Fe(III) > Ag(I) > Pb(II) >

Sb(III), Sn(II) > Bi(III)

They conclude that the factors determining EP and AW properties were different. Two series of salts were studied 4-methyl pentyl-2 and n-hexyl derivatives. Although the nature of the alkyl group was found to have little effect in determining L-C properties. Coy and Jones (29) in their study of ZDDP thermal degradation also investigated the EP performance of the additives. Both heated oil samples and synthesised degradation products were tested on a four ball machine. They concluded that although the rates of thermal decomposition of the three additives were very different, the four ball results failed to differentiate between them.

s-butyl >> n-butyl > iso-butyl decreasing rate of thermal decomposition

The EP performance of the heated oils remained high due to the thermal degradation products having similar EP properties to the original ZDDPs. Many workers (55)-(57) have tried to show that the load carrying propeties of ZDDPs are related to their thermal stabilities. Generally ZDDP thermal stabilities increase as the number of 6-hydrogen on the alkyl group decreases and as the length of the alkyl chain increases (27) (30). This has been explained as being due to increased steric hindrance to nucleophilic attack at the a-carbon atom (30). Also that thermal stability increases along the series (27)

Cu(II) < Ag(I) < Zn(II) < Cd(II) Pb(II)

From these results and those from L-C tests the following general conclusions can be drawn. 42

1) Nature of the metal atom plays an important role in determining load-carrying properties. Possibly in so far as it also determines thermal stability of the additives. 2) AW effectiveness increases as thermal stability decreases. 3) EP effectiveness increases as thermal stability increases. 4) Little difference in L-C properties has been found between different alkyl groups. Although AW activity increases as the number of B-hydrogen atoms on the alkyl group increases.

2.7 Mechanism of ZDDP L-C Action

Despite extensive research in this area there is still no satisfactory mechanism of ZDDP load-carrying action. Many workers have shown that ZDDP degradation products form strongly adherent films on rubbing metal surfaces, which significantly reduce wear and friction (52). There has been no definite identification of these films, the chemical species responsible for their formation, or a satisfactory explanation for the differences between L-C performance of different MDDPs. Some correlation has been established between AW and EP efficiency and thermal stability (55). The protective surface film is usually considered to be an essentially inorganic reaction product of the metal and AW species. Although other types of film have been suggested, eg formation of an in situ polymer in which the metal is not a reactant (50). More recently mixed inorganic/organic films have been postulated (52) (109), with film thicknesses in excess of those suggested previously (109). Georges and co-workers (52) have identified AW behaviour with two types of adherent film. A brown film composed of ZDDP and hydrocarbons adsorbed on a relatively thick inorganic layer. A second blue film is then deposited on the brown film, it was found to be composed essentially of iron and silicon oxides. The authors concluded that the formation 43

of these adherent AW films was a result of the presence of a paste in the contact area, composed of a colloidal agglomeration of surf ace product s. Heat generated by friction leads to degradation of dithiophosphate in the paste to form the brown film. Watkins (91) in his study of ZDDP AW behaviour concluded that the AW function of ZDDPs is due to the formation of fusible glassy compounds in the surface oxide layer. A polymeric zinc polyphosphate (32) is physically adsorbed on the rubbed areas, this,'due to its relatively low melting point is thought to lubricate by forming a fluid glass. Iron sulphide was also identified in the surface layer, formed via a reaction between the oxide layer and ZDDP degradation products (sulphides). The iron sulphide is considered to form a ternary eutectic with iron oxide, which could form a fluid surface film under extreme conditions. Several workers (91) (57) (52) have studied the adsorption of ZDDPs on to iron oxide surfaces. Watkins (91) concluded that a low temperatures, ZDDPs do not chemisorb onto iron oxide surfaces. Adsorption of Zn, P and S is only significant at higher temperatures where decomposition of the ZDDP would be expected to occur. This result conflicts with earlier work in which adsorption studies of ZDDPs on iron-on-sepolite at room temperatue suggest that dithiophosphates are strongly chemisorbed on such surfaces (57). The,importance of impurities in determining ZDDP performa- performance has been demonstrated by Barton and co-workers (58). Activation analysis of a sample of zinc di'-n-hexyldithiophosphate showed that it contained a significant amount of polar zinc and phosphorus containing impurities. The results of four ball tests indicated that the active wear reducing species are acid phosphates and/or thioacid phosphates. A mechanism for the AW action of ZDDPs was postulated in which the following sequences occurred.

1) Preferential adsorption of polar impurities on the surface of the metal. 2) Asperity contact producing relatively high temperatures. This induces : 44

3) Chemical reaction of the polar phosphorus containing impurities with the metal to produce a metal phosphate as a protective layer.

From the review of the literature the following conclusions have been drawn:

1) Film formation by ZDDPs is dependent on temperature and contact conditions under which it is formed. At low temperatures adsorption by ZDDPs onto metal surfaces has been reported although the evidence is not conclusive. At ^80°C a chemical reaction between ZDDP and steel occurs to form iron DDP and zinc (52). 2) Two main types of film have been identified with ZDDP wear surfaces. a) AW film characterised by a relatively high phosphorus, low sulphur content, with a polymeric sometimes layered structure. b) EP film with a relatively high sulphur content, usually identified as iron sulphide.

The mechanism of formation of the AW film has not been fully explained, a number have been postulated: physical adsorption of AW species onto the oxide surface (91), chemical reaction of the AW species with the metal surface (57), or an in situ polymerisation process in which the metal is not a reactant (50). The chemical species in solution responsible for L-C film formation has not been positively identified. A review of the thermal degradation results indicates a number of P/S species which themselves could possess L-C properties. These include alkyl sulphides (mono, di, tri), mercaptans, and hydrogen sulphide. Allum and Forbes (57) showed that the disulphide derived from a ZDDP gave a better EP but poorer AW result than the original ZDDP. Indicating that the metal atom is relatively more important in determining AW rather than EP performance. Coy and Jones (29) also demonstrated that induced ZDDP (synthesised) thermal degradation products had EP properties comparable to those of 45 the original ZDDP. Watkins (91) considered that alkyl sulphides formed the iron sulphide layer necessary for AW action. An alternative view is that free HgS reacts with zinc or iron to form sulphides (52). The importance of AO reaction of ZDDPs in determining L-C activity if not fully understood. Some workers have concluded that the products of ZDDP AO reaction show no AW activity. The products from such reactions include disulphides, zinc oxides, alcohols and water, the disulphides would be expected to possess AW properties. The relative importance of AO and AW reactions is a lubricant system is not fully understood, recent results (41) indicate that the AO mechanism predominates. There is evidence indicating that addition of peroxides to ZDDP solutions does suppress thermal degradation reactions (27). It is also possible that peroxides are important in determining AW film formation (51), in which case good AO efficiency of the ZDDP would be expected to be detrimental to the AW action. 46

CHAPTER 3 PREPARATION OF ORGANOPHOSPHORUS ADDITIVES

3.1 Introduction

Almost all the organophosphorus additives used in this work were prepared and purified in the laboratory. One aim of the project was to determine the effects of polar impurities on the AW behaviour of pure ZDDP's. It was therefore essential to prepare and carefully purify all ZDDP's used. A commercial additive, Lubrizol 1395*, was analysed and used as a 'standard' for all pure ZDDP's and other organophosphorus additives tested. Analysis of 1395 had important implications in how pure ZDDP's were prepared in this work, and is detailed in 3.3.3. Several workers (23) (60) have published papers describing the laboratory preparation of ZDDP's. A two stage process is used involving the synthesis of the parent di-alkyldithiophos- phoric acid [(RO)2 P(S)SH] from which the metal salts are prepared.

3.2 Preparation of 0*0 Dialkyl Dithiophosphoric Acid

3.2.1 Experimental

The first stage in the preparation of ZDDP's is the synthesis of the parent acid. Phosphorus pentasulphide is reacted with the parent alcohol to give the dithiophosphoric acid with hydrogen sulphide as a by-product. The stoicheiometric equation is given below.

RO S

+ 8ROH 4 P + 2h2s RO

dialkyl dithiophosphoric acid

supplied by Elf (Aquitaine) 47

This preparation has been described in detail by other workers (60) (28) (56). Most specify addition of solid pentasulphide to alcohol although Ashford (23) reverses this to avoid formation of triesters. The dithiophosphoric acids were prepared in this study by the following method. The appropriate alcohol (2 moles) was slowly added to phosphorus pentasulphide (0.5 moles), whilst the mixture was stirred and cooled in an ice/water bath. Evolved HgS was adsorbed in an alkali trap. The lower molecular weight alcohols gave a violent reaction, and careful cooling and addition over a long period was required to keep the reaction temperature low (<30°C). For the high molecular weight alcohols (butyl, hexyl, n-octyl), a warm water bath was required to initiate the reaction. When alcohol addition was complete the reaction mixture was heated for one hour at 70°C until hydrogen sulphide evolution ceased. Excess phosphorus pentasulphide was then filtered from the acid.

3.2.2 Purification of Dithiophosphoric Acids

The procedure used was essentially the same as Brazier and Elliott (28). An aqueous solution of 0.5 molar sodium hydroxide was slowly added with stirring to a mixture of 0.5 moles dithiophosphoric acid and distilled water, in order to form an ionic sodium dithiophosphate. Addition of alkali was halted when the reaction mixture reached PH5-6. The resulting solution was allowed to cool and then extracted with petroleum ether (twice) and then di-ethyl ether (once) to remove neutral and some acidic impurities. The aqueous layer was then acidified with concentrated hydrochloric acid to liberate the purified dithiophosphoric acid. After cooling the mixture was extracted twice with di-ethyl ether. The combined extracts were washed twice with distilled water and dried over anhydrous magnesium sulphate. Di-ethyl ether was removed prior to use of the dithiophosphoric acid, by evaporation. 48

3.2.3 Analysis of Dithiophosphoric Acids

Previous workers have estimated the purity of dithiophosphoric acids by titration with alcoholic potash (28), elemental analysis (27), infra-red spectroscopy (23) and nuclear magnetic resonance spectroscopy (23). In the present study, the purified acids were examined by IR, a method not particularly sensitive for detecting impurities. The main impurities were considered to be oxidised phosphorus species. Ashford et al (23) has reported an estimated threshold detection level of these impurities to be 0.8% for P=0 at 1280cm"1 and possibly 4% for P-OH at 900-1000cm-1. No absorption bands attributable to P=0 or P-OH were observed in the IR spectra of the prepared acids. (Chapter 4, Figures 4.3 - 4.9).

3.3 Preparation of ZDDP Salts

The preparation of ZDDP's from dithiophosphoric acid can be achieved in two ways.

1. Reaction with metallic zinc or zinc oxide under anhydrous conditions (23) (56). The stoicheiometric equation for this reaction is given below.

2 (RO)2P(S)SH + ZnO [( RO)2P(S)S ]2 Zn + H20

2. Conversion of dithiophosphoric acid into the sodium salt, followed by a metathetic reaction of this salt with an aqueous solution of zinc chloride.

(RO)2P(S)SH + NaOH (RO)2P(S)S Na + H20

2 (RO)2P(S)S Na + ZnCl [(RO)2P(S)S]2 Zn + 2NaCl 2

In both cases the reaction conditions need to be carefully controlled to avoid formation of the basic zinc double salt analogue of the required neutral ZDDP (60). 49

Wystrach et al (60) first prepared and investigated the basic salt. They found that its formation depended on the presence of excess hydroxyl ions. Reaction conditions can be chosen to yield either the neutral zinc salt or basic double salt. The stoicheiometric equations are given below (60).

++ 3(RO)2PSS~ + OH" + 2Zn — .Zn2 [SSP(OR)2 ] OH

Basic Zinc Double Salt

+ + 2(RO)2PSS" + Zn —* [Zn SSP(OR)2]2

Neutral Salt

The basic double salts are characterised by their higher melting points and insolubility in methanol compared to their neutral analogues. ZDDP's were prepared in the laboratory by both reactions described above. The lower molecular weight (

1. Reaction of purified dialkyl dithiophosphoric acid with zinc The appropriate acid (0.05 mole) in toluene was added dropwise with cooling to a suspension of zinc dust in stoicheiometric excess in toluene. When addition was complete the reaction mixture was heated for one hour at 70°C to complete the reaction. It was then allowed to cool overnight. Excess zinc was filtered from the mixture and the solvent removed. The zinc salt was recovered and purified (Section 3.3.1).

2. Metathetical reaction sodium salt of dialkyl phosphoro- dithioc acid and zinc chloride 0.05 moles of purified dialkyl phosphorodithioic acid was dissolved in a solution of 0.05M sodium hydroxide in 50

distilled water. Additional sodium hydroxide solution was added until the reaction mixture was PH7-8. A filtered aqueous solution of zinc chloride (0.025 moles) was then added with stirring. Both neutral and basic ZDDP's were precipitated as white solids. The PH of the reaction mixture was adjusted using sodium hydroxide to a value dependent on the desired product, PH7-8 for the neutral salt and PH9.5 for the basic salt. Both salts were extracted twice from aqueous solution with diethyl ether, which was then washed with distilled water. The organic solvent was removed and the ZDDP salts recovered. The basic and neutral salts were separated by repeated recrystallisation from methanol.

3.3.1 Purification of Basic and Neutral Zinc Salts

The lower molecular weight neutral ZDDP's (n = 1,2,3,4) were purified by recrystallisation to constant melting point from methanol. Zinc dimethyl dithiophosphate was found to be sparingly soluble and no further purification was attempted after the first crystallisation. Both solid and liquid neutral ZDDP's were also purified by elution through an activated silica gel/alumina column using heptane:acetone solvent. The salts were recovered from solution, air dried and stored in an desiccator' until required. Basic zinc salts were recovered form the residue of the first crystallisation. After washing with methanol and acetone, no further purification was attempted. The higher molecular weight ZDDP's (n = 6,8) were liquid at room temperature. They were purified on a chromatography column as above. This procedure was not found to be entirely satisfactory as final analysis of zinc di-n-hexyl dithiophosphate and zinc di-n-octyl dithiophosphate shows (Appendix 1). A full list of the prepared compounds, certain physical properties and melting points (literature and found) is given in Tables 3.1 and 3.2. 51

TABLE 3.1 PREPARED DIALKYLDITHIOPHOSPHORIC ACIDS

Alkyl Group Appearance Molecular Weight

Di-ethyl pale yellow liquid 186 Di-n-propyl pale yellow liquid 214 Di-iso-propyl pale yellow liquid 214 Di-n-butyl pale yellow liquid 242 Di-n-hexyl pale yellow liquid 298 Di-n-octyl pale yellow liquid 354

TABLE 3.2 PREPARED ZINC DIALKYLDITHIOPHOSPHATES

Alkyl Group Appearance M.p (°C) Molecular (literature) Weight

Di-ethyl white crystals 77.5 (78) 434 Di-n-propyl white crystals 154 490 Di-iso-propyl white crystals 138 (138) 490 Di-n-butyl* white crystals 40 (40) 546 Di-iso-butyl* white crystals 112 (112) 546 Di-sec-butyl* white crystals 52 546 Di-n-hexyl clear liquid liquid at RT 659 Di-n-octyl pale yellow liquid liquid at RT 771

* Samples provided by Shell Research Ltd 52

3.3.2 Characterisation of ZDDP's

The prepared neutral and basic salts were examined by a number of analytical techniques. The identity of the neutral ZDDPs was confirmed by mass spectroscopic (Appendix 1) and infra-red spectral analysis (Section 4.2). Thin layer chromatography and melting point determination provided the simplest guide to the purity of the salts, and these were used routinely to monitor the purification.

Basic zinc salts were examined by IR spectroscopy, the melting points of the two basic salts prepared were found to be far higher than their neutral analogues (Table 3.2).

3.4 Analysis of Commercial Additives

In order to fully understand the behaviour of commercial ZDDP's, and any possible differences between them and the ones used in this work a commercial ZDDP, Lubrizol 1395, was analysed as well as being tested on the friction device (Chapter 6). Analysis of Lubrizol 1395 by thin layer chromatography (Appendix 1), infra-red spectroscopy (Chapter 4) and mass spectroscopy (3.4.1) was undertaken. The suppliers specification for Lubrizol 1395 is given in Table 3.3.

TABLE 3.3 Lubrizol 1395

Type Dialkyl Alcohol Primary Alkyl 50% isopropyl 50% isobutyl Thermal decomposition temperature 205 °C Phosphorus content 9.5% Sulphur content 20.0% Zinc content 10.6% 53

3.4.1 Analysis of Commercial ZDDP's

Thin layer chromatography revealed that, as expected, 1395 was very impure, containing a significant amount of highly polar material. This is probably acidic, possibly an acid phosphate. A small amount of parent phosphor- dithioc acid was observed near the origin. Analysis does indicate the presence of other non-polar organic material, poorly resolved near the solvent front. This was tentatively identified as unreacted parent alcohol from the first stage of the preparation. Identification of the alkyl groups was provided by IR analysis, a detailed discussion of which is provided in Section 4.7. The isobutyl group was definitely identified, but due to the absence of isopropyl (C-H) bands at 1089-1114, 1136-1147, 1170-1190cm"1 (Figure 4.22) the isopropyl group could not be definitely identified, although its P-O-C vibration may be a shoulder observed at 975cm~ . Mass spectral analysis gave a parent ion peak of M/e 575 indicating a proportion of alkyl groups higher than butyl, possibly pentyl or hexyl. This proportion of higher alkyl groups could also explain the broadening of the P-O-C vibration at ^lOOOcm" on the low wavelength side. From the IR and mass spectral data it appeared that the alkyl groups were isobutyl with possibly a small proportion of pentyl and isopropyl.

3.5 Discussion

Commercial additives would therefore appear to contain three distinct types of impurity.

1. Basic zinc salt, unavoidable side product formed during the second stage of the preparation. 2. Acid impurity, either formed during preparation due to oxidation of reactants, or due to unreacted parent phosphorodithioic acid remaining in the product. 3. Parent alcohol, remaining from first stage which could possibly act as an oiliness additive during lubrication. 54

In addition a number of other minor impurities have been reported in the literature. Phosphorus sesqui sulphide (110) has been reported as present in the analysis of phosphorus pentasulphide used for the preparation of the dithiophosphoric acid. This impurity does not react with alcohol although easily oxidised and could act as a oxidising agent in the crude dithiophosphoric acid. Bacon and Lesuer (61) analysed impurities found in crude diethyl dithiophosphoric acid. A tri ethyl ester and a tetra ethyl pyrophosphate were found to be present in significant amounts. Barton et al (58) examined a commercial additive, zinc di-n-hexyl dithiophosphate, using a combination of paper chromatography and neutron activation analysis. The additive was found to contain small amounts of both zinc and phosphorus containing polar impurities. The phosphorus containing compound was identified as a mono or di acid phosphate, and it was postulated that this compound was responsible for determining the wear behaviour of the zinc di-n-hexyl phosphorodithioate. 55

CHAPTER 4 INFRARED SPECTRA AND MOLECULAR STRUCTURE OF ZINC PIALKYLDITHIOPHOSPHATES AND RELATED COMPOUNDS

4.1 Introduction

The prepared organophosphorus compounds were characterised by a number of analytical techniques: thin layer chromatography, mass spectroscopy, and infrared spectroscopy. The following chapter is devoted to a detailed interpretation of the infrared spectroscopy of organophosphorus compounds. The aims of this section of the programme were

i) To develop a full correlation between absorption spectra and chemical structures for the various phosphorus compounds synthesised and tested, ii) To assess the purity of the various synthesised phosphorus compounds. iii) To establish the degree of ionic character of the Zn-S bond.

The first is important in analysing commercial unknown phosphorus compounds and mixtures and also in following the changes that occur both in solutions and on metal surfaces when phosphorus compounds are used in lubrication. Two types of compounds were studied.

1. Dialkyldithiophosphoric Acids. 2. ZDDP's.

4.2 Infrared Spectroscopy

Infrared (IR) spectroscopy is a commonly used analytical technique which provides information on the identity, structure and purity of organic compounds. The general theory of IR adsorption spectroscopy has been treated in cn ctj

3000 2000 1500 1000 500 FREQUENCY CM -1

FIGURE 4.1 Infrared Spectrum of n-Hexadecane 57

detail in many text books (62). Briefly, if a molecule is irradiated with infrared, it will absorb energy at certain wavelengths which correspond to the energy of particular bond vibrations in that molecule. The resulting spectrum is often complex with many absorption bands (Figure 4.1). Examination of the position, intensity and shape of these absorption bands provides information on the type and nature of covalent bonds in the molecule. The infra-red region (A = lOOnm-lpm) is often expressed in terms of wavenumber v. This is defined as the reciprocal of wavelength expressed in centimetres.

-1-1 v = T cm A

This notation has been used throughout the text.

4.3 Literature

The infrared analysis of organophosphorus compounds is well documented, one of the best general reviews being provided by Thomas (63). The first extensive infrared study of ZDDP's was published by Rockett (64). More recently Zimina (65) and Gallopoulos (66) have published systematic surveys of several metal 0,0 dialkyl phosphorodithioates. Gallopoulos obtained IR spectra of MDDP1s in non-polar solvents over a range of 4000-400cm A series of resonance structures for both covalent and ionic MDDP's were proposed to account for absorption band shifts and differences in the physical properties of certain MDDP's. Zimina studied zinc, nickel and lead salts as solid precipates on KBr disc from CCl^ solution. It was observed that with increasing alkyl chain length the intensity of the P=S absorption band decreased. An expression was derived relating the average alkyl carbon number to the integral specific extinction coefficient. A review of the literature indicates some dispute (63) (67) concerning band assignments and the molecular structure 58

of MDDP's.

4.4 IR Spectra Dialkyl Dithiophosphoric Acids

Molecular formula (R0)oP(S)SH

RO .S R = alkyl C H0 . \ s p J n 2n+l n = 1,2,3,4,6,8 RO/' ^SH

4.4.1 Experimental and Introduction to Results

The acids were prepared and purified as previously described. Spectra were recorded as liquid phase capillary films in potassium bromide cells over a wavelength range of 4000-400cm~*. Throughout this work a Perkin Elmer 580B spectrophotometer was employed. This instrument is further described in Chapter 5. Normal slit program and resolution were used. IR spectra of the dialkyl dithiophosphoric acids are shown in Figures 4.3-4.9. The appearance of the C-H vibrations were as predicted and will not be discussied. Interpretation of results is confined to vibrations characteristic of organophosphorus compounds of interest. The most important region for the identification of the thiophosphoric acids is 1500-400cm~ , which contains absorption bands attributed to the P-O-C, P=S and P-S groups. Vibration of the -SH group occurs between 2400 and 2600cm 1 and is considered of less importance. Absorption band characteristics of these four groups will be discussed in turn and compared to the literature.

4.4.2 -S-H Bond

The -S-H vibration is easily recognisable as a weak band occurring between 2400-2600cm~*. The literature suggests that the -S-H band can occur as a doublet depending on the degree and type of hydrogen bonding (69). This was confirmed by Nyquist (68) who 59

studied two dithiophosphoric acids (dimethyl and diethyl) at various concentrations in CC1. solution. For high 4 _ i concentrations (>10%w/w) a weak, broad band at ^2410cm was observed. It was attributed to v(S-H) which is inter- molecularly hydrogen bonding. For dilute solutions (

RO -=s P RO—=a p / \ RO./ \ S RO S-H

Rotational Isomer 1 Rotational Isomer 2 ^2550cm"1 2575-2585cm~1

FIGURE 4.2 Rotational Isomers of (RO)2P(S)SH (68)

Vorsina et al (70) reported three absorption bands in the region 2400-2600cm~1 in the IR spectrum of bis(2-ethyl hexyl) dithiophosphorus acid. A broad band at 2460cm 1 and two sharp maxima at 2500 and 2595cm These were also attributed to the -SH stretch frequency of the dimeric molecule and two rotational isomers of the monomeric molecule. Nyquist (68) further identified two bands in the region 700-900cm~ in the spectra of dimethyl dithiophosphoric acid as being due to 6(S-H) vibrations. The two bands at 782cm"1 and 772cm"1 were found to be temperature dependent and were assigned to rotational isomer 1 and rotational isomer 2 (Figure 4.2) respectively. 60

Results and Discussions

Table 4.1 summarised the position and shape of the -S-H bands. Due to the method of sampling the S-H stretch appeared as a weak broad band in the region 2400-2600cm~ .

1 Alkyl Group v(S-H) cm Comment

2460 Broad/Weak C2H5° 2410 Broad n_C3H7° 2420 Broad/Weak n"C3H7° 2440 Broad n-C4H9°

n-C6H13C 2440 Broad

n-C8H17C 2440, 2480 Doublet

TABLE 4.1

4.4.3 P-O-C Bond

The IR spectra of all organophosphorus compounds examined were dominated by an intense absorption band at ^lOOOcm The shape and position of this was dependent upon the nature of the alkyl group. Thomas (63) has identified three types of vibration in the region 1200-900cm-1 which are attributed to the presence of a P-O-alkyl group in the molecule. Although there is some controversy regarding final assignments of certain absorption bands, this threefold division is retained for discussion purposes.

i. loss-osocm"1 There is an intense absorption band which becomes broad and unresolved as alkyl chain length increases. Thomas (63) subdivided the region depending on structural 61

configuration of the alkyl chain.

Configuration Assignment

1 P-O-CH3 1010-1088crrf

1 P-O-CH2R 987-1042cm" P-O-CHR 950-1018cm"1

Thomas and Chittendon and others (71)C 72) have assigned this band to the P-O-(C) vibration. Other workers have concluded that the band is due to v(C-O-) ( 67) ( 7 3), or from a coupling of the C-O- and P-0 vibrations (74). Nyquist and Muelder (67) suggested that the bands near 1000cm 1 are essentially symmetric and antisymmetric. v(C-O), whereas the v(P-O) vibration occurs near 800cm~ , this assignment has been supported by other workers (73). The bands are not thought to be pure vibrations, since a small amount of coupling probably occurs between v(P-O), v(C-O) and other vibrations (67).

2. oso-ooocm"1 In this region a second absorption band occurs. For diethyl esters it is found to be almost as intense as the vibration at 1040cm 1 (Figure 4.4). As the alkyl chain length increases, intensity of this band decreases so that it might not be detected. Thomas and others (71)(67) assign this band to v(C-C), although v(P-O) (74) and antisymmetric v(P-O) (75) have been suggested.

1 3. 1000-1200cm"9 A group of weaker adsorption bands are observed in this region characteristic of the nature of the alkyl group. They are particularly clearly seen in the isopropyl esters (Figure 4.5). Three sharp absorption bands are observed at 1095-1114;, 1136-1147 and 1170-1190cm"1. Thomas (63) considered these bands to be v(C-O). An alternative view suggested by Nyquist and Muelder from deuteration studies is that they are due to -C-H deformation vibrations (67). Two general schools of thought, concerning band 62

assignment in this region have been noted. These are summarised below in Table 4.2.

Region/cm 1 Thomas (63) (71) Nyquist ( 69)(67 )

1088-950 v(P-O) v(C-O) Two Bands 980-900 v(C-C) v(C-C) 1000-1200 v(C-O) v(C-H) 800 v(P=S)I v(P-O) 1 Rotational Isomer Two Bands

TABLE 4.2

Despite controversy regarding band originsjthe 900-1200cm~ region provides valuable information on the presence of a P-O-C group in the molecule and the nature of the alkyl chain.

Results and Discussion

Absorption band correlation for this region are given for the dithiophosphorus acids examined. Litera- ture values are also given as a comparison were possible, fairly good agreement was found. (Table 4.3).

4.4.4 P=S Thiophosphoryl

There is some controversy (76)(67) regarding the origin of absorption bands due to the P=S group. Various frequency ranges have been proposed (72)(77), the overall one being gOO-GOOcm"1. Popov et al (69) suggests that v(P=S) appears as a doublet due to the presence of rotational isomers, a view supported by other workers (68)(78). Nyquist (68) in his

1 study of (CH3-0-)2P(=S)(-SH) has assigned bands at 670cm" and 659cm 1 to v(P=S) rotational isomers 1 and 2 (Figure 4.2). FIGURE 4.3 IR Spectrum of Di-ethyl Phosphorodithioic Acid FIGURE 4.4 IR Spectrum of Di-n-propyl Phosphorocithioic Acid FREQUENCY CM"1

FIGURE 4.5 IR Spectrum of Di-isopropyl Phosphorodithioic Acid FREQUENCY CM'1

FIGURE 4.6 IR Spectrum of Di-n-butyl Phosphorodithioic Acid FIGURE 5.7 IR Spectra of Di-n-hexylphosphorodithioic Acid FIGURE 4.8 IR Spectrum of Di-n-octyl Phosphorodithioic Acid 69

TABLE 4.3 ASSIGNMENT OF P-O-C SKELETON VIBRATION FREQUENCIES FOR

(RO)2P(S)SH

Alkyl v(C-0)cm v(P-0)cm Comments Group Symmetric Symmetric Asymmetric Asymmetric

1 Ethyl 1040 (sh) 850 813(sh) Band at 850cm" lost 1019 777 on salt formation, possibly 6(SH)

n-propyl 1060 859 999 752

iso-propyl 1000 (sh) 813, 832 Band at 8 1 3 , 83 2cm"1 978 759 possibly 6(-SH)

n-butyl 1021 844, 861 978 788

n-hexyl 1000 865 800

n-octyl 993 852 800 70

Thomas and Chittendon (76) also identified two absorption bands as being associated with the presence of a P=S group. These bands were designated P=S(I), frequency range 730-857cm_1 and P=S(II), frequency range 649-67lcm-1. The P=S(II) band was identified as being due to the P=S valence vibration. Thomas (63) observed that this band was often reported as a doublet because of the presence of isomers. P=S(I) was said to arise from coupling of the P=S vibration with that of another group in the molecule. Support for this view has been provided by Beg and Khawaja (79) in their study of tetramethyldiphosphorus disulphide (TMDPDS) (Figure 4.9) and its metal complexes.

Me^ || ^ Me

P—P FIGURE 4.9a Me ^ II Me

Absorption bands at 833 and 568cm 1 were assigned to P=S(I) and P=S(II) vibrations in TMDPDS. The P=S(I) absorption band was found to be relatively insensitive to co-ordination, undergoing a shift of <10cm~1. The second band at 568cm 1 is split into two, one to a higher and the other to a lower frequency. This is explained as evidence of formation of a chelate structure (Figure 4.9b) with the sulphur atoms placed cis as opposed to trans in the original molecule (Figure 4.9a). There is, however, some doubt as to the validity of the 733cm 1 assignment, due to presence of P-CH^ adsorption in this region.

Me Me \ FIGURE 4.9b / Me \ Me 71

Nyquist and Muelder (67 ) and other workers (73 )( suggest, however, that the absorption band at ^800cm 1 previously identified as v(P=S)(I) is, in fact, primarily due to v(P-O). Nyquist and Muelder (80) in their study of the fundamental vibrations due to CH^-, -C-O-P, and P=S

groups in CH3-0-P(=S)Cl2 and CH3-0-P(=0)Cl2, assigned the bands near 'vSOOcm"1 to symmetric and antisymmetric v(P-O). It is not thought to be a pure vibration, a small amount of coupling is believed to occur between a funda- metal vibration of the -C-0P(=S)C12 solution and a CH3 fundamental vibration. Also a small amount of coupling between v(O-P) and v(P=S) is postulated.

Results and Discussion

Comparison of the spectra of the dithiophosphorus acids and their metal salts indicated that the vibrations at 800cm are more sensitive to a change in alkyl groups than formation of the zinc salt. A result which would support the idea of coupling of CH3 and C-O-P fundamental vibrations. It would be expected that the P=S(I) vibration would be less sensitive to change of alkyl groups than formation of the zinc salt if co-ordination does occur. The positions of the P=S(II) vibrations are summarised in Table 4.5, with changes observed on formation of the zinc salt. Loss of the doublet occurs in some cases. While most authors agree that the band in the frequency range of GOO-SOOcm"1 can be assigned to the P=S valence vibration. The correlation of a second band at 730-857cm"1 as being associated with the presence of a P=S group in a molecule is less dependable. It would appear more likely that the band at 800cm"1 is due to P-O(C) vibration, or 6(S-H). This is supported by the fact that the frequency range for the P-O-(S) vibration is ^OO-OOOcm"1 (63). 72

TABLE 4.4 ASSIGNMENT OF v(P=S) BAND POSITIONS FOR (ROXP(S)SH

Alkyl Group v(P=S)cm 1 v(P=S)cm 1 Acid Zinc Salt

ethyl 660 661

n-propyl 664 662

iso-propyl 654 654,639

n-butyl 661 662

n-hexyl 642 660

n-octyl 665 662 73

4.4.5 P-S Bond

Literature

The overall frequency range for the P-S vibration is reported by Thomas (63) to be 440-613cm-1. It is generally accepted that v(P-S) occurs as a doublet (63) (68). Limited frequency ranges of 526-548cm-1 and 493-526cm-1 have been reported for dithiophosphoric acids (63). Nyquist (68) suggests that the two bands arise from v(P-s) rotational

isomers. In the spectrum of (CHg-O-)2P(=S)(-S-H) the band at 489cm-1 is assigned to v(P-S) rotational isomer 1, and 523cm- band to v(P-S) roational isomer 2 (Figure 4.2).

Results

The band assignments for the dithiophosphoric acids studied are summarised in Table 4.5. Literature values are provided where possible for comparison.

4.5 IR Spectra ZDDPs

This section examines the IR spectra of ZDDPs and relates them to their chemical structure. A feature of the literature concering ZDDPs has been a long standing controversy over the precise structure of the compounds, in particular the extent to which they are ionic or covalent. This study concludes that the Zn-S bond is essentially covalent with a degree of ionic character.

4.5.1 Literature Survey

Crystal and Molecular Structure of MDDPs

The heavy metal salts of dialkylphosphordithioic acid were originally thought by many workers (66) (64) to be entirely covalent (structure I) while the sodium, potassium 74

TABLE 4.5 ASSIGNMENT OF v(P-S) BAND POSITIONS FOR (ROXP(S)SH LITERATURE VALUES ARE GIVEN IN BRACKETS WERE POSSIBLE (111)

Alkyl Group P-S(I)cm~1 P-SdDcm"1

Ethyl 510 (580) 540 (543)

n-propyl 517 (518) 548 (543)

iso-propyl 519 (518) 548 (548)

n-butyl 537 (526) 550 (546)

n-hexyl 464 575

n-octyl 505 - 75

and ammonium salts were ionic in character (63) (structure II).

RO . S S OR RO S \ ^ W / \ / 2+ P P pl " In / ^ / \ /X RO S — Zrt — S OR RO s

FIGURE 4.10 Proposed Molecular Structures of ZDDPs

This assumption was partly based on the observation that the free acid and heavy metal salts were soluble in organic solvents but insoluble in water, whereas the potassium, sodium and ammonium salts were soluble in water but insoluble in organic solvents. The heavy metal salts were also found to have generally lower melting points (64). More recently ionic structures have been favoured for all the dialkylphosphordithioic acid salts (63). Infrared spectra provides evidence for both sides of the argument, .Gallopoulos (66) studied the IR spectra a series of metal and ammonium salts of dialkylphosphorodithioic acids as solutions in non-polar solvents and he concluded that the ammonium and potassium salts were essentially ionic whereas the zinc, nickel, lead and mercury salts were essentially covalent. To explain this transition a series of resonance structures were postulated (Figure 4.11). 76

IONIC SALT R-o s R-o s-

+ + wP ..M . p ..M . P\VG M / \ / \ / \ R-O' R<)- s R-o s

COVALENT SALT R-O S R-O S" R-O S \/ V/ \/ p p yp^

R-c/^S — M R-O7' ^S—M RO S M

FIGURE 4.11 Resonance Structures of MDDPs from Gallopoulos (66)

Metals of high electronegativity form essentially covalent salts, metals of low electronegativity ionic salts. The ionic salt resonance hybrid contains two identical P-S bonds. These should give rise to two absorption bands, symmetric and anti-symmetric stretch vibrations of the coupled PS2~ oscillator. The position of these bands should therefore be higher (anti-symmetric stretch) and lower (symmetric stretch) than that of the isolated P-S bond found in the covalent salts. Gallopoulos (66) showed these effects to explain the observed frequency shift of bands and difference between the spectra of salts of metals of different electronegativity. Thomas (63) however used IR spectral evidence to postulate an ionic structure (II) for all MDDPs, based on the observed loss of absorption bands and frequency shifts associated with salt formation. It is suggested that two distinct bands are associated with the presence of P=S group in a molecule, an absorption band at 730-851cm 1 designated P=S(I) and at 649-671cm~1 designated P=S(II) which is 77

identified as the valence vibration. Formation of the metal salt involves a loss of the band designated P=S(I), which it is suggested should be present if the molecule has a covalent structure in I. An alternative assignment by Nyquist (68) is that the band is due to 6(S-H). On this basis the loss on forming the zinc salt is explicable. The P-S absorption in the free acid is a doublet due to rotational isomers (Figure 4.2), on salt formation one new absorption band is observed. Again loss of the doublet and appearance of a new band can be explained by the loss of rotational freedom about the sulphur atom. In an earlier publication Heilwell (81) in his study of association properties of ZDDPs in hydrocarbon solvents postulated that as the difference between the electronegativity of the sulphur and metal decreased the covalent nature of the S-M bond increased. Also that as the alkyl chain length increase so the ionic character of the M-S bond increased. An internal chelate dimer was proposed to explain the association measurements (Figure 4.12).

\ .s .or

P Zn ' p'

RO S OR

FIGURE 4.12 Equilibrium Chelate Structures of ZDDPs in Solution from Heilwell (81) 78

Lawton and co-workers published a series of papers (82)-(86) investigating the crystal and molecular structure of a number of metal dithiophosphates. X-ray diffraction measurements were used to establish the structures of mercury (82), gold (83), copper (84), lead (85), zinc and cadmium (86) salts. The zinc salt was found to be a bimolecular complex, each metal atom being co-ordinated with four sulphur atoms which assume a distorted tetrahedral configuration about the zinc atom. Each metal atom was associated with two dtp groups, one functions as an intrachelating group bound only to one metal atom. The other is an interchelating group linking two ZDDP molecules together to form a dimer. A simplified diagram of which is shown below (Figure 4.13). The P-S bonds were found to be equivalent and to possess double bond character as did the P-0 bonds. The average bond length for P-S was found to be 1.9 in agreement with previous workers (89). This compares to length of 2.148 and 1.948 for P-S single and double bonds respectively. For P-0 the average bond length was 1. 582 (single 1. 742. double 1. 448 P-0 bond). For each dimer four 'normal' covalent metal-sulphur bonds were postulated, as the average bond length of 2.35(5)8 is in good agreement with the value 2.3 s2 calculated from the sum of the zinc and sulphur tetrahedral covalent radii. The dimers were found to be aligned so that they formed layers of sheets. It is suggested that forces between the layers are less than those within them, allowing the layers to slide over one another and form cleavage planes. The conclusion that normal covalent Zn-S bonds were formed is not fully explained by the evidence presented. Interpretation of these results can be clarified by crystal studies of related MDDPs. Coppens and co-workers (87) determined the crystal structure of potassium dimethylphosphorodithioate, they found that it consists of [S2P(OCH3)2]~ ions and potassium ions arranged in an irregular eight co-ordinate. Each potassium ion is surrounded by six sulphur and two oxygen atoms forming a distorted tetragonal antiprism. As each sulphur in this way takes part in the co-ordinate of three K+ ions, the oxygen atoms are co-ordinated bo one K+ only. FIGURE 4.13 Molecular Structure of the Zinc Di-iso-propyldithiophosphate Dimer from Lawton and Kokotailo (86) 80

Therefore it was suggested that most of the charge of the anion resides on the sulphur atom. The potassium salts are usually considered to be ionic. As a comparison the structure of a 'covalent' salt, tellurium bis dimethyldithiophosphate has been determined (88). Each tellurium atom was found to be a roughly square planar co-ordinate of sulphur atoms. Two different P-S bond lengths were obtained 1. 9248 and 2. 098 these were interpreted as being due to P=S and P-S bonds respectively. Fernando and Green (89) in their study of Nickel (II) di-ethyldithiophosphate reported that the nickel atom forms a planar tetra co-ordinated chelate with the sulphur atoms of the ligand molecules interchelating. Four equivalent P-S bonds were found with an average length of 1. 978. Fernando and Green with reference to Coppens conclusions posulated that most of the negative charge in the nickel salt is located on the sulphur atom, and that the Ni-S bond is predominantly ionic. They conclude that the P-S bond has double bond character and that although the Ni-S distance is normal for a tetra co-ordinated nickel complex, the bond has a degree of ionic character. From the literature reviewed little conclusive evidence can be drawn on the nature of the Zn-S bond. It would appear from the work presented that ZDDPs exist as dimer both in solids and in non-polar solvents. The evidence for an essentially ionic structure is based mainly on IR measurements and there is still some controversy concerning band assignments for the bond vibrations in question. From the X-ray diffraction data it would appear that ZDDPs form a dimeric chelate structure and that the Zn-S bond is essentially covalent with a degree of ionic character. The effect of increased alkyl chain length on the polarity of the Zn-S bond has not been fully investigated, although Heilwell (81) suggests that ionicity of the Zn-S bond increases as alkyl chain length increases. 81

4.5.2 IR Spectra ZDDPs - Experimental

The zinc dialkyldithiophosphates were prepared and purified as previously described (Chapter 3). A commercial additive, Lubrizol 1395, was also examined. Previous workers (65) have expressed doubts as to the consistency of sampling techniques for ZDDPs. Both potassium bromide and nujol mulling methods have been found to give inconsistent results. ZDDPs have been studied as solutions in non-polar solvents (66) and solid precipitates from CCl^ solution (65). In this work zinc di-iso-propyldithiophosphate was sampled as both nujol mull and solid precipitate on KBr disc from both polar (acetone) and non-polar (heptane) solvents. As only small differences were observed in band positions and shapes, it was decided to use solid precipita- tion from acetone on KBr for all solid ZDDPs. Liquid ZDDPs were sampled as capillary films in potassium bromide cells. Spectra were recorded from 4000-400cm-1, normal slit and scan programs were employed.

4.5.3 Results and Discussion

The IR spectra of all ZDDPs examined are given in Figures 4.14-4.22. The IR spectra of ZDDPs and their parent acids were found to be very similar, main differences occuring below 800cm~ and the loss of v(-SH) bands at 2500-2200 . The main discussion of this section will therefore be limited to the position and appearance of bands attributed to P-S/P=S bonds.

4.5.3.1 P-O-C Bonds

IR absorption bands due to the P-O-C group were very similar to those observed for the corresponding phosphorodithioic acids. Twobands in the ^800cm 1 region were assigned to the P-0 symmetric and anti-symmetric stretch. Generally the intensity of absorption for this vibration was Frequency cm 1

FIGURE 4.16 IR Spectrum of Zinc diiso-propy l dithiophosphate FIGURE 4.16 IR Spectrum of Zinc diiso-propy ldithiophosphat e 3000 2000 1500 1000 .500 FREQUENCY

FIGURE 4.16 IR Spectrum of Zinc di iso-propyl dithiophosphate Frequency cm

FIGURE 4.17 IR Spectrum of Zinc di-n-butyldithiophosphate 4.18 ir Spectrum of Zinc di-iso-butyldithiophosphate FREQUENCY cm 1

Figure 4.19 . IR Spectrum of Zinc di-sec-butyl dithiophosphate 3000 2000 1500 1000 5 00 Frequency cm 1

FIGURE 4.20 IR Spectra of Zinc Di-n-hexyldithiophosphate 0 O G oo ct +-> CO -P •H E CO c d P E-"

JL 3000 2000 1500 1000 500 -1 Frequency cm

FIGURE 4.16 IR Spectrum of Zinc di iso-propyl dithiophosphate CO o

I I 1500 1000 500 FREQUENCY CM -1

FIGURE IR Spectrum of Lubrizol 1395 91

found to decrease as the alkyl chain length increased. Absorption band correlations for v(P-O) of the ZDDPs are summarised in Table 4.6.

4.5.3.2 P-S/P=S Bonds

The largest differences in the spectra of phosphorodithioic acids and zinc salts occur in the region below 850cm In view of the forgoing discussion on molecular structure of ZDDPs this spectral region can be presented as a whole. Band positions are summarised below in Table 4.7 for those ZDDPs examined.

Results

Examination of the spectra of the zinc salts shows that on conversion from the parent acid two bands in the region 850-400cm 1 are lost. One band ^800cm-1 is attributed to loss of 6(S-H). In general formation of zinc salt has little effect on the P=S position. Spectra of sodium salts however revealed band shift of % 18cm 1 on salt formation (Figure 4.24). Loss of the other absorption band was at ^500cm attributed to the v(P-S) vibration of one rotational isomer. The absence of this band could be due to loss of rotational freedom due to salt formation.

4.5.3.3 Zn-S Bond

Observation of the position and intensity of the Zn-S bond would provide direct evidence of the degree of ionic character. Little information is available in the literature as the spectral region associated with M-S bonding is below the frequency range of most IR spectrophotometers. Several workers have reported M-S bonds in the region of ^300cm 1 (90). Six of the prepared metal dithiophosphates were examined in the region 400-200cm . They were sampled as 92

TABLE 4.6 P-O-C SKELETON VIBRATION FREQUENCIES FOR ZDDP

Alkyl Group v(C-0)cm v(P-0)cm symmetric symmetric ant isymmetric ant isymmetric

Ethyl 1040 813 1016 780

n-propyl 1059 839 989 739

iso-propyl 1023 890 975 784,758

n-butyl 1020 890 975

iso-butyl 1030 851 998 825

sec-butyl 1031 855 974 825

n-hexyl 1000 860 800

n-octy1 1010 849 999 800 93

TABLE 4.7 ASSIGNMENT OF ZDDP P-S/P=S VIBRATION FREQUENCIES

Alkyl Group v(P=S)cm_1 v(P—S)cm~1

Ethyl 661 540

n-propyl 662 538

iso-propyl 654,639 534

n-butyl 662 545

iso-butyl 666 554

sec-butyl 657 546

n-hexy1 660 -

n-octyl 662 557 94

1. Zi nc di -ethyl dithiophosphate 2 . Zinc di -n-propyl dithiophosphate 3. Zinc di -iso-propyl dithiophosphate 4. Zinc di -n-butyl dithiophosphate 5 . Zinc di -iso-buty1 dithiophosphate 6 . Zinc di -sec-butyl dithiophosphate

FIGURE 4.23 IR Spectra of ZDDP's in 400-200cm 1 region 95

nujol mulls in polyethylene cell, normal scan and slit programs were employed. Due to the sampling method it was not possible to make quantitative measurements, as it was hoped that intensity changes of Zn-S peak for several ZDDPs would provide information on polarity of the Zn-S bond. The spectra are presented in Figure 4.23 and the results summarised in Table 4.8.

TABLE 4.8 SUMMARY OF RESULTS FOR v(M-S)

•1 Alkyl Group v cm

ethyl 295 , 325 n-propy1 300 , 350 (Sh) i-propy1 310 , 279 (Sh) n-butyl 310 i-butyl 265 , 320 s-butyl 270 , 320

4.5.3.4 Discussion of ZDDP Results

Results from MDDP crystal structure determination indicates the following:

1. P-0 bond has double bond character. 2. For all salts except tellurium the P-S bonds are equivalent and have double bond character.

IR evidence on the nature of the Zn-S bond is not conclusive. On salt formation two absorption bands are generally lost from the IR spectra of the parent acid. One at ^SOOcm-1 is attributed to 6<-SH) although an alternative assignment is v(P=S)II. The other at ^500cm_1 is assigned to v(P-S)I. Two sodium salts (diethyl and di-n-octyl) were also prepared and examined in this study, 96

the results for the 900-500cm_1 region are shown in Figure 4.24). Comparison of the sodium salt with its zinc and parent acid analogues indicate the following:

1. Intensity of the v(P=S) absorption increases in the order acid < zinc salt < sodium salt. Increased band intensity in the IR is usually attributed to increased polarity of the bond (62). 2. The position of the P=S vibration frequency is Acid and zinc salt > sodium salt. Increased bond order in the IR causes a shift to higher frequencies. This suggests that on sodium salt formation the P=S bond order decreases. The two vibrations in the 700-400cm-1 range can be attributed to the symmetric and anti-symmetric stretch of the PS~ mesomeric ion (63).

If the zinc salts are essentially ionic then the intensity of the P=S vibration for both the zinc and sodium salts should be the same, this is not found in practice (Figure 4.23). Formation of an ionic salt should give two equivalent P-S bonds, it would be expected therefore that the P=S vibration frequency should decrease and the P-S frequency increase relative to the parent acid. The P=S frequency decrease is observed for the sodium salts (^18cm ) but not for the zinc. The P-S frequency increase is observed for both the sodium and zinc salts (18cm-1 and SOcm"1 respectively). IR spectra of several zinc salts have been examined in the 400-200cm~* range, where M-S absorption would be expected to occur. Although absorption bands have been recorded at ^SOOcm"1 for all salts examined, no definite assignment can be made. From these results it has been concluded that the Zn-S bond is essentially covalent with a degree of ionic character.

4.6 IR Spectroscopic Analysis of Lubrizol 1395

The infrared spectra of Lubrizol 1395 is shown in Figure 4.22. A full list of IR band frequencies with assignments is Di-ethylphosophorodithioic Acid

Sodium of Di-ethyldithiophosphate Zinc Di-ethyldithiophosphate

900 800 700 800 500 000 800 700 600 500

-1 1 Frequency cm Frequency cm

FIGURE 4.24 IR Spectra for Di-ethyl and Di-n octyl Derivatives in the 900-50Qcm 1 Spectral Region 98

given in Table 4.9. Examination of the spectra indicates that a major constituent of Lubrizol 1395 is the isobutyl derivative. Absorption bands at 1370, 1380cm 1 (gem dimethyl), \ 848cm-1 [v(P-O)] and 665cm"1 [v(P=S)] correspond to band positions in the isobutyl spectrum (Figure 4.16). The specified composition of Lubrizol 1395 (Table 3.3) is isobutyl/ iso-propyl. On the IR evidence, significant amounts of di-iso-propyl derivative, were not present. Three peaks at ^1200cm-1 characteristic of the isopropyl group were not identified, although this region is poorly resolved in the spectra. The relative intensity of the v(C-H) vibrations [compared to v(C-O)] indicates that 1395 contains a significant amount of hydrocarbon (non-phosphorus containing) material. This was confirmed by the thin layer chromatographic analysis of the additive. It has been attributed to the presence of a parent alcohol impurity from the first stage of the synthesis. A weak broad band was also observed at 3190cm"1 this is attributed v(OH) of an alcohol, which would confirm this. A second possibility is that the additive contains a small amount of mineral oil, added as a carrier to prevent crystallisation of the ZDDPs. 99

TABLE 4.9 SUMMARY OF LUBRIZOL 1395 IR BAND ASSIGNMENTS

Band Position Assignment Comment

3190 v(OH) weak/broad polymeric alcohol

2980 v antisymmetric CH^

v antisymmetric CH2 2930 v symmetric CH^

1850 combination band broad isobutyl v(C-O) + v(P-O)

1470 6 CHa

1395 go CH2 1380 6 CH3 Doublet gem dimethyl isobutyl 1370 isopropyl

1165 skeletal isopropyl 1130 isopropyl

1040 v symmetric (C-O) 995 v antisymmetric (C-O) isobutyl 915 v(C-C)

848 v symmetric (P-0-) isobutyl 824 v antisymmetric (P-0-) isobutyl 765 * CH2 rock 725 665 v(P=S) isobutyl 660, v(P=S) 588 555 v(P-S) isobutyl 480 100

CHAPTER 5 INFRARED SPECTROSCOPIC ANALYSIS OF THIN FILMS ON METAL SURFACES

5.1 General Introduction

The identification of chemical species formed on metal surfaces during boundary lubrication is of obvious importance in understanding the mechanism of ZDDP AW action. Surface analysis using a variety of analytical techniques has been extensively applied to lubrication research. Previous work in this area has been reviewed in 2.5.

5.2 Analysis of Thin Films on Metal Surfaces

A number of analytical techniques have been developed for the characterisation of thin films on metal surfaces. These include X-ray fluorescence (51), electron probe microanalysis (46), X-ray photoelectron spectroscopy (45) (47) (46), radiotracer techniques (44) and infrared reflection-absorption spectroscopy (51). This work has been reviewed in 2.5.

Most analytical techniques provide information on 1) element identification and spatial distribution 2) film thickness. Few provide identification of chemical species or the structure of the surface films. Of the methods reviewed infrared reflection-absorption spectroscopy was the most convenient technique available for surface analysis. It is a rapid, non-destructive, comparatively simple technique which can give positive identification of surface species. It is recognised that reflection- absorption spectroscopy does not provide a complete description of the metal surface. It also requires large sample areas which makes the examination of wear scars 101

b) Single Reflection

Metal Mirrors

c) Multiple Reflection

FIGURE 5.1 Transmission and Reflection Infrared Sampling Methods 102

impract icable.

5.3 Infrared Reflection-Absorption Spectroscopy

5.3.1 Introduction

IR reflection-absorption spectroscopy can provide direct information on the identity structure and average thickness of thin films on metal surfaces. Films of molecular dimensions 52) have been detected using this technique (92). Developed in the late 1950's IR reflection- absorption (or specular reflectance) spectroscopy is similar in principle to the more usual transmission (absorption) spectroscopy. In IR absorption spectroscopy (Figure 5.1a) radiation is trasmitted through the sample supported in an IR transparent cell. Absorption of radiation occurs at frequencies characteristic of the chemical structure of the sample. Examples of transmission spectra are shown in Chapter 4. In reflection-absorption spectroscopy, radiation transmitted through the sample layer, is reflected at the metal surface and transmitted again through the sample. Either single (Figure 5.1b), or multiple reflections (Figure 5.1c) between parallel metal samples can be used. Greenler(93) first suggested the term reflection- absorption (R-A) spectroscopy as an alternative to specular reflectance spectroscopy. Although the process cannot be successfully modelled either as transmission through a thin layer, especially where the surface layer is thin compared to the amplitude of the incident radiation. Nor is it true reflection spectroscopy which is obtained by reflection from the surface of a bulk sample.

The resulting R-A spectrum is usually similar ( 93) to that obtained by transmission spectroscopy. Absorption bands corresponding to vibrations of different bonds 103

in the molecules of the surface film appearing as bands of reduced energy.

5.3.2 Applications of, Infrared Reflection-Absorption Spectroscopy

Infrared reflection-absorption spectroscopy has been used in many areas of chemical research, these include catalysis ( 94) (95 ), oxide (92) and lubricant film studies (97), corrosion inhibitors (96). Several reviews have been published detailing these applications ( 98) ( 99). The study of oxide films has been extensively reported (92 ) (100) (98 ). Poling (92) used a seven reflection system at 73° incidence to obtain spectra of copper and iron oxide films. Film thicknesses were calculated using spectral band intensities, and were verified by independent determina- tions. For copper the band intensity of CUgO was found to increase linearly with film thickness to a limiting value of 90 08. Similar results have also been obtained for iron oxide films in the range 0-12008. It was reported that using the technique described, sensitivity to detect oxide films in the region of 52 was possible. Infrared reflection-absorption spectroscopy has also been used to study protective films formed by corrosion inhibitors on different metals (96) and oil oxidation products on copper G01). The study by Francis and Ellison (51) of ZDDP films on metal surfaces has already been reviewed in 2.5. They used a four reflection optical system at a 72° angle of incidence. Several components were observed in films formed by the decomposition of ZDDPs. These included zinc sulphate, zinc oxide and adsorbed ZDDP. It was suggested that the sulphate is produced by oxidation of the sulphur in ZDDP molecule in the presence of hydroperoxides . 104

FIGURE 5.2 Model of a Thin Film on a Metal Surface

180° out of phase 90° out of phase

FIGURE 5.3 Orientation of Polarisation Before and After Reflection From a Metal Surface 105

5.3.3 Theory and Development of Infrared Reflection " Absorption Spectroscopy

The theory of infrared reflection-absorption spectroscopy has been treated in detail in many papers (102) (103). A brief review of the development of infrared R-A theory is presented below together with experimental requirements arising from these considerations. Frances and Ellison published one of the first papers in this field (102). They developed the theory of IR reflection-absorption spectroscopy and confirmed their predictions with experimental studies of solidified metal stearate films on metal mirrors. Spectra were recorded of both monomolecular and multimolecular films deposited on metal mirrors using the Langmuir-Blodgett technique. Evidence that reflection spectra can also provide information on the orientation of adsorbed species was also presented. The theory of reflection spectra of thin films on metal surfaces has been developed by a number of workers (104) (102). Greenler in a series of papers (103) ( 93) used theoretical considerations in the development of a sampling system to obtain many reflections between closely spaced mirrors (105). As an illustration of the technique the spectrum of cellulose acetate layer on a silver , mirror was obtained. Sutecka (106) studied preferred orientations of crystals in thin films of urea and thiourea on steel mirrors. He also concluded that reflection spectra should differ little from transmission spectra obtained at an optimum angle of incidence. Several theoretical models have been used to formulate infrared R-A spectroscopy. Discussion of the theory of R-A spectrum is therefore limited to general results and the way they determine experimental variables. When light is incident on a highly reflecting metal surface at near normal incidence (6 - 0) it undergoes a phase shift of nearly 180°. The incident and reflected waves combine to form a standing-wave electric field which has a node at the surface of the metal. Therefore 106

as the amplitude of the electric field is zero at the surface it cannot interact with the oscillating dipoles of adsorbed species. For light at non-normal incidence (Figure 5.3) the phase shift is dependent on both the angle of incidence and the polarisation of the light. For light polarised perpendicular to the plane of incidence the phase change is 180° for all angles of incidence. Figure 5.3a shows the relative phase of the electric vectors before and after reflection for perpendicular polarisation. For all angles of incidence the vectors would cancel each other out. For light polarised parallel to the plane of incidence the phase shift is dependent on the angle of incidence, especially at high angles (Figure 5.4). At grazing incidence the phase shift is 180° and cancelling of the incident and reflected vectors would occur. At higher angles of incidence where the phase change upon reflection is 90°, addition of reflected and incident waves produces an elliptical standing wave with a component of electric vector normal to the metal surface (Figure 5.3b). The ratio of the components of the electric vectors at the surface for perpendicular and parallel polarisation has been estimated as l:icr (J_: | | > .(103) The optimum conditions for observing a thin film of absorbing material located on the surface of a highly reflecting metal is therefore:

a) high angles of incidence (not grazing) b) polarisation of light parallel to the plane of incidence.

It would also be expected that only those molecules where the vibration gives rise to a change of dipole moment perpendicular to the surface should be observed (102). Changes in electric dipole parallel to the surface will not absorb as strongly. This conclusion is supported by experimental results of Francis and Ellison (102) . Eckstromand Smith (95) applied classical theory of metallic reflection to the problem of IR reflection- 107

Angle of Incidence

- phase shift for light polarised parallel to the plane of incidence

<5 i - phase shift for light polarised perpendicular _L to the plane of incidence

FIGURE 5.4 Phase Shift for Light Reflected from a Metal Surface from Greenler (103) 108

absorption. They concluded that the amplitude at the surface is an appreciable fraction of the incident 2 3 amplitude and as much as 10 to 10 times larger than that previously supposed. They therefore suggested that the experimental conditions for obtaining detectable IR absorptions - could be eased, and that lower incident angles could be used. An experimental cell was designed to confirm their theoretical results, incident angles of 75-80° were used to give a large number of reflections (8-16). The IR R-A spectrum obtained should be essentially the same in appearance as the transmission spectra. Greenler et al (93) calculated band shapes and shifts for R-A spectra for strong and weak bands. They concluded that the variation in refractive index for extremely strong bands causes changes from the transmission spectra, whereas weak or moderately strong bands were similar in both transmission and reflection spectra. The depth of an absorption band obtained in reflection- absorption spectroscopy is usually measured as an absorption factor, A (Figure 5.5) where

A = R° " R AR = R° - R R° where R° = reflectance in the absence of an absorption band R = reflectance of absorption band.

Francis and Ellison (102) showed that A can be related to the film thickness by the following expression for both parallel and perpendicular components.

167Td ¥i + 5.1 For radiation Aj_ = Cose A K K

N1k1Sin 6 N^f(N^ e) 16-rrd For radiat ion A Cose 5.2 A 4 N1 Co^% k4 Cos 0 109

Wavelength

FIGURE 5.5 Definition of Absorption Factor (A) 110

where 0 = angle of incidence d = film thickness A = wavelength refractive index of film absorption constant oiA 4tt absorption coefficient in cm 1 refractive index metal absorption constant metal

For | | radiation f(iK,0) is a function of N^ and 6 having a value of the order of unity. These expressions were obtained for isotropic films-only. The expression for the parallel component can be simplified so that measurement of A|| can be used to calculate film thicknesses. A|| has been shown to increase linearly with film thickness to t- 0.0004 1 A (105), e.g. 400A at 1000cm" . Several workers (92 ) (100) have calculated film thicknesses using similar expressions (depending on the reflection system used). Good agreement with results from other techniques has been achieved (100). The advantage of the R-A technique is that quantitative measurements of films as thin as 58 can be obtained, few other techniques are capable of such sensitivity.

5.3.4 Optimisation of Experimental Conditions

Greenler (103) calculated that under optimum conditions the absorption of a thin layer by the reflection-absorption technique was far greater (x25) than by transmission through the unsupported film. To obtain this sensitivity a number of experimental variables have to be determined.

a) Polarisation of Incident Radiation It is generally agreed that the component of radiation polarises parallel to the plane of incidence Ill

in front of the detector should therefore enhance the percentage absorption, and increase the sensitivity of the system. b) Angle of Incidence

The optimum angle of incidence for one reflection has been calculated to be 88° for radiation polarised || to the plane of incidence (Figure 5.6). The absorption 3 factor (A) at 88° is approximately 5 x 10 times greater than at 0° (105).

FIGURE 5.6 Absorption Factor as a Function of Angle of Incidence for Parallel Polarisation from Greenler (105)

Greenler also showed that for two angles of incidence (88°, 80°) the absorption factor varies linearly with film thickness up to values of d/A of 0.0004. The angle of incidence chosen should be used in conjunction with the optimum number of reflections. Eckstrom and Smith suggested that smaller incident angles (^65°) than previously predicted as multiple reflections could be used to give better resolution and signal to noise ratios (95). 112

c) Number of Reflections

Greenler (103) calculated the optimum number of reflections as a function of incidence angle (Figure 5.7)

30 60 90 Angle of Incidence in Degrees

FIGURE 5.7 Optimum Number of Reflections Shown as a Function of Incidence Angle from Greenler (103)

He showed that an optimum number of reflections is required for each angle of incidence. If the number of reflections is varied by ±2 then the value of AR is reduced by 20-30%. The optimum number of reflections is that number which gives the maximum signal to noise ratio, i.e. maximum AR^ where:

N N ARf = (R°) - R

N reflections, AR^ is the final change in reflection resulting from an absorption band. Greenler concludes that for a particular angle of incidence the number of reflections that reduces the background energy to 37% of its initial value is that number which will give the maximum AR^. In conclusion IR reflection-absorption spectra of thin films on metal surfaces can be detected on an ordinary commercial IR spectrophotometer if the following facilities are available 113

1. Polariser 2. Multiple reflections at variable angles of incidence 3. Ordinate scale expansion 4. Wide slit, low noise scan mode

5.4 Experimental

5.4.1 Preparation of Samples

a) Metal Specimens

Mild steel sheet (0.075mn thick) was prepared by light polishing using 125 grade aluminium oxide, and then rinsed in distilled water and dried. The sheet was cut to size (60 x 20mm) which allows 10mm for handling. Cleaned in toluene vapour in a soxhlet for two hours and then stored under acetone until required. All solvents used were of Analar grade.

b) Solutions

Hexadecane was purified prior to use by percolation through activated silica gel/alumina and then stored over silica gel and alumina in a dark glass bottle until required. The organophosphorus additives were purified as previously described (Chapter 3). Three types of additives were tested, neutral zinc di-ethyldithiophosphate, di-ethylphosphorodithioic acid and Lubrizol 1395.

5.4.2 Equipment - IR Spectrometer

In early studies, the results of which are not presented in this work, a Perkin-Elmer 457 IR spectrophotometer was used. Preliminary experiments were performed using the 25-reflection unit described below (5.4.3). A common beam mounted polariser was not available on this spectrometer instead a KRS-5 polariser was mounted in conjunction with the reflectance unit in the sample beam. 114

The experimental set up proved to be unsuitable for the work attempted. The sample polariser, while enhancing the spectra at || polarization gave spurious absorption bands which were difficult to distinguish from the required peaks. In later experiments and all results presented in this work a Perkin-Elmer 580B Infrared Spectrophotometer with data station was used. The Perkin-Elmer 580B is a double beam high energy ratio recording spectrophotometer with a scanning range of 4000cm"''" to 180cm"''". It is microprocessor controlled and provides a wide range of scanning programs, for all reflection studies a low noise/high energy mode was employed. The model available was also equipped with a common beam mounted silver chloride polariser. Data handling facilities were provided by the Perkin- Elmer 3500 data station which was used in conjunction with the IR spectrophotometer. The data station allowed manipulation of infrared spectra run on the P-E 580B. Up to three spectra could be stored in digital form in the data station memory for manipulation. In this way spectra can be ordinate expanded or reduced, smoothed to reduce noise, one spectrum subtracted from another. The resulting spectra are replotted on the spectrophotometer in either transmittance or absorbance as required. Spectra could also be permanently stored on floppy disc until required. This allowed one reference spectrum to be used for all experiments.

Scan Parameters

All reflection spectra were run using the following scan parameters:

Scan mode 3B high energy, low noise mode Scanning region SOOO-eOOcm"1 Resolution 3.0cm 1 Relative noise 0.025 Polariser II orientation Scan averaging usually x 5 115

Most spectra were scanned overnight using an averaging option (95). Repeated scans in this way are averaged to further improve the signal-to-noise ratio.

5.4.3 Sample Optics

Reflection spectra were recorded using a modified multiple attenuated total reflectance (ATR) unit supplied by Analytic Accessories Ltd (Figure 5.9, 5.8). Instead of the ATR crystal and sample holder a multiple specular reflection attachment was used. The system provides variable angles of incidence (30-60°) and sample spacing (l-10mm). The angle of incidence is chosen by rotation of the knurled adjuster until the back face of the sample carriage is in line with the required angle setting. Infrared radiation is concentrated on and collected from the metal samples by four front surface aluminised mirrors. The system could be focussed by rotating the mirrors as required. For multiple specular reflectance use two metal samples 50 x 20mm and 48 x 20mm were required. The flexibility of the system allowed the optimum number of reflections to be used for a given incident angle, by altering the spacing between the samples (equation 5.3). In later work single reflection sampling was used. This proved to be more sensitive and provided better repeatability of results. For single reflection operation, one metal sample was required (50 x 2 0mm). The accessory mirrors were adjusted to give one reflection at a high angle of incidence (estimated at 85°). In all cases, once the system had been focussed and an optimum configuration found this was kept constant for all spectra recorded. A silver chloride polariser supplied by Perkin-Elmer Ltd was installed in the monochromatic section of the spectrophotometer. This was common beam mounted to remove the unwanted component of radiation from both sample and reflectance beams. 116

FIGURE 5.8 Reflectance Unit Infrared Source Detector

1 Reference Beam A Position for single reflection 2 Sample Beam B Position for multiple reflection 3 25 Reflection ATR Unit 4 Mirrors 5 Moveable carriage for adjustment of angle of incidence 6 Metal Samples 7 Polariser

FIGURE 5.9 Schematic Diagram of Sample Optics 118

5.5 Preliminary Experiments

A number of experimental variables were investigated so that optimum sensitivity could be obtained.

1. Polarisation angle. 2. Angle of incidence/number of reflections.

A number of preliminary experiments were performed to establish the sampling configuration used. As a test sample thin films of nujol were smeared on mild steel samples. Spectra were recorded in the 3000-2800cm 1 region to detect absorption bands due to v(C-H). Sampling parameters were chosen to give optimum sensitivity in this region as follows.

1. Polarisation Angle

Spectra were scanned using two polarisation angles 0° (perpendicular) and 90° (parallel) and with no polariser present. Results are shown in Figure 5.10. In agreement with the theory, absorption was increased when the component polarised perpendicular to the metal surface is removed. All subsequent experiments, both multiple and single reflections, were run at 90° polarisation.

+-> 1 Polariser = 0° (J_) •H > 2 No polariser •H 2 -P 3 Polariser = 90° (I O 3 o +4 ft 3000 2800 -1 cm

FIGURE 5.10 Effect of Polarisation Angle on Absorption 119

2. Incident Angle

The importance of incident angle in determining sensitivity has been demonstrated by earlier workers. The reflection unit provided variable angles of incidence, 30-60°, for all multiple reflectance work the maxmimum incident angle 60° was used.

Multiple Reflections

The number of reflections is a function of the angle of incidence and sample length. - It is given by N = number of reflections L = >T t/^- .uA sample length (5 ) N = /tcot6 t = sample spacing 0 = angle of incidence Metal samples were scanned at different separations at 60° incident angle. The results are shown in Figure 5.11. The optimum number of reflections was found to be ^17. The number of reflections was limited by the amount of energy reaching the detector, too many reflections decreased the background radiation to 10-20% with a consequent increase in relative noise. The number of reflections used is therefore a balance between high sensitivity and good signal-to-noise performance.

Single Reflections

For later experiments a single reflection system was developed. One metal sample (48mm x 20mm) was mounted on the outside of the reflectance attachment (Figure 5.9). The mirrors were refocussed to give reflections at a high angle of incidence (^85°). This arrangement was found to be more sensitive than multiple reflections at lower incident angles. As the angle of incidence was not accurately known, the attachment was not altered or refocussed after the reference sample was run. One advantage of the single reflection system is that high energy throughput is obtained allowing poorly polished or rubbed surfaces to be studied. 120

5.6 Experimental Procedure

The metal specimens and test solutions were prepared as previously described (5.4.1). Experiments were carried out at room temperature, 100-150°C and 180-200°C. These were chosen to correspond with regions of interest indicated by friction testing.

5.6.1 Room Temperature Tests

Metal specimens were immersed in the test solution in a stoppered sample bottle. This was shaken occasionally throughout the experiment. After the required time (1-72 hours), the metal samples were removed and rinsed twice with heptane (Koch light, pure grade percolated through silica gel/alumina before use). Then air dried and mounted on cardboard templates using double-sided cellotape. Samples were then cut to the required size. After cleaning and through out testing the samples were handled by one edge using tweezers. The handling edge was removed when the samples are trimmed to size after mounting. The metal specimens were then sampled and scanned on the IR spectrophotometer as described in 5.6.3.

5.6.2 High Temperature Tests

A calibrated oil bath was used to heat test solutions to a maximum of 220°C. This was preheated to the required temperature and a test tube containing the additive solution was suspended in the oil bath. When the solution reached the required temperature the metal specimens were introduced so that they were immersed, and the test tube plugged with glass wool. The metal samples were stirred occasionally throughout the experiment. When the test was complete the samples were removed, washed and sampled as previously described. The test solution was also sampled at the beginning and end of the experiment for IR analysis. 121

3000 2800CM-1

FIGURE 5.11 Appearance of a Reflection-Absorption Band After a Different Number of Reflections 122

5.6.3 Scanning Procedure

The metal specimens were sampled either for multiple or single reflection. The specular reflectance attachment was placed on the locating pins and the entire assembly placed in the sample beam holder. The polariser was switched on. The IR spectra were scanned from 3000-400cm 1 using scan mode 3 with multiplier B. This is a low noise, wide slit, high energy scan. Good signal-to-noise ratios are achieved at a slow scanning speed. To improve signal-to-noise ratios futher, five average scans were run, i.e. the sample was scanned five times in succession and the resulting spectrum averaged. This entire operation took ^18 hours and was usually run overnight. This combination of scan parameters gave signal-to-noise ratios of 0.025 with resolution of 3.0cm"1. After scanning the spectra were stored on floppy disc until required.

5.6.4 Reference Spectra

Most workers (94) (96) have recorded sample spectra differentially to remove absorption bands due to the sampling method. To do this two identical reflection accessories are required, the clean metal sample is placed in the reference beam, the prepared metal in the sample beam. As a second reflectance accessory was not available, reference spectra of clean metal specimens were run prior to prepared samples. These reference spectra were stored on floppy disc and digitally subtracted from the sample spectra. The metal samples used for reference specta were prepared as previously described but scanned without immersion in additive solutions. Reference spectra were repeated periodically to check the efficiency of the cleaning procedure. 123

5.7 Results

5.7.1 Diethylphosphorodithioic Acid

Two concentrations of diethylphosphorodithoic acid in hexadecane were tested, 0.003M and 0.015M.

5.7.1.1 Room Temperature

Steel samples were immersed in the test solutions for 1-24 hours. Representative spectra are shown in Figures 5.12 and 5.13. At low concentrations (5.12a) the acid reacted slowly with the steel surface and relative film thicknesses (measured by A||) increased gradually with immersion time. IR analysis of the film revealed a broad adsorption band at 1000-1300cm 1 and one further intense peak was observed at ^700cm-1. Various assignments can be made for these peaks. The bands at vliso are possibly due to P=0 vibrations (63). The band at 700cm~ is more difficult to assign. One possibility is that it is due to v(P=S) although its frequency is v50cm~ higher than v(P=S) for either the original acid or zinc diethyldithiophosphate. Examination of IR results for the P=S vibration indicates that it is relatively unaffected by salt formation. It would therefore appear unlikely that the vibration at 700cm~ is due to v(P=S) of an iron dithiophosphate. The relatively high intensity of this peak would also make such a correlation unlikely. An alternative assignment is that this peak is due to a P-O-S vibration. Von Lampe (107) (108) in his study of the IR spectra of mixed phosphates and sulphates (structure I) assigned a frequency range of 817-927cm to the assymetric P-O-S vibration and 699-765 to the symmetric P-O-S vibration. It is possible therefore that the 700cm band is due to the P-O-S symmetric vibration. A species similar to that in I is therefore envisaged, the broad band at 1150cm~ being assigned to mixed P=0, S=0 vibrat ions. 124

Frequency cm

FIGURE 5.12 IR Spectra of Steel Surfaces After Immersion in Diethylphosphorodithioic Acid Solution at 20°C 125

1800 1500 1000 600

-1 Frequency cm

FIGURE 5.13 IR Spectra of Steel Surfaces After Immersion in Diethylphosphorodithioic Acid Solution at 20°C TABLE 5.1 SUMMARY OF IR PEAK POSITIONS AND ASSIGNMENTS FOR FIGURES 5.12 AND 5.13

Spectra Peak Position/cm-1 Assignment Comment

5.12a 1000-1200 (Broad) v(P=0)/v(S=0) Phosphate/sulphate 705 v(POS) symmetric

5.12b 1180 - 2 } v(P-O) ionised PO , PO ", POS~ 1105, 1099 1060 v(C-O) Perturbated by POg vibration 970 v(C-C) 799, 815 v(P-O-) P-O-C 640 \>(P=S)

5.12c 1180 2 } v(P-O) ionised PO", PO ", POS" 1105,1099 1060 v(C-O) Perturbated by PO^ 970 v(C-C) 799 v(P-O) P-O-C 640 v(P=S) 622 v(P=S)/vP-S) FeDDP/POS"

5.13b 1200,1150 2 } v(P-O) ionised PO2, PO ", POS" 1105,1070 1038 v(C-O) 980,970 v(C-C) 799 v(P-O) P-O-C 640 v(P=S) 622 v(P=S)/v(P-S) FeDDP/POS" 127

0 0 0 II II II M0-S-0-P-0-S-0M M = metal atom II II II

0 OM 0

At higher concentrations (5.12b) the dtp acid was found to react corrosively with the steel forming a brittle surface layer. Initially a brown adsorbed surface film is formed (5.12a). This is gradually covered by a brown precipitate (5.12b). Further adsorption/reaction then occurs to form a white precipitate (5.13b). Removal of this precipitate revealed a substantially different surface film (5.13a). All spectra examined were characterised by intense absorption bands in the 1000-1200cm_1 region. These are - 2-

assigned to ionised P-0 vibrations indicating P02, PO^ and POS species. Observation of the peak at ^640cm~l attributed to the P=S (stretch) vibration showed that as -the tests proceded the intensity of this peak decreased while a new peak at 622cm"1 was observed. This band at 622cm 1 can be assigned to either the v(P=S) of an iron dtp or more probably to the J-S vibration of the POS" anion. This would indicate that the dtp acid is initially adsorbed (5.12b) but that further reaction forms a brittle surface film containing ionised P-0 and P-S species. 5.7.1.2 100-150 °C

Steel samples were immersed in the test solutions for 1-8 hours, a representative spectrum is shown in Figure 5.14. At both concentrations, dtp acid reacted rapidly with the steel to form a relatively thick essentially inorganic film. This was characteristed by one sharp intense peak at 1218cm"1 with smaller absorptions at 1045, 949 (both shoulders) and 787cm"1. The peak at 1218cm"1 can be assigned to a P=0 vibration, Thomas (63) gives an overall frequency range of 1087-1415cm 1 for the P=0 valence vibration. Within this frequency range more precise assignments can be made as to the chemical species involved. The high intensity of the absorption at 128

Frequency cm

Concentration : 0.0015M Immersion Time : 8 hours Ordinate Expansion : x 2

FIGURE 5.14 IR Spectrum of Steel Surface After Immersion in Diethylphosphorodithioic Acid Solution at 100-150°C 129

1218cm-1, suggests that it is an ionised phosphorus species. Of the phosphorus ions with vibration frequencies in

2 - - the right spectral region, the P0g and POS anion appear to be the most likely. Two other alternatives are possible, - 3 -

P02 PO^ , but on closer examination these can be discounted. The ani°n has "two absorption bands, attributed to the symmetric and assymetric stretching vibrations, within the frequency ranges 1092-1323 and 95-1164cm-1. Both absorption bands are strong in intensity and of similar strength, making the assignment of PO - for 3- the single peak at 1218 unlikely. The tetrahedral PO^ anion is characterised by a strong absorption band at lOOO-lllOcm-1, this has been assigned to the antisymmetric stretch vibration. Francis and Ellison (51) assigned a single sharp band at 1050cm7 obtained by rubbing ZDDP onto a silver mirror to this vibration. The PO„ anion, has two vibration frequencies, -1 asymmetric stretch 970-1242cm and symmetric stretch at 893-1021cm-1 (63). The higher frequency (asymmetric) band is usually strong in intensity while the symmetric band is usually weaker, sometimes occuring as a shoulder. Using this correlation the band at 1218cm-1 can be assigned to the 2 - asymmetric stretching vibration of PO„ anion and the weaker -1 band at 949cm to the symmetric stretch. The smaller peaks at 1040 and 787cm 1 are difficult to assign on the basis of this correlation. The band at 1040cm-1 could be assigned to v(C-O) and that at 787cm-1 to P-O-(C). Absorption bands at ^3000-2800cm71 also observed were assigned to v(C-H). An alternative correlation for this spectra is that of the POS - vibration, overall frequency ranges for v(P-O) of 1050-1240cm~1 and v(P-S) 545-655cm~1 are given by Thomas (63). This frequency range for v(P-O) would appear to be too high in the light of v(P-O) assignments discussed earlier in the text (4.4.3). However if this anion exists in the mesomeric form (I and II), then both the P-0 and P-S would have a bond 130

O 0 / I \

order between 1 and 2. This would explain the relatively high frequency range quoted for both bonds. In the mesomeric form POS ~ is characterised by two strong absorption bands, restricted frequency ranges of 1110-1215 (P-O) and 577-658 (P-S) are given by Thomas (63) for compounds of the type III.

R-0 0 \ / P

R-0 S

III

The band at 1218cm"1 can be assigned to v(P-O) and that indicated at <600cm" to v(P-S). The high frequency of the P-O vibration suggests that the species approximates more to the thiolo form (I) rather than the thiono (II). Using this correlation the band at 949cm"1 can be assigned to P-O-P vibration (63). On this basis diethylphosphorodithioic acid reacts rapidly with the steel surface to form a thick,possibly polyphosphate film. A structure of the type IV is suggested. TABLE 5.2 SUMMARY OF IR PEAK POSITIONS AND ASSIGNMENTS FOR FIGURE 5.14

Peak Position/cm 1 Assignment Comment

1218 v(P-O) ionised po~/pos~

1048 v(C-O) Perturbated by P02 949 P-O-P Polyphosphate species 787 v(P-O) P-O-(C) ^600 v(P-S) POS"

TABLE 5.3 SUMMARY OF IR PEAK POSITIONS AND ASSIGNMENTS FOR FIGURE 5.15

Peak Position/cm 1 Assignment Comment

1460 1220 v(P-O) ionised PO~/POS" 950 P-O-P Polyphosphate species 750 v(P-O) P-O-(C) 132

Frequency cm *

concentration : 0.0015M immersion time : A 1 hour B 4 hours 1 reflection

FIGURE 5.15 IR Reflection Spectra of Steel Surfaces After Immersion in Diethylphosphorodithioic Acid Solution at 200°C 133

Peak position and assignments for Figure 5.14 are summarised in Table 5.2

5.7.1.3 160 °-200 °C

Steel samples were immersed in the test solutions for 1-8 hours, representative spectra are shown in Figure 5.15. Similar results were obtained for both concentrations, the result shown in Figure 5.15 is for 0.015M acid solution at 200°C. Spectra were characterised by an intense absorption at 1220cm~ , film thickness was observed to increase rapidly with them. Generally the absorption band obtained at 1220cm-1 was broader for higher concentrations. Further peaks were observed in the spectrum at 1640, 1460, 950 (sh) and 750cm-1. For prolonged immersion times a black polymeric residue was observed on the metal surface and in solution IR analysis of this indicated a polyphosphate composition, similar to spectra obtained in the 100-150°C temperature range. v(C-H) was also observed at ^3000cm-1 for both the surface film and polymeric residue. Spectra obtained for this temperature range were similar to those described in 5.7.1.2, and similar assignments have been assumed,these are summarised in Table 5.3. 134

5.7.2 Zinc Diethyldithiophosphate

Zinc diethyldithiophosphate was tested at two concentrations in hexadecane, 0.0015M and 0.015M.

5.7.2.1 Room Temperature Tests

Steel samples were immersed in the test solutions for 1-72 hours. A representative result is shown in Figure 5.16. Similar results were obtained to those of Francis and Ellison (51). At low temperatures zinc diethyldithiophosphate appeared to adsorb onto the metal surface forming a thin (possibly monomolecular) thiophosphate layer. Spectra obtained from these experiments were poorly resolved, only weak absorption bands were recorded. Adsorption did not appear to increase with time, beyond a limiting coverage, again in agreement with earlier results (51). From the spectra obtained similar peak positions for the adsorbed ZDDP to the original ZDDP spectrum were observed, for the C-O-(P), C-C and P-O-(C) vibrations. At increased temperatures ru80-100°C slightly different spectra were obtained (Figure 5.16). Two peaks at 1020, 1040cm"1 attributed to v(C-O-) were still visible, but in the OSO-gOOcm"1 region a broad unresolved absorption band was observed. Peak maxima were at 670, 700, 760 (shoulder) and 800 (shoulder) cm"1. A table of peak position and assignments,with those of the original zinc diethyldithiophosphate is given in Table 5.4. Vibration frequency shifts for both the P=S and P-O-(C) bonds were observed for the surface film. This could possibly be attributed to the formation of an iron dithiophosphate. The intense absorption band at 700cm 1 is tenatively assigned to the symmetric P-O-S vibration (107). Frequency cm 1 Concentration : 0.0015M Immersion Time : 2 hours Single Reflection Ordinate Expansion x 10

FIGURE 5.16 IR Reflection Spectrum From a Steel Surface After Immersion in Zinc Di-ethyldithiophosphate Solution at 80-100°C TABLE 5.4 SUMMARY OF IR PEAK POSITIONS AND ASSIGNMENTS FOR FIGURE 5.16

Spectra Zinc Diethyl- Figure 5.16 Dithiophosphate

Peak Assignment Peak Assignment Position Position cm cm"1

1040 , symmetric v(C-O) 1040 , symmetric v(C-O) 1020 asymmetric v(C-O) 1020 asymmetric v(C-O)

962 v(C-C) 970 v(C-C)

813 symmetric v(P-O) 800 v(P-O) symmetric 786 asymmetric v(P-O) 760 v(P-O) symmetric

661 v(P=S) 700 P-O-S symmetric 540 v(P-S) 670 v(P=S) ZDDP/FeDDP 137

5 .7 .2.2 100-150 °C

Steel samples were immersed in additive solutions for 1-8 hours. Zinc diethyldithiophosphate reacted slowly with the steel surface to form a thin surface film, this was characterised by a broad weak absorption band between 1250-900cm-1. This is similar to results presented by Francis and Ellison (51) for this temperature range, they 2- assigned this peak to a SO^ vibration. Lubrizol 1395 formed similar films in this region, an example of which is shown in Figure 5.19. Film formation in both cases was slow and appeared to reach a limiting value after 4-5 hours. Confirmation of this assignment of iron sulphate rather than a zinc/phosphate species has been provided by preliminary Electron Probe Microanalysis studies of these films. Strong sulphur but relatively weak zinc, and phosphorus signals were observed. A rough estimate of limiting film thickness by IR RA calculation supported by EPMA calibration is of 50-100&.

5.7.2.3 150-200 °C

Steel samples were immersed in test solutions for 1-3 hours at 150-200°C. Representative spectra of the results are shown in Figures 5.17 and 5.18. In this temperature range ZDDPs rapidly formed a thick polymeric surface film, characterised by intense absorption bands at gOO-lSOOcm"1. Film thickness was observed to increase rapidly with time, initially a brown film was observed, this was gradually covered by a blue one. At high concentrations (0.015M ) or after prolonged heating, the additive solution formed a brown precipitate. Examina- tion of both the bulk decomposition product (Figure 5.18) and the metal specimen (Figure 5.17) indicated a similar chemical structure.

A full list of absorption band positions and assignments is given in Table 5.5. The blue/brown film was observed to increase in thickness 0 i—I 4H 0 ft

1800 1000 ,400 Frequency cm"1

(A) 1 hour at 200°C (B) 2 hours at 200°C (C) 3 hours at 200°C

FIGURE 5.17 Increase of Film Thickness with Time for Metal Samples in 1% w/w Zinc Diethyldithiophosphate Solution Frequency cm

FIGURE 5.18 IR Transmission Spectrum of the Bulk Decomposition Precipitate of Zinc Di-ethyldithiophosphate in Hexadecane TABLE 5.5 SUMMARY OF IR PEAK POSITION AND ASSIGNMENTS FOR FIGURES 5.17, 5.18

Figure 5.17 Figure 5.18

Spectra Peak Position/ Assignment Peak Position/ Assignment cm cm~A

716 515 f>-0 deformation 789 P-O-(C) stretch 953 P-O-P stretch 926 P-O-P stretch 1075 £-0 (PO„ symmetric stretch) 1056 f>-0 (PO~ symmetric stretch) + _ 2- 2- 1175 1129 P-0(P03 asymmetric stretch) P-0(P03 asymmetric stretch)

A 1222 P=0 stretch P=CKM 1215 P=0 stretch (P=0 H) B 1227 P=0 stretch P=0-*M C 1234 P=0 stretch P=CKM 141

as the test proceeded, no limiting value was found. The absorption band at 1215cm"1 in the spectrum of the bulk decomposition product, is assigned to the P=0 valence vibration, although this at first appears to be too low as Thomas(63) gives a frequency range of 1282- 1307cm-1. It is suggested that the low vibration frequency is due to hydrogen bonding between the phosphoryl group and water of hydration. A broad band at 3100-3600cm 1 was also observed in this spectrum, this is assigned to hydrogen bonded OH stretch. Lowering of the phosphoryl vibration due to hydrogen bonding has been reported by several workers (6) (133). Smith (133) noted that on formation of the sodium salt of di-n-butylphosphate, the P=0 vibration frequency increased from 1220cm"1 to 1242cm 1. He explained this as being due to the release of the P=0 group from hydrogen bonding. In the R-A spectrum the P=0 absorption is observed at 1222cm"1, a shift of 7cm"1. This indicates that although the P=0 group has been released from hydrogen bonding (no bands at 3100-3600 were observed in this spectrum) the vibration frequency is still too low. It is suggested that in this case the phosphoryJL vibration has been perturbated by coordination to the metal. Formation of donor P=0+ metal bonds occurs resulting in a lowering of the vibration frequency. Guilbault and Daas (134) reported a frequency range of 1167-1232cm-1 in their study of the chemisorption of di-iso-propyl methyl phosphonate with the chlorides iron, vanadium and copper, which supports the above assignment. As the film thickness increases, the frequency of the phosphoryl band also increased (Table 5.5). This effect was not observed for the other absorption peaks. This suggested that the influence of the metal atoms decreases as film thickness increases. The two intense bands at 1050 and 1127cm 1 in the transmission spectrum are typical of ionised P-0 vibrations (77). Frequency shifts of 20 and 48cm"1 respectively are observed for those peaks on surface film formation. In general all absorption bands in this R-A spectrum showed a shift to higher frequency on film formation. The band at 142

926cm assigned to the P-O-P stretching vibration was observed to shift +27cm . At present no explanation is offered for this. A polymer structure similar to that reported by Jones and Coy (30) for the bulk decomposition products (I) is suggested for both the spectra obtained.

0 0 0 II II II -0-P-0-P-0-P-SR I I I I -0 0- SR

Chemisorption of this species onto the metal surface is thought to occur forming polymeric film as in II

Zn Zn R 0 0 S

ZnO-P-O-P-O-P-SR 11 II II II 0 0 0 + i i M M M

5.7.3 Lubrizol 1395

Lubrizol 1395 was tested at two concentrations 0.1% w/w and 1.0% w/w in hexadecane.

5.7.3.1 Room Temperature

Similar results were obtained for Lubrizol 1395 to those for zinc diethyldithiophosphate in this temperature range (reported in 5.7.2.1). Lubrizol 1395 was found to form a thin adsorbed layer on the metal surface. Spectra for these results were poor, only very weak absorption bands were recorded. As an alternative technique samples were drained rather than washed with heptane. No significant improvement 143

Frequency cm

Concentration : 1% w/w Immersion Time : 8 hours 1 Reflection Ordinate expansion x 2

FIGURE 5.19 IR Reflection Spectra From a Steel Surface After Immersion in Lubrizol 1395 Solution at 100-150°C 144

in the spectra was observed. From these results it would appear that ZDDPs are only weakly adsorbed onto steel surfaces. Adsorption was not observed to increase significantly with time.

5.7.3.2 100-150°C

Steel samples were immersed in additive solutions (0.1% and 1% w/w) for 1-8 hours. In this temperature range Lubrizol 1395 slowly formed a thin surface film similar to that obtained for zinc diethyldithiophosphate. A representative spectrum is shown in Figure 5.19. Film thickness did not appear to increase significantly with immersion time or concentration beyond a limiting value. Lubrizol 1395 reacted more rapidly than the pure ZDDP forming generally thicker films for the same immersion times. The spectra were characterised by broad absorption bands at 1140, 1050 and 788cm-1, these are attributed to v(P=0) and a sulphate species. Similar results have been obtained by earlier workers (51).

5.7.3.3 150-200 °C

Steel samples were immersed in additive (0.1% and 1% w/w) solutions for 1-3 hours. Similar results were obtained to those for zinc diethyldithiophosphate in the same temperature range. Lubrizol 1395 rapidly formed a thick polyphosphate film on the steel surface with a composition similar to that of the bulk decomposition product (Figure 5.20). A full list of peak positions and assignments is given in Table 5.6. Intense absorption bands were observed in both spectra at 900-1300 and ^600cm-1. The bands at 1220cm-1 were attributed to v(P=0). Comparison of the intensities of v(C=H) and bands at ^1160 for both the bulk and surface products indicates that the metal v(P=0) vibration is far more intense, suggesting that the band is 3000 1500 1000 400 Frequency cm

1. R-A Spectrum. Concentration : 1% w/w, Immersion : 3 hours, Ordinate expansion x 2 2. Transmission Spectrum. Concentration : 1% w/w, Reaction time : 3 hours

FIGURE 5.20 IR Spectra for Lubrizol 1395 Test Results at 200°C 146

in fact ionised. In general in this region the R-A spectra is broader and less resolved than the tranmission sample. Peaks at 1150 and 1060 are clearly resolved in the bulk sample being reduced to a broad shoulder in the R-A spectrum. The peak at ^socm-1 is again assigned to P-O-P stretch (63). Corbridge (71) also assigns a lower frequency absorption band at 670-800cnT1 to the P-O-P group. This could explain the origin of bands at ^750cm-1. As in the previous section (5.7.2.3) a chemical structure such as that proposed by Jones and Coy (30) is suggested for both the bulk decomposition and surface film spectra. Increased intensity of the P=0 vibrations compared to the v(C-H) indicates ionisation of the bonds. This is possibly due to the formation of donor P=CKM bonds. It is therefore suggested that chemisorption of the polymeric species onto the metal surface occurs through complex bond formation. Using the R-A technique film thicknesses were estimated to be in the region of ^10002 for the brown film. Similar thicknesses have been reported by other workers for such films (52). Successive washing of the metal samples with heptane, acetone and methanol indicated that these films are strongly adherent. Small decreases in film thickness were observed (Figure 5.21). A: Heptane B: Acetone C: Methanol

lA , 1200 1000 CM"1 FIGURE 5.21 Effect of Successive Washing of a Metal Sample TABLE 5.6 SUMMARY OF IR PEAK POSITIONS AND ASSIGNMENTS FOR FIGURE 5.20

Spectra Figure 5.20(1) Figure 5.20(2)

Peak Position/ Assignment Peak Position/ Assignment cm-1 cm""1-

1637 1637

1472 CH2 1470 CH2 1220 P=0 stretch (P=0->M) 1220 P=0 stretch (P=0-+M) 1157 F-0 1137 £-0 1058 F-0 951 P-O-P stretch 942 P-O-P stretch 750 P-0-P(?) 730 P-0-P(?) 600 P-S stretch 550 P-S stretch 148

5.8 Discussion

The nature of chemical film formed by both ZDDPs as diphosphorothioic acids during immersion tests has been found to be dependent on temperature. For the dtp acid two general types of film were formed:

1. Low temperature: Acid reacts to form a P-O-S species. With increasing time or concentration further adsorption/reaction occurs forming a brittle surface layer.

2. >100°C: At elevated temperatures a thick polyphosphate film is observed for both concentrations. This film appears to form a protective layer preventing further corrosive reactions.

For the ZDDPs tested both commercial and pure samples formed essentially similar films at all temperatures. Generally film formation was more rapid for the commercial additive at comparable concentrations and reaction times. Three general types of film have been observed. These can be summarised as follows:

1. Low temperature, 20-100°C: Formation of an essentially thiophosphate film possibly by weak adsorption of ZDDP onto the metal surface. At higher temperatures 80-100°C slightly different spectra are observed, possibly due to the formation of an iron dithiophosphate.

2 . 100-180 °C: In this region ZDDPs form an essentially inorganic surface film. Possibly due to sulphate species.

3. >150°C: At high temperatures ZDDPs form a thick chemisorbed surface film with a polymeric structure similar to that of the bulk decomposition product. Film thickness increases rapidly with time, while the colour is observed to change from brown overlaid by 149

blue as the reaction proceeds.

The antiwear function of ZDDPs has been related by many workers (124) (52) (91) to the formation of a polymeric phosphorus-sulphur layer. In this study thick polyphosphate films have been shown to form on metal surfaces at elevated temperatures. The formation of adherent polyphosphate films is also responsible for the observed anticorrision action of ZDDPs. The use of polyphosphate films as passivating layers on metal surfaces has been extensively reported (132). From these results it would be expected that initially, ZDDPs form good AW films only at high temperatures. This has been indicated by results presented in Chapter 6. Decreasing thermal stability would be expected to increase AW efficiency as it is the phosphorus thermal decomposition (Figure 2.6) products that are responsible for polymer film formation. Thermal degradation of ZDDPs has been shown to be autocatalytic, in fact extended immersion experiments at relatively low temperatures, <110°C,indicate that polymer film formation would occur after an induction period of several hours. It would be expected that although fresh ZDDP oils would only provide AW film formation at high stress points eventually the autocatalytic nature of the process would mean that all metal surfaces are covered by the brown polymeric film. This film has been observed in earlier work with ZDDPs on cams and tappets (42). The effect of dialkylphosphorodithioic acid addition is not clear, in solution the acid would be expected to catalyse ZDDP thermal decomposition and the cationic decomposition of hydroperoxide. Solutions of dtp acids at concentrations comparable to ZDDP solutions, react rapidly with metal surfaces at room temperature giving a variety of products. At all concentrations tested diethylphosphorodithioic acid was found to react corrosively with steel forming a brittle surface layer. At high temperatures a polyphosphate structure was indicated, which appeared to have anticorrosive properties. The polyphosphate film obtained for the ZDDPs and acids 150

were significantly different. ZDDP spectra were characterised by a broad, intense, unresolved band at 900-1300cm These are attributed to various ionised P=0 bond vibrationsipossibly ; indicating chemisorptive bond formation (P=CKM). The acid spectra revealed a sharp intense peak at ^1220cm again this was attributed to an ionised P=0 vibration. The ZDDP spectra suggests a more complex structure with a longer polymeric chain. Examination of the results obtained in Chapter 6 indicates that both ZDDPs and acids are capable of forming thick boundary films during HFR testing. Boundary film formation was observed at lower temperatures (100-140°C) for the dtp acids than for the zinc salt. If polyphosphate film formation is a necessary precursor of boundary film formation, these results are in general agreement. The dtp acid tested was found to form polyphosphate films at ^100°C whereas for the zinc salt film formation was at high temperatures ^180°C or longer immersion times. It would appear for these results and those presented in Chapter 6 that the AW functioni of ZDDPs can be related to polyphosphate film formation. In the EP region, surface films with high sulphur content have been reported usually identified as iron/zinc sulphide (Chapter 2). Static tests have indicated that in the 100- 180°C temperature range ZDDPs form thin iron sulphate films. Whether these static films can be related to those formed under rubbed conditions is doubtful, although recent work has indicated that oiliness additives adsorb more strongly on iron sulphate surfaces than iron sulphide (123). The function of an EP layer is to provide an easily sheared protective layer or chemical film onto which other additives can strongly chemisorb (121). In this case it would appear that efficient formation of an iron sulphate is beneficial to EP performance. Hydroperoxide addition has been observed to facilitate sulphate film formation (51) either by oxidation of sulphur species in solution or possibly by oxidation of the iron sulphide film. In this study IR RA spectroscopy has not been applied 151

quantitatively in the analysis of film formation, although several workers have used such film thickness measurement successfully in the past. Film thickness can be calculated using a simple form of equation 5.2. Use of this Equation requires knowledge of the optical constants of the surface film, and the angle of incidence which in this study was not accurately known. The formula was used for the rough estimation of film thickness reported in the text , but it should be emphasised that these figures are approximate and have not been supported as yet by other analytical techniques.

5.9 Conclusions

The following conclusions have been drawn from the foregoing chapter.

1. Dialkylphosphorodithioic acids at low temperatures react rapidly with steel surfaces to form adherent inorganic chemical films. Further reaction then occurs forming a brittle layer which is eventually removed as a brown powder. At higher temperatures, film formation is more rapid forming a thick reacted polyphosphate layer.

2. a) ZDDPs at temperature chemisorb onto steel surfaces forming a thin surface layer. At higher temperatures a reaction takes place forming iron ddp.

b) 100-150°C ZDDPs form an essentially inorganic layer composed of iron sulphate. Film formation is slow and a film thickness of possibly ^50-1008 was found to be limiting. The commercial additive was found to react more rapidly than the pure additive.

c) At high temperatures both commercial and pure ZDDPs rapidly formed a thick inorganic/organic polymer layer with a structure similar to that of the bulk 152

decomposition product. A structure of the form (I) is suggested,similar to that proposed by Jones and Coy (30).

0 0 0 II II II -0-P-0-P-0-P-0- I -0 SR 0-

Wakins (90) has suggested that such a polyphosphate physically adsorbs on iron oxide surfaces # ^g this species which is responsible for AW behaviour. Georges and co-workers (52) also recognised the formation of such thick polymeric layered films as being responsible for AW protection. Formation of this type of film could be responsible for the observed anticorrosion properties of ZDDPs. 153

CHAPTER 6 LUBRICATION OF STEEL BY ZINC DIALKYLDITHIQPHOSPHATES

6.1 Introduction

A major part of this study comprised friction testing of the prepared organophosphorus additives. It was expected that a combination of friction test results, static tests and analysis of bulk oil changes would provide some insight into the mechanism of ZDDP antiwear action.

6.2 Methods of Studying Load Carrying Additives

There are two broad approaches to the study of boundary lubrication using friction and wear tests. The first adopts a fundamental approach, and uses test rigs which are designed to look at simplified, controlled contact conditions rather than simulate any particular application. Early workers (115) used static friction tests in the development of lubrication theories- In later studies kinetic friction was measured. The Bowden-Leben test device was first introduced in 1939 (10), and has been extensively used by previous workers (113) (114) . In this test the kinetic coefficient of friction can be measured at slow speeds (typically O.lcm/sec) as a function of oil temperatures (n 300°C). The second is used mainly in the evaluation of commercial lubricants as a preliminary screening prior to full scale testing. There are many test devices available, these include the four-ball test, pin and disc and Falex EP machines. In these tests two metal surfaces are rubbed together at high loads and speeds. Most devices can be used to assess load- carrying additives in both the AW and EP regions. For both types of testing the effects of a number of parameters on additive performance can be investigated, these include sliding speed, oil temperature, metallurgy and geometry of contact. 154

There are disadvantages associated with both types of testing. In the Bowden-Leben device a large surface area of lubricant is exposed to the air which can cause high oxidation rates at elevated temperatures. It is a single pass test and, as such, sensitive to contamination of the metal surfaces, which could modify friction. Repeatability of test results therefore is poor, even with careful cleaning of metal surfaces (113). The commercial test devices also present problems in evaluating results. It is important that the tests are entirely in the boundary region, hydrodynamic lift generated by high sliding speeds has been reported (116), for some test devices. Also problems due to frictional heating of lubricant and metal surfaces can occur. As contact conditions are uncontrolled the effect of any single parameter, especially temperature on the performance of a lubricant is hard to deduce. It was therefore decided to test the prepared additives on a new friction testing device, the High Frequency Recipro- cating (HFR) machine. This test device provides a repeated pass boundary lubricated contact. Both coefficient of friction and electrical contact resistance can be monitored as functions of oil temperature. Electrical contact resistance measurements have been extensively used in boundary lubrication to monitor metal-metal contact (117) , formation of surface films (118) (52) and oil film thickness (119) . On the HFR rig it is used to observe qualitatively any thick reacted surface films formed during lubrication. The advantages of the HFR rig over other friction devices available is that it provides a controlled contact which nevertheless simulates boundary lubrication conditions found in practice. It is a repeated pass device with out of contact times comparable to those found in reciprocating engines (107). The surface area of lubricant exposed to the air is small, so that temperatures of >300°C can easily be obtained without excessive oxidation. VIBRATOR

FRICTION FORCE

FIGURE 6.1 Schematic Diagram of the High Frequency Reciprocating Device 156

FIGURE 6.2 Overall View of the Test Apparatus 157

6.3 Test Apparatus and Procedures

6.3.1 HFR Rig

The HFR (High Frequency Reciprocating) rig was originally developed in the Lubrication Laboratory in 1978 to study piston ring scuffing. It consists essentially of an upper steel specimen which is loaded and oscillates against a lower steel plate. The whole system is immersed in a heated lubricant bath. The contact is repeated pass, slow speed, boundary lubrication. The slow speed and the geometry of the upper specimen ensure that there are no hydrodynamic effects and also that flash temperatures remain small (n 5°C) (109). The system is traditionally arranged to monitor continuously both friction and electrical contact resistance. The latter provides a qualitative measure of the build up of thin lubricant surface films in the contact.

6.3.2 Description of HFR Device

The test apparatus used is shown in Figures 6.1 and 6.2. It has been fully described in earlier publications (109) (112). It consists of an upper dynamic specimen held in a chuck which can be vibrated against a lower static specimen. The chuck and connecting shaft are made of stainless steel and easily demountable for cleaning. The static specimen is held in a stainless steel oil bath, which is located in a larger aluminium heater box. Both oil bath and static specimen are held rigidly in the heater box by a screw and spring arrangement (Figure 6.1). Rod heaters of 200W are inserted into the aluminium block so that oil bath temperatures of n 300°C can be reached. In the present study the temperature of the test specimen and lubricant were monitored by a stainless steel sheathed chromel-alumel thermocouple placed near the contact area. The output from the thermocouple was observed on a calibrated microvoltmeter and recorded by hand throughout a test. The friction force was measured by a piezo electric crystal force gauge attached to a connecting block fixed under the heater 158

FIGURE 6.3 Oscilloscope Traces 159

1 forward + 4 reverse stroke

High friction, trace unstable

No film formation

Friction reduced, will fluctuate as boundary film forms and breaks down

Partial thick boundary film ECR forming. Tending to break o _ down at end of stroke

Low friction (^O.Im)

Stable thick boundary film formed

FIGURE 6.4 Typical CRO Traces 160

box, which transfers the friction force from the lower test piece. The average friction force was plotted on a two channel chart recorder. In this study, electrical contact resistance across the contact was monitored as a qualitative indication of the existence of a lubricant film. The dynamic specimen was electrically insulated from the apparatus base. A constant voltage source was connected to the dynamic specimen and the potential drop between the contact of the metal specimens recorded. Contact resistance output was monitored by both chart recorder and CR oscilloscope. A permanent record of the average friction and contact resistance trace was obtained. The CR oscilloscope was used to simultaneously monitor friction force and formation of a lubricant film as a function of stroke position. A typical oscilloscope trace is shown in Figure 6.3. Observation of the oscilloscope trace provided more precise information on the nature and formation of a lubricant film and film stability throughout the stroke • (Figure 6.4). Wherever possible this information was used in the interpretation of results, as will become clear later in this chapter.

6.3.3 Test Procedure

Immediately after assembly of the test pieces, the test solution was introduced to immerse the flat specimen. The chuck was then loaded against the static specimen by a hanging weight suspended directly below the contact point. The thermocouple was cleaned and placed in position close to the contact area. The stroke length, load, frequency and heating rate were constant for every test. Their values were determined in a series of preliminary tests described later in this chapter (6.4.2). The sensitivity of the contact resistance measurement was altered as required. Friction and electrical contact resistance traces were zeroed on the two channel plotter. The vibrator was switched on and the friction trace at constant temperature recorded until the trace stabilised (n 5 minutes). The heater was then switched on and the coefficient of friction and contact 161

resistance traces obtained to ^ 240°C, the oil temperature was plotted by hand. Periodically (100°C, 200°C, 230°C) samples of the test solution were withdrawn from the oil bath using a microcapillary tube for infra-red analysis. A sample of the original solution was also analysed, the object was twofold. Firstly to ensure that there was no residual contamination from the test apparatus. Secondly to monitor bulk oil changes throughout the HFR test especially those due to oxidation of the hexadecane.

6.3.4 Cleaning Procedure

After initial testing the following cleaning procedure was adopted. The metal test pieces, chuck, shaft and oil bath assembly were immersed in toluene and cleaned in an ultrasonic bath. They were then removed, washed with acetone, air dried and assembled. Periodically, to prevent build up of contaminants or after testing an ester, the whole test assembly was dismantled and cleaned in toluene vapour in a soxhlet for three hours. After washing in acetone and air drying it was reassembled. All solvents used were of analar grade. After each test the static specimen was polished with 600 grade emery paper (109) so that the previous wear scar was removed. The specimen was then cleaned as previously described A new dynamic specimen was used for each test.

6.3.5 Solvents, Additives, Metals Employed

a) Test solvent

n-Hexadecane (chemical formula CiAH0/1) was used as the 16 34 main test solvent. It fulfilled the necessary conditions of being a non polar, non volatile temperature stable solvent of high purity. Previous investigators have used this lubricant on both the Bowden-Leben (113) (114) and HFR test rigs (109). 162

Mills reported that HFR tests using hexadecane were in the boundary region at all frequencies, no hydrodynamic lift being observed (109).

To remove polar impurities, the hexadecane was purified by percolation through activated silica gel/aluminium oxide. It was then stored over silica gel/aluminium oxide in a brown glass bottle until required. Immediately before use it was filtered through a sintered glass filter under reduced pressure. The purity of the hexadecane used was checked periodically by infra-red spectroscopy. Test solutions were made up just before use.

b) Additives

The preparation and purification of the organophosphorus additives used has already been described (Chapter 3). Table G*2 gives a list of additives used in friction testing, together with certain physical properties. The dialkylphosphoro dithioic acids were found to absorb water very quickly, so immediately after purification they were stored over anhydrous magnesium sulphate and used as soon as possible. All solutions of ZDDP's were initially made up to the same molarity (0.0015M), this corresponded to a concentration of 0.1% w/w for ZDDP's in the middle of the molecular weight range. This initial working concentration was established in a series of preliminary tests which are described below (6.4). The lower molecular weight ZDDP's, methyl, ethyl, propyl were found to be only slightly soluble in hexadecane. Solubility was found to increase as alkyl chain length increased. As a consequence some solutions were warmed (% 60°C) to fully dissolve the additive. For high concentrations of zinc diethyldithiophosphate (1% w/w) a small amount of heptane (^ 5% w/w) was added to aid dissolution. 163

c) Metallurgy

Static Specimen: A strip of EN31 steel (composition below, Table 6.1) 20mm x 10mm plate cut and surface ground from a 50mm diameter round bar was employed as the stationary test piece.

Dynamic Specimen: The moving specimen was a 6.35mm EN31 steel ball bearing supplied by Fag Bearing Company. A new ball was used for each test.

TABLE 6.1 SPECIFIED COMPOSITION OF EN31 STEEL

C(%) Si(%) Mn( %) S(%) P(%) Cr (%) Fe( %)

0.9- 0.10- 0.30- 0.050 0.050 1.0- remainder 1.2 0.35 0.76 (max) (max) 1.6 164

6.4 Preliminary Results

6.4.1 Test Repeatability

Previous workers (109) (112) have shown the HFR tests to be very repeatable. This has been attributed to the early removal of surface contaminents by the repeated passes. The good repeatability of tests is shown in Figure 6.5. Because of this two consecutive tests were run on each batch of lubricant . If an anomalous result occurred the tests were repeated. Although the friction trace was found to be very repeatable the contact resistance measurements were not quite so consistent, therefore only a qualitative picture of the general formation and decay of thick lubricant films could be deduced.

6.4.2 Other Preliminary Tests

A series of preliminary tests were run to establish a number of operating variables. These were: a) Stroke length b) Frequency c) Heating rate d) Load e) Additive concentration.

a) Stroke length

A stroke length of ±0.5mm was used for all tests. Previous workers (109) have shown that a stroke length of 1mm is independent of friction force at frequencies of 50-100Hz. It was thought that a stroke length of 1mm was also large enough to prevent fretting (109).

b) Frequency

Mills (109) showed that a stroke of 1mm and frequency

of 50Hz that lubrication was entirely boundaryt "when the 165

viscosity of the test lubricant was less than 250cs. No evidence of partial hydrodynamic film formation by hexadecane was observed due to its low viscosity. Mills calculated that the midstroke flash temperatures generated using a stroke of 1mm and frequency of 50Hz were of the order of 5°C. A sliding speed of 2.5cm/second was used.

c) Heating rate

A standard heating rate of 7°'C/min was used, so that excessive oxidation of hexadecane at high temperatures was avoided (114).

d) Load

A load of 0.5kgf was used for all testing. The calculated contact pressure for a 6.5mm diameter ball on flat was of the order of 0.6GPa (109).

e) Additive concentration

Preliminary tests on 1395 were carried out using additive concentrations of 0.1%, 0.5%, 1% w/w. The results are shown in Figure 6.16. At low concentrations a thick lubricant film was found to form at high temperatures. It was this film formation which was of interest and all further tests were performed at % 0.1% w/w, although the effect of increased concentration of the additives was also investigated. 166

The test conditions used throughout the testing programme were as follows:

Load 0.5kgf

Contact Pressure 0.6GPa

Frequency 50Hz

Stroke Length ±0.5mn

Heating Rate 8 °C/min

Static Specimen polished EN31 steel strip

Dynamic Specimen 6.35mm EN31 steel ball bearing

Additive Concentration 0.1-1.0% w/w

Base Stock n-Hexadecane 167

p o pa OS c o o w u Test 2 fP (D 3 o Test 1 p cn o -H CD CQ rH (D W OS

3 o •M p o •H ft

P O p

3 (1) 0*1 - Test 2

P P G Test 1 C O

Temperature/°C

Figure 6.5 Test Repeatability of HFR Friction Device. Friction and Electrical Contact Resistance Traces are Shown as Functions of Temperature. Two tests are shown, the Upper Curve is Displaced 1.5cm for clarity. 168

6.5 Main Results

A number of different organophosphorus additives were tested on the HFR device. A full list is shown in Table 6.2. In order to evaluate the action of an individual component of a commercial additive, each was tested separately on the HFR. The various components were then broughttogether in different combinations so that their joint effect on electrical contact resistance and friction could be monitored. Initially all additives were tested at equimolar concentrations (0.0015M), this corresponded to 0.1% w/w for additives in the middle of the molecular weight range. Later the effect of increased concentration, preoxidation of the hexadecane and addition of parent phosphorodithioic acid was investigated. The test programme can be divided into two separate parts.

1) Tests on single additives summarised in Table 6.2. (Described in 6.5.3). 2) Tests on combinations of additives summarised in Table 6.3. (Described in 6.6).

The results are presented graphically as electrical contact resistance (ECR) (upper curve) and coefficient of friction (lower curve) as functions of temperature. A high ECR trace indicates infinite resistance and the formation of a thick non-conducting film in the contact. Zero ECR indicates metal-metal contact . In addition a short description of each test result is given in the text. For a number of tests no graph is provided, only a description. This occurs where the result obtained is similar to one already shown. Due to generally good repeatability , only one representative test result is shown. TABLE 6.2 SUMMARY OF ORGANOPHOSPHORUS ADDITIVE TESTS

Additives Molecular Concentration in Hexadecane (% w/w) Weight

Di-ethylphosphorodithioic Acid 186 0 . 0015M (0 .036), 0.003M (0.072) Di-iso-propylphosphorodithioic Acid 214 0 . 0015M (0 .048), 0.003M (0.09)

Zinc di-ethyldithiophosphate 434 0 . 0015M (0 .084), 0.009M (0.5), 0 .017M (1. 00) Zinc di-n-propyldithiophosphate 490 0 . 0015M (0 .096) Zinc di-iso-propyldithiophosphate 490 0 . 0015M (0 .096), 0.0078M (0.5), 0 .015M (1 .00) Zinc di-n-butyldithiophosphate 546 0 . 0015M (0 .105) , 0.0075M (0.5), 0 .015M (1 .00) Zinc di-iso-butyldithiophosphate 546 0 . 0015M (0 .105), 0.0075M (0.5), 0.015M (1 .00) Zinc di-sec-butyldithiophosphate 546 0 . 0015M (0 .105) Zinc di-n-hexyldithiophosphate 659 0 . 0015M (0 .127) Zinc di-n-octyldithiophosphate 771 0 . 0015M (0 .15), 0.005M (0.5), 0 .01M (1 .0)

Lubrizol 1395 - 0 .1% w/w 0.5% w/w, 1.0% w/w TABLE 6.3 SUMMARY OF MIXED ADDITIVE TESTS

Additive Zinc Dialkyl Dialkyl j Preoxidised Pure Anti-oxidant Mixtures dithiophosphate phosphorodithioic Hexadecane Hexadecane (Toponol OC) (concentration) Acid (concentrat ion) Test (concentrat ion) (Alkyl Group)

6.20a (ethyl) X / (0.003M) / X X 6.20b X / (0.003M) X X (iso-propyl) /

6.21a (ethyl) / (0.0015M) / X / X 6 . 22a / (0.0015M) X X (iso-propyl) / /

6.21b (ethyl) / (0.0015M) X / X X 6.22b / (0.0015M) ' X X X (iso-propyl) /

6.23a (ethyl) / (0.017M) X / X X 6.23b / (0.015M) X X X (iso-propyl) /

6.21c (ethyl) / (0.0015M) / / X X 6 .22c / (0.0015M) / / X X (iso-propyl)

6.24 / (0.0015M) X X / / (1% w/w) 171

ECR

ECR zero throughout the test

M »

0-5 Failed 80°C

0-4

0-3 -

0-2 ~

0-1 -

1 I 50 100 150 200 250 Temperature /°C

Figure 6.6 Pure Hexadecane

ECR

ECR zero throughout the test

0' 5 H

0* 4

0- 3 '

0-2 "

0- 1 -

—I i 5 0 100 150 200 250 Temperature /°C

Figure 6.7 Preoxidised Hexadecane 172

Temperature/°C a) 0.0015M Zinc Diethyldithiophosphate

Temperature/°C b) 0.5% w/w Zinc Diethyldithiophosphate

Figure 6.11 Zinc Dinbutyldithiophosphate 173

c) 1% w/w.Zinc Diethyldithiophosphate

Figure 6.8 Zinc Diethyldithiophosphate 174

Figure 6.9 0.0015M Zinc Dinpropyldithiophosphate 175

Temperature/°C a) 0.0015M Zinc Diisopropyldithiophosphate

b) 0.5% w/w Zinc Diisopropyldithiophosphate

Figure 6.10 Zinc Diisopropyldithiophosphate 176

ECR

1 1 1 1 1- ) 50 100 150 200 250

Temperature/°C

c) 1% w/w Zinc diisopropyldithiophosphate

Figure 6.10 Zinc Diisopropyldithiophosphate 177

a) 0.0015M Zinc Dinbutyldithiophosphate

Tc,rDerature/ °C b) 0.5% w/w Zinc Dinbutyldithiophosphate

Figure 6.11 Zinc Dinbutyldithiophosphate 178

Temperature/°C

c) 1% w/w Zinc di-n-butyldithiophosphate

FIGURE 6.11 Zinc Di-n-butyldithiophosphate 179

a) 0.0015M Zinc diisobutyldithiophosphate

b) 0.5% w/w Zinc diisobutyldithiophosphate

Figure 6.12 Zinc Diisobutyldithiophosphate 180

1% w/w zinc diisobutyldithiophosphate

Figure 6.12(c) Zinc Diisobutyldithiophosphate 181

FIGURE 6.13 0.0015M Zinc Di-secbutyldithiophosphate 182

Temperature / °C

Figure 6.14 0.0015M Zinc Dinhexyldithiophosphate 183

Temperature / °C

Figure 6.15 0.0015M Zinc Dinoctyldithiophosphate 184

a) 0.1% w/w Lubrizol 1395

0-4 -

Temperature/°C b) 0.5% w/w Lubrizol 1395

Figure 6.16 Lubrizol 1395 185

Temperature/°C c) 11 w/w Lubrizol 1395

Temperature/°C d) 1I w/w Lubrizol 1395 with addition of n-hexanol at 230°C

Figure 6.16 Lubrizol 1395 Temperature/°C a) 0.0015M Diethylphosphorodithioic Acid

Temperature/°C b) 0.003M Diethylphosphorodithioic Acid

Figure 6.17 Diethylphosphorordithioic Acid 187

Temperature/°C a) 0.0015M Diisopropyl phosphorodithioic Acid

b) 0.003M Diisopropyl phosphorodithioic Acid

Figure 6.18 Diisopropyl Phosphorodithioic Acid 188

Temperature/°C

a) 0.003M Diethyl phosphorodithioic acid in preoxidised hexadecane

b) 0.003M Diisopropyl phosphorodithioic acid in preoxidised hexadecane

Figure 6..19 Phosphorodithioic Acid Additive Mixtures 189

Temperature/°C a) 0.0015M Zinc diethyldithiophosphate and diethylphosphorodithioic acid

b) 0.0015M Zinc diethyldithiophosphate in preoxidised hexadecane

Figure 6.20 Zinc Diethyldithiophosphate Additive Mixtures 190

c) 0.0015M Zinc diethyldithiophosphate and diethylphosphorodithioic acid in preoxidised hexadecane

Figure 6.20 Zinc Diethyldithiophosphate Addit ive Mixture 191

ECR

I\ As A A

0.5 J

0.4 ^ M 0.3 H

0.2 "

0.1- ^jJ

0 50 100 150 200 250 Temperature/°C a) 0.0015M Zinc Di-isopropyldithiophosphate and Di-isopropylphosphorodithioic Acid

ECR

0.5 A

0.4-1 M 0.3 J

0.2 J

o.i H

lo 100 150 200 250 Temperature/°C b) 0.0015M Zinc Di-isopropyldithiophosphate in Pre-oxidised Hexadecane

FIGURE 6.21 Zinc Di-isopropyldithiophosphate Additive Mixtures 192

Temperature/°C c) 0.0015M Zinc Di-isopropyldithiophosphate and Di-isopropylphosphorodithioic Acid in Pre-oxidised Hexadecane

GURE 6.21 Zinc Di-isopropyldithiophosphate Additive Mixt 193

Temperature/°C

a) 11 w/w Zinc Diethyldithiophosphate in Preoxidised Hexadecane

Temperature/°C b) 1% w/w Zinc Diisopropyldithiophosphate in Preoxidised Hexadecane

FIGURE 6.22 Preoxidised Hexadecane Additive Mixtures 194

ECR

0.4 B

0 .2 - A, J

0.1 - • i i i 50 100 150 200 250 Temperature/°C

0.1% w/w Zinc diethyldithiophosphate and 1% w/w Toponal OC in hexadecane

FIGURE 6.2 3 0.1% w/w ZDDP and Antioxidant 195

6.5.2 No Additives

a) n-Hexadecane (Figure 6.6)

Pure hexadecane was found to be an extremely poor lubricant. The coefficient of friction rose rapidly from an initial value of 0.15, the test failed at 80°C. No evidence of thick film formation was observed, the electrical contact resistance measurement remained zero throughout the test.

b) Preoxidised n-Hexadecane (Figure 6.7)

n-Hexadecane was preoxidised before HFR testing by heating to 200°C for ten minutes in an open glass beaker and then cooled to room temperature before testing. The presence of oxidised hexadecane species was confirmed by IR analysis described in section 6.9. Preoxidised hexadecane gave a generally low friction trace (^O.lp) over the entire temperature range, although unstable particularly at lower temperatures 20-120°C. No film formation was observed throughout the test.

6.5.3 Organophosphorous Additive Tests

6.5.3.1 Zinc Dialkyldithiophosphates

Zinc Di-ethyldithiophosphate (Figure 6.8)

a) 0.0015M (0.084% w/w)

At low concentrations zinc di-ethyldithiophosphate gave an initial friction value of vO.ly. This remained steady until 80-85°C and was associated with the formation of a thin unstable film which decayed at ^70°C. Friction rose rapidly at 85°C to a maximum of 0.35y at 90-100°C. This high average friction value was maintained although the trace was unstable, indicating scuffing (120). At 185°C formation of a thick stable chemical film was 196 recorded. A corresponding decrease in friction (to <0.1p) was observed. The thick film appeared to break down towards the end of the test at ^240°C. These tests were repeated using different heating rates (10°C/min, 20°C/min) in all cases insulating boundary film formation occured at 180-185°C.

b) 0.009M (0.5% w/w)

Initial friction value was O.llp with an unsteady trace to 85°C. No significant film formation was observed in this region. There was a rapid increase in friction coefficient at 85°C to a maximum of 0.4p at 90°C. Average friction force remained high with rapid fluctua- tions of ^ ±0 .1 p to 200°C, then decreased slightly to 0.3u at 250°C. No significant film formation from 85-200°C, then an unstable film observed at 200-220°C.

c) 0.017M (1% w/w)

Initial friction value to O.lp at 20°C, unstable trace with intermittent film formation to 90°C. Rapid increase in friction at 90°C to a maximum of 0.45p at 100°C. The friction trace was very unstable in this region, tests generally failed at ^180°C. In one test the vibrator was switched off and the chuck lifted, exposing the contact. The chuck was then replaced and the vibrator switched on, a thick stable film formed, possibly due to increased oxidation of oil in the contact.

Zinc Di-n-propyldithiophosphate (Figure 6.9)

0.0015M (0.096% w/w)

Steady friction trace 20-80°C, ^O.lp, gradual increase in friction from 80-120°C to maximum of 0.16p. Trace remains generally stable, gradual decrease in friction from 140-180°C, stable to 230°C. Corresponding drop in friction over this region. 197

Zinc Di-isopropyldithiophosphate (Figure 6.10)

a) 0.0015M (0.096% w/w)

Low initial friction value ^O.lp stable to 50°C associated with thin film formation. Friction rise at 50°C associated with the decay of this film, to a friction plateau at 50-200°C, the trace fluctuating over range of 0.25-0.35u. At 200°C the friction coefficient drops to ^O.lu, associated with formation of stable boundary films.

b) 0.0078M (0.5% w/w)

High unstable friction trace increasing to a maximum of .5|j at 100-220°C. Stable film formed at ^220°C reduces friction appreciably.

c) 0.015M (1% w/w)

Initial friction coefficient of 0.1M rose rapidly from 40°C to maximum of ^0.5m at 80°C. Highly unstable trace 0°-80°C, tests failed at 90-100°C. No thick film formation observed.

Zinc Di-n-butyldithiophosphate (Figure 6.11)

a) 0.0015M (0.105% w/w)

Initial friction rise to 'VO.IM decreasing to 0.06M at 80°C. Thick boundary film formed between 40-60°C. Friction increase at 90°C, unstable trace observed rising to a maximum of 0.14M at 140°C. Intermittent film formation from 145°C, with a thick stable film forming from 165°C. Full film conditions reached at 210-230°C then decays at 240°C. Low stable friction trace observed in this region 150-240°C. 198

b) 0.0075M (0.5% w/w)

Initial friction value 0.1m, trace remains steady with unstable film formation at 70-90°C. This boundary film decays at 90-100°C to a thin unstable film which is maintained to 170°C. This is associated with a slight decrease in friction to a minimum value of 0.1M at 120-145°C.

c) 0.015M (1% w/w)

Initial friction value of 0.1M, remaining stable to 80°C, unstable film formation occurs from 20-90°C. Small friction rise observed at 90°C to a maximum of 0.15M, trace then remains fairly constant (0.1M-0.125M) until the end of the test. No thick film formation observed after 90°C, although the friction is relatively low, the trace is unstable.

Zinc Di-iso-butyldithiophosphate (Figure 6.12)

a) 0.0015M (0.105% w/w)

Initial friction 0.1M, gradually increasing after 65°C to a maximum of 0.17M at 108°C. Stable thick film formation 35-65°C. High friction region maintains 105-115°C, although the trace was stable. Thick film formation occurs at 180°C, low friction region maintained at 180-240°C.

b) 0.0075M (0.5% w/w)

Initial friction value of 0.09M maintained to 90°C, with thin unstable film formation decaying at 100°C. Friction increase occured at 100°C to a maximum of 0.3M at 150°C. High friction maintained to 240°C. Trace unstable although slight decrease observed at 210°C, on formation of thin film which decayed at 230°C. 199

c) 0.015M (1% w/w)

Initial friction value of 0.1M, maintained to 100°C with thin film formation. Friction rise at 100°C and 140°C to a maximum of 0.4M at 180°C, unstable trace to 250°C. Intermittent film formation observed, stabilising after 200 °C.

Zinc Di-sec-butyldithiophosphate (Figure 6.13)

0.0015M (0.105% w/w)

Initial friction value of 0.1M, remaining stable to 60°C, some film formation observed decaying at 50-60°C. High friction region maintained 60-170°C maximum of 0.4M at 150°C. Thick film formation observed initially at 170°C stabilising after 200°C. Low stable friction trace (0.125M) observed specially at 200-250°C.

Zinc Di-n-hexyldithiophosphate (Figure 6.14)

0.0015M (0.127% w/w)

Low stable friction trace maintains throughout the test. Thick film formation observed from 25°C to 230°C.

Zinc Di-n-octyldithiophosphate (Figure 6.15)

Zinc di-n-octyldithiophosphate was tested at three concentrations (0.0015M, 0.5% w/w. 1% w/w). All three gave similar friction electrical contact resistance results. Low stable friction trace was maintained through the temperature range, initially 0.1M it generally decreased to ^o.07m at ^200°C. Thick film formation observed throughout the test. 200

Commercial Additive Lubrizol 1395 (Figure 6.16)

Lubrizol 1395 was tested at three different concentrations (0.1% w/w, 0.5% w/w, 1% w/w) and was regarded as a 'standard' for all other additive tests.

a) 0.1% w/w

Initial friction value of 0.1M stable to 70°C, with stable thick film formation to 100°C. Gradual friction rise with increasing instability of trace to a maximum of 0.16M. Intermittent film formation rapidly increasing and stabilising after 160°C, with consequent decrease in friction to 0.1M at 230°C.

b) 0.5% w/w

Low stable friction trace (0.1M) recorded to 120°C with some film formation. Friction rise at 120°C to a maximum of 0.16m, at 140°C. Friction trace gradually decreased after 150°C, thick film formation occuring at 200°C.

c) 1% w/w

Initial friction value of 0.1M remaining fairly stable to 125°C. Thick film formation observed 70-100°C, decaying to an unstable intermittent film to 150°C. Friction rise observed at 125°C to a maximum of 0.19M at 140°C. Friction trace remains unstable to end of test. No thick film formation observed at higher temperatures. Two tests are presented for 1% w/w Lubrizol 1395, in the second test a small amount (^.Olml) of n-hexanol was introduced into the contact area by microcapillary tube. Thick film formation was observed almost immediately with a consequent reduction in friction and a stabilising of the trace. 201

6.5.3.2 Dialkylphosphorodithioic Acids

The dialkylphosphorodithioic acids were tested at two concentrations, 0.003M and 0.0015M. The aim of the experiments was to determine whether the parent acids alone were capable of propagating and maintaining a thick boundary film.

Diethylphosphordithioic Acid

0.003M, 0.0015M (Figure 6.17)

Both acid concentrations gave similar friction/temperature curves. Initial friction coefficient of 0.1M rising gradually to 0.18M at 80°C. High friction region from 80-140°C, rapidly fluctuating friction trace indicating scuffing (120). Thick film formation occurs for both tests at 140°C, film relatively unstable decays at 220°C. Low stable friction trace obtained in this region (140-220°C).

Di-iso-propylphosphorodithioic Acid

0.0015M, 0.003M (Figure 6.18)

The two DTP acid concentrations gave similar friction/ temperature curves. Initial friction value of 0.13m steadily rising to a maximum of 0.25M at 100°C. Thick film formation observed at vi20°C, associated with rapid decrease in friction to a value of 0.12M at 125°C. Low stable friction trace maintained to end of test, although the thick film was observed to decay at 180-190°C.

6.6 Mixed Additive Tests

6.6.1 Introduction

A series of mixed additive tests were undertaken in an attempt to simulate a commercial additive. Chemical analysis of a commercial ZDDP indicated that it contained the following main impurities. 202 a) polar/acid phosphorus impurities including parent phosphorodithioic acid. b) parent alcohol which could possibly act as an oiliness additive in conjunction with the ZDDP, or as easily oxidised organic material. Two pure ZDDPs were used in separate series of mixed additive tests, zinc di-ethyldithiophosphate and zinc di-iso-propyldithiophosphate were chosen as representative normal and branched chain ZDDPs.

The following mixed additive tests were undertaken for both salts (see Table 6.3).

1) ZDDP and preoxidised hexadecane - to determine the effect of polar oxidised hexadecane species on the formation and stability of the boundary film.

2) ZDDP and parent dithiophosphoric acid - to determine the effect of a common acid impurity on the lubrication properties of a pure ZDDP.

3) ZDDP and parent dithiophoric acid and preoxidised hexadecane - with this combination it was hoped to simulate a commercial additive by the inclusion of both acid and polar surfactant species.

Other mixed additive tests were undertaken to complete the programme. This are listed below. a) phosphorodithioic acid in preoxidised hexadecane. b) 0.0015M zinc di-ethyldithiophosphate and 1% w/w anti- oxidant (Toponol OC) in pure hexadecane. 203

6.6.2 Results

6.6.2.1 Diethyl Additives a) Zinc diethyldithiophosphate in preoxidised hexadecane

This test was carried out at two ZDDP concentrations, 0.0015M and 1% w/w.

0.0015M (Figure 6.20b)

Initial friction value of 0.07p steady trace maintained to a maximum of O.llp at 120°C. Intermittent film formation at 20-80°C. Formation of insulating boundary film first observed at 160°C, rapid increase in film thickness and stability at 180°C. Film maintained to 230°C. Low friction trace recorded.

1% w/w (Figure 6.22a)

Low stable friction trace observed for entire temperature range (0.1M). Thick film formation observed at 100°C, stabilising after 120°C. b) Zinc diethyldithiophosphate (0.0015M) and diethylphosphorodithioic acid (.00017M) (Figure 6.2Qb)

Tests were run with equivalents of 5% and 10% weight acid impurity in the ZDDP. Little difference was seen in the test result. Initial friction value of ^0.05p steadily rising at 40°C to a maximum of ^0.25-0.3m at 60°C. High unstable friction trace to 170°C. Thick boundary film formed 170-180°C, low stable friction trace obtained 175-240°C. c) Zinc diethyldithiophosphate (0.0015M) and diethylphosphorodithioic acid (.00017M) in preoxidised hexadecane (Figure 6.20c)

A combination of both acid and oxidised impurities with 204 a pure ZDDP gave a low stable friction trace over the entire temperature range (20-230°C). Coupled with thick boundary film formation, which rapidly increased in thickness 115- 140°C. Maximum film thickness recorded at 140°C maintained until end of test.

6.6.2.2 Di-iso-propyl Derivatives

Zinc Di-iso-propyldithiophosphate and di-iso-propylphosphorodithioic acid (Figure 6.21a)

Initial friction value O.lp, sharp increase occuring at 40°C to a maximum of 0.35p at 60°C. High friction unstable trace obtained to 160°C. After 160°C intermittent film formation occurs causing wildly fluctuating friction trace. Film formation finally established at 210°C, causing reduction to O.lp.

Zinc Di-iso-propyldithiophosphate (0.0015M) and di-iso- propyldithiophosphodithioic acid (.00017M) in preoxidised hexadecane (Figure 6.21c)

Low, stable friction trace (O.lp) observed throughout test. Thick film formation occurring from 50°C, film stabilising after 180°C, slight decrease in friction observed. Film formation observed to decay at 230°C.

Zinc Di-iso-propyldithiophosphate in preoxidised hexadecane

0.0015M (Figure 6.21b)

Low, stable friction trace (O.lp) maintained to 140°C, slight instability observed to 180°C. Thick film formation occurring at 200°C-230°C, slight decrease in friction observed.

0.1% w/w (Figure 6..22,b) i Low, stable friction trace M).lp over entire tempera- 205 ture range. Boundary film formation observed at 80°C, thick stable film found 80-190°C, decrease in film thickness observed at 190-230°C. Low stable friction trace.

6.6.2.3 Organophosphorus/preoxidised Hexadecane Additive Tests

Dialkylphosphorodithioic acids in preoxidised hexadecane

0.003M (Figure 6.17)

Similar results were obtained for both di-ethylphosphorodithioic acid and di-iso-propylphosphorodithioic acid at 0.003M concentration. Relatively low friction M).l-0.15|j was maintained throughout the test, although the trace for both acids was unstable to 140°C. Thick film formation occurred at 140°C for the di-ethyl and 125°C for the di-iso-propyl acids. Similar temperatures were obtained for acid thick film formation in pure hexadecane. The films were observed to decay at ^200°C.

6.6.2.4 ZDDP and Antioxidant Tests (Figure 6.23)

0.0015M Zinc di-ethyldithiophosphate and antioxidant

To observe the effect of antioxidant (radical scavenging) reactions on ZDDP load-carrying ability, 0.0015M zinc di-ethyldithiophosphate in hexadecane solution containing 1% w/w Toponol OC was tested. Toponol OC is a purer form of Toponol 0 which acts as a radical trap and so inhibits autooxidation of hydrocarbons. Addition of 1% w/w Toponol OC was found to inhibit thick film formation. As a consequence no friction reduction was observed at ^180°C. The friction increase at 85°C was also more severe (^0.5n) than in the test with zinc di-ethyldithiophosphate alone (^0.35p). 206

6.7 Discussion

All pure ZDDPs tested gave markedly similar friction/ temperature curves. These consisted of three general regions (Figure 6.24).

(i) Low temperature, low friction (^0.1n)> steadily increasing from room temperature to a critical temperature between 50 and 90°C. This is generally associated with formation of thin unstable film which usually decays at a critical temperature, (ii) A sharp increase in friction occurs somewhere between 50-90°C. The temperature of this increase and its severity depend upon the nature of the alkyl group. As alkyl chain length increases the effect of friction change is found to decrease. It is also higher for branched groups than their straight chain analogues. A high friction trace continues from ^90°C to ^160°C. This is very unstable indicating high wear. Inter- mittent film formation is observed, though this is not effective in reducing friction, (iii) Friction decreases at high temperatures typically i>160°C and generally between 140-200°C. The temperature of the decrease is dependent on the purity, structure and concentration of the ZDDP. No friction decrease is observed for high ZDDP concentrations which often . fail in this region. Friction decrease corresponds to a rapid increase in the (electrical) contact resistance. A thick stable film is established within a few seconds, for low molecular weight straight chain ZDDPs. The film appears to decay at temperatures >230-240°C. A low, stable friction trace is observed.

The results of all additive tests have been summarised in Tables 6.4-6.6 with reference to Figure 6.21. The base oil, pure n-hexadecane proved to be an extremely poor lubricant. Erratic friction traces were obtained and failure occurred at 90-100°C. This could be due 207a

Temperature! °C

T^ - temperature of friction rise

T2 - temperature of film format ion/friction drop M1 - average maximum coefficient of friction

FIGURE 6.24 Typical Coefficient of Friction/ECR Curves for 0.0015M Solution of a Pure ZDDP in Hexadecane 208a

TABLE 6.4 SUMMARY OF FRICTION RESULTS FOR 0.0015M ZDDP SOLUTIONS

Alkyl Temperature Maximum Temperature Group Friction Rise Friction Film Formation T T 1 2

C2H5" 90°C 0.35 185°C

n C3H7- 80 °C 0.18 160 °C iso CgH^- 50 °C 0.3 200 °C

« C4H9" 80-90 °C 0.18 150 °C

iso C4Hg- 95 °C 0.19 180 °C

sec C4H9- 60 °C 0.35 170 °C 1395 70 °C 0.16 160 °C

n C6H13- - 0.09 100°C

n C8H17" - 0.1 75 °C TABLE 6.5 EFFECT OF INCREASED ADDITIVE CONCENTRATION a) Zinc Diethyldithiophosphate (Figure 6.8)

Concentration Initial Temperature Maximum Temperature Final ZDDP M Friction Rise M Film Formation M T1 T2

0.0015M 0.1 90-100 °C 0.35 180-185 °C 0.09 0.5% w/w 0.11 90-100 °C 0.4 185 °C 0.32 1% w/w 0.11 90-100 °C >0.45 Failed 180 °C

b) Zinc Diisopropyldithiophosphate (Figure 6.10)

0.0015M 0.09 50 °C 0.3 200 °C 0.1 0.5% w/w 0.22 50 °C 0 .45 210 °C 0.11 1% w/w 0.1 50°C >0.45 Failed 110°C TABLE 6.5 Continued c) Zinc Di-n-butyldithiophosphate (Figure 6.11)

Concentrat ion Init ial Temperature Maximum Temperature Final ZDDP M Friction Rise U Film Formation M T1 (Mj) T2

0.0015M 0.1 90 °C 0.14 150 °C 0.07

0.5% w/w 0.11 90 °C 0.15 60-100 °C 0.1

1% w/w 0.11 85 °C 0.15 25-100 °C 0.125 d) Zinc diisobutyldithiophosphate (Figure 6.12)

0.0015M 0.09 8 5 °C 0.17 180 °C 0.1 0.5% w/w 0.09 105 °C 0.3 210 °C 0.25

1% w/w 0.1 140 °C 0.4 - 0.4 e) Lubrizol 1395

0.1% w/w 0.1 70 °C 0.16 160 °C 0.1 0.5% w/w 0.1 120 °C 0.16 200 °C 0.12

1% w/w 0.1 125 °C 0.19 - 0.13 TABLE 6.6 SUMMARY OF HFR RESULTS FOR MIXED ADDITIVE TESTS

ADDITIVE MIXTURE Initial T1 T2 Final M M

0.0015M Zinc Diethyldithiophosphate in 0.05 preoxidised hexadecane 0.08 90 °C 0.13 155 °C

0.0015M Zinc Diethyldithiophosphate and 170 °C 0.06 diethylphosphorodithioic acid 0.05 40 °C 0.3

0.003M Diethylphosphorodithioic acid in 50 °C 0.15 140 °C 0.1 preoxidised hexadecane 0.1

0.0015M Zinc Diethyldithiophosphate and diethylphosphorodithioic acid in 0.08 - 0.09 0.05 preoxidised hexadecane

1% w/w Zinc Diethyldithiophosphate in preoxidised hexadecane 0.1 - 0.1 95 °C 0.1 TABLE 6.6 continued

ADDITIVE MIXTURE Initial Ti M1 T2 Final M M

0.0015M Zinc Diisopropyldithiophosphate in preoxidised hexadecane 0.1 - 0.1 205 °C 0.1

0.0015M Zinc Diisopropyldithiophosphate and diisopropylphosphorodithioic acid 0.1 40 °C 0.34 160 °C 0.1

0.003M Diisopropylphosphorodithioic acid in preoxidised hexadecane 0.1 - 0.13 125 °C 0.1

0.0015M Zinc Diisopropyldithiophosphate and diisopropylphosphorodithioic acid in 0.1 - 0.1 50 °C 0.07 preoxidised hexadecane

1% w/w Zinc Diisopropyldithiophosphate - 0 .095 80 °C 0.1 in preoxidised hexadecane 0.09 213a to desorption of physically adsorbed hydrocarbon molecules at the higher temperature (121). Preoxidised hexadecane maintained a low, unstable friction trace throughout the entire temperature range. No thick film formation was observed. Spectroscopic analysis of the preoxidised hexadecane confirmed the presence of polar species (see 6.8). These could act as oiliness addit ives strongly chemisorbing onto the metal surfaces and maintaining an orientated hydrocarbon layer at the higher temperatures (122). They appear to be effective in reducing friction over the entire temperature range. The additive testing programme showed that the friction/ temperature performance of ZDDPs is determined by the following:

1. Initial concentration of the ZDDP. 2. Nature of the alkyl group. 3. Presence of acid impurities. 4. Presence of polar oxidised species.

6.7.1 Effect of Initial Concentration

These results are summarised for different ZDDPs tested in Table 6.5. Generally for all pure ZDDPs tested high initial concentrations (>0.007M) gave poorer friction results especially in the high temperature 90-250°C range. For concentrations of =1% w/w thick film formation by ZDDPs in pure hexadecane was often not observed. The tests generally failed in the 180-200°C range for the lower molecular weight ZDDPs. In the high friction region ^90-200°C a rapidly fluctuating friction trace was obtained, identified as scuffing regime (120). High rates of wear are postulated in this region. Analysis of static films formed by zinc di-ethyldithiophosphate in the temperature range 100-180°C indicated slow formation of a thin inorganic sulphate/ phosphate type film. Load-carrying in this region could possibly be by strongly chemisorbed polar species on the sulphate film (123). As ZDDPs have an anti-oxidant as well as 214a an antiwear function it is quite likely that the high anti-oxidant capacity prevent formation of polar species necessary for load carrying. At high temperatures, ^180-200°C, the low ZDDP concentration <0.1% w/w shows a decrease in friction due to the formation of a stable thick chemical film within the contact. This film does not appear to form outside the contact as increasing the stroke length causes an initial decay in contact resistance followed by a gradual recovery (Figure 6.25). Anomalous results for zinc di-n-hexyldithiophosphate and zinc di-n-octyldithiophosphate is partly attributed to impure salts. The dtp acids (Figures 6.17-6.19) gave a highly fluctuating trace in the 90-140°C temperature range although M was lower than the corresponding zinc salts. IR examination of surface films found by immersion in dtp hexadecane solutions (5.7) has shown the acids to be extremely corrosive. It is suggested that in the 90-140°C temperature range that dtp acids cause corrosive wear of the metal pair. This contributes to the generally lower average friction observed, by forming a brittle surface film possibly of iron dithiophosphate. This surface film is easily removed allowing further attack by the acid resulting in corrosive wear and unstable friction trace indicating a scuffing regime. The role of the metal cation would appeair to be of secondary importance. The parent acids themselves are capable of sustaining thick film formation which occurs at lower temperatures (v20°C) than the corresponding zinc salts. This indicates that the metal cation is important only in that it would determine thermal stability of the MDDP. The zinc cation itself does not appear to play a fundamental role in thick film formation.

6.7.2 Effect of Alkyl Chain Length

The effect of alkyl chain length and branching on the friction/temperature performance of ZDDPs is shown in Table 6.4. Generally the temperature of the first friction rise 215a

T^ decreased as the alkyl chain length increased, T2 (temperature of film formation) also decreased. T^ was higher and T2 lower for straight chain alkyl ZDDPs compared to the branched chain analogues. As the alkyl chain length increases the severity of the friction rise decreases. For the hexyl and octyl derivatives no friction increases at T^ were observed. This is possibly due to impurities in the additives. Generally p^ was lower for straight chain ZDDPs than their branched chain analogues. This is most apparent for the butyl salts, n-butyl gave a slight friction decrease in the 100-150°C region whereas both iso and sec butyl showed friction increases at 100°C and 60°C respectively.

6.7.3 Presence of Acid Impurities

Two dtp acids were tested as solutions in hexadecane (Figure 6.17, 6.18). Both gave similar friction/temperature profiles to comparable S/P concentrations of their zinc salts. Generally a lower p^ was observed and film formation occurred at a lower temperature. The thick films found were not as stable and tended to decay at lower temperatures. Addition of small amounts (^5%) of parent acids to pure zinc salts, (diethyl, diisopropyl) decreased the temperature of film formation by ^15°C.

6.7.4 Addition of Polar Oxidised Species

High concentrations of additives that combine antiwear and anti-oxidant functions have been shown in earlier studies to inhibit thick film formation (109). The accepted capacity of the additive prevents the formation of polar oxidised hydrocarbon species necessary for film formation. Additive tests in preoxidised hexadecane are presented in Figures 6.20b, 6.21b, 6.22. In all cases thick film formation occurred at lower temperatures (20-80°C) depending on the concentration of the additive. Lower p^ was observed for all tests compared to those in pure hexadecane, a result which would be predicted by Figures 6.8 and 6.9. A further additive combination of zinc di+ethyldithiophosphate (0.0015M) and Toponol GC was tested (Figure 6.23). Addition of the anti-oxidant prevented thick film formation for all temperatures to 240°C. In previous tests (6.8a) 0.0015. zinc di-ethyldithiophosphate formed a thick film at ^180-185°C. Thick film formationis also induced by the addition of other polar material, eg hexanol (Figure 6.16).

6.7.5 General Discussion

To summarise thick film formation is generally found to be determined by:

1. Initial concentration of ZDDP. 2. Presence of acid impurities. 3. Addition of oxidised species.

Several combinations of additives were tested to simulate commercial ZDDPs. Addition of dtp acid to a solution of ZDDPs in preoxidised hexadecane was found to give friction/temperature traces comparable to those obtained from similar concentrations of a commercial additive. For the zinc di-ethyldithiophosphate additive mixture (Figure 6.20c) the stability and thickness of the film increases rapidly at ^170°C. This is the same temperature observed for the zinc di-ethyldithiophosphate/di-ethylphosophorodithioic acid (Figure 6.20a) test in pure hexadecane. Low friction in test 6.20c is maintained in the 20- 170°C temperature range by a thinner but relatively stable boundary film this is not observed in the comparable test 6.20a in pure hexadecane. Nor is it observed in test 6.20b (zinc di-ethyldithiophosphate in preoxidised hexadecane), to any extent. It would therefore appear that load-carrying performance is determined by the formation of a stable, relatively thick boundary film throughout the entire temperature range. Several workers (109) 0L24) (125) have observed similar film 217a formation. In suggesting the term "thick boundary film" it is not proposed that any quantitative assessment of film thickness should be made. The term is used qualitatively only. The nature of the film is still under investigation. Infra-red analysis of surface films formed under static conditions has indicated that three general types of chemical films are formed depending on the temperature range. a) Low temperature, 20-100°C

Chemisorption of ZDDP molecules onto the metal surface occurs, at higher temperatures (^70°C) ZDDP has been found to react with the iron surface to form iron dithiophosphate (52). A mixed zinc/iron dithiophosphate surface is found in this region. b) 100-150°C

Reaction of ZDDP degradation products to form a thin inorganic sulphate/phosphate film. Possibly facilitated by the presence of peroxides (51). c) >150°C

At high temperatures a blue/brown film is formed, identified as being similar to the bulk decomposition product of ZDDPs. It is possibly formed by in-situ polymerisation of phosphorus degradation species. Georges and co-workers (52) have identified two films formed by ZDDPs as giving high ECR readings. A brown film is formed initially, which is then covered by a blue film. Boundary film formation is generally observed in the final temperature range, although it is not solely due to the formation of the polymer film. In this study ECR measurements of brown/blue polymer films formed under static conditions were found to be zero. It is suggested, however, that the presence of a polymer layer is a necessary precursor of thick film formation. 218a

If it is due to thermal decomposition products alone, then high ZDDP concentrations would be expected to enhance film formation. The opposite effect is observed, this has been interpreted as being due to the increased anti-oxidant capacity of high ZDDP concentrations inhibiting oxidation of the base oil (109). The polar oxidation products being necessary to thick film formation, a conclusion that appears to be supported by the effect of adding Toponol OC, a radical inhibitor to low concentrations of ZDDPS (Figure 6.23). An alternative conclusion, however, is suggested by examination of the anti-oxidant literature (2.3). Briefly ZDDPs decompose hydroperoxides by a radical, and catonic chain mechanism. High concentrations of ZDDPs and the presence of radical traps (eg Toponol OC) have been found to inhibit the cationic chain mechanism. This process is considered to be catalysed by the parent dithiophosphoric acid. On this basis, therefore, the failure of high additive combinations to form thick films could be due to suppresion of the acid-catalysedhydroperoxide decomposition. Anti- oxidant function is then by a predominantly radical mechanism. The products of the two mechanisms are different, alcohols hydrogen peroxide and disulphide from the cationic chain mechanism, but disulphides, basic ZnO and ketones from the radical mechanism. It is therefore proposed that the products of certain anti-oxidant reactions (iecationic chain mechansims) could be important in thick film formation and hence determining load carrying performance. This result is contrary to the conclusions of previous workers (59). However, in these previous studies zinc di-n-octyldithiophosphate was reacted with peroxy radicals derived from tetraphenyl butane (TPB). Under these conditions (ie predominantly radical decomposition mechanism) reaction products would not be expected to enhance load-carrying performance. The conclusion that alcohols are important in thick film formation is supported by the test result in Figure 6.16d. Addition of a relatively short chain alcohol (n-hexanol) to a 1% w/w solution of Lubrizol 1395 rapidly induces thick film formation at a high temperature. The presence of similar 1 stroke increasing wavelength

ECR

0 -

Full film established Film breaks down at Film reforms from throughout stroke ends of the stroke midstroke

FIGURE 6.25 Diagrammatic CRO Traces Showing the Effect of Increased Stroke Length on Boundary Film Formation 220a alcohols are postulated in commercial additives as impurities from the first stage in the synthesis. This would explain the ability of the commercial additive to form a thick film throughout the temperature range. This is inhibited at high ZDDP concentrations possibly due to the predominantly radical mechanism preventing the cationic formation of more alcohol. In these tests thick film formation was found to decay as the test continued. It is therefore proposed that the products of cationic chain decomposition of hydroperoxides are important in thick film formation. That this process is catalysed by phosphorodithioic acid and inhibited by the addition of radical traps (Toponol OC) or basic compounds.

6.7.6 Nature of the Boundary Film

Several workers have postulated thick film lubrication in the boundary region (126) (12^. Evidence for this phenomenon in this study is solely from ECR measurements. The following observations of thick film formation were made:

1. The boundary film is not formed outside the contact area, since on increasing the stroke length the film is observed to break down at the end of the stroke, followed by recovery after a few seconds. No corresponding severe rise in friction is observed. 2. Switching off the vibrator causes the film to decay immediately to a relatively low but stable value. This indicates a thinner residual film in the contact.

These results would indicate that these thick films are not varnishes nor due to organic debris or paste formation. These would be expected to give high ECR even when the vibrator is switched off. When film formation occurs the friction very rapidly drops to a low level. Further increase of film thickness has 221a no further effect on friction, although an increase in the stability of the film stabilises the friction trace. In some cases friction is observed to drop before film formation is observed on the CRO9 indicating that initial formation of a thinner but conducting film is the important step in reducing friction. It is suggested that formation of the residual film observed above in 2. possibly by chemisorption of polar species on reacted metal surfaces (128) is responsible for forming an anisotropic coherent film The subsequent thickening of the film could be due to long range orientation effects in the bulk fluid by the chemisorbed layer giving rise to aggradation of molecules (126) induced by flow in the contact area The adsorbed surface layer could also be expected to occur outside the contact area (supposing that the reacted metal surfaces are similar both within and outside the contact). This is supported by the observation that on changing the stroke length, or the position of the specimen no severe rise in friction occurs. In these cases a thick film is usually re-established within a few seconds. Thick film formation would therefore appear to be a three stage process:

1. Reaction of ZDDP AW species with the metal surface to form an inorganic film, onto which polar species can strongly adsorb. 2. Adsorption of polar species on to the inorganic film, forming a stabilised coherent film. 3. Subsequent thickening of the film within the contact area induced by long range orientation effects of the adsorbed film.

6.7.7 Comparison of HFR Test Results with Previous Work

Correlation of HFR results with those of previous workers is difficult due to the variety of testing methods and contact conditions of the different friction devices used. Most of the L-C studies on ZDDPs have been carried 222a out on four-ball test machines and it is believed that valid comparisons can be drawn between these results and those obtained on the HFR rig. This work has been reviewed in 2.6 , a summary is presented below.

1. Little difference in AW efficiency of different alkyl groups. Although generally AW activity decreases as thermal stability increases. 2. EP efficiency increases as thermal stability increases.

Comparison of HFR results with those from four-ball tests is possible if a general correlation between increasing load and increasing temperature within the contact can be assumed (12 7). The division between AW and EP regions is poorly defined, it is generally agreed to be temperature dependent (48). In the four-ball test the onset of the EP region is recognised by a sharp rise in friction at a certain load, the initial seizure load (ISL). It should therefore be possible to divide the HFR friction curves into AW and EP regions depending on the operating temperature. It is considered therefore that in Figure 6.24 region 1 can be retermed AW and region 2 EP. Using this division the AW region has a temperature range of RT~ 80/90°C and EP 100°-180°C depending on the additive used. This is in rough agreement with oil outlet temperatures measured in previous four-ball studies (127). Examination of HFR results using this division show that:

1. Increasing chain length decreases T^,therefore this corresponds to a decreasing ISL and therefore a poorer AW performance. 2. Increasing chain length decreases and therefore improves EP efficiency.

3. Increasing chain length decreases T2 preventing test failure, improving EP performance. 4. For the commercial additive increasing the concentration from 0.1% to 1% w/w should give improved AW results, but poorer high temperature results. 223a

On this basis HFR results are in general agreement with those obtained in the four-ball tests.

6.8 IR Analysis of Bulk Oil Changes

Test oils were sampled at various temperatures throughout the HFR tests. The samples were run as liquid capillary films in potassium bromide cells. IR spectra were scanned from 1900-500cm-1 using normal scan and resolution programmes The spectra obtained provided a useful guide to the presence of major contaniments due to inadequate cleaning of the test apparatus. When this was found to occur the rig was dismantled and cleaned in toluene vapour in a soxphlet for three hours, then washed in acetone. This was found sufficient to remove the contanimant. The main purpose of the IR analysis was to qualitatively monitor the oxidation of the hexadecane and, if possible, the degradation of the organophosphorus additives. Hexadecane oxidation was indicated by the appearance of an absorption band at 1723cm 1. Pure hexadecane was found to oxidise fairly rapidly especially above 150°C. As the degree of oxidation increases a second peak is observed at ^1180cm-1. The peak at 1728cm" is attributed to the carbonyl stretching vibration, typically of a saturated aliphaticcarboxylic acid, aldehyde or ketone The peak at 1180cm-1 is attributed to a C-O stretching vibration. Small absorption peaks are also observed in the 1700- 1800cm 1 region, towards the end of the test. The shoulder at 1745cm observed is possibly v(C~0) of a saturated ester. Small absorption bands between 1700-1800cm 1 are possibly due to peroxide carbonyl stretch. Previous workers (113) have reported carboxylic acids, alcohols, aldehydes and as oxidation product of n-hexadecane. The appearance of the peak at 1723cm 1 was usually associated with thick film formation at high temperatures. High concentrations (>0.5% w/w) of ZDDPs were found to prevent the appearance of the v(C=0) vibration as well as inhibiting thick film formation. 224a

Other small changes due to degradation of organophosphorus additives were also observed. Generally as the test proceeded the intensity of absorption bands due to P=S and P-O decreased, indicating additive depletion. No definite evidence as to whether the degradation was due to oxidation or thermal decomposition reactions was obtained. Coy and Jones have recently published a paper indicating that additive degradation due to friction testing on a Amsler machine is mainly due to oxidation reactions (41).

6.9 General Conclusions

The results presented in this chapter are summarised below:

1. The HFR test device can be used to provide coefficient of friction and electrical contact resistance data over a wide range of temperatures. Both AW and EP regions can be investigated. Results have shown that the load carrying performance of ZDDPs is determined by a) nature of the alkyl group b) purity of the additive c) composition of base stock d) initial concentration of additives. Previous studies of load-carrying performance of ZDDPs have generally failed to differentiate between alkyl groups. 2. ZDDPs form stable thick chemical films during lubrication. These films survive pure sliding. 3. For low initial concentrations the formation and stability of the films is dependent upon purity and the alkyl group of the ZDDPs. The most stable films are found by short chain primary alkyl groups. 4. High concentrations of ZDDPs increase wear and friction and often result in failure at high temperatures. They do not form stable thick film. 5. For ZDDPs to form thick films surfactant material, either deliberately added or from oxidation of the base oil must be present. Alternatively the products of 225a some anti-oxidant reactions could provide the surfactant material. Initial concentration of ZDDPs should be lower than previously used. A concentration of ^0.5% w/w is suggested.

Anti-oxidant reactions of ZDDPs play a more important role in determining AW efficiency than previously supposed. 226a

CHAPTER 7 GENERAL CONCLUSIONS AND SUGGESTIONS FOR FUTURE WORK

7.1 Introduction

The main conclusions from this study and suggestions for future work are presented separately for each section of the work. A general mechanism of ZDDP load-carrying action is presented in the light of these conclusions.

7.2 Chapter 4. IR Spectroscopy of ZDDPs

Examination of published results and those presented in this study indicates that ZDDPs contain essentially covalent Zn-S bands with a degree of ionic character. Further results giving a more detailed description of the M-S band would be expected from:

a) Investigation of the effect of the metal atom by examination of a series of MDDPs.

b) Examination of M-S band disruption by polar solvents. The degree of ionic character can be estimated by measurement of M-S band shifts on the addition of polar solvents.

c) Quantitative measurement of M-S band intensities in the 400-200cm~ region for a series of MDDPs.

Examination of a series of MDDPs using Raman Spectro- scopy, which together with the IR results should provide positive identification of the M-S absorption band and the degree of ionicity. Raman intensities decrease as the polarity of the bond increases while the opposite effect occurs in the infrared. A combination of these techniques could therefore be used to determine the effect of the metal atom and alkyl chain length on the polarity of the M-S band. 227a

7.3 Chapter 5. IR Reflection-Absorption Studies of Surface Films 7.3.1 Conclusions IR analysis indicates that three types of chemical film are formed by ZDDPs from hexadecane solution on metal surfaces, under static conditions.

(i) Low Temperatures (RT-100°C)

Dithiophosphate film formed either by adsorption of the ZDDP molecule on the metal surface possibly by a dissociative mechanism or by chemical reaction with the metal surface to form an iron dithiophosphate (52). This type of film is associated with the AW action of ZDDPs.

(ii) 100-160 °C

Thin inorganic film possibly sulphate and/or phosphate formed by reaction of ZDDP decomposition products with the metal surfaces. This may be facilitated by the presence of peroxide radicals (51). This type of film is associated with the EP region.

(iii) 160-230 °C

Golden brown film formed initially gradually turning blue as the test proceeds. Analysis indicates that this film has a polymeric structure similar to that of the bulk ZDDP decomposition precipitate. A structure similar to that proposed by Coy and Jones (30) is indicated

SR 0 SR

"O-P-O-P-O-P-SR II II II ooo

Possibly formed by in situ polymerisation on the metal surface, this film is relatively thick compared to the inorganic film and is probably responsible for the anticorrosion properties of ZDDPs. 228a

7.3.2 Future Work

It is suggested that the following areas should be investigated.

1) IR surface analysis of films formed under both rubbed and static conditions. Large surface areas of rubbed films could be formed on a modified Bowden-Leben device. In this way both static and rubbed surfaces could be simultaneously produced under the same temperature conditions so that direct comparison can be made.

2) Examination of thick films formed during friction testing on the HFR device. Reduction of the IR beam focus using a beam condenser would make examintion of wear scars formed during friction testing possible, after completion of the test.

7.4 Chapter 6. Lubrication of Steel by ZDDPs

7.4.1 Conclusions

The following conclusions have been drawn from the results presented.

1) The HFR test device can be used to provide coefficient of fricion and electrical contact resistance data over a range of temperatures. In this way the effect of film formation in the contact area on wear and friction can be monitored.

2) Results have shown that the load carrying performance of ZDDPs is determined by: a) nature of the alkyl group, b) purity of the additive, c) oxidation of the base stock, d) initial concentration of the additive. Generally AW efficiency was found to decrease with increased thermal stability while EP activity increased. 229a

a) nature of the alkyl group b) purity of the additives c) oxidation of the base stock 3. ZDDPs form stable, thick chemical films during lubrication. These films survive pure sliding. 4. For ZDDPs to form thick films surfactant material must be present. 5. Antioxidant reactions of ZDDps are important in determining AW activity.

7.4.2 Future Work

Further testing of ZDDPs on the HFR rig is suggested, this would include: a) Systematic study of the effect of various oiliness additives on thick film formation by ZDDPs eg, esters, alcohols, organic acids. The effects of alkyl chain length and structure, concentration. b) Addition of peroxides to ZDDP tests and their effect on L-C properties. c) A series of pure metal dithiophosphates, to study the effect of metal cation on L-C properties. d) Effect of basic additives on thick film formation.

It is also suggested that the tests should be combined with examination of the wear scars using scanning electron microscopy, electron probe microanalysis and IR surface analysis at different stages throughout testing.

7.5 Mechanism of ZDDP L-C Action

This thesis has investigated the AW action of ZDDPs, from the results presented the following general conculsions have been drawn:

1) AW action by ZDDPs is associated with the formation of 230a

a polyphosphate film chemically adsorbed on the metal surfaces. 2) Thick boundary films formed during lubrication are found to significantly reduce friction and possibly wear.

The nature and mechanism of formation of these thick films is not fully understood, although a three stage process is suggested. This involves:

1) Chemical adsorption of a polyphosphate species onto the metal surface. 2) Physical adsorption of polar molecules onto this film forming a stabilised coherent layer. 3) Subsequent thickening of this film within the contact area induced by long range orientation effects of the adsorbed film.

The process described above in 1 and 2 has been identified as 'sensitised' adsorption (133). Polar 'surfactant' hydrocarbon molecules are necessary for film formation, suggesting that adsorption to form a solid orientated film stablised by lateral cohesive forces, occurs. Film thickness of several hundred angstroms has been reported for such films (122). Other workers (129) have observed (hydrocarbon) film formation during rubbing tests. These films appear to be different in that they do exist outside the contact area. Chaikin (129) suggested that a free radical process occured in the reactions leading to the formation of the frictional polymer. This could provide an alternative mechanism to observed ZDDP film formation. It might also explain why high concentrations of ZDDP, which can act as a radical scavenger inhibits film formation. The function of acidic impurities in ZDDP AW action is unclear. The presence of small amounts of parent dtp acid has been shown to induce film formation at lower temperatures than observed for the pure salt. Two explanations are offered for this:

1) Preferential adsorption/reaction by acid species on the metal surface forming a polyphosphate film at low temperatures. 230a

2) Catalysis of ZDDP thermal decomposition reactions by acid species.

Hydrolysis of ZDDPs to form the free acid has also been reported (26) indicating that the presence of water is important in such systems. 231a

APPENDIX 1 ANALYSIS OF ORGANOPHOSPHORUS ADDITIVES BY THIN LAYER CHROMATOGRAPHY

Thin layer chromatography (TLC) is an analytical technique which has been extensively used in the separation and identification of organophosphorus additives (130)-(131). TLC provided the simplest method of determining the purity of the ZDDPs prepared. IR analysis is relatively insensitive to small amounts of parent acid or other impurities present. Mass spectroscopy was used to confirm the identity of the ZDDP but gave no information on impurities of a lower molecular weight than the ZDDP. TLC was used for routine monitoring of prepared salts and for analysis of commercial additives.

Experimental and Results

The prepared ZDDPs were characterised using TLC. Analysis was on precoated 0.25mm silica gel plates (Polygram sil G Mancherey-Nagel and Co, supplied by Cam Lab, Cambridge). A heptane-acetone solvent system was used. The results are shown in Figure 1. Visualisation of spots was by iodine and/or ammonium molybdate reagents. ZDDPs give a characteristic ammonium imolybdate? colour of rose pink turning to dark blue on a blue background. Ammonium molybdate solution was found to be unreliable and iodine vapour was more generally used for location of spots. Parent acid impurities were characterised (using iodine vapour) by a white spot near the origin, other polar organophosphorus impurities were observed as brown spots further away from the origin. The six crystalline ZDDPs were found after purification to contain no significant impurities. As evidence of the sensitivity of this technique the results from two batches of zinc di-iso-propy1 dithiophosphate are included. Batch A (Figure Id) (mp = 144°C), analysed by TLC indicated a significant amount of acidic impurities, this was not observed in the IR spectrum of this specimen. Batch B 232a

(Figure le) was after recrystallisation (mp = 138°C), it had no measureable impurities. Zinc di-n-octyl and zinc di-n-hexyl dithiophosphate were not purified successfully. The results show that both "contain significant amounts of acidic species and another impurity poorly resolved towards the solvent front. This is possibly unreacted parent alcohol from the first stage of the synthesis. In addition the n-octyl derivative gave two distinct spots due to ZDDPs, possibly from a mixture of alcohols in the original n-octyl alcohol. Lubrizol 1395 gave three spots, the one close to the origin indicates acidic impurities, possibly parent dithiophosphoric acid. The ZDDP spot was large and poorly resolved indicating a range of molecular weights, although an alkyl chain length of C^-Cg was indicated. One further spot poorly resolved near the solvent front was observed. This was identified as being due to hydrocarbon rather than organophosphorus species, again this could be due to parent alcohol or a small amount of mineral oil added as a carrier fluid to the commercial additive. Solvent Heptane : Acetone 3:1 Sample Alkyl Group Rf value* Solvent Front A ethyl 0.25 B n-propy1 0.27 C iso-propyl 0.365 D iso-propy1 0.365 0 E n-butyl 0.33 0 F iso-butyl 0.35 0 G sec-butyl 0.6 H n-hexy1 0.4 I n-octyl 0.3, 0.5

0 0 J Lubrizol 1395 0.44 o o 0 0 0 0 0 0 o * R Distance moved by spot f Distance moved by solvent

Origin 0 0 0 0 0

ABC D E F G H i J

FIGURE 1 Analysis of by Thin Layer Chromatography 234

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