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The Fading of Basic Dyes in Polymer Substrates

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The Fading of Basic Dyes in Polymer Substrates THE FADING OF BASIC DYES IN POLYMER SUBSTRATES A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY AT THE UNIVERSITY OF NEW SOUTH WALES BY SUSAN ELIZABETH MATTHEWS ·ft. SCHOOL OF TEXTILE TECHNOLOGY · ···JONE .. 1980 i I hereby certify that the work described in this thesis was performed by me in the School of Textile Technology, The University of New South Wales, and has not been submitted previously for any other degree or award. ii ACKNOWLEDGEMENTS I wish to express my sincere thanks to my supervisors, Associate Professor C.H. Nicholls and Dr. M.T. Pailthorpe, for their guidance and support throughout the course of this work. I thank my colleagues and friends for their continued encouragement and interest. I also thank my friend Anne for the typing and presentation of this thesis. Finally, I particularly thank my fiance, Paul, for his invaluable support during this time. iii ABSTRACT The reasons for the higher lightfastness of basic dyes on acrylic substrates compared to other substrates were investigated. Acrylic, nylon and poly(vinyl alcohol) substrates in film and fibre form were dyed and irradiated in order to study this phenomenon directly in the solid state. It was found that there is a difference in the initial quantum yields of dye fading of at least two orders of magnitude between the acrylic and the non-acrylic fibres. Temperature studies indicated that the high rigidity of acrylic polymers, by virtue of their inherently compact structures and their high glass transition temperatures, contributes significantly to the high lightfastness of basic dyes present in them. This is postulated to be due to the relative inhibition of movement in the polymer, which affects both diffusional and conformational processes that are involved in the fading reactions. Oxygen and moisture were found to have little effect on basic dye fading. Very little difference could be found between the rate of fading of dyes bound to sulphonate and carboxylate dye sites of the same pKa. However, the presence of acid caused a significant reduction in the rate of dye fading which appears to indicate that the pK of the dye site affects a basic dye fading. This pH effect on dye fading rates suggests that electron transfer from substrate to dye could contribute iv to the dye degradation. Thus the reduced fading in acrylics might be attributed in part to their being poor electron donors compared with other substrates studied. The presence of -1% cyano groups in an inert substrate was found to significantly reduce basic dye fading, with the effect increasing with an increased cyano concen­ tration of -8%. Since cyano groups comprise approximately 50% by weight of an acrylic polymer this effect would appear to be the single most important factor contributing to the high lightfastness of basic dyes present in them. It was postulated that the formation of an excited complex between the excited dye molecule and one or more cyano groups would either inhibit the usual fading reactions or facilitate deactivation of the excited dye molecule. V CONTENTS DECLARATION i ACKNOWLEDGEMENTS ii ABSTRACT iii CONTENTS V CHAPTER 1 INTRODUCTION 1.1 Preamble 1 1.2 Photochemical Principles 2 1.2.1 Excited States 4 1.2.2 Photophysical Decay Pathways 5 1.2.3 Photochemical Decay Pathways 6 1.3 The Photochemistry of Dyes 9 1.3.1 Typical Photochemical Reactions of Dyes 10 1.3.1.1 Azo Dyes 11 1.3.1.2 Anthraquinone Dyes 15 1.3.1.3 Triphenylmethane Dyes 18 1. 4 Factors Influencing Dye Fading 22 1.4.1 Intensity and Spectral Distribution of Radiation 22 1. 4 .2 The Physical State of the Dye 24 1.4.3 The Nature of the Substrate 26 1.4.4 The Dye-Fibre Bond 29 1.4.5 Oxygen 30 1.4.6 Moisture 31 1.4.7 Temperature 33 1.5 Scope of Present Work 35 vi CHAPTER 2 QUANTUM YIELDS OF BASIC DYE FADING 2.1 Introduction 41 2.2 Fabrics 42 2.2.1 Acrilan 16 42 2.2.2 Nylon 6.6 43 2.2.3 Basic-Dyeable Nylon 43 2.2.4 Chemical Compositions 43 2.3 Dyes 44 2.4 Dyeing Methods 45 2.4.1 Acrilan 16 46 2.4.2 Nylon 6.6 47 2.4.3 Basic-Dyeable Nylon 47 2.5 Determination of Dye Concentration 48 2.6 Irradiation Technique 51 2.6.1 Calibration of Incident Light Intensity 52 2.7 Determination of Initial Quantum Yields of Basic Dye Fading 57 2.7.1 Measurement of Fading Rates 57 2.7.2 Determination of Quanta Absorbed by Dye 59 2.7.3 Calculation of Initial Quantum Yields of Basic Dye Fading 60 CHAPTER 3 THE EFFECT OF THE PHYSICAL STATE OF THE SUBSTRATE ON BASIC DYE FADING 3.1 Introduction 64 3.2 Polymer Film Preparation 65 3.2.1 Nylon 6 66 3.2.2 Poly(vinyl alcohol) 66 3.2.3 Acrilan 16 67 vii 3.3 Measurement of Glass Transition Temperatures 69 3.3.l Experimental Technique 70 3.3.2 Results 72 3.4 Irradiation Technique 74 3.4.1 Method of Mounting and Heating the Film Sample 74 3.4.2 Method of Irradiation 76 3.5 Determination of the Initial Quantum Yields of Basic Dye Fading 77 3.5.1 Measurement of Fading Rates 77 3.5.2 Measurement of Quanta Absorbed 81 3.5.3 Calculation of the Initial Quantum Yields of Dye Fading 83 3.6 Experimental Results 83 3.6.1 The Effect of Temperature 84 3.6.2 The Effect of Oxygen and Moisture 90 CHAPTER 4 THE INFLUENCE OF THE CHEMICAL NATURE OF THE SUBSTRATE ON THE FADING OF BASIC DYES 4.1 Introduction 96 4.2 Irradiation Technique 97 4.3 The Identification of Reaction Intermediates in Basic Dye Fading 98 4.3.1 The Use of Scavenging and Quenching Agents 98 4.3.1.1 Excited State Quenching 99 4.3.1.2 Electron and Radical Scavenging 99 4.3.1.3 Experimental Technique 100 4.3.1.4 Results 102 4.3.2 Luminescence Studies 104 viii 4.4 The Influence of Dye Sites on Basic Dye Fading 104 4.4.1 Experimental Technique 105 4.4.2 Results 106 4.5 The Influence of the Cyano Group on Basic Dye Fading 109 4.5.1 Experimental Technique 109 4.5.2 Results 111 CHAPTER 5 CONCLUSION 114 REFERENCES 120 1 CHAPTER l INTRODUCTION 1.1 Preamble The fading of dyed materials by light, or photo­ fading, refers to the loss of colour or change of hue of the dye caused by its exposure to light. Being of obvious commercial interest in relation to the durability of dyed fabrics, such phenomena have long been the subject of investigation. References 1 to 8 give reviews of this research. There are, however, still vast areas of the subject about which relatively little is known. This is due to the extremely complex nature of the photochemistry of such dye­ fibre systems and the number of different variables which can influence it. Further, as the concentration of dyes on fabrics is usually very low (no more than -3%) and because fading reactions have low quantum yields and occur only at the surface of the fabric only small yields of decomposition products are obtained. Consequently isolation and identification of these products is so tedious that other less direct methods have to be used to establish the mode of reaction. In addition, the reaction sequence of fading may be so complex that identification of the final products may not shed much light on the preceding reactions. Model systems using organic solutions instead of solid substrates and simple instead of the more complex dyes have been used to simplify the systems and much information has 2 been gained in this way. However, great care must be taken in this respect as it is not always possible to apply such results directly to solid substrates. The fading behaviour may not necessarily be the same in the two environments due to differing physical and/or chemical effects. The aim of the present work was to elucidate some aspects of the fading behaviour of one class of dyes in which large differences in lightfastness properties are found between different fibre substrates. Solid substrates in the form of transparent polymer films and fabrics were used in order to study the problem directly and avoid the interpretation difficulties of solution models. 1.2 Photochemical Principles The first step in a photochemical process is the absorption of light by a molecule. This causes an electron to be promoted from its lowest energy level or ground state to a higher energy level or "excited" state. This can be represented by D + hv ~D.* A molecule in this excited state is much more reactive than in its ground state having more energy, a different electron distribution and different molecular geometry. Consequently photochemical reactions of various types, often unusual in ordinary "thermal" reactions, can occur readily. The lifetime of this reactive excited state is critical in determining whether or not a photochemical reaction, leading to dye fading, will occur. The longer the molecule remains in its excited state the greater the 3 probability of such a reaction occurring. There are numerous alternative deactivation pathways by which the molecule can return to its ground state without any chemical change taking place and therefore the relative rates of these "photophysical" processes compared with the "photo­ chemical" processes will determine the rate at which the dye will fade. The initial processes involving the electronically excited molecule are known as the primary photochemical and photophysical processes.
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