Kuan-Chen Huang Master Thesis

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Kuan-Chen Huang

POLYURETHANES AND DIOLS

A Thesis

Presented to

The Graduate Faculty of The University of Akron

In Partial Fulfillment

of the Requirements for the Degree

Master of Science

Kuan-Chen Huang

May, 2021

STUDY THE APPLICATION OF NON-ISOCYANATE APPROACH IN

POLYURETHANES AND DIOLS

Kuan-Chen Huang

Thesis

Approved: Accepted:

______Advisor Department Chair Dr. Qixin Zhou Dr. Lu-Kwang Ju

______Committee Member Interim Dean of the College Dr. Zhenmeng Peng Dr. Craig Menzemer

______Committee Member Dean of the Graduate School Dr. Junpeng Wang Dr. Marine Saunders

______Date

ABSTRACT

Conventional polyurethanes have been widely used over the world for foams, coatings, and food packaging films. However, conventional polyurethanes are produced from isocyanates which are harmful and toxic. Recently, scientists have developed alternative processes to produce polyurethanes without using isocyanates. The most used non-isocyanate method to obtain the urethane group is by the reaction of cyclic carbonates with amines. This method becomes popular is because various cyclic carbonates and amines can be selected. The work of this thesis is the application of the non-isocyanate approach through the cyclic carbonate/amine method.

In this thesis, a non-isocyanate polyurethane (NIPU) and urethane diols were developed using the non-isocyanate approach. In Chapter 3, a zinc phosphate pigmented

NIPU was designed to improve the corrosion resistance of the polyurethane coating. In

Chapter 4, urethane diols were produced by the non-isocyanate approach. And then, the urethane diols were crosslinked by a hexa(methoxymethyl) melamine (HMMM) crosslinker to increase the mechanical property of the coating.

In Chapter 3, the NIPU was successfully synthesized from bisphenol A cyclic carbonate and fatty acid amine. Different weight percentages of zinc phosphate pigments were added to the NIPU. The corrosion resistance of the NIPU coating was investigated by electrochemical impedance spectroscopy and salt spray measurement. The corrosion resistance of the NIPU was increased by adding the zinc phosphate pigments. The NIPU

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coating with 5 wt. % zinc phosphate pigment showed the best corrosion resistance among the whole formulations. This study provides a capability for the application of NIPU in corrosion protective coatings.

In Chapter 4, the urethane diols were produced by the non-isocyanate approach at room temperature without using any catalyst. Two urethane diols with different middle chains were obtained by the reaction of 1,4-diaminobutane or 1,6-diaminohexane with ethylene cyclic carbonate, respectively. Different weight percentages of HMMM crosslinker were mixed with urethane diols. The mechanical property of the coating was investigated by tensile test, adhesion test, and conical mandrel bend test. The HMMM crosslinker can react with urethane diols to form a polymer network to generate a coating film. This study provides a non-isocyanate way to produce urethane diols and successfully used them in melamine-formaldehyde coatings. The urethane diols can be further utilized for producing coatings and polymers.

In general, this thesis successfully synthesized NIPU and urethane diols through a non-isocyanate approach. By further adding the NIPU with zinc phosphate pigments or crosslinking the urethane diols with HMMM crosslinkers, this thesis intends to expand the non-isocyanate method for the application in corrosion protective coatings. This work would encourage the replacement of conventional polyurethanes or urethane diols for green and environmentally friendly alternatives.

ACKNOWLEDGEMENTS

I would like to show my sincere appreciation to Dr. Qixin Zhou for her patience, motivation, research suggestion, and encouragement. Her valuable guideline helped me throughout my entire master’s study. I would also like to express my appreciation to the committee members: Dr. Peng and Dr. Wang for their comments.

I also much appreciate Dr. Lingyan Li and Mr. Will Imes for their help with instrumental support. I would also like to thank all my group members, Haoran Wang,

Cheng Zhang, Weixiu Zeng, and Zichen Ling.

Finally, I would like to express special thanks to my parents, who have always been supporting me in my master’s study.

TABLE OF CONTENTS

LIST OF FIGURES ...... ix

LIST OF TABLES ...... xi

CHAPTER I

INTRODUCTION ...... 1

1.1 Overview of Work ...... 1

1.2 Research Objectives ...... 2

CHAPTER II

BACKGROUND ...... 3

2.1 Organic Coating ...... 3

2.2 Polyurethane Binder ...... 3

2.3 Melamine Formaldehyde Binder ...... 5

2.4 Characterization ...... 7

2.4.1 Electrochemical Impedance Spectroscopy (EIS) ...... 7

2.4.2 Fourier Transform Infrared Spectroscopy (FTIR) ...... 8

2.4.3 Salt Spray Test ...... 8

2.4.4 Tape Adhesion Test ...... 9

2.4.5 Flexibility Test ...... 10

2.4.6 Tensile Test ...... 11

CHAPTER III

ANTI-CORROSION NON-ISOCYANATE POLYURETHANE WITH ZINC PHOSPHATE PIGMENT COATINGS ...... 12

3.1 Introduction ...... 12

3.2 Experimental ...... 13

3.2.1 Material ...... 13

3.2.2 Synthesis of BPA Cyclic Carbonate ...... 14

3.2.3 Synthesis of Amine-terminated NIPU ...... 14

3.2.4 Preparation of NIPU Coatings with Pigments ...... 14

3.2.5 Instrumentation ...... 16

3.3 Results and Discussion ...... 16

3.3.1 Structural Characterization of BPA Cyclic Carbonate ...... 16

3.3.2 Structural Characterization of Amine-terminated NIPU ...... 20

3.3.3 Anti-corrosion Performance Evaluation: EIS ...... 20

3.3.4 Anti-corrosion Performance Evaluation: Salt Spray ...... 25

3.4 Conclusion ...... 27

CHAPTER IV

URETHANE DIOLS VIA NON-ISOCYANATE PROCESS AND ITS CHARACTERIZATION IN THE COATING ...... 28

4.1 Introduction ...... 28

4.2 Experimental ...... 30

4.2.1 Material ...... 30

4.2.2 Synthesis of Urethane Groups Diols ...... 30

4.2.3 Preparation of Films ...... 31

4.2.4 Instrumentation ...... 33

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4.3 Results and Discussion ...... 34

4.3.1 Structural Characterization of Urethane Diols ...... 34

4.3.2 Films Properties ...... 37

4.4 Conclusion ...... 42

CHAPTER V

SUMMARY ...... 44

REFERENCES ...... 45

viii

LIST OF FIGURES

Figure 2. 1 The reaction of isocyanates with alcohols ...... 4

Figure 2. 2 The reaction of cyclic carbonates with amines ...... 4

Figure 2. 3 Acyclic hydrogen bonds and cyclic hydrogen bonds ...... 5

Figure 2. 4 The reaction of MF resins with phenols ...... 6

Figure 2. 5 Three-electrode electrochemical setup for EIS measurement ...... 8

Figure 2. 6 Insides and working diagrams for the salt spray chamber ...... 9

Figure 2. 7 Conical mandrel blend tester ...... 11

Figure 3. 1 FTIR spectra of (a) the BPA cyclic carbonate after the reaction at 130 °C for 96 h; (b) the amine-terminated NIPU synthesized from BPA cyclic carbonate and fatty acid amine ...... 18

Figure 3. 2 (a) 1H and 13C NMR spectra of the BPA cyclic carbonate after the reaction at 130 °C for 96 h; (b) 1H and 13C NMR spectra of the amine-terminated NIPU synthesized from BPA cyclic carbonate and fatty acid amine ...... 19

Figure 3. 3 EIS Bode plot of NIPU coatings in the immersion of 3.5 wt.% NaCl solution for 1, 3, 7, 14, 21, and 28 days. NIPU0 to NIPU20 refers to the formulation in Table 3.1 ...... 24

Figure 3. 4 Images of NIPU coatings under salt spray testing for 1 day (top row), 7 days (middle row), and 14 days (bottom row) exposure ...... 26

Figure 4. 1 The reaction of MF resin with polyols ...... 29

Figure 4. 2 Chemical structure of 1,6-diaminohexane ...... 30

Figure 4. 3 Chemical structure of 1,4-diaminobutane ...... 31

Figure 4. 4 Synthesis of urethane diols from ethylene cyclic carbonate and amines .. 31

Figure 4. 5 Chemical structure of 1,10-decanediol ...... 32

Figure 4. 6 Chemical structure of 1,12-dodecanediol ...... 32

Figure 4. 7 FTIR spectra of the initial reactants (top) and final product (bottom) in the reaction for BHC6 ...... 35

Figure 4. 8 FTIR spectra of the initial reactants (top) and final product (bottom) in the reaction for BHC4 ...... 35

Figure 4. 9 1H spectra of BHC4 ...... 36

Figure 4. 10 1H spectra of BHC6 ...... 36

Figure 4. 11 Tensile strength of samples from the experimental group and the control group. Experimental group refers to the formulation in Table 4.1; Control group refers to the formulation in Table 4.2 ...... 38

Figure 4. 12 Elongation-at-break of samples from the experimental group and the control group. Experimental group refers to the formulation in Table 4.1; Control group refers to the formulation in Table 4.2 ...... 40

LIST OF TABLES

Table 2. 1 ASTM D3359 standard for tape adhesion test ...... 10

Table 3. 1 Formulation of isocyanate-free polyurethane coatings (wt. %) ...... 15

Table 4. 1 Formulation of MF coatings for experimental group (wt. %) ...... 32

Table 4. 2 Formulation of MF coatings for control group (wt. %) ...... 33

Table 4. 3 Dry adhesion based on ASTM D3359 ...... 41

Table 4. 4 ASTM D552 conical mandrel bend test results ...... 42

CHAPTER I

INTRODUCTION

1.1 Overview of Work

Polyurethanes are mostly utilized over the world in commodities and industries.

In the past, the conventional processes to produce polyurethanes were from isocyanates and polyols. However, isocyanates are detrimental and deleterious. Moreover, the main raw material to manufacture isocyanates is phosgene which is virulent to humans. In order to mitigate the problem in the conventional processes, scientists have developed alternative methods to produce polyurethanes over the years. Nowadays, several methods to obtain polyurethanes without isocyanates have been invented such as the reaction of cyclic carbonates with amines and the copolymerization of aziridines with carbon dioxide. The reaction of cyclic carbonates with amine is commonly employed because the selection of cyclic carbonates and amine is various. In this work, the non-isocyanate polyurethane and urethane diols were produced via this method and their properties were investigated.

Pigments are commonly added into a coating for some purposes such as modifying optical properties (color, opacity, and gloss), increasing mechanical or chemical resistance (light, heat, and chemicals), and/or reducing cost. Zinc phosphate pigment is commonly used to improve coatings’ corrosion resistance.

The melamine coating is widely applied as building materials. Conventionally, the melamine resin is curing with polyols to form a network polymer. However, it has some disadvantages. For example, the flexibility and chemical resistance of the melamine coating is inadequate, which limits its application. To enhance their properties, doping the urethane groups into polyols can make the polymer network more flexible. The polyols with the urethane groups are expected to be produced from a safe and green process.

1.2 Research Objectives

The main objective of the research is to expand the non-isocyanate approach in the application of polyurethanes and diols. A non-isocyanate polyurethane (NIPU) and urethane diols will be developed through the non-isocyanate approach. Specifically, in

Chapter 3, the NIPU will be synthesized from bisphenol A (BPA) cyclic carbonate and fatty acid amine. Furthermore, different weight percentages of zinc phosphate pigments will be added to the NIPU to improve the corrosion resistance of the polyurethane coating. In Chapter 4, the diols will be generated by the reaction of cyclic carbonates and amines. The urethane diols will then be crosslinked with hexa(methoxymethyl) melamine

(HMMM) to form the melamine-formaldehyde coating films. Different carbon atoms and different weight percentages of HMMM crosslinker will be studied. Moreover, the property of the coating films will also be investigated by different characterization methods.

CHAPTER II

BACKGROUND

2.1 Organic Coatings

Organic Coatings can be found in our daily life in industry, vehicle, house, and et al. It is employed widely in our world to provide many functions such as surface protection and appearance embellishment. Generally, the coating usually consists of binder, solvent, pigments, and additives. Different components play important roles.

Binders are the materials that form the film that adheres to the substrate. They are organic polymers. Polyurethane and epoxy are commonly used. Solvents are volatile components as a liquid. It can make the coating fluid enough for application and film formation. They evaporate after or during application without chemical reaction. Additives are materials that are included in small quantities to modify some property of coatings. Examples are catalysts, stabilizers, and flow modifiers. Pigments are solid particles, which are the most insoluble in solvents. They are incorporated into organic coatings by dispersion technique to provide color and opacity to the coating film and resistance among other functions.

2.2 Polyurethane Binder

Polyurethane is a chemical substance containing urethane linkages. It could be in different forms such as foam and resin. In the aspect of resins, there is a lot of methods to synthesize polyurethane. Traditionally, the synthesis of polyurethane is from isocyanates and alcohols as shown in Figure 2.1[1]. In this way, polyurethane binders can provide

good mechanical properties and chemical properties such as adhesion strength, chemical resistance, and water resistance. However, the isocyanate and the raw material to produce isocyanate like phosgene are harmful chemicals. Nowadays, the non-isocyanate polyurethane has been developed in recent years, which is employing cyclic carbonate and amines in Figure 2.2 [2].

Figure 2. 1 The reaction of isocyanates with alcohols

Figure 2. 2 The reaction of cyclic carbonates with amines

Another feature of polyurethane binder is the formation of intermolecular hydrogen bonds. There is two types of bonding, which is acyclic hydrogen bond and cyclic hydrogen bond in Figure 2.3 [1]. When the stress is applied, those H-bonds is breaking due to absorb the part of energy. Moreover, those bonds can reform again. With this feature, it can reduce the possibility of film failures.

Figure 2. 3 Acyclic hydrogen bonds and cyclic hydrogen bonds

2.3 Melamine Formaldehyde Binder

It is a resin with melamine rings with hydroxyl groups derived from formaldehyde and the alcohol used for etherification. Based on the ratio of functional groups, it could be classified into two groups. With a relatively high ratio of formaldehyde to melamine and/or two alkoxy methyl substituents connected with the nitrogen, it is called Class I resins. Class II resins, it has lower ratios of formaldehyde and only one substituent with many of the nitrogen.

Class II resins were predominantly used for cross-linking alkyds from 1940 through the 1950s. These resins have enough bridging, and the alcohol is usually n-butyl or isobutyl alcohol. They are readily miscible in alkyd formulations, and they give a wide latitude such as economical product; that is, it doesn’t need an exacting control formulation to produce acceptable application characteristics and film properties in coatings industries.

Class I resins were first commercialized around the 1950s. Class I resins contained methylated are more compatible than butylated resins. In some cases, it provided tougher films. The shift toward Class I resins during the 1970s because of the

introduction of waterborne and high solid coatings. Methylated Class I resins are more miscible. It has a promising application in waterborne coatings. Moreover, Class I resins have a low polymerization value, which can give low viscosity to high solid coatings.

MF resins are used to be a crosslinker with coreactant resins containing hydroxyl, carboxylic acid, urethane (carbamate), and/or amide groups. Acrylic, polyester, alkyd, epoxy, and polyurethane resins are the most commonly coreactant resins.

Polyols are the resins most cross-linked by MF resins. The hydroxyl groups of polyols could react with the activated alkoxymethyl groups by transetherification or methylol groups of MF resins to form new cross-links by etherification. The different polyols reacting with MF resins can get different film properties. For example, the reaction with phenols has the advantage of a C-C bond formation so that the product is stable to hydrolysis in Figure 2.4 [1].

Figure 2. 4 The reaction of MF resins with phenols

Strong and weak acid catalysts are used in the reaction. For strong acid catalysts, it is usually used for Class I resins such as sulfonic acids. The weak acid catalysts such as carboxylic acids are used for Class II resins. The selection of polyols and MF resins depends on the properties that it needs.

2.4 Characterization

2.4.1 Electrochemical Impedance Spectroscopy (EIS)

EIS is an important method for evaluating the coating film’s properties on the metal substrate. It can generate quantitative data that relates to the qualities of a coating.

Moreover, EIS is a sensitive detector that can indicate slight changes of the coating films which is invisible. The signals are often detected by ions migration due to uptake water and the degradation of coating films. Thus, EIS is a powerful tool to predict the lifetime of coatings [3]. Generally, EIS consists of three parts: a working electrode (the coated metal or substrates), a counter electrode (graphite or platinum), and a reference electrode

(Saturated Calomel Electrode, SCE, or Silver Chloride, Ag/AgCl). The typical three- electrode electrochemical setup for EIS measurement is shown in Figure 2.5. When the electrodes have been set up, the electrolyte solution can be poured into the cell, commonly a NaCl solution. EIS is a non-destructive method, which means that it can track the changes when the corrosion occurs on the metal surface [4]. Furthermore, the quantitative data from EIS can be analyzed by different models to know more details such as pore resistance, solvent resistance, and corrosion resistance, etc.

Figure 2. 5 Three-electrode electrochemical setup for EIS measurement

2.4.2 Fourier Transform Infrared Spectroscopy (FTIR)

FTIR is a common tool to identify the chemical composition of a polymer. The change of the chemical structure can be identified in the spectrum. Therefore, it can show that the reaction is completed by the peak of the functional groups in the spectrum.

2.4.3 Salt Spray Test

The salt spray test is an accelerated corrosion testing because it can run the program continuously for 24 hours. It can produce a corrosive attack on the metal or other materials to evaluate the corrosion resistance [5-8]. By this test, it can generate a simulation of natural corrosion in the laboratory. Basically, ASTM B-117 is a common standard to test the metal or coating films. For ASTM B-117, the electrolyte solution is 5

wt. % of NaCl solution and set up the chamber temperature at 35 °C. The diagram for the salt spray chamber is shown in Figure 2.6 [9].

Figure 2. 6 Insides and working diagrams for the salt spray chamber

2.4.4 Tape Adhesion Test

The tape adhesion test is presented by ASTM D3359. Use the crosshatch cutter to create the cross-section on a surface of coating films. The vertical angle cut is first placed on the sample and another angle after that. After the first step, the specific tape is covered on the cross-cut area. The tape adhesion is evaluated based on Table 2.1 [10].

Table 2. 1 ASTM D3359 standard for tape adhesion test

2.4.5 Flexibility Test

The flexibility test is applied to give ideas about resistance to cracking. The conical mandrel tester is commonly used to test coating flexibility as illustrated in Figure

2.7. The coating sample is set up on the tester. The tester blends it to see the coating films depriving or peeling off the metal substrate. The distance from the test demonstrated the flexibility better or worse. For example, the cracking was on the shorter diameter side, which means that the flexibility is better with the same coating thickness for the specimens.

Figure 2. 7 Conical mandrel blend tester

2.4.6 Tensile Test

Instron 5567 test is applied to determine the tensile properties of coating films.

The method follows ASTM 2370. The sample is removed by the blaze from the metal substrate with a width of 10 mm and a length of 20 mm. The sample is fixed in the clamp with a speed of 10 mm per minute at room temperature. All samples are tested by Instron

5567 for tensile properties.

CHAPTER III

ANTI-CORROSION NON-ISOCYANATE POLYURETHANE COATINGS WITH

ZINC PHOSPHATE PIGMENTS

3.1 Introduction

Isocyanate is an essential material for the synthesis of conventional polyurethane

[11-13]. However, there are many drawbacks such as detrimental phosgene-derived production, high toxicity, and high-water sensitivity [14-17]. Leading to the requirements of the fabrication of environmentally friendly polyurethane. Scientists and industrial researchers have developed NIPU starting as early as 2000 [18-24]. Among various methods, the most common method is the synthesis of NIPU through the reaction of cyclic carbonates and amines. Forming the urethane linkage on the polymer backbone.

Moreover, the primary or secondary hydroxyl groups were formed on the side chain, which can utilize the hydroxyl group to achieve better properties of NIPU [20].

Nowadays, NIPU is widely used in the application of the coating, but it has not been well explored the investigation of the mechanical properties, the thermal properties of different NIPU coatings, a major function of NIPU, and corrosion resistance. In the study of Sabnis et al., the improvement of corrosion resistance of the NIPU coatings was prepared using the treated zinc oxide nanoparticles [25]. The surface of zinc oxide was modified by cyclic carbonated 3-glycidoxy propyl trimethoxy silane and formulated into

NIPU coatings. It improved the corrosion resistance of NIPU coatings because the micropores were filled with the treated zinc oxide. Dolui et al. studied the corrosion

resistance of NIPU coatings through the relationship between the structure and properties

[26]. A series of NIPU coatings were obtained from ethylenediamine, diethylenetriamine, isophorone diamine (IPDA), and biobased cyclic carbonate. Consequently, the IPDA- based NIPU coating provided the best corrosion resistance among these samples. There are other developed methods for various purposes in coatings. However, the major goal of NIPU coatings is to develop effective methods for the improvement of anti-corrosion.

The aim of this study is to prepare NIPU coatings with different anti-corrosion pigment concentration and study the effect of its. The zinc phosphate pigment is selected because it is commonly used. The BPA cyclic carbonate was synthesized from BPA epoxy and carbon dioxide. The amine-terminated NIPU was synthesized by BPA cyclic carbonate and fatty acid amine. Then, the NIPU coatings were prepared from amine- terminated NIPU, BPA epoxy, and the zinc phosphate pigment. The chemical structure of synthesized materials was characterized by FTIR,1H and13C NMR. Finally, the corrosion resistance of NIPU coatings was evaluated by EIS and salt spray.

3.2 Experimental

3.2.1 Material

BPA epoxy resin was supplied by Hexion (EPON Resin 828). Fatty acid amine was obtained from CRODA (PRIAMINE 1075). Tetra-n-butylammonium bromide (TBAB,

≥99%), acetone, methanol, and sodium chloride, zinc phosphate pigment were purchased from Sigma-Aldrich. The wetting agent BYK-333 was kindly provided by BYK. A compressed carbon dioxide gas cylinder was purchased from Praxair. Steel substrates were purchased from Q-Lab (QD-36, 3×6 in2). All the materials were used as received without further purification.

3.2.2 Synthesis of BPA Cyclic Carbonate

Base on the previous report [27], the BPA cyclic carbonate was synthesized from

CO2 and epoxy with a catalyst. BPA epoxy (100.0 g) and TBAB catalyst (5.0 g) were put into a 500 ml three-neck round-bottom reaction flask with inlet of CO2 under the normal atmosphere at 130 °C for 96 h. Through the disappearance of the epoxy group signal in

1H NMR spectrum determined the completed reaction. After colling down, the final product was a yellow rigid solid. FTIR, 1H and 13C NMR were used to characterize the chemical structure of BPA cyclic carbonate.

3.2.3 Synthesis of Amine-terminated NIPU

Amine-terminated NIPU was synthesized from BPA cyclic carbonate and fatty acid amine under N2. Usually, this reaction needs to be at elevated temperature. In this study, it doesn’t need any catalyst. Firstly, BPA cyclic carbonate (10.0 g) was heated up to 80 °C in the three-neck reaction flask. After solid BPA cyclic carbonate melt, fatty acid amine (20.0 g) was slowly added into a reaction flask under magnetic stirring and then the temperature was raised to 100 °C. Finally, the reaction was completed at 100 °C for 5 h. The amine hydrogen equivalent weight (AHEW) was determined by the ASTM

D2074 based on primary amine value and secondary amine value (576 g/eq.). FTIR, 1H and 13C NMR were used to characterize the chemical structure of amine-terminated

NIPU.

3.2.4 Preparation of NIPU Coatings with Pigments

NIPU coatings were prepared from amine-terminated NIPU and BPA epoxy with adding different amounts of zinc phosphate pigments from 0 to 20 wt. %. The formulations are listed in Table 3.1. The samples were named by the amount of pigments.

For example, NIPU5 is the NIPU coating with 5 wt. % of pigments. The amount of amine-terminated NIPU and BPA epoxy was determined by the ratio of AHEW/EEW, which was kept at 1:1. The resin and epoxy were dissolved in acetone. When it mixed well, the zinc phosphate pigment was added into the mixture as shown in Table 3.1. In addition, the wetting agent BYK-333 was then added appropriately into the container to avoid defects on the coating surface. The coatings were applied on the steel panels. The samples were cured at 100 °C for 2 h after overnight at room temperature. The cured dry coating thickness was approximately 40 µm. The formulation of all samples was summarized in Table 3.1.

Table 3. 1 Formulation of isocyanate-free polyurethane coatings (wt. %)

Sample Amine-terminated NIPU BPA Epoxy Zinc phosphate

NIPU0 75.6 24.4 0

NIPU5 71.8 23.2 5

NIPU10 68.1 21.9 10

NIPU15 64.3 20.7 15

NIPU20 60.5 19.5 20

3.2.5 Instrumentation

The FTIR spectra was obtain by Nicolet iS10 FT-IR Spectrometer (resolution 4 cm-1; scan number 64). The range of scanning wavenumber was from 4000 to 400 cm-1.

The drops of the liquid samples were dropped on the diamond crystal. After that, the

FTIR testing was carried out at room temperature. The 1H and 13C NMR spectra were obtained by Varian INOVA 400 instrument. Herein, the sample solvent is CDCl3.

EIS was operated by a Reference 600 potentiostat (Gamry Instruments). It used a three-electrode cell system including a working electrode (the coated panel), a reference electrode (saturated calomel), and a counter electrode (niobium platinized mesh with 6.25 cm2 surface area), as shown in Figure 2.5. The electrolyte was a NaCl aqueous solution

(3.5 wt. %). The samples were clamped to the cell body with an O-ring (diameter 32 mm) in order to avoid the solution leakage. The exposed coatings surface area was 8.03 cm2 with a thickness of around 55 µm. The measurements were operated with 10 mV (versus open circuit potential) AC perturbation using a frequency range of 0.01 to 100k Hz at room temperature. The salt spray experiment followed ASTM B117. The coating surface was scribed by a blade. Then, all samples were placed in the fog chamber and the fog was generated from 5 wt. % NaCl solution with the temperature set to be 35 °C, as shown in

Figure 2.6.

3.3 Results and Discussion

3.3.1 Structural Characterization of BPA Cyclic Carbonate

In the synthesis of NIPU, BPA cyclic carbonate is commonly used because it comes from commercially available BPA epoxy resin [28-30]. In this study, BPA cyclic carbonate was synthesized from BPA resin and CO2 with the TBAB as a catalyst. The

cyclic carbonate group and the consumption of epoxied groups were determined by FTIR and Varian INOVA 400 instruments. The appearance of signal at 1803 cm-1 in Figure

3.1(a) is defined to C=O linkage in cyclic carbonate group [31, 32]. Figure 3.2(a) shows the 1H and 13C NMR spectra of BPA cyclic carbonate. From the 1H NMR spectra, there are two signals observed at 4.41 and 4.96 ppm. It could be the H atom of the cyclic carbonate group [33]. In addition, there is not a signal at between 2.50 to 3.10 ppm which is assigned to epoxy group. Because of it, it confirmed that all epoxides were reacted and formed cyclic carbonate group. From 13C NMR spectra in Figure 3.2, there is a strong signal at 155.8 ppm. It could be a the C atom of the carbonyl linkage in the cyclic carbonate group [26].

Figure 3. 1 FTIR spectra of (a) the BPA cyclic carbonate after the reaction at 130 °C for

96 h; (b) the amine-terminated NIPU synthesized from BPA cyclic carbonate and fatty

acid amine

Figure 3. 2 (a) 1H and 13C NMR spectra of the BPA cyclic carbonate after the reaction at

130 °C for 96 h; (b) 1H and 13C NMR spectra of the amine-terminated NIPU synthesized

from BPA cyclic carbonate and fatty acid amine

3.3.2 Structural Characterization of Amine-terminated NIPU

Through FTIR, the formation of the urethane group in amine-terminated NIPU was firstly observed. Figure 3.1(b) shows the FTIR spectra of the amine-terminated NIPU synthesized from BPA cyclic carbonate and fatty acid amine. In Figure 3.1(b), there is a strong signal at 1710 cm-1 which is assigned to the C=O linkage in the urethane group

[34]. The disappearance of the signal of cyclic carbonate group at 1803 cm-1 indicated the reaction between BPA cyclic carbonate and fatty acid amine was completed [35].

Furthermore, the signal of the hydroxyl group synthesized from the reaction of cyclic carbonate and amine was from 3200 to 3600 cm-1 [36]. Figure 3.2(b) shows the 1H and

13C NMR spectra of the amine-terminated NIPU synthesized from BPA cyclic carbonate and fatty acid amine. From the 1H NMR spectra, there are two strong signals at 6.73 and

7.03 ppm, which are determined to the signals of urethane linkage formed by cyclic carbonate/amine reaction. Additionally, the evidence of OH group was found in signal bands from 4.80 to 4.97 ppm, which is formed by the cyclic carbonate/amine reaction.

3.3.3 Anti-corrosion Performance Evaluation: EIS

In this study, improving the corrosion resistance of NIPU coatings is another goal by adding zinc phosphate pigments. Commonly, EIS was used to monitor the corrosion resistance of NIPU coatings for 30 days immersion in 3.5 wt. % NaCl solution. In EIS measurement, impedance modulus usually represents the resistive performance of a coating at the low frequency [37]. For NIPU15 coating, the impedance modulus is from around 108 Ohm cm2 to 107 Ohm cm2 after 28 days immersion at low frequency as shown in Figure 3.5.

The bode plot shows the different performances of NIPU coatings after 28 days immersion. From the bode plots, the NIPU5 coating performed a better anti-corrosion ability after 28 days immersion. On the other hand, NIPU15 coating was the worst after

28 days immersion. Base on bode plots in Figure 3.3, NIPU5 coating had the best corrosion resistance than other formulations.

Figure 3. 3 EIS Bode plot of NIPU coatings in the immersion of 3.5 wt.% NaCl solution for 1, 3, 7, 14, 21, and 28 days. NIPU0 to NIPU20 refers to the formulation in Table 3.1

3.3.4 Anti-corrosion Performance Evaluation: Salt Spray

Salt spray is a common accelerating test to assess corrosion resistance. Figure 3.4 illustrates the images of NIPU coatings after several days of exposure under the salt spray. In the first day exposure, all the coating films on the substrate showed no corrosion products along with the scribe line. However, NIPU0 had a corrosion product on the area which was scribed after 7 days exposure. This phenomenon was ascribed to that the

NIPU0 coating cannot adhere to the metal substrate at the wet condition, so the corrosive ions can attack the metal through the interface between coating and metal. The rest of samples performed same condition as the first day exposure. In 14 days, it’s obvious that corrosion products were on all the samples. Moreover, there was a large corrosion area on

NIPU5 compared to other samples. On the contrary, NIPU20 had a smaller corrosion area. Based on salt spray test, the corrosion resistance increased along with adding pigments. NIPU20 showed the best corrosion resistance. The salt spray results are similar to the EIS results. Although the results from EIS showed that NIPU5 provided the best corrosion resistance, and NIPU20 had a similar corrosion resistance to NIPU5.

Figure 3. 4 Images of NIPU coatings under salt spray testing for 1 day (top row), 7 days

(middle row), and 14 days (bottom row) exposure

3.4 Conclusion

In this study, the NIPU coatings were successfully prepared from amine- terminated NIPU and BPA epoxy. The anti-corrosion performance of the environmentally friendly NIPU coatings was significantly increased by the zinc phosphate pigment. NIPU coatings with 5 wt. % zinc phosphate pigment exhibited the best corrosion resistance which was confirmed by EIS. However, the coating with 5 wt.

% zinc phosphate pigment showed inadequate corrosion protection in salt spray test. although NIPU5 showed poor corrosion protection in 14 days, the EIS results of NIPU5 were more precise and accurate. In general, incorporating zinc phosphate pigments into

NIPU coating can be an effective approach to improve the anti-corrosion performance of

NIPU coatings which is the green alternative to the conventional polyurethane coating.

CHAPTER IV

URETHANE DIOLS VIA NON-ISOCYANATE PROCESS AND THE APPLICATION

IN MF COATINGS

4.1 Introduction

High-solid coatings are widely used as topcoats in the thermoset system in various industries. Most properties are of high-solid coatings derived from the highly cross-linked networks. The heat-cured coating systems are one of the common types. It consists of a base resin with hydroxyl functional groups and melamine-formaldehyde resin as a crosslinker [38, 39]. The hardness and chemical resistance are provided by the heterocyclic ring structure of melamine resin as the base resin aided in flexibility.

Methylolated amine, known as hexa(methoxymethyl) melamine (HMMM), is an alternative crosslinker due to its high reactivity with hydroxyl and carboxylic acid groups. HMMM can react with primary and secondary hydroxyl functional polymer to form the three-dimensional thermoset polymer network [40]. The reaction of melamine with polyols is shown in Figure 4.1 [1].

Although melamine/polyol thermoset systems are widely used for many industrial coatings, there are drawbacks such as poor chemical resistance and the lack of flexibility at high solid applications. Many studies have investigated the effect of different polyols such as polyester and polyurethane on the properties of melamine/polyol thermoset systems [41-44]. For example, Balgude et al. [45] modified the cardanol oil as bio-based

polyols to improve the chemical resistance and the flexibility with the amount of HMMM crosslinker.

Figure 4. 1 The reaction of MF resin with polyols

Polyurethanes are popular material in many different coating applications due to their outstanding properties such as good adhesion, flexibility, and durability [13].

However, the raw material which is isocyanate to produce polyurethane is toxic and harmful to humans and the environment. To overcome this problem, the scientist has developed an alternative method to obtain urethane linkages.

The most common method is the reaction of cyclic carbonate and amines. This method has caught more attention due to the straightforward mechanism and formation of secondary hydroxyl groups with the reason of the ring-opening of cyclic carbonate [46,

47]. Moreover, the selection of cyclic carbonate and amines can be various respected to different goals.

In this study, several non-isocyanate diols were obtained by a non-isocyanate polyurethane process. Cyclic carbonate reacts with linear diamines to form different urethane polyols. The control groups are the linear diamine including the same carbon atoms but without urethane groups. The coatings were prepared by various urethane polyols along with different weight percentages of HMMM crosslinker. The flexibility, adhesion, and tensile strength were studied after the full curing of the coating.

4.2 Experimental

4.2.1 Material

Ethylene carbonate, 1,4-diaminobutane, 1,6-diaminohexane, 1,10-decanediol,

1,12-dodecanediol were purchased from Sigma-Aldrich. HMMM was kindly supplied by

Allnex (CT, USA) and para-toluenesulfonic acid (pTSA; K-cure 1040) was provided from King Industries (CT, USA). Ethanol and methylene chloride were obtained from

Sigma-Aldrich. Steel substrates were purchased from Q-Lab (QD-36, 3×6 in2, OH,

USA). All the materials were used as received without further purification.

4.2.2 Synthesis of Urethane Diols

To synthesize 1,6-bishydroxyalkylcarbamates (BHC6), 26.3 g (0.3 mol) of ethylene carbonate and 17.4 g (0.15 mol) of 1,6-diaminohexane, as shown in Figure 4.2, were mixed well in the methylene chloride solution at room temperature, according to the previously published process [48]. Similarly, to synthesize 1,4- bishydroxyalkylcarbamates (BHC4), 26.3 g (0.3 mol) of ethylene carbonate and 13.2 g

(0.15 mol) of 1,4-diaminobutane, as shown in Figure 4.3, were stirred well in the methylene chloride solution at room temperature. The scheme of the reaction is shown in

Figure 4.4. The reaction was monitored by FTIR characterization.

Figure 4. 2 Chemical structure of 1,6-diaminohexane

Figure 4. 3 Chemical structure of 1,4-diaminobutane

Figure 4. 4 Synthesis of urethane diols from ethylene cyclic carbonate and amines

4.2.3 Preparation of MF Coating Films

The MF coating films with different weight percentages of HMMM crosslinker were prepared from urethane diols. Firstly, urethane diols were dissolved into ethanol in a vial. And then, the crosslinker and the catalyst were added into the vial till mixed well.

The mixture was coated on the steel panel by a film applicator and heated up to 120 °C for two hours. The linear diols without urethane groups were chosen as the control. The chemical structures of the diols are shown in Figure 4.5 and Figure 4.6. They followed the same procedure as mentioned above to prepare the MF coatings. The formulation of the samples for the experimental group and the control group was summarized in Table

4.1 and Table 4.2, respectively. The samples were named by the amount of HMMM crosslinker. For example, in the experimental group, BHC4-15% is the 1,4- bishydroxyalkylcarbamates with 15 wt. % of HMMM crosslinker. BHC6-15% is the 1,6- bishydroxyalkylcarbamates with 15 wt. % of HMMM crosslinker. Similarly, in the

control group, BHC4C-15% is the 1,10-decanediol with 15 wt. % of HMMM crosslinker.

BHC6C-15% is the 1,12-dodecanediol with 15 wt. % of HMMM crosslinker.

Figure 4. 5 Chemical structure of 1,10-decanediol

Figure 4. 6 Chemical structure of 1,12-dodecanediol

Table 4. 1 Formulation of MF coatings for experimental group (wt. %)

Experiment group Diols HMMM Catalyst

BHC4-15% 82% 15% 3%

BHC6-15% 82% 15% 3%

BHC4-25% 72% 25% 3%

BHC6-25% 72% 25% 3%

BHC4-35% 62% 35% 3%

BHC6-35% 62% 35% 3%

Table 4. 2 Formulation of MF coatings for control group (wt. %)

Control group Diols HMMM Catalyst

BHC4C-15% 82% 15% 3%

BHC6C-15% 82% 15% 3%

BHC4C-25% 72% 25% 3%

BHC6C-25% 72% 25% 3%

BHC4C-35% 62% 35% 3%

BHC6C-35% 62% 35% 3%

4.2.4 Instrumentation

The FTIR spectra were obtained by Nicolet iS10 FT-IR Spectrometer (resolution

4 cm-1; scan number 64). The range of scanning wavenumber was from 4000 to 400 cm-1.

The drops of the liquid samples were dropped on the diamond crystal. After that, the

FTIR testing was carried out. The 1H NMR spectra were obtained by Varian INOVA 400 instrument. Herein, the sample solvent was CDCl3. The adhesion property was obtained by the cross-cut tape method according to ASTM D3359, as shown in Table 2.1. The flexibility was evaluated by a conical mandrel tester based on ASTM D522, as shown in

Figure 2.7. The tensile property was carried out by Instron 5567 following the ASTM

D2370 standard. All the tests were repeated at least 3 times.

4.3 Results and Discussion

4.3.1 Structural Characterization of Urethane Diols

As mentioned in the experimental, BHC6 was obtained in the reaction of ethylene carbonate with 1,6-diaminohexane at room temperature without any catalyst. When the reaction was completed, the final product was precipitated as a white powder. From the

FTIR spectrum, as shown in Figure 4.7, there was no absorption band characteristic for a carbonyl group of five-membered cyclic carbonate at 1800 cm-1. Similar to BHC6, no catalyst was needed to synthesize BHC4, and the final product of BHC4 was also a white powder. In Figure 4.8, the disappearance of the peak at 1800 cm-1 indicated that the reaction of ethylene carbonate with 1,4-diaminobutane was completed.

The BHC4 and BHC6 were also characterized by 1H NMR as shown in Figure 4.9 and Figure 4.10, respectively. The spectrum of the reaction products was recorded at room temperature. Two signals of NH groups are found at 7.08 ppm and 6.72 ppm with an intensity ratio around 8 to 2. This is consistent with the previously reported work by

Neffgen et al. [49]. The BHC6 had one more peak at 1.2 ppm due to the additional carbon atom in the middle chain comparing to BHC4.

Figure 4. 7 FTIR spectra of the initial reactants (top) and final product (bottom) in the

reaction for BHC6

Figure 4. 8 FTIR spectra of the initial reactants (top) and final product (bottom) in the

reaction for BHC4

Figure 4. 9 1H spectra of BHC4

Figure 4. 10 1H spectra of BHC6

4.3.2 Films Properties

Figure 4.11 is the comparison of tensile strength between the samples of the experimental group and the control group with different weight percentages of HMMM crosslinker. In the experimental group, by comparing the samples with different carbon atoms, BHC4 samples showed higher tensile strength than BHC6 samples with the same weight percentage of the crosslinker. This is because the short middle chain of the urethane diols could strength the hydrogen bonding as mentioned in Figure 2.3. By comparing the same carbon atoms with different weight percentages of the crosslinker, the tensile strength was enhanced with the increase in the percentage of the crosslinker.

This is caused by the increased crosslinking due to the increased amount of the crosslinker. Both BHC4 and BHC6 behaved similarly with different percentages of the crosslinker.

In the control group, BHC6C samples had a similar tensile strength to the BHC4C samples. It suggests that the atom numbers in the linear diols have a tiny impact on the tensile strength. By comparing the same carbon atoms with different weight percentages of the crosslinker, the tensile strength raised with the increase in the percentage of

HMMM. It is the same reason that HMMM could increase the amount of crosslinking in order to make a more rigid polymer structure.

As expected, the experimental group by urethane diols showed a significant result in tensile strength, compared with the control group by linear diols. Adding urethane groups in the diols is a way to improve the tensile strength of MF coatings. The effect of the HMMM crosslinker on the tensile strength is the same for the experimental group and the control group.

Figure 4. 11 Tensile strength of samples from the experimental group and the control group. Experimental group refers to the formulation in Table 4.1; Control group refers to

the formulation in Table 4.2

Figure 4.12 shows the elongation-at-break of the experimental group and the control group from the tensile test. In the experimental group, BHC6 samples showed a similar elongation-at-break to BHC4 samples with the same weight percentage of

HMMM crosslinker. This indicated that carbon number in the middle chain had a slight impact on the flexibility. The elongation-at-break decreased along with the increase in the weight percentage of the HMMM crosslinker at the same carbon number. This is because the structure of the crosslinked film became more rigid with a higher percentage of crosslinker. More rigid structure and the hydrogen bonding from urethane groups could

make the polymer films lose their flexibility. It shows that the excess of the HMMM could lead to the polymer network too rigid to elongate. Both BHC4 and BHC6 behaved the same way.

However, for the control group, it finds out that the elongation-at-break is positively proportional to the weight percentage of the crosslinker. Moreover, the elongation-at-break of BHC4C samples and BHC6C samples performed similarly with the same weight percentage of the crosslinker. This is because the control groups were linear diols with only carbon-carbon bonding. It shows that the carbon atoms have a slightly impact on the elongation.

Compared to the experimental group and the control group, it is obvious that adding urethane groups into diols could provide malleability of the polymer structure so as to improve its elongation with the proper amount of the crosslinker. The numbers of the carbon atoms in the middle chain influence slightly the flexibility of the experimental group and the control group.

Figure 4. 12 Elongation-at-break of samples from the experimental group and the control group. Experimental group refers to the formulation in Table 4.1; Control group refers to

the formulation in Table 4.2

The adhesion results were judged as mentioned in Table 2.1. All the adhesion results were summarized in Table 4.3. The adhesion of the coatings to the substrate is influenced by unreacted hydroxyl groups and urethane linkages in the polymer matrix.

The experimental groups provided urethane linkages in the polymer matrix resulting in increasing the bonding of the coatings to the substrate. Moreover, BHC4 samples and

BHC6 samples showed pretty similar results in the adhesion test. It implies that the difference of carbon atoms does not affect the adhesion of the coating. The dry adhesion of the control group behaved poorly compared with the experimental group. It is because it only had hydroxyl groups bond to the substrates for the samples in the control group.

The dry adhesion has a significant increase as increasing the amount of HMMM crosslinker in the control group. Compared with the experimental group and the control group, it showed that adding urethane diols can enhance the adhesion of the MF coating.

Table 4. 3 Dry adhesion based on ASTM D3359

Experimental group Dry Adhesion Control group Dry Adhesion

BHC4-15% 5B BHC4C-15% 1B

BHC6-15% 5B BHC6C-15% 1B

BHC4-25% 4B~5B BHC4C-25% 1B

BHC6-25% 5B BHC6C-25% 2B

BHC4-35% 5B BHC4C-35% 3B

BHC6-35% 5B BHC6C-35% 3B~4B

As shown in Table 4.4 for the conical mandrel bend test, all the samples in the experimental group passed the test, no matter the amount of the crosslinker with diols or carbon atoms of diols, which indicated that the urethane diols can improve the flexibility of the MF coating. However, the samples in the control group passed the test only for those with the lower weight percentage of the HMMM crosslinker. This is because adding more HMMM will lead to excess of crosslinking in order to form brittle films and also lose their elasticity adhesion. This conical mandrel bend test suggests that urethane

diols can provide enough force to ensure the structure of the network. However, the control group that only had hydroxyl groups was easily broken when adding more

HMMM crosslinker.

Table 4. 4 ASTM D552 conical mandrel bend test results

Experimental group Results Control group Results*

BHC4-15% Pass BHC4C-15% Pass

BHC6-15% Pass BHC6C-15% Fail

BHC4-25% Pass BHC4C-25% Fail

BHC6-25% Pass BHC6C-25% Pass

BHC4-35% Pass BHC4C-35% Fail

BHC6-35% Pass BHC6C-35% Fail

*Pass: No cracking was on the coating surface and the coating film was attached on

the substrate; Fail: Cracking appeared on the coating surface and the coating film was

detached from the substrate.

4.4 Conclusion

Different urethane diols were synthesized and crosslinked with different weight percentages of HMMM crosslinkers. Characterization results showed that diols with

urethane groups in MF coatings can bring better mechanical properties compared to those from the linear diols. The number of carbon atoms is an important parameter in the tensile strength of MF coatings from urethane diols. In addition, the weight percentage of the crosslinker also influences the tensile strength and the elongation-at-break of MF coatings from urethane diols. Shorter carbon middle chain in urethane diols could make the structure of the polymer network rigid in order to enhance tensile strength. On the other hand, the influence of carbon atoms is not significant in MF coatings from the linear diols. Although the MF coatings from urethane diols showed the enhanced property in the flexibility and tensile strength, excess of the HMMM crosslinker could diminish the elongation of the MF coatings. Based on all the results, urethane diols with

25 wt. % HMMM is an optimized option for the appropriate flexibility and tensile strength of the MF coating. This work provides a non-isocyanate approach to synthesize urethane diols which were successfully used in MF coatings to improve the coatings’ mechanical property.

CHAPTER V

SUMMARY

This thesis studied two applications of the isocyanate-free approach. For isocyanate-free polyurethane, it was synthesized successfully from BPA cyclic carbonate and fatty acid amine. It can avoid using harmful materials to produce urethane linkages through a non-isocyanate method. Zinc phosphate pigments were introduced to the synthesized NIPU to generate polyurethane coatings. Based on EIS and salt spray results, the 5 wt.% zinc phosphate pigments showed the best performance on the corrosion resistance.

Moreover, diols containing urethane groups can enhance the mechanical properties of MF coatings. The HMMM crosslinker and the number of carbon numbers affect the polymer network to be rigid and ductile. The number of the carbon atoms in the linear polymer chain didn’t show significant influence in mechanical property of the MF coatings from linear diols.

Overall, this thesis provides an application of the non-isocyanate approach in

NIPU and urethane diols. It successfully obtained NIPU and urethane diols with this perspective method. Furthermore, by adding the NIPU with zinc phosphate pigments or crosslinking the urethane diols with HMMM crosslinkers, this thesis developed corrosion protective coatings. This work would inspire the substitution of conventional polyurethanes or urethane diols for green and environmentally friendly alternatives.

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