Chapter 4 Acrylic Polyurethane Emulsion Polymers

Transcription

1 Chapter 4 Acrylic Polyurethane Emulsion Polymers In this chapter, isophorone diisocyanate (IPDI), a cycloaliphatic diisocyanate, has been selected to react with the different types of hydroxyl acrylic latex prepared from Chapter 3 (core-shell, non core-shell, with and without IBMA), at different stoichiometric NC-to-H ratios, to form the desired one-component acrylic polyurethane (APU) latex. NC NC Structure of IPDI IPDI has been widely used in the polyurethane industry, e.g. as a monomer to manufacture polyurethane dispersion, as a building block in mixed urethanes (e.g. polyurethane elastomers), as a trimer in liquid paint for rigid cross-linking, or as a blocked trimer in powder coatings. The rigid, fully saturated cycloaliphatic ring of IPDI not only expedites the surface drying, but also imparts UV resistance (nonyellowing) and excellent durability in polyurethane coatings. Moreover, the chemical nature of IPDI is more stable and less prone to hydrolysis than the aromatic counterpart upon storage in aqueous medium. 2 Spectroscopic techniques such as FTIR and XPS are used in this chapter to analyze the resulting APU samples. DSC and MFFT are used to determine the Tg and the film formation properties of the polymers. Figure 4.1 illustrates the typical chemical reaction between the hydroxyl functional group of the acrylic latex with the NC moiety of IPDI. 67

2 RNC + R'H RHN R' Urethane e.g. Acrylic polymer backbone carrying isobornyl functional group [ ] m [ ] n [ ] p R H C N NH Polymer urethane linkage [ ] m [ ] n [ ] p R NH urethane linkage NH Polymer Figure 4.1 Reaction of hydroxyl acrylic latex with IPDI to form the APU polymer. 4.1 Preparation of the APU Latex The procedure to prepare the cross-linked APU latex is briefly described as follows: 50.0g of hydroxyl acrylic latex prepared as in Chapter 3 was weighed into a 100ml beaker. The latex was stirred using a mechanical stirrer at a stirring speed of 250rpm. IPDI was weighed accordingly based on the respective NC-to-H ratios desired and then added drop-wisely into the acrylic latex with constant stirring. The 68

3 mixture was further mixed for an hour at room temperature. After that, 2% active Abex 2535 (based on total polymer weight) pre-dissolved in deionised water was added into the mixture. The addition of this alcohol ethoxylated nonionic surfactant improves the storage stability of the finished APU latex. Deionised water was further added so that the finished APU latex had a final solid content of 35%. This % solid content was chosen because the finished APU latex had a greater tendency to coagulate at higher solid content. In the following, we illustrated some calculations performed to obtain appropriate feed ratios for preparing the APU in stoichiometric equivalence. Isocyanates are usually characterized by their NC content in %, or sometimes by their equivalent weight (EW), i.e. EW (isocyanate) = (MW NC 100) %NC = 4200 %NC n the other hand, the polyols (i.e. the hydroxyl acrylic latex in our case) are described by their hydroxyl value (HV), as %H, or alternatively by EW. HV is defined as the weight in milligrams of KH needed to react with all the H groups contained in 1g of polyol; while EW is the weight of polyol needed to obtain one mole of hydroxyl functionality: HV = (MW KH 1000 %H) (17 100) = %H 33 EW (polyol) = (MW H 100) %H = 1700 %H In our case, the hydroxyl functional groups of the acrylic polymer were contributed by the HEMA monomer used (~22.0g) in the feed in Chapter 3. Since MW HEMA (C 6 H 10 3 ) = 130, %H per mol HEMA = Hence, %H in acrylic polymer = [(amount of HEMA used ) total polymer] 100% = [( ) total polymer] 100, where total polymer = latex weight % solid content of the hydroxyl 69

4 acrylic latex. The various feed ratio needed for reaction are thus calculated and tabulated in Table 4.1. For illustration purpose, an example calculation is given below. Table 4.1 Sample recipes for the preparation of APU latex with different stoichiometric ratios of NC-to-H. NC-to-H ratios Weight of hydroxyl acrylic latex, g Weight of IPDI, g Weight of Abex 2535, g Weight of deionised water, g Total solid weight, g Total weight, g % solid content Example calculations: % solid content of hydroxyl acrylic latex = (determined from Chapter 3) Total weight of hydroxyl acrylic latex yield from the synthesis = g Total weight of polymer yield from the synthesis = = g Weight of hydroxyl acrylic latex used in experiment = 50.00g Weight of polymer used in experiment = = 22.31g From literature, %NC (IPDI) = EW(IPDI) = = %H (hydroxyl acrylic latex) = [( ) ] 100 = 1.44 EW (hydroxyl acrylic polymer) = = For NC-to-H = 1, EW (IPDI) = EW (hydroxyl acrylic polymer) each g IPDI will react with g hydroxyl acrylic polymer 70

5 22.31g hydroxyl acrylic polymer will react with ( ) = 2.10g IPDI. For comparison, a series of APU samples were prepared with different conditions and the sample codes are given as in the flow chart below: Two-stage (core-shell) or one-stage (non core-shell) emulsion polymerization of hydroxyl acrylic latex; with or without IBMA added in the feed With or without final neutralization with NH 3 (25% aqueous) Cross-linking reaction with IPDI and addition of stabilizing surfactant APU1 Core-shell APU latex at neutral ph APU3 Non core-shell APU latex at neutral ph APU2 Core-shell APU latex at acidic ph APU4 Core-shell APU latex with IBMA at neutral ph APU5 Non core-shell APU latex with IBMA at neutral ph The suffix 1N or 2N is further used to denote the ratios of NC-to-H = 1 or 2 respectively in these samples (e.g. APU-1N is the core-shell APU at neutral ph with NC-to-H =1). In our discussion, a comparison between APU1 and APU2 samples will be discussed in Section 4.2.1, while the other three samples are compared in Section

6 4.2 Results and Discussion All the APU latex prepared were kept at room temperature for analysis. At different storage days, i.e. 0, 1, 3 and 7 days, a small amount of each latex sample was cast respectively onto two separate glass-plates using a 250µm gauge. ne of the glass plates was cured at room temperature while the other cured at 60 C. After drying overnight, the polymer films were removed from the glass plates and ground into fine powder. The powder was then vacuum-dried overnight prior to FTIR, XPS and DSC analyses Effect of ph and Curing Temperature In this section, the effect of ph of the core-shell hydroxyl acrylic latex and the curing temperature will be presented. The hydroxyl acrylic latex prepared (Chapter 3) contains carboxyl functional monomer due to methacrylic acid, i.e. 0.5 wt % based on the total monomer feed in the core-polymer and 2.5 wt % based on the total monomer feed in the shell-polymer. In the last step of hydroxyl acrylic latex preparation (before crossing-linking with IPDI), the core-shell hydroxyl acrylic latex was neutralized to ph 7.0±0.1 with aqueous ammonia (samples AP1). For comparison purposes, a batch of core-shell acrylic latex without the final ph adjustment (samples AP2) was also prepared with identical composition and used for the cross-linking reaction. The ph of this un-neutralized latex is measured at ~

7 (a) FTIR Analysis FTIR analysis was employed to analyze the unreacted isocyanate residue in the APU polymer film samples cured at different temperatures. Besides FTIR analysis, dibutylamine titration method using bromphenol blue indicator is also commonly used to determine the isocyanate residue. This method is, however, not suitable for our studies because the level of residual isocyanate in the dried polymer film is difficult to be analyzed by direct titration technique. The FTIR spectra of IPDI, as well as those of the core-shell neutralized (AP1) and acidic (AP2) hydroxyl acrylic latex were obtained respectively for comparison as shown in Figure 4.2. The FTIR spectra of the prepared APU samples kept for various storage periods and curing temperature are presented in Appendix 1. For illustration, some spectra of APU1-2N samples are shown in Figures 4.3 and 4.4. The IPDI spectrum shows a sharp and distinct peak at cm -1 due to the asymmetric stretching vibration of the N=C= group. While there is a very rich vibrational profile in the spectra of the two core-shell samples due to the many functionalities present in the latex (Figure 4.2b and c), region within the NC stretching is relatively clear. As shown in Figures , upon reaction over storage, the characteristic absorption band of NC would gradually diminish. n the other hand, some new absorption bands in the range of cm -1 emerge. These could be attributed to the NHC carbamate and NHCNH urea linkages. The characteristic peak assignments for the FTIR spectra of AP1 and APU1-2N (at 0-day storage, cured at room temperature) are given in Table

8 N=C= (isocyanate band) (a) IPDI (b) neutralized AP1 (c) acidic AP2 Figure 4.2 FTIR spectra of (a) IPDI, (b) neutralized AP1 and (c) acidic AP2 hydroxyl acrylic latex. 74

9 (a) 0 day (b) 3 days (c) 7 days Figure 4.3 FTIR spectra of APU1-2N samples kept for different storage periods cured at room temperature: (a) 0, (b) 3 and (c) 7 days. 75

10 (a) 0 day (b) 3 days (c) 7 days Figure 4.4 FTIR spectra of APU1-2N samples kept for different storage periods cured at 60 o C: (a) 0, (b) 3 and (c) 7 days. 76

11 Table 4.2 Assignments of FTIR spectra of AP1 and APU1-2N at 0-day storage, cured at room temperature. Assignments Wavenumber, cm -1 AP1 APU1-2N C H stretching in alkyl group N=C= stretching C= stretching of ester C= stretching of ester/ urethane C= stretching of urea N H deformation of urea/ urethane =C CH 3 deformation In the case of cabonyl stretching vibration, the N,N -dialkyl substituted ureas has an absorption band in the region of cm This is fairly separated from the C= stretching of secondary urethanes (N-monosubstituted), which overlaps with that of ester at cm -1. Another new absorption band appearing at cm -1 can be attributed to the C NH group vibration, contributed by both urethanes and ureas. 55 In order to confirm the position of the C= stretching band of urea, we have prepared a sample by reacting IPDI with water, and the FTIR spectrum of the product is shown in Figure

12 -N=C= (2263 cm -1 ) -C= of urea (1636 cm -1 ) -C-NH of urea (1558 cm -1 ) Figure 4.5 FTIR spectrum of urea formed from the reaction of IPDI with water. Using baseline method, 47, 48 we attempt to perform a quantitative analysis of the peak area of the functional groups. For example, the amount of NC remained in the APU polymer would be estimated through its characteristic absorption in the region cm -1, with respect to the C-H stretching band in the region cm -1. The peak area ratio, A(NC)/A(CH), thus provides a way to characterize the amount of residual NC present in the sample independent of the overall spectral intensity. The CH band is selected as a reference because it arises from the vibrations of bonds (contributed majority by the hydroxyl acrylic latex) which are not broken or formed during the cross-linking reaction. Most importantly, this reference peak can be easily differentiated from the other bands on the spectra. For illustration, the peak area ratios obtained from Figures 4.3 and 4.4 are presented in Table 4.3. Similar tabulations for other spectra are presented in Appendix 2. 78

13 Table 4.3 Relative FTIR peak area ratios obtained for APU1-2N sample kept for different storage periods cured at room temperature and 60 C, respectively. Storage period Peak Area N=C= Room temperature curing Peak Area -CH- A (NC)/ A (CH) Peak Area N=C= 60 C curing Peak Area -CH- A (NC)/ A(CH) Day Day Day Day The ph of the acrylic latex seems to exert little effect on the rates of NC consumption as shown in Figure 4.6. It seems that the un-neutralized carboxylic acid functional groups in the AP2 samples do not compete actively with the hydroxyl functionality for the cross-linking reaction with isocyanate. In the case of waterborne system, the isocyanate compound has been found to react with water to give a carbamic acid (RNHCH) intermediate that can decompose spontaneously at low temperatures into an amine with C 2 as a co-product. 2 The liberated amine reacts rapidly with another isocyanate molecule to produce a urea derivative: RNC + H 2 RNH 2 + RNC RHN H Carbamic Acid RHN NHR Urea RNH 2 + C 2 79

14 NC/ CH ratio APU1-1N-RT APU2-1N-RT APU1-2N-RT APU2-2N-RT Number of storage days at room temperature NC/ CH ratio APU1-1N-60C APU2-1N-60C APU1-2N-60C APU2-2N-60C Number of storage days at room temperature Figure 4.6 Evolution of the NC/ CH FTIR peak area ratio with storage time for APU1 and APU2 samples cured at room temperature (RT) and 60 o C. Film defects such as pin-holes and blistering observed on the polymer coatings (more for the samples with higher NC-to-H ratios) is a strong evidence of the release of carbon dioxide as a result of the reaction between isocyanate and water (moisture absorption from ambient) during the curing process. ur finding shows that the carboxyl functionality is less reactive towards the isocyanate group as compared with water. This observation is in line with the relative reaction rates of different species with isocyanate: 2 amine > hydroxyl > water > urea > urethane > carboxyl Furthermore, we have also noticed that the rates of NC consumption during film curing is more rapid and drastic for the samples cured at 60 o C than those samples 80

15 cured at room temperature (Figure 4.6). This can be explained in the light that the hydroxyl groups of the acrylic polymer are not necessarily located in close proximity to the isocyanate group in the polymer latex. The formation of urethane linkages in the latex further impairs the mobility of the polymer network and consequently affects the reaction of residual isocyanate with the remaining hydroxyls during film curing. At higher curing temperature, an increase in the mobility of the polymer network will help to facilitate the cross-linking reactions and thus increasing the rate of NC consumption. (b) MFFT Analysis The MFFT of the various APU1 and APU2 samples were determined and shown in Figure 4.7. It is noted that APU2 samples consistently have higher MFFT than APU1 samples at all NC-to-H ratios used. The inclusion of ammonia as a neutralizing agent in the hydroxyl acrylic latex seems to have exerted a plasticising effect on the polymer films. Thus the neutralized APU1 samples form continuous film at lower temperature than the acidic APU2 samples. Moreover, the gradual increase in MFFT with increasing NC-to-H ratios correlates approximately with the degree of urethane and urea cross-linking networks formed in the polymer matrix MFFT, deg C APU1 APU NC: H ratio Figure 4.7 Variation of minimum film formation temperature (MFFT) of APU1 and APU2 samples with different NC-to-H ratios. 81

16 (c) Storage Stability In our experiments, Abex 2535, a nonionic surfactant was used to improve the storage stability of the APU latex. Without the post-addition of Abex 2535, the APU latex coagulated upon overnight storage at room temperature. The storage stability of the latex was determined by storing the latex at 50 C for one week. The particle size distributions (PSDs) of APU1 and APU2 before and after storage were measured and compared as shown in Figures 4.8 and 4.9. For longer storage stability, the APU latex was examined in an accelerated aging process by storing in an oven at 50 C for one month. The storage stability of the APU latex may be influenced by the ph of the latex as well as the NC-to-H ratios (i.e. the degree of cross-linking reaction). Comparing the four samples in Figures 4.8 and 4.9, it is found that APU1 latex prepared with NC-to-H = 1 shows the best storage stability. There was no observable coagulation in the APU1-1N sample even after one-month of storage at 50 C. We suggest that, at neutral ph, the carboxylate anionic groups of APU1 impart colloidal stability to the latex particles via electrostatic stabilization. The repulsive forces between similarly charged particles are effective in preventing flocculation. Thus, the particle size of APU1-1N latex exhibits little increase during storage. n the other hand, when the NC-to-H ratio is increased to 2, the higher degree of crosslinking reaction in the APU1-2N sample may destabilize the APU latex. A bimodal particle size distribution was observed after one-week of storage at 50 C. In the cases of APU2 samples (Figure 4.9), a remarkable increase in the population of coarse particles appears after one-week of storage at 50 C. The polymer particles in these samples tend to coalesce upon storage probably because the acidic groups do not provide sufficient electrostatic stabilization to the latex particles. 82

17 20.00 % population Particle size, micron APU1-1N-0D APU1-1N-1W APU1-1N-1M % population Particle size, micron APU1-2N-0D APU1-2N-1W Figure 4.8 Particle size distributions before and after storage at 50 C at different time period for APU1-1N and APU1-2N samples. Legends: 0D 0 day; 1W 1 week; 1M 1 month. 83

18 20.00 % population Particle size, micron APU2-1N-0D APU2-1N-1W % population Particle size, micron APU2-2N-0D APU2-2N-1W Figure 4.9 Particle size distributions before and after storage at 50 C at different time period for APU2-1N and APU2-2N samples. Legends: 0D 0 day; 1W 1 week Effect of Core-shell Morphology on the Cross-linking Reaction The locus of hydroxyl groups on the polymer particle is an important factor in this study because it affects the extent of urethane cross-linking reaction during the film formation process. Different methods have been adopted to control this factor; while in our case, we employ sequential emulsion polymerization (Chapter 3) in which the hydroxyl functionalized monomer HEMA was incorporated only during the second stage of polymerization. We hope that the hydroxyl groups would be concentrated on the shell of the latex particles by using this two-stage process. In this section, we 84

19 discuss the properties of polymer films prepared from using this core-shell hydroxyl acrylic latex. For comparison purposes, hydroxyl acrylic latices prepared using the one-stage process (in which the hydroxyl groups are distributed throughout the entire volume of the polymer particles) were also cross-linked to form APU samples. Thus, four types of APU polymers with the same NC-to-H ratio, namely APU1-1N, APU3-1N, APU4-1N and APU5-1N, all fully cured at 60 C after a storage period of one week at room temperature will be discussed in this section. All these samples are neutralized samples, with the last two (APU4 and 5) containing the IBMA. (a) Surface Analyses by XPS We first attempt to analyze the surface composition of the polymer using XPS. In the wide scan spectra (Figure 4.10), elemental photoelectron peaks due to C, N and are clearly observed and no other impurities are detected. We aim to provide an estimation of the relative surface composition of the core-shell versus the non coreshell polymers via XPS analyses. It is hence pertinent to perform peak-fitting to abstract the various components enveloped under these elemental peaks. In this curve fitting and peak integration procedure (using the XPS Peak-fit software), a guided guess for the binding energies, peak widths and areas for the expected number of peaks in the spectrum is first performed. An iterative curve fitting procedure is then carried out by the software to determine the best fit to the experimental data through optimizing the peak parameters. As a precautionary practice, consistency within the sets of fitted parameters for the series of samples analyzed is maintained as far as possible. 85

20 Figure 4.11 shows a schematic structure of the core-shell APU polymer with IBMA incorporated (i.e. APU4-1N). In general, we could identify six different types of carbon and two types of nitrogen species in the polymer system. A six-peak fit procedure was thus adopted for the C1s XPS spectra as shown in Figure Wide Scan Spectrum of APU1-1N 1s C1s Counts N1s Binding energy (ev) Figure 4.10 A wide scan spectrum of APU1-1N. a CH 3 a d ( CH 2 C ) bc a d ( CH2 CH ) H f C HN NH b C c a CH 2 C 3 H 7 a CH 3 a d ( CH 2 C ) a (CH2 a CH 3 d C ) m n o p b C b C c CH 3 c CH 2 c CH 2 e C NH a (CH2 a CH ) NH q a CH 3 a d ( CH 2 C ) b C r HN NH Hydrolysis of the NC groups lead to the formation of urea linkages between IPDI Figure 4.11 A schematic structure of the core-shell APU polymer containing IBMA (e.g. APU-4 with NC-to-H = 1). 86

21 NH-C-NH- (288.7eV) (a) APU1-1N -CH 2 -- (286.7) -C*H 2 -C- (285.6eV) (b) APU3-1N -NH-C-NH- (288.7eV) -CH 2 -- (286.8) -C*H 2 -C- (285.7eV) Counts C- (289.0eV) -NH-C- (289.6eV) -CH 2 - (285.0eV) Counts C- (289.3eV) -NH-C- (289.8eV) -CH 2 - (285.0eV) Binding Energy (ev) Binding Energy (ev) (c) APU4-1N (d) APU5-1N Counts NH-C-NH- (288.7eV) -C- (289.2eV) -NH-C- (289.6eV) -CH 2 -- (286.9) -C*H 2 -C- (285.7eV) -CH 2 - (285.0eV) Counts C- (289.4eV) -NH-C- (289.6eV) -CH 2 -- (286.8) -NH-C-NH- (288.7eV) -C*H 2 -C- (285.7eV) -CH 2 - (285.0eV) Binding Energy (ev) Binding Energy (ev) Figure 4.12 Results of C1s peak analysis for the APU samples cured at 60 C. Points represent the experimental data, the dotted lines represent the fitted curves for the various components and the solid line represents the resulting fitted envelope. The various C1s chemical shifts for the fitted components, relative to the saturated hydrocarbon (285.0 ev, a C in Figure 4.11), are tabulated in Table 4.4. We noticed that the fitted values compared fairly well with the literature values for primary and secondary C1s chemical shifts. 56 In addition, the secondary chemical shift of ~0.7 ev for the backbone carbon atoms attached to the C group ( d C in Figure 4.11) is consistent with reported values. 57, 58 Similarly, we have thus performed peak-fitting for the N1s peaks as shown in Figure The two nitrogen components, i.e. HN C and HN C NH, are assigned accordingly since the former species is expected to have higher binding energy due to the withdrawing effect of -C group. 87

22 Table 4.4 C1s chemical shifts (ev) for the fitted components of the C1s XPS peaks, relative to the saturated hydrocarbon (C1s = ev). Mean chemical shifts Type of carbon atom Literature APU1-1N APU3-1N APU4-1N APU5-1N values 56 a C CH b C C c C CH d C *CH 2 C e C HN C f C HN C NH After correction with the various elemental sensitivity factors, the relative elemental ratios of C, N and were obtained from the respective XPS peak areas. The results are compared with the expected values in Table 4.5. Thus, we found that while the experimental contents of C and N are consistently higher than the expected values for all the samples, those of are consistently lower than the expected values. This could be attributed to the fact that the majority of the hydrophobic monomers carrying oxygen atoms (such as butyl acrylate and methyl methacrylate) were buried inside the polymer particles. 88

23 6000 (a) APU1-1N 6000 (b) APU3-1N N-C- (400.5eV) -N-C-N- (399.6eV) N-C- (400.5eV) -N-C-N- (399.6eV) Counts 4000 Counts Binding Energy (ev) Binding Energy (ev) (c) APU4-1N 6000 (d) APU5-1N N-C- (400.4eV) -N-C-N- (399.6eV) N-C- (400.5eV) N-C-N- (399.7eV) Counts 4000 Counts Binding Energy (ev) Binding Energy (ev) Figure 4.13 Results of N1s peak analysis for the APU samples cured at 60 C. Points represent the experimental data, the dotted lines represent the fitted curves for the various components and the solid line represents the resulting fitted envelope. Table 4.5 Expected and experimental contents of C, N and elements obtained from XPS analysis. C1s N1s 1s Sample Theo. % Expt. % Theo. % Expt. % Theo. % Expt. % APU1-1N (core-shell) APU3-1N (non core-shell) APU4-1N (core-shell with IBMA) APU5-1N (non core-shell with IBMA)

24 We then proceed to determine the relative ratios of urethane ( NC ) to urea ( NCN ) linkages in the samples using both the fitted components of C1s and N1s peaks. The values obtained from both sets of data were found to be consistent as shown in Tables 4.6 and 4.7. verall, we observed that the core-shell APU samples contain relatively higher ratio of urethane to urea linkages compared to their non coreshell counterparts. It is generally known that the RH functional group has a higher reactivity towards isocyanate compared to H 2. Since there is a relatively higher concentration of hydroxyl groups at the surface of the core-shell particles, the crosslinking reaction with IPDI is facilitated in these samples. In the case of non core-shell samples, some of the hydroxyl groups are buried inside the polymer particles. Consequently, water in the APU latex and absorbed moisture molecules during the film formation will compete for the reaction with IPDI and resulting in a comparably higher content of urea linkages. Table 4.6 Relative ratios of urethane to urea linkages obtained from XPS C1s peak fitting analysis. -N-C-N- -N-C-- Sample B.E/ ev % Area B.E/ ev % Area Ratios of urethane: urea linkages APU1-1N APU3-1N APU4-1N APU5-1N

25 Table 4.7 Relative ratios of urethane to urea linkages obtained from XPS N1s peak fitting analysis. -N-C-N- -N-C-- Sample B.E/ ev % Area B.E/ ev % Area Ratios of urethane: urea linkages APU1-1N APU3-1N APU4-1N APU5-1N Interestingly, we noted that the core-shell APU latex with IBMA (i.e. APU4-1N) has a relatively lower ratio of urethane to urea linkages compared to sample without IBMA (i.e. APU1-1N). We suspect this is due to the bulkiness of the isobornyl ring, which effectively reduces chain packing and provides more free volume within the polymer network. Since IBMA monomer is incorporated into the shell layer (Section 3.2.4), the shell of APU4-1N latex particle is expected to be slightly more porous. Such a porous structure could have enhanced the up-take of moisture from ambient and led to the formation of more urea linkages (versus urethane) in this sample. (b) FTIR Analysis The FTIR spectra of the series of core-shell and non core-shell APU samples were measured after different storage period and cured at both room temperature and 60 o C. The amount of isocyanate residue remaining in the polymer film was determined. All the FTIR spectra are presented in Appendix 1, together with tables of the relative area ratios (Appendix 2) obtained from the baseline method. 91

26 Figure 4.14 illustrates the rate of consumption of NC with time under both the room temperature and 60 o C curing condition. For comparison, the plots for APU1 samples (Section 4.2.1) were reproduced in the figures. Thus, it can be seen that the amount of isocyanate residue in the APU1 samples having core-shell morphology decreases more drastically with respect to storage time as compared to the corresponding non core-shell APU3 samples. The presence of NC signal is almost undetectable after ~3 days for the APU1-1N sample cured at room temperature. When the curing temperature is elevated to 60 C, however, both the APU1 and APU3 samples show little NC signals after ~ 1 (NC-to-H ratio = 1) or 3 (NC-to-H ratio = 2) days. Thus the increasing network mobility and the enhanced diffusion rate of IPDI into the hydroxyl acrylic polymer at higher curing temperature were found to expedite the urethane cross-linking reactions. Apparently, the higher concentration of hydroxyl groups at the surface of the core-shell acrylic particles will facilitate crosslinking reaction with IPDI and thus faster NC consumption at room temperature. 92

27 NC/ CH ratio APU1-1N-RT APU3-1N-RT APU1-2N-RT APU3-2N-RT Number of storage days at room temperature NC/ CH ratio APU1-1N-60C APU3-1N-60C APU1-2N-60C APU3-2N-60C Number of storage days at room temperature Figure 4.14 Evolution of NC/ CH peak area ratio for APU1 and APU3 samples, with NC-to-H ratio = 1 and 2, cured at room temperature (RT) and 60 o C, respectively. We have also attempted to compare the urea content for the samples using the experimentally determined NHCNH: CH peak area ratios, since the C= absorption band of urea can be distinctively differentiated from those of urethane and ester. Comparisons between the variations of these ratios with time for APU1 and APU3 samples are presented in Figure First, we observe that the urea content in the samples cured at 60 C is always higher than those cured at room temperature. At room temperature curing, both core-shell and non core-shell samples contain comparable amount of urea linkages. However under an elevated temperature curing condition, the urea content in the non core-shell APU3 is higher than the core-shell APU1. This finding is consistent with the earlier XPS analyses in which all the samples are cured at 93

28 60 C after one-week of storage at room temperature. ur XPS results (C1s and N1s) show that APU3-1N has higher urea content than APU1-1N. This suggests that the reaction of IPDI with water/ moisture is indeed a rather slow process at room temperature. The rate of this side reaction is obviously improved at higher curing temperature. Alternatively, we have demonstrated here that it is advantageous to adopt the core-shell morphology such that the hydroxyl groups of the acrylic polymer can be concentrated on the surface. This will enhance the efficiency of the cross-linking reaction between H and the NC of IPDI, at the expense of the side reaction of IPDI with water APU1-1N-RT APU3-1N-RT APU1-2N-RT APU3-2N-RT NHCNH/ CH ratio Number of storage days at room temperature NHCNH/ CH ratio APU1-1N-60C APU3-1N-60C APU1-2N-60C APU3-2N-60C Number of storage days at room temperature Figure 4.15 Evolution of NHCNH/ CH peak area ratio for APU1 and APU3 samples, with NC-to-H ratio = 1 and 2, cured at room temperature (RT) and 60 o C, respectively. 94

29 Apart from the effects of polymer morphology and curing temperature, the internal chemical structure of the hydroxyl acrylic polymer was found to give a small yet discernible influence on the extent of urea formation in the APU film. Thus, the various plots of peak area ratios for APU4 and APU5 samples, in which IBMA has been incorporated, are compared in Figures 4.16 and NC/ CH ratio APU4-1N-RT APU5-1N-RT APU4-2N-RT APU5-2N-RT Number of storage days at room temperature NC/ CH ratio APU4-1N-60C APU5-1N-60C APU4-2N-60C APU5-2N-60C Number of storage days at room temperature Figure 4.16 Evolution of NC/ CH peak area ratio for APU4 and APU5 samples, with NC-to-H ratio = 1 and 2, cured at room temperature (RT) and 60 o C, respectively. 95

30 0.10 APU4-1N-RT APU5-1N-RT APU4-2N-RT APU5-2N-RT NHCNH/ CH ratio Number of storage days at room temperature 0.10 APU4-1N-60C APU5-1N-60C APU4-2N-60C APU5-2N-60C NHCNH/ CH ratio Number of storage days at room temperature Figure 4.17 Evolution of NHCNH/ CH peak area ratio for APU4 and APU5 samples, with NC-to-H ratio = 1 and 2, cured at room temperature (RT) and 60 o C, respectively. When Figure 4.16 is compared to Figure 4.14, we observe that the rate of NC consumption is largely similar for the core-shell samples with or without IBMA (i.e. APU1 and APU4) cured at both room and elevated temperatures. For the non coreshell samples, however, APU5 samples (with IBMA) seem to be undergoing a slightly faster cross-linking reaction than APU3 samples (without IBMA), and this relatively higher rate of NC consumption is more clearly observed for APU5 samples cured at room temperature. We propose that this arises from the bulky isobornyl rings of IBMA, which have effectively reduced chain packing and hence provided more free volume within the polymer network. Such free volume allows better diffusion of reagents within the latex network and increases the reaction rate. The presence of free 96

31 volume is not required for the cross-linking reaction in the core-shell samples, since the active groups in this case are concentrated on the particle surface. As expected also, the free-volume effect is less important when the curing temperature is elevated. The relative amount of urea linkages seems to be affected with the incorporation of IBMA monomer (i.e. Figure 4.17 c.f. Figure 4.15). verall, the area ratios of urea in non core-shell APU3 are higher than those area ratios of non coreshell APU5. Especially at 60 C curing, the relative amount of urea in APU5 is significantly lower than APU3. This again may be attributed to the poor inter-polymer chain packing with the incorporation of IBMA, leading to an increased free volume within the polymer network that enhances the diffusion of IPDI into the polymer particles to react with those buried hydroxyl groups. The diffusion rate of IPDI is further increased at elevated temperature. Hence the side reaction of IPDI with water/ moisture to form urea is reduced. (c) MFFT and Tg Analysis Figure 4.18 depicts the MFFT values of the four different APU samples at various NC-to-H ratios. It is noted that MFFT values of the core-shell samples are consistently lower than those of their non core-shell counterparts for all NC-to-H ratios. We have thus achieved part of our objectives of designing latex with hard coresoft shell morphology, i.e. to form a continuous latex film at lower temperatures. However this is only useful provided the mechanical integrity of the finished polymer film is not impaired. Since film formation is a critical aspect that involves both the coating of a surface and forming a layer with good cohesive properties, it is imperative to provide a more detailed discussion to this process at this juncture. 97

32 20.0 MFFT, deg C NC: H ratio APU1 APU3 APU4 APU5 Figure 4.18 Variation of minimum film formation temperature (MFFT) of the various core-shell and non core-shell APU samples with different NC-to-H ratios. Several stages during film formation have been observed experimentally, and a phenomenological description of the process is generally accepted as illustrated below: 53 Stage 1 Stage 2 Stage 3 Stage 1: Water evaporation and colloid concentration As water evaporates, a uniform shrinkage of the inter-particle distance occurs and particle sliding gradually over each other to fill the voids to form a dense packing. Stage 2: Particle deformation and evaporation of bound water The polymer particles further coalescence and are deformed to pack into honeycomb-like structure. Water molecules bound by the high concentration of surfactants residing at the particle surface are evaporated. 98

33 Stage 3: Interdiffusion of macromolecules The mechanical strength increases and the water permeability of the film decreases. Under certain conditions, the polymer chains can diffuse through the particle boundaries. The honeycomb-like structure disappears and a homogeneous and continuous film is formed. Since the cross-linking reaction occurs at the inter-particle boundaries for our core-shell samples (i.e. Stage 3 above), such enhanced interfacial reaction will lead to the development of a rigid network across the particle-particle interface. In Figure 4.19, we depict the DSC thermographs of APU1 and APU3 samples cross-linked at various NC-to-H ratios. Two distinct inflection points are clearly observed in the control sample, suggesting the presence of heterogeneous phases in the samples. Upon cross-linking, we can see that the inflection point at lower temperature progressively becomes less distinct, indicating the formation of a homogenous phase. Such features are not observed in the non core-shell samples. Similar conclusion can be drawn via comparing APU4 and APU5 samples (DSC thermographs presented in Appendix 3). 99

34 APU1-2N APU1-1.5N APU1-1N APU1-0.5N AP1 APU3-2N APU3-1.5N APU3-1N APU3-0.5N AP3 Figure 4.19 DSC thermographs showing the Tg s of APU1 (core-shell) and APU3 (non core-shell) samples with different NC-to-H ratios cured at 60 C after oneweek storage at room temperature. (d) Storage stability Finally, we need to test the storage stability of the prepared samples. We found that only the core-shell APU latex (APU4 shown in Figure 4.20; APU1 shown in Figure 4.8) could successfully pass the one-month storage at 50 C without any significant sign of flocculation. In the case of APU3 and APU5, the particle size distribution broadens and extends into larger particle size upon aging for just one 100

35 week. These results again confirm the merits of adopting core-shell morphology in the preparation of APU latex % population Particle size, micron APU3-1N-0D APU3-1N-1W 20.0 % population Particle size, micron APU4-1N-0D APU4-1N-1W APU4-1N-1M 20.0 % population Particle size, micron APU5-1N-0D APU5-1N-1W Figure 4.20 Particle size distributions for APU3-1N, APU4-1N and APU5-1N samples before (0D) and after storage at 50 C for one week (1W) and one month (1M). 101

36 4.3 Summary In the first part of our experiments, we discovered that neutralizing the ph of the acrylic latex helps to further improve the storage stability of the final APU latex. This is probably attributed to the electrostatic repulsion of similarly charged particles that leads to stabilization of the colloidal latex. When higher IPDI content is used in the preparation (NC-to-H = 2), coagulation of the neutralized APU latex is accelerated. This is due to the extensive reaction of an excess IPDI with water, leading to the liberation of carbon dioxide and urea linkages as evident by the blistering of the polymer films and FTIR analyses. In the second part of experiments, we compare the properties of films formed by core-shell versus non core-shell acrylic samples, with or without the incorporation of IBMA monomer. ur work has adequately demonstrated that the H groups are concentrated at the surface of the core-shell polymer particles, resulting in the formation of a higher % of urethane linkages as detected by XPS and lower % of urea linkages as confirmed by FTIR analyses. As a result of this concentration, interfacial regions of adjacent latex particles are highly cross-linked, giving polymer film with higher T g. Such cross-linked polymer structure is desirable as it contributes to mechanical properties such as hardness, scratch resistance and solvent resistance of the resultant coating. Despite the high Tg, the soft shell latex of this structure is capable of inducing film formation at lower temperature (lower MFFT). In essence, our preparation and characterization in this Chapter confirm that single-component waterborne APU with desired film-forming behavior, as well as good mechanical and storage stability, can be achieved through having a hard core-soft shell morphology obtained via the two-stage emulsion polymerization. 102