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EMULSION POLYMERIZATION OF A VEGETABLE OIL MACROMONOMER BASED

ON SOYBEAN OIL AND EVALUATION OF THE COATING PERFORMANCE

Article · January 2011

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EMULSION POLYMERIZATION OF A VEGETABLE OIL MACROMONOMER BASED ON SOYBEAN OIL AND EVALUATION OF THE COATING PERFORMANCE

Ethem Kaya, Sharathkumar K. Mendon, James W. Rawlins, and Shelby F. Thames

School of Polymers and High Performance Materials The University of Southern Mississippi, Hattiesburg, MS 39406

james.rawlins@usm.edu, 601-266-5683 Abstract

A soybean oil-based vegetable oil macromonomer (VOMM) was incorporated as a comonomer in an all-acrylic copolymer via conventional semi-continuous emulsion polymerization. The characterized latexes were formulated to semi-gloss coatings for end application testing. VOMMs are ideal for incorporation in latex polymers as the monomer imparts the ability to plasticize the polymer matrix through low starting point glass transition temperatures (Tgs). The monomers facilitate low temperature film formation with minimal or no VOCs and then after application, the oil residue tethered to the polymer backbone begins to auto-oxidize and form crosslinked, higher Tg coatings. Structurally, VOMMs are comprised of long hydrocarbon fatty acid moieties with allylic double bonds which enable auto-oxidative crosslinking at ambient temperature. This paper’s focus is the characterization necessary to validate VOMM allylic unsaturation retention before, during, and after polymerization, and a confirmation of the quantifiable increased Tg affected via crosslinking. These data were acquired quantitatively by solid state 13C nuclear magnetic resonance (NMR) spectroscopy and differential scanning calorimetry (DSC). A commercial control, an experimental latex, and an internal latex (control w/o VOMM) were formulated into semi-gloss coatings. The formulated coatings were compared via characterization and coating specific evaluations of films cast in air and nitrogen, with and without a metal drier for structure-property relationships and thin film performance. Introduction

In recent years, research advances in designing environmentally friendly and sophisticated polymer latex systems have catapulted new developments in the field of coatings. Further utilization of renewable resources as desirable competitors to the previous petroleum derived raw materials has opened an innovative market to the modification of naturally occurring options. Specifically, soybean derivatives have been utilized in many research, development, and commercialization projects.1,2,3,4

Larock et al.5 studied a series of methoxylated soybean oil polyols with different hydroxyl

functionalities for the synthesis of vegetable-oil-based waterborne polyurethane dispersions. Cadiz et al. reviewed developments in biobased polyols and their utilizations in polyurethanes.6 Soybean oil phosphate ester polyols have also been evaluated in low-VOC corrosion resistant coatings.7 Since there has been a growing interest of industry in the use of renewable resources, the number of commercialization projects on soybean derivatives has increased steadily. U.S. Pat. No. 7,691,914 to Abraham et al.8 describes development of soybean oil based polyols for use in the manufacturing of polyurethane foams. U.S. Pat. No. 7,799,895 to Ddamulire et al.9 describes a method of low-VOC bio-based adhesive compositions utilizing drying oils.

In view of these considerations, vegetable oil macromonomers (VOMMs) have been investigated

as comonomers in copolymerization of conventional monomers.10,11,12 VOMMs have three distinct characteristics that are advantageous in the synthesis of environmentally-responsible emulsions: (1) by virtue of its molecular length and large monomer size, it is an excellent plasticizing monomer that facilitates coalescence without the necessity for solvent based coalescing agents, (2) they readily copolymerize with vinyl monomers through the acrylate functionality and reduce minimum filming temperature (MFT) during film formation, and (3) the allylic functionalities within the monomer tail react

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auto-oxidatively during coalescence at ambient temperature, creating films with highly crosslinked networks that enhance mechanical strength through impinged molecular movements from the entangled polymer chains. The synergistic combination of allylic vegetable oil fatty acids and an acrylic backbone provides mechanically stable, self-crosslinking emulsions for utilization in coatings with reduced or zero volatile organic compound (VOC) emissions that result in an unique fundamental structure properties dissimilar to conventional monomers.

These studies elucidate the synthetic methods and structure-property relationships of VOMMs in

all-acrylic copolymers. While one research objective is to establish and advance the knowledge of VOMMs, the second is to recognize the utility of VOMM-based latexes for various coating applications. Experimental Materials

Soybean oil was purchased from Alnor Oil Company Inc., Valley Stream, NY. Sodium bicarbonate, ammonium persulfate (APS), dimethyl sulfoxide (DMSO), methyl methacrylate (MMA), n-butyl acrylate (BA), and methacrylic acid (MA) were purchased from Sigma-Aldrich, St. Louis, MO. Surfactants Rhodapex® CO-436, Igepal® CO-887, Abex® 2020, and Abex® 2545 were obtained from Rhodia, Cranbury, NJ. Cobalt Hydro-Cure II® was provided by OMG Borchers GmbH, Langenfeld, Germany. Characterization

CP/MAS solid state carbon (13C) nuclear magnetic resonance (NMR) spectroscopy was performed on a Bruker MSL 400 MHz spectrometer and the peaks were referenced to the adamantine methane at 29.5 ppm as external standard. Preservation of allylic unsaturation was quantified upon 2048 scans.

Gel content was determined from the amount of insoluble material relative to the initial sample

weight. Approximately 0.1 g sample (either as wet latex or as a dried film) was taken in a scintillation vial with 10 mL of DMSO and placed overnight in a vortex shaker. The solution was subsequently filtered and the insoluble part dried in a vacuum oven. Samples were tested in triplicate and the average was reported for each sample.

Latex particle sizes were determined via the Microtrac® UPA 250 Particle Size Analyzer having a

sample range capability of 0.003 to 6.54 µm (standard deviation ±10 nm) after diluting the latexes with 200 times their volume of deionized (DI) water to a concentration of 2.25 x 10-3 g/mL.

The latex minimum filming temperature was determined visually as the temperature

corresponding to the point at which the latex dried to a clear film on the MFFT-BAR instrument (Rhopoint, Inc., temperature range -5 – 90°C).

The glass transition temperature (Tg) was analyzed by differential scanning calorimetry (DSC)

studies conducted on the DSC Q1000 (TA Instruments) over the temperature range of -90 ºC to 150 ºC at heating rate of 5 ºC/min and cooling rate of 10 ºC/min for two cycles. Latex films utilized in these studies were coalesced separately under nitrogen and air (with 1 wt% cobalt driers) to evaluate auto-oxidation of the VOMM and the overall film formation process. VOMM Emulsion Copolymerization

VOMM latexes were synthesized utilizing a starve-fed, semi-continuous polymerization process. The latexes were formulated to a copolymer composition of SoyAA-1/MMA at 10/90, 20/80, 40/60, 60/40, and 80/20 (by weight) with 0.2 wt% MA at a weight solid content of 45%. Pre-emulsions were

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prepared by blending the monomers with deionized (DI) water, 0.36 wt% Igepal CO-887, 0.54 wt % Rhodapex CO-436, and 0.75 wt% APS. The reaction flask was placed in a water bath at 70 °C and purged with nitrogen. The reaction flask was charged with a solution of DI water, 0.11 wt% Rhodapex CO-436, 0.3 wt% sodium bicarbonate, and 0.25 wt% APS, while the contents were stirred continuously at 190 rpm. Five percent of the pre-emulsion was injected into the reaction flask to form seed particles. The reaction was continued under starve-fed conditions at a pre-emulsion feed rate of 0.023 mL/sec over five hours under continuous stirring and nitrogen blanket. Once the pre-emulsion feed was exhausted, the reaction kettle was maintained in the water bath at 70 ºC for 19 hours, cooled to ambient, filtered, and pH adjusted to 9.0 with ammonium hydroxide. Result and Discussion

Acrylate-functional soybean oil-based macromonomer (SoyAA-1) was synthesized in high yields utilizing sequential amidation and acrylation processes. A representative example of the chemical structure of SoyAA-1 monomer is given in Figure 1. Based on the reported composition of soybean oil,13 i.e., 9% linolenic (C18:3), 51% linoleic (C18:2), 25% oleic (C18:1), 4% stearic (C18), 11% palmitic (C16), the average number of double bond of SoyAA-1 was determined to be 1.5.

Figure 1. Representative Example of SoyAA-1 Chemical Structure. VOMM Copolymer Characterization

Latexes with varying levels of SoyAA-1 and MMA were synthesized to elucidate the effect of VOMMs on the Tg and MFT. All synthesized latexes exhibited particles sizes of 140 ± 10 nm. Figure 2 illustrates the Tg changes for SoyAA-1 latexes containing 10, 20, 40, 60, and 80 wt% SoyAA-1 between zero cure (films dried in nitrogen) and cure in air with 1 wt% cobalt drier for six weeks. With increasing VOMM concentration, the overall Tg decreased in both cure states. Continued monitoring for an additional 4 weeks validated that within the limits of experimental error, cure was complete at 6 weeks of ambient drying.

Figure 2. Tg as a Function of Increasing SoyAA-1 Content at Zero and Final Cure.

∆Tg represent the difference between zero and final cure Tg values.

-40

-20

0

20

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60

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10% 20% 40% 60% 80%

Tg

(°C

)

SoyAA-1 Composition

Zero Cure TgFinal Cure Tg

ΔTg

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The zero cure data represents a coalescence state where the unsaturated groups of SoyAA-1 have not been exposed to oxygen, thus inhibiting the formation of internal crosslinks. The higher Tg values observed upon ambient cure validate that the fatty acid chains crosslinked auto-oxidatively catalyzed by cobalt drier in the presence of oxygen. The difference between zero and final cure Tg, i.e., ∆Tg, is a measure of the auto-oxidation that has occurred in the latex films and corresponds linearly with SoyAA-1 composition (Figure 3). As expected, the ∆Tg increase was more profound for latexes containing 40, 60, and 80% VOMM than latexes containing lesser amounts of VOMMs.

Figure 3. ∆Tg Values as a Function of SoyAA-1 Content.

Tg – MFT Difference

The MFT indicates the temperature at which the forces driving latex deformation exceed the forces that resist deformation.14 For each latex, MFT depends on a number of factors including Tg, plasticization by water and cosolvents, and particle size. Sperry and co-workers suggested that two important stages in the process of latex film formation: evaporation of the aqueous solvent and deformation/compaction of particles leading to void closure, and postulated that either of these steps could be rate-limiting, depending on the process temperature. Near its Tg, the rate-limiting step in film formation is particle deformation. At temperature more than 20K above its Tg, evaporation is the dominant rate-limiting process in film formation.15

Figure 4 illustrates the MFT-Tg profile of control latexes (w/o VOMM) synthesized via

copolymerization of BA and MMA at varying weight ratios. It is clear that the lack of fatty acid unsaturation in the control latexes results in very little difference between the MFT and Tg values.

0

5

10

15

20

25

30

35

10% 20% 40% 60% 80%

5.01

12.23

26.2729.16

30.95ΔT

g(°

C)

SoyAA-1 Composition

198

Figure 4. MFT and Final Cure Tg Values as a Function of BA Content (control latex w/o VOMM).

The MFT-Tg profile of latexes synthesized via copolymerization of SoyAA-1 and MMA is

exemplified in Figure 5. It is apparent that increasing SoyAA-1 content has a significant effect on the difference between the MFT and Tg values. Unfortunately, these results are hindered by the capabilities of the MFT instrument which is limited to a lower temperature of -5°C (and thus freezing all samples) and upper temperature of 90°C. The data indicates the plasticization efficiency of long hydrocarbon fatty acid moieties of VOMMs which enable low temperature film applications.

Figure 5. MFT and Final Cure Tg Values of SoyAA-1 Latexes as a Function of SoyAA-1 Content.

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10% 20% 40% 60% 80%

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ture

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*These results are exceeded by the MFTcapabilities above 90° and below 5 °C.

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*These results are exceeded by MFT capabilities above 90° and below 5 °C.

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Oxidative Curing Solid state 13C NMR spectroscopy is a simple but powerful technique for such a crosslinked

system to reveal structural changes of the polymer matrix and study cure kinetics. Solid state 13C NMR spectra of SoyAA-1/MMA (40/60 w/w) with 1 wt% cobalt drier cured at ambient temperature are illustrated in Figure 6. The peaks appearing between 40-55 ppm are attributed to MMA, while the peaks between 125-135 ppm are associated with allylic double bonds of SoyAA-1. The ratio of allylic double bonds of SoyAA-1 over MMA was quantified via peak integration to determine the percent unsaturation after polymerization and during auto-oxidative curing. Figure 6 shows that the peak area of allylic double bonds of copolymer (SoyAA-1/MMA) decreases with time due to auto-oxidative crosslinking catalyzed by cobalt drier.

Figure 6. Solid State 13C NMR Spectra of SoyAA-1/MMA (40/60 w/w) with 1 wt% Cobalt Drier Cured over a) Initial, b) 1 Day, and c) 7 Days.

The cure kinetics of SoyAA-1/MMA (40/60 w/w) latex films were studied over a 10-week period

and the percent unsaturation was determined by solid state 13C NMR (Figure 7). Latex films utilized in this study were coalesced separately to evaluate auto-oxidation of the VOMM under nitrogen, and under air with and without cobalt drier. The theoretical amount of unsaturation was reported as 100% and the preserved unsaturation was calculated based on this value. An initial rapid decrease in percent unsaturation was observed in all samples, however, the amount of double bonds preserved was almost constant after 1 day cure in nitrogen and air. Latex samples containing 1 wt% cobalt drier lost about 25% of its allylic double bonds after one day of drying. Unlike the samples without drier, samples with cobalt drier retained only 10% of their original double bonds at the end of six weeks, and no change was observed after this period. This result is consistent with the Tg change for SoyAA-1 latexes. The residual unsaturation is probably due to the high degree of crosslinking that limits oxygen diffusion to the deep layers of the latex film as discussed for other systems.16,17

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With Drier Exposed to Air Under N2

% U

nsat

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ion

Time (Days)

Figure 7. Percent Double Bonds Preserved during Cure for SoyAA-1/MMA (40/60 w/w) in Air with

and without 1 wt% Cobalt Drier and Nitrogen as Determined via Solid State 13C NMR.

Latex gel content is directly proportional to its level of crosslinked polymer. Initially, the wet latex was totally soluble in DMSO. However, once dried in the presence of cobalt drier, the films exhibited a gel content of over 70 wt%, and after one day with cobalt drier, the gel content increased to about 80 wt%. Only a slight increase in gel content was observed after the first day of drying. Unlike the samples with cobalt drier, the film samples under air and nitrogen were almost totally soluble initially, and they exhibited a steady increase during their drying period. The highest gel obtained for these samples was about 70% in 10-week period. It is unclear why samples dried under nitrogen exhibit increasing gel content with time (this phenomenon has been observed on multiple occasions) and we have not come across any literature references identifying similar behavior.

The similar trends in both the percent unsaturation and gel content of the film samples cured

under air and nitrogen suggest that the extent of crosslinking requires cobalt catalysis. A number of research articles have discussed the effect of cobalt drier on auto-oxidative curing of pure drying oils18,19 and alkyd paints.20,21 Oyman et al.22 studied model compounds including methyl oleate (MO), ethyl linoleate (EL), and methyl linolenate (MLn) emulsified in water to mimic waterborne alkyd paints. After water evaporation, auto-oxidation was monitored via Raman spectroscopy, ATR-FTIR, and 1H NMR. Almost no change was observed for MO and EL without cobalt drier over 130 hours, but all the esters showed significant changes in the presence of cobalt drier within 3 hours.

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With Drier Exposed to Air Under N2

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Figure 8. Gel Content during Cure of SoyAA-1/MMA (40/60 w/w) Latex in Nitrogen and in Air

with and without 1 wt% Cobalt Drier. Evaluation of End-use Performance

The SoyAA-1 latexes synthesized above were evaluated against a commercial latex and an internal latex (control w/o VOMM) in semi-gloss architectural coatings (Tables 1 and 2). The VOMM latex (SM) was formulated with 45% SoyAA-1 and 55% MMA. The internal control (IC) latex was formulated with BA/MMA (50/50 w/w). The glass transition temperatures of SM and IC are in the same range as that of the CC. Although the latex Tgs are roughly the same, their MFT values are quite different. The coatings were evaluated for their performance properties such as gloss, adhesion, block resistance, and scrub resistance.

Table 1. Latex Properties

Latex Composition (w/w) Particle size (nm) % Solids MFT (oC) Tg (oC) CC - 134 51.0 15.2 13.3 SM SoyAA-1/MMA (45/55) 147 44.2 21.3 16.0 IC BA/MMA (50/50) 187 44.0 6.0 13.3

Table 2. Coating Formulation

Component Amount (lb) Grind Water 125.0 Tamol® 2001 7.8 NH4OH (28%) 2.5 Foamstar® A-36 0.5 Surfynol® CT-111 2.0 Ti-Pure® R-706 210.0 Kathon LX® 1.5% 1.5 Dowanol® DPM 14.0 Sodium nitrite 2.0

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Letdown Water 77.0 Latex 520.0 Texanol® 20.5 Foamstar A-36 0.5 NH4OH (28%) 1.5 Polyphase® 663 10.0 Acrysol® RM-2020 NPR 15.0 Acrysol RM-12W 2.0 Acrysol RM-8W 1.5

Total 1,013.3 PVC 19.1 Weight solids, % 34.5 Volume solids, % 48.0

Gloss is related to the mean particle size of the dispersed pigment and is a measure of the

interaction between the pigments and its interaction with the rest of the coating components.23 For gloss testing, each coating was applied on a Leneta form 605C chart using a 3 mil Bird applicator. The panels were allowed to dry overnight at ambient before determining its optical properties using a BYK colorimeter and gloss meter (Figure 9). All coatings formed clean films that were free from surface defects. As illustrated, samples with CC, SM, and SM/drier (SM with drier) produced films with similar optical properties, however, the gloss of the IC sample was found to be approximately 20 units less.

Figure 9. Optical Properties of Commercial Control and Experimental Latex Coatings.

Block resistance was measured by applying the films to a sealed Leneta panel using a 3 mil Bird

applicator. Films were cured at room temperature for one and seven days before evaluation according to ASTM D 4946. Samples were rated on a scale of 0 to 10, with 10 being the best and 0 being that the samples were severely stuck together, and the film was destroyed upon separation. The results are shown in Figure 10 and indicate that the SM and SM/drier coatings are comparable with the CC in one day block resistance and exhibit superior block resistance after seven days of drying. Block resistance improved upon incorporation of 1 wt% cobalt drier. The IC-based films exhibited the lowest block resistance.

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Optical Properties

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Figure 10. Block Resistance of Commercial Control and Experimental Latex Coatings.

Scrub resistance was measured via ASTM D-2486. All coating samples were applied side by side with the commercial control on a Leneta form P121-10N scrub panel using a 7 mil DOW applicator. The films were allowed to dry for seven and 30 days at ambient prior to testing. Testing was performed in duplicates. As illustrated in Figure 11, scrub resistance of SM samples was inferior to the CC latex-based coatings over seven and 30 days, while the SM/drier samples outperformed the CC samples after 30 days of drying. IC samples, on the other hand, exhibited the best scrub resistance over both seven and 30 days.

Figure 11. Scrub Resistance of Commercial Control and Experimental Latex Coatings.

Adhesion properties of each coating were characterized by using ASTM D 3359. The coatings were applied over a green gloss alkyd coating at approximately 6 mils wet film thickness using a square bar applicator. The coatings were allowed to dry at ambient temperature and humidity for one, seven and 30 days before evaluation. The coatings were evaluated for both dry and wet adhesion. Dry adhesion was evaluated by the crosshatch method as described in ASTM D 3359, while wet adhesion was evaluated by using a modified method. The coatings were covered with a dampened cloth for 30 minutes after crosshatching and then blotted dry before being evaluated as described in ASTM D 3359. Although the VOMM latexes were formulated without any adhesion monomers,24,25 both the SM and SM/drier based coatings outperformed the CC samples in adhesion to gloss alkyd. The SM latex with 1 wt% cobalt drier exhibited the best results over one and seven days period, while the IC samples showed the poorest adhesion overall.

0

2

4

6

8

CC SM SM/Drier IC

3 3 3

1

4

67

4

Block Resistance

1 day 7 day

0

500

1000

1500

2000

2500

3000

CC SM SM/Drier IC

320121 244

1955

345151

491

2794

Scrub Resistance

7 day 30 day

204

Table 3. Green Alkyd Dry/Wet Adhesion

Latex 1 day (Dry/Wet) 7 day (Dry/Wet) 30 day (Dry/Wet) CC 2.5B/2B 5B/4.5B 5B/5B SM 1.5B/2B 4B/4B 5B/5B SM/drier 2B/3.5B 5B/5B 5B/5B IC 0B/0B 0B/0B 0.5B/0B

Conclusions

A soybean oil macromonomer, SoyAA-1, containing both acrylate and allylic functionalities, was successfully copolymerized with MMA at varying weight ratios. Solid state 13C NMR spectroscopy confirmed the preservation of allylic functionalities during emulsion polymerization. ∆Tg and MFT studies indicate a linear correlation with SoyAA-1 concentration and illustrate auto-oxidative crosslinking during application, coalescence and film solidification. Evaluation in a semi-gloss architectural coating formulation compared with a commercial latex formulation confirmed the performance capabilities of the SoyAA-1 monomer-based latex. References 1. Larock R and Lu Y, Biomacromolecules 2007, 8, 3108-3114. 2. Mannari V M and Massingill J L, JCT Research 2006, 3, 151-157. 3. Boyles D A and Al-Omar M S D, US2009/0275715. 4. Behr J M, US2008/0250976. 5. Larock R and Lu Y, Biomacromolecules 2008,9, 3332-3340. 6. Lligadas G, Ronda J C, Galia M, and Cadiz V, Biomacromolecules 2010, 10, 2825-2835. 7. Guo Y, Mannari V M, Patel P, and Massingill J L, JCT Research 2006, 3, 327-331. 8. Abraham, T. W., Carter, J. A., Dounis, D., Malsam, J. US7691914. 9. Ddamulari R K, Raidy J E, and Wright B K, US 7,799,895. 10. Hao G, Mendon S K, Delatte D E, Rawlins J W, and Thames S F, Proceedings of the 35th

International Waterborne, High-Solids, and Powder Coatings Symposium, 2008, 57-68. 11. Thames S F and Rawlins J W, Proceedings of the 36th International Waterborne, High-Solids, and

Powder Coatings Symposium, 2009, 5-26. 12. Thames S F, Rawlins J W, Mendon S K, and Delatte D, US20090143527. 13. Cakmakli B, Hazer B, Tekin I O, and Begendik F C, Biomacromolecules 2005, 6, 1750-1758. 14. Lovell P A and El-Aasser M S, Emulsion Polymerization and Emulsion Polymers; John Wiley &

Sons: New York, 1997, p. 489. 15. Pethrick R A and Cannon L A, Macromolecules 1999, 32, 7617-29. 16. Stenberg C, Svensson M, and Johansson M, Ind Crops and Prod 2005, 21, 263-272. 17. Gardette J, Lemaire J, and Mallegol J, Prog. Org. Coat. 2000, 39, 107-113. 18. Meneghetti S M P, Souza R F, Monteiro A L, and Souza M O, Prog. Org. Coat. 2000, 39, 107-113. 19. Lazzari M and Chiantore O, Polym Degrad Stab 1999, 65, 303-313. 20. Zabel K H, Klaasen R P, Nielen M W F, Hubert J O, and Muizebelt W J, Prog. Org. Coat. 2000, 40,

121-130. 21. Bouwman E and Gorkum R, Coordin. Chem. Rev. 2005, 249, 1709-1728. 22. Oyman Z O, Ming W, and Linde R, Prog. Org. Coat. 2003, 48, 80-91. 23. Butler L N, Fellows C M, and Gilbert R G, Prog. Org. Coat. 2005, 53, 112-118. 24. Singh B, Chang L W, DiLeone R R, and Siesel D R, Prog. Org. Coat. 1998, 34, 214-219. 25. Gonzalez I, Mestach D, Leiza J R, and Asua J M, Prog. Org. Coat. 2008, 61, 38-44.

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