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Industrial Crops and Products 25 (2007) 257–265

Incorporation of drying oils into emulsion polymersfor use in low-VOC architectural coatings

Gregory Booth, David E. Delatte, Shelby F. Thames ∗The University of Southern Mississippi, 118 College Drive #10037, Hattiesburg, MS 39406-0001, USA

Received 7 December 2005; accepted 6 December 2006

bstract

The use of vegetable oil macromonomers (VOMMs) as co-monomers in emulsion polymerization enables good film formationithout the use of traditional coalescing solvents which constitute volatile organic compounds (VOCs). However, the allylic protons

ssociated with the fatty acid double bonds can result in extensive chain transfer, reduced rates of polymerization, and potentialel content. Different vegetable oils were derivatized to yield their respective VOMMs which were subsequently polymerizednto latexes with conventional (meth)acrylate monomers. The degree of ambient crosslinking was related to the extent of chainransfer for the various vegetable oils. The retention of VOMM unsaturation depended on reaction temperature, and the greatestariability between high and low temperatures was exhibited by the linseed oil macromonomer (the highest level of unsaturation).ower reaction temperatures minimized the negative impact of the chain transfer reactions, yielding latexes with higher moleculareights and greater retention of allylic unsaturation. Core–shell polymers were characterized by bimodal particle size distribution

ndicating that the presence of VOMM-rich droplets contributed little to homogeneous VOMM distribution. Optimized single-stageolymerizations resulted in significant preservation of unsaturation, good film-forming qualities, rapid drying, and improved solvent

esistance. The resulting latexes exhibited potential for use in higher performance application than conventional latexes. This studyas demonstrated that drying oils can be incorporated into emulsions in limited quantities as effective reactive monomers for internallasticization and auto-oxidative crosslinking after application. Broader ranges of incorporation require further study of VOMMeaction kinetics as a function of structure and improved process methods for macromonomer incorporation into emulsion polymers.

2006 Elsevier B.V. All rights reserved.

; Chain

eywords: Vegetable oils; Macromonomers; Emulsions; Unsaturation

. Introduction

Drying oils have been primarily used in architecturaloatings in the form of low molecular weight polyesteresins termed alkyds (Hare, 1994). Alkyds develop per-

ormance properties over time via molecular weightevelopment through auto-oxidative cure of unsaturatedouble bonds inherent to vegetable oils.

∗ Corresponding author. Tel.: +1 601 266 4080;ax: +1 601 266 5880.

E-mail address: shelby.f.thames@usm.edu (S.F. Thames).

926-6690/$ – see front matter © 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.indcrop.2006.12.004

transfer; Urethanes; Alkyd-modified emulsions

Increased awareness of the environmental impactof solvent-based polymers has popularized waterbornetechnology making it the standard synthetic route forarchitectural coating polymers (Pourreau et al., 1999).Although the flexibilizing properties and crosslinkingcapabilities of vegetable oils are highly desirable forpigment dispersion, substrate wetting, and film forma-tion, the incorporation of vegetable oil macromonomers

(VOMMs) via emulsion polymerization presents a chal-lenge because their hydrophobicity limits their transportthrough the aqueous phase to the polymerization site(Williams et al., 2003). While diffusion limitations can

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258 G. Booth et al. / Industrial Cr

be overcome by the addition of phase transport catalysts,the allylic protons associated with the oil carbon–carbondouble bonds typically result in extensive chain trans-fer, poor rates of polymerization, and high gel content(Odian, 1991; King et al., 1999). An attempt to over-come these challenges began in the early 1990s withthe development of functionalized castor oil or castoracrylated monomer (CAM) at the University of SouthernMississippi (Thames, 1998).

Latexes containing CAM copolymerized with con-ventional emulsion monomers exhibited good filmformation and high gloss. However, due to the low unsat-uration levels in castor oil, CAM-based latexes were onlyeffective in interior flat coatings formations and wereunsuitable for low-VOC, high performance applications.Alternative functionalities, therefore, were investigatedto alleviate the limitations imposed by chain transferduring emulsion synthesis. Castor oil macromonomerswith urethane functionality were synthesized to uti-lize the urethane hydrogen bonding capabilities forperformance enhancement. However, the performanceproperties were influenced by the level of unsaturationand the urethane functionality was not singularly suffi-cient to improve coatings performance (Booth, 2002).The macromonomer synthesis process, therefore, wasrevised to improve the delineation between the level ofunsaturation and performance. This paper focuses onthe degree of ambient crosslinking versus the extent ofchain transfer while comparing different vegetable oils.Coconut, high oleic safflower, soybean, and linseed oilswere derivatized to obtain the macromonomers termedurethane coconut acrylate monomer (UCoAM), urethanesafflower acrylate monomer (USfAM), urethane soy-bean acrylate monomer (USAM), and urethane linseedacrylate monomer (ULiAM). Each macromonomer wasincorporated into an emulsion polymerization and eval-uated for latex properties.

2. Experimental

2.1. Materials

Chemicals were acquired from several vendorsthroughout this research project, and are organized pre-dominantly by vendor name. All chemicals listed wereused as received unless otherwise noted.

Sodium carbonate, t-butyl hydroperoxide, ammo-nium persulfate, methyl methacrylate, butyl acrylate

(BA), methacrylic acid, 2-hydroxyethyl acrylate (HEA),dibutyl tin dilaurate, coconut oil, safflower oil, soybeanoil, linseed oil, hydroquinone (HQ), and phenoth-iazine (PTZ) were purchased from Sigma–Aldrich,

Products 25 (2007) 257–265

St. Louis, Missouri. Rhodapex® CO-436 and Igepal®

CO-887 were obtained from Rhodia, Cranbury, NewJersey; 5% Cobalt Hydro-Cure®, and 12% ZirconiumHydro-Chem® were obtained from OMG, Franklin,Pennsylvania; and isophorone diisocyanate (IPDI) wasobtained from Bayer, Pittsburgh, Pennsylvania.

2.2. Monomer characterization

Fourier transform infrared spectroscopy (FTIR) wasused to monitor monomer synthesis. Aliquots wereextracted at regular intervals and applied as thin filmsonto sodium chloride plates. Spectra were collected for32 scans on a BioRad FTS 25 with a resolution of2 cm−1. Reactant and product molecular weight analy-ses were determined via gel permeation chromatography(GPC) using a refractive index (RI) detector and atetrahydrofuran (THF) mobile phase with toluene asa peak marker. The RI detector was calibrated usingpoly(methyl methacrylate) standards purchased fromPolymer Laboratories. Proton (1H) nuclear magneticresonance (NMR) spectroscopy was used to qualifymacromonomer structure, and particularly to quantifythe preservation of the allylic unsaturation derived fromvegetable oil fatty acids. NMR spectra were based upon128 scans using a Varian 300 MHz NMR with a relax-ation delay of 1 s. Emulsion particle size was measuredon a Microtrac UPA 250 dynamic light scattering basedparticle size analyzer.

2.3. VOMM synthesis

Castor oil offers the advantage of direct acrylationthrough the ricinoleic acid’s hydroxyl functionality;however, other vegetable oils such as coconut, safflower,soybean, and linseed oils require synthetic modificationsprior to acrylation. The simplest of these modifications isthe introduction of hydroxyl groups through conversionto diglycerides.

For diglyceride synthesis, a round-bottom flask wascharged with a 1:1 molar ratio of oil and glycerol, andnitrogen was bubbled through the mixture for 2 h. Trans-esterification catalyst FasCat® 4100 (butyl stannoic acid)was added at 1% by weight of oil and the reaction mixturewas maintained at 250 ◦C for 4 h. GPC analysis indicatedthat the reaction product did not contain any residual oil.The NMR peak assignments for soybean oil (the basic

peak assignments are also valid for linseed, coconut, andsafflower oils with each oil’s subtle variation) and thecorresponding diglyceride structure are shown in Fig. 1.Changes in peak area and peak positioning between 3.5

G. Booth et al. / Industrial Crops and

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Fig. 1. 1H NMR of soybean oil and soybean diglyceride.

nd 4.5 ppm represent changes to the glyceride struc-ure. Retention of the oil unsaturation was verified viantegration of the CH2 peak adjacent to the ester car-onyl of the triglyceride at 2.4 ppm, whereas the olefiniceak at 5.4 ppm was monitored to identify any remaininglycerol after washing.

A round-bottom flask was charged with 36 g ofsophorone diisocyanate (IPDI) and 2 g of dibutyl tinilaurate. An addition funnel was charged with 100 g ofhe soybean diglyceride. The reaction flask was heatedlowly to 70 ◦C, at which time the crude soybean diglyc-ride was added dropwise over 2 h, followed by dropwiseddition of 18.7 g of HEA over 30 min. The reaction was

onitored by following the disappearance of the iso-

yanate peak (2250 cm−1) via FT-IR. When the reactionas complete, 100 ppm of HQ and 100 ppm of PTZ were

dded to inhibit both acrylate polymerization and oxida-

Fig. 2. 1H NMR peak assignments of urethane

Products 25 (2007) 257–265 259

tive cure of the olefinic double bonds. The product wasthen cooled and characterized via 1H NMR (Fig. 2).

The olefinic double bonds concentration per acrylatewas calculated by determining the ratio of the area underthe acrylate peaks (5.8–6.5 ppm) and the olefinic peak at5.4 ppm (Table 1). The number of double bonds per acry-late group was proportional to the predicted structure.

2.4. Latex synthesis

Single-stage emulsion polymerizations were initiatedby charging the reactor with a small amount of waterand surfactant (kettle charge) and purging with nitro-gen for 1 h. The reaction feed was prepared by blendingwater, surfactant, and buffer. Each monomer was addedsequentially to the water/surfactant/buffer solution, withmethyl methacrylate and butyl acrylate preceding theVOMM. The pre-emulsion was stirred using a Hamil-ton Beach mixer at a rate of 18,000 rpm for 30 min.The average pre-emulsion particle size was 250 nm.Due to the hydrophobicity and VOMM size, smallerpre-emulsion particle size was preferred so that themonomer droplets can compete with the polymer par-ticles for radicals and the effects of VOMM monomerdiffusional limits are minimized. A poorly dispersedpre-emulsion forms multimodal particle size distributionthat indicates the formation of large monomer dropletsor aggregated particles. Such pre-emulsions lead to lit-tle or no polymerization within the monomer dropletand create polymers with poorly distributed functionalmonomers.

Core–shell polymerizations were performed by usingseparate pre-emulsions for each stage with all themacromonomer added in the second stage. This stepwas intended to concentrate the reactive macromonomer

linseed acrylate monomer (ULiAM).

260 G. Booth et al. / Industrial Crops and Products 25 (2007) 257–265

Table 1Number of double bonds per acrylate of urethane coconut, safflower, soybean, and linseed acrylate monomers

Monomer Average integration ofacrylate peak

Integration ofolefinic protons

Normalized to oneproton

Double bonds peracrylate

UCoAM (coconut) 1.00 1.13 0.57 0.57USfAM (safflower) 0.95 4.29 2.15 2.26

00

USAM (soybean) 0.90 5.5ULiAM (linseed) 0.99 7.7

towards the outside of the polymer particle and capitalizeupon its film-forming capabilities. Chain transfer duringpolymerization was monitored using GPC and 1H NMR.

3. Results and discussion

3.1. Acrylated urethane diglyceride latex synthesis

A series of emulsion polymers were synthesized todetermine the effects of macromonomer concentration,reaction temperature, and reaction procedure on latex

performance properties such as molecular weight, allylicdouble bond preservation, and solvent resistance as afunction of cure time as a rough guide to networkdevelopment. Latex molecular weights as a function of

Fig. 3. Control, UCoAM, and ULiA

2.75 3.053.85 3.89

reaction temperature and method are presented in Fig. 3.Each latex consisted of 5 wt% VOMM, with the excep-tion of the control polymers which were VOMM-free.

The reaction temperature and method had little effecton the molecular weight of the control polymers althoughvariations were noted in the molecular weight of theVOMM latexes. Molecular weight of UCoAM-basedlatexes did not vary significantly as a function of reactionmethod at or below 65 ◦C, but there was a marked differ-ence in molecular weight between the single-stage andcore–shell latexes synthesized at 80 ◦C. The data support

the tenet that chain transfer reactions are more preva-lent at higher temperatures, thereby adversely affectingmolecular weight, particularly at high VOMM concen-trations. The same phenomenon was also noted with

M latex molecular weights.

G. Booth et al. / Industrial Crops and

Up

3

ieasamso

Fig. 4. 1H NMR integration of ULiAM latex film.

LiAM latexes, but the temperature effects were moreronounced.

.2. Double bond preservation

Allylic protons are susceptible to abstraction dur-ng latex synthesis. It was important to determine thextent of chain transfer at those locations because post-pplication oxidative crosslinking is dependent on theurrounding architecture. To determine the percent-

ge of residual olefinic bonds, 5 wt% of a specificacromonomer was post-added to the control latex and

tirred at 1800 rpm for 1 h. Peaks associated with thelefinic bonds, methyl methacrylate, and butyl metha-

Fig. 5. Percentage of double bonds preserved in UC

Products 25 (2007) 257–265 261

crylate were integrated to ascertain whether the latexcomposition matched the latex feed composition. Forinstance, the 1H NMR spectra of a ULiAM latex is shownin Fig. 4.

The olefinic bond peaks at 5.4 ppm were quantifiedthrough peak integration to determine the level of unsatu-ration versus reaction time. The amount of double bondspreserved during emulsion polymerization of the differ-ent VOMM latexes processed under varying conditionsis presented in Fig. 5.

For the most part, single-stage polymerizationspreserved a greater percentage of unsaturation thancore–shell reactions at the same temperature. Thecore–shell polymerizations were designed to containhigh macromonomer concentration during the last stagesof polymerization. During this final stage, there isless butyl acrylate or methyl methacrylate present.Because these co-monomers also act as solvents for themacromonomer, their loss caused the macromonomerto become increasingly concentrated, less mobile, andtherefore more likely to participate in chain transfer reac-tions. As expected, increased chain transfer reactionsresulted in lower molecular weights.

3.3. Oxidative curing

Latex film cure kinetics were studied as a function ofcomposition, reaction temperature, and method of poly-merization. Each latex was applied on glass panels using

a drawdown bar at 6 mils wet film thickness, and placedin a dry box under a steady stream of nitrogen. The vari-ous latex films were dried for 1, 2, and 4 weeks at ambientconditions, and the disappearance of olefinic bonds was

oAM, USfAM, USAM, and ULiAM latexes.

262 G. Booth et al. / Industrial Crops and

Fig. 6. 1H NMR spectra of ambient cured VOMM latex.

determined via 1H NMR. An example of 1H NMR spec-tra obtained over 4 weeks of a VOMM latex cured atambient conditions is illustrated in Fig. 6. Typically, themajority of the detectable unsaturation was consumed in

2 weeks.

The loss in unsaturation of seven UCoAM latexes syn-thesized under various conditions are shown in Fig. 7a.Latexes synthesized at 80 ◦C lost over two-thirds of their

Fig. 7. (a) 1H NMR analysis of ambient cured UCoAM. (b) 1H NMR analysUSAM. (d) 1H NMR analysis of ambient cured ULiAM.

Products 25 (2007) 257–265

unsaturation during the polymerization process whilelatexes synthesized at 45 and 65 ◦C lost almost one-halfof their original unsaturation during polymerization. Bythe end of the first week of drying, however, all UCoAM-based latexes retained 20% of the original double bonds.Thus, the latexes synthesized at 45 and 65 ◦C consumedtheir unsaturation faster than the latexes polymerized at80 ◦C. After drying for 4 weeks, the single-stage latexessynthesized at 45 and 65 ◦C had the lowest residualunsaturation, whereas the core–shell (70:30) latex syn-thesized at 80 ◦C had the highest residual unsaturation.This is of particular significance where rapid propertydevelopment is desired.

The USfAM-based latexes behaved similarly to theUCoAM-based latexes except that the latexes processedat 80 ◦C had lower residual unsaturation than the latexessynthesized at 65 ◦C (Fig. 7b).

The USAM-based latexes exhibited the same trend inloss of unsaturation as the UCoAM-based latexes, but theoverall retention of unsaturation during polymerization

was higher than the UCoAM-based latexes (Fig. 7c).

The loss of unsaturation in ULiAM-based latexes(Fig. 7d) was similar to USAM-based latexes. Becauselinseed oil contains more double bonds than soybean

is of ambient cured USfAM. (c) 1H NMR analysis of ambient cured

G. Booth et al. / Industrial Crops and Products 25 (2007) 257–265 263

M latex

oitbr

p

Fig. 8. ULiA

il, more double bonds must have been consumed dur-ng polymerization of ULiAM. After 4 weeks of drying,he single-stage ULiAM latex possessed very few dou-

le bonds, but core–shell latexes synthesized with higheratios in the shell retained higher unsaturation.

Core–shell polymers with ULiAM exhibited bimodalarticle size distribution and the slowest rate of cure. The

Fig. 9. (a) MEK resistance of single-stage latexes synthesized at 45 and

properties.

bimodal particle size distribution is supporting evidenceof residual droplets containing high macromonomerconcentrations during and after polymerization. Upon

application, the ULiAM latex films appeared hazy and“pockets” of macromonomer could be seen. Single-stagepolymerization, however, resulted in unimodal particlesize distribution and exhibited the fastest rate of cure

65 ◦C with 5% VOMM. (b) MEK resistance of ULiAM latexes.

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264 G. Booth et al. / Industrial Cr

of all the macromonomer-based latexes, thus makingthis latex an interesting candidate for further testing inlow-to-no VOC applications. These film-forming andreactive characteristics were most advantageous between5 and 10% ULiAM, above which chain transfer reactionsbegan to limit molecular weight and allylic double bondpreservation (Fig. 8).

The fully dried films were tested via methyl ethylketone (MEK) double rubs to rank crosslink density. Thenumber of MEK double rubs required to break throughthe dried films is a direct measure of polymer crosslinkdensity. Resistance to MEK was quantified versus timeafter application to determine the VOMM efficacy inimproving crosslink density via auto-oxidation. The sol-vent resistance of single-stage latexes synthesized at 45and 65 ◦C is depicted in Fig. 9a.

The control and UCoAM-based latexes demonstratedminimal improvement in solvent resistance, suggest-ing that only minor changes in polymer architectureoccur after application, i.e., the polymers remain ther-moplastic in structure. However, the USfAM, USAM,and ULiAM-based latexes did exhibit MEK resistanceimprovements versus time indicating that auto-oxidativereactions increased the films’ crosslink density. TheULiAM latex, in particular, had a 330% increase insolvent resistance after 4 weeks of drying, and thistransition was attributed to the high level of unsatura-tion preserved during synthesis. Interestingly, however,once the level of ULiAM increased beyond 10 wt%, theMEK resistance remained almost constant at the higherlevels (Fig. 9b).

4. Conclusions

Non-drying, semi-drying, and drying oils were suc-cessfully incorporated into emulsions and propertyimprovements were achieved correlating VOMM struc-ture, level of incorporation, reaction temperature, andpolymerization process variables. The series of novelurethane-modified VOMMs has broadened the under-standing of structure–property relationships in VOMMpolymerization, and provided a better insight into the bal-ance between polymer reaction kinetics, side reactions,and film performance.

The retention of VOMM unsaturation was dependentupon reaction temperature with the greatest variabil-ity between high and low temperatures exhibited whileutilizing ULiAM (which contained the highest level of

unsaturation). Preservation of olefinic bonds decreasedsignificantly at reaction temperatures above 65 ◦C.Lower reaction temperatures minimize the negativeimpact of chain transfer reactions, resulting in higher

Products 25 (2007) 257–265

molecular weights and greater retention of unsaturation.These polymers also maintain the greatest potential forsmooth film formation in the absence of externally addedcoalescing solvents.

The results confirm that higher levels of chain trans-fer may not eliminate the allylic unsaturation, but thelinkages produced during polymerization inhibit or elim-inate the remaining unsaturation from participating in theauto-oxidative process to the same degree. Chain trans-fer tethers molecules into a semi- or microcrosslinkedregion that has limited mobility, and therefore is notadvantageous to the crosslinking process.

Unlike other VOMM latexes, ULiAM latexes weresensitive to the method of polymerization. Core–shellpolymers resulted in bimodal particle size distribution,indicating that the presence of monomer-rich dropletscontributed little to homogeneous VOMM distributionand likely did not improve film integrity after applicationvia auto-oxidative crosslinking. The optimized single-stage polymerizations, however, resulted in a significantpreservation of unsaturation, good film-forming quali-ties, faster drying, and improved solvent resistance. Theresulting latexes have the potential to be used in high per-formance applications and will be further investigated.It is believed that the success of this latex is solely afunction of the monomer combination and the resultingpolymer composition. Beyond 10 wt% macromonomerincorporation, the performance properties reduce dras-tically with respect to concentration as chain transferreactions compete for propagating radicals.

This study has demonstrated that drying oils canbe incorporated into emulsions in limited quanti-ties as effective reactive monomers for auto-oxidativecrosslinking. Higher levels of incorporation require adetailed study of VOMM reaction kinetics as a func-tion of structure and improved process methods ofmacromonomer incorporation into emulsion polymers.

Acknowledgements

This material is based upon work supported by theCooperative State Research, Education, and ExtensionService, U.S. Department of Agriculture, under Agree-ment Nos. 91-38202-5928 and 2001-38202-10424.

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