Predicting the Viscosity of Low VOC Vinyl Ester and Fatty 

Acid-Based Resins 

by John J. La Scala, Amutha Jeyarajasingam, Cherise Winston, 

James M. Sands, and Giuseppe R. Palmese 

ARL-TR-3681 December 2005 

Approved for public release; distribution is unlimited.
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Army Research Laboratory 

Aberdeen Proving Ground, MD 21005-5069 

ARL-TR-3681 December 2005 

Predicting the Viscosity of Low VOC Vinyl Ester and Fatty 

Acid-Based Resins 

John J. La Scala, Amutha Jeyarajasingam, Cherise Winston, 

James M. Sands, and Giuseppe R. Palmese 

Weapons and Materials Research Directorate, ARL 

Approved for public release; distribution is unlimited.
ii

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Predicting the Viscosity of Low VOC Vinyl Ester and Fatty Acid-Based Resins 

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John J. La Scala, Amutha Jeyarajasingam, Cherise Winston, James M. Sands, and 

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14. ABSTRACT 

Styrene and methacrylated fatty acid (MFA) monomers were used as reactive diluents in the vinyl ester resins. The viscosities 

of these resins were measured as a function of reactive diluent content and type, temperature, and vinyl ester molecular weight

to determine the operating window for composite manufacture. The viscosity decreased exponentially with reactive diluent 

content. Styrene and MFA monomers affected the viscosity in the same manner, but not to the same extent. The viscosities of 

resins using both diluents were accurately predicted using a logarithmic rule of mixtures from the two-component viscosity 

functions. The viscosity increased exponentially and predictably as a function of the vinyl ester number average molecular 

weight. Increasing the temperature decreased the viscosity in an Arrhenius manner. The activation energy for viscous flow 

decreased linearly as the diluent content increased, but was unaffected by vinyl ester molecular weight, fatty acid chain length, 

and unsaturation level. Overall, the resin viscosity was modeled as simple functions of the resin temperature, vinyl ester 

molecular weight, styrene content, MFA content, MFA chain length, and MFA unsaturation level, which are all known 

quantities for a formulated resin. Therefore, the operating window for various liquid molding operations was predicted for 

standard DGEBA-based vinyl ester resins and low VOC/HAP resins. 

15. SUBJECT TERMS

low VOC vinyl esters, styrene, viscosity 

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John J. LaScala 

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iii

Contents 

List of Figures iv

List of Tables vi

1. Introduction 1

2. Experimental Procedure 3

2.1 Preparation of Vinyl-Ester Monomers ............................................................................3 

2.2 Fatty-Acid Monomer Preparation .................................................................................10 

2.3 Resin Formulation .........................................................................................................10 

2.4 Rheological Characterization ........................................................................................11 

2.4.1 Steady Flow Experiments..................................................................................11 

2.4.2 Temperature Experiments .................................................................................12 

3. Results 12

3.1 Viscosity of Bimodal Blends of Vinyl-Ester Monomers ..............................................12 

3.1.1 Effect of Vinyl-Ester Molecular Weight and Styrene Content .........................12 

3.1.2 Effect of Temperature .......................................................................................18 

3.1.3 Complete Viscosity Model for Vinyl-Ester/Styrene Resins..............................21 

3.2 Viscosity of Fatty Acid-Based Vinyl-Ester Resins .......................................................22 

3.2.1 Effect of Fatty-Acid Chemical Structure...........................................................22 

3.2.2 Effect of Temperature .......................................................................................26 

3.2.3 Complete Viscosity Model for Fatty Acid-Based Vinyl-Ester Resins..............27 

3.3 Viscosity of Vinyl-Ester Resins Using Both Fatty-Acid Monomers and Styrene as 

Reactive Diluents....................................................................................................................28 

4. Conclusions 31

5. References 34

Distribution List 37 
iv

List of Figures 

Figure 1. The reaction of methacrylic acid with Epon to form vinyl-ester monomer. ...................3 

Figure 2. The FTIR spectra of Epon/methacrylic acid and the resulting vinyl ester formed 

after complete reaction...............................................................................................................6 

Figure 3. SEC chromatographs of vinyl ester 828 and representative vinyl-ester bimodal 

blends. The curves are vertically offset for clarity. ..................................................................6 

Figure 4. The NMR spectrum of (A) vinyl ester 828 with 50 weight-percent styrene, with the 

peaks labeled to correspond to chemical groups of (B) vinyl-ester monomer and (C) 

styrene (5). Note the definition of m (m = 1 represents the lowest possible value of m for 

DGEBA-based resins)................................................................................................................9 

Figure 5. The reaction of glycidyl methacrylate and oleic acid to produce the MOA 

monomer. .................................................................................................................................10 

Figure 6. The resin viscosity as a function of VE Mn for different styrene contents, showing 

that resin viscosity is normalized by VE Mn............................................................................13 

Figure 7. The resin viscosity as a function of styrene content for various VE Mn, showing 

that styrene content affected the resin viscosity to the same extent for all bimodal resins 

with the same Mn, and the effect of styrene content on resin viscosity decreased as Mn

decreased..................................................................................................................................13 

Figure 8. The resin viscosity as a function of VE Mw for resins containing 20 weightpercent styrene, showing that resin viscosity is not normalized by VE Mw. ...........................14 

Figure 9. The bulk viscosities of C5-C16 alkanes and alcohols at 25 °C dependence on 

molecular weight (35)..............................................................................................................15 

Figure 10. The dependence of Me(S) on the styrene weight fraction in bimodal blends of 

vinyl-ester resins. .....................................................................................................................17 

Figure 11. The viscosity-temperature dependence is Arrhenius for vinyl-ester resins with 20 

weight-percent styrene.............................................................................................................18 

Figure 12. The activation energy for viscous flow as a function of styrene content in vinylester resins................................................................................................................................19 

Figure 13. The prefactor as a function of styrene content in vinyl-ester resins............................20 

Figure 14. The activation energy for viscous flow as a function of vinyl-ester molecular 

weight.......................................................................................................................................20 

Figure 15. Viscosity of MFA monomers as a function of fatty-acid chain length. ......................23 

Figure 16. The viscosity of vinyl ester /MFA as a function of fatty-acid chain length at 25 

°C. ............................................................................................................................................23 

Figure 17. Viscosity of vinyl-ester resin as a function of MFA content for MLau fatty-acids, 

showing the exponential fit to the data. ...................................................................................24 

Figure 18. Viscosity of vinyl-ester resin as a function of MLau content, showing the 

exponential fit to the data.........................................................................................................25 
v

Figure 19. The MFA coefficient in equation 18 is a linear function of MFA viscosity. ..............26 

Figure 20. The activation energy as a function of diluent content and fatty-acid type................27 

Figure 21. The pre-exponential constant as a function of diluent content and fatty-acid type.....28 

Figure 22. The minimum MFA content required to achieve vinyl-ester resin viscosities 

below 500 cP as a function of temperature, fatty acid chain length, and unsaturation level. 

The operating window is any MFA content above the curve. Legend: [carbon atoms: 

unsaturation sites]. ...................................................................................................................29 

Figure 23. The viscosity of vinyl ester/MHex/styrene as a function of styrene content in the 

resin for resins containing 55 weight-percent and 65 weight-percent vinyl ester 828, 

along with model predictions...................................................................................................30 

Figure 24. The experimental and model viscosities of vinyl ester/MHex/styrene as a function 

of vinyl-ester molecular weight and vinyl-ester content for resins containing 20 weightpercent styrene. ........................................................................................................................30 

Figure 25. The experimental and model viscosities of VE/MLau/styrene as a function of 

temperature. .............................................................................................................................31 

Figure 26. The minimum styrene content required to achieve vinyl-ester resin viscosities 

below 500 cP as a function of fatty acid chain length, vinyl-ester content, and vinyl-ester 

molecular weight at 30 oC. The operating window is any styrene content above the curve 

for any molecular weight along the curve. Legend: [carbon atoms: unsaturation sites]. ......32 
vi

List of Tables 

Table 1. Epoxy equivalent weight as determined by epoxy titration and literature values (20).....4 

Table 2. The vinyl-ester molecular weights and styrene contents as determined using NMR 

and SEC along with literature/known values. The styrene contents listed for the bimodal 

blends of vinyl ester 828/100XF are the experimentally determined values and the initial 

weight fraction styrene (known) in the reaction mixture of the prepared vinyl ester................8 

Table 3. The fatty acids used to make MFA monomers...............................................................11 

Table 4. The calculated styrene content as a function of vinyl-ester molecular weight and 

temperature that is required to achieve η = 500 cP for DGEBA-based vinyl esters. ..............22 
1

1. Introduction 

Vinyl-ester resins are used to make polymer matrix composites in military and commercial 

applications because of their high modulus, high strength, high glass transition temperature, low 

weight, and low cost (1, 2). These resins have superior properties relative to unsaturated 

polyester systems and are less expensive and easier to process than epoxy systems (1). Vinylester resins contain vinyl-ester monomers and a reactive diluent, such as styrene. The vinyl-ester 

monomers have two terminal free-radically polymerizable unsaturation sites, enabling the resin 

to cross-link. Reactive diluents, such as styrene, have a single vinyl group and act as linear chain 

extenders. The reactive diluent is used because it severely reduces the resin viscosity and 

improves overall polymer performance by allowing for a higher degree of reaction of the 

monomers by delaying the onset of gelation and reducing diffusion limitations (3). These 

properties provided by the reactive diluent also allow for the use of low cost composite 

fabrication processing methods, such as vacuum-assisted resin transfer molding. On the other 

hand, styrene and other commonly used reactive diluents are volatile organic compounds (VOC) 

and hazardous air pollutants (HAP) that are damaging to health and the environment. 

VOCs are liberated during all of the phases of composite fabrication (2, 4). VOC emissions 

occur during the mixing of diluents, catalysts, and initiators into the system. In open mold 

fabrication, composite parts typically have very large surface to volume ratios, which allow 

substantial styrene contents to be lost during the molding stage. During cure, elevated 

temperatures increase the vapor pressure of styrene and thus increase the rate of VOC emission. 

Unfortunately, even after cure during the lifetime of the part, VOC emissions can be substantial. 

Past work has shown that up to 40% of the styrene in vinyl-ester resins is unreacted after cure 

(5). Because styrene is an HAP and a VOC, the Federal Environmental Protection Agency of the 

United States of America introduced legislation to limit styrene emissions from composite 

manufacturing (4). Therefore, liberation of VOCs must be mitigated not only during composite 

processing, but also during curing and fielding of the composite part. Thus, a means of reducing 

the styrene emissions from vinyl-ester resins is desired. Current low VOC vinyl-ester systems 

do exist, such as Dow Derakane 441-400 (6). However, the use of these resins is limited because 

of their low fracture toughness (~100 J/m2

), higher viscosity (400 cP), and higher cost relative to 

standard vinyl-ester systems, such as Derakane 411-350, which have viscosities below 200 cP 

and fracture toughness values above 200 J/m2

 (5–8). 

Unfortunately, decreasing the styrene content in vinyl-ester resins does not offer an acceptable 

solution to this problem. As the styrene content is reduced, the resin viscosity increases, making 

it difficult to use inexpensive molding techniques (1, 9). In addition, Dow Derakane 441-400 

uses 27% less styrene than Derakane 411-350 (6) but has less than half the fracture toughness 

(5, 8). 
2

Various monomers with volatilities lower than that of styrene have been used as styrene 

replacements, such as vinyl toluene (10). These styrene replacements still produce significant 

VOC emissions and are therefore still regulated by the EPA (4). In addition, few monomers 

yield resins with properties comparable to styrene-based resins and even fewer can match the 

low cost of styrene. 

Fatty-acid monomers are derived from plant oils, such as soybean oil, which are composed of 

over 99% triglyceride molecules (11). Triglycerides are composed of three fatty acids connected 

by a glycerol center. The triglycerides in plant oils typically contain 10 or more different fatty 

acids, which range in length, level of unsaturation, and functionality (e.g., epoxide and hydroxyl 

functionality as found in vernonia and castor oils, respectively) (11). 

Fatty acids and triglycerides have been used in a number of polymeric applications. Epoxidized 

and acrylated triglycerides have been used as plasticizers and toughening agents (12). In fact, 

the largest non-food use of triglycerides is the use of epoxidized soybean and linseed oils as 

plasticizers in poly(vinyl chloride) (12). Pressure sensitive adhesives have been made from 

fatty-acid methyl esters (13). In addition, thermosetting liquid molding resins, similar to vinylester resins have been made using plant oils as the cross-linkers (14–17). However, until 

recently, fatty-acids have not been used as reactive diluents in thermosetting liquid molding 

resins. In fact, fatty acid-based vinyl-ester resins use less than 20 weight-percent styrene 

monomer, while maintaining low resin viscosities and high polymer performance (18). 

Simply altering the molecular weight of vinyl-ester monomers does not provide a means for both 

reducing styrene emissions and maintaining resin and polymer properties. Low molecular 

weight vinyl-ester monomers have poor fracture properties because of their high cross-link 

densities (9, 19). High molecular weight vinyl-ester monomers yield resins with high fracture 

properties via matrix toughening. However, low styrene contents cannot be used because the 

resin viscosity increases severely and processing becomes unacceptably difficult (20). On the 

other hand, using a mixture of low and high molecular weight vinyl-ester monomers (i.e., 

bimodal blend) produces high performance polymers with low viscosities along with VOC/HAP 

reduction of up to 38% (21). 

The resins examined in this work have sufficient properties for at least low-tech nondemanding 

composite applications where polyester or vinyl-ester quality performances are sought. Some of 

these resins are appropriate for high-tech Department of Defense applications. The resin 

viscosity governs the manufacturing method, regardless of the end-use application. When the 

resin viscosity is too high, the pressure driving force in liquid molding operations is not enough 

to push the viscous resin into the gaps between the fibers (glass, carbon, flax, etc.) before the 

resin cures. Consequently, viscous resins produce composite parts with high void contents. To 

reduce the void contents, long production times are necessary. As a result, a viscosity of 500 cP 

is considered about the maximum for most inexpensive liquid molding techniques (1, 22). The 

use of bimodal blends of vinyl-esters and fatty-acid monomers to reduce styrene content in 
3

vinyl-ester resins are acceptable solutions. However, the operating window for processing these 

resins has not been established. This report examines the effect of possible solutions (fatty-acid 

monomers and bimodal blends of vinyl-ester monomers) on resin viscosity. Specifically, this 

report will present models that allow predictive determination of resin viscosity as functions of 

reactive diluent content, diluent type, vinyl-ester molecular weight, and temperature for 

DGEBA-based vinyl esters. 

2. Experimental Procedure 

2.1 Preparation of Vinyl-Ester Monomers 

Vinyl-ester monomers were prepared via methacrylation of diglycidyl ether of bisphenol A 

(DGEBA) (figure 1) (1). Epon 828, 834, 836, 1001F, 1004F, 1007F, and 1009F (MillerStephenson, Danbury, CT) were used as the source of DGEBA. To determine the epoxy 

equivalent weight of the Epon resins, epoxy titration was performed as per ASTM D1652-90, 

Procedure B (23). The epoxy resin was dissolved in 10–15 mL methylene chloride (Aldrich, 

Milwaukee, WI), and 10 mL tetraethylammonium bromide solution (Aldrich, 0.4 g/mL) was then 

added to the mixture. The sample was titrated with the perchloric acid/peracetic acid solution 

(Aldrich) until the indicator, 0.1% crystal violet in acetic acid (Aldrich), changed color from blue 

to green. The epoxy titration results agreed well with literature values for the epoxy equivalent 

weight (EEQ) (table 1) (20). 

AMC-2 

1%, 90-100o

C 

CH2 CH

OH

O C

CH3

CH3

CH2 CH2

OH

O CH CH CH2

OH

[ ] CH2 CH2 O

CH3

CH3

O C n C CH2

CH3

C

O

CH2 C O

CH3

C

O

O

Vinyl Ester (VE) 

CH2 CH

O

O C

CH3

CH3

CH2 CH2

O

O CH CH CH2

OH

[ ] CH2 CH2 O

CH3

CH3

O C n CH2 C

CH3

C

O

OH + 

Diglycidyl ether of bisphenol-A (DGEBA) Methacrylic Acid 

Figure 1. The reaction of methacrylic acid with Epon to form vinyl-ester monomer. 

Pure vinyl ester was prepared via methacrylation of Epon 828, 834, or 836 (figure 1). 

Approximately 500-g Epon resin was reacted with 1.01 × the stoichiometric amount of 

methacrylaic acid (i.e., as determined through epoxy titration). AMC-2 (Aerojet Chemicals, 

Rancho Cordova, CA), which is a mixture of 50% trivalent organic chromium complexes and 

50% phthalate esters, was used as a catalyst for the reaction and to prevent epoxy 

homopolymerization (24) in the amount of 1 weight-percent. The reaction was run at ~90 °C, 
4

Table 1. Epoxy equivalent weight as determined by epoxy titration and literature 

values (20). 

Epon Resin Literature EEQ 

(g/mol) 

Experimental EEQ 

(g/mol) 

Calculated MW 

(g/mol) 

828 185–192 186 ± 3 372 ± 6 

834 230–280 255 ± 4 510 ± 8 

836 290–335 307 ± 3 614 ± 6 

1001F 525–550 548 ± 3 1096 ± 6 

1004F 800–950 900 ± 5 1800 ± 10 

1007F 1700–2300 1990 ± 7 3980 ± 14 

1009F 2300–3800 2770 ± 8 5550 ± 16 

but because of the exothermicity of this reaction, water was periodically flowed through cooling 

coils within the reactor to keep the temperature below 95 °C. Acid number titration was used 

during the course of the reaction to measure the amount of free (unreacted) acid in the vinyl-ester 

system. The acid number tests were performed in accordance with ASTM D1980-87 (25). 

Approximately 1 g of the vinyl-ester reaction mixture was dissolved in 5-g acetone. Three drops 

of 0.5 weight-percent phenolphthalein in 50% ethanol were added to the mixture to determine 

the neutralization point. The solution was then titrated with 0.5-N sodium hydroxide until the 

solution remained slightly pink in color for 30 s. An acid number (mg NaOH/g VE) of 5, 

corresponding to ~2% free acid, was the maximum allowable acid number. If the acid number 

was too high, the methacrylation reaction was allowed to continue until future acid numbers 

measurements were below 5. Although the vinyl ester 828 has a small percentage of higher 

molecular weight monomer, it will be referred to as the “monodisperse” vinyl ester throughout 

because the distribution of molecular weights is far smaller than that of the bimodal resins 

prepared. 

Bimodal blends of vinyl-ester monomers (BM-VE) were prepared by methacrylation of blends of 

Epon 828, used as the low molecular weight DGEBA, and Epon 100XF (1001F, 1004F, 1007F, 

or 1009F) used as the high molecular weight DGEBA. Epon 100XF are solids at room 

temperature. The viscosities of these resins are too high to effectively mix during the vinyl-ester 

reaction, even at 100 °C. Therefore, these resins were blended with Epon 828 to prepare 

bimodal vinyl-ester blends. Unfortunately, the solid contents were just too high for only Epon 

828 and the required amount of methacrylic acid to dissolve, even at 100 °C. Therefore, low 

styrene (Aldrich, 99%) contents (< 20 weight-percent) were added to completely dissolve the 

high molecular weight Epon 100XF. Addition of styrene during the methacrylation reaction 

normally causes gelation. Therefore, hydroquinone (Aldrich, 99%) in the amount of 1000 ppm 

based on the entire reaction contents was added to inhibit polymerization. 

Approximately 200-g Epon 828 was added to the reactor. Approximately 100-g styrene 

(20 weight-percent) was added to the reaction vessel. The reaction vessel was sealed and stirred 

vigorously with the mechanical mixer. While heating to 90 °C, the desired content of Epon 

100XF was added to the reactor in aliquots over the course of 1 hr to prevent excessive clumping 
5

of the solid Epon 100XF. In addition, 1.01 × the stoichiometric amount of methacrylic acid 

along with 1 weight-percent AMC-2 catalyst (based on Epon and methacrylic acid masses only) 

were added to the solution. The reaction temperature was kept in the range of 90–95 °C, while 

using the cooling coils for cooling as needed. Once the acid number was ~5, the reaction was 

ended. 

Bimodal blends of vinyl ester were prepared with the specific mass ratios of 828/1001F 29/71, 

828/1004F 42/58, 828/1007F 50/50, and 828/1009F 50/50. These mass ratios only compare the 

low and high vinyl-ester weight fractions and are independent of styrene content. The styrene 

content in vinyl-ester resins is listed as weight percentage of the entire resin. Bimodal blends 

with higher vinyl-ester 828 weight fractions were also desired for this study. These were 

prepared by mixing a specific amount of bimodal blend with the desired amount of pure vinyl 

ester 828 and styrene. 

Derakane 411-350 and 441-400 resins were provided by the Dow Chemical Company 

(Channahon, IL) and used as received. Styrene content and viscosity were provided by the 

manufacturer and verified by measurement. Derakane 411-350 is a recent replacement for 

Derakane 411-C50, and the resin and polymer properties are comparable (6). 

Fourier Transform Infrared Spectroscopy (FTIR) and near-IR were used to measure the 

concentration of unreacted epoxides and attached methacrylate groups (figure 2). A Thermo 

Nicolet Nexus 670 FTIR was used in absorbance mode, taking 16 scans per spectrum with a 

resolution of 4 cm-1. The raw FTIR spectra of these resins showed that the peaks representing 

the epoxide groups (6066 cm-1, 4530 cm-1, 917 cm-1) were no longer visible after reaction, and 

methacrylate groups (6160 cm-1, 942 cm-1) were present in the resin (7, 26). Interference with 

other peaks prevented determining the extent of reaction over 95% conversion, but other 

characterization techniques were also used to quantify the extent of reaction. These results 

indicate that the epoxide groups reacted to near completion with methacrylic acid. 

Size exclusion chromatography (SEC) was run on styrene and the vinyl-ester resins to determine 

vinyl-ester molecular weight and styrene content in the bimodal blends (figure 3). A Waters 515 

GPC was used with two 30-cm long, 7.5-mm diameter, 5-μm styrene-divinyl benzene columns 

in series. The columns were equilibrated and run at 45 °C using tetrahydrofuran (Aldrich) as the 

elution solvent at a flow rate of 1 mL/minute. The column effluent was monitored by two 

detectors operating at 25 °C: a Waters 2410 refractive index detector and a Waters 2487 dual 

absorbance detector operating at 270 and 254 nm (absorbed by phenyl rings). Samples were 

prepared by dissolving 2-mg sample in 1-mL THF. Because high molecular weight species 

cannot diffuse into the packing, they elute first from the column, while lower molecular weight 

species elute later (27). Figure 3 shows that these blends do indeed have bimodal molecular 

weight distributions. The peak at 14.5 min represents the vinyl-ester 828 fraction of the resin, 

while the peak at lower elution times represents the higher molecular weight vinyl-ester 
6

 0.1

 0.2

 0.3

 0.4

 0.5

 0.6

 0.7

 0.8

 0.9

 1.0

 1.1

 1.2

 1.3

 1.4

 1.5

Absorbance

 1000 800 

Wavenumbers (cm-1)

Figure 2. The FTIR spectra of Epon/methacrylic acid and the resulting vinyl ester formed after complete 

reaction. 

10 12 14 16 18 20

Time (min)

VE 828

VE 828/1004F 42/58

VE 828/1009F 54/46

Figure 3. SEC chromatographs of vinyl ester 828 and representative vinylester bimodal blends. The curves are vertically offset for clarity. 

Epoxy peak, 

917 cm-1

Methacrylate 

peak, 940 cm-1

t=0 

t=1.5 hrs 
7

monomers. The peak representing Epon 828 appeared at 15.5 min. The lower retention time of 

the vinyl-ester peak (14.5 min) indicates that the epoxy reacted with methacrylic acid to form a 

higher molecular weight species (i.e., vinyl ester). The peak at 18 min represents low molecular 

weight species, such as styrene, methacrylic acid, and catalysts. Previous work has shown that if 

a significant amount of epoxy homopolymerization occurred, a broad peak appearing at 10 min 

and lower elution times would appear (27, 28). Because no such peak appeared in our prepared 

vinyl-esters, we conclude that no epoxy homopolymerization occurred. SEC analysis of the 

methacrylation of epoxies as a function of time showed that the peak at 18 min decreased with 

reaction time, while the vinyl-ester/epoxy peaks shifted to lower elution times. The peak at 

18 min decreased to less than 95% of its original size when the methacrylation was run without 

styrene, as was done when preparing vinyl ester 828. These results conclusively show that Epon 

reacted with the methacrylic acid to form higher molecular weight vinyl-ester monomers. 

To measure the molecular weights of vinyl-ester resins using SEC, the molecular weight as a 

function of retention time was calibrated using Epon resin samples (29). The number average 

molecular weight of the Epon 100XF resins is known through epoxy titration results. The 

calibration curve relating peak retention time to Epon molecular weight was constructed and 

used to calculate the number average molecular weights of the vinyl-ester peaks. For the 

bimodal blends, the relative areas of the peaks and the two different number average molecular 

weights were used to simply calculate the number average (Mn) and weight average molecular 

weights (Mw): 

high low

high high low low

n n n

n M n M

M

+

+ = , (1) 

and 

high high low low

high high low low

w n M n M

n M n M

M

+

+ =

2 2

, (2) 

where the subscripts high and low refer to the high and low vinyl-ester molecular weight 

species/peak, respectively, M is the molecular weight as determined by the Epon calibration, and 

n is the number of moles. Table 2 shows that there was good agreement between experimental 

and literature values (20). Chromatography gives the volume ratio of low and high molecular 

weight species, from which the molar ratios can be calculated. The Epon standards used had a 

narrow distribution of molecular weights. The effect of these distributions was ignored in 

calculating the vinyl-ester molecular weights. Because we are using the same Epon standards to 

prepare the vinyl-ester monomers, there will be little error in the calculation of Mn, but the 

distribution of molecular weights in the Epon standards should yield significant error in the 

weight average molecular weight. We expect this error to be no more than 10% in the 

calculation of Mw. 
8

Table 2. The vinyl-ester molecular weights and styrene contents as determined using NMR and SEC along 

with literature/known values. The styrene contents listed for the bimodal blends of vinyl ester 

828/100XF are the experimentally determined values and the initial weight fraction styrene (known) 

in the reaction mixture of the prepared vinyl ester. 

Resin Exp. VE Mw

(g/mol) 

Exp. VE Mn

(g/mol) 

Literature 

VE Mn

(g/mol) 

Exp. Styrene 

Content 

(weight-percent) 

Literature/Known 

Styrene Content 

(weight-percent) 

Derakane 411-C50 1320 ± 60 880 ± 50 910 [5,6] 43 ± 3 45-50 [5,6]

Derakane 411-350 1540 ± 60 960 ± 50 NA 45 ± 3 45 [6]

Derakane 441-400 720 ± 50 700 ± 20 700 [6,7] 29 ± 3 28 [6,7]

828 + 30% styrene 550 ± 50 540 ± 10 540a 29 ± 3 30 a

834 740 ± 50 680 ± 10 630-730a NA NA 

836 864 ± 50 790 ± 10 750-840a NA NA 

828/1001F 29/71 867 ± 60 802 ± 20 NA 16.5 ± 2 16.6a

828/1004F 42/58 1370 ± 70 933 ± 20 NA 16.5 ± 2 16.6a

828/1007F 53/47 2257 ± 90 920 ± 30 NA 19.5 ± 2 19.5a

828/1009F 50/50 3130 ± 100 987 ± 20 NA 19.5 ± 2 19.6a

a

Values were taken from product specifications sheets. 

Notes: VE = vinyl ester, and NA = not available. 

SEC shows that pure styrene had a single peak at 18 min. This peak appeared at the same 

retention time regardless of the vinyl-ester resin used. The styrene content was calculated by 

measuring the area of the low molecular weight peak at 18 min relative to the area of the vinylester peaks from 12–16 min (figure 3). However, pure vinyl ester 828 also had a small peak at 

18 min as a result of catalyst and unreacted methacrylic acid. The effect of these components 

were small, but were removed anyway using a calibration procedure. Vinyl ester 828 standards 

with 30–50 weight-percent styrene were prepared and tested to calibrate SEC results. Using this 

calibration curve, the styrene content in commercial resins was measured. In all cases, the 

results agreed within 1% of the amount of styrene added to the bimodal resin system (table 2). 

Nuclear magnetic resonance (NMR) spectroscopy was run on the Epon resins, the prepared vinyl 

esters, and commercial vinyl-ester resins to verify the extent of methacrylation, styrene content, 

and vinyl-ester molecular weight. A Bruker (Billerica, MA) 600-MHz spectrometer with 

spectral window of ±2000 Hz, 16 scans at 293 K, and 90° pulse width was used. The method 

used to analyze the vinyl ester is described in the literature (5, 7). The internal standards for 

vinyl ester are the 4 methylene protons and the 6 methyl protons of the methacrylate groups per 

vinyl ester (figure 4). The area per proton for these standards should be in agreement. The value 

of n for the vinyl ester (figure 1) is calculated based on the area of the 5n+10 isopropyl protons, 

the 8n+8 phenyl protons, or the 6n+6 DGEBA methyl protons. These values of n resulting from 

all three standards were always equal within 3% error. In addition, the calculated values of n

were always within experimental error of the values calculated using epoxy titration. The 

styrene content was calculated by measuring the relative area of the styrene methylene protons 

(5.2 and 5.8 ppm) to the internal standards. The extent of methacrylation was determined by 

measuring the height of the three epoxide peaks at 3.33, 2.88, and 2.73 ppm (26, 30) relative to 

the heights of the phenyl protons and the DGEBA methyl protons before and after 

methacrylation. In all cases, the epoxide peaks were nearly gone, indicating epoxide 
9

CH2 C

CH3

C

O

CH2 C O

CH3

C

O

O CH2

OH

CH CH2 C CH2

CH3

C

O

CH2 O

OH

CH CH2 C CH2

CH3

C

O

CH O

OH

CH2 CH2 O

CH3

CH3

CH O C

OH

CH2 CH2 O

CH3

CH3

O C

m

e e

b b a

a

c

c c c c c g

g

g

g

CH2 C

CH3

C

O

CH2 C O

CH3

C

O

O CH2

OH

CH CH2 C CH2

CH3

C

O

CH2 O

OH

CH CH2 C CH2

CH3

C

O

CH O

OH

CH2 CH2 O

CH3

CH3

CH O C

OH

CH2 CH2 O

CH3

CH3

O C

m

CH2 C

CH3

C

O

CH2 C O

CH3

C

O

O CH2

OH

CH CH2 C CH2

CH3

C

O

CH2 O

OH

CH CH2 C CH2

CH3

C

O

CH O

OH

CH2 CH2 O

CH3

CH3

CH O C

OH

CH2 CH2 O

CH3

CH3

O C

m

e e

b b a

a

c

c c c c c g

g

g

g

(B) 

CH=CH2

f d

h

h

h

CH=CH2

f d

h

h

h (C) 

8 7 6 5 4 3 2 1 0

ppm

(A) 

g 

a 

c 

c 

d d 

e e 

h 

g 

f 

Figure 4. The NMR spectrum of (A) vinyl ester 828 with 50 weight-percent styrene, 

with the peaks labeled to correspond to chemical groups of (B) vinyl-ester 

monomer and (C) styrene (5). Note the definition of m (m = 1 represents 

the lowest possible value of m for DGEBA-based resins). 

conversions greater than 99%, verifying the FTIR results. Derakane 411-350 and 441-400 were 

analyzed in this manner along with the vinyl ester 828 and bimodal blends of vinyl-ester resins. 

The experimental results for Derakane resins and the vinyl ester 828 resin were similar to the 

literature results (table 2) (5–7). Table 2 shows that the agreement between literature and 

experiment was good. Furthermore, vinyl ester 828 with 30 weight-percent styrene was 

determined to have 29 weight-percent styrene using NMR, which proves the validity of this 

method.
10

2.2 Fatty-Acid Monomer Preparation 

A number of synthetic procedures have been established for making fatty acid-based monomers 

to be used as the reactive diluent in vinyl-ester resins (31). Figure 5 depicts the synthetic route 

used to form methacrylated fatty acid (MFA). The carboxylic acid of fatty acids undergoes a 

simple addition reaction with the epoxide group of glycidyl methacrylate (GM) (Aldrich, 

Milwaukee, WI) to form a single product. Stoichiometric quantities of the reactants were mixed 

together and reacted at 70 °C for 2.5 hr using 1 weight-percent AMC-2 catalyst. FTIR and 1HNMR (250.13 MHz, spectral window of ±2000 Hz, 0.427 Hz/pt digital resolution, 16 scans at 

293 K, 90° pulse width) with a Bruker AC250 spectrometer showed that methacrylation of the 

fatty acids went to completion. Each MFA contains one terminal polymerizable unsaturation site 

per molecule. In this way, the fatty-acid monomers act as chain extenders, analogous to styrene, 

in vinyl-ester resins. Various fatty-acids were used to prepare MFA monomers (table 3). Short 

fatty acid chains result in better polymer properties than any long fatty acids (18), but are more 

expensive. Therefore, moderate chain lengths, such as lauric acid, represent a good balance 

between properties and cost. 

Figure 5. The reaction of glycidyl methacrylate and oleic acid to produce the MOA monomer. 

2.3 Resin Formulation 

Fatty acid-based vinyl ester resins (FA-VE) were formulated using 55 weight-percent and 

65 weight-percent vinyl-ester 828 monomers, 0–45 weight-percent styrene (Aldrich 99%), and 

0–45 weight-percent MFA. MFA monomers with fatty acid chain lengths ranging from 4 to 18 

carbon atoms were used with 0–2 unsaturation sites along the fatty acid backbone. 

OH

O

O

O

O

O

O

OH

O

O

+ 

1 wt% AMC-2 

T = 70o

C 
11

Table 3. The fatty acids used to make MFA monomers. 

Fatty Acid Chain Length Unsaturation Sites MFA 

Butyric acid 4 0 MBut 

Hexanoic (Caproic) acid 6 0 MHex 

Octanoic (Caprylic) acid 8 0 MOct 

Decanoic acid 10 0 MDec 

Lauric acid 12 0 MLau 

Myristic acid 14 0 MMyr 

Palmitic acid 16 0 MPalm 

Stearic acid 18 0 MSA 

Oleic acid 18 1 MOA 

Linoleic acid 18 2 MLin 

Resins using bimodal blends of vinyl ester 828/1001F 29/71, 828/1004F 42/58, 828/1007F 

50/50, and 828/1009F 50/50 were formulated. In addition, the viscosities of resins with higher 

vinyl-ester 828 fractions were analyzed by adding pure vinyl ester 828 to these blends. Styrene 

contents ranging from 20–55 weight-percent were used for all blends. Vinyl ester 834 and 836 

were blended with 20–55 weight-percent styrene as a baseline to determine the effect of bimodal 

blends on the viscosity. 

The vinyl-ester resins were heated for 10 min at 70 °C for easy pouring. After heating the vinyl 

ester, ~10 g were carefully poured into a 20-mL scintillation vial. Based on this vinyl ester 

content, the amounts of other monomers were calculated according to their particular 

formulation. If MFA was used, it was added next. Lastly, the styrene was added to minimize 

VOC mass loss during mixing. After all of the monomers were added to the vial, it was heated 

at 70 °C for 5 min and then manually stirred with a glass stirring rod to dissolve the components. 

The clarity of the solution was noted. 

2.4 Rheological Characterization 

2.4.1 Steady Flow Experiments 

The viscosities of the resins were measured using a Brookfield digital viscometer in Couette 

geometry (i.e., concentric cylinders). Approximately 8 ml of the sample were placed into the 

sample holder. Spindle 21 was used in all cases because of its large diameter of 1.5 cm, which 

allows the measurement of the low viscosity samples. Because this viscometer provides more 

reliable numbers when the torque applied is near the middle of the instrument’s range, the shear 

rate was varied, depending on the sample viscosity, to do this. As a result, the rotation rate was 

varied from 2.5 rpm to 100 rpm depending on the sample viscosity. All samples were run at 

30 °C. The bimodal blends 828/1001F 29/71, 828/1004F 42/58, 828/1007F 52.5/47.5, and 

828/1009F 50/50 were tested. Each bimodal blend system was also tested using two higher 

vinyl-ester 828 weight fractions to examine the effect of molecular weight and polydispersity on 

the resin viscosity. The vinyl-ester resins were prepared with styrene contents of 20, 30, 35, and 

45 weight-percent. The viscosities of selected samples were also measured using a TA 
12

Instruments (New Castle, DE) AR2000 Rheometer in steady shear flow experiments using 

parallel plate geometry (40-mm plates) to prove that the resins have Newtonian behavior. The 

shear rate was increased from 1 s-1 to 3000 s-1 and then decreased back to 1 s-1, and 10 

measurements were taken per decade. At a given shear rate, the shear stress was measured every 

2 s. The shear rate and viscosity were recorded when the shear rate stabilized to within 5% 

tolerance for three consecutive intervals. 

2.4.2 Temperature Experiments 

Selected vinyl-ester resins with different molecular weights, reactive diluent, and diluent content 

were used to determine the effect of temperature on vinyl-ester resins. All resins were blended 

with diluent contents ranging from 20–55 weight-percent. The viscosity-temperature 

dependence of pure vinyl ester 828 was also measured. 

The resin viscosity was measured using a TA Instruments AR2000 Rheometer in steady shear 

flow experiments using parallel plate geometry (40-mm plates). The temperature was 

equilibrated at 30 °C for 5 min. The viscosity was measured at shear rates of 30–100 s-1 with 

4 points per decade. The shear rate and viscosity were recorded when the shear rate stabilized to 

within 5% tolerance for three consecutive points. The temperature was incremented by 2.5°, and 

the system was allowed to equilibrate for 3 min. This procedure was repeated until the 

temperature reached 60 °C. The temperature was then decreased by increments of 10–30 °C 

with 5 min equilibration times, and the viscosity was measured. In most cases, there was no 

viscosity-temperature hysteresis. This indicates that the equilibration times were sufficient for 

thermal stabilization. However, in the few cases where hysteresis did occur, the viscosity was 

always higher when decreasing the temperature. This indicates that the resin polymerized 

slightly upon heating. 

3. Results 

3.1 Viscosity of Bimodal Blends of Vinyl-Ester Monomers 

3.1.1 Effect of Vinyl-Ester Molecular Weight and Styrene Content 

Both the viscometer and rheometer results showed that the viscosity of the resins did not depend 

on the shear rate and no shear history was observed. Therefore, the vinyl-ester resins used in this 

work are Newtonian, nonpolymeric fluids (32). 

The viscosity decreased as the molecular weight of the bimodal blend decreased (figure 6). This 

result was expected because the viscosity of epoxy resins increases with the molecular weight of 

the monomer (20). A decrease in the degrees of freedom of the vinyl-ester monomers caused 

this viscosity increase with molecular weight (33). The viscosity decreased exponentially with 

styrene content for all bimodal blends (figure 7). Styrene is a nonpolar small molecule that 
13

Figure 6. The resin viscosity as a function of VE Mn for different styrene contents, 

showing that resin viscosity is normalized by VE Mn. 

Figure 7. The resin viscosity as a function of styrene content for various VE Mn, 

showing that styrene content affected the resin viscosity to the same extent 

for all bimodal resins with the same Mn, and the effect of styrene content on 

resin viscosity decreased as Mn decreased. 

10

100

1000

10000

100000

500 600 700 800 900 1000 1100

VE Mn (g/mol)

Resin Viscosity (cP)

1001F

1004F

1007F

1009F

20% styrene 

30% styrene

45% styrene

35% styrene

1

10

100

1000

10000

100000

10 20 30 40 50

Styrene Content (w t%)

Resin Viscosity (c

P

1001F

1004F

1007F

1009F

Mn = 920 g/mol 

Mn = 580 g/mol 

Mn = 660 g/mol 
14

basically acts as a solvent and increases the degrees of freedom and decreases the intermolecular 

interactions among the larger and more polar vinyl-ester monomers. 

The number average molecular weight of the vinyl-ester monomers governed the resin viscosity. 

For all bimodal blends, the resin viscosity was a single function of the number average molecular 

weight of the vinyl-ester monomers for constant styrene contents (figure 6). In other words, it 

did not matter which bimodal blend was used (e.g., 1001F vs. 1009F); only the overall Mn of the 

vinyl-ester monomers affected the resin viscosity. On the other hand, similar results were not 

found when normalizing with the weight average molecular weight (figure 8). Vinyl-ester resins 

using 1001F, for example, had a different dependence on Mw than vinyl-ester resins using 

1004F. 

100

1000

10000

100000

0 1000 2000 3000 4000

VE Mw (g/mol)

Resin Viscosity (cP)

1001F

1004F

1007F

1009F

Figure 8. The resin viscosity as a function of VE Mw for resins containing 

20 weight-percent styrene, showing that resin viscosity is not 

normalized by VE Mw. 

For polymer melts, the viscosity depends on Mw in a power-law relationship, as shown by Rouse 

and Reptation theory (33). The Mark-Houwink-Sakurada equation shows that the viscosity of 

dilute polymer solutions depends on the viscosity average molecular weight, which is between 

the values of Mn and Mw, in a power law-fashion (29, 33). When using a power-law relationship 

to relate the vinyl-ester resin viscosity to the molecular weight, the power-law exponent had a 

value between 5 and 10, which is much higher than that for dilute polymer solutions (<1) and for 

polymer melts (1 or 3.4) (29, 33). The resins tested in this work were monomers or small 

polymers with a very low degree of polymerization, and they were not in dilute solution. In 

dilute solution theory, the dissolved polymers interact with the solvent only and increase the 
15

viscosity through these interactions. In concentrated vinyl-ester resins, vinyl-ester monomers 

interact with styrene and other vinyl-ester monomers to produce frictional forces. Therefore, 

dilute solution theory is not applicable. The physics and mathematics of concentrated solutions 

are not well understood (34). Analysis of literature data shows that the bulk viscosities of 

C5-C16 alkanes and alcohols at 25 °C depend on molecular weight in a power-law relationship 

with exponents of 2.3 and 1.8, respectively (figure 9) (35), which are considerably higher than 

the value of 1 Rouse theory predicts for nonentangled melts. Rouse theory failed for the vinylesters used in this work because these molecules are Newtonian, and Rouse theory only applies 

to viscoelastic polymer melts (32, 33). It is possible that the viscosity of these resins will behave 

according to Rouse theory when no styrene is present in the resin or for higher molecular weight 

vinyl esters. Reptation is only valid for entangled polymer melts and is not appropriate for these 

low molecular weight vinyl-ester resins (33). Furthermore, weight average molecular weights 

only apply to polymeric molecules and would not apply to Newtonian vinyl-ester resins. 

Therefore, the number average molecular weight should govern the viscosity of vinyl-ester 

resins. 

y = 0.0011x1.8083

R2 = 0.9957

y = 1E-05x2.3144

R2 = 0.9887

0.1

1

10

100

0 50 100 150 200 250

MW (g/mol)

Viscosity (cP)

Alcohols

Alkanes

Power (Alcohols)

Power (Alkanes)

Figure 9. The bulk viscosities of C5-C16 alkanes and alcohols at 25 °C dependence 

on molecular weight (35). 

Styrene affected the resin viscosity in a consistent manner. For different vinyl-ester blends with 

the same number average molecular weight, the resin viscosity was a single decreasing 

exponential function of styrene content (figure 7). Styrene content affected all bimodal blends 

with similar Mn to the same extent regardless of the molecular weight of the individual 

components. However, as the overall number average molecular weight decreased, the effect of 

styrene content on the viscosity decreased, as shown by a less negative slope (figure 7). Most of 
16

the deviation from the best fit lines was due to the fact the various vinyl-ester resins do not have 

identical Mn (±10 g/mol), which can affect the viscosity by as much as 15%. 

The best fit lines in figure 6 represent a simple expression for the resin viscosity (η) as a function 

of number average molecular weight and styrene content. The form of this relationship is 

( ) ⎥

⎦

⎤ ⎢

⎣

⎡ = ⋅ ( ) , / M S

M S M Exp

e

n η VE Sty n η Sty

 [cP]

.

 (3) 

ηSty is the viscosity of styrene (i.e., 0.7 cP, the viscosity when the vinyl-ester molecular weight is 

zero) (35), and Me(S) is an effective molecular weight that is a function of styrene content, S. 

The form of the relationship implies that we are modeling the effect of the addition of a more 

viscous, higher molecular weight species on the styrene viscosity. As shown in figure 10, Me(S) 

is an exponential function of the styrene content: 

M S Exp[ S] e ( [g/mol]. (4) ) = 46.7 ⋅ 2.63⋅

A linear function of styrene weight fraction with a slope of 292.1 g/mol and an intercept of 17.9 

g/mol can approximate this function quite well in the range of the collected data, but its fit was 

not as good as equation 4. Combining equations 3 and 4 shows that the viscosity of bimodal 

blends was a predictable function of the Mn of the vinyl-ester monomers and the styrene weight 

fraction: 

( ) ( )⎥

⎦

⎤ ⎢

⎣

⎡

⋅ ⋅ = ⋅ Exp S

M S M Exp n

VE Sty n Sty 46.7 2.63 , η / η [cP]. (5) 

Equation 5 shows that resin viscosity was proportional to the exponential of the number average 

molecular weight and proportional to the exponential of the inverse of the exponential of the 

styrene fraction. However, because Me(S) can be approximated fairly well by a linear 

relationship, the viscosity was basically inversely proportional to a single exponential function of 

styrene content, as shown in figure 7. 

Interestingly styrene’s molecular weight (104 g/mol) falls right into the range of values of Me(S) 

that increased from 46.7 g/mol at ~0% styrene to 648 g/mol at ~100% styrene. Because of the 

exponential dependence, small changes in styrene content had a larger effect on the resin 

viscosity for low values of Me(S) than they did for high values of Me(S), as can be seen in figures 

6 and 7. The numerical coefficients in equations 4 and 5 are fitting parameters that physically 

relate how well styrene can reduce the interactions among vinyl-ester monomers and represent 

the ratio of the pure vinyl-ester viscosity to that of styrene. Rather than examining the effect of 

polarity, etc., Me(S) allows for an even comparison of vinyl-ester molecular weight to an 

effective styrene molecular weight. In other words, at high styrene concentrations, the vinylester monomers would need to have at least a molecular weight on the order of 650 g/mol 
17

y = 46.676e2.6295x

R2 = 0.9993

10

100

1000

0 0.1 0.2 0.3 0.4 0.5

Styrene Mass Fraction, S (w/w)

Me(S) (g/mol)

Figure 10. The dependence of Me(S) on the styrene weight fraction 

in bimodal blends of vinyl-ester resins. 

for small changes in styrene content to have a significant effect on the viscosity. Because vinylester monomers are more polar in nature than styrene, the value of Me(S) at low styrene 

concentrations was less than the molecular weight of pure styrene. This means that hypothetical 

vinyl-ester monomers with lower molecular weights than styrene could still raise the viscosity of 

the resin because of polar interactions. 

When the resin viscosity is too high, the pressure driving force in liquid molding operations is 

not enough to push the viscous resin into all of the gaps in between the fibers (glass, carbon, flax, 

etc.) before the resin cures. Consequently, viscous resins produce composite parts with high 

void contents. To reduce the void contents, long production times are necessary. As a result, a 

viscosity of 500 cP is considered about the maximum for most inexpensive liquid molding 

techniques (1, 22). Equation 5 can be rearranged to solve for the minimum styrene content 

required to achieve such a viscosity for given values of Mn. The minimum styrene content 

increased approximately linearly with the number average molecular weight of the vinyl-ester 

monomers. For example, to formulate a resin with only 30 weight-percent styrene, an Mn of 

675 g/mol can be achieved by blending as much as 78 weight-percent vinyl ester 828 with 

22 weight-percent vinyl ester 1009F or as little as 62 weight-percent vinyl ester 828 with 

38 weight-percent 1001F. In addition, solving equation 8 shows that a minimum styrene content 

of 20 weight-percent is necessary for the lowest molecular weight DGEBA-based vinyl ester 

(520 g/mol) to achieve a maximum viscosity of 500 cP. Therefore, there is a significant limit to 

the amount that bimodal blends can reduce styrene emissions in vinyl-ester resins (i.e., no less 

than 20 weight-percent styrene in vinyl-ester resins). 
18

3.1.2 Effect of Temperature 

The viscosity of most chemical species follows an Arrhenius relationship (36): 

⎟

⎠

⎞ ⎜

⎝

⎛ = ⋅ RT

E

η A exp , (6) 

where A is the pre-factor, E is the activation energy for viscous flow, R is the ideal gas constant, 

and T is the absolute temperature. Taking the natural logarithm of both sides of equation 6 yields 

RT

E Ln(η) = Ln(A) + . (7) 

Therefore, a plot of Ln(η) vs. 1/T should be a line with a slope equal to E/R and an intercept of 

Ln(A). Figure 11 shows that this plot is indeed linear for vinyl-ester resins. When there was 

viscosity-temperature hysteresis, the viscosity usually deviated from linearity at 55 °C or so. 

These points at 55 °C and higher were omitted from analysis because they did not represent the 

viscosity of the monomeric resin. 

0.1

1

10

100

1000

0.0029 0.003 0.0031 0.0032 0.0033 0.0034

1/T (1/K)

Viscosity (Pa*s)

VE 828/1004F 43/57

VE 828/1007F 75/25

VE 828

Figure 11. The viscosity-temperature dependence is Arrhenius for vinyl-ester resins with 

20 weight-percent styrene. 

The activation energy for viscous flow and the pre-factors were calculated from the Arrhenius 

plots. The activation energy decreased linearly as a function of styrene content (figure 12): 

EVE / Sty = −186* S +114.5[kJ], (8) 

where S is the styrene weight fraction. This indicates the sensitivity to temperature decreased as 

the styrene content increased. This expression is only valid through 50 weight-percent styrene, 

considering that over 61 weight-percent styrene, the calculated activation energy is negative. 

This would indicate that the viscosity should become more dependent on styrene content at 
19

y = -186.12x + 114.54

R2 = 0.958

0

20

40

60

80

100

120

140

0 0.1 0.2 0.3 0.4 0.5 0.6

Styrene Content (wt. fraction)

E (kJ/mol)

Figure 12. The activation energy for viscous flow as a function of styrene content in 

vinyl-ester resins. 

higher styrene contents, which is not physically reasonable. The activation energy decreased 

with styrene content because both styrene and temperature reduce viscosity by disrupting 

intermolecular interactions. As the styrene content increased, it is easier for molecules to slide 

by each other, thereby reducing the activation energy for viscous flow. The range of activation 

energies measured is typical for these monomeric resins (37). The pre-factor increased 

exponentially as a function of styrene content in the resin (figure 13): 

A Exp( S ) VE Sty = ⋅ ⋅ ⋅ − 3.67 10 59 18

/ [Pa*s]. (9) 

This factor was expected to increase because it is related to the frequency of interactions, which 

should increase given that high styrene contents produce resins with lower viscosities. 

Calculating Α involves extrapolating to infinite temperature. An accurate measurement of this 

pre-factor is very difficult and is the main cause for the scatter in the results. The activation 

energy and pre-factor were not functions of vinyl-ester molecular weight (figure 14). Therefore, 

vinyl-ester molecular weight had no effect on the temperature dependence of the resin, but 

chemical differences in the resin did affect the temperature dependence of the viscosity (37). 

Combining equations 6, 8, and 9 yields the viscosity-temperature model: 

( ) ⎟

⎠

⎞ ⎜

⎝

⎛ − ⋅ + + ⋅ ⋅ = ⋅ ⋅ −

RT

S S RT

VE Sty S T Exp 186120 114540 59 , 3.67 10 18 η / [cP]. (10) 
20

y = 3.6651E-18e5.9032E+01x

R2 = 9.4440E-01

1E-18

1E-16

1E-14

1E-12

1E-10

1E-08

1E-06

0.0001

0.01

1

0 0.1 0.2 0.3 0.4 0.5 0.6

Styrene Content (wt. fraction)

A (Pa*s)

Figure 13. The prefactor as a function of styrene content in vinyl-ester resins. 

0

10

20

30

40

50

60

70

80

90

500 600 700 800 900 1000

Mn (g/mol)

E (kJ/mol)

20% Sty

30% Sty

40% Sty

50% Sty

Figure 14. The activation energy for viscous flow as a function of vinyl-ester 

molecular weight. 
21

Therefore, the viscosity-temperature dependence of vinyl-ester resins was only a function of 

temperature and styrene content. Specifically, the viscosity was exponentially proportional to 

the inverse of the temperature. Equation 10 shows the viscosity was both exponentially 

proportional to the styrene content and to the inverse of the styrene content. 

3.1.3 Complete Viscosity Model for Vinyl-Ester/Styrene Resins 

We already have an accurate working model to predict resin viscosity as a function of styrene 

content and vinyl-ester molecular weight at 30 °C (equation 5). The viscosity-temperature model 

only needs to be used to determine the change in viscosity as a result of changing the 

temperature from 30 °C to a higher or lower temperature. The ratio of the viscosities at 

temperatures 2 and 1 (303 K) is 

( ) ⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛ ⋅ − + − ⋅ + =

1 / 2 1

2 186120 114540 186120 114540 , RT

S

RT

S S T Exp

η VE STy

η [cP]. (11) 

Given the viscosity at 30 oC (T1), the viscosity at an arbitrary temperature 2 is 

( ) ⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛ ⋅ − + − ⋅ + = ⋅

2 1

2 / 1

186120 114540 186120 114540 , RTS

RT

S S T Exp η VE Sty η [cP]. (12) 

Substituting in the viscosity model at 30 °C for η1 (equation 5) gives the overall viscosity model: 

( ) ( ) ⎥

⎥

⎦

⎤

⎢

⎢

⎣

⎡

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛ − − ⋅ + ⋅ ⎥

⎦

⎤ ⎢

⎣

⎡

⋅ ⋅ = ⋅ R T K

S Exp Exp S

M

S T M Exp n VE Sty n Sty 303

186120 114540 1 1

46.7 2.63 , ,

2

η / η [cP]. (13) 

This empirical model allows calculation of the viscosity using only the styrene content, vinylester Mn, and temperature. This equation is accurate at temperatures ranging from 25 °C to 

60 °C, Mn ranging from 540–950 g/mol, and styrene contents ranging from 0 to 50 weightpercent. This model probably can be extrapolated to other parameter ranges with a good degree 

of accuracy, but is not valid at styrene contents approaching and above 60 weight-percent 

because of the zero and negative values of the activation energy calculated at these styrene 

contents. An advantage of using equation 13 rather than using equation 10 to calculate the resin 

viscosity as a function of temperature is that the latter equations use the pre-factor, which was 

calculated with less accuracy than the activation energy or the 30 °C viscosity model. This 

viscosity model for vinyl-ester/styrene resins can be used to predict the minimum styrene content 

necessary for a particular liquid molding operation. For example, table 4 shows the required 

styrene content for given operation temperatures and vinyl-ester molecular weights for a 

maximum allowable viscosity of 500 cP. Interestingly, 45 weight-percent styrene is only 

necessary for high molecular weight vinyl esters at fairly low operation temperatures. 
22

Table 4. The calculated styrene content as a function of vinyl-ester molecular weight and temperature that is 

required to achieve η = 500 cP for DGEBA-based vinyl esters. 

Temperature Required Styrene Content to have η = 500 cP in Weight-Percent (°C) Mn = 520 g/mol Mn = 650 g/mol Mn = 800 g/mol Mn = 950 g/mol 

20 25.7 33.0 39.8 45.4 

25 22.9 30.8 38.1 44.2 

30 20.1 28.5 36.4 43.0 

35 17.1 26.2 34.6 41.6 

3.2 Viscosity of Fatty Acid-Based Vinyl-Ester Resins 

3.2.1 Effect of Fatty-Acid Chemical Structure 

The viscosities of the MFA and the reactants used to produce these monomers were measured at 

30 °C using a Brookfield digital viscometer. The viscosities of oleic acid and linoleic acid were 

24 cP (fatty acids including decanoic acid and longer are crystalline solids at 30 °C), while the 

viscosity of glycidyl methacrylate was ~1 cP. The viscosities of MFA monomers (figure 15) 

were higher than their starting materials because MFA contain hydroxy-ester groups, which 

induce hydrogen bonding (37, 38). The viscosities of MFA monomers increased with chain 

length and as the level of unsaturation along the fatty acid backbone decreased. At 30 °C, the 

viscosity was found to be a function of fatty-acid chain length, lf: 

0.0111* 0.3857 * 5.145* 29.684 * 92.351 4 3 2 η MFA = l f − l f + l f − l f + [cP]. (14) 

The level of unsaturation affected the viscosity because these sites put kinks along the fatty-acid 

chain that increase the intermolecular spacing (39), thereby reducing intermolecular interactions 

and the viscosity. An unsaturation site is the equivalent of having a fatty-acid with 3.5 fewer 

carbon atoms according to figure 15: 

l f (U ) = l f − 3.5*U , (15) 

where U is the level of unsaturation. Initially, the viscosities of polyunsaturated MFAs were 

similar to that of monounsaturated MFAs. However, the viscosities of polyunsaturated MFAs 

increase over time. Therefore, it was impossible to quantify the effect of two or more 

unsaturation sites on MFA viscosity, and equation 15 is only valid for U ≤ 1. 

The viscosities of the vinyl-ester resins before cure are shown in figure 16. The viscosities of 

FA-VE resins are considerably higher than that of vinyl-ester/styrene resins (<100 cP) because 

fatty-acid monomers have considerably higher viscosities than styrene. The viscosity went 

through a minimum at 6–8 carbon atoms as the fatty-acid chain length increased. This occurred 

because two factors are affecting the resin viscosity. At short fatty-acid chain lengths, polar 

interactions/hydrogen bonding, due to the MFA hydroxyl group, dominate. However, at longer 

fatty-acid chain lengths, the concentration/effect of this hydroxyl group decreases and chain 
23

y = 0.0111x4 - 0.3857x3 + 5.145x2 - 29.684x + 92.351

R2

 = 1

0

10

20

30

40

50

60

70

80

90

0 5 10 15 20

Carbon Atoms in Fatty Acid

Viscosity (cP)

Saturated Acids

Mono-unsaturated acids

Figure 15. Viscosity of MFA monomers as a function of fatty-acid 

chain length. 

100

1000

10000

0 5 10 15 20

Number of Carbon Atoms

Viscosity (cP)

25% MFA

35% MFA

45% MFA

25% MOA

35% MOA

45% MOA

Figure 16. The viscosity of vinyl ester /MFA as a function of fatty-acid 

chain length at 25 °C. 
24

length effects dominate. Overall, it appears that fatty acids with chain lengths of 6–8 carbon 

atoms are ideal for producing low viscosity FA-VE resins. The presence of unsaturation sites 

along the fatty-acid backbone decreased the resin viscosity (figure 16), as they did for MFA 

monomers alone. 

The effect of fatty-acid content on vinyl-ester viscosity was quantified. Figures 17 and 18 show 

that the resin viscosity decreased exponentially with MFA content. Two functions were used to 

approximate the effect of MFA content on viscosity. The first was simply an exponential fit of 

the data (figure 17), which does a good job of fitting the data, except at low MFA contents. A 

function similar to the vinyl-ester/styrene viscosity model was also used (figure 18). This 

function does an excellent job of fitting the data with the added benefit of taking into account the 

effect of vinyl-ester molecular weight. Unfortunately, we are not able to prepare bimodal blends 

of vinyl-ester monomers with MFA as the only comonomer and therefore cannot verify the effect 

of bimodal molecular weight in this section. However, using blends of vinyl ester, MFA, and 

styrene, we will verify the form of this equation in a later section. 

y = 60899e-9.4091x

R2 = 0.9882

100

1000

10000

100000

0 0.1 0.2 0.3 0.4 0.5 0.6

MFA Content (wt%)

Resin Viscosity (cP)

Figure 17. Viscosity of vinyl-ester resin as a function of MFA content for 

MLau fatty-acids, showing the exponential fit to the data. 
25

100

1000

10000

100000

0 0.1 0.2 0.3 0.4 0.5 0.6

MFA Content (wt%)

Resin Viscosity (cP)

MLau - Exp

MLau - Model

MOA - Exp

MOA - Model

Figure 18. Viscosity of vinyl-ester resin as a function of MLau content, showing 

the exponential fit to the data. 

The best fit curves in figure 18 have the form 

( ) ⎥

⎦

⎤ ⎢

⎣

⎡ = ⋅ ( ) , *

/ M MFA

M MFA M Exp

e

n η VE MFA n η MFA [cP], (16) 

where MFA is the weight fraction of MFA monomer and η

*

MFA = 50 cP, which is the 

approximate viscosity for all MFA monomers at 30 °C. As for styrene, Me(MFA) was an 

exponential function of the MFA content: 

M MFA Exp[f (MFA)] e ( ) = 58.5 ⋅ [g/mol].

 (17) 

f(MFA) is an exact linear function of MFA viscosity (figure 19): 

f (η MFA ) = (2.9007 − 0.0063 ⋅η MFA )⋅ MFA. (18) 

Combining equations 14 and 18, f(MFA) can be related to the fatty-acid chain length and 

unsaturation level: 

f ( MFA ) = [2.9007 − 0.0063 ⋅(0.0111⋅l f − 0.3857 ⋅l f + 5.145 ⋅l f − 29.684 ⋅l f + 92.351)]⋅ MFA 4 3 2 η .(19) 

Combining equations 16, 17, and 18 gives the overall shows that the viscosity of bimodal blends 

was a predictable function of the Mn of the vinyl-ester monomers and the styrene weight 

fraction: 

( ) ( ) ( ) ⎥

⎦

⎤ ⎢

⎣

⎡

⋅ − ⋅ ⋅ = ⋅ Exp MFA

M MFA l U M Exp

MFA

n

VE MFA f n MFA

η

η η 58.5 2.9007 0.0063 , , , *

/

[cP] (20) 

.
26

y = -0.0063x + 2.9007

R2

 = 1

2.54

2.56

2.58

2.6

2.62

2.64

2.66

2.68

2.7

2.72

20 30 40 50 60

MFA Viscosity (cP)

MFA Coefficient - f(

ηMFA)

Figure 19. The MFA coefficient in equation 18 is a linear function of 

MFA viscosity. 

Therefore, the resin viscosity was proportional to the exponential of the inverse of the 

exponential of the MFA. This model is not likely to predict the up-turn in viscosity at short 

fatty-acid chain lengths as seen in figure 16. Overall, this empirical viscosity model does an 

excellent job of predicting the viscosity of vinyl-ester/MFA resins as a function of MFA content 

and MFA type. 

3.2.2 Effect of Temperature 

The viscosities of MFA-based vinyl esters follow an Arrhenius relationship. The activation 

energy for viscous flow and the pre-factors were calculated from the Arrhenius plots. The 

activation energy decreased linearly as a function of MFA content (figure 20) as it did for 

styrene. 

 6 114.1* 114. EVE / MFA = − MFA + [kJ]. (21) 

This indicates the sensitivity to temperature decreased as the MFA content increased. The three 

different monomers have very different viscosities alone and in vinyl-ester resins. Therefore, it 

is expected that all fatty acid-based vinyl esters will have activation energies that fall along this 

line. The activation energy was higher for a given content of MFA monomer relative to styrene, 

because of a lower molar content of MFA monomers. Equation 21 indicates that the activation 
27

y = -114.1x + 114.6

R2

 = 0.9877

0

20

40

60

80

100

120

140

0 0.1 0.2 0.3 0.4 0.5 0.6

MFA Content (wt%)

E (kJ/mol*K)

MHex

MLau

MOA

Figure 20. The activation energy as a function of diluent content and 

fatty-acid type. 

energy is ~0 for MFA monomers alone, which is not correct. Therefore, we expect this curve to 

level out as the MFA content is increased above 55 weight-percent. 

The pre-factor increased exponentially as a function of MFA content in the resin (figure 21): 

A Exp( MFA) VE MFA = ⋅ ⋅ ⋅ − 1.76 10 34.7 18

/ [Pa*s]. (22) 

This factor was expected to increase because it is related to the frequency of interactions, which 

should increase given that high MFA contents produce resins with lower viscosities. For this 

same reason, the value of A is lower for vinyl-ester/MFA resins relative to vinyl-ester/styrene 

resins because of the lower molar concentration of MFA monomers used in this work. 

Combining equations 6, 21, and 22, we obtain the viscosity of VE/MFA as a function of 

temperature: 

( ) ⎟

⎠

⎞ ⎜

⎝

⎛ − ⋅ + + ⋅ ⋅ = ⋅ ⋅ −

RT

MFA S RT

VE MFA T Exp 114100 114600 34.7 1.76 10 18 η / [cP]. (23) 

3.2.3 Complete Viscosity Model for Fatty Acid-Based Vinyl-Ester Resins 

As for vinyl-ester/styrene resins, the viscosity model for vinyl-ester/MFA resins was a simple 

combination of the effects of temperature (equation 21) and the effects of monomer content 

(equation 20), as well as monomer type: 
28

y = 1.76E-18e3.47E+01x

R2 = 9.949E-01

1.0000E-18

1.0000E-17

1.0000E-16

1.0000E-15

1.0000E-14

1.0000E-13

1.0000E-12

1.0000E-11

1.0000E-10

1.0000E-09

1.0000E-08

0 0.1 0.2 0.3 0.4 0.5 0.6

MFA Content (wt%)

Pre-exponential Constant, A (Pa*s)

MHex

MLau

MOA

Figure 21. The pre-exponential constant as a function of diluent content and 

fatty-acid type. 

( ) ( ) ( ) ⎥

⎦

⎤ ⎢

⎣

⎡

⋅ − ⋅ ⋅ = ⋅ Exp MFA

M MFA T l U Exp

MFA

n

VE MFA f MFA

η

η η 58.5 2.9011 0.0063 , , , *

/ . (24) 

 ⎥

⎦

⎤ ⎢

⎣

⎡

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛ − − ⋅ +

R T K

MFA Exp 303114100 114600 1 1 *

2

 [cP] 

The model allows for calculation of the vinyl-ester/MFA viscosity for any given temperature, 

vinyl-ester molecular weight, MFA chain length, MFA unsaturation level, and MFA content. 

Figure 22 shows the operating window for vinyl-ester/MFA resins as a function of temperature, 

fatty-acid chain length, and unsaturation level. Increasing temperature reduces the required 

MFA content, as was expected. Increasing the MFA chain length has little effect on the window 

until above chain lengths of 12 carbon atoms. Above which, the required MFA content 

increased severely with chain length. Unsaturation sites decreased the required MFA content, as 

expected. Interestingly, figure 22 shows that fatty acids of 8 carbon atoms minimize the required 

MFA content. Therefore, the viscosity model does predict the minimum in viscosity as a 

function of fatty-acid chain length (figure 16) at 6–8 carbon atoms. 

3.3 Viscosity of Vinyl-Ester Resins Using Both Fatty-Acid Monomers and Styrene as 

Reactive Diluents 

The viscosities of vinyl-ester resins using MFA were fairly high. Only a small number of 

formulations achieve viscosities of 500 cP or lower at 30 °C using less than 50 weight-percent 

MFA. To reduce the viscosity, blends of MFA and styrene were used as the reactive diluent in 
29

40

45

50

55

60

65

70

75

80

15 20 25 30 35 40

Temperature (oC)

Required MFA Content (wt%)

4:0

8:0

12:0

18:1

16:0

18:0

Figure 22. The minimum MFA content required to achieve vinyl-ester resin 

viscosities below 500 cP as a function of temperature, fatty acid chain 

length, and unsaturation level. The operating window is any MFA 

content above the curve. Legend: [carbon atoms: unsaturation sites]. 

vinyl ester 828. The viscosity of vinyl ester/MFA/styrene was measured at 30 °C and was found 

to decrease exponentially with styrene content for resins containing 55 weight-percent and 

65 weight-percent vinyl-ester 828 resin (figure 23, shown for MHex- based resins). Styrene 

contents of ~10% or more reduced the viscosity of vinyl-ester resins to the acceptable range 

(<500 cP is required for most liquid molding processes [1, 22]). Only low styrene contents are 

necessary to reduce the viscosities of vinyl-ester/MFA systems to acceptable processing 

conditions at 30 °C. 

The best fit curves in figure 23 were derived from a logarithmic rule of mixtures (37): 

exp[ ( ( )) ( ( ))] η VE / MFA / Sty = wS (D) ⋅ Ln η VE / Sty D + wMFA(D) ⋅ Ln η VE / MFA D [cP],

 (25) 

where wS(D) and wMFA(D) are the styrene and MFA weight fractions, respectively, of the total 

diluent content, D, and ηVE/Sty(D) and ηVE/MFA(D) are the viscosities of vinyl-ester/styrene and 

vinyl ester/MFA, respectively, evaluated at the total diluent weight fraction. This logarithmic 

rule of mixtures does an excellent job of predicting vinyl-ester/MFA/styrene resin viscosity as a 

function of composition. On the other hand, a simple rule of mixtures does not accurately fit the 

data, nor does the Fox equation (41). Furthermore, this model is able to accurately predict the 

effect of vinyl-ester molecular weight (figure 24) and temperature (figure 25) on resin viscosity. 
30

Figure 23. The viscosity of vinyl ester/MHex/styrene as a function of styrene content in the resin for 

resins containing 55 weight-percent and 65 weight-percent vinyl ester 828, along with 

model predictions. 

Figure 24. The experimental and model viscosities of vinyl ester/MHex/styrene as a function of 

vinyl-ester molecular weight and vinyl-ester content for resins containing 20 weightpercent styrene. 

1

10

100

1000

10000

0 10 20 30 40 50

Styrene Content (wt%)

Viscosity (cP)

65% VE - exp

65% VE - model

55% VE - exp

65% VE - model

500 cP

100

1000

10000

400 500 600 700 800 900 1000

VE Number Average Molecular Weight, Mn 

(g/mol)

Resin Viscosity (cP)

65% VE - exp

65% VE - model

55% VE - exp

55% VE - model
31

1

10

100

1000

10000

2.9E-03 3.0E-03 3.1E-03 3.2E-03 3.3E-03 3.4E-03

1/T (1/K)

Resin Viscosity (cP)

65/25/10 - Exp

65/25/10 - Mod

65/15/20 - Exp

65/15/20 - Mod

Figure 25. The experimental and model viscosities of VE/MLau/styrene as a function 

of temperature. 

Therefore, this overall viscosity model allows prediction of the viscosity of vinyl-ester resins 

based on the vinyl-ester molecular weight, styrene content, MFA chain length, MFA 

unsaturation level, MFA content, and temperature. 

Using this overall viscosity model, an operating window (i.e., viscosity below 500 cP) can be 

determined for a single variable while keeping the other variables constant. For instance, at 

30 °C, the required styrene content in vinyl-ester/MFA/styrene resins increases as the vinyl-ester 

molecular weight increases (figure 26). However, fatty-acid chain length does not have a strong 

affect until effective fatty acid lengths of 14 are reached. Therefore, using lauric acid to make 

fatty acid-based vinyl-ester resins will not result in significantly higher viscosities than hexanoic 

acid-based resins. 

4. Conclusions 

The viscosities of DGEBA-based vinyl-ester resins are predictable functions of reactive diluent 

content, type, temperature, and vinyl-ester molecular weight. The viscosity decreased 

exponentially with reactive diluent content. Therefore, changes in the diluent content have a 

larger effect on the resin viscosity at low diluent contents than they do at higher diluent contents. 

Two different types of reactive diluents affected the viscosity in the same manner, but not to the 

same extent. The viscosities of resins using both diluents were accurately predicted using a 

logarithmic rule of mixtures from the two-component viscosity functions. The viscosity 

increased exponentially and predictably as a function of the vinyl-ester number average 

molecular weight. Increasing the temperature decreased the viscosity in an Arrhenius manner. 
32

0

10

20

30

40

50

60

500 700 900 1100 1300 1500

VE Mn (g/mol)

Required Styrene Content (wt%)

65% VE, 4:0

65% VE, 18:1

65% VE, 18:0

55% VE, 4:0

55% VE, 18:1

55% VE, 18:0

55% VE, 4:0

45% VE, 18:1

45% VE, 18:0

Figure 26. The minimum styrene content required to achieve vinyl-ester resin 

viscosities below 500 cP as a function of fatty acid chain length, 

vinyl-ester content, and vinyl-ester molecular weight at 30 oC. The 

operating window is any styrene content above the curve for any 

molecular weight along the curve. Legend: [carbon atoms: 

unsaturation sites]. 

The activation energy for viscous flow decreased linearly as the diluent content increased, but 

was unaffected by vinyl-ester molecular weight, fatty-acid chain length, and unsaturation level. 

Overall, the resin viscosity can be modeled as simple functions of the resin temperature, 

vinyl-ester molecular weight, styrene content, MFA content, MFA chain length, and MFA 

unsaturation level, which are all known quantities for a formulated resin. 

This work has a number of potential applications: 

• determining the minimum VOC/HAP content for liquid molding operations, 

• design of low VOC/HAP resins, and 

• emissions modeling. 

Because of Environmental Protection Agency regulations, minimizing VOC/HAP emissions is a 

priority for facilities that work with composite materials. This work allows a predictive method 

for doing so. Considering that this work allows for viscosity prediction of bimodal blends of 

vinyl esters and fatty acid-based vinyl esters, this model can be used to determine the set of 

formulations that would meet the viscosity processing requirements. Emissions modeling work 

so far has shown that styrene emission rates from vinyl-esters are dependent on resin viscosity. 

Therefore, this model allows for better emissions modeling predictions. 
33

Although styrene is damaging to the environment, there is a minimum styrene content required 

in vinyl-ester/styrene resins to achieve acceptable viscosities for liquid molding. At 30 °C, 

20 weight-percent styrene is required in vinyl-ester/styrene resins. The minimum styrene content 

is much lower in fatty-acid-based resins because of the viscosity reduction associated with MFA 

monomers. The exact styrene content required is dependent on the processing temperature, 

fatty-acid monomer used, fatty-acid content, and vinyl-ester content used, but 5–10% is a typical 

minimum styrene content required. Therefore, both bimodal blends of vinyl-ester resins, and 

especially fatty-acid-based vinyl esters, can be used to reduce VOCs, maintaining low resin 

viscosities. 
34

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NO. OF 

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