journal of coatings technology and research, 2010, volume 7, number 5, pages 603-613

Soy-based UV-curable thiol–ene coatings

Zhigang Chen, Bret J. Chisholm, Radhika Patani,

Jennifer F. Wu, Shashi Fernando, Katie Jogodzinski,Dean C. Webster

� FSCT and OCCA 2010

Abstract Novel soy-based thiols and enes were syn-thesized and characterized. Then, soy-based thiol–eneUV-curable coatings were formulated and their coat-ing physiochemical properties were investigated indetail. The use of biorenewable resources, combinedwith environmentally friendly UV-curable technology,provides a ‘‘green + green’’ solution to the stricterregulations in the coatings industry. Novel soy-basedthiols and enes were synthesized through the Lewisacid-catalyzed ring opening reaction of epoxidizedsoybean oil with multifunctional thiols or hydroxylfunctional allyl compounds. FTIR and NMR confirmedthe formation of the target compounds. The soy-basedthiols and enes were formulated with petrochemical-based enes and thiols, respectively, to make thiol–eneUV-curable coatings. Typical coating film properties,thermal properties, and photopolymerization kineticsof these coatings were studied. Soy-based thiol–enecoatings having lower functionality thiols and eneshave poor UV curability and coating properties, whichwas attributed to the lower crosslink density. Soy-based thiols and enes with higher functionality can beUV-cured in combination with petrochemical-basedenes or thiols even without the presence of free radicalphotoinitiators. Better coating film properties wereobtained from these higher functionality thiol–enesystems that were toughened by commercial hyper-branched acrylates.

Keywords Biobased, Soybean oil, UV curable,Coating, Thiol–ene

Introduction

The anticipated depletion of fossil oil reserves andrising oil prices make the utilization of renewable rawmaterials a necessary step toward sustainable develop-ment.1 Soybean oil (SBO) is a bio-renewable resourcewith a high annual production in the US.2 Research onsoybean utilization has been of long-term interest inthe chemical industry in order to meet the need for bio-based materials.3–6 Commercialized SBO derivativessuch as epoxidized soybean oil (ESBO, generic struc-ture shown in Fig. 1) are important starting materialsfor the development and production of biobasedindustrial products such as adhesives, coatings, plast-icizers, lubricants, and composites.2,7–14

Curing of polymers by radiation is an efficient wayto turn suitable liquid, low viscosity reactive materialsinto solid materials having good properties, for appli-cations such as coatings, inks, adhesives, and dentalmaterials.15,16 Curing by light (either visible or ultra-violet, UV) is a form of radiation curing and is alsoknown as photopolymerization. Compared to conven-tional curing mechanisms, radiation curing technologyfeatures extremely fast cure (within minutes or evenseconds, compared to hours or days for conventionalsystems), very low energy consumption, and moreimportantly, is a solventless, ‘‘green’’ process. It is anenvironmentally friendly technology experiencingrapid development and a rapid increase in marketshare. Free radical radiation-curable technology basedon acrylate/methacrylate chemistry comprises over90% of the radiation-curable materials market. How-ever, a major drawback of acrylate/methacrylatechemistry is the inhibition of the photopolymerizationby ambient oxygen.15,17 Fortunately, thiol–ene chemistry

Z. Chen, B. J. Chisholm, R. Patani, J. F. Wu,S. Fernando, K. Jogodzinski, D. C. WebsterCenter for Nanoscale Science and Engineering,North Dakota State University, PO Box 6050, Fargo,ND 58108, USA

B. J. Chisholm, D. C. Webster (&)Department of Coatings and Polymeric Materials,North Dakota State University, PO Box 6050, Fargo,ND 58108, USAe-mail: [email protected]

J. Coat. Technol. Res., 7 (5) 603–613, 2010

DOI 10.1007/s11998-010-9241-x

603

has been developed that enables rapid photopolymer-ization through a unique step-growth polymerizationmechanism, during which ambient oxygen can beturned into a reactive species, as illustrated inScheme 1. Consequently, thiol–ene photochemistry isinsensitive to oxygen inhibition.15–17 This feature, inaddition to the excellent mechanical and physicalproperties of the resultant polymers, has attractedextensive research interest in thiol–ene photochemistryand materials in recent years.16,18–20 UV-curable thiol–ene materials have been explored as protective coat-ings, inks, and adhesives.17,19 Furthermore, the extraor-dinarily high refractive index of thiol–ene materialsenable high value-added applications such as coatingsfor optical lenses and fibers, and as adhesives forphotonic and electronic components.18,20,21

It is highly desirable to incorporate bio-renewablematerials into the ‘‘green’’ UV-curable technologies.Such a combination provides a ‘‘green + green’’ solu-tion to the stricter environmental regulations that thecoating industry is facing. SBO-based materials such asacrylated SBO and ESBO have been investigated inUV-curable materials.7,22–24 However, no previousresearch has been found on the utilization of SBO inthiol–ene photochemistry. The incorporation of SBOinto thiol–ene-based materials is expected to providebio-renewable, lower cost, UV-curable materials withinteresting properties and little or no environmentalimpact. This paper reports progress in the synthesisand characterization of soy-based UV-curable thiol–ene coating materials.

Experimental procedure

Materials used

The chemicals used, abbreviations, and their sourcesare listed in Tables 1 and 2. All chemicals were used asreceived.

Synthesis and characterization

Soy-based multifunctional thiols and enes were syn-thesized by reacting ESBO with thiols (GDMP,TMPMP, and PETMP, respectively) or enes (AAand TAE, respectively), in acetone, using 1 wt% BF3

solution as an acid catalyst. The reactions wereconducted at room temperature with magnetic stirring.The reactant mole ratio of ESBO to thiols or enes was1:4.4 in order to react all of the epoxy groups on theESBO. A typical synthesis procedure using the GDMPand ESBO reaction as an example is: 4.54 g ESBO,4.76 g GDMP, and 4.65 g acetone were charged into aconical flask under magnetic stirring. Then, 0.093 g BF3

solution was mixed with 4.65 g acetone in a beaker,and then this BF3 acetone solution was slowly chargedinto the conical flask. The completion of the reactionwas determined by the disappearance of the epoxypeaks of ESBO in the FTIR spectrum at 823 and843 cm�1. It took approximately 16 h for the comple-tion of GDMP and ESBO reaction, and 6–7 h for otherreactions. After reaction, the acetone and BF3 solutionwere stripped off using a rotary evaporator, undervacuum, at 50�C.

UV-curable coating formulation andcharacterization

To form films for coating physical property testing, theliquid materials were cast on aluminum Q-panels witha wire-wound drawdown rod to form a thin film with�50 lm thickness, followed by UV curing using aFusion LC6B Benchtop Conveyor with an F300 UVAlamp (UVA, intensity �1180 mW/cm2 measured byUV Power Puck� II from EIT Inc.), in air, at

R'

RS

R'

RS

R'

RS

R'

RS

R'

H

RS . +O2

OO.. RSH OOH

+ RS

RSH

+ RS .

Scheme 1: Illustration of free radical step-growth polymerization and oxygen scavenging mechanism of thiol–enephotopolymerization17

O

O

O

O

O

O

( C H 2)7

( C H 2)7

O O

( C H 2)4O

( C H 2)7 ( C H 2)7

( C H 2)4

O O

ESBO

Fig. 1: Generic structure of epoxidized soybean oil (ESBO)


604

Table 1: Chemicals used

Name Abbreviation Source Structure and description

Vikoflex 7170 ESBO Arkema Inc.

O

O

O

O

O

O

( C H 2 ) 7

( C H 2 ) 7

O O

( C H 2 ) 4O

( C H 2 ) 7 ( C H 2 ) 7

( C H 2 ) 4

O O

epoxidized soybean oil, MW 1000 g/mol,Oxirane oxygen 7% min, �4.4 epoxy

groups per ESBO molecule

THIOCURTM

PETMPPETMP Evans Chemetics LP

OO

O

O

O

CH2CH2SHO

CH2CH2SH

CH2CH2SH

O

O

HSCH2CH2

Pentaerythritol tetra-3-mercaptopropionate

THIOCURTM

TMPMPTMPMP

OO

O

O

O

CH2CH2SHO

CH2CH2SH

CH2CH2SH

Trimethylolpropane tri-3-mercaptopropionate

THIOCURTM

GDMPGDMP

O

O

O

O

HSCH2CH2 CH2CH2SH

Glycol di-3-mercaptopropionate

Allyl alcohol AA AldrichOH

Allyl triazine ATN

N

N

O O

O

Allyl isocyanurate AI

N

N

N

O

OO

Tri-allyl ether TAE O

O

OH

O

Tris[4-(vinyloxy)butyl] trimellitate

VE

OO

O

OO

O

O

OO

Boron trifluoridedibutyl etherate

BF3 O

BF3

Lewis acid solution

Irgacure 2022 PI Ciba SpecialtyChemicals

1:4 (by weight) photoinitiator blend of Phenylbis(2,4,6-trimethylbenzoyl)-phosphine oxide) and

2-Hydroxy-2-methyl-1- phenyl-1-propanone


605

RH � 25%. The general curing protocol was 1 passthrough the lamp with a conveyor belt speed of10 in./min. The coatings were tested after being condi-tioned under ambient laboratory conditions for at least24 h. An automated surface energy measurement unit,manufactured by Symyx Discovery Tools, Inc. and FirstTen Angstroms, was used to measure the water contactangle on the UV-cured thin film materials. Droplets ofwater were deposited on the film surface. A CCDcamera was used to image the droplets. Automatedimage analysis was used to determine the contact angle.Three drops of water were used for each measurementand the average contact angle values were reported.The standard deviation of water contact angle mea-surements is ca. ±2�.

Hardness testing was performed using a BYKGardener pendulum hardness tester in Konig mode.The hardness was reported in seconds. Methyl ethylketone (MEK) double rubs were used to assess thesolvent resistance of the cured coatings. A 26 ozhammer with five layers of cheesecloth wrappedaround the hammerhead was soaked in MEK forrubbing. After 100 double rubs, the cloth was re-wettedwith MEK. The number of double rubs needed to marthe coating surface and to expose the substrate wererecorded and reported. Falling weight, direct impacttesting was carried out according to ASTM D2794 witha 2 lb weight. The starting height was increased untilthe film was damaged. The maximum height at whichthe film was intact was recorded. The results werereported in inch-lbs. Tensile tests were performedusing an Instron 5542 testing system (Instron Corp.,Norwood, MA). ASTM D412-D dumbbell specimenswere used. The strain rate was 0.2% s�1.

Dynamic mechanical thermal analysis (DMTA) wasperformed using a TA Instruments Q800 DMA inrectangular tension/compression geometry. Free filmsof the cured materials were obtained by removing thematerial from the aluminum substrate using a razorblade. The sample size was 10 9 5 mm2. The filmthickness was measured using a Micromaster� micro-meter. The analysis was carried out from �50�C to200�C at a frequency of 1 Hz and a ramp rate of 3�Cmin�1. Tg was obtained from the maximum peak in thetan d curves. The crosslink density (me) was calculated

according to equation: E¢ = 3 meRT. The E¢ value wasdetermined in the linear portion at a temperature thatwas at least 50�C greater than the Tg. DSC experimentswere performed utilizing a TA Instruments Q1000DSC with a heat-cool-heat cycle. The sample sizeranged from 4.5 to 5.5 mg. The temperature wasramped from �50�C to 200�C at 10�C min�1 innitrogen.

The real-time FTIR (RTIR) method provides astraightforward way of examining the photopolymer-ization behavior of UV-curable materials.10,11 TheRTIR experiments were performed using a NicoletMagna-IR 850 spectrometer Series II with detectortype DTGS KBr, with a UV optic fiber mounted in asample chamber. The light source was a LESCO SuperSpot MK II 100 W DC mercury vapor short-arc lampwith a UVA bulb. Such a setup directly monitors thefunctional group conversion as the photopolymeriza-tion proceeds. Samples were spin-coated onto a KBrplate at 3000 rpm for 15 s, followed by exposure to UVradiation in the FTIR beam for 60 s. Spectra weretaken over a 120 s period at 2 spectra/s. The resolutionwas 4 cm�1. The UV intensity was �36 mW/cm2

(UVA) as measured by UV Power Puck� II fromEIT Inc. The experiments were performed in air at25 ± 1�C. The thiol, ene, and acrylate conversion of7170-T, AT, and HBA were calculated from thepercent peak height decrement at �2570 cm�1, �3086cm�1, and �810 cm�1, respectively. The functionalgroup conversion at 120 s was reported and compared.

NMR spectra were obtained in deuterated chloro-form using a JEOL 400 MHz ECA400 spectrometer,equipped with a 24 position autosampler. Spectralanalysis was facilitated using Delta software. MALDI-TOF spectra were obtained using a Bruker Daltonics’Ultraflex Series II Matrix Assisted Laser Desorption/Ionization Time-of-Flight (MALDI-TOF) Mass Spec-trometer equipped with a 337-nm nitrogen laser. Thesamples were dissolved in THF (1%) and mixed with asolution of the MALDI-TOF matrix, alpha-cyano-4-hydroxycinnamic acid (HCCA). Trifluoroacetic acid(0.1% in water) was used as obtained as the dopant.

Results and discussion

Synthesis and characterization of novel soy-basedthiols and enes

Four novel soy-based thiols and enes were synthesizedaccording to the procedures described in Section 2.2.The synthesis routes, using reactions of ESBO withtrifunctional thiol and allyl alcohol, respectively, asexamples, are illustrated in Scheme 2. GDMP andTMPMP-functionalized ESBOs (7170-G and 7170-T,respectively) are clear, colorless low-viscosity liquids.The TAE and AA-functionalized ESBOs (7170-TAEand 7170-AA, respectively) are clear, light yellowliquids. The overlay of FTIR spectra for the ESBO,

Table 2: Branched/hyperbranched polyester acrylateoligomers from Sartomer Company Inc.

Abbreviation Acrylatefunctionality

Acrylateequivalent

weight(grams/mol)

Surfacetension(mN/m

at 25�C)

CN2300 A8 8 163 32.6CN2301 A9 9 153 38.4CN2302 A16 16 122 37.8CN2303 A6 6 194 40.3CN2304 A18 18 96 32.6


606

7170-G, and 7170-T is shown in Fig. 2a. After reaction,the epoxy peaks of the ESBO at 823 and 843 cm�1

disappeared. A thiol peak at 2575 cm�1 and a muchstronger hydroxyl peak at around 3450 cm�1 appeared.These indicated the successful ring opening of theepoxy by the thiols. Figure 2b shows the FTIR spec-trum overlay of the ESBO, 7170-AA, and 7170-TAE.The spectral changes are similar to those seen for 7170-G and 7170-T; the exception being that a double bondpeak at 1648 cm�1 appeared instead of the appearanceof the thiol peak, indicating that the attachment of enesto the ESBO molecule was completed. In Figs. 3a and3b, the NMR and MALDI-TOF spectra of reactionproduct 7170-T are shown. In the NMR spectrum, thecharacteristic epoxy peaks of ESBO, at 2.75–3.15 ppm,diminished and were overlapped by the characteristicpeaks of the thiol TMPMP. The FTIR and NMR dataindicated the completion of the epoxy ring openingreaction of ESBO. Figure 3b shows the MALDI-TOFspectrum of 7170-T. The data indicate that the finalproduct 7170-T was a mixture composed of a group ofTMPMP and ESBO reaction products withTMPMP:ESBO ratios ranging from 1 to 5, as indicatedby the molecular mass peak distribution. It wasconcluded from these data that although all of theepoxy groups of ESBO were reacted as determined byFTIR and NMR, the product obtained possessedvarious functionalization degrees, which may be due

to intermolecular ring opening by the multifunctionalthiol TMPMP.

Preliminary observations of UV-cured soy-basedthiol–ene coatings

The synthesized soy-based thiols (7170-G, 7170-T) andenes (7170-AA and 7170-TAE) were combined witheither soy-based or petroleum-based enes and thiolsaccording to theoretical stoichiometric ratios, followedby UV curing with and without 3 wt% PI. A generaltrend was observed that coatings composed of lowerfunctionality soy-based thiols and enes (such as 7170-Gand 7170-AA) could only be UV-cured to a tacky stateeven with the aid of a photoinitiator. Higher function-ality soy-based thiol–ene coatings could be cured to atack-free film with one pass through the UV lampwithout photoinitiator. However, the coating film prop-erties such as solvent resistance, hardness, etc., werepoor, and the glass transition temperature for thesefilms are all lower than �10�C (from DSC, data notshown). The poor film properties are attributed to theflexible SBO structure and the flexible C–S-C bondformed in the crosslinked network. In addition, theexistence of modified ESBO molecules with lowerfunctionalities, as shown by the MALDI-TOF analysis,will not only lower the crosslink density, but also

(a)

R

R2"

R2'

R1"

R3"

R3'

R1'

O

O

O

O O

R'

SH

SHSH

R'

SHSH

R'

S

SHSH

R'

SH

SHS

S

R'

SH

S

SH

R

R2"

R2'

R1"

R3"

R3'

R1'

OH

OHOH

R'

SH

S

SH

OH

OH

+

ESBO

Multifunctional thiols

BF3, acetonen

(n=num ber of oxirane per ESBO m olecule)

(b)

R

R2"

R2'

R1"

R3"

R3'

R1'O

O

O

O O

O

O

O

O

R

R2"

R1"

R3"

R3'

R1'

OH

OH

OH

OH

R2'

OH

O

OH

+

ESBO

BF3, acetone

n

(n =number of oxirane per ESBO molecule)

allyl alcohol

Scheme 2: Synthesis of thiol and ene functionalized ESBO through reaction of epoxy ring opening by (a) multifunctionalthiols; and (b) hydroxyl functional enes


607

plasticize the crosslinked network. When soy-basedthiols and enes were combined with petroleum-basedenes and thiols, as listed in Table 1, tack-free coatingscould be obtained after one pass under the UV lamp,and the coating properties were better than thoseobtained with the all soy-based combinations. Quan-titative physical and thermal analysis of the coatingsrevealed that UV curing 7170-T with enes such as AT,AI, TAE, and VE generated coatings with betterproperties. The best films were obtained from a coatingcomposed of 7170-T, AT, and 3% PI as a result of themore rigid ring structure and lower allyl equivalentweight of AT. However, these film properties were stillnot satisfactory for practical applications.

Hyperbranched acrylate-toughened, UV-cured,soy-based, thiol–ene coatings

In order to enhance further the soy-based thiol–enecoating film properties, a set of five different commer-cial hyperbranched acrylates (HBAs) with polyesterpolyol cores were incorporated into the soy-basedthiol–ene coatings. These HBAs have various degreesof acrylate functionality and surface tension. HBAshave been reported to have high reactivity, give lowerphotopolymerization shrinkage, and yield better adhe-sion to the substrates.25 A UV-cured thiol–ene–acrylate ternary system has been explored previously.It has been shown that the acrylate gives a higher

2.01.91.81.71.61.51.41.31.21.11.00.90.80.7

Abs

orba

nce

Abs

orba

nce

0.60.50.40.3

0.20.1

–0.0–0.1–0.2–0.3–0.4

4000

0.60

0.55

0.50

0.45

0.40

0.35

0.30

0.25

0.20

0.15

0.10

0.05

–0.00

–0.05

3500 3000 2500 2000

ESBO

7170-G

7170-T

7170-TAE

7170-AA

ESBO

Appearanceof thiol peak

Appearanceof double bond peak

Disappearanceof epoxy peak

Disappearanceof epoxy peak

1500

Wavenumbers (cm–1)

1000 500

4000 3500 3000 2500 2000 1500

Wavenumbers (cm–1)

1000 500

(a)

(b)

Fig. 2: Overlay of FTIR spectra of (a) ESBO, 7170-G, and 7170-T; (b) ESBO, 7170-TAE, and 7170-AA


608

+ 1 TMPMP

+ 2 TMPMP

+ 3 TMPMP

+ 4 TMPMP

+ 5 TMPMP

(b)

(a)

7170-T

7170

TMPMP

Fig. 3: (a) NMR spectrum of 7170-T; (b) MALDI-TOF spectrum of 7170-T


609

photopolymerization rate and a higher homopolymer-ization tendency than reaction with thiols as an ene.26

Due to the greater tendency of HBAs to form highercrosslinked UV-cured networks, it was expected that inthe 7170-T + AT + HBA system, two networks com-posed of homopolymer of HBAs and soy-based thiol–ene–HBAs would be formed after UV irradiation.These two networks were expected to have enhancedinter-network adhesion due to the potential reactionbetween residual acrylate groups and soy-based thiols.

Five HBAs were formulated with the 7170-T + AT(the reference formulation) together with a PI. Thecompositions of these formulations are given in Table 3.The soy-based thiol–ene coatings, cured with and withoutthe aid of PI, yielded soft coating films with good adhesionand impact resistance, but very poor solvent resistance, ascompared to a fully petroleum-based thiol–ene coatingmade of TMPMP and AT, which did not show any changeat more than 400 MEK double rubs. The poor solventresistance of the soy-based thiol + AT coating could bedue to the loosely crosslinked network arising as a resultof the higher thiol equivalent weight of the 7170-T(theoretically 312.5 g/mol for 7170-T vs 133 g/mol forTMPMP). In addition, the hazy cured film indicated acertain degree of incompatibility between the oil-basedthiol and the AT, which also contributes to the poorsolvent resistance of the film. As to the HBA-toughenedcoatings, the six acrylate functional A6 does not havegood compatibility with the soy-thiol, as evidenced by thetranslucent film with poor coating properties. Otherwise,the other four HBAs all gave good compatibility with thesoy-based thiol, resulting in transparent films. Theenhanced compatibility of these HBAs with the soy-based thiol may be due to the result of the more polaracrylate ester and more branched structures in themolecules, increasing the solubility of these HBA oligo-mers. Because of the better compatibility, these coatingsall gave enhanced hardness and solvent resistancecompared to those of SOY-T no PI and SOY-T. Inaddition, their adhesion and impact resistance were notsacrificed.

The tensile test results and the glass transitiontemperature and crosslink density data determined fromDMA are shown in Figs. 4 and 5, respectively. For the7170-T + AT samples, the addition of the PI acceleratedthe thiol–ene reaction during photopolymerization, gen-erating a coating material with greater modulus, crosslinkdensity, and glass transition temperature, but less elon-gation. When the HBAs were incorporated into the7170-T + AT formulations, it was found that A8, A9, andA18 increased the glass transition temperature, crosslinkdensity, tensile modulus, and elongation of the coatings.However, A16 and A6 did not impart significant coatingproperty improvements to the soy-based thiol–ene coat-ings. This observation may be a consequence of the loweracrylate functionality and poor compatibility with thesoy-based thiol for A6.

The gloss and water contact angle values for theHBA-toughened soy-based thiol–ene coatings weremeasured, and the data are shown in Fig. 6. All of theT

ab

le3:

Refe

ren

ce

form

ula

tio

nan

dH

BA

-to

ug

hen

ed

form

ula

tio

ns

an

dco

rresp

on

din

gco

ati

ng

ph

ysic

al

pro

pert

ies

Form

ula

tions

SO

Y-T

no

PI

SO

Y-T

SO

Y-6

SO

Y-8

SO

Y-9

SO

Y-1

6S

OY

-18

Description

7170-T

+A

Tw

ithout

PI

7170-T

+A

T+

3w

t%P

IS

OY

-T+

20

wt%

A6

SO

Y-T

+20

wt%

A8

SO

Y-T

+20

wt%

A9

SO

Y-T

+20

wt%

A16

SO

Y-T

+20

wt%

A18

UV

cura

bili

ty(B

elt

speed

10

ft/m

in,

1pass)

Tack-f

ree

hazy

film

Tack-f

ree

hazy

film

Tack-f

ree

translu

cent

glo

ssy

film

Tack-f

ree

transpare

nt

glo

ssy

film

Tack-f

ree

transpare

nt

glo

ssy

film

Tack-f

ree

transpare

nt

glo

ssy

film

Tack-f

ree

transpare

nt

glo

ssy

film

Pendulu

mhard

ness

(s)

25

26

21

81

62

30

49

Cro

ss-h

atc

hadhesio

n(%

)98

92

298

93

100

98

Forw

ard

impact

(in.-

lbs)

160

160

80

160

160

160

160

ME

Kdouble

rubs

(mar/

substr

ate

)1/2

2/4

20/2

445/5

930/5

518/2

614/1

9


610

HBA-toughened coatings have water contact anglesaround 70�. In addition, most of the coatings have ahigh 20� gloss value, the exception being SOY-T-8,which had a value of approximately 85.

The thermal weight loss profiles for the HBA-toughened soy-based thiol–ene coatings are shown inFig. 7. Comparison of coatings SOY-T and SOY-T noPI shows that SOY-T has a slightly higher thermaldecomposition temperature than SOY-T no PI. Thisobservation can be explained by the presence ofenhanced crosslinking and the resultant higher glasstransition temperature caused by the addition of 3%PI. The HBA-toughened coatings all have higherthermal decomposition temperatures than SOY-T,which is a direct result of the improved crosslinkdensity and the diluted fatty acid ester concentration inthe cured film.

Real-time FTIR experiments were conducted toevaluate the photopolymerization kinetics of the soy-based thiol–ene–HBA ternary systems, in comparisonto soy-based thiol–ene formulations, with and withoutthe addition of 3 wt% PI. The functional groupconversion curves for SOY-T no PI, SOY-T, andSOY-T-8 are shown in Fig. 8, and the 120 s functionalgroup conversion for different formulations are shownin Table 4. From Fig. 8 and Table 4 it can be seen that,while the soy-based thiol–ene formulation can beUV-cured to a tack-free film, the conversions of thioland ene during photopolymerization are low, whichcontributes to the poor cured film properties. With theaddition of 3 wt% PI, the thiol and ene conversionswere significantly improved. The ene conversionreached 100% almost instantaneously, while the thiolconversion reached 80%. The greater conversion of theene than the thiol is attributed to homopolymerizationunder radical conditions.26 When the HBAs wereadded to the soy-based thiol–ene formulations, thephotopolymerization kinetics changed. HBA has beenreported to have greater homopolymerization ten-dency via free radical addition than copolymerizationwith thiol via a thiol–ene step-growth mechanism.Further, such homopolymerization was accelerateddue to the elimination of oxygen inhibition because

0

20

40

60

80

100

120

140

SOY-T no PI SOY-T SOY-6 SOY-8 SOY-9 SOY-16 SOY-18

20°

glo

ss

0

10

20

30

40

50

60

70

80

90

Wat

er c

on

tact

an

gle

(°)

20° gloss water contact angle

Fig. 6: Gloss and water contact angle for HBA-toughened soy-based thiol–ene coatings

0

2

4

6

8

10

12

14

16


Cro

ssli

nk

den

sity

(m

mo

l/cm

3 )

-5

0

5

10

15

20

25

30

35

40

Gla

ss t

ran

siti

on

tem

per

atu

re (

°C)Crosslink density

Glass transition temperature

Fig. 5: Glass transition temperature and crosslink densitydata for HBA-toughened soy-based thiol–ene coatings

0

1

2

3

4

5

6

7

8

9

10


Mo

du

lus

(MP

a)

0

10

20

30

40

50

60

Str

ain

at

bre

ak (

%)

Tensile modulus (MPa) Strain at break (%)

Fig. 4: Tensile modulus and elongation data for HBA-toughened soy-based thiol–ene coatings


611

of the chain transfer mechanism that was induced bythe thiol.27 This is evidenced by the lack of aninduction period for the acrylate conversion as indi-cated in the RTIR conversion curve, shown in Fig. 8.The chain transfer mechanism also further consumesand improves the thiol conversion (>95% for HBAadded formulations) in the soy-based thiol–ene–HBA,as shown in Table 4. However, the ene conversion insuch ternary formulation systems is generally lowerthan that in the thiol–ene system due to the con-sumption of thiol in the acrylate homopolymeriza-tion stage and reduced system mobility in the later

thiol–ene copolymerization step. It was noticed thatSOY-16 had a lower ene conversion compared to theother HBA-toughened formulations. This observationmay explain why no significant film property (solventresistance, hardness, tensile strength, and glass tran-sition temperature) improvements were observedcompared to SOY-T. In conclusion, the addition ofHBA to the soy-based thiol–ene system seemed tohave a synergistic effect which resulted in very highfinal conversions of the acrylate, thiol, and enefunctional groups in the formulation, during photo-polymerization.

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 20 40 60 80 100 120Time (s)

Co

nve

rsio

n (

x100

%)

thiol-SOY-T no PIthiol-SOY-Tthiol-SOY-8

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 20 40 60 80 100 120Time (s)

Co

nve

rsio

n (

x100

%)

allyl triazine-SOY-T no PIallyl triazine-SOY-Tacrylate-SOY-8allyl triazine-SOY-8

Fig. 8: Thiol (left), ene and acrylate (right) conversion curve in RTIR experiments for formulations SOY-T no PI, SOY-T, andSOY-8

Table 4: Functional group conversion at 120 s in RTIR experiments for different soy-based thiol-ene-HBA ternaryformulations


Acrylate – – 100 100 100 100 100Thiol 22.5 79.5 100 99.2 99.4 98.8 97.2Allyl triazine 45.3 100 84.9 86.6 87 71.9 90.8

0-0.1

0.4

0.9

1.4

100 200 300 400 500 600

Temperature (°C)

Der

iv. W

eigh

t (%

/°C

)

Universal

SOY-T-6

SOY-TSOY-T no PI

SOY-T-9SOY-T-8SOY-T-18

SOY-T-16

7

Fig. 7: TGA weight loss derivative curves for HBA-toughened soy-based thiol–ene coatings


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Conclusions

Soy-based thiols and enes were synthesized by theacid-catalyzed ring opening reaction of ESBO bymultifunctional thiols or hydroxyl functional enes, asevidenced by FTIR and NMR. Due to the low Tg

feature of the soy-based thiols and enes, tack-freeUV-curable coating films with better film propertieswere obtained only by formulating higher functionalitysoy-based thiols with allyl triazine. The photopolymer-ization conversion and the coating film properties suchas solvent resistance, modulus and elongation, glasstransition temperature, and thermal degradation tem-perature could be further enhanced by the addition ofmultifunctional, hyperbranched acrylates A8, A9, andA18.

Acknowledgment We would like to thank the NorthDakota Soybean Council for sponsoring this research.

References

1. Meier, MAR, Metzger, JO, Schubert, US, ‘‘Plant OilRenewable Resources as Green Alternatives in PolymerScience.’’ Chem. Soc. Rev., 36 1788–1802 (2007)

2. Liu, Z, Erhana, SZ, Xu, J, ‘‘Preparation, Characterizationand Mechanical Properties of Epoxidized Soybean Oil/ClayNanocomposites.’’ Polymer, 46 10119–10127 (2005)

3. Fomin, VA, Guzeev, VV, ‘‘Biodegradable Polymers: Statusand Prospects.’’ Plast. Massy, 2 42–48 (2001)

4. Schroeter, J, ‘‘Biodegradable (Plastic) Materials. A StatusReport.’’ Kunststoffe, 90 64–66 (2000)

5. Mishra, DP, Mahanwar, PA, ‘‘Advances in BioplasticMaterials.’’ Pop. Plast. Packag., 45 68–76 (2000)

6. Wool, RP, ‘‘Development of Affordable Soy-Based Plastics,Resins, and Adhesives.’’ Chemtech., 29 44–48 (1999)

7. Gu, H, Ren, K, Martin, D, Marino, T, Neckers, DC,‘‘Cationic UV-Cured Coatings Containing EpoxidizedSoybean Oil Initiated by New Onium Salts ContainingTetrakis(pentafluorophenyl)gallate Anion.’’ J. Coat. Tech-nol., 74 49–52 (2002)

8. Teng, G, Soucek, MD, ‘‘Epoxidized Soybean Oil-BasedCeramer Coatings.’’ J. Am. Oil Chem. Soc., 77 381–387(2000)

9. Masayuki S, Hiroshi, U, Tadashi, H, Nobuo F, Mamoru S,Toppan Printing Co., Ltd., ‘‘Polyolefin-Based AdhesiveCompositions for Lamination.’’ JP 99-352849 19991213,2001.

10. Lee, SH, Lee, SY, Lim, H, Nam, J, Kye, H, Lee, Y,‘‘Preparation and Properties of Plasticized Cellulose Diace-tate Using Triacetine/Epoxidized Soybean Oil.’’ Polymer(Korea), 30 202–206 (2006)

11. Ali, SM, Mahmoud, M, Saie, AS, ‘‘Mechanical and ThermalProperties of the Plasticized PVC-ESBO.’’ Iran Polym. J., 14769–773 (2005)

12. Choi, JS, Park, WH, ‘‘Effect of Biodegradable Plasticizers onThermal and Mechanical Properties of Poly(3-hydroxybuty-rate).’’ Polym. Test., 23 455–460 (2004)

13. Adhvaryu, A, Erhan, SE, ‘‘Epoxidized Soybean Oil as aPotential Source of High-Temperature Lubricants.’’ Industr.Crop Product, 15 247–254 (2002)

14. Hwang, H, Erhan, SE, ‘‘Modification of Epoxidized SoybeanOil for Lubricant Formulations with Improved OxidativeStability and Low Pour Point.’’ J. Am. Oil Chem. Soc., 781179–1184 (2001)

15. Senyurt, AF, Wei, H, Phillips, B, Cole, M, Nazarenko, S,Hoyle, CE, Piland, SG, Gould, TE, ‘‘Physical and Mechan-ical Properties of Photopolymerized Thiol–Ene/Acrylates.’’Macromolecules, 39 6315–6317 (2006)

16. Lee, T, Roper, TM, Jonsson, ES, Guymon, CA, Hoyle, CE,‘‘Thiol–Ene Photopolymerization Kinetics of Vinyl Acrylate/Multifunctional Thiol Mixtures.’’ Macromolecules, 37 3606–3613 (2004)

17. Hoyle, CH, Lee, T, Roper, TM, ‘‘Thiol–Enes: Chemistry ofthe Past with Promise for the Future.’’ J. Polym. Sci. A, 425301–5338 (2004)

18. Senyurt, AF, Warren, G, Whitehead, JB, Jr, Hoyle, CE,‘‘Matrix Physical Structure Effect on the Electro-OpticCharacteristics of Thiol–Ene Based H-PDLC Films.’’ Poly-mer, 47 2741–2749 (2006)

19. Roper, TM, Kwee, T, Lee, T, Guymon, CA, Hoyle, CE,‘‘Photopolymerization of Pigmented Thiol–Ene Systems.’’Polymer, 45 2921–2929 (2004)

20. Cramer, NB, Beckel, ER, Harant, AW, Davies, T,Williamson, DL, Bowman, CN, ‘‘Formation of a HostNanostructure for Ferroelectric Liquid Crystals UsingThiol–Ene Polymers.’’ Liquid Crystal., 29 1291–1296 (2002)

21. Li, Y, Birnkrant, MJ, Natarajan, LV, Tondigli, VP, Lloyd,PF, Sutherland, RL, Bunning, TJ, ‘‘Polymer Crystallization/Melting Induced Thermal Switching in a Series of Holo-graphically Patterned Bragg Reflectors.’’ Soft Matter., 1 238–242 (2005)

22. Cai, J, Shu, W, Yun, L, Jiu, L, ‘‘Influence of ReactiveDiluents on UV Curing Acrylic Epoxidized Soybean OilSystem.’’ Tuliao Gongye, 36 12–15 (2006)

23. Bajpai, M, Shukla, V, Singh, DK, Singh, M, Shukla, R,‘‘A Study of the Film Properties of Pigmented UV-CurableEpoxidised Soybean Oil.’’ Pigment Resin Technol., 33 160–164 (2004)

24. Desai, D, Rahim, M, Massingill, JL, Jr, ‘‘Cationic UV CuredCoatings Using Epoxidized Soybean Oil.’’ In: Experience theWorld of UV/EB, RadTech 2000: The Premier UV/EBConference & Exhibition, Baltimore, MD, United States,pp. 22–44 (2000)

25. Klang, JA, Radiation Curable Hyperbranched PolyesterAcrylates. Sartomer Company, Inc. Exton, PA, USA.

26. Cramer, NB, Bowman, CN, ‘‘Kinetics of Thiol–Ene andThiol–Acrylate Photopolymerizations with Real-TimeFourier Transform Infrared.’’ J. Polym. Sci. A, 39 3311–3319 (2001)

27. Wei, H, Senyurt, AF, Jonsson, S, Hoyle, CE, ‘‘Photopoly-merization of Ternary Thiol–Ene/Acrylate Systems: Filmand Network Properties.’’ J. Polym. Sci. A, 45 822–829(2007)


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journal of coatings technology and research, 2010, volume 7, number 5, pages 603-613

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