1. IntroductionTraditionally, epoxidized vegetable oils (EVO) areused as poly(vinyl chloride) plasticizers, stabilizersand lubricants. Recently, modified vegetable oilscould be used to synthesize segmented polyure -thane [1], thermoplastic polyurethanes [2], poly(methyl methacrylate)-multigraft copolymers [3],novel thermosets and rubbers [4–5] and ‘green’composites [6]. In recent years, there has been sub-stantial growth in biopolymers from EVO. TheseEVO from biological origin substrates could bringalong numerous advantages and new beneficialproperties, which may not be able to gain from petro-leum-based epoxy resins. For example, EVO havebeen evaluated as ecological and environmentalfriendly alternative for petroleum-based epoxy resinssince they are neutral in the carbon dioxide cycleand are readily biodegradable. Other advantages of

the EVO include cost effectiveness, renewabilityand availability.In general, epoxidized soybean oil (ESO) is a triglyc-eride made up of a complex multi-component mix-ture of functionalized oleic, linoleic and linolenicacid methyl esters as well as saturated fatty acids(i.e. palmitic and stearic acids). It has been recog-nized in the literature that these functionalized fattyacids in ESO could be thermally cured with a ther-mal latent initiator [7], thermally cured with acidanhydride under the catalytic reaction of tertiaryamines catalyst [8] and ultraviolet (UV) cured inthe presence of photo-initiators [9]. However, satu-rated fatty acids in ESO do not take part in the poly-mer network formation. This is due to the fact thatthey do not possess any epoxy functional groups intheir saturated backbone structures.Specifically, in the open literature, there exist numer-ous models to explain the curing mechanisms for

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Thermal properties, curing characteristics and waterabsorption of soybean oil-based thermosetS. G. Tan, W. S. Chow*

School of Materials and Mineral Resources Engineering, Engineering Campus, Universiti Sains Malaysia, Nibong Tebal14300 Penang, Malaysia

Received 20 October 2010; accepted in revised form 10 December 2010

Abstract. Epoxidized soybean oil (ESO) was successfully thermal-cured by using methylhexahydrophthalic anhydride(MHHPA) curing agent, in the presence of tetraethylammonium bromide (TEAB) catalyst of varied concentration (0.3–0.8phr). The polyesterification process of ESO thermoset was proven and supported by Fourier transforms infrared spec-troscopy (FTIR) and gas chromatography-mass spectroscopy analysis (GC-MS). A possible chemical reaction of theMHHPA, TEAB and ESO was proposed based on the experimental work. Differential scanning calorimetry (DSC) anddynamic mechanical analysis (DMA) revealed that there is a positive relationship between the degree of conversion andcrosslink density of ESO thermoset with TEAB concentration. The kinetics of water absorption of the ESO thermoset werefound to conform to Fickian law behavior.

Keywords: biopolymers, epoxidized soybean oil, thermosetting resin, thermal properties, water absorption

eXPRESS Polymer Letters Vol.5, No.6 (2011) 480–492Available online at www.expresspolymlett.comDOI: 10.3144/expresspolymlett.2011.47

*Corresponding author, e-mail: chowwenshyang@yahoo.com© BME-PT

ESO. Jin and Park [7] proposed thermally inducedring opening polymerization route for ESO ther-moset initiated by N-benzylquinoxalinium hexaflu-oroantimonate thermal latent initiator. Ortiz et al.[9] reported that radical induced cationic photo-polymerization of ESO thermoset in the presence ofdiarylinodonium salt photo-initiator follows con-ventional cationic polymerization mechanism andchain reaction mechanism. Gao [10] proposed thatthe curing mechanism of catalytic ESO-anhydridethermosets involves reaction of the tertiary aminewith ESO monomer followed by the ring opening ofthe anhydride functional group with the alkoxide.Although the EVO based thermosetting materialsare more environmental friendly, they tend to sufferfrom shortcomings in terms of long curing sched-ule, high curing temperature, poor thermo-physicalproperties and high degree of water uptake. In orderto overcome some of these limitations, our workaims to chemically synthesize a thermally curableESO thermosetting resin in the presence of thetetraethylammonium bromide (TEAB) catalyst. It ishypothesized that the TEAB-catalyzed ESO ther-mosetting resin can be prepared using shorter cur-ing schedules and lower curing temperature due tothe fast cure rate of the TEAB catalyst. Accord-ingly, the primary objective of this present study isto investigate the effect of TEAB catalyst concen-tration on the curing characteristics, thermal proper-ties and kinetics of water absorption of ESOthermoset. A plausible curing mechanism of thethermal curable ESO thermoset under the catalyticreaction of TEAB catalyst will also be proposed.

2. Experimental2.1. MaterialsEpoxidized soybean oil (ESO; trade name MingchenESO-1) resin with 6.1 wt% epoxy oxirane contentand molecular weight of about 950 g/mol was pur-chased from Shangdong Longkou Longda Chemi-

cal Industry Co., Ltd., China. Industrial grademethylhexahydropthalic anhydride (MHHPA) cur-ing agent was purchased from CAPE TechnologySdn. Bhd., Malaysia. Tetraethylammonium bro-mide (TEAB: code T7012) catalyst was supplied bySigma-Aldrich, Malaysia. The materials designa-tion and composition of the ESO thermoset weresummarized in Table 1.

2.2. Preparation of thermal curable ESOthermoset

MHHPA curing agent was pre-mixed with TEABcatalyst at a predetermined ratio. ESO resin andMHHPA/TEAB mixture were then mixed at roomtemperature and stirred mechanically. The mixturewas then poured into the cavities of mould and sub-jected to thermal curing process in an oven at 140°Cfor 3 hours. The dimension of the mould used to pre-pare the ESO thermoset is 300 mm!"100 mm!"3 mm(length " width " thickness).

2.3. Curing characteristics of ESO thermosetThe curing characteristics of the TEAB-catalyzedESO thermoset were characterized using Fouriertransform infrared spectroscopy (Spectrum 100FTIR, Perkin Elmer, USA), gas chromatography-mass spectroscopy (GC-MS, Perkin Elmer Clarus600T, USA), DSC Diamond Analyzer (Perkin Elmer,USA) and DMA 8000 (Perkin Elmer, USA).The curing characteristics of ESO thermoset whichthermally cured for 1, 2 and 3 hours were deter-mined with FTIR. The FTIR spectra of the thermalcurable ESO thermoset from the wavelength of4000 to 550 cm–1 were recorded as the infraredradiation transmitted through the sample. The car-bonyl index (C.I.) values of the samples were calcu-lated based on the ratio of absorbance (A) at twodifferent wave-numbers as shown in Equation (1):

(1)

where A1700 represents the absorbance band at1700 cm–1 due to the presence of carbonyl stretchof the aromatic acid whereas A1456 represents theband at 1456 cm–1 corresponding to the absorbancefrom the presence of methyl group in MHHPA cur-ing agent, which is taken as a reference band.The pure ESO and extracted molecular fragmentsof the ESO thermoset were analyzed by gas chro-

C.I. 5A1700

A1456C.I. 5

A1700

A1456

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Table 1. Materials designation and composition of ESOthermoset

Materials designation Concentration of TEAB catalyst[phr]

ES_0.3A 0.3ES_0.4A 0.4ES_0.5A 0.5ES_0.6A 0.6ES_0.7A 0.7ES_0.8A 0.8

matography mass spectroscopy (GC-MS). One µlinjection volume using splitless mode was performedon capillary column (30 m"250 µm) with heliumflows at constant pressure. The initial temperature,injection temperature and transfer temperature ofthe GC-MS test were set at 65, 250 and 180°C,respectively. The holding time was set for 10 min-utes at a ramp rate of 8°C/min. The molecular frag-ments of the ESO were determined using Turbo-Mass™ software.The degree of conversion of ESO thermoset wasdetermined using DSC and calculated based onEquation (2). The crosslink density (vc) and molec-ular weight between cross-linking (Mc) of the ESOthermoset were determined using DMA and calcu-lated based on Equations (3) and (4) respectively[11–12]:

(2)

(3)

(4)

where ! is the degree of conversion, #Hc is the totalexothermic heat generated for a fully cured systemand #Hr is the total residual exothermic heat gener-ated during a specified period of time. vc is thecrosslink density for the epoxy network, E$ is thestorage modulus of the thermoset in the rubberyplateau region at Tg + 40°C, R is the gas constant, Tis the absolute temperature, d is the density(1.1 g/cm3) and Mc is the molecular weight betweencrosslink.

2.4. Thermal characterization of ESOthermoset

The curing profile of ESO thermoset was examinedusing DSC Diamond Analyzer (Perkin Elmer, USA).Approximately 20 mg of the sample was placed intothe DSC aluminium pans, followed by thermal scan-ning from 30 to 300°C at a heating rate of 10°C/min,in nitrogen gas atmosphere. The onset curing tem-perature (Tonset), the temperature where maximumcross-linking reaction takes place (Tpeak) and thetotal heat released during curing reaction (#H) ofESO thermoset were determined. The dynamicmechanical properties of ESO thermoset were deter-

mined using DMA 8000 (Perkin Elmer, USA). Thespecimen with the dimension of 25 mm"10 mm"2 mm was heated from –100 to 200°C at a heatingrate of 2°C/min in nitrogen atmosphere. Specimenheld in a single cantilever mode was fixed at oneend and the other end was vibrated by the bendingstress at the frequency of 1 Hz with the displace-ment of 0.05 mm. The glass transition temperature(Tg), storage modulus (E$) and damping properties(tan") of ESO thermoset were examined with DMA.The thermal stability of ESO thermoset was charac-terized using TGA Pyris 6 (Perkin Elmer, USA).Approximately 5 mg of the specimen was heatedfrom room temperature to 600°C at a heating rate of10°C/min under nitrogen gas atmosphere.

2.5. Water absorption of ESO thermosetWater absorption test according to the ASTM D570was conducted on ESO thermoset by immersing thespecimens in distilled water at room temperaturefor three months. The weight gained of the ESO ther-moset at any time t, Mt as a result of water absorp-tion was determined using Equation (5). The waterdiffusion coefficient (D) of ESO thermoset wasdetermined using Equation (6) [13]:

(5)

(6)

where Wd and Ww represent the material dry weightand weight of materials after being exposed to thewater absorption at a period of time t respectively.Mm is the percentage equilibrium or the maximumwater absorption of ESO thermoset, t is the timetaken for the ESO thermoset to reach the saturatedstage, and h is the thickness of the ESO specimen.

3. Results and discussion3.1. Curing characteristics of ESO thermoset3.1.1. FTIR characterizationFigure 1 shows the FTIR spectra of TEAB catalyst,MHHPA curing agent and the mixture of MHHPA/TEAB. One may observe that FTIR spectrum of theMHHPA/TEAB mixture consists of a new 1700 cm–1

shoulder peak. The appearance of this peak is asso-ciated to the carbonyl stretch (C=O) of the aromaticacid, which indicates the asymmetric cleavage of

Mt

Mm5 1 2

8p2

exp c 2 Dth2

p2 d

Mt 5Ww 2 Wd

Wd? 100

Mc 5dvc

vc 5E9

3RT

a 5DHc 2 DHr

DHca 5

DHc 2 DHr

DHc

vc 5E9

3RT

Mc 5dvc

Mt 5Ww 2 Wd

Wd? 100

Mt

Mm5 1 2

8p2

exp c 2 Dth2

p2 d

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the MHHPA functional group by TEAB catalyst toform the zwitterions. The amount of carbonyl groupspresent and the extent of MHHPA ring opening areestimated based on the carbonyl index value obtainedfrom the FTIR study (c.f. Table 2). It is proposedthat the ring opening of MHHPA curing agent bythe TEAB catalyst involves SN2 reaction. The tri-ethylamine formed as a result of the dequaterniza-tion reaction of TEAB catalyst, serves as a nucle-ophile and attacks the MHHPA curing agent to yieldzwitterions. Although the exact absorbance bandrepresenting the zwitterions is undetected fromFTIR spectra, the intensity reduction of the infraredabsorbance bands that appear at 1856, 1775 and887 cm–1 in the FTIR spectra is a good indicator tosupport the phenomenon of MHHPA ring openingby the TEAB catalyst. This is due to the fact thatthese absorbance bands are the characteristic bandsfor the conjugated cyclic anhydride which could beassigned to the C=O and C–O stretches. Consider-ing these phenomena, a plausible assumption that

the zwitterions are generated during the pre-mixedreaction could be made.It is noticed that the intensity of the 1700 cm–1

shoulder peak increases with increasing the TEABcatalyst concentration in MHHPA/TEAB mixture.This incident could be evidenced by the increment inthe carbonyl index calculated as reported in Table 2.While, the absorbance assignable to the C=O andC–O stretches of MHHPA curing agent show adecreasing trend. These findings suggest theincreased anhydride ring opening reaction betweenthe TEAB catalyst and MHHPA curing agent as theTEAB catalyst concentration increases from 0.3 to0.8 phr. This may be due to the increased potentialof TEAB catalyst to facilitate the MHHPA ring open-ing forming more zwitterions species at higherTEAB concentration. However, it is noted that onlylow conversion of zwitterions is achieved duringthe pre-mixed reaction. The lower efficiency of theTEAB catalyst to cleavage the anhydride ring dur-ing pre-mix reaction is strongly believed to be attrib-uted to the relatively stable and low reactivity ofthese quaternary ammonium salts. It is also experi-mentally determined in this study that no MHHPAring opening is detected at low level of TEAB con-centration (<%0.3 phr).During the heating reaction of ESO/MHHPA/TEABmixture at 140°C in the oven, the zwitterions formedwould eventually react with the epoxy rings on theESO backbone chains and subsequently generatethe alkoxide intermediates. The alkoxide intermedi-ates would then cleave another MHHPA curingagent to yield carboxylate anions during the propa-gation stage. These carboxylate anions would chemi-cally react with another ESO resins to yield thereaction intermediates product and propagate thecycles. The polyesterification reaction in ESO/MHHPA/TEAB mixture is completed after 3 hoursof curing reaction. This evidently results in the for-mation of ESO thermoset with 3-dimensional poly-ester-type of linkages. To support the curing trans-formations proposed, the curing characteristics ofESO thermoset at three different curing times (i.e.1, 2 and 3 hours) were evaluated using FTIR studyas shown in Figure 2.As observed in Figure 2, the intensities of absorbancebands corresponding to the C–O and C=O stretchesof MHHPA curing agent show a progressive decreaseas a function of curing reaction time for the ESO/

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Figure 1. FTIR spectra representing the ring opening ofMHHPA with TEAB (Note: Arrow shows the1700 cm–1 shoulder peak)

Table 2. Curing characteristics of TEAB-catalyzed ESOthermoset

acalculated from FTIR analysisbcalculated from DSC analysisccalculated from DMA analysis

Characteristics ES_0.3A ES_0.6A ES_0.8A

Carbonyl indexa

MHHPA/TEABmixture 0.62 0.64 0.79

Curing1 h 1.03 1.69 1.682 h 2.39 2.96 3.553 h 2.96 3.79 3.95

Degree of conversionb [%] 95.6 98.6 99.8Crosslink densityc [10–3 mol/cm3) 0.215 0.260 0.446Mc

c [g/mol] 5116 4230 2471

MHHPA/TEAB mixture. Additionally, it is clearlynoticeable that the stretching vibrations of theC–O–C and C–C–O functional groups in MHHPAcuring agent which in the range of 1300–1100 cm–1

disappear gradually with curing time. The reductionin intensities of these absorbance bands suggest thering opening reaction of MHHPA curing agent andthe polyesterification reaction of MHHPA curingagent with alkoxide. Furthermore, it is discoveredthat the absorbance bands at 1695 and 1161 cm–1

which corresponding to the C=O and O–C–C vibra-tions of ester groups increasingly emerge with cur-ing time. This finding further signifies that the poly-

esterification process has taken place in the ESO/MHHPA/TEAB mixture as these absorbance bandsare the representative bands for esters. Chemicalreactions of epoxy groups in ESO with carboxylateanions followed by reacting with anhydrides duringthe propagation cycle give rise to the formation ofpolyester-type linkages between ESO and MHHPAcuring agents. The polyesterification reaction ofESO/MHHPA/TEAB mixture forming the poly-ester-type of linkages is also verified with the aid ofgas chromatography-mass spectroscopy analysis(c.f. Figure 3).From Figure 2, it is noted that the peaks which arecorresponding to epoxy groups in ESO disappearedand are substituted by the peaks assignable to C=Oand C–O functional groups of the conjugated cyclicanhydride and the C=O and O–C–C of the estergroups after curing process for 3 hours. As shownin Figure 3, it is noted that the peak (i.e. 7.63 min)corresponding to the epoxy group in ESO resin dis-appear after being thermally cured for 3 hours. How-ever, the peaks (i.e. 15.81 and 20.82 min) corre-sponding to the ester functional groups are detectedin the cured ESO thermoset. Furthermore, the peak(i.e. 16.18 min) representing the molecular frag-ments of MHHPA curing agent with ring-openedstructures are also being detected in thermally curedESO thermoset. One possible explanation to thesefindings could be linked to the polyesterificationprocess of ESO/MHHPA/TEAB. Based on the infor-mation obtained from the FTIR spectra and GC-MS, the curing mechanism of ESO thermoset isproposed. Figure 4 shows the proposed mechanismfor the thermally cured ESO thermoset.Furthermore, as shown in Figure 2, it is determinedthat the polyesterification reaction of the ESO/MHHPA/TEAB mixture proceeds at relatively fastmanner at the beginning stage during thermal cur-ing compared to that of at the pre-mix stage. This isowing to the fact that TEAB catalyst tends to expe-rience the chemical reaction of dequaternization atthe temperature range of 100–200°C to generate tri-alkylamine. The tertiary amines formed during theinternal displacement reaction will eventually boostthe polyesterification rate of ESO/MHHPA/TEABmixture. However, it is determined that the poly-esterification reaction of the mixture proceeds atrelatively slow manner after 2 hours of thermal cur-ing. This event occurs as the overall catalytic reac-

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Figure 2. (a) FTIR spectra representing the curing charac-teristics of ES_0.3A thermoset, (b) FTIR spectrarepresenting the curing characteristics of ES_0.6Athermoset, (c)&FTIR spectra representing the cur-ing characteristics of ES_0.8A thermoset

tion in the mixture begins to change from kinetic todiffusion-controlled. The curing process in bisphe-nol-S epoxy resin/phthalic anhydride epoxy systemchanging from kinetically controlled to diffusioncontrolled was also reported by Li et al. [14].Apart from that, the effects of the TEAB concentra-tion on the ESO thermoset are also being displayedin Figure 2. An intensity reduction in representativebands for the MHHPA curing agent and an incre-ment in absorbance bands intensities for esters areobserved when increasing the TEAB concentrationfrom 0.3 to 0.8 phr. These results indicate the poly-esterification rate is higher in thermally cured ESOcontaining higher TEAB concentration. Lower lev-els of TEAB concentration (<%0.3 phr) are also stud-ied in this work, perhaps even a 0-level catalystexperiment. However, it is found that the polyester-ification reactions of the ESO mixture are highlyunfavourable. The mixture is unable to be thermallycured at low level of TEAB concentration. One pos-sible explanation could be linked to the very limited

or even no chemical reaction between the ESOresin and MHHPA curing agent without the pres-ence of a catalyst.

3.1.2. DSC characterizationThe DSC heating thermograms showing the non-isothermal curing profiles of TEAB-catalyzed ESOthermoset are demonstrated in Figure 5. An exother-mic peak representing the epoxy curing reactions isdetected on the DSC heating curves. The enthalpyof ESO/MHHPA/TEAB polymerization can beexamined by integrating the exothermic peak. It isexperimentally proven that the ESO thermoset cat-alyzed with higher TEAB catalyst concentrationshows higher #H value. The differences in the #Hvalues obtained are mainly due to the differentextent of conversion of the liquid ESO monomers toESO thermosets during the thermal heating processin DSC. These findings show that the higher theTEAB concentration, the greater the extent of con-version in ESO thermoset, and the higher the cross-

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Figure 3. (a) GC-MS chromatogram of ESO, (b) GC-MS chromatogram of ES_0.6A thermoset

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Figure 4. Curing mechanism of TEAB-catalyzed ESO thermoset

linking formation seems to be. One possible expla-nation is that the higher conversion of zwitterions isachieved during the pre-mix reaction to initiate thethermal curing reaction of ESO thermoset catalyzedwith higher TEAB concentration.From the DSC thermograms, second exotherm peakor shoulder after the Tpeak is not observable. This maygive us a hint that homo-polymerizations of ESOresin do not take place. According to Boquillon andFringant [12] who studied on the thermal curing ofepoxidized linseed oil under the influence of differ-ent catalysts and anhydride hardeners, second shoul-der exotherm peak after the main exotherm peakappearing at lower temperature could be associatedto the epoxy homo-polymerization. Therefore, it canbe concluded that ESO/MHHPA polymerization isthe only chemical reaction occurs during polyesteri-fication process and the crosslinked ESO thermosetdoes not contain any epoxy homo polymer. Thisfinding is also consistent with the curing mecha-nism proposed (cf. Figure 4) in which the ESO resinwill chemically react with the MHHPA curing agentin the presence of TEAB catalyst forming 3-dimen-sional polyester linkages instead of forming poly-ether linkages among ESO resins. It is also noticedthat the Tonset and Tpeak (c.f. Table 3) of the ESOthermoset shift to lower temperature as the TEABconcentration is increased from 0.3 to 0.8 phr. Theshifts of DSC heating thermograms to lower tempera-ture are presumably due to the increase in thecrosslinking reaction rate. There was a direct rela-tionship between initial curing reaction rate and cat-alyst concentration of a catalyzed reaction [12]. Thisis consistent with the finding by Liu et al. [15] whichstated that the increase in catalyst concentration inepoxy-phenol/montmorillonite nanocompositeswill shift Tonset and Tpeak to lower temperature.

Also, as shown in Figure 5, ESO thermoset under-goes the enthalpy relaxation phenomenon at rela-tively low TEAB concentration. The extent reducesprogressively as the TEAB catalyst concentrationincreases from 0.3 to 0.8 phr. The reduction of thisenthalpy relaxation phenomenon is believed to bedue to the increase in crosslink density resultingfrom an increase in the extent of ESO conversion inESO thermoset. Shin and coworkers [16, 17] alsoreported that the restriction of enthalpy relaxationcan be directly related to an increase in crosslinkdensity and activation energy for the enthalpyrelaxation.The influence of TEAB concentration on the degreeof conversion (!) of ESO thermoset is summarizedin Table 2. The degree of conversion increases about4.2% when the TEAB concentration is increasedfrom 0.3 to 0.8 phr. This result indicates that theefficiency of TEAB to facilitate the ring opening ofMHHPA curing agent and to create more reactivesites on the anhydride (zwitterions) increases withincreasing TEAB catalyst concentration. Theincreased number of zwitterions formation favoursthe polyesterification reaction in ESO thermosetand accounts for the increment in degree of conver-sion eventually. This finding is in good agreementwith the study on the curing kinetics of bisphenol-Fresin using benzyl dimethyl amine as a catalystreported by Shokrolahi et al. [18] who determinedthe positive relationship between the catalyst con-centration and the extent of conversion rate.

3.1.3. DMA characterizationThe crosslink density and the molecular weightbetween crosslink (Mc) of the ESO thermoset as afunction of TEAB concentrations are reported inTable 2. It is found that the crosslink density of

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Figure 5. DSC heating thermograms of TEAB-catalyzedESO thermoset

Table 3. Thermal properties of TEAB-catalyzed ESO ther-moset

adetected from DSCbdetected from TGAcdetected from DMA

Thermalcharacteristics

ESO thermosetting resinsES_0.3A ES_0.6A ES_0.8A

Tonseta [°C] 183.0 162.1 150.7

Tpeaka [°C] 211.9 203.8 200.8

To1b [°C] 200.0 206.0 210.0

To2b [°C] 351.0 342.0 347.0

Tdb [°C] 458.0 454.0 456.0

Tgc [°C] 53.0 59.0 69.8

ESO thermoset increases with TEAB concentrationwhereas Mc exhibits a different trend. Generally,the crosslink density and Mc obtained for the ESOthermosets are found to be lower than the epoxi-dized vegetable oil (EVO)/petroleum-based epoxyblends [19], but close to those triethylamine cat-alyzed ESO/anhydride systems [20]. The increasein TEAB concentration gives rise to an increase inthe number of reactive sites on the MHHPA curingagents. The consequence is an increase in the cat-alyzed reaction rate and the extent of conversion.This incident will attribute to an increment incrosslink density. These findings seem to contradictwith the previous findings reported by Boquillonand Fringant [12] who mentioned that the crosslinkdensity reduces with the catalyst concentration dueto the diffusional restriction of the reagent at highcatalyst concentration. One possible explanationcould be linked to the difference in the polymeriza-tion reaction rate in both epoxy systems. The highconcentration of 2-methylimidazole (2MI) catalystin epoxidized linseed oil (ELO)/ tetrahydrophthalicanhydride (THPA) system leads to a fast gelling ofthe ELO thermoset. Consequently, the diffusion ofthe reagents is slow and results in lower extent ofconversion. While, in ESO thermoset catalyzed withlow TEAB concentrations, the diffusion restrictionassociated to vitrification is less hindered. There-fore, the extent of conversion and crosslink densityincrease with the catalyst concentrations. This wouldinherently result in lower Mc value in ESO ther-moset catalyzed with higher TEAB concentration.

3.2. Thermal properties of ESO thermoset3.2.1. Dynamic mechanical analysisThe storage modulus (E$), damping properties (tan")and the glass transition temperature (Tg) of ther-mally cured ESO thermoset are studied with DMA.The E$ and tan" of cured ESO as a function of tem-perature are shown in Figure 6. Basically, in thetesting temperature range, the ESO thermosettingresin shows three regions of viscoelasticity: theglassy state, the leathery state and the rubbery state.As shown in Figure 6, it is determined that the E$-Tcurve shifts to higher temperature as the TEABconcentration in the ESO thermoset increased from0.3 to 0.8 phr. The increment in E$ as a function ofTEAB concentration is attributed to the incrementin crosslink density and the reduction in the molec-

ular weight between crosslink (Mc) of the ESO ther-moset. The reduction in Mc as discussed in the pre-vious section indicates a reduction in the chain’smobility and consequently an increase in stiffnessand storage modulus. This finding is in good agree-ment with the reduction in storage modulus withincreasing Mc reported by Jin and Park [21].Figure 6 shows the effects of TEAB concentrationon the tan" of the ESO thermosetting materials. Theshifting of tan"-T curve to lower temperature can beassociated to the depression of glass transition tem-perature (Tg). Damping value (tan") of the curedESO catalyzed with higher TEAB concentration islower than that of those with lesser catalyst concen-tration. Higher crosslinked ESO thermosetting resinhas the tendency to render higher elastic stiffness.Generally, the reduction in elastic constant will leadto the improvement in damping properties. There-fore, more flexible ESO thermosetting resins withlower crosslink densities would possess higher tan"peak. In addition, it can be seen that the dampingpeak intensity decreases with increasing TEAB con-centration. The broadening and reduction in thedamping peak height with increasing TEAB con-centration are due to the increase in degree of cross-linking [22, 23]. This is owing to the fact that highlycross-linked network structure can retard and restrictthe chains mobility and subsequently lead to thereduction in the damping peak intensity [24].Table 3 shows the influence of TEAB concentrationon the glass transition temperature (Tg) of curedESO thermoset. It is determined that the Tg valuesobtained for the TEAB-catalyzed ESO thermosetsare comparable to or even higher than the existingepoxy resins based on ESO. An increment in the Tg

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Figure 6. Effect of TEAB concentration on the E$ and tan"of ESO thermoset

value is detected if the TEAB catalyst concentrationincrease from 0.3 to 0.8 phr. These changes in Tgvalues can be related to the increase in cross-linkdensity of the curable ESO thermoset and thereduced amounts of unreacted monomer in the ESOthermoset. The presence of unreacted monomers inthe thermoset would plasticize the specimens dra-matically and lower the Tg value. This is owing tothe fact that the plasticizing effect of unreactedmonomers will add flexibility and enhance thedegree of freedom for movement of the molecularchain in the 3-dimensional crosslink network struc-ture.However, from the DSC characterization as dis-cussed previously, it is determined that the degreesof conversion of TEAB-catalyzed ESO thermosets,except for the ES_0.3A thermoset, approach 99%.Hence, the residual monomer concentration incured ESO thermoset is found to be very low. It istherefore, expected that the observed change in Tgvalue is solely due to the influence of crosslink den-sity in ESO thermoset. The increase in TEAB con-centration gives rise to an increment in crosslinkdensity and a reduction in degree of freedom forchain movements and internal rotation in the net-work structure. Moreover, the increase in TEABconcentration in ESO thermoset is expected toinduce more complete curing reaction in whichabout all the unreacted monomers are built intothree dimensional infusible cross-linked networkstructure of ESO thermoset. Thus, ESO thermosetcatalyzed with higher TEAB concentration tends topossess higher Tg value.

3.2.2. Thermogravimetry analysisThermogravimetry analysis (TGA) was used in thispresent study to determine the thermal stability ofTEAB-catalyzed ESO thermoset. Figure 7 presentsthe TGA and DTG curves as a function of tempera-ture for the cured ESO thermoset. Table 3 summa-rizes the TGA results [i.e. onset decompositiontemperature (To) and the maximum decompositiontemperature (Td)] for the cured ESO thermoset. It isclearly noted in Figure 7 that ESO thermoset expe-riences two-stage thermal decomposition whenbeing heated up to 600°C. TEAB-catalyzed ESOthermoset passes through To1 at nearly 200°C, To2 at350°C and Td at 450°C. With approximation 25 wt%of thermal curable ESO decomposes during the first

stage of thermal decomposition and about 71 wt%is lost during the second stage of decomposition.The remaining 4 wt% of the ESO residuals decom-pose totally as the heating temperature is furtherincreased up to 500°C.The first stage of decomposition is believed to bedue to the decomposition of the low-molecular-weight components such as MHHPA curing agentor catalysts. It is believed that, generally, the low-molecular-weight compound will show lower ther-mal stability as the thermal stability is determinedto be proportional to molecular weight. The secondstage of the major decomposition which takes placeat the temperature range of 350 to 400°C could beattributed to the thermal decomposition of ESOcomponent. Similar decomposition behaviour wasreported by Gerbase et al. [20] for the ESO/triethy-lamine/dodecenylsuccinic anhydride mixtures,where two different thermal decomposition stagespresent. Also, Hazer et al. [25] found that the decom-position temperature of the soya components inmicrobial elastomers is at around 310°C. Referringto Figure 7, the first stage effect of ESO thermosetwith different TEAB concentration is found to bediverged from each other. ESO thermoset withhigher TEAB concentration is determined to bemore thermal stable than those thermosets withlower concentration. This finding is believed to beattributed to the increase in degree of crosslinkingand crosslink density as the TEAB catalyst concen-tration is increased. Foussier and Rabek [26] alsoreported in their study that polymer system withhighly cross-linked network structure commonlyexhibit greater thermal stability than those systemswith lower crosslinking.

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Figure 7. TGA and DTG traces of TEAB-catalyzed ESOthermosets

3.3. Kinetics of water absorption of ESOthermoset

Figure 8 shows the effect of TEAB concentrationon the water uptake of the ESO thermoset after sub-jected to water absorption test. It is seen that thewater uptake of all TEAB-catalyzed ESO ther-mosetting resins reach the saturation stage at approx-imately 2000 hours. This is owing to the fact that thewater uptake of TEAB-catalyzed ESO thermosethas achieved the equilibrium condition. The wateruptake would reach the real saturation at equilib-rium stage, in which all the micro-voids in the 3-dimensional crosslinked network structure are filledwith water molecules [27]. Water uptake of curedESO thermosets is attributed to the presence of freevolume and hydrophilic functional groups such ashydroxyl groups in their backbone structure. Thehydroxyl groups in cured ESO thermoset tend toattract water molecules which are polar in nature.The water molecules absorbed in ESO thermosetmay exist as bound water or unbounded cluster.It is clearly noted that the maximum water uptake(Mm) and water diffusion coefficient (D) of thecured TEAB-catalyzed ESO thermoset decreasewith increasing TEAB concentration as shown inTable 4. It is determined that increase the number ofcrosslink in cured ESO thermoset would reduce thepenetration of water molecules into the ESO net-work structure. This is owing to the fact that theintense crosslink network would hinder the watermolecules to diffuse into the free volume followedby residing in the volume available in the ESO ther-moset. Research study conducted by Saijun et al.,[28] also showed that polymeric systems withhigher crosslink density will possess lower free vol-ume and lower water uptake. The water absorption

behaviour is highly dependent on the free volumeand chemical nature of polymer material. The highTEAB concentration in ESO thermoset would giverise to an increase in crosslinking and a decrease infree volume. This phenomenon will eventually leadto a reduction in the capability of the water mole-cules to penetrate and reside in the free volume.Therefore, the maximum water uptake and waterdiffusion coefficient of the ESO thermoset decreasewith increasing TEAB concentration.

4. ConclusionsBased on the work devoted to study the effect oftetraethylammonium bromide concentration on thecuring characteristic, thermal properties and kinet-ics of water absorption of ESO thermoset, the fol-lowing conclusions can be drawn:The polyesterification rate, degree of conversionand crosslink density were higher for ESO ther-moset catalyzed with higher TEAB concentration.From the DMA and TGA studies, it was found thatESO thermoset with higher TEAB concentrationpossessed higher storage modulus, glass transitiontemperature and thermal stability. It was found thatthe water absorption behaviour of ESO thermosetobeyed Fickian law. The maximum water absorp-tion (Mm) and water diffusion coefficient (D) of theESO thermosets were in the range of 1.77 to 2.94%and 0.48 to 1.09·10–12 m2/s respectively. Wateruptake and diffusivity of ESO thermoset can bereduced by increasing the crosslink density in thethermoset.

AcknowledgementsAuthors would like to express their appreciation to Univer-siti Sains Malaysia for the Incentive Grant (8021013) andthe Research University Postgraduate Research GrantScheme (USM-RU-PRGS 8043019). Authors also thankMinistry of Science, Technology and Innovation (MOSTI),Malaysia for the National Science Foundation fellowship.

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Figure 8. Effect of TEAB concentration on water uptake ofESO thermoset

Table 4. Maximum water absorption (Mm) and diffusioncoefficient (D) of ESO thermoset

ESO thermoset Mm [%] D [10–12 m2/s]

TEAB-catalyzed

ES_0.3A 2.94 1.09ES_0.4A 2.60 0.87ES_0.5A 2.34 0.73ES_0.6A 2.28 0.70ES_0.7A 2.09 0.51ES_0.8A 1.77 0.48

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