Effect of Epoxidized Oils on Properties of Epoxy Resins

Effect of Biodegradable Epoxidized Castor Oil on

Physicochemical and Mechanical Properties

of Epoxy Resins

Soo-Jin Park,* Fan-Long Jin, Jae-Rock Lee

Advanced Materials Division, Korea Research Institute of Chemical Technology, P.O. Box 107, Yusong,Daejeon 305-600, South KoreaFax: (þ82) 42-861-4151; E-mail: [email protected]

Received: May 25, 2004; Revised: July 15, 2004; Accepted: July 15, 2004; DOI: 10.1002/macp.200400214

Keywords: biodegradable; cationic polymerization; crosslinking; mechanical properties; thermal properties

Introduction

Recently, replacement of petroleum-derived raw materials

with vegetable oil-based polymeric materials has become

important from the social and environmental viewpoints.[1,2]

Vegetable oils are complex multi-component mixtures of

different triacylglycerols, that is, esters of glycerol and fatty

acids.[3] Vegetable oils containing unsaturated fatty acids can

be valuable in chemical industry because they can be used in

polymerizations to make biobased polymers.[4–6]

Among them, castor oil represents a promising raw ma-

terial based on its low-cost, low toxicity, and its availability

as a renewable agricultural resource. Castor oil is used

commercially in large amounts, and its major constitution,

ricinoleic acid (12-hydroxy-cis-9-octadecenoic acid), is a

hydroxyl fatty acid.[5] There have been many studies on the

synthesis and characterization of a wide variety of polymers

based on vegetable oil. Patel et al. and Zlatanic et al.

reported interpenetrating polymer network from castor oil-

based polyurethanes with poly(methyl methacrylate) and

4,40-diphenylmethane diisocyanate and polyols based on

vegetable oils.[7,8] Hu and co-workers also examined poly-

urethaneurea/vinyl polymer hybrid aqueous dispersions

based on renewable material.[9] Diamond et al. studied

modeling the polymerization behavior of vegetable oil

derived macromonomers in solution by computer simula-

tion.[10] Srivastava and co-workers showed gel point pre-

diction of metal-filled castor oil based polyurethanes

system.[11] Thames et al. examined cationic ultraviolet cur-

able coatings from castor oil glycidyl ether and epoxy

Summary: Biobased epoxy materials were prepared fromdiglycidyl ether of bisphenol A (DGEBA) and epoxidizedcastor oil (ECO) initiated by a latent thermal catalyst. Thephysicochemical and mechanical interfacial properties of theDGEBA/ECO blends were investigated. As a result, the ther-mal stability of the cured epoxy blends showed a maximumvalue in the presence of 10 wt.-% ECO content, which wasattributed to the excellent network structure in the DGEBA/ECO blends. The storage modulus and glass transition tem-perature of the blends were lower than those of neat epoxyresins. The mechanical interfacial properties of the curedspecimens were significantly increased with increasing theECO content. This could be interpreted in terms of the addi-tion of larger soft segments of ECO into the epoxy resins andthus reducing the crosslinking density of the epoxy network,which results in increasing toughness in the blends.

KIC values of the DGEBA/ECO blends as a function of ECOcontent.

Macromol. Chem. Phys. 2004, 205, 2048–2054 DOI: 10.1002/macp.200400214 � 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

2048 Full Paper

resins.[12] And, Gultekin and co-workers reported styrena-

tion of castor oil and linseed oil by macromer method.[13]

Recently, Park et al. studied synthesis and thermal proper-

ties of epoxidized vegetable oil initiated by a thermal latent

catalyst.[14]

Latent catalysts are inert under normal conditions, such

as ambient temperature and room light, but they show acti-

vity with external stimulation, such as heating or photo-

irradiation. The use of latent catalysts increases the storage

stability and handling of thermosetting resins.[15] There-

fore, latent catalysts are important in the field of thermo-

setting resins, such as epoxy resins and multi-functional

vinyl ethers.[16]

The use of liquid rubbers, such as carboxyl-terminated

butadiene acrylonitrile (CTBN) and amine-terminated buta-

diene acrylonitrile (ATBN), as impact modifiers to increase

the toughness of the epoxy resins, can lead to a reduction of

final thermal and mechanical properties.[17] Thus, it is more

interesting that the introduction of biobased epoxidized

castor oil into the epoxy network leads to an improvement

in the toughness of the epoxy resins without degrading the

thermal properties and elastic modulus.

In this work, novel biobased epoxy blends were prepared

from diglycidyl ether of bisphenol A (DGEBA) and

epoxidized castor oil (ECO) initiated by N-benzylpyrazi-

nium hexafluoroantimonate (BPH) as a latent thermal

catalyst, and the effect of the ECO contents on the phy-

sicochemical and mechanical interfacial properties of the

DGEBA/ECO blends was studied by using differential

scanning calorimetry (DSC), thermogravimetric analysis

(TGA), dynamic mechanical analysis (DMA), an universal

test machine (UTM), and scanning electron microscopy

(SEM).

Experimental Part

Materials

Epoxy resins used in this study were DGEBA supplied byKukdo Chem. of Korea (YD-128), which had an epoxideequivalent weight of 185–190 g � equiv.�1 and a density ofabout 1.16 g � cm�3 at 25 8C. ECO was synthesized by reactionof castor oil with glacial acetic acid and hydrogen peroxideusing Amberite IR-120 as a catalyst.[14] BPH, which was usedas a thermal cationic latent catalyst, was synthesized by aknown procedure.[18] The chemical structures of the DGEBA,ECO, and BPH are shown in Figure 1.

Synthesis of Epoxidized Castor Oil

Castor oil (91.8 g, 0.14 mol), glacial acetic acid (21.0 g,0.35 mol), Amberlite (23 g), and toluene (40 g) were chargedinto in a round, four-necked 500 mL flask equipped with amechanical stirrer, thermometer sensor, and reflux condenser.The mixture was heated to a constant temperature of 55 8C.Then, 30% H2O2 (56.7 g, 0.5 mol) was added slowly and allow-

ed to react at 55 8C for 7 h. After the reaction was complete, thecrude product was filtered and washed with a saturated solutionof NaCO3 and distilled water, and then dried with anhydroussodium sulfate. Finally, the toluene was removed by distillationunder a vacuum oven at 80 8C for 2 h (yield, 84%).

FT-IR (KBr): 3 009 cm�1 (C C), 822, 833 cm�1 (epoxidegroup).

1H NMR (CDCI3): d¼ 5.3 (2H, C C), 2.9–3.1 (2H,epoxide group).

13C NMR (CDCI3): d¼ 129.7–130.2 (C C), 54.0, 54.3(epoxide group).

Sample Preparation

DGEBA and ECO were mixed to obtain the biobased blends ata weight ratio of 100:0 to 60:40. DGEBA and ECO were mixedin an oil bath at 80 8C for 30 min and then 1 wt.-% of the latentthermal catalyst (BPH) was added to the mixture. The mixturewas completely mixed by a mechanical stirrer and degassed ina vacuum oven to eliminate air bubbles before measuring. Thepreparation of the specimens for physicochemical and mech-anical tests was as follows: bubble-free mixtures were pouredinto the mold and cured at 120 8C for 1 h, at 160 8C for 2 h, andfinally postcured at 200 8C for 1 h in a convection oven.

Characterization and Measurements

The curing of the DGEBA/ECO blends was performed at aheating rate of 10 8C �min�1 under the nitrogen gas with a dy-namic differential scanning calorimeter (Perkin Elmer, DSC6).

The temperature dependence of the loss fact (tan d) for theblends was obtained from their dynamic mechanical propertiesusing a dynamic mechanical analyzer (RDS-II, RhemetricsCo.) in a three-point bending mode. The measurements wereperformed at a frequency of 1 Hz and the temperature rangedfrom ambient temperature to 250 8C at a heating rate of 5 8C �min�1. The specimen dimensions were 3 mm� 12 mm�35 mm.

Thermogravimetric analyses were carried out with a duPontTGA-2950 analyzer to measure the weight loss of the cured

Figure 1. Chemical structures of DGEBA, ECO, and BPH.

Effect of Biodegradable Epoxidized Castor Oil on Physicochemical and Mechanical Properties of Epoxy Resins 2049

Macromol. Chem. Phys. 2004, 205, 2048–2054 www.mcp-journal.de � 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

DGEBA/ECO blends from 30 to 850 8C at a heating rate of10 8C �min�1 in a nitrogen atmosphere.

The fracture toughness parameter, critical stress intensityfactor (KIC), of the DGEBA/ECO blends were measured usinga single-edge-notched (SEN) test in a three-point flexural test,which was conducted according to the ASTM E-399 using anuniversal test machine (Instron Model 1125 mechanical tester)at a cross-head speed of 2 mm �min�1. The flexural propertiesof the blends were surveyed with an Instron Model 1125 mech-anical tester according to ASTM D-790. All mechanical pro-perty values were obtained by averaging of five experimentalvalues.

To investigate the phase morphology of the specimens, thefractured surfaces after KIC test were observed by means of ascanning electron microscope (SEM, JEOL JXA 840A).

Results and Discussion

Physicochemical Properties

To investigate the curing behavior of the DGEBA/ECO

blends, the thermal latent properties, peak maximum tem-

perature, and cure activation energy were investigated by

using dynamic DSC. Figure 2 shows the dynamic DSC scan

curves for the blends initiated by BPH at a heating rate of

10 8C �min�1 and the results are given in Table 1. The Tp

shows a maximum value at 10 wt.-% ECO and decreases

with increasing the above this content. This result shows

that the blends with 10 wt.-% ECO content forms good

network structures at relative high temperature.

The fractional conversion as a function of temperature

for the DGEBA/ECO blends is shown in Figure 3. Both the

polymerization of neat epoxy resins and the blends initiated

by BPH do not proceed below 140 and 100 8C, res-

pectively. But, both of the polymerizations are performed

rapidly above these temperatures. BPH is, therefore, an

excellent thermally latent initiator in the absence of co-

initiators. The polymerization of the blends is initiated at

low temperature, which can be attributed to the polymer-

ization of ECO at relative low temperature.[14] The poly-

merization range of the blends is larger than that of the neat

epoxy resins, which is due to the feature of internal epoxy

groups in the trifunctional ECO.[19]

The cure activation energy (Ea) of the blends were

determined by DSC scans at different heating rates, based

on Flynn–Wall–Ozawa’s equation, as follows [Equation

(1)]:[20]

ln q ¼ lnAEa

R

� �� ln gðaÞ � 5:33 � 1:052

Ea

RTP

ð1Þ

Ea is the cure activation energy, q the heating rate, A the

Arrhenius frequency factor, Tp the peak maximum temper-

ature, g(a) the function form of a, and R the gas constant.

Table 1 shows the Ea determined by Equation (1) as a

function of ECO content. TheEa shows a maximum value at

10 wt.-% ECO and decreases with increasing the above this

content. From these results, it is expected that the addition

of 10 wt.-% ECO to DGEBA can lead to an excellent

network structure in the DGEBA/ECO blends.[21]

The thermal degradation behavior of the DGEBA/ECO

blends was studied with TGA at a heating rate of 10 8C �min�1 in a nitrogen atmosphere and the TGA thermograms

are shown in Figure 4. As observed, the decomposition

Figure 2. Dynamic DSC thermograms of the DGEBA/ECOblends.

Table 1. Peak maximum temperature and cure activationenergies (Et) of the DGEBA/ECO blends as a function of ECOcontent.

DGEBA:ECO 100:0 90:10 80:20 70:30 60:40

Tp 205 206 205 201 196

8CEa 78 110 107 97 94

kJ �mol�1

Figure 3. Conversion of the DGEBA/ECO blends as a functionof temperature.

2050 S.-J. Park, F.-L. Jin, J.-R. Lee


curve behavior of the blends is largely similar to those of

neat epoxy resins. Thermal stability factors, including

initial decomposed temperature (IDT; 5% weight loss) and

temperatures of maximum rate of degradation (Tmax) of the

blends, can be determined from the TGA thermo-

grams.[22,23]

The results of the IDTand Tmax of the blends are listed in

Table 2. It can be seen that the decomposition of the blends

commences near at 358 8C and rapidly continues until

433 8C. From these results, it can be confirmed that the

biobased blends containing 40 wt.-% ECO still show

excellent thermal stabilities. And, the IDT and Tmax of the

blends show maximum values at 10 wt.-% ECO, which can

be attributed to the excellent network structure in the

DGEBA/ECO blends.[21]

Dynamic Mechanical Analysis

To evaluate the effect of the ECO content on glass transition

temperature and crosslinking density of cured epoxy resins,

the storage modulus and loss factor (tan d) measurement

were conducted by DMA in a wide temperature range 0 to

250 8C at a heating rate of 5 8C �min�1. Figure 5 shows the

temperature dependence of the tan d and storage modulus of

cured DGEBA/ECO specimens. From the DMA spectra,

the blends show a single relaxation and locate at a lower

temperature than that of neat epoxy resins with increasing

the ECO content. It confirms that the ECO is partially

miscible with the epoxy resins.[24] The glass transition tem-

perature (Tg) value was recorded as the maximum value of

the tan d curve.

The crosslinking density (r) of cured specimens was

calculated from the equilibrium storage modulus in the

rubber region over the a-relaxation temperature according

to the rubber elasticity theory [Equation (2)].[25]

r ¼ G0�fRT ð2Þ

Tg is the a-relaxation temperature,G0 the storage modulus at

Tgþ 30 8C, f the front factor, R the gas constant, and T the

absolute temperature at Tgþ 30 8C.

The obtained Tg, G0, and r are summarized in Table 3.

The Tg and r of the DGEBA/ECO blends are systematically

decreased with increasing the ECO content. This is caused

by the addition of larger soft segments of ECO into the

epoxy resins and decrease the rigidity of the epoxy network,

Figure 4. TGA thermograms of the DGEBA/ECO blends.

Table 2. Thermal stabilities of the cured DGEBA/ECO blends.

DGEBA:ECO 100:0 90:10 80:20 70:30 60:40

IDT 378 384 371 360 358

8CTmax 431 438 437 435 433

8C

Figure 5. tan d (a) and storage modulus (b) of the DGEBA/ECOblends as a function of temperature.



leading to an increase the motion of segments of the macro-

molecules in the blends.[21] The loss factor intensities of the

blends gradually increase with increasing the ECO content.

And, Tg and G0 of the blends are 131 8C and 1.15 GPa at

40 wt.-% ECO, respectively. Therefore, the biobased epoxy

blends are still maintaining relative higher Tg and G0.

Mechanical and Mechanical Interfacial Properties

The mechanical and mechanical interfacial properties for

the DGEBA/ECO blends were determined, in terms of the

fracture toughness, flexural strength (sf), and elastic modu-

lus in flexure (Eb). Fracture toughness of cured specimens

was measured by three-point bending tests for the critical

stress intensity factor (KIC), as follows [Equation (3)]:[26]

KIC ¼ P � Lb � d3=2

� Y ð3Þ

P is the rupture force, L the span length between the

supports, a the depth of notch, b the specimen width, d the

specimen thickness, and Y the geometrical factor.

Figure 6 shows the obtained KIC values of the DGEBA/

ECO blends for different content of ECO up to 40 wt.-%. As

observed, the neat epoxy resins have high crosslinking

density and brittleness, with a KIC value of 1.7 MPa �m1/2.

The toughness is significantly increased with increasing the

ECO content and shows a maximum value of 3.5 MPa �m1/2

at 30 wt.-% ECO content. The addition of larger soft seg-

ments of ECO into the epoxy resins leads to an increase of

the flexible properties of the network structure, which is

probably due to the reduced crosslinking density in the

blends.[21,27]

The flexural strength (sf) and elastic modulus (Eb) in

flexure values of the blends were obtained with the three-

point bending tests and were calculated as follows

[Equation (4) and (5)].[26]

sf ¼3PL

2bd2ð4Þ

Eb ¼ L3

4bd3� DPDm

ð5Þ

P is the applied load, L the span length, b the specimen

width, d the specimen thickness, DP the change in force in

the linear portion of the load-deflection curve, and Dm the

change in deflection corresponding to DP.

Figure 7 shows the result of flexural strength (sf) values

of the blends as a function of ECO content. The sf values of

the blends indicate that flexural strength increases from

87.6 to 117.1 MPa with increasing ECO content of 30 wt.-

%. This result means that the addition of larger soft seg-

ments of ECO into the epoxy resins increases the ductility,

resulting in increasing the flexural strengths of the

blends.[21,27] The elastic modulus (Eb) in flexure values of

the blends are shown in Figure 8. TheEb values are constant,

Table 3. Dynamic mechanical analysis of the cured DGEBA/ECO blends.

DGEBA:ECO Tg Storage modulus (GPa) r

8C Glassyregiona)

Rubberyregionb)

10�3 mol � cm�3

100:0 197 1.27 0.102 4.6190:0 169 1.19 0.077 3.6880:20 158 1.22 0.065 3.1870:30 150 1.15 0.051 2.5460:40 131 1.15 0.041 2.13

a) Storage modulus at 30 8C.b) Storage modulus at Tgþ 30 8C.

Figure 6. KIC values of the DGEBA/ECO blends as a function ofECO content.

Figure 7. sf values of the DGEBA/ECO blends as a function ofECO content.



although the ECO content is increased. From the results of

the mechanical and mechanical interfacial properties, it is

confirmed that the addition of ECO into the epoxy resins

leads to the increase in fracture toughness and flexural

strength without decreasing the elastic modulus in the

blends.

SEM was used to examine the morphology on fractured

surfaces of the specimens after KIC tests. In Figure 9(a), the

SEM micrograph of the neat epoxy resins shows regula-

tive cracks in the fracture surface, indicating a brittle

fracture surface. On the other hand, the micrographs of the

blends show tortuous cracks and exhibit many ridges with

increasing the ECO content, indicating a reinforced mor-

phology, as shown in Figure 9(b)–(e). This is why the

DGEBA/ECO blends exhibit higher mechanical and mech-

anical interfacial properties than those of the neat epoxy

resins.Figure 8. Eb values of the DGEBA/ECO blends as a function ofECO content.

Figure 9. SEM micrographs of the DGEBA/ECO blends after KIC tests: (a) 100:0; (b)90:10; (c) 80:20; (d) 70:30; (e) 60:40 (magnification of 500).



Conclusion

In this work, the epoxy blends were newly prepared from

commercial DGEBA and biobased ECO initiated by BPH

as a latent thermal catalyst, and the effects of ECO content

on the cure behavior, thermal stabilities, and mechanical

properties of the DGEBA/ECO blends were discussed. The

cure activation energy and initial decomposed temperature

of the blends showed a maximum value at 10 wt.-% ECO

content, which was attributed to the excellent network

structures in the DGEBA/ECO blends. The glass transition

temperature and storage modulus of the blends were decre-

ased with increasing the ECO content. And, the mechanical

and mechanical interfacial properties of the blends were

significantly increased with increasing the ECO content.

This could be attributed to the addition of larger soft seg-

ments of ECO into the epoxy resins and thus increased the

flexible properties of the epoxy network, resulting in an

improvement in the toughness of the blends.

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Effect of Epoxidized Oils on Properties of Epoxy Resins