Epoxidized Glycidyl Ester of Soybean Oil as Reactive Diluent For

7/27/2019 Epoxidized Glycidyl Ester of Soybean Oil as Reactive Diluent For

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Epoxidized Glycidyl Ester of Soybean Oil as Reactive Diluent for

Epoxy Resin

Rongpeng Wang and Thomas Schuman

Department of Chemistry, Missouri University of Science and Technology, Rolla, MO 65409

Abstract

Epoxidized glycidyl esters of soybean oil (EGS) have been synthesized and used as reactive diluents for partial

replacement of a commercial, bisphenol A-based epoxy resin (DGEBA). The EGS merits include a higher epoxy

content and lower viscosity than the epoxidized triglyceride soybean oil (ESO). Thermosetting resins were

fabricated from DGEBA systems blended with various amounts of EGS and ESO, using 4-methyl-1,2-

cyclohexanedicarboxylic anhydride as a curing agent and 2-ethyl-4-methylimidazole as catalyst. The curing

behavior and glass transition were monitored by differential scanning calorimetry (DSC), the performance of

thermosetting resins was studied by measurement of thermal stability and flexural properties. The results

indicate that EGS resins provide better compatibility, intermolecular crosslinking, and yield materials that are

stronger than materials obtained using ESO. However, the EGS resin systems significantly reduce viscosity

compared to either pure DGEBA or ESO-blended DGEBA counterparts. Therefore, EGS derived from renewable

sources holds potential to enable fabrication of complex, shaped epoxy composites for structural applications.

1. IntroductionEpoxy resin is one of the most important thermosetting polymers and widely used in coatings, adhesives

and composites due to its excellent mechanical strength, outstanding chemical resistance, good thermal and

electrical properties, and low shrinkage upon cure.1 Most commercially epoxy resins are relatively high

viscosity liquids or solids. Cured pure epoxy resins are rigid and brittle materials with low impact resistance.

Adding linear elastomeric or thermoplastic additives can increase the toughness; however, this invariably

results in a corresponding decrease in resin flow and processing difficulty.2

Reactive diluents are used for reducing and controlling the viscosity of epoxy resins to improve wetting

and handling characteristics because in the liquid-moulding technologies like resin transfer moulding or

pultrusion, the viscosity and resin flow are critical to achieving a quality laminate.3 Recent trends toward lower

Correspondence to: T. Schuman ([email protected])


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VOC, higher solids epoxy formulations have also resulted in increased utilization of reactive diluents.1 Phenyl

glycidyl ether and n-butyl glycidyl ether are efficient and widely used diluents, but they have been losing

interest due to toxicity, volatility or obnoxious odor issues. Industrial trends show increasing interest in longer

chain reactive diluents, e.g., C12C14 alkyl glycidyl ethers, neopentyl glycol diglycidyl ether, diglycidyl ether of

polypropylene glycol. These longer chain reactive diluents can also behave as flexibilizing agents to increase

thermosetting polymer elongation and impact resistance, though there is often a tradeoff of tensile strength,

glass transition temperature and chemical resistance.1

Since petroleum resources are limited, polymers based on vegetable oils are of great interest because they

are renewable and can significantly contribute to a more sustainable development.4-6

Epoxidized soybean oil

(ESO) has attracted great interest because of a plentiful soybean supply in the United States and therefore of

relatively low cost. ESO can be crosslinked into thermosetting polymers by various curing agents.7

However,

due to a low oxirane content and sluggish reactivity of internal epoxy groups, the cured ESO normally has a lowcrosslinking density, resulting from partially unreacted ESO and saturated fatty acid chains that act to plasticize

and degrade the thermal and mechanical properties of cured resin.

Most ESO industrial uses are limited to nonstructural applications such as plasticizers or stabilizers for poly

vinyl chloride (PVC)8, oil-base coatings

9with low strength requirements.

10ESO has a moderate viscosity and

offers good miscibility with epoxy resins.11 So ESO or their derivatives can be used as reactive diluents for the

partial replacement of epoxy resin to decrease the overall cost and improve the processability.2,12,13 Generally,

the mechanical strengths of ESO blended resins are not comparable to those of pure, non-modified epoxy

resins, while their toughnesses are better due to the introduction of a two phase structure.14-17

Only those oils of poly-unsaturated fatty acid content, especially soybean or linseed oils that can produce

dense epoxy functionality resins, are capable to produce satisfactory properties.7,18,19 Epoxidized vegetable oils

(EVO) of low oxirane values either are not reactive or impart waxy, non-curing properties to the resin system. In

this research, epoxy resins of epoxidized glycidyl ester (EGS) derived from soybean oils were synthesized and

examined. The goals were to remove as much as possible the plasticizing effect of saturated components, to

further increase the oxirane content, and to minimize viscosity to permit use as an efficient reactive diluent.

We reason that EGS with the addition of a terminal epoxy group (glycidyl), which is readily accessible to

nucleophilic attack, should further support reactivity compared with standard, commercial ESO. The increase

in oxirane content, consequent reduction of the ESO molecular size, and a facilitated removal of the saturated

component should each provide a denser intermolecular crosslinking structure and yield a thermosetting resin

material with improved properties.


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2. Experimental2.1. Materials

Refined, food grade soybean oil was purchased from a local grocery store. ESO was purchased from Union

Carbide Corporation. Epichlorohydrin, methylene chloride, meta-chloroperoxybenzoic acid (MCPBA), sodium

carbonate, sodium bicarbonate, sodium sulfite, and anhydrous sodium sulfate were purchased from Fisher

Scientific. Cetyltriethylammonium bromide (CTEAB), 2-ethyl-4-methylimidazole, and 4-methyl-1,2-

cyclohexanedicarboxylic anhydride (MHHPA) were purchased from Aldrich. Commercial DGEBA was supplied

by Momentive with trade name EPON Resin 828. Mold release agent Chemlease 41-90 EZ was purchased

from Chem-Trend, Inc.

2.2. Soap and free fatty acid preparationSoybean oil derived free fatty acids (FFA) were made via acid neutralization of soap. Vegetable oil was

reacted with sodium hydroxide to generate soap, then acidified with sulfuric acid to pH


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Glycidyl ester (GE) and sodium carbonate were mixed with methylene chloride. MCPBA dissolved in

methylene chloride was added dropwise at a reaction temperature below 15C for 4 hours to complete

epoxidation. The reaction mixture was washed with 10% sodium sulfite and then by 10% sodium bicarbonate

and water. Methylene chloride was removed by in vacuo rotary evaporation and the product EGS was dried

using anhydrous sodium sulfate. Epoxidation also was accomplished by an analogous performic acid process

except using toluene as solvent instead of benzene.20

2.4. CharacterizationInfrared spectra (IR) were conducted on a Nicolet Nexus 470 E.S.P. spectrophotometer.

1H NMR spectra

were obtained on a Varian VXR 400 MHz spectrometer using DMSO-d6 as solvent. Iodine value measurements

were based on ASTM Method D554-95. Oxirane value was measured using AOCS Method Cd 9-57. Viscosity

was tested on a Brookfield LVDV-III+ Ultra Rheometer at 25C.

2.5. Curing reactionThe weight ratios of EGS/ESO to DGEBA resin blend chosen for the present work were 0:100 (pure DGEBA),

10:90, 30:70, 50:50, 70:30; and 90:10. Stoichiometric weight of MHHPA curing agent and 1 wt% (based on

epoxy part) of 2-ethyl-4-methylimidazole were added to the epoxy resin blend. After mixing by a high shear

mixer for 10 min, the mixture was degassed for 30 min, then poured into a mold sprayed with a mold release

agent. Curing was performed at 145o

C for 15 hrs for all blends except ESO-DGEBA (90:10) blend, which was

firstly pre-cured at 145oC for 10 min then mixed again and poured into the mould for fully cure at 155

oC for 15

hrs, because of the low reactivity of ESO. The postcure for all samples were performed at 175oC for 1 hr.

The mixtures weighing 2 to 3 mg encapsulated in aluminum hermetically sealed pan were also cured on a

differential scanning calorimetry machine at a heating rate of 10 oC/min from 40-250 oC to study the cure

behavior of the different formulations.

2.6. Thermal TestsDSC (Q2000, TA Instrument) was used to determine the glass transition temperature (Tg) of cured resin.

Measurement was carried out over a temperature range from -40 to 180 C at a heating rate of 20 C/min.

Samples were first preheated to 180 C and quenched with liquid N2 to remove any thermal history.

TGA (Q50, TA Instrument) was used to determine the thermal stability of cured resin. Measurement was

performed from 30 to 750 C at a heating rate of 10 C/min under an ambient air flow environment.


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2.7. Mechanical TestsThe flexural properties were determined according to the ASTM method D790 on an Instron 4469

universal testing machine. The modulus was determined in a three-point bending mode, with a sample

dimension of 102 mm12.7 mm3.2 mm. The span was 50.8 mm, the crosshead speed was set at 12.7

mm/min.

3. Results and Discussion3.1. Preparation of Epoxidized Glycidyl Ester

Scheme 1. Synthetic route to EGS. (Soybean oil and fatty acids are shown as simplified structures

containing only oleic acid though they also contain saturated and polyunsaturated fatty acids. See the text for

detail.)

Scheme 1 shows the synthetic route to EGS, oleic acid generalizing a soybean fatty acid chain. Preparation

of mixed FFA from triglyceride is straightforward and well-developed. Acetone was used as a low boiling

recoverable solvent to prepare soap and effect low temperature crystallization. A slight excess of NaOH with

higher concentration was desirable when preparing soap from FFA because unsaturated FFAs are prone to

dissolve in acetone rather than react with base and also unsaturated FFA soap is more soluble in water.21

Carefully dried and finely powdered soap resulted in greater yields of glycidyl esters of fatty acids.22


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A low solubility of soap in epichlorohydrin suggested a phase transfer catalyst was needed to accelerate

the reaction. With CTEAB catalyst, the consumption of soap was completed within half an hour under reflux

condition. Glycidyl esters can also be prepared directly from FFA in EPCH rich medium, and then subsequently

dehydrohalogenating with alkali, but the yield and purity were lower than the soap process.21

The epoxidation

of glycidyl ester of soybean oil (GES) was carried out using MCPBA or in situ generated performic acid. The

former was more efficient, however, due to the low solubility of MCPBA in methylene chloride, large amounts

of recoverable solvent was required for the epoxidation.

Figure 1. IR spectra of soybean oil, mixed-FFA, GES and EGS

Figure 1 shows the FT-IR spectra of soybean oil, mixed FFA, GES and EGS. The band at 3008 cm-1

was

attributed to the C-H stretching of =CH in unsaturated fatty acid, such as oleic acid, linoleic acid or linolenic acid.

New bands at 910 cm-1 and 852 cm-1in the spectrum of GES, with the disappearance at 937 cm-1 in spectrum of

mixed-FFA means the occurrence of glycidyl group. The conversion of double bonds to epoxy was confirmed by

the disappearance of the 3008 cm-1 band in GES, and the appearance of band at 752 cm-1 in EGS.


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Figure 2. 1H NMR spectrum and structural assignments of FFA, GE and EGS.

Table 1. General physical properties of epoxy resins

Epoxy ResinOxirane oxygen

(g/100g sample)EEW

Viscosity at 25oC

(mPaS)

EGS 10.1 158 70

ESO 6.9 232 430

DGEBA 8.6 186 13000

Figure 2 shows the1H

NMR spectra of mixed-FFA, GES and EGS, where linoleic acid was used as a

generalized compound for structural assignments. The spectra showed no evidence of side reactions in

preparing GES using soap process, nearly quantitative conversion of double bonds to epoxy groups, and no

oxirane ring opening during the epoxidation of GES to EGS using MCPBA, i.e., showed complete conversion but

a lack of side reactions.

General properties of EGS product compared to ESO, ELO, EGL and DGEBA is shown in Table 1.


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3.2. Viscosity reducing ability

Figure 3. Viscosity of DGEBA with various EGS/ESO concentrations.

It was found that EGS had inherently lower viscosity than ESO. EGS has an extra glycidyl group and lower

molecular weight compared to ESO, which is a triglyceride and has oligomeric behavior. The viscosity reducing

abilities of EGS and ESO were tested at different concentrations replacement of DGEBA resin, which has a high

viscosity at 13000 mPaS (see Figure 3). ESO and EGS all showed good miscibility with DGEBA; however, EGS

exhibited a much better viscosity reducing ability than ESO. Only 30 wt% of EGS reduced the DGEBA resin

viscosity to value below 1000 mPaS, which is indispensable for many applications. At least 50 wt% of ESO was

needed to reduce DGEBA resin to the same viscosity.


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3.3. Curing of Reaction

Figure 4. Dynamic thermograms of DGEBA-EGS/ESO-MHHPA systems


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Table 2. DSC results of curing DGEBA-EGS/ESO-MHHPA systems

Composition H, (J/g)*H, (kJ/mol)

Tonset, (

oC) Tpeak,(

oC)

Pure DGEBA 355.3 66.4 133.7 151.910wt% EGS 347.7 63.8 138.0 156.6

30wt% EGS 348.2 61.8 138.1 157.7

50wt% EGS 340.2 58.4 136.9 159.2

70wt% EGS 336.6 55.9 136.5 160.9

90wt% EGS 334.8 53.9 136.7 163.3

Pure EGS 321.5 52.5 141.3 167.6

10wt% ESO 359.9 68.5 134.4 153.9

30wt% ESO 320.6 63.4 135.1 156.0

50wt% ESO 300.2 61.7 140.3 159.8

70wt% ESO 281.4 60.3 143.2 167.8

90wt% ESO 250.1 55.9 147.6 205.4

Pure ESO 230.0 52.6 182.6 215.9

* based on the total number of epoxy groups.

Differential scanning calorimetry was applied to study the curing behavior of the blended epoxy resins, as

shown in Figure 3, the exothermic peaks were characteristic of the epoxy and anhydride curing reactions.

Integration of these peaks allows the determination of the enthalpy of curing reaction(H), onset curing

temperature (Tonset) and exothermic peaks (Tpeak). The results are shown in Table 2.

From Figure 3, the pure DGEBA and ESO all show single exothermic reaction peaks at 152oC and 216

oC,

respectively. The higher Tpeakvalue of ESO means a slower reaction rate, which was also confirmed by a lower

H value in Table 2. A lower oxirane content of ESO and also internal epoxy function groups react sluggishly

with MHHPA curing agent.

The addition of ESO to DGEBA lead to a shifting ofTpeakand Tonset to higher values. With a decrease ofH

value, two partially overlapped peaks were clearly observed, especially for 50 wt% ESO or higher replacement,

which suggested that there are decreasing levels of ESO miscibility in the DGEBA. Non-homogenous mixing will

prevent complete cure of the epoxy resin. ESO has internal, hindered epoxy groups whereas DGEBA has

glycidyl groups of less steric hindrance and more reactive than the internal epoxy groups of ESO. Similar results

have also been reported by Altuna23

and Boquillon.24


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The prepared EGS resin showed a quite different and interesting curing behavior. The neat EGS shows two

overlapped peaks, similar to the blend of DGEBA and ESO, which is believed to be due to the inherent different

reactivity of glycidyl and internal epoxy function groups. The Tpeakand Tonsetvalue of EGS were more than 40oC

lower than ESO, which indicated EGS is much more reactive than ESO. Increased addition of EGS to DGEBA also

lead to shifting ofTpeak to higher values, but the Tonset remained nearly constant, and only a 16oC increase of

Tpeak was observed for 90 wt% EGS replacement compared to pure DGEBA, while it was 54oC for 90 wt% ESO

replacement.

The H (J/g) also followed a similar trend. The higher oxirane content of EGS, which bears glycidyl groups

like DGEBA, appears to facilitate a more homogenous three dimension polymer structure upon curing

compared to ESO blends. One note of interest was that a small amount of replacement, e.g., 30 wt% EGS or

below, or 10 wt% ESO, had little effect on the H or Tpeak values of DGEBA, which may be related to

homogeneity and compatibility with the DGEBA.

3.4. Thermal Properties

Figure 5. The change of glass transition temperatures with various ESO/EGS contents


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The glass transition temperature (Tg) is considered one of the fundamental characteristics as it relates to

polymer properties and processing. For a polymer to serve as a useful plastic, its Tg must be appropriately

higher than the temperature of its intended work environment.25

The aliphatic amine,7,26

or boron trifluoride

diethyl etherate27,28

cured ESO polymers generally have very low Tg (even below 0oC). Aromatic amine

29,

cycloaliphatic amine,2thermal latent initiator30 or anhydride31 cured ESO polymers generally have higher Tg, but

it is still rare to see a cured ESO6with Tg above 60

oC. Low Tg represents a low crosslink density, as mentioned

above. Due to the low oxirane content of ESO, sluggish reactivity of internal epoxy groups with nucleophilic

curing agents, the self intermolecular crosslinking of ESO, and unreactive saturated components like stearic

acid, palmitic acid or myristic acid that act as plasticizers, which further decrease the polymer Tg.

DSC and Dynamic Mechanical Analysis (DMA) are widely used to characterize the Tg. It is necessary to note

here that for most thermosetting plastics, the DMA measurement based on the tan peak at a frequency of 1

Hz generally occurs at a temperature as much as 15-20

o

C above Tg as measured by dilatometry or DSC.

32

Thechange of cured epoxy resin Tg as measured by DSC is shown in Figure 5.

Not surprisingly, the MHHPA cured pure EGS had a much higher Tgat 88oC, which was nearly 40

oC higher

than ESO though still lower than the pure DGEBA resin.

Addition of ESO or EGS lead to a decrease ofTg,for smaller replacement, e.g., below 30 wt%, the Tg values

of ESO-DGEBA or EGS-DGEBA systems were quite similar, which indicated the Tg behavior was mainly

determined by the DGEBA structure. Further increase the replacement contents of ESO or EGS, theTg values

decreased rapidly, especially for the ESO system. The inherent aliphatic long chain structure of ESO and the

higher saturated content and lower epoxy groups make it difficult to produce a densely crosslinked structured

polymer as some segments will vibrate more freely upon thermal stress.12

It was also found that neat ESO or higher replacement (above 50 wt%) of ESO-DGEBA thermosetting

polymer showed a broad transition from the glassy to the rubbery state. Similar behavior was also found in

ELO replacement of di-glycidyl ether of bisphenol F (DGEBF) resin,33

which was not found in EGS-DGEBA system.

The plasticizing effect of saturated fatty acids in the network32and/or the different reactivity of ESO and DGEBA

lead a broad distribution of polymer molecular weight and indicat a heterogeneous polymer network was

formed.34

Figure 6 presents the TGA curves as a function of temperature for the cured epoxy resin. Since ESO-

DGEBA resin had similar thermal stability with EGS-DGEBA resin, only the latter is shown here. TGA results

indicated all cured EGS-DGEBA resins appear thermally stable to temperatures below 300 oC. Addition of EGS

lead to an earlier onset of degradation. All the resins similarly presented two stage degradation behavior. The

first stage of decomposition, from 350 to 450oC, is believed to be due to the pyrolysis of the crosslinked epoxy


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resin network, decomposition of unreacted MHHPA, and dehydration of hydroxyl groups. The second loss

stage from ~450 to 600 oC was considered to be the complete decomposition of the smaller fragments like

cyclized or aromatic degradation byproducts as indicated by a decrease of char residue when EGS component

was increased in DGEBA.

Figure 6. TGA analysis of cured EGS-DGEBA blends compared to pure EGS and pure DGEBA.

3.5. Mechanical PerformanceThe flexural properties of the cured resin system with varying ESO/EGS content were determined and the

results were listed in Table 3. As can be seen from Table 3, 10 wt% of ESO or EGS replacement of DGEBA lead

to an improvement in flexural modulus, although further increases of ESO/EGS content lead to a decrease of

flexural modulus. Similar results were also reported in an amine cured soy-based epoxy resin system.34

The

flexural stress of EGS-DGEBA exhibited a gradual decrease until 50 wt%, then followed an abrupt degradation.

While for ESO-DGEBA, such degradation occurs at 30 wt%.

As discussed previously, two overlapping peaks in the curing curves determined by DSC were observed

when high contents of EGS/ESO were added into DGEBA. Different reactivity or reduced compatibility of


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ESO/EGS with DGEBA, may ultimately prevent the ESO/EGS resin from fully participating in the crosslinking.

Therefore, the resulting crosslinked thermosets may become increasingly plasticized by partially reacted

ESO/EGS resins at high contents, leading to a decrease of flexural strength. EGS has higher oxirane value and

was observed to be more reactive with DGEBA compared to ESO, so a higher content of EGS replacement of

DGEBA was achieved with less degradation of the mechanical strength. Due to the inherent lower epoxy

content and sluggish reactivity of internal epoxy function groups, a lower ESO content replacement was

required to prevent additional sacrifice of mechanical performance.

Table 3. Flexural properties of cured DGEBA-EGS/ESO resin

Composition Flexural StrengthMPa Flexural ModulusMPa

Pure DGEBA 138.1 2945.0

10wt% EGS 133.0 3162.130wt% EGS 125.2 2837.8

50wt% EGS 121.9 2829.1

70wt% EGS 91.0 2608.2

90wt% EGS 81.3 1687.6

10wt% ESO 124.2 2977.2

30wt% ESO 120.8 2822.8

50wt% ESO 107.0 2471.0

70wt% ESO 81.4 2270.8

90wt% ESO 60.0 1652.8

4. ConclusionEGS resin materials were produced from soybean oils with reduced saturated FFA fraction content. The

products were characterized and showed high oxirane contents and were more reactive than ESO. The EGS

blends were cured by MHHPA and their thermosetting polymer Tgs measured in comparison to control ESO

cured in similar fashion. The EGS resin systems had significantly reduced viscosity compared to their pure

DGEBA or ESO-blend epoxy counterparts. The products displayed glass transitions that were a fairly simple

function of oxirane content with some added influence of glycidyl versus internal oxirane reactivity. The

products displayed improved Tgs and mechanical properties compared to their ESO counterparts and, in

addition to an inherently low viscosity and efficient viscosity reduction, should therefore be more attractive as


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a reactive diluent. For instance, EGS derived from renewable sources could further enable defect-free

fabrication of complex, shaped epoxy composites for structural composite applications.

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Epoxidized Glycidyl Ester of Soybean Oil as Reactive Diluent For

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