Epoxidized Soybean Oil-Based Epoxy Blend ... - ACS Publications

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Epoxidized soybean oil based epoxy blend cured with Anhydride based crosslinker: Thermal and Mechanical Characterization Sudheer Kumar, Sushanta Kumar Samal, Smita Mohanty, and Sanjay Kumar Nayak Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b03879 • Publication Date (Web): 26 Dec 2016 Downloaded from http://pubs.acs.org on December 26, 2016

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Industrial & Engineering Chemistry Research

Epoxidized soybean oil based epoxy blend cured with Anhydride based crosslinker: Thermal and Mechanical Characterization Sudheer Kumar*, Sushanta K. Samal, Smita Mohanty, Sanjay K. Nayak Laboratory for Advanced Research in Polymeric Materials (LARPM), Central Institute of Plastic Engineering & Technology (CIPET), B/25, CNI complex, Patia, Bhubaneswar 751024, Odisha, India, Fax: +91-674-2743863; Tel: +91-674-2742852 *Corresponding Author, E-mail id: [email protected] ABSTRACT The present research is based on, a comparative study of anhydride cured bio-based and petroleum based epoxy network. The effect of epoxidized soybean oil (ESO) bioresin on petroleum

based

epoxy

(DGEBA)

at

varying

composition

cured

with

methylhexahydrophthalic anhydride (MHHPA) as curing agent and 2-methyl imidazole (2MI) as the catalyst has been investigated. The tensile strength of virgin epoxy (42.94 MPa) increased to 48.62 MPa with the addition of 20% of ESO. The fracture toughness parameters; critical stress intensity factor (KIC) and critical strain energy release rate revealed enhancement of toughness in the bio-based blends. Differential scanning calorimetry (DSC) studies confirmed an enhancement in the peak temperature and a reduction in the heat of curing in virgin epoxy on incorporation of ESO content. The thermomechanical and fracture morphological properties of virgin epoxy, ESO and its bio-based blends were investigated by thermogravimetric analysis (TGA), dynamic mechanical analysis (DMA), atomic force microscopy (AFM) and scanning electron microscopy (SEM) respectively. Keywords: Bio-epoxy resin; Epoxidized soybean oil; Mechanical and thermal property; Morphological analysis.

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1. INTRODUCTION Depletion of petroleum resources and increase in awareness of environmental concerns associated with petroleum derived products has inspired the polymer researchers to design and synthesize bio-based materials from renewable resources. Among the thermoset polymers, diglycidyl ether of bisphenol-A (DGEBA) based epoxy has been extensively in electronics, automobile, aerospace, adhesive and coatings industries.1–5 However, its hazardous aspect, non-renewable nature, and brittleness characteristics have been the major impediment in various applications. Hence development of a sustainable green material bestowed with desired attributes and comparable performance with petroleum based (DGEBA) resin has gained considerable research interests in the recent years.6–8 Plant oils are triglyceride molecules of glycerol and constitutes of the long aliphatic chain of saturated, unsaturated fatty acids, with one or more double bonds, which, can be functionalized to form crosslinkable groups.9,10 Thus, renewable resource based plant oils are used as an alternative feedstock of petroleum based materials after functionalization through epoxidation11,12 transesterification13,14 acrylation15–17 etc. The degree of unsaturation of the double bonds was defined by the iodine value. Considering the iodine value and availability, soybean oil is a suitable drying oil (iodine value < 90-130) for formation of pre polymers because of its unique composition (linoleic acid 51%, oleic acid 25%, pammitic acid 10%, linolenic acid 7% and stearic acids 5%) and easy availability.18–20 In the recent years, many researchers reported the preparation and characterization, properties of epoxidized soybean oil (ESO) incorporated bio-based thermoset materials, (Sahoo21,22 Ratna,23 Gupta,24 Jin et al.25 ) The epoxidized soybean oil (ESO) based bioresin toughen the thermoset matrix significantly acting as impact modifiers or plasticizers for epoxy.26 However, the incorporation of epoxidized plant oils reduces the stiffness and strength of


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amine cured epoxy drastically because of lower crosslink density and phase separated network which is undesirable for industries. The properties of bio-based epoxy network depend on the nature and functionality of the epoxy and curing agent. Generally amine and carboxylic acid anhydride are used as curing agent for bio-based epoxy resins. But the aromatic polyamine cured network is poorly crosslinked while anhydride cured bio-based epoxies are more rigid and have a higher glass transition temperature and less curing shrinkage.27–29 Further, as compared with aromatic amine curing agent, anhydride based are superior in respect low viscosity, are readily miscible with epoxy resin, exhibit very low exotherm, very long pot life, and less hazardous.30 Also the anhydride cured product gives good electrical, chemical, and mechanical properties. The anhydride curing agents are commonly used with cycloaliphatic and ESO olefin resins, because of its rapid reaction with anhydride as compared to amine.31,32 In the recent years, many researchers have investigated the effect of different anhydride curing agent and catalyst on properties of cured epoxidized vegetable oils and their blends.33– 36

Among the anhydride based curing agents, methylhexahydropthalic anhydride (MHHPA)

are highly preferable because of high reactivity, liquid at room temperature with low viscosity, short molecular chain and rigid structure. MHHPA cured epoxy resins and its blends yield a high transparent product for high performance applications. Pin et al.37 earlier reported MHHPA cured epoxidized linseed oil (ELO) catalyzed by 2methylimidazole with good thermal stability and higher damping factor. Anhydride is a high temperature curing-agent. The used of 2-metylimidazole (2-MI) as a catalyst balance the pot life and significantly reduced the curing temperature of the reaction, and increases the mechanical, thermal properties of the blends because this is more active initiators for the faster cross-linking.38 Furthermore, 2-MI was more effective than 1-methylimidazole, tertiary amines, 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU), or dimethylaminopyridine in initiating 3 ACS Paragon Plus Environment


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the polymerization of epoxidized soybean oil and epoxy resin and the resulting network exhibited a higher anhydride conversion.39 Tan and Chow investigated the increase in mechanical and thermal properties of ESO/DGEBA blends with an increase in ESO content when cured with MHHPA in the presence of 2-ethyl-4-methyl-imidazole catalyst.40 Ma et al.41–43 reported MHHPA cured itaconic acid based bio-epoxy with higher mechanical performance and thermal stability. No work has been reported on ESO based bio-epoxy cured with MHHPA catalyzed by 2-Methyl imidazole till date. Thus, in current work, MHHPA cured virgin epoxy, ESO and epoxy/ESO based blends were prepared in the presence of 2-methylimidazole (2-MI) catalyst.44 The comparative study of petroleum based DGEBA and ESO-based product is presented describing various properties. 2. EXPERIMENTAL 2.1 Raw Materials The diglycidyl ether of bisphenol-A (DGEBA) with an epoxide equivalent weight (EEW) of 185-190 g eq-1 was purchased from Sigma Aldrich Company Bangalore, India. Methyl hexahydrophthalic anhydride (MHHPA >99%, equivalent weight 168) and 2-methyl imidazole (>99% 2-MI) were obtained from TCI Chemicals (India) Pvt. Ltd. The commercial grade of epoxidized soybean oil (EEW=222-230 g/mol, iodine value=2.0 max) was purchased from Chemsworth (India). All other chemicals were used as received. The structures of the used chemicals are shown in Figure 1.


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Figure 1. Chemical structure of (a) DGEBA, (b) ESO, (c) MHHPA and (d) 2-MI. 2.2 Preparation of DGEBA-ESO bio-based blends network. Epoxy/ESO bio-based blends were prepared by blending epoxy and ESO at varying ratio of epoxy: ESO (100: 0, 90: 10, 80: 20, 70: 30 and 0: 100). The mixtures of the resin blends were mixed using an overhead mechanical stirrer, followed by ultrasonication for 30 minutes. Then the mixture was kept in a vacuum oven at 70 ºC for 15 minutes to remove the air bubbles. Subsequently, stoichiometric amount of MHHPA (0.85 wt% of total epoxy resin) and 0.90 wt% of 2-MI catalyst (total weight of epoxy resin and curing agent) were added to the resin blends and continuously stirred to get a homogeneous mixture. Then, the mixture was degassed in a vacuum oven at 70 ºC for 10 minutes and poured into a stainless steel mould sprayed with silicon spray. The samples were then cured at 130 ºC for 1 h and 150 ºC for 1 h to achieve complete curing.



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2.3 Characterization 2.3.1 FTIR Analysis FTIR spectra of virgin epoxy resin and bio-based epoxy blends were recorded using an FTIR spectrometer (Thermo Scientific, Nicolet 6700, USA). Each spectrum was obtained by coadding 60 scans with a resolution of 4 cm-1 within the range of 500-4000 cm-1. The degree of conversion of epoxy resin was calculated from the area of the transmittance peak of epoxy group at 822-846 cm-1. The degree of curing conversion (α) of epoxy resin was determined using the equation (1) and explained in the next section. ( )

= 1 − (

)

.......................................................... (1)

Where, A is the area of the peak of the epoxy group of the resins at 822-846 cm-1. In case of epoxy/ESO blends, the band corresponding to phenyl group at 1604 cm-1 was taken as a reference with respect to the epoxy band at 914 cm-1 and degree of curing conversion was calculated by using the following equation (2).22 [(

= 1 − [(

) ( )]

) ( )]

........................................ (2)

2.3.2 Mechanical properties. Tensile and flexural properties of virgin epoxy and its blends were determined using an Universal testing machine (UTM) in according to the ASTM-D 3039 and ASTM-D 790. For measurement of tensile strength, the cross head speed was kept at 2 mm/min while for the flexural test was carried out of cross head speed 5mm/min and span depth of 50 mm. The izod impact strength test was carried out according to ASTM-D 256 standard using an impactometer (M/s Tinus Olesan, USA). Five replicate samples of each batch was taken for


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the measurement of mechanical property and the data reported are the average of six. Further the tests were carried out at standard laboratory condition of 23 ± 5 ºC and 55% RH. 2.3.3 Single-edge notch-bend (SENB) fractures toughness. The critical stress intensity factor (KIC) and critical strain energy release rate (GIC) of the virgin epoxy and its bio-based blends were obtained by the single-edge-notched three-point bending test (SEN-3PB) in accordance with ASTM D 5045 standard. The KIC, GIC values were calculated using the equation 3 and 4 respectively.

= 6/ ⁄ .......................... (3) Where KIC = Critical stress intensity factor = 1.93(%⁄ )⁄ − 3.07(%⁄ )(⁄ + 14.53(%⁄ ),⁄ − 25.11(%⁄ ).⁄ + 25.80(%⁄ )0⁄ ,

Y is the geometrical factor, % the average per-crack length, P the maximum load (in kN), B the sample thickness (in mm), and W the specimen width (in mm). The critical strain energy release rate (GIC) was determined the following equation.

1 = /2 .................................. (4) Where, E is the tensile modulus 2.3.4 Differential scanning calorimetry (DSC) The glass transition temperature (Tg) of the cured epoxy and bio-based epoxy blends were determined employing DSC analyzer (Q20, M/s TA Instruments, USA) at a scanning temperature of 30 ºC to 200 ºC and heating rate 10 ºC/min under a nitrogen atmosphere. The curing behaviour of virgin epoxy and its blends were also analysed from 30 ºC to 250 ºC at a



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heating rate of 10 ºC/min. The corresponding peak temperature and curing enthalpies of the samples were determined and reported. The scans reported are from the second heating cycle. 2.3.5 Thermogravimetric analysis (TGA) The thermal degradation of virgin epoxy and its bio-based blends were carried out using thermogravimetric analyzer (TGA Q 50 TA Instruments, USA) as per ASTM-E-1868. The weight of samples 6-8 mg was scanned at 30 to 800 ºC at a 10 ºC/min heating rate in the nitrogen atmosphere. 2.3.6 Dynamic mechanical analysis (DMA) Dynamic mechanical analysis (DMA) in three point bending mode was carried out using dynamic mechanical analyser (M/s TA instruments, DMA Q 800 USA). All the samples were scanned from 30 ºC to 200 ºC at a heating rate of 10 ºC/min and frequency of 1 Hz corresponding storage modulus (E'), loss modulus (E'') and tan δ of virgin epoxy and its biobased blends have been reported. 2.3.7 Optical properties The optical properties of virgin epoxy and bio-based epoxy blends were determined using a hazemeter (EEL 57D, RDM) 1.2 v 20 w halogen lamp corresponding equipped with an optical sensor in accorandance with ASTM D1003, haze and transmittance of transparent and translucent materials, were measured. 2.3.8 Water contact angle (WCA) Water contact angle of the specimen were measured with the benchtop Phoenix measurement system (PHX300, SEO, South Korea) at room temperature using the sessile drop method. The samples were kept in a sample holder and 2ml deionized water was poured on the surface


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automatically from a needle joined to a syringe pump. The hydrophilic and hydrophobic nature of the virgin epoxy, ESO, and bio-based blends were studied from contact angle measurements. 2.3.9 Morphology analysis using scanning electron microscopy (SEM) The morphological study of the impact fractured specimens was carried out employing SEM (EVO MA 15, Carl Zeiss SMT UK). Prior to analysis the fractured surfaces of the specimens were sputtered coated with platinum and dried at 70 ºC for half an hour in the vacuum. 2.3.10 Atomic force microscopy analysis (AFM) The surface topography and three dimensional phase morphology of virgin epoxy and biobased epoxy blends were analysed using atomic force microscopy (PARK XE-100 AFM, Korea). The surface profile was scanned in non-contact mode and captures topography, 3D phase morphology images. 3. RESULTS AND DISCUSSION 3.1 FTIR spectra of virgin epoxy, ESO and its bio-based blends In the FTIR spectra of uncured and cured virgin epoxy, ESO and epoxy/20% ESO blends, and their crosslink network cured with MHHPA are shown in Figure 2(a) and (b) respectively. In the FTIR spectra of uncured ESO, the C-H stretch was observed at 2922 and 2853 cm-1, C=O stretching, vibration appears at 1742 cm-1 is attributed to triglyceride carbonyl group of ESO. The transmittance peak at 1241 cm-1 and 822-843 cm-1 corresponds to C-O-C stretching from the oxirane vibration of epoxide group.26 The absence of O-H stretching peak indicates that the sample is free from moisture or contaminants and the epoxy group are not opened. The other peak observed at 1100 cm-1, 1155 cm-1, 1378 cm-1, 1459 cm-



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1

corresponds to C-O-C asymmetric stretching, ester asymmetric stretching, –CH3 asymmetric

stretching and –CH3, asymmetric deformation respectively. It was observed that the absorption peak at 822 cm-1 corresponding to epoxide ring present in uncured ESO disappeared completely after curing. This confirms that the epoxide group completely react with zwitterions generated from the curing agent (MHHPA). The intensity of the carbonyl stretching peak of the triglyceride ester gradually increases and shift to lower frequencies at 1732 cm-1 for the cured ESO. The new peak appears at 1159 cm-1 corresponding to the C=O and C-O stretching, vibration from ester moieties. Similar results reported by Pin et al.37 The FTIR spectra of uncured virgin epoxy resin shows a characteristic peak at 2964 cm-1 and 2871 cm-1 corresponding to C-H stretch. The absorption peak at 1608 cm-1, 1506 cm-1, 1456 cm-1 of the C=C bond on the benzene ring. The transmittance peak of ether bond (C-O-C) stretching from the oxirane vibration appears at 1031 cm-1 and 914 cm-1. 45 The peak at 827 cm-1, is assigned to the C-H bending vibration of multi-substituted phenyl ring. After curing the epoxide ring at 914 cm-1 of virgin epoxy was absent because of the zwitterions generated which opens the epoxy group and forms a crosslinked network and appear of the ester peak at 1732 cm-1 on the cured epoxy system. Further, the alkoxide intermediates produced from the zwitterions cleaved the other MHHPA to yield the carboxylate anions which can again react with epoxy and ESO to form the intermediate products. The peaks at 1039 cm-1 correspond to (C-O-C) groups of the crosslinked epoxy network. 21, 22 Similarly, in case of epoxy/20%ESO blend, the epoxide ring band at 915 cm-1 disappeared, completely. The (C-O-C) stretching indicates ether bond observed at 1042 cm-1. No significant change was observed between the virgin epoxy and epoxy/20%ESO blend due to 10 ACS Paragon Plus Environment

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overlapping of the peaks of crosslinked ESO with that of epoxy. The peak of 1732 cm-1 corresponds to the C=O stretching arises from triglyceride in ESO. The intensity of the absorption peak corresponding to a carbonyl group (C=O) in the cured epoxy/20%ESO blends system, increase after the complete curing process as displayed in Fig. 2(b) and indicates that the ring opening of the anhydride (MHHPA) group.46 The degree of conversion of epoxide group of virgin epoxy resin, ESO and epoxy/ESO blend by FT-IR spectroscopy are 0.973, 0.779 and 0.939 respectively. The conversion of an epoxide group in blend was comparable with virgin epoxy resin. Detail reaction mechanism in virgin epoxy, ESO and epoxy/ESO blend has been described in the next section.

Figure 2. FTIR spectra of (a) uncured system and (b) cured system. 3.2 Mechanical properties The tensile and flexural properties of cured virgin epoxy resin, ESO and epoxy/ESO blend are shown in Table 1 and the curve depicted in the Figure S1. The tensile strength of virgin ESO was found to be 13.01 MPa while that of epoxy is 42.9 MPa. Similarly, the modulus of



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ESO (351 MPa) is much lower than that of epoxy (1845 MPa). This significant difference is because of the flexible aliphatic structure of the triglyceride chain of ESO and the rigid structure of aromatic ring present in DGEBA. In case of blends, the tensile strength of epoxy increased up to 20% ESO content while there is a marginal decrease in modulus. However, the epoxy/20%ESO blends demonstrates enhanced tensile strength of 48.62 MPa, which is 13.23% higher than the virgin epoxy due to the incorporation of ESO into brittle epoxy resins can dramatically reduce the internal stress. The increase in the tensile strength by incorporation of suitable amounts of ESO attributed to the reduction of internal stress in the epoxy/20%ESO bend system by incorporation of the soft segment into the brittle epoxy network. The flexible aliphatic chain of the ESO with ester linkages are able to change the bond angle and hence the effectively reduce the internal stress.47,48,49,50 The nature of substitute side chain and plays a significant role in the mechanical properties. Increasing the amount of polar side chain groups in the polymer backbone tends to result in an enhanced in the tensile strength due to the better interaction between epoxy polymer and ESO plasticizer. Because of epoxide ring of ESO, they form random copolymer network at critical concentration of ESO. The tensile strength increased at 20 phr due to strong interfacial bonding with the virgin epoxy because of its end functionality. Exact similar findings has been reported for Methyl nadic anhydride cured epoxy/20% ESO blend by Chen et al. 55. Further 10% petro based diluent AGE exhibited similar behavior (increase in tensile strength and decrease in modulus) when added in to the virgin epoxy as reported by Sahoo et al.

22

.

Addition of ESO reduced the stiffness of virgin epoxy with increase in tensile strength on account of micro phase separation. ESO acts as reactive plasticizer homogeneously dispersed


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in the interphase between the epoxy resin chains which increases the interaction at the phase boundaries and improves the flexibility of epoxy/ESO blends. This result in strong interphase interaction, which reduces the stress concentration point when tensile load is applied on the blends and consequently produced higher mechanical strength. At higher content of ESO (30%), a deterioration in both tensile strength and modulus was observed to the tune of 9% and 5%, respectively, because of the reduction in cross linking density as well as the rigidity of benzene and other groups in the molecular structure with the increase is ESO content. The higher content of ESO resin shows a more ductile behaviour due to the presence of long aliphatic chain structure. Similar results have also been reported by other researchers.21,22,32 It is notable that elongation at break of ESO was extremely much higher than that of virgin epoxy due to the presence of residual monomer in triglyceride based polymer which can serve as a plasticizer to soften the resulting thermosetting resins. Consequently, the cause for the higher elongation in ESO systems might be the plasticizing effect of the un-reacted monomer or non-crosslinked components. 51 The flexural strength and modulus of virgin epoxy resin is much higher than that of ESO. In the case of epoxy/10% ESO blend composition exceptionally the flexural strength increases from 95.36 MPa of epoxy to 116.36 MPa with the addition of 10 wt% of ESO due to the microphase separation but decreased at higher content of ESO on account of macro-phase separation or inverse phase separation. Similar results and explanation also reported by Sahoo et al.21 and Jin et al.25. However, the flexural modulus of blend decreases steadily because the incorporation of ESO within the epoxy resin results a decrease in rigidity of the epoxy structure.



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Table 1. Mechanical properties of virgin epoxy, ESO and epoxy/ESO blends

Tensile

Tensile

Elongation

Flexural

Flexural

strength

modulus

at break

strength

modulus

(MPa)

(MPa)

(%)

(MPa)

(MPa)

Epoxy

42.94 ± 2

1845.21 ± 50

2.9 ± 0.1

95.36 ± 3

2839.11 ± 36

ESO

13.01 ± 2

351.85 ± 22

13.8 ±0.2

17.80 ± 2

472.36 ± 29

Epoxy/10% ESO

45.42 ± 0.4

1822.36 ± 19

3.3 ± 0.4

116.36 ± 5

2754.23 ± 14

Epoxy/20% ESO

48.62 ± 1

1779.28 ± 10

3.7 ± 0

80.58 ± 1

2735.89 ± 19

Epoxy/30% ESO

38.79 ± 0.5

1744.95 ± 17

3.8 ± .06

71.98 ± 3

2382.42 ± 47

Name of sample

3.3 Impact strength Notched impact strength and fracture toughness of the cured virgin epoxy resins, ESO and epoxy/ESO blends have been summarised in Table 2. The impact strength of ESO was found to 24.18 J/m, which is 39% higher than the virgin epoxy resin. However, the pure ESO shows lower critical stress intensity factor (KIC), revealing the lower toughness value than virgin epoxy resin due to the curing shrinkage and stiffened ester group of ESO backbone and residual unreacted MHHPA contributing brittleness. But in case of blend, both epoxy and ESO takes part in crosslinking process forming random co-polymer. Epoxy being higher reactive having terminal epoxy groups, got cured to a greater extent while ESO being less reactive not able to cure properly. The unreacted part of ESO plasticizes the epoxy matrix and improved the ductile property. Similarly, the critical strain energy release value (GIC) is 94% lower than virgin epoxy. It is observed that the impact strength of epoxy/ESO blends increase with the addition of ESO and 14 ACS Paragon Plus Environment

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attained the maximum value is 28.61 J/m at 20 wt% of ESO which is above 65% higher than virgin epoxy. The increase in impact strength up to 20 wt% of ESO is primarily due to the plastic deformation between ESO and epoxy which is well agreement with Ratna et al. 52 The improvement in impact strength due to plastic deformation is also well explained in the morphological section (Figure 6c). Beyond the 20 wt% the impact strength decreases due to increase in plasticization effect. The plasticization effect could be ascribed by the fact that the reactivity of epoxy group in ESO is lower than that of DGEBA based virgin epoxy because of steric hindrance the DGEBA based virgin epoxy-MHHPA induced plasticization effect is the virgin epoxy/ESO blend. However the amount of ESO reacted with epoxy remained in the continuous phase in parts plasticizing effect which in turn decreases the impact strength. The stress intensity (KIC) of 10 and 20% ESO content is calculated to be 3.54 MPa m1/2, 3.59 MPa m1/2 respectively, improved the fracture toughness and correlate with prior findings.26 The epoxy/20%ESO exhibit a higher KIC value than epoxy/10% ESO due to the brittleness, caused by ester groups. Therefore, the epoxy/20%ESO blend demonstrate the 65% higher impact strength and comparable fracture toughness with a KIC value of 3.59 MPa m1/2 than virgin epoxy resin (3.41 MPa m1/2). Table 2. Notched impact strength and fracture toughness of virgin epoxy, ESO and epoxy/ESO blends

Notched impact

Critical stress intensity

Critical strain energy

Strength (Jm-1)

factor (KIC) (MPa m1/2)

release rate (GIC) (KJ m2)

Sample

Epoxy

17.28 ± 0.1

3.41

6.30

ESO

24.18 ± 0.6

1.10

3.44

25.25 ± 1

3.54

6.87

Epoxy/10% ESO



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Epoxy/20% ESO

28.61 ± 2

3.59

7.24

Epoxy/30% ESO

15.24 ± 0.7

1.71

1.67

Similarly, the GIC value was observed 7.24 KJ m-2 in case of epoxy/20% ESO blends. The higher value of GIC of epoxy/20%ESO blends is mainly due to the reduction in modulus. GIC provides a measure of the critical energy required to extend a crack over a unit area.53 The KIC and GIC value gradual increase in the addition of ESO bioresin up to 20wt% and successfully toughened the epoxy matrix with the proper balance of stiffness and toughness. 3.4 Curing reaction mechanism The curing reaction mechanism of virgin epoxy, ESO with the MHHPA as curing agent described in the initiation and propagation steps. The ring opening of the virgin epoxy and ESO was takes place through SN2 reaction mechanism in presence of MHHPA curing agent and 2-MI as catalyst. In the initiation step, the 2-MI having a lone pair of electron on N-atom acts as a nucleophile and reacts with the electron deficient electrophilic carbon of the anhydride group in MHHPA. This result in zwitterions with a positive change of the N-atom and negative change on O-atom as shown in reaction Scheme 1. The ESO formed zwitterion, then reacts with the epoxy ring of ESO and forms an alkoxide anion. The alkoxide anion reacts with another molecule of the MHHPA curing agent to form a carboxylate anion. Finally, the carboxylate anions react with the ESO and an epoxy monomer to generate the reaction intermediate product and after 2 hr curing at 150 ºC the polyesterification in the epoxy/MHHPA/2-MI, ESO/MHHPA/2-MI, the mixture is completed and forming a polyester type linkage. Tan et al.39 Tao et al.54 also reported the similar curing


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mechanism of an ESO and epoxy with the anhydride curing agent in the presence of 2-ethyl4-methylimidazole (EMI) catalyst.



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Scheme 1. Curing reaction mechanism of (a), ESO and (b) virgin epoxy with MHHPA initiated by 2-MI.


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3.5 DSC Analysis 3.6 Study of curing behaviour The non-isothermal curing of virgin epoxy, ESO and epoxy/20%ESO blends was carried out with MHHPA as a curing agent and catalysed by 2-methylimidazole. As anhydride is a hightemperature curing agent, imidazole-type catalysts are frequently used to reduce the curing temperature of the reaction as reported by Tao et al.50 The curing curves of virgin epoxy, ESO and epoxy/20%ESO blends are depicted in Figure 3 and the curing parameters are summarized in Table S1. The ESO system exhibited a single exothermic peak with a high peak temperature of 200 °C and lower heat of reaction of 113.2 J g-1 which is higher than that of the virgin epoxy system with peak temperature of 142.64 °C and heat of reaction of 278.3 J g-1. Higher peak temperature and heat of reaction is mainly due to the internal epoxy rings present in the middle of the long aliphatic chain of ESO that leads to decrease in the reactivity of ESO with MHHPA as compared with epoxy which contains external epoxy groups at both the ends.33,34

Figure 3. DSC curing curves of virgin epoxy, ESO and epoxy/20% ESO blends cured with MHHPA 19 ACS Paragon Plus Environment


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In case of the epoxy/20%ESO blend, a slightly higher exothermic peak temperature at 144.85 °C was obtained and lower heat of reaction 268.3 J g-1 compared to virgin epoxy was observed (Figure 3). This result also attributed to the low reactivity of ESO in the blend due to the presence, an internal epoxy group in the long chain ESO backbone. 40 DSC analysis was also used to determine the (Tg) of virgin epoxy, ESO and epoxy/20%ESO blends are shown in Figure S2. It was observed that the cured epoxy resin exhibited higher Tg value (144.4°C) than ESO (63.6°C) which is due to the presence of long aliphatic chain in the latter. Incorporation of ESO into the epoxy matrix significantly decreases the Tg value of the polymer in the case of epoxy/20%ESO blends system (130.9°C). This behaviour is due to the fact that ESO increases the flexibility as well as the degree of freedom for movement of the molecular chain in the epoxy/20%ESO blends system.21,22,38 3.7 Thermogravimetric analysis (TGA) TGA analysis was used in this study to determine the thermal stability and degradation behaviors of virgin epoxy, ESO and its blends Figure 4 (a) and (b) shows the TGA and DTG curves of the anhydride cured virgin epoxy, ESO and epoxy/20%ESO blend system, at a 10 °C/min heating rate in a nitrogen atmosphere. The thermal stability parameters such as, Toneset, T5, T10, T50, Tendset, and char residue weight (%) at 800 °C (R800) are presented in Table 3. The anhydride cured ESO system was demonstrated a two-stage thermal degradation temperature with Tonset degradation occurs at 213 °C and final degradation (Tendset) occurs at 575 °C with 0% char residue. The initial stage degradation can correlate with the release of the low molecular weight compound such as unreacted MHHPA or 2-MI shows a lower thermal stability.45 While the cured virgin epoxy resin exhibited higher initial and comparable


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final degradation temperature of 283 °C, 596 °C respectively, due to the pyrolysis of the cross-linked epoxy resin network. Similarly the second step degradation of ESO started at the temperature range of 427 °C to 800 °C is much lower than virgin epoxy of (478 °C) due to the presence of a rigid benzene structure in the cured epoxy, provides a greater thermal stability than ESO. Similar results have been reported by Chen et al.55 The thermal properties of epoxy/ESO blend containing 20 wt% ESO slightly higher than the virgin epoxy resin demonstrating the 0.31 wt% residue due to the high viscosity and poor liquidity of the virgin epoxy resin, voids and internal residual stress are easily formed during the curing process with the addition of the flexible 20% ESO, voids and internal residual stress can be partially reduced

49.

While the ESO showing 0% residue due to the complete

decomposition of unreacted long chain fragments of ESO as reported by Park et al.27. In the case of epoxy/20%ESO blend, also showed the two-stage thermal degradation. The onset degradation starts earlier at 195 °C and final degradation at 591 °C with 0.38% char residue percentage at 800 °C, which indicates the higher thermal stability in the cured epoxy/20%ESO blend than virgin epoxy and ESO bioresin system, which indicates the pyrolysis of the virgin epoxy resin system similar to the epoxy matrix.



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Figure 4. TGA curve of virgin epoxy, ESO and epoxy/20% ESO blends (a), TGA and (b) DTG graph. Furthermore the 5%, 10%, 50% weight loss thermal degradation (T5,T10,T50) of the ESO and virgin epoxy is observed at 237 °C , 272°C and 359 °C respectively, much lower than that of virgin epoxy (341 °C, 366 °C, 404 °C) because of reduced the crosslink density of the ESO bioresin system. Similarly, in the case of epoxy/20%ESO blends demonstrate the higher or comparable thermal degradation temperature of 50% and a final weight loss (T5, Tendset) of the cured virgin epoxy resin. The similar results have been reported by Sahoo et al.21, Mustata et al. 56 Table 3. The thermal degradation behaviours of the cured virgin epoxy, bio-based epoxy and its blends network

Tonset a

T5 b

T10 c

T50 d

Tp1 e

Tp2 f

Tendsetg

R800h

(°C)

(°C)

(°C)

(°C)

(°C)

(°C)

(°C)

(%)

Epoxy

283

341

366

404

406

531

596

0.31

ESO

213

237

272

359

356

524

575

0

Sample


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Epoxy/20% ESO a

195

283

321

403

409

530

591

0.38

Initial degradation temperature starts. b 5% weight loss temperature. c 10% weight loss temperature. d 50% weight

loss temperature. e First peak degradation temperature. f Second peak degradation temperature. g Final degradation temperature. h Residue weight (%) at 800 °C.

In the DTG graph of thermogram, it is found that in the case of ESO, a double peak corresponding to the maximum effect on the rate of degradation (Tp1) occurs at 356 °C and (Tp2) 524 °C while the virgin epoxy also exhibits two corresponding peaks Tp1 and Tp2, at 406 °C and 531 °C, respectively. Similarly, epoxy/20%ESO blend shows two peaks in the DTG curve at 409 °C and 530 °C corresponding to the Tp1 and Tp2. The thermal degradation rate of epoxy/20%ESO blend is comparable with virgin epoxy. 3.8 Dynamic mechanical analysis (DMA) The storage modulus (E'), loss modulus (E") and damping factor (tan δ) of virgin epoxy, ESO and epoxy/20%ESO blends are shown in Figure 5 (a), (b) and (c). The various parameters calculated from these DMA curves are summarised in Table 4. It was observed that all the sample passes through three states of viscoelasticity namely glassy, leathery and rubbery state. Glassy state plateau was observed with higher E' at low temperature and a rubbery state plateau with low E' at high temperature was observed in all samples. The virgin epoxy at 30 °C shows a E' of 2411 MPa which is higher than ESO with E' of 648 MPa. The E' of epoxy/20%ESO blends was observed to be lower than epoxy which is mainly due to the presence of a long aliphatic flexible chain of ESO.21 The high E' of epoxy among all is attributed to the rigid benzene ring of bisphenol-A. Above Tg such as Tg + 30 ºC, there is no significance difference in E' which suggests the slightly lower cross-linking densities of both epoxy and epoxy/20%ESO blends.



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The Tg values are determined from the maximum peak value of tan δ curve. It was found that virgin epoxy has maximum Tg value of 156 ºC where as for ESO, the Tg value is 76 ºC. Tg of epoxy decreased to 143 ºC with the incorporation of 20% ESO in the epoxy/ESO blend. The shifting of Tg towards lower temperature with the addition of ESO is primarily attributed to the plasticization effect of ESO due to the flexibility of long aliphatic chain in the backbone of the later. The flexibility and elastic nature of ESO in epoxy/ESO blends allowed easy chain segmental movement and results in lower Tg. From Figure 5 (c), it can be observed that peak height of epoxy decreases and at the same time the width of peak increases with the addition of ESO in the blend. This result indicates the decrease in Tg of epoxy in a blend system which is in line with the observation made from tan δ curve and DSC analysis as well.

Figure 5. Variation of (a), storage modulus (b), loss modulus and (c) loss tangent (tan δ) curve of the virgin epoxy, and its blends with temperature. 24 ACS Paragon Plus Environment

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Table 4, shows that the crosslink density and E' of bio-based epoxy blends decreases due to the incorporation of ESO. Similar results were also reported by Sahoo et al.22 The crosslinked density of cured bio-based epoxy blends were determined from the kinetic theory of the rubber elasticity equation (5) shown in the following.

2 ′ = 334 56 ............................ (5) Where E' is the storage modulus in the rubbery plateau region at T= (Tg + 30) ºC constant temperature, R is the gas constant and 34 = crosslink density. The crosslink density directly related to the material stiffness as reported by Samper et al.58 Table 4. DMA values of virgin epoxy, ESO and epoxy blends

a

Tan δ e

E'a (MPa)

E"b (MPa)

Tg c

E'a(MPa) at

νed (×103

at 30(°C)

at 30(°C)

(°C)

(Tg+ 30) ºC

mol m-3)

Epoxy

2411

418

156

19.35

1.69

0.70

ESO

648

101

76

12.01

1.37

0.34

Epoxy/20% ESO

2184

336

143

18.39

1.65

0.74

Sample

Storage modulus find out from DMA.

b

Loss modulus.

c

Glass transition temperature measured by DMA.

d

Crosslink density. e

There are numerous theoretical and empirical equations to explain to the dependence of glass transition temperature on composition. The Flory-Fox and Gordon-Taylor equation was applied to description for Tg of epoxy/20%ESO blend composition system.59,60 The theoretical Tg value was calculated by using equation (6) and (7). Fox equation:

Gordon–Taylor equation:

78

9

9

= 7 + 7 ; ...................................................... (6) :

68 =

:;

9 7: ? surface tension of the liquid and (θ) is the contact angle. Work of adhesion of the virgin epoxy, ESO and epoxy/20%ESO was obtained as 109 mJ/m2, 102 mJ/m2 and 93 mJ/m2 respectively, which confirms a declination in WA in the presence of ESO bioresin. Similarly, wetting energy also decreased from 36 mJ/m2 to 24 mJ/m2 with the addition of 20wt% of ESO in the blend. 3.11 Fracture morphology study SEM micrographs of the impact fractured specimens of virgin epoxy, ESO and epoxy/20%ESO blend are shown in Figure 6 (a), (b), and (c). The SEM micrograph of virgin cured epoxy resin shows a very smooth surface in the fracture which indicates a brittle



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fracture due to the poor impact resistance.25 The poor toughness order due to the lower resistance to crack propagation which is attributed to the high chemical cross-linking in the epoxy resin network. The fracture surface of the epoxy/20%ESO curve with MHHPA is rough with massive ridges which contains ductility of the crack and leads to lower brittleness and shows a single phased morphology no phased separation indicates that ESO compatible with the epoxy matrix.45,63 The increases in surface area of the crack were observed due to deviation of crack from its rough plane results higher energy requirement from crack propagation. The higher energy absorption suggested plastic deformation and enhancement of toughness and impact strength. The plastic deformation blunts the crack tip, which reduces local stress content reduction and allows the internal support to higher loads before failure cross.

Figure 6. SEM micrographs of fractured surface of (a), virgin epoxy (b), ESO and (c) epoxy/20% ESO.


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3.12 Atomic force microscopy (AFM) The surface topography and heterogeneity of the virgin epoxy and its bio-based epoxy blends were studied by using AFM. The Figure 7 (a), (b), and (c) shows the topography images of the epoxy, ESO and epoxy/20%ESO blends where as the representative 3D images are shown in Figure 7 (d), (e), and (f) respectively. The ESO is well dispersed throughout the epoxy matrix in epoxy/20%ESO blend system and forming dual-phase continuity Altuna et al.33 also reported two-phase structure of epoxy/ESO transparent blends by AFM analysis. The bright regions are nodules and the dark regions are interstitial domains. Both epoxy and ESO phase are likely intermingled.45 The bright areas are smaller in the blends as compared with the virgin epoxy and ESO. The bright areas in epoxy/ESO blends confirmed the increase in hydrophobicity nature, based on the measured contact angle. The roughness of the virgin epoxy and ESO calculated as 4.53 nm, and 3.54 nm, respectively, which decreased to 2.06 nm, for epoxy/20%ESO blend. The 3D image shows like the arrangement ridge and valley-like structure. The valley regions are relatively smooth where as the ridge region are mainly consisting of many crystals like structure with certain orientations.



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Figure 7. AFM image of (a) virgin epoxy topography, (b) ESO topography, (c) epoxy/20% ESO blend topography, (d) epoxy 3D phase, (e) ESO 3D phase and (f) epoxy/20% ESO blend 3 D phase morphology. 4. CONCLUSIONS Epoxy/ESO blends were successfully prepared by using MHHPA as a curing agent and 2-MI as catalyst. The tensile strength, modulus, and impact strength of the epoxy/20%ESO blends was increased with the addition of ESO bioresin. The epoxy/20%ESO blend exhibits higher fracture toughness compared with virgin epoxy resin. The SEM and AFM results indicate the two-phase network formation in the epoxy/20%ESO blend. DSC analysis indicates a decrease in Tg with the addition of ESO due to an increase in flexibility and molecular chain movement. The study of curing behaviour shows a single exothermic peak at 144.85 ºC and slightly decrease in the heat of reaction of about 268.35 J g-1 than epoxy resin. Improvement in the thermal stability also observed in case, epoxy/20%ESO blend. AFM and WCA results confirm the decrease in hydrophilicity on the epoxy/20%ESO blend. The entire study


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concludes that the ESO bioresin is a most possible potential candidate to partially replace the virgin epoxy resin (DGEBA). ACKNOWLEDGEMENTS Authors would like to thanks Department of Chemicals and Petrochemicals, Govt., of India for financial support under CoE-II projects. Supporting Information Curing parameter, tensile strength and strain curve, DSC scans and experimental, theoretical glass transition temperatures and Transmittance % of virgin epoxy, ESO and its blends. WCA contact angle and surface free energy of virgin epoxy matrix, ESO and epoxy/ESO blends. REFERENCES (1)

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