Toughening of Petroleum Based (DGEBA) Epoxy Resins with Various

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Review

Toughening of Petroleum based (DGEBA) Epoxy Resins with various Renewable Resources based Flexible Chains for high performance Applications: A Review Sudheer Kumar, sukhila krishnan, Sushanta Kumar Samal, Smita Mohanty, and Sanjay Kumar Nayak Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b04495 • Publication Date (Web): 07 Feb 2018 Downloaded from http://pubs.acs.org on February 7, 2018

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Industrial & Engineering Chemistry Research

Toughening of Petroleum based (DGEBA) Epoxy Resins with various Renewable Resources based Flexible Chains for high performance Applications: A Review Sudheer Kumar*, Sukhila Krishnan, Sushanta K. Samal, Smita Mohanty, Sanjay K. Nayak Laboratory for Advanced Research in Polymeric Materials (LARPM), Central Institute of Plastics Engineering & Technology (CIPET), B/25, CNI Complex, Patia, Bhubaneswar 751024, Odisha, India, Fax: +91-674-2743863; Tel: +91-674-2742852 *Corresponding Author, E-mail: [email protected]

Abstract: Petroleum based diglycidyl ether of bisphenol A (DGEBA) epoxy resin is one of the most extensively used epoxy resin for various industrial applications such as paints, coatings, adhesive and structural applications owing to its excellent mechanical, thermal properties, low curing shrinkage and good chemical resistance. However, the serious drawbacks in term of brittleness or poor fracture energy significantly restricted its extensive utilization. Various renewable resources based flexible chains were used to blend with DGEBA for toughening and to significantly improve the fracture properties without sacrificing the mechanical properties. This review mainly focuses on toughening of DGEBA with various renewable polymers and the effect of its concentration on its toughening mechanism. Keywords: Petroleum based epoxy resin (DGEBA), Renewable resource, Toughening, Modification, Fracture morphology.

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1. INTRODUCTION Owing to the fast depletion of the fossil resources and to the growing concern about environmental issues such as global warming, CO2 emission, there is an increasing interest in the exploitation of the renewable resource based feedstock in the development of biobased materials and products without compromising the properties such as tensile strength, fracture toughness and impact strength.1-4 In this prospect, the synthesized biobased materials can partially or completely replaces DGEBA epoxy resin in some fields.5 As DGEBA epoxy resin demonstrates rigid and brittle behaviour and poor resistance to crack propagation due to its higher crosslinking. As rigidity and strength are required for the several engineering applications6,7, but the brittleness or lack of crack growth resistance reduces the extensive exploitation of DGEBA epoxy resin.8 Therefore, the toughening of DGEBA epoxy resin is a necessity to ensure the feasibility of these materials for practical applications. By the addition, of flexible chain, materials into the DGEBA epoxy resin such as plant oils,912

lignin,13-15 tannic acid16,17 and cardanol,18-20 the fracture toughness of the DGEBA epoxy

resin enhances. In the last decade, several modifiers such as rubber block copolymer, clay, hyperbranched polymer, carbon nanomaterials and thermoplastic was incorporated in the toughening process of the DGEBA epoxy resin.21-24 Numerous researchers reported three different routes for toughening of the DGEBA epoxy resin such as: (1) elastomeric toughening, 25 (2) thermoset toughening 26 and (3) thermoplastic toughening 27 amongst others. Elastomeric toughening based on DGEBA epoxy monomer is generally carried out by employing soft rubber particles in different shapes in a DGEBA matrix and was optimized on the basis of toughening properties. Several researchers studied the toughening of DGEBA, with rubber particles as a toughening agent to enhance the fracture toughness.28,29 Yee et al.30


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employed various types of carboxyl-terminated butadiene nitrile liquid rubber to evaluate the toughening mechanisms in elastomer-toughened epoxies. To get reinforced into the DGEBA epoxy resin network the particle size plays a extremely crucial role on the deformation mechanism. Thermoset is usually in liquid or malleable form prior to curing and cannot be reheated after it has been cured. Normally, polyester, vinyl ester or polyurethane were used as a filler into the thermoset modification of DGEBA because of their low manufacturing cost, easy processing and high molecular weight properties. The mechanical properties of the DGEBA epoxy resin are improved owing to the low viscosity of the unsaturated polyester as reported earlier.31-33 Chozhan et al.34 investigated the fracture toughness properties of vinyl esters incorporated DGEBA epoxy resins systems with different ratios of vinyl ester oilgomer content such as 5, 10 and 15 wt%. The results demonstrated the improvement in the mechanical properties owing to the formation of a complicated crosslinked network. Thermoplastic generally has a reversible cycle, altering from the solid state to the liquid state. Thermoplastic was used as a toughening modifier for DGEBA due to its high stiffness and fracture toughness compared to several elastomer and thermoset modifier, while decrease in the thermal and mechanical properties.35-38 The oftenly utilized thermoplastics are poly(amide-ester), poly(acrylonitrile-co-butadiene-styrene) (ABS), poly(aryl ether ketone), polybutadiene (PB), polybutylene terephthalate (PBT), polycarbonate (PC), poly(ether ketone) (PEEK) and poly(etherimide) (PEI). The main aim of this review paper is to provide better information on the research progress of toughening of DGEBA with various biobased flexible chain polymers and its mechanical properties and fracture toughness performance. Figure 1 shows the growing interest by the



academicians towards toughening of DGEBA with various renewable resources based materials year by year and its involvement in various high performance applications.

600 Published articles

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Figure 1. Published research articles on toughening of DGEBA epoxy resin with renewable resource based materials from 2005-06 to 2015-16 (based on Web of Science). 2. Toughening of petroleum DGEBA epoxy resin with renewable resource based materials Various renewable materials such as plant oils and their derivatives, natural rubber and its modified products, plant oil, biobased copolymers, rice husk nanosilica, carbon dioxide lignin, tannic acid, hyperbranched polyether and cardanol have been widely used for blending with DGEBA epoxy resin with the aim of toughening without altering its sustainability depicted in Figure 2. As most of the renewable substances direct blending with DGEBA epoxy resin, usually leads to poor performance DGEBA blends with poor interfacial adhesion and coarse phase morphology. So, both are vital factors for determining the mechanical


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properties of DGEBA epoxy blends. Therefore, the key issue in toughening DGEBA epoxy resin with renewable substances is to maintain interfacial adhesion and phase morphology. The strategies utilized for enhancing compatibility and phase morphology are different for various blending systems. In the following section, we had described toughening of DGEBA epoxy resin with renewable substances.

Figure 2. Renewable resources based materials used for toughening of DGEBA epoxy resin protect the environment. 2.1 Plant oils toughened DGEBA epoxy resin Sahoo et al.39 synthesized epoxidized soybean oil (ESO) based bioresin and toughening of the DGEBA epoxy resin with different ratios of ESO. In addition to the incorporation of ESO into the DGEBA epoxy resin, ESO was further transesterified by a base catalysed process to



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form epoxy methyl soyate (EMS) without hindering its oxirane content. It was investigated that the modified DGEBA blends exhibit high impact strength owing to the phase separated network. As depicted in the Figure 3, the blends with 20 wt% of ESO demonstrated the maximum impact strength and KIC in comparison to other blends with different ESO content because at higher ESO content, the inverse phase separation occurs. ESO behaves like a continuous matrix phase in which DGEBA networks are dispersed over the matrix in inverse phase separation. Therefore, the toughness of DGEBA blends at 30 wt% or higher ESO quantity

leads

to

the

deterioration

of

the

shear

yielding

process.

However,

DGEBA/20wt%ESO blend shows greater KIC value as compared to DGEBA/20wt%EMS blend due to the presence of ester groups in the ESO. On the other hand, impact strength and fracture toughness of DGEBA/30wt% EMS blend has been reported to be improved with KIC value of 1.96 MPa m1/2 in comparison to neat DGEBA with KIC value of 1.36 MPa m1/2. In addition to this, EMS based blends shows no inverse phase separation, even at higher ratio of EMS, which results in an increased impact strength. Similarly, DGEBA/20wt%AGE based blends demonstrate enhanced KIC (2.69 MPa m1/2) value in contrast to the neat DGEBA epoxy resin. Enhanced interfacial bonding and better miscibility of AGE resulted to a nonphase separated network, which attributes to toughening of DGEBA blends by multiple crazing and shear yielding procedures.


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Figure 3. Impact strength and fracture toughness (KIC) of the DGEBA and its biobased blends. In the same way, enhanced value of GIC of the DGEBA/ESO and DGEBA/AGE blends as shown in Figure 4 is as a result of extensive decrease in modulus. The studies observed that as AGE ratio increases, KIC and GIC values continuously rises signifying that Alkyl glycidyl ether (AGA) bioresin toughens the DGEBA epoxy matrix much efficiently shown in Figure 5.



Figure 4. Fracture toughness (GIC) of the DGEBA and its bio-based blends. It is reported that the toughened ESO blend leads to extensive decline in tensile strength and modulus with a raising content of ESO bioresin. At 20 wt% of ESO bioresin, the tensile strength and modulus of the neat DGEBA are decreased by 46% and 34%, respectively. On the other hand, the elongation at break enhances to about 26% with the same ESO amount. Increasing proportion of ESO bioresin exhibits improved ductile behavior with the one step curing process attributed to its aliphatic long chain structure. On the contrary, at 20 wt% EMS, the blend shows a restrained decrease in tensile properties but with 22% decline in tensile strength and 16% in the case of modulus along with 37% increase in elongation at break. This signifies that in the crosslinking reaction, the EMS takes part instead of behaving as a plasticizer or toughening agent. It also illustrates that the soyate resin improves the


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reactivity of the resin by decreasing the viscosity thus causing in better reaction of the resin with the crosslinker. Whereas, at 10% AGE in the neat DGEBA, the tensile strength is increased due to good interfacial bonding with the neat DGEBA owing to its end functionality. Additionally, the effective crosslink density is caused due to the presence of similar groups (glycidyl ether and DGEBA ring) in both the neat DGEBA epoxy resin and AGE, which leads to strong interpenetrating network. The tensile strength and modulus of the DGEBA/20%AGE blend reduces to the tune of 20% and 28%, respectively, in comparison to neat DGEBA along with increased elongation at break. It is examined that both the blends shows a single glass transition temperature, revealing that all the reactive diluents are completely miscible. The amine group concurrently reacts with the epoxide rings of both the neat DGEBA epoxy resin and reactive diluents, leading to a free radicals in case of direct mixing. As the reactivity of the neat DGEBA and the bioresins vary significantly and are both less than 1, that leads to random copolymers.40 Random copolymer mostly exhibits a single glass transition temperature, that is confirmed from the DSC and DMA analysis.41 The bioresin improves the flexibility and mobility of the molecular chains within the DGEBA blend network. The decreased Tg may also be confirmed from the mechanical results in which there is a reduction in the modulus with increment in the elongation. Conversely, abrupt decrease in Tg was observed with 30 wt% EMS, showing its excess plasticization effect. The Tgs are highly reduced in comparison to ESO even though both have similar oxirane values, which may be associated with the transesterification that stimulates all the fatty acid carbons along with the carbons in the epoxy groups into pendant chains. Despite, ESO is partially crosslinked via glycerol. Besides, the released saturated esters acts as plasticizers in the matrix to enhance the chain mobility.42 The EMS based blends exhibited a lowest Tg owing to the existence of methyl stearate and methyl palmitate, that doesn’t have oxirane functionalities and which remains unreacted with the curing agents.43 However, the



maximum Tg in case of the DGEBA/AGE blend in comparison to the DGEBA/EMS blends may be due to the comparatively high reactivity and limited mobility of the mono epoxy functional glycidyl ether. It is observed that the neat DGEBA shows a higher Eʹ (storage modulus) than the ESO as well as EMS based DGEBA blends and a significantly greater value than the AGE based DGEBA blend, because of the bulky bisphenol groups within its polymeric networks and a more degree of crosslinking owing to the amine curing agent.44 The Eʹ remains constant at lower temperatures (glassy region), after which a sudden decrease along with a rubbery plateau is noted as temperature increases. The neat DGEBA exhibits a decreased Eʹ at temperature more than 90˚C, while for the blends, the modulus reduces at a temperature lower than 70˚C for DGEBA/ESO and 60˚C for DGEBA/EMS system. The incorporation of the bioresins forms a soft rubbery material at room temperature because of their long flexible aliphatic chain structure. Moreover, at room temperature in comparison to neat DGEBA, the DGEBA/EMS blend shows Eʹ 14% less, 12% more than that of the ESO based DGEBA blends and 43% more than the DGEBA/AGE blend. The decrease in the storage modulus demonstrates the improvement in the flexibility of the blends. In analogous to the DGEBA/AGE blend system, the DGEBA/EMS system also shows an sudden decrease of the matrix modulus above Tg attributed to the existence of unreacted ester groups, while there is a marginal decrease for the ESO based blend, which indicates the presence of various regions with various conversions or degrees of entanglement.


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Figure 5. Schematic representation of DGEBA toughening of (a) DGEBA, (b) bioresin and its (c) blends systems Sudha et al.45 had calculated the critical stress intensity factor (KIC) and critical strain energy release rate (GIC) of neat DGEBA, ECO and its biobased blends as depicted in the Figure 6. It demonstrated that KIC value of DGEBA blends increases in presence of epoxidized castor oil (ECO). However, 20 wt% ECO was optimized with maximum KIC value, beyond which, KIC value decreases because of the plasticizing effect of ECO as shown in Figure. 7. Whereas, GIC values exhibits a continuous increment with increasing content of ECO in DGEBA blends from 0-50 wt% indicating enhanced toughness of blends in presence of ECO. The brittle nature of DGEBA declines with the incorporation of ECO owing to the plasticizing effect and reduced crosslink density of ECO based DGEBA blends. In presence of ECO, the tensile strength and modulus of the biobased DGEBA blends reduces whereas elongation at break increases because of the plasticizing effect of ECO. It was noted that the tensile strength and modulus reduces as the ECO amount increases from 0 to 10, 20, 30, and 50 wt%. This is due to the reaction of aromatic DGEBA to aliphatic long chain ECO backbone and low crosslink density of ECO. As crosslink density influences the mechanical properties that results to the decrease in tensile strength and modulus of biobased DGEBA



blends. Consequently, the elongation at break shows an increment with as ECO content raises which signifies a more plasticizing effect of ECO in the biobased DGEBA blends. Elongation at break enhances from 2.77 to 3.86% as ECO content varies from 0 to 50 wt%. In addition, it has also been reported that decrease in the tensile strength and modulus was till 20 wt% content of ECO, which observes a sudden decline beyond 20 wt%. Due to the excess plasticization or inverse phase separation in the biobased DGEBA blends, Tg reduces with the increasing ECO content. Meanwhile, it should be noted that the E' of the biobased DGEBA blends lowers with increasing ECO content. E' at room temperature was found to be 2240, 1993, 1936, 1428, and 772 MPa for the biobased DGEBA blends with 10, 20, 30, and 50 wt% of ECO respectively. This is as a result of the replacement of hard segment of neat DGEBA with the soft segments of ECO that leads to a decrease in the E'.

Figure 6. Effects of the ECO toughening DGEBA in the fracture toughness.


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Figure 7. Fracture toughness parameter of DGEBA/ECO blend system. Park et al.46 also reported the toughening properties of the DGEBA epoxy resin by the incorporation of soft segment of the epoxidized castor oil (ECO). Figure 8 demonstrated the fracture toughness of the cured DGEBA/ECO blend systems for various content of ECO. The DGEBA epoxy resin exhibited lower KIC (1.7 MPa.m1/2) value due to the higher crosslink density and brittleness. The fracture toughness of DGEBA epoxy resin significantly enhanced after addition of ECO content. The Tg of the DGEBA/ECO blends are steadily reduced with increasing ECO content. Tg and Eʹ of the blends are 131˚C and 1.15 GPa at 40 wt % ECO, respectively indicating the biobased DGEBA blends resulting in to higher Tg and Eʹ. Further, the storage modulus of the DGEBA blends was decreased as the ECO content increases. Whereas, the mechanical and mechanical interfacial properties of the blends were considerably improved as the ECO content increases which may be due to the presence of large soft segments of the ECO into the DGEBA epoxy resin and thus, increases the flexible



properties and reduces the crosslinked density of the blend network structure. Figure 9 demonstrated the crack propagation of DGEBA/ECO biobased blend.

KIC (MPa.m1/2)

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4 3.5 3 2.5 2 1.5 1 0.5 0

Figure 8. KIC values of the DGEBA/ECO blend system as function of ECO content (wt%).

Figure 9. Represent the crack propagation of DGEBA/ECO biobased blend system. ACS Paragon Plus Environment

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Scanning electron microscopy (SEM) was used to determine the fracture morphology of the fracture surface of the DGEBA/ECO blend system. The DGEBA epoxy resin shows the regular cracks in the fracture surface, demonstrating a brittle fracture surface. Conversely, the blend systems reveal tortuous cracks and exhibit many ridges with enhanced ECO content and increased fracture toughness. The biobased epoxidized palm oil (EPO) incorporated into the DGEBA epoxy resin also enhances the toughness of the blend system as reported by Tan et al.47 Figure 10 indicates that fracture toughness of the DGEBA/EPO blend system increases with enhanced of EPO content in the system. EPO plays a significant role to toughen DGEBA blend system and provides a flexibility and enhanced freedom for the motion of the molecular chain in the DGEBA blend network structure. Introduction of the flexible chains replaces the rigid backbone chains of DGEBA and enhance the ductility of the systems. As depicted in Figure 10, the KIC improves to 16.2% with the incorporation of 12% EPO in DGEBA/EPO blend in comparison to neat DGEBA.



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Figure 10. Effect of EPO incorporation of the DGEBA epoxy resin blend on the fracture toughness. Similar results have also been reported where the fracture toughness increases with increasing EPO content due to the plasticizing effect of the EPO and thus declining in the crosslink densities. Levita et al.48 explained the relationship between fracture toughness and crosslink density based on the reported research articles to improve the toughness of the DGEBA system by reducing the crosslink densities of the system. According to Zhang et al.49 highly crosslinked DGEBA system exhibit lower fracture toughness because highly crosslinked structure fails to absorb sufficient amount of energy during the fracture. Sahoo et al.50 synthesized epoxidized linseed oil (ELO) by in-situ polymerization and studied the effect of its incorporation into the DGEBA based epoxy matrix with various amounts of (10-30%) ELO and then cured it with biobased phenalkamine (PKA) crosslinker. The elongation at break and notched impact strength increases with increasing the ELO wt%. Although ELO demonstrated good toughening effect in the DGEBA matrix, tensile strength, modulus and glass transition temperature reduces as compared to the commercial DGEBA epoxy resin. However, cardanol based PKA cured biobased DGEBA blend can be employed as

a

potential

eco-friendly

material

for

coating

application

and

balance

the

stiffness‐toughness. 2.2 Biobased block copolymers toughened DGEBA epoxy resin There are very few commercially accessible petroleum based block copolymers in the market, in which green block copolymers is negligible. Unavailability of block copolymers prevents the usage of copolymers as tougheners. Henceforth, it is researchers liability to introduce industrially feasible ways to prepare green block copolymers. With advancement in synthesis procedures, consisting atom transfer radical polymerization (ATRP),51 nitroxide-


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mediated controlled radical polymerization (CRP) and reversible addition-fragmentation transfer (RAFT),52 preparation of block copolymers from among all vinyl monomers is achievable. In this criteria, biobased monomers like acrylated epoxidized soybean oil (AESO) derives more attention from researchers and industries. Besides, lignin derived monomers like guaiacol and vanillin can acts as an substitute to styrene in copolymer synthesis.53 As these monomers are a product of the pyrolysis of lignin and a modification is needed to introduce a methacrylate group within the monomer.53 Cochran et al.54,55 reported ATRP and RAFT to synthesize poly(styrene-AESO-styrene) block copolymers which were used to modify asphalt. The biobased copolymer shows properties like petroleum based copolymers that were found to be a better material to toughen DGEBA epoxy resin. Other biobased building blocks derived from the conversion of starch/sugar, which can be used for the synthesis of block copolymers to toughen DGEBA are Poly(α-methylene-g-butyrolactone) (PMBL), lactide and poly(D-glucose carbonate) (PDGC).53 Fully biobased block polymer from menthide and α-methylene-γ-butyrolactone (MBL or tulipalin A) using ring-opening trans-esterification (ROTEP) and ATRP methods was also attempted by Shin et al.56. Menthide is a biobased monomer originated from Mentha arvesis and MBL is a biobased monomer originated from Tulipa gesneriana L.56 These block copolymers exhibits high elongation at break in comparison to commercially available block copolymers. Another biobased block copolymer through ring opening polymerization of poly(propylene carbonate) (PPC) and ε-caprolactone (ε-CL) was reported by Chen et al.57. Poly(propylene carbonate) is among the attractive polymers in relation to carbon fixation that is synthesized by the reaction of carbon dioxide and propylene oxide in the presence of a catalyst and shows misciblity with DGEBA. It is amorphous in nature with a low Tg.57 The amphiphilic block copolymer (PCLPPC-PCL) was inserted into the DGEBA matrix and the effect of block copolymer content (5-30%) was investigated. The neat DGEBA epoxy resin shows tensile toughness and



elongation at break of about 4.7 MJ/m3 and 13%, respectively. Whereas, with the addition of 5, 10, 20 and 30% of biobased block copolymer elongation at break enhances to 15, 48, 107 and 323%, respectively. Besides, it was noted that tensile toughness and fracture toughness improves as the content of biobased copolymer raises and the increment in toughness was attributed to reaction induced phase separation. The addition of biobased block copolymer suddenly decreases the tensile strength and Tg of DGEBA epoxy resin. Further enhancement of strength and thermal properties of toughened DGEBA was done by adding chemically reduced graphene oxide into the sample consisting 30% block copolymer. According to the study, the incorporation of 0.25% chemically reduced graphene oxide improves its tensile strength by 64.8%, although fracture toughness remains constant.58 Among the leading commercial block copolymers in the market Polystyrene-polyisoprenepolystyrene (SIS) and polystyrene-polybutadiene-polystyrene (SBS) are mostly utilized as thermoplastic elastomers.59 Most common building blocks in block copolymers is Isoprene that can be prepared from Bacillus bacteria. BioIsoprene as commercialized by Genecor and Goodyear is a good biobased copolymer which can excel in toughening DGEBA epoxy resin.60 2.3 Carbon dioxide (CO2) toughened DGEBA epoxy resin Carbon dioxide (CO2) is one of the most abundant, non-toxic and non-flammable gas which has drawn more interest in the recent years. As its presence had caused global warming in the atmosphere, the utilization of CO2 is of great interest. CO2 as a novel renewable resource is a good substitute to hydro-carbonic resources.61,62 Five-membered cyclic carbonates, synthesized via CO2 fixation reaction of epoxy group, have attained much interest on preparing novel green technologies. Cyclic carbonates as polar aprotic solvents have great


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applications in industries due to their high solubility, high boiling and flash points, nontoxicity, and biodegradability.61,63,64 Ghanbaralizadeh et al.65 studied toughening of an DGEBA epoxy resin via chemical modification of the DGEBA structure (not the DGEBA network) through converting oxirane ring by CO2 fixation method to 5-membered cyclic carbonate that is a molecular structure modification (MSM) method mostly suggested for toughening DGEBA. CO2 fixation was applied in order to get toughened DGEBA at various carbonation amounts as shown in Figure 11. The urethane bonds and a high number of H-bonds (secondary network), while curing with diethylenetriamine, resulted in CO2 fixed modified DGEBA epoxy resin (CFME) samples. In accordance to DMTA analysis, the damping factor and the magnitude of brelaxation, an extent criteria of toughness, was improved. The fracture toughness test observed an enough improvement in the toughness of the resins modified by CO2 fixation reaction. Critical stress intensity factor (KIC) raises to one order of magnitude without affecting its strength. The increasing viscosity in this procedure due to the formation of a large number of H-bonds hinders the CO2 fixation reaction and lead to high carbonation content. The inclusion of CO2 as a cost-effective raw material resulted to a 100% toughness improvement for 31% CFME. This green technology based method of DGEBA toughening exhibits much potential in the manufacturing of super-toughened engineering polymers.



Figure 11. CO2 fixation toughened DGEBA epoxy resin curing with DETA. In another study, a bifunctional DGEBA resin was modified via CO2 fixation process and followed by the addition of Cloisite 30B to obtain CFME/clay nanocomposite (CFMEN) reported by Khoshkish et al.66 The cure kinetic behavior was extensively affected by CO2 fixation and nanoclay. The viscosity variation reveals that the number of H-bonds in CFMEN is more in comparison to unmodified DGEBA/nanoclay mixture. The DMA observed that the addition of nanoclay in CFMEN causes an increment in Tg, whereas in case of unmodified DGEBA reduction in Tg was noted. SEM illustrated a smooth and flat fracture surface patterns for CFMEN and a non-smooth fracture surface patterns for CFME. Anita et al.67 proposed a green and supramolecular method to improve the transparency, toughness and adhesive strength of DGEBA crosslinked network as depicted in the Figure 12. A monocyclic carbonate (MCC) is prepared by chemically reacting CO2 and an DGEBA


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epoxy monomer in moderate pressure. By inserting one-end reactive MCC into DGEBA network which consists of triamino oligoetheramine and DGEBA, elongation increases up to 67% and Tg reaches around 90 °C, DGEBA network with 10 wt.% MCC displays more than 100% increment in toughness in comparison to neat DGEBA. The FESEM and AFM analysis indicates that no phase separation occurs in the networks. The adhesive strength of DGEBA networks enhances from about 17 to 22 MPa because of the H-bonding interactions. These networks can acts as self-standing films because of their flexibility.

Figure 12. CO2 derived cyclic carbonate functional molecule toughened DGEBA epoxy resin network. 2.4 Rice husk silica nanoparticles toughened DGEBA epoxy resin Rice husks are one of the most plentifully available biobased waste materials which can be acquired from the rice-milling process without any cost and are mostly treated by burning. The main ingredient of rice husk contains cellulose, lignin, and ash with the content varying with climate and geographic location of its growth.68 Many researchers had synthesized nanosilica from rice husk. The formation, physical properties and structure of nanosilica from



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rice husk was highly influenced by the experimental condition. Ouyang et al.69 characterized the microstructure of rice husk silica prepared by burning the rice husk at 600˚C in air and found that nanosilica particles of about 50 nm stick loosely to each other. The nanosilica particles and nanopores make the silica in rice husk a highly surface specific and highly active. In the recent years, several works on the progress of DGEBA nanocomposites with different nanosized fillers were observed, within which DGEBA nanocomposites, nano (1-100 nm) silica filled epoxy materials have been widely studied due to their great potential in adhesives, functional coatings and packaging materials for electronic devices.70,71 Amorphous rice husk nanosilica of high purity was prepared through sintering acid leached rice husk at 600 ˚C in air and then modifying it with coupling agent.72 Addition of these nanosilica into epoxy resin can enhance the toughness, strength and stiffness of the materials depending on the various silica content. The higher amount of silica particles creates more fracture surfaces by the formation of rough surfaces that prevents crack propagation and showed improved matrix toughness. Furthermore, the rice husk silica is embedded within the composites matrix indicating better compatibility of husk silica with the epoxy that causes very high extent of stiffness and strength. Pham et al.73 proposed a new procedure to toughen DGEBA epoxy resin with nanosilica fabricated from rice husk through a thermal treatment process with a particle size of 40-80 nm. The nanosilica amount was varied from 0.03-0.10 phr in the epoxy resin. The mechanical studies determined that with the incorporation of 0.07 phr of rice husk based nanosilica, the fracture toughness of the neat epoxy resin improves 16.3% from 0.61 to 0.71 MPa m

1/2

as

shown in Figure 13. The tensile strength and Young’s modulus of the DGEBA-silica resin improves slightly with increasing silica content. The DMA analysis reveals that Tg of a 0.07


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phr nanosilica dispersion in DGEBA epoxy resin improves from 140 to 147 °C in comparison to neat DGEBA epoxy resin. SEM also signifies that the nanosilica particles dispersed within the DGEBA hinders and altered the progress of crack along with a alteration in the fracture surface morphology of cured DGEBA epoxy resin.

Figure 13. Fracture toughness (KIC) of DGEBA with various rice husk nanosilica contents. The addition of rice husk ash (RHA) in DGEBA epoxy paints can improves wear resistance, scratch resistance, and elongation as reported by Azadi et al.74 It was found that this type of filler in DGEBA epoxy paints improves paint plasticity. The incorporation of white ash is good in enhancing the wear resistance due to the presence of more silica. These types of fillers, that is cost effective and highly available, can tailor some mechanical properties of DGEBA epoxy paints in addition to lowering air pollution by burning rice husks. Finally, a green product was obtained in the paint industry.



Different proportions of surface functionalized rice husk ash reinforced bismaleimide (BMI) toughened DGEBA nanocomposites (0.5, 1.0, and 1.5 wt.% RHA/DGEBA-BMI) were prepared and their mechanical, thermal, dielectric and surface morphology were characterized by Kanimozhi et al.75 The fabricated composites show improved thermo-mechanical properties because of excellent covalent interaction among them. RHA/DGEBA-BMI posses higher values of Tg, slow degradation of the composites, higher char amount and good flame retardant behaviour indicating that the composite materials can resist at higher temperature in comparison to neat DGEBA matrix. The hydrophobic nature of the composite samples was illustrated from the higher value of contact angle and the surface roughness of the composites shown by the RHA and the BMI. Mantry et al.76 had fabricated jute-DGEBA composites with the addition of silicon carbide (SiC) derived from rice husk. These fillers improve the tensile and flexural of the juteDGEBA composites. The micro-hardness and density of the composites are also highly varied by the amount of fillers. Recently, an effort has been made to prepare natural fibers reinforced polymer composites with rice husk and fly ash as fillers.77 The amount of rice husk changes from 10, 15 and 20 wt% but the amount of fly ash remains constant at 5% in DGEBA matrix. They determined that composite with 10% rice husk is having maximum tensile strength of 33 MPa. In the same way, the flexural modulus for 10% rice husk composite was 4881 MPa. At 20% rice husk, the impact strength and hardness of composite was maximum. The water absorption was highest in case of composite with 10% rice husk. It was found that composite with 10% rice husk and 5% fly ash with DGEBA matrix will be most appropriate for structural applications. However, composite with 15 and 20 % rice husk may be utilized for automobile applications, although, unsuitable in wet environment. Conversely, composite with 10% rice


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husk will be appropriate for structural applications like packaging, interior decorations, body of washing machines and domestic appliances etc. With caprolactam as a toughener for diglycidyl ether of bisphenol A (DGEBA) epoxy resin (caprolactam epoxy (CPE)) and rice husk based glycidyl-functionalized bio silica (GRS) as a filler to prepare hybrid nanocomposites with enhanced properties (Figure 14).78 Caprolactam (20 wt%) and DGEBA (80 wt%) have been added with varying GRS of 0.5, 1.0 and 1.5 wt% cured with diaminodiphenylmethane. The mechanical studies signifies that the tensile strength, flexural strength and impact strength of 1.5 wt% GRS-reinforced caprolactamtoughened DGEBA blend composites was increased to 135, 77 and 162%, respectively, in contrast to neat epoxy matrix. In the same trend, the Tg and char residue shows an increment to 21 and 22%, respectively, while maintaining inherent surface and insulating behaviour. As a result, the hybrid composite materials obtained have extensive prospective in coatings, adhesives, sealants, matrices and composites for various industrial and engineering applications by replacing commercial DGEBA composites for enhanced properties and high longevity.



Figure 14. Preparation of glycidyl-functionalized bio silica (GRS) and DGEBA (caprolactam epoxy (CPE) composites.


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3. Other renewable resource toughened DGEBA epoxy resin 3.1 Lignin toughened DGEBA epoxy resin Lignin is one of the main bio-based components for the synthesis of eco-friendly polymers. It is obtained from sustainable sources such as trees, grasses, and agricultural crops. It is generally a polyphenolic material acquiring an amorphous structure, which occurs from an enzyme-initiated dehydrogenative polymerization of p-coumaryl, coniferyl and sinapyl alcohols.79 The toughness of the DGEBA matrices can be enhanced by covalent insertion of rigid macromolecular segments into DGEBA networks through decreasing crosslinking density whereas, declining the reduction of the glass transition temperature and mechanical strength.80 Lignin is a good option for aforesaid macromolecular toughening agents. Earlier lignin has been recommended as a toughening agent for thermosetting polymers.81 As a nonreactive additive, predissolved

butyrated lignin (which is an liquid lignin with the

incorporation of functional group) that can actively toughen acrylated epoxidized soybean oil-styrene thermosets critical stress intensity factor (KIC) and critical strain energy release rate(GIC) to about 86 and 180% respectively.82,83 The preparation of lignin based DGEBA epoxy resin or copolymerization of modified lignin with DGEBA has been investigated by many researchers based on its curing kinetics and thermal properties instead of mechanical properties.84,85 But Liu et al.86 had reported the involvement of stiff lignin in toughening the DGEBA epoxy resin and had established a relationship among toughness and key properties like stiffness, strength, and thermal properties. Lignin used either from hard or soft wood is immiscible with the majority of polymers as it forms an aggregate due to intermolecular Hbonding, weak Vander Waals interactions amid polymer chains and π−π stacking of aromatic



groups.87,88 Thereby, lignin chemical modification is an important method to enhance the solubility of lignin in polymers and to initiate reactive sites.89,90 Liu et al.86 had esterified the hydroxyl groups of lignin in presence of anhydride (Aradur 917) and had used one-pot method to simultaneously mix it with anhydride-cured epoxy networks to obtain the toughened epoxy resin. The lignin incorporated tough epoxy matrix exhibits good mechanical properties without affecting its tensile properties. In comparison to neat epoxy, the incorporation of 1 wt% of carboxylic acid functionalized-alkali lignin increases its the critical stress intensity factor (KIC) and critical strain energy release rate (GIC) at the tune of 68 and 164% respectively whereas the sample with 2.0 wt % AL-COOH exhibits maximum average modulus of about 2485 ± 68 MPa, an enhancement of around 10% compared with that of neat DGEBA epoxy resin as depicted in Figure 15, 16.The costeffective and eco-friendly lignin toughened epoxy resins shows extensive applications towards coating materials or matrix materials intended for high performance composites.

0.95

1

0.85

0.9 0.75

0.8 KIC (MPa.m1/2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.7 0.6

0.55

0.5 0.4 0.3 0.2 0.1 0 AL-COOH (0 wt %)

AL-COOH (0.5 wt %)

AL-COOH (1.0 wt %)

AL-COOH (2.0 wt %)

Figure 15. KIC value of DGEBA and DGEBA-lignin with different AL-COOH content.


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318

350 300 245

262

250 GIC (J.m-2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


200 150

120

100 50 0 AL-COOH (0 wt %)

AL-COOH (0.5 wt %)

AL-COOH (1.0 wt %)

AL-COOH (2.0 wt %)

Figure 16. GIC value of DGEBA and DGEBA-lignin with different AL-COOH content. 3.2 Biobased tannic acid toughened DGEBA epoxy resin Tannic acid (TA) is polyphenolic high molecular weight water-soluble compound usually derived from plants and microorganisms, chemically represented as C76H52O46 which can be correlated to decagalloyl glucose. Its chemical structure is identical to hyperbranched aromatic polyester with ample reactive terminal phenolic hydroxyl groups due to which it was extensively used in wide-range of applications like coatings, adsorption and antibacterial materials, and nanomaterials.91-97 Liu et al.86 had investigated many UV-curable antibacterial resins based on TA whose antibacterial properties was highly dependable on the amount of phenolic hydroxyl groups. TA-based methacrylates antibacterial ability was analyzed by zone of inhibition tests. Besides, the coatings based on UV-curable TA based antibacterial resins illustrates improved hardness, adhesion, chemical resistance, and diverse antibacterial ability. These results describes that TA-based methacrylates with better antibacterial ability have extensive application in UV-curable antibacterial coatings, like food packaging, medical



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devices, and other end-use areas related to infection and contamination. The resin with the maximum phenolic hydroxylgroups displays the best antibacterial property. Many researchers have recently revealed the improvement in the toughness of DGEBA epoxy resin along with diminishing the reduction of Tg and its strength by the addition of hyperbranched

aromaticpolyesterthroughphase-separated

or

non-phase-separated

mechanism.98-101 In this perspective, TA is a good alternative as an DGEBA epoxy toughener. But, without modification TA is immiscible in DGEBA epoxy resin resulting into a precipitation during curing attributed to intermolecular hydrogen bonds, Vander Waals interactions and π−π stacking of aromatic groups. Hence, TA needs to be chemically modified in order to retard the intermolecular interaction and enhance the miscibility of TA in DGEBA matrix. Fei et al.102 had prepared a dodecane functionalized tannic acid (TA-DD) through ring-opening reaction between TA and 1,2-epoxydodecane in presence of triphenylphosphine

catalyst,

which

consequently

acts

DGEBA/anhydride system shown in Figure 17.


as

toughening

agent

for

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Figure 17. Modification of tannic acid and its modified thermoset.103 TA-DD effortlessly gets dispersed in DGEBA epoxy resin and creates DGEBA matrix with phase separation through the curing process, resulting in a micro-scaled separated phases with improved interfacial adhesion, mainly due to its modification. The incorporation of TADD drastically boosts the toughness of cured thermosets with a great improvement in its impact strength at minimum loading content, and concurrently increasing the Tg and strength. Even at 0.5 wt% of TA-DD, the impact strength raises to about 196 % in comparison to its



counterpart neat DGEBA epoxy resin which demonstrates its great potential towards high performance applications demonstrated in Figure 18.

40 Impact strength KJ/m2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

35 30 25 20 15 10 5 0 DGEBA

TA-DD TA-DD (0.25 wt%) (0.5 wt%)

TA-DD (1.0 wt%)

TA-DD (2.0 wt%)

Figure 18. Impact strength of TA-DD toughened DGEBA epoxy resin. The neat DGEBA epoxy resin reveals a plain surface with nominal deformation which attributes for its low impact strength. Whereas, TA-DD modified DGEBA exhibits spherical domains of TA-DD embedded within the continuous epoxy resin matrix which verifies that during curing process, the phase separation occurs. Mostly the size of these separated phases is homogeneous. The toughness is prominent if there is a homogeneous distribution of separated phase within the matrix which permits the yielding process to function all over the matrix.103 However, better adhesion within TA-DD and the DGEBA matrix was observed due to the absence of cavitations in TA-DD phase. So, an improved interphase compatibility leads to transfer of stress between the TA-DD and DGEBA matrix resulting to the plastic deformation and crack formation, attributing to the toughness of the thermosets.104,105


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Another Carboxylic acid-modified tannic acid (TA-COOH) modification has also been reported.106 TA-COOH was synthesized by esterification of TA and methylhexahydrophthalic anhydride in presence of catalyst (ethyl triphenyl phosphonium bromide) and consequently used as a modifying agent to the DGEBA/anhydride curing system. The DGEBA epoxy resin with 0.5, 1 and 2 wt% of TA-COOH was prepared. TA-COOH quickly gets dispersed in epoxy resin and exhibits enough interface interaction within TA-COOH and epoxy matrix, which is attributed to its chemical modification. The DGEBA with 1 wt% TA-COOH exhibits the maximum crosslinking density. Below 1 wt%, Tg increases with TA-COOH and attains a maximum of 146.7 °C, that is 10 °C more than that of neat DGEBA. However, for the DGEBA with 2 wt% TA-COOH content, Tg decreases. Usually, reducing the crosslink density causes weak material rigidity, thereby decreasing Tg. Though, the impact strength was markedly increased with the incorporation of TA-COOH. It achieves the maximum of 33.9 kJ/m2 at 1 wt% content that is 159% more than that of neat DGEBA. Increasing TACOOH content above 1 wt%, results in a marginal reduction in its impact strength, which can be attributed to the crosslinking density reduction effect. Additionally, the elongation at break enhances constantly from 3% to 5.9% as TA-COOH content raises from 0 to 2 wt% which signifies better interface interaction due to a chemical reaction between the end carboxyl group of TA-COOH and DGEBA matrix. Usually DGEBA material shows increasing elongation at break along with decreasing tensile strength. But, a constant increase in tensile strength was found as the amount of TA-COOH increases. The DGEBA system with 2 wt% TA-COOH shows a maximum tensile strength of 67 MPa, that is around 42.5% more than that of neat DGEBA which may be due to the aromatic structure and better interface interaction among TA-COOH and DGEBA matrix. The results demonstrated a good improvement in its impact strength, tensile strength, elongation at break and glass transition temperature, even at low content of TA-COOH.



3.3 Cardanol based phosphate flame retardant toughening agent for DGEBA epoxy resin. Cardanol is among one of the most usually used, economical and readily accessible renewable material, that is extracted from cashew nut shell liquid. It is chemically modified due to the presence of reactive phenolic hydroxyl group and unsaturated long alkyl chain. Various studies have been reported based on the cardanol and its derivatives in the areas of bio-composites,107,108 coatings,109-112 curing agents,113,114 and antioxidant.115 The utilization of cardanol and its derivatives as a plasticizer has been reported recently, which has been showed its great potential towards green chemistry in the field of polymer and rubber industries. The application of cardanol shown in Figure 19.

Figure 19. Application of cardanol based polymeric materials in various fields.


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Mohapatra et al.116 had reported the cardanol grafted natural rubber which exhibits considerable plasticizing properties. Besides, cardanol acetate, epoxidated cardanol acetate, 117

and epoxidized cardanol glycidyl ether118 have also been noted to be a quite effective

plasticizers for poly(vinyl chloride). Moreover, very low impact resistance and high flammability, which is the main cause behind fire accidents occurring in most household or industrial applications are the main limitations of DGEBA epoxy resins like most of the synthetic organic polymeric materials. So, flame retardant modification has been emerging as a main concern to reduce or even eradicate fire hazards of DGEBA epoxy resins. Nowadays, for a better flame retardant properties of DGEBA epoxy resins, various methods like boroncontaining92 phosphorus,119,120 nitrogen,121 silicon,122 compounds as well as nanomaterials,123126

have been investigated. 9, 10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO)

and its derivatives are regarded as a class of highly effective and efficient flame retardant for commercial DGEBA products amongst these all compounds.127,128 Therefore, merging of cardanol with DOPO was considered to be a good option to retrieve bio-based flame retardant toughening agent for DGEBA materials. Recently, Wang et al.129 had synthesized a phosphaphenanthrene

groups-comprising

triscardanyl

phosphate

(PTCP)

through

dehydrochlorination, epoxidation and ring opening process from bio-based cardanol. By simple mixing method, epoxy resins with various ratios of PTCP were fabricated. It was observed that, as the PTCP content increases, the flame retardant properties also showed an improvement. With 30 wt% of PTCP, the composite reported a limiting oxygen index of 30.5 %.Whereas, its peak heat release rate, total heat release and average effective heat of combustion values were reduced by 50%, 27% and 32%, respectively, in contrast to those of neat DGEBA epoxy resin. The improved flame retardant behavior was due to the good quality of char remnants that efficiently prevented the flammable volatiles, oxygen and heat transfer among degradation zone and flame zone. The impact strength enhances to 19.14



kJ/m2 for 30 wt% of PTCP composite from 14.85 kJ/m2 for neat DGEBAepoxy resin, implying the toughening effect of PTCP on DGEBA depicted in Figure 20. After adding PTCP, the Tg of DGEBA composites exhibits a reduction as PTCP content increases, indicating the plasticization effect of PTCP on DGEBA epoxy resin. The decreased Tg of DGEBA/PTCP composites may be due to the fewer crosslinking density derived from the long alkyl chains and enough aromatic structures within PTCP molecules. High amount of PTCP molecules prevents the formation of crosslinking networks, and hence the Tg reduces to 87 ˚C for 30 wt% of PTCP composite as compared to neat DGEBA with 137 ˚C. The results demonstrated that PTCP can be utilized as an efficient flame retardant toughening agent for DGEBA epoxy resins to prevail over their disadvantages of intrinsic brittle and high flammability.

Figure 20. Tensile properties and impact strength of DGEBA and DGEBA/PTCP composite.


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Henceforth, PTCP synthesized will be an alternative for the growth of flame retardant toughening agents for industrial applications in future. 4. Hyperbranched polyether toughened DGEBA epoxy resin The benefits of hyperbranched polyether (HBPs) in comparison to usual toughening agents are that HBPs contributes to very less prepolymer viscosity due to their spherical arrangement and absence of chain entanglement and firm adhesion with the matrix assigned to the existence of high density of surface functional groups. Ratna et al.130 reported the ability of HBP materials towards toughening a trifunctional DGEBA epoxy resin and had discussed the curing of an DGEBA-functionalized HBP and a trifunctional DGEBA blends. They found that triglycidyl p-amino phenol (TGAP)-HBP blends exhibits improved impact strength till 10% HBP concentration and continues up to 20%. At 10% HBP, the impact strength of the blend is 1.3 kJ/m2 that is around100% more in comparison to the unmodified TGAP network (0.7 kJ/m2). Two definite phases are observed for the fractured surfaces of the modified networks: globular HBP particles scattered in continuous TGAP matrix. In the case of 10% HBP-modified sample, the HBP particles are uniformly distributed all through the matrix. This signifies that HBP-modified TGAP shows higher impact strength as compared to the unmodified TGAP. The Tg value of modified-HBP TGAP up to 10% there is no change and after increasing the concentration of HPB (15%, 20%) Tg value slightly reduced due to the dissolution of certain amount of HBP in the DGEBA matrix. Luo et al.131 had synthesized new DGEBA-ended hyperbranched polyether (HBPEE) with aromatic skeletons via one-step proton transfer polymerization. It was reported to be very effective modifier in toughening and reinforcing DGEBA matrix. In contrast to other hyperbranched modifiers, the glass transition temperature (Tg) was also improved. In comparison to neat DGEBA, the hybrid curing systems illustrates good mechanical



performance during 5 wt% HBPEE content. The excellent enhancements were assigned to high crosslinking density, rigid skeletons, and the molecule-scale cavities caused by the highly reactive HBPEE, that were verified by dynamical mechanical analysis (DMA) and thermal mechanical analysis (TMA). In addition to this, due to the reactivity of HBPEE, the hybrids tend to form a uniform system after the curing. DMA and scanning electron microscopy (SEM) results verifies that no phase segregation happened in the DGEBA/HBPEE hybrid systems by the incorporation of reactive HBPEE. Besides that, SEM proves that the addition of HBPEE improves the toughness of DGEBA materials as verified from fibril pattern. The presence of HBPEE in DGEBA matrix leads to rough surface with more fibrils, which signify its improved impact strength. It has been reported that the discontinuous fracture surface or fibrils are assigned due to the coalescence of microcracks. High fibrils on the fracture surfaces at high content of HBPEE resulted in an excellent improvement in toughness. Brocks et al.132 compared the hyperbranched poly(butylene adipate) (HPBA) polymer with a commercial dendritic polyol (HPOH), that acts like a toughening agent for a commercial onepart DGEBA epoxy resin. At weight percentages of 1, 3, 5, and 10%, these two modifiers were blended with epoxy system. Toughness of the blend shows an improvement without effecting the thermomechanical properties of the DGEBA epoxy resin. Marginal reduction in viscosity was shown by both modifiers, resulting to easier infusion processability. SEM micrographs does not shows any sign of new chemical linking even though phase separation was verified which shows that only interfacial linkage occurs among modifiers and DGEBA chains. In addition to this, SEM results clearly indicates the fracture surface altering from brittle to ductile in presence of modifiers.


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Recently, renewable resources like sorbitol and castor oil were utilized to prepare novel hyperbranched DGEBA epoxy resins. Castor oil based three hyperbranched DGEBA epoxy resins with different weight percentage of sorbitol cured with poly(amido amine) has been investigated. The cured DGEBA epoxy resins exhibits good mechanical properties with impact strength to 34.5 kJ/m2, which is associated to the presence of distinctive combination of aromatic and aliphatic linkages in suitable proportions. With increasing content of sorbitol in the hyperbranched moiety, the tensile strength of the thermoset also shows a significant improvement up to 31 MPa due to its high functionality. As sorbitol acts as a branching moiety in this case, the pattern of non-entangled globular structure of the resin suggests best probable crosslinking in the structure which leads into high tensile strength and toughness. Additionally, elongation at break also increases to about 37%. High molecular weight of the hyperbranched DGEBA due to higher functionality of the branch generating moiety along with the plasticizing effects of the hardener, sorbitol and monoglyceride of the oil contributes to enhanced elongation at break of the thermoset. The thermosets revealed better chemical resistance in various chemical environments with adequate biodegradability. Consequently, the sorbitol modified castor oil based hyperbranched DGEBA thermosets has been reported to be better in toughness, biodegradability, elongation, chemical resistivity etc. in contrast to a biobased epoxy without sorbitol as well as earlier investigated thermosets synthesized from biobased materials such as vegetable oil, starch and glycerol. Henceforth, these biobased thermosets acquire immense prospective to be utilized as renewable materials for diverse kind of applications.133



5. CONCLUSION AND OUTLOOK The main outline of this research was on toughening of DGEBA epoxy resin from renewable resource based polymers and to emphasize its great potential to partially replace the petroleum based (DGEBA) epoxy resin. In the recent years, toughening of DGEBA epoxy resin was growing importance because of the requirement of high performance biobased materials in various applications along with higher tensile strength, impact strength and fracture toughness. However, the fracture toughness of DGEBA epoxy resin significantly improved after incorporation of higher biobased content flexible chains polymers without affecting the processability and other mechanical properties. Therefore, biobased polymers flexible chains is considered as a promising modifier for enhancing the toughness of the DGEBA epoxy resin for high performance application ACKNOWLEDGEMENTS The authors are thankful to the Department of Chemicals and petrochemicals, Govt. of India for the finance support of the research work. REFERENCES

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