Green Approaches To Engineer Tough Biobased Epoxies: A Review

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Cite This: ACS Sustainable Chem. Eng. 2017, 5, 9528-9541

Green Approaches To Engineer Tough Biobased Epoxies: A Review Ghodsieh Mashouf Roudsari,†,‡ Amar K. Mohanty,*,†,‡ and Manjusri Misra*,†,‡ †

School of Engineering, Thornbrough Building, University of Guelph, 50 Stone Road East, Guelph, Ontario N1G 2W1, Canada Bioproducts Discovery and Development Centre, Department of Plant Agriculture, Crop Science Building, University of Guelph, 50 Stone Road East, Guelph, Ontario N1G 2W1, Canada

‡

ABSTRACT: Epoxy resins possess a variety of excellent properties including adhesion, mechanical performance, electrical insulation and chemical resistance; however cured epoxy resin is brittle and typically petroleum based. Rising concerns about depletion of nonrenewable resources and climate change have resulted in attempts to find green alternatives for petroleum based materials and mitigate greenhouse gas emissions. The present review is aimed to discuss green approaches to overcome epoxy resins brittleness and deal with ongoing research strategies to make tough biobased epoxies. First, the key toughening modifiers such as rubbers, thermoplastics, nanofillers, dendritic and block copolymers are briefly discussed and pros and cons of each method are presented. Then, the studies that followed green approaches are thoroughly reviewed. The utilization of epoxidized vegetable oils, biobased hyperbranched polymers and biobased copolymers in epoxy matrix are discussed. The challenges for commercialization of biobased modifiers are assessed and the present and prospective status of research and development of the tough biobased epoxies are explored. KEYWORDS: Epoxy resin, Toughness, Biobased modifiers, Impact strength, Block copolymer, Hyperbranched polymers, Nanofillers, Epoxidized vegetable oils

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performance requirement, final properties of thermosets can be tailored by changing type of epoxy resin or curing agent. Due to the superior properties of epoxy resins, they are used in coatings, adhesives, composites, electrical insulators and electronic encapsulation materials.6 The US epoxy resin market revenue is projected to be around USD 1400 million by 2020.8 In 2014, the North American market was valued at over USD 1003.6 million.8 The epoxy resins’ global market size was forecasted at USD 7.1 billion in 2014 and it is projected to reach USD 10.5 billion by 2020.8,9 The consumption of epoxy resin by sector demand can be found in Figure 1. As it can be seen, the most dominant application of epoxy resin is in the coatings industry. The fastest growing application of epoxy resin is composites and it is estimated that the global compound annual growth rate (CAGR) of epoxy composite segment will reach 7.3% from 2015 to 2022.8 Use of epoxy composites in structural applications including wind turbine blades and transportation vehicle parts (such as airplanes, automobiles) is increasing due to the importance of producing green energies, reducing fuel consumption and making lightweight vehicles. For example, the Boeing 787 is claimed to be 20% more fuel efficient than the Boeing 767 and up to half of its primary structure including fuselage and wings are made of lightweight composites.10 Also,

INTRODUCTION

Thermoset resins (also called thermosets) are a class of polymers that form three-dimensional cross-linked networks through curing reactions. Worldwide consumption of thermosets was 27 million tonnes in 20021 and the thermoset composite market is projected to reach 13.8 billion by 2021.2 Epoxy resins, phenolic resins, polyurethanes, acrylics, alkyds, furans, polyimides, vinyl esters and unsaturated polyesters are typical examples of thermosets in the market. Epoxy resins were commercialized around 1950.1 They consist of oxiran rings (epoxy groups) in their backbone. They can make a cross-linked network through reaction with a second component (known as hardener or curing agent). Also, an epoxy chain can react with another epoxy chain through anionic or cationic mechanism. Multifunctional amines, anhydrides, acids, thiols and polyols are common hardeners.3 A major group of epoxy resins are synthesized by reaction of epichlorohyrdin (1-chloroprene 2-oxide) and bisphenol A (bis(4- hydroxy phenylene)-2,2 propane) in the presence of sodium hydroxide.4 Epoxy resins exhibit higher strength, stiffness and better creep, heat and solvent resistance than most thermoplastics.5 In addition, shrinkage in epoxy composites is lower than many unsaturated polyesters.6 Furthermore, the required composite fabrication pressure for epoxy resins is lower than other thermosets.7 Various viscosities of epoxy resin ranging from low viscous liquid to solid are available in the market. Depending on © 2017 American Chemical Society

Received: May 6, 2017 Revised: July 15, 2017 Published: September 25, 2017 9528

DOI: 10.1021/acssuschemeng.7b01422 ACS Sustainable Chem. Eng. 2017, 5, 9528−9541

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ACS Sustainable Chemistry & Engineering

where load bearing is a key requirement. This highlights the importance of toughening epoxy resin. The present review is aimed to discuss green approaches to overcome epoxy resins brittleness and deal with ongoing research strategies to make tough biobased epoxies.

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TOUGHNESS IN EPOXIES The ability of a material to withstand applied load under specific conditions is known as toughness and it is a measure of its ability to absorb energy and resist failure. Various methods have been implemented to improve epoxy resin toughness. Use of rubber particle in epoxy matrix was the first approach taken by scientists, the first results reported by McGarry at 1970.21 Since then, many approaches have been implemented to make a tough epoxy network. These approaches can be divided into two main categories, namely inclusion of a soft phase such as rubber and addition of hard phase such as nano particles into the epoxy matrix. Due to extensive research in these areas, there are review papers mainly devoted on epoxy toughness.22−25 Since the main focus of the present review is on green approaches to toughen epoxy resin, the key toughening methods of toughening including rubber, thermoplastic, nanofiller, dendritic and block copolymer are briefly discussed. Effective toughening mechanisms, pros and cons of each method are concisely discussed. Then, studies that followed green approaches are reviewed thoroughly. Rubber Toughened Epoxies. Rubber toughening can be divided into two methods, namely reactive rubber and core− shell particles addition.23 Addition of reactive rubbers is the oldest approach in toughening epoxy resins. In this approach, functionalized acrylonitrile-butadiene rubber such as carboxylterminated butadiene-acrylonitrile (CTBN), amino terminated butadiene acrylonitrile (ATBN),26 epoxy terminated butadiene acrylonitrile (ETBN)27 and vinyl terminated butadieneacrylonitrile (VTBN)28 will be dissolved in epoxy resin. Dissolving the rubber in the epoxy matrix and their separation during curing (i.e., reaction induced phase separation) is one of the main requirements of a rubber to be considered as epoxy toughener.25 In addition, functional groups of rubber should react with epoxy groups.25 It is worth mentioning that CTBN is the most extensively used rubber modifier.25,29 Difficulty in handling, working with rubbers and controlling the dispersed particle size are drawbacks of these modifiers.29 Core−shell rubber (CSR) particles are indeed the next generation of reactive rubbers. CSR modifiers are preformed rubber particles covered by a thin glassy shell. Incorporation of these modifiers facilitates controlling dispersed phase size. The presence of glassy skin prevents merging rubber particles due to their stickiness. It results in better thermo-mechanical properties than reactive rubber toughening. Core−shell toughening agents are usually synthesized by emulsion polymerization and the size of particles can vary depending on polymerization conditions. 30 Shell materials include epoxy compatible polymers and poly(methyl methacrylate) (PMMA) is the most common one. Core materials are usually made of butadiene31 and acrylate-polyurethane rubbers.32 Hayes and Seferis reviewed the effect of preformed CSR particles in epoxy resin and composites.33 Localized shear band yielding around the rubber phase and cavitation of rubber particles accompanied by void growth in the matrix are two accepted toughening mechanisms in rubber toughened epoxies.30 Size of rubber particles,34 particle size distribution,35 interparticle distance,36 volume fraction of rubber phase,23

Figure 1. Global demand of epoxy resin base on application [adopted with permission from Ding, C.; Matharu, A. S. Recent developments on biobased curing agents: A review of their preparation and use. ACS Sustain. Chem. Eng. 2014, 2 (10), 2217−2236. Copyright 2014 American Chemical Society].

Airbus the A350XWB is claimed to produce 25% less CO2 emission than previous Airbus aircrafts and the A350 XWB’s fuselage is made of carbon fiber composites.11 Most epoxy resins in the market are petroleum based. Overall, 90 billion pounds of petroleum based polymers are used in various industries such as coatings, textiles, and automotive; and production of the mentioned amount of polymers requires 300 million tons of the oil and natural gas world supply.12 The volatility and uncertainty of oil price and its negative effect on investment and durable consumption13 have impelled both industry and academia to develop biobased products. Moreover, binding regulations including mandatory CO2 emission regulation in Europe and Corporate Average Fuel Economy (CAFE) in the USA are forcing industries to move toward a sustainable economy. In December 2015, 196 countries in United Nation Framework Convention on Climate Change (UNFCCC) in Paris committed to keep global temperature increase under 2 °C and aimed at keeping temperature rise below 1.5 °C.14 Researchers and industries are under social, economic and political pressures to reduce use of petroleum based materials, lower greenhouse gas emissions and increase biobased content of products. Green chemistry market is projected to reach $98.5 billion by 2020.15 There are several review papers dealing with biobased thermosets. In 2010, Raquez et al.16 reviewed thermoset resins including phenolics, epoxy resins, polyurethanes, and polyesters derived from renewable resources. Auvergne et al.3 had a comprehensive review on biobased epoxies and the review was mainly devoted to different approaches to synthesize epoxy resin from tannins, cardanols, vegetable oils, woody biomass, lignin, terpenes, terpenoids and resin acids. Ma eta al.17 wrote a mini-review discussing epoxy resins and unsaturated polyesters derived from renewable resources. Ding et al.15 reviewed biobased curing agents for epoxy resins including biobased acids and anhydrides, rosin acids, modified plant oils, biobased amine and phenols, terpenes and lignin. Gandini et al.18 reviewed progress in furans, polysaccharides and vegetable oils derived polymers including thermosets. Considering extensive available review papers on biobased epoxies, the current review does not restate the progress in synthesis methods and the readers are encouraged to study the cited reviews. Despite all the choice properties of epoxy resin, it is inherently brittle and it suffers from poor impact resistance and low crack initiation energy due to its high cross-link density. Fracture energy of epoxy resins is significantly lower (i.e., two or 3 orders of magnitude) than metals.19 This drawback results in constraints on design parameters20 and failure of epoxy resin 9529


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ACS Sustainable Chemistry & Engineering concentration of curing agent,20 matrix ductility,37 kinetic of reaction23 and rubber/matrix interface38 are key parameters in this approach. Although addition of rubber modifier usually increases fracture toughness of epoxy resin by several orders of magnitude, it reduces strength and stiffness of matrix.23 In fact, there is a linear relationship between the reduction of strength and stiffness and the addition of rubber modifiers.39 If the phase separation of rubber is not completed, it may lead to a reduction of the glass transition temperature and an increase in the coefficient of linear thermal expansion (CLTE)23 accompanied by a reduction in transparency, which is an important characteristic for coatings.40 Thermoplastic Toughened Epoxies. Thermoplastic modified epoxies were introduced by Bucknall and Partridge in 198341 to overcome drawbacks of rubber toughened epoxies, i.e., reduction in strength and modulus and inefficiency of rubber modifiers in highly cross-linked epoxies.42 Due to the high glass transition temperature of thermoplastics, high modulus and rigidity and also their ductility, thermoplastics are considered as good candidates for epoxy toughening.24 The thermoplastic is dissolved in epoxy resin and solvent (if required) and then similar to rubber modifiers, phase separation occurs during curing.42 Similarity in chemical structure of epoxy and modifier is a key requirement to choose a thermoplastic for epoxy toughening. Polyether sulfone,43 polyether ether ketone (PEEK), 44 polyphenylene oxide (PPO),45 polystyrene (PS),46 poly(methyl methacrylate) (PMMA)47 and poly(ether imide) (PEI)48 are the most common thermoplastic tougheners. Phase separation of thermoplastic phase is another main requirement for epoxy toughening and thermoplastics with high miscibility are not favorable toughening modifiers.49 Generally, the presence of many reactive end groups resulting in miscibility increasing can be considered a disadvantage for epoxy toughening.24,49,50 Thermoplastic content, chemical structure including molecular weight and functional groups, ductility of epoxy matrix,24 interfacial bonding between thermoplastic and epoxy matrix,51 and kinetics of curing and morphology of system24 have significant effects on toughness. Although rubber toughening is more effective in epoxy matrix with lower cross-link density, thermoplastics exhibit better performance in epoxy resins with higher cross-link density. Epoxies Toughened Using Rigid Particles. Addition of inorganic rigid particles such as glass beads, silica and alumina into epoxy matrix is another approach to overcome the mentioned drawbacks of rubber toughened epoxies (i.e., low modulus, low glass transition temperature and high viscosity). It was found that micrometer size inorganic particles (4−10 μm) improved fracture toughness of epoxy resin.52−54 Localized plastic deformation,55 crack deflection,55 crack pinning,52,53 crack tip blunting,55 particle deformation at crack tip,55 particle−matrix debonding54−56 and particle pull out54−56 were the main toughening mechanisms. Micrometer size inorganic filled epoxies suffered from filtration problem for fiber composite production and they also resulted in high viscosity.57 Moreover, micrometer size inorganic modifiers moderately improve fracture toughness of highly cross-linked epoxies.55 In the past decades, reinforcing polymers with nanomaterials has become a promising approach to improve the mechanical, optical, thermal and electrical properties of polymers. Due to the small size and large surface area of nanoparticles, a small

amount of them is required and this results in lower viscosity in comparison to micrometer size particles. Various nanofillers including silica,58 alumina,59 carbon nanotube (CNT),60 carbon black,61 graphene oxide,62 halloysite nanotubes,63 titanium oxide,59 fullerene64 and nanoclay,65 were used to improve toughness of epoxy resin. It was reported that well dispersed nanoadditives can improve elastic modulus, tensile strength and ductility and fracture toughness of matrix.55 Although crack pinning and crack deflection were considered as dominant toughening mechanisms by Wetzel et al.59 in epoxy/alumina and epoxy/titanium oxide composites, they were excluded in epoxy matrix modified with nanosilica.57,66 Plastic deformation by localized shear band, debonding of nanoparticles accompanied by void nucleation and growth are key toughening mechanisms in epoxy nanocomposites.55,57,66 Study of epoxy modified with nanofibrous materials showed that crack propagation and void growth were reduced by bridging mechanism and it resulted in an increase in toughness.60 In the case of nanotubes, it was reported that pull-out of nanotubes has a key effect on fracture toughness improvement.67 In epoxy reinforced by layered nanomaterials, including nanoclay, crack deflection and formation of micro voids were distinguished as key toughening mechanisms.68 The extent of the effectiveness of a nanoadditive is related to several parameters such as size, volume fraction, shape, aspect ratio, dispersion and surface treatment of nanoadditive, crosslink density and glass transition temperature of matrix.55,67 It was found that an increase in concentration of nanoparticles increases fracture toughness.59,69 On the other hand, there are controversial reports on the effect of particle size on the fracture toughness of epoxy resin. Some studies showed that particle size does not have a significant effect on fracture toughness69 whereas the other showed positive or negative effects from size reduction.70 In general, it is difficult to distinguish the complex effect of all parameters individually.55,67 Hsieh et al.57 reported that nanosilica improved fracture toughness of epoxy resin with a low glass transition temperature and lower cross-link density more than epoxy resin with high cross-link density. Surface treatment of nanoparticles can have a significant effect on curing kinetics of epoxy resin and can change cross-link density and fracture toughness,71 however the correlation between these parameters has not been reported. Since void growth and propagation were reported as one of the dominant mechanisms in toughening, it was found that lower interfacial adhesion between nanoparticles and the matrix favors toughening.57 In the case of nanotubes, it has been reported that better interfacial adhesion improves fracture toughness of the epoxy matrix and amine treated nanotubes exhibited higher fracture toughness than pristine and carboxylated nanotubes.71 Epoxies Toughened Using Block Copolymers. Advances in chemistry have resulted in development of block copolymers as toughening agents for epoxy resin. Using block copolymers gives the ability to reach desired properties by selecting an appropriate block and adjusting the domain functionality.72 The first report on using block copolymers for toughening epoxy resin was reported by Könczöl et al. in 1994.73 This new generation of modifiers can be divided into three subgroups. Self-assembling block copolymers consisting of epoxy-philic (epoxy miscible block) and epoxy-phobic blocks (epoxy immiscible block) are one class of block copolymers.74 In the first subclass, self-assembled nanostructure heterogeneities (around 10−20 nm) formed in the precure stage are fixed 9530


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ACS Sustainable Chemistry & Engineering during the curing stage.75 In the second subclass, epoxy miscible blocks have functional groups reacting with either the epoxy resin or the curing agent and the chemical bonding of copolymer and matrix results in better toughening of epoxy with a lower concentration of copolymer.76 The next group consists of miscible blocks. In this group, the reaction induced phase separation of one of the blocks happens during curing and toughens epoxy resin.22 Nanostructure morphology of copolymer in the epoxy matrix is a significant parameter. Several morphologies including spherical micelles, cylindrical micelles (worm-like) and vesicles have been observed in previous studies.77 The best toughness can be achieved in systems with wormlike micelles morphology.22 Also, vascular morphology is preferred to a micellar morphology.22 Poly(ethylene oxide)-b-poly(propylene oxide)-b-poly(ethylene oxide) (PEO-PPO-PEO) is the most extensively used block copolymers in toughening epoxy resin. Poly(methyl acrylate-co-glycidyl methacrylate)-b-polyisoprene (P(MA-coGMA)-PI),76 Polybutadiene-b-poly(epoxy-1,4-isoprene-ran1,4-isoprene) (PB-ePI),76 Poly(ethylene oxide)-b-polyisoprene (PEO-PI)78 and poly(styrene-isoprene-styrene) (SIS)79 are other examples of toughening modifiers for epoxy resin. Hyperbranched and Dendrimer Modifiers. Dendritic polymers have been introduced to achieve toughened epoxy with good processability and without sacrificing thermomechanical properties.20 Generally, high molecular weight linear tougheners are avoided due to the substantial unfavorable increase in viscosity; although, dendritic macromolecules are spherical high molecular weight polymers that do not increase viscosity, unlike their linear counterparts.20 Dendritic molecules can be divided into three main categories, namely dendrimers, dendrons and hyperbranched polymers (HBP).80 Dendrimers and dendrons are high molecular weight polymers with wellcontrolled size and shape.80 They are usually synthesized by multistep reactions. Furthermore, excessive isolation and purification is required to synthesize them.80 On the other hand, HBPs are high molecular weight branched polymers that can be polymerized by one step polymerization of ABx type monomers.80 They are usually synthesized through noniterative steps and have irregular structures.81 HBPs are usually more preferable than dendrimers and dendrons due to their easier polymerization. Due to the interesting properties of dendritic polymers, they have been used in liquid crystals,82 soluble functional supports,83 catalysts,84 sensors,85 coatings,86 delivery devices85 and nanofoams87 in low dielectric materials. The extensive number of functional groups on the shell, high interaction potential and good compatibility of these macromolecules makes them good candidates for toughening epoxy resin.20 Various studies focused on using HBPs into epoxy matrix and toughening mechanisms.88−92 Miscibility of HBP in matrix, interfacial adhesion between modifiers and matrix (chemical nature of shell), domain size of modifier and volume fraction of HBP are the key parameters in effective toughening. Mezzenga et al.93 reported that HBPs with particle size in the range of 1 μm exhibited the most efficient toughening effect. Cavitation of HBP particles and shear banding as a result of cavitation are predominant toughening mechanisms.93 Improvement in toughness can be achieved by tailoring of appropriate functional groups.20 HBPs give the ability to tailor phase separation and postpone phase separation to the curing stage.20

The main drawback of dendrimers and HBP modifiers is their cost. Although HBPs are much cheaper than dendrimers, both are too expensive due to their synthesis method (i.e., stepwise process).93 Many synthesis methods require catalysis and neutralization steps that adds to the cost of HBPs. Recently, scientists tried bulk synthesis to overcome this drawback.94,95 Green Approaches for Epoxy Toughening. a. Epoxidized Vegetable Oils. Oils are triglycerides (i.e., esters of glycerol and fatty acids) and can be derived from plants (e.g., soybean) or animals (e.g., fish).96 Vegetable oils (VOs) are used in food97 (over 80% of vegetable oils consumption) and industrial applications such as coatings98 and biodiesel.97 Vegetable oils have gained increasing attention in both academia and industry due to their abundance, low price and biodegradability.96 Double bonds in the backbone of vegetable oils can be economically transformed into many functional groups. The presence of functional groups creates the opportunity to use VOs as monomeric feedstock for the plastic industry.12 Many regulations such as EISA (i.e., Energy Independence and Security Act of 2007) resulted in less price fluctuation of vegetable oils than many petroleum based monomers such as butadiene ($1000/metric ton fluctuation for the former vs $4000/metric ton for the later).12 All of the mentioned benefits have made vegetable oils a great monomeric feedstock for over 20 years.12 Soybean, rapeseed, palm and sunflower oils are the most important vegetable oils with 80% market share.99 Many studies have been devoted to conversion of VOs into epoxidized vegetable oils with high yield.99−102 Among all of vegetable oils, soybean oil and linseed oil are the best candidates for epoxidation due to their high content of unsaturation (double bonds) and high iodine value.103 Chemically modified vegetable oils such as epoxidized soybean oil (ESO) have been used as plasticizers and they can be used as a toughening agent in epoxy resin.104 It was reported that epoxidized vegetable oils have three main advantages in epoxy toughening, in comparison to reactive rubber, including better toughening ability, lower cost and lower sodium content (key characteristic in electronics).105 The first report on use of epoxidized natural oil to toughen epoxy resin goes back to the 1980s.106 In 1990s, epoxidized vegetable oils as epoxy toughening agents were extensively studied by Frischinger et al.105,107,108 In their first attempts, epoxidized vegetable oil was simultaneously mixed with epoxy resin and amine curing agent. Due to the higher reactivity of diglycidyl ether of Bisphenol A (DGEBA) than that of ESO, separation of ESO (average size of 1 μm) occurred and the droplets did not react with the curing agent, even at elevated temperatures. Instead, ESO was dissolved in DGEBA and acted as a plasticizer. Thus, phase separation that is favorable for rubber toughening did not happen when ESO was directly added. The next approach implemented by the group105,107,108 was prepolymerization of epoxidized soybean oil or vernonia oil and preparing biobased rubber. One phase interpenetrated network (IPN), two-phase IPN or phase inverted IPN was observed depending on the content of epoxidized vegetable oil (EVO) and type of curing agent. After the mentioned publications, many scientists incorporated VOs and their derivatives into epoxy matrix and studied their toughening effect. Table 1 shows summary of studies along with range of achieved fracture toughness and impact strength, if applicable. 9531


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decreased as the amount of ELO increased.120 The addition of up to 60% ELO did not have any significant effect on Izod impact strength of samples, and further increase in ELO content resulted in a decrease in Izod impact strength. The same trend was observed for fracture toughness and the addition of more than 50% ELO, which reduced the fracture toughness of thermosets. The reduction in fracture toughness and Izod impact strength can be related to an increase in crosslink density. It was found that the addition of 70% and more ELO increased cross-link density of samples.120 Also, the damping factor and heat deflection temperature decreased as the amount of ELO increased.120 Miyagawa et al.114 studied the effect of type of epoxidized vegetable oil (i.e., ESO and ELO) and ELO content (30 and 50%) in diglycidyl ether of bisphenol F based epoxy matrix. ESO was a more effective toughness modifier than ELO and the addition of 30% of ESO significantly increased the impact strength and fracture toughness of epoxy. Figure 2 shows SEM images of impact-fractured surface of modified epoxies. The sample containing 50 wt % ELO did not show any phase separation while the fractured surface of ESO modified sample indicated the phase separation of soft ESO domains in the epoxy matrix. The improvement in toughness was related to the phase separation of ESO in the thermoset. Also, the effect of ESO on toughening can be due to the lower cross-link density and glass transition temperature of ESO modified samples. ELO has lower epoxide equivalent weight (i.e., higher functionality) than ESO (∼7 for the former vs ∼6 for the latter). The higher functionality of ELO results in a slightly higher glass transition temperature for ELO samples. The glass transition temperature of ESO and ELO modified samples were slightly lower than neat epoxy (around 7−9 °C). Raghavachar et al.122 studied the effect of the addition of epoxidized crambe oil on the mechanical properties of an amine cured epoxy resin. Epoxidized crambe oil was synthesized. 5% and 10% of epoxidized crambe oil were incorporated into epoxy matrix. Neat epoxy had a fracture toughness of 0.76 MPa m1/2. The addition of 5% and 10% epoxidized crambe oil increased the fracture toughness to 1.51 and 1.64 MPa m1/2, respectively. Neat epoxy exhibited a tensile strength and modulus of 93.4 MPa and 3.93 GPa and addition of epoxidized crambe oil reduced the tensile strength and modulus to 75.4 MPa and 3.6 GPa, respectively. The reduction in glass transition temperature

Table 1. Summary Table for Use of Epoxidized Vegetable Oils and Its Derivatives for Epoxy Toughening Vegetable oil type

Content wt %

Fracture toughness (MPa m1/2)

Epoxidized soybean oil Epoxidized methyl soyate Epoxidized allyl soyate Epoxidized castor oil Epoxidized linseed oil Epoxidized crambe oil Epoxidized lunaria oil Vernonia oil Carbonated soybean oil Acrylated epoxidized castor oil

0−60

1.36−2.07

13.9−28.3

0−30

1.36−1.96

17.6−37.6

0−30

-

-

0−40

1.7−3.5

-

0−100

0.4−0.5

10−20

0−50

0.76−1.64

11−13

104, 113, 117 103, 118, 119 106, 114, 120, 121 106, 122

0−50

-

11−13

106

0−50 0−40

-

8.5−32.7

107, 108 98

0−100

0.7−2.2

20−29 (kJ m−2)

123

Impact strength (ASTM D256) (J/m)

ref. 103, 105, 107−116 110

In order to overcome low reactivity of ESO, carbonated soybean oil was synthesized and incorporated into epoxy matrix by Parzuchowski et al.98 Carbonated ESO was synthesized using carbon dioxide. Using carbon dioxide can be considered a green method that can help in carbon fixation. The samples with higher amounts of carbonated ESO i.e., 20% or 40% had higher impact strength than neat epoxy.98 Also, tensile toughness was increased by addition of biobased modifier. The addition of up to 10% of carbonated ESO increased tensile strength and further addition of carbonated ESO resulted in decrease in tensile strength. Also, all modified samples exhibited lower glass transition temperature than neat epoxy. Moreover, samples with higher amounts of carbonated ESO had phaseseparated morphology that was effective in making tough samples. Miyagawa et al.120 studied the effect of addition of epoxidized linseed oil (ELO) on thermal and mechanical properties of epoxy resin. Various amounts of ELO (0 to 100%) were added into a diglycidyl ether of bisphenol F (DGEBF) based epoxy resin. Storage modulus and glass transition temperature

Figure 2. SEM micrograph of impact fractured surface of ELO (left image) and ESO (right image) modified epoxy resin [Reprinted with permission from Miyagawa, H.; Misra, M.; Drzal, L. T.; Mohanty, A. K. Fracture toughness and impact strength of anhydride-cured biobased epoxy. Polym. Eng. Sci. 2005, 45 (4), 487−495, Copyright 2005 John Wiley and Sons, License number: 4170320831901]. 9532


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Mesua ferrea oil, but the effect of incorporation of these biobased HBPs into epoxy resin has not been studied. Barua et al.129 synthesized a glycerol based hyperbranched epoxy resin. The synthesized biobased HBP was used as matrix and cured using poly(amido-amine). The cured samples had good impact resistance (>1 m for drop ball impact) and tensile strength (24.6−48.6 MPa) and high tensile toughness (483− 611 MPa). Saikia et al.130 synthesized a biobased and biodegradable hyperbranched epoxy from sorbitol and castor oil. The mechanical properties of various hyperbranched coatings were compared to epoxy coatings. Biobased hyperbranched polymer had higher tensile strength (22−31 MPa), tensile toughness (5.56−8.11 MJm−3), impact strength (17−34 kJ/m) and elongation at break (33−37%) and slightly lower glass transition temperature than neat epoxy, and mechanical properties were varied with amount of sorbitol in the structure.130 Parzuchowski et al.91 synthesized a hyperbranched polymer from glycidol and studied the effect of the addition of 10, 20 and 40% of HPB into an amine-cured epoxy. Glycidol can be derived from glycerol (i.e., byproduct of biodiesel industry).131 Impact strength, elongation at break and tensile toughness increased as the amount of HBP increased and a phase separation in the range of 65−350 nm was observed in modified thermosets.91 It should be mentioned that the addition of HBP drastically reduced tensile and flexural strength of thermosets. Fröhlich et al.132 reported the effect of the addition of a biobased hyperbranched polyester with epoxidized fatty acid ester end groups in epoxy matrix. Investigation of various end group functionalities revealed the significance of the compatibility of modifier and matrix. Figure 3 shows transmission

was also observed as the amount of epoxidized crambe oil increased.122 Park et al.118 synthesized epoxidized castor oil (ECO) and studied the effect of its addition into the epoxy matrix. The amount of epoxidized castor oil was changed between 0 and 40% and a latent cationic catalyst was used to cross-link the chains. It was found that incorporation of ECO can increase fracture toughness of epoxy resin from 1.7 to 3.5 MPa m1/2. Also, addition of ECO improved flexural strength of epoxy resin and reduced modulus and glass transition temperature of epoxy resin. Sahoo et al.110 synthesized epoxidized methyl soyate (EMS) by esterification reaction of epoxidized soybean oil. Various amount of EMS or ESO (10−30%) was added into DGEBA based epoxy matrix. The toughening effect of EMS was compared with a commercial petroleum based modifier (i.e., alkyd glycidyl ether). Elongation at break, notched Izod impact strength and critical stress intensity factor increased as the amount of ESO or EMS increased.110 Although ESO and EMS showed good toughening effect in the epoxy matrix, the tensile strength and modulus, the flexural strength and modulus and glass transition temperature were lower than petroleum based modifier. b. Biobased Hyperbranched Polymers. As mentioned earlier, hyperbranched polymers are good candidates for toughening epoxy resin due to their high reactivity (functionality), low viscosity and facile synthesis (one step polymerization). Duarah and Karak124 synthesized a biobased hyperbranched epoxy and compared mechanical properties of biobased HBP with a DGEBA based thermoset. A poly(amido-amine) based curing agent was used in this study and tensile toughness of the cured hyperbranched thermoset was at least twice that of the epoxy resin (DGEBA) but the tensile strength and glass transition temperature of biobased samples were lower than DGEBA based samples. De et al.125 synthesized a novel biobased HBP from castor oil and glycerol using a simple poly condensation reaction. The authors took advantage of aliphatic polyester and ether linkage flexibility and aromatic rings stiffness as well as the hyperbranched structure. They were able to improve all the mechanical properties including tensile strength, tensile toughness and impact strength. Hazarika and Karak95 synthesized a waterborne biobased hyperbranched polyester and hyperbranched epoxy. Citric acid, glycerol and polyethylene glycol were used as monomers. Also, the absence of catalyst and neutralizing agent is an advantage of the proposed method. The developed formulations exhibited good toughness (up to 1742 MPa) and impact strength (>1 m for drop ball impact test) although no comparison between hyperbranched samples and neat epoxy was presented. Das et al.126 studied the effect of the addition of biobased hyperbranched urethane in a biobased epoxy coating. Both HBP and epoxy resin were synthesized from Mesua ferrea L. seed oil. Tensile strength, elongation at break and impact strength increased by addition of biobased HBP and it was found that the developed epoxies are biodegradable. It is worthy of note that incorporation of clay into the matrix did not show any promising effect and it decreased impact strength and elongation at break. Other scientists such as Karak et al.127 and Kalita et al.128 also synthesized biobased hyperbranched polyurethane from various vegetable oils including castor oil,

Figure 3. TEM of epoxy resin containing 10 wt % of hyperbranched polymer [Reprinted with permission from Fröhlich, J.; Kautz, H.; Thomann, R.; Frey, H.; Mülhaupt, R. Reactive core/shell type hyperbranched blockcopolyethers as new liquid rubbers for epoxy toughening. Polymer (Guildf). 2004, 45 (7), 2155−2164, Copyright 2004, Elsevier, License number: 4170321291377].

electron microscopy (TEM) of one of modified epoxies. Addition of hyperbranched polymer (Boltorn E) showed phase separation after curing. The addition of only 5% of Boltorn E increased fracture toughness of epoxy resin by 50%, whereas the samples with no phase separation exhibited no significant improvement. The improvement in toughness was related to the phase separation of the modifier with particle sizes of 200 9533


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ACS Sustainable Chemistry & Engineering nm.132 The tensile modulus of modified samples decreased slightly and the tensile strength was almost unaffected by addition of hyperbranched polymer. However, the addition of biobased modifier drastically decreased glass transition temperature.132 Glycerol is a suitable material to synthesize HBPs. Due to the expansion of biodiesel, production of crude glycerol, a byproduct of the biodiesel industry, increased drastically and resulted in saturation of the market and significant cost drop (i.e., by a factor of 10 in recent years133). Scientists including Wyatt and Strahan,134 Stumbe and Bruchmann135 and Wyatt136 synthesized biobased hyperbranched polyesters from pure glycerol and biobased carboxylic acids. Recently, Valerio et al.137 introduced biobased HBPs from crude, analytical and refined glycerol and succinic acid without the use of solvent and catalyst. The hyperbranched polyester showed promising results in toughening of poly(butylene succinate).138 Coativy et al.139 used microwave synthesized biobased hyperbranched polyester (from glycerol and sebacic acid) to toughen poly(lactic acid). Unfortunately, there is not sufficient developed knowledge and cost analysis about using biobased hyperbranched polyesters in epoxy matrix. The main focus of most of the mentioned publications was the use of novel biobased hyperbranched polymers in coatings and biomaterial applications and introducing facile synthesis routes. Although it can be assumed that incorporation of biobased hyperbranched polymers into the epoxy matrix for structural applications (i.e., samples with higher thickness) results in improvement of toughness, there are not enough studies devoted to these applications. c. Green Block Copolymers. More than 50 years after the introduction of block copolymers, they mainly belong to a specialty market apart from commodity polymers.72 There are not many commercially available petroleum based block copolymers in the market, not to mention green block copolymers. Low availability of block copolymers limits potential application of copolymers as tougheners. Therefore, it is researchers’ responsibility to use industrially viable approaches to synthesize green block copolymers. By advances in synthesis methods, including atom transfer radical polymerization (ATRP),140 nitroxide-mediated controlled radical polymerization (CRP) and reversible addition− fragmentation transfer (RAFT),141 synthesis of block copolymers from almost all vinyl monomers is possible. In this aspect, biobased monomers including acrylated epoxidized soybean oil (AESO) can get more attention from academia and industry. Also, lignin based monomers such as guaiacol and vanillin can be used as an alternative to styrene in copolymer synthesis.142 These monomers are a product of the pyrolysis of lignin and a modification step is required to insert a methacrylate group in the monomer.142 Recently, Cochran et al.143,144 used ATRP and RAFT to synthesize poly(styrene-AESO-styrene) block copolymers. The novel triblock copolymers were used to modify asphalt. The biobased copolymer exhibited similar properties to petroleum based copolymers in the studied media. Transmission electron microscopy (TEM) image of triblock copolymer showed a periodic structure (Figure 4) in which darker sections are related to styrene. As was discussed before, it can be considered as good candidate to toughen epoxy resin. Poly(α-methylene-g-butyrolactone) (PMBL), lactide and poly(D-glucose carbonate) (PDGC) are other examples of biobased building blocks that can be used in synthesis of block

Figure 4. TEM image of poly(styrene-AESO-styrene) block copolymer [Reprinted with permission from Hernández N.; Williams R. C.; Cochran E. W. The battle for the “green” polymer. Different approaches for biopolymer synthesis: bioadvantaged vs bioreplacement, Org. Biomol. Chem. 2014, 12 (18), 2834−2849, Copyright 2014, Royal Society of Chemistry, License number: License number: 4170321507360].

copolymers to toughen epoxy resin. They can be derived from the conversion of starch or sugar.142 Shin et al.145 synthesized fully biobased block polymer from menthide and α-methylene-γ-butyrolactone (MBL or tulipalin A) using ring-opening trans-esterification (ROTEP) and ATRP methods. Menthide is a biobased monomer derived from Mentha arvesis and MBL is a biobased monomer derived from Tulipa gesneriana L.145 The elongation at break of biobased block copolymer is higher than the elongation at break of some other available block copolymers. Chen et al.146 synthesized a biobased block copolymer via ring opening polymerization of poly(propylene carbonate) (PPC) and ε-caprolactone (ε-CL). Poly(propylene carbonate) is one of the interesting polymers in respect to carbon fixation. It is synthesized from a reaction of carbon dioxide and propylene oxide in the presence of a catalyst and is miscible with epoxy resin. PPC is amorphous and has a low glass transition temperature.146 The amphiphilic block copolymer (PCL-PPC-PCL) was incorporated into the epoxy matrix and the effect of block copolymer content (5−30%) was evaluated. Figure 5 shows tensile strength, elongation at break and tensile

Figure 5. Tensile strength, elongation at break and tensile toughness of epoxy with biobased block copolymer [Reprinted with permission from Chen, S.; Chen, B.; Fan, J.; Feng, J. Exploring the application of sustainable poly(propylene carbonate) copolymer in toughening epoxy thermosets. ACS Sustain. Chem. Eng. 2015, 3 (9), 2077−2083, Copyright 2015, American Chemical Society]. 9534


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ACS Sustainable Chemistry & Engineering Table 2. Dimension of Cellulosic Fillersa

toughness of thermosets. The neat epoxy resin had tensile toughness and elongation at break of 4.7 MJ/m3 and 13%, respectively. Addition of 5, 10, 20 and 30% of biobased block copolymer increased elongation at break to 15, 48, 107 and 323%, respectively. Also, it was found that tensile toughness and fracture toughness increased as the amount of biobased copolymer increased and the improvement in toughness was related to reaction induced phase separation. The incorporation of biobased block copolymer drastically reduced tensile strength and glass transition temperature of epoxy resin. In order to improve strength and thermal properties of toughened epoxy, Liu et al.147 incorporated chemically reduced graphene oxide into the sample containing 30% block copolymer. It was reported that the addition 0.25% chemically reduced graphene oxide increased tensile strength by 64.8%, whereas it did not change fracture toughness significantly. Polystyrene−polyisoprene−polystyrene (SIS) and polystyrene−polybutadiene− polystyrene (SBS) are dominant commercial block copolymers in the market and are mainly used as thermoplastic elastomers.77 Isoprene is one of the common building blocks in block copolymers and it can be synthesized from Bacillus bacteria. In 2008, Genecor and Goodyear had a joint agreement to commercialize biobased isoprene called “BioIsoprene”.12 If the economic concerns of biobased building blocks such as isoprene are overcome, there will be more chances for biobased copolymer to shine in toughening epoxy resin. Another obstacle in expanding block copolymer market is that no synthesis methods, even the most advanced, can be ideal and the presence of undesired homopolymer is a big challenge. The presence of homopolymers can change the preferred expected morphology. In this regard, ABC triblock copolymers are more favorable since they can accept larger amount of imperfection than ABA copolymers.77 d. Nanofillers from Renewable Resources. Nanocomposites are not excluded from biobased motion and extensive research has been performed to develop cellulosic nanomaterials. Incorporation of cellulosic fillers in nanocomposites has plentiful advantages, including low density, reactive surface for surface treatment and unlimited availability of cellulosic sources (e.g., bacteria, agricultural crops and by products).148 Cellulose nanofibrils present 25% of the strength of carbon nanotubes. Using such material is a good biobased choice considering its potential costs are 10 to 100 times lower.149 Despite the mentioned merits, cellulosic nanomaterials exhibit high moisture absorption, incompatibility with various polymeric matrixes due to their hydrophobic nature and limitation of processing temperature.148,150 Two types of nanosized cellulose can be used as the reinforcement in thermosets, namely cellulose nanocrystals (cellulose nanowhiskers (CNW)) and microfibrillated cellulose (MFC). Table 2 shows the dimension parameters of different cellulose fillers.151 Masoodi et al.152 studied the effect of addition of cellulose nanofiber in a commercial biobased epoxy resin (SuperSap, Entropy resin) and compared the mechanical properties including toughness with glass fiber/epoxy composites. The results showed that surface treatment is required to improve interfacial adhesion between fiber and matrix and achieve mechanical properties comparable with glass fiber composites. Gabr et al.153 incorporated 0.5−2 wt % of MFC into epoxy carbon fiber composite. 2 wt % of MCC resulted in significant improvement in initiation and propagation fracture energy (by

Cellulose structure Microfibril Microfibrillated Cellulose (MFC) Cellulose nanocrystals (Cellulose nanowhisker) (CNW) Microcrystalline cellulose (MCC)

Diameter (nm)

Length (nm)

Aspect ratio (L/d)

2−10 10−40 2−20

>10000 >1000 100−600

>1000 100−150 10−100

>1000

>1000

∼1

a

[adopted with permission from Siró, I.; Plackett, D. Microfibrillated cellulose and new nanocomposite materials: A review. Cellulose 2010, 17 (3), 459−494, Copyright 2010 Springer Science+Business Media, License number: 4170330366227].

80% and 44%, respectively). The addition of nanocellulose increased glass transition temperature but it did not significantly increase tensile strength and modulus. It should be mentioned that addition of higher amount of MCC was not possible due to high viscosity. Phong et al.154 studied the effect of alkali treated micro/ nanosize cellulose into epoxy/carbon fiber composite and found increment in tensile strength and fracture toughness by addition of 0.8 wt % micro/nanocellulose derived from bamboo. Lu et al.155 incorporated bamboo cellulose nanofibers into the epoxy matrix and investigated the effect of surface treatment (silane and sodium hydroxide treatment) and fiber loading (10, 20 and 30%) on mechanical properties of epoxy network. Regardless of the type of surface treatment, samples with 20% nanocellulose had the highest tensile strength, elongation at break and impact strength. Silane treated nanocellulose resulted in better tensile properties and samples with 20% of silane treated cellulose had tensile strength and elongation at break of 71% and 53% higher than neat epoxy, respectively. Also, De et al.156 introduced a novel approach to toughen epoxy resin by using vegetable oil modified nanoclay. Crude neem (Azadirachta) seed oil modified nanoclay (2.5 wt %) was incorporated into a hyperbranched epoxy matrix. Toughness and elongation at break increased by 5.5- and 3-fold, respectively and the nanocomposite exhibited a higher tensile strength than neat epoxy (50% improvement). Emami et al.157 used two commercial available surfactants (block copolymer of poly(ethylene oxide) (PEO) and poly(propylene oxide) (PPO)) to modify cellulose nanocrystals and investigated the effect of the addition of 2% modified and unmodified cellulose nanocrystals on mechanical properties of epoxy resin. The addition of modified cellulose nanocrystals resulted in higher fracture toughness and tensile modulus. Since the physical modification was used in the study, it is possible that the surfactant was not completely absorbed by cellulose and unabsorbed copolymer micelles acted as toughening modifier in the matrix. Abraham et al.158 adopted an environment friendly method to make nanocrystalline cellulose (CNC) epoxy nanocomposites. Recycled cellulose from wood pulp industry was used and the acetylation solvent (i.e., acetic anhydride) was a reactant in the cellulose modification reaction so no solvent was required in the process. The effect of loading (0.01−1%) of modified CNC in epoxy matrix was studied. It was reported that the addition of acetylated CNC increased tensile properties of epoxy resin. Samples with 0.5% CNC exhibited tensile strength and modulus of 45.2 MPa and 1.6 GPa, respectively and they had almost twice the tensile strength and modulus of neat 9535


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improve toughness. Considering the various required steps for purification and the higher cost of pure glycerol, use of pure glycerol will add to cost of expensive HBPs and it is not feasible to commercialize the mentioned products. As mentioned earlier, the effect of the addition of hyperbranched polymers derived from glycerol into epoxy matrix is unknown. It is the responsibility of academia and industrial R&Ds to develop costcompetitive biobased HBPs and investigate the effect of the addition of biobased HBPs in toughening epoxy resin and synthesis methods should have potential for scale-up to industrial scale. The block copolymer industry has a long way ahead as it becomes a mature industry and many mutual academic and industrial efforts should be devoted to synthesizing cost competitive block copolymers. Most of available block copolymers in the market are too expensive and are mostly used for toughening other type of polymers such as poly(lactic acid) (PLA). Regulations for limiting greenhouse gas emission may shift the trend of R&D sections to use biobased monomers as blocks. Biobased monomers including acrylated epoxidized soybean oil (AESO), lignin based monomers and poly(propylene carbonate) (PPC) can replace the common monomers in block copolymer synthesis. There are not enough studies on the effect of biobased block polymers in toughening epoxy resin, but the studies on petroleum based epoxies revealed the efficiency of block copolymers as toughening modifiers. Although the research in biobased nanofillers is an ongoing field of study and many aspects of biobased nanocomposites are unknown, the addition of nanofillers showed promising results on improving fracture toughness without sacrificing other mechanical properties. If the commercialization obstacles are overcome, the biobased nanofillers are good candidates to make tough epoxy composites. In summary, the incorporation of vegetable oils derivatives seems the most feasible approach in the current market status. Due to the observed negative effect of their incorporation on other mechanical properties such as strength, modulus and glass transition temperature, it is predicted that other approaches such as biobased hyperbranched polymers, block polymers and nanofiler are the future trend in toughening epoxies. There are many challenges for biobased material to meet physical, mechanical specifications and economic characteristics of their petroleum based counterparts. Also, the cost competitive scaleup of tough biobased materials and mass production of renewable based materials are ongoing challenges. Despite significant studies, further studies need to be devoted to overcoming obstacles to the sustainable thermoset economy.

epoxy. Also, an increase in tensile toughness was observed with addition of crystalline nanocellulose. Despite of all attempts made to develop tough epoxy nanocomposites, there are many obstacles to replacement of glass fabric or carbon fabric with biobased nanofillers. Low interfacial adhesion of the reinforcement and high moisture absorption resulting in poor mechanical properties are key factors and should be taken into account. Also, manufacturing processes and surface treatments or other required modifications should be cost competitive with available reinforcement in the market.

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CONCLUDING REMARKS AND OUTLOOK Worldwide consumption of thermosets including epoxy resin is increasing. Various methods have been developed to overcome epoxy resin brittleness and make it a better candidate for coatings and composites in structural applications and the automotive industry. Binding legislation and policies such as Lead Market Initiative and BioPreferred force industries to shift toward sustainable economy and resulted in increase market share of biobased polymers. Since there are not many fully biobased epoxy resins available in the market, implementation of other strategies including replacement of petroleum based additives for modifiers with biobased counterparts is required. Use of vegetable oils derivatives is one of the most feasible strategies and cost competitive approaches. It has been successfully implemented in automotive companies including Porsche and Subaru. Among various vegetable oils, derivatives of soybean oil are the most studied biobased modifiers. Derivatives of soybean oil exhibited the highest impact strength and fracture toughness among the vegetable oil derivatives. They can be noted as one of the most effective biobased modifiers however the balance between increase in toughness and decrease in other mechanical properties such as glass transition temperature, strength and modulus is an ongoing challenge. Apart from the performance improvement, other factors such as availability of biobased feedstocks and the effect of use of the biobased feedstock on food chain are crucial for commercialization of the modifiers. Due to the increase in use of epoxy resin in structural applications, the demand for use of high cross-link density epoxy resin has increased. Considering the high efficiency of biobased block copolymers and hyperbranched polymers in high cross-link density epoxies, they can be good candidates for epoxy toughening in structural applications. One of effective green approaches toward toughening of epoxy resin can be incorporation of biobased HBPs. In general, the review of the effect of HBPs on epoxy resin revealed their ability in improving fracture toughness without sacrificing strength and modulus. On the other hand, there are not many studies on the effect of biobased HBPs on toughening of epoxy resin. There are few commercial HBP and dendrimers in the market including DSM (Hybrane), Perstrop (Boltorn, a dendritic from fatty acid) and Nanopartica (dendritic polyglycerol). Glycerol is one of the promising materials to synthesize HBPs. This low-cost coproduct can be used to synthesize cheaper HBP and can simply outweigh the high cost of HBP synthesis. Most of available studies focused on synthesizing biobased HBPs with oxiran functionality and evaluating their properties. Instead of using high purity glycerol and converting it to hyperbranched epoxy as matrix, the inexpensive biobased HBPs obtained from crude glycerol can be added in small amounts into the epoxy matrix and probably

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AUTHOR INFORMATION

Corresponding Authors

*M. Misra. E-mail: [email protected]. *A. K. Mohanty. E-mail: [email protected]. ORCID

Amar K. Mohanty: 0000-0002-1079-2481 Manjusri Misra: 0000-0003-2179-7699 Notes

The authors declare no competing financial interest. 9536


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ACS Sustainable Chemistry & Engineering Biographies

Dr. Misra is a professor in the School of Engineering and holds a joint appointment in the Dept. of Plant Agriculture at the University of Guelph. Dr. Misra’s current research focuses primarily on novel biobased composites and nanocomposites from agricultural and forestry resources for the sustainable bioeconomy targeting the development of biobased and eco-friendly alternatives to the existing petroleum based products. She has authored more than 500 publications, including 281 peer-reviewed journal papers, 25 book chapters and 14 granted patents. She was an editor of the CRC Press volume, “Natural Fibers, Biopolymers and Biocomposites,” Taylor & Francis Group, Boca Raton, FL (2005); American Scientific Publishers volume “Packaging Nanotechnology”, Valencia, California (2009); “Polymer Nanocomposites”, Springer (2014) and “Fiber Technology for Fiber-Reinforced Composites”, Woodhead Publishing (2017). She was the chief editor of “Biocomposites: Design and Mechanical Performance” Woodhead Publishing (2015). She was the 2009 president of the BioEnvironmental Polymer Society (BEPS). She is one of the associate editors of the journal “Advanced Science Letters” and serves in the editorial board of “Journal of Applied Polymer Science”. In 2012, Dr. Misra received the prestigious “Jim Hammer Memorial Award” from BEPS and University of Guelph’s Innovation of the year award in 2016 for the involvement in developing the “compostable single-serve coffee pods”. Total citations: 19 811; hindex: 65; i10-index: 225 (Google Scholar, July 12, 2017). ResearchGate (RG) Score: 45.24 (higher than 97.5% of ResearchGate members’) (ResearchGate, July 12, 2017).

Dr. Ghodsieh Mashouf Roudsari received her Ph.D. in Biological Engineering from University of Guelph, Ontario, Canada. She received her Bachelor’s and Master’s degrees in Polymer Engineering from Amirkabir University of Technology (Tehran Polytechnic), Iran. The primary focus of her research is synthesis and characterization of thermoset polymers, biocomposites and nanocomposites. She has published 6 peer reviewed journal articles.

Dr. Amar Mohanty is a professor and Premier’s Research Chair in Biomaterials and Transportation at the University of Guelph and a former Michigan State University professor. He is an international leader in the field of bioplastics and biobased materials. He holds the University Research Leadership Chair Professor and is the director of the Bioproducts Discovery & Development Centre (BDDC) at the University of Guelph. He has around 740 publications to his credit, including 315 peer-reviewed journal papers, and 51 patents awarded/ applied. Five of his recently invented technologies have been commercialized. The total citation number of his research articles is 21 096, h-index is 67 and i10index is 220 (Google Scholar, July 12, 2017) and his ResearchGate Score of 45.61 is higher than 97.5% of the 11 million ResearchGate members (Research Gate, July 12, 2017). His work was recognized by the Lifetime Achievement Award from the BioEnvironmental Polymer Society, USA. Professor Mohanty has also received the Andrew Chase Forest Products Division Award from the Forest Products Division of the American Institute of Chemical Engineers and was the holder of the Alexander von Humboldt Fellowship at the Technical University of Berlin, Germany. Currently, he holds the Director (elect) position of Forest Products Division of the American Institute of Chemical Engineers.

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ACKNOWLEDGMENTS We acknowledge the financial support from (1) Ontario Ministry of Agriculture, Food, and Rural affairs (OMAFRA)New Directions Research Program (Project # 050155), (2) OMAFRA-University of Guelph Bioeconomy-Industrial Uses Theme (Project # 200283), (3) Grain Farmers of Ontario (GFO) (Project # 049182), (4) Natural Sciences and Engineering Research Council (NSERC)-Collaborative Research and Development (CRD) (Project # 400515), (5) Ontario Ministry of Research and Innovation (MRI), currently known as the Ontario Ministry of Research, Innovation and Science (MRIS) (Project # 052644 and # 052665), (6) FedDev Ontario (Project # 050993), (7) Canada Foundation for Innovation (CFI) (Project # 460214), (8) Bank of Montreal (BMO) and (9) numerous University of Guelph’s Alumni.

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REFERENCES

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Green Approaches To Engineer Tough Biobased Epoxies: A Review

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