A renewable approach to synthesize highly toughened bio-epoxy from

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A renewable approach to synthesize highly toughened bio-epoxy from castor oil derivative-epoxy methyl ricinoleate and cured with bio-renewable phenalkamine Sushanta Kumar Sahoo, Vinay Khandelwal, and Gaurav Manik Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b02043 • Publication Date (Web): 27 Jul 2018 Downloaded from http://pubs.acs.org on July 27, 2018

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Industrial & Engineering Chemistry Research

A renewable approach to synthesize highly toughened bio-epoxy from castor oil derivative-epoxy methyl ricinoleate and cured with bio-renewable phenalkamine Sushanta K Sahooa, Vinay Khandelwala, Gaurav Manika* a

Department of Polymer and Process Engineering, Indian Institute of Technology

Roorkee, Saharanpur Campus, Saharanpur 247001, UP, India *Corresponding Author mail id: [email protected]

Abstract Epoxidation and transesterification of castor oil were carried out to synthesize less viscous epoxy methyl ricinoleate (EMR) and confirmed by FTIR and proton NMR analysis. Both the bio-resins, epoxidized castor oil (ECO) and EMR, were copolymerized at different compositions (10, 20 and 30 wt%) with DGEBA-epoxy resin using biorenewable phenalkamine (PKA) crosslinker for better processibility and superior toughening. On incorporation of 20 wt% EMR, the viscosity of epoxy got reduced significantly. The tensile and impact properties showed that the bio-epoxy blend with 10 wt% of EMR possess greater stiffness and strength with higher toughness compared to its ECO counterpart. The increased peak intensity or broadened tan δ curve of bio-epoxy blends confirmed higher damping ability. FE-SEM micrographs showed single phase morphology of epoxy/EMR blends and phase separated network of epoxy/ECO blend which ensured better toughening. Preliminary biodegradability of the epoxy/EMR blend has been studied through vermi-compost burial.

Keywords: Bio-based epoxy; Castor oil; Bio-renewable crosslinker; Toughening; Thermomechanical Properties

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Industrial & Engineering Chemistry Research 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

INTRODUCTION The overuse of non-renewable resources for polymer or pre-polymers production has created major concerns among the researchers across the globe as these resources are rapidly diminishing, and also creating serious environmental issues during synthesis.1 Polymer and composite industries have also suffered significantly due to the raised cost of petroleum based resources and environment regulations. In this context, plant oils are the most commendable renewable feed stock, consisting of unsaturated fatty acids from which pre-polymers or polymers can be synthesized through varieties of chemical modifications.2 However, cured functionalized oils are incapable of delivering the desired properties as petro-based materials for industrial applications.3–5 Liu et al. developed sixarmed bio-based resin epoxidized hexa (linoleoyl hydroxymethyl) melamine (EHL) using linoleic acid and Hexamethylol melamine (HMM) as the raw materials which exhibited lower mechanical properties compared to petro-epoxy.6 Petro-based epoxies have rigid aromatic moieties which offer stiffness to the material wherein bio-based thermosets are devoid of aromatic groups thus exhibit inferior properties. Thus, bio-resins can be used as secondary component as a toughening additive in petro based polymers to improve their processability and specific properties.7,8 DGEBA-epoxy resin due to its versatility finds use in diverse applications like adhesives, high-performance composites, coatings, and electronics applications.9 As reactive diluents, epoxides of soybean, linseed, sunflower oil etc. have attracted growing attention of researchers in the last decade to toughen petrobased epoxies. Recently, fatty acids of different chain lengths were grafted on epoxidized soybean oil (ESO) through esterification process to formulate the bio-rubber in order to toughen vinyl ester as reported by Yadav et al..10 However, the wide use of edible oils in thermoset and thermoplastic polymer industries may create immense threat to food market and global edible oil supply. Among promising non-edible oils, castor oil is competently used to form pre-polymers or polymers in many chemical industries. While being inexpensive, it has unique functionalities like unsaturated double bonds, hydroxyl groups and longer shelf life. Castor oil consists of unsaturated functionality and ricinoleic acid (12-hydroxy-9-cisoctadecenoic acid) which covers 85-90% content.11,12 Such multiple functional sites 2 ACS Paragon Plus Environment

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make it more reactive and versatile material compared to other plant oils. Epoxidized castor oil (ECO) is being widely used by researchers worldwide as bio-based plasticizer, coating material, pre-polymers, lubricants, additives, and adhesives due to its multiple functionality.8,13–17 However, ECO is inefficient as a diluent for epoxy and fails to fulfil the physical properties as per technical requirements. Commercial multifunctional aliphatic reacting diluents exhibit viscosity within the range 100-1000 cPs18,19 while ECO has viscosity range of 4000-6000 cPs.17 Cayli et al. synthesized less viscous epoxidized methacrylated castor oil (EMETCO) monomer and copolymerized with styrene (ST) to from EMETCO-ST copolymer with improved thermophysical properties.20 The reduction of viscosity and Newtonian flow behavior of resins are important to ensure better processability for compression molding, injection, resin transfer moulding, pultrusion etc. and also helps to meet the end-use property the material.21 Modifications like transesterification, methoxylation, maleation etc. strongly reduce the viscosity and enhance the reactivity of epoxidized oil to enable ease of copolymerization with base polymer.7,11,17,22,23 Wang and Schuman prepared glicidyl esters of epoxidized fatty acids (EGS) with larger internal epoxy content through transesterification and epoxidation reactions which showed improved properties compared to ESO when copolymerized with DGEBA.24 Pan et al. reported epoxidized sucrose esters of fatty acids (ESEFAs) based materals with significantly higher Tg and acceptable mechanical properties for coating application when cured with petro-based anhydrides.25,26 The anhydride and amine based crosslinkers play a vital role to form rigid, dimensionally and thermally stable networks through polyaddition or copolymerization reactions.27,28 Especially, amine based curing agents are widely used in industrial applications due to their ability of curing at room temperature and higher reactivity.29 However, these have serious environment related issues due to their volatility, toxicity and hazardous nature and also from brittle networks.30,31 In this circumstances, bio-based curing agents are essential for preparing green and sustainable products.29,32 Phenalkamines (PKA) are commonly used as curatives which can be cured at lower temperature and humid conditions.33,34 Recently, these are mostly synthesized through Mannich reaction from cardanol, obtained from cashew nut shell liquid (CNSL).34



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In the current work, we have prepared epoxidized castor oil (ECO) and epoxy methyl

ricinoleate

(EMR)

through

in-situ

epoxidation

and

base

catalyzed

transesterification. Some of the green chemistry principles were followed during epoxidation and transesterification process.7 Both ECO and EMR were used as sustainable reactive diluent to lower the viscosity of the epoxy matrix and toughen it through crosslinking by copolymerization. Bio-renewable PKA has been used as an ecofriendly curative to achieve optimum toughening. The novelty of this study is to synthesize an efficient low viscosity, functional bio-based diluent (EMR) and use it to develop and characterize the beneficial bio-epoxy-petro epoxy blends through determination of its flow behavior, mechanical, thermal and thermomechanical properties. Preliminary studies of biodegradation of the bio-epoxy blend through vermicompost burial, and subsequent FE-SEM analysis, presents possibilities of useful biodegradation potential.

EXPERIMENTAL Materials Diglycidyl ether of bisphenol A (DGEBA) epoxy (Araldite LY 556, with epoxy equivalent weight (EEW) of 190) was supplied by Huntsman. Castor oil was purchased from Himedia, India. Acetic acid (99 wt%) and hydrogen peroxide (30 wt%) were received from Rankem, India. Seralite (SRC-120) amberlite resin, was supplied by SRL Pvt. Ltd, India. Phenalkamine (PKA) was procured from Cardolite Specialty Chemicals, Bangalore, India.

Synthesis of epoxidized castor oil Epoxidized castor oil was synthesized using the process as reported earlier for castor and linseed oil by Sahoo et al.35 and Khandelwal et al.36 76.5 gm of castor oil was placed in a flask and then 15 gm of acetic acid and 19.05 gm of SRC-120 catalyst were added to it, subjected to stirring for 20 minutes. After this, 56.5 gm of H2O2 was added drop-wise through the dropping funnel and stirred for 5 hr at 500 rpm. At the end of the reaction, the mixture was filtered through a cloth to separate the seralite catalyst. After


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that, the epoxidized oil was washed and filtered as reported earlier and then dried using magnesium sulphate.

Synthesis of epoxy methyl ricinoleate Transesterification of 100 g ECO was carried out using 1 wt% of sodium methoxide catalyst in 30 mL methanol. This reaction was carried out at 50 °C as reported for epoxidized soybean oil earlier.7,37 After 30 min, the reaction mixture formed two layers (EMR and glycerol layer) and the epoxy methyl ricinoleate (EMR) was separated using a separatory funnel. The possible transesterification reaction is shown in Scheme 1.

Scheme 1. Preparation of epoxy methyl ricinoleate (EMR) from epoxidized castor oil (ECO) In the epoxidation and transesterification process, green byproducts like water and glycerol are produced which can be used for other relevant industries.

Development of cured bio-epoxy blends Synthesized ECO and EMR were used as secondary component with DGEBA-epoxy in different ratios (10, 20 and 30 wt%) to develop bio-based epoxy blends. The homogeneous mixture of resins were stirred mechanically at 500 rpm and then bio-based 5 ACS Paragon Plus Environment


curative, PKA, was added to the resin mixture in a stoichiometric ratio33 and subsequently the resin blend was poured into a release agent coated steel mold. The preliminary crosslinking was performed at room temperature for 24 hr and post curing carried out at 120 °C for 2 h and 150 °C for 6 hr. The hand layup method technique was implemented to fabricate bio-based epoxy blends. The crosslinked network of epoxyECO and epoxy-EMR is shown in Scheme 2 and Scheme 3. Epoxy, epoxy/10%ECO, epoxy/20%ECO, epoxy/10%EMR and epoxy/20%EMR are coded as EP, EPECO10, EPECO20, EPEMR10 and EPEMR20 respectively.

Scheme 2. Crosslinked network of epoxy-ECO cured with PKA


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Scheme 3. Crosslinked network of epoxy-EMR cured with PKA

Characterization Chemical group analysis Oxygen oxirane content (OOC) reveals the conversion of unsaturation in castor oil to epoxide groups during epoxidization as well as retention of oxirane rings during transesterification. The OOC and EEW of ECO and EMR were determined by ASTM D 1652. Crystal violet was taken as indicator and 0.1 N HBr solutions in glacial acetic acid were used as titrant. Fourier Transform Infrared spectra of bio-resins were taken on FTIR spectrophotometer (PerkinElmer FT-IR C91158, UK) with a 4 cm-1 resolution. The bio-resin samples were placed between two KBr windows, then spread on their surface for analysis. Proton NMR spectra of ECO and EMR was obtained on a spectrometer (JEOL, Japan) and CDCl3 was used as a solvent during recording. Synthesized EMR resin was analyzed for molecular weight (Mn and Mw) through Gel Permeation Chromatography (Waters 717 plus Autosampler and Water 600s Controller) using HPLC grade tetrahydrofuran (THF) as eluent with flow rate of 1ml/min. 7 ACS Paragon Plus Environment


Mechanical properties The tensile modulus and strength of DGEBA-epoxy and its blends were conducted as per ASTM-D-3039 standard employing Universal Testing Machine, (Instron 3382, UK). The dimensions of each specimen were 160 mm x 25mm x 3 mm and the test performed at a crosshead speed of 1 mm/min keeping the gauge length of 70 mm. Flexural properties of all specimens of dimension 127 mm × 12.7 mm × 3 mm were determined for all the samples as per ASTM-D-790. Izod impact strength of notched specimens was evaluated using Izod impact tester (Tinius Olsen, UK) as per ASTM-D256. The specimens of dimensions 63.5 mm × 12.7 mm × 3 mm were used to determine the impact strength of toughened epoxy blends. Notch depth of 2.54 mm and notch angle 45º were made using a notch cutter (Tinius Olsen, UK). Five specimens were tested for each sample for the tests. Flow behavior of resin blends The rheological behavior of uncrosslinked petro-based epoxy (DGEBA) resin and its blends was studied in a parallel plate rheometer (MCR 102, ANTON PAAR, USA). The tests were carried out at 25 °C within 0−500 s-1. Differential Scanning Calorimetry (DSC) The samples were heated from 25 to 200 °C in a nitrogen atmosphere with a scanning rate of 10 °C/min to compute the glass transition temperature (Tg) using DSC (NETZSCH, Germany). Thermogravimetric Analysis (TGA) Pure epoxy and its co-polymers were analyzed using thermogravimetric analyzer (Exstar TGA/DTG 6300) to investigate thermal stability. Samples of 9-11 mg weight were heated within the range of 40 to 800 °C at a scanning rate of 10 °C/min in a N2 atmosphere. T5, T10, T20 and T50 (the temperatures corresponding to 5, 20 and 50 wt% sample degradation respectively) and Tendset (the temperature where degradation stops) were calculated from TGA thermograms. The temperature derivative of degradation rate was analyzed by DTG thermogram. Dynamic Mechanical Analysis (DMA)


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The thermophysical characteristics of the cured samples were investigated using DMA analyzer (NETZSCH, Germany) at a frequency of 1 Hz and a strain rate of 0.09%. The samples of dimensions 60 mm x 12.7 mm x 3 mm were subjected to heating within temperature range from 30 °C to 250 °C, at a scanning rate of 10 °C/min. Field Emission-Scanning Electron Microscopy (FE-SEM) Analysis The micrographs of fractured samples were taken using FE-SEM (MIRA 3 LM) to analyze the morphology. Prior to test, the fractured samples were coated with goldpalladium alloy sputtering process. Biodegradation In order to analyze the biodegradation potential of bio-epoxy blend, it was buried in vermi-compost for a long term study. After 90 days, the micrographs of the sample were taken after washing in distilled water and drying. RESULTS AND DISCUSSION Functional group analysis Oxirane content of ECO and EMR were determined to be 4.4 ± 0.1 % and 4.2 ± 0.2 % respectively, obtained through epoxy titration according to ASTM D1652 standard. The marginal decrease in OOC of EMR may be attributed to the ring opening reaction and side reactions by residual solvents and byproducts like glycerol. From OOC values, EEW has been determined to be 363 g/mol and 381 g/mol for ECO and EMR, respectively. FTIR spectra of ECO and EMR is depicted in Figure 1 to confirm the epoxidation and transesterification.



Figure 1. FTIR spectra of epoxidized castor oil (ECO) and epoxy methyl ricinoleate (EMR) The characteristic C–O–C stretching band due to the epoxide vibration appeared within the range 820-845 cm-1.16 The most significant signal at 843 cm-1 confirmed the epoxy group of ECO as reported by Hernandez et al,11 Sudha et al.38 and Borugadda et al.39 Likewise, the signals C–O–C stretch from epoxy ring at 845 cm-1 in EMR confirms the retained epoxide group after transesterification.38 The broad band appearing at 33003650 cm-1 is attributed to OH moiety of ricinoleic acid. The bands noticed at 2927 and 2856 cm-1 correspond to C–H stretch of ECO which slightly shifted to 2930 and 2857 cm1

in EMR after transesterification. Highly intense band at 1743 cm-1 is attributed to C=O

stretch of the formed aliphatic ester. Additionally, consecutive three bands at 1173, 1199 and 1248 cm-1 are detected which are the distinctive methyl esters of EMR.17,39 The angular deformation vibration of aliphatic C–H group is noticed at 1460 cm-1. The 1H NMR spectra (Figure 2 (a)) of ECO shows the occurrence of secondary (-CH-) and primary glyceral proton (-CH2) at 5.10 -5.20 ppm and 4.10-4.30 ppm, respectively. The peaks centered at 2.9-3.12 ppm signify the presence of epoxide protons (–CH–O-CH-) of the epoxide ring of ECO. The absence of the olefinic protons (-CH=CH-), signals in the range 5.34-5.43 ppm further confirmed the epoxidation. The signal attributed to hydroxyl (–OH) group is shifted to 3.85 ppm after epoxidation. Similar spectra was noticed for synthesized ECO reported earlier.11,17,38 10 ACS Paragon Plus Environment

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1 4

3 2

Figure 2 (a). Proton NMR of epoxidized castor oil (ECO)

3

1 2

Figure 2 (b). Proton NMR of epoxidized methyl ricinoleate (EMR)

A discrete and highly intense methyl ester peak is observed at 3.65 ppm in NMR spectra of EMR depicted in Figure 2(b) as observed earlier38 confirming success transesterification of ECO. Additionally, no traces of glycerol moieties has been found



within 4.12-4.26 ppm revealing further synthesis of EMR.17,38 Similarly, peak attributed to unsaturation at 5.1-5.4 ppm is disappeared. However, it is noticeable that the signals at 2.9-3.1 ppm ascribed to epoxide ring are still retained even after esterification as mentioned in FTIR analysis. The rest of the peaks can be correlated well with epoxy methyl esters derived from castor oil.39 Epoxy methyl ricinoleate (EMR) can easily undergo ring opening reaction probably due to the presence of glycerol moiety. During transesterification in presence of methanol, small amount of methoxylated methyl ricinoleate and methoxylated castor polyol as impurities might have been generated which is difficult to remove due to its hydrogen bond association with the methyl ricinoleate.17 Thus, transesterified ECO (EMR) was analyzed through GPC and the plot has been provided as supplementary file (Figure S1) to find the molecular weight of the sample and presence of impurities. The number average molecular weight (Mn) of EMR is found to be 482 Dalton which is close to the expected molecular weight of 328 g/mol determined previously through ESI-MS spectra by Sankaranarayanan et al..17 Further, the retaining of epoxy moieties at 2.9-3.1 ppm in NMR spectra revealed that the formation of EMR is convincing and the mentioned impurities are less in quantity.

Mechanical performance Tensile properties The nature and molecular architecture of a polymeric network play an important role in predicting its mechanical properties. At required high temperature, both the epoxide and hydroxyl groups of ECO and EMR reacted with the amine hydrogen of PKA during crosslinking of epoxy. The molecular chains in each of the reactive monomers, covalently link with each other to form perfectly crosslinked network and such systems exhibited more strength and stiﬀness. The tensile strength, modulus and maximum elongation of all the samples are summarized in Table 1.


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Table 1. Mechanical properties of epoxy and bio-epoxy blends Name of

Tensile Strength

Tensile Modulus

Elongation at

Sample

(MPa)

(MPa)

break (%)

EP

55 ± 3

1379 ± 51

3.5 ± 0.2

EPECO10

42 ± 1

1390 ± 52

4.2 ± 0.1

EPECO20

35 ± 2

1233 ± 23

7.0 ± 0.2

EPECO30

26 ± 1

1040 ± 63

6.2 ± 0.1

EPEMR10

44 ± 2

1449 ± 65

4.1 ± 0.2

EPEMR20

39 ± 1

1081 ± 56

8.5 ± 0.3

EPEMR30

31 ± 1

830 ± 33

11.4 ± 0.5

From Table 1, it can be seen that PKA cured DGEBA-epoxy exhibited similar values of tensile strength and 29-34 % lower modulus compared to triethylene tetramine (TETA) crosslinked one but displayed higher elongation at break.14,40 This could be attributed to the long alkyl chain present in PKA which adds ductility and softens the epoxy matrix. At 10 wt% of ECO and EMR loading, the tensile strength of pure epoxy reduced by 20-23% but Young’s modulus remained almost the same (EPECO10) or slightly increased (4% in case of EPEMR10) along with an increase in maximum elongation. This may be attributed to adequate reduction in viscosity of the resin system by 10 wt% of ECO and EMR which resulted in better diffusion of reactants and ease of crosslinking with negligible plasticization. The minor increase in strength and modulus and decrease in elongation of EPEMR10 compared to EPECO10 is due to the stiffening nature of epoxy fatty acid esters and better reactivity of epoxide groups which increased the crosslink density.7 Similar increase in tensile modulus with moderate strength was observed in case of epoxy/10 % EMS7 and unsaturated polyester resin (UPR)/5-10% castor oil pentaerythritol glyceride maleates (COPERMA) blends developed earlier.22 However, at higher content (for both ECO and EMR), the strength and modulus suffered significant reduction but with benefits of improved ductility (up to 200-300 % increase in elongation). Reduction in tensile strength and modulus of pure epoxy was noticed to the tune of 52.7% and 24.5% respectively at 30 wt % ECO loading. Similar trend in strength (36-37% decrease) and modulus (40-46% decrease) was observed for TETA cured 13 ACS Paragon Plus Environment


DGEBA/30%ECO blends as reported previously.8,14 Likewise,43.6% and 39.8% reduction in strength and modulus was noticed respectively when 30wt % EMR was incorporated in DGEBA-epoxy. This reduced the stiffness and a higher ductility was observed because of over plasticization caused by long flexible aliphatic chains present in oil epoxides and a larger replacement of rigid moiety DGEBA by soft and low reactive bio-resin. It is notable that low molecular weight EMR based blends exhibit higher tensile strength and elongation compared to its ECO based counterparts but at a sacrifice of modulus. Particularly, EPEMR20 and EPEMR30 blends show relatively lower modulus than their ECO based counterparts because of lower epoxide content and less entanglement. From the elongation at break study, it was revealed that an effective toughening of epoxy occurred at 20 wt% of ECO and EMR with a moderate strength and modulus obtained that may suit specific applications. At 30 wt% of EMR, elastomeric nature of epoxy blend is observed with inferior modulus and much higher elongation owing to undesired plasticization. Similar trend and explanations were reported earlier for TETA cured ESO and EMS based epoxy blends by Zhu et al.41 and Sahoo et al.7,40

Toughening properties For further study on toughening capacity of bio-resin, the influence of ECO and EMR on impact strength of epoxy has been presented in Figure 3. The impact strength of pure epoxy progressively increased with addition of ECO and EMR due to larger replacement of rigid bisphenol groups by soft aliphatic segments/glyceride linkages and methyl ester chain respectively .15


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Figure 3. Impact strength of epoxy and modified castor oil based epoxy blends In ECO based epoxy blends, maximum impact strength (8.62 kJ/m2) was noticed at 30 phr of ECO content, which is 146 % higher than that of pure epoxy. In earlier reports, maximum increase of 80.2 % in impact strength values was observed on addition of 20% ECO into TETA cured epoxy, which got reduced at 30% ECO loading because of inverse phase separation or over plasticization.8 Liu et al.42 and Miao et al.43 reported nearly 61 % and 96 % increase in un-notched impact strength at 5% EHBPES (epoxideterminated hyperbranched polyether sulphone) of low and medium Mn value respectively. It is interesting to note that no significant difference in impact strength was observed between EPECO10 and EPEMR10 sample owing to formation of stiffened network which can be correlated with similar strain at break and increased stiffness as explained in mechanical section. Similar findings were noticed in 5-10 % highly functionalized COPERMA modified UPR.22 When more than 10 wt % of the epoxy resin was substituted by the EMR bio-resin, the impact strength starts to increase gradually. At 2030 wt % EMR content, epoxy blends exhibited higher impact strength or energy absorbing ability than that of corresponding EPECO counterparts which is already confirmed earlier from larger elongation at break in tensile tests. Out of all the samples, EPEMR30 displayed highest impact strength (14.5 kJ/m2) which is 68 % higher when compared to EPECO20 (7.45 kJ/m2)

and 315 % higher when compared to neat epoxy

(3.49 kJ/m2). Similar increased trend in impact strength was observed for epoxy/EMS and 15 ACS Paragon Plus Environment


UPR/COPERMA. Higher bio-resin loading initiated inverse phase separation and incorporated substantial ductility within epoxy network and toughened the matrix effectively with increased impact strength. Among all the samples, epoxy containing 20 wt % ECO and 20 wt % EMR exhibited substantial impact values with reasonable mechanical strength and modulus applicable for structural or building applications. Thus, it can be perceived that a maximum 20 wt% of bio-resin can be incorporated within epoxy without sacrificing much strength and modulus. Hence, only epoxy blends with 20 wt% of ECO and EMR were considered for further studies.

Rheological behavior The flow behavior of epoxy, ECO, EMR, EPECO20 and EPEMR20 has been studied as a function of shear rate which is a prerequisite for molding technique. Shear stress-shear rate graph of all samples is shown in Figure 4(a) which shows that EP, ECO and EPECO20 resin display non-linear relationship revealing pseudoplastic or shear thinning flow behavior while EMR and EPEMR20 exhibit linear relationship confirming Newtonian flow. Such flow performance is further confirmed from viscosity-shear rate graph as displayed in Figure 4(b). It was observed earlier that viscosity of ECO at zero shear gets reduced from 6.76 Pa.s to 0.10 Pa.s and also behave as perfectly Newtonian fluid after transesterification.7 It can also be observed from Figure 4 (b) that EP, EPECO20 and EPEMR20 have zero shear viscosity of 11.45 Pa.s, 7.19 Pa.s and 1.54 Pa.s respectively. It is notable that EMR and EPEMR20 samples displayed constant viscosity irrespective of shear rate signifying no rheological changes under the effect of stress. On the other hand, the viscosity of ECO and EPECO20 blends displayed slight variation at lower shear rate and then became constant at higher shear values. This disparity is observed because of dissimilar structural and physical changes occurring in molecular entanglement at different shear rates. Addition of ECO reduced the pseudoplastcity of epoxy to some extent, but not completely, whereas incorporation of EMR eliminated the peudopalstic nature entirely. This is primarily since ECO has higher viscosity than transesterified ECO (EMR) because of higher molecular weight, more entanglements and larger intermolecular forces, as also reported previously for ESO and transesterified ESO.7 16 ACS Paragon Plus Environment

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Consequently, EPECO20 blend exhibited shear thinning revealing a non-Newtonian behavior and apparently had much higher viscosity than that of EPEMR20 at all shear rates. Significant reduction in viscosity and flow transition from non-Newtonian to completely Newtonian is noticed as 20 wt% EMR is incorporated into epoxy. This is attributed to short chain structure, less branching of EMR and reduced degree of entanglement of the blend.

(b) (a)

Figure 4. Rheological behavior of pure epoxy and bio-epoxy resin mixture represented through plot of (a) shear stress versus shear rate, and (b) viscosity versus shear rate

Thermal stability The thermal decomposition behavior of cured epoxy, EP/ECO and EP/EMR samples was investigated by TGA and DTG as displayed in Figure 5 (a) and (b). The effect of ECO and EMR (10-20 wt%) on the thermal stability parameters are depicted in Table 2. It is worthy to see that all samples show a similar multiple degradation throughout the temperature range considered for degradation. Cleavage of -C-C-, -C-Nbonds and breakdown of long aliphatic chains of PKA takes place in the initial stage of degradation.44 The second stage of mass loss is assigned to the decomposition of aromatic moiety of DGEBA epoxy as well as of PKA. In the final stage, degradation occurs up to 650 °C which is assigned to complete decomposition of char residues of un-reacted components.22 It is notable that T10, T20 and T50 values of EP remain almost unaffected on addition of ECO and EMR. Among all the samples, EPECO10 shows enhanced thermal 17 ACS Paragon Plus Environment


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stability having higher T5 and EPEMR20 exhibits lower stability but the difference is minor. Relatively inferior stability (lower T5 value) of EPEMR20

is owing to

degradation of methyl ester chain of EMR at lower temperature which was seen in EPEMS20 blend reported earlier by Sahoo et al.7 and for UPR/COPERMA20 by Liu et al.22 However, the obtained thermal parameters are acceptable for structural applications.

(b)

(a)

Figure 5. (a) TG and (b) DTG thermogram of modified castor oil based bio-epoxy blends

Table 2. Thermal degradation parameters of ECO and EMR based epoxy blends Sample

T5 (oC)

T10 (oC)

T20 (oC)

T50 (oC)

Tend set (oC)

EP

308

338

357

415

638

EPECO10

317

340

360

414

651

EPECO20

307

335

355

412

644

EPEMR10

308

339

358

413

634

EPEMR20

297

338

357

411

629

The DTG graph is provided for better understanding of the rate of degradation. All the blends show multiple peaks corresponding to different stages of degradation corresponding to degradation of different crosslinked segments of PKA and functional moieties.34,44 18 ACS Paragon Plus Environment

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Dynamic Mechanical Analysis In order to have more insight on toughening and additional effect of ECO and EMR observed, the thermomechanical properties of epoxy were measured by DMA. The obtained temperature dependent storage modulus (E′) and loss tangent (tan δ) is shown in Figure 6 (a) and (b). The estimated E′ values at different temperatures (before and after Tg) is presented in Table 3. In both glassy and rubbery region, E′ of the epoxy/ECO blends were found to have lower value compared to unmodified epoxy as shown in Table 3. This decrease in storage modulus value can be ascribed to the flexible long chain and polyol content present in ECO, which plasticize the epoxy matrix and reduced the rigidity of the material. On the contrary, EPEMR10 exhibit higher modulus compared to its ECO based counter parts and to unmodified epoxy at 30 ºC because of the stiffening effect imparted by epoxy methyl esters. A maximum increase of 27.1% increase in storage modulus was observed for EPEMR10 compared to neat epoxy. This stiffening effect of 10 wt% EMR is noticeable, particularly in glassy region and this effect diminishes as the temperature was raised under dynamic loading condition. Intermolecular forces between crosslinked ester molecules impart stiffening effect to the matrix which loses its significance at higher temperature resulting in decrease in moduli. Similar trend was noticed previously for UPR/COPERMA blend at 5-10% COPERMA loading.22 However, at 20% ECO and 20% EMR content, this modulus got deteriorated significantly due to undesired over plasticization and larger amount of soft segments.

Because of the

presence of relatively more rigid aromatic groups and highly crosslinked network, pure epoxy showed higher moduli in rubber plateau region compared to all types of bio-based blends.33 The replacement of highly reactive and rigid DGEBA with less reactive and flexible chained bio-resin chain decreased the modulus due to formation of poor crosslinked network.45 It is notable that PKA cured epoxy and its blends with ECO showed higher E′ than the blends when cured earlier with TETA and BPH counterparts.8,15 Thus, it is concluded that EMR can well replace ECO with improved properties when used as reactive diluent or secondary component in epoxy based system. Similarly, the bio-based crosslinker PKA can also replace petroleum based curing agents to more effectively crosslink bio-based epoxy system as the obtained thermomechanical properties are found to be better or similar under vibrating conditions. 19 ACS Paragon Plus Environment


(a)

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(b)

Figure 6. (a) Storage modulus and (b) loss factor of ECO and EMR based epoxy blends

It can be observed from the loss factor (tan δ) curve depicted in Figure 6(b) that all bio-epoxy blends show single relaxation at lower temperature than epoxy with increase in bio-resin content. The temperature corresponding to the peak of tan δ is considered as glass transition temperature (Tg) or α-transition point where Brownian motion of the main polymer chains and the relaxation of associated dipoles occurs.46 Crosslink density of the cured specimens was evaluated using equilibrium storage modulus applying theory of rubber elasticity (eq. (2)) and summarized in Table 3 where E' is the storage modulus at T = (Tg + 50) °C, R = gas constant and νe = crosslink density. E ΄ = 3ν e R T


(2)

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Table 3. DMA parameters of epoxy and bio-epoxy blends Name of the

E' at 30 ºC

E' at Tg+ 50 ºC

Crosslink Density

Tg determined

Sample

(MPa)

(MPa)

(νe) x 103 (mol/m3)

from tan δ curve (ºC)

EP

3062

61.67

5.89

101

EPECO10

2870

58.47

5.56

98

EPECO20

2982

52.50

5.10

89

EPEMR10

3894

65.71

6.24

99

EPEMR20

3143

56.22

5.59

80

The Tg values of epoxy/ECO and epoxy/EMR blends are found to be lower than that of unmodified epoxy, which gradually decreased with increase in bio-resin content as shown in Table 3. This is because of presence of flexible and soft segments which plasticizes the matrix. However, the reduction in Tg is not much high and the samples are still acceptable for specific application. Similar decreased trends have been reported for TETA and BPH cured ECO based epoxy blends earlier.8,15 Similarly, νe is decreased for all blends with exception at EPEMR10. The raised νe value of EPEMR10 may lead to a noticeable improvement on the mechanical and dynamic mechanical modulus. This enhancement is observed as the addition of 10 wt% of EMR stiffens the system by increasing the reactivity of the epoxy system and reducing the viscosity for better contact of resin with crosslinkers. In the last decade, addition of other bio-based additives like ESO, ELO, ECO, EMS and EML, etc. was reported to decrease the epoxy modulus. However, in current work, incorporation of EMR raised the modulus of epoxy as similar to the improvement seen earlier by COPERMA addition to UPR.22 However, incorporation of a higher amount of EMR over plasticizes the matrix and reduces its rigidity. The peak intensity and broadening of loss factor of the blends gradually increased with increase in ECO and EMR content. The peak shift towards lower temperature and broadening of the tan δ curve may be attributed to the reduced crosslinking density of the blends, molecular entanglements and variable relaxation times of molecules. EMR based blends display relatively more broadening because of its wide distribution of molecules and heterogenous chain structures. Similar trend has been noticed for epoxy/EMS blends reported earlier by Sahoo et al.7 and Area underneath the 21 ACS Paragon Plus Environment


tan δ curve represents the energy dissipated during viscoelastic relaxations22 which can be correlated with the increased impact strength of the blends mentioned earlier in mechanical properties section. This may be ascribed to the plasticizing effect caused by the long alkyl chains of ECO and EMR. The decreasing tendency in Tg at higher wt% of epoxidized oils and epoxy methyl esters have been reported earlier for both petro-based crosslinkers by Miyagawa et al.47, Kumar et al.48, Sudha et al.8, Park et al.15, Paluvai et al.14, Zhu et al.41, and Sahoo et al 7. Similar decreasing trend in Tg was also observed in DSC as shown in Figure S2 (supplementary file).

Morphological study of epoxy and blends

(a)

(b)

(c)

(d)


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(e)

(f) (f)

(g)

(h)

(i)

(j)

Figure 7. FE-SEM micrographs captured at 2K magnification for epoxy and its blends (a) EP (b) EPECO10 (c) EPECO20 (d) EPECO30 (e) EPEMR10 (f) EPEMR20 (g) 23 ACS Paragon Plus Environment


EPEMR30, and micrographs at 10 K magnification (h) EPECO20 (i) EPECO30 (j) EPEMR30

The micrographs of fractured surfaces of samples were taken at lower (2 K x) and higher (10 K x) magnifications and shown in Figure 7 (a-g) and Figure 7 (h-j) respectively. A relatively glossy and featureless surface with elastic deformation was seen in PKA cured epoxy matrix (Figure 7(a)) confirming reduced brittleness compared to TETA cured epoxy.49 Polymerization induced phase separation is an important mechanism to toughen the epoxy matrix. At 10 wt% of ECO (Figure 7 (b)), many line ridges are formed which resist deformation and crack propagation, responsible for excellent impact strength of DGEBA-ECO based blends network. While at higher loadings of ECO, highly viscous and relatively poorly miscible and highly entangled ECO could not crosslink completely due to steric hindrance and formed cavities. Hence, phase separation starts as is evident from Figure 7 (c) and (d). These small and uniformly dispersed cavities seen in EPECO blends contribute to toughening through shear yielding mechanism. The micro cavities produced during co-polymerization through crosslinking of epoxy and ECO resulted in molecular scale phase separation are very apparent in case of EPECO20 and EPECO30 blend at higher magnification (10 Kx) as shown in Figure 7 (h) and (i). Similar morphology of micro cavities were seen in amine cured bio-epoxy blends with higher content of epoxidized oil earlier.40 On the other hand, no cavities or phase separation were found on the fractured surfaces of EPEMR blends, (Figure 7 (e-g)) indicating that low viscous EMR forms a single phase and well crosslinked copolymerized structures due to its miscibility, low molecular weight and short chain structure. Microphotographs of EPEMR blends show tortuous cracks and many ridges with increase in EMR content, revealing effective toughening with a reinforced morphology. The roughness of fractured surface of the EPEMR blends increased when EMR loading increased from 10-30 wt%. Similar tortuous morphology with higher fracture roughness was also noticed in case of BPH cured DGEBA/ECO blends reported by Park et al.5,15 The presence of higher number of micro-ridges confirmed the increased roughness on surface and thus resulted in greater ability to absorb energy. The toughness decreased with increase in crosslink density 24 ACS Paragon Plus Environment

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whereas a rise in roughness is associated with the release of higher impact energy owing to shear yielding.22 When the roughness effect overcomes the crosslink density effect on toughness, the impact energy of the thermoset material gets raised.22 Consequently, the enhancement in toughness was observed when 10-30 wt% ECO and EMR were added to epoxy which is similar to the phenomena

noticed earlier in case of 10-20%

COPERMA/UPR blend.

Biodegradation potential Partially crosslinked or un-crosslinked epoxidized oils might decompose readily in compost soil when kept for several months as lipase secreted by bacteria or microorganisms can attack easily the ester linkages.50 As per the previous reports, Comamonas sp., Streptomyces sp., Pseudomonas sp., Bacillus sp., and Acinetobacter sp. are efficient soil microbes that can degrade the epoxidized oil based bio-thermosets in the compost soil environment.51 The samples were kept in vermicompost to observe the long term biodegradation test. The FE-SEM micrographs of EPEMR30 sample were taken after 90 days which are illustrated in Figure 8 (a) and (b) respectively. It is observed that sample starts degrading with time as low molecular weight methyl esters degrade easily. This reveals the biodegradation potential of the bio-epoxy blend which may be fully degraded in vermi-compost after several months.

0 day

90 days

Figure 8. FE-SEM micrographs of EPEMR20 after burial in vermi-compost after (a) 0 day (b) 90days



Page 26 of 32

CONCLUSION Non-edible castor oil was epoxidized and transesterified to form epoxy methyl ricinoleate which was employed as a reactive diluent for epoxy matrix and the properties compared with epoxy-ECO blends. EMR improved the processibility by reducing the viscosity as well as toughened the epoxy matrix effectively. Renewable and eco-friendly curing agent, cardanol based phenalkamine, supported an additional toughening of matrix along with moderate strength and modulus. The mechanical strength and moduli were found to be superior for EMR based epoxy blends. Higher impact strength and elongation at break of epoxy blends confirmed an effective toughness with an appropriate stiffnesstoughness balance. The thermal stability of epoxy was marginally affected by addition of ECO and EMR. A significant improvement in damping ability of toughened epoxy was observed due to incorporation of EMR as compared to ECO. FE-SEM micrographs revealed a single phase morphology for EPEMR blend samples and phase separated network for EPECO blend at higher content (>10 %) of bio-resin. Nanoscale cavities formed by ECO contributed to improvement in toughening through shear yielding whereas increased number of tortuous cracks and ridges act as toughening agents in EMR based blend. The biodegradation potential of the bio-epoxy blends was quite evident through morphological changes seen in FE-SEM graphs of a post 3 months vermicompost burial. Thus, low viscous EMR can be utilized as a potential green and sustainable diluent to epoxy which can replace commercial diluents applicable for its coating and structural application.

SUPPORTING INFORMATION GPC plot of EMR has been provided in Figure S1 for determination of molecular weight. The number average molecular weight (Mn) of EMR is found to be 482 Dalton. DSC thermogram of cured samples has been provided in Figure S2 for Tg determination and it is found that the Tg decreased with increase in bioresin loading.

ACKNOWLEDGEMENT First author is grateful to Science and Engineering Research Board (SERB), Govt of

India

for

the

National

Post-Doctoral

Fellowship


project

(File

number:

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PDF/2015/000705). Cardolite Corporation India Pvt Ltd is highly acknowledged for giving free sample of phenalkamine.

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Graphical Abstract 175x131mm (300 x 300 DPI)

ACS Paragon Plus Environment

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A renewable approach to synthesize highly toughened bio-epoxy from

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