Biobased Epoxy Matrix from Diglycidyl Ether of Bisphenol A and

Article pubs.acs.org/IECR

Biobased Epoxy Matrix from Diglycidyl Ether of Bisphenol A and Epoxidized Corn Oil, Cross-Linked with Diels−Alder Adduct of Levopimaric Acid with Acrylic Acid Fanica R. Mustata,* Nita Tudorachi, and Ioan Bicu “P. Poni” Institute of Macromolecular Chemistry, Aleea Grigore Ghica Voda, No. 41 A, Iasi-700487, Romania S Supporting Information *

ABSTRACT: Epoxidized corn oil (ECO) was synthesized and used as reactive diluent for diglycidyl ether of bisphenol A (DGEBA)/Diels−Alder adducts of rosin acids with acrylic acid (AcAAbA) mixtures. The influences of ECO amount on the curing kinetics and thermal properties of cured DGEBA/ECO/AcAAbA mixtures were investigated. From scanning electron microscopy images, a uniform distribution of ECO in the composite structure can be observed. The kinetic analysis of the curing reactions was evaluated, using the variable peak exotherm methods of Flynn−Wall−Ozawa, Kissinger, and the Netzsch Thermokinetics software. The cross-linking reactions were identified to take place in four steps. Kinetic analysis of the thermal degradation of the DGEBA/ECO/AcAAbA was evaluated with the Netzsch Thermokinetics software and occurs in three steps, depending on the chemical structure of the sample. On the basis of the analysis of evolved gases by thermal decomposition coupled with Fourier transform infrared and mass spectrometry techniques, a schematic mechanism for the thermal degradation process was proposed. fields. However, these cross-linked resins are brittle and have poor resistance to crack propagation, which limits their uses. To compensate for these defects, they are mixed with other materials, such as rubbers or plasticizers, as a second component. In the past decade, epoxidized derivatives obtained from vegetable oil (EDVO) have been successfully utilized in the preparation of thermoset materials or liquid molding resins, with the aim to substitute a part of the synthetic resins, change their toughness, improve the thermal and rheological properties, and reduce volatile emissions and the risk of environmental degradation.20−23 Using only pure EDVO for producing the polymer matrix is not recommended because the obtained cross-linked products have poor mechanical, thermal, and chemical properties as a consequence of their chemical structure.24 In the present paper, ECO was used as reactive diluent for diglycidyl ether of bisphenol A (DGEBA)/Diels−Alder adduct epoxy resin mixtures. The curing and thermal and morphological properties of these mixtures are investigated.

1. INTRODUCTION In the past decade, because of repeated oil crises and increases in the requirement and price generated by reduced global stocks as well as restrictions imposed by the environment and human health, the need for a new economic alternative to petrochemicals, by replacing them with others based on renewable natural resources, has grown. The most utilized renewable raw materials to manufacture the new chemicals and fuels (bioalcohols or biodiesel type) are carbohydrates and fats (vegetable or animal fats and waste oil used in cooking). The oils used for transformation into biodiesel are mainly soybean, rapeseed, sunflower, and palm oils, waste cooking oil, and animal fats (chicken, beef, or pork fats).1−5 The utilization of these compounds only as fuels does not add too much extra value. When they are used as raw materials for the chemical industry and consumer products for different applications such as polymers, inks, paints, plasticizers, and detergents, these oils are a good alternative to replace the petroleum-based products because are they inexpensive and are produced from renewable and environmentally friendly resources.6−11 Mainly, oils and fats are composed of glycerol esters with various fatty acids (saturated or unsaturated). Most of the fatty acids found in their structures are stearic, palmitic, oleic, linoleic, linolenic, eurucic, and ricinoleic. These triglycerides contain some chemical reactive groups (double bonds, ester, hydroxyl, and epoxy groups) that by different chemical reactions (epoxidation, acrylation, urethanation, maleinization, and hydroxylation) can be transformed into monomers used in the synthesis of new polymers (epoxy resins, polyesters, polyurethanes, polyamides, etc.).7,12−19 Because of their special properties (chemical solvents resistance, mechanical strength, thermal and electrical resistance, adhesion to different substrates), epoxy resins based on petroleum are one of the most important materials used in the domestic and industrial © 2013 American Chemical Society

2. MATERIALS AND METHODS 2.1. Materials. In the present work, corn oil (CO) (Bunge, Hungary), containing ester of glycerides with saturated acids (palmitic acid - 11 wt %, stearic acid - 2 wt %), unsaturated fats (oleic acid - 28 wt %), and polyunsaturated fats (α-linolenic acid - 1 wt %, and linoleic acid - 58 wt %), double bond number = 4.206, and an iodine number of 120 g of I2/100 g, was obtained from the local market. Rosin (acid number, a.n. = 171 mg KOH·g−1) and DGEBA (Sintofarm, Bucharest) (average Received: Revised: Accepted: Published: 17099

July 12, 2013 October 4, 2013 November 6, 2013 November 6, 2013 dx.doi.org/10.1021/ie402221n | Ind. Eng. Chem. Res. 2013, 52, 17099−17110

Industrial & Engineering Chemistry Research

Article

epoxy equivalent weight of 198 g·eq−1) were commercial products. Rosin acid (RA) (acid number, a.n. = 181 mg KOH·g−1) was obtained from commercial rosin by recrystallization from acetone. The Diels−Alder adduct of rosin acids with acrylic acid (AcAAbA) (a.n. = 299 mg KOH·g−1, mp = 221 °C) was obtained as described in our previous published articles.25,26 Acetic acid (AA), sulfuric acid (H2SO4), 30% aqueous hydrogen peroxide (H2O2), hydroquinone (HQ) (Fluka) acrylic acid (AcA), triethylbenzylammonium chloride (TEBAC) (Aldrich), anhydrous sodium sulfate, and all solvents were analytical grade products and used as received. 2.2. Methods. Acid number (a.n.) for rosin and AcAAbA was obtained by direct titration of the samples dissolved in acetone (5% w/v) with 0.1 N ethanolic KOH solution in the presence of phenolphthalein. The melting points (m.p.) were measured by means of Pyris Diamond DSC, Perkin-Elmer differential scanning calorimeter, at 10 °C·min−1 heating rate. The epoxy equivalent weights (EEW) were obtained using a method given in the literature (the pyridinium chloridepyridine method) and expressed in g·eq−1.27 The FT-IR spectra were measured with a Vertex 70 spectrophotometer (BrukerGermany) on KBr salt disks, and the frequencies were expressed in wavenumbers [cm−1]. FT-IR spectra are recorded in the 400−4000 cm−1 domain with a resolution of 4 cm−1. 1 H NMR spectra were obtained on an Avance DRX 400 (Bruker-Germany) apparatus at room temperature. The samples were dissolved in CDCl3 with tetramethylsilane (TMS) as internal standard (NMR chemical shifts were expressed in ppm). The double bonds of corn oil was calculated using 1H NMR spectroscopy by means of the equation from literature data, using as internal standard the signal of glycerol, located in the range of 4.1−4.3 ppm.28 Db(CO) = 0.5(A − Nf )/Nf

The next equations were used. The mathematical equations underlying these methods are ln(β /Tp2) = Ea /RTp − ln(AR /Ea) (Kissinger equation) (2)

and ln β = C − 0.4567(Ea /RTp) (Ozawa equation)

(3)

where A - pre-exponential factor, β - the linear heating rate, C - a constant, Ea - the activation energy of the curing reaction, Tp - temperature of the exothermic peak, R - the universal gas constant. The kinetic analysis of the cross-linking reactions was obtained using Netzsch Thermokinetics 3 software. For starters, the estimation of activation energy and pre-exponential factor was obtained, using the free-estimation models of Friedman and Ozawa−Flynn−Wall. These methods allow the calculation of the two parameters without defining a specific equation for the reaction model. If the apparent values of the activation energy changes with the conversion degree increase, they indicate that the crosslinking reactions are complex and take place in several stages. It can be assumed that the crosslinking process consists of several consecutive reactions. In the first stage, at relative low temperature, the carboxylic groups open the epoxy ring, thus resulting the ester linkage; the hydroxyl groups are obtained and the first prepolymer appears. Further, in the next steps the unreacted carboxyl groups react with another epoxy ring from prepolymers and with the newly formed hydroxyl groups, resulting a crosslinked material. Then, with the activation energy and pre-exponential factor values previously calculated, the multivariate nonlinear regression was employed to obtain the corresponding kinetic parameters by running of the experimental data for α situated between 0.20 and 0.80. The kinetic model with the successive reactions can be written as

(1)

where Db(CO) is the double bonds number, A is the area of the olefinic protons located in the range of 5.2−5.4 ppm, and Nf is the normalization factor, obtained by dividing the area of glycerol with 4. The scanning electron microscopy images were obtained with SEM/ESEM − EDAX − QUANTA 200 apparatus, with the following parameters: field emission filament 30 kV accelerating voltage, high vacuum mode and maximum magnification of 1 000 000×. All the samples for the acquisition of images were fractured at liquid nitrogen temperature and coated with fine gold layer. The kinetic parameters of the curing reactions of the DGEBA/ ECO mixtures cured with Diels−Alder adducts of resin acid were carried out on a 912 Du Pont instrument apparatus using nitrogen as gas purge (3.5 l·min−1), at different heating rates (5, 10, 15, 20 °C·min−1) in the 20−400 °C range. The instrument was calibrated using the melting point of high purity indium (onset temperature for the melting point is 156.66 °C). Samples (ECO/DGEBA/Diels−Alder adducts of resin acid/ TEBAC at different molar ratios), mixed vigorously and degassed in the oven at 40 °C, with weight of about 5−10 mg, were enclosed in standard aluminum crucibles and scanned on the temperature range in the presence of an empty aluminum crucible, as a standard. Kissinger and Ozawa multiple heating rate methods (using the exothermic peak temperatures) were utilized to calculate the kinetic parameters, such as apparent activation energy (E) and the pre-exponential factor (log A) of the curing reactions.29,30 These methods allow estimation of the kinetic parameters without using a predefined model for the chemical reactions.

A‐1 → B‐2 → C‐3 → D with t:f,f reaction code

where A are the initial reactants, B and C are the intermediate products, and D is the final crosslinked product, t:f,f represent the three-steps successive reaction schemes, and 1, 2, 3 represent the reaction steps. The following reaction models were used: (i) reaction order nth model Fn: f(α) = (1 − α)n where n is the reaction order; (ii) Avrami−Erofeev reaction model An: f(α) = n(1 − α)[−ln(1 − α)](n − 1/n) (iii) three-dimensions diffusion of Jander’s type D3: f(α) = 1.5(1 − α)1/3[(1 − α)−1/3 − 1]. The thermal stability studies of cross-linked samples were evaluated by the thermal gravimetric analysis (TGA) on a STA 449 F1 Jupiter apparatus (Netzsch-Germany) in the temperature range 30−600 °C with nitrogen as purge gas (at the heating rates of 5, 10, 15 °C·min−1), coupled with a Vertex 70 spectrophotometer and Aeölos QMS 403C mass spectrometer (Netzsch-Germany) for the evolved gases analyses. The samples, having a weight between 7 and 10 mg, were placed in Al2O3 crucibles and were thermally degraded at three heating rates. The gases released during the thermal decomposition were transferred from the thermobalance to the FT-IR spectrophotometer and mass spectrometer for identification. The gases were introduced into the TGA-IR external modulus, equipped with a liquid-nitrogen cooled mercury cadmium telluride (MCT) 17100

dx.doi.org/10.1021/ie402221n | Ind. Eng. Chem. Res. 2013, 52, 17099−17110


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Scheme 1. Chemical Reactions That Took Place in the Synthesis of ECO and the Possible Crosslinked Structure of DGEBA/ AcAAbA Epoxy Resin Modified with ECO

detector, and FT-IR spectra were recorded on 600−4000 cm−1 domain, with a resolution of 4 cm−1. The MS operating parameters were electron impact ionization energy of 70 eV, quartz capillary 75 μm diameter for transferring gas to mass spectrometer, heated at 290 °C, and vacuum 10−5 mbar. The data were scanned in the range m/z = 1−200, and measuring time was 0.5 s for one channel, resulting in time/cycle of approximately 100 s. The experimental data were processed with Netzsch Thermokinetics 3 software in order to determine the kinetic parameters of the thermal degradation.31 In the first instance, the model free methods of Ozawa−Flynn−Wall and Friedman were used to estimate the activation energy dependence of the degradation processes, versus the conversion degree in order to determine what type of degradation processes have taken place. All the thermal degradation

processes are considered irreversible and as successive reactions. The model of the thermal degradation mechanism can have the following form: A(solid) → B(solid) + C(gaseous)

Subsequently, with the activation energy and pre-exponential factor values previously calculated with model free methods, the multivariate nonlinear regression was employed to obtain the corresponding kinetic parameters, running the experimental data for α between 0.20 and 0.80. The next process mechanisms with two successive reactions were assigned to the above models: A‐1 → B‐2 → C of type d:f An, Fn A‐1 → B‐2 → C of type d:f Fn, Fn 17101



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where A is the initial product, B is the solid intermediate product, and C is the final solid residue, d:f represent the two-step successive reaction schemes, and 1, 2 represent the reaction steps. The following reaction models were used:

Sample 2. DGEBA/ECO/AcAAbA (0.9/0.1/1): 0.9 g DGEBA, 0.1 g of ECO, 0.9 g of AcAAbA, 0.02 g TEBAC Sample 3. DGEBA/ECO/AcAAbA (0.8/0.2/1): 0.8 g DGEBA, 0.2 g of ECO, 0.86 g of AcAAbA, 0.019 g TEBAC Sample 4. DGEBA/ECO/AcAAbA (0.5/0.5/1): 0.5 g DGEBA, 0.5 g of ECO, 0.73 g of AcAAbA, 0.017 g TEBAC Sample 5. ECO/AcAAbA (1/1): 1.03 g of ECO, 1 g AcAAbA, 0.02 g of TEBAC

(i) Avrami −Erofeev reaction model An: f(α) = n(1 − α)[−ln(1 − α)](n − 1/n) (ii) reaction order nth model Fn: f(α) = (1 − α)n where n is the reaction order. 2.3. Synthesis of ECO. The ECO was obtained by oxidation of CO using a peroxyacid, generated in situ from H2O2 and AA in the presence of catalytic amounts of H2SO4 (2% based of H2O2/AA mixture weight), at moderate temperature (65 °C). The molar ratio of oil double bonds/acetic acid/ hydrogen peroxide was 1/0.5/2. Cyclohexane was used as solvent with the aim to minimize the opening of the epoxy ring and facilitate the epoxidation reaction because peracetic acid is soluble both in cyclohexane and aqueous phases.32−34 The synthesis was performed in a 0.5 L four-neck roundbottom vessel, immersed in a thermostatted bath, equipped with a thermometer, condenser, and mechanical stirrer. 100 g of CO, 14.1 g of glacial acetic acid, 1 g of H2SO4, and 50 mL of cyclohexane were added in the vessel and mixed at room temperature (20 °C) for 30 min. Afterward, 107 g of H2O2 (30 wt %) was slowly added, maintaining the temperature of the reaction system under 20 °C for 1 h. Subsequently, the temperature of the mixture was slowly increased at 65 °C and maintained at this level for 5 h. Afterward, the reaction mixture was vigorously stirred, washed twice with distilled water, and separated by decantation. Subsequently, the reaction mass was washed with Na2CO3 aqueous solution (5 wt %) to neutralize the residual acidity, decanted, separated, washed again until neutral pH with distilled water, and dried on anhydrous sodium sulfate overnight to remove the traces of water. Then cyclohexane was removed by distillation, under a vacuum, and ECO was dried at high vacuum in an oven and 60 °C for 16 h (yield: 84 g, average EEW = 361 g·eq−1). 2.4. Sample Preparation for DSC and TGA Studies. To evaluate the effect of ECO content on the kinetic parameters of the DGEBA/AcAAbA systems, the next percentages of ECO were used for the curing study: 10, 20, 50 wt % based on DGEBA content. ECO and DGEBA in different ratios were mixed under vigorous stirring at room temperature and then mechanically stirred on the water bath at 60 °C, in order to obtain homogeneous mixtures. Then, the epoxy mixtures were mixed with AcAAbA as hardener (stoichiometric epoxy/ carboxylic proton ratio) in the presence of TEBAC as catalyst (1 wt % on the basis of epoxy monomer weight). The mixtures, which appeared as a white-matte paste, were placed for 30 min at 50 °C in a vacuum oven to release the air bubbles. After the passage of time, the samples were cooled to room temperature, and a part of them were used to record the DSC curves in nitrogen atmosphere, at four heating rates. The remaining samples were thermally cured in an oven with hot-air circulation, first at 100 °C for 1 h, then at 150 °C for 3 h, and finally at 180 °C for another 4 h. The cured products were cooled in liquid nitrogen, milled with an electric mill, and used for thermal studies.

of of of of of of of

3. RESULTS AND DISCUSSION The epoxidation of vegetable oils with different systems to obtain epoxy oils and methyl esters is already known.34−39 Above, we presented a different version of this synthesis with the aim to produce a high quality epoxy derivative, using cyclohexane as the reaction medium which prevents the epoxy ringopening. The chemical reactions involved in the chemical transformation of CO are presented in Scheme 1. 3.1. FT-IR and 1H NMR Characterization. In Figure S1 (from Supporting Information) are presented the Fourier transform infrared (FT-IR) spectra of ECO, AcAAbA, DGEBA, cross-linked ECO, cross-linked DGEBA, and cross-linked DGEBA/ECO (50/50 w/w). It can be observed that the main signals recorded in all spectra are located in the region 3100−2850 cm−1 being specific to CH, CH2, CH3 aliphatic groups, at 1744 cm−1 associated with the carbonyl stretching vibration υ(CO) of the ester group, in the range of 1110− 1245 cm−1 assigned to the C−O−C ester group and at 722 cm−1 specific to the CH2 aliphatic group. On the other hand, in the ECO spectrum at 3468 cm−1 a weak absorption band appears specific to υOH stretching vibrations (indicating that the epoxy ring has been partially opened) and the peak located at 3008 cm−1 specific to υCC stretching vibrations disappears, demonstrating the epoxidation reaction achievement.

Figure 1. DSC curves recorded at 10 °C·min−1 for (a) DGEBA/ AcAAbA (molar ratio 1/1), (b) DGEBA/ECO/AcAAbA (molar ratio 0.9/0.1/1), (c) DGEBA/ECO/AcAAbA (molar ratio 0.8/0.2/1), (d) DGEBA/ECO/AcAAbA (molar ratio 0.5/0.5/1), (e) ECO/AcAAbA (molar ratio 1/1) (the symbols represent the experimental values, and the straight lines represent the calculated values).

Sample 1. DGEBA/AcAAbA (1/1): 1.06 g of DGEBA, 1.0 g of AcAAbA, and 0.02 g of TEBAC 17102



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Table 1. Kinetic Parameters of DGEBA/ECO/AcAAbA Systems from DSC Scans (Stoichiometric Epoxy/Carboxylic Proton Ratio) Calculated from First Exotherm heating rate (°C·min−1) 5

10

15

20

activation energy of curingb(kJ·mol−1)

resin system (molar ratio)

curing agent

TMa

TMa

TMa

TMa

EOzawa

EKissinger

frequency factorc ln A (s−1)

DGEBA DGEBA/ECO (0.9/0.1) DGEBA/ECO (0.8/0.2) DGEBA/ECO (0.5/0.5) ECO

AcAAbA AcAAbA AcAAbA AcAAbA AcAAbA

128.7 132 130.4 142.7 221.5

138.6 144.6 144.6 149.9 227

146 152 153.7 156.2 237

153 156 160.9 158.5 247

81.6 81.8 106.9 125.6 139.4

74.4 74.8 103.1 118.5 131.4

9.8 9.6 18.6 21.3 18.6

TM maximum peak temperature °C. bEOzawa, EKissinger activation energies of curing reaction calculated with eqs 1 and 2. cCalculated with Kissinger equation.

a

maximum peak temperature specific to each sample and with the heating rate increases, the maximum temperature shifts to higher values. By processing the data from Table 1 with eqs 2 and 3, specific to the multiple heating rate method, the activation energy and frequency factor of the curing reactions were obtained. From Table 1 it can be observed that DGEBA crosslinked with AcAAbA has lower values for Ea in comparison with ECO cross-linked with AcAAbA, a fact that can be attributed to the large sizes of the ECO molecules which present steric hindrance. The activation energy of the curing process for DGEBA/ECO mixtures has different values that vary between DGEBA/AcAAbA energy values and ECO/AcAAbA values and is situated in the range 81.6−139.4 kJ·mol−1, depending on the chemical structure of the mixtures. These differences can be explained in terms of the chemical reactivity (DGEBA is more reactive in comparison with ECO). The obtained values of the activation energies of curing are comparable with the data reported in the literature for epoxy resins/EDVO, cured with p-aminobenzoic acid.40 Because the DGEBA/ECO/AcAAbA mixtures have a complex nature, the nonlinear regression method was used to obtain the kinetic parameters for the curing process. The variation of the cross-linking activation energy calculated with the Friedman method (Figure 2) for α

The presence of the epoxy group can also be evidenced by the signals recorded between 820 and 841 cm−1. The chemical structure of ECO and AcAAbA is also confirmed by 1H NMR spectra (1H NMR spectra were not shown for reasons of brevity) (Figures S2 and S3 from Supporting Information). The terminal CH3 groups from fatty acids moieties are located in the range of 0.86−0.91 ppm, and the signals of glycerin protons are located in the range of 4.1−4.3 ppm. The new signals specific to epoxy ring are recorded in the range 2.85−2.97 ppm. The weak signals located at 3.05− 3.15 ppm may be assigned to the OH groups, confirming that some epoxy rings have been partially opened. The signal located in the range of 5.2−5.4 ppm, specific to protons of the double bonds, disappear in the ECO spectrum, due to the epoxidation reaction. The synthesis and chemical structure of Diels−Alder adduct, used as a hardener in this work, were described in our previous papers.25,26 In Figure S1 (from Supporting Information) is shown the FT-IR spectrum of AcAAbA. The peak located in the range of 2855−3010 cm−1 can be assigned to CH, CH2, CH3 and CHCH groups from the hydrophenanthrene moieties. The peak recorded near 1695 cm−1 can be assigned to the carbonyl groups (υCO). The peak specific to isopropyl group bonded to the hydrophenathrene nucleus is observed near 1465 cm−1. In 1H NMR spectrum (not given for reason of brevity) of AcAAbA (Figure S3 from Supporting Information), the signals assigned to CH, CH2, and CH3 groups are located in the range of 0.6−0.9 ppm; the signals specific to isopropyl group bonded to the hydrophenanthrene moieties appear between 0.9 to 1.09 ppm, CH3 protons at 1.45 ppm, aromatic CHCH between 5.42 and 5.6 ppm, and COOH proton at 11.44 ppm. 3.2. Kinetic Analysis. As a result of the reaction between carboxyl and epoxy groups, as well as the formation of the cross-linked products, the FT-IR (in Figure S1 from Supporting Information) absorption signal specific to carboxyl groups located at 1695 cm−1 and the absorption signal specific to epoxy ring located at 914 cm−1 disappeared completely, and the new band, specific to hydroxyl groups, appeared at 3468 cm−1. Also, the new signal recorded at 1722 cm−1 (specific to the CO group in ester moieties) and between 1080 and 1244 cm −1 (specific to ester type bounds −C−O−C−) appeared. However, the absorption signals at 824 cm−1 and 841 cm−1 specific to the epoxy ring in ECO are still present in the cross-linked DGEBA/ECO mixtures and in cross-linked ECO and indicate that the cross-linking reaction is not complete. From Figure 1 and Table 1, it can be seen that the crosslinking reaction is exothermic one. The DSC curves present a

Figure 2. Dependence of the activation energy on the conversion degree for (□) ECO/AcAAbA (molar ratio 1/1), (●) DGEBA/ AcAAbA (molar ratio 1/1), (■) DGEBA/ECO/AcAAbA (molar ratio 0.9/0.1/1), (▲) DGEBA/ECO/AcAAbA (molar ratio 0.8/0.2/1), (○) DGEBA/ECO/AcAAbA (molar ratio 0.5/0.5/1) cured at stoichiometric epoxy/carboxyl proton ratio, calculated with the Friedman method. 17103



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Table 2. Kinetic Parameters and Statistics, Determined after Nonlinear Regression for the Most Probable Mechanism of Curing Process, by Applying a Three-Step Kinetic Model with Consecutive Reactions on the Temperature Interval between 80 and 220 °C (Stoichiometric Epoxy/Carboxylic Proton Ratio) sample

parametersa E1 (kJ·mol−1) log A1 (s−1) react ord n1 E2 (kJ·mol−1) log A2 (s−1) dimension d2 react ord n2 E3 (kJ·mol−1) log A3 (s−1) dimension d3 react ord n3 FollReact 1 FollReact 2 coreln coeff t-critical (0.95)

DGEBA

DGEBA/ECO (0.9/0.1 molar ratio)



ECO

mechanism scheme A-1 → B-2 → C-3 → D





kinetic model type t:f:f; Fn, An, An

kinetic model type t:f:f; Fn, An, An

kinetic model type t,f:f; Fn, Fn, Fn

kinetic model type t:f,f; D3, Fn, An

kinetic model type t:f,f; D3, Fn, Fn

110.2 12.1 1.75 72.3 4.7 1.0

116.4 13.2 1.23 50.2 6.75 0.3

90.1 8.1 2.02

89.4 8.9 0.8

0.817 0.004 0.999583 1.958

0.004 0.265 0.99868 1.957

110.7 12.7 2.3 149.5 19.5 1.55 50.2 4.7 1.9 0.07 0.31 0.999492 1.956

104.3 8.1

86.3 6.9

85.9 8.1

71.6 6.9

0.32 187 23.4 0.97

0.6 105.3 8.85

0.01 0.1 0.99845 1.955

0.90 0.1 0.28 0.99587 1.955

a

E1, E2, E3 - activation energy of cross-linking for each step, log(A1, A2, A3) - pre-exponential factor for each step. n1, n2, n3 - reaction order, d2, d3 dimension coefficients acording with Avrami−Erofeev model, FollReact 1, FollReact 2 - share of reaction step 1 (A → B), step 2 (B → C), step 3 (C → D) in the total mass loss.

Figure 3. TG and DTG curves recorded at 10 °C·min−1 for (★) DGEBA/AcAAbA (molar ratio 1/1); (▼) DGEBA/ECO/AcAAbA (molar ratio 0.9/0.1/1); (○) DGEBA/ECO/AcAAbA (molar ratio 0.8/0.2/1); (∇) DGEBA/ECO/AcAAbA (molar ratio 0.5/0.5/1); (▲) ECO/AcAAbA (molar ratio 1/1) (the symbols represent the experimental values and the straight lines represent the calculated values).

Figure 4. Evolution of the apparent activation energy of thermal degradation with the conversion degree for (□) ECO/AcAAbA (molar ratio 1/1); (●) DGEBA/AcAAbA (molar ratio 1/1); (■) DGEBA/ ECO/AcAAbA (molar ratio 0.9/0.1/1); (▲) DGEBA/ECO/AcAAbA (molar ratio 0.8/0.2/1); (○) DGEBA/ECO/AcAAbA (molar ratio 0.5/0.5/1), calculated with the Friedman method.

situated between 0.20 and 0.80 shows multiple peaks or inconstant values. This fact is a serious indicator of the presence of a multiple-step process. The Thermokinetic 3 software was used to obtain the kinetic parameters using the nonlinear regression method with a sixth degree Runge−Kutta process in a modified Marquardt procedure.31 This method allows the overlapping of the proposed model onto the experimental data using the initial values of the kinetic parameters obtained with the Friedman method. For each mixture, the conversion

functions and the optimal models were chosen (based on F-test), considering the discrepancy between the experimental DSC curves, the calculated data, and the correlation coefficients values. The best results were obtained by running of the experimental data using the three-step reaction models with consecutive reactions of t:f,f type. The calculated results are in good agreement with the experimental results and indicate that the chosen models describe with sufficient correctness the crosslinked reactions (Figure 1). 17104


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a E1, E2 - activation energy of cross-linking for each step, log(A1, A2) - pre-exponential factor for each step, n1 - reaction order according with Fn, d1 - dimension coefficient acording with Avrami−Erofeev model, FollReact 1-share of reaction step 1 (A → B), step 2 (B → C) in the total mass loss.

254.7 17.6 0.28 75.5 3.5 0.27 0.12 0.999860 1.955 171.1 10.9 0.49 97.9 12.1 0.92 0.402 0.999428 1.955 E1 (kJ·mol ) log A1 (s−1) dimension n1 E2 (kJ·mol−1) log A2 (s−1) react ord n1 FollReact 1 coreln coeff t-critical (0.95)

90.6 4.9 0.12 298.9 21.1 2.98 0.223 0.999363 1.955

206.1 13.6 0.71 236.1 14.4 2.98 0.922 0.999596 1.955

238.4 16.6 0.46 216.8 15.1 2.98 0.331 0.999798 1.955

kinetic model type d:f; An, Fn kinetic model type d:f; An, Fn kinetic model type d:f; An, Fn kinetic model type d:f; Fn, Fn parametersa

−1

kinetic model type d:f; An, Fn

ECO

mechanism scheme A-1 → B-2 → C







DGEBA


sample

Table 3. Kinetic Parameters and Statistics, Determined after Nonlinear Regression for the Most Probable Mechanism of Thermal Degradation Process, by Applying a Two-Step Kinetic Model with Consecutive Reactions on the Temperature Interval between 30 and 600 °C


Figure 5. Fit of the measurements through two-step method d with the conversion functions d,f; An, Fn for DGEBA/ECO/AcAAbA (molar ratio 0.5/0.5/1) sample at (☆) 10 °C·min−1, (Δ) 7.5 °C· min−1, and (•) 5 °C·min−1 (the symbols represent the experimental values and the straight lines represent the calculated values).

In the presence of the catalyst and the hardener, with a temperature increase, DGEBA, DGEBA/ECO mixtures, and ECO are converted in linear and cross-linked products. Figure 1 presents the typical DSC curves (with 10 °C·min−1) for the curing reactions of pure reagents and their mixtures in the presence of AcAAbA as hardener. DSC curves for pure DGEBA at all heating rates show only a maximum on the entire domain of temperature, while pure ECO shows two peaks because it contains more than two epoxy groups (it is possible to obtain a complex three-dimensional structure from the early stages of the reaction). The mechanism of curing reaction of DGEBA, DGEBA/ECO mixtures, and ECO with AcAAbA can be considered as a sequence of consecutive reactions. At the beginning, for DGEBA or ECO in the presence of TEBAC, the epoxy ring reacts with the carboxyl protons of the hardener and the OH groups appear. In this stage, the prepolymers that contain both hydroxyl and carboxyl groups appear. Over time, at high temperature complex reactions can take place between epoxy ring and hydroxyl or unreacted carboxyl groups, resulting in the cross-linked networks. Also, and in the case of DGEBA/ ECO mixtures, the curing reaction takes place by a consecutive reaction mechanism. For these mixtures, at low heating rates, the cross-linking reactions appear simultaneously for both monomers, resulting in heat curves with one exothermic peak. With a heating rate increase and at higher concentrations of ECO, DSC curves show two exothermic peaks. The fact that at higher reaction rates the DSC curves for the different mixtures present two peaks relatively distant from each other suggests that the curing reactions occur in the first phase between the most reactive products and continuing with the least reactive. This type of behavior is confirmed by the literature data for some mixtures of epoxy soybean oils and DGEBA epoxy resin.40 At low heating rates, DGEBA and DGEBA/ECO mixtures show a single exotherm peak situated under 180 °C suggestsing that the linear polymerization and the crosslinking reactions occurred at the same time. The peak values (for DGEBA or DGEBA/ECO mixtures) less than 160 °C suggest that the reactions between hydroxyl and epoxy groups do not take place and arise only between the carboxyl groups and 17105



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Figure 6. Stacked plot diagrams, FT-IR and MS spectra of pyrolysis products at the maximum temperature of evolved gases (430 °C) recorded with 10 °C·min−1 for DGEBA/AcAAbA (molar ratio 1/1), DGEBA/ECO/AcAAbA (molar ratio 0.5/0.5/1), and ECO/AcAAbA (molar ratio 1/1).

most important stage of thermal decomposition, that is very fast, is located in the temperature range of 330−480 °C for the sample with little or no ECO, and in the temperature range of 300−480 °C for the sample with more ECO or with 100% ECO. By means of the multivariate nonlinear regression method (Thermokinetics 3), the reaction models presented below were fitted to the experimental data (the thermal degradation data of cured samples were obtained on the temperature interval 30−600 °C) and Ea, log A, and reaction order parameters were calculated. The kinetic and statistical parameters obtained from the above presented mechanism selected on the basis of the minimum sum of least-squares, the maximum of correlation coefficient, and maximum critical F-value (F crit-0.95) are shown in Table 3. As it can seen in the Table 3, for each sample are the twostep reaction models of type: A-1 → B-2 → C with consecutive

epoxy ones. The complex cross-linked products appear only for DGEBA/ECO mixtures as a consequence of the chemical structure of ECO.41 The possible cross-linked structures of DGEBA epoxy resin and DGEBA/ECO mixtures are presented in Scheme 1. 3.3. Thermal Analysis. The thermogravimetric curves (TG and DTG), recorded under nitrogen flow, at 10 °C·min−1 heating rate for DGEBA/AcAAbA, ECO/AcAAbA and DGEBA/AcAAbA/ECO cross-linked resins, and the thermal parameters are shown in Figure 3, and Table S1 (from Supporting 1nformation) presents the thermal parameters. The curves measured for other rates are relatively similar for all resins and are not present for reasons of brevity. On the basis of the TG-DTG curves and of Ea versus conversion degree (Figure 4), it can be considered that the degradation process presents a complex reaction path with a multiple-step process.31,42 The 17106



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Figure 7. The variation of the FT-IR absorbance with temperature for the main gases obtained at the thermal degradation of (•) DGEBA/AcAAbA (molar ratio 1/1), (○) DGEBA/ECO/AcAAbA (molar ratio 0.5/0.5/1), and (▼) ECO/AcAAbA (molar ratio 1/1).

reactions that lead to a higher fit quality of the experimental data (correlation coefficient situated between 0.999363 and 0.999860). The calculated curves based on the parameters presented in Table 3 and showed in Figure 5, approximate very well the experimental data (the other curves were not shown from reasons of brevity). These results imply that the chosen formal kinetic models describe the system with accuracy. However, these models have a formal character and do not give precise information on the chemical reactions that occur in reality. The real thermal degradation processes are very complex. In order to propose a possible degradation mechanism, the evolved gases were identified using FT-IR and MS analysis. In Figure 6 are presented the stacked plot diagrams, FT-IR and MS spectra of the pyrolysis products recorded at maximum volume of the evolved gases (430 °C) for 100 wt % DGEBA, DGEBA/ECO (50/50), and 100 wt % ECO samples. As it can be seen from the figure, the most important signals can be assigned to water (3550−3750 cm−1), various aliphatic hydrocarbons (around 2850−3010 cm−1 and 1410−1530 cm−1), carbon dioxide (between 2300−2400 cm−1), acids, esters, or anhydride derivatives (in the range of 1690−1820 cm−1), aromatic hydrocarbons (1605 cm−1), esters or phenols (1100− 1250 cm−1), and para-substituted benzene (in the range of 700−850 cm−1). The components of the evolved gases were identified on the basis of FT-IR spectra, available in the spectral libraries of the NIST.43 On the other hand, the presence of the aromatic hydrocarbons in these samples can be explained on the basis of dehydrogenation and condensation reactions of the hydrophenanthrene moieties that constitute the hardener. The chemical composition of the gases was also evaluated using MS analysis. In Figure 6 are shown the mass spectra obtained at 10 °C·min−1 heating rate, for the evolved gases at thermal degradation of DGEBA/ECO/AcAAbA where total ion current of the fragment ions versus m/z are presented. The significance of the main signals was attributed using the spectral libraries of

the NIST in correlation with FT-IR spectra recorded during the thermal decomposition of samples.43 As can be seen from Figure 6 all major signals m/z detected at the maximum peak of the degradation process are water (m/z = 18), CO2 (m/z = 44), alkane derivatives (ethane m/z = 30, propane m/z = 44, butane m/z = 58) and aromatic products (benzene m/z = 78, toluene m/z = 92, phenol m/z = 94, isopropyl benzene, benzoic acid m/z = 120, 122, ethyl phenol m/z = 122). The water, CO2, and alkane derivatives (ethane, propane, butane) come mainly from the thermal decomposition of ECO and the hardener, and from the thermal decomposition of DGEBA. The aromatic products (benzene, toluene, phenol, isopropyl benzene, benzoic acid, ethyl phenol) result from the degradation of DGEBA and by the thermal dehydrogenation and condensation of AcAAbA moieties. The ratio between gaseous components differ (as seen from figure) and depend on the initial composition of the sample; those that do not contain DGEBA have a very small amount of aromatic compounds. From the FT-IR stacked plot diagrams (Figure 6) were extracted the absorbance at the main wavenumber specific to evolved gases. The absorbance variation with temperature increase is shown in Figure 7. From this figure, it can be observed that the amount of aliphatic hydrocarbons is higher for the sample that consists only of ECO and can be attributed to its chemical composition. On the basis of the results presented above, a mechanism can be suggested for thermal decomposition of the cured products. The cured products contain aliphatic and aromatic chains linked together by the ester groups. On the main chains a large number of OH groups appeared as a consequence of epoxy rings opening. The degradation of cured products is a multistage process that can begin with the breaking of ester groups with removal of water, carbon oxide, carbon dioxide, and then followed by the macromolecular chains breaking. At temperatures up to 330 °C, the macromolecular chains begin to break and the main degradation components can be glycerol derivatives, acids, aliphatic and aromatic hydrocarbons, phenol derivatives, 17107



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Scheme 2. Probable Mechanism of the Thermal Degradation Process for DGEBA/ECO/AcAAbA Crosslinked Sample

3.4. Morphological Study of Cured Composites. In Figure 8 are presented the SEM micrographs that show the fractures of pure DGEBA and DGEBA/ECO samples crosslinked with AcAAbA. The fact that the fractured surface of pure DGEBA is smooth and presents a great number of discontinued regular cracks, distributed on the entire surface, suggests that the fracturing is of elastic type, indicating a brittle material, specific to the epoxy resins with poor toughness. The fractured surfaces of the DGEBA/ECO mixtures have a flat form and are furrowed by smooth edges with tortuous cracks, which become rare when the ECO percentage increases. This result suggests resistance to crack propagation and a good impact strength. With ECO concentration increase is observed a decrease of the fractures number suggesting a better distribution in the entire matrix, showing a good miscibility of the components. The presence of ECO into the DGEBA network also leads to an increase of dissipation of the internal forces throughout the network, as a consequence of the cross-linking density decrease.

4. CONCLUSIONS Synthesis of epoxidized corn oil and the influences of different amount of ECO on the curing kinetics and thermal properties of DGEBA epoxy resin cured with AcAAbA were investigated. Synthesis of epoxidized corn oil and the influences of different amount of ECO on the curing kinetics and thermal properties of DGEBA epoxy resin cured with AcAAbA were investigated. Introduction of ECO in the polymer matrix reduces the cost and increases the environmental compatibility, due to its natural origin. The thermal stability of the cross-linked samples decreased with ECO content increases due to its aliphatic structure, reduction of the cross-linking density, and the plasticizing

Figure 8. SEM micrographs: DGEBA/AcAAbA (molar ratio 1/1); DGEBA/ECO/AcAAbA (molar ratio 0.9/0.1/1); DGEBA/ECO/ AcAAbA (molar ratio 0.8/0.2/1); and DGEBA/ECO/AcAAbA (molar ratio 0.5/0.5/1) at a magnification of 2000.

and others. When the temperature increases to 450 °C, the main components are of aromatic type, and at 600 °C a carbonic residue appears. Using the FT-IR and MS data, the following degradation mechanism can be proposed (Scheme 2). 17108



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(16) Sharmin, E.; Ashraf, S. M.; Ahmad, S. Epoxidation, hydroxylation, acrylation and urethanation of Linum usitatissimum seed oil and its derivatives. Eur. J. Lipid Sci. Technol. 2007, 109, 134. (17) Metzger, J. O. Fats and oils as renewable feedstock for chemistry. Eur. J. Lipid Sci. Technol. 2009, 111, 865. (18) Rupilius, W.; Ahmad, S. Palm oil and palm kernel oil as raw materials for basic oleochemicals and biodiesel. Eur. J. Lipid Sci. Technol. 2007, 109, 433. (19) Liu, Z. S.; Erhan, S. Z.; Xu, J.; Calvert, P. D. Solid freeform fabrication of epoxidized soybean oil/epoxy composites with di-triand polyethylene amine curing agents. J. Appl. Polym. Sci. 2004, 93, 356. (20) Miyagawa, H.; Mohanty, A. K.; Misra, M.; Drzal, L. T. Thermophysical and impact properties of epoxy containing epoxidized linseed oil, 1 Anhydride-cured epoxy. Macromol. Mater. Eng. 2004, 289, 629. (21) Mustata, F.; Bicu, I.; Cascaval, C. N. Rheological and thermal behaviour of an epoxy resin modified with reactive diluents. J. Polym. Eng. 1997, 17, 491. (22) Gupta, A. P.; Ahmad, S.; Dev, A. Modification of novel biobased resin-epoxidized soybean oil by conventional epoxy resin. Polym. Eng. Sci. 2011, 51, 1087. (23) Czub, P. Characterization of an epoxy resin modified with natural oil-based reactive diluents. Macromol. Symp. 2006, 245−246, 533. (24) Chen, J.; Soucek, M. D.; Simonsick, W. J.; Celikay, R. C. Synthesis and photopolymerization of norbornyl epoxidized linseed oil. Polymer 2002, 43, 5379. (25) Bicu, I.; Mustata, F. Polymers from a levopimaric acid−acrylic acid Diels−Alder adduct: Synthesis and characterization. J. Polym. Sci.: Part A: Polym. Chem. 2007, 45, 5979. (26) Mustata, F.; Bicu, I. Polyhydroxyetheresters from resin acids. Polimery (Warsaw, Pol.) 2005, 50, 10. (27) Cascaval, C. N.; Mustata, F.; Rosu, D. Viscosity characteristics of some para-nonylphenol formaldehyde novolac epoxy resins. Angew. Makromol. Chem. 1993, 209, 157. (28) Nicolau, A.; Samios, D.; Piatnick, C. M. S.; Reiznautt, Q. B.; Martini, D. D.; Chagas, A. L. On the polymerisation of the epoxidised biodiesel: The importance of the epoxy rings position, the process and the products. Eur. Polym. J. 2012, 48, 1266. (29) Kissinger, H. F. Reaction kinetics in different thermal analysis. Anal. Chem. 1957, 29, 1702. (30) Ozawa, T. A modified method for kinetic analysis of thermoanalytical data. J. Therm. Anal. 1976, 9, 369. (31) Netzsch Thermokinetics-3, version 2008.05. (32) Wanc, S.-M.; Tsianc, R. C.-C. Epoxidation of partially hydrogenated styrene-butadiene block copolymers using peracetic acid in a cyclohexane/water heterogeneous system. J. Polym. Sci.: Part A: Polym. Chem. 1996, 34, 1483. (33) Gan, L. H.; Ooi, K. S.; Gan, L. M.; Goh, S. H. Effects of epoxidation on the thermal oxidative stabilities of faty acid esters derived from palm olein. J. Am. Oil Chem. Soc. 1995, 72, 439. (34) Sander, M. M.; Nicolau, A.; Guzatto, R.; Samios, D. Plasticizer effect of oleic acid polyester on polyethylene and polypropylene. Polym. Test. 2012, 31, 1077 ies. (35) Doll, K. M.; Bantchev, Gr. B.; Murray, R. E. Bismuth (III) trifluoromethanesulfonate catalyzed ring-opening reaction of mono epoxy oleochemicals to form keto and diketo derivatives. ACS Sustainable Chem. Eng. 2013, 1, 39. (36) Petrovic, Z. S.; Zlatanic, A.; Lava, C. C.; Sinadinovic-Fiser, S. Epoxidation of soybean oil in toluene with peroxoacetic and peroxoformic acids−kinetics and side reactions. Eur. J. Lipid Sci. Technol. 2002, 104, 293. (37) Sharmin, E.; Ashraf, S. M.; Ahmad, S. Epoxidation, hydroxylation, acrylation and urethanation of Linum usitatissimum seed oil and its derivatives. Eur. J. Lipid Sci. Technol. 2007, 109, 134. (38) Espinoza-Perez, J. D.; Nerenz, B. A.; Haagenson, D. M.; Chen, Z.; Ulven, C. A.; Wiesenborn, D. P. Comparison of curing agents for epoxidized vegetable oils applied to composites. Polym. Compos. 2011, 32, 1806.

effect introduced by its large dimension. The thermal degradation mechanism of the DGEBA/ECO mixtures has been proposed on the basis of evolved gases analysis, by FT-IR and MS techniques. From the SEM images, a uniform distribution of ECO in the composite structure can be observed.

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ASSOCIATED CONTENT

S Supporting Information *

In Figure S1 are presented the FT-IR spectra of ECO, AcAAbA, DGEBA, cross-linked ECO, cross-linked DGEBA, and crosslinked DGEBA/ECO (50/50 w/w). In Figures S2 and S3 are presented 1H NMR spectra of ECO and AcAAbA. Table S1 presents the thermal parameters of the cross-linked samples. This information is available free of charge via the Internet at http://pubs.acs.org/.

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AUTHOR INFORMATION

Corresponding Author

*Fax: +40-232-211299. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

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