Epoxy Resins Cross-Linked with Rosin Adduct Derivatives. Cross

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Ind. Eng. Chem. Res. 2010, 49, 12414–12422

Epoxy Resins Cross-Linked with Rosin Adduct Derivatives. Cross-Linking and Thermal Behaviors Fanica R. Mustata* and Nita Tudorachi “P. Poni” Institute of Macromolecular Chemistry, Aleea Grigore Ghica Voda, No. 41 A, Iasi 700487, Romania

Rosin adduct derivatives were obtained by chemical reaction of the Diels-Alder adduct of rosin acid with maleic anhydride (RAMA) with aspartic acid (ASP) and p-aminobenzoic acid (p-ABA), at 1/1 molar ratio. The structure of the obtained monomers was established by means of elemental analysis, FT-IR spectroscopy, and 1H NMR. These acids were used as cross-linking agents, in the presence of triethylbenzylammonium chloride as catalyst (TEBAC) for diglycidyl monomers (diglycidyl ether of bisphenol A, DGEBA, and diglycidyl ether of hydroquinone, DGEHQ). Using differential scanning calorimetry (DSC) at different heating rates and the literature methods, the kinetic parameters of cross-linking reactions were obtained. The thermal stability of the cross-linked polymers investigated by using thermogravimetric analysis (TGA) showed they are reasonably thermostable. 1. Introduction Most of the synthetic polymers that are currently produced are based on petroleum-derived raw materials. As a consequence of repeated oil crises (raw material with relatively high price and relatively small production) in recent years, much more attention has been directed to obtaining raw materials from renewable and inexpensive natural resources, such as cellulose, starch, vegetable and animal oils, proteins, latex rubber, carbohydrates, turpentine, and rosin. These materials (with low price, easy availability, and possible biodegradability) have used environmentally friendly chemistry in the synthesis of green chemistry type products. Rosin, a solid resinous material, was obtained naturally from the fractional distillation of oleoresin secreted by pine trees or as a byproduct from paper pulp production (tall oil). It is wellknown that rosin is a complicated mixture of organic acids (90% abietic, neoabietic, palustric, levopimaric, etc.) and neutral materials (10%, in special turpentine).1,2 Rosin acids have in their basic molecular structure one carbonyl group, eight CH2 groups, and two double bonds, groups which can react with different chemical substances and can be transformed in di- or multifunctional monomers, which can be subsequently used in the synthesis of new polymers. These polymers are used in the formulation of paper sizing agents, varnishes, adhesives, alkyd resins, odorants, binders for printing inks, etc. Levopimaric acid, as a diene product, can readily react with dienophile substances (maleic anhydride, acrylonitrile, acrylic acid, etc.) by Diels-Alder reaction, and new monomers were obtained, extensively used as raw materials in the synthesis of polymers. The large majority of the Diels-Alder adducts of rosin acids proved to be suitable for polycondensation reactions. The syntheses of polyamides, poly(esterimide)s, poly(amideimide)s, poly(esteramide)s, and saturated and unsaturated polyesters by condensation of diacid Diels-Alder adducts with proper diamines or diols have already been reported.3-19 In recent years, there have been some reports regarding the use of rosin acid derivatives as epoxy resins or as effective epoxy curing agents.3,15,20-26 The thermosetting resins as epoxy resins are the most important group of polymers, which after cure display excellent mechanical strength, good thermal and chemi* To whom correspondence should be addressed. Fax: +40-232211299. E-mail: [email protected].

cal resistance, good bonding to substrates, and good electrical resistance, and are used in numerous industrial fields such as fiber composites, metal coating compositions, friction molding materials, insulation materials for electric and electronic devices, etc. These cross-linked products have good bonding properties but are not environmentally friendly because they are not biodegradable. The aim of this paper is to continue the investigations regarding opportunities of using the Diels-Alder adducts of abietic acid as substitutes for petroleum-derived products and as possible cross-linking agents for epoxy resins. The kinetics of curing and the thermal stability of cured products are also emphasized. 2. Materials, Methods, and Synthesis 2.1. Materials. Maleic anhydride (MA), hydroquinone (HQ), aspartic acid (ASP), p-aminobenzoic acid (p-ABA), triethylbenzylammonium chloride (TEBAC), and organic solvents were analytical grade products. Rosin acid (RA) (acid number (a.n.) ) 181 mg of KOH · g-1) was separated from commercial rosin and purified by twice recrystallization from acetone. The Diels-Alder adduct of rosin acids in levopimaric form with MA (RAMA) was obtained as described in a previously published article.27 Carboxyphenylmaleimide (CPMI) was obtained as in the literature.28 Epoxy resins (a) diglycidylether of bisphenol A (DGEBA) (Sintofarm, Bucharest) was a commercial product with an average epoxy equivalent weight of 345 g · equiv-1 and (b) diglycidyl ether of hydroquinone (DGEHQ) was synthesized according to the procedure described in our previous article and has an average epoxy equivalent weight of 205 g · equiv-1.28 2.2. Methods. The epoxy equivalent weights were obtained using the literature data (by the pyridinium chloride-pyridine method) and expressed as grams per equivalent (g · equiv-1).29 Acid numbers for RA, RAMA, and RAIMID were obtained by dissolving the samples in acetone to make a 5% (w/v) solution and were directly titrated with 0.1 N ethanolic KOH solutions in the presence of phenolphthalein. Nitrogen content was determined in accordance with the Kjeldhal method.30 Fourier transform infrared (FT-IR) spectra were recorded on a Bruker Vertex 70 FT-IR spectrometer using KBr pellets. The vibrational transition frequencies were reported in wavenumbers (cm-1).

10.1021/ie101746v  2010 American Chemical Society Published on Web 11/19/2010

Ind. Eng. Chem. Res., Vol. 49, No. 24, 2010

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Scheme 1. Syntheses of RAIMID1 and RAIMID2

1

H NMR spectra of the synthesized monomers were run on an Avance DRX 400 (Bruker, Reinstetten, Germany) at 50 °C. Samples were dissolved in deuterated dimethyl sulfoxide (DMSO-d6) with tetramethylsilane (TMS) as an internal reference (NMR chemical shifts were expressed in parts per million (ppm)). The thermal stability of cross-linked polymers was evaluated by thermogravimetric analysis (TGA) on an STA 449 F1 Jupiter apparatus (Netzsch-Germany) in the temperature range 25-600 °C under static air atmosphere, at a heating rate of 10 °C · min-1, coupled with a Vertex 70 Bruker FT-IR and Netzsch-QMS 403C spectrometer for TG-IR and quadrupole mass spectroscopic (QMS) analyses, respectively. Sample weights were in the range 5-10 mg, and Al2O3 crucibles were used. The Netzsch Thermokinetics-A software module for kinetic analysis of thermal measurements program was used for processing the initial TG and differential thermogravimetric (DTG) curves. The standard procedure suggested by this program is to estimate the activation energy of the degradation process versus the degree of conversion using the isoconversional methods of Friedman (FR) and Ozawa-Flynn-Wall (OFW) and by use of the obtained values as initial values for the nonregression procedure.31-34 Model-free kinetic analyses have the advantage of avoiding the need to specify the certain kinetic model type and hence any dependence on this choice. The gases released during the thermal decomposition processes are transferred by two isothermal transition lines to FTIR and mass spectrometers. The FT-IR transferring line is made of polytetrafluorethylene; it is 1 m long and 1.5 mm in diameter and is heated at 190 °C. The gases are introduced in the TGAIR external modulus, equipped with a liquid nitrogen cooled MCT detector (mercury cadmium telluride), and FT-IR spectra are recorded in the 600-4000 cm-1 domain, with a resolution of 4 cm-1. The acquisition of FT-IR spectra in three dimensions was done with OPUS 6.5 software. The transfer gas line to the QMS spectrometer is made of a quartz capillary, with 75 µm diameter and 2 m length; it works at 10-5 mbar vacuum, with electron impact ionization energy of 70 eV, and at 290 °C. The acquisition of the data was achieved with Ae¨olos 7.0 software, in the range m/z ) 1-200, with measuring time of ca. 0.5 s for one channel, resulting in a time/cycle of approximately 100 s.

Studies of the glass transition temperature and curing were carried out using a 912 Du Pont differential scanning calorimetric (DSC) instrument at heating rates 5, 10, and 15 °C · min-1 in the range 20-400 °C, in a nitrogen atmosphere (2.5 L · min-1) with pure indium as standard. Kinetics parameters were estimated from DSC thermograms using the variable peak exotherm methods of Kissinger and Ozawa.35,36 This calculation mode for the activation energy of cross-linking reactions is possible without previous knowledge of the reaction order. The following equations were used: Kissinger equation: ln(β/TM) ) Ea /RTM - ln(AR/Ea)

(1)

and Ozawa equation: ln β ) -0.4567(Ea /RTM) + C

(2)

where A is the preexponential factor, β is the heating rate, C is a constant, Ea is the activation energy for the curing reactions, TM is the peak exothermic temperature, and R is the gas constant. From graphs of ln β versus 1/T and ln(βTM-2) versus 1/T, the energies of curing and the preexponential factor were obtained. 2.3. Synthesis. The imido acids of rosin acid derivatives (RAIMID) were obtained. Details of the synthesis of imidotricarboxylic acid from the Diels-Alder adduct of rosin acids with maleic anhydride and aspartic acid (RAIMID1) are given in section 2.3.1, and two synthesis methods for the imido diacids of Diels-Alder adduct of rosin acids (RAIMID2) are given in section 2.3.2. 2.3.1. Synthesis of the Imidotricarboxylic Acid from the Diels-Alder Adduct of Rosin Acids with Maleic Anhydride and Aspartic Acid (RAIMID1). In a 0.5 L threenecked round-bottomed reaction flask, equipped with an oil bath, mechanical stirrer, thermometer, N2 inlet, and water trap connected to a water-cooled condenser, were placed 100 g (0.25 mol) of RAMA, 33.25 g (0.25 mol) of L-aspartic acid, and 100 mL of DMF. The heating was carried out under a nitrogen stream at 165 °C and kept at this temperature for 2.5 h with

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Scheme 2. Cross-Linking of Epoxy Resins with RAIMID1 and RAIMID2

stirring. Then, the reaction mixture was heated at 180 °C and kept at this level of temperature for another 4 h. Finally, the mixture was cooled to room temperature, diluted with 100 mL of acetone, and poured into a large amount of natural snow, where a pale brown solid was obtained. The fine-grained precipitate was filtered and washed twice with large quantities of distilled water. The obtained product was dried overnight under vacuum at 105 °C (yield 94%). The imidotricarboxylic acid was recrystallized twice from the concentrated solution of acetone and dried for 24 h at 65 °C under vacuum (pale brown, a.n. ) 206 mg of KOH · g-1, N ) 2.68%, 54% yield). (See Scheme 1.) 2.3.2. Synthesis of the Imido Diacids of Diels-Alder Adduct of Rosin Acids (RAIMID2). Two preparation methods for RAIMID are given in sections 2.3.2.1 and 2.3.2.2.

2.3.2.1. Synthesis of the Imidodicarboxylic Acid from Diels-Alder Adduct of Rosin Acids with Maleic Anhydride and p-Aminobenzoic Acid (RAIMID2). RAIMID2 was prepared as follows: 60 g (0.15 mol) of RAMA, 20.55 g (0.15 mol) of p-ABA, and 80 mL of DMF were mixed for 20 min in a reaction flask equipped with a thermometer, mechanical stirrer, N2 inlet, and water condenser with a Dean-Stark trap and oil bath, at reflux, until the mixture became homogenous. Then, the reaction mass was maintained for 2 h when the temperature was raised to 165 °C. This level of temperature was maintained for another 3 h, when it was raised to 180 °C and maintained for another 1 h. Finally, the mixture was cooled to room temperature, diluted with 50 mL of acetone, and poured into a large amount of natural snow, where a grayish white solid was


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Figure 2. 1H NMR spectra for (a) RAIMID1 and (b) RAIMID2.

Figure 1. FT-IR spectra for (a) RAIMID1, (b) DGEBA cross-linked with RAIMID1, (c) DGEHQ cross-linked with RAIMID1, (d) RAIMID2, (e) DGEBA cross-linked with RAIMID2, and (f) DGEHQ cross-linked with RAIMID2.

obtained. The obtained slurry was filtered, washed twice with distilled water, and dried under vacuum at 105 °C for 16 h (93% yield). The fine grains of RAIMID2 were dissolved in ethyl ether and precipitated with petroleum ether (bp 20-40 °C). Finally, the slurry was dried at 60 °C under vacuum and a pale brown product was obtained (84% yield). (See Scheme 1.) 2.3.2.2. Synthesis of the Imidodicarboxylic Acid from Rosin Acids and Carboxyphenylmaleimide (RAIMID2). Into the reaction flask equipped as above, 30.2 g (0.1 mol) of rosin acids (RA) (a.n. ) 181 mg of KOH · g-1), 21.7 g (0.1 mol) of CPMI, 1 g of hydroquinone, and 50 mL of DMF were charged. Then the reaction mixture was drained with N2, heated at reflux, and maintained at this level of temperature for 6 h under a slow stream of N2. The reaction mass was cooled to room temperature, diluted with 40 mL of acetone, and poured into a large quantity of natural snow, where a pale brown precipitate was obtained. After filtering and washing twice with distilled water, the precipitate was dried at 60 °C under vacuum, overnight. The solid mass was purified as above and dried at 100 °C under vacuum for 16 h (pale yellow, a.n. ) 208 mg of KOH · g-1, N ) 2.88%, 65% yield). (See Scheme 1.) 2.4. Sample Preparation for DSC and TGA Studies. The epoxy resins and imido acids (RAIMID1 and RAIMID2) (as concentrated solution in chloroform), accurately weighed, were vigorously mixed in a proportion corresponding to a molar ratio r ) 1, where r represents the carboxyl group/epoxy ring, and a homogeneous paste was obtained. The obtained mixture was dried under vacuum at 60 °C for 6 h. A small quantity of the

Figure 3. TGA and DTG curves of cross-linked systems obtained at 10 °C · min-1 heating rate: (2) DGEHQ/RAIMID1; (O) DGEHQ/RAIMID2: (0) DGEBA/RAIMID1; (9) DGEBA/RAIMID2.

mixture was taken for DSC studies. The remaining samples were cured at 120 °C for 1 h and at 160 °C for 4 h, and postcured at 180 °C for another 3 h. The cured products were powdered and used for TGA studies in the dynamic scan. The probable chemical structures of the cured products are presented in Scheme 2. 3. Results and Discussion 3.1. Synthesis and Characterization. Scheme 1 presents the method applied for synthesis of monomers and polymers. Monomaleamic acids were cyclodehydrated to the corresponding monomaleimide using catalysts (fused sodium acetate or ptoluenesulfonic acid) in the synthesis of CPMI or azeotrope for the elimination of the water in the synthesis of rosin imido acids. The monomers were purified by recrystallization. The IR spectra of RAIMID1 and RAIMID2 are shown in Figure 1. In Figure 1a (IR spectrum for RAIMID1), the presence of the peaks in the domain 2870-2940 cm-1 are specific to CH, CH2, and CH3 aliphatic groups that can be assigned to the hydrophenanthrene

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Table 1. Kinetic Parameters of Epoxy Systems/RAIMID from DSC Scans (Equivalent of Carboxyl Proton/Epoxide Group ) 1/1) TMa heating rate heating rate activation energy freq factor, (first exotherm) (°C · min-1) (second exotherm) (°C · min-1) of curingb (kJ · mol-1) ln Ac (min-1) epoxy resin curing agent DGEBA DGEHQ DGEBA DGEHQ

RAIMID1 RAIMID1 RAIMID2 RAIMID2

5

10

15

5

10

15

ref 36

ref 35

ref 35

glass transition tempd (°C)

144.2 131.7 148.8 135.5

155.5 149.2 159.2 152.3

163.6 157.4 167.5 161.5

304 328 314 332

326 352 336 357

365 355.6 369 366

86.4 61.1 90.8 55.6

79.2 54.1 83.6 48.6

14.15 14.15 16.11 5.61

136 126 140 128

a TM ) maximum peak temperature (°C). b EOzawa, EKissinger ) activation energies of curing reactions calculated with eqs 1 and 2. c Calculated with Kissinger equation. d Measured after cross-linking at 200 °C, 3 h.

and hydroxypropyl groups. The broad absorption bands situated in the range 3200-3370 cm-1 and the sharp band located at 1710 cm-1 reveal the presence of COOH groups, while the peaks situated at 1776 and 1388 cm-1 are specific to CO groups and CN groups defining the cyclic imide groups. The band characteristic of the double bonds in the hydrophenanthrene moiety specific to rosin acid located at 1460 cm-1 has disappeared as a consequence of the Diels-Alder reaction between rosin acid and maleic anhydride. Other characteristic bands specific to imide ring appear in the range of 1180 cm-1 (imide III). The bands specific to RAIMID2 (Figure 1d) are similar to those of RAIMID1, except for the signals at 1607 and 770 cm-1 specific to para-substituted aromatic ring. The IR spectra of cross-linked polymers (Figure 1b,c,e,f) do not show any peak at 915 cm-1 specific to an epoxy ring. In exchange, at 3450, 1705-1710, and 2230 cm-1 new signals are present (signals specific to secondary OH groups and ester groups), which are a consequence of chemical reactions between the epoxy ring and carboxylic protons. In the 1H NMR spectra of both monomers one can observe a lot of peaks located in the range of the 0.71-2.16 ppm chemical shift, representing the aliphatic protons belonging to hydrophenanthrene moieties (Figure 2). The signals situated in the range of the 0.51-0.9 ppm chemical shift are probably the protons from the isopropyl methyl group bonded to the hydrophenthrane ring. The CH3 groups bonded directly to the hydrophenanthrene units of RA appear as a singlet at 1.39 ppm. The olefinic protons located in the hydrophenanthrene moieties (after Diels-Alder reaction) appear as weak peaks at the 5.52

Figure 4. Dependence of the activation energy of degradation process versus degree of conversion, for cross-linked DGEHQ/RAIMID1 system in 100-600 °C interval: (b) evaluated with the Ozawa-Flynn-Wall method; (9) evaluated with the Friedman method.

ppm chemical shift. The aromatic protons situated in the p-aminobenzoic acid structure are presented in the range of the 7.2-7.28 ppm chemical shift (Figure 2b). The carboxylic acid protons appear as a broad band with chemical shifts of about 12.24 ppm for RAIMID1 and 12.46 ppm for RAIMID2 (Figure 2). 3.2. Curing Behavior. In the present study, RAIMID1 and RAIMID2, the two rosin acid derivatives containing imide ring in their structures (with free carboxylic group and one sterically hindered abietic acid carboxylic group), were used as crosslinking agents for epoxy resins (DGEBA, DGEHQ). The curable systems were obtained by mixing these epoxy resins with rosin acid derivatives at the molar ratio 1/1 (epoxy group/carboxylic proton) for RAIMID1 and at the molar ratio 1/0.66 (epoxy group/carboxylic proton) for RAIMID2. The obtained formulations were cross-linked under the action of temperature, in the presence of TEBAC as catalyst. In Scheme 2 the possible chemical reactions and the network formation are shown. The reactions between acidic protons and epoxy ring are autocatalytic in nature and imply a complex mechanism.37,38 The dynamic DSC scans of the systems presented above were registered at different heating rates, and the curing exotherms were recorded. As a consequence of the molar ratio between the epoxy ring and acidic protons, and as unequal reactivities of the carboxylic groups, in the first step, at relatively low temperature, the carboxylic protons attached to imide moieties react with epoxy ring and OH groups appear. In the second step, along with the increase of temperature, the new OH protons and hindered carboxylic group attached to hydrophenanthrene moieties can react with another epoxy ring and the cross-linked structure can appear. As can be seen in Figure 3 and Table 1, the epoxy/ RAIMID1 system presents a first exotherm peak centered in the range 132-163 °C, while the epoxy/RAIMID2 system

Figure 5. DTG curves for DGEHQ/RAIMID1 system in 100-600 °C interval at (1) 5, (2) 10, and (3) 20 °C · min-1 heating rate.


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Table 2. Kinetic and Statistical Parameters Obtained from the Most Probable Mechanism of Thermal Degradation Process of Cured Resins in the Interval Temperature between 100 and 600 °C sample DGEHQ/RAIMID1

parametera log A1 (s-1) E1 (k J · mol-1) n1 log Kcat1 log A2 (s-1) E2 (k J · mol-1) n2 log Kcat2 log A3 (s-1) E3 (k J · mol-1) n3 follReact1 follReact2 correln coeff t-critical (0.95)

DGEHQ/RAIMID2

DGEBA/RAIMID1

DGEBA/RAIMID2

mechanism scheme mechanism scheme mechanism scheme mechanism scheme A-1 f B-2 f C-3 f D; A-1 f B-2 f C-3 f D; A-1 f B-2 f C; A-1 f B-2 f C; kinetic model type t:f,f An, Fn, An kinetic model type t:f,f An, Fn, An kinetic model type d:f CnB, Fn kinetic model type d:f CnB, CnC 13.67 195.7 0.38 18.4 184.7 2.84 10.36 150.2 2.96 0.34 0.58 0.999 1.956

18.84 268.4 0.24 17.36 200.4 2.96 27.5 265.3 0.34 0.39 0.14 0.999 1.956

10.18 163.9 1.11 1.34 16.76 97.9 2.47 0.21 0.9984 1.955

11.17 170.9 1.41 0.24 15.3 76.7 0.2 0.24 0.5 0.9992 1.955

a E1, E2, E3 ) decomposition activation energy for each step. log A1, log A2, log A3 ) logarithm of the preexponential factor for each step. n1, n2, n3 ) order of reaction for each step. log Kcat1, log Kcat2 ) autocatalytic order of reaction for each step. follReact1, follReact2 ) share of the each step in the total mass loss.

first exotherm, but from the TGA data (Figure 4) we can conclude that, at maximum peak temperature, it is possible the degradation process took place. This fact may change the energy values by summarization of the degradation effect. The kinetic data (activation energy of curing reactions and frequency factor, calculated from the first exotherm) are listed in Table 1 and are situated in the range 61-86 kJ · mol-1 for

Figure 6. Curves for experimental measurements and theoretical results versus temperature, obtained for the step method d:f CnB, Fn (6) for DGEBA/ RAIMID1 system (O, experimental; b, theoretical) at 5 °C · min-1, (3, experimental; 1, theoretical) at 10 °C · min-1, and (0, experimental; 9, theoretical) at 20 °C · min-1.

presents a first exotherm peak centered in the range 136-168 °C, which may be due to the reaction between carboxylic protons bonded to imide moieties and epoxy ring. The second exotherms are centered in the range 304-365 °C for the epoxy/ RAIMID1 system and between 314 and 369 °C for the epoxy/ RAIMID2 system, behavior that can be attributed to the reactions between secondary OH protons or hindered carboxylic protons bonded to the hydrophenanthrene ring with epoxy ring moieties. The exothermic temperature peak is specific to each epoxy resin/RAIMID system and has the minimum value for the DGEHQ/RAIMID system (Table 1). This fact can be explained by the great mobility of DGEHQ resin (with lower molecular weight), which has less viscosity in comparison with DGEBA. These data show that, for all systems, the maximum peak temperature is shifted to great values with the increase of heating rate. For the second exotherm, the energy of crosslinking reactions has higher values than that obtained from the

Figure 7. FT-IR spectra of gaseous products arising from degradation of the cross-linked products (obtained at 10 °C · min-1 heating rate and at maximum peak temperature): (a) DGEHQ/RAIMID2, (b) DGEBA/ RAIMID2, (c) DGEBA/RAIMID1, and (d) DGEHQ/RAIMID1.

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Figure 8. QMS of gaseous products at 410 °C for DGEHQ/RAIMID1 system.

epoxy/RAIMID1 systems and between 55 and 91 kJ · mol-1 for epoxy/RAIMID2 systems, depending on the chemical structure of epoxy resins. These values are in agreement with other data reported in the literature for epoxy resins cross-linked with rosin acid derivatives.21,24 The activation energy of curing process and the preexponential factor obtained from the first exotherm have different values which vary significantly with the chemical nature of the epoxy resin; they have the smallest value for the systems with DGEHQ. The preexponential factor generally reflects the collision of the reactant molecules, and its small values for DGEHQ systems can be explained as a consequence of the fact that DGEHQ epoxy resins have low values of viscosity and the collision frequency of reactant molecules can be higher. According to the literature data, the cross-linking degree increases the glass transition temperature.38 As can be seen in Table 1, the glass transition temperature increases for both epoxy resins when the curing agent is RAIMID2. This may be attributable to the higher degree of cross-linking due to three carboxyl groups. 3.3. Thermal Analysis. TG and DTG results for epoxy resins cross-linked with RAIMID1 and RAIMID2 are shown in Figure 3. For higher heating rates, the onset and end of degradation process were shifted to higher temperatures. As can be seen in Figure 4, the activation energy of degradation increases with the degree of conversion. For a given value of R the activation energy of degradation obtained with the Friedman method is higher than the activation energy obtained by Ozawa-Flynn-Wall and presents a strong growth in the range 0-0.20 and in the range of 0.8-1.0 degree of conversion. This dependence shows that the process is complex. From the DTG curves we can observe that the nonisothermal decomposition process consists of three single steps for DGEHQ/RAIMID1 (Figure 5) and two single steps for DGEBA/RAIMID2. Because the individual mass losses of each step were independent of the heating rate, a kinetic model with two or three consecutive steps was chosen for each system. The probable mechanism of the degradation process and kinetic parameters were obtained with a multivariate nonlinear regression program, using the 18 different models

included in the Neztsch procedure. Using the non-isothermal data recorded at three heating rates, the differential equations of the reaction rates were numerically acquired. The best results were obtained by running experimental data for R values between 0.1 and 0.90 specific to each reaction model, selected on the basis of the discrepancy between experimental and calculated values and the correlation coefficient. The following kinetic models were used: nth-order reaction model, Fn: f(R) ) (1 - R)n

(3)

where n is the reaction order. Avrami-Erofeev reaction model, An: f(R) ) n(1 - R)[-ln(1 - R)](n-1)/n

(4)

nth-order autocatalytic model, Cn: f(R) ) (1 - R)n(1 + KcatR)

(5)

where Kcat is the autocatalytic order. The following process mechanisms were assigned to the above models: d:f CnB, Fn A-1 f B-2 f C

(6)

A-1 f B-2 f C

(7)

A-1 f B-2 f C-3 f D

(8)

d:f CnB, CnC t:f,f An, Fn, An where A, B, C, and D are solid products, d:f and t:f,f represent the reactions schemes, and 1, 2, and 3 denote the mechanism steps. The best results of the kinetic and statistical parameters obtained from the above-presented mechanisms of the thermal degradation processes of cured resins in the interval temperature between 100 and 600 °C are given in Table 2. As an example,


for these mechanisms, the experimental data obtained for DGEBA/RAIMID1 hardened system, at three heating rates, are in good agreement with the simulated curves (Figure 6). Footnote a of Table 2 lists the specific parameters of each tested kinetic model. From Table 2 and Figure 6, we can see that the assigned reaction models presented above have activation energy values of the degradation process in good agreement with the values obtained from FR and OFW analyses (Figure 4). The values of the activation energy of degradation, for the systems with DGEHQ in the structure, are greater than those for the systems with DGEBA in the structure. This can be explained by the more rigid structure of DGEHQ in comparison with DGEBA. Use of the TG-IR technique can identify the significant volatilized products formed during thermal degradation of the cured products. In Figure 7, the FT-IR spectra of the gas products of degradation of epoxy/RAIMID cross-linked systems at the maximum peak temperature of degradation and 10 °C · min-1 are presented. The important FT-IR peaks were observed in the range 3500-3800 cm-1, around 3260 cm-1, at 2360 cm-1, in the range 1500-1710 cm-1, and in the range 670-1060 cm-1. These signals can be attributed to the main degradation products which can be ammonium derivatives (NH3, NO2-, 3500-3800 cm-1), water (3240 cm-1), alkane derivatives (2970 cm-1), CO2 (2360 cm-1), acid or anhydride derivatives (1710-1800 cm-1), and aromatic compounds (1500-1600 and 670-900 cm-1). The formation of CO2 and H2O may be assigned to the degradation of ester groups, the anhydride can be assigned to the reaction between acid molecules (resulted in the degradation process), and the aromatic compounds can result from degradation of epoxide moieties. The main peaks of degradation for the main pyrolysis products of cross-linked resins detected by QMS 403C Ae¨olos technique at 10 °C · min-1 confirm the data obtained by FT-IR. In Figure 8 the QMS spectrum obtained for the DGEBQ/ RAIMID1 system is presented. The spectra obtained for the other systems are relatively similar. It can be found that a lot of m/z peaks are detected. These peaks can be associated with destruction of the epoxy chains and diacid moieties, where CO2, water, ammonium derivatives, acidic derivatives, alkane derivatives, and aromatic products are detected at this temperature. The main products are the following: water (m/z 17, 18), CO2 (m/z 16, 28, 43, 44), alkane derivatives (m/z 15, 16, 26, 27, 31), and aromatic products (benzene, toluene, phenol, phenol derivatives) (m/z 39, 66, 77, 78, 105, 132). In the range 450-550 °C the amount of the aromatic products is increased and the phenols and condensed aromatic compounds are evolved as the scission of epoxy resin and curing agent’s moieties are present. 4. Conclusions Diacids and triacids based on the levopimaric Diels-Alder adduct with maleic anhydride, were synthesized, characterized, and used as cross-linking agents for epoxy resins. The structures of these monomers were confirmed by IR spectroscopy, 1H NMR, and elemental analysis. The curing reactions of these multifunctional monomers with DGEBA and DGEHQ are a complex process due to the different functionalities of the monomers. The energies of cross-linking reactions obtained from DSC thermograms are situated in the range 56-86 kJ · mol-1 and are in good agreement with previous literature data.21,23,24 The dependence of the activation energy of the degradation process Ea versus the degree of conversion for the cross-linked products, calculated by the Friedman method, shows that the process is complex and is divided into several stages. The

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simulated curves, obtained by using the above-proposed kinetic models (eqs 3-5), are in good agreement with the experimental curves. The cured resins possess good thermal stability, with apparent activation energies of the degradation process having values in the range 164-196 kJ · mol-1, comparable with the literature data.39-41 The presence of imide ring increases the thermal stability of the cross-linked resins. The presence of rosin derivatives in the structure of these cross-linked products suggests that these materials can be considered as environmentally friendly products. Literature Cited (1) McSweeney, E. E., Arlt, H. G., Jr., Russell, J., Eds.; Tall Oil and Its Users; Pulp Chemical Association Inc.: New York, 1987; Chapter II. (2) Sadhar, S.; Founds, I. S.; Gray, C. N.; Koh, D.; Gardiner, K. Clophonysuses, healts effects, airbone measurement and analysis. Ann. Occup. Hyg. 1994, 38, 385. (3) Penczek, P.; Matynia, T. Estry glicydylowe kwasu maleopimarowego jako cykloalifatyczne zywice epoksydowe. Polimery 1974, 19, 609. (4) Ray, S. S.; Kundu, A. K.; Maiti, M.; Ghosh, M.; Maiti, S. Polymers from renewable resources, part 7. Synthesis and properties of polyamideimide from rosin-maleic anhydride adduct. Angew. Makromol. Chem. 1984, 122, 153. (5) Maiti, M.; Adhicari, B.; Maity, S. Polymers from renewable resources. Part 6. Polyesterimides from rosin. J. Polym. Mater. 1988, 5, 201. (6) Hoa, L. T. N.; Pascault, J. P.; My, L. T.; Son, C. P. N. Unsaturated polyester prepolymer from rosin. Eur. Polym. J. 1993, 29, 491. (7) Bicu, I.; Mustata, F. Crosslinked polymers from resin acids. Angew. Makromol. Chem. 1996, 234, 91. (8) Hutter, G. F. Rosin ester-amide support resins for acrylic latexes. U.S. Patent 5,656,679, 1997. (9) Kim, S. J.; Kim, B. J.; Jang, D. W.; Kim, S. H.; Park, S. Y.; Lee, J. H.; Lee, S. D.; Choy, D. H. Photoactive polyamideimides synthesized by the polycondensation of azo-dye diamines and rosin derivative. J. Appl. Polym. Sci. 2000, 79, 687. (10) Bicu, I.; Mustata, F. Water soluble polymers from Diels-Alder adducts of abietic acid as paper additives. Makromol. Mater. Eng. 2000, 280/281, 47. (11) Kim, W.-S.; Jang, H.-S.; Hong, K.-H.; Seo, K.-H. Synthesis and photocrosslinking of poly(vinylbenzyl) abietate. Macromol. Rapid Commun. 2001 22, 825. (12) Lee, J. S.; Hong, S. I. Synthesis of acrylic rosin derivatives and application as negative photoresist. Eur. Polym. J. 2002, 38, 387. (13) Atta, A. M.; Mansour, R.; Abdou, M. I.; Sayed, A. M. Epoxy resins from rosin acids: synthesis and characterization. Polym. AdV. Technol. 2004, 15, 514. (14) Barabde, U. V.; Fulzele, S. V.; Satturwar, P. M.; Dorle, A. K.; Joshi, S. B. Film coating and biodegradation studies of new rosin derivative. React. Funct. Polym. 2005, 62, 241. (15) Atta, A. M.; Mansour, R.; Abdou, M. I.; El-Sayed, A. M. Synthesis and characterization of tetra-functional epoxy resins from rosin. J. Polym. Res. 2005, 12, 127. (16) Kundu, A. K.; Roy, S. S.; Adhikari, B.; Maiti, S. Polymer blends 2. Compatibility and thermal behaviour of blends of novolac and polyamidimide from rosin. Eur. Polym. J. 1986, 22, 369. (17) Ray, S. S.; Kundu, A. K.; Maiti, S. Polymers from renewable resources. XII. Structure property relation in polyamideimides from rosin. J. Appl. Polym. Sci. 1988, 3, 1283. (18) Mustata, F.; Bicu, I. Hydroxyesters of resin acids modified with o-cresol/p-nonylphenol formaldehyde resins. Polimery 2009, 54, 627. (19) Mustata, F.; Bicu, I. A novel route for synthesizing esters and polyesters from the Diels-Alder adduct of levopimaric acid and acrylic acid. Eur. Polym. J. 2010, 46, 1316. (20) Matynia, T. Badania nad wlasnosciami cycloalifatycznych zywic epoksydowych typu estrow glicydylowych. Polimery 1975, 20, 7. (21) Liu, X.; Xin, W.; Zhang, J. Rosin-derived imide-diacids as epoxy curing agents for enhanced performance. Bioresour. Technol. 2010, 101, 2520. (22) Liu, X. Q.; Xin, W. B.; Zhang, J. W. Rosin-based acid anhydrides as alternatives to petrochemical curing agent. Green Chem. 2009, 11, 1018. (23) Mustata, F.; Bicu, I. The effect of some Diels-Alder adducts of resin acids on the process of epoxy resin curing. Polimery 2008, 53, 24.

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(24) Mustata, F.; Bicu, I. Lactic ester of maleoabietic acid as crosslinking agent for epoxy Resins. Mater. Plast. 2009, 46, 249. (25) Wang, H.; Liu, B.; Liu, X.; Zhang, J.; Xian, M. Synthesis of biobased epoxy and curing agents using rosin and the study of cure reactions. Green Chem. 2008, 10, 1190. (26) Wang, H.; Liu, X.; Liu, B.; Zhang, J.; Xian, M. Synthesis of rosinbased flexible anhydride-type curing agents and properties of the cured epoxy. Polym. Int. 2009, 58, 1435. (27) Mustata, F.; Bicu, I. Polyhydroxyetheresters from resin acids. Polimery 2005, 50, 10. (28) Mustata, F.; Bicu, I. Polyhydroxyimide from resinic acids. Polimery 2000, 45, 258. (29) Cascaval, C. N.; Mustata, F.; Rosu, D. Viscosity charactheristics of some p-nonylphenol formaldehyde resin. Angew. Makromol. Chem. 1993, 209, 157. (30) Cheronis, N. D.; Ma, T. S. Organic Functional Group Analysis by Micro and Semimicro Methods; Interscience Publishers, Wiley: New York, 1964. (31) Friedman, H. L. New methods for evaluating kinetic parameters from thermal analysis data. J. Polym. Sci., Part B: Polym. Lett. 1969, 7, 41. (32) Flynn, J. H.; Wall, L. A. A quick, direct method for the determination of activation energy from thermogravimetric data. J. Polym. Sci., Part B: Polym. Lett. 1966, 4, 323.

(33) Ozawa, T. Kinetic analysis of derivative curves in thermal analysis. J. Therm. Anal. 1970, 2, 301. (34) Opfermann, J. Kinetic analysis using multivariate non-linear regression. J. Thermal. Anal. Calorim. 2000, 60, 641. (35) Kissinger, H. F. Reaction kinetics in different thermal analysis. Anal. Chem. 1957, 29, 1702. (36) Ozawa, T. A modified method for kinetic analysis of thermoanalytical data. J. Therm. Anal. 1976, 9, 369. (37) Boutevin, B.; Parisi, J. P.; Robin, J. J.; Roume, C. Synthesis and properties of multiacrylic resins from epoxy resins. J. Appl.Polym. Sci. 1993, 50, 2065. (38) Matsuoka, S. Relaxation Phenomena in Polymers; Hanser Publishers, Springer Verlag: Munich, 1992. (39) Mustata, F.; Bicu, I. Lactic ester of maleoabietic acid as crosslinking agent for epoxy resin. Mater. Plast. 2009, 46, 249. (40) Arasa, M.; Ramis, X.; Salla, J. M.; Mantecon, A.; A. Serra, A. A study of the degradation of ester-modified epoxy resins obtained by cationic copolymerization of DGEBA with γ-lactones initiated by rare earth triflates. Polym. Degrad. Stab. 2007, 92, 2214. (41) Chen, K. S.; Yeh, R. Z. Pyrolysis kinetics of epoxy resin in a nitrogen atmosphere. J. Hazard. Mater. 1996, 49, 105.

ReceiVed for reView August 19, 2010 ReVised manuscript receiVed October 22, 2010 Accepted November 3, 2010 IE101746V

Epoxy Resins Cross-Linked with Rosin Adduct Derivatives. Cross

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