Synthesis and Properties of Multiallyl Maleate Resins from Epoxy

Feb 12, 2008 - Formaldehyde resins at various molar ratios of monomers (aniline/o-cresol/cyclohexanone) were synthesized under acid catalysis...
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Ind. Eng. Chem. Res. 2008, 47, 1355-1364

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APPLIED CHEMISTRY Synthesis and Properties of Multiallyl Maleate Resins from Epoxy Formaldehyde Resins Fanica R. Mustata* and Ioan Gh. Bicu “P. Poni” Institute of Macromolecular Chemistry, Aleea Grigore Ghica Voda, No. 41 A, Iasi 700487, Romania

Formaldehyde resins at various molar ratios of monomers (aniline/o-cresol/cyclohexanone) were synthesized under acid catalysis. The resultant resins were reacted with a large excess of epichlorohydrin, and multifunctional epoxy formaldehyde resins were obtained. These resins were reacted with monoallyl ester of maleic acid in the presence of benzyltriethylammonium bromide as catalyst, and multiallyl maleate resins were obtained. The resins were characterized by elemental analysis and by spectroscopic methods (IR and NMR). The curing and decomposition behavior of cross-linked resins were studied by DSC and TGA techniques. The presence of allyl maleate as pendant groups on polymeric chains confers on them the improvement of the possibilities for obtaining thermosetting resin systems based on bismaleimide derivatives. The cross-linked products showed good thermal properties, high glass transitions, and low water absorption. 1. Introduction Cross-linkable polymers (unsaturated polyesters, multifunctional epoxy resins, multifunctional maleimide resins, vinyl ester resins, etc.), which have a high number of reactive groups on/ in the polymeric chain (that can chemically, photochemically, or thermally polymerize without the formation of voids), are an important type of material, used in many industrial applications owing to their properties (obtained after cross-linking) such as good thermal and chemical resistance, good adherence to many substrates, and superior mechanical and electrical behavior, in addition to good processing (before cross-linking). These resins have been used as structural adhesives and matrixes for fiber composites in numerous applications including aerospace and hydrospace engineering, the car industry, the electrical and electronic industry, and so on. Unfortunately, after cross-linking, these resins become brittle and with poor resistance to crack propagation. Modification of the chemical structure of these polymers by incorporation of a second reactive diluent (diallyl bisphenol A, acrylic elastomers, butadiene homopolymers) or by chemical transformation of polymeric chains, with the aim of enhancing properties, has received considerable attention from the research community.1-16 The objective of the present work is to report the results on the synthesis and cross-linking polymerization of multiallyl maleate resins, which possess many polymerizable carboncarbon double bonds with different reactivities, one of which is the vinylene groups of maleate types and the other is the allylic type, and their characterization by Fourier transform infrared (FT-IR) spectroscopy, NMR, and elemental analysis. Thermal properties of the cross-linked products were also investigated. 2. Materials, Methods, and Synthesis 2.1. Materials. Maleic anhydride (MA) (Aldrich), allyl alcohol (AlA) (Aldrich), aniline (AN) (Chimopar, Romania), o* To whom correspondence should be addressed. E-mail: fmustata@ icmpp.ro.

cresol (o-Cz) (Fluka), cyclohexanone (CHx) (Chimopar, Romania), 4-aminobenzoic acid (p-ABA) (Aldrich), 4,4′-diaminodiphenylmethane (DDM), epichlorohydrin (ECl) (Merck), triethylbenzylammonium bromide (TEBAB) (Fluka), acetic anhydride (AcA) (Aldrich), hydroquinone (HQ) (Aldrich), and sodium and potassium hydroxide (NaOH, KOH) (Chemapol, Czech Republic) were purchased as analytical grade products and used as received. Paraformaldehyde (p-FA) was was a commercial source, with 98% purity. Hydrochloric acid (HCl, 35%) and all organic solvents were chemically pure reagents and used as received or purified by distillation. Diaminodiphenyl ether bismaleimide (DDEBMI) was prepared from 4,4′-oxidianiline according to the method reported in the literature.17,18 Carboxy phenylmaleimide (CPMI) was obtained according to literature data and has a melting point of 225-227 °C.19 The epoxy resin (diglycidylether of bisphenol A) (DGEBA) (SC Sintofarm SA, Romania) with an average epoxy equivalent weight of 340 g‚equiv-1 was a commercial product and was used without further purification. 2.2. Methods. The hydroxy number and the epoxy equivalent weight were determined using literature methods.20,21 Nitrogen content was obtained according to the Kjeldhal method.22 Average molecular weight was obtained by a cryoscopic method using DMSO as solvent.23 Infrared spectra (FT-IR) were recorded using a Bio-Rad DigiLab Division (Portmann Instruments) spectrophotometer on KBr pellets. 1H NMR and 13C NMR spectra were obtained on an Avance DRX 400 (BRUKER, Rheinstatten, Germany) at 50 °C using (D6)DMSO as solvent and tetramethylsilane as internal standard (NMR chemical shifts were expressed in parts per million). Thermogravimetric analysis (TGA) was performed by a Paulik, Paulik-Erdey thermogravimetric analyzer of MOM (Budapest) type, at a heating rate of 10 °C‚min-1 in air and a temperature range between 25 and 700 °C. The kinetic parameters and activation energies of degradation reactions for the obtained resins were calculated using the Swaminathan and Modhavan and the Coats and Redfern equations.24,25 The general equations used are

10.1021/ie0705913 CCC: $40.75 © 2008 American Chemical Society Published on Web 02/12/2008

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dR/dT ) A exp(-Ea/RT)[Rm(1 - R)n][-ln(1 - R)p] (Swaminathan & Modhavan equation) (1) and

log[1 - (1 - c)1-n]/(1 - n)T2 ) log(AR/φEa) 2.303(Ea/RT) (Coats & Redfern equations) (2) where R is the conversion degree (ratio of the weight loss at time t and at the end of the process), A is the preexponential factor, c is the conversion, Ea is the activation energy of decomposition, n is the reaction order, m and p are the exponents of the conversion function, T is the temperature, φ is the heating rate, and R is the gas constant. The extent of curing and glass transition temperature were evaluated by means of a Mettler TA Instruments DSC 12E at different heating rates (5, 10, 15 °C‚min-1), at the range 20-400 °C in a nitrogen atmosphere (3 L‚min-1). The instrument was calibrated using pure indium as a standard. Runs were always carried out using an empty cell as a reference. Kinetic parameters of cross-linking reactions were estimated from DSC (differential scanning calorimetry) thermograms using the variable peak exotherm method of Kissinger and Ozawa. Based on the linear relationship between the reciprocal of the exotherm peak temperature (TM) and the logarithm of the heating rate (log β) and ln(φ/TM2), the activation energy of cross-linking reactions and the preexponential factor were calculated.26,27 This calculation mode of the activation energy of cross-linking reactions is possible without knowing the reaction order previously. The following equations are

ln(φ/TM2) ) Ea/RTM - ln(AR/Ea) (Kissinger equation) (3) and

ln φ ) C - 0.4567(Ea/RTM) (Ozawa equation)

(4)

where A is the preexponential factor, C is a constant, Ea is the activation energy for the curing reactions, TM is the peak of exothermic temperature, R is the gas constant, and φ is the heating rate. The water absorption was determined by placing the cured disks (20 mm diameter, 1.5 mm thick) in cold water at 25 °C for 14 days and in hot water at 100 °C for 1 h. Then the disks were removed, dried with filter paper, and weighed. The quantities of the absorbed water were determined as the ratio between the final weight after water absorption and the initial weight. 2.3. Synthesis. 2.3.1. Synthesis of Multiallyl Maleate Resins Based on Epoxy Formaldehyde Resins. The products were prepared by the following steps: (1) synthesis of formaldehyde resins; (2) synthesis of epoxy formaldehyde resins; (3) synthesis of monoallyl maleate; and (4) synthesis of multiallyl maleate resins (Schemes 1 and 2). 2.3.1.1. Synthesis of Formaldehyde Resins (FR). The novolak resins containing different amounts of AN, o-Cz, and CHx in their structures were obtained at the molar ratio monomer/formaldehyde 1/1, in the presence of HCl as catalyst (3% based on monomer weight) and DMF/toluene mixture (1/ 1, v/v) as carrier for reaction water using the standard procedure.28-31 In a typical experiment (Table 1), a mixture of 18.6 g (0.2 mol) of AN, 19.6 g (0.2 mol) of CHx, 12 (0.4 mol) of p-FA, and 20 mL of DMF/toluene was charged into a 500 mL fournecked round-bottom flask, equipped with an oil bath, mechanical stirrer, nitrogen inlet, and Dean-Stark trap fitted with a

water condenser and a thermometer and stirred at room temperature. Then, the mixture was heated to 75-80 °C and maintained at this level for 10 min, and 4.3 mL of the catalyst (HCl 35%) was added dropwise over 10 min. After the catalyst addition, the reaction mass became transparent and the temperature was raised by 17 °C, and maintained at reflux temperature for 2 h (for the synthesis of Mannich mono- and dibases). Then, 21.6 g (0.2 mol) of o-Cz, 6 g (0.2 mol) of p-FA, 2 mL of HCl, and 20 mL of DMF/toluene was added, and the reaction mass was maintained at reflux for 2 h. Under the temperature and catalyst action, the Mannich mono- and dibases became unstable and rearranged into oligomers with CH2 bridges between aromatic and aliphatic rings. This time, the generated water was extracted as a water/toluene azeotrope using a DeanStark trap. As a consequence of water extraction, the temperature slowly rose to 145-155 °C in 1 h. Finally, the toluene was extracted under vacuum and the melting resin was transferred into an aluminum mold and cooled at room temperature. Then, the resin was cooled, ground to a fine powder, and extracted twice with ethylic ether. The final mass was filtered and the cake was placed under light vacuum at 60 °C to remove the volatile components, resulting in 30.5 g (yield 91%) of bright reddish resin. 2.3.1.2. Synthesis of Epoxy Formaldehyde Resins from Novolak Resin (ENR). For obtaining low-molecular-weight multifunctional epoxy resins, the reaction was conducted in a large excess of epichlorohydrin.29-35 To a 1 L four-necked round-bottom flask, equipped with a water bath, thermometer, mechanical stirrer, and dropping funnel, was added 0.2 mol of novolak resin obtained as above and 2 mol of ECl. The resin was dissolved at room temperature, and 1 mol of water was added. The solution was heated at 8085 °C and kept at this level for 8 h. Then, the temperature was decreased to 55-60 °C and 28 g (0.7 mol) of NaOH of aqueous solution (33 wt %) was added under vigorous stirring over 6 h, at the same level of temperature. After the addition of NaOH solution, the mixture was kept at this level for another 1 h. The resultant mass suspension was mixed with 700 mL of warm distilled water (50 °C), and the inorganic salt was removed. The organic layer was decanted, mixed with another 700 mL of water, and again decanted. Finally, the solution was distilled as an azeotrope of toluene/water and as toluene/epichlorohydrin. The remaining mass was cooled, powdered as fine grains, and extracted twice with ethylic ether. A reddish brown resin was obtained (yield 90%). 2.3.1.3. Synthesis of Monoallyl Maleate (MAMA). MAMA was synthesized according to methods described in literature data.36,37 Into a 0.5 L four-necked round-bottom flask equipped as above were charged 116 g (1 mol) of AlA, 98 g (1 mol) of MA, and 1.5 g of HQ. The mixture was heated 15 min at 85 °C when an exothermal reaction was initiated. Then the reaction mass was heated at 140 °C for 3 h to form 1 mol of MAMA. 2.3.1.4.SynthesisofMultiallylMaleateResins(MAMAENR). To a 0.5 epoxy equivalent of epoxy resin, synthesized as above, 0.5 equiv of MAMA, 4 g of hydroquinone as a polymerization inhibitor, and 50 mL of DMF were added. The reaction mixture was purged with nitrogen and heated to 80 °C when 0.05 mol of TEBAB as catalyst was added. The temperature was increased to 110 °C and held for 5 h with an increase of temperature at 10 °C/h while stirring. Then, 50 mL of DMF was added, and the solution was cooled to room temperature and poured into a large excess of ice/water mixture under vigorous stirring. The polymer was separated, washed with a large amount of water, and dried under vacuum at 50 °C for 8 h. The reaction was

Ind. Eng. Chem. Res., Vol. 47, No. 5, 2008 1357 Scheme 1. Preparation of Epoxy Allyl Maleate Resins

Scheme 2. Proposed Reaction Schemes for MAMAENR Curing

controlled by FT-IR spectroscopy. The obtained resin was divided as fine grains, extracted twice with hot methanol, and dried under vacuum at 50 °C overnight (Scheme 1).

2.4. Cross-Linking Process of Multiallyl Maleated Resins. The resins containing allyl maleate moiety units obtained as above were cured in the presence of DGEBA, DDEBMI, CPMI,

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Table 1. Initial Monomer Composition and Some Physical-Chemical Characteristics of the Resulting Resins

sample

monomer ratio (AN/o-Cz/CHx/ p-FA/ECl/MAMA) (mol/mol)

FR 1 FR 2 EFR 1 EFR 2 MAMAENR 1 MAMAENR 2

1/1/1/3/0/0 1/2/1/4/0/0 1/1/1/3/3/0 1/2/1/4/4/0 1/1/1/3/3/3 1/2/1/4/4/4

a

numberaverage mol wta

epoxy equiv wt (exptl/calcd) (g‚equiv-1)

hydroxy no. (exptl/calcd) (mg of KOH‚g of polym-1)

nitrogen content (%)

yield (%)

color

112/124 117/128

3.92 2.97 2.74 2.21 0.98 0.77

92 96 91 93 88 91

reddish resin pale reddish resin pale reddish brown pale reddish brown brown brown

760 948 195/176 236/215

Cryoscopic method, DMSO as solvent.

and DDM (Scheme 2). Two kinds of bismalmaleimide were used: hydroxybismaleimide obtained in situ by reaction between CPMI and DGEBA;30,31,38 DDEBMI synthesized as in the literature.17,18 2.5. Sample Preparation for DSC and TGA Studies. 2.5.1. Sample 1. One gram of DGEBA, 1.08 g of CPMI, and 2.33 g of MAMAENR were mixed in a beaker and stirred to obtain a paste. A small part of the mixture (4-10 mg) was used for the DSC studies, and a large part was cured at 140 °C for 1 h and at 250 °C for 4 h and used for TGA studies. 2.5.2. Sample 2. One gram of DGEBA, 0.5 g of DDM, and 1.35 g of MAMAENR were mixed as above, and a paste was obtained. The mixture was used for DSC studies, and a part was cured at 140 °C for 1 h and at 200 °C for 4 h and used for TGA studies. 2.5.3. Sample 3. One gram of DDEBMI, 2 g of MAMAENR, and 1.5 mL of dioxane were charged in a beaker, placed in an oil bath, and heated at 80 °C when a homogeneous mixture was obtained. The mixture was poured into an aluminum pan, placed in a vacuum oven, and heated at 100 °C, when the solvent was removed. A part of the obtained mixture was used for DSC studies, and another part was cured as above and used for TGA studies. 3. Results and Discussion The chemical reactions involved in these syntheses are presented in Schemes 1and 2. 3.1. Synthesis of Formaldehyde Resins (FR). FR resins with different molar ratios of initial monomers were synthesized by copolymerization with formaldehyde in the presence of acidic catalyst. The condensation conditions and the main characteristics of the resins are presented in Table 1. The obtained resins are solid brittle products with colors varying from pale reddish orange for FR with more o-Cz monomers to reddish brown for FR with more AN. The resins are soluble at room temperature in medium and high polar solvents (DMF, dioxane, NMP, DMSO) and insoluble in ethers (ethylic, petroleum) and n-hexane. The chemical reactions involved in the synthesis of these resins present in the first step an aminobenzyl alcohol intermediate. Under the action of temperature, CH2OH groups react with the enolic forms of CHx and split off the water, resulting in Mannich mono- and dibases.39,40 In the second step in the presence of o-Cz, CH2O, and catalyst, under the effect of temperature, the Mannich base becomes unstable and rearranges into oligomers with methylene bridges between aromatic or aromatic/aliphatic rings, resulting in the formaldehyde resins.41 The chemical structures of FR resins were confirmed using elemental analysis (% N), 1H NMR, and FTIR spectroscopic methods. The FT-IR spectrum (Figure 1a, sample FR 1, Table 1) shows a strong absorption band in the range of 3190-3450 cm-1 specific to OH and NH2 groups

Figure 1. FT-IR spectra for (a) formaldehyde resin (AN/o-Cz/CHx, 1/1/ 1), (b) epoxy formaldehyde resin (AN/o-Cz/CHx/ECl, 1/1/1/3), and (c) allyl maleate epoxy formaldehyde resin (AN/o-Cz/CHx/ECl/MAMA, 1/1/1/3/3).

located in AN and o-Cz moieties. At 2858 and 2930 cm-1 an intense band (more intense for the resins with more CHx in composition), specific to CH2 and CH3 groups located both in the CHx ring and in methylene bridges between aromatic and aliphatic rings and bonded to the benzene ring of o-Cz, is observed. At 1705 and 1890 cm-1 the peaks specific to the CO group in cyclohexanone ring are evident. The peaks specific to the aromatic ring are located in the range of 1510-1620 cm-1, and at 754, 815, and 840 cm-1, which are specific to parasubstituted benzene. The chemical structure was also confirmed by high-resolution 1H NMR and 13C NMR spectra of FR. In Figure 2a (1H NMR spectra, sample FR 1, Table 1), the CH3 protons bonded to o-Cz appear as a singlet in the range of 1.62 ppm and the protons assigned to CH2 groups located in the CHx ring appear as multiplets in the range of 1.62-2.14 ppm chemical shift. The methylene bridges between aromatic or aromatic/aliphatic rings specific to formaldehyde resins are presented as a single peak at 2.51 ppm chemical shift and are overlapped by the peak of solvent (DMSO). The primary amine protons presented in the aniline moiety appear at the singlet at 3.64 ppm. The aromatic protons assigned to AN and o-Cz appear as complex multiplet peaks in the range of 6.66-7.02 ppm, and chemical shifts at 7.5-7.64 ppm denote the aromatic protons ortho to the amino groups. The chemical structures were confirmed also by 13C NMR. In the 13C NMR spectra the characteristic signal attributed to CH3 bonded to the o-cresol ring appears in the range of 15 ppm chemical shift while the peaks assigned to the CH2 group from cyclohexanone moieties are presented in the range of 38.8-40 ppm chemical shift. The peak assigned to the CH2 located in the neighborhood of the CO group is presented at 42 ppm chemical shift and is overlapped by the solvent signal. The peaks specific to CH groups of the aromatic rings can be seen in the range of 114-126 ppm, and the methylene group linked between aromatic and aliphatic rings is presented in the range of 128-131 ppm. The signal presented at 146 ppm can be attributed to C-NH2 and the signal

Ind. Eng. Chem. Res., Vol. 47, No. 5, 2008 1359

Figure 2. 1H NMR spectra of (a) formaldehyde resin (AN/o-Cz/CHx, 1/1/1), (b) epoxy formaldehyde resin (AN/o-Cz/CHx/ECl, 1/1/1/3), and (c) allyl maleate epoxy formaldehyde resin (AN/o-Cz/CHx/ECl/MAMA, 1/1/1/3/3).

of C-OH is presented in the range of 155 ppm chemical shift, respectively. The peak specific to the CO group situated in the cyclohexanone moiety appears at 211 ppm chemical shift. 3.2. Synthesis of Epoxy Formaldehyde Resins (EFR). The chemical reactions associated with the synthesis of novolak epoxy resins are shown in Scheme 2. The conversion of OH and NH2 groups to epoxy groups proceeds to about 90% in 8 h if epichlorohydrin/novolok resin is used in a molar ratio 1/0.1, in the presence of TEBAB at 85-90 °C (Table 1). The structure of obtained epoxy novolak resins was confirmed by the epoxy equivalent weight, as well as by FT-IR and NMR spectra. In the IR spectra (Figure 1b, sample EFR 1, Table 1) the peak specific to the epoxy ring appears at 915 cm-1. As a consequence of reaction between NH2 and ECl, in the range of 3450 and 1350 cm-1 appear the peaks specific to C-N-C groups. In the 1H NMR spectra (Figure 2b, sample EFR 1, Table 1), the glycidyl structures are confirmed by the peaks located in the range of 2.51-3.84 ppm chemical shift (2.51 ppm specific to CH2 from the epoxy ring, 3.42 ppm specific to CH from the epoxy ring, and 3.62 and 3.88 ppm specific to O-CH2 groups). In the 13C NMR spectra the C-O-C group from the epoxy ring is presented in the range of 50 ppm chemical shift. The value of the epoxy equivalent weight represents a yield of approximately 93% based on the total number of NH2 and OH groups. This result confirms a relatively high conversion of NH2 and OH group reaction with epichlorohydrin.

3.3. Synthesis of Multiallyl Maleate Resin (MAMAENR). MAMAENR was obtained by the reaction between multifunctional novolak epoxy resins obtained as above with MAMA in the presence of TEBAB as catalyst (3% based on MAMA weight). The chemical reaction between the epoxy ring and the carboxyl group was recorded by watching the decrease of the acid index values and the intensity of the absorption peak at 915 cm-1 in IR spectra (Figure 1c, sample MAMAENR 1, Table 1) specific to the epoxy ring. This fact is confirmed by the increase of the peaks specific to the tertiary OH group and the ester group situated in the range of 3450-3500 and 1150 cm-1, which appear as a consequence of the above-mentioned reaction. On the other hand, the peaks assigned to the ester group are presented in the range of 1715 cm-1 (symmetrical CdO stretch) and in the range of 1248 cm-1 (C-O-C stretch). The allylic double bonds introduced by MAMA also appear in the range of 1647-1658 cm-1. In the 1H NMR spectra (Figure 2c, sample MAMAENR 1,Table 1) the protons assigned to CHx moieties are presented as broad peaks in the range of 0.9-1.8 ppm chemical shift. The peaks situated in the range of 3.7-3.92 ppm chemical shift can be assigned to the glycerol moieties which appear as a consequence of reaction between the epoxy ring and the COOH group of MAMA. In the range of 5.25 ppm (CH2 protons) and 5.8 ppm (CH protons) the allylic double bounds are presented, and in the range of 6.5-7.15 ppm chemical shift, a lot of peaks are presented and can be assigned to the aromatic protons from AN and o-Cz. The minor peaks

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Figure 3.

13C

NMR spectrum of MAMAENR 1.

Figure 4. FT-IR spectrum for cross-linked DGEBA/CPMI/MAMAENR 1 system.

present in the range of 7.6-7.8 ppm chemical shift can be attributable to the aromatic protons in the ortho position of AN. In the 13C NMR spectra (Figure 3, sample MAMAENR 1, Table 1) CH3 groups of o-Cz appear in the range of 16.2 ppm while the CH2 groups and methylene bridges appear in the range of 28 ppm chemical shift. A signal at 45 ppm chemical shift can be attributed to the tertiary C where OH (appearing from the reaction between the epoxy ring and a MAMA proton) is linked. The double bonds of maleic and allylic type are presented in the range of 119 and 126 ppm chemical shifts and are probably overlapped by the C-H signals of aromatic rings. 3.4. Curing of MAMAENR. It is well-known that polymaleimides are good matrix resins in the area of composites, but they have a major disadvantage because the cross-linked products are brittle. For the improved toughness, these resins are mixed before cross-linking with allylic derivatives, diamines, olefinic compounds, cyanate ester resins, epoxy resins, etc. The modified resins obtained in the present work contain allylic and maleic double bonds as cross-linkable groups. The curable systems were obtained by mixing the synthesized multifunctional resins with bismaleimides (obtained by direct synthesis or in situ by reaction between CPMI and epoxy resin) and DDM at the molar ratio 1/1 (allyl proton/maleimide group and allyl proton/maleate group) and cross-linked under the action of

Figure 5. DSC thermograms for (a) (CPMI + DGEBA + MAMAENR 1) and (b) (DDEBMI + MAMAENR 1) at heating rate of 10 °C/min.

temperature. The most important literature data show that the proposed chemical reactions for the curing, which took place

Ind. Eng. Chem. Res., Vol. 47, No. 5, 2008 1361 Table 2. Kinetic Parameters for Cured Multiallyl Maleate Resins from DSC Scans (Equivalent of Amine Hydrogen Atoms/Epoxide and/or Maleimide Equivalent ) 1/1)a obtained resins (molar ratio of monomers AN/o-Cz/CHx/ p-FA/ECl/MAMA) sample (mol/mol) 1 2 3 4 5 6 7 8

1/1/1/3/3//3 (MAR 1) 1/1/1/3/3//3 (MAR 1) 1/1/1/3/3//3 (MAR 1) 1/2/1/4/4/4 (MAR 2) 1/2/1/4/4/4 (MAR 2) 1/2/1/4/4/4 (MAR 2)

activation energy of curing reaction (kJ‚mol-1)

heating rate (°C‚min-1)

water absorption (%)

first exotherm second exotherm first exotherm second exotherm formulation (mol/mol) DDEBMI DGEBA/CPMI (1/2) DGEBA/CPMI/MAR 1 (1.5/3/1) DGEBA/DDM/MAR 1 (1.5/1.5/1) DDEBMI/MAR 1 (1.5/1) DGEBA/CPMI/MAR 2 (1/2/0.5) DGEBA/DDM/MAR 2 (2/2/1) DDEBMI/MAR 2 (2/1)

5 10 15 T M T M TM

5 TM

284 106 101 79 145 102 76 142

190 207 160 200 206 149 198

298 117 126 93 157 120 96 156

182 125 132 103 171 127 107 167

10 TM 202 227 165 217 224 155 210

15 TM

ref 27 ref 26

206 245 174 240 238 170 229

94.20 72.74 51.47 54.31 67.52 53.27 38.70 66.95

ref 27

Tgb ref 26 (°C)

86.23 182 66.30 120.05 113.57 167 41.65 64.61 51.87 153 44.36 131.53 113.23 144 61.56 54.69 46.10 157 47.61 70.03 61.81 158 32.68 74.33 67.08 151 59.85 63.90 56.29 163

c

d

0.44 0.98 1.38 1.23 0.60 1.55 1.59 0.51

0.41 0.78 1.11 0.97 0.47 1.19 1.13 0.43

a T , maximum peak temperature (°C). b Measured after cross-linking at 120 °C, 1 h, and at 200 °C, 4 h, by DSC method, at a heating rate of M 10 °C‚min-1. c Measured after 14 days at 25 °C. d Measured after 1 h at 100 °C.

in the obtained systems, implied homopolymerization of double bonds of maleic and allylic type, Diels-Alder reaction, alternating copolymerization,“ene” reaction type, and conversion of the maleimide moieties in succinimide groups1,4-8,11-14,42-44 (Scheme 2). In the new systems, whereas the cross-linking reaction took place in the presence of DDM, a new chemical reaction can appear, namely Michael addition, which implies the transfer of amine protons to the oxygen of maleic ester moieties, followed by enol-keto tautomerization.45 In the systems where CPMI and epoxy resins are presented, the most important reaction took place between epoxy rings and carboxylic groups. The reaction between carboxylic proton or amine protons and the epoxy ring has been clarified by literature data.46-50 In the first step at low temperature, the carboxylic proton reacts with the epoxy ring and tertiary OH groups appear. In the second step, at high temperature, the OH protons can react with the epoxy ring or with the carboxylic group and/or etherification reaction and esterification reaction can take place. After the precuring reaction between the epoxy ring and the carboxylic proton, the main curing reaction will occur between allyl proton and maleimide double bonds via Michael addition reaction and aspartamide

groups appear. Also, it is possible, with the increase of temperature, for the polymerization and copolymerization reaction between allylic and maleic double bonds to be performed. These possible reactions are confirmed by the IR spectra of cross-linked polymers (Figure 4) (missing the peak at 915 cm-1, the peak specific to epoxy ring, and the increase of the absorption peak in the range of 3400 cm-1, specific to tertiary OH groups). Moreover, formation of covalent bonds among maleimide functional groups is confirmed by the increase of the intensity of the absorption peak at 1178 cm-1 along with the decrease of the absorption peak of the maleimide ring at 1135 cm-1. All kinds of chemical reactions can exist simultaneously, resulting in highly cross-linked polymers. The thermal cross-linking behaviors of DGEBA/CPMI/ MAMAENR 1 and 2, DGEBA/DDM/MAMAENR 1 and 2, and DDEBMI/MAMAENR 1 and 2 systems were investigated using the DSC technique. As can be seen in Figure 5 and Table 2, the cross-linked reaction (at 10 °C‚min-1), for the systems with MAMAENR 1 and 2 in composition, presents two exotherms. The first exotherm for systems with MAMAENR 1 is centered in the range of 72-225 °C, and the second exotherms are

Figure 6. TGA thermograms for cured resins: (b) (DDM + DGEBA + MAMAENR 1), (O) (CPMI + DGEBA + MAMAENR 1), (g) (DDM + DGEBA + MAMAENR 2), (2) (CPMI + DGEBA + MAMAENR 2), and (9) (CPMI + DGEBA).

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Figure 7. TGA thermograms for cured resins: (+) (DDEBMI + MAMAENR 1), (4) (DDEBMI), and (0) (DDEBMI + MAMAENR 2).

centered in the range of 160-245 °C, as a function of heating rates and chemical composition (the maximum peak temperatures obtained for these systems are given in Table 2). The first exotherms of the systems with MAMAENR 2 is also centered in the range of 76-167 °C, and the second exotherm is centered in the range of 149-238 °C. The first exotherm can be attributable to the Michael addition reaction (for DDEBMI/ MAMAENR 1 and 2 systems) and the reaction between amine protons and the epoxy ring and the second exotherm can be due to esterification of OH groups and to auto-cross-linking of double bonds of maleic and allylic types (for DGEBA/CPMI/ MAMAENR 1 and 2). The exothermic temperature peak is specific to each system and has the minimum value for DGEBA/ DDM/MAMAENR 1 system (Table 2). These data show that, for all systems, the maximum peak temperature is shifted to great values with the increase of heating rate. Using eqs 1 and 2, the activation energy of polymerization and the preexponential factor were calculated (Table 2). The obtained energies have values comparable with the literature data.12,32,45,46 From Table 2, it can be seen that the energy of polymerization does not essentially depend on the chemical structure of the formaldehyde resins. Anyway, the system with MAMAENR 1 in composition has small values for the activation energy of the curing reaction in comparison with the systems with MAMAENR 2.This may be due to the fact that MAMAENR 2 resins possess many double bonds in comparison with MAMAENR 1. The activation energy of curing reactions is situated in the range of 39-68 kJ‚mol-1 (calculated from the first exotherm) and between 55 and 132 kJ‚mol-1 (calculated from the second exotherm), depending on the chemical structure of systems. The higher values of activation energies of the curing process obtained from the second exotherm peaks demonstrate that this process is composed by a sum of the exotherm effect of autopolymerization of double bonds from maleimide moieties, of Michael addition of amine to double bonds from maleimide, and of the esterification process of OH groups with residual epoxy ring. These values are in agreement with literature data reported for multifunctional epoxy maleimide resins.11,29,30,34,35,51 The influence of structure of these cured modified resins upon their thermal behavior was also examined by measuring the glass

transition temperatures (Tg’s). As can be seen in Table 2, the Tg values of cross-linked resins with MAMAENR 2 in structure are higher in comparison with the resins with MAMAENR 1. This can be attributed to the fact these resins have high functionalities and induce a high density of cross-linking resulting in rigid structures in comparison with resins with MAMAENR 1. On the other hand, the MAMAENR 2 systems have more OH groups (hydrophilic in nature) and have large moisture absorption in comparison with systems containing MAMAENR 1. 3.5. Thermal Behavior of the Cured MAMAENR Systems. Six kinds of cross-linked resins were obtained: epoxy resins cross-linked with CPMI and MAMAENR 1 and 2, epoxy resins cross-linked with DDM and MAMAENR 1 and 2, and MAMAENR 1 and 2 cross-linked with DDEBMI. The thermal stability of cross-linked resins was evaluated by TGA analysis. The TGA curves are presented in Figures 6 and 7, and the main parameters of the degradation process are presented in Table 3. The relative thermal stability of cross-linked resins was quantitatively estimated using TG parameters (T10, temperature at 10% weight loss; T50, temperature at 50% weight loss; WL500, weight loss at 500 °C) and the activation energies of the degradation process (obtained with eqs 3 and 4). As can be seen in Table 3, for DGEBA/CPMI/MAMAENR 1 cross-linked system (samples 3 and 4), T10 is situated in the range 339-370 °C and T50 has values in the range 515-525 °C, whereas the resins with DGEBA/CPMI/MAMAENR 2 (samples 6 and 7) have smaller values with 5 °C for T10 and with 20 °C for T50. These values are comparable with the literature data.51 This can be attributable to the aliphatic structure of FR (with more CHx in composition), which is less stable in comparison with FR with aromatic moieties in the structure. On the other hand, the systems with MAMAENR 2 (samples 7 and 8, Table 3) containing more OH groups in comparison with the systems with MAMAENR 1 (samples 4 and 5, Table 3) displayed slightly less thermal stability as a consequence of increased content of OH groups which are more easily degradable. The char yield for resin cross-linked with CPMI and DDEBMI enhances with the increase of CPMI and DDEBMI content. The increase of CPMI and DDEBMI moieties has a significant effect

Ind. Eng. Chem. Res., Vol. 47, No. 5, 2008 1363 Table 3. Thermal Parameters of Cross-Linked Resins temperature (°C) at wt loss sample

formulation

10%

50%

WL500 (%)

ln Aa (min-1) ref 25

1 2 3 4 5 6 7 8

DDEBMI DGEBA/CPMI (1/2) DGEBA/CPMI/MAR 1 (1.5/3/1) DGEBA/DDM/MAR 1 (1.5/1.5/1) DDEBMI/MAR 1 (1.5/1) DGEBA/CPMI/MAR 2 (1/2/0.5) DGEBA/DDM/MAR 2 (2/2/1) DDEBMI/MAR 2 (2/1)

375 380 339 360 370 335 355 364

540 530 515 525 520 400 505 506

42 41 45 47 45 56 47 48

8.14 8.78 10.49 7.48 7.50 5.41 4.95 7.69

a

reaction order ref 25 1.08 1.22 1.35 0.94 1.14 0.82 0.53 0.96

decomposition activation energy (kJ‚mol-1) ref 25

ref 24

60.26 62.60 70.29 55.40 54.41 53.05 41.43 55.98

50.05 48.63 60.05 47.56 43.13 52.87 38.73 44.55

Preexponential factor.

upon the activation energies of the degradation process. Accepting the T10, T50, WL500, and activation energies of the degradation process as the criteria of thermal stability, the crosslinked resins, which contain MAMAENR 1, can be considered more stable in comparison with the systems which contain MAMAENR 2. The activation energies of the degradation process (Table 3) have values situated in the range 40-70 kJ/ mol, comparable with literature data.30-32,34 3.6. Moisture Absorption. The moisture absorption of cured multifunctional systems are presented in Table 2. The presence of CPMI and epoxy resin in the MAMAENR systems (samples 3 and 6, Table 2) produces after cross-linking a large number of OH groups which are hydrophilic in nature. These systems exhibit a great moisture absorption in comparison with the systems cross-linked with DDEBMI (samples 5 and 8, Table 2), systems that do not contain OH groups (samples 5 and 8, Table 2), and have a moisture absorption comparable with the literature data for the bismaleimide O,O′-diallyl-bisphenol A systems.43 4. Conclusions Thermosetting resin systems were obtained in which the formaldehyde resins were epoxidated and modified with allyl maleate, and thermally cured in the presence of BMI, DGEBA, and DDM. The cross-linked resins showed good thermal stability, high glass transitions, and low water absorption. These resins may be used as structural adhesives and matrixes in glass fabric-reinforced composites and in the field of the electrical and electronic industry. Literature Cited (1) Liang, G.; Gu, A. New Bismaleimide Resin with Improved Tack and Drape Properties for Advanced Composites. J. Appl. Polym. Sci. 1997, 64, 273. (2) Morgan, R. J.; Jurek, R. J.; Yen, A.; Donnelan, T. Toughening Procedures, Processing and Performance of Bismaleimide-Carbon Fibre Composites. Polymer 1993, 34, 835. (3) Chattha, M. S.; Dikie, R. A.; Carduner, K. R. Epoxy-Modified Diallylbisphenol A and Bis(maleimidophenyl)methane Thermoset Compositions: Composition and Dynamic Mechanical Thermal Analysis. Ind. Eng. Chem. Res. 1989, 28, 1438. (4) Phelan, J. C.; Sung, C. S. P. Cure Characterization in Bis(maleimide)/ Diallylbisphenol A resin by Flourescence, FT-IR, and UV-Reflection Spectroscopy. Macromolecules 1997, 30, 6845. (5) Phelan, J. C.; Sung, C. S. P. Flourescence Characteristics of Cure Products in Bis(maleimide)/Diallylbisphenol A Resin. Macromolecules 1997, 30, 6837. (6) Chattha, M. S.; Dikie, R. A. Dynamic Mechanical Analysis of Bismaleimidodiphenyl Methane and Diallylbisphenol-A Crosslinked Polymers. J. Appl. Polym. Sci. 1990, 40, 411. (7) Mijovic, J.; Andjelic, S. Study of the Mechanism and Rate of Bismaleimide Cure by Remote in Situ Time Fiber Optic Near Infrared Spectroscopy. Macromolecules 1996, 29, 239.

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ReceiVed for reView April 26, 2007 ReVised manuscript receiVed October 11, 2007 Accepted December 23, 2007 IE0705913