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The Curing Reaction and Glass Transition Temperature of Maleimide Resin Containing Epoxy Groups

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The Curing Reaction and Glass Transition Temperature of Maleimide Resin Containing Epoxy Groups Polymer Journal, Vol. 20, No. 2, pp 125-130 (1988) The Curing Reaction and Glass Transition Temperature of Maleimide Resin Containing Epoxy Groups Akira NAGAI, Akio TAKAHASHI, Motoyo WAJIMA, and Kenji TSUKANISHI * Hitachi Research Laboratory, Hitachi Ltd., 4026 Kujo-cho, Hitachi, lbaraki 319-12, Japan *Shimodate Works, Hitachi Chemical Co., Ltd., 1500 Ogawa, Shimodate, lbaraki 308, Japan (Received July 15, 1987) ABSTRACT: Investigation was made of the glass transition temperature (T.) and the curing reaction mechanism of a new maleimide resin containing epoxy groups. It was found that the T. of the cured product depended on the reaction temperature and a good heat-resistant product could be obtained under appropriate conditions. This T• was above 200°C. The reaction mechanism is discussed using measured activation energy and infrared spectra. The primary reaction changed at about 180°C. Both the addition of diamine to the double bond and epoxy group occurred below 180°C. The polymerization of double bonds of bismaleimide took place simultaneously with the above-mentioned two reactions above 180°C. KEY WORDS Maleimide / Glass Transition Temperature / Activation Energy / Curing Reaction Mechanism / The maleimide resin formed from bismale­ ethanol or 2-butanone and can be final-cured imide and diamine is a good heat-resistant at a low temperature in a short reaction time. polymer. It is suitable for use over a wide This cured product has very good properties, region as a composite material since it does not similar to those of general maleimide resins. produce volatile byproducts which cause Now, we have studied the relationship be­ voids, as other conventional polyimides do. In tween the curing conditions and glass tran­ particular, it has been applied as multilayer sition temperature ( Tg) of products derived printed wiring boards for large-scale com­ from this new prepolymer. The curing reaction puters and FRP for aerospace materials. 1 - 3 mechanism of the prepolymer containing sev­ Generally, though, maleimide prepolymers can eral different functional groups was also ex­ be dissolved only in high boiling point solvents amined. such as N-methyl-2-pyrrolidone and have a poor curability which needs a high tempera­ EXPERIMENTAL ture and long reaction time. Therefore they have poorer workability than epoxy or phenol Reagents resins which have also been applied to the N,N' -Bismaleimide-4,4' -diphenylmethane same use. (BMI, Mitsui Toatsu Chemical) was used as Several years ago, we developed a new bismaleimide component. 4,4' -Diaminodi­ maleimide resin containing epoxy groups.4 phenylmethane (DDM, Mitsui Toatsu Chem­ This new prepolymer can be dissolved in low ical) and 2,4-diamino-6-phenyl-1,3,5-triazine boiling point solvents, such as 2-methoxy- (BG, Tokyo Kasei) were used as aromatic di- 125 A. NAGAI et al. amine components. Phenol novolac type ep­ value in the curing curve, and the activation oxy resin (DEN438, epoxide equivalent 180, energy was obtained from the Arrhenius plot. molecular weight 750, Dow Chemical) was Infrared spectra were obtained using a 215 used for introducing epoxy groups into the type Infrared Spectrometer (Hitachi) by the prepolymer. The solvent for synthesis of the KBr method. The sample was a pressed disk prepolymer was 2-methoxyethanol (Waka contammg prepolymer (2 mg) and KBr Pure Chemical). All these chemicals were (240 mg) powders. Before the curing reaction, used without further purification. a spectrum was recorded to obtain an initial value (D0 ). Next, the sample disk was placed in Synthesis of Aminobismaleimide Prepolymer a heat-bath at the desired temperature for the Three mo! of BMI, I mo! of DDM, and 7.5 curing reaction. The spectrum of the cured mo! (epxoide equivalent) of DEN 438 were disk was measured (D). The ratio (D/D0 ) was mixed in 2-methoxyethanol and heated at regarded as the degree of change caused by the 95°C for 90 min. After cooling to room tem­ curing reaction. perature, I mo! of BG was added to the solution with stirring. Next, the solution was RESULTS AND DISCUSSION dried in vacuo for 24 h to remove the solvent. The aminobismaleimide prepolymer contain­ The Glass Transition Temperature of Cured ing epoxy groups was obtained as a yellow Products powder. The molecular weight distribution of The glass transition temperature (Tg) of the the prepolymer obtained was measured by the aminobismaleimide containing epoxy groups gel permeation chromatography. The prepoly­ was measured at various reaction tempera­ mer contained 0.3 mo! of 2: I (BMI: DDM) tures. The results are shown in Figure 1. When product, 0.3 mo! of 1 : 2 product, 0.1 mo! of the reaction time is short, Tg has a lower value 1 : 1 product, and 2 mo! of unreacted BMI. because of insufficient curing. By adequately These products were reacted each epoxy resin long time curing, the Tg reaches a constant molecule. It was found that there are 0.17 mo! value, showing the product to be final-cured. of double bonds, 0.18 mo! of active hydrogens In this case, the Tg depends on the reaction and 0.17 mol of epoxy groups in 100 g of the temperature and it is not higher than the obtained prepolymer powder. Apparatuses The glass .transition temperature of cured I)~ products was measured with a Thermal 200 /- ~___;;;:. Mechanical Analyzer TMA-1500 (Shinku Riko ). The heating rate was 2°C min - i. The sample was prepared by pre-heating the pre­ polymer powder at l 30°C for 25 min, and then 100 forming a disk of cured product using a mold heated at the desired temperature. The disk had a diameter of 10 mm and a thickness of about 2mm. The curability was measured with a JSR­ 50 100 150 reaction time (min) type Curelastmeter (Japan Synthetic Rubber). Figure 1. Effect of curing reaction time on glass tran­ The vibration angle was 0.5 degree. The curing sition temperature of cured products at various reaction rate was calculated from the slope at the 50% temperatures. O, l 70°C; (), 180°C; (), 200°c. 126 Polymer J., Vol. 20, No. 2, 1988 Maleimide Resin Containing Epoxy Groups reaction temperature. Further crosslinking re­ bismaleimide resins. 5 Moreover, the curing action does not occur because the mobility of time to obtain the final-cured product is just the polymer chain is severally limited below Tg. 40-50 min at 200°C. This is attributed to The Tg of products cured at various tem­ introducing epoxy groups which have a high peratures for 90 min are shown in Figure 2. reactivity toward the aminobismaleimide. T0 The higher the reaction temperature, the obtained the final-cured product at 170cc, higher is the Tg of the cured product and it which is a comparatively low temperature, this reaches the curing temperature value. When prepolymer needs about l00min. These are the the reaction temperature is under 200°C, the same conditions as for conventional epoxy Tg is IO"C higher than the reaction tempera­ resins. But aminobismaleimide, without epoxy ture. It was considered that the sample might groups, shows hardly any reaction at such a be experiencing a higher curing temperature low temperature. This difference is due to than the set temperature due to exothermic epoxy groups which react with diamine and crosslinking reaction. The cured product, with form a crosslinking network in the product its high Tg, is a good heat-resistant thermoset­ structure. ting resin having better properties than usual Reaction Mechanism The curability of the prepolymer was mea­ 250 sured by a curelastmeter. The results are shown in Figure 3. The time at the invariable curelast stress (the reaction time when the degree of curing becomes unity) is the same as the time at which Tg becomes constant as shown in Figure 1. For example, both the degree of curability and Tg are constant at about 40 min for 200°C. At this time, it seems that the curing process is completely finished. 150 According to Figure 3, the higher the reaction 150 200 250 reaction temperature (°C) temperature, the more rapid is the curing rate and the prepolymer is final-cured in a shorter Figure 2. Relationship between glass transition tem­ perature of cured products and reaction temperature for reaction time. a 90 min curing time. The Arrhenius plot which represents the 1D C: ·;:"' ::, 0" ., 0.5 e l reaction time (min) Figure 3. Curing curves at various temperatures. I, l 50°C; 2, l 70°C; 3, l 80°C; 4, 200°C; 5, 220'C. Polymer J., Vol. 20, No. 2, 1988 127 A. NAGAI et al. u u u u u u 10' x 1/T (°C1 ) Figure 4. Arrhenius plots of curing reaction. relationship of the curing rate and curing temperature is shown in Figure 4. The curing rate is obtained from the slope at the half­ cured point in Figure 3. According to Figure 4, 1600 1400 1200 1000 800 600 400 200 the activation energy (E.) change near 180°C wave number (cm-1) and there are different primary reactions for Figure 5. Infrared spectra of prepolymer (A) and different temperature regions. At lower tem­ products cured at 150°C (B), and 220°c (C). perature, a reaction with lower apparent Ea (10.6 kcalmol- 1) occurs mainly. On the other 1) and the ring-opening reaction of the epoxy hand, a reaction of higher apparent Ea (14.0 group by the amino group (eq 2) take place kcal mol - 1 ) proceeds at higher temperature. simultaneously. The value of Ea at the ring­ These reactions are assumed to be the follow­ opening reaction of an epoxy compound by an ing, based on consideration of the prepolymer aromatic amine is 10-12 kcalmol- 1 .6 The composition.
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