The cure kinetics and activation energy (Ea) of bismaleimide homopolymer and modified bismaleimide resin systems with different chain extenders were investigated. The bismaleimide resin under investigation was bismaleimidodiphenyl methane (BMPM) and the chain extenders were (i) O-O′ diallyl bisphenol A (DABA) and (ii) methylenedianiline (MDA). Dynamic multiheating DSC method was used to study the kinetics of the curing process. Activation energies were determined for both unmodified and modified resins from nonisothermal multiheating rate DSC tests by using Ozawa and Kissinger models. Activation energy for BMPM homopolymer increased from 95 kJ/mol to 125 kJ/mol as a function of conversion. For the MDA-modified system the activation energy was independent of percentage conversion, at 108 kJ/mol. In the case of DABA-modified bismaleimide the activation energy increased steadily at 6 kJ/mol from 10 to 100% conversion.
High performance thermosets are of great interest as matrix resin for composites. Bismaleimide- (BMI-) and Polyimide- (PI-) based systems are among the more thermally stable thermosetting resins that are fast replacing the widely used epoxy resins. The unique properties of BMI resins, such as low moisture absorption, high crosslink density, good chemical resistance, and high glass transition temperature (Tg) make these resins suitable for prepregs, adhesives, electrical packaging, and other composite applications [
According to the principles of molecular design, introducing a flexible linkage or chains into BMI can effectively improve the toughness of the resin [
The objective of this work was to compare the curing kinetics of different chain extended BMI resins. The studies were focused on homopolymerisation of (i) 4,4′ bismleimidodiphenyl methane (BMPM), which is one of widely used resins in the bismaleimide (BMI) category (ii) 4,4′ bismleimidodiphenyl methane (BMPM), with O-O′-diallyl bisphenol A (DABA), and (iii) 4,4′ bismleimidodiphenyl methane (BMPM) with Methylenedianiline (MDA). Nonisothermal multiheating rate method was adopted to determine the kinetic response and activation energies of BMI resin systems.
The resin systems 4,4′ bismleimidodiphenyl methane (BMPM) resin (Figure
Chemical structure of BMI resin.
Chemical structure of O-O′diallyl bisphenol A (DABA).
Chemical structure of methylenedianiline (MDA).
The thermal data were obtained by using Mettler 823 differential scanning calorimeter (DSC) calibrated with an Indium standard. The thermal data of the resin was based on nonisothermal, multiheating rate method. The sample was placed in the DSC cell and equilibrated at 40°C. The samples were heated at different heating rates (5, 10 and 20°C/min). A stream of nitrogen at a flow rate of 20 mL/min was used to purge the DSC cell.
Cure kinetics of resins systems are studied using non-isothermal and isothermal methods, the former method includes a single and multi-heating rate and the later involves studying the reaction mechanism at a specific temperature as a function of time. The nonisothermal single heating rate method measures the curing process at a constant heating rate, while the multiheating rate method is an isoconversional method and is suitable for systems with multiple reactions. The two kinetic analysis models are extensively used to understand and predict the cure behaviour of the resin systems. The basic assumption for the application of DSC technique to the cure of the thermosetting resins is that the rate of kinetic process (
Figure
Key temperature values from DSC plot.
Heating rate (°C/min) | Endotherm temperature (°C) | Initial cure temperature (°C) | Peak exotherm temperature (°C) | Percentage conversion at peak temperature |
---|---|---|---|---|
BMPM | ||||
5 | 162 | 180 | 220 | 58% |
10 | 162 | 200 | 240 | 60% |
20 | 162 | 210 | 250 | 59% |
BMPM with DABA (1 : 1) | ||||
5 | 120 | 188 | 235 | 55% |
10 | 120 | 200 | 255 | 59% |
20 | 120 | 217 | 260 | 57% |
BMPM with MDA (3 : 2) | ||||
5 | 90 | 180 | 214 | 56% |
10 | 90 | 188 | 226 | 56% |
20 | 90 | 198 | 233 | 58% |
DSC plots of BMPM, BMPM-DABA, and BMPM-MDA at different heating rates.
The DSC plots precisely follow the assumptions stated in the MFK model. In the case of BMPM the endotherm temperature appeared exactly at 162°C irrespective of the heating rate; however, the magnitude of heat flow corresponding to endotherm peak was more intense at –3, −7, and –10 mW, signifying a complete melting at a heating rate of 20°C/min. As expected and tabulated in Table
The shift in the peak exotherm temperature as a function of heating rates is tabulated in Table
The pure bismaleimide (BMPM) resin undergoes only homopolymerization while curing and this occurs above 180°C and continues up to 295°C. This is the reason behind the relatively sharp peak of the curing reaction. In the second resin system, comprising of BMPM-DABA, it is observed that the peak temperature of the reaction shifts by 15°C to higher temperature range. This is because bismaleimide undergoes different chemical reactions with DABA, like Ene reaction and Diels-Alder reaction, while curing as shown in Figure
Proposed curing mechanisms of BMI/DABA resins.
Proposed curing mechanisms of BMI/MDA resins.
The percentage conversion for different heating rates as calculated from the DSC data is shown in Figure
Percentage conversion of BMPM, BMPM/DABA, and BMPM/MDA as a function of temperature.
The multiheating rate technique adopted in this study is an isoconversional method that assumes that the activation energy (
The activation energy,
Activation energy (
Each data point on the
The
In case of BMPM where complex curing reaction occurs, the activation energy remains constant at 100 kJ/mole up to 60% conversion. Beyond 60% conversion the
The cure kinetics of bismaleimide resins with different chain extenders were investigated by conventional DSC under nonisothermal multi heating rate method. In the case of BMPM/DABA, the activation energy increased steadily from 55~60 kJ/mol to 110~115 kJ/mol, as a function of conversion following the model-free kinetics model, while in the case of BMPM/MDA resin system the activation energy remained constant at 108 kJ/mole as per the
The authors wish to thank the authorities of Naval Research Board, New Delhi, for providing the financial support for this research work under the project titled “innovative approaches for improving the hot-wet performance of bismaleimide/carbon composites” (NRB-134/MAT/07-08).