Synthesis and characterisation of epoxy–novolac/bismaleimide networks

Abstract

Epoxy–novolac resin was modified with 1,1^′-(methylenedi-4,1-phenylene) bismaleimide (BMI), and cured with an aromatic amine. Cure behaviour of these blends was studied using differential scanning calorimetry (DSC). The thermograms indicated a unimodal exothermic peak for blends containing lower percentage of BMI. The cured blends showed much higher glass transition temperatures than that of the unmodified epoxy. Thermal stability of the cured epoxy resin was also improved with BMI addition. A homogeneous structure (with no phase separation) of the blends was confirmed both by DSC analysis and scanning electron microscopy (SEM).

Introduction

Epoxy resins are the most versatile class of thermosetting polymers among the contemporary plastics. Because of their versatility, they are used as adhesives for honeycomb structures, automobiles and for fabric-cut prototype moulds. Epoxy based solution coatings are used as maintenance and product finishes. They are also used as additives for a variety of other plastic materials such as vinyl, acrylic resins, natural and synthetic rubbers.

The use of epoxy resins in high performance structural materials has been increasing recently. Multifunctional epoxies are used in applications for high performance adhesives and advanced composite matrix materials in the aerospace and electronic industries. The combined mechanical properties of high modulus and high strength at relatively high temperatures make the material suitable for advanced applications. These resins can be easily modified to get tailor-made materials. However, the major draw back of epoxy resins are the brittleness and moisture absorption. The first problem can be overcome by toughening the resin by incorporating a second component which is rubbery in nature. The addition of these elastomers enhances the impact resistance of the epoxy resins. The toughening of epoxy resins by carboxy or amino terminated butadiene acrylonitrile and other thermoplastics has been studied extensively by researchers [1], [2], [3], [4], [5], [6]. However addition of these elastomers causes reduction in glass transition temperature (T_g) and also lowers the hot–wet property retention.

To overcome the deterioration in properties due to moisture uptake recently a new approach has been practiced consisting of a formulation of themoset/themoset blend to balance between the thermal and mechanical properties of the resin systems [7], [8], [9], [10], [11], [12]. These systems belong either to the class of intercrosslinked or interpenetrated networks. Owing to these types of structures these networks exhibit synergistic effects which result in the improvements in some properties with respect to those of individual components [11]. However, these systems are highly viscous and are difficult to process.

In the present study modification is carried on epoxy–novalac (EPN) resin having moderate functionality using 1,1^′-(methylenedi-4,1-phenylene) bismaleimide (BMI). BMI exhibits a superior rigidity to the epoxy and hence shows higher T_g and thermo oxidative stability. On the other hand it is very crystalline and difficult to process as the thermal curing starts just above the melting point. A suitably blended EPN and BMI should exhibit higher T_g and less moisture uptake than epoxy and much better processing than that of BMI. This work describes the preparation of blends containing EPN/BMI characterisation of the blends through scanning electron microscopy (phase behaviour), cure and thermal behaviours, using differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA).