EFFECT OF MICROSPHERE CONTENT ON FIRE PERFORMANCE AND THERMOMECHANICAL PROPERTIES OF PHENOLIC RESOLE SYNTACTIC FOAM COMPOSITES

摘要

In recent years, there has been a growing interest in the use of syntactic foam [1, 2] for many thermostructural applications. They exploit several features typical of syntactic foams such as the good thermal insulation properties and the high specific mechanical properties. Syntactic foams are composite materials in which hollow microspheres are dispersed in a polymer binder. The hollow spheres may be made up of glass, metals, polymers or ceramics. The structure of a syntactic foam is entirely different from the conventional cellular foams like polyurethanes, PVC etc. In syntactic foams majority of voids are enclosed within rigid walls and therefore isolated from each other and hence are called "closed-cell foams". Syntactic foams are essentially isotropic materials due to the randomness of the microstructure. Otherwise, there is an increased awareness for the need to improve fire standards in these areas where public safety is important. An approach to improving the fire performance of these composites is through the use of resins that are less susceptible to heat and flame. In this way, the use of phenolic syntactic foams seems to be a better compromise of mechanical strength, isolation and fire behaviour. The phenolic resins are a class of compounds that have superior fire and flammability properties compared to other thermoset resins such as the polyesters and epoxies. Because of their unique chemical structure which primarily comprises C-C bonds in aromatic rings, phenolic resins have intrinsic resistance to ignition, low generation of smoke and relatively low cost [3]. When phenolic resin based structures do burn, a porous carbon char is rapidly formed, as an effect of the thermal degradation reaction, which insulates and protects underlying material. Phenolic polymers, characterized by high aromatic ring content, decompose into aromatic fragments that fuse via condensation reactions to produce a high amount of char. It is known that 40-60% of the resin mass is transformed to char, resulting in a much lower yield of flammable volatiles compared to many other polymers (epoxy, polyesters). The char acts as a thermal insulation layer because the thermal conductivity of char can be lower than the conductivity of the virgin composite material [4]. For example, Fanucci [5] reports that the thermal conductivity of solid char at room temperature is 0.17 W.m~(-1)K~(-1). Low density and highly porous chars tend to provide the best thermal insulation. The char layer reduces the conduction of heat to the underlying virgin material and thereby slows the decomposition reaction of the polymer matrix. An additional valuable property derived from the phenolic resin is the ability to retain its physical properties at high temperatures [4]. Due to their fire properties, it might appear reasonable to expect phenolic syntactic foam to show good structural performance in fire.