Eleventh International Conference of the Polymer Processing Society, 1995?
S F Bush, with J M Methven, D R Blackburn and M Esfandeh
Introduction
Moulding compounds are generally mixtures of a cross linkable resin, a polymerisable monomer, fillers and discrete fibres, and a thickening agent. The most common composition is one in which the resin is an unsaturated polyester with residual carboxylic end groups, the monomer is styrene and the fibres are glass. The thickening agent is designed to turn a viscous fluid into a leathery sheet which can be rolled up, transported, and cut to fit a compression mould. The thickening agents are conventionally finely dispersed Group II metal oxide powders such as MgO. Among other interations hydrogen bonding occurs between -OH groups on the powder surface and the carboxylic acid groups in the resin. This bonding creates a complex network of resin and powder which typically increases the viscosity of the mixture from under 1 Pas to more than 104 Pas over 48 hours. The bonding achieved is very sensitive to the concentration of acid groups, moisture content, and MgO particle size, thus imposing appreciable constraints on the production process. When a sheet of this material is placed in a mould and heated above about 110-120 oC the oxide-resin network dissolves and the resin flows freely through the mould cavity.