Infra-red estimation of interactions between base resin and saturated crystalline additives in the formation of low pressure thermoset moulding blends
June 10th, 1997
Paper to the Polymer Processing Society 13th Annual Meeting, Secaucus, New Jersey, USA, 6k , 10th June 1997.
S F Bush with M Esfandeh and J M Methven
Introduction
Sheet moulding compounds have been established for a considerable time as convenient materials for the compression moulding of large open structures. They have typically been 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. When a sheet of this material is placed in a mould and heated above about 110-120 oC, it softens sufficiently to allow the resin and glass to flow to the boundaries of the cavity, so that a faithful moulding is obtained. Thompson1 describes the use of crystalline unsaturated polyesters such as poly(neopentyl glycol fumarate) as thickening agents in place of the metal oxides. Gibson and Payne2 have described the use of these materials for injection moulding. In 1988 the present authors3 described the use of blends of saturated crystalline additives and crosslinkable base resins such as unsaturated polyesters and uracrylates. These additives are typically polyadipate, polyamide, or polyglycol oligomers with 8-20 repeat units and melting points in the range 40-120 oC. They are selected by their degree of compatibility with the base resin in the molten state. The process and blends are now available commercially.
Process and Blends
The thickened sheets are made by a process in which the molten blend of resin and additive is cooled to room temperature from just above the additive melting point. During the cooling process the blend remains a single phase, changing from a transparent liquid to an opaque solid which is malleable as a low tack sheet. As seen on the hot-stage microscope the cooling process is accompanied by the formation of a network of microcrystalline domains which provides the thickening effect(4, 5). When the sheet material is placed in the compression mould and heated above the additive melting point, the network disappears, the viscosity drops dramatically and resin plus additive chains flow easily to the boundaries of the mould. The moulding viscosity is much lower than that of conventional SMCs and this permits either smaller presses for a given moulding area, or increased areas at given press sizes, or a combination of both: National Composites in the USA have moulded complete garage doors on standard presses. Moreover the blends appear to be intrinsically shrink resistant in that they do not require low profile additives5. The resultant resins are conveniently referred to as Viscosity and Shrinkage Controlled Reactive (VISCOR) blends.
References
[1] Thompson S J, GB Patent 2111513 (20 July 1983)
[2] Gibson A G and Payne D J, Fib Rein Comp Conf, London (1988) p 11.1
[3] Bush S F, Methven J M, Blackburn D R, Networks as the basis of pre-thickening sheet moulding compounds, Biol and Snth Networks, Elsevier (1988) p 321-334
[4] Bush S F, High Perf Polym 8 (1966) 67-82
[5] Bush S F, US Patent 5,496,873 (5 March 1996)
[6] Coleman M M et al, Macromolecules 21 59-69
See also the section on Development of New Products & Processes.