Paper published in the Journal of High Performance Polymers (1996) 8 67-82.

S F Bush with J M Methven and D R Blackburn

Abstract

This paper describes a new class of thermosetting polymer blends. The blends are now used commercially in the manufacture of sheet moulding compounds (SMC) where they exhibit important advantages compared with conventional systems. The new compositions comprise a cross-linkable base resin and unsaturated monomer in which the base resin is dissolved and a saturated additive resin which has a crystalline melting temperature T_m below the temperature T_r at which cross-linking occurs. Between T_r and T_m the additive and base resin form a single liquid phase, which on cooling thickens reversibly to a leathery sheet with no bulk phase separation. The paper describes the mechanisms underlying this viscosity switch phenomenon and the partial compatibility criterion used to select the resin-additive combinations which exhibit this behaviour.

Eleventh Annual Meeting of the Polymer Processing Society, Seoul, Korea, 27th-30th March 1995, paper 08-10

S F Bush, with J M Methven

Synopsis

In Ref 1 the authors describe a novel process for sheet moulding compounds in which the standard Group II metal oxide thickening agents are replaced by organic compounds chosen to have what may be termed partial compatibility with an unsaturated base resin. The purpose of the thickening agent in both cases is to produce from a liquid resin with viscosity typically 1 poise, a leathery moulding compound of viscosity in the range 10⁵ to 10⁶. This blend is usually in the form of a sheet which may be cut at a later time to fit into a compression mould. The new process is the subject of world-wide patenting (Ref 2) and the resultant products entered commercial production earlier this year.

The present paper describes recent experimental results and discusses the mechanisms underpinning the huge reversible changes in viscosity which are obtained over a 15 ^oC temperature interval. Such a viscosity switch principle enables reinforcing fibres to be thoroughly wetted at basically the resin viscosity before a modest lowering of temperature produces a material which can be cut and manipulated. At the heart of the mechanism is the formation of a network made up of the additive polymer chains connected at intervals by a series of micro crystalline domains formed by the same additive. When the thickened sheet is placed in the mould and heated to the reaction temperature of the unsaturated base resin, the micro crystalline domains melt and the resin then flows freely to the internal surfaces of the mould.

Experiments show that the requisite partial compatibility of resin-additive polymer pairs can be determined in many cases by requiring their solubility parameter differences Δδ to lie within a given range. If Δδ is too small, the additive polymer remains in solution with the base resin; if Δδ is too large it separates completely. The paper discusses the advantages and limitations of the Δδ approach to estimating partial compatibility.

References

[1] S F Bush, J M Methven, D R Blackburn, Networks as the basis of pre-thickening sheet moulding compounds, Biol Synth Poly Networks, ed O Kramer, Elsevier 1987, pp 321-334.

[2] S F Bush, J M Methven, D R Blackburn, Polymer Compositions, Intl Patent WO 8906258, 13 July 1989 and European Patent EP 0396625, 14th November 1990.