Paper to the 17th Annual Meeting of the Polymer Processing Society, Montreal, Canada

M Esfandeh with S F Bush and J M Methven

Abstract

This paper describes a Differential Scanning Calorimetry (DSC) study of a new class of thermosetting polymer blends. The blends are made by physical thickening in which a particular crystalline additive is capable of forming a thickened system when it is melt blended with a thermoset resin. The blends are now used commercially in the manufacture of sheet moulding compounds (SMC) where they exhibit important advantages over conventional systems. In previous reports from this laboratory the morphology of these blends was studied[6] and on the basis of secure physical foundations, a model for blend morphology was presented. In this paper the effect of the presence of resin on melting and crystallisation temperature of thickening additive is studied using DSC technique. The corresponding enthalpies of transition are also measured and are compared with values expected from dilution effect. Finally the interaction parameter between resin and additive is calculated from a depression in the melting point of the blend.

[6] Bush S F, Esfandeh M and Methven J M, “New Blend Morphologies for Low Pressure Moulding Compounds”, Polymer Processing Society, 15th International Meeting, Hertogenbosch, The Netherlands, May 31st-June 4th (1999)

Paper to the Polymer Processing Society, 15th International Meeting, ‘s-Hertogenbosch, Holland

S F Bush with M Esfandeh and J M Methven

Abstract

This paper describes the morphology of 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 over the conventional systems. Previous reports from this laboratory have described a model for blend morphology of these compounds in which a limited and constrained crystallisation of an additive occurs in distributed microcrystalline domains connected severally by chains of additive threading their way through a liquid reactive base resin. This paper puts the model of the morphology on secure physical foundations. Hot stage microscopy and Wide Angle X-ray scattering techniques are used to investigate the morphology changes of the blend as a result of interaction between blend components.

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.

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.