Many applications for automotive and appliance components have been developed for low profile sheet molding (SMC) and bulk molding (BMC) compounds to use the advantage of moderate cost and excellent product quality. Efficient processing of fibre reinforced SMC and BMC materials is essential to meet the stringent requirements for consistent product quality with high volume production rates.

Processing SMC and BMC involves the application of heat and pressure in a mold. The processing characteristics of the molding compound depend upon the thermal and rheological properties of the compound in the mold. The thermal behaviour of the compound is complicated by the exothermic nature of the reaction while curing. (H. Kobuta, Journal Applied Polymer Science, 19, 2279-2297).

A typical resin formulation suitable for use as SMC or BMC is shown below:

Material				Quantity		Remarks
Unsaturated Polyester		Propylene Glycol		4 parts
		Dipropylene Glycol
		Maleic Anhydride		1 part		Converted to Fumaric Acid Double Bond
		Acid Number		25 mg KOH
		Molecular Weight		2700 g/mol
Reactive Diluent		Styrene		50.0 Wt%		Constant Mole Ratio
		VT		53.7 Wt%
		TBS		60.7 Wt%
Initiator		Di-tert-Butylperbenzoate		1.2 Wt%
Low Profile Additive		Cellulose Acetate
		Cellulose Butyrate
Filler		Calcium Carbonate				200 Parts by Weight SMC/BMC
		Magnesium Oxide

The mole ratio of styrene monomer, VT or tert-Butylstyrene (TBS) reactive diluent is held constant for the comparison. Effect of the diluent on curing characteristics was determined by DSC at 4.2 N/mm² 10 ^oC/min. Kinetic results of study are shown in the figures below:

	Effect of Reactive Diluents on Curing Rate
dQ/dt
	Temperature oC

	Effect of Reactive Diluents on Curing Conversion
Conversion
	Temperature oC

Use of VT as the reactive diluent results in improved mechanical properties and reduced shrinkage of cured resin compared to styrene monomer. In addition the volatile emissions released during the molding process are significantly reduced.