Additives for molding compounds composed of unsaturated polyester

In the field of molded compounds, we target resin-based materials, fillers and glass fibers, during production and processing. Since all these ingredients are used in the production of mixtures, we want to describe here the additives covering these applications.

Sheet Molding Compound (SMC) and Bulk Molding Compound (BMC) are fiber-reinforced materials consisting essentially of a thermosetting resin, mainly based on styrene-unsaturated polyester, glass fiber reinforcement and fillers.

Fillers are mainly calcium carbonate-based for general applications, and aluminum trihydrate-based (ATH) for flame-retardant formulations.

Glass fibre reinforcements guarantee superior mechanical properties.

In SMC, glass fibers are 25 and/or 50 mm long, and their quantity varies from 20 to 60% by mass in the formula. In BMCs, the fibers are shorter (6 to 12 mm) and their dosage is lower (10 to 18% by mass).

SMC and BMC are molding compounds, processed by compression or injection, at high temperatures (130 to 170°C) and under high pressure (2 to 12 MPa).

The use of composites based on thermosetting resins has increased in recent years. Products made from composites such as SMC and BMC are showing remarkable growth not only in the automotive sector, but also in the electrical and construction industries.

The designer uses composites for products requiring long-term stability, high-temperature use (e.g. painting line), low thermal expansion coefficient, high mechanical properties, integrated parts and an attractive price/performance ratio.

To improve the performance and/or application of these materials, components are added to the formula. These include :

• Low Profile (LP)/Low Shrink (LS) additives;

• thermoplastics such as PS (polystyrene) ;

• PMMA (polymethyl methacrylate),

• PVAc (polyvinyl acetate) ;

• saturated polyesters and/or SB (styrene butadiene) co-polymers;

• reaction initiators ;

• thickeners and release agents.

The total organic part represents less than 30% of the formula's total mass.

Different formulas are used, depending on the specifications to be met.

To identify the different types of SMC/BMC formulations, we distinguish between Standard, Low Shrink (LS) and Low Profile (LP) formulations. These three types of formula can be differentiated according to the composition of the mixture, leading to variable...