Low-shrink additive

Some thermoplastic polymers commercially used as the low-profile, low-shrink additives to SMC and BMC composites are... 

A low profile, low shrink additive suitable for moulding compound manufacture and pultrusion. 

A PSM-based low-shrink additive for SMC BMC, having high compatibility with PS. 

Low-profile additives, which control shrinkage, have emerged as a distinct science and class of additive. Unsaturated polyester resins, as do all thermosetting polymers, shrink when cured. Low-profile additives are a major class of additives used to control shrinkage, which vastly improves surface quality. This science is credited with the opening of automotive markets where surface quality is of prime importance. In exterior automotive body panels, Class A surfaces are required for market acceptance. 

Some new types of S-B block copolymers are now being used as low-shrink/low-profile additives in sheet molding compounds made from unsaturated polyesters. 

Polarite. CC Int L] Surface-treated minerals low-profile additives for low and zero>shrink DMC and BMC moldings, high gloss prods. 

Shrinkage Any filler will decrease shrinkage the most commonly-used fillers are silica, clay, calcium carbonate, alumina talc, powdered metals, and lithium aluminum silicate. Low-shrink or low-profile additives are used in TS systems such as SMCs and BMCs to minimize shrinkage during and after molding, and improve the surface appearance of a molding. 

Low-shrink polypropylene fibers requires presence of additive that nucleates polymer crystals within the target polypropylene after exposure to sufficient heat to melt the initial pelletized polymer and allowing such a melt to cool." The preferred rigidifying compounds include dibenzylidene sorbitol based compounds, as well as less preferred compounds, such as sodium benzoate, certain sodium and lithium phosphate salts (such as sodium 2,2 -methylene-bis-(4,6-di-tert-butylphenyl)phosphate, otherwise known as NA-11)."... 

Rnbber or elastomeric additives have been found to be effective low-shrink/low-profile additives for sheet and bnlk molding compounds (SMC and BMC). These additives include butadiene/styrene and styrene/butadiene/styrene. For maximum surface properties, 15 weight percent of the additive gives good molded appearance even in large, relatively fiat, thin panels. 

Low shrink offers minimum surface waviness in the molded part (as low as 1 mil/in. mold shrinkage) low profile offers no surface waviness from 0.5 to 0 mil/in. of mold shrinkage. Low-profile additives can also be used for improved surface appearance. 

Reduction of shrinkage — low-profile or shrink additives (usually thermoplastic polymers dissolved in styrene)... 

A low shrink, low profile resin containing vinyl ester for SMC and BMC applications, with high heat resistance and high tensile elongation. Can be employed as a low profile additive - see section H8. 

A highly reactive, high viscosity orthophthalic resin which in combination with low shrink and low profile additives, is suitable for SMC manufacture and then car body, furniture and electrical mouldings, b. 72% c. 3900-4910... 

Because of their lesser ability to control shrinkage, the non-polar polymers such as polystyrene and polyethylene are often classified as low shrink rather than low profile additives. Usually, low profile additives are supplied as 30-40% polymer solutions in styrene monomer. Polyester resin manufacturers also package the low profile additives dissolved in their resins. These are referred to as one pack systems. As the industry has expanded, other thermoplastics have been identified which have shrinkage control properties. These are also now used commercially in a variety of applications. Examples of these other polyers are saturated polyesters, polyurethanes, stryene-butadiene copolymers and polycapro-lactones. Polyfvinyl acetate) based materials are probably still the most used low profile additives, being useful with the broadest range of unsaturated polyester resin structures. Relative proportions of the organics used in most formulations are 30-50% polyester alkyd, 10-20% thermoplastic and 40-50% styrene. 

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