Low shrink profile

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. 

Refers to low profile RJ surfaces. These are special TS polyester compounds for RPs that are combinations of thermoset and thermoplastic materials. Although the term low profile and low-shrink are sometimes used interchangeably, there is a difference. Low-shrink contain up to 30wt% TP, while low-profile contain from 30-50wt% TP. Low-shrink offers minimum surface waviness on molded parts, as low as 25 pm (1 mil) per in mold shrinkage whereas low-profile offers no surface waviness of 0-12.7 pm (0-0.5 in) per in mold shrinkage. 

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 (LS) and low profile (LP) systems have been developed over the past 20 years for large-surface parts in car bodies such as bumpers, trunk lids, doors, fenders, etc. 

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

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. 

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 medium to highly reactive isophthalic base resin suitable for low profile/low shrink SMC/BMC manufacture. Thickenable. 

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... 

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

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. 

The tendency of thick sections of DMC structural parts to crack has been overcome by using low-profile polyester resins (or low-shrink resins). These are prepared by making a blend of a thermoplastic (e.g., acrylic) polymer-styrene system with a polyester-styrene system. Moldings of this blend cured at elevated temperatures exhibit negligible shrinkage and minimal warpage and have very smooth surface, to which paint may be applied with very little pretreatment. 

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