This invention pertains to improved polyester moldings and more particularly to those having improved surface characteristics.
A technical improvement that has made a significant contribution to commercial polyester molding technology is the use of low profile additives to reduce shrinkage during the curing reaction, and to thereby improve dimensional stability and surface smoothness. Low profile additives are thermoplastic polymers such as vinyl acetate polymers, polystyrene, acrylic polymers, and polycaprolactones. There are a number of theories that seek to explain the low profile or anti-shrinkage action of these polymers, but the one that seems to best explain the phenomenon is the following:
The low profile additive is at least partly soluble in the uncured polyester/styrene solution. As the polyester/styrene mixture crosslinks, the thermoplastic polymer becomes incompatible or less soluble and at least partly comes out of solution. This action causes a volume expansion that compensates for the shrinkage that occurs when the polyester/styrene mixture crosslinks.
The development of low-profile unsaturated polyester compounds has led to a wide acceptance of these materials by the transportation industry because of their good surface appearance, dimensional stability, physical properties, assembly consolidation and potential weight savings. However, as new applications developed standards have been raised making it desirable for even better surface appearance and the elimination of ripples and waviness that sometimes develop, particularly in relatively large appearance sensitive areas.
There is, therefore, a need to provide low-profile unsaturated polyester compounds which afford improved surface appearance in the molded parts obtainable therefrom.