enhanced lubricating properties and good heat stability upon processing are imparted to thermoplastic materials such as polyvinyl chloride , polypropylene and acrylonitrile - butadiene - styrene resins by the incorporation therein of a polyglycerol ester of a c 8 to c 22 fatty acid . the ester is characterized by a degree of polymerization ranging from 2 to 8 and a degree of acid substitution ranging from 1 to 10 .