They can be made from several different things:
Butene Copolymer
Copolymer - HDPE (High Density Polyethylene)
Ethylene Vinyl Acetate
HDPE - Reclaim
LLDPE (Linear Low Density Polyethylene) -Hexene Copolymer
Homopolymer - HDPE
Homopolymer - LDPE (Low Density Polyethylene)
Homopolymer - PP (Polypropylene)
PP-IM (Impact)
LDPE-Reclaim
LLDPE-OC (Octene Copolymer)
LDPE Copolymer
For more information go to http://www.townsendpolymer.com