Oxygen transmission rate. OTR of plastic materials varies considerably with humidity, therefore it needs to be specified. Standard conditions of testing are 0, 60 or 100% relative humidity. Units are cc./100 square inches/24 hours, (or cc/square meter/24 Hrs.) (cc = cubic centimetres) - Polypropylene. Has much higher melting point, thus better temperature resistance than PE. Two types of PP films are used for packaging - cast, (see CAPP) and oriented (see OPP).