Semicrystalline polymers change their mechanical and optical properties significantly even after the conversion process, especially in case of exposure to elevated temperatures. In case of polypropylene (PP) the polymer type as well as the conversion technology and the respective temperature range determine direction and extent of these changes. Extensive experimental results for different combinations of materials and processes are presented. While in case of homopolymers and random copolymers practically always embrittlement takes place, in case of heterophasic (impact) copolymers the toughness can also be increased. For applications requiring thermal post-treatment like e.g. sterilization such materials are of special interest.