Polypropylene is a thermoplastic resin obtained by polymerizing propylene. According to the position of the methyl group, it is classified into three types: isotactic polypropylene, atactic polypropylene, and syndiotactic polypropylene.
The methyl group is arranged on the same side of the main chain of the molecule as isotactic polypropylene. If the methyl group is disorderly arranged on both sides of the main chain of the molecule, it is called atactic polypropylene. When the methyl groups are alternately arranged on both sides of the molecular main chain, Gauge polypropylene. In general industrial production of polypropylene resin, the isotactic structure content is about 95%, and the rest is random or syndiotactic polypropylene. Industrial products are based on isotactic substances. Polypropylene also includes copolymers of propylene and a small amount of ethylene. Usually a translucent, colorless solid, odorless and non-toxic. The melting point can be as high as 167 ° C due to the high degree of crystallisation due to structural regularity. Heat-resistant, corrosion-resistant, steam-sterilizable products are its outstanding advantages. The low density is the lightest general purpose plastic. The disadvantage is that the low temperature impact resistance is poor, and it is easy to age, but can be overcome by modification.
The copolymer type PP material has a low heat distortion temperature (100 ° C), low transparency, low gloss, low rigidity, but has stronger impact strength, and the impact strength of PP increases with the increase of ethylene content. . The Vicat softening temperature of PP is 150 °C. Due to the high degree of crystallinity, this material has excellent surface stiffness and scratch resistance. There is no environmental stress cracking problem in PP.
The melt mass flow rate (MFR) of PP is usually from 1 to 100. Low MFR PP materials have better impact resistance but lower elongation. For materials of the same MFR, the impact strength of the copolymerization type is higher than that of the homopolymer type. Due to crystallization, the shrinkage rate of PP is quite high, generally 1.6 to 2.0%.
On October 27, 2017, the World Health Organization's International Agency for Research on Cancer published a preliminary list of carcinogens, polypropylene in the list of three types of carcinogens.