Twenty Kinds of Plastics and Their Properties—(16) PP

16.PP polypropylene

Typical application range:

The automobile industry (mainly used with metal additives: PP fender, ventilation pipe, fan etc.), equipment (dishwasher door gasket, dryer ventilation pipe, machine frame and washing machine cover, refrigerator door liner), consumer goods (lawn and garden equipment such as lawn mowers and sprinkler etc.).

Injection molding process conditions:

Drying process: dry process is not required if the storage is appropriate.
Melting temperature: 220~275 °C, be careful not to exceed 275 °C.
Mold temperature: 40~80 °C, it is recommended to use 50 °C. The degree of crystallization is mainly determined by the temperature of the mold.
Injection pressure: can be as large as 1800bar.
Injection speed: usually, high speed injection can be used to minimize internal pressure. If defects appear on the surface of the products, low speed injection molding at higher temperatures should be used.
Runner and gate: for cold flow, the typical flow range is 4~7mm. It is recommended to use an injection port and a flow channel with a circular body. All types of gates can be used. The typical gate diameter range is 1~1.5mm, but it is also possible to use a gate that is small to 0.7mm. For the edge gate, the minimum gate depth should be half of the wall thickness; the minimum gate width should be at least two times the thickness of the wall. The PP material can be completely used in the heat flow system.

Chemical and physical properties:

PP is a semi crystalline material. It is harder than PE and has a higher melting point. Because the PP temperature of homopolymer type is more than 0 °C, it is very brittle. Therefore, many commercial PP materials are irregular copolymer with 1~4% ethylene or a higher proportion of ethylene content. The copolymer type PP materials have lower thermal distortion temperature (100 °C), low transparency, low gloss and low rigidity, but have stronger impact strength. The strength of PP increases with the increase of ethylene content. The softening temperature of VEKA in PP is 150 °C. Because of high crystallinity, the material has good surface stiffness and scratch resistance. There is no environmental stress cracking in PP.

PP is usually modified by adding glass fiber, metal additives or thermoplastic rubber. The flow rate of PP is MFR in the range of 1~40. The low MFR PP material has better impact resistance but lower extension strength. For the same MFR material, the strength of the copolymer type is higher than that of the homopolymer type. Because of crystallization, the shrinkage rate of PP is quite high, which is generally 1.8~2.5%. And the direction uniformity of the shrinkage is much better than that of PE-HD and other materials. The shrinkage rate can be reduced to 0.7% by adding 30% glass additives. The homopolymer and copolymer type PP materials have excellent anti hygroscopicity, acid alkali corrosion resistance and solubility resistance. However, it has no resistance to aromatic hydrocarbons (such as benzene) solvents, chlorinated hydrocarbons (carbon tetrachloride) solvents and so on. PP does not have antioxidation at high temperatures as PE does.