Polypropylene (PP) has a high melting point and excellent comprehensive properties. It is the plastic with the smallest relative density among various resins. Generally, the relative density of polypropylene is 0.90~0.91. Compared with other general-purpose thermoplastics, it has low price, small specific gravity, excellent mechanical properties such as yield strength, tensile strength, surface strength, outstanding stress cracking resistance and wear resistance, good chemical stability, and molding. With the characteristics of easy processing and wide application range, it has been widely used in chemical, electrical, automotive, construction, packaging and other industries. This article will focus on the classification and characteristics of different polypropylenes.
Polypropylene is divided into isotactic polypropylene (iPP), syndiotactic polypropylene (sPP), atactic polypropylene ( aPP) Three stereoconfigurations of polypropylene. In homopolymer polypropylene products, the weight percentage of isotactic polypropylene in the whole polymer is called isotacticity. This is an important indicator to measure the quality of polypropylene products. Isotacticity has a great influence on product properties. Different uses require polypropylene to have different isotacticity. However, it is generally required that the isotacticity be greater than 90%. The homopolypropylene commonly used in the film industry is isotactic polypropylene.
Homopolymeric isotactic PP has high crystallinity and good properties, and is the most valuable in homopolymeric PP. Secondly, random PP cannot be crystallized, similar to unvulcanized rubber, and has low melting point, hardness and mechanical properties. The properties of syndiotactic PP are between isotactic and random PP. The disadvantage of homopolymer PP is poor impact resistance at low temperature, high embrittlement temperature, low melt strength, narrow melting limit, and certain processing difficulties. Therefore, on the basis of homopolypropylene, copolymerized polypropylene appeared.
There are also copolymers in polypropylene polymers, such as copolymers with propylene as the main monomer and a small amount of ethylene as the second monomer (or comonomer), which is called copolymerized polypropylene. Copolymers can be divided into random copolymers, block copolymers and graft copolymers according to the regularity of their three-dimensional structure. The purpose of preparing the copolymer is to improve some properties of the homopolymer (such as cold resistance, temperature resistance, impact resistance, etc.) to meet the needs of special purposes.
Random copolymer PP
Also known as propylene-ethylene copolymer. The addition of ethylene reduces the crystallinity of homo-PP. When the ethylene content reaches 30%, it is already completely amorphous. Compared with homopolymeric PP, the impact strength, flexibility, transparency and heat sealability of random copolymerized PP are significantly improved; the embrittlement temperature is decreased.
block copolymer PP
Block copolymer PP is a copolymer of block copolymer polyethylene on polypropylene branches. Because PP and PE are incompatible, the crystallinity of PP and PE parts is almost unchanged, and the rigidity, impact resistance and creep resistance of PP polypropylene film will be better. Block copolymer PP greatly improves brittleness and can be used in Use below -10℃.