Introduction to the types of polypropylene
Polypropylene is generally divided into three types:
1. Homopolymer
2. Random copolymers
3. Block copolymers.
The term comonomer is often used with ethylene. When ethylene propylene rubber or EPDM is added to polypropylene homopolymers (thermoset polymers), their low temperature impact strength (LTIS) can be increased. As randomly polymerized ethylene monomers are applied to polypropylene homopolymers, the crystallinity of the polymer decreases, the melting point decreases, and the polymer becomes more transparent.
Polypropylene can be divided into:
1. Random polypropylene, expressed as PP-at.
2. Cotactic polypropylene, denoted as PP-st.
3. Isotactic polypropylene, expressed as PP-it.
The methyl groups (-CH3) are uniformly arranged in atactic polypropylene, alternately arranged in syndiotactic polypropylene, and uniformly arranged in isotactic polypropylene. This affects the crystallinity (amorphous or semi-crystalline) and thermal properties of the material. On the other hand, atactic polypropylene is amorphous and lacks regularity, making it difficult to crystallize.
The crystal structures of various polypropylenes are as follows:
1. Isotactic polypropylene formed in industrial products has high crystallinity. About 30% - -60%.
2. Syndiotactic polypropylene is less crystalline, while atactic polypropylene is amorphous (non-crystalline).
3. Isotactic polypropylene comes in a variety of crystal forms, each with its own molecular arrangement of the polymer chains. In iPP, -modification is the most common. Folded chain-like sheets are used to make these crystals. The lamellae are arranged in a "cross hatching" pattern, which is a distinguishing feature.
Since atactic polypropylene is amorphous, it lacks a crystalline structure. Due to its lack of crystallinity, it dissolves easily even at moderate temperatures, which allows it to be extracted as a by-product of isotactic polypropylene.