Properties and Production Process of Polypropylene
Polypropylene (PP) is a general-purpose thermoplastic with excellent properties such as high transparency, non-toxicity, low relative density, easy addition, high impact strength, chemical resistance, good flex resistance, and good electrical insulation. , and it is easy to be modified by T process measures such as copolymerization, blending, filling, and strengthening, with excellent comprehensive performance and reasonable price. and many other fields. Among polyolefin resins, it has become the third largest product after polyethylene and polyvinyl chloride, and has the status of saving more and more fireflies. Polypropylene is a polymer obtained by polymerizing propylene as a monomer. Because the main chain of polypropylene contains asymmetric carbon atoms, the methyl groups on the tertiary carbons have different arrangements in space. Thus there are three different stereostructures of polypropylene, namely isotactic, syndiotactic and atactic. Isotactic polypropylene has high rigidity, strength, hardness and thermal deformation resistance; regular polypropylene has good toughness, impact strength, transparency and low heat sealing temperature; atactic polypropylene has excellent hydrophobicity, chemical resistance properties, electrical insulation, adhesion and compatibility; sometimes in order to provide the impact resistance of polypropylene, a small amount of ethylene is added during polymerization to form a rubber phase of ethylene propylene copolymerization in polypropylene, which is called impact polypropylene.
(1) Crystallization properties
Polypropylene has 4 crystal forms, α, β, γ and pseudo-hexagonal. Different crystalline polypropylene products have different properties, such as:
(2) Mechanical properties
Polypropylene has good mechanical properties, tensile strength and rigidity are relatively good, but the impact strength strongly depends on the temperature, the impact strength is higher above room temperature, but the impact resistance is poor at low temperature. The mechanical properties of polypropylene are related to relative molecular mass, spherulite size and crystallinity. When the relative molecular weight is low, the crystallinity is high, and the spherulite size is large, the rigidity of the product is high and the toughness is low. In addition, if the article is formed with orientation or stress, the impact strength can also be significantly reduced. Although the impact strength is poor. However, after modification such as filling or reinforcement, its mechanical properties can compete with higher-cost engineering plastics in many fields.
Due to the mutual repulsion of the side methyl groups of the isotactic polypropylene crystalline chain, its main conformation is like a helix, that is, isotactic polypropylene macromolecules always crystallize in a helical conformation that can accommodate larger side groups, which makes polypropylene with High bending fatigue resistance, the hinge made of it will not be damaged after 70 million times of folding and bending.