Post-processing process characteristics of polypropylene

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Characteristics of the processing techniques used in downstream applications of polypropylene: (1) Polypropylene has a very low water absorption rate; after being immersed in water for 1 day, its water absorption rate is only 0.01%–0.03%. Therefore, there is no need to dry the pellets before molding. (2) The melt of polypropylene is close to a non-Newtonian fluid; its viscosity is sensitive to shear rate, and increasing pressure can improve the flowability of the polypropylene melt. (3) The melt has low viscosity, making it easy to form thin-walled, long-length products. (4) Since polypropylene is a crystalline polymer, it has a relatively high molding shrinkage rate, generally ranging from 1% to 2.5%, and exhibits significant post-shrinking. Orientation is likely to occur during the processing stage; therefore, these factors must be taken fully into account when designing the mold and determining the process parameters. (5) Polypropylene is prone to oxidative degradation when heated, and it is particularly sensitive to oxygen at high temperatures. To prevent thermal degradation during processing, antioxidants are generally added during the resin synthesis stage. (In addition, the heating time should be minimized as much as possible, and contact with oxygen and copper during heating should be avoided.) ) (6) Polypropylene has excellent one-step formability, and almost all molding methods can be applied to it; among these, injection molding and extrusion molding are the most commonly used.