Polypropylene (PP) is a non-toxic, odorless, milky white highly crystalline polymer, ranking among the lightest plastics currently available. It exhibits outstanding water resistance, with a water absorption rate of merely 0.01%. In addition, PP boasts favorable heat resistance; finished products can be sterilized at temperatures above 100°C and remain undeformed at 150°C without external force.
Nevertheless, when applied under high-temperature environments with unavoidable external loads, PP is prone to warpage and deformation. Under such circumstances, modification for enhanced stiffness and heat resistance becomes necessary. The mainstream modification approaches for PP stiffness and heat resistance are listed as follows:
1. Filling Modification
Filling modification refers to incorporating non-metallic mineral powders or other additives into PP resin, including calcium carbonate, talc powder, mica powder, kaolin, wollastonite powder, barium sulfate, etc. This process can markedly boost PP’s stiffness, heat resistance, dimensional stability, high-temperature creep resistance and hardness. Most fillers also serve as extenders to cut production costs.
A core challenge in filling modification lies in achieving uniform distribution and dispersion of fillers within the PP matrix. Meanwhile, fillers require appropriate surface treatment to gain favorable compatibility with polypropylene. The surface modification technique and coupling agent selection are decisive factors for successful filling modification. At present, numerous filler manufacturers supply customized grades to meet client demands. For high-end product requirements, filled masterbatches are recommended.
2. Reinforcement Modification
Reinforcement modification involves blending fibrous reinforcements into PP to elevate its mechanical strength, an effect also achievable with high-aspect-ratio mineral fillers. Common fibrous reinforcing materials for PP include glass fiber, carbon fiber, inorganic whiskers and metallic whiskers.
Glass fiber acts as the primary reinforcing agent, substantially improving PP’s tensile strength, flexural strength, modulus (stiffness), heat resistance and dimensional stability. Glass fiber reinforced PP can substitute engineering plastics in numerous applications, such as fan blades, heater grilles, pump impellers, lamp housings, electric furnace and heater casings. Long glass fiber reinforced PP has gained widespread popularity in recent years.
3. Nucleating Modification
Nucleating modification introduces a small dosage of nucleating agents into PP to accelerate its crystallization rate, raise crystallinity and refine crystal grains, thereby improving stiffness and heat resistance. PP nucleating agents fall into two categories: inorganic nucleators such as ultra-fine talc, silica and nano calcium carbonate; and organic nucleators represented by sorbitol and its derivatives.
Inorganic nucleating agents tend to compromise the surface gloss and transparency of finished products, while organic nucleators can significantly enhance transparency and surface smoothness.
With continuous improvement of living standards, higher requirements have emerged across cultural, entertainment, food, medical, material and interior decoration industries, driving rising demand for transparent materials. Transparent PP possesses unique advantages and broader development prospects compared with conventional PP, PVC, PET, PS and other plastics.
