Polypropylene (PP) is a thermoplastic resin produced by coordination polymerization or free radical polymerization of propylene monomers. It is a core member of the polyolefin family. Due to its low density, chemical resistance, ease of processing, and low cost, it has become one of the world's largest-produced and most widely used general-purpose plastics, playing a fundamental role in packaging, automotive, construction, textiles, and daily necessities.
Structurally, PP molecules consist of repeating -CH(CH₃)- units. Based on stereoregularity, it can be divided into three types: isotactic (iPP), syndiotactic (sPP), and atactic (aPP). The mainstream industrial product is isotactic polypropylene, where methyl side chains are regularly arranged on one side of the main chain, forming a highly crystalline structure (crystallinity approximately 50%-70%), giving the material high rigidity, melting point (approximately 160-170℃), and heat resistance. Atactic polypropylene, due to its disordered structure, low crystallinity, and poor strength, is mostly used as a modifying agent. This crystallinity gives PP both lightweight properties (density 0.90-0.91 g/cm³, the lightest among general-purpose plastics) and certain mechanical strength (tensile strength 30-40 MPa, flexural modulus 1000-1500 MPa), while maintaining good toughness within a temperature range of -20℃ to 120℃, and a low-temperature embrittlement temperature as low as -30℃.
PP's chemical resistance is particularly outstanding, exhibiting excellent resistance to water, dilute acids, dilute alkalis, salt solutions, and most organic solvents (such as alcohols and oils). It may only swell or degrade in strong oxidizing agents (such as concentrated nitric acid) or high-temperature aromatic hydrocarbons. This characteristic makes it an ideal material for food packaging, chemical containers, and medical devices. Simultaneously, PP has good electrical insulation properties, with a volume resistivity exceeding 10¹⁶ Ω·cm, unaffected by humidity, making it widely used in electrical appliance housings, cable insulation, and other applications.
Processability is another core advantage of PP. PP has a wide melting temperature range (180-220℃), and its melt flowability increases with decreasing molecular weight, making it suitable for various processes such as injection molding, extrusion, blow molding, spinning, and thermoforming. Injection molding can efficiently manufacture thin-walled products (such as food containers and bottle caps), extrusion is suitable for pipe, sheet, and fiber production, and blow molding is used for hollow containers (such as detergent bottles and automotive fuel tanks). PP has a low shrinkage rate (1.0%-2.5%), and its crystallization shrinkage and orientation shrinkage are controllable, which helps improve the dimensional accuracy of products.
Through copolymerization, blending, or filler modification, PP can be derived into a variety of functional grades. Block copolymer PP (PP-B) incorporates a small amount of ethylene monomer, resulting in an impact strength 2-3 times higher than homopolymer PP, making it suitable for impact-resistant applications such as automotive bumpers and appliance housings. Random copolymer PP (PP-R), due to the random distribution of ethylene monomers, exhibits excellent transparency and heat resistance (long-term operating temperature 70℃), making it a mainstream material for drinking water pipes. Glass fiber reinforced PP (GF-PP), with 10%-40% glass fiber added, boasts a flexural modulus exceeding 5000MPa, suitable for heavy-duty applications such as automotive structural components and industrial pallets. Flame-retardant PP, by adding phosphorus-nitrogen or magnesium hydroxide flame retardants, can achieve UL94 V-0 rating, meeting the fire protection requirements of electronic and electrical appliances.
In terms of applications, PP has a wide range of applications: in the packaging industry, it is used to manufacture woven bags, films (such as BOPP film), disposable lunch boxes, and liquid packaging bottles, ensuring food and daily necessities safety due to its lightweight and oil-resistant properties; in the automotive industry, it is used in bumpers, dashboards, door panel liners, and battery casings to achieve lightweighting and cost optimization; in the construction industry, PP-R pipes replace traditional metal pipes, significantly improving corrosion resistance and ease of installation; in the textile industry, it is melt-spun into polypropylene fibers for carpets, ropes, and functional clothing.
As a general-purpose plastic that combines performance, cost, and sustainability, the life cycle management of PP is also a focus. It can be recycled through physical recycling (melt regeneration) or chemical recycling (pyrolysis to olefins), with recycled materials achieving over 80% of the performance of virgin materials. With the advancement of research and development of bio-based PP (using renewable resources such as sugarcane bagasse as raw materials) and biodegradable PP composites, PP's role in green manufacturing will be further strengthened.
In summary, polypropylene, with its lightweight, high toughness, chemical resistance, ease of processing, and great modification potential, has become an indispensable basic material for modern industry, continuously providing cost-effective solutions for various fields, and constantly expanding its application boundaries through technological innovation.