Polypropylene Overview
Polypropylene, abbreviated as PP and with CAS number 9003-07-0, is one of the five major general-purpose plastics. It is a semi-crystalline thermoplastic resin obtained by the catalytic polymerization of propylene monomers and is also the lightest mainstream plastic in terms of density. Based on molecular structure, PP is classified into three main categories: homopolymer PP (PP-H), random copolymer PP (PP-R), and block (impact-resistant) copolymer PP (PP-B). PP is non-toxic and odorless, resistant to acids and alkalis as well as organic solvents, and can withstand high-temperature sterilization; its heat resistance is superior to that of PE, and its processing methods are flexible, allowing for injection molding, extrusion, spinning, and blow molding. Its disadvantages include poor low-temperature toughness and weak resistance to UV aging; therefore, it generally requires the addition of antioxidants and light stabilizers for modification. It is widely used in packaging, nonwoven fabrics, automotive applications, household appliances, piping, food contact materials, medical consumables, and other fields.
Basic Parameters
|
Item |
Specification |
|
CAS number |
9003-07-0 |
|
molecular formula |
(C₃H₆)ₙ |
| density | 0.90～0.91 g/cm³ |
| surface | Natural-colored translucent granules/powder; odorless and non-toxic |
| melting point | Homopolymer: 164–167 °C; random copolymer: 130–150 °C; block copolymer: 160–165 °C |
|
Vika softening point |
>140°C (homopolymer) |
| water absorption | 0.01% (extremely low) |
| solubility | Insoluble in water, alcohols, weak acids, and weak bases at room temperature; soluble in hot aromatic hydrocarbons and halogenated hydrocarbons. |
| characteristic | Chemically corrosion-resistant with excellent insulation properties; prone to brittleness at low temperatures and not UV-resistant without modification. |
| Melt Flow Rate (MFI) | Broad range: 0.5–100+ g/10 min (230 °C/2.16 kg); classified by grade |
Production Processes
Mainstream industrial routes: liquid-phase, gas-phase method, and slurry method; the catalysts used are predominantly Ziegler–Natta catalysts and metallocene catalysts.
Liquid-phase(China mainstream) involves the direct polymerization of liquid propylene within the reactor, eliminating the need for large amounts of solvent. This process features a short workflow, low energy consumption, and high output, enabling the production of homopolymer and impact-resistant copolymer PP. Most refining and petrochemical facilities in China adopt this process. Advantages: low cost and high production capacity; Disadvantages: moderate flexibility in grade switching.
Gas-phase process (Unipol and other processes): Propylene is polymerized in a gas phase within a fluidized-bed reactor under mild pressure and temperature conditions, allowing flexible switching between product grades; metallocene PP is typically produced using the gas-phase process, enabling the production of high-transparency, low-smell premium-grade materials. Advantages: Wide product range and high flexibility; Disadvantages: High requirements for equipment control.
Slurry process (legacy technology): polymerization is carried out in an inert solvent, resulting in low ash content in the product; its share is now gradually declining.
Product Grades
|
Level / Type |
Key Features |
Typical Applications |
|
Homopolymer PP (PP-H) |
High rigidity, high crystallinity, good heat resistance, and poor low-temperature impact resistance |
Wrapping tape, BOPP film, injection-molded rigid boxes, spinning, non-woven fabric |
|
Random copolymer PP (PP-R) |
Ethylene features random access, high transparency, good toughness, and a low melting point. |
Transparent food containers, water cups, food containers, PPR hot-water pipes |
|
Block anti-impact copolymerized PP (PP-B) |
Ethylene–ethylene-propylene rubber is phase-dispersed, providing excellent low-temperature impact resistance. |
Car bumpers, home appliance enclosures, turnover boxes, and suitcases |
|
fibre-gradePP |
Low ash content, good纺可纺性, and relatively high MFI |
Spinning, non-woven fabrics, melt-blown fabric for masks, geotextiles |
|
Film-grade PP |
Smooth and slippery, non-stick, and thermally stable |
BOPP film, CPP cast film, packaging laminated film |
|
Food-grade PP |
Complies with GB 4806.7; low leaching and odorless |
Food containers, food preservation containers, straws, food packaging |
|
Medical-grade PP |
Medical-grade; sterilizable by ethylene oxide / steam; low leachability |
Single-use syringes, infusion sets, and medical consumables |
|
Modified PP (talc powder / glass fiber reinforced) |
High rigidity, low shrinkage, and enhanced heat resistance |
Automotive interiors and household appliance structural components |
Note: Standard industrial materials must not be used directly in food or medical applications; only the corresponding compliant grades may be used.
Upstream and downstream industries
Upstream raw materials
Propylene (from naphtha cracking and propane dehydrogenation, PDH); supporting catalysts, antioxidants, slip agents, nucleating agents, and color masterbatches. China has sufficient PDH production capacity, a large supply of propylene, and a complete PP industry chain.
Downstream applications
Packaging industry: BOPP films, CPP films, food containers, and injection-molded packaging;
Textile nonwovens: spunlace fabrics, melt-blown fabrics, geotextiles, and rope nets;
Automotive industry: bumpers, door panels, instrument panels, and battery housings (modified PP);
Piping material: PPR hot and cold water supply pipes;
Home appliances and daily necessities: washing machine drum, storage bins, plastic tableware;
Medical: single-use medical consumables;
Other: injection-molded toys, strapping bands, and wire and cable insulation layers.
Industrial chain: crude oil / propane → propylene → PP production units → granules / powders → modified-fiber plants, film plants, injection-molding plants, and nonwoven fabric plants.
Safety, Storage & Transportation
Polypropylene is a combustible solid and is not classified as a hazardous chemical; its primary risks are dust explosions and ignition during melting.
Packaging: 25 kg, lined with a PE bag and wrapped in a film-coated woven bag;
Storage: Keep in a cool, well-ventilated storage area; store away from open flames and heat sources, and keep separate from oxidizing agents; for powdered materials, take particular care to prevent static-induced dust;
Transportation: General cargo transport; protect against rain and sunlight to prevent packaging damage;
Processing and protection: During high-temperature processing, avoid inhaling hot decomposition fumes; ensure adequate ventilation in the workshop;
Emergency response: Dry powder or foam may be used to extinguish a fire; contact of molten material with the skin can cause burns.
FAQ
Q: How should you choose homopolymer, random, or block PP?
A: For applications requiring high rigidity, select homopolymer for BOPP films; for applications requiring transparency and flexibility, select random copolymer for hot-water pipes; for applications requiring drop resistance and low-temperature impact resistance, select block impact-copolymerized material.
Q: What is the difference between PP and PE?
A: PP offers greater rigidity and heat resistance and can withstand steaming at temperatures above 100 °C; PE is softer and exhibits better low-temperature toughness; PP is more suitable for rigid containers, whereas PE is commonly used for flexible films and flexible tubes.
Q: What are the advantages of the polypropylene industry in China?
A: China is the world's largest producer of PP, with ample supporting refining and processing facilities, large production capacity, and a comprehensive range of grades, ensuring stable supply of fiber-grade, injection-grade, and pipe-grade products; China boasts a short supply chain and fast delivery, offering new materials, transparent materials, impact-resistant materials, and medical-grade food-grade materials in multiple specifications; long-term contracts provide stable spot supply, making it suitable for global procurement.
Sources and authoritative references
- PubChem,ChemicalBook(CAS 9003-07-0)
- GB 4806.7-Standards for plastic materials and articles for food contact and pharmaceutical raw materials
- HG/T petrochemical industry standards, MSDSs from leading petrochemical manufacturers, and product technical manuals
- Research Report on Process and Capacity in the Polypropylene Industry
- Data scope note: Indicators vary significantly across different catalysts and modified grades; the final specifications shall be subject to the supplier's COA and MSDS.
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