Properties of commonly used plastics
As an important polymer material, plastics are widely used across various industries due to their excellent comprehensive properties, including mechanical, thermal, chemical, and processing properties. Different types of plastics exhibit significant differences in performance, and understanding these characteristics is essential for selecting the right plastic. Whether used in everyday items, automotive parts, or electronic and electrical products, the properties of plastics directly impact their performance and lifespan. Therefore, in-depth analysis of the properties of commonly used plastics is of great practical significance.
Polyethylene (PE) is one of the most widely produced plastics. Its properties vary depending on density and can be categorized into low-density polyethylene (LDPE), high-density polyethylene (HDPE), and linear low-density polyethylene (LLDPE). LDPE offers excellent flexibility and transparency, with a density of approximately 0.91-0.925 g/cm³ and relatively low tensile strength (7-15 MPa), but boasts a high elongation at break of 300%-600%, making it suitable for making films, hoses, and other products. HDPE, with a density of 0.941-0.965 g/cm³ and high crystallinity, possesses high rigidity and hardness, with a tensile strength of 20-30 MPa and excellent chemical resistance. It is commonly used in blow-molded bottles, turnover boxes, and other products. LLDPE combines the flexibility of LDPE with the strength of HDPE, offering excellent puncture resistance and making it an ideal material for heavy-duty packaging films. PE also has poor heat resistance, typically operating at temperatures not exceeding 60-80°C. It also exhibits poor light aging resistance and is susceptible to degradation from prolonged exposure to sunlight.
Polypropylene (PP) is a high-performance general-purpose plastic with a low relative density (0.90-0.91 g/cm³) and well-balanced mechanical properties. PP boasts a tensile strength of 20-30 MPa and a flexural strength of 25-40 MPa. It also exhibits excellent fatigue resistance and can withstand tens of thousands of bending cycles without damage, making it suitable for parts requiring repeated movement, such as hinges and gears. PP also boasts superior heat resistance to PE, with an operating temperature of around 100°C. Modified PP can withstand temperatures exceeding 120°C, making it commonly used in microwaveable lunch boxes and hot water pipes. However, PP exhibits low-temperature brittleness, prone to brittle fracture at temperatures below 0°C. Toughening with elastomers can effectively mitigate this flaw. PP exhibits excellent chemical stability and resistance to acids, alkalis, salts, and other chemicals. However, its resistance to organic solvents is poor, and it is easily swollen by aromatic hydrocarbons and chlorinated hydrocarbons.
Polyvinyl chloride (PVC) is a versatile plastic that can be divided into rigid polyvinyl chloride (UPVC) and flexible polyvinyl chloride (SPVC) depending on whether plasticizers are added. UPVC contains no or minimal plasticizers and exhibits high rigidity and hardness, with a tensile strength of 40-50 MPa, excellent chemical resistance, and good flame retardancy. It is widely used in building pipes, door and window profiles, and other applications. SPVC, with the addition of 30%-50% plasticizers, exhibits excellent flexibility and elasticity, and its hardness can be adjusted based on the plasticizer content. It is suitable for products such as wire and cable sheathing and artificial leather. However, PVC has poor heat resistance, with operating temperatures typically limited to 60-70°C. It also releases hydrogen chloride gas during processing, requiring the addition of stabilizers to inhibit degradation. Furthermore, PVC is difficult to recycle and can easily pollute the environment after disposal, limiting its application in certain areas.
Polystyrene (PS) and its modified varieties possess unique properties and are widely used in packaging and electronics. General-purpose polystyrene (GPPS) offers high transparency (light transmittance of 88%-92%), but is brittle and has low impact strength (only 10-20 kJ/m²), making it suitable for manufacturing disposable tableware, packaging trays, and other applications. High-impact polystyrene ( HIPS), modified by adding styrene-butadiene rubber to PS, increases its impact strength to 20-50 kJ/m² while maintaining good processing properties. It is widely used in manufacturing television casings, toys, and other products. Styrene-acrylonitrile copolymer (SAN) offers high rigidity and heat resistance, with a tensile strength of 70-80 MPa, making it suitable for instrument casings and containers. Acrylonitrile-butadiene-styrene copolymer (ABS), combining the advantages of all three monomers, offers excellent impact strength, heat resistance, and surface gloss, making it a common material for automotive parts and appliance casings. However, its weatherability is poor, and it is prone to aging and discoloration after long-term outdoor exposure.
Engineering plastics such as polycarbonate (PC), polyamide (PA), and polyoxymethylene (POM) offer superior mechanical properties and heat resistance, making them suitable for high-end manufacturing applications. PC is a transparent engineering plastic with a light transmittance of 85%-90%, an impact strength of 60-80 kJ/m², and excellent heat resistance (operating temperature range -40-120°C). It is widely used in products such as eyeglass lenses, optical discs, and bulletproof glass. PA, commonly known as nylon, offers excellent wear resistance and self-lubrication, with a tensile strength of 60-90 MPa and good fatigue resistance, making it suitable for transmission parts such as gears and bearings. However, PA is highly hygroscopic, which can reduce dimensional stability and necessitates moisture-proofing. POM is a crystalline engineering plastic with excellent rigidity, fatigue resistance, and a low coefficient of friction, making it suitable for precision gears, valves, and other applications. However, its heat resistance is average, and its operating temperature does not exceed 100°C. While these engineering plastics offer excellent performance, they are costly and difficult to process, making them typically used in applications with specific performance requirements.
In summary, commonly used plastics have varying properties, each with its own unique advantages and limitations. In practical applications, the appropriate plastic type must be selected based on a comprehensive consideration of factors such as the product’s operating environment, mechanical requirements, processing conditions, and cost budget. If necessary, the plastic’s properties can be improved through modification methods such as blending, filling, and reinforcement to meet specific application requirements. With the advancement of materials science, new plastic types are constantly emerging, and their performance is constantly improving, providing more options for development across various industries.
