Types and properties of transparent plastics
Transparent plastics are a type of polymer material with excellent light transmittance (transmittance is usually ≥80%). They are widely used in optical lenses, packaging containers, electronic display screens and other fields. The diversity of its varieties and performance provides the possibility of precise selection for different scenarios. The light transmittance of transparent plastics comes from its amorphous structure or regular crystalline form. Amorphous plastics have a generally high light transmittance (such as PMMA up to 92%) due to the disordered arrangement of molecules. Some crystalline plastics can also achieve high light transmittance by controlling the crystallinity (such as PET crystallinity ≤30%). In addition to light transmittance, transparent plastics have significant differences in impact resistance, heat resistance, chemical resistance and other properties. For example, the impact strength of polycarbonate (PC) is 5-10 times that of polystyrene (PS), while polymethyl methacrylate (PMMA) has better weather resistance than most transparent plastics. These characteristics determine their application boundaries.
Polymethyl methacrylate (PMMA, commonly known as organic glass) is one of the most commonly used transparent plastics. Its light transmittance reaches 92%, approaching that of ordinary glass (90%). It also exhibits excellent weather resistance, remaining yellow-free for 5-10 years outdoors. Its density is 1.19 g/cm³, half that of glass, and its impact resistance is 6-10 times greater, making it suitable for making eyeglass lenses, advertising light boxes, instrument housings, and other applications. PMMA has excellent processability and can be formed using injection molding and extrusion processes. Its melt flow rate (MFR) is 3-10 g/10 min (230°C/3.8 kg), and its molding temperature is 180-230°C. However, PMMA has moderate heat resistance, with a heat deflection temperature (HDT) of 78-105°C, making it unsuitable for use in high-temperature environments. Its low surface hardness (Rockwell hardness M80-90 ) makes it easily scratched, requiring surface coatings (such as SiO₂ ) to improve its wear resistance.
Polycarbonate ( PC) is a high-performance transparent engineering plastic with a light transmittance of 89%-90%. It possesses excellent impact resistance, with a notched impact strength exceeding 60 kJ/m², three to five times that of PMMA. It is known as “transparent metal.” It also boasts excellent heat resistance, with an HDT of 120-140°C (1.82 MPa), and can be used for extended periods in environments ranging from -40°C to 120°C. It is suitable for applications such as baby bottles, bulletproof glass, and automotive headlights. PC has a high melt viscosity (MFR of 1-10 g/10 min at 300°C/1.2 kg), requiring high temperatures (260-300°C) and pressures during molding. It is also highly hygroscopic and requires drying at 120°C for 4-6 hours before processing. However, PC’s chemical resistance is poor, making it susceptible to corrosion by organic solvents (such as alcohol and gasoline). Long-term use can also lead to stress cracking, requiring copolymerization (such as PC/ABS alloys) to improve its properties.
Polyethylene terephthalate (PET) is translucent in its unstretched state. After biaxial stretching, its transmittance can reach 88%-90%. It also exhibits excellent mechanical properties, with a tensile strength of 50-70 MPa, approaching that of PC. PET also has good heat resistance, with an HDT of 80-85°C, making it suitable for beverage bottles, food packaging films, and more. PETG, produced through crystallization modification (such as the addition of nucleating agents), has a transmittance of up to 91%, a heat distortion temperature of 70°C, and excellent chemical resistance, making it suitable for cosmetic packaging. PET has moderate melt flow (MFR of 15-30 g/10 min at 280°C/2.16 kg) and a molding temperature of 260-290°C. However, its slow crystallization rate requires controlled cooling to maintain transparency, with mold temperatures typically set between 20-40°C.
Polystyrene (PS) is a low-cost, transparent plastic with a transmittance of 80%-88% and a density of 1.05 g/cm³. It also offers excellent processability, with an MFR of 5-40 g/10 min (200°C/5 kg) and a molding temperature of 150-200°C. This makes it suitable for the mass production of toys, disposable tableware, CD cases, and other products. However, PS is brittle, with a notched impact strength of only 2-3 kJ/m². It also has poor heat resistance (HDT of 60-80°C) and is susceptible to corrosion by organic solvents. High-impact polystyrene (HIPS) was developed to improve its performance, but this reduces its transmittance to 50%-70%. Styrene-acrylonitrile copolymer (SAN), on the other hand, offers a transmittance of 80%-85%, a 50% increase in impact strength compared to PS, and improved heat resistance to 80-100°C. It is often used in transparent containers and instrument housings.
Transparent polyamide (PA) is a new high-performance transparent plastic with a light transmittance of 85%-90%. It combines the chemical resistance of PA with the impact resistance of PC, with a notched impact strength of 40-50 kJ/m² and a heat deflection temperature of 120-160°C. It can be used for short periods at 150°C, making it suitable for the production of oil cups, medical devices, and other applications. Its melt flow is poor (MFR of 5-15 g/10 min at 275°C/2.16 kg), requiring a molding temperature of 280-320°C. Its high hygroscopicity requires drying at 120°C for 8-12 hours before processing. Transparent PA offers superior weather resistance to PC, resists yellowing under UV light, and exhibits excellent resistance to oils, greases, and solvents. It is an emerging material in the automotive and electronics sectors. With the advancement of copolymerization technology, the cost of transparent PA is gradually decreasing, making it a promising alternative to traditional transparent plastics in a wider range of applications.
