Polystyrene plastics refer to a class of plastics containing styrene in the macromolecular chain, including PS, HIPS and ABS.
(1) Polystyrene plastic
Polystyrene refers to the polymer synthesized from styrene monomer through free radical polycondensation reaction, abbreviated as PS in English. PS includes common polystyrene (GPPS), expandable polystyrene (EPS) and metallocene polystyrene (m-SPS), among others.
1. Structural properties of polystyrene plastics
(1) Structure of PS
The macromolecular backbone of PS is a saturated hydrocarbon polymer, which has good electrical insulation and can be used in humid environments due to its low moisture absorption. The molecular structure of PS is asymmetrical, the movement of macromolecular chains is difficult, PS is rigid and brittle, and the products are prone to internal stress. The molecule of GPPS is randomized, resulting in PS being an amorphous polymer with high transparency. Due to the presence of phenyl group, the product becomes yellow and brittle after long-term outdoor use, but the presence of phenyl group makes it have high radiation resistance.
(2) Performance of PS
(1) General performance PS is a colorless and transparent particle, which emits thick smoke and turpentine odor when burning, and can be stretched by blowing out. The quality is hard like glass, and it makes a metal-like sound when it hits the ground or hits it. It can be broken but not bent, and the fracture is mussel-colored silver. The water absorption rate of PS is 0.05%, which is slightly greater than that of PE, but it has little effect on the strength and dimensional stability of the product.
(2) Optical performance: Good transparency is the biggest feature of PS, and the light transmittance can reach 88%-92%, which is the best transparent plastic variety like PC and PMMA. The refractive index of PS is 1.59-1.60, but due to the existence of benzene ring, its second 222 birefringence is large and cannot be used in high-grade optical instruments.
(3) Mechanical properties PS is hard and brittle, non-extensible, and fractures when stretched to near the yield point. The tensile strength and flexural strength of PS are the highest among general-purpose thermoplastics, but the impact strength is small, making it difficult to use as an engineering plastic. PS has poor wear resistance and average creep resistance. The mechanical properties of PS are greatly affected by temperature.
(4) Thermal performance PS has poor heat resistance, the heat deflection temperature is only 70-90 °C, and it can only be used in the range of 60-80 °C for a long time. PS also has poor low temperature resistance, with a embrittlement temperature of -30°C. The thermal conductivity of PS is low, the coefficient of linear expansion is large, and the difference between PS and metal is huge, so the product is not easy to have metal inserts.
(5) Electrical properties PS has excellent electrical insulation, and is not affected by temperature and humidity. The tangent value of dielectric loss is small, it can withstand appropriate corona discharge, has good arc resistance, and is suitable for high-frequency insulating materials.
(6) Environmental performance PS has good chemical stability, can resist general acids, alkalis, salts, mineral oils and lower alcohols, etc., but can be attacked by many hydrocarbons, ketones and higher fatty acids, soluble in aromatic hydrocarbons (such as benzene, toluene, ethylbenzene and styrene, etc.), chlorinated hydrocarbons (such as carbon tetrachloride, chloroform, dichloromethane and chlorobenzene) and esters. PS has poor weather resistance, and its light resistance and oxidation resistance are poor, so it is not suitable for long-term outdoor use. However, PS has good radiation resistance.
2. Molding and processing of PS plastics
(1) Processing characteristics
PS is an amorphous resin, with no obvious melting point, and a wide melting temperature range, which can become a fluid between 120 °C and 180 °C. The thermal stability is good, and the decomposition temperature is above 300°C. PS melt is a non-Newtonian fluid, and the viscosity is strongly dependent on the change of shear rate, but the influence of temperature is also obvious. PS has excellent fluidity and is an easy-to-process plastic.
PS generally does not need to be dried before processing, and is dried only when there are special needs (such as requiring high transparency of products), with a drying temperature of 70°C-80°C, 1.5-2 hours. PS is prone to internal stress in processing, in addition to selecting appropriate process conditions, improving product design and reasonable mold structure, heat treatment should also be carried out on the product to eliminate stress. The shrinkage rate of PS molding is relatively low, generally only 0.4%-0.7%.
(2) Processing method
Both injection molding and extrusion can be carried out on ordinary injection molding machines and extruders.
PS foam products are the main uses of its resin, and there are two specific molding methods:
(1) Expandable PS resin (EPS) is used as raw material. EPS is a GPPS bead under heating and pressure conditions, and about 6% of the low-boiling point physical foaming agent such as n-pentane, petroleum ether or isopentane penetrates into the beads to make them swell, which is the expandable PS beads. EPS can produce foam products using methods such as steam molding, extrusion and injection molding.
(2) GPPS is used as raw material. The one-step extrusion process is adopted, and the foaming agent is directly mixed with PS or the physical foaming agent is injected into the PS melt in the extrusion melting stage, and the foaming and cooling can be extruded.
3. The application range of polystyrene plastics
(1) Electrical products PS has both transparency and good insulation, and can be used for various electrical accessories, housings and high-frequency capacitors.
(2) Transparent products PS has excellent transparency and can be used for general optical instruments, transparent models, lampshades, instrument covers, packaging containers, etc.
(3) Daily necessities PS has good coloring and gloss, and can be widely used in the manufacture of daily necessities, such as children's toys, furniture handles, decorative boards, pen holders, stationery, combs, toothbrushes, etc.
(4) Packaging materials PS foam products have good shockproof and thermal insulation properties, and can be used as packaging and thermal insulation materials. Packaging materials are mainly used for the packaging of electrical appliances, precision instruments, handicrafts, glass products, ceramic products, etc., and thermal insulation materials include partition panels and roof sandwich panels.
4. Metallocene polystyrene
Metallocene polystyrene, abbreviated as m-SPS, is a metaisostructural PS resin synthesized under the action of metallocene catalysts. The product has high melting point (270 °C), hydrolysis resistance, heat resistance (heat deflection temperature is 251 °C), chemical corrosion resistance, low density, no drying before processing, small shrinkage, good dimensional stability and other similar properties to engineering plastics PBT, PA and PPS, which is an ideal alternative to traditional reinforced engineering plastics.
m-SPS can be molded by injection molding, mainly used for heat-resistant plastic products, and the amount of m-SPS ranks second after PPS among many high-temperature resistant plastic materials.
(2) High impact polystyrene
The English abbreviation of high-impact polystyrene is HIPS. HIPS is essentially an impact-modified variety of PS, which is composed of PS and rubber. According to the magnitude of HIPS modification, it can be divided into three categories: medium impact PS, high impact PS and ultra-high impact PS.
The appearance of HIPS is white opaque bead-like particles, in addition to excellent impact performance, it also has most of the advantages of PS, such as good rigidity, easy processing, high gloss of products, easy coloring, etc., but the tensile strength and transparency are reduced.
As with PS, HIPS has good processability, the fluidity of the melt is higher than that of ABS, and the molding shrinkage is close to that of ABS, so the mold for molding ABS is also suitable for HIPS molding.
HIPS can replace ABS in some areas.
HIPS is widely used in the housings of televisions, vacuum cleaners, telephones, air conditioners, washing machines, photocopiers, lighting appliances, instrumentation, office supplies and auto parts.
HIPS's low-foaming products have the sawable, planing, and nailing properties of wood, and are similar in appearance to wood, and can be used for furniture, packaging, and building materials.
HIPS can be made into plates, sheets, and used to blister into containers, cups and plates, etc. It can also be made into pipes, medical and textile equipment, etc.
(3) ABS plastic
ABS is a polymer composed of acrylonitrile (23%-41%), butadiene (10%-30%) and styrene (29%-60%). There are four types of synthetic ABS: medium impact type, high impact type, ultra-high impact type and heat-resistant type. Due to its advantages of toughness, rigidity and hardness, the application range has far exceeded PS and has become an independent plastic variety. ABS can be used for both ordinary plastics and engineering plastics.
1. Performance of ABS
(1) General performance The appearance of ABS is opaque ivory-colored granules, and its products can be coated into various colors and have 90% high gloss. The relative density of ABS is 1.05 and the water absorption rate is low. ABS has good adhesion to other materials and is easy to print, coat and coat. ABS has an oxygen index of 18.2, is a flammable polymer, has a yellow flame, black smoke, burns but does not drip, and gives off a special cinnamon smell.
(2) Mechanical properties ABS has excellent mechanical properties, its impact strength is excellent, and it can be used at very low temperatures. Even if the ABS product is damaged, it can only be tensile failure and not impact failure. ABS has excellent wear resistance, good dimensional stability, and oil resistance, and can be used for bearings under medium loads and speeds. The creep of ABS is greater than that of PSF and PC, but less than that of PA and POM. The flexural strength and compressive strength of ABS are among the poorest among plastics. The mechanical properties of ABS are greatly affected by temperature.
(3) Thermal performance The heat deflection temperature of ABS is 93 °C-118 °C, and the product can be increased by about 10 °C after annealing. ABS can still show some toughness at -40°C and can be used in the temperature range of -40°C to 100°C.
(4) Electrical performance ABS has good electrical insulation, and is almost not affected by temperature, humidity and frequency, and can be used in most environments.
(5) Environmental performance ABS is not affected by water, inorganic salts, alkalis and a variety of acids, but it is soluble in ketones, aldehydes and chlorinated hydrocarbons, and will be stressed cracking by glacial acetic acid, vegetable oil, etc. ABS has poor weather resistance, is easy to degrade under the action of ultraviolet rays, and the impact strength is reduced by half after being placed outdoors for half a year.
2. Molding and processing of ABS plastics
ABS, like PS, is a thermoplastic with excellent processability and can be processed using common processing methods. The fluidity of ABS melt is better than that of PVC and PC, but worse than that of PE, PA and PS, similar to POM and HIPS. The flow characteristics of ABS are non-Newtonian fluids, and the melt viscosity is related to the processing temperature and shear rate, but it is more sensitive to the shear rate.
ABS has good thermal stability and is not prone to thermal degradation. ABS has high water absorption and should be dried before processing. The drying condition of general products is 80°C-85°C, 2-4 hours, and for special products (such as electroplating), 70°C-80°C, 10-18 hours.
ABS products are prone to internal stress in processing, such as too much stress or the product is absolutely forbidden to stress cracking, should be annealed, the specific method is to place the product in the 70 °C-80 °C hot air circulation drying oven for 2-4 hours, and then cool to room temperature.
ABS can be molded by injection molding, extrusion, calendering, blister and blow molding, and the injection molding method is the most widespread, followed by the extrusion method.
3. Modified varieties of ABS plastic
The modified varieties of ABS are mainly its alloys, with a wide variety and a wide range of applications.
(1) ABS/PVC can improve the flame retardancy of ABS, and can also improve tear strength and corrosion resistance, and can be made into soft or semi-hard products.
(2) ABS/PC can improve the heat-resistant temperature of the product, generally up to about 120 °C, and can also appropriately improve the mechanical properties such as strength and hardness.
(3) ABS/TPU This alloy has the advantages of impact and wear resistance of TPU, rigidity, comprehensive performance and low price of ABS.
4. The application range of ABS plastics
Housing material is widely used in the manufacture of telephones, mobile phones, televisions, cassette recorders, photocopiers, fax machines, washing machines, power tools, kitchen supplies and children's toys housing.
Mechanical accessories can be used to manufacture gears, impellers, bearings, handles, pipes, pipe fittings, battery slots, etc.
The specific varieties of auto parts include steering wheels, instrument panels, fan blades, fenders, handles and armrests.
Other products All kinds of chemical corrosion-resistant pipes, gold-plated products, stationery, imitation wood products, etc.
