Polystyrene, referred to as PS. PS is a thermoplastic non-crystalline resin, which is polymerized from styrene monomers. It can be polymerized by a variety of synthetic methods. The main methods used in industry are bulk polymerization and suspension polymerization. PS is mainly divided into general-purpose polystyrene (GPPS, commonly known as transparent polystyrene), high-impact polystyrene (HIPS, commonly known as modified polystyrene) and expanded polystyrene (EPS).
Nature Introduction
General-purpose polystyrene is a thermoplastic resin that is a colorless, odorless, tasteless, shiny, transparent beaded or granular solid. Density 1.04-1.09, transparency 88%-92%, refractive index 1.59-1.60. Under stress, birefringence occurs, the so-called stress-optical effect. The melting temperature of the product is 150-180°C, the thermal decomposition temperature is 300°C, the heat deformation temperature is 70-100°C, and the long-term use temperature is 60-80°C. At 5-6°C lower than the heat deformation temperature, after annealing, the stress can be eliminated and the heat deformation temperature can be increased. If a small amount of α-methylstyrene is added during the production process , the heat resistance grade of general-purpose polystyrene can be improved.
It is soluble in aromatic hydrocarbons , chlorinated hydrocarbons, aliphatic ketones and esters, but can only swell in acetone . It is resistant to the effects of certain mineral oils, organic acids, alkalis, salts, lower alcohols and their aqueous solutions. It has low water absorption and can still maintain its mechanical properties and dimensional stability in a humid environment. Its optical properties are second only to acrylic resins. It has excellent electrical properties, high volume resistivity and surface resistivity, and is not affected by changes in temperature and humidity, nor by corona discharge. It also has good radiation resistance. Its main disadvantages are that it is brittle and easy to crack, has low impact strength, poor heat resistance, cannot withstand boiling water, and can only be used at lower temperatures and lower loads. It has poor sunlight resistance and is flammable. It emits black smoke and a special odor when burning.
Preparation
General-purpose polystyrene is produced by free radical polymerization or ionic polymerization using styrene as a monomer. The production methods include bulk polymerization, solution polymerization, suspension polymerization and emulsion polymerization. Currently, suspension polymerization and bulk polymerization are mainly used in industrial production.
1. Bulk polymerization method: put the styrene monomer into the prepolymerization kettle, add a small amount of additives and initiators, heat and stir at 95-115℃ for prepolymerization, and after the conversion rate reaches 20%-35%, send it into a tower reactor with a stirrer for continuous polymerization. The polymerization temperature is gradually increased to about 170℃ to achieve complete conversion. A small amount of unreacted styrene is released from the top of the tower and can be recycled. The polymer is continuously discharged from the bottom of the tower, and the finished product is obtained by extrusion and granulation, and then shipped out of the factory after packaging.
2. The suspension polymerization method uses styrene as the monomer, water as the medium, gelatin or starch, polyvinyl alcohol, hydroxyethyl cellulose and other protective colloids or insoluble inorganic salts such as magnesium carbonate, magnesium silicate, calcium phosphate and the like as dispersants, maleic anhydride-styrene copolymer sodium salt as the auxiliary dispersant, and benzoyl peroxide as the initiator, and initiates the polymerization at about 85°C. It is also possible to use no initiator and perform high-temperature polymerization at a high temperature of more than 100°C in a high-pressure polymerization kettle. The polymer is washed, separated and dried to obtain a colorless and transparent fine bead resin.
Processing
Polystyrene has good fluidity, good processing performance, easy coloring, and good dimensional stability. It can be processed into various parts by injection molding, extrusion molding , blow molding, foaming, thermoforming, bonding, coating, welding, machining, printing, etc., and is particularly suitable for injection molding.
During injection molding, materials can generally be used directly without drying. However, in order to improve the quality of the product, it can be pre-dried in a 55-70℃ blast oven for 1-2 hours. The specific processing conditions are roughly: barrel temperature of about 200℃, mold temperature of 60-80℃, injection temperature of 170-220℃, 60-150 MPa, and compression ratio of 1.6-4.0. In order to eliminate internal stress, the molded product can be treated at a constant temperature of 70℃ for 2-4 hours in an infrared lamp or blast oven.
During extrusion molding, the screw length-to-diameter ratio L/D is generally 17-24, air cooling is used, and the extrusion temperature is 150-200°C.
During blow molding, the desired product can be obtained by blow molding the parison obtained by injection molding and extrusion. The blow molding pressure is generally 0.1-0.3MPa.
Applications
Polystyrene has a broad application prospect in food packaging because it is transparent and has excellent rigidity, electrical properties and printing properties, especially good hygiene and low price. It has been widely used in the electromechanical industry, instrumentation, communication equipment industry, etc. as various instrument housings, lampshades, optical parts, instrument parts, transparent windows, transparent models, chemical acid storage tanks, acid delivery tanks, telecommunication parts, high-frequency capacitors, high-frequency insulation pads, brackets, inserts and frozen insulation materials. It is also widely used in various daily necessities, such as bottle caps , containers, decorations, buttons, combs, toothbrushes, soap boxes, cigarette boxes and toys.
Security
The raw materials used to produce resins can cause different degrees of irritation to human skin and mucous membranes, which can cause skin allergic reactions and inflammation. At the same time, attention should be paid to the harm of resin dust to the human body. Long-term inhalation of high-concentration resin dust can cause lung lesions. Most resins have common hazardous characteristics: they are flammable when exposed to open flames and high temperatures, and they are at risk of burning when in contact with oxidants. Therefore, operators should improve the operating environment, consciously separate the operating area from the non-operating area, make it as automated and closed as possible, and install ventilation facilities.
Polystyrene resin is usually packed in paper-plastic composite bags or other packaging bags, with a net weight of 25kg per bag. Other large packaging of different quality can also be used. It should be stored in a ventilated and dry warehouse, away from fire and direct sunlight, and should not be piled in the open air. Polystyrene resin is a non-dangerous product. It is strictly forbidden to use sharp tools such as iron hooks during loading and unloading, and it is forbidden to throw to avoid damaging the packaging bags. During transportation, it should not be exposed to the sun or rain, and should not be mixed with toxic, corrosive, flammable and explosive products.
Environment and Recycling
GPPS is cheap and is often made into various daily necessities, such as convenience bags. Since GPPS is difficult to degrade, it is easy to cause environmental pollution when the recycling system is not yet perfect, which is often called “white pollution”. Nowadays, people pay more and more attention to environmental protection issues. Therefore, when using such products, they should be recycled in separate categories. The recycling symbol of polystyrene is 6.
