Polystyrene (PS) is a polymer synthesized from styrene monomers by free radical addition polymerization, and its chemical formula is (C8H8)n. It is a colorless and transparent thermoplastic with a glass transition temperature higher than 100°C, so it is often used to make various disposable containers that need to withstand the temperature of boiling water, as well as disposable foam lunch boxes.

On October 27, 2017, the World Health Organization’s International Agency for Research on Cancer published a preliminary list of carcinogens for reference, and polystyrene was included in the list of Class 3 carcinogens.

History

Polystyrene is different from other plastics. As early as the second half of the 15th century, people have used its natural product, the resin of coniferous trees called “balsam”. However, it was not studied from a chemical perspective until 1836, when Simon of Germany separated styrene monomers from balsam resin by distillation and named it “Styrene”.

Simon obtained polystyrene by polymerization of styrene in 1839, and he thought it was a product of oxidation. In 1845, Blyth and Hoffman denied this oxidation theory and thought it was a solid styrene, thus naming it “Metastylene”.

In 1869, Berthelot of France discovered that styrene can be synthesized from benzene and ethylene. Later, in 1920, Staudinger of Germany conducted experiments on the polymerization and cracking of styrene, and proposed that polystyrene is a linear polymer formed by styrene monomers linked together, and used it as evidence to confirm the concept of polymers, thus establishing the polymer theory.

The industrialization of polystyrene was based on its interest in being a glassy, ​​transparent insulating material, but the industrialization of the synthetic raw material styrene was relatively difficult. On the other hand, in 1933, in the research on synthetic rubber conducted in Germany, the copolymerization of butadiene and styrene was successfully used to prepare styrene-butadiene rubber, which was valued as a strategic material, thus quickly promoting the industrialization of styrene. In 1934, styrene was successfully synthesized by dehydrogenation of ethylbenzene, and a year later, in 1935, the industrialization of polystyrene was also successful.

Synthesis process

Polystyrene can polymerize in the presence of an initiator or catalyst by a free radical mechanism or an ionic mechanism. Industrially produced polystyrene is polymerized by a free radical mechanism using an initiator. The polymerization can be carried out in bulk, suspension, solution or emulsion.

Production

As people’s quality of life improves, the consumption level of polystyrene products continues to rise, and the market demand for polystyrene has increased. In 2016, China’s polystyrene production was 2.2078 million tons, 2.4313 million tons in 2017, and 2.5624 million tons in 2018. China’s polystyrene production capacity has remained stable. In 2018, the annual production capacity of the polystyrene industry was 3.39 million tons.

Physical and chemical properties

Density: 1.05g/cm³

Conductivity : 10-16 S/m

Thermal conductivity : 0.08 W/(m·K)

Young’s modulus : 3000-3600 MPa

Tensile strength : 46–60 MPa

Elongation: 3-4%

Charpy impact test : 2–5 kJ/m²

Glass transition temperature : 80-100℃

(Glass transition temperature : 100°C (or 105°C) for atactic polystyrene and 100°C for isotactic polystyrene)

Thermal expansion coefficient : 8×10^-5/K

Heat capacity : 1.3kJ/(kg·K)

Water absorption : 0.03–0.1

Degradation: 280℃

The glass transition temperature of polystyrene is 80-105℃, the amorphous density is 1.04-1.06g/ cm3, the crystal density is 1.11-1.12g/cm3, the melting temperature is 240℃, and the resistivity is 10^20-10^22 Ω·cm. The thermal conductivity is 0.116W/(m·K) at 30℃. Ordinary polystyrene is an amorphous random polymer with excellent thermal insulation, insulation and transparency. The long-term use temperature is 0-70℃, but it is brittle and easy to crack at low temperatures. In addition, there are isotactic, syndiotactic and atactic polystyrene. Isotactic polymers are highly crystalline, and syndiotactic polymers are partially crystalline.

Toxicology

Minimum lethal dose (TD L0) for rat injection: 200 mg/kg.

Acute poisoning symptoms: Toxicity is related to the amount of unpolymerized monomer, namely styrene, in the polymer, and is mainly a strong irritant to the respiratory tract.

Emergency measures

First aid measures

Skin contact: Take off contaminated clothes and rinse with running water.

Eye contact: Lift the eyelids and rinse with running water or saline. Seek medical attention.

Inhalation: Leave the scene to fresh air. If breathing is difficult, give oxygen. Seek medical attention.

Ingestion: Drink plenty of warm water and induce vomiting. Seek medical attention.

Leakage treatment

Isolate the leaked contaminated area and restrict access. Cut off the fire source. It is recommended that emergency response personnel wear dust masks (full-face masks) and protective clothing. Collect with a clean shovel into a dry, clean, covered container and transfer to a safe place.

Large spills: Collect and recycle or transport to a waste disposal site for disposal.

Firefighting methods

Firefighters must wear gas masks and full-body fire suits and extinguish fires in the upwind direction.

Fire extinguishing agents: water spray, foam, dry powder, carbon dioxide, sand.

Classification

Polystyrene (PS) includes ordinary polystyrene, expanded polystyrene (EPS), high impact polystyrene (HIPS) and syndiotactic polystyrene (SPS). Ordinary polystyrene resin is non-toxic, odorless, colorless transparent particles, glass-like brittle materials, and its products have extremely high transparency, light transmittance can reach more than 90%, good electrical insulation performance, easy coloring, good processing fluidity, good rigidity and good chemical corrosion resistance. The disadvantages of ordinary polystyrene are brittleness, low impact strength, easy stress cracking, poor heat resistance and inability to resist boiling water.

Property

Ordinary polystyrene resin is an amorphous polymer. The side groups of the polystyrene macromolecular chain are benzene rings. The random arrangement of the large-volume side groups of benzene rings determines the physical and chemical properties of polystyrene, such as high transparency, high rigidity, high glass transition temperature, and brittleness. Expandable polystyrene is made by impregnating ordinary polystyrene with a low-boiling point physical foaming agent. It foams when heated during the processing and is specially used to make foam plastic products. High-impact polystyrene is a copolymer of styrene and butadiene, with butadiene as the dispersed phase, which improves the impact strength of the material, but the product is not transparent. Syndiotactic polystyrene is a syndiotactic structure produced using metallocene catalysts. It is a new type of polystyrene with good performance and belongs to engineering plastics.

Material characteristics

PS generally has a head-to-tail structure, with a saturated carbon chain as the main chain and a conjugated benzene ring as the side group, which makes the molecular structure irregular, increases the rigidity of the molecule, and makes PS a non-crystalline linear polymer. Due to the presence of benzene rings, PS has a higher Tg (80-105°C), so it is transparent and hard at room temperature. Due to the rigidity of the molecular chain, it is easy to cause stress cracking.

Polystyrene is colorless and transparent, can be freely colored, and its relative density is second only to PP and PE. It has excellent electrical properties, especially good high-frequency characteristics, second only to F-4 and PPO. In addition, its light stability is second only to methacrylic resin, but its radiation resistance is the strongest among all plastics. The most important feature of polystyrene is that it has excellent thermal stability and fluidity when melted, so it is easy to mold and process, especially injection molding, and is suitable for mass production. The molding shrinkage is small and the dimensional stability of the molded product is also good.

Mechanical properties

Polystyrene molecules and their aggregated structure determine that it is a rigid and brittle material, which exhibits brittle fracture under stress.

Thermal properties

The characteristic temperatures of polystyrene are: brittle temperature of about -30°C, glass transition temperature of 80-105°C, melting temperature of 140-180°C, and decomposition temperature of more than 300°C. Since the mechanical properties of polystyrene decrease significantly with the increase of temperature and its heat resistance is poor, the continuous use temperature is about 60°C and the maximum should not exceed 80°C. The thermal conductivity is low, 0.04-0.15W/(m·K), and is almost unaffected by temperature, so it has good thermal insulation.

Electrical properties

Polystyrene has good electrical properties, with volume resistivity and surface resistivity as high as 10 16 ～ 10 18Ω·cm and 10 15 ～ 10 18Ω respectively. The dielectric loss tangent is extremely low and is not affected by changes in frequency, ambient temperature, and humidity, making it an excellent insulating material.

Optical performance

Polystyrene has excellent optical properties, with a light transmittance of 88% to 92% and a refractive index of 1.59 to 1.60. It can transmit all wavelengths of visible light, and its transparency is second only to acrylic polymers such as organic glass among plastics. However, due to its poor weather resistance, polystyrene will become turbid and yellow when exposed to sunlight and dust during long-term use or storage. Therefore, when using polystyrene to make highly transparent products such as optical components, it is necessary to consider adding an appropriate type and amount of antioxidant.

Chemical properties

It has good corrosion resistance, but poor solvent and oxidation resistance.

Polystyrene is resistant to various alkalis, salts and aqueous solutions. It is also stable to low-level alcohols and certain acids (such as sulfuric acid, phosphoric acid, boric acid, 10% to 30% hydrochloric acid by mass, 1% to 25% acetic acid by mass, and 1% to 90% formic acid by mass ). However, concentrated nitric acid and other oxidants can destroy it.

Polystyrene can be dissolved in many solvents with similar solubility parameters, such as acetone, tetrachloroethane, styrene, benzene, chloroform, xylene, toluene, carbon tetrachloride, methyl ethyl ketone, esters, etc. It is insoluble in mineral oil, aliphatic hydrocarbons, ether, phenol, etc., but can be swollen by them. Many non-solvent substances, such as higher alcohols and oils, can cause stress cracking or swelling of polystyrene.

Polystyrene is prone to aging under heat, oxygen and atmospheric conditions, causing breakage and coloration of the macromolecular chains. It is more susceptible to aging when the system contains trace amounts of monomers, sulfides and other impurities. Therefore, polystyrene products will turn yellow and become brittle after long-term use.

Production Application

Processability

The first step is pre-foaming or simple foaming, which sets the density of the final product. In this process, polymer particles containing a blowing agent are softened under heating conditions and the blowing agent evaporates. As a result, expansion occurs within each bead, forming a number of cells. The number of cells (and thus the final density) is controlled by the heating temperature and the heating time. During this process, the beads must remain dispersed and free-flowing.

In industrial production, the foaming process is to place the expandable PS directly in steam. Generally, the reaction is completed by continuous mixing of beads and steam in a stirred tank. The reaction equipment (such as a pre-foaming machine) is open to maintain the external pressure at normal pressure, and the expanded beads are allowed to overflow from the top. Some manufacturers use intermittent reactors to ensure a more balanced residence time or when certain expandable DPS require a relatively high temperature. After foaming, the beads must be aged to allow air to be gradually incorporated into the pores.

Step 2. First, place the matured pre-foamed beads into a mold with a specific cavity. For small and complex structure products, a venturi action device (such as a filling gun) should be used for molding. The beads are blown into the mold cavity with the help of air flow. Large products can fill the mold cavity by their own gravity. The mold cavity filled with granules is sealed and heated, and the beads soften due to the heat, causing the bubbles to expand. The beads foam and expand to fill the gaps between each other and bond into a uniform foam body. At this time, the foam body is still soft and withstands the pressure of the hot gas in the pores. Before removing the product from the mold, the gas must be allowed to seep out of the pores and the temperature must be lowered to stabilize the shape of the product. This is generally done by spraying water on the inner wall of the mold.

Since the molding mold is double-walled, the molding of foamed PS is called “steam chamber molding”. The size of the inner wall of the mold is the size of the actual product, and there are pores on the inner wall of the mold to allow steam to pass through the foam and diffuse the hot air. The space between the double walls forms a steam chamber, in which steam is introduced to heat the beads. For most products, the molding pressure of foamed PS is less than 276kPa. The mold is made of aluminum and cast into a certain shape according to the product requirements. The molding of foamed PS is an economical production method because of the low molding pressure and low molding equipment cost.

Applications

Polystyrene is often used to make foam plastic products. Polystyrene can also be copolymerized with other rubber- type polymer materials to produce various products with different mechanical properties. Common applications in daily life include various disposable plastic tableware, transparent CD boxes, etc. In the use of foamed polystyrene in building materials, it has been widely used in hollow floor sound insulation and heat insulation materials since 2003.

High Impact Polystyrene (HIPS)

High-impact polystyrene is an impact-resistant polystyrene product produced by adding polybutyl rubber particles to polystyrene. This polystyrene product adds micron-sized rubber particles and connects the polystyrene and rubber particles together through grafting. When impacted, the tip stress of the crack propagation will be released by the relatively soft rubber particles. Therefore, the crack propagation is hindered and the impact resistance is improved.

Styrene-acrylonitrile copolymer (SAN)

SAN is the abbreviation of Styrene Acrylonitrile, which is a copolymer of styrene and acrylonitrile. It is a colorless, transparent, polypropylene- based engineering plastic with high mechanical strength. The chemical stability of SAN is better than that of polystyrene. The transparency and UV resistance of SAN products are not as good as those of polymethyl methacrylate products, but the price is relatively cheap.

Acrylonitrile butadiene styrene copolymer (ABS)

ABS is the abbreviation of Acrylonitrile butadiene styrene, which is a copolymer of acrylonitrile, butadiene and styrene. It has the characteristics of high strength and low weight and is one of the commonly used engineering plastics.

SBS Rubber

SBS rubber is a three-block copolymer of poly(styrene – butadiene -styrene) structure. This material has the characteristics of both polystyrene and polybutadiene and is a durable thermoplastic rubber. SBS rubber is often used to make tires.

Use

Polystyrene is easy to process and has the advantages of transparency, low price, rigidity, insulation, good printability, etc. It can be widely used in the light industry market, daily decoration, lighting indication and packaging, etc. In the electrical field, it is a good insulating material and heat insulation material, and can be used to make various instrument housings, lampshades, optical and chemical instrument parts, transparent films, capacitor dielectric layers, etc.

It can be used in powder and emulsion cosmetics. It has good compressibility when used in pressed powder and can improve the adhesion of powder. It gives skin luster and lubricity and is a high-grade filler that replaces talcum powder and silica.

Environmental issues

Due to its low mass (especially in expanded form) and low residual value, polystyrene is not easily recyclable. Polystyrene is not normally recycled. However, the industry has made great progress in the recycling of expanded polystyrene, with new methods of densifying it. This method increases its density, usually by 15 slugs/ft³ (Translator’s note: 1 slugs/ft³=1.94055g/cm³) and forms the center of suitable recycling operations on clean polystyrene.

Material Type

Crafting Materials

plastic:

Polyethylene; polyvinyl chloride; polystyrene; polyvinyl alcohol; polypropylene; polyacrylic acid; polybutylene; polyisobutylene; polysulfone; polyoxymethylene; polyamide; polycarbonate; polylactic acid; polytetrafluoroethylene; polyethylene terephthalate; epoxy resin; phenolic resin; polyurethane

Synthetic Rubber:

Butadiene rubber; Styrene butadiene rubber; Nitrile rubber; Chloroprene rubber

synthetic fiber:

Polypropylene; polyester; nylon; acrylic; spandex; vinylon; nylon; dacron; keflon

Impact resistant

Impact polystyrene is an amorphous polymer formed by graft polymerization of styrene monomer and rubber, or a physical blend of polystyrene and rubber (usually polybutadiene rubber). The resulting polymer is tough, usually white (transparent grades are also available), and is very easy to extrude and mold. Its toughness is mainly determined by the ratio and amount of the rubber component. The representative properties of impact-resistant PS are: flexural strength and tensile strength are 13.8~48.3MPa (depending on the content of rubber and additives); elongation is 10~60%; gloss is 5~ 100%. Visual transparency ranges from excellent to poor, shrinkage is about 0.006, and the thermal expansion coefficient is the same as that of transparent PS. The performance of impact-resistant PS does not change after γ-ray sterilization, and it has the same solvent resistance as transparent PS. The melt index of impact-resistant PS is 1~10g/min, and the Vicat softening point is 215°F. The commercial production of impact-resistant polystyrene with enhanced performance has broad market prospects. Some of the special grades available include: ultra-high gloss grade, high transparency grade, wear-resistant grade, environmental stress resistance grade- ESCR grade, high modulus grade, low gloss grade, and grade with low residual monomer styrene content.

The outstanding properties of high-impact polystyrene are easy processing, excellent dimensional stability, high impact strength and high rigidity. For HIPS, it is only heat resistant. Oxygen permeability, UV stability and oil resistance have certain limits. Chemistry and properties High-impact polystyrene is made by dissolving polydiene rubber in styrene monomer before polymerization. Although HIPS can be made by suspension polymerization, the main method currently used in the industrial production of HIPS is bulk polymerization. In the bulk polymerization process, the mixture of styrene monomer/rubber/additives passes through a series of reactors with a conversion rate of 70-90%. The polymerization reaction needs to be heated or an initiator is added to complete the reaction, and then heated in a vacuum to remove volatile residual monomers from the resin, and then pelletized for sale.

The continuous development of polystyrene technology has enabled manufacturers to produce grades with more outstanding properties than standard PS. Many properties of polystyrene cannot be obtained at the same time. For example, to improve impact strength, you have to sacrifice gloss. Some new resins have appeared, which have the gloss of ABS and also have high toughness. Some grades have been developed that are resistant to various oils and fats when packaging food and chlorofluorocarbon ( CFC ) blowing agents when used in refrigerators. Flame retardant grades (UL V-0 and UL 5-V) of impact polystyrene have been produced and widely used in TV housings, commercial machines and electrical products. These resins are easier to process than many flame retardant engineering resins and are cheaper.

Developable

Used to make everything from tea cups to household insulation materials. The properties of foamed plastics (such as density and impact strength) depend on the size and distribution of the cells, which are controlled by the dispersion, percentage and volatility of the added foaming agent. Representative foaming agents are pentane and isopentane. Flame-retardant expanded polystyrene uses halogenated hydrocarbons as flame retardants and is widely used as sound insulation layers in buildings and in engineering. The foaming agent SAN has been used to make floating products and other gasoline-resistant products.

Copolymer

They have very good toughness. The main varieties are: styrene- acrylonitrile copolymer (SAN), styrene-maleic anhydride copolymer (SMA), styrene- butadiene – styrene copolymer (SBS), styrene- acrylate copolymer and their modified bodies. SAN has a higher heat deformation temperature than transparent PS, and its solvent resistance is also improved, with excellent anti-permeability. Rubber-modified SAN includes resins such as acrylonitrile-butadiene-styrene copolymer (ABS) and acrylonitrile- styrene -acrylate copolymer (ASA). S-MA has a higher heat deformation temperature than transparent PS, up to 40°F, and it has excellent transparency and gloss. SMA can be modified with rubber or reinforced with glass fiber. SBS and various SBS modified bodies can be used as components to improve impact resistance, flexibility and fluidity for the production of products with viscosity and bending resistance such as cement, shoe soles, asphalt felt, etc. SBS is also used to produce transparent impact-resistant PS. Styrene can be copolymerized with acrylic elastomers to produce transparent impact-resistant PS with excellent physical properties.