Polystyrene Encyclopedia

Polystyrene (abbreviated PS) refers to a polymer synthesized from styrene monomer through radical addition polymerization reaction, the 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.

Polystyrene chemical formula (C8H8)

Melting point: 212 ℃

Boiling point: 293.4 ℃

Density: 1.05 g/cm³

Appearance: colorless and transparent thermoplastic

Application: description of the safety of electronic appliances

development path

In 1829, the Germans first extracted polystyrene from natural resin.

In 1930, BASF began commercial production of polystyrene in Germany.

In 1934, Dow began producing polystyrene in the United States.

In 1954, Dow began to produce polystyrene foam.

Physical and chemical properties

Simple structure:

Simple structure of polystyrene

Simple structure of polystyrene

Density: 1.05 g/cm3

Conductivity: (σ) 10-16 S/m

Thermal conductivity: 0.08W/(m·K)

Young's modulus: (E) 3000-3600 MPa

Tensile strength: (σt) 46–60 MPa

Elongation length: 3–4%

Charpy impact test: 2–5 kJ/m2

Glass transition temperature: 80-100℃

Coefficient of thermal expansion: (α) 8×10-5/K

Heat capacity: (c) 1.3 kJ/(kg·K)

Water absorption: (ASTM) 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 1020～1022Ω·cm. The thermal conductivity is 0.116 watts/(m·K) at 30°C. Normal polystyrene is an amorphous random polymer with excellent thermal insulation, insulation and transparency. The long-term use temperature is 0-70°C, but it is brittle and easy to crack at low temperature. There are also isotactic and syndiotactic and atactic polystyrene. Identical polymers have high crystallinity, and syndiotactic polymers have partial crystallinity.

classification

Polystyrene in food packaging

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, like a glass-like brittle material, its products have extremely high transparency, light transmittance can reach more than 90%, good electrical insulation performance, easy to color, and process Good fluidity, good rigidity and good chemical resistance. The disadvantages of ordinary polystyrene are its brittleness, low impact strength, stress cracking, poor heat resistance, and resistance to boiling water.

Attribute editing voice

Ordinary polystyrene resin is an amorphous polymer. The side group of polystyrene macromolecular chain is benzene ring, and the random arrangement of bulky side group is benzene ring determines the physical and chemical properties of polystyrene, such as high transparency. . Rigidity. High glass transition temperature, brittleness and so on. Expandable polystyrene is made by impregnating low-boiling physical foaming agent in ordinary polystyrene. It is heated and foamed during processing, and is specially used to make foam plastic products. High-impact polystyrene is a copolymer of styrene and butadiene, and butadiene is the dispersed phase, which improves the impact strength of the material, but the product is opaque. Syndiotactic polystyrene has a syndiotactic structure and is produced using metallocene catalysts. The developed new polystyrene varieties have good performance and belong to engineering plastics.

Features

PS generally has a head-to-tail structure, the main chain is a saturated carbon chain, and the side group is a conjugated benzene ring, 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 the benzene ring, PS has a high Tg (80～105℃), 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 to F-4 and PPO. In addition, it is second only to methacrylic resin in terms of light stability, but its radiation resistance is the strongest among all plastics. The most important feature of polystyrene is that it has very good thermal stability and fluidity during melting, so it is easy to mold and process, especially easy to injection mold, 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 they are rigid and brittle materials, which exhibit brittle fracture under stress.

Thermal performance

The characteristic temperature of polystyrene is: embrittlement temperature of about -30°C, glass transition temperature of 80 to 105°C, melting temperature of 140 to 180°C, and decomposition temperature of 300°C or more. Since the mechanical properties of polystyrene decrease with the increase of temperature and the heat resistance is poor, the continuous use temperature is about 60℃, and the highest should not exceed 80℃. The thermal conductivity is low, ranging from 0.04 to 0.15W/(m·K), and it is hardly changed by temperature, so it has good thermal insulation.

Electrical properties

Polystyrene has good electrical properties, with volume resistivity and surface resistivity as high as 1016～1018Ω·cm and 1015～1018Ω, respectively. The dielectric loss tangent value is extremely low, and it is not affected by frequency, environmental temperature, and humidity changes. It is 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 visible light of all wavelengths. The transparent material is second only to acrylic polymers such as plexiglass in plastics. . However, because polystyrene has poor weather resistance and is exposed to sunlight and dust during long-term use or storage, turbidity and yellowing will occur. Therefore, when polystyrene is used to make high-transparent products such as optical parts, it is necessary to consider adding appropriate varieties and The amount of antioxidant.

Chemical properties

Good corrosion resistance, poor solvent resistance and oxidation resistance.

Polystyrene is resistant to various alkalis, salts and aqueous solutions. It is resistant to lower alcohols and certain acids (such as sulfuric acid, phosphoric acid, boric acid, 10% to 30% hydrochloric acid, and 1% to 25% acetic acid. , Formic acid with a mass fraction of 1% to 90%) is also stable, but concentrated nitric acid and other oxidants can destroy it.

Polystyrene can be dissolved in many solvents similar to its solubility parameters, such as acetone, tetrachloroethane, styrene, benzene, chloroform, xylene, toluene, carbon tetrachloride, methyl ethyl ketone, esters, etc., and insoluble in minerals Oils, aliphatic hydrocarbons, ether, phenol, etc., but can be swelled 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 macromolecular chains to break and develop color. When the system contains trace monomers, sulfides and other impurities, it is more likely to age. Therefore, polystyrene products are long-term It will turn yellow and brittle during use.

Production application

Expandable processing

The first step is pre-foaming or simple foaming to set the density of the final product. In this process, the polymer particles containing the blowing agent soften under heating, and the blowing agent volatilizes. As a result, swelling occurs in each bead, forming many cells. The number of cells (final density) is controlled by heating temperature and heating time. During this process, the beads must remain dispersed and free-flowing.

In industrial production, the foaming process is carried out by placing 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 (like a pre-foaming machine) is designed to maintain the outside world. The pressure is atmospheric and open, and the foamed beads overflow from the top. Some production plants use intermittent kettles to ensure a more balanced residence time or when higher temperatures are required for certain expandable DPS. After foaming, the beads must undergo aging treatment to gradually incorporate air into the cells.

The second step. First, put the matured pre-expanded beads into a mold with a specific cavity. For small and complex structure products, venturi action equipment (such as a filling gun) should be used when forming. The beads are blown into the mold cavity by means of air flow. Large-scale products can fill the cavity according to their own gravity. The mold cavity filled with pellets is sealed and heated, the beads are softened by the heat, and the cells expand. The beads expand to fill the gaps between each other and bond to form a uniform foam. At this time, the foam is still soft and bears the pressure of the hot gas in the cells. Before taking out the product from the mold, the gas must be allowed to ooze out of the cells 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 die 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. In order to make the steam penetrate the foam and make the hot air diffuse out. The space between the double walls forms a steam chamber into which steam for heating the beads is passed. For most products, the molding pressure of foamed PS is lower than 276kPa. The mold is made of aluminum and cast into a certain shape according to the requirements of the product. The molding of foamed PS is an economical production method due to its low molding pressure and low cost of molding equipment.

application

Polystyrene is often used to make foam plastic products. Polystyrene can also be copolymerized with other rubber-type polymer materials to produce products with different mechanical properties. Common applications in daily life include various disposable plastic tableware, transparent CD boxes and so on. Expanded polystyrene is used in building materials and has been widely used in sound insulation and heat insulation for hollow floor slabs 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 will add micron-sized rubber particles and connect the polystyrene and rubber particles together by grafting. When subjected to an impact, the tip stress of the crack propagation will be released by the relatively soft rubber particles. Therefore, the propagation of cracks is hindered, and the impact resistance is improved.

Styrene-acrylonitrile copolymer (SAN)

SAN is the abbreviation of Styrene Acrylonitrile. It is a copolymer of styrene and acrylonitrile. It is a colorless and transparent polypropylene-based engineering plastic with high mechanical strength. The chemical stability of SAN is better than that of polystyrene. The transparency and anti-ultraviolet properties of SAN products are not as good as 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. With high strength and low weight, it is one of the commonly used engineering plastics.

SBS rubber

SBS rubber is a three-stage 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 shape, and has the advantages of transparency, low cost, rigidity, insulation, and good printability. It can be widely used in the light industry market, daily decoration, lighting instructions and packaging. In the electrical aspect, it is a good insulating material and heat insulation material, which can be used to make various instrument casings, lampshades, optical chemical instrument parts, transparent films, capacitor dielectric layers, etc.

It can be used in powder and emulsion cosmetics. Used for pressed powder, it has good compressibility and can improve the adhesion performance of powder. It is a high-grade filler that replaces talcum powder and silicon dioxide to give the skin luster and smoothness.

environmental issue

Due to its low quality (especially foam type) and low residual value, polystyrene is not easy to recycle. Generally, polystyrene cannot be recycled by the kerbside method. However, the industry has also made great improvements to the reuse of expanded polystyrene, and many new methods for densification have emerged. This method of increasing its density usually increases the density by 15 slugs/ft3 (Translator’s Note: 1slugs/ft3=1.94055g/cm3) and forms the center of a suitable regeneration operation on clean polystyrene.

The discarded polystyrene cannot enter the biogeochemical cycle through biodegradation and photolysis. Due to the low specific gravity of expanded polystyrene, it floats on the water or drifts with the wind, causing subjective landscape damage. According to the California Coastal Commission (California Coastal Commission) survey, polystyrene has been the main marine drift. Ingestion of such plastic marine organisms can cause damage to their digestive system.

material type

Synthetic material

plastic:

Polyethylene; Polyvinyl Chloride; Polystyrene; Polyvinyl Alcohol; Polypropylene; Polyacrylic Acid; Polybutene; Polyisobutylene; Polysulfone; Polyoxymethylene; Polyamide; Polycarbonate; Polylactic Acid; Polytetrafluoroethylene; Poly Ethylene terephthalate; epoxy resin; phenolic resin; polyurethane

synthetic rubber:

Butadiene rubber; Styrene butadiene rubber; Nitrile rubber; Neoprene

synthetic fiber:

Polypropylene; Polyester; Nylon; Acrylic; Spandex; Vinylon; Nylon; Dacron; Keflon

Impact resistant

Impact-resistant 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 has toughness, usually white (there are also transparent grades), extrusion and molding is very easy. Its toughness is mainly determined by the ratio and usage of rubber components. The representative performance of impact-resistant PS is: flexural strength and tensile strength of 13.8-48.3MPa (different with the content of rubber and additives); elongation 10-60%; gloss 5-100%. The visual transparency ranges from excellent to poor, the shrinkage rate is about 0.006, and the thermal expansion coefficient is the same as that of transparent PS. The impact-resistant PS has no change in its performance after being irradiated by γ-ray sterilization and has the same solvent resistance as the 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 properties has broad market prospects. Some of the existing special grades include: ultra-high gloss grade, high transparency grade, abrasion resistance grade, environmental stress cracking resistance (ESCR), high modulus grade, low gloss grade, and grades with low residual monomer styrene content .

The outstanding characteristics of impact-resistant polystyrene are easy processing, excellent dimensional stability, high impact strength and high rigidity. For HIPS, it is only in heat resistance. There are certain limits in oxygen permeability, UV stability and oil resistance. Chemical and performance Impact polystyrene is made by dissolving polydiene rubber in styrene monomer before polymerization. Although the suspension polymerization method can be used to prepare HIPS, the bulk polymerization method is currently used in the industrial production of HIPS. In the bulk polymerization process, the mixture of styrene monomer/rubber/additive passes through a series of reactors with a conversion rate of 70-90%. In the polymerization reaction, heating or adding an initiator is required to complete the reaction, and then heating in a vacuum to remove volatile residual monomers from the resin, and then pelletizing and selling.

The impact polystyrene performance test is divided into several levels according to its relative impact strength:

The impact strength of notched cantilever beams of medium impact resistance grade is generally 0.6-1./i;

The impact resistance of high-impact grades is 1.5-2.5ftlb/in;

The impact strength of the extremely high impact resistance level is >2ftlb/in

Some HIPS grades have an impact strength value of up to 6ftlb/in, but this resin is usually used in blended resins to improve the impact strength of low-strength grade resins.

Other important performance testing matters of standard HIPS are as follows: bending strength 13.8-55.1MPa; tensile strength 13.8-41.4MPa; elongation at break 15-75%; density 1.035-1.04 g/mL; Vicat softening point 185- 220°F.

The only industrially mixed HIPS alloy is its blend with polyphenylene ether. The heat resistance and toughness of this blend are outstanding, but the product price is much higher than that of HIPS alone.

The continuous development of polystyrene technology allows the production plant to produce a grade with more outstanding performance than the standard PS. Many properties of polystyrene cannot be achieved at the same time. If you want to improve the impact strength, you have to sacrifice gloss and so on. Some new resins appearing at present have the glossiness of ABS and high toughness at the same time. Some grades, such as those that can withstand various oils and fats when packaging food, and chlorofluorocarbon (CFC) foaming agents when used in refrigerators, have also been developed. Flame-retardant (UL V-0 and UL 5-V), impact-resistant polystyrene has been produced and widely used in TV housings, business machines and electrical products. The processing operation of these resins is easier than many flame retardant engineering resins, and the price is lower.

Occurrence type

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

The following and good grades of low temperature impact resistance.

Copolymer

Their toughness is very good. The main varieties are: styrene-acrylonitrile copolymer (SAN), styrene-maleic anhydride copolymer (SMA), styrene-butadiene-styrene copolymer (SBS), styrene-acrylate copolymer and Modified body with them as the base material. The heat distortion temperature of SAN is higher than that of transparent PS, its solvent resistance is also improved, and it has excellent penetration resistance. Rubber-modified SAN includes resins such as acrylonitrile-butadiene-styrene copolymer (ABS) and acrylonitrile-styrene-acrylate copolymer (ASA). The heat distortion temperature of S-MA is higher than that of transparent PS, which can reach 40°F. It has excellent transparency and gloss. SMA can be modified with rubber or reinforced with glass fiber. Various modifications of SBS and SBS can be used as components to improve impact resistance, flexibility and fluidity for the production of viscous and anti-bending products such as cement, shoe soles, asphalt felt, etc. SBS is also used to produce transparent impact resistance PS. Styrene can be copolymerized with acrylate elastomer to produce transparent impact-resistant PS with excellent physical properties.