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Polystyrene is produced by the polymerization of the monomer styrene, which is a derivative of petroleum. If you look at the chemical structure of polystyrene, you'll see that it is composed of carbon and hydrogen atoms only. Thus, it is classified as a hydrocarbon. Now, if you observe the bonds in its chemical structure, you'll see that the carbon atoms are linked to one another by covalent bonds. Every alternate carbon atom on the polystyrene chain has a phenyl group (name given to benzene ring) attached to it. It is a long-chain hydrocarbon, and its chemical formula is C8H8)n. Given below is the chemical structure of polystyrene.

polystyrene

Styrene is an aromatic monomer, commercially manufactured from petroleum. Polystyrene is a vinyl polymer, manufactured from the styrene monomer by free radical vinyl polymerization.

Properties of Polystyrene

Now that we have seen the structure of polystyrene, let us delve a little deeper into its properties. Here we shall learn about the physical, mechanical, optical, thermal, electrical, and chemical properties of polystyrene.

Physical Properties

► The density of polystyrene can vary from 10kg/m3 to 50kg/m3.

► Unfilled polystyrene is amorphous, and has a glassy, sparkling appearance. It is also known as crystal polystyrene.

► An important property of extruded polystyrene is its buoyancy or ability to float in water. This makes it the perfect choice for making floating boards. If you've ever been to the swimming pool and noticed the colorful boards, you'll know what we're talking about!

► The viscosity of polystyrene, like all other non-Newtonian fluids, depends on the shear rate. It is the ratio of the shear stress to shear rate.

Here are the values for the physical properties of general purpose polystyrene (GPPS).

Physical Properties
Property Unit Value
Specific Gravity g/cm3 1.03 to 1.06
Apparent Density g/cm3 0.60 to 0.65
Water Absorption % 0.03 to 0.10

The physical properties of polystyrene are due to the presence of weak van der Waals forces between the chains of polymer. On heating, the forces weaken further, and the chains slide past one another. This is the reason polystyrene is highly elastic and softens when heated beyond its glass transition temperature.

Mechanical Properties

The mechanical properties of a polymer include its strength, elongation, modulus, impact strength, and toughness. Crystal forms of the polymer polystyrene have low impact strength. Polystyrene polymers get degraded on exposure to sunlight, due to photo oxidation, which affects its mechanical properties. The following table shows the value of the mechanical properties of general purpose polystyrene (GPPS).

Mechanical Properties
Property Unit Value
Tensile Modulus or Young's Modulus MPa 3000-3600
Tensile Strength MPa 30-60
Tensile Elongation % 1.0 to 5.0
Shear Modulus MPa 1400
Flexural Strength MPa 76
Flexural Modulus MPa 3200

Optical Properties

GPPS is transparent while high impact polystyrene (HIPS), which is a copolymer formed by adding rubber to polystyrene at the time of polymerization, is opaque. However, HIPS has gloss, which is measured by the percentage of light reflected by the surface of the polymer. Given below are the values of the optical properties of GPPS.

Optical Properties
Property Unit Value
Refractive Index - 1.58 to 1.59
Transmittance % 88 to 90
Haze % 0.10 to 1.1

Thermal Properties

Thermal properties are the properties exhibited by the substance when it is subjected to heat. These include the heat distortion temperature, glass transition temperature, thermal conductivity, etc. Polystyrene is a rigid, transparent thermoplastic, which is present in solid or glassy state at normal temperature. But, when heated above its glass transition temperature, it turns into liquid form that flows and can be easily used for molding and extrusion. It becomes solid again when it cools off. This property of polystyrene is used for casting it into molds with fine detail. Given below are the values of the thermal properties for GPPS.

Thermal Properties
Property Unit Value
Glass Transition Temperature °C 100
Specific Heat Capacity J/Kg-K 1250
Thermal Conductivity W/m-K 0.14
Thermal Expansion (20°C to 100°C) μm/m-K 120
Vicat Softening Temperature °C 100

Electrical Properties

Electrical properties are the properties of a substance that determine its response to an electric field. Given below are the values of these properties for GPPS.

Electrical Properties
Property Unit Value
Dielectric Strength MV/m 20
Dielectric Constant (at 1MHz) - 2.5
Volume Resistivity ohm-cm >1016
Arc Resistance sec 70

Chemical Properties

► Polystyrene is chemically inert, and does not react with most substances.

► It dissolves in some organic solvents. It is soluble in solvents that contain acetone, such as most aerosol paint sprays and cyanoacrylate glues.

► The transformation of carbon-carbon double bonds into less reactive single bonds in polystyrene, is the main reason for its chemical stability. Most of the chemical properties of polystyrene are as a result of the unique properties of carbon.

► It is highly flammable and burns with an orange yellow flame, giving off carbon particles or soot, as a characteristic of all aromatic hydrocarbons. Polystyrene, on complete oxidation, produces only carbon dioxide and water vapor.

Other Forms of Polystyrene

Unfilled polystyrene is also known as crystal polystyrene (PS) or general purpose polystyrene (GPPS). However, since crystal polystyrene is brittle, other polymers are added to polystyrene to improve its strength, and copolymers are formed as a result. One such copolymer is high impact polystyrene (HIPS), which is produced by adding polybutadiene rubber to polystyrene during the polymerization process. HIPS is tougher and has a greater impact strength than unfilled polystyrene.

There are different types of polystyrene. Extruded polystyrene (XPS) is one form of the polymer that has high tensile strength, with good elasticity. It is commonly known as Styrofoam™. The other common form of polystyrene is expanded polystyrene (EPS). Both EPS and XPS are made from similar materials but there are difference between the two. XPS has a higher density as compared to EPS, due to the absence of air passages between its cells. The higher density makes XPS more rigid and strong. Also, XPS is waterproof and is an effective thermal insulator.

Uses

Polystyrene was first produced in Germany, in the year 1930, by I.G. Farben. Since then, it has come a long way, and today, it is one of the most widely manufactured polymers in the world, second only to polyethylene. A key reason for this is the fact that it is a thermoplastic. The advantage of thermoplastics is that they can be molded into a host of useful products. Also, being clear and transparent, it allows for the addition of various colors. These colors are added to the plastic in its liquid state. One of the major uses of polystyrene is in the manufacture of polystyrene foam for packing objects for shipment. It is also used to manufacture disposable cutlery, plates, cups, etc. Medical and pharmaceutical equipment are also manufactured using this polymer.

In the market, you'll find polystyrene in both, pellet and sheet form. Extruded polystyrene has insulating properties, and is used in the making of common household items and toys. Polystyrene is not a toxic product, and is approved by the FDA for use in the manufacture of food containers. However, like all other plastics, it is not biodegradable. However, it can be easily recycled.