POLYSTYRENE

Polystyrene  a synthetic aromatic polymer made from

the monomer styrene. Polystyrene can be solid or foamed. General purpose polystyrene is

clear, hard, and rather brittle. It is an inexpensive resin per unit weight. It is a rather poor

barrier to oxygen and water vapour and has a relatively low melting point. Polystyrene is one

of the most widely used plastics, the scale of its production being several billion kilograms

per year. Polystyrene can be naturally transparent, but can be coloured with colorants. Uses

include protective packaging (such as packing peanuts and CD and DVD cases), containers

(such as "clamshells"), lids, bottles, trays, tumblers, and disposable cutlery

As a thermoplastic polymer, polystyrene is in a solid (glassy) state at

room temperature but flows if heated above about 100 °C, its glass transition temperature. It

becomes rigid again when cooled. This temperature behaviour is exploited for extrusion, and

also for moulding and vacuum forming, since it can be cast into moulds with fine detail.

Polystyrene is very slow to biodegrade and is therefore a focus of

controversy among environmentalists. It is increasingly abundant as a form of litter in the

outdoor environment, particularly along shores and waterways, especially in its foam form,

and also in increasing numbers in the Pacific Ocean.

HISTORY OF POLYSTYRENE

Polystyrene was discovered in 1839 by Eduard Simon, an apothecary from

Berlin. From storax, the resin of the Turkish sweet gum tree Liquidambar orientalis, he

distilled an oily substance, a monomer that he named styrol. Several days later, Simon found

that the styrol had thickened, presumably from oxidation, into a jelly he dubbed styrol oxide

("Styroloxyd"). By 1845 Jamaican-born chemist John Buddle Blyth and German

chemist August Wilhelm von Hofmann showed that the same transformation of styrol took

place in the absence of oxygen. They called their substance metastyrol. Analysis later showed

that it was chemically identical to Styroloxyd. In 1866, Marcelin Berthelot correctly

identified the formation of metastyrol from styrol as a polymerization process. About 80

years later it was realized that heating of styrol starts a chain reaction that

produces macromolecules, following the thesis of German organic chemist Hermann

Staudinger (1881–1965). This eventually led to the substance receiving its present name,

polystyrene.

The company I. G. Farben began manufacturing polystyrene

in Ludwigshafen, about 1931, hoping it would be a suitable replacement for die-cast zinc in

many applications. Success was achieved when they developed a reactor vessel that extruded

polystyrene through a heated tube and cutter, producing polystyrene in pellet form.

In 1941, Dow Chemical invented a Styrofoam process. Before 1949, the chemical engineer

Fritz Stastny (1908–1985) developed pre-expanded PS beads by incorporating aliphatic

hydrocarbons, such as pentane. These beads are the raw material for moulding parts or

extruding sheets. BASF and Stastny applied for a patent that was issued in 1949. The

moulding process was demonstrated at the Kunststoff Messe 1952 in Düsseldorf. Products

were named Styropo

The crystal structure of isotactic polystyrene was reported by Giulio Natta.In

1954, the Koppers Company in Pittsburgh, Pennsylvania, developed expanded polystyrene

(EPS) foam under the trade name Dylite. In 1960, Dart Container, the largest manufacturer of

foam cups, shipped their first order. In 1988, the first U.S. ban of general polystyrene foam

was enacted in Berkeley, California.

STRUCTURE

In chemical terms, polystyrene is a long chain hydrocarbon where in

alternating carbon centres are attached to phenyl groups (the name given to the aromatic

ring benzene). Polystyrene's chemical formula is (C8H8)n; it contains the chemical elements

carbon and hydrogen.

The material's properties are determined by short-range van der

Waals attractions between polymers chains. Since the molecules are long hydrocarbon chains

that consist of thousands of atoms, the total attractive force between the molecules is large.

When heated (or deformed at a rapid rate, due to a combination of viscoelastic and thermal

insulation properties), the chains are able to take on a higher degree of conformation and slide

past each other. This intermolecular weakness (versus the high intramolecular strength due to

the hydrocarbon backbone) confers flexibility and elasticity. The ability of the system to be

readily deformed above its glass transition temperature allows polystyrene (and thermoplastic

polymers in general) to be readily softened and molded upon heating.

Extruded polystyrene is about as strong as an unalloyed aluminium, but

much more flexible and much lighter (1.05 g/cm3 vs. 2.70 g/cm3 for aluminium).

POLIMERIZATION

Polystyrene results when styrene monomers interconnect. In the

polymerization, the carbon-carbon pi bond (in the vinyl group) is broken and a new carbon-

carbon single (sigma) bond is formed, attaching another styrene monomer to the chain. The

newly formed sigma bond is much stronger than the pi bond that was broken, thus it is very

difficult to depolymerize polystyrene. About a few thousand monomers typically comprise a

chain of polystyrene, giving a molecular weight of 100,000–400,000.

A 3-D model would show that each of the chiral backbone carbons lies at

the centre of a tetrahedron, with its 4 bonds pointing toward the vertices. Consider that the -

C-C- bonds are rotated so that the backbone chain lies entirely in the plane of the diagram.

From this flat schematic, it is not evident which of the phenyl (benzene) groups are angled

outward from the plane of the diagram, and which ones are inward. The isomer where all of

the phenyl groups are on the same side is called isotactic polystyrene, which is not produced

commercially.

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.

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).

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).

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

General purpose polystyrene (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.

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 general purpose

polystyrene (GPPS).

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 general purpose polystyrene (GPPS).

PROPERTY UNIT VALUE

Dielectric Strength MV/m 20

Dielectric Constant (at

1MHz)

- 2.5

Volume Resistivity ohm-cm >10^16

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 vapour.

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THE USES OF POLYSTYRENE

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 moulded into a host of

useful products. Also, being clear and transparent, it allows for the addition of various

colours. These colours 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.