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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.
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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.
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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).
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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.
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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.
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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
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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
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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.
Read more at Buzzle
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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.