By,

JEFF JOSERoll No:101sem Msc.Biopolymer scienceCBPST,COCHIN

polystyrene

INTRODUCTION

Polystyrene (PS) is a synthetic aromatic polymer madefrom the monomer styrene, a liquid petrochemical. In chemical terms, polystyrene is a long chain hydrocarbon wherein alternating carbon centers are attached to phenyl groups (the name given to the aromatic ring benzene). Polystyrene's chemical formula is C8H8

Polystyrene was discovered in 1839 by Eduard Simon, from Berlin

General purpose polystyrene is clear, hard and brittle.It is a very inexpensive resin per unit weight

As a thermoplastic polymer, polystyrene is in a solid (glassy) state at room temperature but flows if heated aboveabout 100 °C, its glass transition temperature.

It is very slow to biodegrade and therefore a focus of controversy,since it is often abundant as a form of litter in the outdoor environment,particularly along shores and waterways especially in its foam form.

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 colourful boards, you'll know what we're talking about!

► Specific gravity 1.03 to 1.06 g/cm3 ► Apparent density .60 to .65 ► Water absorption 0.03-0.10 %

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).

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.

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 moulding and extrusion. It becomes solid again when it cools off. This property of polystyrene is used for casting it into moulds with fine detail. Given below are the values of the thermal properties for GPPS.

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.

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.

► High impact polystyrenes are susceptible to solvent crazing at the interface between the rubber particles and the polystyrene phase. The resistance of polystyrene to this crazing is referred to as environmental stress crack resistance (ESCR).

► For food-packaging applications, such as butter tubs and deli containers, polystyrenes with high ESCR properties are desirable. Increasing the percentage of gel, percentage grafting, and rubber particle size can increase stress crack resistance. Residual levels of low molecular weight materials are also important to polystyrene performance.

Some of the chemical impurities in the polystyrene are styrene monomer and ethyl benzene solvent. Residual levels of styrene below 200 ppm and ethyl benzene levels below 30 ppm are obtainable for very specialized applications

PREPARATION OF THE MONOMER

The bulk of commercial styrene is prepared by the Dow process or some similarsystem. The method involves the reaction of benzene and ethylene toethylbenzene, its dehydrogenation to styrene and a final finishing stage. It istherefore useful to consider this process in each of the three stages.1) Preperation of ethylbenzeneEthylbenzene is prepared by reaction of ethylene and benzene in the presence ofa Friedel -Crafts catalyst such as aluminum chloride at about 95°C

2)DehydrogenationStyrene is produced from the ethylbenzene by a process of dehydrogenation

3)Styrene purificationThe dehydrogenation reaction produces 'crude styrene' which consists ofapproximately 31.0% styrene, 61% ethylbenzene and about 2% of aromatichydrocarbon such as benzene and toluene with some tarry matter.

POLYMERIZATION

The polymer maybe prepared by mass, suspension, solution and emulsion methods.

MASS POLYMERIZATION

Mass polymerization has advantages of apparent simplicity and gives a polymer of high clarity and good electrical insulation characteristics Styrene is polymerized by heating in a pre polymerization kettle at 80*for two days until a 33-35%conversion to polymer is reachedThe monomer mixture is run into a tower about 25mtr high . The tower is fitted with heating and cooling jackets and internally with a number of heating and cooling coils. The top of the tower is maintained at a temperature of 110* C . The high bottom temperature not only ensure a high conversion but boils of the residue styrene from the polymer. The base of tower forms the hopper of the extruder fro which the melt emerges as filaments which are cooled, disintegrated and packed as shown

TOWER PROCESS

SOLUTION POLYMERIZATION

Styrene and solvent are blended together and pumped through number of reactors. At temperature of 225* the solvent and polymer is fed into extruder unit, extruded as filaments, granulated and stored. Residual monomer and some very low molecular weight polymers are removed.

SOLUTION POLYMERIZATION

• In this polymerization monomer is suspended in droplets. • The reaction is initiated such as benzyl peroxide• Suspension agents such as PVA talc are added to prevent cohering• Particle size depend on quality suspension agent and speed of agitation• After polymerization unreacted monomer are distilled and polymer is washed and dried

STRUCTURE PROPERTY RELATION

• Ps is linear hydrocarbon being thermoplastic in nature• Specific gravity of polystyrene is 1.054• Amorphous in nature as the specific position of benzene ring however sufficiently enough to inhibit crystallization• Due to benzene ring causing stiffening effect• Solubility parameter is 18.6Mpa hence soluble in benzene,styrene,toluene. The presence of benzene makes it more reactive than polyethylene• Due to phenyl group poly styrene undergoes chlorination, nitration, hydrogenation and halogenations.• Excellent electrical properties

PROCESSING CONDESIRATIONS

•The negligible water absorption avoids the need of pre-drying•Specific heat of PS is less, therefore higher plasticizing machines should be used•Due to amorphus in nature, polymer gives low mould shrinkage

PROCESSING TECHINIQUES

INJECTION MOULDING

•The material is fed into a hopper located at the rearand top of the machine

•Each successive cycle makesthe plunger move forward, forcing the material into theheating cylinder.

•The material progressively movescloser to the nozzle, which is removable for fast, easycleaning and changeover.

The mold is kept at a constant temperature, usually justlow enough to solidify and cool the material. At the righttime in the cycle, the die opens and a part is removedeither mechanically or by hand.

EXTRUSION

High impact strength polystyrene is widely used to make sheets used in thermoforming. An extruder screw have L/D ratio 25:1 and 30:1 and compression ratio 2:1 to 3:1 are recommended to ensure a uniform melt temperature .recommended process temperature is about 150* - 220*

COMPRESSION MOULDING

Compression molding is the term applied to thetype of molding wherein the molding compound isplaced in an open mold cavity, the mold is closed, thenpressure and heat are applied until the material is softenedand fills out the entire cavity. After the softeningprocess, with the pressure applied, the mold is cooledand the material hardens into its exact shape. In a specifiedamount of time, the mold is opened to eject a finishedpart. Compression molding is not generally used withPolystyrene. However, under certain conditions andneeds, and with the proper techniques, it can and is beingdone.

THERMOFORMING

HIPS is one of the major thermoforming materials. Solid sheets of HIPS is extruded and shaped by a variety of forming methods In this process vacuum is drawn between sheet and mould or pressure is applied to the mould plug while air is drawn out from the space the sheet and mould.Typical forming temperature is around 130-180*C and pdt removal temperature about 85*C

CASTING

Casting comprises pouring a molten mass into an open form in which it cools into a single faced shape. With the rapid growth in electronic .EMI &RMI shielding has become important.PS is shielded by coating molded solid and structural foam parts with paints made conductive by metal such as copper, zinc, silver, or with graphite

SOME COMMON POLYSTYRENE TRADE NAMES

ARCO Polymers, Inc., Dylene®Philadelphia, PA

Bacon Felt Company, Inc., Lustrex®East Taunton, MA

Dow Chemical Company, Styron®Midland, MI

Georgia-Pacific Resins, Bakelite®Atlanta, GA

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