Polystyrene:
Polystyrene was first produced commercially by Dow chemical company in 1937.
Polystyrene was first initially used for its excellent dielectric and optical properties.
Monomer Preparation:
Styrene is produced from the ethyl-benzene by a process of dehydrogenation at 630°C.
- Like many aromatic compounds, it has pleasant smell in pure form but looses the same due to traces of ketones and aldehydes if allowed to oxidize by exposure to air.
- It is a solvent for polystyrene and many synthetic rubbers, including SBR, but has only a very limited mutual solubility in water.
- It has a strong tendency to polymerize on heating or on exposure to UV light.
Polymerization
Polymerization methods
- Mass
- Suspension
- Solution
Mass Polymerization (Tower Process)
- Styrene is pre-polymerized by heating (without initiators) at 80°C for two days in pre-polymerization kettle.
- Monomer-polymer mixture is then run into a tower which is fitted with heating and cooling jackets and the top of the tower is maintained at temperature of 110°C, the temperature of about 150°C in between and bottom temperature at 180°C.
- High temperature at bottom ensures a high conversion.
- Base of the tower forms the hopper of an extruder from which melt emerges as filament which are cooled, disintegrated and packed.
Solution Polymerization
- Styrene and solvent are blended together and pumped through number of reactors with different heating zones.
- From the last reactor the polymer is run into devolitilising vessel.
- At a temperature of 225°C, the solvent and the polymer are removed, condensed and polymer is fed into extruder units, extruded as filaments, granulated, and stored.
- Polymerization in solution reduces the exotherm but may lead to problem of solvent recovery and solvent hazards.
Suspension Polymerization
- In this process the monomer is suspended in droplets.
- The reaction is initiated by monomer soluble initiators such as Benzoyl peroxide.
- It is necessary to coat the droplets effectively with some suspension agents e.g. Poly (vinyl alcohol),talc to prevent them cohering.
- The resulting particle size depends on the quantity of suspension agent and speed of agitation.
- Following polymerization, unreacted monomer may be removed by steam distillation and the polymer is washed and dried.
Structure Property Relationship
- PS is linear hydrocarbon polymer.
- Due to steric hindrance of benzene ring causing stiffening effect, the Tg. of commercial polymer is 90° to 110°C. Consequence of this Tg value and amorphous nature of material make it hard and transparent at room temperature.
- It is soluble in benzene, styrene, toluene. The presence of benzene ring in polystyrene having greater reactivity than Polyethylene.
- Due to phenyl group Polystyrene is having limited chemical resistance.
General properties of PS
General properties of Polystyrene are,
- Hard, rigid and transparent thermoplastic.
- Low cost, good mouldability.
- Low water absorption, good dimensional stability.
- Good electrical insulation properties, colourability.
- Incorporation of 1% of saturated aliphatic amines, cyclic amines or aminoalcohols has been found to improve greatly the resistance to weathering.
- It is brittle in nature.
- It is enable to withstand the temperature of boiling water.
- It is having mediocre oil resistance.
Properties of Polystyrene
Processing Considerations
- Unmodified grades have negligible water absorption.
- Specific heat of PS is less, therefore higher plasticizing capacity machines should be used.
- Melts have good stability at processing temperatures.
- Generally no need to purge while shutting down.
- Recycling percentage should not be more than 15-20% in injection moulding.
- Due to amorphous innature, polymer gives low mould shrinkage.
- Polystyrene melts are of medium viscosity but highly pseudo plastic.
Processing Techniques
- Injection Moulding
- Plastic temperature in the process range from 200° C to 250°C for GPPS and 180° to 250°C for HIPS grades.
- Injection pressures are of 30 to 275 MPa depends on the grade of the material.
- Typical mould temperature is 10-80°C.
Extrusion
Typical extrusion conditions
- Temperature profile 150-200°C
- Recommended screw L/D ratio 25:1 to 30:1
- Recommended compression ratio 2:1 to 3:1
Grading of Polystyrene
General purpose polystyrene (GPPS)
- Good balance is maintained to have good heat resistance, reasonably high setting-up temperature, good flow properties and moderate impact strength.
High molecular weight polystyrene (HMPS)
- HMPS gives impact strength without the loss of clarity.
Heat resistant grade
- By reducing monomer content from 5% to 0%, softening point may raised from 70°C to 100°C.
Easy flow grade
- It can be obtained by using low molecular material, by using internal lubricant , by using external lubricant and by controlling size and shape of granules.
Expanded Polystyrene
- Bead (Suspension )Polymerization is generally used for manufacturing expanded polystyrene.
- Blowing agents may be incorporated before polymerization or used to impregnate the bead under heat and pressure in a post-polymerization operation.
- The impregnated beads may then be processed by two basically different techniques.
Steam moulding process.
- Direct injection moulding or extrusion.
- The beads can be expanded to about 40 times their previous size with densities as low as 16 kg / cm3.
Applications of PS
Household
- Items like power boxes, combs, toys, bangles, decorative gift articles, ball pen, water jug, mugs, plates, trays, racks, boxes, jar, etc.
Automobile
- In automobile industries automatic parts like reflector, doom lights, display signs and automotive penal covers.
Electrical/Electronics
- Many electrical and electronic items like light diffusers battery cases, electrical coil forms,TV and transistor cabinets, refrigerator, door and body liners, floppy storage boxes.
Medical
- Medical applications like disposable syringe.
Packaging
- Thin walled packaging, containers, disposable transparent containers, bottles, utility boxes, packaging of fish, bottle caps.
Industrial
- Battery cases, filling cabinets, quick dry emulsion paints, blade dispenser, foot rules and lay flat produce boxes.
Building
- Bathroom accessories like toilet seats, Flooring & ceiling channels and profiles, wall tiles, towel racks, window envelop and building
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