Polymer Technology

PS and HIPS molding materials are processed using molding processes generally used for thermoplastics. About 50% of the total quantity is processed with screw injection molding machines at 200- 280°C. Films and sheets are produced by extrusion at 200°C with broad-slit dies. Thermoforming is used to produce articles such as refrigerator linings from sheets and drinking cups and disposable tableware from films.
The main areas of application of PS and HIPS molding materials are in packaging, technical items, household and consumer goods, and refrigeration equipment. Technical items include radio, television housing, cassettes, and computer accessories. Household and consumers goods include toys, containers, furniture, disposable tableware, and cups.

In suspension polymerization styrene is dispersed in water in the form of small droplets by stirring The addition of suspension agents such as polyvinyl alcohol, alkaline phosphates, or celluloses is necessary to stabilize the organic phase Polymerization must be started with radical-forming initiators because the process temperatures (80100°C) are considerably lower than those of continuous bulk polymerization.
The main advantage of suspension polymerization is that the heat of polymerization can easily be removed via aqueous phase. A high degree of conversion can also be achieved. However the polystyrene beads must be repelletized after the separation of the water and drying to satisfy marker requirements. This makes suspension process uneconomical. Very few of the older plants based on this technology are still running commercially.

By polymerizing styrene in solution many problems associated with heat transfer and the physical movement of viscous mass are reduced. But the solvent recovery and chain transfer reactions pose new challenges. In 1955 Distrene Ltd. (UK) started production of PS by solution polymerization process.
Styrene and solvent (toluene or ethylbenzene) are blended together and then pumped to the top of the first reactor which is divided into three heating zones. In the first zone the solution is heated to start up the polymerization reaction. The polymerization reaction in the second and the third zones of the first reactor and the three zones of the second reactor is exothermic and cooling coils are used to take heat out of the system By the time reaction mixture reaches the third reactor the polymerization reaction starts to slow down, so the reaction mixture is reheated.

From the third reactor the polymer is then run into a devolatilising vessel in the form of thin strands. At a temperature of 225°C the solvent, residual monomer and some very low molecular weight polymers are removed, condensed and recycled. The polymer is then fed to extruder units, extruded as filaments, granulated, lubricated and stored to await dispatch.

Solution-modified bulk polymerization
a) Reactors; b) Degasser; c) Dye mixer; d) Extruder; e) Water bath; f) Granulator; g) External lubrication; h) External lubricating agent; i) Polymer; j) Pure styrene; k) Monomer and solvent

All industrial bulk polymerization processes are carried out continuously and are cost effective. The polystyrene product has consistent quality, high purity, and low residual monomer content The most difficult task is the control of the highly viscous melt

Tower Process for mass polymerisation of styrene

Most bulk processes used today are a variation of that developed by Wolff in Germany. In this process the styrene is prepolymerized by heating without initiators in a prepolymerization kettle at 80C for two days until a 33-35% conversion to polymer is reached The monomer polymer mixture is then run into a tower about 25 ft 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 about 100C, the centre at about 150C and the bottom of the tower at about 180C. The high bottom temperature ensures higher conversion and boils off the residual styrene from the polymerfrhe base of the tower forms the hopper of an extruder from which the melt emerges as filaments which are cooled and palletized

The annual consumption of polystyrene is 10.5×106 t; thus it is one of the quantitatively important polymers. Styrene was first isolated in 1831 by Bonastre from the resin of the amber tree. In 1839 E. Simon first described the polymer. Around 1925 the development of an industrial production process for polystyrene began. This work achieved success in the plants of the IG Farbenindustrie in Germany in 1930. In USA polystyrene was first produced on commercial scale in 1938 by the Dow Chemical Company.
Polystyrene molding materials are hard, transparent with high gloss. Below 100°C PS molding materials solidify to give glass like material with adequate mechanical strength and resistance towards a large number of chemicals. The mechanical properties of the relatively brittle PS molding materials can be considerably improved by adding rubbers, generally polybutadiene. Styrene-butadiene molding materials are generally referred to as high-impact polystyrene (HIPS) They are also known as toughened PS or rubber-modified PS or impact-resistant polystyrene (IPS).
Styrene can be copolymerized with many other monomers. Styrene-acrylonitrile molding materials, in particular, have achieved great economic importance in transparent and rubber-modified forms known as ABS. Compared with the pure styrene polymers they have

advantage with regards to hardness, strength, and resistance to heat distortion and environmental stress cracking.

Electron micrograph of HIPS
The dispersed rubber particles are embedded in the polystyrene matrix.

Copolymers of styrene and maleic anhydride have a softening point that is up to 30°C higher than that of PS. These products are used in the form of foams in the automotive industry.
Expandable polystyrene (EPS) is the starting material for PS hard foam materials. It is produced by addition of 6% of low-boiling hydrocarbon (pentane) as a foaming agent. Extruded polystyrene (XPS) foams are produced from polystyrene and halogenated hydrocarbons as blowing agents.

Industrial Production

Monomer

Styrene is produced from ethyl benzene by a process of hehydrogenation at 63.00C.This is an endothermic reaction in which a volume increase accompanies dehydrogenation. The reaction is therefore favored by reduced pressure. By use of selected catalysts such as magnesium oxide and iron oxide a conversion of 35-40% per pass with 90-92% yield may be obtained.

The dehydrogenation reaction produces crude styrene. The crude styrene is first passed through a pot containing elemental sulphur which dissolves and acts as a polymerization inhibitor. The benzene and toluene are then removed by distillation. The ethyl benzene is then separated from the styrene and tar by passing through two distillation columns, each with top temperature about 500 C and bottom temperature 900C under vacuum of about 35 mmHg The tar and sulphur are removed by a distillation column and the styrene is permanently inhibited by addition of ppm of t-butylcatecol.
Styrene is colourless liquid with BP 145.2°C

Polymerization

Styrene can function as an electron donor or an electron acceptor. It can therefore be polymerized by radical, cationic, or anionic mechanisms. The industrial polymerization of styrene to PS is carried out exclusively by free radical mechanism. Styrene itself can form polymerization-initiating radical. The propagation mechanism for chain growth proceeds by addition of further monomer to the radical chain end. Growth of the chain is mainly terminated by recombination. The growth of a chain proceeds rapidly with the result that removal of the liberated heat of polymerization presents one of the main problems of the industrial process. Average molecular masses between 100 000 and 400 000 are obtained within short time.

The polymer may be prepared by :

1. bulk polymerization
2. suspension polymerization
3. solution polymerization
4. emulsion polymerization

The first two process are most important. Emulsion polymerization is rarely used since the soap used seriously affects the clarity of PS product. Suspension processes are mailnly used for the production of expandable polystyrene.