38. 1. Introduction

All the processing stages between manufacture of the crude polymer and the final molding step are called the compounding processes. These include storage, transportation, metering, mixing, plastication and granulation. Sometimes additives may be added to the polymer mix to allow production on an industrial scale. Addition of plasticizers renders flexibility and ductility to rigid and brittle polymers. In a similar way, fillers can enable to enlarge the volume and also has influence upon the polymer properties.

38. 2. Processes

The various compounding processes are –

1. Mixing : Mixing allows obtaining a homogeneous mixture of polymer and additives. The additive is distributed uniformly throughout the crude polymer, without any unnecessary stressing . Mixing can occur under the action of gravitational forces in a tumble mixer ; or in a ribbon mixer , where materials are mixed in a horizontal barrel in a spiral manner. Such mixers are collectively known as cold mixers.

Another class of mixers are the hot mixers, wherein the polymers are heated by contact with heated metal components. Such mixers are also known as high – speed or turbo mixers. This method is widely used in the plastic industry , especially in the compounding of poly vinyl chloride (PVC).

2. Rolling and Kneading : Before processing , the plastics are compacted, melted, homogenized and modified with additives. These include rolling, in which the plastic is passed between two counterrotating , heated rolls wherein it is pressed into a thin layer. It is an open process in which additives can be added, and other scrap materials may be reintroduced. PVC is one plastic which extensively employs this process. Moreover, the material temperature can be controlled precisely and cleaning of the machine is easy.

3. Pelletizing : It is the method of producing granules of equal size and shape. This enables the optimum feed behaviour on the processing equipment. Pelletizing can be done in hot or cold conditions.

Using a pelletizing die, strands, ribbons or sheets are formed ; and after solidification, they are chopped into the desired shapes. This is one procedure strictly followed in cold pelletizing. However, in hot pelletizing , the plasticated material is sent through a die , and a blade that rotates on the surface of the die plate chops the exiting strands. The melt is then cooled ; and subsequently the water is removed.

4. Shredding and Grinding : In thermoplastics processing, a variety of semifinished and finished products are generated. These products, inorder to be made suitable for processing, must be cut to produce a pellet form. A range of particle sizes is obtained in the process.

Regranulating is achieved through cutting mills. The material is added through a feed system wherein a rotor revolves with high momentum. The system is further equipped with multiple cutting heads that operate against stationary cutting edges. The rotating cutting heads are divided and displaced with respect to each other in some other machines. As soon as the desired particle size is obtained, the material is ejected through a sieve , the pores of which determine the size of the particles. The housing and rotor may be equipped with water cooling apparatus to remove the excessive heat generated.

5. Storage and Transportation : The storage and transportation of base polymers and intermediate products are achieved through automatic equipments. These equipments have an edge over the manually operated ones as they ensure improved working conditions , lower work place , lower contamination of materials and subsequent reduction losses. Moreover, accident hazards are also reduced. Large external silos are used for the storage of polymers, either in powder form , or in the pellet form. By incorporating vibrating frames, bunker cushions, slotted shelves or similar techniques as ventilation, the formation of granule bridges is avoided.

The materials are transported via road or rail. Reserve materials and additives are transported in ships in bulk containers, drums or sacks. Pneumatic equipments are generally preferred for conveying powders and pellets. The conveyors used may be high pressure conveyors, medium pressure conveyors or low pressure conveyors. Fill level sensors help in controlling the transport interval.

Inorder to meter liquids such as plasicizers or other additives for compounding, oscillating displacement pumps are used. Metering of soilds can be done using gravimetric metering equipments or volumetric metering equipments.