From Resins To Thermoplastic Products

Thermoplastic resins are available to the processing industry as pellets of resin. Converting the raw material into useful products can involve separate segments of the plastics industry. As Figure 2.10 suggests, the resin might be compounded by a custom compounder and formed into the final product by a processor or a fabricator. The compounding can also be carried out by the processor in an in-house facility.

The resin raw material needs to be mixed intimately with a variety of chemical additives to impart specific properties to the end product. Additives are used widely in the plastics industry, in nearly all types of plastic products. The use of common plastics in consumer products would not be possible without the use of additives. For instance, vinyl plastics (particularly PVC) undergo easy thermal and photodegradation; no useful products can be made with it if stabilizer additives designed to protect the resin during thermal processing and use were not available. Selecting the appropriate set of additives called for by a given product and mixing these in correct proportion with the resin is referred to as compounding. Table 2.7 lists the common classes of additives used in manufacturing plastic products. To ensure adequate mixing or dispersion of the additive in the mass of plastic, these may be added into the virgin resin as a concentrate (or a masterbatch) already dispersed in the same or a compatible resin. Some additives may also be added directly to the resin mix. In any event, the mixing is accomplished by passing the resin and additive mixture at a temperature

Plastic Antioxidant Diagram
Figure 2.10. Flow diagram illustrating components of plastics industry.

Table 2.7 Common Plastics Additives and Their Functions

Antiblocking agent Antioxidants Antistatic agent Biocide Blowing agent Inert filler Reinforcing filler Coupling agent

Curing agents Flame retardant

UV stabilizers

Impact modifier Lubricant

Pigments

Prevents sticking of thin plastic sheets to each other, or "blocking" Reduces the rate of autoxidation of the plastic at service temperature Prevents charges on polymer surface leading to static discharge Prevents growth of microorganisms on plastics Used to create polymeric foams.

Reduces the cost of formulation and changes the color Increases the modulus and other properties of a polymer Promotes better adhesion between phases in filled and glass-fiber-reinforced plastics Crosslinks the polymer

Reduces the flammability of plastics products

Minimizes the solar UV-B induced degradation of plastics outdoors Increases the impact resistance of plastics Minimizes internal and external friction leading to degradation Colors plastic products

Plasticizer Softens the plastic and makes it more processable

*AIBN = 2,2' azobisisobutyronitrile.

Quartz or silica in polyethylene

Metal deactivators, peroxide decomposers.

Quaternary ammonium salts in rigid PVC

Phenols and chlorinated phenols in coatings

Inert gases and AIBN* that decompose into N2 on heating.

Chalk used in plastic formulations

Carbon black in rubber formulations

Organosilanes, titanates, and zirconates

Sulfur or organic sulfur compounds in rubber Borates, and organophosphorous compounds Hindered amines and light absorbers

Rubber and thermoplastics in epoxy resin Ethylene(bis)stearamide used in rigid PVC

Inorganic pigments, carbon black, and organic pigments Phthalates in rigid PVC compounds high enough to melt the thermoplastic, through a mixing screw in an extruder (a compounding extruder). Care is taken not to overheat or overshear the mix to an extent to cause chemical breakdown of the plastic itself or the additive materials. The now "compounded" resin with the additives evenly distributed within its bulk is repelletized, cooled, dried (where the pelletization is carried out under cooling water), and stored for subsequent processing.

Processing is the final step that converts the compounded material into a useful plastic product. Basically, the compounded resin needs to be melted into a liquid and heated to a temperature that allows easy handling of the fluidized plastic or the "melt." This melt is fed into molds or dies to force the material into required shapes and quickly cooled to obtain the product. Usually, some minor finishing is needed before the product is made available to the consumer. The different processes used and the equipment employed are determined by the type of product being manufactured. A detailed discussion of the various processing techniques available for common thermoplastics is beyond the scope of this discussion. However, the basic principals involved in common processing methods associated with high-volume products will be discussed briefly below.

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