Processing of plastics

As noted previously, an important factor favouring the use of plastics in any application is the easy shaping for even the most recalcitrant polymer, shaping can be accomplished at moderate temperatures with equipment fabricated from conventional materials. There is the added advantage that many of the shaping processes are suited to automatic operation. 

Most of the synthetically manufactured macromolecular substances are used as plastic materials in the materials science sense. In order to make a plastic material of a macromolecular substance, it must be equipped with antioxidants, fillers, lubricants, etc. Such protective additives are intended to improve the mechanical, electrical, and/or chemical properties of the plastic, simplify its processing, and give it a suitable, acceptable appearance or finish. Compounding is the controlled addition of such additives to the macromolecular substance. 

The type and extent of the processing and the protection needed depend partly on the type of the plastic and partly on the intended application. Normally, plastics are classified according to their mechanical and thermal behavior as thermoplasts, elastomers, thermosets (duroplasts), and fibers (see also Section 11.1). According to the application, further distinctions are made among construction materials, insulating materials, adhesives, floor sealants, paints, films, soundproofiing media, etc. 

Batch polymerizations give products which often vary slightly from batch to batch despite the use of the same recipe. These batches are blended in order to be able to deliver products with the same specifications to the 

When two polymers are intimately mixed, a polyblend (polymer alloy) is produced. Polyblends are usually thermodynamically unstable (see Section 6.6.6). Because of the very low diffusion coefficient, however, the demixing process is extremely slow, so that polybends appear kinet-ically stable. 

To give suitable protection, it is often sufficient to work in just a small amount of an additive to a plastic. Since it is difficult to mix small amounts of additive efficiently with the polymer, use is made of what is called a master batch. This is a concentrate, so to speak, of the additive in the polymer to be compounded. The true working-in is then effected by diluting the master batch with the required amount of polymer. 

In commercial polymer production, devolatilisation is also carried out through an opening for venting volatile products. 

Films and sheets consisting of layers of two or more different polymers can be produced by mixing the molten streams from a similar number of extruders in a multi-manifold die. By this process, it is possible to combine materials to provide combinations of properties that cannot be obtained in a single polymer. 

The extrusion of film can also be carried out by casting, as in sheet extrusion or by the blown film process. 

In the moulding process, finely divided plastic is forced by the application of heat and pressure to flow into, fill, and conform to the shape of a cavity (mould). Moulding can 

In compression moulding, the polymer is put between stationary and movable members of a mould. Under heat and pressure the material becomes plastic, flows to fill the mould and becomes homogeneous. Thermal and rheological properties of the polymer determines the necessary pressure and temperature, which typically is 150 °C and 6.9-20.7 MPa. A slight excess of material is placed in the mould to ensure its being completely filled. 

---

Differences among the processes have a major impact on the use of the products. Products from a particular process or manufacturer may dominate one market, while products from a different process may be preferred in a different appHcation. Major uses include hot-melt adhesives for appHcations requiring high temperature performance, additives to improve the processing of plastics, sHp and mb additives for inks and paints, and cosmetic appHcations. 

Principles of the Processing of Plastics 8.2.6 Thermal Properties Affecting Cooling... 

In this book, the process of plastic deformation and the related crystal defects liave been discussed repeatedly. In Section 2.1.6, the distinction between continuum... 

---