This paper focuses on the material properties, flow characteristics and phenomena present in the processing of materials in a twin-screw extruder.

Screw extruders generally have good pumping characteristics for viscous fluids, and these characteristics have become very important for processing applications of polymer melts and their blends.

Polymer Characterization 

Polymers are compounds with very high molecular weights, typically up to 20,000 to 200,000 or higher. Polymers are generally linear molecular chain structures of single or multiple repeating molecular units. The most common polymers (or homopolymers) contain only one repeating molecular unit of their own, such as polyethylene, polypropylene, polyvinyl chloride and polystyrene.

Twin Screw Extruder Polymers 

Twin Screw Extruders are also widely used in the food processing industry, where the materials being processed are also polymers, such as carbohydrates, proteins, etc., which are often polycondensates with different molecular units, protein-polyamide copolymers.

Typical molecular weights of commercial copolymers range from 10,000 to 200,000, depending on the various polymers. The width of the molecular weight distribution also varies widely, typically between 1.5 and 10, depending on the polymerization method. However, the molecular weight distribution can be designed to suit the needs. For example, linear polyolefins with narrow molecular weight distributions are more stable to production during melt spinning than those with wide molecular weight distributions.

Thermodynamic Characterization of Polymers 

In general, pure low molecular weight compounds exist in three forms, the crystalline state in a highly three-dimensional conformation at low temperatures, the molten state at moderate temperatures, and the gaseous state at high temperatures.

Of the important commercially available ethylene homopolymers, polyethylene, polypropylene (iso-gauge), and polybutene-1 (iso-gauge) are crystalline, while polystyrene (amorphous) is amorphous.

1,2-Polybutadiene already has an intergauge structure and is a crystalline material, and its atactic structure is amorphous. Polyvinyl chloride has enough intergauge content to form a very small crystallinity. However, it is often regarded as a transparent material.

Regarding other types of polymeric materials. Polyamides are crystalline materials. Compared to aliphatic vinyl polymers and polyamides, polymers containing a benzene ring in the main chain crystallize at a much faster rate than polybutylene terephthalate, which crystallizes much faster than polyethylene terephthalate, but both are crystalline materials. Similarly, the crystallization rate of polyether ether ketone is very slow; polycarbonate is much more difficult to crystallize, and is generally considered to be an amorphous material. This article specifically lists the transition temperature of several common polymer materials, see the table below.

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