Classification of plastics

2-1.Thermo-plastics and thermosetting plastics

Some of the thermosetting plastics (thermosetting plastics) may be liquids from the beginning, or some begins to melt with the application of heat. In any case, they turn solid during continuous heating and then never returns to liquid form. On the other hand, thermoplastics melt by heating and become moldable. They become solid upon cooling, but melts again upon reheating. Due to this feature, thermoplastics are divided into general purpose plastics and engineering plastics. In addition, engineering plastics are further divided into general purpose engineering plastics and super engineering plastics.

2-2.General purpose plastics and engineering plastics

General-purpose plastics include PE (polyethylene), PP (polypropylene) and PVC (polyvinyl chloride) , which are called three major general-purpose plastics. Each of them has balanced mechanical properties, excellent moldability and relatively low cost. Therefore, the are widely used ranging from daily necessities to industrial materials. Engineering plastics have a broad definition, but one of the general guidelines that they have the heat resistance of 100 ℃ and above. The main use is for industrial purpose, and in that engineering plastics are collectively referred to as the plastics used in the fields of structural materials and functional components. The history of engineering plastics started from polyamide (=nylon), while polycarbonate and polyacetal appeared after that. Among all engineering plastics, these three types of engineering plastics have the highest demand. Engineering plastics made large contribution in the development of low cost new products as they are more compact, lighter and highly functional as alternative to metals.

2-3.Crystalline plastics and non-crystalline plastics

In the plastic made from polymer molecules, crystals refer to regular arrangement of structural polymer molecules. PA (polyamide=Nylon), POM (polyacetal) and PPS (polyphenylene sulfide) are some of the main crystalline plastics, while PC (polycarbonate) and PVC (poly-vinyl chloride) are some of the main non-crystalline plastics. Main differences in properties of crystalline and non-crystalline plastics are, 1. Non-crystalline plastics are transparent, while crystalline plastics are generally opaque, 2. Since crystals in crystalline plastics shrink at temperatures below melting point, molding shrinkage is large, 3. Glass transition points are significantly different.