General Classification and Types of Polymers

There are a variety of ways to classify polymers according to their molecular structure and these will be covered in more detail later in Chapter 4. However, there are two general types that should be mentioned here. Most polymers can be broadly classified as either thermoplastics or thermosets. The fundamental physical difference between the two has to do with the bonding between molecular chains - thermoplastics have only secondary bonds between chains, while thermosets also have primary bonds between chains. The names are not only associated with the chemical structure of each but their general thermal and processing characteristics as well since 

Thermoplastic Polymers Thermoplastic polymers may be either amorphous or crystalline. Crystallinity (or morphology) will be discussed in more detail in the next chapter but it is important to point out here that the degree of crystallinity is low by standards for crystalline metals, ceramics and other materials. That is, polymers are rarely over 50 % crystalline. Crystalline polymers are often more dense than amorphous polymers due to closer packing of their long chain molecules and, in general, the following properties are enhanced. 

Corrosion resistance and/or resistance to environmental stress cracking 

An example of a much-used crystalline thermoplastic polymer is polyethylene. LDPE (low density polyethylene) is considered to be semicrystalline while HDPE (high density polyethylene) or UHDPE (ultra high density polyethylene) are considered to be highly crystalline. LDPE is one of the most widely used plastics accounting for more than 20% of the total polymer market and is used extensively for milk containers and other packaging operations. HDPE and UHDPE are used extensively in water 

Polyamides (nylon) Polytetrafluoroethylene (PTFE) Polyetheretherketone (PEEK) 

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