Commercial plastics are known as resins in North America, and are made of polymers. These polymers have been compounded with modifying or stabilizing additives. Typically based on the element carbon, polymer molecules are made from simple, oil-based raw materials. The starting materials for polymers are called monomers and they are small molecules. These small molecules go through a process called polymerization which combines them and forms very large molecules or polymers.
The reason behind the term polymer is due to the final product consisting of several, identical, repeating molecule units. The polymer is sometimes referred to as a ‘high polymer’ or, as a ‘macromolecule’ because usually, the final size (or length) of these molecules and therefore their molecular weight, or mass, is huge.
All plastics are polymers, however, not all polymers are plastics. Several natural products, like cellulose, are polymers, but they are unable to be processed like a commercial plastic material until they are altered from their natural form.
Types of Plastics Material
The definition of a plastic material is a compound of polymer plus additives that has the capabilities of being shaped or molded into a valuable product. This is done in conditions of moderate temperature and pressure. Commonly, plastics have a high stiffness/modulus and a lack of reverse elasticity, allowing them to be distinguished from rubbers or elastomers.
The two key categories of plastic are thermoplastics and thermosetting plastics (thermosets). Thermoplastic products have the ability to be continually softened, melted and reshaped/recycled, for instance in injection molding or extrusion resins. However, thermoset products do not have this property. In terms of tonnage, thermoplastics are undoubtedly the more significant. If the polymer in the plastic is based on one monomer, it uses the term ‘homopolymer.’ If the polymer used is based on two or more monomers, it is referred to as a ‘copolymer.’
Amorphous and Crystalline
It is possible to further divide thermoplastic materials into two key categories: amorphous and semi-crystalline. An amorphous thermoplastic material has no specific molecular structure and is normally a solid, clear, stiff material with low shrinkage, for example, polystyrene. In comparison, a crystalline polymer has an systematized, “crystal-like” structure, but also comprises of amorphous material.
It is usually known as a semi-crystalline thermoplastic material. These types of plastics are commonly tougher and less fragile compared to amorphous thermoplastics. It is also common for them to have a higher heat distortion temperature.
Additionally, these plastics are translucent, or opaque, and have a high shrinkage and a high specific heat. Polyethylene is the best-known example of a semi-crystalline, thermoplastic material. (Clear polystyrene is sometimes referred to as “crystal polystyrene” and it is an amorphous polymer. However, this name refers to the fact that it is “crystal clear” and not a crystalline thermoplastic.) Both amorphous and semi-crystalline thermoplastics are frequently tested by rheometry and are extensively used in rheological studies.
Names and Abbreviations for Thermoplastics
There are two main types of plastics: thermoplastics and thermosets. Thermoplastics are by far the ones most studied in rheology, therefore, this section will focus only on the major types of thermoplastic material.
Common Names for Thermoplastics
Most thermoplastics have the prefix ‘poly’, which means ‘many.’ This is followed by the name for the monomer from which the plastic is derived, so the name is source-based. As a result, we get names like polystyrene and polyethylene for homopolymers.
Sometimes the plastic material has more than one word in the name, and when this occurs parentheses may be put around the words, for instance poly (vinyl chloride). This practice is, however, not universal and one may also see the same term without the brackets i.e. polyvinyl chloride.
Other names like ‘acetals’ and ‘cellulosics’ may also be met since source-based nomenclature is not universally used. In addition to this, several plastics are known by more than one name. For instance, acetals may be known as polyformaldehyde or polyoxy-methylene.
Polymers are often referred to by abbreviations because of the complex, chemical names used to describe them. These abbreviations are formed using a short string of capital letters, in which each capital letter denotes a part of the common name. In the case of the plastic material beginning with ‘poly’, the first letter is P and the other letter(s) are taken from the monomer unit. To give an example, polystyrene is shortened to PS and polyethylene is shortened to PE respectively.
On the other hand, copolymers are frequently referred to through initials, in which the monomers used are represented; without a “P” for “poly.” For instance, the copolymer formed from styrene and acrylonitrile is known as styrene acrylonitrile copolymer, or as SAN. When two or more polymers are combined, they are known as blends or alloys, and they are normally represented by the abbreviations used for the individual materials. Each abbreviation is separated by an oblique stroke, such as SAN/EVA.
There are several standards organizations that are responsible for issuing standards that specify the letters used when naming plastics. The American Society for Testing and Materials (ASTM) and the International Standards Organization (ISO) are just two that are responsible.
Standard and Non-Standard Abbreviations
It is important to note that both standard and non-standard abbreviations are used. Frequently, materials are formed, and a name is used for them prior to the standards committee suggestions being issued. Consequently, a ‘non standard’ abbreviation may become recognized, or more than one abbreviation may be referred to for the same material. For instance, the thermoplastic elastomer, polyether ester elastomer, is often referred to as PEEL, as COPE (from copolyester), as TEEE (thermoplastic elastomer ether ester) or, as YBPO (an American suggestion).
End of part 1.