Before moving on to the definition and classifications of plastics, we have to understand the building blocks of plastics. Those are called polymers. In short polymers are large molecule chemical compounds made of repetitive units called “monomers”. They could have linear, branched or cross-linked structure.

We can classify the polymers alone by many criteria – listed below are some of them:

Classification by physicochemical properties:

• Thermoplasts – materials that become soft when heated, and become hard after a decrease of temperature.
• Thermoset (duroplasts) – after being formed they stay hard, they do not become soft with the influence of temperature.
• Elastomers – materials, which we can be stretched and squeezed and are able to reshape back to their original form when the applied stretching and squeezing force is removed.

Classification by origin:

• Synthetic polymers – originate from chemical synthesis (addition polymerization, poly-condensation copolymerization).
• Natural polymers – produced and degraded in nature e.g. cellulose, protein, nucleic acids.
• Modified natural polymers – those are natural polymers, chemically changed to receive new functional properties e.g. cellulose acetate, modified protein, thermoplastic starch.

Classification by origin of raw materials, which polymers are made of:

• Renewable sources (plant and animal sources)
• Non-renewable/Fossil sources (oil, natural gas, coal)

Classification by susceptibility to microorganism enzymatic attack:

• Biodegradable (polylactide – PLA, polyhydroxyalkanoates – PHA, regenerated cellulose, starch, linear polyesters).
• Non-biodegradable (polyethylene – PE, polypropylene – PP, polystyrene - PS).

There are, of course, many more types of classifications of polymers available; however it is important to know that in industrial applications polymers alone are often not enough. Most plastics contain other organic or inorganic compounds blended in. Those are called additives and they can provide new properties to plastics.

Therefore:

Polymer + Additives = Plastic

The amount of additives ranges from very small percentages for polymers used to wrap foods to more than 50 % for certain applications. Such polymers with additives in technical and industrial usage are called plastics.

Some examples of additives include: plasticizers - oily compounds that confer improved rheology, fillers that improve overall performance and reduce production costs, stabilizers that inhibit certain chemical reactions such as fire retardants additives decreasing flammability, antistatic agents, colouring agents, sliding agents and many more.

The world of plastics is immense, given the broad range of different polymers and additives that can be compounded. This in turn gives a wide range of possibilities to transform and process plastics. Most basic techniques in plastics processing are: extrusion, blow extrusion, injection, compaction/compression, pressing, board/slab forming,rolling and calendaring, and die-casting.