Types of Plastics
There are three major classes of plastics – thermoplastics, thermosets, and elastomers.
Thermoplastics are those, which once shaped can be softened by the application of heat and can be reshaped repeatedly, till it loses its property. e. g. Polyethylene, Polypropylene, Polyvinyl chloride, Polystyrene, Polyamides, Polyesters, and Polyurethanes.
High temperature thermoplastics include polyether ether ketones, liquid crystalline polymers, Polysulfones, and polyphenylene sulfides.
Commodity plastics account for about 80% of plastics used for medical devices in applications like lab ware, tubing, containers and molded connectors.
Thermoplastics can be further categorized into amorphous and semicrystalline polymers. Amorphous thermoplastics are long chain molecules that have no order. Semicrystalline (or crystalline) polymers have short range order and can align together to form ordered, crystalline structures within amorphous regions.
The extent of alignment and the amount of crystallinity are determined by the polymers chemical structure, which determines how well the polymer chains can align with each other. Semicrystalline thermoplastics are typically opaque compared to amorphous thermoplastics. Semicrystalline polymers have better chemical resistance than amorphous thermoplastics because they have a lower amount of amorphous regions that are permeable to chemicals and solvents. Thermosets are formed when two or more components chemically react with each other under ambient conditions or when induced by radiation or heat to form a highly cross linked network. The formation of thermosets isan irreversible process. Thermosets are typically hard and rigid.
Elastomers are loosely cross linked polymers. They have the characteristics of rubber in terms of flexibility and elasticity e.g. natural rubber, styrene butadiene block copolymers, polyisoprene, polybutadiene, ethylene propylene rubber, silicone elastomers, fluoroelastomers, polyurethane elastomers, and nitrile rubbers.
Poly (ethylene terephthalate) – PET or PETE
It is clear, tough and has good gas and moisture barrier properties making it ideal for carbonated beverages, food containers and life saving fluid bottles etc.
Poly (vinyl chloride) – PVC or V
In 1912 German chemist Fritz Klatte at Greisheim Electron unknowingly made the first PVC in an attempt to create uses for large quantities of acetylene gas fuel lamps. He had reacted acetylene with hydrochloric acid.
Not knowing what to do with the new material, it was stored for some time and polymerization took place. It has excellent transparency, chemical resistant, long term stability, and stable electrical properties. Vinyl products can be broadly divided into rigid and flexible materials. The application varies from water pipes and fittings to windows, laminates, blood bags, medical tubing etc.
Polypropylene – PP
It has excellent chemical resistance and thus commonly used in packaging. It has high melting point making it ideal for hot fill liquids. Like other plastics PP has excellent resistance to water, salt and acid solutions that are destructive to metals. Typical applications include ketchups bottles, medicine bottles and automobile battery casings.
Polystyrene –PS
PS is used to make a wide variety of containers, including those known as Styrofoam‘‘ plates, dishes, cups, biomedical devices etc.
