Thermoplastics (also referred to as thermo-engineering plastics) are high molecular weight polymers that become liquid upon heating and glassy solids on cooling. Bonding between molecules of a polymer could be of varying strength, thus resulting in different types of thermoplastics, i.e., polythenes that have relatively weak attractive forces (van der Waals forces), stronger hydrogen bonds in nylon, and very strong stacked aromatic ring bonds in polystyrene. Thermoplastics are essentially addition polymers because they are formed by an addition reaction, in which smaller molecules combine together to form a larger molecule and are therefore chemically inert, non-biodegradable, and difficult to recycle.
Thermoplastics can be broadly classified as either being amorphous or semi-crystalline, based on their molecular arrangement. Amorphous thermoplastics are characterized by their high brittleness and stiffness. They are also clear solids in their normal state. They can undergo a transformation of their molecular arrangement at high temperatures and on application of stress. Examples of amorphous thermoplastics are polyamideimide, polyethersulphone and polyetherimide. Semi-crystalline solids, on the other hand, only partially exhibit such characteristics (i.e., brittleness and stiffness), given their partially amorphous state. Examples of semi-crystalline thermoplastics include polyetheretherketone, polytetrafluoroethylene, and polyamide 6.6.
The specific gravity of various thermoplastics typically varies from about .92 for polyybutylene to 1.72 for poly-vinylidene fluoride. The tensile yield strength (defined as the engineering stress that causes irreversible deformation) of thermoplastics has a wide variance, as low as 2.8 (_10^3 pounds per square feet) for cross-linked polythene to as high as 8.0 (_10^3 pounds per square feet) for polyvinyl chlorides.
Characteristics of Specific Thermoplastics
Acrylonitrile butadiene styrene is a strong thermoplastic that is also resistive to most bases and acids. Chlorinated hydrocarbons can still, though, corrode or damage this polymer. It is usable only at about 71 C temperature, and is used in drainage and vent pipes.
Polybutylene and polyethylene (normal as well as cross-linked varieties) are flexible polymers that find their most common application in pressurized water systems. Although polybutylene doesn’t get affected by extreme water temperatures, polyethylene cannot be used for hot water.
Polypropylene is a light-weight polymer and can be used up to about 82 C temperatures while also being highly resistive to most acids, bases, and solvents. It is primarily used in laboratory plumbing.
Polyvinyl chloride is also a strong and highly resistive plastic, with the maximum usable temperature being 60 C. It finds applications in piping but should be avoided in hot water applications. A variant polymer, chlorinated polyvinyl chloride can be used for higher water temperatures (up to 82 C).
Polyvinylidene fluoride is an extremely strong and tough thermoplastic, which is also resistant and non-abrasive. It can be used up to temperatures 138 C, and its most common application is in laboratory plumbing.