Plastic pipes, though developed before World War II, did not come into general use until the 1950s. They are of two types – thermoplastic and thermosetting. Thermoplastic materials soften on heating, and stiffen and return to their former state on cooling. Thermosetting materials, while they initially soften with heat to allow molding, harden by the application of continued heat, and cannot be reprocessed once they have set.
The principal thermoplastic pipes in general use are made from unplasticized polyvinyl chloride. (uPVC) and polyethylene (PE). The latter material is produced in low, medium and high density.
Thermosetting pipes are made from epoxy or polyester resins reinforced with glass fibers, and are known as glass-reinforced plastic pipes (GRP).
Thermoplastic pipes, whether of uPVC or PE, are generally produced by a continuous extrusion process. Material in powdered or pellet form is fed through a hopper into a barrel where it is heated, and the hot material is forced through a tube-forming die. The tube is cooled either in a water bath or by water spray, and cut into the required lengths.
GRP pipes are made from epoxy or polyester resins (mostly from polyester) and glass fiber in the form of cloths, rovings, chopped strands or chopped-strand mats. Pipes of these materials can resist moderate pressures, even if they are made with extremely thin walls, so inert fillers such as sand are often added to provide increased thickness,making the pipe more rigid and easier to handle.
GRP pipes are made in diameters up to 80 in. (2000 mm) and are used for both pressure and non-pressure duties. Pressure pipes resist pressures up to about 220 psi (15 kg/cm2).