Plastic Pipe Glossary

Plastic -- A high polymer material, often synthetic, that during manufacturing or processing can be extruded, molded, cast, drawn, or laminated into objects of all sizes and shapes by application of heat or pressure, by chemical condensation, or by casting during polymerization of monomers, which can retain its new shape under conditions of use.

Polymer -- A substance, synthetic or natural, consisting of large complex molecules formed by the chain-like chemical union of five or more identical combining monomers. (Low molecular weight primary molecules are called monomers.)

Polypropylene -- A thermoplastic polymer of propylene resembling polyethylene, and used for making molded and extruded plastic products such as water pipe, tubing, and fittings.

Polycarbonate -- A thermoplastic polymer resin that is linear polyester of carbonic acid. Polycarbonate is a transparent, nontoxic, non-corrosive, heat resistant, high impact strength plastic; it is generally stable, but may be subject to attack by strong alkalis and some organic hydrocarbons. It can be molded, extruded, or thermoformed, and is commonly used for numerous applications, such as non-breakable windows, household appliances, tubing, piping, and cartridge filter sumps.

Thermoplastic -- Materials, such as certain synthetic resins and plastics, that soften or fuse when heated; and harden and fuse when cooled. These materials can often be reheated and cooled time after time with no appreciable changes in their physical or chemical properties.

Thermoset -- Certain plastics and synthetic resins that once solidified will not soften or fuse when heated. Thermoset materials may decompose at high temperatures, but will not soften or melt.

Visco-elasticity problems -- All plastics and specifically PVC are visco-elastic materials. You may be familiar with Silly Putty, which is an example of a visco-elastic material. Visco-elasticity allows a material, like Silly Putty, to be stretched considerably if you pull it slowly. However, if you pull quickly, the material will break with a snap.

Visco-elasticity has both advantages and disadvantages in piping systems. The ability of PVC pipe to deform or "creep" is a major benefit when you install a sprinkler system along with a curved sidewalk or when you need to compensate for changes in elevation or grades. Also, a visco-elastic material can more easily tolerate temperature-related expansion and contraction.

The two most common types of system failures related to visco-elasticity are catastrophic burst and long-term yield (stretch). You can identify burst failures by the sharp, jagged breaks that almost always leave the system in multiple pieces. Pressure surges, especially those caused by water hammer, produce burst failures. It is not uncommon for the failure to spread away from the origin and travel into adjacent components, such as pipe or fittings.

Long-term yield failure will remain localized and often result in areas of plastic erosion around the point of origin. You can see in a close up examination of these areas that the plastic has stress marks and a very glossy surface. These long-term failures usually will be located in the most highly stressed area of a fitting or component. In typical PVC T's and elbows, these areas can be the inner corners of the direction change.

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