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CBD-220. Thermoplastic Pipe

Originally published November 1981.

The principal plastic piping material, with respect to both quantity used and range of applications, is thermoplastics. This class of materials includes many that differ significantly in their properties, characteristics and suitability for various uses. Thus, the designer and user should become familiar with the nature of these different materials to avoid misapplications.

This Digest, which describes briefly the major thermoplastics used in piping, is a sequel to an earlier Digest explaining the general use of plastics as piping material¹.

General

Thermoplastics are widely used, primarily because of their low cost and great ease of fabrication (usually by extrusion). This has fostered considerable laboratory and field experience and provided a wide body of general knowledge, technical data and recommendations on the design, installation, use, limitations, and physical and chemical properties of common thermoplastic piping materials^2-6.

The most common thermoplastics used in piping systems are poly(vinyl chloride) (PVC), polyethylene (PE), acrylonitrile-butadiene-styrene (ABS) and polybutylene (PB). The typical properties and general characteristics and applications of thermoplastic pipe are summarized in Tables I and II.

Table I.  General properties and applications of some thermoplastic pipe.

Poly(vinyl chloride) (PVC)

This plastic has the broadest range of applications in piping systems and its use has grown more rapidly than that of other plastics. PVC has good chemical resistance to a wide range of corrosive fluids, but may be damaged by ketones, aromatic and some chlorinated hydrocarbons (Table I). There are two principal types of PVC used in the manufacture of pipe and fittings, Type I and Type II (ASTM D 1784). Type I, also called unplasticized or rigid PVC, contains a minimum of processing aids and other additives and has maximum tensile and flexural strength, modulus of elasticity, and chemical resistance; however, it is more brittle. It also has a maximum service temperature under stress of about 65°C (150°F), lower thermal expansion than Type II, and does not support combustion. Type II PVC, which is modified with rubber to render it less rigid and tougher, is also called high-impact, flexible or non-rigid PVC. It has lower tensile and flexural strength, lower modulus of elasticity, lower heat stability and less chemical resistance than Type I. The improvements made through research and the availability of product standards for special uses have increased its acceptance by designers, contractors and building code officials^2-4. PVC pipe is available in both schedule number, which is determined by the expression 1000 x P/S, where P is the service temperature and S, the allowable stress (both expressed in the same units) and standard dimension (SDR) sizes, obtained by dividing the outside diameter of the pipe by its wall thickness. It is used in drain-waste-vent (DWV) applications, in storm, sanitary, water-main, and natural gas distribution, and in industrial and process piping. The fastest growing application in North America is currently for municipal water and sewerage systems. PVC pipe is also used as a conduit for wiring (both electrical and communications).

Chlorinated PVC (CPVC)

The basic resin in this plastic is made by post-chlorination of PVC. CPVC has essentially the same properties as Type I PVC material, but has the added advantage of withstanding temperatures up to 100° C (212°F), approximately 33 deg C (59 deg F) more than PVC plastic. Although it is suitable for the same piping applications as Type I PVC, the higher cost of CPVC restricts its use to that of conveying hot fluids. A plumbing system in which CPVC pipe of the same diameter as copper pipe is used for water distribution lines can handle 690 kPa (100 psi) working pressure at 82°C (180°F). Consequently, CPVC pipe is now replacing copper pipe in many areas of Europe and the U.S.A.

Polyethylene (PE)

Pipe made from PE has a relatively low mechanical strength (Table II), but exhibits good chemical resistance and flexibility and is generally satisfactory for use at temperatures below 50°C (122°F). The temperature limitation is, however, offset by good flexibility retention down to -55°C (-67°F). Polyethylene piping plastics are classified into three types on the basis of density: low density (Type I), medium density (Type II) and high density (Type III). The most popular are Types II and III. The mechanical strength, chemical resistance and temperature resistance increase with density, whereas creep diminishes as the density increases.

PE pipe is available in both schedule number and standard dimension (SDR) sizes. Its principal applications are: irrigation and sprinkler systems, drainage, chemical transport, gas distribution pipe and electrical conduit systems.

Table II.  Typical properties* of common thermoplastic pipe materials.

Specialty PE Pipes

A relatively new development in PE piping is the introduction of ultrahigh molecular weight (UHMW) PE and cross-linked PE plastic piping materials. The UHMW PE has considerably higher resistance to stress-cracking but is more costly than conventional PE piping material. It offers an extra margin of safety when used in sustained pressure conditions in comparison with pipe made from lower molecular weight resin. It is suitable for certain applications in the chemical industry where stress-cracking resistance has been a limiting factor for the conventional PE pipe.

Cross-linked PE piping material, when compared to ordinary PE pipe, displays greater strength, higher stiffness and improved resistance to abrasion and to most chemicals and solvents at elevated temperatures up to 95°C (203°F). Pipe made from cross-linked PE also has high-impact resistance even at sub-zero temperatures. It is used in applications too severe for ordinary PE pipe. The joining technique used is threading.

Acrylonitrile-butadiene-styrene (ABS)

ABS plastic is a copolymer made from the three monomers described in the heading, and contains at least 15 per cent of acrylonitrile. It is a rigid plastic with good impact resistance at lower temperatures down to -40°C (-40°F) and can be used at temperatures up to 80°C (176°F). ABS is utilized mainly for drain-waste-ventilation (DWV) pipe and fittings but it is also used in solvent cement for installing pipe in various applications. ADS pipe can be joined by solvent welding or threading.

A new development in the ABS-DWV piping industry is the co-extruded foam-core ABS pipe. It consists of a foam core sandwiched between solid skins and can be used for sewer, conduit and duct pipe. The foam-core pipe, with its lower resin requirements, could make ABS more price-competitive with existing materials in these applications.

Polybutylene (PB)

Polybutylene piping has practically no creep and has excellent resistance to stress cracking. It is flexible, and in many respects similar to Type III polyethylene, but is stronger. Polybutylene plastic piping is relatively new, and thus far its use has been limited to the conveyance of natural gas and to water distribution systems. Its high temperature grade can resist temperatures of 105-110°C (221-230°F).

Polypropylene (PP)

Polypropylene-based piping is the lightest-weight plastic material (density = 0.90 g/cm³) and generally has better chemical resistance than other plastics. PP is used in some pressure piping applications, but its primary use is in low pressure lines. Polypropylene plastic pipe is used for chemical (usually acid) waste drainage systems, natural-gas and oil-field systems, and water lines. The maximum temperature for non-pressure piping is 90°C (194°F). Pipe lengths are joined by heat fusion, threading (i.e., with heavy pipe) and mechanical seal devices.

Other thermoplastics

Other thermoplastics used in the manufacture of pipe include poly(vinylidene chloride), poly(vinylidene fluoride), cellulose acetate butyrate (CAB), acetal homopolymer resins, rubber-modified systems, polytetrafluoroethylene (PTFE), and fluorinated ethylene-propylene (FEP) copolymer. All of these materials are relatively expensive and are used only for very special applications.

References

1. Blaga, A., Use of plastics as piping materials, Division of Building Research, National Research Council of Canada, Ottawa, 1981 (CBD 219).
   
2. Wyly, R. S. et al., Investigation of standards, performance characteristics and evaluation criteria for thermoplastic piping in residential plumbing systems, National Bureau of Standards (NBS), US Dept. of Commerce, NBS Building Science Series 111, Washington, D.C., 1978.
   
3. Crowder, J. R. and Rixon, R., Trial of plastic pipes for hot-water services, British Plastic 40, 1161, 1967.
   
4. Hucks, R. T., Jr., Performance of PVC pipe in water distribution systems, Modern Plastics 40 , 112, 1972.
   
5. Kent, A. D., Plastic pipe in buildings, Housing Note No. 26, National Research Council of Canada, Division of Building Research, Ottawa, 1973.
   
6. Chasis, D. A., Plastic piping systems, Industrial Press, Inc., New York, 1976.