UNDERSTANDING BASIC TYPES OF PLASTICFor most folks, plastic piping materials are harder to understand than the metal varieties. This is because steel and copper are specific product categories - what it's called is what you get - no other flavors to worry about (except for the option of galvanization, in the case of steel). In the case of plastics, however, we have a general term covering a number of different subcategories. So while many would understand the general properties and advantages of the two main metal types, they often lack a comparable grasp of the specific materials within the plastics field. Needless to say, it is just as essential to know your way around the proper application of plastics as it is in the case of metals. (If Emmitt Smith's football helmet had been made of low density polystyrene rather than ABS or polycarbonate, for example, there's a good chance he might stand a few inches shorter today.While the first material is relatively brittle, the second two can take a pounding all day.)
Last month we covered some of the specific types of plastic materials used for supply piping - PVC, CPVC, PE and PEX. Now let's back up a step and take a look at the two most basic categories into which each of these fall.
Thermoplastics: This category of materials is formed into shape by means of elevated pressures and temperatures, a process which is reversible. In other words, once molded or extruded, it can be returned to a molten state by subjecting it to the same factors - heat and pressure. In the case of piping, reversibility is important not because it is likely that, once molded, the material would be melted down and formed into something else, but because applications of the product which involve excessive heat and pressure could cause the material to partially revert, resulting in a loss of desired properties (such as strength). Each individual type of thermoplastic material has its own characteristics in this regard, involving recommended levels of heat and pressure to which it can safely be subjected. When it comes to the joining of certain types of plastic piping materials (i.e., pipe and fittings), this reversible characteristic becomes an advantage in that a partial breaking down of the piping surface permits a strongly bonded joint through one of several welding processes. (Certain types of thermoplastics can also be "melted" by chemical reaction, the basis of solvent welding.)
Thermoset Plastics: Unfortunately, the name for this other basic category of plastic resins is not very different from the first one, which can sometimes cause confusion. Thermoset plastics can also be formed by means of elevated temperatures and pressures, though there are some specific types that are essentially cast at room temperature. The significant difference in thermosets as a broad category, however, is that the process in this case is irreversible. Once formed, such materials cannot be returned to a "moldable" state again. It might be helpful to remember the tail end of the term, thermoset - the three letters, "set" - as a means to establish the definition in your mind. Once this material is formed, it is set that way for good.
Of the plastic materials commonly used in supply pipe applications today, PVC, CPVC and PE are thermoplastics, PEX is a thermoset plastic.
CLASSES, SIZES & WEIGHTSSince metal piping materials came first, their systems of sizing and weight classifications have been largely followed by the plastic varieties. In categorizing plastic pipe, the first thing to determine is whether you are dealing with IPS (iron pipe size) or CTS (copper tube size) classifications. Most plastic pipe is made to conform to one of these two basic systems. (There are exceptions with specialized types of plastic tubing.) Properly labeled plastic pipe will always indicate which basic sizing system is involved, with an appropriate "IPS" or "CTS" included in the data given.
Once this basic sizing classification is determined, the specific nominal sizes of plastic pipe available are essentially the same as those covered on our tables for steel and copper piping last section. In other words, IPS plastic pipe is available in most of the nominal sizes listed for steel pipe, while CTS plastic pipe is available in nominal sizes comparable to copper tube. This is a general statement, however, and does not mean that each manufacturer of plastic pipe makes its material in all possible sizes. Check the manufacturer's catalog in each case to determine specific information on weight class, sizing system, and nominal sizes.
Like metal varieties, most types of plastic pipe are offered in more than one weight grade (meaning there is a selection of wall thicknesses available for various pressure requirements). The subject of weight classes isn't quite as simple in this case, however, since there are two different systems by which plastic pipe can be designated - by schedule number, or, by standard dimension ratio.
Schedule System: With many types of plastic pipe, the graduated differences in weight (wall thickness) are designated by "schedule number." The common schedule designations for plastic pipe are 40, 80 and sometimes, 120. Under this system, Schedule 40 is the lightest weight grade (thinnest walls), Schedule 120 is the heaviest (thickest walls). It is important to understand that while the system makes provision for all these classifications, this does not necessarily mean that each manufacturer offers its piping materials in all such schedules. These weight grades of plastic pipe roughly parallel those we described for steel pipe (Standard, Extra Strong, Double Extra Strong). As with steel pipe, the graduated weight schedules in plastic become thicker toward the inside, maintaining a consistent outside diameter in each of the nominal sizes involved. For example, all schedules of 1/2-inch nominal plastic pipe have an outside diameter of .840 inches, but the inside diameters become increasingly smaller as you move from the lightest to the heaviest grades (40 to 120).
Standard Dimension Ratio System (SDR): Plastic pipe that is not designated according to the schedule system is assigned a weight classification based on what is called a "standard dimension ratio," or "SDR." This is a system based directly on a pressure rating value, rather than on an arbitrary dimensional factor. As a result, all pipe within a particular SDR series has the same pressure capabilities, regardless of diameter. (This is not true of the schedule number system, in which pressure capability generally drops with increased diameter.)
There are some types of plastic pipe that fall within this SDR system of weight classification which, unlike those in the schedule system, maintain a consistent inside diameter through the various weight grades of a nominal size. In this case, for example, all SDR classifications of 1-inch nominal pipe will have an inside diameter of 1.049 inches, while the outside diameter becomes increasingly larger as you move from the lightest to the heaviest grades. Plastic piping materials that are joined by means of insert fittings use this system, since it is necessary that the inside diameters relate consistently to the mating dimensions of the inserts, regardless of wall thickness.