Polyethylene pipe comes in both standard and high density formulations. After PE, the next number in the ASTM grade number, an international standard, is the density. There are several grades, shown as numbers from 0 to 8, although 0 is not used. A density grade of 1 indicates .925 grams of material per square centimeter or lower, while a grade of five means .955 grams per square centimeter or less. Higher grades will indicate exact density.
The next number is the melt index. This indicates the weight of a PE resin that would flow from a standard die for 10 minutes. The melt index is important so that pipes will only be used for long-term exposure to heat if they are able to handle it.
This is the next number in the ASTM grade. The flexural modulus number indicates the amount of stress necessary to put a 2 percent strain on the outer fiber of the pipe. The higher density the pipe, the higher the flexural modulus number, generally speaking.
The fourth number is the tensile strength of the pipe. This indicates the point at which the PE pipe will become deformed beyond the point where it can snap back into place, or the plastic strain region.
Slow Crack Growth
Poly pipe in general is highly resistant to slow crack growth in its typical use in potable water systems. High density PE pipes are particularly resistant. The fifth number indicates the level of resistance, with many of the modern pipes ranking 6 or higher on this property.
Hydrostatic Design Basis
To find the sixth number in the ASTM grading label, the PE pipe is tested at two temperatures: 73 and 140 degrees F. This measure allows users to calculate the amount of water pressure the pipe can handle.
The seventh space in a ASTM grade for PE pipe is a letter indicating the color and UV stabilizer found in the pipe. A means natural poly pipe, B is colored, C is black with minimum 2 percent carbon black, D is natural with UV stabilizer, and E is colored with UV stabilizer.