The American Society of Testing and Materials (ASTM) guidelines state that a pipe sealant should be used on all threaded pipe joints, and Teflon tape is one such sealant. Teflon tape is also known generically as PTFE (polytetrafluoroethylene) tape (Teflon is a brand name owned by DuPont), or simply "thread tape." Thread tape is easy to use, and helps lubricate threaded joints as they are connected in addition to forming a tighter seal.
The tape comes in various grades. Your choice of tape will depend on the function, nature, type and composition of the pipe fittings you are using.
Function of the Pipe
The tape grade you use will be determined in part by the function of the pipe you will be sealing. The main types of pipe will carry hot or cold water, fuel oil or natural gas. For oxygen lines, the thread tape must be grease-free to avoid combustion. Thread tape is not suitable for vacuum or compressed-air systems.
Nature of the Pipe
Your choice of tape grade will also be determined by the nature of the junction. A permanent joint will be part of the infrastructure of the house or building, such as a water main or gas line, and will require little or no maintenance. A semi-permanent joint will for joints that require routine maintenance, such as kitchen or bathroom plumbing. Thread tape works well for semi-permanent joints, while pipe sealant would be a better choice on permanent joints.
Types of Pipe and Fittings
Thread tape is preferable as a sealant for threaded plastic fittings because there is little risk of a chemical reaction. Thread tape works well on metal fittings also, since it serves to lubricate the metal threads for a tighter fit and seal. The thread tape should be wrapped in the direction of the pipe threads to form the tightest seal.
Tape Grade, Color and Density
Thread tape comes in different colors for different uses.
White tape is single density and intended for water pipes of 3/8 of an inch or less. It can sometimes be found in a silver color to match the pipes.
Red tape is triple density, for pipes one-half inch to 2 inches in diameter. It is designed for pipes with larger joints, such as water distribution pipes. Note that the container is red, but the tape itself looks pink.
Yellow tape is double density, and designed for gas and fuel lines.
Green tape is grease and oil free, and designed for pipes carrying oxygen.
Please note that these tape colors refer to specifications in the United States only.
Thread tape should be a minimum thickness of 3.5 milli-inches, with a minimum PTFE purity of 99 percent, and able to withstand an extreme range of temperatures, from 550 degrees Fahrenheit to 450 degrees below zero.