Technical Bulletin: Plastic pipe threaded fittings

2. Thread sealants for plastic pipe

Thread sealants are an integral part of making a threaded connection with plastic pipe fittings. Without a sealant a thread will always leak, even at almost zero pressure. There are many different types of thread sealant available on the market, and preference is often a matter of habit, rather than performance.

The most common thread sealant used is PTFE tape. Generally white in colour, PTFE tape is supplied in rolls and wound onto the male thread only. The number of turns around the fitting is dependent on the size of the fitting. Prior to applying the tape the thread should be cleaned of any grease or debris by wiping with a cloth soaked in pvc cleaner. The outside of the thread should then be very slightly roughened using a flat file. These two steps help bind and bed the PTFE tape in, to the male thread.

PTFE tape should be wound onto the thread in the same direction as the thread. This will ensure that the tape does not unwind as the plastic pipe fittings are connected together. If the tape is wound on the other way, it will begin to unravel as soon as the fittings are threaded together.

There is a lot of debate over how many turns of PTFE tape should be applied to a thread. The generally accepted amount is between 3 and 5 turns.

Cord products

Cord type thread sealant products such as Loctite 55 are relatively recent additions to the market compared to PTFE tape. One of the key advantages of Loctite 55 over PTFE such cords, as claimed by the manufacturers, is that the thread can be “backed off” by up to ¼ of a turn without any fear of leakage. This enables more precise positioning of fittings, especially in a situation where you are connecting to fixed fittings.

The other advantage of cords such as Loctite 55 is that they can be cleaned off threads very easily if the fitting needs to be removed and reinstalled.

Paste compounds and hemp

Paste type compounds such as Boss white and a myriad of other trade names are generally, for the plastic pipe industry, regarded as having been superseded by PTFE tape and cord. Although they can provide a good seal, care must be taken that the compound, which is usually applied with some strings of hemp (a plant based material used to make natural fibre ropes), are suitable for the application they are being used in. This is especially so where potable water or chemical s are concerned.

The use of paste compounds is also very messy in comparison to PTFE tape and cord products and it is very difficult to make a joint that looks sanitary or with a professional finish to it.