Sealants for Threaded Connections

Sealants

For sealing threaded connections, various products can be used such as Teflon tape, Teflon cord, and liquid thread sealants. The most ideal situation, however, is to use threaded fittings with an O-ring seal or a PTFE coating.

Many pneumatic components such as push-in fittings are available with BSPP (parallel thread) including O-ring sealing, or BSPT (tapered thread) with PTFE coating. These mount quickly, without mess, and are ready for use immediately.

Teflon Tape

If fittings are not provided with an O-ring or PTFE coating, the thread must be sealed to prevent leakage. A commonly used method is PTFE tape (Teflon tape). It is easy to apply and (if properly installed) immediately airtight.

The disadvantage is that often too much or too little tape is used. When tightening, pieces of tape can shear off and end up inside the component. While a piece of tape in an air line may seem harmless, it will eventually reach valves, quick couplings, and filters. We often see failures in valves and compressed air quick couplings caused by residual tape particles.

Threadlockers and Thread Sealants

A better solution is to use liquid threadlockers and thread sealants. Many types are available, and it is important to distinguish clearly between a threadlocker and a thread sealant. Unfortunately, these are often confused.

Although some curing time is required, most sealants can already withstand pressure shortly after assembly. It is important, however, to check whether the thread size matches the specifications of the sealant manufacturer. Not all sealants are suitable for thread sizes larger than 1”.

• For fine threads up to ¾”, we recommend Loctite 542 (LOC-542), a sealant with a breakaway torque of 8.5 Nm.

• For coarser threads from 1” up to 3”, we recommend Loctite 577 (LOC-577), technically a threadlocker with a breakaway torque of 11 Nm.

Always check the manufacturer’s website for detailed specifications.

Make sure to check the expiry date, and thoroughly degrease the sealing surfaces to ensure proper adhesion. Always apply the sealant to the male thread, evenly around the circumference. After tightening, remove any excess sealant — this will not cure and will remain sticky. Most sealants are anaerobic (“without oxygen”), meaning they only cure once oxygen is displaced.

Combining Different Materials

If two different materials are combined, for example a nylon fitting in a nickel-plated brass ball valve, it may be advisable to use an epoxy adhesive. These two-component adhesives cure through a chemical reaction. Processing time is usually short, with curing occurring within minutes. However, for the joint to withstand full loads, several hours of curing are usually required.

An activator such as Loctite 7649 can speed up this process, although it can also make the processing time too short — which must be tested in advance.

Activators

As mentioned earlier, drying time can be influenced by using an activator. This accelerates the chemical reactions between the components. Apply the activator to the female thread (after degreasing) and let it flash off properly before applying the sealant.

Sealants vs. Threadlockers

The difference is in the name: a sealant seals, a threadlocker locks. However, the two are often used interchangeably, which increases the risk of leakage. This typically happens when a connection is re-tightened after the sealant has cured, breaking the bond and causing leakage.

The reverse is also true: a sealant may be able to lock a connection, but it is not designed to withstand high forces. For larger projects, it is advisable to run a test to ensure that the chosen product is suitable for the application.