Explainer: threadlockers, sealants and anti-seize compounds

Andy Bardon, a Senior Application Engineer at Henkel Corporation, outlines the main characteristics of threadlockers, sealants and anti-seize compounds, and explains how these different thread treatment options benefit machine builders, end users and maintenance technicians.

Proactively treating threaded pipes and fasteners with thread treatment products is crucial to efficient and cost-effective manufacturing. Loosening of threaded fasteners, which is a major cause of industrial equipment failure, results in unscheduled downtime costing millions of euros each year. Similarly, millions of litres of industrial fluids are wasted annually through leakage. Beyond the value of lost fluids and gases, leaks also affect toxicity, emissions, safety, contamination and personnel.

Threaded fasteners set and hold tolerances on assemblies ranging from light-duty equipment to heavy machinery. To ensure reliable performance, it is imperative that the specified tolerances hold during the equipment's entire service life.

To lubricate, protect, seal and hold threaded fasteners to their original tolerances, various forms of thread treatments are used to increase an assembly's reliability. Thread treatments can be divided into three categories: thread sealants, threadlockers and anti-seize materials.

Threadlockers

Threaded fasteners are subjected constantly to various types of differential stress. Stresses such as vibration and shock, thermal expansion and contraction, and micro-movement of the fastened parts can all reduce clamping force and, ultimately, cause machine failure. Many mechanical devices - such as spring washers, castellated nuts and locknuts - were created in an effort to prevent uncontrolled fastener loosening. These mechanical locking methods add significant cost to the fastener assembly, but are unable to perform reliably due to the side-sliding motion that causes self-loosening. Furthermore, they do not seal or prevent corrosion within the assembly and must be sized appropriately for the specific fastener.

Liquid threadlocking adhesives are one of the most reliable and cheapest ways to ensure that a threaded assembly remains locked and leak-proof for its full service life. Applied as drops to fastener threads, liquid anaerobic products fill the voids in the threads and cure to a hard thermoset plastic when exposed to active metal ions in the absence of air. Locking the threads together prevents unwanted movement or loosening of the fastener, and seals the threads, thus preventing leakage or corrosion. Excess threadlocker is wiped away easily since it remains liquid.

Although a properly prepared surface ensures the most consistent threadlocker performance on bonded assemblies, advances in anaerobic technology have resulted in products that limit the need for thorough cleaning and surface preparation. New threadlocker formulations now cure on inactive metal surfaces, tolerate oily surfaces and can be operated at increased temperatures. In addition, surface primers are only required when using conventional liquid products on challenging applications and substrates.

Threadlockers can help prevent common failures such as misalignment (where there is a loss of clamp load). Regarding shafts for gearboxes and engines, the mounting bolts must be torqued properly to maintain correct alignment. However, over time these mounting bolts can loosen due to vibration, thermal expansion and contraction, or shock. This results in a loss of clamp load and, ultimately, misalignment. In such cases, proactive use of liquid threadlockers helps to maintain the clamp load and prevent misalignment.

Threadlockers, which are available in various strengths to suit different applications, allow threaded fasteners to maintain the clamp load even in extreme environments. These advanced adhesives offer high shear strength, very good temperature resistance, rapid curing, easy dispensing and excellent resistance to vibration. New anaerobic formulations are available, including surface-insensitive varieties and high-temperature formulations for short-term exposures up to 230degC.

Selection criteria

Several factors need to be considered when selecting the right threadlocking adhesive for an application. Contrary to popular belief, any bolt previously locked with threadlocking adhesive may be reused simply by removing any old adhesive before applying new threadlocking material and commencing reassembly.

Threadlockers are available in low-strength formulations for easy removal, medium-strength grades that can be removed using common hand tools, and high-strength formulations that offer the highest holding abilities. However, no threadlockers are completely permanent; even the highest strength threadlockers can be removed using standard hand tools following direct exposure to temperatures of 230-260degC for about five minutes.

It is possible to remove significant amounts of old threadlocker residue by using a wire brush. Low levels of threadlocker residue are compatible with the next application of threadlocker, as long as they do not restrict the reinstallation of the fastener.

Thread sealants

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The potential for costly, dangerous leaks exists in all fluid systems - gas, vapour or liquid. The degree to which a leak is tolerated varies significantly with each application. For example, a few drops of water per hour leaking from a pipe fitting in a drainage hose can be ignored. However, allowing a similar amount of water leakage on to an electrical switchboard could be disastrous.

Most leaks can be traced to pipe joints. Threaded joints in piping are necessary evils. Pipe system designers seek to reduce the number of joints wherever possible, but without joints that can be dismantled, each repair would require the removal of lengthy pipe links.

Despite the standards created to maintain uniform fittings, tapered pipe threads are imprecise. Over the course of use and repair, threads can become damaged and even more vulnerable to leakage. The area where the crest and the root of the thread meet forms a spiral leak path. No amount of tightening will eliminate this. Thread sealing is an attempt to block this leak path in pipe connections. Mechanical thread sealants include a variety of products, such as sealing tapes, dopes, pastes, O-rings and cone fittings. While effective, these pipe sealing methods can create problems over time.

Tapes act only as a lubricant and can shred, clog pipes or even cause over-tightening, which may damage threads. Tapes are applied manually, limiting automation of the application, and offer poor vibration resistance due to their slick surface. Most pastes contain solvents and exhibit creeping and shrinking problems as they dry. Such issues limit their chemical and vibration resistance. O-rings require large inventories and a special joint design, and may be damaged easily during handling or assembly. Machined cone fittings require costly machining and are also easy to damage.

Easy to apply

Thread sealant adhesives are liquid to paste-like products that are applied easily and cured through an anaerobic reaction. Like anaerobic threadlockers, thread sealant formulations do not contain any volatile solvents that evaporate out of the threads over time and affect long-term performance of the joint. These materials use formulation modifiers to lubricate and instantly seal the joint while the cure is in process.

Once cured, anaerobic thread sealants will not melt, thus eliminating leak paths. These materials seal and lock threads at the same time, and act as a lubricant during assembly to promote tightening while ensuring consistent assembly torque. Furthermore, uncured thread sealant dissolves, eliminating the potential for contamination. At the time of application, these materials provide instant low-pressure seals; after curing, many formulations are rated to seal to pressures of 1000bar. Thread sealants also improve the sealing function in compression fittings, and provide exceptional fluid compatibility and sealing ability.

Threaded hydraulic fittings provide a common leakage point. This is because alternative sealing products fail over time due to shrinkage (which results in a leak path) or loosening, since the common alternative sealant is really just a lubricant and does not seal the voids. Anaerobic thread sealants provide the best of both worlds by providing lubrication to help assemble the fittings and by sealing without shrinking to provide long term seals.

Anti-seize compounds

Anti-seize materials protect threaded and slip-fitted metal parts from rust, corrosion, galling and seizing at high temperatures. They also reduce friction, wear and breakage on critical parts in severe operating environments.

These high-performance pastes (formulated with or without specific types of solids such as copper flakes or graphite powder) help ensure that fasteners are easy to assemble and disassemble. At the same torque, a consistent bolt tension is always achieved using anti-seize materials, even in performance extremes.

Specific formulations cater for the type of metal used for the threads, as well as temperature extremes. Many products perform well in excess of 540degC and some perform in temperature extremes up to 1315degC. As a result, anti-seize materials can be used on a furnace door hinge, for example, to prevent seizing in the heat.

These products are typically brush-applied to parts. This requires a reservoir and a separate applicator to spread the material. Dipping a fastener into the material is not recommended since it results in over-application, contamination and an inconsistent, messy assembly. Aside from the brush top package, some products are also available in an aerosol can.

New technology

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Recent advances in the stability and reactivity of threadlocking and thread sealing materials have enabled the development of semi-solid 'stick' formulations that complement liquid counterparts. Semi-solid threadlocking, thread sealing and anti-seize products work well in applications not previously considered because liquids were found to be too messy or could potentially migrate into areas where a cured adhesive might create problems. For example, sealant sticks are excellent alternatives in overhead or difficult-to-see areas where fluids and tapes are cumbersome to use. Moreover, anti-seize stick technology allows direct use on specific parts and minimises over-application.

The sticks are very useful when an assembly operation is time-sensitive or must be staged. Here, the threadlocking material is applied in advance without having to worry about it running off the part. Treated parts are then assembled all at once without time-consuming matching of mechanical locking devices or dropwise application of liquid thread treatment.

More about thread treatments

Follow the link for more information about the Loctite Success Secured campaign; the microsite has customer case studies, videos, an adhesive selection 'wizard', webinars and a wealth of other useful information.

Henkel Ltd

Wood Lane End
Hemel Hempstead
HP2 4RQ
UNITED KINGDOM

+44 (0)1442 278000

technicalservice.loctite@uk.henkel.com

www.henkel.com

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