How do anaerobic adhesives cure? It is a puzzle, they stay liquid in the bottle, you tip them out and they remain liquid and don’t appear to dry. They are non-tacky to the touch. How can they possibly be an adhesive?!
Well, the cure mechanism of an anaerobic adhesive (or anaerobic sealant as they are often referred to) is triggered when it comes into contact with a metallic surface. This will cause the anaerobic adhesive to gel and cure. To facilitate full cure, the anaerobic adhesive needs to have exclusion of oxygen in combination with contact with a metal. With both these boxes ticked, the reactive molecules inside the liquid adhesive become activated, which triggers the curing mechanism, causing the monomers to polymerize and create a solid.
Anaerobic adhesives and sealants come in a variety of strengths to suit the application requirements. They are generally fairly rigid with a high tensile and compressive strength making them ideal for threadlocking nuts and bolts, retaining bearings into housings, gasketing between two metal flanges or pipe sealing threaded pipe joints. They are less suited to structural bonding, where there could be large flat areas for bonding where flexibility or peel forces acting on the joint.
Firstly gap fill – due to the fact air must be excluded from the joint for the adhesive cure mechanism to kick in, it means that component parts must be close fitting. Anything larger than 0.5mm (0.02”) is going to be a problem, as too much air is floating around wreaking havoc! With this in mind, consider too that the interface of the joint where the anaerobic adhesive meets the outside world will remain liquid and wet to the touch (this often gives the impression that the industrial adhesive hasn’t cured but the adhesive inside the joint has cured). Uncured surface adhesive can be easily washed off and shouldn’t cause a problem inside pipework, valves or injector nozzles. Specialty grades of industrial adhesives are available for hydraulic systems.
Temperature also affects speed – Chemical reactions are normally affected by temperature – if you increase the temperature, the molecules inside the liquid go crazier and will cause a faster cure! Generally an increase of 8°C will cause the anaerobic adhesive cure twice as quickly.
Lastly, the reactivity of the surface – the type of metal being bonded or sealed will factor heavily in the cure speed of an anaerobic adhesive.
| Super Active | Active | Less Active | Passive |
| Copper Brass | Steel Nickel Iron Aluminium Zinc | Anodized aluminium Cadmium finishes Chrome finishes Passivated metals Stainless steel Titanium | Ceramics Glass Plastics Painted finishes Lacquered finishes |
At normal room temperature, a general purpose threadlocker would typically take 10-15 minutes to cure on steel, less than 5 minutes to cure on copper, but significantly longer on less active surfaces. Some users choose to use a surface activator on low-activity and passive surfaces. These work by leaving a microscopic film of copper on the surface (which is of course highly reactive) and trick the industrial adhesive into curing.
1) Check that the joint is properly sealed – if the gap is too big or there is too big a surface area in contact with the air, the industrial adhesive will remain liquid
2) Is your surface active enough? Sometimes metal alloys are quite inactive, and the anaerobic adhesive will take longer to cure. Using an activator or heating the parts will speed the adhesive up
3) Sometimes it is hard to see but surfaces may have a lacquer coating. This stops the industrial adhesive from coming into contact with the metal beneath, thus preventing cure. If this is the case, or if you have a painted surface, it is best to remove the lacquer or paint before bonding. This will also help you achieve higher bond strength performance
For further help and information on bonding an anaerobic adhesive, please contact the Permabond technical team here.
