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Anaerobic Adhesive : Curing Mechanism & Activation

Being the Loctite distributor, we are frequently asked by our customers when promoting the Loctite anaerobic adhesives - some similar questions, again and again...

' the xxx brand glue ( usually referred to cyanoacrylate adhesives or instant adhesives ) that I bought not long ago  hardened  like a stone, would this be the same? '

" how long can I keep this ( anaerobic ) glue? I had bad experience with *glue hardening in an unopened bottle '

After explaining the curing mechanism of anaerobic adhesive and showing them the Loctite technical bulletins, they were delighted and  even amazed to know the fact that the adhesive remains in liquid state as long as it is in contact with atmospheric oxygen.


Anaerobic adhesives are adhesives that cure to a tough solid ( thermoset plastic ) when they come into contact with metal(s), and under the condition of exclusion of oxygen supply. This refers to the situation that the adhesive is enclosed in a joint gap, either a bolt - nut fastening, bearing - shaft assembly, threaded fitting, flanges assembly which separating from oxygen supply.

See the following diagrams for illustration :

1. In an oxygen-riched condition ( green rings ), the monomers ( white )  in the adhesive remain in free flowing liquid state by the peroxide molecules ( red ).




2. Once the adhesive is separated from the constant oxygen  (green ) supply when it is confined in a joint gap by mating the parts, the peroxides  change in to free radicals ( red )  under the catalytic effect of metal ions ( rectangles ).

A cross section of a bolt-nut assembly showing the anaerobic adhesive ( red ) is confined in close fitting metal surfaces, shown below.






3. Free radicals ( red ) initiate the formation of monomer chains ( black rings ) -- polymerization begins and subsequently  a macro molecular substance of thermoset plastic is formed, holding ( bonding ) the two mating parts with very high shear strength ( achieves 4,500 psi in cylindrical retaining in some cases ). See technical specifications of  Loctite 635,  Loctite 648.
Note : The diagrams above are meant for illustration only, and not reflecting the actual molecular size ratio, configuration, and chemistry stoichiometry.



Under the circumstances where
1. Speed curing of anaerobic adhesive is desired  for faster return to service,
2. Large gaps and deep threads,
3. Cold parts
4.  Mating metal parts are of the *inactive surfaces; priming with a liquid activator is needed.


* Inactive surfaces commonly  encountered :  stainless steel, high alloy steel, galvanized steel,  nickel, aluminium, zinc to name a few.



LOCTITE® 7649™ Activator N -- used where increased cure speed of LOCTITE® anaerobic products is required. It is especially recommended for applications with passive metals or inert surfaces and with large bond gaps. LOCTITE® 7649™ is particularly recommended when prevailing temperature is low (<15 °C). ( Loctite 7649 has long on-part life, up to 30 days, after applying onto the parts )


LOCTITE® 7471™ Activator T-- a solvent based primer used where increased cure speed of LOCTITE® anaerobic products is required. It is especially recommended for applications with passive metals or inert
surfaces and with large bond gaps.
( Loctite 7471 has on-part life of approximately 7 days after applying onto the parts )