What is Anaerobic Adhesives

Anaerobic adhesives, commonly referred to as anaerobic glues, do not cure when exposed to oxygen or air. However, once isolated from air, they can rapidly polymerise and cure at room temperature—either on a metal surface or in the presence of a catalyst—forming a strong bond and seal with the substrate.

Characteristics of Anaerobic Adhesives

① Anaerobic; 
② Most anaerobic adhesives are single-component, and their viscosity, strength, and initial cure time can be adjusted as needed; 
③ Excellent performance, with good weather resistance and resistance to various media; 
④ Minimal shrinkage during curing; nearly 100% of the components participate in the curing reaction, resulting in excellent sealing performance; 
⑤ Stable storage; generally storable for more than 1 year; 
⑥ They have a wide range of applications, including locking, sealing, bonding, securing, and filling.

Formulation Composition of Anaerobic Adhesives

Anaerobic adhesives are balanced systems in which initiators and inhibitors coexist; currently, there are three types in the industry. The first type is a general-purpose anaerobic adhesive, typically formulated with monomers, initiators, accelerators, inhibitors, thickeners, dyes, and other components; the second type is a pre-coated anaerobic adhesive, whose formulation is divided into Component A and Component B. Component A primarily consists of methacrylic diesters, acrylic emulsions, accelerators, and inhibitors, while Component B consists of microcapsules containing dibenzoyl peroxide initiator encapsulated in urea-formaldehyde resin. The third type is a penetrating adhesive, whose formulation consists of methacrylic diesters, initiators, accelerators, polymerisation inhibitors, and other components.

Functions of the Various Components in Anaerobic Adhesives

Monomers

Commonly used monomers include polyethylene glycol dimethacrylates of various molecular weights, ethyl methacrylate or hydroxypropyl methacrylate, epoxy resin methacrylates, polyol methacrylates, and polyurethane acrylates. Since these monomers contain two or more double bonds that can participate in the polymerisation reaction, they can serve as the main components of anaerobic adhesives. To improve the performance of anaerobic adhesives, prepolymers that increase bond strength and thickeners that alter viscosity may also be added.

2-Hydroxyethyl methacrylate CAS 868-77-9

2-Hydroxypropyl Methacrylate CAS 27813-02-1

Triethylene Glycol Dimethacrylate 109-16-0

Isobornyl Methacrylate CAS 7534-94-3

1,4-Butanedioldimethacrylate (BDDMA) CAS 2082-81-7

Bisphenol A Ethoxylate Dimethacrylate CAS 41637-38-1

Initiators

The curing reaction of anaerobic adhesives is a radical polymerisation reaction; most use hydroxyisopropylbenzene peroxide as an initiator, with an initiator concentration of approximately 1% to 5%.

Cumyl Hydroperoxide CAS 80-15-9

N,N-Dimethylaniline CAS 121-69-7

N,N-Dimethyl-p-toluidine(DMPT) CAS 99-97-8

Accelerators

In the curing reaction of anaerobic adhesives, an appropriate amount of saccharin, tertiary amines, or similar substances is added as reducing agents to promote the decomposition of peroxides; the amount of accelerator used ranges from 0.5% to 3%.

Plasticizers

Not all anaerobic adhesives require high strength. Adjusting the cured strength of anaerobic adhesives to facilitate later disassembly and maintenance is typically achieved by adding a certain proportion of plasticisers. Additionally, for anaerobic adhesives used for sealing, adding plasticisers can enhance the flexibility of the cured adhesive layer and improve the sealing performance. When using plasticisers, priority should be given to compatibility with the base materials (such as monomers), following the principle that “like dissolves like,” to prevent separation during storage and migration of plasticisers after curing.

Bis(2-ethylhexyl) sebacate CAS 122-62-3

Diisononyl phthalate CAS 28553-12-0

Polyethylene glycol monolaurate CAS 9004-81-3

Polyethylene glycol monooleate CAS 9004-96-0

Triethylene Glycol Monomethyl Ether CAS 112-35-6

Polyethylene Glycol CAS 25322-68-3

Polymerization Inhibitors

To improve the storage stability of anaerobic adhesives, it is necessary to add certain polymerisation inhibitors, such as hydroquinone and p-benzoquinone. Table 26-5 lists the effects of some of these inhibitors on the curing time and stability of anaerobic adhesives.

When it comes to the formulation of anaerobic adhesives, achieving a system that cures rapidly yet remains highly stable requires not only the careful selection of monomers and oligomers but, more importantly, a proper balance between initiation and inhibition.

Other Additives

Co-accelerators are generally imides and carboxylic acids, such as o-phthalimide (commonly known as saccharin), o-phthalimide, triphenylphosphine, ascorbic acid, and methacrylic acid. The most widely used and effective co-accelerator is o-phthalosulfimide, followed by ascorbic acid. The typical dosage of co-accelerators ranges from 0.01% to 5%, and their effectiveness varies depending on the specific compound. It is particularly important to note that the same co-accelerator and initiator will produce different results when used with different monomers and initiators. Table 26-6 lists the effectiveness of several co-accelerators.

Ascorbic Acid CAS 50-81-7

Saccharin CAS 81-07-2