I. Definition

Anaerobic adhesive, often referred to simply as anaerobe, is also known as oxygen-impermeable adhesive, anaerobic sealant, threadlocker, or mechanical adhesive. It is a single-component sealing and bonding agent formulated based on the principle that oxygen inhibits free-radical polymerization. It can be used both for bonding and for sealing. When the coated surface is isolated from air and subjected to catalytic conditions, the adhesive rapidly polymerizes and cures at room temperature. The term "anaerobic" means that this adhesive does not require oxygen during use.

The composition of anaerobic adhesives is relatively complex. While unsaturated monomers constitute the main component, they also include aromatic amines, phenols, aromatic hydrazines, peroxides, and other substances. In recent years, overseas formulations of anaerobic adhesives have been continuously innovated and refined, becoming increasingly sophisticated and gaining widespread favor in the mechanical industry.

II. Solidification Principle

The initiator decomposes to generate free radicals, and the abundant oxygen acts as an inhibitor of polymerization. Once oxygen is isolated, the trace amounts of oxygen dissolved in the anaerobic adhesive are completely consumed by the generated free radicals, thereby initiating polymerization and crosslinking for curing.

Anaerobic adhesives have two curing conditions: 1. Contact with metal; 2. The formation of an oxygen-deficient environment.

III. Characteristics

1. Simple operation, low glue consumption, easy storage, high compliance, and convenient for continuous production processes.

2. It has low curing shrinkage, resulting in lower stress at the bonded interface.

3. It cures rapidly at room temperature. The standard product is suitable for operating conditions ranging from -55°C to 150°C; exceptional models can withstand temperatures up to -30°C and exhibit exceptionally excellent resistance to various media.

4. The shear strength has a wide range of variation, from 4 N/mm. ² Up to 50 N/mm ²。

5. Excellent impregnation, vibration absorption, and sealing properties; also provides anti-rust protection for threaded roots.

6. Non-flammable, non-explosive, solvent-free, non-volatile, and harmless to the environment during operation.

7. The adhesive squeezed out from the assembly gap will not cure when it comes into contact with oxygen (air), making it relatively easy to remove any excess adhesive.

8. Suitable for use with water, oil, and gas media; not suitable for oxygen, ozone, chlorine gas, liquid chlorine, or strongly oxidizing media. Not suitable for porous materials, large castings, or non-metallic materials.

9. Most are monomorphic, with a wide range of viscosity variations and numerous varieties, making them easy to select from.

10. No need to weigh, mix, or prepare the adhesive—extremely easy to use and readily adaptable to automated operations.

11. Cures at room temperature, with fast curing speed, high strength, energy savings, low shrinkage, and excellent sealing performance. It can be disassembled after curing.

12. Excellent performance, with good resistance to heat, pressure, low temperatures, chemicals, impact, vibration damping, corrosion, and fogging.

13. The excess sealant outside the joint does not cure and is easy to remove.

14. Solvent-free, low toxicity, minimal hazards, and non-polluting. Widely applicable for sealing, locking, fastening, bonding, leak-stopping, and other uses; exhibits stable storage properties, with a typical shelf life of three years for the adhesive liquid.

IV. Applicable Materials

1. Active surfaces: steel, iron, copper, manganese, aluminum alloys, etc.;

2. Inert Surfaces: Pure aluminum, stainless steel, zinc, chromium, cadmium, titanium, silver, gold, glass, ceramics, etc., require pre-coating with a surfactant to accelerate adhesion.

3. Hydrophobic Surfaces: Certain anodized, oxidized, or plated surfaces require a pre-coating of a surfactant before they can be cured.

V. Curing Properties

1. Generally speaking, anaerobic adhesives can be effectively used on metal surfaces. Compared to other materials, they are more reactive, and their curing speed can vary—faster or slower—depending on the type of material.

2. The curing time of anaerobic adhesives: Anaerobic adhesives are solvent-free (solvents must evaporate). For anaerobic adhesives to cure, they must come into contact with metal ions and be in an oxygen-deficient environment. Outside the bonding area, the anaerobic material cannot fully cure. Inside the bonding area, the curing rate depends on the specific product and the type of accelerator used. Heating can speed up the curing process.

3. Anaerobic adhesives can cure rapidly at room temperature. Standard products are suitable for temperatures ranging from -55°C to 150°C; special models can withstand temperatures up to 230°C and exhibit exceptionally excellent resistance to various media.

4. Anaerobic adhesive is a single-component product that eliminates the cumbersome weighing required for other adhesives. It offers stable quality, is easy to use, requires minimal adhesive application, and facilitates streamlined production processes.

5. The relationship between the curing speed of anaerobic adhesives and metal type and temperature.

As can be seen from the two tables above, the higher the metal activity, the faster the anaerobic adhesive cures; similarly, the higher the temperature, the faster the curing speed.

In addition, the curing speed of anaerobic adhesives is also influenced by oxygen-deficient conditions. When the mating clearance is extremely tight—resulting in excellent oxygen exclusion—the curing reaction will accelerate accordingly.

6. Generally, full curing takes 24 hours; the initial curing time for parts varies—depending on the adhesive formulation, the initial curing time can be as short as 10 minutes. Anaerobic adhesives come in a variety of formulations; for specific curing times, refer to the product instructions.

7. Using an anaerobic adhesive accelerator can speed up the curing process; most products can cure on metal surfaces without an accelerator. On non-metallic and non-reactive surfaces, it is recommended to use an accelerator to achieve the desired application speed.

VI. Composition

Anaerobic adhesive is composed of acrylate monomers, initiators, accelerators, and stabilizers. Other additives, such as fillers, dyes and pigments, thickeners, plasticizers, thixotropic agents, and UV absorbers, can also be added as needed.

Acrylate monomers are the primary components of anaerobic adhesives, accounting for more than 90% of their total formulation. These monomers include diesters of acrylic acid and methacrylic acid, as well as certain specialized acrylates such as hydroxypropyl methacrylate.

7. Applications

Anaerobic adhesives are widely used in industries such as aerospace, ordnance, automotive, machinery manufacturing, home appliances, hardware, casting, and pipeline installation engineering. For example, they are employed for locking and preventing loosening of mechanical threaded components, ensuring tight seals to prevent leaks; securing and holding bearings, bushings, gears, and other parts during assembly; sealing flanges or flat mating surfaces; plugging micro-pores in castings and welded joints; and sealing pipe threads in gas, water supply, fire protection, and HVAC systems. Anaerobic adhesives can be applied to a variety of tasks including locking, sealing, holding, bonding, and leak-stopping. As a result, anaerobic adhesives have become indispensable liquid tools in the mechanical industry. They find extensive applications in sectors such as aerospace, defense, automotive, machinery, electronics, and electrical engineering.

1. Locking and anti-loosening

Metal screws are prone to loosening or coming loose under the influence of impact and vibration. Traditional mechanical locking methods are often unsatisfactory, whereas chemical locking methods offer an inexpensive and effective solution. If screws are coated with anaerobic adhesive before assembly, after curing, a strong, resilient, plastic adhesive film will form in the thread gaps, securely locking the screw and preventing it from loosening.

2. Sealed and leak-proof

No flat surface can achieve perfectly tight contact without leakage-proof sealing. Traditionally, this has been accomplished using gaskets made of materials such as rubber, asbestos, or metal; however, these gaskets tend to leak quickly due to aging or corrosion. By contrast, anaerobic adhesives can replace solid gaskets: once cured, they ensure a tight fit and provide far more durable sealing performance. Anaerobic adhesives are highly effective in sealing threaded pipe joints and threaded plugs, sealing mating surfaces of flanges, and sealing joint surfaces of mechanical housings, delivering excellent leak-prevention results in all these applications.

3. Firmly maintain positioning

Cylindrical component assemblies—such as bearings and shafts, pulleys and shafts, gears and shafts, bearings and housing bores, bushings and holes—have traditionally been assembled using interference fits achieved through methods like hot fitting or cold pressing, often supplemented by keys and pins. This conventional fixing method demands extremely high machining accuracy and, due to differences in thermal expansion coefficients, can lead to wear, corrosion, and eventual loosening. By using anaerobic adhesives, however, the mating gaps can be completely filled; once cured, these adhesives provide a strong, durable, stable, and reliable bond. Adopting anaerobic adhesives for fastening significantly reduces the requirements for machining accuracy, simplifies assembly operations, boosts production efficiency, and helps save energy and processing costs.

8. Classification

Anaerobic adhesives are composed of a variety of components, particularly monomers that exhibit tremendous diversity. Each change in these components can potentially yield new properties, which is why there are numerous types of anaerobic adhesives and no uniform method for classifying them. Generally, they can be classified according to the structure of the monomers, the type and strength of the monomers, and their viscosity; some classifications also group them by application. A relatively common classification method is based on the structure of the monomers and their intended use. Specifically, according to their structure, they can be divided into four categories.

1. Ether-type

A structure represented by bis(methacryloyloxyethyl) trimethylene glycol ester.

2. Alkyd ester

Common examples include bis(methacryloyloxyethyl) polyethylene glycol ester; hydroxyethyl methacrylate or hydroxypropyl methacrylate, and others.

3. Epoxy ester

It is a product obtained by reacting epoxy resins of various structures with methacrylic acid. Common examples include bisphenol A epoxy esters (such as the domestically produced Y-150 and GY-340, which are mixtures of epoxy esters and polyethylene glycol esters).

4. Polyurethane

It is a reaction product of isocyanates, hydroxyalkyl phenols methacrylates, and polyols.

In fact, many anaerobic adhesives are mixtures or complex formulations that are difficult to classify simply.

9. Instructions for Use

Anaerobic adhesive application involves several steps, including surface preparation, adhesive application, assembly, curing, and cleanup.

1. Surface Treatment

First, clean the surfaces to be bonded by removing oil, rust, and other contaminants, followed by appropriate sanding, and finally ensuring that the surfaces are thoroughly cleaned and dried. For inert, non-metallic surfaces, apply a primer or activator to enhance adhesion. Surface cleanliness is crucial for bonding; vapor-phase degreasing provides the best results. Commonly used solvents include trichloroethane, trichloroethylene, methyl ethyl ketone, and methanol. It’s advisable to avoid using gasoline, kerosene, naphtha, fuel oil, or hydrocarbon solvents, as these can leave behind an oily film on the surface. A conventional degreasing method involves washing twice with solvent gasoline, which is generally considered ideal. Lightly sanding the surfaces to be bonded can significantly improve bonding strength.

2. Apply glue

The anaerobic adhesive can be applied directly to the surfaces to be bonded using the original packaging, or it can be applied by brushing or scraping. It works best when both surfaces to be bonded are coated with adhesive. Even if only one surface is fully coated and then assembled, satisfactory results can still be achieved. For bolts, apply the adhesive only to the bolt end and the threaded portion; as you tighten the bolt, the adhesive will fill the entire gap. If the joint involves cracks or areas that are difficult to disassemble, you can also use a soaking method to allow the adhesive to penetrate. It’s best to apply enough adhesive to completely fill the gap; the optimal adhesive layer thickness is 0.03 mm. Applying an excessively thick layer may reduce the strength of the bond.

3. Assembly

During assembly, rotate the parts back and forth to ensure even distribution of the adhesive. Position the parts as quickly as possible. If a catalyst is used, adjust the position only within the allowable time frame (1–5 minutes).

4. Parts assembled after applying adhesive generally achieve partial curing at room temperature (within 5–20 minutes). In general, the larger the bonding area and the smaller the gap, the faster the curing process (for materials of the same type). After 3 hours of curing, the bond strength can reach 50%–65% of its rated value, and it reaches its maximum value after 24 hours. As the temperature rises, the curing time shortens and the bond strength increases. At low temperatures, even extending the curing time to 48 hours makes it difficult to achieve the rated strength. The newly developed fast-curing anaerobic adhesive can be positioned within just a few seconds to a few minutes, and its shear strength can reach over 80% of its maximum value.

X. Frequently Asked Questions and Troubleshooting

1. What solvents can be used to remove liquid anaerobic adhesive products?

Answer: Most organic solvents are effective at removing anaerobic and acrylic products. Chlorine-based solvents are the most commonly used.

2. How long does it take for anaerobic adhesive products to cure?

Answer: Anaerobic adhesives do not contain solvents (solvents need to evaporate). For anaerobic adhesives to cure, they must come into contact with metal ions and be in an oxygen-deficient environment. Outside the bonding area, the anaerobic material cannot fully cure. Inside the bonding area, the curing rate depends on the specific product and accelerator used; heating can speed up the curing process.

3. Why are the bottles of 50 ml and 250 ml anaerobic adhesives filled only halfway?

Answer: In fact, the bottles already contain 50 ml and 250 ml products. Filling the bottles only halfway is intended to allow air to act as a barrier, preventing the anaerobic adhesive from curing. The 50 ml and 250 ml bottle designs also permit some air to seep in, enabling the anaerobic adhesive to “breathe.”

4. What do colors signify?

Answer: Anaerobic adhesives are often referred to as “red or blue items.” For thread-locking adhesives, the color indicates strength. Typically, red signifies high strength, blue represents medium strength, and purple denotes low strength. Other colors may indicate different strength levels in various product categories.

5. Does anaerobic adhesive become thicker in winter and cure more slowly?

Answer: This is an inherent property of oxygen-inhibited adhesives—when the temperature drops, viscosity increases and curing time lengthens. Below -5℃, if used on non-reactive metals, incomplete curing may occur. As the temperature rises, viscosity decreases and curing time shortens. The issue can be resolved by moving the work area to a warmer environment.

6. Some users have reported that the oxygen-deficient adhesive they use frequently has suddenly experienced a drop in locking strength.

Answer: Upon investigation, it was found that the issue arose because some customers applied rust-preventive oil or other types of oil to threaded components in order to prevent them from rusting, or requested suppliers to coat the threaded components with grease. Furthermore, no degreasing treatment was performed during assembly. After carrying out degreasing treatment, the problem was resolved.

XI. Precautions

1. Anaerobic adhesives must not be stored in airtight containers made of metal, glass, or other non-breathable materials. Instead, they should be stored in breathable containers (made of low-density polyethylene), and the containers should be filled to no more than two-thirds of their capacity.

2. Anaerobic adhesives should be stored in a cool, dry place and must not be exposed to direct sunlight.

3. This adhesive is suitable for bonding metals but not appropriate for porous materials such as plastics, wood, or paper. On active metal surfaces like steel, copper, and their alloys, the adhesive cures quickly and exhibits high strength. On metals such as stainless steel, zinc, and cadmium, however, curing is slower and the resulting bond strength is lower.

4. The curing conditions must meet the following two requirements: isolation from oxygen, with a gap generally required to be less than 0.2 mm; and the presence of active initiation centers, such as metals or accelerators.

5. If the adhesive force is too strong during disassembly, you can heat the component to 200–300°C and remove it while it’s still hot. Alternatively, you can soak the component in an anaerobic adhesive remover or acetone for an extended period before proceeding with disassembly.