1. Field of the Invention
This invention relates to anaerobically curable compositions, their use and method of preparation. More particularly, this invention relates to anaerobically curable compositions which exhibit rapid curing irrespective of the type of surface, thereby rendering them surface-insensitive.
2. Brief Description of Related Technology
Anaerobic adhesive compositions are well known for their ability to remain in a liquid, unpolymerized state in the presence of oxygen and to cure to a solid state upon the exclusion of oxygen. Early work on anaerobic adhesive compositions concentrated on developing a cure system which improved the speed and/or bond strength of the adhesive composition. Various cure systems for anaerobic adhesive compositions have been developed to efficiently perform the redox reaction, which is the basis for anaerobic chemistry. Hydroperoxides were found to serve as a catalyst for the generation of a free radical. For example, U.S. Pat. No. 2,895,950 to Krieble, discloses the inclusion of hydroperoxides in amounts of 0.10 to 10% by weight in anaerobic adhesive compositions to achieve faster cure times. Amines, used in amounts up to about 10 percent by weight, are also disclosed in the '950 patent as accelerators to generate free radicals of the peroxide.
In many thread-locking applications, it is desirable for the composition to have a relatively low viscosity to permit penetration and wicking into the thread-engaging spaces. Viscosities which are too high to exhibit sufficient flow characteristics, may not sufficiently fill the voids in this area.
Many anaerobic threadlocking applications, such as in the electronics industry, require compositions which require relatively low strength to permit disassembly and replacement parts. Low strength anaerobic compositions, typically have less reactive monomer content than those anaerobic compositions where reaction products exhibit higher strength. As a result, the cure speed of low strength anaerobic compositions is slower than that of higher strength anaerobic compositions containing a greater level of reactive monomer component.
On surfaces, such as stainless steel, zinc, dichromate, and cadmium, which are considered “slow” or relatively inactive materials for anaerobic cure, primer compositions have been considered necessary for quick fixture and cure times required of many applications. The cure speed on these materials is significantly slower when compared to typical ferrous-containing surfaces, regardless of the reactive monomer content.
To enhance the cure speed of anaerobic compositions, and particularly low and medium strength threadlocking compositions, various surface preparations and primers have been used. While such primers are often very effective, their use requires the additional step of applying the primer with a certain degree of care to prevent migration and contamination of surrounding parts. This requires considerable attention in applications where small and/or sensitive components are involved. Additionally, many primers contain undesirable solvents which may be harmful to component parts and/or the surrounding environment.
Currently, the addition of viscosity modifiers, such as thickeners or fillers, are added to control the viscosity and curb the tendency of adhesive to migrate. This is usually accomplished, however, at the loss of a certain amount of cure strength, since the addition of viscosity modifiers usually replaces a portion of the reactive monomer component.
Other (meth)acrylate-based curable compositions have used a combination of uv curable components to achieve the desired cure properties. For example, Japanese Laid-Open Patent Publication Nos. 10-130601 and 10-130602 reportedly assigned to Nippon Kayakuco, Ltd., disclose uv curing adhesive compositions for use as protective coatings on optical discs which employ a uv curable ethylenically compound, such as a (meth)acrylate compound, a photoinitiator, an organic peroxide and an amino accelerator compound. Among the disclosed curable compounds are 2-hydroxyethyl (meth)acrylate, benzyl (meth)acrylate, isobornyl (meth)acrylate and phenyl oxyethyl (meth)acrylate. Another example of a uv curing (meth)acrylate-based composition is disclosed in U.S. Pat. No. 5,908,873, which contains an aliphatic urethane acrylate oligomer, a reactive (meth)acrylate monomer, a release agent and a photoinitiator. This composition is used to affix coated and inked optical fibers in a ribbon configuration.
There is a need for curable compositions which can fulfill applications requiring a range of cure strengths, i.e., low to high strength, and cure more rapidly without the use of surface treatments, such as primers. There is also a need for a curable composition that can achieve rapid cure on inactive of surfaces and reach a high percentage of its final cure strength in a relatively short time.