Anaerobic adhesives formulation

Some of the chemistry of common accelerators used in curing anaerobic adhesives is discussed in detail. Emphasis is placed on the reactions of aromatic amines particularly in the presence of hydroperoxides. Product studies are presented for the reaction of a series of amines with cumene hydroperoxide (CHP) and plausible mechanisms for product formation are proposed. Relationships are drawn between these results and the chemistry which occurs in anaerobic adhesive formulations. 

Analysis of anaerobic adhesive formulations during aging studies was carried out by GC. Important peaks in the GC traces were identified by co-injection of authentic compounds. 

Anaerobic adhesive formulations vary substantially depending upon the properties sought in the cured and uncured composition. Typical formulations contain (meth)acrylate... 

A recent variation of the microencapsulation technique involves separating the components of the cure system by immobilizing the entire anaerobic adhesive formulation within a low-melting-point wax. The wax slurry can be applied at relatively low temperatures in the presence of oxygen and allowed to solidify in the threaded area of a fastener. Assembly with the attendant exclusion of oxygen mixes the components and initiates the curing process. 

A key factor in selecting any additive for an anaerobic adhesive formulation is the effect which that additive exerts on the delicate balance between activity and stability. Additives... 

Most anaerobic adhesive formulations are based on oligomeric acrylic esters, inherently low-toxicity materials. Various formulations contain additives such as adhesion promoters or cure components, and after excessive or repeated skin contact may cause skin irritation in sensitive persons. Adhesives should be washed away with water after skin contact. Nonaque-ous hand cleaners assist in removal of adhesive. Skin contact should be avoided by the use of appropriate application equipment. 

Activation by a metal surface also takes place in the commercially important anaerobic adhesives. These one-part adhesives are stable in the package, but cure quickly in an oxygen-free environment such as a tightly controlled bond line. Important applications include thread-locking, sealing, retaining, and some structural bonding [111]. A representative model formulation has recently been described [112] (Fig. 3). 

Sample preparation of anaerobic adhesives for metal content is an important step, be it by destructive or non-destructive methods. Inactive metal salts are added directly to anaerobic formulations as fillers or for thixotropic reasons. Generally, active transition metals are not added directly to anaerobic adhesives but are prepared as activators in aerosol solvents to be applied to inactive surfaces as part B of an adhesive formulation. In the majority of cases trace metal analysis of anaerobic adhesives is only required for batches with problematic stability and is best done using destructive methods. 

The use of monomethacrylates in anaerobic formulations was disclosed in a patent assigned to Loctite. Specifically mentioned were hydroxyethyl (XIX), hydroxypropyl (XX), cyclohexyl (XXI), tetrahydrofurfuryl (XXII), dimethylaminoethyl (XXIII), and glycidyl methacrylates (XXIV), and cyanoethyl acrylate [26]. Methacrylate esters containing residual carboxylic add groups were prepared by the reaction of hydroxyethyl methacrylate with phthalic anhydride (XXV), pyromellitic dianhydride (XXVI), and benzophenonetetracarboxylic add dianhydride (XXVII). The residual acid provided improved adhesion [27,28]. The reaction product of hydroxyalkylmethacrylates with maleic anhydride (XXVIII) also produced monomers with residual acid as well as additional curable unsaturation [29]. The dimethacrylates of the bisglycol esters of dicarboxylic adds were used to formulate anaerobic adhesives. Among the dicarboxylic acids mentioned were phthalic (XXIX), maleic (XXX), fumaric (XXXI), and malonic (XXXII) [30]. 

Methacrylate esters have been prepared by the reaction of methacrylic acid with epoxies such as the diglycidyl ethers of bisphenol A (XXXVII) [37]. Methacrylate esters suitable for anaerobic adhesives have also been prepared by the reaction of glycidyl methacrylate (XXXVIII) with a hydroxyl-terminated polyester [38]. The reaction of isocyanatoethyl methacrylate (XXXIX) with polyols resulted in monomers that could be formulated into anaerobic adhesives and sealants [39]. 

Storage-stable anaerobic formulations can be prepared with no hydroperoxide if the methacrylate resin is aerated in the presence of an amide and a tertiary amine [42]. Anaerobic adhesives have been formulated with alkyl hydroxyethyl peroxides such as t-butyl-2-hydroxyethyl peroxide (XLVII) [43]. An adhesive formulated with t-butylperoxymaleic acid has improved surface adhesion (XLVIII) [44]. 

The monomers used in anaerobic adhesives and sealants generally contain at least one free-radical stabilizer, such as hydroquinone or />-methoxyphenol. It was found that ben-zoquinone, naphthoquinone, and similar compounds provided improved shelf stability without retarding the anaerobic cure [56]. It was also found that anaerobic formulations could be stabilized with a stable nitroxide free radical such as di-/-butyl nitroxide (LIV) [57]. The use of a soluble metal chelating agent such as tetrasodium EDTA (V) was found to be an effective method of stabilizing an anaerobic formulation against small amounts of metal contamination [58]. 

The wide variety of applications of anaerobic adhesives and sealants is made possible by the modifications that make the viscosity appropriate to the application. An application that requires penetration into close-fitting parts should have very low viscosity, while a produet used with large, loose-fitting parts should have a high viscosity. A styrene aerylate eopolymer could be used to increase the viscosity [59]. Polymethacrylates, eellulose esters, butadiene-styrene eopolymers, acrylonitrile-butadiene-styrene copolymers, poly(vinyl ehloride), copolymers of vinyl chloride and vinyl acetate, poly(vinyl aeetate), eellulose ethers, polyesters, polyurethanes, and other thermoplastic resins have also been used to eontrol the flow eharacteristics of anaerobic sealants [60]. The flow eharaeteristies of anaerobic formulations can also be controlled by the addition of fumed siliea and other solid additives whieh can impart thixotropic properties [61]. 

One of the tests is maintaining strength through a gap. This can be affected by air inhibition, as well as by the diffusion of accelerators. In another test, air was beaten into several adhesive formulations. The aerobic adhesives were only marginally affected by air inclusion when cured between surfaces. Like anaerobics, fillets or adhesive squeezed outside of a bonded joint will remain uncured unless exposed to UV light. 

Selected examples have been cited to demonstrate that areas of acrylic, anaerobic and radiation-curable adhesive developments have begun to consider telechelic polybutadiene/acrylonitrile liquids as useful formulating ingredients either in direct admixture or in the preparation of rubber-modified oligomers serving as an adhesive formulating base. These cited adhesive examples cover structural, semi-structural, gap-filling and pressure-sensitive types. 

One property common to UV-cure.d acrylate formulations as was discussed earlier is relatively poor surface-cure due to oxygen inhibition (see Fig. 1). This effect is most predominant at low photoinitiator concentrations, in thin coatings. Adhesive formulations can benefit from this property by enhancing surface tack via the presence of residual uncured resin. Post-cure, from residual cross-linking, or other anaerobic chemistry, can later result in enhanced bond strength. 

Tertiary aromatic amines hydroperoxides and sulfonimides are important components of many of the common anaerobic adhesive cure systems. While various formulative aspects of these compounds are well understood, a detailed explanation for their dramatic effect on the rate of polymerization of the adhesive has been lacking. Our approach to the problem has been to study the chemistry of the isolated components of this cure system under well defined conditions and to apply the results to understanding the mechanism by which these compounds accelerate the polymerization of anaerobic adhesives. Herein, we report some of the results of our studies of the reactions of N,N-dimethylaniline derivatives, which are typical amines used in anaerobic formulations, with cumene hydroperoxide (CHP). Connections will be made between the chemistry of the isolated systems and that which occurs in anaerobic formulations, both during storage and cure. 

Radiation curable adhesives can be based on raw materials that are not greatly different from those used in acrylic adhesives (first, second, and third generations), epoxy adhesives, urethane adhesives, and anaerobic adhesives. The formulation approach, however, is somewhat different. Very important is that no solvents are used to thin the resins a monomer reactive diluent is used instead. The monomer must be matched with the resin to give the desired set of properties with respect to adhesion, substrate, flexibility or stiffness, cure behavior, and durability of the cured product. Most of the radiation curable adhesives have an acrylate (or methacrylate) basis many of the acrylics are modified. 

Anaerobic adhesives can be made from tetraethylene glycol-IEM adducts. Hoffman (31) formulated this adduct with cumene hydroperoxide and dimethylaniline and applied it to cap screws and nuts. Upon full cure a torque of 3.5 ft-Ibs was needed to remove the nut. Polyamines were also reacted with lEM by Frisch Jr. (32,33) without the use of catalysts to give similar anaerobic adhesive resins. 

Properties of cured anaerobic adhesives are related to the formulation chosen for a given application area. For example, anaerobic threadlocking formulations cure to very hard materials for studlocking applications and to relatively soft solids for locking precision screws. Usually, cured anaerobic prodncts are highly cross linked and form strongly adhesive, but somewhat, brittle solids. They are resistant to water and solvents and perform well under extremes of temperatnre (—50°C to -I-150 °C). 

Owing to their curing properties, anaerobic adhesives remain uncured outside the joint assembly this allows removal with solvent Some formulations now include sensitizers, which allow drying of the excess with UV light. 

Nevertheless, experience shows that, for the vast majority of like-to-like, co-axial assemblies with diametric clearances around 0.05 mm, temperatures between -55°C and +80°C will be readily accommodated. Adhesives with a higher Tg will cope up to 120°C without major strength loss, and those designed for elevated temperatures perform, on collar and pin assemblies, up to 200°C. This is seen clearly in Figure 2.19, which shows the performance of a highly cross-linked anaerobic adhesive specifically formulated for maximum performance on collar and pin assemblies at elevated temperatures. It cannot be emphasised too strongly that the data given relate only to collar and pin assemblies and must not be applied to lap joints. 

Anaerobic adhesives, based on the acrylic polyester resins, are produced in viscosities ranging from thin liquids to viscous, thixotropic pastes. Within each viscosity band, individual formulations are available possessing specific strength characteristics. The whole family is unique in being the only one where interrelated strength/viscosity characteristics are provided by manufacturers. 

Because of this family s versatility, it is probably more useful to summarise its limitations. They can be brittle, cure rate varies enormously with formulation and their viscosity can make use difficult on very small assemblies. In general, their naturally very high strength may not be modified (reduced) readily. This, coupled with their viscosity problem, has prevented their use in the assembly of mechanisms - especially when dismantling is required - and explains the ubiquitous use of anaerobic adhesives here. 

Silicone methacrylates have been formulated into anaerobic adhesives by Dow Coming [156], Toshiba Silicones [157], and Loctite [158], [159]. 

Early anaerobic adhesives suffered from unstable curing, mainly as a result of the problems of maintaining the oxygen-level in the liquid adhesive prior to its use. This was subsequently solved by modification of formulations. Current versions no longer suffer from this problem, or the associated need for highly-controlled storage. 

Further improvements in the reactivity of anaerobic adhesives were obtained with the introduction of hydrazide accelerators. However, incorporation of more active cure systems was made possible only by concurrent advances in stabilization chemistry. Early formulations employed quinone polymerization inhibitors which were effective at levels of 10-1000 parts per million. As more active formulations were developed, attempts to combat premature polymerization by addition of greater quantities of inhibitor served only to reduce performance. A significant breakthrough occurred when a method of removing trace amounts of contaminant metals from anaerobic formulations was... 

In situations where the rate of assembly demands extremely rapid cure times, or where the surfaces to be bonded are inherently unreac-tive, treatment of substrates with a primer is often necessary. Primers consist of compounds which accelerate the curing reactions. Since they would destabilize the adhesive if added directly to the formulation, they are supplied and used as a separate component. The criteria for an acceptable primer include compatibility with the adhesive, the ability to accelerate the rate of curing, and lack of any adverse effects on bond strengths. Various thiazoles, butyral-dehyde-aniline adducts and thioureas " were found to meet these criteria. Since trace levels of transition metals accelerate anaerobic adhesive cure, primers containing complexed copper have been employed successfully. In another system, acidic primers are used which react with ferrocene in the adhesive to release the required metal ions. ... 

Tables 1-4 list selected commercially available anaerobic adhesive products, illustrating the variety of uncured and cured physical properties which results when formulations are tailored to meet specific application needs. Many products are color-coded for easy recognition and on-part identification.

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