Anaerobic adhesives cure mechanism

Anaerobic adhesives and sealants have been developed primarily in industrial laboratories, and most of the published literature are patents. A number of papers have been published within the last two decades which discuss the reaction mechanisms of anaerobic adhesive cure [10-20]. 

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. 

A number of papers discuss the reaction mechanisms of anaerobic adhesive cure [138]-[142]. The polymerization mechanism of anaerobic adhesives is similar to that of other radical initiation systems except for the special way in which the inhibiting effect of oxygen is used to delay the polymerization and in the chemical activation which occurs at the metal surface. 

Curing acrylic adhesives are distinctly different from anaerobics, cyanoacrylates, and acrylic solution adhesives and emulsions. These related chemistries use different formulating materials, cure via different curing mechanisms, and often possess minimal high performance properties over long periods of time, or when exposed to aggressive environments. 

Fig. 13. Curing Mechanism of Anaerobic Adhesives (Ref W. A. Lees, Brit. Polymer J., 11, 64 (1979).)...

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. 

A new type of nonvolatile reactive acrylic adhesive, bridging the gap between anaerobic acrylic and volatile reactive acrylic adhesives, has recently been developed. The term "aerobic" acrylic adhesives has been coined solely to identify and to set apart these adhesives from other acrylics it does not necessarily describe their cure mechanism. Their term "aerobic" will be used to refer to a diminished sensitivity to air inhibition of thick layer curing properties and the ability to cure between two surfaces regardless of the presence or the absence of air. 

Aerobic acrylics will not cure by themselves due to the absence of air. They require the use of pre-applied activators to initiate the cure mechanism. This property is distinct from anaerobic adhesives which are intrinsically single-component products. Even when accelerators or primers are used to increase their cure rates to meet the demands of rapid assembly techniques, anaerobic adhesives require the absence of air as a necessary condition of cure (1). ... 

Dual curing adhesives offer more than one curing mechanism. They are designed for applications with shadow areas which are not accessible to the UV hght. Full cure in shadow areas will be achieved by anaerobic cure in the absence of oxygen with metal contact, or by adding heat. 

Increasing use of UV-curable acrylates is found in the assembly of medical, automotive, electrical, electronic and optical devices. Dual cure mechanism (UV and anaerobic) adhesives are often used for the assembly of ammunition and automotive air bag inflators. 

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. 

The hybrid joints prepared with the FT-EP, AC and PU adhesives presented an increase in the adhesion strength with respect to the unbonded samples. The use of the FT-EP adhesive provided values of maximum decoupling load fom times higher than those of the interference alone. In the case of the PU adhesive, the interference decoupling load was doubled in the hybrid joints. The AC adhesive presented the highest resistance improvement in the presence of the interference-fit compared to its performances in clearance condition because of its anaerobic curing mechanism (Gallio et al., 2013). 

Anaerobic adhesives derive their name from the characteristic of requiring a relatively oxygen-free environment for proper cure, such as found in closely mating assemblies. Oxygen inhibits free radical polymerization by the mechanism shown in Eq. (1). The active radical R reacts with molecular oxygen to form an inactive hydroperoxy radical before initiation or chain propogation can occur. Anaerobics can be used as one-part systems, relying on reactions with the active metal surface of the substrate to provide the redox initiation. 

Accelerators are additional compounds that serve to speed-up the curing process. Many such compounds have been developed over the years, but the most widely used are aromatic amines, orthobenzoic sulfimide (saccharin) and acetyl phenyl hydrazine. Sineokov and coworkers reviewed the initiation mechanism of the curing of anaerobic adhesives [9]. 

Dual cure. A combination of UV curing and another curing mechanism (anaerobic, thermal, moisture activated) is able to cure the adhesive film at points which are not accessible to the UV radiation ( shadow curing ). The UV absorption normally takes place in the wavelength region between 320 and 450 nm. 

In recent years, the range of adhesive materials used in automotive manufacture has expanded to include polyurethanes, plastisols, phenolics, hot melts, anaerobics, cyanoacrylates, toughened acrylics and epoxies (see Structural and Hot melt adhesives). Selection criteria are based principally upon the nature of the adherends, the mechanical properties required under service conditions and application and curing characteristics. 

Essentially limited as a class to co-axial mechanical assembly, retention and sealing, they also make good general purpose gasketting media. The cure rate depends upon surface activity and may require a supplementary catalyst. The family copes with the gaps of normal engineering practice. As clearances increase, the anaerobics capacity to cope well falls rapidly. The majority of materials in the family are only suitable for use in lap joints as gasketting media or to seal a gap. Only the special anaerobic materials can be considered to be true adhesives and suitable for use on unsupported lap joints. 

The curing process depends on the radical polymerisation of the acrylic vinyl group giving total liquid-to-solid conversion. In anaerobic variants, this may be by the same mechanism described in Section 5.1.2 but all respond to a catalytic primer - either a surface initiator or mixed directly into the adhesive. 

These adhesives are quickly set by a UV cure and are more fully cured by a second mechanism involving the introduction of heat or moisture or the elimination of oxygen (anaerobics). In EB-curable adhesive, the depth of EB penetration is limited by the density of the material, rather than its opacity. 

Pinch valves are used for low-viscosity adhesives which are processed in the pressure range up to 10 har (O Fig. 38.9). The tube cross section is pinched in such a way by a pneumatically driven lever that the flow of adhesive is stopped. The advantage is that the actual valve mechanism does not come into contact with the adhesive. This type of valve is the preferred for processing anaerobically curing adhesives and cyanoacrylates. 

Many small-volume users dispense adhesive directly from the manufacturer s bottles and tubes, which are specifically designed for this purpose, but when mechanical application is required the anaerobic nature of the adhesive demands equipment which will not cause spontaneous curing within the apparatus. This must be obtained from the adhesive manufacturer. However, it may be coupled with a variety of supplementary mechanisms ranging from simple guidance systems to fully automated, self-checking dispensers. The ingenious methods often used to apply these adhesives have led to the development of some of the sophisticated equipment to be found in the adhesives field. 

Epoxy adhesives such as Huntman s Araldite AW 134 with HY 994 hardener (cured for 15 min at 120°C) and Araldite AV 1566 GB (cured for 1 h at 230°C) give the best results with this engineering resin. Other adhesives that can be used are cyanoacrylate (Loctite 414 with AC primer), anaerobics (Loctite 638 with N primer), and silicone sealant (Loctite Superflex). The highest lap-shear strength was obtained with Araldite AW 134. This adhesive has balanced properties, good resistance to mechanical shock, thermal resistance to 100°C, and reasonable stability in the presence of aliphatic and aromatic solvents. Some solvents, particularly chlorinated hydrocarbons, will cause deterioration of the bond [30]. 

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