Sealing applications, anaerobic

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). 

Anaerobic adhesives mainly serve the purpose of protecting threads from undesired loosening as a consequence of vibrations or dynamic stress and represent a preferred alternative to mechanical locking devices. Furthermore, they enable shaft-hub connections, for example, when mounting a gear wheel on a shaft (Figure 11.6). They have also delivered optimal performance as liquidly applied surface seals alternatively to the solid seals required in the respective dimensions. Figure 4.6 shows these application possibilities. 

These anaerobic adhesives fill all surface irregularities and tolerance gaps and effectively seal clearances up to 0.76 mm. They can be applied by high-speed applications in moving production lines. The cured film has excellent chemical resistance to most liquids and gases within an operating temperature range of -54°C to 232°C. 

Anaerobic adhesives are not generally stringy in nature and contain no solid catalysts, so they are suited for application on many relatively small parts -particularly co-axial assemblies where they can be placed with ease. Their characteristics ensure that they readily fill the minute gaps always found in interference-fitted joints. Thus, they can be used both to seal and supplement the overall performance of such joints. 

Appropriate forms of anaerobic composition make very successful gasket replacements - particularly because they are virtually incompressible. Furthermore, the same material may be used to seal several different assemblies unlike the conventional gasketting techniques which are very application specific. 

Anaerobic adhesives are frequently used as sealants in various applications. Of these, perhaps the most useful illustrations are the sealing of pipes, whether unions are intended to be permanent or dismountable for maintenance. 

Joint sealing and retention of co-axial components - often both in the same application - are prime examples. Components can be threaded (screws or pipes), or splined or smooth (eg bearings). Special versions of these adhesives (often not truly anaerobic - in that primers may have to be used) give much higher levels of effective adhesion than the normal materials and so may be used in lap joints. Other versions are formulated as gasketting media. 

Traditional uses for anaerobic adhesives, such as threadlocking, sealing, and bonding of bearings and shafts, have been sufficiently reviewed, and will not be discussed here. Table VI summarizes some recent bonding applications. 

The first successful application of anaerobic adhesives was to replace lock washers in threaded fastener assemblies (e.g., nuts and bolts), where they fulfil the dual function of locking and sealing the assembly. Frequently referred to as liquid lock washers, anaerobics adhere strongly to the metal surfaces and prevent loosening from vibration. They also completely fill the void space in a threaded assembly and protect it against corrosion. This remains the most... 

Other important applications for anaerobics include the sealing of threaded pipe joints, gasketing, structural adhesives and the bonding of slip fitted cylindrical parts (often called retaining compounds). 

Numerous applications exist for anaerobic threadlockers and sealants in engines, transmissions and axles to create vibration-proof fasteners and to seal fluid joints. 

Impregnation sealants are very low viscosity liquids which, when simply painted onto a surface or applied under vacuum as described earlier, migrate into pores, cracks, and surface imperfections in castings, welds, or powdered metal parts. Anaerobic sealants have replaced older types of sealant because of the relative ease and cleanliness of their application. Cure occurs only where desired and excess resin is easily removed. Castings sealed with the resins exhibit improved mechanical properties. One of the most impressive benefits of the impregnation process is the improvement in machin-ability which results from the smoothing of surfaces due to the filling of voids and crevices. 

The great benefit of anaerobic compounds, when they are used as gasketing media, is that they allow metal/metal contact between the surfaces being sealed, which means that dimensional stability is immediately obtained and retained. This is particularly important in the application illustrated in Fig. 7.28 where correct alignment is clearly important. 

The use of anaerobic adhesives has become increasingly popular in appliance assembly applications, as they provide a number of manufacturing benefits. Typically used to augment the seal or holding force of a mechanically joined appliance assembly, anaerobic adhesives serve as threadlockers, thread sealants, retaining materials, and flange sealants [19]. 

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