Specifications anaerobic adhesives

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]. 

Where specification of the metal is considered to be important, polarographic analysis may be utilized. In the case of primers based on oil-soluble iron salts for use with anaerobic adhesives, the Fe /Fe ratio may be determined by differential pulse polarography in a supporting electrolyte based on ammonium pyrophosphate buffer adjusted to pH 9.0. The iron(II) form is known to be significantly more effective than the iron(III) form in the redox-based curing process. 

Figure 2.2 Fatigue performance of a typical toughened anaerobic adhesive. This S/N curve (load v cumulative number of loading cycles to induce joint failure) for a toughened anaerobic adhesive shows that it will not fail through fatigue if the load applied is less than 50 per cent of the ultimate strength in shear of the adhesive - in this specific joint configuration.

Anaerobic adhesives offer an additional advantage in being available in a specific range of strengths based on the requirements of the MoD s DTD 5629-33 inclusive. So bonded assemblies may be made suitable for disassembly using conventional workshop equipment, or made permanent. 

Figure 2.19 Effect of temperature on the shear strength of conventional and highly cross-linked anaerobic adhesives. The poorer performance of the conventional version as measured on a standard collar and pin assembly (MoD specification DTD 5628) is quite marked.

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 were created specifically for assembling a wide variety of mechanical engineering components. The impact their properties and use have on fundamental design concepts is discussed in relation to other types of adhesives in Chapter 2. What this approach does not demonstrate is the overall value of these unique materials to the mechanical engineer in a wide range of situations, nor does it show the degree to which the characteristics of individual grades are matched to the intended end use. 

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. 

Testing of Anaerobic Adhesives. Testing standards and performance specifications for anaerobic adhesives and sealants have been established by government agencies and industrial organizations in several countries. In the United States there are military specifications for thread lockers, sealants, and retaining compounds. Many of these specifications are being superseded by commercial standards. 

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.

Anaerobic adhesives are typically applied using bottles, cartridges etc., made for specific applications. Since air exclusion is imperative for premature polymerization, metering and... 

As with the anaerobic adhesives, moisture-curing adhesives, such as cyanoacrylates, are typically applied using bottles, cartridges etc., made for specific applications. Since moisture exclusion is imperative for premature polymerization, metering and dispensing equipment is designed specially to prevent contact of moisture-curing adhesives with air, prior to use. 

Until relatively recently the most widely used specifications for anaerobic adhesives were the American MIL specifications which have been criticised because they were of little relevance to quality control. The Ministry of Defence has therefore issued specifications DTD5628-5633 which cover test procedures and performance requirements for sixteen materials. Five strength bands and a wide range of viscosity levels are defined. 

Gas and water institutes in a number of countries have issued specifications which are widely recognised within their respective industries. Where this has happened the use of, and respect for, anaerobic adhesives and sealants has been considerably enhanced the Veg-Gasinstituut of Apeldoom, in the Netherlands, is a good example of this. By comparison the UK lagged behind until the MoD specifications were issued. 

It is now common for anaerobic adhesives to be formulated to meet the specific needs of both the application and the equipment required to apply it — a process which has now resulted in several new and significant product developments. 

Finally, care must be taken, either in the choice of adhesive or in the assembly technique, to ensure that the adhesive is not wiped from the surface as the parts are placed together. This is likely where, for example, one part is slid into position over another the sliding action tends to wipe the adhesive from the joint area, forming pockets and reducing overall performance. In this context, co-axial assemblies must of course be slid together because of the very nature of the joint. This can cause severe problems for some adhesives but presents no difficulties for the anaerobic varieties, which are formulated specifically to cope with this type of work. 

The paper deals with the response of a high strength, single eomponent adhesive that cures in absence of oxygen (anaerobic). Its commercial name is LOCT1TE648 , a specifically developed product for cylindrical joints with or without interference. The typical product performances are reported in [16]. 

In industries that are bonding plasticized materials, there are some specific tests to measure for instance peel strength before and after accelerated aging test. The A and S suppliers, who know the raw materials included in their formulations, can tell quickly whether a given adhesive should be sensitive or not to oils, greases and plasticizers. Some adhesives are sensitive to oils (vinyl emulsion glues), some are not (epoxies, anaerobics, etc.). 

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. 

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