Surface anaerobic adhesives

The reluctance of acrylic monomers to polymerise in the presence of air has been made a virtue with the anaerobic acrylic adhesives. These are usually dimethacrylates such as tetramethylene glycol dimethacrylate. The monomers are supplied with a curing system comprising a peroxide and an amine as part of a one-part pack. When the adhesive is placed between mild steel surfaces air is excluded, which prevents air inhibition, and the iron present acts as a polymerisation promoter. The effectiveness as a promoter varies from one metal to another and it may be necessary to use a primer such as cobalt naphthenate. The anaerobic adhesives have been widely used for sealing nuts and bolts and for a variety of engineering purposes. Small tube containers are also available for domestic use. 

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). 

Fillers, stabilisers and colorants can be inorganic or organic, and are added as part of identification, stability and durability, they must be non-leaching and meet health requirements. Some plastics products are required to keep moisture and air out of sensitive products and these are usually high-density plastics with low permeation properties. Some products require plastics with high permeability to allow air (oxygen) as part of product stability, e.g. anaerobic adhesives. Healthcare products require that some containers do not allow air or moisture to permeate the surfaces (bacteria or other micro-organisms) and in some extreme cases may require that the inside be coated with an inner layer of approved lacquer or other very low permeable material. 

Anaerobic adhesives require confinement on an active metal surface. Substrates such as plastic, cadmium and zinc are slow but can be improved with the use of accelerators or primers. 

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 contact with metal surfaces that, in the case of anaerobic adhesives (Section 4.3.4), enables the reaction to an adhesive layer if, at the same time, the liquid adhesive (isolated by the adherends) is no longer in contact with the oxygen of the air. 

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. 

Anaerobic adhesives for surface sealings and thread-locking devices, furthermore,... 

Activators Chemical compounds able to trigger chemical reactions not possible without such compounds (e.g., activators as means of pretreatment of hard-to-bond plastic surfaces with anaerobic adhesives). In contrast to catalysts, activators participate directly in chemical reactions. 

Glass plates varying in the surface roughness were used as model substrates in our experiments. The roughness was smoothed out by replication of supersmooth etalon surfaces of polished Si wafers. For replicate layers we used thin films of acryl anaerobic adhesives and photopolymer compounds. 

It has been suggested that one of the functions of the saccharin in anaerobic cure is to dissolve metal ions from the surfaces in order to catalyze the decomposition of CHP. X-ray photoelectron spectroscopy of a model anaerobic adhesive cured in contact with a metal surface indicates that trace amounts of metal or metal ions are found at the interface [19]. 

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]. 

Anaerobic adhesives Absence of oxygen and metal contact Gaskets Flat surface bonding Adhesive bonding of electric and electronical components Fastener locking Shaft/hub bonding... 

Anaerobic adhesives cure as single-component products only on certain "active" metal surfaces when ions from the surface act as accelerators for the rate of adhesive cure. Single-component, anaerobic threadlocking adhesives are examples, deriving their speed of cure from both the anaerobic catalysts and the accelerating effect of transition metal ions derived from copper, iron, or steel surfaces. 

For most other surfaces, however, accelerators of various kinds similar to the activators of two-component acrylics are often required to achieve cure rates of practical lengths. Structural anaerobic adhesives cured without the use of pre-applied accelerators or primers frequently cure too slowly for rapid assembly techniques (3). ... 

Atomic spectroscopy (including atomic absorption spectrometry, atomic emission spectrometry, and atomic fluorescence spectrometry) is of use across the span of reactive adhesive technologies. For example, the cure of anaerobic adhesives on non-reactive surfaces is usually assisted by the use of an active metal-based primer. Similarly, the cross-linking of silicone adhesives is promoted by the use of organometallic salts of cobalt, tin, iron, lead, and platinum. In the case of polyurethane adhesives, the key condensation reactions are catalyzed by tin salts (e.g., dibutyl tin dilaurate and stannous octoate). 

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. 

Two of the most difficult types of adhesives to dispense are Cyanoacrylate adhesives and Anaerobic adhesives. Cyanoacrylates cure by interaction with the surface moisture on a component part. Anaerobics remain liquid in the presence of oxygen but automatically cure once enclosed in a joint. 

Adhesives may also be classified by the way they are applied or cured. Hence, anaerobic adhesives are adhesives that set only in the absence of air, for instance, when confined between plates or sheets. A contact adhesive is one that is apparently dry to the touch but will adhere to itself instantaneously on contact (also called contact bond adhesive and dry bond adhesive). A heat-activated adhesive is a dry adhesive film that is made tacky or fluid by application of heat or heat and pressure to the assembly. A pressure-sensitive adhesive is a viscoelastic material that in solvent-free form remains permanently tacky. Such materials will adhere instantaneously to most solid surfaces with the application of very slight pressure. Room-temperature setting adhesives are those that set in the temperature range of 20-30°C. These are usually two-component adhesives that must be mixed before application. A solvent adhesive is an adhesive that has a volatile organic liquid as a vehicle and sets or becomes tacky after the solvent has evaporated. A solvent-activated adhesive is a dry adhesive film that is rendered tacky just before use by applicafion of a solvent. 

Figure 2.17 Idealised relationship between surface finish and shear strength for a high-strength anaerobic adhesive.

Figure 2.18 Effect of surface finish and gap size on the shear strength of an anaerobic adhesive. Adhesive DTD 5633/G2 tested to DTD 5628 using given finish and gap.

Anaerobic adhesives of appropriate viscosity are often used to augment and standardise the performance of shrink and interference-fitted components, where they can reduce hoop stress in the outer component and reduce its tendency to crack or display fretting corrosion. These benefits stem from the ability of anaerobics to fill the voids that are present in even the best-prepared surfaces. This spreads loads evenly over the whole of the faying surfaces instead of concentrating them on the engaged peaks, which represent only a small proportion of the theoretical contact area. 

If it is necessary to maximise performance, the engaged area should be as large as allowed by the design and the plastic s surface should be chemically treated or roughened if at all possible. Finally, a high-performance adhesive of low modulus - preferably toughened - should be used. The selection procedure of Section 5.2 helps locate a suitable material. Note in the presence of a steel part, most high quality anaerobic adhesives will set satisfactorily - even if one surface is non-metallic - but two non-metallic surfaces will invariably require a primer if an anaerobic adhesive is to cure. 

The accommodation of surface contamination shown by some adhesives depends upon two distinct factors. First, as the anaerobic adhesives rely almost exclusively on their jamming action, any further reduction in their levels of true adhesion is unimportant provided that the contamination is not so gross that the hardened film of adhesive slides on the oil film. Situations as bad as this are rare and, when likely, must be either prevented or countered by cleaning. 

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. 

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