Anaerobic adhesive

Anaerobic adhesives find their origins in the 1950s and early 1960s. Research by many companies over the past 20 years has resulted in hundreds of patents and publications on these adhesives and their uses. Reviews covering the early literature, physical properties, and applications are numerous. 

The adhesives are based mainly on multifunctional methacrylate monomers such as dimethacrylates and trimethacrylates. The term anaerobic was originally used to signify that the adhesives would cure spontaneously if air was excluded, and the adhesives are frequently referred to as simply anaerobics . The adhesives are free-radical redox systems in which a hydroperoxide initiator reacts with transition metals on the substrates, typically iron or copper, to generate free radicals, for example  

The free radicals RO and ROO then polymerise the monomers. However, as with most free-radical systems, atmospheric oxygen is a strong retarder and thus the systems polymerise rapidly in an assembled joint, but will not cure if placed on a surface in the 

Anaerobics typically comprise Monomers Initiators Accelerators Plasticisers Stabilisers Thickeners Fillers 

Although the range of useable additives is quite large, all additives have to be chosen and tested carefully. Anaerobics can be destabilised by certain types of impurities, for example, traces of transition metals (e.g., iron or copper), peroxides and amines. 

Anaerobics are usually formulated with di- or trifunctional methacrylate monomers that can be polymerised rapidly to form a tightly crosslinked thermoset polymer. Typical examples are triethylene glycol dimethacrylate, and ethoxylated bisphenol A dimethacrylate. Other monomers are used to modify the properties. Examples are hydroxyethyl or hydroxypropyl methacrylate and acrylic or methacrylic acid these help to adjust viscosity, cure speeds and adhesive strength as seen in Equations 2.3-2.S  

Common variations on this basic cure mechanism are  

Colour Break Torque Prevail Torque LSS SG Vise. (Pas) Service ro Flash Point (°C) Gap Filling mm max) Cure Pg- No. 

Canning Hylomar Grade 520 (Feb 95) Wm. Canning Hylomar Grade 550 Screw Lock (Feb 95)  

Manufecturers Comments High strength. Manufacturers Comments Low strength.  

Typical Use Fixing and locking of components. Typical Use Thread locking of items that require dtemantling.  

Depending on their chemical basis and the resulting thermosetting crosslinkage ratio, the adhesive layers are thermally resistant and thus perfectly suitable for transmission and engine manufacturing. If repair is required, the bonded joints can be removed by heating them to approximately 120-150 °C. 

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Small amounts of TAIC together with DAP have been used to cure unsaturated polyesters in glass-reinforced thermo sets (131). It has been used with polyfunctional methacrylate esters in anaerobic adhesives (132). TAIC and vinyl acetate are copolymerized in aqueous suspension, and vinyl alcohol copolymer gels are made from the products (133). Electron cure of poly(ethylene terephthalate) moldings containing TAIC improves heat resistance and transparency (134). 

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). 

Anaerobic adhesive chemistry has been reviewed [113], so only a few recent developments will be covered here. Many of the components of anaerobic adhesives. 

Organic peroxide-aromatic tertiary amine system is a well-known organic redox system 1]. The typical examples are benzoyl peroxide(BPO)-N,N-dimethylani-line(DMA) and BPO-DMT(N,N-dimethyl-p-toluidine) systems. The binary initiation system has been used in vinyl polymerization in dental acrylic resins and composite resins [2] and in bone cement [3]. Many papers have reported the initiation reaction of these systems for several decades, but the initiation mechanism is still not unified and in controversy [4,5]. Another kind of organic redox system consists of organic hydroperoxide and an aromatic tertiary amine system such as cumene hydroperoxide(CHP)-DMT is used in anaerobic adhesives [6]. Much less attention has been paid to this redox system and its initiation mechanism. A water-soluble peroxide such as persulfate and amine systems have been used in industrial aqueous solution and emulsion polymerization [7-10], yet the initiation mechanism has not been proposed in detail until recently [5]. In order to clarify the structural effect of peroxides and amines including functional monomers containing an amino group, a polymerizable amine, on the redox-initiated polymerization of vinyl monomers and its initiation mechanism, a series of studies have been carried out in our laboratory. 

ISO 10123 1990 Adhesives - Determination of shear strength of anaerobic adhesives using pin-and-collar specimens... 

Anaerobic adhesives Anaerobic adhesives consist of mixtures of dimethacrylates and hydroperoxides (initiators) that polymerize in the absence of oxygen. They are used for anchoring bolts. 

Because the vast majority of the monomers react at both ends, these adhesives are thermosets and brittle. Thus, such adhesives are not suitable for areas requiring flexibility. Because they are thermosets, they are typically resistant to oils, water, and solvents. Thus, the adhesive plays two roles bonding and protection. These properties make anaerobic adhesives suitable for automotive and truck motor mounts and electric engines. 

Different mixtures of anaerobic adhesives are available that offer a variety of curing times, strengths, and compatibilities. A typical cure time is 20 min for cure to begin, vibration resistance in about 1 h. Some fast cure systems will have about 20% cure in a few minutes and are almost cured in about 2 h. Some instant cure systems are ready for use within seconds. 

Polyester resins and anaerobic adhesives and sealants have also exhibited high radiation resistance. Anaerobic adhesives have several years of long-term exposure in radiation environments due to their use as thread locking sealants in nuclear reactors and accessory equipment. 

Thread locking grades of anaerobic adhesives have sustained 2 x 107 rads without molecular change or loss of locking torque.77... 

Brennan, M.C. and Svehla, G. (1992) Novel Electroanalytical and Atomic Spectrometric Techniques in the Characterisation of Anaerobic Adhesives, Cork University of Cork, pp247-340. 

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 contain reactive monomer(s), accelerators, stabilisers or inhibitors, fillers, colorants, modifiers, tougheners (rubbers, butadiene graft, etc.). The chemistry of anaerobic adhesive is complex but the schematic brief of the reaction may be shown as follows ... 

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. 

Anaerobic adhesives are designed to be metal-sensitive, especially with respect to the mono- or divalent transition metals, e.g. Cu, Fe, Co, Ni, Mn, etc. these elements function by homolysing the hydroperoxide compound into free radicals which initiate polymerisation. It is important to be able to measure trace concentrations (ppb levels) of these metals as such levels may cause instability in these products. Contamination can be caused by raw materials or metal from the plant components used in the manufacturing process. 

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. 

To study methods of sample preparation for metal analysis of anaerobic adhesives the following laboratory-prepared adhesives containing the usual additives and spiked with and without metals were used for comparison ... 

Type A. Anaerobic adhesive containing monomers, stabilisers, initiators, promoters,... 

Type B. Anaerobic adhesive containing Type A with thixotropic agents, inorganic fillers and rubbers. 

Dry ashing is a means of combusting the organic components in anaerobic adhesives and at the time retaining the metals for quantification by ICP-OES. 

Ashing Method of Type A and Type B Anaerobic Adhesives... 

Figure 6.3 Ramping and hold stages for ashing anaerobic adhesives...

Results. Microwave acid digestion is carried out as a means of destroying the organic components in the anaerobic adhesive product and retaining the metals of interest in the... 

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