Curing anaerobic adhesives

Humphreys, of the Loctite Corporation, reported on the chemistry of accelerators for curing anaerobic adhesives. He showed that the reaction between N,N-dimethylaniline derivatives and cumene hydroperoxide is relatively slow even at lOO C. He concluded that the accelerated polymerization of anaerobic adhesives of ambient temperatures caused by cure systems containing combinations of tertiary aromatic amines, hydroperoxides, and sulfonimides does not result from the hydroperoxide-amine reaction. 

Chemistry of Accelerators for Curing Anaerobic Adhesives-Reaction of N, N-Dimethylaniline Derivatives with Cumene Hydroperoxide... 

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. 

Properties of cured anaerobic adhesives are related to the formulation chosen for a given application area. For example, anaerobic threadlocking formulations cure to very hard materials for studlocking applications and to relatively soft solids for locking precision screws. Usually, cured anaerobic prodncts are highly cross linked and form strongly adhesive, but somewhat, brittle solids. They are resistant to water and solvents and perform well under extremes of temperatnre (—50°C to -I-150 °C). 

Anaerobic adhesives are generally acrylate monomer-based adhesives. They are essentially monomeric, thin liquids that polymerize to form a tough plastic bond when confined between closely fitting metal joints. They generally require a metal substrate because the metal ions catalyze the reaction process. However, catalyzed primers are available for curing anaerobic adhesives on nonmetallic substrates. 

The application areas into which anaerobic adhesives have made their most recent inroads are those in which the adhesive is used to assemble flat, structurally bonded parts. Impetus for the replacement of more conventional structural adhesives by anaerobic products can be attributed to the ease of use, lack of mixing or pot-life problems, reduced toxicity, and rapid cure associated with the anaerobic materials. The incorporation of novel synthetic resins has extended the range of properties available in cured anaerobic adhesives, leading to improvements in tensile strength, impact strength, and peel strength. Primers or heat can be used to accelerate cure when necessary. 

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). 

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. 

The reaction rate of anaerobic adhesives can be increased by heating, making it possible to adapt the adhesion process to other production processes. At temperatures under 10°C the curing reaction (nearly) stops. 

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. 

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]. 

An important factor in the initiation of anaerobic adhesive cure is the redox reaction between a hydroperoxide and transition metals with adjacent oxidation states [10]. 

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]. 

The monomers used in anaerobic adhesives and sealants generally contain at least one free-radical stabilizer, such as hydroquinone or />-methoxyphenol. It was found that ben-zoquinone, naphthoquinone, and similar compounds provided improved shelf stability without retarding the anaerobic cure [56]. It was also found that anaerobic formulations could be stabilized with a stable nitroxide free radical such as di-/-butyl nitroxide (LIV) [57]. The use of a soluble metal chelating agent such as tetrasodium EDTA (V) was found to be an effective method of stabilizing an anaerobic formulation against small amounts of metal contamination [58]. 

Solid fillers are added to some anaerobic adhesives and sealants for various purposes. Mica, talc, and other mineral fillers can help to provide an instant seal capability to anaerobic pipe sealants. The sensitivity of the anaerobic cure system to metal contamination requires that these fillers be chosen very carefully. 

In Germany, standards have been published describing the Compression Shear Test (DIN 54452), Dynamic Viscosity Determination of Anaerobic Adhesives by Rotational Viscometer (DIN 54453), Initial Breakaway Test at Bonded Threads (DIN 54454), and Torsion Shear Test (DIN 54455). DIN 54455 is particularly interesting since it is one of a very few tests in which a nut and bolt (MIO) are seated to a measured torque before the anaerobic sealant is allowed to cure. 

Uses Urethane-acrylic for radiation and peroxide curing uses incl. anaerobic adhesives, optical/paper/plastic/PVC floor/wood coatings, and inks Features Chem, water, and heat resist. nonyellowing Properties APHA25 clear liq. dens. 9.262 Ib/gal vise. 590 cps... 

Specificallly for ultra-violet light-cured glass-metal joints Anaerobic adhesives on threaded fasteners... 

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