16 сентября, 2015 
Pokraskin Polymerization adhesives harden through radical or ionic polymerization of the monomers; at the same time, graft polymerization or cross-linking of the dissolved, still polymerizable polymers also may occur.
Two-Component Polymerization Adhesives. The best known types are polymerization adhesives based on solutions of unsaturated polyesters in styrene or (meth)acry — lates. Peroxides are added as hardeners to the resin component; amines or heavy-metal salts are used as accelerators.
More important and with better properties are the cold-setting, two-component acrylate adhesives, which contain methacrylates or acrylates, sometimes mixed with styrene and methacrylic acid as monomer and, in addition, various polymers. The polymers used are primarily synthetic rubbers, such as polychloroprene, styrene-butadiene rubber, butyl rubber, polystyrene, polymethacrylates, and acrylate graft polymers of these polymers. Amines are used as accelerators, and benzoyl peroxide in the form of plasticizer pastes or a powder mixture with fillers is preferred as hardener [43].
Trade Names. In Germany: Agomet, Pattex Stabilit Express; in Japan: Hard-Lock, Sumikitt; in the USA: Versilok.
One-Component Polymerization Adhesives. Cyanoacrylate Adhesives. Methyl, ethyl, butyl, and methoxyethyl esters of cyanoacrylic acid are used for cyanoacrylate adhesives [44]; soluble polymers and plasticizers are incorporated to regulate viscosity and for elastification. Cyanoacrylate adhesives rapidly polymerize by an ionic reaction mechanism initiated simply by weak bases to form high molecular mass, but largely uncross-linked polymers. In most cases, atmospheric moisture or the film of moisture on the substrate is sufficient to initiate polymerization, because the adhesives are applied in very thin layers. However, this sensitivity to atmospheric moisture means that the adhesives must be stored in tightly sealed form, usually in polyethylene bottles. Cyanoacrylates are used for bonding small items of nearly all substrates. In case of polyethylene or polypropylene special primers are available. Setting is complete with rubbers in seconds, with aluminum in less than one minute.
Cyanoacrylate adhesives are also used in surgery [45]. They enable parts of the body to be joined together and, therefore, must be used carefully.
Anaerobic adhesives are mixtures of methacrylic esters which remain liquid when exposed to air but harden when confined between metal surfaces. These mixtures can be used for a large number of industrial purposes such as locking threaded fasteners, sealing threaded pipe connections, retaining cylindrical machine components, sealing flange joints, bonding structural components, sealing porous metal castings, welds and powdered metal parts and many other applications which are still being found more than forty years after the initial invention [134]. Several reviews have been published which describe anaerobic adhesives and their applications [135]-[137].
All anaerobic-curing adhesives consist of a monomer, an initiator, one or more accelerators, and stabilizers. The properties of the cured and uncured adhesives can be modified to control viscosity, color, or fluorescence, reduce strength, increase toughness or heat resistance, provide lubrication and reduce settling of solid fillers. In some applications anaerobic sealants cure more rapidly if the surface is treated with chemicals which catalyze their polymerization.
A number of papers discuss the reaction mechanisms of anaerobic adhesive cure [138]-[142]. The polymerization mechanism of anaerobic adhesives is similar to that of other radical initiation systems except for the special way in which the inhibiting effect of oxygen is used to delay the polymerization and in the chemical activation which occurs at the metal surface.
Initiation:
I —> Г Г+ M’
Inhibition:
M’ + 02M00′
Propagation:
M + M’ —> M’
Termination:
M’ + M’ —> M2 M’ + I’ —> MI
The reaction rate of oxygen with free radicals is very high, and the resulting peroxy radical is a relatively poor initiator. When the supply of oxygen is used up within a thin bond line the propagation step can provide rapid development of adhesive strength.
An important factor in the initiation of anaerobic adhesive cure is the redox reaction between a hydroperoxide and transition metals with adjacent oxidation states [138].
Fe2+ + ROOH —> Fe3+ + RO’ + OIT Fe3+ + ROOH —> Fe2+ + ROO’ + H+
Other transition metals react similarly, and copper is particularly active.
The use of saccharin and (V,,:V-dimethyfp-toluidine results in a substantial acceleration of the initiating reaction. Although each of these components is an accelerator by itself the combination has a strong synergistic effect. It has been suggested that a charge-transfer complex is formed by these materials [139]. It has been suggested that one of the functions of saccharin in anaerobic curing is to dissolve metal ions from the surface that then catalyze the decomposition of CHP [142]. The role of the bonding surfaces and the effect of different types of accelerators account for most of the literature on the cure mechanism of anaerobic adhesives.
The first patents on anaerobic adhesives mentioned only the polyglycol dimethacrylates as monomers, mainly tetraethyleneglycol dimethacrylate [143], [144]. The use of acrylic or methacrylic acids to improve adhesion to smooth surfaces was mentioned in [145]. A series of polyurethane polyacrylates were prepared by reaction of toluene diisocyanate, other isocyanates, and isocyanate-terminated oligomers with hydroxyalkyl methacrylates [146]. These monomers could be tailored to provide the strength and toughness for some structural adhesive applications. The incorporation of hard and soft segments into the polyurethane backbones provided significant improvements in toughness, cure-through-gap, and cryogenic strength properties [147].
The use of monomethacrylates in anaerobic formulations was disclosed in a patent assigned to Loctite. Specifically mentioned were hydroxyethyl, hydroxypropyl, cyclohexyl, tetrahydrofurfuryl, dimethylaminoethyl, and glycidyl methacrylates and cyano — ethyl acrylate [148].
Methacrylate esters containing residual carboxylic acid groups were prepared by the reaction of hydroxyethylmethacrylate with phthalic anhydride, pyromellitic dianhydride, and benzophenone tetracarboxylic acid dianhydride. The residual acid provided improved adhesion.[149], [150] The reaction product of hydroxyalkylmethacry- lates with maleic anhydride also produced monomers with residual acid as well as additional curable unsaturation [151].
Three Bond Company used trimethylolpropane trimethacrylate [152] and ethoxy — lated bis-phenol A dimethacrylate [153] in anaerobic formulations. These monomers have some advantages in providing improved heat resistance.
At Henkel dicyclopentadienyl methacrylate was used in anaerobic formulations with high strength [154]. Rohm & Haas disclosed the use of dicyclopentenyloxyethyl acrylate
and methacrylate in anaerobic formulations [155]. These monomers provide good cure strength on metal parts which have not been degreased and also have lower odor and volatility than the corresponding dicyclopentadienyl esters.
Silicone methacrylates have been formulated into anaerobic adhesives by Dow Corning [156], Toshiba Silicones [157], and Loctite [158], [159].
The most commonly used initiator for anaerobic adhesives is cumene hydroperoxide (CHP). Many other hydroperoxides have been disclosed such as fert-butyl hydroperoxide, p-menthane hydroperoxide, diisopropylbenzene hydroperoxide, pinene hydroperoxide and methyl ethyl ketone hydroperoxide [160].
Storage-stable anaerobic formulations can be prepared without hydroperoxide if the methacrylate resin is aerated in the presence of an amide and a tertiary amine [161].
The first accelerator used in an anaerobic adhesive was tri-«-butylamine.[162] Saccharin was also found to be an effective accelerator [163] and the combination of saccharin and АГД-dimethyl-p-toluidine was particularly effective if properly stabilized [164].
Many patents have been issued on various accelerators and combinations thereof. l-Acetyl-2-phenylhydrazine [165], [166], benzenesulfonyl hydrazide [167], dibenzene — sulfonamide [168], and other similar compounds have been disclosed.
The use of saccharin is of particular interest, and a number of compounds have been prepared which have a similar chemical structure and reactivity. The reaction of sulfuryldiisocyanate with acetic acid gives a disulfonamide which is an effective accelerator [169]. Similar reactions of p-toluenesulfonyl isocyanate and chlorosulfonyl isocyanate can be used to prepare many different compounds which are active accelerators [170], [171]. These methods allow the preparation of accelerators with improved solubility.
The monomers used in anaerobic adhesives and sealants generally contain at least one free radical stabilizer such as hydroquinone or p-methoxyphenol. Benzoquinone, naphthoquinone, and similar compounds provide improved shelf stability without retarding the anaerobic cure [172].
The use of a soluble metal-chelating agent such as tetrasodium ethylenediamine tetraacetate effectively stabilizes an anaerobic formulation against small amounts of metal contamination [173]. The wide variety of applications of anaerobic adhesives and sealants is made possible by the modifications which make the viscosity appropriate to the application. An application which requires penetration into close fitting parts should have very low viscosity while a product used with large, loose fitting parts should have a high viscosity. Polymethacrylates, cellulose esters, butadiene — styrene copolymers, acrylonitrile — butadiene — styrene copolymers, polyfvinyi chloride), copolymers of vinyl chloride and vinyl acetate, polyfvinyl acetate), cellulose ethers, polyesters, polyurethanes, and other thermoplastic resins have been used to control the flow characteristics of anaerobic sealants [174].
The flow characteristics of anaerobic formulations can also be controlled by the addition of fumed silica and other solid additives which can impart thixotropic properties [175].
Many anaerobic adhesives and sealants may be required to develop a relatively low strength so that the components can be disassembled for repair or replacement. Many liquid plasticizers have been used for this purpose [176], but the use of a low molecular mass polyester is advantageous [177].
The “toughness” properties of anaerobic adhesives can be enhanced by the addition of a reactive elastomer [178]. The heat resistance of anaerobic adhesives and sealants can be enhanced by the addition of a bis-maleimide [179]. These maleimide additives appear to be relatively unreactive during the initial anaerobic cure. As the adhesive is exposed to high temperatures, the methacrylate backbone degrades and the methacrylates can then copolymerize with the maleimides to form a more heat resistant matrix [180].
The addition of dyes to anaerobic adhesives and sealants assists in identification and inspection of the products. Automated inspection procedures are made possible with dyes that fluoresce under ultraviolet light. Titanium dioxide pigments can make the sealants opaque and more visible.
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.
Powdered graphite, polytetrafluoroethylene, and polyethylene can function as lubricants in pipe sealants and threadlocking compounds. This lubrication can prevent galling in close-fitting pipe threads [181]. Lubricating additives in thread-locking sealants can provide control of the clamping force exerted by a fastener at a given tightening torque.
The cure speed and adhesion of an anaerobic sealant can be increased by treatment of the surface with a solution of a primer or activator. Early anaerobic thread lockers were strongly affected by part cleanliness, and degreasing the parts with a chlorinated solvent improved performance dramatically. The condensation product of an aldehyde and a primary or secondary amine, a sulfur-containing radical accelerator, or a compound of an oxidizable transition metal were some of the materials used to activate the anaerobic cure [182]-[184]. Due to the ozone-depleting potential of chlorinated solvents, acetone, alcohols, or hydrocarbon solvents are generally used at present.
Modified acrylics, also referred to as second-generation acrylics or no-mix adhesives, are composed of a modified acrylic adhesive and a surface activator. Typical modified acrylics are based on cross-linked polymethyl methacrylate grafted to vinyl-terminated nitrile rubber. Carboxyl-terminated rubbers have also been used.
Unlike epoxies, which cure by an ionic polymerization mechanism, modified acrylics cure by free-radical addition. Therefore, careful proportioning of components is not required. In two-component systems, no mixing is required because the adhesive is applied to one substrate, the activator to the other, and the substrates are joined. Handling strength is rapidly achieved with this fast-curing system.
Trade Names. In Germany: Agomet, Multi-Bond; in Japan: Diabond SGA, Cemedine SGA; in the USA: Versilok.
A/В acrylates are a new class of acrylate adhesives. They are two-component adhesives that cure rapidly after application. They avoid using a solvent-based primer as in modified acrylics. The acrylates used are mainly acrylated polyurethane oligomers, compounded with catalyst in one component and accelerator in the other. The two components can be applied 1/1 without mixing as beads side by side. Mixing is performed by coating the materials to be bonded or by a static-mixer applicator. Curing starts immediately after mixing. Handling strength is attained rapidly, and final curing is complete after several hours. An advantage over modified acrylics is the lesser odor due to the use of oligomers instead of monomers.
Trade Names. In Germany: Omnifit A/В; in the USA: Loctite.
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