Anaerobic polyurethane adhesive

So-called anaerobic polyurethane adhesives also belong to this category. This system combines urethane poly-addition chemistry with free-radical initiated addition polymerization. A polymerizable alcohol such as j -hydroxyethyl methacrylate is reacted with an equivalent amount of a diisocyanate such as TDI or with an isocyanate-terminated urethane prepolymer. An organic hydroperoxide is then added to such intermediates... 

The use of synthetic adhesives in the past twenty-five years (1) has grown/ particularly the use of eight classes of polymers polyvinyl acetate/ polyolefins/ styrenic block copolymerS/ acrylicS/ cyanoacrylates/ anaerobicS/ polyurethanes/ and epoxy resins. Some of these polymers are Still in high demand as specialty adhesives (2). During the last several yearS/ however / other polymers have been added to this list/ e.g. / polyamides/ polyimideS/ and polyesters. Today/ synthetic adhesives account for 75% of the adhesives produced and 85% of the sales/ while the market share of natural products has steadily declined. 

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). 

Thermoset materials Epoxy resins, phenolic resins, polyurethanes (highly cross-linked), anaerobic adhesives. 

Urethanes have also been used to toughen vinyl-terminated acrylic adhesives for improved impact resistance. Thus rubber-toughened urethane acrylates [79,80], water-dispersible urethane acrylates [81], and high-temperature-performance urethane-acrylate structural adhesives have been reported [82]. Polyurethanes terminated with acrylic functionality are also used for anaerobic or radiation-cured adhesives with improved toughness [83]. 

The wide variety of applications of anaerobic adhesives and sealants is made possible by the modifications that make the viscosity appropriate to the application. An application that requires penetration into close-fitting parts should have very low viscosity, while a produet used with large, loose-fitting parts should have a high viscosity. A styrene aerylate eopolymer could be used to increase the viscosity [59]. Polymethacrylates, eellulose esters, butadiene-styrene eopolymers, acrylonitrile-butadiene-styrene copolymers, poly(vinyl ehloride), copolymers of vinyl chloride and vinyl acetate, poly(vinyl aeetate), eellulose ethers, polyesters, polyurethanes, and other thermoplastic resins have also been used to eontrol the flow eharacteristics of anaerobic sealants [60]. The flow eharaeteristies of anaerobic formulations can also be controlled by the addition of fumed siliea and other solid additives whieh can impart thixotropic properties [61]. 

Adhesives and sealants are manufactured from a variety of polymers. Their selection and their combinations used impact solvent selection. Most solvent systems are designed to optimize the solubility of the primary polymer. Adhesives can be divided into ones which bond by chemical reaction and ones which bond due to physical processes. Chemically reactive adhesives are further divided into three more categories for those that bond through polymerization, polyaddition, or polycondensation. Physically bonding adhesives include pressure sensitive and contact adhesives, melt, or solution adhesives, and plastisols. Polymerization adhesives are composed of cyanoacrylates (no solvents), anaerobic adhesives (do not contain solvents but require primers for plastics and some metals which are solutions of copper naphthenate), UV-curable adhesives (solvent-free compositions of polyurethanes and epoxy), rubber modified adhesives (variety solvents discussed below). 

The use of adhesives/sealants in both the industrial and consumer spheres has increased dramatically in the past 20 years. In the industrial segment, both reactive and nonreactive systems are used in a wide variety of applications. The increasing use of reactive systems has, however, tended to overshadow that of the nonreactive systems. Most prominent amongst the reactive systems favored in industrial applications are anaerobic sealants (methacrylate ester based), instant adhesives (alkyl cyanoacrylate ester based), acrylic (toughened) adhesives, epoxy resin adhesives, polyurethane/isocyanate-based adhesives, silicone adhesives/sealants, and phenolic resin adhesives. 

Adhesives recommended include epoxies, polyurethane, silicone, nylon-epoxy, nitrile-phenolic, neoprene-phenolic, acrylic, cyanoacrylate, anaerobics, and partially hydrogenated polybutadiene (for bonding copper to polyethylene). 

Adhesives recommended by the manufacturer (Phillips Chemical Company) include anaerobics (Loctite 306), liquid two-part epoxies (Hughson s Chemlok 305), and a two-part paste epoxy (Emerson Cuming s Eccobond 104). Also recommended are USM s BOSTIK 7087 two-part epoxy and 3M Company s liquid two-part polyurethane EC-3532. ° ... 

The most common thermoset adhesives are epoxies, phenoUcs and thermoset polyurethanes. The most widely used thermoplastic adhesives are acrylics (including anaerobics, hot melts, cyanoacrylates) and thermoplastic polyurethanes. A brief description of some adhesives is given in the EUROCOMP Handbook, 5.3.4. 

Most of the adhesive families have either a thermoset or thermoplastic base. This is also the primary and the most traditional way of categorising adhesives, although within some adhesive families, such as polyurethanes, both thermoset and thermoplastic adhesives may be found. Thermoset adhesives form bonds that are essentially infusible and insoluble after curing and they typically have a much higher load-bearing capability than thermoplastic adhesives. Thermoplastic adhesives are fusible, soluble, soften when heated and their creep resistance is lower than that of the thermoset adhesives. The most common thermoset adhesives are epoxies, phenolics and polyurethanes, while the most widely used thermoplastic adhesives include acrylics (including anaerobics, hot melts and cyanoacrylates) and thermoplastic polyurethanes. A brief description of these adhesives (both thermoset and thermoplastic) is given below from reference 5.20 and 5.28. 

In recent years, the range of adhesive materials used in automotive manufacture has expanded to include polyurethanes, plastisols, phenolics, hot melts, anaerobics, cyanoacrylates, toughened acrylics and epoxies (see Structural and Hot melt adhesives). Selection criteria are based principally upon the nature of the adherends, the mechanical properties required under service conditions and application and curing characteristics. 

BEARING FIT (Anaerobic) FORTEFLEX series (Polyurethane) GLASS BONDING ADHESIVE (Misc.) HiGH STRENGTH RETAINER (Anaerobic)... 

Toughening of anaerobics by using urethane methacrylate monomers or by incorporating rubbers has been moderately successful, to create (although expensive) structural adhesives. However, in contrast to other tough adhesives such as reactive acrylics and polyurethanes, they remain relatively brittle materials. 

Cyanoacrylate Adhesives Anaerobic Compounds Epoxies Industrial Sealants and Adhesives Construction Silicones Construction Polyurethanes Latex Sealants Firestop Products Pool Spa Sealants Silicone Greases and Compounds Aerosols Cleaners... 

Although overall adhesive volume increased by 50% in the 1975-87 period, there were seven adhesive materials that experienced growth of at least 100%. These high performers are acrylics, cyanoacrylates, anaerobics, polyvinyl acetate, ethylene-vinyl acetate, styrenic block copolymers, and polyurethanes (Tables 7 and 8). 

Reactive hot melt polyurethane Epoxies Cyanoacrylate Anaerobics Reactive acrylics Radiation-cured adhesives. 

Adhesive choice can therefore be influenced by actual joint tolerance control and dimensions and where a cyanoacrylate or an anaerobic adhesive might be ideal (polymeric substrates or a threaded metallic joint, respectively), the measured bond gap may be too large and an alternative adhesive such as a polyurethane or an epoxy may be better suited. 

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