Cyanoacrylate adhesives hardening

Grinder-Polishers with extra wheels and Minimet (Buehler), the latter with slide holder for thin sections 2 Glass plates (300x300 mm) Vacuum impregnator Epoxy and hardener and polyethylene cups (several varieties of each) Cyanoacrylate adhesive (Super Glue )... 

Other examples are less obvious and may go unrecognized. For example, cyanoacrylate adhesive is a solvent for some polymer surfaces, and may actually dissolve and mix with some of the surface before it hardens. As a result, the bonds exhibit excellent durability toward moisture and other environmental stresses, seemingly in contradiction to the adhesive s poor reputation for service under these conditions. The interpenetration provides durability which is absent when the same adhesive is used on an inorganic substrate. 

An adhesive may be defined as a material which when applied to surfaces of materials can join them together and resist separation. Adhesive is the general term and includes cement, glue, paste, etc. and these terms are all used essentially interchangeably. Various descriptive adjectives are often applied to indicate certain characteristics. For example, to indicate the physical form of the adhesive, e.g. liquid adhesive, liquid two-part adhesive, film adhesive its chemical form, e.g. epoxy adhesive, cyanoacrylate adhesive, polychloroprene adhesive to indicate the type of materials bonded, e.g. metahto-metal adhesive, paper adhesive, wood adhesive or to show the conditions of use, e.g. solvent based adhesive, cold-hardening, or -curing, adhesive, hot-melt adhesive. 

Figure 1.4 Fast, room-temperature hardening, single-part cyanoacrylate adhesives are widely used for bonding metallic, plastic, rubber and wooden substrates, such as in attaching the rubber recoil pad to this hardwood gun stock.

Anaerobic acrylic sealants or cyanoacrylates are known sensitizers, while the modified acrylic structural adhesives that cure in air more rarely cause allergy. Tosti et al. (1993) reported of three carpenters, each of whom became sensitized to wood paints and glues with butyl acrylate, 2-ethoxyethyl methacrylate, or a phth-late. Epoxy resin compounds are another commonly used group of adhesives, which contain resins (generally included in patch-test screening batteries), hardeners, and reactive diluents, as well as many other potentially harmful ingredients, such as tar, fillers, colorants, and other plastics. 

While both cyanoacrylate and toughened acrylic adhesives contain no solvents - all the liquid present is converted to solid - their common base material is an excellent solvent in its own right and so both function well on unprepared surfaces. Toughened acrylics are noteworthy in this respect, though the tolerance of the cyanoacrylates is partially limited because contamination may inhibit hardening. Nonetheless, the cyanoacrylates generally cope well with the unprepared surfaces of the small plastics, rubber and metal parts that they are usually used on. 

Polymerization adhesives based on a-cyanoacrylates (see p. 30) are suitable for special bonds involving rubber, because they enable bonds of high strength to be obtained quickly and easily. Medium-viscosity types are preferred for rubber-to-rubber bonds by virtue of the minimal hardening of the joint. In the rubber industry, for example, the adhesives are used for bonding profiles to one another and also for repair work. 

The versatility of epoxy-resin systems arises from the many combinations of epoxy resins and hardeners, each of which gives a different cure profile and results in a different molecular structure in the resulting polymer. Compared with other adhesive systems such as cyanoacrylates or anaerobics, epoxies are not very sensitive to impurities. This gives tremendous scope for modification of their properties by additives, modifiers, fillers, rubber tougheners,... 

Where the hardening of an adhesive or sealant proceeds by chemical reaction, the rate will increase with temperature in accordance with the Arrhenius equation. Such materials include epoxides, structural acryhcs, cyanoacrylates, silicones, and high temperature adhesives. Hence, pot life and cure time will show the following proportionality ... 

For thermodynamic reasons, some adhesives may not cure at all above a critical temperature, which is known as the ceiling temperature, and may only partially cure at lower temperatures. This particularly applies to adhesives which harden by polymerization of monomers with C=C bonds, such as acrylates, methacrylates, and cyanoacrylates. 

Cyanoacrylate. These adhesives are also based on acrylic resins but unlike the anaerobic types they harden — in thin films — because of the catalytic effect of surface moisture. Generally, hardening takes place in a few seconds and for this reason they are frequently used in the assembly of small plastic and rubber parts — which they normally bond well. However, caution is needed when it is intended to use them in conjunction with metal components in warm moist conditions. 

Epoxy adhesives such as Huntman s Araldite AW 134 with HY 994 hardener (cured for 15 min at 120°C) and Araldite AV 1566 GB (cured for 1 h at 230°C) give the best results with this engineering resin. Other adhesives that can be used are cyanoacrylate (Loctite 414 with AC primer), anaerobics (Loctite 638 with N primer), and silicone sealant (Loctite Superflex). The highest lap-shear strength was obtained with Araldite AW 134. This adhesive has balanced properties, good resistance to mechanical shock, thermal resistance to 100°C, and reasonable stability in the presence of aliphatic and aromatic solvents. Some solvents, particularly chlorinated hydrocarbons, will cause deterioration of the bond [30]. 

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