Thermally resistant cyanoacrylates

Crosslinking has been claimed to improve thermal resistance of the cyanoacrylate adhesive [18]. However, in other reports [6], little or no improvement in thermal resistance of the adhesive was demonstrated by the addition of a difunctional monomer. As seen in Fig. 2, the addition of varying amounts of crosslinker 7 provided no improvement in the tensile adhesive strength of ethyl cyanoacrylate on steel lapshears after thermal exposure at 121 °C for up to 48 h. 

To minimize the gradual embrittlement that can occur on aging of cyanoacrylate adhesives, plasticizers are added. Some of the materials, which have been used as plasticizers, include phthalates, phosphonates, acyl esters, succinates, and cyano-acetates. The use of allyl, methallyl, and crotyl phthalates is also claimed to improve thermal resistance properties in addition to plasticizing the adhesive [23]. 

The cyanoacrylate adhesives are more rigid and less resistant to moisture than acrylate acid diester adhesives. They are available only as low-viscosity liquids that cure in seconds at room temperature without the need of a primer. The cyanoacrylate adhesives bond well to a variety of substrates, as shown in Table 7.25, but have relatively poor thermal resistance. Modifications of the original cyanoacrylate resins have been introduced to provide faster cures, higher strengths with some plastics, and greater thermal resistance. 

Long term thermal resistance of cyanoacrylate adhesives where constant exposure to temperatures greater than 60-80 C is required remains a problem. As already mentioned, an early approach was to substitute an allyl group in the side chain in place of the saturated alkyl group. 

Increased thermal resistance was claimed via a two-stage cure mechanism, whereby thermally induced crosslinking occurs after anionic polymerization of the cyanoacrylate double bond. In practice, however, the crosslinking is very slow, and assembled parts might have to be supported until it takes place. 

Anhydrides of polyfunctional carboxylic acids, as well as the acids themselves, have been reported to improve the thermal resistance as well as impact toughness of cyanoacrylate adhesives.Bond hot strengths are improved substantially, even though the thermal decomposition temperatures of the polymer remain unchanged. 

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]. 

Addition of typical crosslinking agents,such as 20% diallyl phthalate, 10% ethylene glycol dimethacrylate, 1% maleic anhydride or 1% itaconic anhydride, to the isobutyl 2-cyanoacrylate so that after curing a more rigid,insoluble, hydrolytically stable polymeric adhesive might be formed,does not increase the strength of the dentin-poly(methyl methacrylate) joint. Perhaps the cross-linked adhesive possesses decreased resistance to the thermal shock encountered by the test specimens. 

Ordinarily solvent cementing or thermal welding is used with PMMA. These methods provide stronger joints than adhesive bonding. Adhesives used are cyanoacrylates, second-generation acrylics, and epoxies, each of which provides good adhesion but poor resistance to thermal aging. "... 

Disadvantages of cyanoacrylate adhesives include poor thermal and moisture resistance on metals and glass, brittleness, sensitivity to surface preparation and poor cure through... 

Some work has also been done on copolymerizing cyanoacrylates with fcw-cyanocar-boalkoxy butadienes which claims significant improvements in thermal and solvent resistance." ... 

Cyanoacrylate adhesive can be made from different acrylate monomers such as methyl, ethyl, butyl, isopropyl, and so on. These molecules differ in size and adhesives and exhibit different physical properties. Methyls are the smallest molecule and seem to work best on metal and rubber parts while ethyls work best on plastic parts. Many modifications can be made to the monomers to alter or improve their properties as adhesives. They can be toughened with rubber or formulated to have low odor, resistance to thermal cycling, or less sensitivity to surface conditions which tend to stabilize the adhesive and slow down the cure. ... 

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