Cyanoacrylates resistance

Cyanoacrylate adhesives (Super-Glues) are materials which rapidly polymerize at room temperature. The standard monomer for a cyanoacrylate adhesive is ethyl 2-cyanoacrylate [7085-85-0], which readily undergoes anionic polymerization. Very rapid cure of these materials has made them widely used in the electronics industry for speaker magnet mounting, as weU as for wire tacking and other apphcations requiring rapid assembly. Anionic polymerization of a cyanoacrylate adhesive is normally initiated by water. Therefore, atmospheric humidity or the surface moisture content must be at a certain level for polymerization to take place. These adhesives are not cross-linked as are the surface-activated acryhcs. Rather, the cyanoacrylate material is a thermoplastic, and thus, the adhesives typically have poor temperature resistance. 

Tetraethylene glycol may be used direcdy as a plasticizer or modified by esterification with fatty acids to produce plasticizers (qv). Tetraethylene glycol is used directly to plasticize separation membranes, such as siHcone mbber, poly(vinyl acetate), and ceUulose triacetate. Ceramic materials utilize tetraethylene glycol as plasticizing agents in resistant refractory plastics and molded ceramics. It is also employed to improve the physical properties of cyanoacrylate and polyacrylonitrile adhesives, and is chemically modified to form polyisocyanate, polymethacrylate, and to contain siHcone compounds used for adhesives. 

Cyanoacrylate adhesives are particularly valuable because of their speed of action, which allows the joining of intricate parts without the need for complex jigs and fixtures. Within very broad limits the more monomer that is used to make a joint the less will be the strength. These adhesives have in fact no gapfilling ability, nor can they be used on porous substrates. Whilst they have good heat and solvent resistance their weathering behaviour is limited and joints should not be in frequent contact with water. 

Solvent swelling experiments, with CH2CI2 and ECA polymer crosslinked with 7, demonstrate that the addition of a difunctional cyanoacrylate monomer does improve solvent resistance [6], shown in Fig. 1. 

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]. 

Because of the high resistance to chemicals, gluing is not possible with solvents but suitable cyanoacrylate, epoxy and polyurethane adhesives can be used leading to strengths significantly inferior to those of PPS. Preliminary tests are essential. 

Vauthier, C., C. Dubernet, et al. (2003). Drug delivery to resistant tumors the potential of poly(alkyl cyanoacrylate) nanoparticles. J Control Release 93(2) 151-60. 

Many cyanoacrylates give rather rigid bonds which may in consequence be brittle and not able to resist peeling forces. However, in recent years more flexible toughened grades have been developed for applications in which resistance to peeling is required. 

Since they are thermoplastic the cyanoacrylate adhesives have limited resistance to heat they do not resist moisture, and can be softened by highly polar solvents like ketones. They are expensive but since only a small quantity is necessary to form a bond, their overall economy in use is good. 

In most instances the resistance of cyanoacrylate bonds to humid aging (at 40 °C, saturated humidity) was found to be quite good when there was a reduction of strength on exposure to these conditions no further diminution occurred within three months. 

For adhesive bonding of PES to itself or to other materials, epoxy adhesives are generally used. Cyanoacrylates provide good bond strength if environmental resistance is not a factor. Parts made from PES can be cleaned using ethanol, methanol, isopropanol, or low-boiling petroleum ether. Solvents that should not be used are acetone, MEK, perchloroeth-ylene, tetrahydrofuran, toluene, and methylene chloride. 

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. 

When using cyanoacrylate adhesives it should be considered that the resistance of the bonded joints against water, especially at higher temperatures, is not comparable to that of epoxy joints. In the case of common household joints, unlimited dishwasher safety cannot be taken for granted. Heat resistance is usually limited to approximately 80 °C. 

Chemical properties Polyether-based materials have higher resistance to hydrolysis but are more vulnerable to photo-oxidation than polyester types. Good chemical resistance. Solvents cause reversible swelling but not solution. PUs can be adhered with polyurethanes, nitriles, epoxies and cyanoacrylates. 

Cyanoacrylates are one-part, highly polar thermoplastic polymers. The resin monomers cure in seconds when in contact with a weak base such as the moisture that is present on most surfaces. Many cyanoacrylate-adhesive formulations are commercially available, but not widely used in electronics assembly because of their poor resistance to solvents and moisture at elevated temperatures (>70 °C). Cyanoacrylates have relatively low impact and peel strengths and may be brittle unless toughened by the addition of elastomeric resins. 

A fuse manufacturer replaced a 30 min epoxy used in a heat cure process to assemble stainless steel inserts into phenolic holders. By utilizing an aerobic adhesive bonding process, the assembly was complete in 30 s at room temperature. The resulting increase in productivity more than offset the higher adhesive cost. A cyanoacrylate bonding process was even faster, but did not resist aging in a moist atmospheric environment or pass a drop test for impact resistance. 

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