Cyanoacrylates humidity resistance

With the incorporation of the new surface-insensitive additives which allow rapid curing on acidic and porous substrates such as wood and paper, and the marketing of these versatile adhesives in the consumer sector, the number of applications where cyanoacrylates are used is limited only by the ingenuity of the user. The new toughened adhesives with improved heat and humidity resistance and dynamic performance are already encroaching into areas where epoxies were once the only choice. Versatility... 

The ability of cyanoacrylates to resist attack from moisture when bonded to polymeric substrates can be most drastically tested by subjecting bonded assemblies to autoclaving. The autoclaving process combines the environmental stresses of high temperature, high-pressure and humidity. As such, it provides a good indicator of the ability of adhesives to withstand exposure to moisture. 

Epoxies cure to become thermoset adhesives and so show excellent humidity resistance, especially if one of the substrates is metal. For long-term, durable adhesively bonded metal-to-metal joints, toughened epoxies are hard to beat [6] but where plastics are involved, some of the other technology adhesives invariably offer benefits in terms of ease of application and cure speed whilst still retaining satisfactory environmental resistance. Indeed epoxies do not often show good adhesion to elastomers and for the thermoplastic elastomers cyanoacrylates will generally show the best environmental 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. 

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. 

Sterilization. All devices implanted in the body or that come into contact with body fluids or blood must be sterilized and be resistant to the sterilization conditions. Adhesive-bonded parts are not used in direct contact with the body fluids. They are generally part of an electronic module that is hermetically sealed or otherwise encapsulated or shielded. However, the sterilization conditions for the module must not degrade the internal adhesive-attached components. Some adhesives such as cyanoacrylates andUV-cured acrylics degrade at the elevated temperatures and high humidity... 

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