Cyanoacrylates surface wetting

A common application for adhesives is where co-axial (cylindrical parts) require to be joined. Figure 5.11 shows a typical application where the black polypropylene spigot is to be bonded and sealed into the red elastomeric (thermoplastic elastomers) housing. In this case, both these plastics are low-surface-energy materials and so will require either a surface primer or some form of surface treatment to increase the surface wetting (see Chapter 6). In this application a medium viscosity ethyl cyanoacrylate with a primer was selected. 

Cyanoacrylate has been used for sealing corneal perforations, but it is not smooth or malleable, and it exhibits a hard tissue-scratching surface and requires the patient to use a soft contact lens for comfort. Another surgical limitation when using cyanoacrylate is the immediate polymerization on a wet or moist corneal surface, and that property reduces the time allocated for and increases the difficulty in applying the glue to a precise location in a well-controlled manner. 

Ethyl cyanoacrylate is a little less polar than methyl cyanoacrylate, and has the ability to wet plastic surfaces more readily, and is a better solvent for plastics. With this added ability to make intimate contact with the surface, the bonds on plastic are stronger with ethyl cyanoacrylate than with the methyl ester. This difference in performance gives rise to the adage that methyl is for metal and ethyl is for everything else. Sometimes this difference can be utilized in reverse to good advantage to avoid stress cracking on such sensitive plastics as polycarbonate and polyacrylate. 

Application of cyanoacrylates to the bonding surface is simple, for these are one-part, 100% reactive adhesives. The adhesive is applied as a drop or bead to one surface, then the other adherend is used to spread the adhesive in a thin film. Due to the volatility of the monomer and the fast cure, no more than three or four square inches of bondline should be open at one time. Enough adhesive should be applied to provide a slight fillet when the adherends are mated. This will ensure that the adherends are thoroughly wetted and that any air bubbles are removed. On impervious, well-fitted substrates, two drops (25 mg) of adhesive per square inch is sufficient coverage. Too much adhesive will create a weak bond, as the surface-initiated cure may not extend throughout a thick adhesive layer. Also, excessive adhesive can wash the initiators out of the bondline, causing a slow or incomplete cure. 

Silicone rubber is difficult to bond with cyanoacrylates. The adhesive does not wet the surface properly without special surface treatment, due to the very low surface energy, and therefore will not bond. Polypropylene (PP), polyethylene (PE), polytetrafluoro ethylene (PTFE) and acetal plastics and Santoprene rubber also fall into this category and cannot be bonded without prior surface preparation. 

The a-alkyl cyanoacrylate super glues ( one drop holds 5000 lbs ) are now a familiar part of the consumer market. Originally, the monomers had extremely low viscosities and so could crawl into narrow crevices and wet the adherend surfaces rapidly. On the other hand, they would not fill gaps and were absorbed into porous adherends, giving poor bonds. Newer versions are available with higher viscosities to overcome these drawbacks. Cyanoacrylates can polymerize in seconds by an anionic addition reaction believed initiated by hydroxyl ions from water adsorbed on the adherend surfaces ... 

Semisynthetic and synthetic surgical adhesives include gelatine— resorcinol—formaldehyde (GRF), urethane prepolymers, and cyanoacrylates. These adhesives have some shortcomings such as low bioabsortion and metabolic rates, cytotoxicity (low biocompatibility), low adherence to wet surfaces, and chronic inflammation induced by the release of formaldehyde from GRF and cyanoacrylate polymers and aromatic diamine from aromatic polyurethanes [11]. 

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