Polycyanoacrylates, adhesion

A unified science of adhesion is still being developed. Adhesion can result from mechanical bonding between the adhesive and adherend and/or primary and/or secondary chemical forces. Contributions through chemical forces are often more important and illustrate why nonpolar polymeric materials such as polyethylene are difficult to bond, although polycyanoacrylates are excellent adhesives. Numerous types of adhesives are available such as solvent-based, latex, pressure-sensitive, reactive, and hot-melt adhesives. 

Cyanoacrylate esters (generally the methyl or ethyl ester) are colorless, monomeric liquids sold commercially as quick-setting, high-strength adhesives (e.g.. Superglue ). These esters polymerize rapidly at ambient temperature via an anionic mechanism in the presence of weak bases such as alcohol and water (Figure 2). The reaction product, polycyanoacrylate, is a hard white solid. 

The thermal instability of polycyanoacrylate is of particular importance both to the manufacturer and the consumer of cyanoacrylate adhesives. The degradation of cyanoacrylate oligomer has been described in general terms previously, as an important step in the production of cyanoacrylate monomer. Heating PECA above 150°C results in the steady production of cyanoacrylate monomer in high yield. This process is end group-initiated... 

The resistance to hydrolysis of a polycyanoacrylate can also be improved by copolymerization with another monomer. In fact, the cyanopentadienoates cited above should also be considered comonomers. An electron-rich vinyl monomer like styrene and a cyanoacrylate ester will spontaneously copolymerize a short time after mixing. The product is a one-to-one alternating copolymer. This reaction has been used to make a two-part adhesive with better hydrolytic stability than the standard cyanoacrylate. " The carbon dioxide evolved from a suspension of the polymer in boiling water was measured to follow the degradation. After eight hours, polymethyl cyanoacrylate had lost 28% carbon dioxide by hydrolysis and decarboxylation of the polycyanoacrylate ester groups, while the copolymer had lost none. However, this improvement in moisture durability is achieved at the expense of the convenience of a one-part adhesive. 

Durability. Cyanoacrylates suffer from poor heat and moisture durability. This failing is pronounced on metal adherends, but minimal on most plastic or rubber adherends. Poor heat resistance is due to several causes the thermoplastic nature of the polycyanoacrylate, the tendency to retropoly-merize, and the loss of adhesion experienced on heat aging of cyanoacrylate bonds. The poor moisture resistance is due in part to the hydrolytic degradation of the polymer and in part to the loss of adhesion caused by exposure to moisture. 

Whilst the glass transition is of very great importance in adhesives, crystalline melting is not. This is because the polymers in most adhesives are totally amorphous, which in turn is due to a lack of molecular regularity. Stereoregular polymers can be made from vinyl and vinylidene monomers with the employment of organometallic initiators, but these are not used when such monomers are converted into polymers which are used in adhesives. The latter are atactic and examples are MMA copolymers in structural acrylic adhesives, amorphous polypropylene, polyacrylic pressure-sensitive adhesives, and polycyanoacrylates. 

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