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Tuesday, August 24, 2010

Alkyl Cyanoacrylate Adhesives

It is recorded that when a drop of highly purified ethyl 2-cyanoacrylate was placed between the prisms of an Abbe refractrometer in order to determine its refractive index, the prisms could not be pulled apart. That led to the evaluation, patenting and development of alkyl cyanocrylates as adhesives, and to the marketing of the solvent less adhesives composition based on methyl 2-cyanoacrylate known as Eastman 910 Adhesive.
The monomer is unusually interesting in that it polymerises spontaneously when spread thinly on the surface of the adherents. Polymerisation occurs by an ionic mechanism and is catalysed by water or other weak bases. The minute amount of water that is normally present on the surface of most minute amount of water that is normally present on the surface of most materials is sufficient to effect polymerisation throughout the thickness of the glue film provided the adhesive is spread thinly. Only contact pressure is required. To what extent the polymer is truly thermoplastic or marginally thermosetting is not entirely clear.

It is stated that the low shrinkage on polymerisation makes possible the formulation of gap filling adhesives, but this assertion is not easy to reconcile the need to spread thinly.

Alkyl 2-cyanocrylates are prepared by condensation of an alkyl cyanoacetate and formaldehyde. The reaction gives the polymer which is then depolymerised to give the alkyl 2-cyanoacrylate. The methyl monomer is a low viscosity liquid and requires the addition of a thickening agent for most applications, and since the polymer tends to be somewhat brittle a plasticizer is generally added.

Methyl 2-cyanoacrylate is capable of giving good bond strength with a very wide range of materials including glass, wood, rubber, many metals and plastics including polyesters, polyurethanes and nylons. But like most other adhesives it does not stick well to silicone plastics. On the other hand it is said to gives weak bonds to polystyrene, polyethylene and fluorohydrocarbon plastics presumably with their surfaces untreated. Water resistance is only fair but heat resistance is stated to be good up to about 80oC. The simplicity of use and high speed of setting, from a few seconds to a few minutes, depending on the materials being bonded, are the outstanding properties of the adhesives that make it attractive for the assembly of small objects difficult to clamp in position.