Cyanoacrylates, A Brief Overview:
By: Steven D. Russell
Eurowood Werks Woodturning Studio
The Woodlands, Texas U.S.A.
Posted with permission of the Author
Monofunctional 2-cyanoacrylates were first patented in 1949, but the first viable production process did not evolve until 1954. In the early 1950's, scientists at Eastman Kodak were working on thermal polymerization and discovered the rapid room-temperature cure and excellent adhesion properties of 2-cyanoacrylates, quite by accident.
While working on a freshly prepared monomer, the scientists discovered that the glass prisms of the refractometer has become tightly bonded. Extensive work thereafter, found that many different types of substrates bonded in the same manner. Subsequently in 1958, Eastman 910 debued, the first in a large family of 2-cyanoacrylate ester adhesives.
2-cyanoacrylates polymers spontaneously form (via an anionic/radical mechanisms) when their liquid precursors, or monomers are placed between two closely fitting surfaces. The great utility of these adhesives arises from the electron-withdrawing character of the groups adjacent to the polymerizable double bond.
The high reactivity (cure rate) and their polar nature, enables the polymers to adhere quite tenaciously to a wide variety of substrates. Low humidity and/or acidic groups on the substrate surface will slow or inhibit the cure reaction. To extend the usable shelf life, free-radical stabilisers such as quinones or hindered phenols are used.
Methyl, Ethyl, Butyl, Allyl and Methoxyethyl esters are available with various setting characteristics and rheological properties. However, the Methyl and Ethyl esters dominate the commercial industrial market. The vinyl structure of 2-cyanoacrylates makes them prone to spontaneous polymerization. The chain propagation can be initiated by ionic or radical mechanisms.
The rate of polymerization depends on temperature, humidity, light and the presence of accelerators, like peroxides or bases. In addition to polymerization, 2-cyanoacrylates undergo reactions typical of vinyl compounds, such as addition.
2-cyanoacrylates can be manufactured by many different methods. The base monomers are too thin for convenient use, so thickeners, stabilisers or property-modifying additives may be added. The viscosity's are available from wicking grades (water thin) to thixotropic gels that range from 20,000 to 50,000 mPa*s for large gaps.
The acrid odour of 2-cyanoacrylates, can be effectively mitigated by the substitution of an alkoxyalkyl ester side chain, for the normal alkyl group. Products so modified, are practically odour free, but are slightly less effective as adhesives.
The basic method to manufacture 2-cyanoacrylate esters involves preparation via the Knoevenagel condensation reaction (the corresponding alkyl cyanoacetate reacts with formaldehyde in the presence of a basic catalyst, to form a low molecular weight polymer). The resulting polymer slurry is acidified and the water is removed.
The polymer is then cracked and redistilled at high temperatures onto a suitable stabiliser combination to prevent repolymerization. Protonic or Lewis acids are typically used in combination with small amounts of a free-radical stabiliser.
Although the methods and processes have continually changed and evolved over the years, this is the standard method to manufacture these esters. One recent and significant advancement in the manufacturing process is a continuous process where the condensation is carried out in an extruder. By-products are then removed in a degassing zone and the molten polymer (mixed with stabilisers), is cracked to yield a raw monomer.
Recent advances have lead to flexible 2-cyanoacrylate formulas, which remain somewhat flexible when cured. These types of esters are particularly useful to turners when bonding dissimilar materials like stone/metal and wood. The dissimilar expansion and contraction rates of these materials, can cause subsequent failure of the bond when using traditional cyanoacrylates that feature non-flexible, or brittle bonds.
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