Cyanoacrylates

Cyanoacrylates are a family of strong fast-acting adhesives with industrial, medical, and household uses. They are various esters of cyanoacrylic acid. The acryl groups in the resin rapidly polymerize in the presence of water to form long, strong chains. They have some minor toxicity. Specific cyanoacrylates include methyl 2-cyanoacrylate (MCA), ethyl 2-cyanoacrylate (ECA, commonly sold under trade names such as 'Super Glue' and 'Krazy Glue', or Toagosei), n-butyl cyanoacrylate (n-BCA), octyl cyanoacrylate and 2-octyl cyanoacrylate (used in medical, veterinary and first aid applications). Octyl cyanoacrylate was developed to address toxicity concerns and to reduce skin irritation and allergic response. Cyanoacrylate adhesives are sometimes known generically as instant glues, power glues or superglues. The abbreviation 'CA' is commonly used for industrial grade cyanoacrylate. The original patent for cyanoacrylate was filed in 1942 by the B.F. Goodrich Company as an outgrowth of a search for materials suitable for clear plastic gun sights for the war effort. In 1942, a team of scientists headed by Harry Coover Jr. stumbled upon a formulation that stuck to everything with which it came in contact. The team quickly rejected the substance for the wartime application, but in 1951, while working as researchers for Eastman Kodak, Coover and a colleague, Fred Joyner, rediscovered cyanoacrylates. The two realized the true commercial potential, and a form of the adhesive was first sold in 1958 under the title 'Eastman #910' (later 'Eastman 910'). During the 1960s, Eastman Kodak sold cyanoacrylate to Loctite, which in turn repackaged and distributed it under a different brand name 'Loctite Quick Set 404'. In 1971, Loctite developed its own manufacturing technology and introduced its own line of cyanoacrylate, called 'Super Bonder'. Loctite quickly gained market share, and by the late 1970s it was believed to have exceeded Eastman Kodak's share in the North American industrial cyanoacrylate market. National Starch and Chemical Company purchased Eastman Kodak’s cyanoacrylate business and combined it with several acquisitions made throughout the 1970s forming Permabond. Other manufacturers of cyanoacrylate include LePage (a Canadian company acquired by Henkel in 1996), the Permabond Division of National Starch and Chemical, which was a subsidiary of Unilever. Together, Loctite, Eastman and Permabond accounted for approximately 75% of the industrial cyanoacrylate market. As of 2013 Permabond continued to manufacture the original 910 formula. In its liquid form, cyanoacrylate consists of monomers of cyanoacrylate ester molecules. Methyl 2-cyanoacrylate (CH2=C(C≡N)COOCH3) has a molecular weight of 111.1 g/mol, a flashpoint of 79 °C (174 °F; 352 K), and a density of 1.1 g/mL. Ethyl 2-cyanoacrylate ((CH2=C(C≡N)COOCH2CH3)) has a molecular weight of 125 g/mol and a flashpoint of more than 75 °C (167 °F; 348 K). To facilitate easy handling, a cyanoacrylate adhesive is frequently formulated with an ingredient such as fumed silica to make it more viscous or gel-like. More recently, formulations are available with additives to increase shear strength, creating a more impact resistant bond. Such additives may include rubber, as in Loctite's 'Ultra Gel', or others which are not specified. In general, the acryl groups rapidly undergo chain-growth polymerisation in the presence of water (specifically hydroxide ions), forming long, strong chains, joining the bonded surfaces together. Because the presence of moisture causes the glue to set, exposure to normal levels of humidity in the air causes a thin skin to start to form within seconds, which very greatly slows the reaction; hence, cyanoacrylates are applied as thin coats to ensure that the reaction proceeds rapidly for bonding. Cyanoacrylate adhesives have a short shelf life—about one year from manufacture if unopened, and one month once opened. The reaction with moisture can cause a container of glue which has been opened and resealed to become unusable more quickly than if never opened. To minimise this reduction in shelf life, cyanoacrylate, once opened, should be stored in an airtight container with a package of silica gel desiccant. Another technique is to insert a hypodermic needle into the opening of a tube. After using the glue, residual glue soon clogs the needle, keeping moisture out. The clog is removed by heating the needle (e.g. with a lighter) before use. The polymerisation is also temperature-dependent: storage below freezing point of water, 0 °C (32 °F; 273 K), stops the reaction, so keeping it in the freezer is also effective. Cyanoacrylates are mainly used as adhesives. They require some care and knowledge for effective use: they do not bond some materials; they do not fill spaces, unlike epoxies, and a very thin layer bonds more effectively than a thicker one that does not cure properly; they bond many substances, including human skin and tissues. They have an exothermic reaction to natural fibres: cotton, wool, leather, see reaction with cotton below.