Cyanoacrylates are a family of strong fast-actingadhesives with industrial, medical, and household uses that are derived fromethyl cyanoacrylate and relatedesters. The cyanoacrylate group in themonomer rapidlypolymerizes in the presence of water to form long, strong chains.[1]
Specific cyanoacrylates includemethyl 2-cyanoacrylate (MCA),ethyl 2-cyanoacrylate (ECA, commonly sold under trade names such as "Super Glue" and "Krazy Glue"),n-butyl cyanoacrylate (n-BCA),octyl cyanoacrylate, and2-octyl cyanoacrylate (used in medical, veterinary and first aid applications). Cyanoacrylate adhesives are sometimes known generically asinstant glue,power glue, orsuper glue. The abbreviation "CA" is commonly used for industrial grade cyanoacrylate.
The originalpatent for cyanoacrylate was filed in 1947 by theB.F. Goodrich Company[2] as an outgrowth of a search for materials suitable for clear plasticgun sights forthe war effort. In 1942, a team of scientists headed byHarry Coover Jr. stumbled upon a formulation that stuck to everything with which it came in contact.[3] The team quickly rejected the substance for the wartime application, but in 1951, while working as researchers forEastman Kodak, Coover and a colleague, Fred Joyner, rediscovered cyanoacrylates, then applied for a patent in 1954 which was issued in 1956.[4] The two had realized its true commercial potential, and a form of the adhesive was first sold in 1958 under the name "Eastman #910" (later "Eastman 910").
During the 1960s, Eastman Kodak sold cyanoacrylate toLoctite, 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 theNorth 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 (aCanadian company acquired byHenkel in 1996), the Permabond Division of National Starch and Chemical, which was a subsidiary ofUnilever. Together, Loctite, Eastman, and Permabond accounted for approximately 75% of the industrial cyanoacrylate market.[5] As of 2019[update] Permabond continued to manufacture the original 910 formula.[6]
The most common monomer isethyl cyanoacrylate. Several related esters are known. To facilitate easy handling, a cyanoacrylate monomer is frequently formulated with an ingredient such asfumed silica to make it moreviscous or gel-like. Formulations are available with additives to increaseshear strength, creating a more impact resistant bond. Such additives may includerubber, as in Loctite's "Ultra Gel", or others which are not specified.
In general, the C=C groups rapidly undergochain-growth polymerization in the presence of water (specificallyhydroxide ions), forming long, strong chains, joining the bonded surfaces together. Because the presence of moisture causes the glue to set, exposure to normal levels ofhumidity in the air causes a thin skin to start to form within seconds, which greatly slows the reaction; hence, cyanoacrylates are applied as thin coats to ensure that the reaction proceeds rapidly for bonding. Cyanoacrylate adhesives generally have a shortshelf life—about one year from manufacture if unopened, and one month once opened, but this can be extended by refrigeration.
Cyanoacrylates are mainly used as adhesives. Thin layers bond effectively, thick layers much less so. They bond many substances, including human skin and tissues, natural fibres, cotton, wool, and leather.
Cyanoacrylate glue has a lowshearing strength, which has led to its use as a temporary adhesive in cases where the piece needs to be sheared off later. Common examples include mounting a workpiece to a sacrificial glue block on alathe, and tightening pins and bolts. It is also used in conjunction with another slower, but more resilient, adhesive as a way of rapidly forming a joint, which then holds the pieces in the appropriate configuration until the second adhesive has set.
Cyanoacrylate-based glue has a weak bond with smooth surfaces and as such easily gives to friction; a good example of this is the fact that cyanoacrylates may be removed from human skin by means of abrasives (e.g. sugar or sandpaper).
Cyanoacrylate glue is widely used in human and veterinary medicine.[7] It was in veterinary use for mending bone, hide, and tortoise shell by the early 1970s or before. A cyanoacrylate spray was used in theVietnam War to reduce bleeding in wounded soldiers until they could be taken to a hospital.[8]
n-Butyl cyanoacrylate has been used medically since the 1970s. In the US, due to its potential to irritate the skin, theU.S. Food and Drug Administration (FDA) did not approve its use as a medical adhesive until 1998 with Dermabond (2-octyl cyanoacrylate).[9] A 1986 independent study suggests that cyanoacrylate can be safer and more functional for wound closure than traditionalsuturing (stitches).[10] Although cyanoacrylates are widely used for sutureless wound repair, "existing adhesives still lack sufficient adhesion strength to completely replace sutures and staples."[7]
Somerock climbers use cyanoacrylate to repair damage to the skin on their fingertips.[11][12] Similarly, stringed-instrument players can form protective finger caps (typically, when they lose theircalluses due to inactivity or accidents) with cyanoacrylates. While the glue is not very toxic and wears off quickly with shed skin, applying large quantities of glue and its fumes directly to the skin can causechemical burns.[13]
While standard "superglue" is 100% ethyl 2-cyanoacrylate, many custom formulations (e.g., 91% ECA, 9%poly(methyl methacrylate), <0.5%hydroquinone, and a small amount of organicsulfonic acid,[14] and variations on the compoundn-butyl cyanoacrylate for medical applications[10]) have come to be used for specific applications. Three cyanoacrylate compounds are available[where?] as topical skin adhesives: 2-Octyl cyanoacrylate is marketed as Dermabond, SurgiSeal, and LiquiBand Exceed.n-Butyl cyanoacrylate is marketed as Histoacryl, Indermil, GluStitch, GluSeal, PeriAcryl, and LiquiBand. The compound ethyl 2-cyanoacrylate is available as Epiglu.[15]
The ester substituent has several effects upon the biocompatibility of the adhesive. Longer side chains result in higher flexibility of the cured adhesive allowing a better match for the flexibility of skin, and can result in a slower release offormaldehyde after curing.[16] Alkoxy-ethyl substituents have also been investigated and have shown some promise as a medical adhesive.[17]
Cyanoacrylate is used as aforensic tool to capturelatent fingerprints on non-porous surfaces like glass,plastic, etc.[18] Cyanoacrylate is warmed to produce fumes that react with the invisible fingerprint residues and atmospheric moisture to form a white polymer (polycyanoacrylate) on the fingerprint ridges. The ridges can then be recorded. The developed fingerprints are, on most surfaces (except on white plastic or similar), visible to the naked eye. Invisible or poorly visible prints can be further enhanced by applying a luminescent or non-luminescent stain.
Cyanoacrylate is used in the cosmetology and beauty industry as an eyelash extension glue, or a "nail glue" for someartificial nail enhancements such as nail tips and nail wraps, and is sometimes mistaken foreye drops causing accidental injury (adhesion and/orchemical eye injury).[19]
Cyanoacrylates are used to assemble prototypeelectronics (used inwire wrap), flyingmodel aircraft, and as retention dressings fornuts andbolts. Their effectiveness in bonding metal and general versatility have made them popular amongmodeling andminiatures hobbyists.[20]
Cyanoacrylate glue's ability to resist water has made it popular withmarine aquarium hobbyists forfragmenting corals. The cut branches ofhard corals, such asAcropora, can be glued to a piece oflive rock (harvested reef coral) orMilliput (epoxy putty) to allow the new fragment to grow out. It is safe to use directly in the tank, unlikesilicone which must be cured to be safe. However, as a class of adhesives, traditional cyanoacrylates are classified as having "weak" resistance to both moisture and heat[21] although the inclusion ofphthalic anhydride reportedly counteracts both of these characteristics.[22]
Cyanoacrylate glue is also used frequently inaquascaping both freshwater and marine aquariums for the purpose of securing therhizomes of live plants to pieces of wood or stone. Thin cyanoacrylate glue has application inwoodworking.[23] It can be used as a fast-drying, glossy finish. An oil, such asboiled linseed oil, may be used to control the rate at which the cyanoacrylate cures. Cyanoacrylate glue is also used in combination withsawdust (from a saw or sanding) to fill voids and cracks. These repair methods are used on pianosoundboards, wood instruments, and wood furniture. Cyanoacrylate glue is also used in the finishing of pen blanks (wooden blanks for turning pens) that have been turned on a lathe by applying multiple thin layers to build up a hard, clear finish that can then be sanded and polished to a glossy finish.
Some hobby applications call for cyanoacrylate glue mixed with inorganic fillers such as baking soda (sodium bicarbonate)[24] andcalcium oxide.[25]
Cyanoacrylate is used inarchery to gluefletching to arrow shafts. Some special fletching glues are primarily cyanoacrylate repackaged in special fletching glue kits.[26] Such tubes often have a long, thin metal nozzle for improved precision in applying the glue to the base of the fletching and to ensure secure bonding to the arrow shaft.
Cyanoacrylate is used infencing to glue the blade wire into the groove of theépée orfoil.[27]
Cyanoacrylate adhesives may make body parts adhere, and may cause skin to be torn off trying to separate them.[28][29] Without force, however, the glue will spontaneously separate from the skin over time (up to four days). The glue can also cause chemical burns, and exposed skin should be washed with soap and warm water.[30] Separation can be accelerated by applyingvegetable oil oracetone near, on, and around the glue.[31] Glue can cause serious injuries if it gets in the eyes; in such cases the eye(s) should be rinsed with lukewarm water for 15 minutes and medical care obtained.[30]
Heating causesdepolymerization of the cured polymers producing gaseous products that are highly irritating. They are immediately polymerized by the moisture in the membranes and becomeinert. These risks can be minimized by using cyanoacrylate in well-ventilated areas. About 5% of the population can becomesensitized to cyanoacrylate fumes after repeated exposure, resulting influ-like symptoms.[32] Cyanoacrylate may also be a skin irritant, causing anallergic skin reaction. TheAmerican Conference of Governmental Industrial Hygienists (ACGIH) assign athreshold limit value exposure limit of 200parts per billion. On rare occasions, inhalation may triggerasthma. There is no singular measurement of toxicity for all cyanoacrylate adhesives because of the large number of adhesives that contain various cyanoacrylate formulations.
The United Kingdom'sHealth and Safety Executive and the United StatesNational Toxicology Program have concluded that the use of ethyl cyanoacrylate is safe and that additional study is unnecessary.[33] The compound 2-octyl cyanoacrylate degrades much more slowly due to its longer organic backbone (series ofcovalently bondedcarbon molecules) and the adhesive does not reach the threshold of tissue toxicity. Due to the toxicity issues of ethyl cyanoacrylate, the use of 2-octyl cyanoacrylate for sutures is preferred.[34]
Applying cyanoacrylate to some natural materials such ascotton (jeans,cotton swabs,cotton balls, and certainyarns or fabrics),leather orwool results in a powerful, rapid,exothermic reaction. This reaction also occurs withfiberglass andcarbon fiber. The heat released may cause serious burns[35] or release irritating white smoke.Material safety data sheets for cyanoacrylate instruct users not to wear cotton (jeans) or wool clothing, especially cotton gloves, when applying or handling cyanoacrylates.[36]
Acetone, commonly found as a fraction ofnail polish remover (or at hardware stores in pure form), is a widely available solvent capable of softening cured cyanoacrylate.[37] Other solvents includenitromethane,dimethylformamide,[38]dimethyl sulfoxide, andmethylene chloride.[39] Another viable solvent for removing cured cyanoacrylate isgamma-butyrolactone.[40] Commercial debonders are also available, many based onpropylene carbonate.[citation needed]
Cyanoacrylate adhesives have a short shelf life. Date-stamped containers help to ensure that the adhesive is still viable. One manufacturer supplies the following information and advice:
When kept unopened in a cool, dry location such as a refrigerator at a temperature of about 55 °F (13 °C), the shelf life of cyanoacrylate will be extended from about one year from manufacture to at least 15 months. If the adhesive is to be used within six months, it is not necessary to refrigerate it. Cyanoacrylates are moisture-sensitive, and moving from a cool to a hot location will create condensation; after removing from the refrigerator, it is best to let the adhesive reach room temperature before opening. After opening, it should be used within 30 days. Open containers should not be refrigerated.[41]
Another manufacturer says that the maximum shelf life of 12 months is obtained for some of their cyanoacrylates if the original containers are stored at 35 to 40 °F (2 to 4 °C).[42] One manufacturer has given a very detailed explanation in answer to a question about shelf life, including the fact that cyanoacrylate adhesives contain an inhibitor to slow curing in the bottle; the amount of inhibitor affects shelf life (but a glue with more inhibitor can require accelerant to be applied for some applications).[43]
Polymerization is nearly stopped, leading to an almost unlimited shelf life, by storing unopened at −4 °F (−20 °C), the typical temperature of a domesticfreezer, and allowing the contents to reach room temperature before use.[44] Opening a container while chilled may cause moisture from the air to condense in the container.
As cyanoacrylates age, they polymerize, become thicker, and cure more slowly. They can be thinned with a cyanoacrylate of the same chemical composition with lowerviscosity.[32]
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