Cyanoacrylate

General uses

Cyanoacrylate is a tenacious adhesive, particularly when used to bond non-porous materials or those that contain minute traces of water. It is also very good at bonding body tissue, and while this can be a bothersome (or even dangerous) side effect during everyday use, it has been exploited for the benefit of suture-less surgery.

Cyanoacrylate glue has a low shearing strength, which has also led to its use as a temporary adhesive in cases where the piece can easily be sheared off at a later time. Common examples include mounting a workpiece to a sacrificial glue block on a lathe and also tightening pins and bolts.

Cyanoacrylates are often used to assemble prototype electronics (see wire wrap), flying model aircraft, and as retention dressings for nuts and bolts. Their effectiveness in bonding metal and general versatility have also made them popular amongst modeling and miniatures hobbyists. They are used to re-harden the boxes and shanks of ballerinas' pointe shoes as well.

Cyanoacrylate glue's ability to resist water has made it popular with marine aquarium hobbyists for fragging corals. The cut branches of hard corals such as Acropora can be glued to a piece of live rock or Milliput substrate to allow the new frag to grow out.

Standard cyanoacrylate adhesive is not recommended for use with glass, although there are special formulations which are more suitable. However it may be used in conjunction with glass fibre mat or tissue to reinforce joints or to fabricate small parts, reinforcing pins may also be cut from solid fibreglass rod.

When added to baking soda (sodium bicarbonate), cyanoacrylate glue forms a hard, lightweight filler/adhesive (baking soda is first used to fill a gap then the adhesive is dripped into the baking soda). This works well with porous materials that the glue doesn't work well with alone. This method is sometimes used by aircraft modelers to assemble or repair polystyrene foam parts. Note that the reaction between cyanoacrylate and baking soda is very exothermic (heat producing) and also produces noxious vapors.

One non-adhesive use for cyanoacrylate is as a forensic tool. Fumes from warmed CA can develop latent fingerprints on non-porous surfaces like glass, plastic, etc. The invisible fingerprint residues react with the CA fumes and atmospheric moisture to form a white polymer (polycyanoacrylate) on the fingerprint ridges and can then be recorded. The developed fingerprints are, on most surfaces (except on white plastic or similar), visible to the naked eye. Non-visible or poorly visible prints can be furthermore enhanced by applying a luminescent or non-luminescent stain.

Thin CA glue is also used as a wood finish, particularly among woodturners. Its fast drying time and glossy finish make it ideal for small applications which generally look best when glossy (such as pens), although it is messy and somewhat expensive. A common mistake made by novice users is to use an accelerator, which can cloud and thus ruin the finish.

Some rock climbers use cyanoacrylate to repair damage to the skin on their fingertips. Similarly, stringed-instrument players can form protective finger caps (in addition to callouses) with cyanoacrylates. Acetone (nail polish remover) removes it.

Superglue was in veterinarian use for mending bone, hide, and tortise shell by at least the early 1970s. The inventor of cyanoacrylates, Harry Coover, said in 1966 that a superglue spray was used in the Vietnam war to retard bleeding in the wounded, whilst they were being transported to hospital. As it can irritate the skin, the U.S. Food and Drug Administration did not approve superglue's civilian medical use until 1998 when a variant called 2-octyl-cyanoacrylate was developed. The latest generation of 2-octyl cyanoacrylates, such as SurgiSeal, have greater tensile strength, greater moisture and oxygen transmission to the wound, and do not require a catalyst/initiator. Superglue's use in the Vietnam War is referenced in the film Dog Soldiers.

Some glues, e.g. Super glue and Krazy glue, are believed to be 100% ethyl cyanoacrylate. Other glues may be a composition, e.g. 91% ECA, 9% poly(methyl methacrylate), <0.5% hydroquinone, and a small amount of organic sulfonic acid.

Properties

In its liquid form, cyanoacrylate consists of monomers of cyanoacrylate molecules. Methyl-2-cyanoacrylate (CH2=C(CN)COOCH3 or C5H5NO2) has a molecular weight equal to 111.1, a flashpoint of 79 °C, and 1.1 times the density of water. Ethyl-2-cyanoacrylate (C6H7NO2) has a molecular weight equal to 125 and a flashpoint of >75°C. To facilitate easy handling, adhesives made with cyanoacrylate are usually formulated so that the glue is more viscous and gel-like.

Generally, cyanoacrylate is an acrylic resin which rapidly polymerises 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 moisture in the air can cause a tube or bottle of glue to become unusable over time. To prevent an opened container of glue from setting before use, it must be stored in an airtight jar or bottle with a package of silica gel.

Another important trait is that cyanoacrylate sets quickly, often in less than a minute. A normal bond reaches full strength in two hours and is waterproof. Accelerators such as toluidine trigger setting in two or three seconds, with some loss of strength.

Acetone, which is commonly found in nail polish remover, is a widely available solvent capable of softening cured cyanoacrylate. Nitromethane is also an excellent solvent. Methylene chloride is the most effective solvent, but is toxic. Gamma-butyrolactone is also effective at removing cured cyanoacrylate, and has low toxicity.

Low temperatures cause cured cyanoacrylate to become brittle. Cyanoacrylate's bonds can be weakened (allowing disassembly) by placing a glued object in a household freezer for several hours. Opened containers of cyanoacrylate glue can also be delayed from prematurely setting by storing the containers in the household refrigerator.