Cyanoacrylates

Cyanoacrylates are extremely rapid curing adhesives known as instant adhesive or Super glue. These adhesives were discovered by accident by Dr. Harry Coover´s group at Eastman Chemical Company while doing some basic research on characterizing certain polymers derived from a chemical called ethylene.

A cyanoacrylate adhesive is a very rapid curing adhesive also from the acrylic family tree, but having a completely different cure system. Cyanoacrylate monomer is made from a complex chemical process. The monomer produces a very reactive polymerization. The reaction or polymerization process is stabilized and the monomer kept in the liquid state by the addition of a small amount of an acid stabilizer material.

When a drop of cyanoacrylate adhesive is put on the surface of a part, the acid stabilizer molecules react with the water molecules present on the surface of the part from the relative humidity in the air. The reaction of the water and acid causes the acid stabilizer to be neutralized. The cyanoacrylate molecules then react with each other and form polymer chains without crosslinking.

The amount of stabilizer molecules in cyanoacrylate is very small--measured in parts per million--and very little moisture molecules are required to cause rapid polymerization. Cyanoacrylates begin to form polymer chains immediately on contact with the water vapor on the surface of the part. If parts are moved during initial contact, the polymerization process and polymer chains are stopped. The process must start again at a new catalyzed site.

Cyanoacrylate adhesive can be made from different acrylate monomers, such as methyls, ethyls, butyl, isopropyl, etc. These molecules differ in size and adhesives made from them exhibit different physical properties. Methyls are the smallest molecule and seem to work best on metal and rubber parts while ethyls work best on plastic parts. Many modifications can be made to the monomers to alter or improve their properties as adhesives. They can be toughened with rubber such as in Black Max"!, thickened such as Quick Gel™ or formulated to have low odor, resistance to thermal cycling, or less sensitivity to surface conditions which tend to stabilize the adhesive and slow down the cure, such as in the PRISM™ series of products.

Cyanoacrylate adhesives, also known as "super glues", are useful around the home in the bonding of common substrates (e.g., dishes, toys, etc.), which can take place in a matter of seconds. Yet their usefulness is limited when bonding wood because the cure time (the time it takes for an adhesive to undergo polymerization and become capable of holding the two substrates together) in this instance is much longer. Cyanocrylates tend to be brittle thus they are vulnerable to impact and dramatic changes in temperature.

To reduce these shortcomings, small amounts of finely ground rubber has been used as filler. The rubber introduces flexibility thus reducing brittleness. In addition, cyanocrylates are attacked by polar solvents. Polar solvents will weaken cured cyanocrylate bonds over time. Therefore, applications involving water, alcohols, or other polar solvents should be avoided. Polymerization is the reaction of small molecules combining to form very large molecules.

The reason for the longer cure time in the bonding of wood has to do with chemical reactions (between adhesive and substrate) that affect the rate at which the cyanoacrylate molecules polymerize. A typical cyanoacrylate product is a low viscosity liquid that readily flows over a substrate's surface. When the cyanoacrylate molecules encounter a basic environment, the small molecules within the adhesive polymerize.

Many common surfaces tend to be basic, so a cyanoacrylate adhesive works fine in these instances; wood, however, tends to have an acidic surface, and because an acidic environment will inhibit polymerization, cure time is increased.

Cyanoacrylates are not appropriate for the bonding of the steel parts of an automobile for example, because of the environments that the car will be exposed to. Those environments include such things as rain, variations in temperature, exposure to solvents (such as gasoline, oil, and windshield washer solution), ozone, acid rain, salt spray, and ultraviolet light from the Sun.

Sources/Properties:
Ethyl Cyanoacrylate adhesives cure through reaction with moisture held on the surface to be bonded.
They need close fitting joints and usually solidify in seconds.
Cyanoacrylates are suited to small plastic parts and to rubber.
They are a special type of acrylic resin.
It works best on smaller surfaces, using a very small amount of glue.
It bonds instantly with a colourless and transparent bond that is very strong (except for shear forces). Originally for non-porous surfaces, gel versions are now available for porous surfaces.

Common Uses:
Cyanoacrylates are very rapid curing and provide high bond strengths on plastic and rubber materials. The versatility of these adhesives make them highly useful in all industries.
Some of the larger application areas are in electronics for printed circuit board wires and components, and in medical technology for disposable plastic medical devices.
Other OEM applications exist in the toy, small and large appliance, automotive, and cosmetic packaging. MRO applications exist in all industries for repair of all rubber and plastic parts as well as some metal parts.
The consumer market is a large volume user of cyanoacrylate adhesives for repairing everything in the home from wallpaper tears to broken toys to torn and false fingernails.