Cyanoacrylate adhesives (CNAs), also known as ‘‘Superglues" or "Krazy Glues", are single component liquids that exhibit rapid cure and excellent shear strength. Although very similar in curing characteristics to anaerobic adhesives, cyanoacrylates are much more rigid and less resistant to moisture. This class of adhesives is available as low-viscose liquids that cure in seconds at room temperature. Like many one-part urethane adhesives, they require moisture to initiate polymerization and are applied in thin bondlines. In fact, a very thin layer bonds more effectively than a thicker one that might not cure properly.
Cyanoacrylates bond many different substrates, including metals,
plastics, and glass. Their general versatility
and fast cure have made them very popular adhesives despite heir relative high cost.
However, they have poor heat and moisture resistance, do not fill
gaps, and have
relative low peel and impact strengths. Another major drawback is
their short shelf life (typically one month once opened, but one year
unopened). Recently, a new generation of cyanoacrylate adhesives has
been developed that has overcome some of these problems. These
adhesives have higher shear strength and improved toughness, creating a more impact resistant bond.
To facilitate easy handling, cyanoacrylate adhesives are
frequently formulated with thickeners such as fumed silica to make
them more viscous.
Commercially important cyanoacrylate monomers include methyl 2-cyanoacrylate, ethyl-2-cyanoacrylate, n-butyl cyanoacrylate and 2-octyl-2-cyanoacrylate. The majority of super glues are either methyl or ethyl cyanoacrylate based. When bonding metals and other rigid polar surfaces, methyl cyanoacrylate is usually prefered because it forms stronger bonds and is more impact resistant than ethyl cyanoacrylate, whereas ethyl cyanoacrylate is preferred on rubber and many plastic substrates. Octyl cyanoacrylate was introduced to address (minor) toxicity concerns and to reduce skin irritation and allergic response. This monomer is used in medical, veterinary and first aid applications.
Cyanoacrylates rapidly polymerize in the presence of water, to be more specific in the presence of (trace amounts) of hydroxide ions. For this reason, super glues typically contain acidic stabilizers to prevent premature polymerization. Any slightly alkaline surface with trace amounts of adsorbed water will first neutralize the acidic stabilizers in the adhesive and then initiate rapid polymerization, whereas acidic surfaces may delay or even prevent curing.
Since the presence of trace amounts of moisture causes cyanoacrylates to cure, exposure to normal levels of humidity in the air causes a thin skin to start to form within a few seconds. This greatly slows down polymerization. For this reason, cyanoacrylates have to be applied thinly to ensure that the reaction proceeds rapidly and a strong bond is formed within a reasonable time.
In summary, cyanoacrylates are normally used where there is minimal environmental stress and fast setting times are required.
Cyano acrylates are sold by many adhesive companies. Two well known trade names are Super Glue® and Krazy Glue®. Other important brands are Eastman 910® (Permabond) and Loctite 4XXX® (Henkel).