What are instant adhesives?
Cyanoacrylates or instant adhesives are one-part chemistries that cure within an instant at room temperature to provide high shear strength. Instant adhesives provide more simplified processing which offsets the limitations of performance like shallow depths of cure and poor peel strength. Light-cure acrylics cure in seconds upon exposure to sunlight and offers good strength properties but this adhesive is only used when one substrate can transmit light through to the bond line. Silicones have longer cure times but they have less cohesive strength; however, they are able to maintain their properties over a range of temperatures and offer excellent resistance to environmental conditions.
The importance of surface preparation
Whether structural or instant adhesives are used, surface preparation is critical to ensure that a lasting adhesion is achieved. Bond strength is determined by the degree of adhesion between the substrate and the adhesive but if there is surface oxidation or rust, it can impede adhesion. Oxidation and rust must be removed. Metals like carbon steel are often treated with oil and other rust preventatives but they have a negative effect on the bonding process which requires a preliminary cleaning process. However, there are certain adhesives that can provide excellent bond strength even with the presence of contaminants. Joint failure is usually a result of inadequate surface cleaning and poor design and not adhesive strength. Often the applicator that makes a choice for the adhesive makes a poor selection of the adhesive material based on the operating environment.
Advantages of instant adhesives
90% of instant adhesives that is currently used for general purposes are ethyl cyanoacrylate with the remaining percentage as methyl cyanoacrylate. The common characteristic is that they become rigid and unyielding when dried. Glass transition temperature is one of the important properties of cyanoacrylate which is the temperature at which the adhesive becomes malleable. This temperature is governed by the flexibility of the side monomer groups; the more flexible the group the lower is the temperature. This is the characteristic that is often used to distinguish polymers that behave similarly such as ethyl and methyl cyanoacrylate.
When buying super glue from the stores, they are in small tubes with finely tipped, thoroughly sealed nozzle. What is inside every tube of super glue is the monomer of either ethyl or methyl cyanoacrylate supplemented by various chemicals in miniscule amounts that will prevent the instant adhesives from curing before the seal is broken. In industrial applications, chemicals exist that are added to the reaction to halt its progress.