Cyanoacrylate is an instant adhesive with three (3) principal chemical components, namely: methyl, ethyl, and butyl. Cyanoacrylate is generally a clear liquid with a viscosity from 3 cps to 2000 cps. The higher viscosity cyanoacrylates are referred to as “gap-filling”. There is also a cyanoacrylate, called Black-Max, with a rubber component to provide elasticity. Bonding strength is 15 Kg per cm².
The polymerization of cyanoacrylate is a reaction which occurs when the adhesive comes into contact with humidity. Polymerization reaction starts at the surface of the adhesive toward the center of the deposit. In order to have good adhesion (bond) between two sufaces, the deposit of the cyanoacrylate must be thin. For example: If a large glob of glue is dispensed on a flat service, it will stay in liquid form because only a very thin layer of adhesive contacts the surface and will polymerize.
Blooming may occur when a large amount of cyanoacrylate remains in liquid form. Some of the liquid material may vaporize and react with humidity. The result is a polymerization, which appears as a white dusty marking on the surface near the adhesive. To solve (or reduce) blooming:
- Ventilate to eliminate vapor
- Use an “activator” to polymerize at a faster rate
An activator is generally a liquid which is volatile and can be applied before or after the assembly of the two parts being joined. When using an activator, the total polymerization process occurs in seconds and not over a longer period if relying on humidity alone to propagate the polymerization. The negative effect when using an activator is that it slightly reduces the strength and adhesion of the bond. Other conditions that can affect the bond strength and rate of polymerization are:
- Too great a clearance between the two pieces requiring an excessive amount of material
– Solution – use gap-filling material or activator
- An irregular surface, which provides limited surface contact between the two parts being joined
– Solution – use a higher viscosity or gel-type cyanoacrylate
- Room air is too dry
– Solution – control the level of humidity or use an “activator”
The materials to be bonded can influence polymerization. Since Teflon is a material that “repels” humidity on its surface, it cannot be glued with cyanoacrylate. This is a reason for using Teflon-lined tips when dispensing cyanoacrylates.
Polyethylene and polypropylene are also materials which cannot be easily glued with cyanoacrylates (only with a special primer).
Cyanoacrylates generally become hard and brittle after curing. For this reason, cyanoacrylates should not be used to bond materials in cases where:
- The two materials have a different coefficient-of-expansion due to temperature changes.
- The assembly will be exposed to water.
- The assembly will be exposed to vibration.
Cyanoacrylates are generally shipped in 20 gram, 50 gram or 500 gram high-density polyethylene bottles. Gel cyanoacrylate is generally packed in tubes because of its viscosity. Placing the bottles into a refrigerated environment will eliminate moisture and extend the life of the fluid.
Part Two of our Instant Adhesives Briefing will discuss some Cyanoacrylate dispensing methods. In the meantime, you can view them here.