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Facts & Questions
How Do Conventional CAs cure?
Cyanoacrylate adhesives are acrylic resins that polymerize or cure rapidly when exposed to alkali surfaces. Water, which is found in trace amounts on virtually all surfaces, contains hydroxyl ions that trigger the rapid polymerization, whereby CA molecules link up to form a very strong and durable thermoplastic material. CAs reach fixture strength, or the strength where assemblies can be handled without falling apart, very quickly – typically in 5 to 20 seconds depending on the assembly. CAs reach their full strength in 24 hours, often reaching levels of 2,000 to 3,000 psi depending on the materials bonded. Very dry or acidic surfaces can also be bonded using Loctite® “Surface Insensitive” cyanoacrylate products. Cyanoacrylate adhesives that do not contact a surface, but when left in open air will cure slowly due to atmospheric moisture. In this case accelerators can sprayed over the adhesive to rapidly complete the curing process.
How Do Light Cure CAs cure?
Light curing cyanoacrylates are uniquely versatile as they offer the benefits of a conventional cyanoacrylate as well as the benefits of a light cure adhesive. These products will cure as described above typically reaching fixture strengths in 5-20 seconds. They can also reach fixture strengths in 2-5 seconds with the introduction of the proper light source. Light cure CAs contain additives called photo-initiators which react with light of the proper wavelength and intensity to initiate the polymerization process extremely rapidly. Light that reaches the adhesive, either through a transparent substrate or an exposed bond line, allows manufacturers to tackle assemblies together literally in a few seconds, followed by the continued curing of the conventional CA in the bond line that was not exposed to light.
What is Blooming?
Blooming is a phenomenon unique to cyanoacrylates that manifests itself as a rainbow haze or whitish haze surrounding a bond line. Unreacted cyanoacrylate molecules can leave the surface of the dispensed adhesive and become airborne, only to later fall back onto the surface of the substrate. When they land, the molecules react with surface moisture and adhere themselves to the substrate. A small concentration of cyanoacrylate particles on the surface can be diffracted by light causing the rainbow effect, while higher concentrations can result in a whitish haze on the substrate. A number of options exist to minimize or eliminate blooming as listed below:
- Use Low Odor/Low Bloom Products - Designed with a heavier backbone molecule, thus having a lesser tendency to become airborne.
- Avoid Large Fillets - An adhesive fillet is where the adhesive is exposed to the open air and provides an open window for blooming to occur. Attempts should be made to keep the adhesive within the bond line.
- Increase Air Flow - Cross ventilation is useful in carrying airborne CA molecules away from the bond area. Avoid assembling parts and immediately placing in an airtight environment such as a bag or shipping container.
- Avoid Acidic Surfaces - Acidic surfaces slow down the CA cure mechanism, thus keeping the adhesive uncured longer. In the event of such a surface, either rinse to reduce acidity or choose a product and/or accelerator designed for such substrates (i.e. surface insensitive products).
- Avoid Extremes in Relative Humidity - If RH is too low, the adhesive can remain uncured longer. Similarly, if RH is too high, the curing reaction may occur too quickly, thus allowing for blooming due to the exothermic reaction. (40 - 80% RH is recommended).
- Avoid Extremes in Temperature - Elevated temperatures can drive off unreacted monomers from within the adhesive, causing a blooming effect.
- Avoid Vacuum Environments - A vacuum environment can pull unreacted monomers from within the adhesive.
- Avoid the Use of Old Material - Old product does not cure as quickly as fresh material.