Ask the Engineer

Light Cure Adhesives

By Christine Salerni Marotta

New and Revived: Your questions on adhesives usage answered by Christine Salerni Marotta at Henkel-Loctite.

 1. I noticed that there are now “visible” light adhesives. Can these be cured with traditional light cure systems?  

There are new visible light products on the market today that will only respond or cure with higher wavelength output curing systems. Most current light cure adhesives react with light in the UV (~ 250-399 nm) or low end visible (~ 400-405 nm) range of the electromagnetic spectrum. True visible systems require light above 450 nm and this is delivered by special systems.

2. How can I be assured that the light cure adhesive I am using is fully cured?
Unfortunately, there is no magic on-line signal to determine extent of cure . The best way to ensure complete and consistent cure results is to establish limits on process variables, part composition and adhesive quality through  thorough testing of parts assembled under various conditions when the application is designed, followed by regular monitoring of the critical parameters.

There are analytical techniques that can be used with some light cure adhesive technologies but such methods are not conducive to production line use.

3.  I used an instant adhesive to assembly my plastic components and noticed a white residue around the bondline?  What is this and how do I get rid of it?
Un-reacted cyanoacrylate molecules that are outside of a bondline can leave the surface of the dispensed adhesive and become airborne. These unreacted molecules are heavier than air and tend to fall onto the surface of the substrate. When they land, the molecules react with surface moisture and adhere themselves to the substrate. This phenomenon is referred to as “blooming” and is mainly an aesthetic concern. 

Some methods to reduce or eliminate blooming include: Using low-odor/low-bloom cyanoacrylate adhesives; avoiding large fillets/squeeze out of the adhesive; increasing air flow; avoiding assembling parts and immediately placing in an airtight environment such as a bag or shipping container; avoiding acidic surfaces; avoiding extremes in relative humidity and extremes in temperature; avoiding vacuum environments; and avoiding the use of old material. 

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