Instant Adhesives for High Speed Production Lines
Production Line Adhesives Need Precise Cure Times
I recently had a call from a gentleman trying to write a process for high speed production line. They had tested one of our cyanoacrylates (instant adhesives) and found it extremely fast. The adhesive developed strength so rapidly they planned to implement an inline packaging operation within seconds of bonding. The call came because he noted on the technical data sheet that the product took 24 hours to achieve full cure.
“So which is it INSTANT? Or 24 HOURS?” Well the answer is Yes and Yes.
Have you ever bonded your fingers with instant adhesive? You know it happens in an instant! Don’t try this – but if you left the bond alone for 24 hours it would be even stronger.
Instant Adhesive Cure Time vs. Handling Strength Explained
If you bond something -other than your fingers please…say a steel coupon, with instant adhesive, then test that sample on an Instron a few seconds later – you will see that the bond is strong. If you bond another steel coupon and clamp it into the Instron 24 hours later the bond would be even stronger. Then try one more – bond the coupons and let them sit for 48 hours – you’ll have the same result as you did at 24 hours. So we know that strength continues to develop for 24 hours. This doesn’t mean you have to sit and look at it for 24 hours. If sufficient strength is achieved for handling, the cure can and will continue while you are not looking.
Production Line Adhesive – Strength, Depth of Cure and Tack Free Time
In other cases the production line adhesive strength development is not as important as the tack free or skin over time. For example MS Polymers can take 72 hours to cure, however, skin over time is much faster. Cure times with moisture cure products are often listed in terms of depth per hours. But again, you don’t need to sit and watch. Identify strength required to handle or identify tack free time and use that as the determining time for the next process to occur in your manufacturing facility.
Production Line Adhesive –Additional Strength Development
Another phenomena in strength development occurs with any polymer created. For example UV curing adhesives are fully cured quickly with high intensity UV light. They do not continue to cure once the light is removed but they continue to develop strength after they are fully cured. Keep in mind, that unless there is some secondary cure mechanism like anaerobic, or moisture, UV curable adhesives ONLY cure while exposed to UV light. So why do they continue to develop strength? The slight strength development that continues to occur for up to 24 hours can’t really be called curing – but strength continues to increase. All those polymer chains just need some time to relax into the proper place.
Production Line Adhesive Quality Control
Full strength testing should be done for the production line adhesive after the manufacturers listed cure time. Then correlative testing can be done via an inline QC test. For example if full strength is 3000psi after 24 hours – what does that correlate to at your QC test time?
When it comes to packaging a product that has not completely cured, it is good to note that depending on the type of production line adhesive, different by-products of curing can be given off. It is a good idea to test the packaged product to ensure sensitive plastics or electronics will not be adversely affected if packaged prior to complete cure.