Written by Caleb Shaw
All adhesives have open times, varying from seconds (polyurea, cyanoacrylates) to permanent (pressure sensitive adhesives). In this blog, I’m going to explore how open times can affect you, and what you can do about it.
Most data sheets provided with any product will state an open time. This is the length of time from which you apply the adhesive to when the substrates must be joined together. If you go beyond this time, you are likely to see adhesive failure. This is known as ‘extension of open time’.
A key indicator that you have extended your open time is when the adhesive stays bonded to the substrate it’s applied to but comes away from the secondary substrate. Some MS polymer adhesives can skin over yet are still able to bond for a limited period – this is known as an ‘active skin’. (Hence the skin over time and open time may differ).
The open time of an adhesive is stated at a given temperature and relative humidity, but these can be manipulated and changed. For example, if you have an open time of 20 minutes at 23C, you will likely find that the more you exceed the temperature requirement, the quicker your open time will be. The same applies for a reduction in temperature – the lower the temperature in comparison to the requirements, the longer your open time.
Some adhesives (such as hotmelt) form a crystalline structure as they cure. If the bond is disturbed before the crystalline structure has formed, the final strength will be impacted.
If there is a risk of extended open time when using polyurethane or an MS polymer, you should use a triangulated nozzle. This will form a triangulated bean of adhesive which, if starting to skim over, will burst when the substrates are brought together revealing fresh adhesive at the bond interface.
If you are having any issues with your open time or require some advice, feel free to contact one of our specialists on the number above for some friendly, non-biased advice.
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