Urethane outgassing

I am encapsulating LED circuitry in a PVC tube using a two-part, optically clear urethane adhesive. I am having a problem with the urethane outgassing during curing and bubbles permanently forming. This gives an unfinished look and has negative effects on the light output. The urethane is being cured at room temperature, but the problem also occured when I tried curing at a raised temperature. Are there any other methods or tricks I might try to get rid of the bubbles, or any other optically clear materials I could use for this application?

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Yep ... for that reason i would even avoid using a water aspirator for the vacuum, since it will bring water vapor into yer system... not much, but noit having it will still help. Do you have the possibility of doing the sealing in more than one steps? Thinner layers that won't entrap gas bubbles in their bulk?

Good luck from me too :-)


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There are some new UV curing LED encapsulants available; details on our website.


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Peter Swanson

Ooops, forgot this sig! Peter

-------------------------------------------------------- Peter Swanson snipped-for-privacy@intertronics.co.uk INTERTRONICS

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INTERTRONICS is dedicated to providing quality material, consumable and equipment solutions to high technology assembly industries, with the highest levels of technical support and customer service.

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Peter Swanson

The isocyanate portion of the two part cure will absorb a lot of CO2 from the atmosphere. Thus it helps sometimes to degas this portion(perhaps at an elevated temperature to lower viscosity, then cool down again) before mixing(the better urethane compounders will do this before they ship, but don't count on it). This will allow you to degas some without using up any of the work life, you will still need to degas the mixed product but much of the work will have already been done. If your application will tolerate a moderate reduction in physical properties, you may want to consider adding up to 10% of a low viscosity plasticizer such as butyl oleate or texanol isobutyrate(Eastman Chemical), make sure it is urethane grade, no free alcohols, low acid# & almost no water. This will aid degassing by lowering viscosity and extending work life. Make sure you check your stoichiometry. An excess of isocyanate has a tendency to "find" any trace water, react with it, and produce CO2 as a byproduct. More bubbles.-Jitney

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