Thread: Build Log light weight project
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Old Dec 20, 2012, 03:07 AM
quincross is offline
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Joined Dec 2007
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Quote:
Originally Posted by Charlee View Post
The point made in that forum was that CA was much easier to sand if you don't wait an hour or more before doing it. Someone then speculated that CA continues to get more brittle over time, which is not true to a significant degree. After 12 hours (usually less), CA will be as "hard" as it will ever be. Tests have shown that quality CA will loose about 15% of its strength after a ten year period.
Both CA and Epoxy are structural engineering adhesives - and I used to use both when researching adhesion technologies for the A380.

Epoxy (in general) has better shear strength and chemical resistance, CA is very weak in shear but (in general) forms has the broadest material bonding range (it sticks anything to anything).

What matters for us is that CA cannot bond well with glass - it only forms a mechanical bond rather than a chemical one. So GF with a CA matrix already has millions of tiny cracks around each fibre in the cloth that will ultimately grow into big ones and destroy the material. Great if you're sticking on a DLG tail that needs to be replaced after a period, really really bad if you use it for a DLG peg (high stress/shear area). So use the right technology for the job.

However, glass bonding aside, it all comes down to the individual chemical composition of the system employed - not all CA's and Epoxies are equal. The epoxide number is important for epoxy's. CA actually performs well in water, if phthalic anhydride additive is used. You increase the shear strength of both with the addition of a rubberising agent etc. Guess what I'm saying is that one is only easier to sand over the other, or degrades over time or cures this way.......for a particular brand of glue. Basically always use a professional system for critical jobs (i.e. not something from hobbystore/DIY shop shelf) and always, always download and view the datasheet from the manufacturer.

I'm pretty sure the degradation you mention for CA is down to chemical degradation (probably moisture ingress) in an unprotected formula. It's the same with Epoxy and other adhesives and 15% is a very low number in any practical modelling application and 10 years is a very long time for an airframe .....at least in my stable!

(PS. if you want some fun, put some decent quality CA on a cotton wool ball and run away - just make sure everything is fireproof and well ventilated first - never use CA on cotton )
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Last edited by quincross; Dec 20, 2012 at 03:36 AM.
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