Glues & Resins for Aircraft Building

Choosing glues and resins for building an airplane wasn't all that easy. Much depends on prior builder experience and on how you want to paint the ship when that time comes.
Originally I had a paint job in mind that consisted of a few dark colors (dark

greyish silver, a bit of red and 'royal blue'). The problem with these dark colors is that in bright sunlight, structures below the paint heat up quite a bit.

Structures heat up in sunlight

From my yachting experiences I know that in extreme situations, depending on surface color a structure can heat up up to 75-80 °C when exposed to direct sunlight. Because of this I initially considered glues that stood the test of time (Aerodux and others) instead of more modern epoxies.
Then I read the spec sheet of the Styrodur foam [95 KB] specified for the canard- and main wing.
The physical properties of this foam deteriorate above 75 °C. The Styrodur spec sheet says to use this material up to 75 °C only. Also remember that these specs were written with building construction in mind...
For aviation use all of this means danger zone when allowing the foam to heat up, so out went the planned darkish paint job.
Since a future paint job obviously needs to based on rather bright colors to keep temperatures within the structure low, this was a chance to consider more modern resins after all...

Epoxy resin glue

I've decided to use an epoxy resin produced by SP Systems. This product, called SP-106 [73 KB] , will be used to assemble the entire main structure.
This epoxy resin system is around for a few decades, is well tested and I have had the pleasure of using it for sailing yacht repair jobs. I know how it behaves. Fully cured, it is still somewhat more flexible than newer epoxy formulations that are intended for high modulus carbon fibre laminates. This higher flexibility makes it more compatible with lower modulus wood fibres, which - theoretically - should postpone wood fibre cracking directly adjacent to the glue line. A somewhat more gentle failure mode should be the result.
Since I don't like to rush anything worth doing ;o) , I'm using the slow hardener for all glueing/laminating jobs.

Glueing slabs of Styrodur before wirecutting

Glueing slabs of Styrodur or Jackodur together with epoxy resin is perhaps not a good idea, because the cured epoxy might wreak havoc when wirecutting the foam to shape.
The builders' manual specifies PPU-100, which is a polyurethane glue produced by International, a subsidiary of AKZO Nobel.
So far I couldn't source this product. I'll keep you posted.

UPDATE on PPU-100:

Wolfgang Feichtner, an Austrian boatyard owner/operator kindly informed me about PPU-100 not being in production anymore. This polyurethane glue was originally distributed in France only, due to health and environmental issues. Because of this International ceased production completely quite some time ago and apparently isn't offering a replacement product.
For us this means that if you still find PPU-100 on sale somewhere, it has been shelve-ware for a rather long time already. Avoid it!

Summa summarum: I now have to source and test yet another suitable replacement product... As before: I'll keep you posted.

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Glues & Resins for Aircraft Building