Glass bubbles

Not many recipies are found when it comes to filling honeycomb cores with glass bubbles thickened resin. R&G provides here a list of reciepies for usual additives. They advice to use 30 g of 3M 0.2 g/cm³ glass bubbles for 100 g of resin. We will denominate this as 30% fraction from here on, though officially the mass fraction is only 23%.

The recipie of R&G is too thick to fill the small channels of a 3.2 mm honeycomb core. As a result, air is trapped in the core weakening it. Take, for example, this carbon sandwich:


Using less bubbles provides a better mixture, which fills easily the channels. However, less glass bubbles means also more weight. So what is the best mixture?

To answer this question, we made some systematic tests with a low-viscosity resin from HP-Textiles designed for vacuum resin infusion (HP-E300RI). Four different fractions were tested:


Cross sections show a separation of the mixture into phases of pure resin and glass bubbles:


This means that the mixture was oversaturated. Pure resin does not only add unnecessary extra weight, but also tends to have additional shrinkage leading to marks on the finished surface.

An estimation of the volume fractions and extrapolation delivers the optimal amount of glass bubbles (about 18.5%):


Having a figure for the optimal recipie, we made a further test in which the effect of adding a thixotropic agent (1.5% fraction) was investigated:


It looks that we found the best recipie!

With a good feeling, we filled the core of Schneewittchen:


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