Composite Material Characterization

The composite material was put together by mixing the commercially available hollow glass microspheres with silicone prepolymer in planetary mixers. The figure below shows the fabrication process in snapshots.

After mixing in the planetary mixers, the bead/silicone composite mixture was poured into large Petri dishes and thermally cured. Consequently, these casts were in the shape of discs.

Next, we characterized these discs in a specially built setup shown in the picture above. The composite disc was positioned on an aluminum table that itself was positioned to wade inside a mixture of water and ice at the bottom of a pressure vessel, to stand for freezing water or very cold seawater. A heater element was positioned on top of the composite disc and further insulated at the top with additional spare discs of composite. The wire from the heater element was run through the hatch of the pressure vessel through an air-tight port to an electronic controller box that supplied voltage and current to the heater while also measuring the temperature of the heater. The control box was set to maintain 37degC to mimic the external temperature of the human body. As heat passed from the heater through the test sample disc into the aluminum heat sink, the voltage and electric current readings on the control box would indicate the power dumped into the heater, over a fixed temperature difference, when a steady state was achieved. This information was then sufficient to calculate the thermal insulance of the sample.

With the lid closed and secured, pressure was applied to the pressure vessel from an air compressor, so that the elevated air pressure would simulate hydrostatic pressure at depth. This allowed the measurement of thermal insulance as a function of applied ambient pressure, simulating depth in seawater. These measurements were done separately for both the composite sample and a sample of commercial neoprene.
 

In the figure above, the empty-circle datapoints show the results from a piece cut out from the chest section of a commercial 8mm AquaLung neoprene suit. The neoprene degrades its insulance with ambient pressure as the bubbles shrink. In contrast, the solid-black datapoints show the results from measurements of our composite containing hollow glass microspheres embedded in silicone, demonstrating that the composite material retains virtually all insulance with depth.

The above comparison was fair, because a thick sample from an actual top-of-the-line commercial diving suit was used. The results showed that while the two materials were comparable at sea level, the neoprene insulation rapidly degraded with depth, while the composite insulation remained about the same.

The source of thermal protection was the insulating properties of the air inside the glass of the microspheres. The thermal resistivity of air was much higher than that of glass and silicone. Hence, the more air could be packed in the composite, the more protection would be achieved. To measure and study this performance, we prepared a series of composite discs at various concentrations of microspheres. We then measured them in the setup described above, but the measurements were done at atmospheric pressure to save time and because we already knew the material would not shrink significantly. Because the thickness was varied and known, instead of calculating the thermal insulance (a property of the sample), we calculated the resistivity (a property of the material), which is obtained by dividing the measured thermal insulance by the measured thickness of the sample. The results are shown in the figure below.

These results confirm that when used at maximal volumetric density, the composite material is comparable in thermal resistivity to state-of-the-art top-of-the-line neoprene. This is very important in several ways. First, neoprene protection worsens with increasing depth, so the composite material only gets better comparatively with increasing depth. Second, while neoprene is limited in maximal thickness by the need to be flexible enough to wear, the composite is not limited in thickness in the same way, since the intention is to use segmented pieces of composite anyway. Hence, even at the same resistivity as neoprene at the water surface, the composite can be deployed at greater thickness than neoprene, and so the composite will have superior insulance even at zero depth and would only get better the deeper the diver descends in the water.

The above results also show that the practical volumetric density achieved is up to around 55%. This result makes sense because the microspheres are not uniform and not arranged perfectly in a lattice. Even if those two conditions had been satisfied, the resulting best lattice would have been the FCC (face-centered cubic), whose maximal volumetric density would be around 72%. Since the composite microspheres are not uniform and cannot be arranged perfectly at the microscopic scale using macroscopic techniques, a practical limit of 55% is both reasonable and still very useful practically. This number is also a good design parameter to keep in mind regarding practical samples.