SEARCH. The aerogels of silica, these materials are very lightweight and super-insulating, are the subject of many works, but their fragility is an obstacle to a practical use. The development of aerogels derived from starch, more resistant and totally green, could solve this problem. Interview with Lucile Druel, phd student at Cemef-Mines ParisTech, specialist of the question.
The aerogels are materials that offer great prospects in terms of isolation : it is acted in fact of gels in which the liquid is replaced by air, which results in an extremely lightweight material with thermal properties remarkable. Lucile Druel, a young researcher at the Centre for Implementation in the Form of Materials (Cemef), Mines Paris-Tech, explains : “Their thermal conductivity is lower than that of the air, hence their belonging to the category of super-insulating. Thus, one may consider using these materials in the thermal insulation of buildings“. However, the silica aerogel has a big disadvantage : it is extremely fragile. The idea is to use bio-aerogels made from natural polymers, such as cellulose, pectin or starch.
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Very isolating, but still too sensitive to moisture
Dr. Tatiana Budtova we had revealed, to 2015, the secrets of the aéropectine end of citrus zest. Today, his phd student discusses the work carried out on the starches of peas and corn : “The aerogel-starch is different from that of pectin. But it is also a super thermal insulator, which presents the same problem : it is very hygroscopic, that is to say, it takes moisture, although a little less sensitive. In contrast to the silica aerogel, it is not fragile at all, and its thermal insulation is slightly more low“. The coefficient of thermal conductivity is estimated to be around 0,021 W/m. K, where the air is 0.025. Rock wool and polystyrene are much less effective, evolving to be 0.035 W/m. K.
The work are therefore to improve the characteristics of these biomaterials. Lucile Druel list the possible changes : “The composition of the starch varies depending on whether it comes from peas, or corn, with varying proportions of amylose and amylopectin, the two ingredients of the polymer. The corn starch contains, for example, a lot of amylose. It offers good performance, but that remains lower than that of pea starch“. She continues : “It is possible to use modified starch chemically, but its dissolution will become very difficult. The goal would be to functionalize the polymer chain or cover it with a coating to make the material more hydrophobic“.
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- Soon an insulation with a base of citrus zest
Another line of research, the improvement of the thermal performance, this time by modifying the manufacturing process : the starch is first dissolved in water at certain conditions of pressure and temperature, under mechanical agitation to disperse and break up the grains. The solution is then placed at 4 °C for the stage of “demotion” in which the gel is formed, and of which the duration can be changed. In this gel, the water is then replaced by a solvent, during a drying step supercritical. Again, it would be possible to substitute the acetone to ethanol used, which would alter the characteristics of the final product. “There is an ecological aspect : the ethanol, or acetone are certainly flammable, but not hazardous to the environment. The process is, therefore, bio-sourced and green“, we learn from the phd student. In the final step, the solvent is replaced by air.
However, the research is now halted, because of lack of funds. The project has not been selected by the French National Research Agency. However, the applications in the thermal insulation could be interesting. Lucile Druel assures us : “Everything can go very quickly, in 1 to 2 years, as has been the case for aerogels of silica, which are passed from the laboratory to the industrial scale“. All that the competition does not remain inactive : a laboratory of Hamburg (Germany) will also work on the issue. The investment partners or financial industry will therefore be crucial to develop bio-materials, super-insulation.