A Thermal Super-Insulator
- Fig. 1: Cogelation with apple-derived pectin reinforces silica aerogels by forming an interpenetrating network of long pectin chains and silica nanoparticles.
- Fig. 2: Photograph of silica-pectin hybrid aerogels with increasing pectin content from left to right (top) and Transmission Electron Microscopy (TEM) images of a silica aerogel (bottom left) and a silica-pectin aerogel hybrid (pH 1.5, 20:100 pectin:silica mass ratio) (bottom right). In silica aerogels, silica nanoparticles link together to form a “pearl-necklace” structure with weak inter-particle necks. Cogelation with apple-derived pectin leads to a mechanically stronger, neck-free microstructure with thicker struts.
- Fig. 3: Thermal conductivity versus final compressive strength: the silica pectin aerogels exhibit the best combination of low thermal conductivity and high final compressive strength reported to date for any aerogel material.
Building insulation is the most cost-effective method to reduce our overall energy consumption and CO2 emissions. Unfortunately, the thermal conductivity of the most commonly used insulation materials decreased only fractionally over the last decades. As a result, better building insulation equates to thicker building insulation.
For new buildings, this poses strong design limitations and comes at a significant reduction in living area. For building renovation, the situation is even worse as the necessary space to install thick insulation layers often is just not available. Here, we present mechanically strong biopolymer-silica aerogel composites that have a thermal conductivity that is half of that of conventional insulation, allowing for a thinner insulation layer.
How to Engineer Better Insulation Materials?
Most insulation products are porous materials that conduct heat along three major pathways: through the solids around the pores, through the gas inside the pores and through radiation. Of these three, the gas phase conduction dominates. The most drastic way to reduce the thermal conductivity is to remove the gas completely by creating a vacuum, for example in vacuum insulation panels (VIP). This technology works and VIPs are now very common in high-end refrigerators. One drawback in the building sector is that they cannot be cut to size at the construction site. Another method to lower the thermal conductivity is to change the composition of the gas, for example in pentane-blown polyurethane foams. Unfortunately, pentane is flammable and the gasses with the lowest thermal conductivity cannot be used because they are either extremely expensive (such as krypton and xenon) or damage the environment (CFC’s). The third option to lower the gas phase conductivity is by making the pores extremely small, i.e. smaller than the mean-free path length of the air molecules. This limits the thermal conductivity because the gas molecules are more likely to collide with the pore walls than with each other.
Silica aerogels are light-weight, nanoporous solids with a large specific surface area, about a football pitch for 7 grams of aerogel (ca.
800 m2/g), and a thermal conductivity that is half of that of conventional insulation: 12-15 mW/(m·K) compared to 33 mW/(m·K) for mineral wool for example. They are synthesized through a sol-gel process in which colloidal silica nanoparticles gel together to a cohesive network. Their main application area is as pipe and reactor insulation, but with a growing niche in the building market. For many applications, silica aerogels are too brittle and fragile. Many different reinforcement agents such as polyurethane, polyacrylamide, polyurea or polyvinylpyrrolidone have been tested, but none of these managed to strengthen the material without simultaneously increasing the density and thermal conductivity.
Silica-pectin Hybrid Aerogels
We developed silica-pectin hybrid aerogels by dissolving apple-derived pectin, a common gelation agent in the food industry, in silicic acid followed by gelation, a hydrophobization treatment and finally drying from supercritical CO2. The pH of gelation determines the aerogel microstructure and at pH 1.5, the silica-pectin hybrid aerogels display a mechanically more stable microstructure compared to standard silica aerogel (Fig. 2). At this pH, the pectin is dispersed as molecular strands on the silica surfaces as shown by Nuclear Magnetic Resonance spectroscopy (NMR) and elemental mapping. The co-gelation with pectin and the resulting change in microstructure reinforce the aerogels: the hybrids are no longer brittle and can withstand a deformation of at least 80% without rupture. In addition, the final compressive strength increases more than ten-fold, and the materials are less dusty. Importantly, this strengthening comes with only a very small penalty in thermal conductivity: the silica-pectin hybrid aerogels have the best combination of low thermal conductivity and high mechanical strength reported to date (Fig. 3). Despite the high content of hydrophilic pectin and the open-porous network structure, the hybrid aerogels are hydrophobic: water droplets have a high contact angle to the aerogel surface and do not penetrate the material.
Aerogel Insulation Applications
The main application area of aerogels today is in industrial insulation, for example for oil and gas pipelines or for reactor vessels and piping at chemical processing factories. There is also a small but growing niche market in the building sector. Combined with the ongoing work on the cost reduction, the availability of more robust aerogels prepared from renewable resources will drive the growth of aerogel insulation in the construction sector. With their superinsulation properties, aerogels are particularly attractive in markets where the necessary space for conventional insulation is not available (building renovation) or where space is extremely expensive (inner city environments).
S. Zhao, W. J. Malfait, A. Demilecamps, Y. Zhang, S. Brunner, L. Huber, P. Tingaut, A. Rigacci, T. Budtova, M. M. Koebel, Strong, thermally superinsulating biopolymer-silica aerogel hybrids by cogelation of silicic acid with pectin, Angew. Chem. Int. Ed. 54, 14282-14286 (2015) – DOI: 10.1002/anie.201507328