Thermal insulation foams

logo_smallIt is always interesting to see how other look to the market you like to get. Lorna Gibson, famous MIT professor, gives a lot of courses about cellular solids. The course notes are public domain and are nicely organized in an MIT-website.

In this post, we discuss the notes about thermal insulation. The paper gives a list of thermal conductivities with solid materials like polystyrene, polyurethane, glass, with gases and also the resulting foams. It shows that glass foams have a higher thermal conductivity because glass itself has a higher thermal conductivity.

gibson2The paper discusses the different heat transport mechanisms and shows that foams with 1mm cell size have no contribution to convective heat transfer. The paper explains also why at too low densities the thermal conductivity increases. It seems that the contribution of radiation becomes too important.

As a consequence, it seems that not only the compressive strength but also the thermal conductivity induces an under limit on the density. It seems that a relative density of 0.04 or 100 kg/m³ density is an under limit.

Lorna Gibson

Prof. Dr. Lorna Gibson

Further improving cellular glass

logo_smallToday, the best cellular glass is for example produced by ZES FOAMGLASS. ZES 500 has an average thermal conductivity of 0.038 W/mK at a density of 110 kg/m³ and an average compressive strength of 500 kPa. Other thermal insulations can have thermal conductivities halve the above value. How can we improve cellular glass?

gibson1Lowering further the density is probably not an option. Even if we keep a fine cell structure, the compressive strength lowers quadratically with the density like shown in this graph from the paper from Lorna Gibson, famous MIT professor and cellular solids expert. Indeed, we can assume the compressive strength is linear with the Young modulus because glass breaks at a certain strain above the static fatigue limit, independent of the density of the cellular glass.

Besides the solid heat conduction in the glass, we have the gas thermal conduction and radiation. Radiation can  be improved by more walls absorbing and radiating or an improved cell structure.

Gas thermal conduction is sensitive to the weight of the molecules. If we assume that we have only CO2 in the cells with a carbonaceous foaming agent like carbon black or glycerin, we can improve with a receipe generating SO2. Indeed, CO2 has a thermal conductivity 0.015 and SO2 0.009 W/mK like shown here. But I doubt that an SO2 containing foam will be popular.

XPS-vacuumInducing a vacuum in the cells could be the solution. For example, open cell XPS, evacuated to 10 Pa has a large improvement like shown in the graph of this interesting paper of the EMPA institute in Switzerland. It looks simple but a vacuum of 10 Pa is not easy to generate in a fine cells foam. This low pressure is needed to get a situation where the mean free path of the gas molecules is a lot larger than the cell size.

But the graph shows that the thermal conductivity of XPS with air (0.031 W/mK) decreases to 0.007 W/mK with the vacuum. The difference = 0.024 W/mK is the thermal conductivity of air. If we apply this on a cellular glass (0.038), filled for 100% with CO2 (0.015), we arrive at the lowest thermal conductivity = 0.023 W/mK possible for cellular glass based on soda lime glass.

In principle, we can dream about the following material:

  • stable thermal conductivity = 0.023 W/mK
  • compressive strength = 500 kPa or permanent load = 250 kPa
  • absolute vapor tight
  • free from water absorption
  • non-combustible
  • extreme good ecology balance due to the use of waste glass without remelting
  • 150€/m³

I guess this nice dream is a nightmare for the other thermal insulation materials and after 70 years cellular glass, I doubt it is realistic. But it is maybe not impossible ….