Another book for your library: Cellular Solids, Structure and Properties

logo_smallThis book (ISBN: 9780521499118) at a 2nd edition is written by Lorna J. GibsonMassachusetts Institute of Technology and Michael F. Ashby from the University of CambridgeThe last author is famous by the following statement:

When modern man builds large load-bearing structures, he uses dense solids; steel, concrete, glass.When nature does the same, she generally uses cellular materials; wood, bone, coral. There must be good reasons for it.

The book contains much more than thermal insulation and gives a theoretical foundation for the elastic properties of foams. These properties are used in cellular glass engineering and process calculation.

I learned that the stress relaxation behavior does not depend on the geometry / density of the cell structure but solely on the basic material, glass in this case. But the Poisson ratio depends completely on the geometry and is 0.33 for an open cell structure with regular round cells, while it is 0.28 for solid glass.

Both facts are important for the calculation of the annealing temperature curve for cellular glass, already one reason to buy this excellent book.


Waste glass conversion to cellular glass was already hot in 1977

logo_smallAlmost 37 years ago, a report about the production of foam glass from waste glass was published. This report describes three foaming systems:

  • After water absorption by the waste glass powder
  • After mixing the waste glass powder with carbon (black)
  • After mixing the waste glass with milled limestone

The report learns that up to 6% water can be absorbed in waste glass by using an autoclave and the addition of NaOH in the water. This glass can be foamed but only small pellets are possible. The glass with 6% water has a low viscosity and during reheating, we have steam as the driving gas for the foaming. When the water has left the glass, the viscosity increases and the foam freezes.

But also carbon black can be mixed to the waste glass. In this case, also bentonite is added to able to form green pieces. This clay also seems to reduce the water absorption of the foam. Between the carbon blacks we find exotic ones and the ASTM defined ones, produced mainly in China. The paper assumes that the absorbed gas in the carbon is inducing the foaming, while we know that the sulfate in the glass is also important (Demidovich p12). Last but not least, the importance of a reducing atmosphere is mentioned.

CaCO3 was also mixed with the glass together with again bentonite. It was found that milled limestone gives the best results and larger dimensions are possible. A thermal conductivity as low as 0.052 W/mK is reported with this recipe at 160 kg/m³. Controlling temperature and foaming agent allows to foam a closed cell structure.

The report is in favor for the carbon black method but gives only thermal conductivity measurements for the CaCO3 system. Personally, I think that this last system deserves more attention.

Preparation of the foaming powder

logo_smallThe production methods, mentioned in this block are examples of the powder method. Indeed, the glass is ground to a powder, a foaming agent is included and the powder mixture becomes sintered later on at about 650°C. At this point, we have glass with a built in foaming agent.

Grinding of the glass is an important part of the production process. For the grinding of glass and other brittle materials, we have a lot of different possibilities but the old fashioned ball mill is the most popular one. Engineers will study and choose between the different equipment to find the most efficient ones. Scientists will more try to change the glass surface in the direction of more efficient grinding. This blog is written by a scientist and for that reason, the concept grinding aid is mentioned.

The readers, hitting the “grinding aid” link, have download a nice (but old) paper about grinding additives. The paper learns that

  • Only about 1% of the energy in a ball mill goes to actual surface creating.
  • 20 kWh/ton is a typical energy consumption.
  • Less than 1% carbon black improves the grinding of cement with 30%.

Carbon black is already mentioned in this blog. It is used as a foaming agent for glass because it is very fine carbon, which can be well distributed over the glass surface. Because it is also a grinding aid, it makes sense to add it already in the ball mill.

In the gravel methodology, it is the common practice to grind the glass without any additive and to mix the foaming agent with the glass powder afterwards with special equipment like for example Eirich mixers.

Some people doubt that a ball mill can  mix with the same quality as an efficient mixer but others mix in the ball mill and do not invest in these mixers. I am sure about one thing: this knowledge is the key to a superior cell structure like for example the Neoporm ware shows to us. Mixing and grinding is still 90% empirical knowledge, which means that the solution is hard laboratory work.

Foaming agents which are also effective grinding aids are favorites in this business. That is probably the reason why carbon black was hard to replace.

Composition, synthesis and properties of foam glass obtained from waste container glass

logo_smallThis paper, about a year ago published is another proof that the race for the best cellular glass, based on waste glass without remelting is really going on. The research is done at the university in Sofia in Bulgaria. Indeed, East Europe is doing a lot of effort for cellular glass today.

The paper reports on the use of glycerin and water glass for the foaming of the container glass. What do we learn?

  • They start from 6000 cm²/g glass powder while another source mentions 8000cm²/g.
  • Glycerin is used as foaming agent.
  • Also water glass is needed although the role is not clear.

But they also report a thermal conductivity of 0.02 W/mK. I doubt that this is possible and for that reason, I have my doubts about the measuring system C-Therm TCI. These dynamic cheaper systems are popular these days but BELGLAS advises to work with the stationary systems for thermal conductivity measurements. In a future blog, we will report again on that subject.

I am looking forward to measure a real 0.042 W/mK on a cellular glass plate, foamed from not remelted waste glass. In my opinion, that is the way to go.

The inverted roof reinvented

logo_smallThe inverted roof has its water proofing membrane under the thermal insulation. On the contrary, a cellular glass roof has in principle this membrane on top of the thermal insulation. For example GLAPOR cellular glass blocks are “swimmed” in hot bitumen to get a vapor (and water) tight system. This is nicely shown in the following YOU TUBE movie.

And indeed, the water proofing membrane is the weak point in the construction. UV-light and large temperature variations are causing the rather short life time of the membrane. In case of a leak, not visible inside, freeze and thaw on the cellular glass will damage irreversibly the roof in a few months over a large thickness.

On top of that, some cellular glass insulation becomes rather expensive when a large thermal resistance is requested, which is the standard today.

The inverted roof with XPS does not know these problems. Current producers are DOW, BASF and KINGSPAN. But this roof has another possible problem. During rain fall in the winter, cold water may reach the water proofing membrane, cooling the structure under the water membrane proofing. This may induce condensation inside if the structure is not very heavy. For this reason, BASF, synonym for German quality, requests to have a minimum thermal resistance  of 0.15 m²K/W.

This thermal resistance is already availbale with 1cm GLAPOR PG700 cellular glass but 4cm is needed to have enough mechanical strength for installing. For just 6€/m² extra material cost, you have the best of both worlds.

  1. An absolute protection against humidity problems inside
  2. A very well protected water proofing membrane
  3. A huge thermal insulation with cheap XPS without any problem possible

6€/m² or less than 600€/m² for a 200m² house (one stock) is giving you absolute protection against humidity with a cheap very well insulating inverted roof, with its never to replace water proofing membrane.

Indeed, this is the inverted roof reinvented or the best of two worlds: CG + XPS (CG = cellular glass; XPS = extruded polystyrene). It is the conviction of BELGLASCZ that the combination of different thermal insulations is giving you the best systems.