Other applications for cellular glass

logo_smallThe first application of cellular glass was a floating device. During the war, harbors must be protected with curtains against submarines. The Germans had occupied Portugal, the major source of cork and closed cell cellular glass was the best alternative.

Later on, cellular glass became popular as a thermal insulation and this is today the real market. But it can also used as an acoustic absorber, today sold under the name REAPOR®. An acoustic absorber avoids echo, which is for example always a problem in open offices.

A less known application is to use it as kinetic energy absorber. Kafka, the famous Czech writer invented the principle of a helmet during his work for an insurance company. But a helmet can not avoid that the brains have an internal recoil, although your head is well protected. This recoil is a consequence of the elastic behavior of the helmet.

But if this helmet should have been constructed from cellular glass, your head will crush the cellular glass but there will be no internal  recoil and brain damage would be less likely. The use of cellular glass as a kinetic energy absorber was already published in  US2981317 patent of 1961 about safety seats at line 49. The specification non-elastic crushable is clearly given on line 46.  The same is needed when a Formule 1 car looses control and hits the tires along the road. The recoil of the brains of the pilot can be the reason of medical problems afterwards although the outside damage can be relatively small. I am curious to know   what density the cellular glass needs to have and which thickness is requested to let stop a car at 120 km/h without even a headache  for the driver. I guess we will find reasonable values. The calculation can not be difficult: the kinetic energy of the car is used to crush the cellular glass = thickness * compressive strength * cross section of the car.  The maximum allowed deceleration gives the compressive strength you need. Solution will be given in a next post.


Thermal conductivity measurements

logo_smallAlthough cellular glass can have many applications, 99.999999% is going to thermal insulation. A good bench-marking of thermal insulation needs good equipment to measure and well trained operators. In this post, I give a short introduction with emphasis on the assumptions, which are made when a certain thermal conductivity is published.

Quality control (QC) needs a fast but reliable measurement for a decent quality assurance (QA). Indeed, the more measurements are done, the better the distribution of the thermal conductivity values around the average can be described. This becomes awarded with an improved declared thermal conductivity.

A very fast equipment is the hot wire method. This transient method is measuring in fact the thermal conductivity and volumetric heat capacity. The method is sensitive for only a small area of the sample (a few cm³) and in that way very sensitive to inhomogeneities and contact resistance. For a cellular material with cells on the scale of a mm, this looks not the appropriate method for the QA of cellular glass. On the other hand, it looks like a fine system to gather the thermal data needed to calculate the annealing curve.

TaurusThe heat flux meter method (HFM) is a more reliable method because the sensor measures typically up to 30 x 30 cm from the sample and the full thickness is taken into account. Typical samples are measured in a few hours with an accuracy up to 0.5%. The system uses a heat flow sensor, built as a many serially connected thermocouples. A sample is installed between a hot and cold sink. The sensor is mounted on the cold or hot sink or both. Because some heat leak to the outside can not be avoided, the system needs to be calibrated. HFM systems built in a cabinet with a stable and controlled temperature are not dependent on the laboratory conditions and have my favor. They need less calibration and more time is available for the actual measurements.

Taurus and Netzsch have both reliable systems, when properly calibrated, need about 2 hours to measure a 4 cm thick sample. Both give confident measurements in a bench marking but I would also put some attention on the robustness of the equipment. Measuring cellular glass can be very hard for this kind equipment due to the scratching surfaces and the unavoidable dust.

The quality of a HFM-measurement depends strictly on the quality of the calibration procedure and so on the quality of the calibration samples. It is well known that a calibrated sample changes slightly after each measurement due to thickness changes. Each confident cellular glass producer should have access to a system, which allows a perfect absolute measurement. In that way, confident calibration samples can be used.

NetzschThe Guarded Hot Plate method guarantees that all the measured heat flows through the sample and does not escape to the outside. This is done by working with a guard ring, kept on the same temperature as the actual measuring range. This system needs much more time to measure one sample (about 12 hours) but does not need calibration if certified thermocouple wire and self calibrated volt meters are used. Such a complicated system should be able to measure a large temperature range. In that perspective, I am charmed by the GHP 456 Titan® ,if adapted for larger samples.

The very small error of the GHP-system is calculated with finite elements by L. Troussart in “Analysis of errors in Guarded Hot Plate measurements as compiled by the finite element method”.

Patent study: RU2004000415, a STES patent

logo_smallThis STES patent looks very interesting because they make a comparison with the best cellular glass today on the market at that time. Like we already mentioned in another post about NEOPORM,  they claim the best quality without remelting the glass, which should induce a huge cost reduction.

The abstract is written in English and French, but the content was only Russian. I used GOOGLE TRANSLATE which gives a poor understandable English. (Russian patent).

I understood that it is possible to start from waste glass with different compositions. Water glass is added in a rather high percentage and the mixture is stirred. After some time, a gasifier, containing carbon (I gamble on glycerin) is added and the stirred mixture is heated to 530°C until all water has disappeared. The mixture is further ground to a fine powder with a maximum grain size of 15 micron. This powder is put in molds and heated to about 800°C for 90 minutes.

While for the highest quality cellular glass, it is needed to melt the recycled glass with additional raw materials, this patent discloses a method to obtain this quality without remelting at high temperatures. It is well known that a melting furnace is a huge investment and has only a short life between 5 and 15 years (depending on the oxidizing components in the glass)  while the extra heating is about 1250-550°C=700°C. But we also know that the measured thermal conductivity depends strongly on the used methodology. I am not sure we are comparing the “same thermal conductivity”.

I would be surprised that Andrei’s patent is fake and therefore I am hopeful that cheaper high quality cellular glass can be put on the market while the investor will be pleased. On top of that, the problem of the unsorted piles of waste glass in Russia will be solved. In the next days, I will have a good translation of the patent.

The cold war has given us not only two very different cultures but also two different very competitive processes for cellular glass. And free competition is all what a fair market needs, although free is relative in this case, the patent is valid for another 10 years.

The dimensions of cellular glass

logo_smallRecently, I was in a small glass company. These people make small but old fashioned hand made glass plates as window and for other applications. Since 5 years, I regularly visit float glass plants for CNUD EFCO, where glass plates up to several meters width and height can be produced.

And suddenly a question came up? Why did this not happen with cellular glass? Cellular glass started (in the metric system) from about 45 cm by 30 cm, scaled up to 45 cm by 60 cm and recently 120 cm by 80 cm became available. I can imagine that the thickness is limited because annealing time goes linear with inverse of the thermal conductivity and with the square of the thickness. But I do not see a reason why several meters length and width are not available if I watch the cold end of a float glass line as installed for example by Grenzebach or Lisec.

But this week I visited a plant and the owner was proud to show me what he could produce. The actual dimensions are still secret but they are huge. And suddenly, I had a very nice application in mind for the world of tomorrow.

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Foamed glass gravel

logo_smallSince already many years, foamed glass gravel is on the market as a loose fill thermal insulation. The thermal conductivity is roughly the double of the best cellular glass for the same compressive strength. This thermal conductivity is quite high but when there is enough space for a large thickness, we have an interesting material. The material is foamed from recycled glass with a minimum of treatment. Indeed, in principle a mixture of different glasses (different compositions) can be used without too much cleaning. Two foaming agents are well known in this perspective: glycerin and silicon carbide.


The business seems already well organized because the Czech company Refaglas is delivering also the glass powder with the requested particle size. Indeed, it makes sense to transport the ground glass to a foaming plant as close as possible to the customer to reduce transport cost. The grinding equipment will also be more efficiently used, reducing CAPEX of the different plants.

For the grinding equipment, I learned that today Hosakawa-Alpine is the major grinding equipment supplier, delivering a ball mill with efficient classifier.

It must be hard for a sales man to sell boards for floor insulation if gravel is still possible to install. But on the other hand, the difference between a gravel production line and a board line is only an extension with a new recipe, an annealing furnace and some finishing equipment. It must be very seductive for investors to make that relative small extension to increase the value of the output. And at the end, the invisible hand of the market competition will bring the price of high quality cellular glass down to about 200€ /m³, the price GLAPOR is handling since July 2015.

About this post, I got a reaction from Arjen Steiner:

There is another supplier/maufacturer with two factories in Germany. The name is Veriso (www.veriso.de). The sales office is in Berlin. The production sites are close to Hannover (www.schaumglas-husum.de) and close to Würzburg (www.schaumglas-steinach.de). The total production capacity is 130.000 m^3 per annum. Welcome to competition!

Mineral rendering on cellular glass

Cellular glass is in the first place known as the best  thermal insulation in the roof but is  less popular on (in) facades. And within the facade world, it was for a long time not supported as thermal insulation under a rendering. The reason for this is quite clear:

Most (if not 99%) rendering products for application on walls and thermal insulation contain cement as bonding agent. Cement based products hardens in a totally not flexible way and have a tendency to let shear off the cellular glass. During drying, the rendering shrinks and induces an enormous tensile stress on the cellular glass. In case the layer is thicker than a few millimeter, the cellular glass shears off because the tensile strength of the cellular glass is exceeded. In reality, a rendering must be at least 1 cm thick to get a long life and cellular glass became obsolete in this application.

(Portland) cement changed our life with its great strength and short settling time and let us create strong but still elegant constructions. But there was a time before Portland cement where almost pure hydraulic lime was used as bonding agent. Buildings made with this bonding agent were able to deform without creating a crack, they were (are) in a certain way flexible. Foundations were much smaller or non-existent while today a house without proper foundations will show important cracks when the masonry is performed with ordinary Portland cement.

Rendering, properly based on this old recipe, don’t shear off from cellular glass, even if the layer has a few cm thickness. A thick  layer with coarse grained sand is put on the cellular glass and the finishing is done with fine grained layer. The fine grained layer will dry the wet thick base layer by the difference in surface tension.

Both systems are a perfect solution for those customers who want a rendering on thermal insulation which resists a foot ball, feels like a real wall and does not burn. Both solutions are guaranteed by an ETA and are insured.


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