Measuring the open / closed porosity of cellular glass

logo_smallFactory made standard cellular glass for thermal insulation is assumed to have 100% closed cells like described in the standard  EN 13167. Indeed, the mu-value is not allowed to be lower than 40000. Measurements on materials with such a high mu-value easily take 3 months, which means that this method cannot be used for daily quality control.

pycnometerOn the other hand, the gas – pycnometer is a nice and simple instrument to measure the volume of the closed cells. If this volume is in line with the geometric volume, we know that the cells are closed and that the mu-value will be above 40000. Because this method takes a few minutes, it is the ultimate instrument for QC in factories where open (acoustic ) and closed cell material (thermal insulation) is produced.

The following paper describes nicely the principle of the measurement.


An over pressure in the sample chamber is released into the (calibrated) reference chamber. The pressure before and after release, together with the known volume of the reference chamber allows to calculate the volume of the sample chamber. If this volume is also known, we can calculate the volume of the sample. From this sample volume, we can calculate the amount of closed / open cells.

The instrument can be quite simple like shown here under:


For about 2000€, this instrument can be built and connected to a PC. It is not clear why the above commercial instrument costs about 16000€. For larger samples, a pycnometer with compressor in situ is also available on the market like shown in this leaflet.


The battle for waste glass

logo_smallIn Europe, we are going from huge piles of waste glass to a higher demand than available. A long time ago, manufacturers were paid absorbing waste glass but today they have to pay.

But there is still one type of glass for which the demand is in fact not existing. Indeed, since the Cathode Ray Tube (CRT) televisions are replaced by LCD (Liquid Crystal Display), the CRT´s are not remelted anymore. And as a consequence, a huge pile of glass is available for other applications.

DownloadOne function of CRT glass is to absorb the X-ray radiation, generated by the high velocity electrons bombarding the front panel with phosphor. For this important function, PbO and BaO are introduced in the glass. PbO is introducing a brown color into the screen after long use. For that reason, the PbO is replaced by SrO in the front panel of the color television CRT. This and more is well explained in the following paper about CRT´s.

The source of CRT-panel glass waste seems to be very large like found in the following reports:

  • The following  paper describes the worldwide situation and even mention cellular glass as a way to recycle CRT glass.

Foam glass is a lightweight and handy product that is especially used where heat and sound insulation is necessary [122]. Its use has increased in recent years due to it being nonflammable, waterproof, a good insulator and having a long
life. Foam glass is produced at temperatures of 700 °C and 900 °C [123]. In the production of these products, CRT glass components with a lower melting temperature can reduce the product’s melting temperature in some cases [12].
Although first applications were made entirely of pure glass, the rate of these industrial waste products being used has increased to 98% today [56]. In order to improve the mechanical properties of these products, different materials have
been used besides CRT [124]. Investigations have shown that the amounts of lead leaked through the glass foams are acceptable [125]. Foam glass is usually produced by adding a gas producing material to powdered glass and then baking it to trap the gas bubbles in the glass. These products are generally used as foaming agents, carbon-containing materials, organic compounds, and carbonates [126], [127]. Depending on the area to be used and the desired properties, other minerals can be added to the mixture and the mixture is then sintered. During sintering, the foaming agent content reacts to increase the volume and obtain the glass foam [128]. The foaming agent added to alter the properties of the produced glass foam varies depending on the grain size of the glass used and the cooking temperature [129].

  • Another paper gives the situation in the UK for CRT glass recycling and also mentions cellular glass:

Foam glass is an insulating material which can be made from post-consumer waste glass. Experience in Norway indicates that it is feasible to incorporate at least 20% CRT panel glass in foam glass. There are no known technical  barriers to using CRT glass and no adverse environmental impacts compared with using other types of waste glass. Demand for foam glass in the UK, however, is limited and there is currently no production capability. Production facilities
are being considered, but the projected demand for CRT glass in this application is low, starting at 3,000 tonnes per annum and rising to a maximum of 9,000 tonnes per annum.

  • separatorAnother  paper  mentions the recycling problems of Pb-glass from CRT and the different laws about this in Europe.
  • Further, we also found equipment to separate the panel and other parts of the CRT in the following leaflet.
  • A research paper  warns up for the eventual leach out of CRT-glass and so cellular glass based on this.
  • Last but not least, we found a another paper about the foaming of CRT-glass with SiC and TiN.



A nice introduction in vacuum technology

logo_smallVacuum cellular glass is still fascinating some people, like can be found on this link of the Technical University in Freiberg, Germany. I give a citation here under:


tu-freiberg-logoDas Ziel des Projektes liegt in der Entwicklung eines nachhaltigen, brandfesten und tragfähigen vakuum-isolierten Bauelementes (VIP), mit erheblich verbesserten Eigenschaften im Vergleich zum Stand der Technik. Dazu sollen die derzeitig für den Stützkern verwendeten Werkstoffe durch Schaumglasstrukturen substituiert werden.

Zwei Wege werden untersucht, um die Dämmeigenschaften konventioneller VIPs mit Schaumglas zu erreichen. Einerseits soll ein offenzelliges Schaumglas entwickelt werden, welches im Nachgang evakuiert wird. Andererseits ist ein geschlossenzelliges Schaumglas Ziel der Entwicklungen, welches bereits im Schäumungsprozess mit einem sehr geringen Poreninnendruck versehen wird.

imagesAlso in this case, it is mentioned to develop a recipe for open cell cellular glass, which will be evacuated later on. The evacuation has to be done to a very low pressure because the mean free path of the gas molecules must be larger than the cell size. A very nice introduction to vacuum technology, written by Leybold, can be found here.

Breathing walls, does it make sense?

logo_smallBreathing walls are requested due to the “sick building syndrome“. This illness is attributed (without real proof) to air conditioning, out-gassing of some materials (VOC), too small intake of fresh air, mold and ozone. We may assume that depending on the person some of these products are indeed not positive for the health. Generally, there are two solutions:

  • The passive housing system with air tight walls and forced ventilation
  • The breathing wall system

passive-house-illustrated-simplicityThe passive housing system is the well known. Air tight walls with a very large thermal resistance (U=0.1 W/m2K) where all the fresh air comes from forced ventilation. The intake air is heated by the air leaving the building with a heat exchanger between both flows. Due to the low heat flow, the wall may be a source of interstitial condensation, generating mold while also the heat exchanger needs regular cleaning to avoid mold. On top of that, the limited diameter of the ventilation channels is the reason of high air speed regions in the house, which is not comfortable. It is clear that passive housing is not the favorite for people suffering from the sick building syndrome.

1-s2.0-S2212609013000046-gr1On the other hand, the breathing wall system is solving all the above problems. In a previous post, we used another definition: Dynamic insulation. In this case, cold outside air is sucked through the wall and heated with (low value) heat, which tries to leave the building. This sucking happens with a fan and the heat is stripped of the (warm) air by a heat exchanger and returned into the building. In this way, we have a low energy building with a very low air velocity ventilation and no possibilities for interstitial condensation in the wall, while the cleaning of the heat exchanger remains. A serious disadvantage is that wind on the building creates pressure differences in the building and also outside odor comes into the building.

An Hungarian paper lists the advantages of breathing walls (or dynamic insulation) while a more technical paper treats the dynamic insulation as a heat exchanger. Theory and experiments are given to know the real efficiency of dynamic insulation.

7f6d2b30dc77e32e369d51a258707ec3-Alma_22It is clear that open cell cellular glass can play a role in this dynamic insulation but I do not believe in breathing (dynamic insulation) walls due to wind problems. However, a breathing ceiling would be a nice application. Fresh air is pumped through a ceiling, constructed with open cell cellular glass, generating a ventilation over a large surface with extremely low air velocity. Besides ventilation, the open cell cellular glass serves also as acoustic absorber. I can imagine that this a perfect system for a student restaurant at the university. Ventilation and acoustic absorption, besides food are there the main issues.