Thermal insulation works with the principle of keeping still air (gas) without allowing any convection. The molecules only move and collide randomly without any organized behavior (convection). Glass as base material has a lot of advantages compared to polymer. Glass is not combustible; diffusion of gases in the glass is negligible at room temperature; the glass does not creep or deform under a load at room temperature. With glass, two forming methods are possible to produce thermal insulation. It is possible to form the glass in fibers or as foam.
Fibrous versus closed cellular structure
The structure on the left is clearly the fibrous structure. This structure is flexible because the fibers are kept together by a binder. The fibers don’t allow convection of the air in case of sufficient density. The point contact between the fibers guarantees that only small solid heat conductivity is present. On the other hand, point contacts do not allow a large compressive stress. The structure on the right is a cellular structure with closed cells.
This last structure is suited to sustain a high compressive load like for example wood. The closed cellular structure allows enclosing a low thermal conductivity gas without any diffusion out through the glass walls. This guarantees a stable thermal conductivity for several hundreds of years. On top of that, also water molecules are not diffusing in the glass structure, making the glass cellular structure absolutely water- and vapor tight. Internal condensation and so an increasing thermal conductivity is not possible. For cellular glass, the thermal resistance in real life does not differ from the artificial laboratory experiments and remains during the life time of the insulated construction.
Foaming: what and how?
The powder process is the standard method used for the foaming of the commercial available cellular glass of today. Ground glass cullet is mixed with a foaming agent like glycerin, silicon carbide or carbon black. It is possible to foam directly recycled glass at the expense of an increased thermal conductivity due to highly conductive H2 and CO in the cells. But also a special glass composition can be melted with an increased sulfate content and other multivalent oxides, needed to eliminate the H2 and CO in the cells.
In case the foaming agent acts as a grinding aid, mixing is done in the grinding equipment (for example ball mill) while in the other cases mixing happens after grinding. In the latter case, grinding efficiency can be greatly increased by using classifier equipment, separating fine particles from coarser one. The latter ones are resent to the grinding equipment.
Silicon carbide foaming can be done in a neutral atmosphere. Also glycerin foaming is possible in a neutral atmosphere if water glass is also mixed with the glass powder. Carbon black foaming involves a reducing atmosphere during foaming to avoid that carbon burns away before being actually used for the foaming. Silicon carbide and glycerin are the favorite foaming agents used for foamed glass gravel while carbon black is mainly used for boards.
After foaming at 800°C, a long annealing time is needed to attain cellular glass boards, which do not break in the lehr and that resist the thermal shock of hot bitumen above 180°C. The latter one is used to install cellular glass in flat roofs, the main application of cellular glass today. In the case of foamed glass gravel, the cooling is accelerated with a water spray to get gravel of the right size.
Foaming can be done in molds (batch) or as a continuous ribbon. Most investments of the last ten years are done with continuous foaming. In the case of mold foaming, the buns are annealed in a hollow glass type lehr. For continuous foaming, a radiation lehr without any forced convection can be used.
After annealing, the buns or ribbon have to be finished to rectangular blocks. A ribbon can be first finished on top and bottom and later on sawed in different length / width / thickness sizes. This is an interesting opportunity because larger dimensions are more and more required.
Today, it seems that continuous foaming will become the standard. Direct waste glass foaming at the expense of a 20% higher thermal conductivity is a booming market due to a very interesting CAPEX and OPEX. Cellular glass based on a special glass composition easily needs a double CAPEX and OPEX for a 25% better thermal conductivity.
Where is cellular glass used today?
Cellular glass directly foamed from waste glass has a comparable price with mineral wool of equivalent density and becomes the preferred sandwich partner for flat roofs with polymer foams to increase fire resistance and hardness under the water proofing membranes. But also the inverted roof gets a new future when a thin layer of low cost cellular glass is installed under the waterproofing membrane.
It is clear that during the last years cellular glass started a worldwide revival due to fire and ecology issues and due to the development of the continuous process. CNUD EFCO has all the knowledge to design, construct, install and commission foaming and annealing (lehr) furnaces for the continuous foaming method for any recipe.
Read more about efficient cellular glass production.