In a previous post, we mentioned already PhD work on cellular glass and also one on foaming of CRT-glass. However, the thesis in this post has a lot of attention for the methodology of searching for a new glass foaming system. Hereunder the abstract of the thesis is given.
Foam glass has been used for over 70 years in construction and industry for thermal insulation. Foam glass is mainly made of recycle glass. Strict energy policy motivates foam glass manufactures to improve the thermal insulation of foam glass. The effort to understand the making of foam glass with good insulation ability is scarcely reported. The goal of this Ph.D. thesis is to reveal the underlying mechanism of foaming reaction, foam growth and the heat transport of solid foam glass. In this thesis, the panel glass from cathode ray tubes (CRT) will serve as a key material to reveal the mechanisms.
Foaming is commonly achieved by adding metal oxides or metal carbonates (foaming agents) to glass powder. At elevated temperature, the glass melt becomes viscous and the foaming agents decompose or react to form gas, resulting in foamy glass melt. Subsequent cooling to room temperature, lead to solid foam glass. Metal carbonates decompose due to surface reaction. Based on Na2CO3, we show the reaction is fast and the glass transition is changed considerably. We propose the reaction rate is dependent on contact area between glass melt and Na2CO3, melt viscosity and Na+ diffusion.
A method is developed for optimising process parameters. Characteristic temperatures are derived from a deformation curve and the deformation rate curve. Maximum expansion rate was linked to closed porosity. Using this knowledge the method is applied to literature data to analyse for optimal conditions. The resulting conditions were in agreement with industrial conditions. Since no foam glass properties are necessary to measure, the method allows fast investigation of process parameters.
The melt viscosity is an important parameter for foam growth. We compared bubble- and crystal free melt viscosity with foam density and show in order to minimise the foam density, the heat-treatment should be performed in the viscosity regime of 103.7-106 Pa s.
The thermal conductivity of foam glass made is often reported to be linear dependent on porosity or foam density. Foam glasses made from CRT panel glass and different foaming agents confirm this trend at high porosity level (85-97%). The experimental data suggests the solid conductivity is dependent on the foaming agent applied.
The research is part of a project to build “passive houses” where the cellular glass is a structural element and not only thermal insulation. In fact, the cellular glass takes all the load while thin bricks are just creating a standard house look. Houses built from cellular glass was already mentioned in a previous post.