Foaming of CRT panel glass powder with Na2CO3

Recycling of cathode ray tube (CRT) glass remains a challenging task. The CRT glass consists of four glass types fused together: Funnel-, neck-, frit- and panel glass. The three former glasses contain toxic lead oxide, and therefore have a low recycling potential. The latter on the other hand is lead-free, but since barium and strontium oxide are present, panel glass is incompatible with most common recycling methods. However, foam glass production is a promising approach for the recycling of panel glass waste, since the process parameters can be changed according to the glass waste composition in question. In this work, we foam panel glass cullet using sodium carbonate (Na2CO3) as a foaming agent, and study the foaming mechanism. When heat treating Na2CO3 powder with cullet powder, Na2CO3 reacts with the glass melt and is decomposed into Na2O and CO2. The released CO2 foams the glass melt, while Na2O becomes incorporated into the glass structure. We have quantified the melt expansion through density measurements and the Na2O incorporation is indicated by the decrease of the glass transition temperature (Tg) of the final foam glass. The glass foaming quality depends on the foaming temperature (i.e., the melt viscosity) and the concentration of Na2CO3. We reveal the origin of these dependencies.