Reformulation of container glasses for environmental benefit through lower melting temperatures

The chemical composition of container glass has evolved over many years to become almost generic, providing a cost effective material which fits its purpose very well. Container glass compositions are essentially the cheapest acceptable formulations in terms of raw materials, but increasingly stringent environmental legislation may soon cause changes in the economic balance. A large body of work has been published regarding melting aids and new raw materials in order to increase melting efficiency. However there is far less published work dealing with the specific aim of modifying the chemical composition of container glass to reduce its melting temperature, whilst at the same time maintaining its desirable characteristics. Environmental benefits resulting from reformulation may include reductions in melting energy and lower emissions of CO2, NOx, SOx, particulates and heavy metals. This paper discusses the development of a number of reformulated container glass compositions, with the main emphasis on the physical properties of the new glasses and estimated reductions in thermal NOx generation. Compositional changes ranged from single substitutions to complex alterations involving several components. Computer calculation of the high temperature viscosity of the new compositions was carried out and the results agreed very well with measured values. High temperature viscosity, liquidus temperature, chemical durability, thermal expansion coefficient, density, dilatometric softening point, glass transition temperature and fluxline corrosion were measured on a selection of the new glasses. The results were analysed and evaluated against a set of property criteria based on a benchmark glass composition representative of current UK container glass compositions. Reformulated glass compositions exhibited reductions in melting temperature, defined as the log 2 viscosity, of up to 115°C. Several of the new glasses also fulfilled the benchmark criteria for chemical durability and liquidus temperature. It was estimated that the reduced furnace temperatures which these compositions would enable could cut generation of thermal NOx by approximately 20–40%. Reductions in melting energy and emissions of CO2, SOx, particulates and heavy metals may also result, however they were not quantified in this study as their accurate estimation requires consideration of the effects of raw materials and cullet. Further work is currently underway to study these effects and the behaviour of the glasses on a larger industrial scale.