Soda-lime-silica glasses

MODERN GLASSES

Soda-lime-silica glasses used in container sand flat glass have changed little over the past centuries. Great improvements in glass quality were achieved by changes in the raw materials, the compositions (slight changes actually) and the melting and forming operations. During the17th century Deslandes from Saint Gobain added up to 6% of lime to increase the resistance of plate glass to moisture. Around 1880 Schott suggested a few per cent of aluminium to improve chemical durability and resistance to devitrification. These composition refinements were of utmost importance for improving glass quality and resistance. In 1854, the Siemens brothers transformed coal into CO gas that was further burnt in a furnace while they added regenerators that alternately stored and released heat wastes. The fabricated glass was thus preserved from coal dust contamination, improving its quality, while regenerators, still used nowadays saved energy and allowed much better production yields. By 1822, the young factory proposed new silica-lime-based refractory to build furnaces instead of clay-based ones. These achieved much lower thermal expansion and hence the thermo-mechanical resistance and the fatigue resistance of furnaces were much improved.

The furnace life could be increased accordingly. Towards the end of the 19th century, container fabrication was greatly changed when Ashley (UK) and Boucher (France) proposed semi-automatic machines to produce bottles. At the same time an American engineer, Michael Owens (1859–1923), invented an automatic bottle blowing machine. It was a successful process, which arrived in Europe after the turn of the century and considerably transformed the containers industry.

A further improvement of the process is due to Hartford Company which invented the feeder to provide the forming machines with a thermally conditioned glass gob. Up to this time the glass parison was taken from the glass melt inducing thermal heterogeneities. The feeder worked continuously like an extrusion machine allowing for the production of a gob of a given size at a given frequency. The feeders are still employed nowadays and with the invention of individual section (IS) machines by1925, they deliver several forming machines. As in the containers industry, the glazing process was progressively and strongly automated. Up-stream conditioning of the glass melt benefited as well from refractory developments and thermal waste regeneration. By 1903, Window Glass (USA) had developed the Lubbers process which used automatic blowing. After 20 minutes of blowing, a cylinder 1mindiameter could be produced. Later operations remained manual. This was a great improvement in terms thermal and thickness homogeneity. The float process was developed after the Second World War by Britain’s Pilkington Brothers Ltd. It was a revolution in flat glass production since polishing of the plates was no longer necessary. It was a big astonishment since many glass manufacturers had focused their research and technology on improving glass polishing while this new process allowed excellent surface quality and transparency without this operation. In1959, after seven years of experimentation and an investment of £7 million Pilkington Ltd introduced this economical means to produced is portion-free glass. Nowadays almost 90% of flat soda-lime-silica glass is exclusively produced in this way. The process is based on the idea of using a flat surface of molten metal to support a continuous ribbon of glass. The metal to be used has to be liquid over the temperature range 600–1050˚C corresponding to the processing of glass.

Дата добавления: 2015-01-03