Thursday, January 3, 2013

Glass is Hard for Digital Signage II

Pyrex is a familiar name; what is it. To the technical folk at Corning, if you were discussing Pyrex, you were discussing alumino-borosilicate glass. It had a relatively high melting temperature and relatively low coefficient of thermal expansion (CTE). If you were discussing Pyrex with the marketing people, Pyrex® was a trade name that could be applied to anything. Since Corning sold its consumer products group, Pyrex has taken on more of the latter definition with tempered soda-lime glass being used in bakeware instead of boro-silicate. As the article states, the Pyrex you can buy today is “Not your grandmother’s Pyrex”.

Until the mid 1990’s all LCDs were made with Corning 7059 glass. “7059” was one of the compositions out of the Corning catalogue, the “Blue Pages.” Like the technical reference to Pyrex, 7059 meant a very specific composition. The Blue pages, only about a dozen copies ever existed, usually specified the type of furnace the glass was melted in and occasionally how the furnace was fired. Glasses need to matched to specific furnace refractories and even in the same furnace, changing the way you fired the furnace may give a different end composition and will give a different oxidation state of the glass… a different glass.

LCD substrates were manufactured exclusively on 7059 substrate not because of its composition or particular performance properties but because 7059 formed well on Corning’s fusion draw which made glass with virtually perfect flatness. Although it did not contain any group 1 elements (semiconductor poisons), 7059 had a number of drawbacks. Like most glasses, it formed a bit frothy and had to be compacted before use less the dimensions changed during the high temperature LCD processing. One of its other drawbacks was the high amount of arsenic contained in the glass. Arsenic is one of the elements that get added to glass to increase its formability and 7059 had a lot of it as the fusion process was extremely demanding. 7059 was replaced by 1737 and then by 1734, there was no particular order or logic behind the assignment numbering of glass compositions. Of course all of the newer glasses have names rather than numbers and many of the drawbacks have been addressed.

Having a name rather than a number does not mean that a technical glassmaker can or would ship anything or allow glass composition or processes to vary outside of established norms. Indeed it takes years to develop a new glass composition and depending on the element, parts per billion can radically impact its performance and fit for use. On occasion for CRTs, days worth of glass production were thrown out for parts per billion of Fluorine (a phosphor poison).

Although users of digital signage do not need to be concerned with chemical interactions, some knowledge of glass chemistry would be useful, particularly in selecting a cover glass, if one is required. In deciding between float vs. chemically strengthened, bonded vs. not, different selections will give different result in terms of cost, optical performance, fracture resistance, and what hazard is presented by the broken pieces should a fracture occur. The best advice is to know what you want, are getting, and to have a known supply chain that you trust.


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  2. Very good information related to Digital Signage II.
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