Saturday, August 11, 2012

Glass can be Hard for Digital Signage

Tutorial: Samurai Swords
Many crystalline structures actually get stronger under compression rather than weaker. A samurai sword is actually made from a straight piece of metal. It is the heat treating that causes the distinctive curvature and it is the heat treating that is responsible for the incredible sharpness. Iron forms different crystal structures at different temperatures and, like water, forming a new crystal structure at a lower temperature sometimes mean a volumetric expansion rather than contraction as in water to ice. The samurai sword heat treating process involves freezing in one crystal structure on the cutting edge of the blade while the body of the sword first contracts (forming the curvature) then goes through a final transition where it wants to expand. This leaves the edge under permanent compression and enables the edge to be polished to incredible levels of sharpness. Even though the process is centuries old, the metallurgy was not understood until the 1970s as nothing is at equilibrium. The image above is from the movie “Kill Bill” which provides ample demonstration of just how sharp samurai swords can be.

This principle of having the surface under compression from the internal structure is also used in glass. Correlle dinner ware is made from two separate glasses, a high expansion inner glass and a lower expansion outer glass. After the plates are formed from molten glass, the inner glass wants to contract more than the outer glass leaving the outer glass under compression. In chemically strengthened glass the outer layer compression is accomplished by substituting some of the atoms in the surface layer with atoms of a larger ionic diameter. Stuffing in these larger atoms leaves into the glass lattice leaves the surface under compression. In tempered glass, the compressive layer is due to freezing in an expanded, high temperature structure on the outside, while the interior cools more slowly and wants to contract.

Just on its own, glass can be pretty hard. Making it thicker makes it more rigid and less likely to break. The extra thickness can be either a monolithic structure or a laminate. It can also be over coated with materials that are even harder. Finally, for many applications, you can use a transparent plastic. Each approach has pluses and minuses.

Until now, glass has been the only material used in mainstream high information content displays. The displays themselves are going into different environments and designers of both digital signage and mobile devices (solar as well) are having to comprehend issues that they have never had to deal with before. This posting delves into some of these issues.

Coverglass Choices
Chemically strengthened glass can be made ultra-thin and is most commonly used on mobile devices where thinness is at a premium. However, the forming processes for very thin glass are limited and the ion exchange process to do the chemical strengthening is slow and therefore expensive. Also, the chemical strengthening must be done after cutting the pieces to size, further adding to the expense. (An earlier version of this post inadvertently stated the opposite. However the point is that the processing must be dome with individual parts rather than a motherglass, adding to the expense.) A Correlle-like structure could be tried but if you have ever owned Correlle, you may be aware that although the bi-glass structure works well, it does not work particularly well on the edges or any corners. Also, when the glass does break, due to the stored energy of the purpose built-in stress, it tends to shatter. (A well known book on WWII, concerns the Japanese defeat at Midway, “Shattered Sword”. The title of the book refers to the tendency of a well made samurai swords to shatter rather than just chip or bend.) Particularly for digital signage applications having a cover glass shatter rather than just break could be a consumer issue.

Tempered glass can not be used for LCDs due to the pattern of birefringence the tempering leaves in the glass. Birefringence is a polarization dependent optical effect that would interact with the polarized light coming from an LCD. An LCD viewed through thermally tempered glass would appear to have a mottled image. Coating the glass with an even harder substance is an option. But here again, the product would have optical issues. As far as I know, every material that you could coat the glass with has a higher index of refraction than glass. This would result in increased surface reflections.

Why not use Plastic?
Years ago, I read an article title “The Dream of an All-Plastic Car”. The article was published in ine of the chemical industry magazines and covered opportunities such as plastic bodies and plastic windows. However, it turns out that the dream of an all plastic car was the chemical companies’ dream, not the auto makers or consumers. There were a few models made with plastic bodies and one made with one plastic window (the side window on the back of a CRX. However, plastic does not age very well in the sun and has very very limited abrasion resistance. The auto industry did largely move to plastic headlamp covers. However, these often turn cloudy after a few years and the the cloudy appearance would not be acceptable for a window or auto-body.

Plastic does not have nearly the hardness of glass and can’t handle even moderate levels of abrasion without a grossly diminished finish. It also degrades under UV.

Plate Glass for Digital Signage
Currently, in the digital signage industry, most use plate glass to protect their signage where it is required (anywhere the public can actually touch the screen). UL provides a minimal level of guidance as to the thickness with its ball drop test. Meeting UL 6950 requires about 2mm of chemically strengthened glass or about 2.5mm of plate glass. Other architectural or automotive standards (ASTM or SAE) or practices may be applied leading to thicker glass and site architects my specify vandal-proof or even bullet-proof levels of protection. Thicknesses in actual use in the industry range up to 13mm, frequently either bonded to the LCD or two separate pieces of glass laminated together. The lamination adds strength and also protects the public from broken pieces should he cover glass do break.

The UL standard is a strength standard rather than a dimensional one that buyers of digital signage can test for, non-destructively. It provides minimal protection. Other standards may come from the architectural community, the materials suppliers, or groups like the SAE that are used to dealing with glass safety. However, it would be beneficial if the digital signage industry were to agree internally on standards for both product and consumer protection.


1 comment:

  1. There can be degrees of strengthening with differing results. The more a glass is strengthened by internal stresses, the more stored energy, the more complete the shattering if the part does break. Corning formerly had a product line, referred to as Frangibles, that were strengthened to the point where they shattered into powder. The typical function of product within the frangibles product line was less as a cover glass and more as a disposable encapsulate. At one point, after the Tylenol poisonings, frangibles were considered for encapsulating medicines. The extreme level of strengthening necessitated a long time in the ion-exchange bath and most of the product line that I am aware of was for the military due, to the expense. Judging from what I have seen of broken mobile product with chemically strengthened coverglass, current practice is to strengthen the glass well below this level.


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