The New York Times has discovered materials science! The newspaper recently published a nice story on the scientific advancements being made to strengthen glass for the ever expanding use of the material in architecture. The glass gurus interviewed include ACerS members Harrie Stevens, director of the Center for Glass Research at Alfred University, Carlo Pantano, professor of materials science at Penn State and William LaCourse, professor at Alfred.
Structural projects almost exclusively use soda-lime glass, made largely from sodium carbonate, limestone and silica. “For years, the basic composition of soda-lime glass has not changed much,” said Stevens.
And one property of glass has remained constant: glass begins to lose its strength the moment it’s made.
Even one gas molecule can break a silicon-oxygen bond in glass, generating a defect, said Pantano. While glass is very strong in compression, tensile stresses will make these tiny fissures start to grow, bond by bond. “That’s what makes glass break,” he said. “And if it doesn’t break, it weakens it.”
Protective coatings are one way to avoid new cracks, although they can affect transparency, which is the main reason for using glass in the first place. Changing the glass recipe can also make it harder for cracks to form and propagate.
For structural purposes, glass is often strengthened the old-fashioned way — by tempering. This puts the surface under compression, so that even more tensile force is needed for cracks to grow.
For flat glass, heat tempering is most often used. LaCourse said the process takes advantage of one property of glass — that when it cools slowly it becomes denser. By rapidly cooling the exterior of a sheet (usually with air), the surface stays less dense. “Inside it’s still hot, and tries to cool to a more dense structure,” he said. “This pulls the surface into compression.”
Tempered glass may take longer to crack, but it can still break. A primary concern when building with glass is what happens if and when a component breaks. That’s where lamination comes in. In a typical project, multiple glass sheets (one-half-inch thick in the Sears Tower project) are bonded with thin polymer interlayers. The interlayers add strength and, should one of the glass layers break, keep the structure together, and the pieces from falling.
Already, some engineers are using different glass shapes to reduce the dependence on metal. Other designers think about using different kinds of glass. Using a glass that does not expand much when heated, for example, would enable components to be welded together, forming, in effect, a continuous piece of glass. Conventional soda-lime glass expands too much, so welding introduces stresses that can lead to failure.
Researchers have experimented with welding glass components. But low-expansion glass is much costlier than soda-lime glass. Other engineers are starting to use adhesives to join glass directly to glass.