Strengthening Glass

The rate of cooling directly affects the strength of glass. The regular process of cooling – or annealing – float glass results in a slow rate. Stronger glass can be produced by changing the rate of cooling. Two types of stronger glass are:
· Heat-Strengthened Glass
· Tempered Glass

Heat-strengthened glass is cooled at a rate faster than regular annealed glass. Tempered glass, in turn, is cooled at a faster rate than heat strengthened glass. Another way to strengthen glass is to use more than one lite of glass in the application. Laminated glass consists of two or more lites of glass, joined by a layer of plastic.

In many modern buildings, the glass must be as strong as possible. Three basic reasons to strengthen glass are to:
· Increase Wind Load
· Increase Impact Resistance
· Combat Thermal Stress

Architects and designers must consider the force of wind on a building or installation when choosing glass. Wind causes glass to deflect. This deflection strains not only the glass itself but the entire glazing system: the framework, gaskets and sealants.

Impact resistance is closely related to wind load because the wind carries such things as hailstones, dust, small stones and other debris. During tornadoes and hurricanes, the wind carries many larger objects.

As glass heats, it expands. The center portion of a lite gets hotter and expands at a greater rate than the edges. The stresses on the edges are usually greater at the center of each edge and decrease toward the corners. The imbalance strains the edges. This is called thermal stress. The edge strength of the lite, therefore, greatly determines its ability to resist breaking. Clean-cut edges offer the greatest edge strength. This is particularly crucial with heat-absorbing glass. A well-designed glazing system also reduces stresses on the glass.

Heat-strengthened glass is made by heating annealed glass uniformly, then cooling it at a slower rate than tempered glass.
Characteristics include:
· Is about twice as strong as regular annealed glass of the same size and thickness.
· Is more resistant to wind loading and impacts than regular annealed glass though less resistant than tempered glass.
· Fractures into large, jagged pieces, similar to annealed glass.

Heat-strengthened glass is generally used in high-rise buildings to help the glass resist thermal stress. It is also used in the making of spandrel glass. Spandrel glass is obscure glass that is used in non-vision areas. Because heat-strengthened glass fractures into large jagged pieces, it does not qualify as a safety glazing material. All building codes require safety glazing for shower doors, commercial doors and store fronts for safety purposes.

Glass gains considerable strength from tempering. A lite of tempered glass is about four times stronger than a lite of annealed glass of the same size and thickness. Characteristics include:
· The only characteristic of the annealed glass affected by tempering is its bending or tensile strength:
· Tempering increases the tensile strength of glass.
· This makes tempered glass better able to resist the forces caused by heat, wind and impact.
· Tempering does not change:
· The color, chemical composition, or light transmission characteristics of the annealed glass.
· Its compression strength (the ability of the glass to resist crushing forces)
· The rate at which the glass conducts and transmits heat.
· The rate at which the glass expands when heated.
· The stiffness of the glass.

The main reasons to use tempered glass are:
· Tempered glass, when broken, is designed to shatter into cube-shaped particles. It therefore qualifies as a safety glazing material.
· Tempered glass offers greater strength against deflection, and thus, better resistance to the force of wind, than heat-strengthened glass. It is more effective if placed within a well-designed, overall glazing system.
· Tempering increases the ability of glass to survive the impact of objects that may strike the building. When tempered glass does break, it shatters into small cubes, reducing the likelihood of serious injury on impact.
· Tempering increases a lite’s edge strength. Thus tempered glass is specified when designers anticipate high thermal stresses.

Tempered glass is made by heating annealed glass uniformly. The glass can be from 1/8″ to 3/4″ thick. The annealed glass is then cooled rapidly by blowing air uniformly onto both surfaces at the same time. This is known as air quenching. Rapid cooling increases the compression forces on the surface and the tension forces inside the glass. Two processes are used to temper glass:
· Vertical tempering
· Horizontal tempering

In vertical tempering tongs are used to suspend the glass from its top edge. It moves vertically through the furnace in this manner. In horizontal tempering the glass moves through the furnace on stainless steel or ceramic rollers. Of the two processes, horizontal tempering is the more common. Tempered glass is identified by a permanent label, called the bug, which is placed into the corner of each tempered lite. Tempered glass cannot be cut, drilled or edged. These processes must be performed on the glass before tempering.

Laminated glass, sometimes called “lami,” is made by placing a layer of polyvinyl butyral (PVB) between two or more glass lites. The PVB can be clear or tinted and commonly varies in thickness from .015″ to .090″, but it can be as thick as .120″ for special applications. The entire unit is then fused under heat and pressure in a special oven called an autoclave. The laminating process can be performed on clear, tinted, reflective, heat-strengthened or tempered glass. Characteristics include:
· When laminated glass breaks, the glass particles adhere to the PVB and do not fly or fall. Certain combinations of glass and PVB thicknesses qualify as safety glazing materials under the health and safety standards set by the American National Standards Institute (ANSI). For example, laminated glass with a .030 PVB layer sandwiched between two pieces of two-millimeter annealed glass meets the minimum requirement for safety glazing.

Applications-In addition to safety glazing, laminated glass has many specialty applications, including sound reduction and security.