09-08-2012, 04:20 PM
Heat-Strengthened Glass
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Types of Glass
In addition to annealed float glass there are five other types of float glass available: heat-strengthened glass, fully tempered glass, laminated annealed glass, laminated heat-strengthened glass and laminated fully tempered glass. These glass types can be used individually, or in combinations, for various architectural applications. Each has its own specific properties and performance characteristics that can be related to the requirements established by the design community.
Annealed Glass
Annealed glass has the surface strength that provides the wind-load performance and thermal-stress resistance needed in most architectural applications. In areas of high wind loads, or in conditions where higher-than-normal thermal stresses occur, heat-treated glass may be required.
Annealed glass in standard thickness does not meet the safety glazing standards of the Consumer Product Safety Commission (CPSC) 16 CFR 1201 or the American National Standards Institute (ANSI) Z97.1.
ASTM C1036 “Standard Specification for Flat Glass” is the standard that specifies the required thickness, dimensional tolerances and characteristics of annealed glass.
Heat-Strengthened Glass
Heat-strengthened glass is produced by heat-treating annealed glass under regulated thermal conditions. In this process, annealed glass that has been cut-to-size is carefully heated in a furnace that is controlled between 1100–1500 degrees Fahrenheit (593–815 degrees Celsius) and then quickly air-cooled. This sudden cooling causes a compression envelope around the glass surface and edges, along with a balanced tension stress within the glass itself. This equilibrium of stresses increases the strength of the glass to approximately two times that of the original annealed product when tested under uniform pressure such as wind loads. In addition, when broken, glass that has a low to moderate degree of heat strengthening will generally exhibit few cracks and tends to break into large pieces that initially may remain in the glazed opening. (Note: glass should be removed and replaced as soon as possible after breakage.) As the degree of heat-treating increases, the break pattern of the glass will more closely resemble that of tempered glass.
A significant advantage of heat-strengthened glass is its ability to withstand high thermal stresses due to partial shading and heat build-up from solar loading. With its edge compression levels in excess of 5500 pounds per square inch (38 MPa) and surface compression levels in the 3500 to 7500 psi range, heat-strengthened glass has performed well in demanding architectural applications, such as in direct contact with insulation or with dark applied frit (durable, colored ceramic material) in spandrel areas. This ability to withstand high thermal stresses, and its wind-loading resistance, make heat-strengthened glass a preferred choice in many architectural applications.
The increased toughness of heat-strengthened glass also reduces the likelihood of glass breakage during shipment, handling, installation and in-service use. Heat-strengthened glass, because of its break pattern, does not meet the safety glazing standards of CPSC 16 CFR 1201 or ANSI Z97.1.
ASTM C1048 “Standard Specification for Heat-Treated Flat Glass” is the standard that specifies the required tolerances, characteristics and compression levels for heat-strengthened glass.
Fully Tempered Glass
Fully tempered glass is created in a process that is similar to heat-strengthened glass. Cut-to-size, annealed, float glass is heat-treated and air-cooled, creating an edge compression greater than 9700 psi (67 MPa) and a surface compression greater than 10,000 psi (69 MPa). Fully tempered glass may show more visual distortion of reflected images than heat-strengthened glass. Its key performance characteristics are increased strength and the ability to meet the requirements of safety glazing standards (i.e., CPSC 16 CFR 1201 or ANSI Z97). Fully tempered glass when fractured tends to break into small irregular shaped fragments that meet the criteria of the aforementioned safety glazing standards.
Heat Tempered Glass - Relative Strength
The strength of heat tempered glass is proportional to the surface compression introduced by processing. A surface compression level of 10,000 PSI yields a part that is approximately twice the strength of an equivalent heat strengthened part and four (4) times that of an annealed glass part of similar thickness, size and fabrication processing when placed under uniform static pressure loads.
Heat Tempered Glass - Mechanical Properties
Bow and warp are introduced due to the slight differences in the heating and quenching parameters of the glass surfaces. These differences are also influenced by tinting of the material, coatings and the fabrication of holes, slots or notches in the glass. Thinner materials as well as larger sizes of glass will have more bow and warp introduced than thicker materials and smaller parts. This increased bow and warp is due to the parts heating and cooling from the edges as well as the surfaces and in addition, heating and cooling airflow creates uneven boundary layers on the surfaces during heat tempering.