15-05-2013, 03:06 PM
Toughened glass
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INTRODUCTION
Toughened glass acquires a degree of strength for excess of the strength of normal glass sheet or plate glass, which if broken shatters into small and comparatively harmless pieces. It is claimed that the resistance to mechanical stock of toughened plate glass is 4 to 5 times more than that of ordinary plate glass. A toughened glass has better resistance to the vibration, mechanical shock and abrasion.
Background
For many applications such as buildings requiring large spans of glass, toughened glass is the only acceptable alternative. These glasses have exceptional strengths compared to standard annealed float glasses.
These improved properties are a result of the stress profile that is induced in the glass by the toughening heat treatment process. When performed correctly, the glass surface is in compression, while the centre is in tension. This stress profile in the glass is successful as most failures start at the surface from tensile loads. In toughened glass, the applied tensile load must overcome the compressive stress at the surface before the surface can go into tension and fail.
Process of Manufacture
The glass plate is heated to a temperature above its softening point and then subjected to rapid cooling. The glass is suddenly chilled and in this process contracts towards the core. It stretches until it has solidified and is no longer able to contract further at this stage the core
is still soft. It contracts against restrained exercise by the solidified upper layer of the glass. This compression is responsible for the strength of the glass sheet, which is limited to about 20,000 lb/sq.inch. Thus it is highly stressed and the resultant force is able to nullify the external impact. The intensity of the stresses depends on the rate of cooling, co-efficient of expansion, thermal conductivity of the glass, its specific heat, elasticity, and certain other physical properties.
Toughening Process
The raw plate glass sheet which is free from waviness, distortion etc., is cut to required size and shape and then all the edges are ground and polished as per end use of the product. This is called edge grinding and polishing and is very important for toughening because it will lead to breakages during process. No glass sheet can be toughened without edge grinding and polishing.
Washing and Drying
After the edge grinding and polishing the glass sheets are washed manually or by machine and then dried. The glass sheets are fed into the furnace (Electrically operated). The sheets are kept in the furnace above its softening point, which varies according to the composition of glass. After attaining required temperature the glass sheets are removed out of the furnace and placed in the air blowing quenching boxes for 20 to 25 seconds. After quenching glass sheet is toughened. For bend glass toughening, the glass sheets passes through a set of dies (as per shape) after furnace and then to the quenching boxes.
Properties
Toughened glass is physically and thermally stronger than regular glass. The greater
contraction of the inner layer during manufacturing induces compressive stresses in the
surface of the glass balanced by tensile stresses in the body of the glass. For glass to be
considered toughened, this compressive stress on the surface of the glass should be a
minimum of 69 MPa. For it to be considered safety glass, the surface compressive stress
should exceed 100 MPa. The greater the surface stress, the smaller the glass particles will be
when broken.
It is this compressive stress that gives the toughened glass increased strength. This is because
any surface flaws tend to be pressed closed by the retained compressive forces, while the core
layer remains relatively free of the defects which could cause a crack to begin. However, the
toughened glass surface is not as hard as annealed glass and is therefore somewhat more
susceptible to scratching. To prevent this, toughened glass manufacturers may apply various
coatings and/or laminatesto the surface of the glass.
Uses
Safety approval markings on an automobile vent window made for a Chrysler car by PPG.
Toughened glass is used when strength, thermal resistance and safety are important considerations. The most commonly encountered tempered glass is that used for side and rear windows in automobiles, used for its characteristic of shattering into small cubes rather than large shards. (The windscreen or windshield is instead made of laminated glass, which will not shatter when broken.)
Manufacturing
Toughened glass is made from annealed glass via a thermal tempering process. The glass is placed onto a roller table, taking it through a furnace that heats it above its annealing point of about 720 °C. The glass is then rapidly cooled with forced air drafts while the inner portion remains free to flow for a short time.
An alternative chemical process involves forcing a surface layer of glass at least 0.1mm thick into compression by ion exchange of the sodium ions in the glass surface with the 30% larger potassium ions, by immersion of the glass into a bath of molten potassium nitrate.