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Name: Chanya
Status: student
Grade: 9-12
Country: Thailand
Date: Spring 2014


Question:
What makes safety glass different from regular glass? In other words, what makes safety glass, safety glass?



Replies:
Chanya,

Safety glass is glass that has had additional treatment or augmentation so that it becomes less likely to break and/or if it does break is less likely to be dangerous. For example, car windshields (the glass in the front part of the car) contain glass that has been bonded to a plastic sheet. The sheet that holds the broken glass to the plastic so that there is less chance of large broken glass breaking off and hurting car passengers. On the other hand, tempered glass has been carefully heat treated so that it is tough but also tends to break into small granular pieces that are less likely to cause injury - so this is the glass used on the car door windows.

Greg (Roberto Gregorius) Canisius College


There are different kinds of safety glass; the main criterion is that it does not shatter into long sharp pieces that could cut or stab someone. Two common designs are safety plate, in which a layer of plastic is sandwiched between glass to keep it together when it breaks, and tempered glass, which is more difficult to break than ordinary glass but when it does break makes tiny pieces without sharp angles.

Richard E. Barrans Jr., Ph.D., M.Ed.


Hi Chanya,

There are two common types of safety glass. One type (that is used on the front window of all cars) is actually two layers of ordinary glass with a thin layer of a sticky plastic sheet between. This type of safety glass is no stronger than ordinary glass, but if it is broken, the sticky plastic sheet keeps glass splinters from breaking free and causing damage to passengers. The broken pieces of glass all remain attached to the plastic sheet.

The second type of safety glass is called "Tempered Glass". This type of glass is used (for example) in all windows of cars EXCEPT the front window. It is also used in many large windows in houses and buildings. It is made by taking a sheet of ordinary glass, and heating it until it is close to its melting point, then cooling it VERY suddenly by immersing it in oil, or by directing a powerful blast of air at it. This results in the outer surfaces of the glass sheet being in a high state of compression, which makes the glass much more resistant to breakage. Note that glass is very strong when exposed to compressive stresses, but very weak and easy to break when exposed to tensile stresses.

When tempered glass DOES receive a large enough impact to cause it to break, the compressive stresses in the outer surfaces are suddenly released, causing the entire sheet of glass to break up into millions of tiny pieces that are usually too small to be very dangerous.

Regards,

Bob Wilson


Hi Chanya,

Thanks for the question. There are many types of safety glass available. Some types of safety glass can stop a projectile such as a bullet or fragments of metal. Other types of safety glass can withstand chemical attack by resisting the action of corrosives. One type of safety glass can stop rifle bullets in one direction, while allowing a bullet to pass in the opposite direction.

A common form of safety glass is tempered (heat-treated) glass that will shatter into small pieces if the glass is broken. These smaller pieces are much safer than larger and razor sharp pieces. I should mention that the smaller pieces are also sharp but tend to be a bit safer to clean up. Windows in your car and home have this type of safety glass.

Safety glass that can resist projectiles (such as 9mm bullets) usually has alternating plastic laminate and glass layers. The plastic laminates absorb the glass fragments and dissipate the energy deposited when a projectile strikes the glass.

I hope this helps. Thanks Jeff Grell


Chanya,

In order to consider what makes safety glass "safe," let us first consider what makes glass "unsafe" in general. When glass is cooled, it is usually cooled slowly, in a process called "annealing." The resulting material can be cut, ground, and otherwise made to fit our purposes, such as in windows. Such glass, however, has a very dangerous tendency to break into shards--long glass pieces with very sharp edges and knife-like edges. Handling these pieces is dangerous as it can result in serious cuts. If you are unlucky enough to fall through such glass and break it, the result can be many pieces that can injure you severely. Further, you do not actually have to cause the glass to break to be injured. A rock thrown through a window, for example, can result in many pieces of sharp glass coming at you like tiny knives.

Safety glass tries to stop or minimize injury from broken glass. There are essentially two common ways to make glass safe. The first keeps the glass in place once it is broken and prevents pieces from flying everywhere. This type of glass is typically made by taking two pieces of glass, sandwiching a piece of plastic between them, and heating the glass pieces until the plastic melts and sticks to the glass. Any broken pieces are held in place by the plastic.

The second common method of making safety glass is to "temper" the glass. Tempering causes the glass to shatter into small pieces rather than sharp shards. The small pieces are less likely to cut or injure you. To temper glass, you set up the final cooling process of creating it in a special way. In this case, as the glass is finally cooled, the outer surface of the glass is cooled faster than the inside of the glass. Cooling the outer surfaces causes it to shrink and compresses the inside of the glass material. The glass finally cools completely, but internally, the glass still has this force remaining. In some ways, it is similar to stuffing a rubber ball into a tube too small to slide it in. The ball pushes back, and this force pushing on the tube remains. If you cut the side of the tube, the ball may suddenly force its way out. Similarly, if you cut the surface of tempered glass, or break it in any other way, the internal force suddenly overcomes the compressed surface holding the glass piece together. The glass shatters suddenly into many small pieces.

Processing glass using these methods (lamination and tempering) can reduce injury if the glass suddenly breaks. You should realize, though, that even safety glass can cause injury when broken. Use gloves and wear eye googles when handling any glass that might break during handling. Also realize that scratching or nicking tempered glass anywhere (including its edges) can cause it to shatter into tiny pieces. I discovered this effect the hard way when adjusting an old shower door that no longer fit well in the frame. One light tap (a mistake) to adjust the frame caused the glass to shatter suddenly leaving me surrounded by bits of broken glass.

As a side note, you may wonder how all those actors easily (and safely) fall and jump through windows without getting cut. They are not actually using real glass, but something called "sugar glass" or "candy glass." This material is simply sugar that has been molded to look like glass. It breaks without creating dangerous shards and makes the process of jumping through windows seem safe in real life. It is not.

Kyle J. Bunch, PhD, PE


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