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Tempered Glass
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| Notice how the glass breaks into granular pieces |
Why Does It Break In Such Small Pieces?
Ashish
From coffee tables and skyscrapers to
house windows and car windshields, we are surrounded by glass.
It is a terrific and highly useful
material – transparent, reflective and reasonably strong (depending on the type
of glass).
However, strength is not the only thing that varies depending
on how the glass is
made; there’s another rather unusual trait that
separates certain glass types from others – the way the glass
breaks.
You have surely seen movie
scenes where automobiles (like cars or trucks) blow up or get smashed
against something.
During any such scene, have you
noticed that the glass in car windows doesn’t break like regular glass (that we
use in our daily lives)? Instead, it breaks up into very tiny, granular pieces.
The question is, why does that happen?
Tempered glass
Judging by its appearance and
how it feels to the touch, it’s easy to tell the glass they use in automobiles
apart from regular glass.
The former is called tempered
or toughened glass, and is used in shower doors, refrigerator trays,
architectural glass doors and tables, bullet-resistant glass and smartphone
screen protectors (a rather recent development).
It is known to be 4-5 times
stronger than regular glass and can withstand temperatures up to 295 degrees
Celsius.
Apart from its strength, tempered glass is also known for the
characteristic way it breaks.
Unlike regular glass, which shatters into sharp shards that can
potentially cause injuries, tempered glass breaks into smaller pieces that
interlock with neighboring pieces and therefore don’t fall readily.
Why does tempered glass break
into such tiny pieces?
As mentioned earlier, the characteristics of any glass type depend
on the materials it is made with and the chemical and mechanical processes
it undergoes during its manufacture.
The way that tempered glass breaks into much smaller pieces is
specifically attributed to the manufacturing processes.
Glass (with nickel sulphide inclusions) is tempered by heating and
then rapidly cooled to room temperature.
Cooling takes place much faster at the center of the glass than at
the surface, causing compressive stresses in the surface, whereas contraction
and tensile stresses are caused in the center.
The compressive stress gives toughened glass its increased
strength; the higher the stress, the smaller pieces that the glass breaks into.
The benefit of this differential heating comes in the form of
increased endurance of the glass; it means that such glass can be stretched or
pulled to a certain limit without breaking, which is a good thing to have
in many forms of glass.
In addition to that, the glass is also treated with hydrochloric
acid to eliminate any scratches on the surface and increase its strength.
The nickel sulfides expand over time and impart that ‘spontaneous
breaking’ ability to tempered glass.
Due to the aforementioned manufacturing processes, tempered glass
is able to withstand more stress and bend further before finally breaking.
Laminated glass
Note that toughened glass, i.e. the glass used in
side windows, is different from the glass used in the windshield.
The type used in the latter is called laminated glass, which is
made by sandwiching a laminate (usually polyvinyl butyral or PVB plastic)
between two (or more) layers of glasses and bonding them using high pressure
and heat.
In addition to being very strong, this also offers a unique
advantage. Upon breaking, the broken pieces of glass don’t usually fall out; in
fact, they stick to the laminate and tend to stay in the frame. This is
why applying significant force on the windshield will make the whole
thing pop out of its frame in a single piece.
Due to their strength and the ability to withstand a lot of
pressure, laminated doors are widely used in office complexes and public
buildings. It also offers decent soundproofing.
All in all, both types of glass are strong and have their own
characteristic properties, so choosing the ‘right’ type of glass depends
entirely on your requirements.
Ashish is a
Science graduate (Bachelor of Science) from Punjabi University (India). He
spends a lot of time watching movies, and an awful lot more time discussing
them. He likes Harry Potter and the Avengers, and obsesses over how thoroughly
Science dictates every aspect of life… in this universe, at least.
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| Tempered glass is used in bulletproof glasses |
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| The glass does not shatter out of the frame; instead, it converts into tiny granular pieces and remains intact |
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| For toughened glass, the compressive stress on the surface has to be more than 10,000 psi, whereas for safety glass, it must be more than 15,000 psi. |
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| Cars’s windshields have laminated glass |
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| Notice how the glass comes out in one piece when it’s kicked |
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