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Automotive Glass
How
Automotive Glass Works
BY CHRISTOPHER NEIGER
When
car companies run ads on television touting their vehicle's new safety
features, they rarely mention the car's windshield or the surrounding windows,
but the glass surrounding you in those vehicles has been designed and
manufactured with your safety in mind.
Although automotive glass looks the same as any
other type of glass, it functions very differently.
In
most homes, the windows in each room are made from a standard type of glass
that will shatter into large shards when it breaks.
With
the exception of a sliding glass door or front door, these home windows don't
receive the same amount of strain that an automotive window does.
A
car, on the other hand, will encounter many potholes, rocks and fender benders
in its lifetime.
Because
of this, automotive glass is manufactured into two different types of safety
glass to protect both the structure of the vehicle and the occupants
inside.
The
first type of glass is called laminated
glass, which is for the windshield.
The
second type of glass is known as tempered
glass, which is used for the vehicle's side and back windows.
Later
on, we'll learn how glass makers insert a thin layer of film between two layers
of glass and fuse them together through heat and pressure to make laminated
glass.
We'll
also take a look at how tempered glass gains its strength through a process of
heating and rapid cooling.
Without
these different styles of manufacturing and strengthening, automotive glass
would be little more than a simple barrier between us and the elements outside.
Laminated
and tempered glass each have different functions, but together, they keep you
inside the vehicle in an accident, shield you from flying sharp glass, retain
the roof's rigidity in a rollover and allow the side air bag to protect you
when it's deployed.
Let's
go on to the next page and learn when these types of glass were first used and
why.
The History of Automotive
Glass
During the early 20th century, horseless carriages
started using glass to protect drivers from harsh winds. However, the standard
form of glass used in those times didn't adequately protect occupants from
flying debris.
It also posed a risk to the occupants if an object
struck the glass or if the vehicle was involved in an accident.
In 1903, French chemist Edouard Benedictus stumbled
upon the secret to shatter-resistant glass when he dropped a glass flask filled
with a dried collodion film.
He
found that the glass coated with the film cracked, but kept its original shape.
However, this laminated glass wouldn't be implemented in automobiles until the
1920s [source: Time].
Automakers used laminated glass in their windshields
to optimize occupant safety during accidents and to protect passengers from
projectiles during normal driving conditions.
For
all its benefits, though, the first types of laminated glass offered limited puncture
resistance.
Today's
laminated glass consists of a thin layer of polyvinyl
butyral (PVB) inserted between two layers of solid glass.
In addition to laminated glass, automakers began to
use tempered glass in the late 1930s.
This
type of glass is used in the vehicle's side and back windows and gains its
strength through a heating and rapid cooling process that strengthens the
glass' outer surface as well as its core.
By the 1960s, the American public had become
increasingly aware that automobiles needed to be designed for more than just
looks.
This
realization derived partly from consumer crusader Ralph Nader’s work to expose the dangers posed by certain
vehicles and the need for government safety standards. In response, the U.S
government formed the National Highway Traffic Safety Administration (NHTSA) in
1970 [source: Bowen].
Since then, NHTSA has implemented regulations
affecting all areas of vehicle safety, including automotive glass.
Some
of the Federal Motor Vehicle Safety Standards (FMVSS) for automotive glass
include:
· FMVSS
205 -- This set clear standards for automotive window
transparency and the strength of automotive glass required to keep occupants
inside the vehicle during accidents.
· FMVSS
212 --This windshield mounting standard was established to ensure
a certain level of windshield retention strength during accidents.
· FMVSS
216 -- This legislation implemented a standard for roof
rigidity in case of a rollover.
· FMVSS
219 -- This standard states that no part of most passenger
vehicles can penetrate the windshield more than 6 millimeters (0.24 inches) in
a crash.
Now that we know how automotive glass came to be,
let's find out how it's made.
TIPS FOR BUYING AND REPLACING AUTOMOTIVE GLASS [source: ABC News]
· Replacing your windshield is a major safety
concern. If your windshield has been severely damaged, don't put off getting it
replaced.
· Make sure the technician replacing your
windshield is certified to do so by the National Glass Association.
· Once your windshield has been replaced, make
sure to wait the recommended time before driving the vehicle. In some cases,
this can last up to 10 hours.
Laminated Glass and PVB
Laminated glass is made by sandwiching a layer of
polyvinyl butyral (PVB) between two pieces of glass. The glass and the PVB are
sealed by a series of pressure rollers and then heated.
This
combination of pressure and heat chemically and mechanically bonds the PVB to
the glass.
The
mechanical bond occurs through the adhesiveness of the PVB, while the chemical
bond is created through hydrogen bonding of the PVB to the glass.
That
inserted layer of PVB is what allows the glass to absorb energy during an impact
and gives the glass resistance to penetration from flying projectiles. It also
deflects up to 95 percent of ultraviolet (UV) rays from the sun [source: Reuters].
Laminated
glass can break and be punctured, but it will stay intact because of its
chemical bond with the PVB.
The
strength of laminated automotive glass allows it to perform two very important
functions in cars.
First,
it allows the passenger-side air bag to deploy correctly.
Driver's
side air bags tend to fly straight toward the driver from the steering wheel,
but when the passenger air bag is deployed, it bounces off the windshield
toward the passenger.
An
air bag deploys with incredible speed --1/30th of a second -- and can withstand
2,000 pounds (907 kilograms) of force.
The
windshield has to absorb both the speed and force of the air bag in order
to protect the passenger in an accident.
Because
of its strength, laminated glass can keep occupants inside the car during an
accident.
In
the past, occupants could be ejected through the windshield because the glass
wasn't strong enough, but today's windshields provide more security.
In
addition to absorbing the force of deployed air bags and keeping passengers
inside the vehicle, laminated windshields also provide strength to a car's
roof.
Windshields
keep the roof from buckling and crashing down on passengers completely during a
rollover.
Without
the rigidity and strength of laminated glass windshields, many roofs would pose
greater risks to passengers in certain kinds of accidents.
If
you happen to find a small chip in your windshield, don't worry. You don't have
to go out and get your entire windshield replaced to retain its strength.
Some
small chips can be quickly and easily repaired with a windshield chip repair
kit.
Most
automotive stores carry these kits for around $10 and allow you to inject resin
into the trouble spot and remove excess air in the problem area.
Once
you've repaired the glass, you'll barely be able to notice your patch job.
Let's
move on to the next page to find out how tempered glass is made and how it
protects.
Tempered Glass
Tempered glass is just as important to a
vehicle's safety as laminated glass, but it differs greatly in both form and
function.
This
type of glass is used for the surrounding windows of car (also called the sidelites) and the back window (or backlite).
Tempered
glass is created by heating and then rapidly cooling the glass to room
temperature by ushering it through a system of blowers.
The surface of the glass cools much faster than the
center of the glass and contracts, causing compressive
stresses, while the center of the glass expands because of its temperature,
producing tensile stresses.
What
does that mean? Imagine a piece of glass that could be pulled or stretched to a
certain length (tensile stress), while being pushed down and compressed
(compressive stress) simultaneously.
Both
the pulling and pushing stresses achieved through the heating and cooling
process give tempered glass its tensile and compressive strength.
The
differences between these two give the glass 5 to 10 times the amount of
strength it originally had.
The edges on a typical piece of tempered glass are
very weak. This is caused in part by the rapid release of heat during the
cooling phase of the tempering process.
To
help compensate for this weaker area, the glass is ground down on the edges.
When tempered glass breaks, it shatters into small, dull pieces.
The
differences between the compressive and tensile stresses are what enable the
glass to break in this way.
The
pulling and the pushing of the glass produce a significant amount of energy
during the tempering process. When the glass breaks, this energy is released
and causes the glass to break into small pieces [source: AIS Glass Solutions].
Because of its strength, tempered glass can withstand
the daily use of automobile driving.
Without it, our cars would be filled with glass every time we encountered a
pothole, got into a fender bender or closed a door.
Future Developments in
Automotive Glass
Because of its strength and solid safety record, some
car manufacturers are considering the implementation of laminated glass into
all areas of their cars.
It's
already in use in some larger vehicles: General Motors has installed
it in the back windows of their passenger vans to keep occupants inside the
vehicle during major accidents.
Some
manufacturers, like BMW, have already placed laminated glass in the sidelites
of some of their models as an extra protection from theft.
In
addition to the safety boost it provides, laminated glass also acts as a good
sound dampener because of the PVB inside it [source: Allen].
However, there's one problem with implementing
laminated glass throughout a vehicle: In an emergency, an occupant needing to
exit the vehicle quickly couldn't break the laminated glass without help.
Due
to its strength, laminated glass can take 10 times longer to break than
tempered glass, which can make escape difficult for a weakened and injured
passenger [source: Allen].
This
dilemma hasn't stopped automotive designers from devising new ways to get more
laminated glass into our cars.
For
example, cielo roofs (the name
comes from the Spanish word for "sky") have been popping up all
over the concept car circuit. Cielo roofs extend a car's windshield behind the
driver's head, converting the entire roof into a single piece of laminated
glass [source: Allen].
Automotive glass isn't only being designed for safety
and comfort. Glass makers and car manufacturers are also trying to find avenues
for recycling the glass.
Although
some excess glass produced during the making of automotive glass does get
recycled, once the automotive glass is fitted for a car, it becomes harder to
recycle because of additions like coatings and heating elements.
Despite these dilemmas, glass manufacturers continue
to explore new ideas for making glass stronger, safer and adaptable for new
vehicles.
You
might not think about it much, but our vehicles wouldn't be as safe as they are
without modern tempered and laminated glass.
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