About fifteen years ago, my Commodore VL turbo developed a crack in the windscreen. It was only a small crack, starting at the edge of the glass and extending about 50mm towards the middle of the screen. In those days I used to live in the near-Outback, South Australia - and I used to drive very fast. In top speed, at times about as fast as the car could go.
Hmmm, I thought, eyeing the crack, I wonder how much that has weakened the windscreen? The vague thought of the whole thing imploding at about 220 km/h was not attractive.
So I wandered along to a local windscreen supplier and suggested my concerns to the manager. Like many country folk, he was a man of few words - but lots of action.
"Hmmmph," he grunted. "Come with me."
He armed himself with a large hammer and took me by the arm, heading out to the back of the workshop where discarded windscreens were junked. Stopping in front of one, he pointed to the crack that had obviously resulted in the previous owner having the windscreen replaced. Pausing long enough to make sure that I had noted the crack was similar in length to that which graced my car's front glass, the proprietor suddenly attacked the windscreen with the hammer.
Bang! Bang! Bang! bounced the hammer off the screen. The glass barely wilted under the sustained attack. Sure, the outside surface developed some cracks - but the whole windscreen crumple and fall inwards? Not a chance! Point made, the workshop man grunted again and then went back inside...
I've never forgotten that graphic demonstration of the strength of laminated glass...
Windscreens are made from a glass sandwich - two thin layers of glass with a clear layer of polyvinyl butyral (PVB) within. In addition to enormous strength (which protects the occupants from frontal attack and also helps stop their ejection in a major accident), the laminated glass also decreases the chance of glass splinters flying when the screen is broken, reduces noise transmission, and prevents easy penetration by thieves or vandals. It is also lighter than conventional automotive safety glass of the same thickness - by a factor of 11 per cent.
So, with that list of positives, shouldn't it be being used for the side and rear glass as well? The short answer to that is yes - and in fact it already is being used in that application in some cars. Lexus, Audi, Peugeot, BMW, Daimler-Chrysler and some other manufacturers are employing it primarily in order that noise transmission to the cabin is reduced. That's why when you raise the side window in one of these cars, the outside world goes so uncannily quiet.
US company Solutia is at the forefront of this technology, and much of this article is sourced from their engineering papers and publicity materials. Solutia call their laminated glass that's suitable for side and rear applications "EPG", for Enhanced Protective Glass.
Noise
Monolithic glass (ie glass that is solid) has very low noise damping properties. This means it vibrates readily and continues to vibrate once it is excited. The vibrations of the glass create movements in the air on the side of the glass distant from the sound source, so transmitting the noise. Therefore, giving the glass better damping - so that the vibrations are dissipated as a minor heat build-up rather than continuing relatively unabated - will reduce noise transmission through the glass.
The most effective way of increasing the damping properties of glass is to add a special plastic inner layer. Then, when the outer glass layer bends in response to the sound waves, the viscous inner layer creates sheer strains within itself - turning the vibrations into heat.
How effective the damping is depends on the sound frequency. When 4.8mm solid glass is compared with a sandwich of 2.3mm glass / 0.76mm PVB / 2.3mm glass (total thickness 5.4mm), below 2kHz there isn't much difference in sound transmission. However, from 2kHz to 5KHz there is a 2-5dB improvement.
Developments in laminate glasses that have occurred only over the last few years have now made it possible for laminates to be used in side window applications. Specifically, better weather sealing now allows exposed edge glass to pass required durability standards, while the ability now exists to produce thin, bent, heat-strengthened glass in laminate form.
So, how good is this glass in side windows? Two identical model Volvo S80 sedans were tested - one with laminated glass side windows and one with conventional tempered glass. The laminated glass was 4.96mm thick, and the tempered glass 4.85mm in thickness. The cars were equipped with artificial heads, featuring microphones mounted within ear-like protrusions. Each car was then driven alongside a semi-trailer at a variety of speeds and the in-cabin noise levels measured. Measuring sound is notoriously difficult - witness mufflers that 'sound' louder but objective measurement shows to be quieter - but in summary, there were drops in truck noise by as much as 6dB, and a decrease in wind noise by as much as 3dB.
This graph shows the noise level measured next to the truck at 72 km/h, with the red trace being the vehicle with the conventional tempered glass, and the green trace being for the laminated glass.
Security
Without a doubt, the easiest way into most cars is through the side glass. Increasing the security of door locking systems - even by the use of deadlocks - is a little quaint when a smash-and-grab of contents can carried out with access typically gained in less than 10 seconds. In fact, US statistics show that more than 60 per cent of all illegal car entries occur through the glass. Obviously, any glass which slows or even prevents this access improves vehicle security.
Apparently, thieves gain access through the side glass in primarily two ways - using a spring-loaded centre-punch to shatter fully tempered glass, and the use of a large suction cap to pull the glass outwards that is normally flush-mounted or in frameless doors (ie the glass is not held securely in place by the presence of a full frame that positively engages it). Resistance to pull-out and the prevention of the formation of a hole large enough to allow the insertion of an arm (or, in the official tests, a 40mm sphere) are two tests that are used to evaluate the security aspects of door glass.
Tempered glass, which is 4-5mm thick, is the traditional material of use in door glass. This is because it is robust enough to withstand door slamming and has a high bending strength. However, as indicated above, its security aspects are somewhat suspect.
All tests show that laminated glass has a better resistance to puncturing, while its ability to withstand pull-outs depends very much on the way it is held in place. If the door frame strength is adequate to withstand pull-out stresses and strains, laminated glass improves security under this sort of attack as well.
Aspects affecting the strength of laminated glass in security applications include the thickness of the internal PVB layer (more = better), and the adhesion between the PVB interlayer and the glass layers (less = better the resistance to puncturing, as the plastic layer can more easily stretch without tearing). However, there is a minimum adhesion necessary if the sandwich isn't to become undone!
Other Benefits
- An infrared reflecting layer can be added to EPG glass, reducing solar energy transmission by up to 60 per cent. This can reduce the temperature inside a vehicle exposed to the sun by 15 to 20 degrees C. The infrared reflecting layer is either bonded between two layers of PVB, or coated onto the inside surface of one of the pieces of glass and laminated with one layer of PVB.
- EPG at similar thickness weighs less than standard tempered glass. For a 5mm thickness, EPG weighs almost 10 per cent less than standard tempered glass, which gives a weight saving of 3.5kg for all side and rear glazing of an average size vehicle (3 square metres of glazing).
Conclusion
With major manufacturers already using the special glass in side, rear and - in some cases - the sunroofs of their upper models, it's only a matter of time before it is more widely adopted. Rather than going to the complexity and weight disadvantages of double-glazing, or simply increasing the thickness of monolithic glass, laminated glass represents an effective answer to noise, security and some other concerns.
http://www.solutia-autoglass.com/index.jsp
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