Why are car windows & windshields glass and not a kind of plastic?
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Plastic would just melt in hot conditions. Also, clear plastic that large without being flexible is not something I have ever see, not saying it doesn’t exist though.
Your car is basically an easy-bake oven that would melt most plastics that can be made transparent. The ones that aren’t, like plexiglass and aerogel, are cost prohibitive to manufacture, are not easily mended or recycled, and bend light more than glass (distort vision & magnify heat).
Windshields are laminated glass to prevent them from shattering in your face. The other windows are treated so they explode into little bits instead of dangerous shards.
We don’t just use plastic because it discolors from UV exposure over time and can’t be broken away during an emergency.
It also has an annoying tendency to creep over time, deforming under its own weight.
Comments about melting are incorrect. Lexan is often used for windscreens on race cars to reduce weight and doesn’t melt. We have plenty of plastics that tolerate heat well. Plastics cannot have near the level of durability and scratch resistance of glass, maintaining clear vision. Glass is generally stronger, yet more brittle. Side and back glass are tempered which increases strength and, when broken, causes the glass to shatter into tiny, relatively harmless pieces. Lots of terrible injuries in auto and home accidents before tempered glass became common. We don’t want windshields to shatter like the rest, so it is standard glass with a layer of plastic in the middle, so if it breaks, the plastic still holds the glass together in one piece and you don’t have dangerous shards of glass around.
Primary issues with plastic: in the windshield/window application:
– Effects of UV exposure: which tend to “fog up” plastics that originally start out as transparent. This is frequently seen in older plastic car headlights or solar cells on cheap garden lights. Becoming non-transparent is a bit of a safety issue in this application
– Lack of scratch resistance: plastics scratch/scuff easily compared to glass which is an issue for windshields especially which are frequently subjected to small rocks and other debris
– Softening/deforming when heated: as others have noted, plastics tend to melt or deform at certain temperatures, which would could create issues.
Ultimately, a windshield/window needs to be able to handle arguably rough conditions while still being durable and transparent. Part of the reason we have so many types of plastics is because they all tend to have different properties that make them good for different applications, but in this situation it’s tough for plastic meet all these requirements and beat out good old fashioned tempered glass.
Plastic would quickly become all scratched up. Your windshield takes an ungodly amount of abuse from tiny bits of dirt and sand and pebbles kicking up from the tires of the vehicles in front of you. Over a long time, many years, a glass windshield will get pitted and need to be replaced because you can no longer see well through it. Plastic would become unusable after a very short amount of time, in comparison.
Glass is significantly harder than plastics, resisting scratches (even the strongest plastics aren’t hard enough), can be tempered, increasing safety when broken (tiny dull beads vs. sharp slicey pieces), is chemically inert in all normal circumstances and won’t melt before the rest of the car.
While polycarbonate is tougher (can survive a stronger impact), tempered and laminated glass is strong enough for the job, while providing the above benefits. Even the best, most chemically and UV resistant plastics will discolour and weaken if kept in the elements and sun for decades, glass won’t.
It’s also cheap to make, like plastics, so no downside there.
1. Plastic generally doesn’t last as long under sunlight, UV rays break it down, causing discoloration and weakening the structure.
2. Load, plastic isn’t as sturdy and rigid as glass, you’d almost certainly need reinforcing structures to support a clear windshield sized piece of plastic, which obviously becomes a vision problem. And/or it needs to be thicker which now becomes both a weight AND a vision problem as light passing through a thicker lens is going to be deflected.
3. Heat. Cars can get really hot. Plastic does not handle heat nearly as well as glass. If the plastic doesn’t outright melt it will probably deform and become weaker.
4. Cold. Plastic also doesn’t handle cold as well as glass. Brittle cold plastic getting impacted by small debris won’t fare as well as glass.
5. Speaking of small debris, plastic scratches more easily and all the small debris abrading the plastic impairs the visibility.
Glass is basically stronger, stays clearer, lasts longer, and is overall better for the needs of car windshields.
The glass used in cars is extremely strong, much stronger than a similarly sized piece of plastic.
Basically, this isn’t like old windows, it’s a highly engineered prodcut designed for use in a car.
The windshield of a car is made from laminated glass. This process makes the glass strong and if it is broken, it will not break into shards and will instead hold its shape (ish). This is preferred because if a brick or something hits a car, the glass might break but it will not turn into dangerous shards and will likely still stop the brick.
Considering the motion of a car, high velocity things are likely to come at the front.
Laminated glass is also good at absorbing energy. If you hit a pedestrian, the glass will collapse under them cushioning the blow.
The passenger windows are tempered. Tempered glass is HARD. Like seriously, where the windshield has some give, the side windows have NONE.
This protects from them getting damaged from general wear and tear.
Also another feature of tempered glass is that is can be shattered by using a small focused point of energy. This means that it is easy to shatter using a simple tool which is helpful in getting people out of the car.
With the side windows, there is less of a concern of a high velocity object flying through the window and hitting a passenger. The windows are smaller (the frame of the car protects you) and that’s just not the direction where shit flies at you from.
Just for story… If you ever see a movie where they are filming through the windshield and there is no shade, no frit, no mirror, those are plastic. A guy is Cali custom formed those for the industry. He passed a few years ago (RIP) but I always think of him when I catch one of those cars in a show.
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