Why is plexiglass not used for car windows?

Question

Back in 2014 someone broke the right side window of my VW Golf.

I was in a hurry and short on cash so I "temporarily" cut a 4 or 5mm thick piece of plexiglass ($8), bent it with hot air and placed it in my car thinking that a few weeks later when it will be worn out I would replace it with a proper window.

It's been closing in two years and it is still crystal clear (which is why I kinda forgot about it), though with microscratches if you look really close. Still transparent as ever.

I'm bringing this up because yesterday two guys on a bike attempted to steal my briefcase. Stopped right next to me on a red light and one of them tried to smash through the glass with a hammer. The tool bounced right off the window and hit the driver of the bike on the helmet. They almost fell and fled ungracefully.

For a very long time I thought that plexiglass is not fit for automotive purposes but now I am reconsidering.

Apart from the windshield (which would degrade faster due to direct impact of microparticles), what do you think are the reasons that plexiglass is not considered for car windows?

Answer 1

Plexiglass is flammable. While it doesn't release toxic gases or excessive amounts of smoke, it is still rated B2 (normally flammable) and thus forbidden as interior material in motor vehicles, including windows.

Here is a relevant US standard if you're interested in details.

Collision behaviour mentioned by Thomas is also a crucial property, especially for bigger pieces like windows. If that guy on a bike had a bigger hammer, he could have popped your window inside the vehicle, where its shards would have released their kinetic energy by hurting the driver and the passengers. The same thing could have happened on a side collision with another car.

Answer 2

Because of how it reacts in a collision. Auto glass is designed to shatter into small bits that are fairly dull, as opposed to large, sharp daggers. In the US, your car must be outfitted with glazing material that meets certain specs, and material type is not specified. If anyone could get plexiglass to satisfy those specs, I am sure it would be lower cost and popular. So I assume plexiglass cant meet the specs.

Also, might have to do with the gasses released if it is melted.

Aside from that, over time plastic fogs from UV. Most headlights these days are plastic. One of the first things some enthusiasts do is replace them with glass. When you look at cars barely 10 years old with fogged and pitted headlights, that is exactly what plexiglass would look like it it was used on the windows.

Answer 3

In addition to to the other factors mentioned. Plexiglass is not opaque to UV radition like silica glass. Passengers would get sunburned driving around exposed to sunlight.

Used to be, when not all cars in hot climes had air conditioners, you could tell those drivers who did not because they'd roll down the driver's side window and sometimes put their elbow on the door. The left arm would tan while the right arm, under the UV blocking glass would not.

In the third world, they're taking advantage of most plastics UV transparent to sterilize water. They just fill up an old hand-sized water bottle and toss it it up on a corrugated tin roof for a week or so and when they take it down, all the microbes have been fried. Somebody should have gotten the nobel prize for that.

Answer 4

Plastics are awesome. One of their characteristics is that they easily degrade in UV / sunlight. In my experience that is the weakest link in its usage. With that said, there are a whole lot of plastics used on modern cars.

Generally headlamp lenses are made of polycarbonate. The polycarbonate material has a thin coating sprayed on top of the lens to help provide extra UV protection.

Red tail lamps lenses are virtually all acrylic. It's got decent properties, molds well and passes light consistently. And plexiglas is basically Acrylic.

As for why plexiglass is not used in cars? My guess is you really need to follow the dollar bill $$$$$$$$. Glass forming dies aren't nearly as expensive as molds necessary for things like headlamp lenses. The molds (Dies) for these products is crazy expensive. You can't imagine.

I did a whole lot of engineering for a US based Original Equipment Manufacturer (OEM) for both exterior glass and exterior lighting products. I will say the molds for polycarbonate headlamp lenses were some of the most highly engineered (and super expensive) tooling we had to deal with. Because of the size of the molds you really can only have one person working on a mold at a time. Its not possible to put ten people on the job and have it go ten times faster. That steel is super expensive grade, super hard. If you do something wrong or try to go too fast you can crack a mold.

Some of those tools can take up to year to produce from start of design till first parts come out of the press. From my experience, headlamp design and tooling is on the critical path for a vehicle's development cycle.

No way you'd want that hassle or tooling investment cost for door glass or windshields or rear window. Float glass is still expensive if you had to build up a glass factory from scratch, but once the factory is built glass is very reasonably priced. Heck, the raw material for glass is essentially sand. Float glass has been around for a long time. True you can only shape glass as a ruled surface but designers have worked with that for a long time.

And glass doesn't degrade in UV light. Not one bit.

For the question "Why did the manufacturer do that?" the answer is often.. because its cheaper that way. (Follow the money!)

Answer 5

I think in answering this question you need to answer the initial question which is fundamentally, is plexiglass used for glazing in road cars. The answer to this question is that it is a very definite yes, it is used.

One example of a production vehicle that uses plastic windows is the Porsche 911 GT3 (991 GT3 R). Taken from wiki information about the car available here

All windows – and for the first time ever, the windscreen – are made from polycarbonate to cut weight

Local legislation may prevent the use of plexiglass for road car windows, which is reflected in the same article which states

The American version ... has a standard rear window (not plexiglas) ... to comply with rules of SCCA

Ths 911 GT3 was the first example I thought of but some further research reveals that more humble road cars, such as the Smart Fortwo roadster uses plastic windows.

Answer 6

it scratches too easily. thus, it cannot be used in a road environment

Answer 7

Apart from collision/fire behavior, automotive glazing has to resist damage from road debris - stones and such. It also has to resist abrasion from dusty/dirty windshield wipers. That requires a hard material which plexiglass is not. As well, you need an undistorted view of the road ahead, which requires that the windshield be of uniform thickness and free of ripples. That can be hard to do with plastic.

Answer 8

In Israel people who live in dangerous areas can have their car windows replaced for free. They replace the front windshield with bullet proof glass. The side windows are replaced with some type of plastic to protect from rock throwing. The side windows do scratch and turn a yellowish color after a few years and then they can be replaced.

Answer 9

One insight I got in a similar question about house windows on a different SE site:

The assumption that you want unbreakable windows in a car might be wrong. If you crash your car in a way that jams your doors - e.g. someone rams you against a wall from the side - the side and rear windows provide a quick method of escape if you have an emergency hammer designed to break through the tempered glass used in side windows. (Which is designed to shatter into small, blunt shards.) The windshield is made out of laminated glass which won't break anyway, precisely so road debris doesn't destroy it.