A future for Polycarbonate Automotive glazing? 1

[HDS]

I am sure that most of you have seen some of the news about Polycarbonate Automotive glazing. GE and Bayer will be spending a lot of money to promote this over the next few years. What do y’all think? Will this become popular? Will it deliver everything they promise? Check out

http://www.exatec.de/default.aspx,

http://www.manufacturing.net/dn/index.asp?layout=article&articleid=CA324734,

for more information.

My only window design experience is not on cars but on cell phones. For the phones acrylic is a much better choice than PC. It is more scratch resistant. Doesn’t have birefringence problems from molded in stress. I see these same issues as being a problem for car glazing as well. Even the samples they were showing at NPE had birefringence. Lots of people use polarized sun glasses when driving. So this could be a noticeable problem.

UV resistance will also be a big problem. Just look at how much UV damage there is to automotive accessories like bug guards. The coatings Exatec has developed should help but they will have to be applied to both sides of the glass and the edges.

A lot of the advantages they describe like colors and curvature are not really limits of glass but of cost and optical quality.

There are some real benefits if they can solve the other problems. Obviously better impact strength has some advantages but it could also be a disadvantage if it increases loads on crash victims that come in contact with it. The ability to mold in details like ribs and snaps that would be impossible with glass is nice. The increased flex ability might help with sealing. It could be lighter weight and could also add some damping to the structure.

I do see it being used in sunroofs or windows in lift gates that do not have wipers. Interestingly Exatec never mentions replacing the only current use of plastic windows in cars. Convertible tops.

 

[patprimmer]

As far as I know, cast acrylic is still the material of choice for aircraft windows.

The main weaknesses of PC are vs Acrylic are, UV, flex mod (I think), cost, scratch, light transmission, fatigue and solvent stress crack. Coatings improve some of these qualities, the question is to what extent.

Solvent stress crack and fatigue would be my main areas of concern.

The main advantages I see in plastic windows (acrylic or PC)are weight saving, increased functional design flexability, and improved asthetic design flexability. Acrylic needs little in the way of UV protection, and can also be coated for improved scratch. Impact is the weakness of acrylic.

If anyone seriously want's to know, I could probably find some old references to felex mod, surface hardness, taber abrasion resistance, UV resistance, %age light transmission, haze factors and refractive indicies of both. I don't know what happens to the light at the plastic matrix to coating interface.

Regards
pat

 

[HDS]

How big of an advantage does plastic have in design freedom? It seems to me that the main limits the glass in current products is the tooling cost.

The one tool for a window that I have seen was a very cost effective solution. Probably 5% of the cost of an injection molding tool that size would be. I am sure that you could make more complicated parts with more expensive tooling.

Is there a process similar to injection molding for glass? There are some extremely viscous plastics that can be injection molded?

 

[CoryPad]

I can't add a lot to the discussion. My opinion is that there is a large opportunity for plastic glazing, mainly for safety improvement, but also lower mass and better design flexibility. Scratch resistance, UV resistance, etc. are not big problems - cost will be the big one for automotive applications.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[patprimmer]

Scratch and UV resistance are not a problem, short to mid term, at least with the technology used with plastic headlights. They seem to last about 8 to 10 years, at least in Sydney. If more than 10 years is considered advantageous then they may well be a problem. As lights are exp[osed to extra UV and heat from the bulb, they will suffer more than windows, but moveing window glasses might be more subject to scratching.

The cost of headlight replacement is high enough to consider scrapping an otherwise servicable 10 year old car. having to also replace windows would cause the scrapping of 10 year old cars that are otherwise in very good condition.

I know in some areas, cars are never in good condition after 10 years, but in dry warm climates, they can easily last 20 or 30 years.

I wonder what the net impact is of lighter more fuel efficient cars vs shorter life of the car?

Regards
pat

 

[HDS]

Cory
Where so you see the safety improment coming from? The great impact restance of plastic would prevent intrusion of objecets into the vehicle. However, it could also increase the loads on a person that impacts the window. I think this is why they do not plan to make windshields from it.

 

[CoryPad]

HDS,

With airbags as standard equipment, it is rare for people's heads to contact the windshield. I see polycarbonate as safer than glass because 1) it can absorb crash energy through plastic deformation and 2) it won't have sharp edges when it does fracture. Though neither are large problems now, every little bit helps.

Regards,

Cory

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[CoryPad]

I forgot about another big benefit to polycarbonate glazing - decreased vandalism and theft threats.

On a personal note, another benefit is decreased probability that the glazing will fracture during normal use. My passenger door window fractured in the down position during my drive to work today. I had to drive for about 30 minutes with the window open while the outside temperature was ~ 3 degrees Celcius.

Regards,

Cory

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[ProEpro]

That reminds me of a story I heard about plastic and windshields. It was a Cadillac windshield. It had some kind of plastic film on the inside. When someone went to scrape off the emissions sticker they scraped through the film. They said the windshield popped and had to be replaced.

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[JTreglio]

Interesting point on the UV absorption of acryllic. Many years ago I was developing a detector for elementary particles where I had to collect light signals on a photomultipier tube. I had to use acryllic for the light pipe. However, I couldn't use the standard stuff because it had UV absorbers added. I had to pay more to get material without the UV absorbers.

On the windshield issue, they may be faced with the light reflectance/transmittance issue. The higher the transmittance the lower the windshield angle can be, hence the lower the drag. They may not achieve adequate transmittance with plastics.

Jim Treglio
Molecular Metallurgy, Inc.

 

[patprimmer]

As far as I know, some large telescope lenses were made from acrylic because it had better light transmission than glass. It also (from memory) has a higher refractive index, allowing lenses to be either more powerfull, or thinner at the same power.

I expect a higher refractive index might actually be a disadvantage in windscreens, but probably doesn't matter in other windows.

Re light transmission, as far as I know all automotive windows have some tint to reduce light transmission. I expect this is selective to filter more from the UV and infra red areas so as to reduce damage to the interior from UV and heat as well as for improved passanger comfort, and dare I say it, for fashion.

Regards
pat

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[JTreglio]

Actually, we used acryllic for the light pipe because it had a high refractive index, yielding more internal reflectance and less leakage. The opposite is needed for the windshield -- low refractive index -- to keep reflection down, transmission up in the visible light range.

Jim Treglio
Molecular Metallurgy, Inc.

 

[ProEpro]

For the telescopes it might also have been easier to cast the acryllic than the glass.


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