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Glass is fragile and impractical to transport, install and repair. Even worse, glass kills and hurts people when it breaks. Falling to the streets like guillotines during earthquakes and bomb raids. During wars people put tape on their windows to prevent shattering. When that meteor exploded over Chelyabinsk, people got hurt by standing inside of a window watching the sky when the shock wave hit them.

There are perfectly transparent plastics, for example the PET material used to make coca cola bottles. Why aren't windows made out of that instead of glass (fragile ceramics)? It seems to be much cheaper, safer and more practical to handle. Is there any advantage at all to make windows out of glass? Is this a billion dollar business idea, and if so, why haven't anyone realized it yet?

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    $\begingroup$ If you want to reinvent architecture so that it can cope with bomb raids, war, and meteor impact as typical use cases, what about flammability of plastics? $\endgroup$
    – Hagen von Eitzen
    Commented Feb 13, 2016 at 16:21
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    $\begingroup$ Glass is sturdier than you make it out to be. Consider that a beer bottle is notoriously stronger than a human skull. Fragility is part of the "cost" to pay for hardness - basically, glass will either stay mostly unaffected by stress, or it will fail completely. (Where plastics would scratch, bend, buckle, etc.) Dumping glass it because of those failure modes as opposed to addressing them directly seems like throwing the baby out of the bathwater. $\endgroup$
    – millimoose
    Commented Feb 13, 2016 at 19:09
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    $\begingroup$ @millimoose > since you can put your hands into an oven hotter than that for a few seconds molten plastic will transfer heat far more effectively than hot air. It has the added 'bonus' of probably sticking to your skin. $\endgroup$
    – Bob
    Commented Feb 13, 2016 at 20:24
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    $\begingroup$ There is also the consideration that in an emergency you can break a domestic glass window by hitting it with a chair to provide an escape route or at least access to fresh air, In this case the risk of cuts from broken glass is probably still acceptable if the alternative is dying from smoke inhalation. $\endgroup$
    – Chris Johns
    Commented Feb 13, 2016 at 20:38
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    $\begingroup$ There's is an issue I don't see anyone mentioning. Windows are already a key point in homes where insulation is lost (and hence energy and money is wasted). Do potentially plastic windows have similar properties in terms of heat transfer as their glass counterparts? $\endgroup$
    – PVAL
    Commented Feb 14, 2016 at 3:35

9 Answers 9

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There are two main reasons why glass is still preferred over say PMMA.

The first is durability. As long as it isn't broken, the glass in a window can easily last for hundreds of years in good condition. In particular it is a lot more resistant to scratches than comparable plastics and isn't really subject to much in the way of environmental degradation. Windows are very prone to getting scratched when they are washed as they accumulate small particles of grit on their surface which gets rubbed around the surface during cleaning. Even with scratch resistant coatings no transparent plastics get anywhere near the hardness of glass.

Most glasses are also much more resistant to environmental degradation from sunlight and various chemicals in the environment. Even the most resistant plastics start to discolour and become brittle over time.

The second factor is stiffness. Glass has a much higher Young's Modulus than PMMA. In bottles etc which are stiffened by their shape this doesn't matter much but, as windows tend to be large, flat, thin panels stiffness is a big issue, affecting their ability to be sealed into their frames and their optical properties. So a plastic window would need to be substantially thicker than a glass one to have the same stiffness with consequences for optical quality and cost.

There may also be issues with gas permeability in the context of double glazed windows.

In addition many of the safety concern raised in the question are addressed by laminated and tempered glass. Tempered glass is heat treated to control internal stresses, making it significantly stronger than float glass with the additional benefit that if it does break the entire plate fractures into small granules rather than sharp shards. Laminated glass consists of alternating layer of glass and a polymer film, producing a composite sheet with very high strength and toughness, potentially to the point where it can be usefully bullet resistant.

Another aspect of this is that side and rear windows in vehicles are often required to be made from tempered glass for safety reasons as they can be safely broken to allow access and extraction of passengers in an accident if doors are jammed or inaccessible.

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    $\begingroup$ +1 good answer! The only things I would add to this are environmental properties, specifically UV degradation of polymers (they turn yellow and crack), and creep and sagging from elevated temperatures. Both can be mitigated, but not prevented entirely in bare sunlight. Silica based glass suffers from neither problem on human timescales. $\endgroup$
    – do-the-thing-please
    Commented Feb 13, 2016 at 23:22
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    $\begingroup$ Forgive the naive question, but how does laminated glass avoid introducing the problem you mentioned of having a more scratchable, less weather-proof surface? Also are tempered or laminated glass commonly used in modern building construction? $\endgroup$
    – user56reinstatemonica8
    Commented Feb 15, 2016 at 10:11
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    $\begingroup$ The other layers are glass so a hard surface is maintained, it is essentially a layer of plastic sandwiched between two thin glass layers. The inner layer may also have others films to provide tints or control reflectivity etc. Toughened glass is widely used for glass doors in particular. Usage will depend on specific design decisions and local building regulations but windows in 'skyscrapers' are very likely to be laminated glass. $\endgroup$
    – Chris Johns
    Commented Feb 15, 2016 at 10:21
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    $\begingroup$ @starrise "Silica based glass suffers from neither problem on human timescales" - maybe, maybe not: consider the purple windows in Boston's Beacon Hill neighborhood. $\endgroup$
    – davidbak
    Commented Feb 16, 2016 at 19:24
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    $\begingroup$ Neat find, I seem to recall that certain eras of glass bottles can turn blue/green in sunlight as well. I suppose my statement should be amended to state "properly manufactured modern window glass" instead. libanswers.cmog.org/mobile.php?action=5&qid=283712 $\endgroup$
    – do-the-thing-please
    Commented Feb 16, 2016 at 19:48
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Here is a plastic (most likely PMMA) window, in a boat, after only 37 years.

enter image description here

In addition to the obvious scratches, the outer surface has developed a cloudiness : possibly from degradation due to UV light, and (towards the LH end) you can see a cubelike pattern of stress cracks, rather like a toughened glass windscreen after a pebble hit it.

You really can't tell what you're looking at through it.

In this application, a PMMA window is still the best solution, rather than the difficulty of making a glass window fit the curve of the hull. But it shows the limitations compared to glass, which remains usable for centuries.

As far as replacing glass windows, the cost of a specialist will be high for any type of repair in any technology.

But absent custom curves, the raw material (glass) is cheap : about £1/sq foot, much cheaper than perspex or polycarbonate, and much easier and faster to cut. (Watching a professional exploit the special fracture properties of glass is impressive. Contrast with the difficulty of cutting perspex or polycarbonate!)

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    $\begingroup$ Any window as thin as a soda bottle won't survive a winter, and if it's as easy to replace as wash, it's not well fixed or sealed - OR that's a very impressive windowframe! (The metal was fine : the paint was cheap household paint in a damp environment, the original lining - more plastic! - having died long ago) $\endgroup$
    – user_1818839
    Commented Feb 14, 2016 at 15:12
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    $\begingroup$ This is a great answer. Personally, I have a hard enough time trying to keep windows and doors sealed at home when (such as now) the temps are single-digit outside. I think you sacrifice the efficiency of such a seal if you make the pane easily replaceable, no matter how good your design. There's a reason we still seal our tubs, sinks, sills, etc. with caulk or silicone, for example. $\endgroup$
    – trpt4him
    Commented Feb 14, 2016 at 16:03
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    $\begingroup$ @LocalFluff : then you can demonstrate the idea with low cost plastic sheeting and whatever outer frame you choose, applied over existing windows and frames. If you can get it to survive 5 or 10 years, no misting over, no water leakage, cheap and easy replacement (even upstairs) you may be on to something. $\endgroup$
    – user_1818839
    Commented Feb 14, 2016 at 16:27
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    $\begingroup$ I'm curious how this window would react to a heat-gun. I suspect it'd fix it up, but there's a good chance it'd do the exact opposite. $\endgroup$
    – Dewi Morgan
    Commented Feb 15, 2016 at 20:25
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    $\begingroup$ @DewiMorgan Heat gun might clear the cloudiness (if you're lucky and don't melt the surface) and maybe smooth out the scratches to some extent, but I don't see it doing the stress cracks (right through the interior of the material) any good. $\endgroup$
    – user_1818839
    Commented Jun 10, 2016 at 17:43
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What are the requirements for window glass?

  • Resistance to moisture
  • Resistance to UV radiation
  • Resistance to cleaning agents
  • Very high transmission in the visible spectrum
  • (There are many more, from an engineering point of view)

Most transparent plastics are not resistant enough. Plexiglas is one example which fulfills the requirements enough to be used that way. See this brochure from evonic where they guarantee you 30 years without noticeable yellowing.

The biggest problem with PMMA for household use is its "softness" (more prone to scratches) and mediocre resistance to cleaning agents as compared to glass. Now, there are many different flavours of PMMA, and you can coat a Plexiglas sheet to make it more durable.

PMMA has many advantages over soda-lime glass:

  • Higher transmission in the visible spectrum
  • Lower density
  • Easier processing
  • ...

It is at the moment probably just not cost effective enough to make PMMA as resilient as glass for use as window glass (for household use!, PMMA is used in many different application as a better alternative to the classical inorganic glass). This may change in the near future, as material science and the transparent plastics industry progresses.

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    $\begingroup$ I think scratch resistance is the killer feature. I use plastic lenses on my perscription glasses for shatter resistance, but eventually the lenses get so scratched beyons use. Coatings will also eventually break as heat variation occours. In practice i have ever only broken one window in my life even that was due to building element shift. $\endgroup$
    – joojaa
    Commented Feb 13, 2016 at 16:57
  • $\begingroup$ I can imagine replacing the outer plastic window at no higher cost or trouble than cleaning a glass window. Like a tape or curtain. I put some kind of wax on my car windows in order to prevent dirt from sticking to them. Cost efficiency should come with economies of scale, considering how many billion of windows there are in the world. $\endgroup$
    – LocalFluff
    Commented Feb 14, 2016 at 11:52
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I actually did make windows out of plexiglas for an outbuilding. I could work them to the shape I wanted and they were lightweight. However, I discovered that they indeed scratch easily: just trying to get the protective paper off I both scraped and melted the plastic.

We want hard windows for houses and primary buildings. So what you describe would be addressed by laminates, as used in automobiles. I'm sure you can buy that. But most people find having shutters to be cheaper and satisfactory if severe weather may cause breakage, along with breakage warranty and insurance, or the cheapness of replacing plain glass panes.

There are regulations for requiring tempered glass for some windows that a child may fall into. Tempered glass in general addresses the dangers you raise. I don't know about regulations for glass clad skyscrapers where earthquakes are prone, but I'll bet they don't fall even if shaken because they are hung and can move independantly.

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After some research,

we are using glass windows as it was used traditionally. Actually plastic windows exists. Though it may take time for most of us to adapt to it.

While the reasons to consider glass windows are:

  • Up to 80% of all recycled glass can be reclaimed.

  • Recycled glass uses 40% less energy than manufacturing new glass.

  • Recycling doesn’t compromise glass’ quality or structure and no toxins are produced in its recycling.

Reference

Plastic windows availablity

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  • $\begingroup$ I actually think that recycling is yet another argument against glass windows. A couple of times a window has broken for me, and it has been expensive to call a specialist to reinstall a new one. But I have consumed thousands of plastic transparent soda bottles and none of them ever broke. Nor have I heard of anyone ever getting hurt by a plastic bottle. There's an elaborate consumer recycling system for PET bottles, melting and reforming them, although they really only need to be cleaned and refilled since they are practically unbreakable. $\endgroup$
    – LocalFluff
    Commented Feb 13, 2016 at 8:46
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    $\begingroup$ @LocalFluff ive also reclaimed many glass bottles and most of them didnt break. I also broke a plexiglass window which also needed a specialist that was hard to find and even more expensive than glass windows. Also glass has near infinite recyclability, that is it does not degrade as it is recycled. $\endgroup$
    – joojaa
    Commented Feb 13, 2016 at 16:49
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    $\begingroup$ @LocalFluff, reglazing a glass window (at least a traditional wooden-framed one) is something anyone can learn to do in an hour or so. Why do you think repairing a glass window requires more of a specialist to do than repairing a plastic one? $\endgroup$
    – The Photon
    Commented Feb 15, 2016 at 23:46
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    $\begingroup$ @LocalFluff: PET doesn't make good windows, and PMMA isn't easy to recycle. $\endgroup$
    – MSalters
    Commented Feb 15, 2016 at 23:59
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    $\begingroup$ I've diy-installed both glass and plastic windows (plastic for safety, glass for durability and clarity) and I've found that glass is easier to work. Polycarbonate needs a special saw blade or a lot of elbow grease with a scoring tool and it's really easy to mar or scratch up. $\endgroup$
    – Adam Brown
    Commented Feb 16, 2016 at 21:33
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Another issue is that the coefficient of thermal expansion of many plastics is higher than that of window glass. Frames for plastic windows would have to be able to accommodate more expansion / contraction than those for glass windows.

http://www.eplastics.com/Plastic/plastics_library/Coefficients-of-Thermal-Expansions-of-Plexiglass

http://www.sdplastics.com/acryliteliterature/1121DFFPhysicalProperties%5B1%5D.pdf

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Hardness aside, weathering effects (from e.g. rainfall, snow, hail, temperature and moisture variations etcetera) and UV degradation are of course important to consider for windows. In general, glass is much more resistant to such effects compared to plastics.

There is also something called creep, which is essentially time-dependent deformation during long times (months or years). You might have seen it if you've ever left something heavy on a cardboard box for a long time. Plastics creep pretty easily. I don't know if this actually would be a major problem for a regular window perpendicular to the ground, but it might not be good for a window that will carry static loads.

Plastics will also whiten due to deformation. They might also discolor.

Of course, these properties are modifiable (to some extent) through various additives.

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Plastic is not used for windows mainly because it is not durable, in fact:

  • it is degraded by UV radiation, which breaks atomic bonds, yielding a matt and thinned (and fragile) surface, not so desirable for a window;

  • because of the previous point, plastic generates micro- and nanoparticles which diffuse inside the house and inhaling them is not healthy;

  • it is less hard than glass, then it can be easily scratched with wider variety of materials;

  • it is flammable(!) and can produce dioxin as it is burnt (especially if it contains chlorine atoms in its structure), an extremely toxic and carcinogenic compound.

Moreover, glass is able to shield approximately 10% of solar radiation and almost 100% of infrared radiation (emitted by human body as heat), so it is a powerful insulating material, which helps keep your house relatively fresh in summer and warm in winter.

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Glass can be very dangerous when it is shattered. Years ago glass on windows should have been banned because it the edges of it could be very sharp like a steak knife. Plexiglass should replace glass especially on windows if someone throws a baseball at a window, so they won't break accidentally.

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  • $\begingroup$ This problem can be easily solved by reinforced glass or by tempered glass. While plexi can be stratched by anything (including micro dust particles), causing it to slowly lose its transparence (looks unimaginibly bad). $\endgroup$
    – peterh
    Commented Sep 7, 2020 at 9:03

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