Can this be accomplished?

[keepflowing]

Let me start by stating that I am new to the forum. This project that I am working on is not my area of expertise. Here is the problem. Finding a film to provide sacrificial protection for flat plexiglass or acrylic and glass with exposure to outdoor elements. Based on some research the problems that are created are as follows. The coefficient of thermal expansion differences between plexiglass and glass are different enough to cause film lift. So it looks like I cannot use a film meant for glass on plexi and vice versa. Also from what I understand some plastics off gas when exposed to heat(sun exposure) and that would also cause film lift. Is there a film out there that will work for both materials? Due to project demands that same film must be used regardless of the material. The other catch is that once the covering is damaged it must be removable without damage to the base material. I have been racking my brain to come up with an answer, but it is eluding me. If there are other materials/processes that I am not thinking of please let me know. Thank you in advance for any help or insight that you can offer.

 

[MacGyverS2000]

First thing that comes to mind is the film they put on the faces of electronic devices (cellphones, iPods, etc.). Very stretchy (so differences in expansion coefficients isn't an issue), very scratch resistant, and it can be removed. Seems to fit the bill quite nicely.

Dan - Owner

http://www.Hi-TecDesigns.com

 

[Demon3]

I agree, you need an elastomeric film for two reasons. They give good scratch resistance and they stretch so they will not lift due to expansion and contraction. Even something PE or EVA based could well work. If you need more elasticity add some Kraton or Styroflex to it.

Chris DeArmitt PhD FRSC CChem

 

[keepflowing]

Thank you very much for the replies. It looks as though the answer has literally been touching my ear this whole time. Would something like a PET film have the elasticity required? Anything in particular to watch out for(pitfalls)?

 

[patprimmer]

PET film seems to stay pretty well in place on car windows when used as a tint layer.

Regards
Pat
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[keepflowing]

But how would PET film do on plastics?

 

[patprimmer]

Better as it has a similar co-efficient of expansion as it is a plastic.

Regards
Pat
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[Demon3]

A close friend did his PhD on a very closely related topic. I.e. understanding and quantifying the force that's trying to delaminate a coating from the substrate.

You can think of that force as a spring. The substrate is glass and the coating is your protective layer.

If the protective layer is PET then it's stiff but its CTE mismatch with the glass is small. That means that you have a stiff spring that's only stretched slightly so the delamination force (modulus x deformation) is moderate.

If you have a protective layer of elastomer it's true that the CTE mismatch with the glass is higher compared to the PET case above. However, the elastomer has a low modulus so the delamination force (modulus x deformation) is no larger than it was for PET and is very probably smaller.

Plus elastomers are great for scratch / abrasion resistance (that's why car tyres are rubber and why all PVC flooring is coating in polyurethane elastomer to protect it from trafic) and PET is not.

Overall go for an elastomer solution as it protects the substrate better and will not have a greater tendency to delaminate from the glass.

Chris DeArmitt PhD FRSC CChem

 

[keepflowing]

Demon, I think that I am following you. It will be a balancing act between the two different types of substrates. Unfortunately the film has to be compatible with both. Is there a particular elastomer product or makeup that comes to mind? The PET came to mind to more closely match the plastic side of things. When I originally looked at this, I was looking at safety film. Well it was quickly apparent that it was not compatible with plastics. That's when I started my search for an alternative film for both substrates.

The other question in the back of my mind is off-gassing. Will this be a problem? One item that these will be affixed to has a graphic overlay. I have never seen these overlays delaminate. From what I have read the polycarbonate and acrylic lenses that are used shouldn't have a problem with this. What are your thoughts?

It is unfortunate that when I call and talk to the film manufacturers, the salesmen get very quiet when I start explaining what I am trying to do. If it were possible, I think that I could hear their eyes glaze over. Then they give me a line about seeing what they have that apples, and I never hear back. Ugg. What appeared to be a simple project, is turning into a exercise in patience.

Thanks for the great information!

 

[Demon3]

No balancing act needed. Elastomers provide better protection and they won't delaminate because although the thermal mismatch is higher their low modulus means there is very little delamination force at play.

Weatherability is a challenge and for that many elastomers are not good.

In your shoes I'd get some PMMA with protective film (that's standard as delivered or from the hardware store), test that outdoors and in your application and if it works analyze to see what it is. No need to reinvent the wheel.

Chris DeArmitt PhD FRSC CChem