p.e.t. film- birefringence pattern help?

[tomotomotomo]

Hello all, just a quick question which I really hope someone can help with.
A 125 micron thick transparent p.e.t. film is to be used as part of a glass laminate but is showing a circular pattern under polarised light.
The image i've attached was taken using a circularly polarised light source, and sandwiched between 2 pieces of flat glass (as not to induce any stress due to bending in the film).
I'd really appreciate any sort of explanation as to why the pattern radiates specifically from a single point, and basically if anyone else has had problems with this and how it was remedied/reduced.
I've already tried heating the film to above its softening point and letting it cool but the pattern remains.

Thanks again
James

 

[Pud]

Is the glass itself stress free and optically flat?

I would guess that the weight of glass pressing on the film is stressing it..or the glass.

As your test is qualative rather than quantitative what are you looking for?

Cheers


Harry

 

[tomotomotomo]

Hi yes the glass is flat and shows no distortion of light.
Also, the film shows this pattern even when it isn't between glass, I only put it between the glass to support it and rpevent further stresses.
I think it may be due to the orientation of the polymer chains, but that's as far as I've got :s

 

[tomotomotomo]

SorryI forgot to mention, I'm looking for a way to reduce/eliminate this effect, and also an explanation of why its happening

Thanks

 

[Demon3]

You're right to suspect built in stresses in the PET. When you flow a polymer the chain orient in the direction of flow and when cooled quickly they don't have time to relax back to their natural random coil conformation. The consequence is that you have a material that is anisotropic, showing different refractive index in different directions. However, the orientation should not be in a circular direction unless the film was made somehow by injecting the polymer in the centre of the film. The orientation should in the direction the film was stretched during manufacture.

This makes me suspect you polarizers. Polarizers can work either in a plane or they can be circular polarizers. So, in conclusion I suspect the fact that you see stresses is due to polymer orientation but the circular pattern is not due to your film but due to the type of polarizer you're using. You can look for example at Amazon or other sites to see that polarizers come in different types. Try the other type and see if you pattern changes.

Another experiment would be to cast a PET film from solvent. This will give PET with no built in stresses. See if that gives you a pattern under polarized light. I shouldn't.

As an aside. I once worked on ceramics and we were able to measure the level of stress in them using the colors we saw under cross-polarized light.

Hope that helps.

There is not any memory with less satisfaction than the memory of some temptation we resisted.
- James Branch Cabell

 

[IRstuff]

Trying to get uniformity of deposited layers on a small pieces is nontrivial. There are always annoying edge effects no matter what you do in the deposition.

In semiconductor manufacturing, photoresist (which is a polymer) deposition involves spinning the wafers at a particular speed that uses centrifugal acceleration to balance the radially outward flow in conjunction with the viscosity characteristics of the photoresist.

You might relook at your deposition process and see if there's anything you can do to minimize the nonuniformities in the deposition environment.

TTFN

FAQ731-376

 

[patprimmer]

You might try annealing the film. I expect you will substitute the birefringence with distortion of the film.

So called casting film by the calendering process or making film by blowing it, both induce some stress.

I do not know a solvent that is possible to use to cast PET film from solvent.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[tomotomotomo]

wow thanks for all the replies, I didn't expect much of a response so thanks!
Just to clear things up a little:
1. I've used circularly polarised light, which shows the concentric rings, and ive also used plane polarised light which shows the same thing (there is a slight difference but i don't think its important-imagine a pie made from coloured concentric rings. now take 2 slices out of the pie so there are 2 equal sized pieces left facing each other...)

2. the pattern shows itself even in sunlight/fluorescent light when it is part of the glass laminate. i know this is because some of the refracted light is plane polarized by the glass.
SO!
It's to be used as part of an interlayer in a glass roof for a car, so it's really obvious in certain conditions (showroom/outdoors..)
Any optical physicists out there!?
Thanks for the manufacturing suggestions guys, I'll also look into those myself.

Cheers,
James

 

[Pud]

I thought glass was usually laminated with polyvinyl butyral (or is it butyrate?)(PVB), or I have seen polycarbonate used for ballistic glass.


Cheers


Harry

 

[MikeHalloran]

I think the film (_bo_PET?) is trying to tell you that it's bi-refringent or maybe tri-refringent.

http://en.wikipedia.org/wiki/Birefringence

http://en.wikipedia.org/wiki/PET_film_(biaxially_oriented)

If you don't want it to be optically active, use a different film.



Mike Halloran
Pembroke Pines, FL, USA

 

[Demon3]

What properties do you need from the film? Perhaps we can recommend something other than PET. I recently saw a presentation on colourless polyimide for example from a company called ManTech (formerly SRS).

There is not any memory with less satisfaction than the memory of some temptation we resisted.
- James Branch Cabell

 

[jedward]

is this seen in all samples ?
if not, have you considered that it could be due to an inclusion in or on the film?
variable spacing would occur and cause similar observables

 

[tomotomotomo]

Unfortunately I can't use any other material as the pet is part of an interlayer for variable transmission glazing.
we've had different batches of film and they've all displayed the same pattern, even when fresh out the box. it's not due to inclusions of any sort, as i said before i think its to do with the anisotropic properties of extruded plastic.
i know that pet is naturally birefringent, but i'm just curious to know specifically why the pattern is so perfectly circular even when the film is flat?
there must be a reason why!

If I understand why the pattern is that shape then maybe i can find a way to reduce the effect, and if not at least I'll have a valid explanation for the customer

once again thanks for all of the replies

James

 

[MikeHalloran]

See how the pattern changes when you move your light source.



Mike Halloran
Pembroke Pines, FL, USA