Polymer Cracking

[sandip179]

I am trying to spin-coat a solution of PMMA (MW=15k) on a elastomeric material, but I find that the polymer cracks. Is there any polymer that wont crack? Or can I add any additive to my polymer to prevent cracking?

 

[patprimmer]

PMMA is a very clear very hard abrasion resistant coating, but it has quite low elongation at break. Plastics also typically have about 10 times the coefficient of expansion as metals and to compound the problem, the coefficient of expansion is higher during the phase change from liquid to solid.

You need something with higher elongation at break. There are impact modified acrylics or copolymers, polycarbonate, polyurethanes and polyesters to consider.

Regards

eng-tips, by professional engineers for professional engineers
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[sandip179]

Thanks. Can you suggest any particular solvent soluble polymers that can be used? and where can I get them. I looked at the website of Sigma Aldrich, and didnt find anything worthwhile.

 

[Pud]

This might be silly, but try your local model aeroplane shop. Cellulose (Nitrate or Butyrate type) is still used, as are some of the more "modern" coatings such as PU or Epoxy.

These give thin, tough coatings.

Cheers


Harry

 

[Demon3]

What is your goal? Does the coating need to be transparent? What will it be used for?

As mentioned above, you need a polymer with higher elongation to break.

Some ideas. Buy some clear nail polish/varnish and dilute that with nail polish remover (acetone or similar) and try that. Afterall it is made to form durable coatings. Model shops will also sell clear varnishes and you can buy polyurethane varnished and dilute them as well.


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

 

[sandip179]

Let me state my entire application. I want to coat a thin layer of polymer on an elastomeric material, then transfer this polymer layer onto a metallic substrate. After some time I would like to remove the entire polymeric layer by washing or degrading it. It is not necessary for the polymeric layer to be transparent, though it needs to be resistant to acids. When I try spin coating PMMA on the elastomer, I notice cracks in it, and also the transfer from elastomer to metal is not uniform.

The problem is a little challenging, I would highly appreciate your inputs.

 

[Demon3]

Hmm. Seems clear that the polymer will crack on the elastomer due to different shrinkage between the two. I don't see how to make it work without knowing more.


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

 

[sandip179]

the elastomeric material is cured PDMS (polydimethylsiloxane). The metallic substrate in question is smooth aluminium. Let me know if you need any more details.

 

[Demon3]

What is the function of the polymer layer? How soft can it be? The only way I see to make it not crack is to make it soft and / or elastomeric so it matches the properties of the PDMS.

If you use strong corona treatment or flame treatment on the PDMS you can make the surface turn glassy. Will that do?


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

 

[sandip179]

The function of the polymer is to act as acid resist on selected areas of the metallic substrate. It can be soft, but it should be soluble in a solvent so that it can spin or spray coated onto the elastomer.
I am corono treating the elastomer for 30 s. that does not help. maybe i can treat it for a longer amount of time.

 

[Demon3]

PDMS turns to glass after severe exposure to corona. This effect is seen for example on the insulators for high voltage cables where the PDMS has been seen to do this. A friend of mine did his PhD on it.

For a soft, soluble acid resist you could try SBS, such as Kraton D (

http://www.kraton.com/)or

Styroflex (

http://www.styroflex.com).

Acetone should dissolve it.




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

 

[sandip179]

Hey,
A butadiene styrene rubber (MW=38000) did work for my application. One thing is that the polymer is not very resistant to acids. Any idea as to how to improve its chemical inertness without modifying its properties?

 

[Demon3]

Great, I am glad that the styrene butadiene I suggested worked! I don't see why it would not resist acids though unless you are using some very strong acid. What resistance do you require?

One tip is to try the hydrogenated SBS types, that way the double bonds are gone and so the rubber will be more chemically stable (i.e. other Kraton types).


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

 

[sandip179]

I require pretty strong resistance. I am talking of exposure of concentrated acids (30%) at 100C for 1-2 minutes. However the exposure is just one-time, and even if the polymer film partially survives one-time exposure, its ok.
Btw, the idea of using hydrogenated SBS is great. I was just wondering if they would still retain their ductility.

 

[Demon3]

Yes the hydrogentated SBS is still an elastomer i.e. ductile.


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

 

[sandip179]

Thanks.
Some of the SBR polymers I have tried exhibit the problem of cracking. What makes some polymers crack when they are spin-coated on another elastomer? Is it due to different hardness or Young's modulus? Any help in trying to understand the cracking issue would be highly appreciated.

 

[patprimmer]

They contract or shrink at different rates to each other and to the substrate. This is especially noticable as they change from liquid to solid state.

Regards

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