Transparent, Abrasion Resistant Polymer For Durable Microstructures 1

[mestew790]

I need to mold microstructures in a thin film of material where the microstructures consist of an array of microposts that are 40um in diameter, 40-60um tall, and spaced by 80um. The structured thin film must be transparent (>90% transmission over visible wavelengths), durable, and scratch resistant (not abraded by cheesecloth).

I have tried using hard and elastic urethanes. Microstructures made of hard, brittle materials tend to fracture, whereas microstructures made of more tough, elastomeric urethanes do not break, but become hazy when scratched by cheesecloth.

Can anyone suggest materials (and/or fillers) to use in this application to make a tough, durable, scratch-resistant, and transparent microstructured surface?

 

[Demon3]

Try silicones but I don't think it will work.

Brittle materials are out because they will break. Elastomers should work because they deform and bounce back undamaged.

The PU probably becomes hazy because it picks up lint from your cloth. Does it regain transparency when you clean it thoroughly? I.e. water, detergent and an ultrasonic bath.

Chris DeArmitt

"Knowledge has no value except that which can be gained from its application toward some worthwhile end."
Think and Grow Rich - Napoleon Hill

 

[mestew790]

Thank you Demon3 for your reply.

I forgot to mention that we did try polydimethylsiloxane, but the posts tended to tear from surface during the durability/scratch tests.

Rinsing and sonicating the PU in isopropyl alcohol removed some of the haze. We looked at the PU under a microscope and it appeared to have been physically scratched by the cheesecloth.

Any thoughts on how to make the current PU more scratch resistant? Are there any elastomers or PUs you might suggest trying?

 

[Demon3]

A good elastomeric polyurethane is not easily scratched even by sand so I am surprised. What I suggest is a quick screening test that I have used over the years. It's called the ball mill abrasion test. First, you will save time and money because you can screen a lot of flat samples so you don't have to make microstructured samples to begin with.

Place all plaques flat on the inside of a drum (old biscuit tin etc), add sand or other abrasive, in your case some shredded cheese cloth plus some ceramic milling beads for weight). Then rotate the whole thing at your expected abrasion speed and measure loss in gloss over time. This reveals the most abrasion resistant materials fast. I have introduced this to two companies who now swear by it.

Then take your top candidates and test with the microstructured surface. Cross-link density and Tg of the elastomer are critical. Materials that are elastomers may no longer be elastomeric at the speeds used in your abrasion test (see the WLF equation). This is really important.

Chris DeArmitt

"Knowledge has no value except that which can be gained from its application toward some worthwhile end."
Think and Grow Rich - Napoleon Hill

 

[Demon3]

This paper is really good if you can get it.

??R.M.Evans and J.Fogel?J. Coat. Technol., 49?634?,
p.50?1977?

Chris DeArmitt

"Knowledge has no value except that which can be gained from its application toward some worthwhile end."
Think and Grow Rich - Napoleon Hill

 

[patprimmer]

Have you tries scratch resistant polycarbonate sheet thermoformed or PC injection moulded then coated with the same coatings they use on car headlights for instance.

Regards
Pat
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[Demon3]

Hard SiOx coatings may work. They are applied by plasma. Although hard, they are thin enough to be very flexible.

Chris DeArmitt

"Knowledge has no value except that which can be gained from its application toward some worthwhile end."
Think and Grow Rich - Napoleon Hill

 

[unclesyd]

I would look for a coating based on Methyl Methacrylate chemistry. These coatings are extremely tough as some are used on flooring. I would give Ellsworth a call as they did have a very good technical staff. The Permalac Lacquer is a little different approach.

http://www.ellsworth.com/display/productdetail.html?Tab=Vendors&productid=781

http://www.permalac.com/technicalfacts.asp