Friday, March 5, 2010

Anti-reflective coating options for Polycarbonate

There are several options available for improving anti-reflective performance of Polycarbonate sheet. The correct choice depends on a number of factors including the level of anti-reflection required, the size of the part, the number of parts required and the cost sensitivity of the application. In this blog entry we will discuss how to make the correct choice for the application.

Anti-reflective coatings are typically applied to Polycarbonate that has an abrasion resistant coating applied to the surface. The abrasion resistant coating provides a better surface for the anti-reflective coating to adhere to than the uncoated Polycarbonate. The finished product is therefore more durable. The abrasion resistant coating itself also improves the anti-reflective properties of the Polycarbonate sheet, as discussed in a previous blog post.

There are essentially two broad types of anti-reflective coatings, liquid anti-reflective coatings and vapor deposition anti-reflective coatings. Liquid anti-reflective coatings are applied to the sheet in a solution and are then cured using either ultraviolet light or heat. Vapor deposition coatings are applied using a sputtering process.

Level of anti-reflection achieved.

The following table shows the amount of reflection from each surface of the sheet with each of the anti-reflective options. These figures are over the visible light range of 420-680 nm.

Uncoated Polycarbonate sheet 5.1%

Abrasion resistant coated Polycarbonate sheet 3.9%

Liquid anti reflective on Polycarbonate sheet 2.0%

Vapor deposition anti-reflective on Polycarbonate sheet 0.75%

If a very low level of reflection is required a vapor deposition anti-reflective is normally used. However, it is often possible to use a liquid anti-reflective or even just an abrasion resistant coated sheet for applications not needing such a low level of reflection.

Cost of anti-reflective solutions.

A liquid anti-reflective coated sheet typically sells for about five times the price of a standard abrasion resistant coated Polycarbonate sheet.

A vapor deposition coated anti-reflective sheet would sell for about five times the price of a liquid anti-reflective sheet.

These broad pricing guidelines obviously depend on a number of factors including part size and the number of parts required, but they do give some indication of what you can expect to pay for increasing levels of anti-reflective performance. Often only very high technology applications can justify the cost of a vapor deposition anti-reflective coating.

Part size and minimum order quantity.

One of the problems of vapor deposition technology is the limitation on the size of the part. Parts of up to 14” x 18” can be produced on a standard sputtering machine in reasonably small quantities. However, once you get above this size you need to use a very large sputtering machine that requires large set up costs and thus large production runs. Parts up to 24” x 36” are easily possible but may require production of at least 1000 parts at a time; this makes it very difficult to obtain a couple of parts for a prototype development if parts over 14” x 18” are required. Once you require parts of over 24” x 36” you need very specialized equipment and the cost is extremely high.

For liquid anti-reflective coatings it is possible to easily coat sheets of 48” x 96” or larger and the minimum production size is much smaller. The easier production makes liquid anti-reflective materials much easier to obtain for prototype development. For large parts we typically recommend that liquid anti-reflective coatings are evaluated first, before trying the expensive vapor deposition anti-reflective coatings.

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