Tuesday, November 3, 2009

Transparent conductive coatings explained.

HighLine Polycarbonate can apply transparent conductive coatings to the surface of Polycarbonate sheet and film.  These products are used in the electronics industry to conduct an electrical charge over the surface of the sheet.

Usually Indium Tin Oxide (ITO) is used for this type of coating in either a standard form or an index matched form to improve light transmission.

When specifying this type of product, one of the most important pieces of information is the required surface resistivity.  Surface resistivity is measured in Ohms per square.  One question that is often asked is do we mean Ohms per square inch, Ohms per square foot, Ohms per square centimeter or Ohms per square meter?  The answer to this question is that it doesn’t matter, they are all the same. 100 Ohms/square inch is the same as 100 Ohms/square foot, so the common terminology is 100 Ohms per square.

A few companies can apply what is known as an anti-static coating to conduct a small charge to prevent static build up.  These products typically have a high resistance of around 1,000,000,000 Ohms/square of higher.  While HighLine Polycarbonate can offer these anti-static products, we concentrate on uses requiring a much lower resistance, which need to conduct significantly more current.

With Indium Tin Oxide we are able to achieve surface resistance of anywhere between 10 and 3,000 Ohms/sq.  A resistivity of 3,000 Ohms/sq is achievable with a very thin layer of ITO.  As the required surface resistance is decreased and the conductivity of the sheet is increased, an ever-increasing thickness of ITO is required to conduct electricity.  As a result, achieving lower surface resistance is more expensive than achieving higher surface resistance.

Another issue is that there is always a trade off between the thickness of the ITO layer and the light transmission properties of the product.  It is possible to mitigate these effects by using anti-reflective coatings and index matching the ITO, but these solutions do add cost to the final product.

In summary, to specify Polycarbonate sheet with a transparent conductive coating it is important to know the required surface resistivity and the required light transmission.  As discussed in the last blog, it is also often important to know the amount of reflection that can be accepted.

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