Transparent Heaters built from ITO coated Polycarbonate

Today we have been working with two customers, both of which are considering using ITO coated Polycarbonate sheet as a transparent heater for windows. One of the customers currently uses wires laminated in the sheet to heat the windows. They have recognized that using ITO coated Polycarbonate could be a cheaper option than laminating the wires into a window and the visual appearance of the product would also be much better.

The question that keeps coming up for this application is: “If I need to heat a window with X Watts/square inch, can I use ITO coated Polycarbonate?” Typically the value for X is between 0.2 and 0.8 depending upon the customer’s requirements.

As you might expect, this question is not a yes-no type question, but it involves some simple calculations. In order to carry out the calculation we need some simple information: the voltage (V) that is available for heating and the size of the window to be heated (both the width (W) between the two bus bars and the length (L) of the window/bus bar.

The first step is to calculate the total power requirement for the window, we will assume for this example that 0.5 Watts/square inch is needed.

Power (Watts) = 0.5 (watts/square inch) x W (inches) x L (inches)

We then need to calculate the Heater Resistance (R) where:

R (ohms) = [V (volts)]² / Power (Watts)

We then need to calculate the Surface Resistance (SR) of the sheet where:

SR (ohms/sq) = L (inches) x R (ohms) / W (inches)

Combining these equations into one simple equation:

SR (ohms/sq) = [V (volts)]² / ( 0.5 (watts/square inch) x [W (inches)]²

The limiting factor for Polycarbonate is that the minimum practical Surface Resistance is 10 Ohms/sq. This limitation means that reasonably high voltages will be required for wide heating elements. Smaller heating elements can be achieved with correspondingly lower voltages.

As an example, we will calculate whether a 9” wide x 12” long window requiring 0.5 watts/square inch heating from a 24 volt circuit can be produced from ITO coated Polycarbonate:

Surface Resistance (ohms/sq) =(24 volts x 24 volts) / (0.5 watts/square inch x 9” x 9”)

= 14 ohms/sq

Also the power requirement would be around 40 watts.

The 14 ohms/sq is easily achievable with ITO coated Polycarbonate in this application.

At HighLine Polycarbonate LLC we have a simple Excel spreadsheet to carry out this calculation. If you would like a free copy, please send us an email at info@highlinepc.com requesting a copy and we will send you one.

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.