Saturday, November 14, 2009

Polycarbonate sheet - Temperature stability

When sputtering materials onto Polycarbonate sheets it is important to know what temperature the sheet can withstand. The sputtering process imparts energy to the sheet, which causes the temperature of the sheet to rise. The amount of temperature rise is dependent upon both the length of time and the amount of energy used in the sputtering process.

For example, laying down a relatively thin surface of ITO to give a surface resistance of 200 Ohms/square requires less time than laying down a thicker surface to give a surface resistance of 10 Ohms/square. The sheet will therefore reach a higher temperature when producing a 10 Ohms/square coating, all other things being equal.

This heating of the sheet is why it is relatively easily to put ITO onto glass, as it is not affected by temperature. It is also why it is much more difficult to put ITO onto acrylic than it is to put it onto Polycarbonate, because Polycarbonate is much more resistant to temperature than acrylic.

The ability of plastics to withstand temperature is typically reported on data sheets as the “Heat distortion temperature” as measured by either ISO 75 or ASTM D648. These tests measure the temperature at which deformation of the sheet first occurs when subject to a load. A typical reported result for Polycarbonate sheet would be:

“Heat distortion temperature under a load of 65 psi – 288 F / 142 C
Heat distortion temperature under a load of 260 psi – 264 F / 129 C”

As can be seen, the higher the load, the lower the temperature that the sheet deforms at.

These results, while useful for more traditional processing such as forming sheets, are not really relevant for sputtering type applications for two reasons:

- During a sputtering process the sheets are not subjected to a load but are usually located vertically in a rack.
- When sputtering we are not only interested in deformation of the sheet but, more importantly, in preventing a degradation of the optical properties.

Most Polycarbonate sheet available on the market has not been designed for high optical sputtering type applications. Instead they have been designed for commodity applications such as architectural glazing; in these applications they are not subjected to high temperatures. When these types of Polycarbonate sheet are heated by vacuum coating, some of the inherent stresses in the sheet can lead to optical distortion. This distortion is often unacceptable in optical applications.

HighLine Polycarbonate has specifically designed a range of Polycarbonate sheet for vacuum deposition applications. The material is formulated to be resistant to higher temperatures, has the required optical properties and has reduced internal stresses in the sheet. In a future blog we will discuss how these internal stresses can be both visualized and measured in both commodity Polycarbonate sheet and our advanced materials.

For vacuum deposition processing we have found that our sheet can easily withstand continuous temperatures of 145 C / 293 F for over three hours without any loss in optical performance or distortion of the sheet.


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