Friday, February 12, 2010

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 requesting a copy and we will send you one.

Monday, February 1, 2010

Regrind and the cost of quality

One subject often comes up in discussions with our customers: regrind material. In this blog post we will explain the different types of regrind and why regrind is important to quality.

A Polycarbonate resin plant typically produces 50,000 MT of resin per year. These lines are a continuous production process, operating 24 hours a day, 7 days a week. Each line may produce a number of different grades and when they transition between the grades they do not stop producing. Instead they produce something known as transition material; transition material is between the specification of the initial grade and the grade being transitioned into. Up to 10% of the production of a line may be classified as a transition material.

As the production line is a continuous process, operational problems can also lead to off-specification production. Off specification production may account for another 10% of the production. Between transition material and off specification production as much as 20% of the line output may not be within specification – representing 10,000 MT per year. Obviously this figure varies between manufacturers and also depends on the types of grades being produced on any particular line.

A manufacturer of resin must then decide what to do with this out of specification resin. One option is to sell the material at a discounted price. However, the preferred option is to melt the resin pellets and feed them back into the production process. The amount of resin that is reprocessed again depends upon the manufacturer; but if we look at the figures as much as 10,000 MT of off specification material could be used to make the 50,000 MT of saleable prime resin.

When the resin is then used to make Polycarbonate sheet we also have a similar situation. Changes between different grades and sizes of Polycarbonate sheet can lead to off-specification production. Also when Polycarbonate sheet is being produced the edges are normally not flat and so are trimmed off – this material is then known as edge trim. Both off-specification production and edge trim can then be broken into small pieces in a grinder and then recycled into the sheet extrusion line; this material is known as regrind. In some cases of commodity sheet production as much as 60-70% of the sheet can be composed of regrind material.

Recycling of material, both in the resin and sheet production, can help keep the cost of sheet down – particularly for commodity sheet. However, recycling of material does come at a cost. Polycarbonate is degraded by heat and the more times heat is applied to the material (especially at temperatures high enough to melt and mix the material), the greater the degradation. This degradation manifests itself in three ways, black specks, yellowing and deterioration of mechanical properties. The greater the level of recycled material in the final product (whether from resin or sheet), the greater the possibility and magnitude of problems such as black specks, yellowing and deterioration of mechanical properties. For many commodity applications the price of the sheet is of great importance and the benefits of recycling during production significantly outweigh the consequences. In some applications it may indeed be acceptable to use low priced sheet made from 100% regrind.

At HighLine Polycarbonate we concentrate on high-tech applications requiring exceptional optical and mechanical properties where black specks and yellowing cannot be tolerated. While we take many steps to ensure the quality of the product, we do pay particular attention to recycling. We use only the very best resin from Teijin. Teijin is a Japanese company and is the world’s third largest Polycarbonate resin producer. The grade of resin that we use has no recycled resin in it. Also, most of Teijin’s resin lines are relatively new having been installed in the last ten years. Having modern resin lines also improves the quality.

When we produce the sheet we also do not recycle any off specification material or use any edge trim material. Many other manufactures claim to not use regrind material, but often recycle edge trim material. By using only the very best resin and not using any regrind we are able to produce the very best material for the most demanding applications. Of course there is a cost to quality, but there is also a benefit in some applications.