Wednesday, November 25, 2009

Specific gravity and what every purchaser should know.

On most Polycarbonate (and other polymer) technical data sheets there is a number called the specific gravity.   The specific gravity of Polycarbonate is 1.2 and is one of the most important numbers on the data sheet for purchasers.

Specific gravity is the ratio of the density of the material to the density of water.  As the density of water is 1000 kg/m3, this means that the density of Polycarbonate is 1200 kg/m3.

To convert the density from kg/m3 to lbs/ft3 you need to multiply by 0.0624.  This calculation gives a density of Polycarbonate of 74.88 lbs/ft3.  In this way we can easily calculate the density of Polycarbonate in lb/ft3 from the specific gravity figure on the data sheet.  

The density is an important value for purchasers of Polycarbonate sheet.  It can help answer questions such as "If a supplier says that the Polycarbonate resin prices have gone up 10 cents per pound, what effect would that have on the cost of 1/8" sheet?"

To answer this question we need to be able to convert from $/lb to $/sqft.  We can do this conversion with the help of a simple equation:

Cost ($/sqft) = Cost ($/lb) x Density (lb/ft3) x Sheet thickness (inches) / 12 

So to answer our question on the effect of an increase of $0.10/lb for resin on the price of 1/8" sheet, we can plug the numbers into our equation:

Cost ($/sqft) = $0.10/lb x 74.88 lb/ft3 x 0.118 inches /12 

Cost ($/sqft) = $0.074 /sqft

So a 10 cent per pound increase in Polycarbonate resin price will increase the cost of 1/8" sheet by 7.4 cents per sqft.  

[Note: Nominal 1/8" Polycarbonate has a thickness of 0.118"]

Saturday, November 21, 2009

Polycarbonate sheet - stresses and shrinkage explained

To understand the concept of shrinkage it is important to know how extruded Polycarbonate sheet is produced.

Polycarbonate resin is melted in an extruder and then is passed through a die to form the shape of a sheet.  At this stage the sheet is molten because it is above its glass transition temperature of 150 C / 302 F.  The molten sheet is then pulled though chrome polishing rolls to give it a high quality surface. The sheet is then cooled below its glass transition temperature to give it a rigid structure.

The process of pulling the sheet through the chrome rolls stretches the sheet in the extrusion direction and puts stresses in the sheet.  When the sheet drops below the glass transition temperature these stresses remain in the sheet.

Pulling the sheet through the rolls is only one way in which stresses are put into the sheet. Another is uneven cooling of the sheet.  Usually there are more stresses in the direction of extrusion than in the perpendicular direction.

These stresses in the sheet can cause problems for customers.  When the sheet is heated above the glass transition temperature of 150 C / 302F, these stresses are able to relax.  During this process the sheet behaves a bit like a rubber band and the sheet "shrinks" in the extrusion direction as the stresses are released.  One standard test to measure shrinkage is to take a 12"x12" piece of sheet and draw a 10" diameter circle on the sheet with a marker pen.  The sheet is then placed in an oven that is just above the glass transition temperature for a few hours.  When the sheet is removed and cooled, it is easy to see that the sheet has shrunk in the extrusion direction (as seen in the above diagram).  The percentage change in the diameter of the circle in the extrusion direction is know as the "percentage shrinkage".

The percentage shrinkage is particularly important for customers that thermoform sheet because if the sheet is cut too small, the shrinkage could cause the sheet not to fill the mold after processing.  Using a larger sheet than required is one method of solving this problem, but this leads to increased scrap and increased costs.
For thermoformers it is important not only to have low shrinkage, but also consistent shrinkage from one batch to the next.  With modern sheet extrusion lines, regular monitoring and an experienced production team it is easily possible to control shrinkage to below 1.0%, in fact shrinkage levels of 0.5% should be the target.
Unfortunately not all Polycarbonate sheet producers regularly control shrinkage and some often advise that thermoformers should use a 5% overage on the sheet.  To resolve this issue, thermoformers should ask their suppliers to report the shrinkage test results and ask for them to supply sheet with less than 2.0% shrinkage which can be easily achieved without additional cost.

One of the other problems that can be caused by the stresses in the sheet and high shrinkage is warping of the sheet.  This is often observed as the sheet not remaining flat on a pallet, but hills and valleys forming along the length of the sheet.  Often high humidity can cause the stresses in the sheet to relax and as a result, the sheet will often warp.  One way to mitigate this problem is to keep all pallets of Polycarbonate sheet wrapped until they are used to prevent moisture reaching the sheet.  If a sheet is taken from the pallet, the pallet should then be re-sealed. However, a much better way to resolve the problem is to ask the supplier to supply sheet with low shrinkage. 

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.


Monday, November 9, 2009

Selecting Abrasion and Scratch resistant coatings

Polycarbonate is a reasonably soft plastic and can be prone to scratching and damage in some applications. To solve this problem, there is a wide range of anti-scratch and abrasion resistant coating options available for Polycarbonate sheet. It is important to understand not only the application but also the test methods used when deciding on which coating to select.

The performance of coatings are usually quantified according to two very different test methods:

- Taber Abrasion under a test method such as ASTM D1044
- Pencil Hardness under a test method such as ASTM D3663

The Taber Abrasion test is conducted by placing the coated sheet on an abrasion tester. A 250g, 500g or 1000g load is then placed on top of an abrader wheel and the wheel is allowed to spin a certain number of revolutions. Different abrasion wheels can be used for harder or softer materials, often for Polycarbonate a CS-10F wheel is specified. A haze measurement is taken before and after the test and the percentage difference is reported. When comparing test results it is important to check the wheel type, the weight attached to the wheel and the number of revolutions of the wheel.

The Pencil Hardness test is conducted by placing the coated sheet on a firm, horizontal surface. A pencil is then held firmly against the sheet at a 45-degree angle with the point of the pencil facing away from the tester. The pencil is then pushed away from the tester to give a 0.256” stroke. A range of pencils of different hardness is used for the test. The test starts with the hardest pencil and then continues with progressively softer pencils. Once a pencil that will not cut or mark the coating is found, the test is complete and the pencil hardness is reported as the test result. In practice the test can show significant variability. In order to minimize this variability, a set of reference pencils should be selected as the brand of pencils can affect the result.

It is important to determine which test method is representative of the application for the Polycarbonate sheet. If the sheet is likely to be subject to continuous abrasion over an extended period of time, the Taber Abrasion test may be appropriate. If the sheet is likely to be subject to individual knocks and scratches, the Pencil Hardness test may be more appropriate. Unfortunately, in many situations, neither test is totally representative of the wear and tear that the sheet will experience in the real world.

When selecting the coating for an application there is often a trade off between the wear or scratch resistance and the processing properties of the sheet. Typically as a coating becomes harder, it also becomes more brittle and prone to cracking if bent. There are three broad groups of coating that a user can consider:

Formable coatings – these coatings can easily be bent or formed without cracking the coating. However, this formability is achieved by sacrificing some of the abrasion resistance, as the coating is softer. While the abrasion resistance is much better than uncoated Polycarbonate it is not quite as good as the more traditional hard coatings.

Standard Hard Coats – these coatings tolerate a small amount of bending and can easily be machined. They have good Taber Abrasion Resistance and pencil hardness of 2H or 3H.

Super Hard Coats – these coatings generally crack if bent or machined, as the coating is more brittle than standard hard-coats. Typically parts are usually machined prior to coating. However, the anti-scratch coating performance is excellent with pencil hardness values of 4H or even 5H.

One new development in the scratch resistant coating field is the self-repairing hard-coat. These materials are able to withstand real world damage better than the more traditional coatings and any knocks and scratches that do occur repair themselves within a few seconds. Sheets produced with these coatings can also to be bent and formed without damage to the coating. Unfortunately due to the physical structure of these coatings, the Taber Abrasion test is not accurately able to represent how these coatings perform in real world situations.

When specifying Polycarbonate sheet for an application that needs scratch resistance or abrasion resistance it is important to discuss the various options with the sheet supplier. Often a standard hard-coat is not the best choice for the application.
A full range of formable, standard hard-coat, super hard-coat and self-repairing coatings is available from HighLine Polycarbonate.

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.