EMI/RFI shielding of Polycarbonate

Electronics systems can cause problems by emitting electromagnetic radiation or they can fail to perform due to electromagnetic radiation in the environment. This electromagnetic radiation is often a combination of noise and information. Leakage of information can be of great concern in applications requiring secure communication. Emissions of electromagnetic radiation can interfere with other systems and may have health and safety implications.

To protect against problems caused by both emission and receipt of electromagnetic radiation, systems can be shielded; this process is known as Electromagnetic Interference (EMI) or Radio Frequency Interference (RFI) Shielding.

To shield against EMI/RFI it is necessary to install a conductive ground plane, which will ground some of the electromagnetic radiation. In applications requiring shielding for transparent Polycarbonate, such as screens and windows, this conductive ground plane can be either applied as a coating to the surface or laminated between two sheets. In this article we will discuss the merits of these two options.

To apply a ground plane using a coating we would typically use a transparent conductive oxide such as Indium Tin Oxide (ITO) or Index Matched Indium Tin Oxide (IMITO). It is also possible to use a thin metal layer such as Gold. With these products we have the option of varying the resistance by varying the amount of oxide applied to the surface. The lower the resistance achieved, the better the ground plane achieved and therefore the better the shielding of the finished product.

Using a 10 Ohms/square surface resistivity we can typically achieve a 20 dB reduction in EMI/RFI over the frequency range of 30 MHz to 1 GHz. A 20 dB reduction is about 100 times reduction in noise. If using ITO, this reduction in EMI/RFI does have a trade off, the ITO does lower the light transmission of the Polycarbonate from 89% down to 82%. One option to resolve this loss in light transmission is to use the more expensive IMITO, which allows a light transmission of 94% to be achieved.

The other solution for shielding is to laminate a wire mesh between two sheets of Polycarbonate. Obviously the visible appearance of a fine wire mesh may not be suitable in all applications. There are many options for the mesh including material of construction, mesh density and diameter of the wire; all of these properties will influence both the shielding effectiveness, visible appearance and light transmission of the finished product. For full details of the technical options for wire mesh shielding you will need to contact your supplier or HighLine Polycarbonate. For a simple comparison with the ITO option we will give some technical data for a couple of wire mesh structures.

For a stainless steel 50 Mesh using 0.0012” diameter wire, we would expect a light transmission of 82% with a 30-40 dB reduction in EMI/RFI over the range of 30 MHz to 1 GHz. A 30 dB reduction is about 1000 times reduction in noise.

If we are prepared to tolerate a lower light transmission, we can use a blackened copper mesh which would give a 50-60 dB reduction over a range of 30 MHz to 1 GHz, but the light transmission would drop to around 70%.

The following table summarizes the results:

	Shielding 30MHz–1GHz	Light Transmission
ITO	20 dB	82%
IMITO	20 dB	94%
50 Mesh SS Wire	30-40 dB	82%
Blackened Copper Wire	50-60 dB	70%

As with most projects, there are trade offs to be made between different attributes and overall cost. This article is intended to give a basic understanding of what needs to be considered when specifying Polycarbonate in EMI/RFI shielding applications.

Anti-reflective coating options for Polycarbonate

There are several options available for improving anti-reflective performance of Polycarbonate sheet. The correct choice depends on a number of factors including the level of anti-reflection required, the size of the part, the number of parts required and the cost sensitivity of the application. In this blog entry we will discuss how to make the correct choice for the application.

Anti-reflective coatings are typically applied to Polycarbonate that has an abrasion resistant coating applied to the surface. The abrasion resistant coating provides a better surface for the anti-reflective coating to adhere to than the uncoated Polycarbonate. The finished product is therefore more durable. The abrasion resistant coating itself also improves the anti-reflective properties of the Polycarbonate sheet, as discussed in a previous blog post.

There are essentially two broad types of anti-reflective coatings, liquid anti-reflective coatings and vapor deposition anti-reflective coatings. Liquid anti-reflective coatings are applied to the sheet in a solution and are then cured using either ultraviolet light or heat. Vapor deposition coatings are applied using a sputtering process.

Level of anti-reflection achieved.

The following table shows the amount of reflection from each surface of the sheet with each of the anti-reflective options. These figures are over the visible light range of 420-680 nm.

Uncoated Polycarbonate sheet 5.1%

Abrasion resistant coated Polycarbonate sheet 3.9%

Liquid anti reflective on Polycarbonate sheet 2.0%

Vapor deposition anti-reflective on Polycarbonate sheet 0.75%

If a very low level of reflection is required a vapor deposition anti-reflective is normally used. However, it is often possible to use a liquid anti-reflective or even just an abrasion resistant coated sheet for applications not needing such a low level of reflection.

Cost of anti-reflective solutions.

A liquid anti-reflective coated sheet typically sells for about five times the price of a standard abrasion resistant coated Polycarbonate sheet.

A vapor deposition coated anti-reflective sheet would sell for about five times the price of a liquid anti-reflective sheet.

These broad pricing guidelines obviously depend on a number of factors including part size and the number of parts required, but they do give some indication of what you can expect to pay for increasing levels of anti-reflective performance. Often only very high technology applications can justify the cost of a vapor deposition anti-reflective coating.

Part size and minimum order quantity.

One of the problems of vapor deposition technology is the limitation on the size of the part. Parts of up to 14” x 18” can be produced on a standard sputtering machine in reasonably small quantities. However, once you get above this size you need to use a very large sputtering machine that requires large set up costs and thus large production runs. Parts up to 24” x 36” are easily possible but may require production of at least 1000 parts at a time; this makes it very difficult to obtain a couple of parts for a prototype development if parts over 14” x 18” are required. Once you require parts of over 24” x 36” you need very specialized equipment and the cost is extremely high.

For liquid anti-reflective coatings it is possible to easily coat sheets of 48” x 96” or larger and the minimum production size is much smaller. The easier production makes liquid anti-reflective materials much easier to obtain for prototype development. For large parts we typically recommend that liquid anti-reflective coatings are evaluated first, before trying the expensive vapor deposition anti-reflective coatings.

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.

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.

Polycarbonate sheet - the cost of custom production

Polycarbonate is extruded into sheet by large production lines. The more specialty lines typically extrude 3,000 lbs/hr, while commodity lines extrude 5,000-10,000 lbs/hr. For some of the large lines, if they take ten minutes to change dimensions, they could easily generate over 1,000 lb of off specification material, which will either need to be recycled or scraped.

Non-standard width

The maximum width of the sheet is governed by the width of the die installed on the line; most large lines have a die that can produce 96” wide sheet. The extrusion lines produce most efficiently when they are running at maximum throughput, which means running the maximum width of 96”. One option that sheet extruders have is to cut the sheet in half while the sheet is being produced, this process will give two sheets of 48” wide. The widths of 48” and 96” are some of the most common widths. Because they can be produced cheaply they have become industry standards and are nearly always in stock at the major producers and distributors.

If a customer needs a non standard width, this can be achieved by extruding 96” wide material through the die and then cutting down the edges by in line saws. Any off cut material can then either be recycled or sold as scrap. If a customer needs dimensions such as 95” wide or 47” wide, there will not be much scrap generated, even though the material would still need to be custom produced. If the customer needed a width such as 75”, proportionally more scrap would be generated and the cost to produce would go up. For widths of say 60”, the scrap ratio would be too high and the producer could stop the line and block off part of the die to limit the width of the sheet being produced. The stoppage would obviously lead to lost production and then the machine would be run at a lower, more inefficient rate because the die width would be lower. All of these factors add to the cost.

Non-standard length

The length of the sheet is more easily controlled. During production an in-line cross cut saw is used to cut across the sheet. The length can be set to almost any value (as long as it is not too small). It is therefore possible for a producer to make custom lengths without too much additional cost.

Non-standard thickness

For stocking purposes the major producers have standardized on a number of thicknesses – 0.060”, 0.118”, 0.177”, 0.236” are some examples. These sizes are normally carried in stock in both 48” x 96” and 72” x 96”. Adjusting to another thickness really only involves some minor changes to the die and chrome polishing rolls; these changes are quick and do not generate much off specification production. As a consequence, non-standard thicknesses are not difficult to produce, but manufacturers normally insist on a reasonable minimum order size. Also manufacturers will normally only produce non-standard thicknesses against an order, as they do not have a general need for the material. Because the material is custom produced, manufacturers often quote a long lead-time.

Non-standard color

For custom colors, introducing a new color to the line causes a lot of scrap changeover material between the production of the old color and the production of the new color. The lines are not shut down and cleaned between color changes, as that would be too inefficient. Manufacturers dislike frequently changing colors, and often plan large color runs to improve efficiency. Asking a manufacturer to stop the production of a clear material to produce a few hundred pounds of a red material is not likely to be received well, as the change-over produced from going from clear to red and then back to clear is likely to be many thousands of pounds. A manufacturer can not offer a competitive price if they produce ten or more pounds of scrap for every pound of good product.

Summary

Standard production sizes and colors have been established to improve efficiency and reduce cost. If a customer needs a non-standard product it is likely to be more expensive and require greater lead-time and larger minimum orders. Non-standard colors are the most costly, followed by non-standard width, followed by non-standard thickness, with non-standard lengths being reasonably cheap to produce. Considering these factors during product design can help in minimizing later costs and ensuring availability for the customer.

Anti Fog coatings for Polycarbonate explained

Fogging can often occur on the surface of a Polycarbonate window when moisture from the air condenses onto the cold surface of the sheet or window. The condensed moisture forms droplets that then obscure the view through the window. This fogging effect also occurs on glass and is most often seen on the windshield of a car on a cold morning or on a bathroom mirror if the room is filled with steam.

There are a number of ways to prevent fogging on Polycarbonate and most of them involve some type of coating. Most of these coatings have hydrophilic properties. The term hydrophilic comes from the Greek words “hydros” meaning water and “philia” meaning friendship. A hydrophilic surface attracts the water. This attraction means that the water condensation, instead of forming droplets, forms a thin water layer along the surface of the sheet or window. This thin film of water will then not obstruct vision in the way that water droplets obstruct vision. Hence a hydrophilic material has anti-fog properties. The thin film of water is also able to evaporate quickly if there is air movement near the window.

One problem for some high-tech applications such as lenses and optical sights is that the thin film of water can cause focusing problems. These issues can become severe in environments where the thin film of water is allowed to build up such as telescopes where the users eye is placed against an eyecup. The moisture from the users eye can condense on the lens and because there is an enclosed space with no air movement, the water film can build up with a hydrophilic coating.

One potential solution is a super-hydrophobic coating. A super-hydrophobic coating actually works in the opposite way to a hydrophilic coating and it actually repels water. This effect means that when water condenses on the window or sheet, it will actually bead up into an almost perfect sphere and immediately roll off the surface rather than sit on the surface as a drop, which obscures vision. At the moment super-hydrophobic coatings exist for opaque surfaces, but there are no commercially available options for transparent plastics.

HighLine Polycarbonate has a range of hydrophilic coatings available for Polycarbonate sheet and we have applied these to a number of commercial products. During 2010 we will be looking at the opportunity to develop a super-hydrophobic anti-fog coating for high-tech Polycarbonate applications.

Nominal thickness and thickness specifications

With ever increasing demands to lower production costs, extruded Polycarbonate sheet manufacturers become ever more inventive in their ways of achieving these cost savings. These costs savings sometimes have no affect on the quality of the product, however, sometimes they do.

One of the well-known ways that manufacturers reduce cost is using regrind in the product instead of 100% prime virgin resin. Sometimes the level of regrind can be as high as 60 or 70%. Because the regrind level significantly affects the optical and mechanical properties of the sheet, at HighLine Polycarbonate we refuse to use any regrind in our product.

One of the less well known ways that manufacturers save costs and decrease product quality is by using thickness control. Many manufacturers do this in two ways:

The first way is by selling what has been termed a nominal thickness. If you ask for 1/8” thick Polycarbonate sheet, the thickness that you will be supplied is not 0.125” but rather 0.118”. The manufacturers refer to 0.118” sheet as nominal 1/8” sheet. In the same way, if you ask for ¼” sheet, you will be supplied 0.236” sheet. This nominal thickness means that the manufacturers use about 5.6% less material to make the sheet and as a result save a significant amount of money. Often this nominal thickness is not an issue for the customer, especially if they understand that they will be receiving 0.118” sheet instead of 0.125” sheet. However, if your application requires 0.125” sheet, make sure that you specify that the sheet must be 0.125” when you purchase it.

The second way that manufacturers use thickness control to save cost is by using a thickness specification. The argument is that it is impossible to produce exactly 1/8” thick sheet and that they need a thickness specification range. Some Polycarbonate sheet manufacturers claim a specification range of -5% to + 5% while some claim -10% to + 10%. With the 5% figure, this means that a nominal 1/8” sheet could be between 0.112” and 0.124”. Although this specification sounds reasonable on the surface, it should be understood that manufacturers have sheet thickness control developed to an art form. If the manufacturers claim to have a thickness specification of -5% to +5%, they actually target production in the range of -5% to -4%. Modern sheet extrusion lines can very accurately control sheet thickness and this can lead to huge savings for the sheet manufacturers. As a result of this thickness control, it is actually very rare to receive sheet thicker than 0.113” when ordering 1/8” sheet; these two methods of saving costs actually save sheet manufacturers around 10% on Polycarbonate resin costs. When they purchase tens of millions of pounds of resin a year, the savings are huge.

While all of these savings sound harmless, it should be remembered that Polycarbonate sheet is often used for protective screens, machine guards, and layers in bullet resistant laminates. Shaving material off the thickness of the sheet could, in some circumstances, compromise the application if the customer does not properly understand both nominal thickness and the thickness specification.

At HighLine Polycarbonate, we have taken two measures to prevent problems. Firstly, we make clear that we are supplying 0.118” sheet and that if the customer wants 1/8” sheet, we supply 0.125” sheet instead.

Secondly we have a thickness specification of -0% to +10% [Although in the interest of complete disclosure we typically run in the range of -0% to +1%]. Both of these measures mean that the minimum thickness that you will get if you order 1/8” sheet is 0.125” and not the 0.113” that some other manufacturers supply.

Our advice is to make sure that you understand what thickness sheet that you actually need for your application and then discuss both the nominal thickness and thickness specification with your supplier to ensure that you receive the sheet that you need.

UV Resistance and Warranties - Read the small print

In the last post we discussed UV light and how it can cause Polycarbonate sheet to weather. This weathering damages the Polycarbonate causing it to loose impact strength, yellow and have a lower light transmission. Manufacturers can protect Polycarbonate sheet from the effects of weathering by blocking some or all of the UV light; this protection can be achieved in a number of ways and we will discuss some of them in a future blog post.

How successfully sheet manufacturers protect their sheet against the effects of UV can often be determined from the limited warranties that the manufacturers provide to their customers. As with all legal documents it pays to read the small print in these warranties before deciding which product to purchase. Often the marketing claims in the brochures are not directly related to the details in the limited warranties. In this blog we shall examine in detail the warranties of two manufacturers of weather resistant Polycarbonate sheet; by looking at these examples, hopefully it will give you an idea of the items to look for when deciding on which Polycarbonate sheet to buy for applications requiring weather resistance.

It is also worth noting, that for most of the warranties to be honored by the manufacturers, a warranty agreement must be signed at the time of purchase and proof of purchase must be retained. Manufacturers often rely on the fact that only a small percentage of customers complete a warranty form or even know that they exist. When buying a product with a warranty, make sure that you complete the paperwork at the time of purchase and store the necessary documentation in a safe place.

Our first example is a Polycarbonate sheet with marketing documents claiming that the product “offers long-term weatherability and is backed by a 15 year performance warranty.” From this statement a customer might infer that the product will be replaced if it weathers within 15 years. When we inspect the standard warranty we find:

- If the sheet breaks within 15 years it will be replaced.

- If the yellow index of the material increases by more than 5 in a five-year period it will be replaced.

- If the yellow index of the material increases by more than 10 in a ten-year period it will be replaced.

- If the yellow index of the material increases by more than 15 in a ten to fifteen year period the material will be replaced in accordance with the table below.

- If the light transmission of the material reduces by more than 5% in a five-year period it will be replaced.

- If the light transmission is reduced by more than 7% in a ten-year period it will be replaced.

- If the light transmission is reduced by more than 10% in a ten to fifteen year period the material will be replaced in accordance with the table below.

Month 121 Customer must pay 67% of original purchase price

Month 150 Customer must pay 83% of original purchase price

Month 170 Customer must pay 94% of original purchase price

A sheet would normally be considered as badly weathered if the yellow index increase by 15 and if the light transmission is reduce by 10%. If this type of damage occurs after only 10 years and one day and the sheet would only be replaced if the customer paid 67% of the original purchase price, any customer that inferred that the sheet would be replaced free of charge for fifteen years if it weathers would not be correct.

Our second example is a Polycarbonate sheet that has marketing documents claiming: “A ten-year limited warranty backs the material against excessive yellowing, loss of light transmission and breakage.” When we read the standard warranty we find:

- The material will be replaced if it breaks from impact with only hand thrown objects within seven years.

- If the yellow index of the material increases by more than 6 in a seven-year period it will be replaced.

- If the yellow index of the material increases by more than 10 in a seven to ten-year period it will be replaced in accordance with the table below.

- If the light transmission is reduced by more than 6% in a seven-year period it will be replaced.

- If the light transmission is reduced by more than 6% in a seven to ten-year period the material will be replaced in accordance with the table below.

Year 7 Customer must pay 0% of original purchase price

Year 8 Customer must pay 55% of original purchase price

Year 9 Customer must pay 70% of original purchase price

Year 10 Customer must pay 85% of original purchase price

Again, these conditions are very different than an assumption that the material will be replaced free of charge for any weathering damage for a ten year period. The sheet will only be replaced for free for seven years and then only if it weathers more than the amount quoted in the warranty.

Reading these warranties can be very useful in deciding which material to use. If for example, preventing loss in light transmission of more than 6% for ten years is an important parameter then the second material may be the better option. However, it would still be best to talk to the manufacturer about the application and see if they could extend the full replacement cost warranty out to the ten years.

Another example is breakage; if 15 years against breakage is required, the warranty covering the first material is a much better option.

Warranties are a much better method of evaluating how well a manufacturer believes their material performs than marketing brochures. Time taken understanding the warranty and filling in the paperwork can ensure that material is correctly specified and that you are protected if the material does not perform to the expected level.

UV light and Polycarbonate

We are very familiar with the visible light spectrum. This spectrum ranges from Violet light with wavelengths of 380-450 nanometers to Red light with wavelengths of 620-750 nanometers. We can see this spectrum as a rainbow when visible light is split into its component colors. Below the visible spectrum there is UV light and above the visible spectrum there is Infrared light.

In this article we will be discussing the UV range of the spectrum and its importance for Polycarbonate. The UV spectrum is classified into three broad groups:

UVA Wavelengths of 320-400 nanometers

UVB Wavelengths of 280-320 nanometers

UVC Wavelengths of 100-280 nanometers

You may be familiar with these terms from Sun tan lotion or Sunglasses.

Planck’s constant is an important number when finding the relationship between a Photon of light and its wavelength. The relation is known as the Planck relation and is expressed as:

Energy of a Photon = speed of light x Planck’s constant / Wavelength of light.

From this relation, we can see that higher wavelengths of light have lower energy. Therefore visible light has less energy than UVA light. UVA light also has less energy than UVB light. It is this energy that is destructive and it is why we need to protect our skin, eyes and Polycarbonate from the effects of UV light.

It is not just the energy of different wavelengths that is important, but it is also the quantity of UV light exposure that is important.

In the upper atmosphere light consists of 1.3% UVB and 6.7% UVA giving a total of 8.0%. The atmosphere is reasonably good at absorbing the UV radiation, particularly UVB, so by the time light reaches sea level, it consists of 0.3% UVB and 5.7% of UVA giving a total of 6.0%.

The absorbtion of UV light varies with latitude, at higher latitudes more UVB is absorbed. This latitude affect is why skin protection is particuarly important near the equator.

Also absorbtion of UV varies significantly with altitude, as there is much less atmosphere for light to pass through. This altitude affect is one of the reasons why skin protection is necessary while skiing (in addition to reflection by snow). At a 20 degree solar elevation, UVB increases by about 20% for every 1000 meters increase in altitude and UVA increases by about 12%.

Polycarbonate becomes damaged by UV wavelengths below 300 nanometers and is particularly vulnerable to wavelengths in the range of 280-290 nanometers. The UV light below 300 nanometers starts to cause micro-cracks in the surface, over time it weakens the strength of the Polycarbonate and causes the material to turn yellow. Increasing the amount of UV exposure, particularly to the higher energy UVB will increase the rate of degradation of the Polycarbonate. Also thermal cycling and rain can help to increase the rate of degradation caused by UV exposure.

Because the amount of UV light and the energy of UV light are important factors in the weathering process, we can see how the location of the Polycarbonate installation is likely to affect the amount of degradation due to weathering.

Polycarbonate sheet installed on a mountain near the equator is likely to degrade much quicker than sheet installed at sea level in Alaska. This effect is due to the fact that the amount of UV light in the 280-290 nanometer range is significantly lower at locations at sea level and higher latitudes. Location is an important factor in specifying what type of UV protection to add to sheet as the ratio of UVA to UVB light can vary significantly. Asking your Polycarbonate sheet manufacturer to design a UV protection package for your location is important for preventing damage due to weathering.

In a future article we will discuss how Polycarbonate sheet manufacturers can protect Polycarbonate sheet against weathering and what to look for in manufacturer's warranties concerning UV resistance.

Acrylic vs. Polycarbonate (Part 1)

Acrylic sheet and Polycarbonate sheet are two of the most widely used plastics for optical applications. 

Acrylic sheet trade names include Plexiglas and Perspex.  Polycarbonate sheet trade names include Lexan and Makrolon.

In Part 1 of this occasional series comparing Acrylic to Polycarbonate, we will look at five of the most obvious differences in the properties.  Over the coming months we will come back and look at some of the lesser known differences between the materials.

It should be remembered that both materials have their advantages and it is not a case of which one is better.  As always, it is important to select the right material for the application.

1. Breakage

Polycarbonate is well known for its strength and resistance to impacts.  When it is hit with an object it is almost impossible to break.  This property makes it ideal for machine guards and protective screens.  It is one reason why face shields and protective goggles are often made from Polycarbonate.  Front headlights on cars are also often made from Polycarbonate as they can resist damage from stone chips.

Acrylic does not have the same strength and resistance to impacts as Polycarbonate.  If it is hit with sufficient force it will shatter. 

2. Weathering

Acrylic has excellent resistance to weathering.  UV light does very little damage to Acrylic over time and so Acrylic is often a good choice for outdoor applications.  The rear tail-lights of a car are often made from Acrylic because the colors are very stable and resistant to UV and the potential damage from stone chips is low at the rear of the car.  Acrylic has an almost unlimited resistance to weathering. 

Polycarbonate weathers when exposed to UV light.  This weathering often takes the form of yellowing and micro-cracking of the material.  It is possible to reduce the effects of weathering by either adding a cap layer of UV absorbers or a coating loaded with UV absorbers.  These solutions do however add to the cost of the Polycarbonate sheet and will only protect the product for 10 to 15 years.  There are some advanced solutions to protect Polycarbonate for 25+ years from HighLine Polycarbonate but these are very expensive and are often cost prohibitive for most applications. 

3. Light Transmission

Acrylic has a light transmission of 92%.  Polycarbonate has a light transmission of 88%.  The reason for the difference is the refractive index of the two materials.  Acrylic has a refractive index of 1.49 and Polycarbonate has a refractive index of 1.585.

If the higher light transmission of Acrylic is an important property for an application and some of the other properties of Polycarbonate are not required, Acrylic is a good choice of material.

However, as discussed in previous posts, it is possible to raise the light transmission of Polycarbonate to 90% with a simple abrasion resistant coating.  It is also possible to use advanced anti-reflective coatings to raise the light transmission of Polycarbonate to 98.5%.  The choice of which material to use for optical applications depends upon both cost and the other properties that are required. 

4. Heat stability

Acrylic has a heat distortion temperature under a load of 260 psi of 200 degrees F, whereas Polycarbonate has a heat distortion temperature of 264 degrees F.  A full explanation of heat stability can be found under the previous blog post discussing the subject.

Polycarbonate is much more resistant to temperature than Acrylic.  This means that if the application involves a higher temperature environment where the structural integrity of the material is required, Polycarbonate may be a better choice.  The Heat Stability is also important in vapor deposition of coatings such as Indium Tin Oxide.  It is possible to apply more conductive surfaces onto Polycarbonate than Acrylic. 

5. Scratch and Abrasion Resistance

Acrylic is more resistance to scratches and damage by abrasion than Polycarbonate.  This is one of the well-known weaknesses of Polycarbonate.  To overcome this problem there are a number of solutions including applying an abrasion or scratch resistant coating.  The previous blog post on scratch and abrasion resistance discusses this subject in more detail.  Adding a coating to Polycarbonate will also increase the cost of the material.

 

In conclusion, Polycarbonate sheet has significant advantages over Acrylic in terms of Strength and Heat Stability. 

Acrylic has significant advantages over Polycarbonate in terms of weathering and scratch resistance.  It is possible to improve both the weathering and scratch resistance of Polycarbonate by a number of methods, but these do add cost.

Acrylic does have a marginal advantage over Polycarbonate in terms of light transmission, but this can easily be overcome in high-tech applications with anti-reflective coatings.