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.

Why the Light Transmission of Coated Polycarbonate sheet is higher than Uncoated sheet

This topic is a follow up of previous blog on the 28^th October 2009 – “Transmission – Anti Reflectives and Anti Glare”.  The previous blog gives an introduction Refractive Index and Reflection. 

One question that we are often asked is why the Light Transmission of our abrasion resistant coated polycarbonate sheet (90%) is higher than the Light Transmission of our uncoated polycarbonate sheet (88%)?  

This question is asked because there is a belief that the coating should reduce the “optical properties” of the sheet.  Some people even believe that we must be using a “purer” base sheet for our coated product. 

The answer to the question is related to the reflection of light.  As we discussed in our previous blog post, light is reflected from uncoated sheet on the front surface and the back surface. 

Uncoated sheet.

At the front surface, the light passes from the air (with a refractive index of 1.00) to the Polycarbonate (with a refractive index of 1.585).  Using the Fresnell Equations, the reflection can be calculated as 5.1%.  [See the previous post for details of the Fresnell equations].

At  the back surface, the light passes from the Polycarbonate (with a refractive index of 1.585) to the air (with a refractive index of 1.00).  The reflection from this surface is also 5.1%.

The total reflection is 10.2% giving a light transmission of 89.8%.  Typically we report a light transmission of 88% to be conservative. 

Coated sheet.

In the case of one side coated sheet we introduce another layer – the coating.  The coating material typically has a refractive index of 1.49.  With this information we can calculate the transmission of the coated sheet.

At the front surface, the light passes from the air (with a refractive index of 1.00) to the coating (with a refractive index of 1.49).  The reflection from this surface can be calculated as 3.9%

The light then passes from the coating (refractive index of 1.49) to the Polycarbonate (refractive index of 1.585).  The reflection from this surface can be calculated as 0.1%.

At the back surface, the light passes from the Polycarbonate (with a refractive index of 1.585) to the air (with a refractive index of 1.00).  The reflection from this surface is again 5.1%.

The total reflection is 9.1% giving a light transmission of 90.9%.  Typically we report a light transmission of 90% to be conservative.

It can be seen that adding a coating actually increases the Light Transmission of the Polycarbonate sheet.  The application of a layer with a Refractive Index between that of air and Polycarbonate is actually the theoretical basis of advanced reflective coatings.