One question that we are often asked about Polycarbonate is what causes the rainbow effect patterns on coated sheet and how can they be eliminated.
The answer is not simple and we will need to answer the question over three posts. There is also a lot of confusion in the industry about what causes the effect. Often people try to explain the effect using the wrong terms.
Birefringence and anisotropic materials
The first term that we will discuss is Birefringence or the double refraction of light when it passes through an Anisotropic material. At this stage, don’t worry too much about these terms; we will explain them as we go. Birefringence is often the term that is incorrectly used to explain the rainbow effect patterns seen on the surface of some coated Polycarbonate sheet. As we will explain, Birefringence can allow us to see stresses in the sheet using polarizing filters – they allow us to see the stresses, which will appear as rainbow like effects. However, birefringence is not the cause of the rainbow like effect, which can be seen with the eye on the surface of hard coated Polycarbonate sheet.
To explain birefringence and anisotropic materials we will start with a discussion about the structure of Polycarbonate. Polycarbonate is a long molecule containing Carbon, Hydrogen and Oxygen atoms. A simple web search can give details of the chemical formula. When Polycarbonate is heated and allowed to cool without being subject to any stresses, these molecules will be arranged randomly.
During the production of extruded sheet, the Polycarbonate is melted and then extruded through a wide die into a sheet format. The sheet is then pulled out of the die by some pull rollers through some chrome polishing rolls to create a smooth surface on the sheet. The pull rolls create some stress in the sheet in the direction of extrusion, but not in the direction perpendicular to the extrusion. The sheet is cooled and allowed to “set” while still being pulled by these rolls. This difference in stress in the sheet between the extrusion direction and the direction perpendicular to extrusion is commonly referred to as shrinkage. We have discussed shrinkage in more detail in previous blog posts; shrinkage is able to be controlled below 1%, although often it is possible to find sheet with high levels of shrinkage of 10% or more.
Polycarbonate sheet stresses can be eliminated by annealing the sheet. Annealing is the process of heating it above its glass transition temperature and then allowing it to cool. Also stresses can often be added to the sheet by some fabrication methods.
The more shrinkage that the Polycarbonate sheet has, the more stress it has in the extrusion direction and the more the Polycarbonate molecules are aligned in the extrusion direction. This alignment of the Polycarbonate molecule chains causes the Refractive Index of the Polycarbonate in the direction of the extrusion to be different than the Refractive Index in the direction perpendicular to the extrusion. As explained in previous blog posts, the refractive index is a measure of how fast light travels in a material. The difference of refractive index in the two directions causes extruded Polycarbonate to become what is known as an Anisotropic Material – where the speed of light traveling through the material is dependent upon the direction of the material.
If a Polycarbonate sheet is produced without any stress or 0% shrinkage, it would not be Anisotropic.
The difference in the Refractive Index between the two directions can be calculated using the Stress Optics Law:
(RI1 – RI2) = C x (Stress1 – Stress2)
RI1 = Refractive Index in extrusion direction
RI2 = Refractive Index in direction perpendicular to extrusion
C = Stress Optic Constant
Stress1 = Stress in extrusion direction
Stress2 = Stress in direction perpendicular to extrusion.
If the Refractive Index in one direction is different than the Refractive Index in the other direction, the components of the waves of light moving through the Polycarbonate in one direction will travel at a different speed than the light in another direction. The more Polycarbonate that the waves travel through, the more the one wave will lag behind the other. This effect is known as Retardation of the wave.
The retardation of the wave can be calculated using the following formula:
Retardation = C x thickness of Polycarbonate x (Stress1 – Stress2)
The amount of retardation of the wave is therefore proportional to both the thickness of the sheet and the differences in the stresses in the two directions. The retardation will be much lower on thin sheet with low shrinkage.
When the components of the light in the two directions emerge from the sheet they will recombine. However, how they recombine will be a function of the phase difference caused by the retardation of the light. There could be constructive or destructive recombining of the waves at different wavelengths.
In the next post on this subject we will look at how these waves combine. We will also look at how we can use a polarizer to look at the stresses in the sheet using an experimental method known as Photo-elasticity.
We are often asked about the difference between bullet resistant windows installed in 24hrs stores or banks and the transparent armor used by the military. The bullet resistant windows in convenience stores and banks are often made of cell cast acrylic sheet or a combination of acrylic and Polycarbonate. They are often about 1.25″ to 1.375″ thick and are designed to protect against threats that are likely to be encountered in that environment. Typical bullet resistant ratings of UL.752 Level 1 to Level 3 are encountered. But what does a UL.752 Level 1, Level 2 or Level 3 mean and how does it compare to the transparent armor of military applications? The answer lies in the physics concept of kinetic energy
A UL.752 Level 1 material is designed to stop 9mm FMCJ rounds weighing 8.0 grams traveling at a velocity of up to 394 meters/second.
A UL.752 Level 2 material is designed to stop 0.357 Magnum JSP rounds weighing 10.2 grams traveling at a velocity of up to 419 meters/second.
A UL.752 Level 3 material is designed to stop 0.44 Magnum rounds weighing 15.6 grams traveling at a velocity of up to 453 meters/second.
Calculating the kinetic energy
But what does this mean? One of the most important factors in determining whether a bullet resistant structure will stop a ballistics round is how much Kinetic Energy does the ballistics round have. Using the equation for Kinetic Energy:
Kinetic Energy (Joules) = 1/2 x Mass (Kilograms) x Velocity (meters/second)^2
Calculating the Kinetic Energy for the UL.752 Level 1 ballistics round we find:
Kinetic Energy = 1/2 x 0.008 x 394 x 394 = 620 Joules
For the three UL.752 Levels we get:
Level 1 620 Joules
Level 2 895 Joules
Level 3 1600 Joules
Military transparent armor
We can see as the weight and the velocity of the round increase the Kinetic Energy of the round increases. The bullet resistant material needs to be able to resist a larger amount of Kinetic Energy. We can now look at the military grades to compare the amount of Kinetic Energy they are designed to stop. Military grades of transparent armor are composed of multiple layers of glass and polycarbonate. The glass can be of various types. In some cases advanced materials such as Spinel and ALON are also used. Often the structures can be many inches thick.
For US military grades a standard known as ATPD.2352 is used. The different rounds that the material must stop is listed but the velocities are classified. The fact that the velocities are classified makes it difficult to calculate the required Kinetic Energy that must be absorbed; it would be possible to take an educated guess at the velocities, but for the purposes of this blog post, we do not need to do this is we can use the NATO standard AEP55 STANAG 4549 Volume 1.
STANAG 4549 has 5 protection levels for Light Armored Vehicles. For the purposes of the discussion on transparent armor we will just look at Levels 1 and 4.
Level 1 material is designed to stop a 7.62 mm x 51 NATO ball round weighing 9.65 grams traveling at 833 meters/second.
Level 4 material is designed to stop a 14.5 mm x 114 API/B32 round weighing 64 grams traveling at 911 meters/second.
A Level 1 round has a Kinetic Energy of 3,348 Joules
A Level 4 round has a Kinetic Energy of 26,557 Joules
You can see that the energy that a UL.752 Level 1 material needs to stop is over 40 times less than a STANAG 4549 Level 4 material. The reason for this difference is that the type of ballistics rounds likely to be encountered at a convenience store are likely to be very different from those encountered by the military. Indeed the deterrence factor of bullet resistance glass in commercial applications should not be underestimated. It should be noted that this discussion is very much a simplification and is only meant to compare the Kinetic Energy of the different rounds used for the different tests. There are a number of parameters that have not been discussed in this blog post such as the multi shot spacing and the shape of the round.
Polycarbonate sheet is extruded 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.
The maximum width of the polycarbonate 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.
The length of the polycarbonate 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.
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 polycarbonate sheet 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.
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 polycarbonate sheet 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.
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.
Polycarbonate sheet readily absorbs moisture from the air. Eventually the water content will reach 0.2% by weight. In most applications this water content is not a problem, however, in applications where you need to process the sheet above a temperature of 250F, this water can vaporize within the sheet during processing and lead to small bubbles forming. As little as 0.05% water can cause these bubbles. Two processes that require the sheet to be heated above 250F are lamination of the Polycarbonate and thermoforming of the Polycarbonate. Both of these processes can have problems with bubbles if the sheet is not dried correctly. This article discusses drying polycarbonate sheet before either lamination or thermoforming.
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 obtain EMI RFI shielding 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|
|50 Mesh SS Wire||
|Blackened Copper Wire||
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.