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The Rainbow Effect Part 3

The Rainbow Effect Part 3

In the final blog post of this trilogy we will discuss Iridescence and how it can cause a rainbow effect on abrasion resistant coated Polycarbonate sheet. We will also discuss how the rainbow effect can be minimized.

Iridescence is the rainbow or oil slick type pattern that often appears on the surface of a Polycarbonate sheet particularly under artificial lighting conditions.

 

Coated Polycarbonate sheet has a thin film of coating on the surface of the sheet in order to protect the sheet against abrasion damage. It is this thin film of coating material that causes the problem, in much the same way that a thin film of oil on the surface of a pool of water exhibits the rainbow patterns. The effect is due to the process known as interference.

 

As discussed in previous blog posts, whenever light travels from a material with one refractive index to another material with a different refractive index, some of the light is reflected. In the case of the coated Polycarbonate sheet, when light moves from the air into the coating some of the light is reflected. Then, when the light moves from the coating into the actual Polycarbonate, some more of the light is reflected. When the light that is reflected from the first surface comes into contact with the light that is reflected from the second surface the light waves recombine.

 

Depending on how thick the coating layer is, the light waves may be in sync when they recombine or may be out of sync when they recombine. If they are in sync the two waves will added together and will have constructive interference. If they are out of sync the two waves will start to cancel each other out and will have destructive interference.

 

Since visible light has wavelengths of 380nm(violet) to 750nm (red) and a typical hard coat has a thickness of 4 to 7 microns [4000 to 7000 nm], the coating thickness is an order of magnitude thicker than the wavelengths of visible light. A small percentage variation in the coating thickness can therefore change whether the constructive interference or destructive interference occurs. If there is variation of coating thickness over a small area of sheet, even if the variation is only tens or hundreds of nanometers, then there will be areas of constructive interference and areas of destructive interference. This variation in the interference patterns is part of the cause of the iridescence or rainbow effect.

 

The question then becomes, how do we eliminate the variation in coating thickness? Abrasion resistant coatings are often added to sheet by a process known as flow coating. The sheet is hung vertically and coating solution is allowed to run down the surface of the sheet from top to bottom under gravity. The solvents are then allowed to evaporate. If the sheet is allowed to move before most of the solvents have evaporated, the coating surface can become uneven. However, we need to remember that the coating surface is not the only surface that we need to be concerned about – there is also the sheet surface that is reflecting light. The sheet is extruded between large chrome rolls which are powered by motors. If there is any variation in the motor speed of these motors or the motors pulling the sheet, there can be variation in the thickness of the sheet. While the variation in the thickness will be small, it only requires very small variation to cause iridescence.

 

The reality is that neither the sheet producers or the abrasion resistant coaters have the ability to control their processes to the level of 10-100nm thickness. If we look at sheet producers, many of them state that their thickness specification is plus or minus 10%. On a 0.118″ thick sheet that corresponds to 300,000 nanometers. While this is an overall thickness tolerance and not a measure of local variation of thickness, it does give some idea of the magnitude of the problem. Using this information we can determine that the sheet producers and coaters cannot prevent the problem. In many cases sheet producers often blame the coaters for the problem and coaters often blame the sheet producers. This then leaves us with the question of how do we solve the problem?

 

To answer the question we will briefly move to another topic – different types of lights. Traditional incandescent light bulbs have a relatively smooth light spectrum across the visible region and are similar to sunlight in this respect. When sunlight is split into its component wavelengths (such as in a rainbow) there is a smooth transition from violet through the various colors to red. There are no wavelengths missing. An incandescent light bulb behaves in the same way (as do some full spectrum LED bulbs).

Fluorescent bulbs, mercury bulbs, sodium bulbs and non full spectrum LEDs are different. When the light is split into its component parts, there are peaks at some wavelengths and gaps at other wavelengths. For example, a low sodium bulb emits an almost monochromatic light source at 589.3nm and a standard fluorescent bulb has 22 peaks with the main four being Mercury at 437nm, Terbium at 543nm, Mercury at 547nm and Europium at 611nm. These wavelengths of a fluorescent bulb combine to yield a light that looks like natural light but has discrete wavelengths rather than the continual spectrum of natural light.

 

Having a light source composed of discrete wavelengths rather than a continuous spectrum is a major problem for iridescence; when the light is reflected from the two surfaces the discrete wavelengths make the problem much larger as there are no intermediate colors to cancel out the iridescent effect. In short, the light source can make the problem of iridescence much greater.

 

The best way to reduce the effect of iridescence is to change the lighting source to a full spectrum light source such as incandescent bulbs or full spectrum LEDs. If the only option is to use fluorescent bulbs, it is better to use a bulb with more emission peaks to more closely resemble full spectrum light.

 

Another option to completely resolve the problem is to use what is known as an index matched abrasion resistant coating. The Polycarbonate sheet has a refractive index of 1.585 and most coatings have a refractive index of 1.49. If an abrasion resistant coating with a refractive index of 1.585 is used, the light will treat the coated Polycarbonate sheet as a single layer material and the effect of iridescence will be completely eliminated. While this process sounds great (and HighLine Polycarbonate can offer index matched abrasion resistant coated products) there is a significant downside – index matched coatings are very expensive. In most applications it is better to install full spectrum bulbs to reduce the problem.

 

Finally, to illustrate the effect of lighting on the visual appearance of iridescence we will recount a case study about the problem. A rail car manufacturer was experiencing oil slick like patterns on the Polycarbonate windows of their railcars. The manufacturer of the windows was inspecting the windows prior to sending them to the rail car manufacturer to try and identify the problem. They were unable to detect the issue as their factory was lit with incandescent lights. When the windows were installed in the railcars, the oil slick appearance was easily visible because the internal lights on the rail car were fluorescent bulbs.

The most practical solution would have been to change the bulb type on the railcar, but unfortunately the window manufacturer did not understand the problem. They told the rail car manufacturer that the problem was due to birefringence, which, as anyone who has read these three blog posts knows, was not the cause of the problem. By understanding the cause of the problem it is easier to recommend a solution to the customer.

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The Rainbow Effect Part 2

The Rainbow Effect Part 2

In the last post we discussed how stresses in Polycarbonate can cause the material to become Anisotropic and exhibit Birefringent properties. Light waves parallel to the stress direction will travel through the sheet at a different speed than the light waves perpendicular to the stress direction.

 

It is possible to visualize the stresses in the sheet due to the birefringent properties of the sheet. A technique known as Photoelasticity is often used. In this method light is first passed through a polarizing filter, in order to block all components of the light not vibrating in the direction of the plane. The light coming through the filter is then known as polarized light. The light is then allowed to pass through the Polycarbonate part being examined. The birefringent properties caused by the stresses cause the polarized light to be split into two perpendicular components each moving at different speeds, which are governed by the amount of stress in each direction. The components of the light waves recombine on leaving the Polycarbonate. When this light is then viewed through a second polarizing filter it is possible to see the effect of the retardation of the light in the form of “rainbow” like patterns. There is a lot of theory that can be explored on the method of Photoelasticity and this theory can easily be researched by carrying out a web search. In this blog we do not plan to go into advanced theory of how the light waves recombine, but rather discuss how the method of Photoelasticity can be practically used.

 

In the picture at the top of this blog post is a photograph taken of a piece of Polycarbonate with a hole drilled through it. The photograph was taken with a simple phone camera and two polarizing filters bought from a camera shop for $25 each. One filter was put behind the Polycarbonate part and one filter was put in front of the part. Although this cheap set up does not compare with advanced equipment for visualizing and measuring Photoelasticity, it does provide a simple practical tool for visualizing stresses in Polycarbonate parts.

 

In the Photo it can be seen that there are high levels of stresses on each side of the hole. We suspect that these stresses were caused by poor drilling technique using the wrong drill bit for Polycarbonate and operated at the wrong speed. It is also possible that the drill was started while in contact with the sheet. The technique of Photoelasticity allows us to visualize these stresses and therefore allows us to adjust fabricating methods to minimize stresses. This information is particularly important, as we know that areas of increased stress are prone to cracking and damage, especially when exposed to certain solvents.

 

We invite readers who are involved in fabricating Polycarbonate parts to try this test method themselves to see the stress areas on the parts. All you need to do is buy two Polarizing filters from a camera shop.

 

In this section of the trilogy of blog posts on the subject of the rainbow effect, we have seen how stresses in Polycarbonate sheet can lead to birefringence and that these stresses can be visualized through polarizing filters as a rainbow type pattern.

However, it should be understood that rainbow effect seen on some hard coated Polycarbonate sheet without the use of polarizing filters is not due to the birefringence of the material. These rainbow type patterns on hard coated sheet are often very easy to see with just the eye and can cause the visual appearance of the sheet to seem very poor. In the last post on this topic, we will discuss what causes the rainbow effect on coated sheet and how its effect can be minimized.

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The Rainbow Effect Part 1

The Rainbow Effect Part 1

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)

Where:

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.

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