Clearfix - Repairing Polycarbonate sheet scratches

The above video shows how scratches in both uncoated and abrasion resistant Polycarbonate sheet can be easily repaired using a product developed by 3M and Clearfix Aerospace.  The product was initially developed to repair military helicopter windows; however, HighLine Polycarbonate has worked with 3M and Clearfix Aerospace to evaluate and test the product on Polycarbonate sheet used on transparent armor laminates as well as other applications.

The product works equally well on repairing scratches and other damage on both coated and uncoated Polycarbonate sheet.  Not only can the product be used to repair scratches on in service vehicles but it can also be used to repair scratches on production damaged laminates.  Laminates that would otherwise need to be scrapped can now be repaired allowing manufacturers and users to significantly reduce costs.

The product can be purchased from HighLine Polycarbonate LLC as we are now a primary distributor of Clearfix.  Potential users should contact us to schedule a demonstration at their facility.

Temperatures for thermoforming Polycarbonate

When Polycarbonate is cooled below 150 C / 302 F, it transitions from a flexible structure to a rigid structure that locks into what ever shape it is in; this temperature is known as the glass transition temperature. Conversely, when Polycarbonate it heated above its glass transition temperature it becomes flexible and can be bent into various shapes. This property is used in the process of thermoforming.

Thermoforming can be carried out at any temperature above the glass transition temperature and below the melt temperature of 267 C / 512 F, although in practice the Polycarbonate becomes more flexible the higher the temperature and it is not necessary to approach the melt temperature. The Polycarbonate actually becomes difficult to use much above a temperature of 215 C / 450 F.

There are three broad categories of forming – Cold forming, Low temperature thermoforming and high temperature thermoforming.

Cold forming.

Cold forming uses a frame to hold the Polycarbonate sheet in the desired shape. The sheet is then heated to between 302 F and 340 F for several hours until the entire sheet (interior and not just the surface) rises above the glass transition temperature. The sheet is then cooled below the glass transition temperature to set the shape. Cold forming is a simple process, but can only be used for relatively simple shapes (often two dimensional) without tight radius bends.

Low temperature thermoforming.

Low temperature thermoforming is carried out between 350 F and 370 F. This process is often used for simple shapes where the Polycarbonate sheet drapes over a mold or into a mold. While it is possible to achieve relatively simple 3D shapes with low temperature thermoforming, complex shapes with lots of detail are not possible. One advantage of low temperature thermoforming is that pre-drying of the sheets is not necessary.

High temperature thermoforming.

High temperature thermoforming is carried out between 370 F and 420 F. Complex shapes, sharp details and deep draws are all possible with high temperature thermo-forming. Many thermoforming processes use vacuum to achieve some of the complex shapes. One of the disadvantages of high temperature thermoforming is that all moisture must be removed from the sheet by drying the sheet prior to thermoforming. If this drying is not done, the higher temperatures will cause moisture evaporation bubbles to appear in the sheet during thermoforming.

Drying needs to be carried out above the boiling point of water and it is recommended that the sheet is heated to 120 C / 250 F to dry the material. The drying time is dependent upon the sheet thickness. For 0.118” thick sheet about 10 hours of drying is recommended, for 0.236” sheet, this can increase to closer to 30 hours. After drying the sheet should be used within a reasonably short time frame to prevent the sheet re-absorbing moisture from the air.

Hard coatings.

One thing to remember with thermoforming Polycarbonate sheet is that raising the temperature above the glass transition temperature will make the sheet flexible; any hard coating on the sheet will probably not be flexible and will crack during the thermoforming process. When purchasing Polycarbonate sheet for thermoforming it is important to use only hard coatings designed for thermoforming. These coatings are slightly more expensive than standard hard coats, but are considerably cheaper than the alternative of post coating any thermoformed parts.

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.

Selecting Abrasion and Scratch resistant coatings

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



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

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


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

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


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

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

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

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

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


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

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