Polycarbonate and acrylic sheets are the primary materials used in a number of applications. To improve their performance and reliability, anti-scratch coating for plastics was developed.
Generally, polycarbonate sheets are inherently susceptible to scratches. This was the greatest challenge manufacturers had to deal with even though, it was impact resistant and stronger than glass.
So far, a number of anti-scratch products with varying performance have been developed. However, not all are formulated as per the required manufacturing standards. That is, they don’t meet the required quality standards.
There was need to come up with reliable testing mechanisms that could guarantee quality and consistency in the manufacturing process. At the moment, there are 5 different test mechanisms that are commonly used:
- Bayer test
- Taber test
- Steel wool test
- Eraser test
- Tumble test
This test is used to determine the quality of anti-scratch coating for plastic lenses, enclosures and transparent windows. The test conforms to the ASTM F735-81 test standard that is recognized globally.
During this process, a plastic lenses to be tested is mounted on a special Bayer test kit. This kit holds the lens such that, the curved section protrudes above the bottom of the tray.
An abrasive material (Alundum ZF – 12) is fitted in the tray, after which the tray is reciprocated back and forth at a distance of about 4 inches. With the frequency maintained at 150 cycles per minute, the lens is subjected to this condition for about 4 minutes. The abrasive material used in this case is similar to sand.
After 4 minutes, the lens is removed and the degree of abrasion is measured based on the change in haze. This is determined using a haze meter. The value obtained is compared to the initial value before abrasion. A greater difference in haze value will imply less resistance to scratches.
That is, it involves comparing the haze value of a polycarbonate lens coated with anti-scratch for plastics and the uncoated. The uncoated lens is used as a standard to abrasion (CR-39).
The haze gain between uncoated lenses to the coated lens is in the form of a ratio called Bayer ratio. During this test, a Bayer ratio of:
- 1 implies abrasion coating has same abrasion resistance as an uncoated lens (CR – 39)
- 5 implies uncoated (CR-39) has five times the haze gain as coated lens.
Taber abrasion test
This anti-scratch coating for plastics test was approved by the ASTM in 1982. All processes in this test conform to the ASTM D1044-82 test standard. Like the Bayer test, it was mainly used for durability test of paint on surfaces. Basically, the test used for plastics is a modification of the initial test standard.
The test process involves the use of different materials depending on nature of plastic surfaces. These include:
Conical Diamond tools – they are used to scratch the surface. This is basically to test for ordinary hardness
Diamond Scratch tool – mainly used for unusually hard coating.
1.0mm Hemispherical Scratch tip – this is a perfect choice for fragile surfaces.
The quality of anti-scratch coating is evaluated based on minimum load needed to penetrate the surface of the material under test. It can be obtained by calculating specified load results.
Steel wool abrasion resistance test
This test was adopted in 1975. The evaluation process mainly focused on the loading force that could penetrate the plastic coating through to the polycarbonate sheet. This test is commonly used in the optical industry.
The whole process has improved over time with the test focusing on rubbing a plastic surface with steel wool for a specified period of time at a pre-determined pressure.
Like the Bayer test, the haze gain is measured to determine the degree of scratch resistance of that material. Companies in this industry have improved these tests to get more consistent results.
Tumble abrasion resistance test
The tumble test was developed in 1973. Here, testing criterion was mainly based on obtaining results from equivalent to 1 year of typical wearing conditions.
Anti-scratch coating for plastic are subjected to an off-site field test of 258 subjects for about 2 years. The results obtained were mainly used to verify the tumble test.
During the testing process, samples of polycarbonate sheets and abrasive materials are put in a chamber. They are then tumbled for a given period of time. The haze gain of polycarbonate sheets is then determined. Of course, the process of determining haze gain still remains the same.
Eraser abrasion tests
This test was known as Military Standard MIL-E12397A. It was mainly designed for reflecting-reducing films for optical materials.
In this test, a polycarbonate or acrylic sheet is rubbed back and forth using a standard pencil eraser. The eraser is made of rigid particles that cause scratching effect. During the process a consistent amount of force is applied on the plastic material via pencil eraser. Again, the scrubbing process takes place in line within a given period of time, at controlled pressure and speed.
For consistency and more accurate results, companies are currently using machines to scrub the surface of these sheets. The evaluation process is also based on comparing two surfaces.
Over the past decades, there are a number of standards adopted to ensure anti-scratch coatings meet the required performance standards. These include ASTM, DIN, ISO and JSA. This is mainly to ensure compliance and reliability of all these products.
You will realize that advanced testing process, additional quality evaluation criteria such as adhesion or shear tests. It is possible to achieve this when you adopt the Taber test. This is the reason why it is also referred to as Taber shear/scratch test.
All the five tests discussed above are reliable and they can be used independently to verify performance and reliability of anti-scratch coating for plastics. However, as a rule of thumb, you should use at least two different tests to verify the quality of your polycarbonate or acrylic sheets.