Monday, November 9, 2009
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.