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PENCIL HARDNESS NOT UP TO SCRATCH
The Pencil Hardness test, which is used to determine how well a hardcoat film will survive when in use, is often used within the film manufacturing, printing and converting industries as it requires no expensive equipment and provides an immediate visual result. The surface of the coating can be tested with a range of leads, 9H being the hardest and 6B the softest. The tester performs a number of strokes, typically five, with the pencil on the surface of a film; if the surface is not scratched then the next hardest pencil is used, continuing until the sample is scratched and damaged by the pencil. If a coating can survive scratching by a 3H pencil or better then it is generally considered to be a hard coating. The study, which was commissioned by MacDermid Autotype’s research and development department, to determine the effectiveness of the Pencil Hardness test, showed that variations of 1H can be expected from the test, even when carried out by highly trained individuals, with good eyesight, following a strict protocol and using the correct specification of pencil. These variations can be even greater under laboratory conditions or when carried out by different suppliers or end users. There are a number of reasons why this test has so many variables. For instance, it is common for the test to be carried out by hand by many different people, without a controlled holder, while testing is often carried out with the coated film placed upon a variety of surfaces, ranging from glass to rubber. As part of the study, MacDermid Autotype examined a variety of pencils from different suppliers and discovered that these also displayed large variations in test performance. For example, a Japanese manufactured 3H pencil can be very different from a European 3H one, which likewise can be different from an American 3H. The explanation for this is clear, yet often ignored, as pencils are designed for use by artists and draughtsmen and not for objective scientific tests. Most importantly, the results of this test highlight the many far reaching implications this can have on the film substrate industry. For example, manufacturers need to agree on common standards for pencil hardness testing under controlled conditions in order to identify the most accurate and repeatable methodology. In addition, customers should be made aware of these issues and be encouraged to work closely with manufacturers in order to decide which product meet their specifications, based on tests carried out in controlled circumstances. Ultimately, the study concludes that in today’s highly competitive marketplace, with exacting quality standards, the pencil hardness test is an ineffective and unreliable mechanism on which to base development or purchasing decisions. Alternative tests, such as the Taber test is more relevant to most display applications as it is a surface scratch test rather than a gouge test. The Taber test has been examined and found to be objectively more reliable as well as being more relevant to the final application of film in the print and converting sectors.
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