Hardcoated films improve performance and protect against abrasions, scratches and chemicals, either in finished products or during manufacturing processes. Testing for hardness is complicated and somewhat subjective. Carestream applied the knowledge gained from our work in performance-critical industries to devise a stringent hardness testing process for hardcoat films used in electronic, display, graphic and optical applications.
Hardcoat projects undergo a rigorous production release test, which includes hardness testing as well as evaluations of coating thickness, chemical resistance, flexibility, cleanliness and optical properties. These exacting guidelines apply to every roll in the production environment – ensuring that the process and product quality meet customer expectations.
Typical hardcoat film constructions consist of a substrate film and hardcoat layer. The substrate film is commonly composed of polyethylene terephthalate (PET), polycarbonate (PC), polymethyl methacrylate (PMMA), polyethylene naphthalate (PEN) or cyclic olefin polymer (COP). The hardcoat layer often contains polymers, monomers that are later cured, nanoparticles or various other chemistries.
There are several standards for testing hardness of coatings including the International Organization for Standardization’s ISO 15184, ASTM International’s ASTM D3363, and Japanese Standards Association’s JIS K-5600. In these tests, a pencil with a lead of specified dimensions, shape, and hardness is pushed across a hardcoated film surface to evaluate the surface’s resistance to marking or other defects. Test parameters for each protocol are subtly different. For example, the ASTM test does not specify a fixed applied load on the pencil, whereas the ISO standard requires a sled with a standard load.
At Carestream, we understand the intricacies of each test and tailor our approach for every project. The most commonly used method focuses on ISO 15184 testing. Professionals control as many variables as possible while conducting the ISO 15184 test to ensure consistency and adherence to the published standards. Variables include the fixed applied load on the pencil, sled weight, and pencil type/age/position.
Carestream’s Hardness Testing Approach
Carestream uses a metal block fitted with two wheels, one on each side, to conduct hardness testing. The middle of the metal block has a cylindrical hole inclined at a 45° angle, which holds a Mitsubishi pencil. Careful attention is placed on the wear and age of the pencils used, as well as how the pencils are sharpened. We use a mechanical sharpener to remove approximately 5 mm to 6 mm of wood from the point of each pencil before every test. The tip of the lead is squared by holding the pencil in a vertical position and moving the pencil back and forth over abrasive paper while maintaining a 90° angle. This process is continued until a flat, smooth, circular cross-section is obtained. The pencil tip must be free from chips or nicks in the edges.
A level is mounted on top of the metal block apparatus to ensure the testing device is horizontal. This position allows the tip of the pencil to consistently exert a load of (750 ± 10g) on the coating surface.
We then take a representative sample of each film substrate to be tested. In accordance with the ISO standard, sample panels are free from distortion. The substrate samples are also positioned so that the panel remains horizontal throughout the hardness test.
The tests are conducted at a temperature of (23 ± 2°C) and a relative humidity of (50 ± 5%). The pencil is inserted into the testing instrument and clamped so the pencil tip rests on the surface of the film. The test panel is then pushed away from the operator at a speed of 0.5 mm/s to 1 mm/s for a distance of 7 mm. We apply five scratches on each test panel and may sample multiple panels across a coated web. The coating is then inspected with the naked eye for defects.
If there are no markings on the test panel, Carestream professionals repeat the test without overlap of the test areas, and move up the hardness scale until marking occurs over a distance of at least 3 mm. If there are markings after the initial test, the test is repeated down the hardness scale until no markings occur. The hardness of the hardest pencil that does not mark the coating determines the coating’s hardness level. These directional findings are also used to fine-tune research and development work. To confirm results, the test is performed again in the same manner. If the two results differ by more than one unit of hardness, the results are discarded. If the results match, the hardness rating is confirmed.
The Carestream Contract Manufacturing Difference
Carestream’s hardcoated PET film construction features a proprietary hardcoat formulation and delivers the reduced iridescence required for optical products. Attributes of Carestream Tollcoating’s hardcoat PET film include:
· Abrasion resistance
· Scratch resistance featuring typical 3H pencil hardness
· Optical clarity with less than 0.3% haze typical and neutral color
· Chemical resistance
· Wide range of constructions and alternate substrates
· Manufactured in a clean room environment
Potential applications for Carestream’s hardcoat projects include:
· Protective films used for touch screens and flat panels
· Abrasion resistance for membrane switch, graphics and white boards
· Additional value-added layers and performance, including UV protection, anti-microbial and photovoltaic uses
Carestream’s contract coating services for hardcoated PET films and other substrates can dramatically improve the performance and economics of display, electronic, optical, and graphic coatings. Contact Brian Pahl for more information or to get started.