Hardcoats enhance finished product performance and protect against abrasion, scratches and chemicals on the finished product or during manufacturing. Hardness testing is complex and somewhat subjective. Carestream applies the knowledge gained from our work in performance-critical industries to design rigorous hardness testing processes for hardcoat films used in electronics, display, graphics and optical applications.
Hardcoat items undergo rigorous production release testing, including hardness testing as well as evaluation of coating thickness, chemical resistance, flexibility, cleanliness and optical properties. These strict guidelines apply to every volume in the production environment - ensuring process and product quality meet customer expectations.
A typical hard coating structure consists of a base film and a hard coating. Base films are usually made of polyethylene terephthalate (PET), polycarbonate (PC), polymethyl methacrylate (PMMA), polyethylene naphthalate (PEN) or cycloolefin polymers (COP) composition. Hardcoats typically contain polymers, monomers that cure later, nanoparticles, or various other chemicals.
There are several standards for testing the hardness of coatings, including ISO 15184 from the International Organization for Standardization, ASTM International Standard ASTM D3363, and JIS K-5600 from the Japanese Standards Association. In these tests, a pencil of the specified size, shape, and hardness is pushed across a hardcoated surface to evaluate the surface's resistance to marking or other imperfections. Each protocol has slightly different test parameters. For example, the ASTM test does not specify a fixed applied load on the pencil, while the ISO standard requires a slide with a standard load.
At Carestream, we understand the intricacies of each test and tailor our approach to each project. A common approach focuses on ISO 15184 testing. Professionals control as many variables as possible when conducting ISO 15184 testing to ensure consistency and adherence to the published standard. Variables include the applied load fixed on the pencil, sled weight and pencil type/age/position.
Carestream's Hardness Test Method
Carestream uses a metal block with two wheels, one on each side, for hardness testing. In the middle of the metal block there is a cylindrical hole inclined at a 45° angle to accommodate a Mitsubishi pencil. Pay careful attention to the wear and age of the pencils you use, and how the pencils are sharpened. We used a mechanical sharpener to remove about 5mm to 6mm of wood from each pencil before each test. Square the tip of the lead by holding the pencil in a vertical position and moving the pencil back and forth across the sandpaper while maintaining a 90° angle. Continue the process until a flat, smooth, round cross-section is obtained. The pencil tip needs to have no chips or nicks on the edge.
Install a spirit level on top of the metal block apparatus to ensure that the test apparatus is level. This position allows the tip of the pencil to exert a load of (750 ± 10 g) on the coated surface at all times.
We then take a representative sample of each film substrate to be tested. According to the ISO standard, the sample plate was not deformed. The substrate samples were also positioned so that the panels remained horizontal throughout the hardness test.
The test is carried out at a temperature of (23±2°C) and a relative humidity of (50±5%). Insert the pencil into the test apparatus and clamp it so that the tip of the pencil rests on the film surface. The test board is then pushed away from the operator by a distance of 7mm at a speed of 0.5mm/s to 1mm/s. We apply five scratches to each test panel, and multiple panels can be sampled on the coated web. The coating is then inspected visually for defects.
If there are no marks on the test panel, the Carestream professional repeats the test without overlapping the test area and moves the hardness scale up until the marks occur at least 3 mm apart. If there are marks after the initial test, repeat the test along the hardness scale until no marks appear. The hardness of the hardest pencil that does not mark the coating determines the level of hardness of the coating. These directed research results are also used to fine-tune research and development efforts. In order to confirm the result, the test was performed again in the same way. If two results differ by more than one hardness unit, the results are discarded. If the results match, the hardness grade is confirmed.
Carestream Contract Manufacturing Differences
Carestream's hard-coated PET film construction utilizes a proprietary hard-coat formulation that reduces the iridescence required for optical products. Features of Carestream Tollcoating hard-coated PET films include:
·Abrasion resistance
Scratch resistance, with a typical 3H pencil hardness
· Optical clarity, haze less than 0.3% typical and neutral color
·Chemical resistance
Wide range of structures and alternative substrates
· Manufactured in a clean room environment
Potential applications for Carestream's hard coating program include:
·Protective films for touch screens and tablets
Abrasion resistance of membrane switches, graphics and whiteboards
Additional value-added layers and properties including UV protection, anti-microbial and photovoltaic use
