1. Adhesion. Proper adhesion is critical to the success of the coating, and it can impact other performance aspects such as abrasion resistance, UV blocking, stain inhibiting and water repellency. The key to achieving acceptable adhesion is to formulate the coating to have a lower surface tension than that of the substrate. ASTM D3359 is the Standard Test Method for Measuring Adhesion by Tape Test, and a good test to indicate the level of adhesion to the substrate. Other relevant tests include ASTM D751-06, which covers adhesion testing for coated fabrics, and ISO 2409, a good qualitative test method for assessing the resistance of paint coatings to separation from substrates.
2. Stain and Chemical Resistance. As a coated material acquires dirt, water repellency can decrease. Coating chemistries can be custom-formulated to help withstand this by pulling water into the fabric to lift out stains, then pushing water back out of the substrate. The automotive industry has a specific test for evaluating the stain performance of a coating on upholstery or seating. They follow the ASTM D4966-98 Martindale Abrasion Test, in which unwashed denim is rubbed against the upholstery substrate. Each manufacturer has their own variance to this test and differs in their interpretation of an acceptable degree of blue dye transfer. Stain resistance also can be tested with AATCC TM 8, Colorfastness to Crocking, which is applicable to all textiles. Stain resistance often correlates with chemical resistance. Therefore, another important stain resistance test is covered under the ASTM D1308 Standard Test Method for Effect of Household Chemicals on Clear and Pigmented Organic Finishes. The test determines the effect of household chemicals, including discoloration, change in gloss, blistering, softening, swelling, and loss of adhesion. One caveat to obtaining stain resistance is after incorporating stain inhibiting compounds, gloss is affected unless properly formulated by a coatings engineer.
3. Abrasion and Durability. Coatings on substrates can be damaged by abrasion during manufacturing and service. While “abrasion resistance” (often stated in terms of the number of cycles on a specified machine, using a specified technique to produce a specified degree or amount of abrasion) and “durability” (defined as the ability to withstand degradation or wearing out in use, including the effects of abrasion) are frequently related, the relationship may vary with different end uses. Two relevant tests used to calculate abrasion resistance are ASTM D4157 Standard Test Method for Abrasion Resistance of Textile Fabrics and ASTM D4060 Standard Test Method for Abrasion Resistance of Organic Coatings by the Taber Abraser.
4. Light Aging. Sunlight, moisture and heat can induce property changes in coated materials, which can lead to reduced overall performance, color fading and changes in the stiffness or softness of the material. One way to test for the effects of light aging is with ASTM G155, Standard Practice for Operating Xenon Arc Light Apparatus for Exposure of Non-Metallic Materials. The American Association of Textile Chemists and Colorists (AATCC) has a similar test method, TM16.3, to test colorfastness to light with a xenon arc light source. A related standard, ASTM D4329, addresses fluorescent ultraviolet (UV) lamp exposure of plastics and polymer coatings.
5. Flexibility. Matching the flexibility of the coating to the substrate can be a challenge. Consider vinyl: in the winter, vinyl can expand and contract from extreme temperature fluctuations. In the summer, it can warp and stretch under high temperature and humidity. If the coating is too hard, it may check or crack, then lose adhesion after being exposed to the environment. One test that will help determine the proper hardness of a cured coating formulation is pencil hardness, tested under ASTM D3363.
The right coating chemistry can make the difference between an automotive component that provides long-lasting durability and one that fades, degrades and fails before its designed lifetime is over. Finding a hands-on expert in engineering coating chemistries — one that is willing to work collaboratively with the OEM to achieve required performance — is critical for success.
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