To ensure long-term colorfastness and yellowing resistance in the color printing process of PVC decorative sheets, the key is to carefully select the printing pigments. The industry prefers pigments with excellent weather and light resistance, such as inorganic pigments (such as titanium dioxide and iron oxide) or highly weather-resistant organic pigments (such as quinacridones). These pigments have a stable molecular structure and are less susceptible to decomposition of their chromophores under UV exposure or temperature fluctuations. Inorganic pigments also exhibit excellent chemical resistance, making them resistant to weak alkaline and acidic detergents encountered during daily cleaning, preventing color loss due to chemical reactions. Highly weather-resistant organic pigments, with their specialized molecular structures, mitigate UV damage to their molecular chains, reducing the risk of fading at the source.
Pre-printing substrate pretreatment is crucial for enhancing color stability. The surface of PVC decorative sheet substrates may contain residual oil, release agents, or small impurities from the production process. Left untreated, these can impair the adhesion of the ink to the substrate, leading to subsequent ink loss or partial fading. Therefore, during the pretreatment phase, degreasing and cleaning are performed to remove surface oils and contaminants, followed by corona treatment or plasma treatment to increase the surface roughness and polarity of the PVC substrate. The treated surface allows for closer physical and chemical bonding with the ink, preventing peeling and flaking due to insufficient adhesion. It also reduces the penetration of moisture and oxygen into the ink layer, indirectly delaying yellowing and fading.
Optimizing the ink system formulation plays a key role in preventing fading and yellowing. In addition to high-quality pigments, the binder in the ink must be a resin that is highly compatible with the PVC substrate and highly weather-resistant, such as acrylic resin or vinyl chloride-vinyl acetate copolymer. These resins form a dense film that securely encapsulates the pigment particles, isolating them from environmental damage. UV absorbers and antioxidants are also added to the ink. UV absorbers preferentially absorb ultraviolet energy from sunlight, converting it into harmless heat and preventing direct UV damage to the pigment and resin. Antioxidants capture free radicals generated by oxidation reactions in the environment, preventing the resin from aging and yellowing, and the pigment from oxidative fading, providing a dual layer of protection.
Controlling process parameters during the printing process directly impacts the quality and stability of the ink film. Currently, mainstream PVC decorative sheet printing processes include screen printing and digital printing. Regardless of the process used, curing temperature, curing time, and printing pressure are strictly controlled. For example, during screen printing, the ink must be fully cured at the set temperature to form a complete and dense film. Insufficient curing can create microscopic pores in the ink layer, making it susceptible to moisture and contaminants, leading to fading. Overcuring can embrittle the resin, leading to cracking later and compromising color integrity. Furthermore, automated equipment precisely controls printing pressure to ensure uniform ink layer thickness, avoiding areas of thin ink that can lead to uneven color or premature fading.
Post-printing post-processing can further enhance color durability. Some PVC decorative sheets are coated with a transparent protective film or UV-curable coating on the printed surface. Transparent protective films are often made of weather-resistant PET or PVC, providing physical insulation against UV rays, dust, and moisture, reducing the direct impact of external factors on the printed layer. UV-curable coatings rapidly cure under ultraviolet light, forming a hard, weather-resistant, transparent protective layer. This not only enhances the decorative sheet's wear resistance but also, through its UV-resistant ingredients, further blocks UV damage to the printed layer, slowing yellowing. It also makes the surface easier to clean, minimizing potential damage from detergents.
Adapting weather-resistant design to different usage scenarios is also crucial to preventing fading and yellowing. For example, the printing process for PVC decorative sheets intended for outdoor use or exposure to direct sunlight may increase the proportion of UV absorbers or select a more light-resistant pigment combination. For products intended for humid environments like kitchens and bathrooms, mildew inhibitors are added to the ink to prevent mold growth and color dulling or discoloration. These scenario-specific process adjustments ensure that the decorative sheet maintains color stability even in specific environments, preventing premature fading due to environmental factors.
Rigorous weathering tests before shipment ensure long-term colorfastness and resistance to yellowing. Samples from each batch of PVC decorative sheet undergo accelerated aging tests, simulating extreme environments such as high temperature, high humidity, and strong UV exposure. After a specified period of testing, specialized instruments are used to measure the degree of color change (such as the color difference ΔE value) and the yellowing index. If test results exceed preset standards, adjustments are made to the pigment selection, ink formulation, or curing parameters used in the printing process, and production is resumed after optimization. Only products that pass weathering tests are released to the market, ensuring they retain their vibrant colors over time and resist fading and yellowing.
