In the field of packaging and printing, PET films have become commonly used substrates for flexographic printing due to their excellent mechanical properties, optical properties, and chemical stability. However, the problem of fading of water-based flexographic inks on PET films not only affects the appearance of printed products but may also lead to customer complaints and economic losses. In-depth exploration of the causes of fading is crucial for improving printing quality, and the following analysis is carried out from multiple aspects.

Compatibility Issues between Inks and PET Films

Unreasonable ink formula

Water-based flexographic inks are mainly composed of resins, pigments, additives, and water. Among them, resins, as binders, play a key role in the adhesion of inks to PET films. If the affinity between the resins in the ink and the surface of the PET film is insufficient, it will directly lead to a decrease in adhesion. For example, if the molecular structure of the acrylic resin used in some water-based inks does not match the polarity of the PET film, a strong bond cannot be formed, and the ink is prone to peeling and fading under the influence of external factors. In addition, the dispersion stability of pigments in the ink cannot be ignored. If the pigments are poorly dispersed, agglomerated pigment particles cannot adhere evenly to the surface of the PET film, and fading is likely to occur over time.

Improper Surface Treatment of PET Films

PET films have low surface polarity and low surface energy, and they are non-absorbent materials, making it difficult for inks to adhere directly. To improve the adhesion effect, it is usually necessary to corona-treat the PET film to increase its surface polarity and surface energy. However, if the corona treatment is ineffective, such as insufficient corona intensity or uneven treatment, the adhesion of the ink in some areas of the film surface will be insufficient, resulting in fading. Generally, the surface energy of the PET film after corona treatment should reach more than 38 dynes to meet the adhesion requirements of most water-based inks. In addition, if the PET film is stored for too long or the storage environment is poor, the surface will adsorb contaminants such as dust and grease, which will also affect the bonding between the ink and the film and cause fading.

Printing Process Factors

Substandard Drying Process

Water-based flexographic inks use water as a solvent, and the drying process is crucial. If the drying temperature is too low or the drying time is insufficient, the water in the ink cannot be fully volatilized. Residual water will affect the bonding force between the ink and the PET film, causing the ink to become sticky and prone to fading during subsequent processing, transportation, or use. For example, during the printing process, if the drying temperature is set at 60 – 70°C but the printing speed is too fast, and the ink stays in the drying channel for too short a time, insufficient drying will occur. Conversely, if the drying temperature is too high, the surface of the ink will form a film quickly, creating a hard shell that hinders the continuous volatilization of internal moisture, resulting in incomplete drying inside the ink, which also affects adhesion and causes fading problems.

Uneven Printing Pressure

In flexographic printing, the uniformity of printing pressure significantly affects the ink transfer and adhesion effect. When the printing pressure is uneven, the contact between the PET film and the printing plate is inconsistent. In areas with excessive pressure, excessive ink transfer may occur, leading to ink buildup and poor drying; in areas with insufficient pressure, insufficient ink transfer results in a thin and unstable ink film. Both situations are likely to cause fading. For example, local wear of the printing plate or unevenness of the printing equipment rollers can lead to uneven printing pressure and affect the adhesion quality of the ink on the PET film.

Environmental and Post-processing Factors

Influence of Environmental Factors

During the storage and use of printed PET film products, environmental factors have a significant impact on ink fading. High-temperature and high-humidity environments will accelerate the aging and decomposition of resins in the ink, reducing the adhesion between the ink and the PET film. For example, during the plum rain season in southern China, with high air humidity, stored printed products are prone to fading. In addition, light also affects the ink. Ultraviolet rays can cause photochemical reactions in the pigments of the ink, leading to pigment fading and subsequent color loss.

Improper Post-processing

After the printing of PET film products is completed, post-processing such as lamination, die-cutting, and folding is often required. If the post-processing process parameters are set unreasonably, such as excessive lamination pressure or temperature, the ink layer will be damaged, causing the ink to separate from the PET film and resulting in fading. During die-cutting and folding, if the operation is improper, the ink layer will be subjected to excessive friction and stretching, which can also easily lead to fading.

The fading of water-based flexographic inks on PET films is caused by the combined effect of multiple factors. From the compatibility between the ink and the PET film, the printing process, to environmental and post-processing factors, each link may affect the adhesion effect of the ink. Only by comprehensively analyzing the causes of fading and optimizing each link targeted can printing enterprises effectively solve the fading problem, improve the quality of printed products, and meet market demands.