How to improve the adhesion performance of offset UV ink for plastic film on non-polar substrates?
Release Time : 2025-09-24
The adhesion of offset UV ink for plastic film on non-polar substrates is a core challenge affecting print quality and product durability. Due to the symmetrical molecular chains and low surface energy of non-polar plastic films (such as PE and PP), their compatibility with polar offset UV ink for plastic film is poor. This makes it difficult for the ink to wet and spread on these surfaces, leading to problems such as shedding and agglomeration. To address this challenge, a comprehensive adhesion enhancement system must be developed through coordinated improvements across multiple dimensions, including substrate pretreatment, ink formulation optimization, process parameter adjustment, and the application of supporting technologies.
Substrate surface pretreatment is fundamental to improving adhesion. Non-polar plastic films have smooth and chemically inert surfaces, requiring physical or chemical methods to increase surface roughness and polarity. Corona treatment is the most commonly used technique in the industry. High-voltage ionized air generates high-energy ions that bombard the film surface, introducing polar groups such as carbonyl and carboxyl groups, significantly increasing the surface energy. For example, corona treatment can increase the surface energy of PE film from 30mN/m to 40-50mN/m, reducing the contact angle of offset UV ink for plastic film and improving wettability. Flame treatment uses a high-temperature flame to instantly melt the film's surface, removing oil stains and creating a microscopic roughness, enhancing mechanical anchoring. This method is suitable for irregularly shaped or high-temperature-resistant substrates. For materials that cannot be corona or flame treated, chemical primers offer an alternative. The specialized functional groups in the primer can bind to polar groups on the film's surface, forming a transition layer and eliminating the problem of corona effect attenuation caused by additive migration. This is particularly suitable for substrates such as PET and acrylic.
Ink formulation optimization is crucial for improving adhesion. For non-polar substrates, the ink resin system and additive ratio must be adjusted. Selecting resins containing polar groups (such as hydroxyl or carboxyl groups) can enhance the chemical bonding between the ink and the film. For example, chlorinated polypropylene (CPP) resin, due to its poor compatibility with PE and PP, tends to accumulate on the film surface, forming a polar transition layer that significantly improves adhesion. Adding adhesion promoters (such as silane coupling agents and titanate coupling agents) can further improve interfacial bonding. The silanol groups generated by hydrolysis of silane coupling agents can react with the hydroxyl groups on the film surface to form chemical bonds. At the same time, their organic groups are compatible with the ink resin, building a "molecular bridge" structure. Furthermore, controlling ink viscosity and fluidity is crucial. Excessively high viscosity can lead to poor ink transfer, while too low viscosity can easily cause bleeding. Therefore, the viscosity must be adjusted to an appropriate range based on the film's properties.
Adjusting process parameters directly impacts ink curing quality and adhesion stability. Matching UV lamp power, printing speed, and curing time is crucial. Insufficient power or excessive speed can lead to "false drying" of the ink—curing on the surface while not fully curing within. This can cause stress concentration within the ink layer and reduce adhesion. Conversely, excessive power or prolonged curing time can cause over-curing of the offset UV ink for plastic film, making the ink layer brittle and reducing impact resistance. Therefore, the optimal process window must be determined through experimentation based on the ink type and film properties. For example, for printing with high ink thickness, a staged curing strategy can be adopted: pre-curing at low power to reduce internal stress, followed by a full cure at high power, ensuring a balanced adhesion and ink layer flexibility.
The application of auxiliary technologies provides additional support for improving adhesion. Heating the film before printing can reduce its surface hardness and enhance ink penetration. For example, near-infrared light heating can raise the film surface temperature to 60-80°C, promoting the diffusion of ink molecules into microscopic pores and forming mechanical entanglements. Applying a protective varnish after printing seals the ink layer surface, preventing adhesion loss caused by friction and chemical corrosion. The crosslinker in the varnish reacts with the ink resin to form a three-dimensional network structure, improving the ink layer's wear and weather resistance. Furthermore, controlling the humidity and cleanliness of the printing environment is crucial. High humidity can easily cause the film to absorb moisture and deform, affecting overprint accuracy. Dust, oil, and other contaminants can prevent direct contact between the ink and the film, requiring cleanroom work and electrostatic dust removal equipment.