Achieving the best adhesion for UV ink is not easy. The challenge starts with choosing the right substrate. Whether it’s a glass cup or a stainless-steel wine tumbler, each material requires a unique process. The substrate determines the type of ink to use. The substrate will also dictate whether both a primer and pre-treatment are necessary.
Specific substrates demand unique UV inks. Factors like surface tension and drying time are important when choosing the right UV ink.
- The surface tension of both the substrate and the ink must be compatible.
- Mismatched surface tensions can result in the ink beading off the surface.
- The surface tension of the ink should be lower than that of the substrate for optimal adhesion.
What Makes UV Ink Printing Different
UV ink printing stands apart from other ink types due to its unique polymerization process. During curing, a chain reaction of monomers and oligomers forms the polymer. Monomers serve as liquid-ink carriers. They are low-viscosity liquids, eliminating the need for water or solvent.
This characteristic makes UV-cured inks 100 percent solid, offering an excellent alternative to solvent inks. Oligomers, larger reactive molecules, play a crucial role in constructing the binder.
Polymerization initiates with a brief exposure to UV light, specifically near UV, with a wavelength of 390-200 nanometers. The categorization of this UV light is as follows:
- UV-A (long wave).
- UV-B (medium wave).
- UV-C (short wave).
The UV-reactive components, or photo initiators, absorb energy and produce highly reactive chemical species. Depending on the type, these species are free-radical or cationic. These species bond to monomers or oligomers, initiating a chain reaction to build the polymer. This process is efficient, making UV ink printing a solid choice for various applications.
Why Pre-Treatment Is Necessary for Achieving Optimal UK Ink Adhesion
Surface pretreatment has emerged as the most effective way of treating ink adhesion issues. It enhances the substrate’s surface energy and creates sites for oxidative chemical bonding.
Flame pre-treatment is a widely adopted method. It adjusts the surface tension to amplify ink adhesion.
Gas-phase plasma surface oxidation pretreatments achieve chemical activation. These pretreatments help address inkjet adhesion challenges linked to low surface energy.
Adhesion promoters are another option to enhance ink-to-substrate bonding. Printing a clear or varnished under base before the ink can also improve adhesion on plastics with marginal ink adhesion.
These strategies have proven effective in improving adhesion in UV ink printing applications.
Pretreatment Methods
Prepare the printing surface for pre-treatment. This involves eliminating contaminants and increasing adhesion. Alternatively, an adhesion promoter can strengthen the bond between inks and the surface.
The following steps must be taken for the best outcomes.
Ensure surface cleanliness.
Begin by cleaning the surface thoroughly. Wiping the surface with paper towels is the basic step, but this may not eliminate all contaminants. Besides, it could leave behind lint.
A more effective option is to utilize lint-free towels or clean cloth rags along with isopropyl alcohol. High-percentage alcohol can clean the surface thoroughly and remove moisture.
Conduct a surface test.
Checking the surface is important to know if it’s ready for printing. A surface with optimal energy is necessary to ensure proper ink adhesion and prevent issues like pinhole gaps and bubbles. Wetting describes good surface energy.
You can test the surface using water or special test pens. The pens must display a minimum reading to be considered suitable for printing. Also, an unbroken, flat line indicates a positive result. A surface with gaps in the drawn line or ink bunching suggests it is unfit for printing.
Thoroughly cleaning the surface may eliminate the need for pretreatment for certain materials such as PVC, acrylics, Di-Bonds, certain plastics, untreated wood, and cardboard. If you continue to experience poor wettability, you must explore additional pretreatment methods.
Pretreatment Alternatives
Depending on the material used for printing, there are several other available pretreatment options.
Flame Treatment
Flame treatment is a pretreatment method that modifies the molecular structure of surfaces like plastics, glass, or metals, enhancing their receptivity to inks or coatings. It is particularly crucial for ensuring the adhesion, durability, and abrasion resistance of UV inks.
However, the necessity of flame treatment varies depending on the substrate and the desired adhesion properties.
This method is versatile, compatible with water-based, solvent-based, and UV inks, and applicable across various printing processes such as digital inkjet, pad, screen, flexographic, and gravure printing.
Pyrosil Flame Treatment
The Pyrosil flame treatment applies a silicate-coating treatment to enhance ink adhesion, especially on substrates with low surface energy, such as plastics, glass, and metals.
During this process, the substrate is passed through a gas flame enriched with pyrosil, a silicon-containing precursor material. During this process, the substrate is passed through a gas flame enriched with pyrosil, a silicon-containing precursor material.
- Pyrosil combusts in the flame.
- This results in the deposition of a thin yet dense silicon layer on the surface.
- The deposited layer creates nanoscale roughness.
- The result is improved surface energy and better ink adhesion.
The key advantages of using this method are that Pyrosil improves durability, increases resistance to scratches and damage, and makes UV-printed materials safe for cleaning, even in dishwashers.
Adhesion Promoters
UV adhesion promoters are important in improving the adhesion and durability of UV ink printing on diverse substrates. They are particularly beneficial for challenging surfaces like glass, metal, aluminum, and plastic.
These promoters prepare the surface for UV-LED ink printing, ensuring outstanding adhesion and durability. Various factors influence the quality of adhesion, including.
- The type of UV ink.
- The optimal pre-treatment solution (chemical or mechanical).
- Knowledge of the substrate in relation to the ink.
- Available post-treatment coating options.
Commercially available adhesion promoters cater to different substrates like glass, metal, aluminum, acrylics, and plastics. You can also find adhesion promoters specifically designed to enhance adhesion for UV prints on metals and acrylics.
Jettable adhesion promoter
Jettable adhesion promoter is a specialized primer for enhancing the adhesion of UV inks to various materials. One applies it directly to the substrate using a printer. This method is particularly effective for surfaces like rubber-coated metals and rubberized plastic substrates. An additional UV-LED light source cures the surface.
This cost-effective primer eliminates the need for manual application. They are available in various forms, such as sprays and wipe-ons.
Corona Treatment
Corona treatment enhances the adhesion of UV inks, adhesives, and coatings to treated surfaces by increasing the surface energy of substrates.
This process uses high voltage and frequency to alter the polarity of non-absorbent materials, boost surface energy, and facilitate bonding with UV inkjet ink. Corona treatment is effective for substrates like plastics and glass, offering increased surface energy, improved printability, and stronger bonding.
Conclusion
There are no foolproof methods to ensure perfect UV printing. However, the techniques detailed in this article are known for their demonstrated effectiveness. Many people widely use these techniques. Continuing advancements in printing technology, combined with improved quality of inks and adhesion promoters, have also improved print quality. The perfect UV ink can adhere to any surface without pretreatment while maintaining color vibrancy for a long time. Given the rate of technological advancement in the industry, the perfect UV ink may soon become a reality, according to many experts.
