The rheology of ink plays a central role in ink distribution and transfer in Flexographic printing process.
UV inks are specially formulated to dry only when exposed to ultraviolet (UV) light. Hence the printed image resembles an overall better dot and print effect, resulting in a high-quality finish. UV ink is cured by chemical reactions called polymerization.
UV inks are usually 100% non-volatile materials, so there is no concern for evaporating any volatile materials like solvents or water-based inks. Rheology is defined as deformation and flow behaviour of a material. The rheology of UV inks is determined by the components that are added to the ink.
Ingredients of UV Inks:
Oligomer: The main component used as a binder in ink formulations. These are highly viscous and contain active groups for cross-linkage.
Monomer: A molecule that forms the basic unit for polymers. Monomers provide lower viscosity and can influence rheology, cure rates and adhesion.
Photoinitiator: UV-sensitive components, and under UV radiation, the polymerization reaction is initiated by its decomposition into free radicals.
Pigments: Imparts colour to an ink and insoluble in ink vehicles
Additives: Additives like Stabilizers, Silicon (Rub-Resistance), Hardeners (Mechanical Resistance), Etc., are often added in the UV Inks to enhance specific performance of an ink to meet end use requirements.
It is essential to understand the relationship between printability and rheological behaviors induced by the interfacial interactions between different components within the ink as it directly affects printability. Adding various elements to the ink determines the rheology of inks. Variations in the degree of interfacial interaction between components lead to changes in the rheology of the ink suspensions. The ink transfer between the palate, anilox roller and substrate is dependent upon the type of monomer and the transfer fraction of ink which in turn has an effect on print density.
Mirror-coated paper is a commonly used substrate for label printing, food packages, and other consumer goods. This substrate is printed with both water-based ink and UV ink with Flexographic Printing Press. Water-based ink is mainly used for absorbent substrates. UV ink is used mainly for non-absorbent substrates. In flexographic printing, more solid ink density is achieved by UV inks than water-based inks because the polymerization process dries UV ink with negligible absorption. In contrast, more print contrast is achieved by the water-based inks because water-based ink is dried by the absorption and evaporation process, and it penetrates in the pores of the substrate. In the case of 50% and 100% dot gain, UV inks have more dot gain than water-based inks due to their drying process that is polymerization.
The excellent chemical resistance and physical properties of Polyvinyl Alcohol (PVA) resins makes it suitable as an excellent adhesive and possess solvent, oil, and grease resistance properties. The viscosity of formulated inks increases with increase in binder percentage. The formulation has an effect on adhesion and pigment particle distribution on varying substrates. News print paper shows an even distribution of the pigment particles with less or no air bubbles. Bond paper exhibits good result with some air bubbles for the formulations while Art paper shows much air space with uneven pigment particles distribution. Therefore, it is important to optimize the formulation for better adhesion and particle distribution for different substrates.
Polymer film materials are used to print labels and packaging through the flexography printing process. The polarity and surface energy of films increases due to the decontamination of polar groups and results in good adhesion properties. The decrease in the cosine of the contact angle reduces ink adhesion. The polarity of the surface has the presence of polar groups in inter-molecular interactions and provides an adequate adhesive bond. Maximum spreading and maximum adhesion of UV-inks are possible at high surface energy and a polar component of the surface film.
The rheological properties of suspensions greatly depend on the degree of flocculation. The flocculated structure depends on the shape, size, and surface characteristics of the dispersed particles and the interaction between the dispersed medium and the particles. To understand and control the rheological properties of UV flexo inks, it is necessary to accurately analyze the specific rheological properties produced by the chemical and physical interactions of the ink components. The viscosity of pre-polymer increases with an increase in the shear rate. The dynamic viscosity of pre-polymer is dependent on the molecular weight of the oligomer. Dynamic viscosity of the colorant is increased as the strain amplitude increased. Every ink manufacturer should know these factors to achieve the desired ink formulation.
UV flexo inks are specially formulated with bio-renewable resin technology for lower migration in food packaging applications. The usage of UV lamps to one third of the exposure level contributes to energy saving and reduction of CO2 during the printing process. Understanding the rheological properties of ink including viscosity, high color strength, adhesion to multiple substrates is very important for better print quality in flexographic printing process. Excellent rub resistance, chemical resistance, exceptional color consistency and superior gloss led the printers to use UV inks instead of traditional inks. UV printing has the ability to convey superb effects to eye-catching special substrates with shine or feel properties. The ink dries within fraction of second and hence speeds up the finishing operations of a job.
Written by – Siddharth Adagatla (Student) under the guidance of Dr. Akshay V. Joshi (Printing Engineering Department – P. V. G’s College of Engineering and Technology & G. K. Pate (Wani) Institute of Management, Pune
