Printing ink is a mixture formed by the uniform dispersion of components such as resin binder, pigment (filler), additives and solvents. Resin binder, as the "heart" of ink, mainly plays the role of dispersing pigments and connecting substrates.
Polyurethane Resin as an
Ink Binder has the following advantages:
(1)Flexibility
In the field of flexible packaging printing, as the printing substrate is flexible materials such as PE, OPP, and PET, the ink film should not be too brittle after formation. Polyurethane resin can endow the ink with excellent flexibility. In fact, by adjusting the proportion and structure of soft and hard segments in the polyurethane formula, polyurethane resins of different hardness can be synthesized to meet the requirements of various types of inks.
(2) Resin compatibility
To achieve the optimization of ink performance, the resin binders in most ink formulations are often composed of a mixture of several resins, which requires good matching and compatibility among different resins. Polyurethane resin is compatible with most resins such as chlorinated vinyl acetate resin, nitrocellulose resin, acrylic resin, polyketone resin, chlorinated polypropylene resin, etc., thus enabling the preparation of different types of polyurethane inks.
(3) Adhesion
Due to the presence of polar groups such as carbamate (-NH-COO-) and ester group (-COO) in the molecular structure of polyurethane, it has good adhesion to the surfaces of paper, plastic and metal. As a polyurethane binder for ink, it usually increases the molecular weight of the resin (about tens of thousands) and polar groups, thereby enhancing the adhesion of the resin to the substrate.
(4) Pigment dispersibility
Pigments are insoluble in the ink medium. The finer the pigments are dispersed, the better the tinting strength of the ink will be. This requires that the resin has good dispersibility for pigments. Generally, introducing urea groups into polyurethane molecules to form polyurethane-polyurea resin (PUU) can significantly enhance the resin's wetting and dispersing properties for pigments.
(5) Low odor
For some special packaging, the printing ink is required to have a low odor. However, for some products such as acrylic resin and maleic acid resin, which have a strong odor themselves, it is difficult to meet the customer's requirements after being made into ink. Polyurethane ink binder is a low-odor resin and can meet the requirements of low-odor ink.
Ink with polyurethane as the main resin binder is called polyurethane ink. The molecular structures of different types of polyurethane resins vary, and thus the types of polyurethane inks prepared from them also differ. At present, polyurethane inks can be classified into several types based on their resin structure composition, solvents and curing principles, such as one-component volatile polyurethane inks, two-component reactive polyurethane inks, UV-curable polyurethane inks and water-based polyurethane inks.
(1)Single-component volatile polyurethane ink
Single-component volatile polyurethane ink works by the evaporation of organic solvents in the ink, causing the resin and other solid components in the ink to form a film and adhere to the surface of the printed film. This type of ink features simple application, stable performance and good reproducibility. It is suitable for various printing requirements and is mainly used in plastic packaging printing and other aspects.
(2) Two-component reactive polyurethane ink
Two-component ink is also known as chemically reactive ink. This type of ink is composed of two components: ink (main agent) and curing agent. During printing, they are mixed evenly in a certain proportion. After printing and curing, the polyurethane resin, curing agent and printing substrate undergo chemical cross-linking. This type of polyurethane ink is more complex to operate compared to single-component ink and has a limited usage time. However, it can enhance the adhesion and high-temperature resistance of the ink and is mainly suitable for printing inks in packaging with higher requirements. It is worth mentioning that some one-component polyurethane inks can also be added with curing agents to become two-component polyurethane inks.
(3) Water-based polyurethane ink
Water-based polyurethane ink uses water as the dispersion medium. Among them, the polyurethane binder is insoluble in water but uniformly dispersed in water in the form of particles. Due to the fact that water-based polyurethane ink does not contain volatile organic solvents and has the characteristics of being non-flammable, non-toxic, odorless, energy-saving and environmentally friendly, it has attracted increasing attention in recent years and has broad prospects for development. However, water-based polyurethane ink uses water as the solvent, is not volatile, has high requirements for printing equipment and affects production efficiency. Therefore, in order to balance environmental protection and actual production needs, people have developed alcohol-water-based polyurethane ink, which has now been widely applied in printing.
As people pay more and more attention to the safety and environmental friendliness of the packaging field, the research direction of printing inks is gradually expanding towards being green, safe and pollution-free. Due to the diversity and safety in molecular structure and performance, the application of polyurethane resin in ink has attracted increasing attention. Currently, some related enterprises have launched alcohol-water polyurethane ink and water-based polyurethane ink. With the continuous strengthening of research on polyurethane resin, polyurethane ink will also become increasingly important in the field of packaging printing.
