Solvent-based polyurethane binder for ink

Solvent-based polyurethane binder for ink has the advantages of easy use, stable performance, strong adhesion, excellent gloss, and good heat resistance, and can meet the requirements of various printing methods, especially suitable for screen printing, plastic packaging, and composite films.

Polyurethane (PU) is a kind of polymer with -NHCOO- repeating structural unit in the main chain. It is polymerized from isocyanate (monomer) and hydroxyl compound. Due to the presence of a strong polar carbamate group, it does not dissolve in non-polar groups and has good oil resistance, toughness, wear resistance, aging resistance, and adhesion. Materials suitable for a wide temperature range (-50-1500 °C) can be produced with different raw materials, including elastomers, thermoplastic resins, and thermosetting resins. It is not resistant to hydrolysis at high temperatures and is also not resistant to alkaline media.

Adding polyurethane resin in the ink can improve the wettability of the ink to the pigment, and the film-forming property is good, the ink film is firm and wear-resistant, which is mostly used in the manufacture of offset and water-based inks, but the price is relatively high.

The polyurethane resin used in the ink is generally formed by the reaction of polyester/polyether polyol and isocyanate, with a molecular weight of about 20,000 to 40,000. Its solvents mainly use benzene, ketone, and ester solvents as the main solvents. In the research and development process, according to the environmental protection needs of the ink factory and the printing factory, ketone-ester solvents or alcohol-ester solvents can be used to prepare corresponding benzene-free ink resins.

Characteristics of polyurethane resin for ink

① Excellent yellowing resistance. Polyurethane resin for ink is mainly synthesized with aliphatic polyester and aliphatic isocyanate as the main raw materials during the preparation process, which has excellent optical stability compared to aromatic polyurethane, and the formed film has excellent yellowing resistance.

② Excellent adhesion to thin film substrates. The molecular chain segment of the polyurethane resin for ink contains polar groups such as carbamate, ureidocarbamate, ester bond, and ether bond, forming hydrogen bonds with the polar groups on the surface of various polar substrates such as PET and PA plastics, thereby forming a joint with a certain connection strength. After this polyurethane resin is made into ink and printed on the surface of a polar plastic substrate, it has a very good adhesion.

③ Good affinity and wettability with pigments/dyes. Polyurethane resin for ink is generally prepared from polyester or polyether polyol, alicyclic diisocyanate, and diamine/diol chain extender. Due to the introduction of urea bonds in the PU resin, that is, the formation of polyurethane-urea resin (PUU), it has good dispersion and wetting performance for pigments.

④ Good resin compatibility. Polyurethane resin for ink has good compatibility with aldehyde-ketone resins, chlorovinyl acetate resins, etc., and can be appropriately added in its own process formula according to the actual situation to improve the comprehensive performance of the ink.

⑤ Excellent film-forming performance. Polyurethane resin for ink is different from that used in other fields in structure. Traditional polyurethanes are mainly prepared by the reaction of polyester polyol/polyether polyol and isocyanate to form hydroxyl-terminated polyurethane resin. The polar groups in the molecular structure are mainly carbamate, and the internal cohesion of the molecule is not enough to meet the film-forming performance requirements of the ink resin. Therefore, polyurethane resin for ink introduces urea groups on the basis of traditional polyurethanes to greatly improve the internal cohesion strength and film-forming property of the resin itself.

⑥ Wide compatibility with organic solvents and good solvent release. The dissolution of organic solvents on the resin is through the polar attraction of solvent molecules to solute molecules, that is, the so-called like dissolves like. Traditional polyurethane resins have wide compatibility with organic solvents, and non-alcoholic organic solvents such as ketones, esters, and benzenes are its excellent solvents.

However, in the process of making ink, in order to adjust the fluidity and viscosity of the ink, alcohol organic solvents need to be added, which for traditional polyurethane resins will greatly reduce the stability of the resin system, and there will be incompatible phenomena such as turbidity and flocculent precipitation. However, due to the presence of urea groups, the polyurethane resin for ink can be compatible with alcohol, but it should be noted that alcohol solvents are still pseudo-solvents. Under microscopic conditions, alcohol solvents wrap the polyurethane resin molecules instead of true solvents, where the molecular polarity runs through the molecules, making the ink made with polyurethane resin have good fluidity.