The Recent Development of New Pigment Binders (2016)

Pigment coatings are applied onto paper and paperboard to improve their appearance and printability. For pigmented coatings, pigments and binders are the most important ingredients so their selections are critical.\nPigment binders not only perform the basic required role of binding pigment particles to each other and bonding the base sheet, but also significantly influence the rheology, coater runnability, and drying behaviors of the coating formulation and the optical, viscoelastic, and printing properties of coated paper and paperboard products.\nWhen considering the relative amount of binders to use in a coating, one may speak about main binder, co-binder and sole-binder. By sole-binder it is meant that a single binder alone can perform all the desired functions of the binder in a coating. Usually the binder systems consist of a combination of two binders, in which the main binder is responsible for the binding function.\nConventional starch is inexpensive, but it is rarely used as a sole binder. Instead, it is mostly used as a co-binder. The main reason is coating solid and viscosity. The highest dry solid for cooked starch is about 42%, but conventional latex is 50%. Eventually, conventional starch lowers the dry solid. Viscosity increase with a large portion of starch is too high for paper coatings. However, biobased latex can be used as a dry form and the viscosity is reduced by crosslinking.\nAs a co-binder, biobased latex was used for offset coating formulations, in which their rheological and water retention properties were investigated. This study provided an interpretation the unknown basic nature of water-swollen starch nanoparticles and their colloidal behaviors scientifically in comparison with conventional latexes. Especially, serum replacement experiments showed that starch latexes are complex systems of particles and a minor fraction of soluble polymers.\nConventional pigments are inexpensive, but they are rarely used in inkjet coating formulations. Instead, silica is commonly used for ink-jet coatings since it provides a large surface area for quick ink absorption. However, silica grades are excluded in modern hybrid printing presses, because it is rather poorly applicable to printing processes except ink jet, so other lower-cost types of coating are being sought to replace the silica grades.\nCalcium carbonate was replaced partially to silica pigment in an effort to balance coating solids, viscosity. At high solids content, a minimum viscosity was observed in mixtures of different sizes particles, so a high solid inkjet coating was feasible. The incorporation of amine functional polyvinyl alcohol with conventional silica pigments for an ink-jet coating were examined. It was determined that the silica pigment binding strength was improved with the addition of cationic copolymers, which are produced by the hydrolysis of copolymers of vinyl acetate monomer (VAM) and cationic monomers, in comparison to the conventional homopolymers, which are produced by the hydrolysis of polyvinyl acetate, due to the chemical coated paper and ink interactions. However, due to flocculation experienced during the make down and handling of these coatings, it is strongly recommended that coatings be formulated with cationically dispersed silica pigment when cationic PVOH is used.\nGenerally, the structure as well as chemical differences of coating layers determine the final quality of the inkjet printed image.