Synergetic Effect of Cationic Starch (Ether/Ester) and Pluronics for Improving Inkjet Printing Quality of Office Papers

Improving the printability of paper is still a relevant challenge, despite the fast development of digital communications. While it is well-known that cationic starches enhance ink density, their commercial paper-grade forms are limited to ethers with low degree of substitution. This work addresses the underexplored potential of highly substituted cationic starch for paper coating and its combination with tri-block polymers, namely Pluronics (P123 and F127), taking advantage of their supramolecular interactions with amylose chains. For that purpose, cationic starch ether and ester (starch betainate), both with a degree of substitution of 0.3, were synthesized by alkaline etherification and by transesterification, respectively. Paper without any surface treatment was subjected to one-side bar coating with suspensions encompassing those products and Pluronics, besides other common components. Black, cyan, yellow and magenta inks were printed on all coated papers through an inkjet printer. Key properties of printing quality such as the gamut area, gamut volume, optical density, print-through, inter-color bleed and circularity were measured in a controlled temperature-humidity environment. For instance, a formulation with cationic starch (ether/ester) and P123 improved the gamut area by 16–18% in comparison to native starch-coated paper sheets. Interestingly, the individual assessment of each component showed that cationic starch ether, starch betainate and P123 only improved the gamut area by 5.6%, 8.9% and 6.8%, respectively. Finally, but not less importantly, starch betainate was found to quench optical brightening agents to a lesser extent than cationic starch ethers.

Psychophysical tests for visual-numerical correlation of whiteness formulas

Measures of whiteness are important to the paper industry. Such measures must have good correlations with visual observations. In this study, psychophysical tests were conducted to determine if a visual correlation could be found between the International Commission on Illumination (CIE) whiteness formula and a formula previously derived by our group. The lightness value of the substrate has a significant effect on the perception of whiteness, so observer trials were carried out at approximately the same lightness levels. Perceptions of whiteness and their changes with different light sources were studied. Psychophysical tests were conducted using D50 and D65 light sources for visual correlations, under controlled viewing conditions at a 2° field angle. The psychophysical results were then compared with the two formulas. Considering available light sources for observer experiments and the standards used by the graphic arts industry, along with the effect of optical brightening agents on whiteness perception, recommendations for use of a different light source by the paper industry are provided.

The contribution of pulp brightness and optical brightening agents to paper whiteness

The desired optical properties of a paper can be achieved through a variety of tools: pulp brightness, optical brightening agents, dyes, or fillers. In this work, we explore the interaction between pulp brightness levels and optical brightening agents (OBA) addition on paper whiteness. Conditions were chosen that approximate an uncoated freesheet grade. Papers were prepared using eucalyptus kraft pulps bleached to three different brightness levels. A constant filler level and optimized retention chemistry were used in the preparation. The addition of OBA was evaluated as a wet end only application and as a wet end/size press application. Dye addition was not a part of this evaluation. We also surveyed the optical properties of a range of commercially available uncoated freesheet paper. We found that the initial brightness of the pulp furnish sets the limit for whiteness level achievable with OBA during papermaking. It is not possible to compensate for low initial pulp brightness (<85.4% ISO) by the addition of OBA when aiming for CIE D65 whiteness above 135. We also evaluated the economics of pulp brightness and OBA addition to show how these two variables can be balanced. Based on the conditions used in this study, it is most cost effective to use pulp having 90% ISO brightness or higher when targeting CIE D65 whiteness above 135.

Chromatic Adaptation Transforms to Accommodate Printed Color Matching in Paper with Optical Brightening Agents (OBA)

The color reproduction is based on visual perception. The psychophysics experiments present the light source; illuminants and view background affect the results of color matching. Paper is a typical substrate for printed color matching and effects printed color greatly. Now almost all of print papers contain optical brightening agents, which changes significantly the color appearance of printed color and makes color shifting of printed color matching. This paper used CAT02 chromatic adaptation models to transform the measurements of printed color in paper with and without OBA, compared the effect of printed color matching between before and after transforming with display, present the method using chromatic adaptation transforms to improve the accurate of printed color matching.