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Improving optical properties of printing papers with dyes: A theoretical study

In this study we examined how optical properties of paper may be improved by adding dyes. Our aim was not to study the properties of any real papers or dyes, but the general principles of dying. Different dyes absorb at different wavelengths, and consequently have very different effects on optical properties of the paper. The Kubelka-Munk theory was used to study how much opacity and other optical properties of paper could be improved with dyes. Two different modeled paper grades were used as a basis of the calculations. One of the papers represented yellowish newsprint paper and the other an achromatic uncoated woodfree fine paper. Seven colors and three distributions of dye (paper evenly dyed, dye applied to the surface and middle third of the paper dyed) were studied. If dye is applied to the surface of the paper, less dye is needed for achieving the desired color than in the case of the even distribution. In addition, it is possible to make more colorful paper without losing too much of the lightness. On the other hand, if relatively high lightness or whiteness is required, it may be problematic to achieve good opacity. When the middle third fraction of the paper is dyed, higher opacity could be achieved than in case of the uniform dye distribution, if lightness, whiteness or the desired color of the paper are the limiting factors. On the other hand, more dye is needed than in case of the uniform distribution, and it is not possible to manufacture very colorful paper. Regardless of the distribution, yellow dye is the best solution for making highly colorful paper without sacrificing lightness. The whiteness of the paper may be improved with blue dye and maximum achievable whiteness is quite the same regardless of the distribution. If the amount of dye is constant, the distribution has a rather low effect on opacity. If whiteness or the lightness of the paper is the limiting factor, black dye is the best and blue dye the second-best solution for improving opacity.