Paper Functional Chemicals :Colored Paper Requirements

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Colored Paper Properties
Rub resistance is required for album and wrapping paper and for cover board. Direct dyes are suitable for this purpose.
Acid resistance is required for parchment and vulcanized fiber base paper, writ¬ing, and printing paper. Organic and inorganic pigments, selected representatives of all colorant groups, are suitable for this purpose.
Solvent resistance is required for labeling paper (packaging of perfumes, medi¬cines, and spirits). Special pigments are suitable for this purpose.
Heat resistance is required for cable and core paper and laminate paper. Organic and inorganic pigments are suitable for this purpose.

Optical Brightening Agents (OBA) – Fluorescent Whitening Agents (FWA)
OBAs, also called FWAs, represent 3 % by value and only 1 % of the total dry amount of specialty chemicals . They increase the whiteness/ brightness of paper and are preferably added to the stock. They are very effective when used with highly bleached pulps, and much less effective, or even ineffective, when applied to unbleached chemical pulps and mechanical pulps. OBA are also used in surface applications such as surface sizing and paper coatings.

OBA absorb light in the ultraviolet spectrum range (below 370 nm) and re-emit the light in the visible blue range (peaking at 457 nm). This results in a fluorescent effect with bright white in daylight masking the inherent yellowness of the raw materials. Any material that absorbs ultraviolet light will lower the efficiency of fluorescent whiteners. For example, lignin absorbs ultraviolet light and the higher the lignin content of the pulp, the less effective is the OBA. Hence, mechanical pulps and unbleached pulps are less susceptible to whitening with OBA. Some filler clays tend to counteract the fluorescence and reduce the effect of OBA. Fillers such as calcium carbonate and aluminum trihydrate reflect ultraviolet light, thereby enhancing the effect of OBA. A high pH (above 6) also helps to achieve maximum whiteness. On the other hand TiO2 absorbs UV, thus OBA cannot be used in conjunction with high TiO2 loading.

Substances of different chemical composition and different biological origin are used for fluorescent whitening of paper. Derivatives of diaminostilbene disulfonic acid have proved the most popular in industry because of their fastness properties. The active ingredient content is generally between 20 and 27 %. Their central part is disulfonated diaminostilbene. The types differ in the number of sulfonic acid groups in the side groups. The high-substantivity types with only 2 sulfo groups make up about 11 % of the market while tetrasulfonated derivatives with medium substantivity are about 80 %. The rest is low-substantive, hexasulfonated types.

The paper industry today needs fluorescent whitening agents to obtain high degrees of whiteness at reasonable cost, especially if the FWA (OBA) are combined with shading dyestuffs. This in turn leads to sharper contrasts in the printed image and thus helps to reduce the toner consumption of copiers, for example. Similarly, the color brilliance of color prints is improved. For application in the stock, high or medium-substantivity types are generally used, being 1 % of the commercial-grade product. For surface application in the size or film press, medium or low-sub-stantivity types are used, the normal addition being 1.5 % and in exceptional cases up to 3.5 %. Low or medium-substantivity FWA are also the preferred products for coating application, the amount required being up to 1.5 % up to 3.5 % in excep¬tional cases. A maximum amount of FWA can be used in each application as the shade becomes greener. This will lead to a graying effect on the brightness.

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