The Manufacture of Paper / With Illustrations, and a Bibliography of Works Relating to Cellulose and Paper-Making

by R. W. Sindall

In all such cases, the dyes must be carefully selected in order to obtain a coloured paper which will show a minimum alteration in tint by exposure to light or by contact with chemical substances. This is particularly necessary in coloured wrapping paper used for soap, tea, cotton yarn, and similar goods.

Unevenness of Colour.—The different affinity of the various paper-making fibres for dyes is apt to produce an uneven colour in the finished paper. This is very noticeable in mixtures of chemical wood pulp or cellulose and mechanical wood pulp. The ligno-cellulose of the latter has a great affinity for basic dyes, and if the required amount of dye is added to a beater containing the mixed pulps in an insufficiently diluted form, the mechanical wood pulp becomes more deeply coloured than the cellulose. If the former is a finely ground pulp, the effect is not very noticeable, but if it is coarse, containing a large number of coarse fibres, then the paper appears mottled. The defect is still further aggravated when the paper is calendered, especially if calendered in a damp condition. In that case the strongly coloured fibres of mechanical wood are very prominent.

When dyes have been carelessly dissolved and added to the beating engine without being properly strained, unevenness of colour may often be traced to the presence of undissolved particles of dye.

Irregular Colour of the two Sides.—Many papers exhibit a marked difference in the colour of the two sides. When heavy pigments are employed as the colouring medium, the under side of the sheet, that is, the side of the paper in contact with the machine wire, is often darker than the top side. The suction of the vacuum boxes is the main cause of this defect, though the amount of water flowing on to the wire, the “shake” of the wire, and the extent to which the paper is sized are all contributory causes. By careful regulation of these varying conditions the trouble is considerably minimised.

The under surface of the paper is not invariably darker than the top surface. With pigments of less specific gravity the reverse is found to be the case. This is probably to be explained by the fact that some of the colouring matter from the under side is drawn away from the paper by the suction boxes, and the pigment on the top side is not drawn away to any serious extent, because the layer of pulp below it acts as a filter and promotes a retention of colour on the top side.

It is interesting to notice that this irregularity sometimes occurs with soluble dyes, as for example in the case of auramine. The decomposition of this dye when heated to the temperature of boiling water is well known, and the contact of a damp sheet of paper coloured by auramine with the surfaces of steam-heated cylinders at a high temperature brings about a partial decomposition of the dye on one side of the paper. Generally speaking, acid dyes are more sensitive to heat than basic dyes.

The presence of china clay in a coloured paper is also an explanation of this irregular appearance of the two sides. China clay readily forms an insoluble lake with basic dyes, and when the suction boxes on the machine are worked with a high vacuum the paper is apt to be more deeply coloured one side than another.

The Machine Backwater.—Economy in the use of dyes to avoid a loss of the colouring matter in the “backwater,” or waste water from the paper machine, is only obtained by careful attention to details of manufacture on the one hand and by a knowledge of the chemistry of dyeing on the other. The loss is partly avoided by regulating the amount of water used on the machine, so that very little actually goes to waste, and further reduced by ensuring as complete a precipitation of the soluble dye as possible.

The acid dyes generally do not give a colourless backwater, and all pulps require to be heavily sized when acid dyes are used.

The basic dyes are more readily precipitated than the acid dyes, particularly if a suitable mordant is used, even with heavily coloured papers. The addition of an acid dye to pulp first coloured with a basic dye is frequently resorted to as a means of more complete precipitation.