Bleaching of Vegetable Textiles.—By far the largest of these industries are those which are engaged in producing the 'pure white' on cotton and flax goods. The process, considered chemically, is simply that of isolating a pure cellulose, and we endeavoured to give due prominence to this view in the original work. It is important to insist upon it for the reason that this view gives the due proportion of chemical value to the several contributory treatments—alkaline hydrolyses (caustic lime and soda boils), hypochlorite oxidations, and incidental acid treatments (souring). The first of these is by far the largest contributor of 'chemical work,' though the second, by being the agent for the actual whitening effect or bleaching action proper, occupies a position of often exaggerated importance.
In bleaching processes there has been no radical change of system on the large scale since the introduction of the 'Mather' kier in 1885, and the associated change from lime and ash boiling to the caustic soda circulating boil with reduced volume of lye, which this mechanical device rendered practicable. It is outside the scope of this work to follow up this branch of technology in any detail, and we cannot discuss the evolution of systems on variations of detail where no essential principle is involved. But we have to notice a very recent development which has only just begun its industrial career, and which does give effect to a principle of treatment not previously applied. This is tersely stated by its originator, William Mather,[13] in the expression, 'it is more economical to make liquids pass through cloth than to make cloth pass through liquids.' The starting point of this development is the invention of a complete self-contained machine in which a rolled batch of cloth can receive a succession of chemical treatments, with accessory washings—the solutions, or wash waters, being circulated through the cloth. The essential fact on which this system is based is that a perfect liquid circulation can be maintained from selvedge to selvedge through the folds of a tightly rolled batch of cloth. Such circulation is therefore quite independent of the diameter of the batch. If we consider a cloth under chemical treatment with solutions, it is clear that the reactions and interchanges of soluble matters within the cloth, within the twisted elements of the yarn, and in the last grade of distribution within the actual ultimate fibres, are subject to capillary transmission, and osmotic exchange. There is a mixture of these molecular effects, with the circulation in mass, sweeping both faces of the cloth. It is obvious that for the mass effect a relatively very small volume of circulating liquid is necessary to maintain uniform conditions of action. In the actual disposition of the machine the rolled batch of cloth nearly fills the cylindrical space of what we may call the reaction chamber, and the circulation of the liquid is maintained by a circulating pump and a differential pressure in the horizontal plane across and through the folds of the batch. This is in the meantime kept in slow revolution. For a full description of these mechanical details the reader is referred to the original patent specifications [Engl. Pat. 23,400, 23,401; 1900, W. Mather]. If we again consider the principles involved, they are very much as set forth in our original work (pp. 288-291). Boiling processes in which a relatively large volume of liquid is used are wasteful of steam, the active agent is unnecessarily diluted or used in superfluous quantity, and the soluble by-products, being continually removed as formed, cannot so effectively contribute by secondary actions to the chemical work. The new mechanical appliance enables us to further reduce the volume of liquid required in the alkaline-hydrolytic treatment of vegetable textiles, and where advantageous to bring the treatment down (or up) to a process of steaming with the active agent dissolved in a minimum proportion of water relative to the cloth. This concentration of effect is of importance in flax cloth, and especially linen treatment, where the peculiarly resistant cutocelluloses have to be attacked and a considerable proportion of waxy by-products to be removed. These points are the basis of the special process of Cross and Parkes [Engl. Pat. 25,076/ 99] for steaming flax (and cotton) goods with an emulsion containing, in addition to the special hydrolysing agent—caustic soda—mixtures of soap with 'mineral' or other oils, the presence of which effectually aids the removal of the by-products in question.
A complete system on these lines is now working on the industrial scale in the Belfast district. The results are not merely economical in largely reducing the number of alkaline boiling treatments required on the old plan of pan or 'pot' boiling, but are visible in the strength and finish of the linens so treated.
For cotton bleaching the costs may be put down at a fraction of those of the Irish linen bleach. The economical advantages of the new system are obviously less in relation to the lesser total costs. But there are other points which have come into more prominent influence. The mechanical wear and tear on the cloth is considerable in the ordinary process, more especially in the mangle-washes. As a result the adjustment of warp and weft is more or less disturbed. These defects are absent from a system which operates on the cloth in a fixed position.
But as we are mainly concerned with the purely chemical factors we cannot pretend to deal with textile questions. We have to notice the remaining element of chemical economy as it involves a fundamental principle. The practice of washing residues or products of reaction free from reagents and soluble by-products involves a well-known mathematical law, under which the rate of purification is a function rather of the number of successive changes of washing liquid than of the volume of the latter. The ordinary practice of textile washings entirely ignores this principle, and the consumption of water in consequence may reach many thousand times the economic minimum. With supplies of water often in indefinite excess of requirements, even in this most wasteful method, bleachers are in no need to consider the question of consumption. But leaving aside particular and local considerations of advantage the fact is that the new system gives control of the practice of washing, enabling the operator to adapt an important element of the daily routine to a fundamental principle which has been almost universally ignored.
In the oxidising processes which follow the alkaline treatments, the hypochlorites are still the staple agents. Owing to the steady relative fall in the selling prices of the permanganates these are coming into more extensive use, but the consumption is still small, and they are mainly used for certain special effects, chiefly in linen or more generally flax cloth bleaching.
Paper-pulp Spinning.—Paper is a continuous web or fabric produced by the interlocking of the structural fibrous units of the well-known short length. In Japan and other countries paper is made to serve for all or some of the purposes for which we employ string or twine, and to give the necessary tensile strength the paper is twisted or rolled on itself. Such twisting, however, adds nothing to the intrinsic tensile qualities of the original paper.
A new technical effect is realised in this direction by the treatment of paper-pulp in the process of its conversion into a continuous web: The pulp is formed into continuous strips of convenient breadth (usually from 2 to 8 mm.), these receive a 'rolling-up' treatment immediately following the squeeze of the press rolls by which the superfluous water is removed: they are then further but incompletely dried, and in this condition are subjected to a final spinning or twisting treatment on ring-spinning machinery of special construction.
Such a process was originally patented by C. Kellner in this country (E.P. No. 20,225/1891), and is fully described in his specification. Later improvements in detail were patented by G. Türk (E.P. 4621/1892).
A joint system is now being industrially developed in Germany by the Altdamm-Stahlhammer Pulp and Paper Company under the technical direction of Dr. Max Müller, and there appears to be every prospect of the product taking a position as a staple textile.