Researches on Cellulose Part 21

(Soc. of Arts, 1898.)

(p. 304) The Report of a Representative Committee appointed by the Society of Arts to inquire into the question of qualities of book papers in relation to their several applications, and more especially for doc.u.ments of permanent value.

The report first discusses the two directions of depreciation of papers in use: (1) Actual disintegration shown by loss of resistance to fracture by simple strain, and by loss of elasticity--i.e. increase of brittleness; (2) discolouration. These are independent effects, but often concurrent. They are the result of chemical changes of the cellulose basis of the paper, brought about by acids or oxidants used in the process of manufacture, and not completely removed from the pulp, or by acid products of bleaching--e.g. oxycelluloses or chlorinated derivatives; again, by the changes of starch used as a 'sizing' agent, or by oxidations induced by rosin const.i.tuents when the rosin is used in excess. Discolouration is an attendant phenomenon of these changes, but is more frequently due to the presence of the lower-grade celluloses (esparto and straw) and the lignocelluloses (mechanical wood-pulp).

The physical and chemical qualities of papers depending primarily upon their fibrous or pulp basis, and in a secondary degree upon the kind and proportion of the const.i.tuents added for the purpose of filling and 'sizing,' the report concludes with the following recommendations, positive and negative, under these heads:

The Committee find that the practical evidence as to permanence fully confirms the cla.s.sification given in the Cantor Lectures on 'Cellulose,'

1897 [J. Soc. Arts, xlv. 690-696], and which ranges the paper-making fibres in four cla.s.ses:

(A) Cotton, flax, and hemp (rhea).

(B) Wood celluloses, (a) sulphite process and (b) soda and 'sulphate' process.

(C) Esparto and straw celluloses.

(D) Mechanical wood-pulp.

In regard, therefore, to papers for books and doc.u.ments of permanent value, the selection must be taken in this order, and always with due regard to the fulfilment of the conditions of normal treatment above dealt with as common to all papers.

The Committee have been desirous of bringing their investigations to a practical conclusion in specific terms--viz. by the suggestion of standards of quality. It is evident that in the majority of cases there is little fault to find with the practical adjustments which rule the trade. They are, therefore, satisfied to limit their specific findings to the following--viz. (1) normal standard of quality for book-papers required for publications of permanent value. For such papers they specify as follows:

Fibres: Not less than 70 p.ct of fibres of cla.s.s A; cla.s.s D excluded.

Sizing: Not more than 2 p.ct. rosin, and finished with the normal acidity of pure alum; starch excluded.

Loading: Not more than 10 p.ct. total mineral matter (ash).

(2) With regard to written doc.u.ments, it must be evident that the proper materials are those of cla.s.s A, and that the paper should be pure and sized with gelatin, and not with rosin. All imitations of high-cla.s.s writing-papers which are, in fact, merely disguised printing-papers, should be carefully avoided.

_Appendix._--To the Report is added 'Abstracts of Papers' in 'Mittheilungen aus den Koniglichen Technischen Versuchsanstalten, Berlin,' for the years 1885-1896 inclusive--which is, in fact, a summary of the investigations of the Inst.i.tution in connection with paper and paper-standards.

(p. 273) ~Special Industrial Developments.~--From the point of view of the chemist there has been a very large development of the cellulose industries during the last five years. This is not so much marked by the gradual and progressive growth of the well-established industries, as by the success of the newer ones, with the attendant forecast of enormous developments of the industries in artificial products, the manufacture of which rests upon a purely chemical basis. We can, of course, only treat them from this limited standpoint, and so far as they involve and elucidate chemical principles.

~I. Chemical Treatments of Raw Materials.~

(a) ~Flax-spinning.~--The treatment of the roving on the spinning-frame by the addition of reagents to the macerating liquid--otherwise and usually hot water--continues to be justified by results. The technical basis of the process and the reactions determined in the spinning-trough by the alkaline salts used--chiefly sulphite and phosphate of soda--is set forth in the original work, p. 280. Since that time a sufficient period has elapsed to judge the effects, both technical and industrial, by the results of a commercial undertaking based on the exclusive use of the process. Such a concern is the Irish Flax Spinning Company of Belfast. At this mill the experience is uniform and fully established that by means of the process the drawing, i.e. spinning, quality of inferior flaxes is very considerably appreciated, enabling the spinner to use such flaxes for yarns of fineness which are unattainable by the ordinary method of spinning through hot water. Notwithstanding the success of this undertaking the development of the method is still inconsiderable. It is none the less a further and forcible demonstration of the existence of margins of increased technical effect which it is the work of the scientific technologist to exploit.

(b) ~Wood-pulp and Methods of Manufacture.~--There is a steady growth in the consumption of wood-pulps (cellulose) relatively to other materials.

In regard to the paper-trade of the world, this continues to be one of the most prominent characteristics of its evolution. In the United Kingdom the conditions of its compet.i.tion are of a more special kind by reason of the firm foothold of esparto, which is a most important staple in the manufacture of fine printings. Whereas the consumption of esparto remains nearly stationary at about 200,000 tons per annum, the importation of wood-pulps has shown the extraordinary rate of increase of doubling itself every five years. But in the group 'wood-pulps' the trade returns have until recently included the 'mechanical' or ground wood-pulps. From 1898 we have separate returns for the chemical or cellulose pulps, and in 1899 the tonnage reached nearly to that of esparto, with a total money value about 80 p.ct. greater. When it is remembered that this is one of the newer chemical industries in cellulose products, and that these large commercial results have been accomplished during a period of twenty years, we are impressed with the scope of the industrial outlook to the chemist, afforded by the arts of which cellulose is the foundation.

It may be noted that there have been no important developments in the purely chemical processes involved in the several systems of preparing cellulose from wood. The acid methods (bisulphite processes) have developed much more extensively than the alkaline, the latter including the caustic soda and the mixed sulphide ('Dahl') process. The bisulphite processes depended in the earlier stages upon the efficiency of lead-lined digesters. But the problem of acid-resisting linings has been much more perfectly solved in later years in the various types of cement and other silicate linings now in use. The relative permanency of these linings has had an important effect on the costs of production. Further economies result from the use of digesters of enormous capacity, dealing with as much as 100 tons of wood at one operation. As a combined result of economic production and active compet.i.tion, the selling prices of 'sulphite pulp' have moved steadily downwards in relation to other half-stuffs and raw materials. As a necessary consequence the prices of those which it has gradually displaced have depreciated, and a study of the price and tonnage-equilibrium as between rags, esparto, and wood-pulp over a series of years forms an interesting object-lesson in the struggle for survival which is an especial mark of modern industry.

For these matters the reader is referred to the special literature of the paper-making industry.[12]

It is not a little remarkable that the main by-product of these bisulphite processes--the sulphonated derivatives of the lignone const.i.tuents of the wood--is still for the most part an absolute waste, notwithstanding the many investigations of technologists and attempts to convert it to industrial use (see p. 149). Seeing that it represents a percentage on the wood pulped equal to that of the cellulose obtained, it is a waste of potentially valuable material which can only be termed colossal. Moreover, as a waste to be discharged into water-courses, it becomes a source of burden and expense to the manufacturer, and with the increasing restrictions on the pollution of rivers it is in many localities a difficulty to be reckoned with only by the cessation of the industry. The problem in such cases becomes that of dealing with it destructively, i.e. by evaporation and burning. In this treatment the obviously high calorific value of the dissolved organic matter (lignone) appears on the 'credit' side. But where calcium and magnesium bisulphites are used, the residue from calcination is practically without value. It appears, however, that by subst.i.tuting soda as the base the alkali is recoverable in such a form as to be directly available for the alkaline-sulphide or 'Dahl' process. As a more complicated alternative the soda admits of being recovered on the lines of the old black-ash or Leblanc process, and the sulphur by the now well-established 'Chance' process, for which, of course, an addition of lime is necessary to the fully evaporated liquors previously to calcining. The engineering features of the system, so far as regards evaporating and calcining, are the same. For economic working there is required (a) evaporation by multiple effect and (b) calcining on the continuous rotary principle. For the latter a special modification has been devised so that the draught of air is concurrent with the movement of the charge in the furnace, securing a progressively increasing temperature within the furnace. This interesting development of the chemical engineering of wood-pulp systems has been elaborated by two well-known technologists, Drewson and Dorenfeldt, and readers who wish to inform themselves in detail of these developments are referred to the various publications of these inventors.

a.s.suming the present necessity of a destructive treatment of the by-products of the bisulphite processes, the scheme has many advantages.

The soda-bisulphite liquors are more economically prepared; the pulp obtained is superior in paper-making quality to that resulting from the lime or magnesia (bisulphite) processes: it is more economically bleached.

Then, as pointed out, the soda may on the one plan be obtained in a form in which it is immediately available as a powerful hydrolysing alkali in the manufacture of a 'soda' pulp. These two systems become, therefore, in a new sense complementary to one another. Lastly, it is obvious that the employment of soda as the base opens out a new vista for developing the electrolytic processes of decomposing common salt.

The authors have a.s.sisted in preparing plans for a comprehensive industrial scheme combining all these more modern developments. In this scheme it is only the combination which is novel, and as it involves no new principles in the chemical treatments of the materials we are not further concerned with it than to have briefly sketched its economic basis. This may be summed up in result in the important question of cost and selling price, and the estimate is well grounded that by means of this scheme _bleached wood-pulp_ can be sold on the English market at 10l. a ton. It is important to note this figure and to compare it with the prices of twenty years ago. The fall has been continuous, notwithstanding the influence of the opposing factors of increasing consumption, exhaustion of accessible supply of timber, and relative appreciation of the essential costs of steam, chemicals, and labour. It is important in forecasting the future, since the youngest and apparently most promising of the 'artificial' cellulose industries employs wood-cellulose by preference as its raw material (see p. 173).

As a last point it must be considered that as chemists we are bound to antic.i.p.ate the realisation of value in the soluble by-products of the bisulphite processes. Outside the intrinsic interest attaching to the solution of this problem, it carries with it the promise of a further economy in the production of wood-cellulose.

~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 a.s.sociated 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 pa.s.s through cloth than to make cloth pa.s.s 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 ma.s.s, sweeping both faces of the cloth. It is obvious that for the ma.s.s 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 s.p.a.ce 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 quant.i.ty, 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 j.a.pan 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. Turk (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 Muller, and there appears to be every prospect of the product taking a position as a staple textile.

The process has only the incidental interest in connection with our main subject, that it employs chiefly the 'chemical' pulps or celluloses as raw materials. The industrial future of the application must, of course, be largely determined by costs of production, as the directions of application in the weaving industries will be limited by the necessarily inferior grade of tensile strength belonging to these products and the degree by which this is lowered on complete wetting. All these questions have been duly weighed by those engaged in this interesting development, and the conclusion of those qualified to judge is that the new industry has vindicated for itself a permanent position.

~II. The Chemical Derivatives of Cellulose~, in their industrial aspects, have come to occupy a profoundly important position in the world's affairs. In the way of any essential alteration of the perspective from that obtaining in 1895 we have nothing to chronicle. No new derivatives of industrial importance have been added in that period; but certain new methods incidental to the preparation of well-known compounds or for converting them into more generally available forms have been introduced, and these are contributing to the rapid expansion of the 'artificial' cellulose industries.

Of the cellulose esters the nitrates are still the only group in industrial use. There uses for explosives have attained immense proportions, and their applications for structural purposes are continually on the increase. The manufacture of smokeless powders on the one hand, and of celluloid and xylonite (both in the form of films and solid aggregates) on the other, has taken no new departure. The industry in 'artificial silks' or 'l.u.s.tra-celluloses,' by the collodion processes also, whilst presenting features of unusual interest attaching to rapid expansion, has been barren of contribution of fundamental scientific or technical importance. The tetracetate is now manufactured on the large scale, but the product has yet to make its market.

The process of mercerising cotton yarns and cloth has been developed to an industry of colossal dimensions, and the growth has been especially rapid during the last five years. Significant of the technical progress in these two industries, with their common aim of appreciating cellulose in the scale of textiles by approximating its external properties in those of silk, is the appearance of a monograph of the technology of each, notices of which have been previously given (pp. 22-26).

There is little doubt, however, that the question of the future industry in the various forms of cellulose, thread, film, structureless powder or solid aggregate, obtainable by artificial means, mainly turns upon cost of production. Irrespective of cost, there would, no doubt, be a market for all these products, based upon such of their properties or effects as are indispensable and not otherwise obtainable. As an ill.u.s.tration, we may cite the extraordinary selling prices of 40-50 fr.

per kilo, for the 'artificial silks' (collodion process) which ruled some three years ago; and we may note that for a special application of viscose the dissolved cellulose is paid for at the rate of 10 per lb. These facts are certainly worthy of mention, and should be borne in mind as an index of some special features of modern manufacturing industry. But with a material like cellulose rendered available in a new shape the question which always arises more prominently than that of limited uses at high prices is that of consumption on the extensive scale which marks the older and well-known products. That question is rapidly solving itself in this country as regards the 'artificial silks.' There is at present a limited market at 9s.-10s. per lb., a price which on the one side excludes extensive consumption, and on the other practically bars manufacture in this country by any of the collodion systems. It will appear from a very elementary calculation of what we may call the theoretical costs that the above selling price would not have a remunerative margin. The theoretical costs are made up of

Raw materials[14] {Cotton. Nitrating acid. Ether-alcohol (solvent).

{Denitrating chemicals.

{(a) Nitrating and preparing collodion. Denitrating { and bleaching.

Labour {(b) Textile operations. Spinning. Winding and twisting.

{Rewinding.