Читать книгу The Art of Paper-Making - Alexander Watt - Страница 4
CHAPTER I.
ОглавлениеCELLULOSE.
Cellulose.—Action of Acids on Cellulose.—Physical Characteristics of Cellulose.—Micrographic Examination of Vegetable Fibres.—Determination of Cellulose.—Recognition of Vegetable Fibres by the Microscope.
Cellulose.—Vegetable fibre, when deprived of all incrusting or cementing matters of a resinous or gummy nature, presents to us the true fibre, or cellulose, which constitutes the essential basis of all manufactured paper. Fine linen and cotton are almost pure cellulose, from the fact that the associated vegetable substances have been removed by the treatment the fibres were subjected to in the process of their manufacture; pure white, unsized, and unloaded paper may also be considered as pure cellulose from the same cause. Viewed as a chemical substance, cellulose is white, translucent, and somewhat heavier than water. It is tasteless, inodorous, absolutely innutritious, and is insoluble in water, alcohol, and oils. Dilute acids and alkalies, even when hot, scarcely affect it. By prolonged boiling in dilute acids, however, cellulose undergoes a gradual change, being converted into hydro-cellulose. It is also affected by boiling water alone, especially under high pressure, if boiled for a lengthened period. Without going deeply into the chemical properties of cellulose, which would be more interesting to the chemist than to the paper manufacturer, a few data respecting the action of certain chemical substances upon cellulose will, it is hoped, be found useful from a practical point of view, especially at the present day, when so many new methods of treating vegetable fibres are being introduced.
Action of Acids on Cellulose.—When concentrated sulphuric acid is added very gradually to about half its weight of linen rags cut into small shreds, or strips of unsized paper, and contained in a glass vessel, with constant stirring, the fibres gradually swell up and disappear, without the evolution of any gas, and a tenacious mucilage is formed which is entirely soluble in water. If, after a few hours, the mixture be diluted with water, the acid neutralised with chalk, and after filtration, any excess of lime thrown down by cautiously adding a solution of oxalic acid, the liquid yields, after a second filtration and the addition of alcohol in considerable excess, a gummy mass which possesses all the characters of dextrin. If instead of at once saturating the diluted acid with chalk, we boil it for four or five hours, the dextrin is entirely converted into grape sugar (glucose), which, by the addition of chalk and filtration, as before, and evaporation at a gentle heat to the consistence of a syrup, will, after repose for a few days, furnish a concrete mass of crystallised sugar. Cotton, linen, or unsized paper, thus treated, yield fully their own weight of gum and one-sixth of their weight of grape sugar. Pure cellulose is readily attacked by, and soon becomes dissolved in, a solution of oxide of copper in ammonia (cuprammonium), and may again be precipitated in colourless flakes by the addition of an excess of hydrochloric acid, and afterwards filtering and washing the precipitate. Concentrated boiling hydrochloric acid converts cellulose into a fine powder, without, however, altering its composition, while strong nitric acid forms nitro-substitution products of various degrees, according to the strength of the acid employed. "Chlorine gas passed into water in which cellulose is suspended rapidly oxidises and destroys it, and the same effect takes place when hypochlorites, such as hypochlorite of calcium, or bleaching liquors, are gently treated with it. It is not, therefore, the cellulose itself which we want the bleaching liquor to operate upon, but only the colouring matters associated with it, and care must be taken to secure that the action intended for the extraneous substances alone does not extend to the fibre itself. Caustic potash affects but slightly cellulose in the form in which we have to do it, but in certain less compact conditions these agents decompose or destroy it."—Arnot.[1]
Physical Characteristics of Cellulose.—"The physical condition of cellulose," says Mr. Arnot, "after it has been freed from extraneous matters by boiling, bleaching, and washing, is of great importance to the manufacturer. Some fibres are short, hard, and of polished exterior, while others are long, flexible, and barbed, the former, it is scarcely necessary to say, yielding but indifferent papers, easily broken and torn, while the papers produced from the latter class of fibres are possessed of a great degree of strength and flexibility. Fibres from straw, and from many varieties of wood, may be taken as representatives of the former class, those from hemp and flax affording good illustrations of the latter. There are, of course, between these extremes all degrees and combinations of the various characteristics indicated. It will be readily understood that hard, acicular[2] fibres do not felt well, there being no intertwining or adhesion of the various particles, and the paper produced is friable. On the other hand, long, flexible, elastic fibres, even though comparatively smooth in their exterior, intertwine readily, and felt into a strong tough sheet. … Cotton fibre is long and tubular, and has this peculiarity, that when dry the tubes collapse and twist on their axes, this property greatly assisting the adhesion of the particles in the process of paper-making. In the process of dyeing cotton, the colouring matter is absorbed into the tubes, and is, as will be readily appreciated, difficult of removal therefrom. Papers made exclusively of cotton fibre are strong and flexible, but have a certain sponginess about them which papers made from linen do not possess."
Linen—the cellulose of the flax-plant—before it reaches the hands of the paper-maker has been subjected to certain processes of steeping or retting, and also subsequent boilings and bleachings, by which the extraneous matters have been removed, and it therefore requires but little chemical treatment at his hands. "Linen fibre," Arnot further observes, "is like cotton, tubular, but the walls of the tubes are somewhat thicker, and are jointed or notched like a cane or rush; the notches assist greatly in the adhesion of the fibres one to another. This fibre possesses the other valuable properties of length, strength, and flexibility, and the latter property is increased when the walls of the tubes are crushed together under the action of the beating-engine." From this fibre a very strong, compactly felted paper is made; indeed, no better material than this can be had for the production of a first-class paper. Ropes, coarse bags, and suchlike are made from hemp, the cellulose or fibre of which is not unlike that of flax, only it is of a stronger, coarser nature. Manilla[3] yields the strongest of all fibres. Jute, which is the fibre or inside bark of an Indian plant (Corchorus capsularis), yields a strong fibre, but is very difficult to bleach white. Esparto fibre holds an intermediate place between the fibres just described and those of wood and straw. … The fibre of straw is short, pointed, and polished, and cannot of itself make a strong paper. The nature of wood fibre depends, as may readily be supposed, upon the nature of the wood itself. Yellow pine, for example, yields a fibre long, soft, and flexible, in fact very like cotton; while oak and many other woods yield short circular fibres which, unless perfectly free from extraneous matters, possess no flexibility, and in any case are not elastic.
Micrographic Examination of Vegetable Fibres.—The importance of the microscope in the examination of the various fibres that are employed in paper manufacture will be readily evident from the delicate nature of the cellulose to be obtained therefrom.[4] Amongst others M. Girard has determined, by this method of examination, the qualities which fibres ought to possess to suit the requirements of the manufacturer. He states that absolute length is not of much importance, but that the fibre should be slender and elastic, and possess the property of turning upon itself with facility. Tenacity is of but secondary importance, for when paper is torn the fibres scarcely ever break. The principal fibres employed in paper-making are divided into the following classes:—
1. Round, ribbed fibres, as hemp and flax.
2. Smooth, or feebly-ribbed fibres, as esparto, jute, phormium (New Zealand flax), dwarf palm, hop, and sugar-cane.
3. Fibro-cellular substances, as the pulp obtained from the straw of wheat and rye by the action of caustic ley.
4. Flat fibres, as cotton, and those obtained by the action of caustic ley upon wood.
5. Imperfect substances, as the pulp obtained from sawdust. In this class may also be included the fibre of the so-called "mechanical wood pulp."
Determination of Cellulose. For the determination of cellulose in wood and other vegetable fibres to be used in paper-making Müller recommends the following processes:[5] 5 grammes weight of the finely-divided substance is boiled four or five times in water, using 100 cubic centimètres[6] each time. The residue is then dried at 100° C. (212° Fahr.), weighed, and exhausted with a mixture of equal measures of benzine and strong alcohol, to remove fat, wax, resin, &c. The residue is again dried and boiled several times in water, to every 100 c.c. of which 1 c.c. of strong ammonia has been added. This treatment removes colouring matter and pectous[7] substances. The residue is further bruised in a mortar if necessary, and is then treated in a closed bottle with 250 c.c. of water, and 20 c.c. of bromine water containing 4 c.c. of bromine to the litre.[8] In the case of the purer bark-fibres, such as flax and hemp, the yellow colour of the liquid only slowly disappears, but with straw and woods decolorisation occurs in a few minutes, and when this takes place more bromine water is added, this being repeated until the yellow colour remains, and bromine can be detected in the liquid after twelve hours. The liquid is then filtered, and the residue washed with water and heated to boiling with a litre of water containing 5 c.c. of strong ammonia. The liquid and tissue are usually coloured brown by this treatment. The undissolved matter is filtered off, washed, and again treated with bromine water. When the action seems complete the residue is again heated with ammoniacal water. This second treatment is sufficient with the purer fibres, but the operation must be repeated as often as the residue imparts a brownish tint to the alkaline liquid. The cellulose is thus obtained as a pure white body; it is washed with water, and then with boiling alcohol, after which it may be dried at 100° C. (212° Fahr.) and weighed.
Recognition of Vegetable Fibres by the Microscope.—From Mr. Allen's admirable and useful work on "Commercial Organic Analysis"[9] we make the following extracts, but must refer the reader to the work named for fuller information upon this important consideration of the subject. In examining fibres under the microscope, it is recommended that the tissues should be cut up with sharp scissors, placed on a glass slide, moistened with water, and covered with a piece of thin glass. Under these conditions:—
Filaments of Cotton appear as transparent tubes, flattened and twisted round their axes, and tapering off to a closed point at each end. A section of the filament somewhat resembles the figure 8, the tube, originally cylindrical, having collapsed most in the middle, forming semi-tubes on each side, which give the fibre, when viewed in certain lights, the appearance of a flat ribbon, with the hem of the border at each edge. The twisted, or corkscrew form of the dried filament of cotton distinguishes it from all other vegetable fibres, and is characteristic of the matured pod, M. Bauer having found that the fibres of the unripe seed are simply untwisted cylindrical tubes, which never twist afterwards if separated from the plant. The matured fibres always collapse in the middle as described, and undergo no change in this respect when passing through all the various operations to which cotton is subject, from spinning to its conversion into pulp for paper-making.
Linen, or Flax Fibre, under the microscope, appears as hollow tubes, open at both ends, the fibres being smooth, and the inner tube very narrow, and joints, or septa,[10] appear at intervals, but are not furnished with hairy appendages as is the case with hemp. When flax fibre is immersed in a boiling solution of equal parts of caustic potash and water for about a minute, then removed and pressed between folds of filter-paper, it assumes a dark yellow colour, whilst cotton under the same treatment remains white or becomes very bright yellow. When flax, or a tissue made from it, is immersed in oil, and then well pressed to remove excess of the liquid, it remains translucent, while cotton, under the same conditions, becomes opaque.
New Zealand Flax (Phormium tenax) may be distinguished from ordinary flax or hemp by a reddish colour produced on immersing it first in a strong chlorine water, and then in ammonia. In machine-dressed New Zealand flax the bundles are translucent and irregularly covered with tissue; spiral fibres can be detected in the bundles, but less numerous than in Sizal. In Maori-prepared phormium the bundles are almost wholly free from tissue, while there are no spiral fibres.
Hemp Fibre resembles flax, and exhibits small hairy appendages at the joints. In Manilla hemp the bundles are oval, nearly opaque, and surrounded by a considerable quantity of dried-up cellular tissue composed of rectangular cells. The bundles are smooth, very few detached ultimate fibres are seen, and no spiral tissue.
Sizal, or Sisal Hemp (Agave Americana), forms oval fibrous bundles surrounded by cellular tissue, a few smooth ultimate fibres projecting from the bundles; is more translucent than Manilla, and a large quantity of spiral fibres are mixed up in the bundles.
Jute Fibre appears under the microscope as bundles of tendrils, each being a cylinder, with irregular thickened walls. The bundles offer a smooth cylindrical surface, to which the silky lustre of jute is due, and which is much increased by bleaching. By the action of hypochlorite of soda the bundles of fibres can be disintegrated, so that the single fibres can be readily distinguished under the microscope. Jute is coloured a deeper yellow by sulphate of aniline than is any other fibre.
