Mechanical Wood Pulp in Paper.
The presence of mechanical wood pulp in paper is detected by means of several reagents, which produce a definite colour when applied to a sheet of paper containing mechanical wood. The depth of colour obtained indicates approximately the percentage present, but considerable practice and experience is necessary to interpret the colour exactly. A more reliable method of estimating the percentage of mechanical wood in a paper is by microscopic examination.
The reagents which can be used are—
(1) Nitric Acid.—This produces a brown stain on the paper, but it is not a desirable reagent for ordinary office purposes.
(2) Aniline Sulphate.—A solution of this is prepared by dissolving 5 parts of aniline sulphate in 100 parts of distilled water. When applied to the surface of news a yellow coloration is produced, more or less intense according to the amount of mechanical wood present. It can only be used with white papers, or papers very slightly toned.
(3) Phloroglucine.—This sensitive reagent, which gives a rose-pink colour when brushed on to the surface of the paper, is prepared by dissolving 4 grammes of phloroglucine in 100 c.c. of rectified spirits, and adding to the mixture 50 c.c. of pure concentrated hydrochloric acid.
There are several other aniline compounds which give colour reactions of a similar character, but they are not often used. The phloroglucine reagent fails as a test for mechanical wood in papers which have been dyed with certain aniline colours, for example, metanil yellow. Paper which has been coloured with this dye will, when moistened with the phloroglucine reagent, give an intense pink colour, even if no mechanical wood is present. This is due to the fact that the dye itself is acted upon by the hydrochloric acid in the test reagent. The same colour is produced on the paper with hydrochloric acid per se.
There is little difficulty in distinguishing between the colour arising from the presence of such a dye, because the effect is instantaneous, whereas the coloration due to mechanical wood develops gradually. Moreover, the reaction due to the presence of metanil yellow gives a perfectly even coloured surface, whereas with mechanical wood pulp the fibres appear to be more deeply stained than the body of the paper.
Output of a Paper Machine.—The quantity of paper which can be produced on the paper machine is readily calculated from the following data:—
| Speed of machine in feet per minute | F |
| Nett deckle width in inches | D |
| Width of sheet of paper in inches | W |
| Length of sheet of paper in inches | L |
| Number of sheets in ream | S |
| Weight of paper per ream | R |
The general formula for the output of paper per hour is
| Output in lbs. per hour = | 720 × F × D × R | . |
| S × L × W |
When the number of sheets in the ream is 480, this formula simplifies to
| Output in lbs. per hour = | 1½ × R × F × D | . |
| L × W |
The term “nett deckle width” applies to the width of the trimmed finished paper at the end of the machine. The formula takes no account of the allowance required for trimming edges. In most cases the deckle width of the machine is arranged so that the paper is cut into strips of equal width when leaving the calenders, e.g., a deckle of 80 inches will give 4 sheets, each 20 inches wide.
Fig. 39.—Paper Machine showing Wire, Press Rolls, and Drying Cylinders.
The method by which the general formula is obtained may be explained by an example.
What is the output of a machine having a speed of 100 feet per minute, with an 80-inch deckle, producing a sheet of paper 20 inches by 30 inches, weighing 30 lbs. per ream of 480 sheets?
The machine produces every minute a sheet of paper 100 feet long and 80 inches wide.
Hence output per minute in square inches
| = 12 × 100 × 80. |
Output per hour in square inches
| = 60 × 12 × 100 × 80. |
Now each (20 × 30 × 480) square inches is area of one ream.
Output of paper per hour in reams
| = | 60 × 12 × 100 × 80 | . |
| 480 × 30 × 20 |
Output of paper per hour in lbs.
| = | 720 × 100 × 80 × 30 |
| 480 × 30 × 20 | |
| = | 600 lbs. |
The general formula may be applied for the purpose of calculating the speed at which the machine must be driven.
Example.—A machine with 75-inch deckle is required to produce 6 cwts. per hour of a paper 25 inches by 18 inches (500 sheets), weighing 19 lbs. to the ream. At what speed is the machine to be driven?
Output in lbs. per hour
| = | 720 × F × D × R |
| S × L × W | |
| 672 = | 720 × F × 75 × 19 |
| 500 × 18 × 25 | |
| F = | 148 feet per minute. |
[CHAPTER VI]
BROWN PAPERS AND BOARDS
Common Browns.—The raw material used in the manufacture of common brown papers is chiefly jute and waste fibres of every description, such as waste cuttings from boxboard factories, old papers, wood pulp refuse, and other substances of a like nature. The jute, in the form of sacking or old gunny bags, and the hemp refuse, in the shape of old rope and string, are subjected to a slight chemical treatment just sufficient to isolate the fibres to a condition in which it is possible to work them up into paper. The bagging and string are cut up in a rag chopper and boiled in revolving boilers with lime or caustic soda for several hours at a pressure of 20-30 lbs., the lime being used when it is desired to manufacture a harsh paper, and the caustic soda being employed for the production of paper having a softer feel. The pulp is not always washed very completely after the process of digestion, as is the case with white papers, and it is often possible to extract from brown papers of this class a considerable proportion of the alkaline matter which has not been thoroughly removed from the boiled pulp. The presence of this alkaline residue does not affect the quality of ordinary brown paper, but is frequently a serious defect in the case of middles or straw boards, which are afterwards utilised for boxes and covered with coloured papers. The colour of the paper pasted on to such incompletely washed boards is frequently spoilt by the action of the alkali when moistened with the paste used, many aniline dyes being susceptible to the small proportion of alkali present.
The stronger materials, such as jute or old rope and string, are either used by themselves or blended with inferior raw material according to the quality of the paper being made. The jute and hemp fibres are generally beaten by themselves in the engine before the other materials are added. The pulp is mixed with the required amount of loading, while the sizing and colouring operations are carried out in the usual way.
The common brown papers are known by a variety of trade names which at one time indicated the nature of the fibrous constituent, but at the present day the name is no guide or indication of the material used for the manufacture of the paper. The common heavy brown used for wrapping sugar and sundry groceries made in heavy grey and blue shades is a coarse paper made from cheap materials and containing a large proportion of mineral matter. It is usually supplied under the trade name of royal.
A somewhat lighter and stronger wrapping paper of a white or buff colour, used for wrapping groceries, tea, and cotton goods, is that known as casings, a name probably derived from the application of this paper originally to the lining of cases.
Manila papers so called were originally made from rope, but the term is now applied to papers which may be made entirely of wood pulp.
Rope browns are common papers made of fairly strong material of a miscellaneous character, this name having been derived from the fact that rope and similar fibre were at one time used exclusively.
Wood Pulp Wrappers.—Most of the papers of the present day are made from wood pulp, this material giving a thin, light, tough paper, which is pleasant to handle and forms a great contrast to the dense, opaque, heavily loaded, and inartistic specimens produced some years ago. Paper of this kind, though apparently more expensive than common browns, is really more economical in use. The paper is not only stronger, but it is possible to obtain a larger number of sheets for a given weight. The great advantage in the improvement of brown papers dates from the introduction of the now well-known kraft papers, which are of comparatively recent origin.
Kraft Paper.—The term Kraft, meaning “strength,” is applied to a remarkably strong cellulose paper prepared from spruce and other coniferous woods by the soda treatment, the special feature of the process being an incomplete digestion of the wood.
The wood previously chipped into pieces 1 inch to 1½ inches in length, is boiled with caustic soda, the digestion being stopped before the wood pulp has been quite softened, and while the pulp is still too hard to be broken up into isolated fibres by simple agitation in water. The pulp after thorough washing is disintegrated by means of an edge-runner, or some form of breaking engine, the first mentioned probably giving the most satisfactory results, and converted into paper by the usual methods.
The wood can also be reduced by the sulphate process, in which case the chipped wood is boiled in a liquor to which about 25 per cent. of spent lye from a previous cooking is added.
The best results are obtained by attention to the cooking process to ensure an under-cooked pulp, by careful isolation of the fibres in a kollergang, or edge-runner, which machine is capable of separating the fibres without shortening them, and by proper manipulation on the paper machine.
The paper produced under favourable conditions in this direction is wonderfully tough and strong and may be quoted as the most recent example of the fact that the latent possibilities of wood pulp have by no means been exhausted or even thoroughly investigated.
Imitation Kraft Paper.—If wood is boiled in water at high temperatures the fibre is softened and much of the resinous matter is removed. Such wood, if ground in the same way and by the same methods as ordinary mechanical wood pulp, is readily disintegrated, and a long-fibred pulp may be obtained. The process of boiling short 2 feet logs of wood in a digester under a pressure of 20-50 lbs. has long been known. The wood after boiling is partly washed and then worked up into pulp by the usual mechanical process. The wood is easily ground and yields pulp containing long fibres which in their physical properties closely resemble those of pure wood cellulose, but the original constituents of the wood are present almost unchanged, just as in mechanical pulp. The product obtained by grinding is a very tough flexible material of a brownish yellow colour, and the paper is known as Nature brown. It is chiefly used for the preparation of tough packing papers, for the covers of cheap pocket-books, and other miscellaneous purposes. When this brown mechanical wood pulp paper is glazed on both sides it is then known as ochre glazed, the word ochre referring to the colour. When made up into light weight papers it is sold as imitation kraft paper.
A great variety of wrapping papers are now made from wood pulp, such as sealings, sulphite browns, manilas, sulphite caps, but the distinctions between these papers relate chiefly to the amount of finish, the colour and size of the sheet. The methods of manufacture only differ in small details as indicated by these distinctions.
Fine Wrappings.—The papers used for packing small goods such as silver ware and other delicate articles are generally tissues, the better qualities of which are made from rag, and the cheaper qualities from wood pulp. These papers are known as tissue, crêpe, crinkled tissue, manila tissue, and by a variety of trade terms.
Fig. 40.—Single Cylinder or Yankee Machine.
Many of the fine wrappings of the tissue class and the somewhat heavier papers known as M. G. Caps are manufactured on the single cylinder machine, which produces a paper having a highly polished surface on one side and a rough unglazed surface on the other side.
In the single cylinder machine the beaten pulp passes from the stuff-chest on to the wire of the ordinary Fourdrinier machine and through the press rolls, but instead of being dried over a number of cylinders the paper is led over one single cylinder of very large diameter which is heated internally with steam. The paper is usually pressed against the surface of the cylinder by means of a heavy felt, which is, however, sometimes omitted. The side of the paper coming into contact with the cylinder becomes highly polished, the surface in contact with the felt remaining in an unfinished rough condition. This paper is said to be machine glazed and is known as an M. G. paper.
Fig. 41.—Section of Wet Press, or Board Machine.
Boards.—Cards, millboards, middles, boxboards, carriage panels, and similar paper products are manufactured either on a single board machine, by means of which single sheets of any required thickness can be obtained, or on a continuous board machine, which is capable of producing cards and plain or duplex boards of moderate thickness.
The raw material used consists, as in the case of browns and wrappers, of every conceivable fibrous substance mixed with mineral matter and then suitably coloured. The preliminary processes for the treatment of the pulp are exactly the same as those employed in the case of brown papers up to the point at which the beating has been effected.