The Constituents Of Paper - Part 2
( Originally Published 1920 )
CHEMICAL WOOD-PULPS.—Chemical wood-pulps are obtained by a variety of processes, all of which have as their object the isolation of the pure cellulose fiber by the dissolution of non-cellulose components. The same principles are applied to the treatment of esparto straw or other plants. The character of the pulp depends not only upon the nature of the wood, but also upon the solvents used and the duration and severity of the cooking.
The preparatory steps to any process by which chemical wood-pulp is made are identical with the preparation of trees for ground wood, only after the logs have been "barked," they are reduced to chips by a mechanical "chipper." The ordinary practice in America is to sort out any knotty or imperfect logs as they pass on a conveyor from the "barker," and if the log it too faulty it is discarded. As it is desirable to have a uniform size of chips, the chips are passed through a screen for this purpose.
The chips are stored in bins convenient to the digesters. The digesters are of two types, rotary and stationary. The rotary type is horizontal and the stationary is vertical.
After the digester has been loaded with chips, the chemicals are introduced and the "cook" is carried on by means of high steam pressure. The strength of the chemicals, pressure of steam, and duration of cooking, are the principal factors in determining the result from any particular wood. Slow cooking at low temperatures yields the best results.
SODA PULP.—Soda pulp takes its name from the caustic soda which is used as a solvent. Rotary digesters are employed in its manufacture. The principal wood used for making soda pulp is poplar, though chestnut and aspen are also used. Soda pulp is soft in texture and of no great strength, but in combination with harder stocks it lends mellowness to the sheet. It is almost one-third cheaper than bleached sulphite pulp, quotations for February, 1915, being $2.20 to $2.35 per hundredweight, whereas bleached sulphite was quoted at $2.80 to $2.95 per hundredweight. The prices since the war have risen over 100% and were quoted in September 1919 at $4.75 to $5.00 and $5.75 to $6.50 respectively. One reason for the difference in price between soda and sulphite pulps, is that the soda is recovered from the spent liquor, whereas in the sulphite process the liquors go to waste.
SULPHATE PULP.—The solvent used in making sulphate pulp is a mixture of caustic soda, sulphide of soda and sulphate of soda. Sprucewood, largely, is used and the pulp produced is exceedingly strong. Unbleached sulphate pulp is used, notably, in the making of Kraft wrapping-paper. The soda is recovered from the spent liquors.
SULPHITE PULP.—Su'phite pulp is produced by the use of bisulphite of lime; this, being acid, necessitates a special brick lining in the digesters, which are of the vertical type. Sprucewood is the best raw material and yields a strong, fairly long fiber, capable of being bleached to a good white color.
MITSCHERLICH PULP.—A special method for making sulphite pulp was invented in Germany by Professor Mitscher-Lich. It varies from the ordinary process in that the cook is continued over four times as long under lower steam pressure, and yields a fiber of greater strength.
The steps subsequent to cooking chemical pulps of all kinds are similar. After emptying the digesters, the soft, discolored mass of fibers is washed and bleached. The yield of cellulose fiber is close to fifty per cent of the air-dry weight of the wood. The shives and undigested particles are re-moved by screening, and the pulp is either run out like ground wood on wet machines, or made up into rolls, or sheets, on a paper-machine. The soda pulp is shipped in rolls and the sulphite in sheets, as this is the most favorable form in which to handle them at the paper-mill. If the pulp is to be used on the premises, it is made up into laps on the wet machine and is not artificially dried. The so-called "air dry" pulp contains about 10% moisture, and pulp containing not over this amount of moisture is billed at its actual weight.
ESPARTO AND STRAW.—Esparto pulp is made by the soda process from a grass obtained in the circum-Mediterranean countries, and is used most extensively in England and some-what on the Continent, but freights have been prohibitive for American manufacturers.
Straw pulp is similarly made, and while occasionally used on medium grades of writing-papers, its principal use in this country is for strawboard and cheap wrappings It is expensive to reduce to a clean, bleached pulp on account of its knots, and the large quantities of silicious matter it contains.
WASTE PAPERS: The next largest source of paper-making fibers to wood is the waste paper, such as old books, magazines, newspapers, binders' waste, paper shavings and miscellaneous waste. This stock is collected by regular packers, sorted, and sold by grade to the mills.
The poorest grade consists of a mixture of miscellaneous papers of all colors and description. It is only used in the production of boxboards, sheathing paper, and other coarse varieties, and without undergoing any preliminary treatment it is shoveled right into the beaters.
A higher grade consists only of mixed papers, printed or unprinted. Next is a grade containing no ground wood or colored papers, and above this are graded old ledger and writing papers.
Paper trimmings are divided into four classes, white and mixed, soft and hard "shavings," and are especially available, as they may be used after sorting and dusting without under-going further treatment, but it is customary to macerate them in some sort of a pulper before placing with other stock in the beaters. The printed waste must be boiled in a solution of soda ash. This makes the ink removable. After about six hours' boiling, the stock is transferred to washers and treated like rags. The ink and dirt having first been removed, bleaching solution is introduced, and finally the stock is let down into drainers. In some mills the draining is omitted, the excess bleach is washed out and an antichlor added; then the stock is pumped over to a beating engine to be mixed with the other ingredients preparatory to manufacture. This process is less thorough, and there is more danger of getting residues of bleach into the paper, as it is rather a nice matter to exactly neutralize the bleach in the washer, and the maintenance of a uniform color is endangered.
Printers, or others, who accumulate large quantities of waste papers, will find that it pays to keep the various grades in separate receptacles, as a better price may be obtained for it in this way. Furthermore, by means of a baling press, the papers may be set aside in compact bales, which occupy less room and are not so great a fire risk as loose accumulations. The fact that 21.4 per cent of the paper-making fibers, according to United States Census Report, 1909, are derived from waste papers, indicates their importance as raw materials, while their use lessens the drain upon our forests.
THE NON-FIBROUS CONSTITUENTS OF PAPER
The non-fibrous constituents of paper are the mineral fillers, the ingredients for sizing, and the coloring pigments and dyes. Mineral fillers should not be regarded as adulterants. They are used, not as a means for adding weight, but for the sake of certain effects which are requisite in many papers. No filler is used on good writings or ledgers, as the printing requirements do not call for a closely filled surface or a mellow texture.
In book papers a varying percentage of clay is used, as it improves the printing quality by filling up the interstices between the fibers and increases opacity. Papers for half-tone printing require more filling, in order to have smooth, level surfaces.
There are several kinds of filler in common use. The most common is China clay, of which the cleanest and finest grades are obtained principally in England. No equally good deposit has yet been successfully developed in this country. Clay is a product of the natural disintegration of feldspar. It is soft, plastic, and non-crystalline.
Agalite and talc, which are silicates of magnesia, are also used. They are cheaper and less desirable, both on account of color and their crystalline nature, which is more or less damaging to cutter knives and printing-plates. These fillers are used widely in the cheaper book-papers, and can often be detected by holding a sheet against the light, as the little, translucent crystalline particles then appear like pinholes.
Sulphate of lime, commercially known under such names as gypsum, pearl hardening, satinite, etc., is a white, crystalline substance. This is used to some extent in paper-making, but principally as a coating.
Barium sulphate, prepared chemically, and known as blanc fixe, is used largely for coating papers because of its brilliancy and purity of color.
SIZING MATERIALS. — Starch was one of the earliest materials used for sizing paper, and is used considerably in addition to other materials, as it adds a hard, tinny character desired by the trade on certain grades. Silicate of soda is also used to impart similar characteristics.
Gelatine, or animal size, is obtained by boiling down suitable animal tissues. As a sizing agent, it is applied after the paper is made by passing the web of paper through a vat containing the hot liquid size.
Casein, which is sometimes used as sizing, Is more important in its functions as an adhesive for the making of coated paper. It is prepared by treating skim milk with weak acid.
Rosin size, the most widely used size, is produced from rosin by cooking with soda ash, which produces a soft soap. The soap when mixed with water by agitation assumes a milky appearance. In this condition it is poured into the beater after all other ingredients have entered, and is precipitated by the addition of alum as a resinate of alumina.
IMPURITIES IN PAPER.—Impurities, either chemical or physical, are sometimes found in paper, owing to lax methods or inferior materials.
Free acid occasionally occurs, and in some cases would be very deleterious. In papers that are to be bronzed, for example, this acid would tarnish the bronze. Needle papers, and paper for wrapping steelware, must be acid-free, other-wise they will cause rusting. The presence of free acid may only be determined by an analyst.
Sulphur, which may give rise to the formation of sulphuretted hydrogen, exists sometimes as an impurity in paper. It causes a brownish halo to appear around printed letters, because of its action on printing-ink. It would also cause oxidization of jewelry, mounted upon cardboard containing sulphur residues.
Free chlorine, or chlorine compounds, the result of inadequate draining of the stock, may cause final disintegration in the paper. It is the duty of manufacturers to guard against this and the other deficiencies noted.
Mineral impurities in paper are not uncommon. Minute particles of iron worn off the machinery, or getting into the stock in the shape of wire stitching, can often be discovered by the use of a magnet test. In photographic papers this must positively be excluded, but in most papers, if the particles do not show as specks, and are not large enough to make trouble for the printer, they are not a serious menace.