Sizing.
Rosin (colophony) consists of a mixture of pinic and sylvic acids, which, when heated with a solution of carbonate of soda, combine with the alkali to form a soap, carbonic acid being evolved. The soap is prepared in the following manner:—
Ordinary rosin, the quality depending upon the quality of paper, is boiled in a jacketed pan for from two to three hours with a solution of carbonate of soda, until a sample of the soap formed is completely soluble in water. The quantity of carbonate of soda (Na2CO3 10 H2O) required is about 30 per cent. of the weight of the rosin. It is usual to employ crystallised carbonate of soda (soda-crystals), but {138} soda-ash of good quality, or even caustic soda, might very well be employed, in which case of course the proportion would be different.
A very convenient form of carbonate of soda has lately been introduced by Messrs. Gaskell, Deacon & Co., which goes by the name of “Crystal Carbonate.” It is obtained in the form of minute crystals of the mono-hydrate, Na2CO3 + H2O. It contains 50 per cent. of alkali (Na2O), and possesses this advantage over soda-ash, that is dissolves readily in water without forming a hard cake. It is much cheaper in proportion than soda crystals, as the expense of crystallisation and carriage is saved. The latter contain only 21·68 per cent. Na2O, 62·93 per cent. of their weight consisting of water of crystallisation. One hundred parts of soda crystals are equivalent to 43·36 parts of the “Crystal Carbonate.”
An excess of soda should be carefully avoided, as it consumes an equivalent quantity of alum (see below): on the other hand, it is very essential that the rosin should be completely dissolved, otherwise small particles are sure to find their way into the pulp, and would form clear transparent specks in the finished paper.
The boiling being completed, the charge is run off into iron tanks and allowed to settle; the soap forms a semi-solid mass, while a dark-coloured liquor, containing the impurities of the rosin, rises to the surface, and can thus be removed. The soap so purified is next dissolved in hot water containing a small quantity of carbonate of soda, in case complete solubility has not been attained, and is then mixed with a quantity of starch paste prepared in a separate vessel by dissolving starch in boiling water. The mixture is then carefully sieved and is ready for use.
The proportion of starch to rosin differs in nearly every mill, and also the quantity of size to be added to the beater varies according as the paper is required to be soft or hard-sized. About 3 parts of starch to 1 of rosin, and between 3 or 4 lbs. of the mixture to 100 lbs. of pulp may be considered an average quantity. Some manufacturers prefer to add the {139} starch and rosin size to the engine separately; others again do not dissolve the starch in water, but merely make it up to a thin paste, in which state it is added to the pulp. This method is employed by makers of very fine papers, as it is said to give a certain feel to the paper which cannot be obtained in any other way. The method is costly, as only a small proportion of the starch added is actually found in the paper.
Some papers, which are not intended to be sized, such as blotting and filter-papers, are made with the addition of starch only, this being used to bind the fibres together to some extent. The presence of the starch does not prevent the paper from being absorbent. Papers which are intended to be tub-sized only are usually made up with a small quantity of starch, and are sometimes sized to a small extent with rosin also. The addition of a certain amount of ordinary soap to the rosin soap is said to give good results; it enables the paper to take a higher finish when calendered. Sizing with animal size (gelatine), or Tub-sizing as it is called, will be described in Chapter X.
The mixture of size and starch may be added directly to the engine, or it may be previously dissolved in water; the latter method is perhaps preferable. After allowing it to mix thoroughly with the pulp, a solution of alum is run in. It is made up with boiling water in either lead or copper tanks. Iron or zinc vessels must be avoided, as the solution acts rapidly on these metals.
Alum consists of a double sulphate of aluminium and potassium or ammonium, Al2 3 SO4, K2SO4 + 24 H2O; its function being to form with the rosin acids insoluble soaps which are precipitated in intimate mixture with the pulp, the sulphate of potash taking no part in the reaction. The choice of a suitable alum is a matter of very great importance; it should be free from excess of sulphuric acid and from iron. The former is deleterious on account of its action upon the colouring matter used to tone the paper, some colours being completely discharged by it; and because {140} of its effect upon any metal work with which it may come in contact, especially upon the brass wire-cloth on which the paper is made. The iron is objectionable as it forms a dark red precipitate of oxide of iron.
Alum as supplied by makers of repute is generally sufficiently pure for even the best classes of paper.
Many years ago a substitute for alum was introduced, called Pochin’s aluminous cake. It consisted principally of sulphate of alumina Al2 3 SO4, and was, provided it was pure, as suitable for the papermaker as the more expensive potash alum. It was more economical at equal price, as it contained about 14 per cent. of alumina, whereas potash alum, even if absolutely pure, could only contain 10·85 per cent. Pochin’s aluminous cake however, was liable to contain a considerable amount of free sulphuric acid and soluble sulphate of iron. Of late years, a number of different varieties of sulphates of alumina have been introduced into commerce, of so pure a nature, and at such a low price, that it is a matter of wonder that they have not entirely superseded the more expensive alum. They are prepared either from very pure native alumina (bauxite), or from China clay of good quality. Methods have been discovered of eliminating almost the whole of the iron and free sulphuric acid. The following analyses of different specimens sufficiently indicate their purity.[12]
| Alumina (Al2O3) | 14·84 | 14·70 | 14·95 | 14·85 | 16·00 | 16·20 |
|---|---|---|---|---|---|---|
| Ferric oxide (Fe2O3) | ·06 | ·12 | ·05 | trace | nil | trace |
| Sulphuric acid (SO3) | 35·00 | 34·60 | 36·09 | 34·94 | 38·00 | 38·00 |
| Free sulphuric acid | ·32 | ·40 | nil | ·29 | nil | nil |
| Lime (CaO) | ·11 | ·11 | ·17 | ·14 | ·16 | ·14 |
| Water | 49·42 | 49·95 | 48·72 | 49·60 | 45·50 | 45·43 |
| 99·75 | 99·88 | 99·98 | 99·82 | 99·66 | 99·75 |
[12] Journ. Soc. Chem. Ind., Jan. 1886, p. 16.
Sulphate of alumina possesses this advantage over alum, that it is more soluble in water, and thus a stock solution of considerable strength can be prepared. It is soluble in {141} 2 parts of cold water, alum requiring 18 parts for complete solution.
The amount of alum or sulphate of alumina added to a pulp is largely in excess of the quantity necessary to precipitate the rosin soap; as a matter of fact, in the case of esparto or straw pulps, for the bleaching of which considerable quantities of bleach have been employed, and which therefore contain a certain amount of basic lime, together with calcium chloride, complete precipitation of the size is effected without the addition of alum. A certain amount is also required to precipitate the starch. The excess of alum appears to be necessary, however, not only to brighten the colour of the paper, but also to render it capable of resisting the action of ink. From experiments made by the authors, it appears that one part of rosin requires 2·9 parts of alum for complete precipitation from its solution in soda. One part of starch requires 0·40 part alum.