Rosin, sometimes called colophony, is obtained from the sap of certain firs and pine trees. This on distillation yields spirits of turpentine, leaving behind as a residue the mixture of substances to which is given the name rosin. It behaves as an acid, and therefore will combine with certain alkaline oxides, producing soluble resinates.
The nature of the rosin soap used in the paper mill varies according to the conditions under which the size is prepared. If a large proportion of rosin is used, then the size obtained consists of a mixture of resinate of soda together with free rosin dissolved in the solution. If the proportion of rosin is small compared with the amount of carbonate of soda, the composition of the final mixture is quite different. The difference in treatment results in the formation of—
(A) Neutral Size, prepared by boiling a known weight of rosin with sufficient alkali to combine with it and form a neutral resinate of soda. Theoretically this may be obtained by using 630 parts of rosin to 100 parts of soda ash. It is doubtful how far the reaction is completed so as to produce an exactly neutral solution containing only resinate of soda.
(B) Acid Size.—When the proportion of rosin is largely increased the soda becomes converted into the alkaline resinate, and the excess of rosin is gradually dissolved in the resinate formed.
The practical operations necessary for the preparation of the size are comparatively simple. In the case of size containing relatively small percentages of free rosin, the boiling is conducted in open vessels, but for the manufacture of rosin size containing large proportions of free rosin boiling under pressure in closed vessels must be resorted to.
With the open pan process a steam jacketed pan is used, and the required quantity of alkali, dissolved in water, is placed therein and heated to boiling point. The rosin well powdered is added in small quantities from time to time, this being effected cautiously in order that the carbonic acid gas set free during the process may readily escape. The rosin is generally completely saponified after four or five hours' boiling. It is then passed through strainers into store tanks, from which it is drawn into the beating engines as required.
In the case of rosin boiled under pressure a cylindrical vessel provided with a manhole at the top is used. The correct amounts of alkali and water are put into the digester, and also the rosin in a powdered form, the digester being fitted with a perforated plate placed about two feet above the bottom of the vessel in order to prevent the rosin forming into a hard mass at the bottom of the digester.
It is possible in this way to manufacture a thick size containing 30 or 40 per cent. of free rosin and a comparatively small proportion of water. Many paper mill firms prefer to purchase such size ready made.
The most recent modification of the ordinary rosin size is a compound prepared by treating rosin with silicate of soda. This alkali dissolves rosin readily, and the soap obtained when suitably diluted with water decomposes in the beating engine on the addition of aluminium sulphate, with the precipitation of a gelatinous silica which assists in hardening the paper.