The salts of Chromium, Manganese, Nickel, Iron, Cadmium, Cobalt, Strontium, Copper, all gave precipitates carrying an excess of abietic acid. The one exception was the Salt of Aluminum, which under exactly the same conditions produced a basic aluminum abietate. From this it is proven that a neutral sodium resinate solution when poured into a dilute solution of sulphate of alumina, will always produce a precipitate of basic aluminum resinate.
On the other hand, the investigations of Naugebauer, republished in Paper XI., 10-17, shows that a neutral resinate when precipitated with a considerable excess of sulphate of alumina, will produce an acid precipitate containing approximately 33 per cent. of rosin excess, and with the maximum amount of alum the rosin acid in the precipitate does not exceed 41 per cent.
If we can accept the results of this investigator then it is evident that insoluble acid resinates containing a limited amount of rosin acids can be produced from a neutral sodium resinate by precipitation with even an aluminum salt.
The results produced with sulphate of aluminum therefore, will depend largely upon the mass action of the materials, chemical equilibrium being established in accordance with the relative amount and acidity of the alum used. In short, if 100 grs. of rosin in the form of a neutral resinate is precipitated with approximately 33 grs. of alum, we will have as a result, a basic alum resinate. If, on the other hand, it is precipitated with 330 grs. of alum, we would have an acid resinate of alumina containing approximately 40 per cent. of rosin acid. With less alum excess the amount of rosin acid in the precipitate will be proportionately less.
If the basic aluminum resinates were a satisfactory water repellant then the problem of paper sizing would be a very simple one, and all that would be necessary in practice would be to use the size and alum in proper chemical equivalents. All experience shows however, that when using a neutral resinate for sizing, it is necessary to use a large alum excess in order to obtain a sufficiently water repellant condition in the paper. The inference is that the insoluble acid resinates are essentially the agents which impart to the paper that resistance to aqueous penetration called “Sizing.”
Remington and his associates claim that resinate of alumina only, is formed when a neutral sodium resinate is precipitated with alum, even if the alum is used in excess, but that it is decomposed by extraction with alcohol and that this fact leads others to believe that a portion of the rosin is uncombined. These investigators publish the result of 50 tests for sizing paper, from which they draw their conclusions, but it would seem that their methods of making the tests were quite inefficient, inasmuch as they used not less than 5 per cent. of rosin, and as high as 12 per cent. without always getting sizing results. Now, in mill practice, a very poor size should give results with 3 per cent. of rosin, while an efficient size should produce a very hard-sized paper with an equivalent amount. It would seem unwise to form any fixed conclusion from tests which gave such unsatisfactory results.
Other investigators such as, Emil Meuser and Naugebauer, (Paper, June 25th, 1913), and also Otto Kress & Struthers (Paper April 16th, 1913), have demonstrated by exhaustive tests that rosin acids are liberated from a neutral resinate when alum is used in excess and that the amount of these rosin acids may be from 33 per cent. to 41 per cent. of the total rosin, depending upon the alum excess used.
If an acid resinate of alumina containing 40 parts of rosin acids, can be produced from 100 parts of neutral resinate of soda and 330 parts of sulphate of alumina, then 20 parts of alum will be required to produce the same results from an acid resinate of soda, containing 40 per cent. of rosin acids, or with 200 parts of alum one can produce from this an aluminum resinate with 64 per cent. of rosin acid.
These highly acid resinates are found to be very colloidal in character and have great capacity for distribution within the paper pulp. They also show considerable resistance to dehydration and are thus able to retain their plastic character while the paper is being dried. Such are the properties that these highly acid resinates seem to possess in addition to their water repellant characteristics.
It has been demonstrated in paper mill tests that the rosin acids alone are thrown out of solution from a rosin soap by means of acid, can also produce sizing results providing that the rosin acids precipitated have a similar colloidal character to the aluminum precipitate. The practical difficulty, however, of obtaining colloidal precipitates when using acid, makes this practice a very uncertain one, for it would be only under very favorable circumstances that this practice could be carried out with success. The same difficulty is experienced when other metallic salts, (e.g.,) the salts of iron or calcium, are used to replace the aluminum sulphate, for the precipitates from these are much more dense and granular than those derived from aluminum.