Paper read by J. A. DeCEW at the meeting of the American Chemical
Society.
It is quite to be expected that in any class of chemical reactions
which occur under such varying conditions as those existing during the
precipitation of rosin size in paper mill practice, there will be among
the various investigators a certain amount of disagreement regarding
the theoretical explanation of what actually takes place. There is
also some confusion resulting from the termology which is commonly
used in describing sodium resinate compounds existing in rosin size,
as for example, in the use of the phrase “free rosin”. The purpose
of the following remarks is not to recapitulate the work of other
investigators, nor to criticize their conclusions, but to submit a
short discussion of the effect upon the chemical reactions involved, of
the various physical conditions in which the material may be used.
It is a well known fact that a rosin soap will easily dissolve an
equivalent amount of rosin to that which has been saponified, and
this extra rosin, whether in solution in the size wax, or whether in
suspension in a diluted solution, is still called “free rosin” in the
termology of the trade. In order, however, to distinguish between the
various states in which the rosin might exist, it should be divided
into three classes, consisting first, of dissolved rosin, second,
colloidal rosin, and third, rosin in suspension. The reason for this is
that before the rosin soap can be used in the art of paper making it
must first be brought into dilute aqueous solution.
If the soap should be readily soluble, then dilution may take place
in cold water and consequently the diluting can be carried out within
the beater itself. On the other hand, if the rosin soap is not
readily soluble in cold water, owing to the fact that it contains a
considerable quantity of dissolved rosin, it is necessary to bring
it into a sufficiently dilute condition so that no further material
separation of rosin will take place when it comes in contact with the
paper stock. Obviously the difficulty of carrying out this operation
increases in proportion with the amount of extra rosin which is held in
solution in the rosin soap.
Authorities disagree as to whether abietic acid is mono-basic or
dibasic and it cannot be stated definitely whether rosin which is in
complete solution in a rosin soap is there in the form of an acid
resinate, or whether it is merely dissolved rosin. It seems to the
writer that a fairly intelligible conception is obtained by assuming
that a sodium resinate containing rosin in solution, is in fact an acid
resinate of the alkali metal and that from this solution insoluble acid
resinates of the heavy metals can be produced.
Some interesting data on this subject is recorded by E. O. Ellingson in
a paper before the American Chemical Society, 1914, the subject being
“Abietic acid and some of its salts.” In this investigation he shows
clearly that certain insoluble acid abietates were formed when a dilute
aqueous solution of sodium abietate was added in small proportions to a
dilute solution of a metallic salt.
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.
It would seem therefore that the real necessity for the use of sulphate
of alumina for precipitating the rosin is not so much the necessity
for forming acid aluminum resinates, but the fact that the rosin
precipitated in this way has a more colloidal character, than that
thrown out of solution by other coagulants, and consequently will
have greater covering power and efficiency as a water repellant. This
explanation is opposed to the theory that rosin acids in the form of
emulsion or suspensions are efficient sizing agents, for it is obvious
that visible floating rosin has lost its colloidal character and its
covering power.
The fact is that the so called free rosin emulsions, when properly
made, contain but a very small amount of rosin acid in the emulsified
form, practically all of it remaining in solution in the dilute soap.
The art of preparing good rosin size emulsions (using the term as
generally understood) is therefore the ability to dilute a solution of
rosin acids, without the actual liberation of rosin in the emulsified
form. The difficulty in doing this will explain the erratic results
obtained by Remington and other investigators when endeavoring to
determine the effect of rosin size containing dissolved rosin.
The laboratory difficulties involved are shown by an article by Otto
Kress and R. T. Struthers, published in Paper, April 1913. Their
results show that from a rosin saponified with 15% of sodium carbonate,
over 98% was obtained by them in hot dilute aqueous solution, and
that from a rosin saponified with 10% of sodium carbonate, only 50.6%
was brought into actual solution in hot water. It is quite possible
however, to dilute such a rosin soap holding in solution about 45%
of rosin acids, to an aqueous solution of 2% solids, without having
any of the rosin become insoluble. In this condition all of the rosin
acids can enter into chemical reactions with other solutions and will
precipitate from solution in a very bulky colloidal mass.
Between the extremes of physical condition just described, there are
a great number of intermediate stages. The rosin acids may be partly
liberated by dilution in the form of small visible floating particles
and coarse granular masses and a part may be in a state of colloidal
solution. It is safe to say that all rosin particles which are
sufficiently coarse to be classed as suspensions, have lost the greater
part of their sizing value. That portion of the rosin acids which is in
colloidal solution is still effective for sizing purposes because it
has the property of becoming fixed upon the fibres by absorption. This
action can only take place, however, when the paper stock is free from
such electrolytes as may discharge the colloid before it reaches the
fibre. Dilute acid resinate solutions may contain variable proportions
of dissolved acid resinates, colloidal rosin, and rosin suspensions,
and the relative proportion of these is what determines the basis of
its waterproofing possibilities. Assuming that the sizing value of
these solutions varies directly with the amount of rosin acids that are
in true and colloidal solution, we have a measure of efficiency which
checks very closely with actual mill results.
The maximum amount of rosin acids that can be held in stable solution
in a diluted rosin soap of from 1% to 2%, total solids, is about 50%
of the total rosin content. In such a solution there is always a
slight tendency towards hydrolysis which increases with the amount of
dilution, but the fact that these solutions when once prepared can be
then boiled without decomposition, shows that the solutions are fairly
well stabilized and also that there can be very little rosin then
present in the colloidal form.
The conclusions which it is desired to submit as offering a
satisfactory explanation of practically all the phenomena in connection
with sizing paper with rosin is as follows:--
(1). That the rosin acids which are precipitated from dilute solution
by means of a coagulant which will deposit the rosin in a colloidal
mass, is the material which when properly incorporated into the paper
stock and dried therein, produces the water resistant characteristic
known as sizing.
(2). That the results obtained from a given quantity of material are
largely dependent upon the character of the rosin colloid and its
treatment during the process of manufacturing the paper.
(3). This product can be obtained in limited quantity from a neutral
resinate, by the use of large excess of alum, or it may be obtained in
large proportions from an acid resinate and a relatively less excess
of alum. The maximum obtained from a neutral resinate being about 40%
of the total rosin, and from an acid resinate about 70% of the total
rosin, when a sulphate of alumina containing no free acid is used.