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.