_COAGULATION_
Whether it is necessary to employ any coagulant, or whether latex should be
allowed to coagulate naturally, will not be discussed at this stage.
Neither will mention be made of any patent processes of coagulation which
employ other than acid mediums. These subjects will be treated in a
subsequent section of the book.
CHOICE OF COAGULANTS.--It is not proposed here to enter into a discussion
as to the merits of the dozens of known coagulants. Suffice it to state
that acetic acid, although the oldest general coagulant, still remains the
best and safest at the present time. There is a deal to be said in favour
of the use of another organic acid, formic acid. It is equally as safe as
acetic acid, and quite efficacious; the only drawback is that, taking all
things into consideration, it is very slightly more expensive. Acetic acid,
therefore, will always be implied in this chapter when the word "acid" is
used.
STRENGTH OF ACID SOLUTION.--In the old days it was the rule rather than the
exception to find pure, undiluted acid used in coagulation. In many cases
no harm resulted, for the simple reason that, owing to the large proportion
of water in the latex, the acid was thereby very much diluted. The estates
had to thank the over-dilution of the latex for the non-injury of the
resulting rubber.
Some estates make up a stock solution of 1 part acid to 20 of water, and
use this with success because of the fair amount of added water present in
the latex.
It must be understood that what is being referred to now is not the
absolute quantity necessary for coagulation, but the proportions--_i.e._,
the respective volumes of acid and water in the solution of acid made up
every day. That the strength of the acid solution, as well as the quantity
used, has an effect upon coagulation can be easily demonstrated in the
following way:
Take separate and equal lots of the same latex, and to each add the same
quantity of pure acid, but in each case diluted with varying quantities of
water. It will be found that coagulation is quickest where pure acid is
employed, and slowest where the acid is most dilute. It will also be found
that, providing the quantity of acid employed was sufficient for
coagulation, the best and most uniform coagulation is obtained from the use
of the most dilute acid, within limits. It will often be found that where
pure acid has been employed coagulation is local--_i.e._, we have lumpy
coagulation, and often a very milky remaining liquor. This is due to the
fact that, as coagulation is immediate upon the spot which is first touched
by the pure acid, a deal of the acid is enclosed within the rubber at that
spot, and hence other portions of the latex are deprived of acid. It is in
such cases that most air-bubbles are enclosed.
As the dilution of the acid solution is increased the mixing is more
thorough and uniform. Coagulation is slower, and air-bubbles can escape to
the surface.
METHOD OF MAKING STOCK SOLUTION.--Experiments have been repeatedly made in
the laboratory with acid solutions of varying dilution, from pure acid down
to 1 part of acid in 500 parts of water. While it has been found that a 1
in 5 solution can be used where the latex is very dilute (say, 1 part of
latex to 5 parts of water), and a 1 in 20 solution may be used in fairly
dilute latex (for crepe-making), it is undoubtedly a fact that for latex as
generally "standardised" on estates a much more dilute solution of acid
should be used--_e.g._, 1 in 100, or even 1 in 200, of water. It must be
borne in mind that the quantity of acid necessary for coagulation is not
changed, but merely the dilution. Let us take a concrete case to illustrate
the point:
On an estate at present the stock solution is made up by diluting 1
pint of acid with 20 pints of water, and 1 gallon of this is necessary
to coagulate 50 gallons of pure latex.
It is desired to use a stock solution of 1 pint of acid to 100 pints
of water. Evidently, therefore, 5 gallons of this stock solution
contain only the same quantity of pure acid as 1 gallon of the old
solution contained, and it will be necessary to add 5 gallons for
every 50 gallons of pure latex. Thus:
1 to 20; 1 gallon necessary for 50 gallons pure latex.
1 to 100; 5 gallons necessary for 50 gallons pure latex.
It may be pointed out that the quantities worked out in the foregoing
examples are not absolutely and mathematically correct, but they are quite
close enough for all practical purposes.
It may be advanced by someone that if a dilute solution of acid, such as 1
in 100, is used the bulk of this stock solution (5 gallons to 50 gallons of
latex) is very great, and might be injurious to the quality of the
resulting rubber. A moment's consideration will show that, after all, the
volume of acid solution is only one-tenth that of the volume of latex. This
can have no effect upon the quality of the rubber. Even dilution of the
pure latex with half its bulk of water in the factory will have no effect
upon the quality of the resulting rubber. It is to be remembered that,
except in cases where the proportion of added water to latex is absurdly
large, the main argument against putting water into the latex-cups is
against the possible poor quality of the water rather than against the
actual small quantity theoretically added. It is acknowledged that, where
the water to be put into the cups can be guaranteed to be of good quality,
no great objection would be raised against placing the smallest possible
quantity of such water in the cups. But how many estates have such good
water easily available to the coolies, and how many estates can be sure
that only that smallest possible quantity would be used? It is a notorious
fact that, even on estates where the quantity of water used was supposed to
be a minimum, the proportion of water to latex in some cups often exceeded
even three or four to one. In any case it may be stated as an elementary
truism that the absence of water is more to be desired than water of
doubtful quality.
QUANTITY OF ACID.--As a result of repeated experimental work it has been
found that, for pure average latex, the quantity of acid necessary for
complete coagulation, reckoned in parts of pure acid to parts of latex, is:
1 part pure acid; 1,000 parts average latex.
Where the latex is rather richer than average (above 30 per cent. dry
rubber) probably a little more acid would be required, and similarly if the
dry rubber content is lower the quantity of acid must be less.
It used to be a common belief that the more dilute the latex the greater
the quantity of acid necessary, but this would only apply to cases of
extreme dilution of latex.
As a matter of fact, up to certain limits of added water, the reverse is
actually the case--_i.e._, the more water in the latex the less acid must
be added, assuming that for pure latex the proportion of pure acid to latex
is taken as 1 part to 1,000 parts. This was found to be the case up to
dilutions of three or four times the volume of latex. To take concrete
examples which will perhaps make the truth more clear:
Assuming we commence by making up our stock solution of acid by adding
100 parts of water to 1 part of pure acid, this gives us a mixture of
1 to 100. For 1 gallon of pure latex it would be necessary to add
one-tenth of its volume of the above mixture--_i.e._, 16 ozs.
Suppose we take a gallon of pure latex and add a gallon of water, we
now have 2 gallons of so-called latex. But we still have only 1 gallon
of real latex present in the diluted latex, and it is only necessary
to add sufficient acid to coagulate this gallon--_i.e._, 16 ozs.
Further, if 1 gallon of latex be diluted with 2, 3, or even 4 gallons
of water it is still only necessary to add 16 ozs. of the acid
mixture.
At dilutions beyond this limit, however, it is necessary to add a
little more acid to obtain complete coagulation.
In the process of preparing sheet rubber it is very necessary to see that
the minimum quantity of acid is used, otherwise visible defects are caused.
But in coagulating latex intended for preparing crepe, where the rubber
undergoes protracted washing on the machines, the presence of a slight
excess of acid in coagulation is not calculated to cause any deterioration
in the quality of the rubber. Advantage must not be taken of this statement
to argue that more than a slight excess may be used without injury to the
rubber, as it can be shown that the use of a large excess of acid results
in an inferior rubber.
QUANTITIES NECESSARY FOR MODERN REQUIREMENTS.--It may be commended to the
notice of the beginner that any further experimental work as to the
quantity of acetic acid necessary for complete coagulation would only
involve a waste of time and energy.
The general figure given in a preceding paragraph (1 part pure acid to
1,000 parts of latex) may be accepted as the rough basis for working. In
modern practice, however, undiluted latex is usually diluted to a standard
which may vary on different estates from 1-1/4 lbs. to 1-1/2 lbs. dry
rubber per gallon.
Latices of these strengths can be coagulated at a ratio of 1 part pure acid
to 1,200 parts of standardised latex; and this quantity need not be
exceeded, except in cases where an appreciable amount of some
anti-coagulant is present in the latex. The proportion may then be raised
to 1 in 1,000.
If considered advisable the acid may be used in a 1/2 per cent. solution
for sheet preparation; but in any case it is advised for the sake of
uniformity that a 1 per cent. solution should be employed in the
preparation of both sheet rubber and crepe rubber. In most modern
factories, measuring vessels of various capacities are to be found, and it
is always more satisfactory to have the solution made up in approximately
correct strength at the rate of 1 oz. of pure acid to 5 pints of water.
Often, however, on some estates European supervision of this work is not
possible, and the preparation of the acid solution has to be left in the
hands of a (more or less) skilled coolie. It is thus necessary to find some
less fine, but still approximately correct, method of procedure. In the
East the kerosene tin is in universal favour for the carriage of water, and
there is no reason why it should not be utilised as a standard measure for
preparing the dilute acid solution, _providing it is not allowed to become
rusty_. The capacity of the tin is 4 gallons (640 fluid ozs.), so that a
one-hundredth part would be approximately 6-1/2 ozs. It is suggested that
this quantity should be measured out by means of a glass graduated vessel,
and then that an aluminium cup should be cut down so as to hold the exact
quantity.
This would reduce the making of a solution, sufficiently approximate to 1
per cent. strength for all practical purposes, into a simple operation of
mixing pure acid and water in the ratio of one cupful of acid to 1 kerosene
tin of water.
The actual quantity of solution required for the coagulation of any volume
of standardised latex can be calculated easily from the ratio 1:1,200. As
the strength of solution is 1:100 it will be seen that the quantity to be
taken is _always one-twelfth_ that of the volume of latex--_e.g._:
(_a_) If the latex tank holds 90 gallons of standardised latex, 7-1/2
gallons of dilute acid solution are required.
(_b_) A tank containing 120 gallons of latex would need 10 gallons of
the 1 per cent. acid solution.
It is assumed that all estates, not only in the preparation of sheet
rubber, but also in the making of crepe rubber, always employ the system of
standardising latex in order to obtain uniformity. They are ill-advised if
they do not follow this practice; but in case average undiluted latex is
treated in coagulation, the quantity of acetic acid to be used should be
calculated from the ratio 1:1,000.
If the acid solution is to be employed in 1 per cent. strength, _one-tenth_
of the volume of latex to be treated will indicate the required quantity of
solution necessary for complete coagulation unless anti-coagulants have
been used, when the quantity must be increased as experience directs. It
will be recognised, of course, that undiluted latex may only be used in any
case for the preparation of crepe rubber; or in some exceptional case, such
as the special preparation of "slab" rubber.
CARE IN MIXING.--It is essential that the mixture of dilute acid and latex
should be thoroughly intimate. This can only be attained by careful
manipulation, especially in the case of sheet preparation. Where crepe
rubber is to be made it may be permissible to employ a solution stronger
than 1 per cent., but it is not advised. The acid should be poured into
the latex while stirring, and the agitation should continue for such a
period as to ensure thorough mixing in all parts.
It will be appreciated that in the preparation of sheet rubber this period
may not be unduly prolonged, otherwise the latex will have begun to
coagulate before skimming and the placing of the partitions in their
respective slots can be effected. Furthermore, while in the preliminary
treatment for crepe rubber, the formation of enclosed bubbles and surface
froth is immaterial. For sheet preparation it is essential that the
stirring shall be done so carefully as to try to avoid internal bubbles and
to reduce surface froth to a minimum. For crepe-making a perforated board,
with handle attached at right angles to the face of the board, may be used;
but in shallow sheet-coagulating tanks, broad paddles (which may or may not
be perforated) give good results as long as there is a sufficient number
used to cover the area of the tank in reasonable time. Obviously also,
where the area of any tank or compartment is of any appreciable size, the
dilute acid solution should be poured in from various points so as to
obtain a good even distribution. In some cases the acid is distributed from
a sprinkling can, but this is a refinement which experience shows to be
unnecessary. In actual practice, working on a tank measuring 12 ft. by 4
ft., no difficulty is found if coolies pour in acid solution from four
points. The degree of success depends entirely upon experience and
efficient supervision. This remark applies equally to the use of various
devices, such as rakes with broad teeth, used as stirring implements. There
is room for display of ingenuity in this direction, and it is found often
that, while they are used successfully on one estate, they may be condemned
on another.
[Illustration: TWO VIEWS OF DILUTION AND MIXING TANKS.
Below, on the right, coagulating tanks. At the far end strainers. Each
dilution tank is of equal capacity to the corresponding coagulating tank.]
USE OF SODIUM BISULPHITE.--Some few years ago a demand for pale crepe
rubbers sprang up, and this demand has been maintained. The total quantity
of pale rubber put on the market previously could only have amounted to
very little, and that little was obtained by luck and various tricks in
manipulation. It must be premised that if coagulation is allowed to take
place, either naturally or with the aid of acetic acid, the resulting
rubber will almost inevitably oxidise on the surface, except in the cases
of very dilute or young latices. Even supposing that this darkening of the
surface does not take place in the wet stage, it is often found that a
rubber expected to dry to a pale colour does not fulfil expectations, and a
dull neutral shade results. This darkening of crepe rubber may be
attributed to a slow process of oxidation, which continues until the rubber
is dry. From these remarks it will be seen that the process of oxidation is
a natural one, and that any pale rubber formerly shipped was the outcome of
circumstances outside the control of the estate, except in such cases where
boiling of the coagulum, etc., was resorted to. The fact that one rubber
happened to be a shade darker than another was absolutely no criterion as
to the value of the rubber, but apparently the market thought, and still
thinks, otherwise, although the actual necessities of manufacturers for a
pale crepe to be employed in special processes must be comparatively small.
The prevention of this natural oxidation was a problem which exercised the
minds of all responsible for the preparation of pale rubbers, and much time
and thought were expended upon it. Various theories were propounded, and
the chief conclusion arrived at was that the darkening of rubber was to be
prevented by excluding all the light possible from the drying houses. To
this end windows were to be kept shut, or else they were provided with
ruby-coloured glass, which incidentally kept out the air. In spite of these
precautions, little success attended the expenditure of so much energy and
thought. It was absolutely necessary that some chemical agent should be
discovered which would make the preparation of pale crepe possible for any
estate. This chemical would have to fulfil several requirements before it
could become popular:
1. It must be a simple substance capable of being easily handled.
2. It must be very soluble, so that solutions could easily be made up
by inexpert workers.
3. It must be cheap.
4. It must be quite innocent of any harmful effect upon the quality of
the rubber.
After months of investigation into the properties of other chemicals the
writers decided that the only one which satisfactorily answered all
requirements was sodium bisulphite. The writers make no pretension to any
claim of having discovered the properties of this substance, which was a
common chemical, and widely known. Even its action on latex was suspected
before they engaged upon the work. These matters are only mentioned because
the credit, if any, should be given to the laboratories of the Rubber
Growers' Association.
As soon as it began to be known on the market that sodium bisulphite was
being used in the preparation of pale crepe, a great outcry was made, and
estates were warned that no more rubber prepared in this way would be
accepted. It was said that the chemical would destroy the "nerve" of the
rubber,[2] and it was definitely stated that rubber prepared with this
chemical was brittle. It must be remembered that brokers had some
legitimate excuse in raising objections to the introduction of new and
strange chemicals for preparing rubber, as they were quite without means of
judging whether the rubber had suffered harm or not. Still, on the other
hand, private tests had been made in conjunction with Messrs. Beadle and
Stevens for fully eight months before the name of the chemical was
mentioned in reports, and they had decided from the results of
vulcanisation tests that the chemical was quite innocuous. Then, and only
then, did we consider it advisable to recommend the use of sodium
bisulphite in general estate practice. Owing to the initial prejudice
against rubber prepared with sodium bisulphite, the results of our
preliminary work were published by permission of the Rubber Growers'
Association.[3] The original instructions to estates regarding the proper
employment of this chemical were given in the private reports issued by the
Rubber Growers' Association in 1911. At the present time it is probably
accurate to state that it is now used by all estates preparing fine crepes.
Representatives of manufacturers have sometimes given us to understand
that the question of paleness of colour in such rubber is of no such
importance as is impressed upon us as producers. While we are prepared to
believe, we can only plead that from our point of view the supply arises
from the demand. Such are the conditions governing the sale of rubber that,
irrespective of the requirements of the ultimate user, we have to market
rubber which is valued almost completely upon its appearance at the time of
sale.
[2] Williams, International Rubber and Allied Congress, London, 1914.
[3] "The Employment of Sodium Bisulphite in the Preparation of Plantation
Rubber," Beadle, Stevens, and Morgan, _India-rubber Journal_, August 2,
1913.
As long as such conditions prevail estates must continue to adopt any
device of proved harmlessness, in order to obtain the best possible price
for their product, and not because we desire to continue a practice which
some assure us to be unnecessary, and which, moreover, adds somewhat to the
cost of production.
QUANTITIES OF SODIUM BISULPHITE.--It must be premised that, although sodium
bisulphite is employed on some few estates in the preparation of sheet
rubber, we do not advise the practice. It is unnecessary, and may lead to
some little trouble and delay in drying. In any case, sodium sulphite gives
the results desired for sheet rubber (see following). It must be
understood, therefore, that we are concerned here, in the case of sodium
bisulphite, with its employment in the preparation of fine pale crepe only.
As the dry rubber contents of latices vary with the age of the trees, the
general health of the trees, the seasons and general climatic conditions,
the relative strain imposed by depletion of reserves through tapping, etc.,
it will be clear that the effect produced by a definite quantity of sodium
bisulphite in any given volume of latex will also vary--_i.e._, the effect
depends upon the potential amount of rubber present. A dilute latex needs
less sodium bisulphite than a richer latex to produce the same effect in
colour.[4]
[4] Incidentally there are certain occasions, as in the opening of areas of
bark rested for long periods, when the latex is of a rich yellow colour.
Sodium bisulphite will not "bleach" this colour, and it is well to remark
again at this stage that the action of the chemical is only to avoid or
arrest oxidation (darkening).
Hence it follows that if in any factory uniform quantities of the solution
are used for any given volume of undiluted latices from different areas of
the estate, the effect upon the dry rubbers will vary. This explains why
some estates obtain different shades of rubber in their fine pale crepes.
The remedy obviously is to reduce the variation in latices by diluting them
all to a standard rubber content as is done in sheet preparation. One is
thus assured that the prescribed quantities of sodium bisulphite will meet
requirements in every case, and that waste will be avoided.
Working with a standard of 1-1/2 lbs. dry rubber per gallon the following
formula should serve as a _maximum_:
_Formula for Use of Sodium Bisulphite._
(_a_) Dissolve sodium bisulphite in water at the rate of 1 lb. to 10
gallons.
(_b_) Of this solution use 1 gallon to every 10 gallons of latex.
MAKING A SOLUTION.--The making of a solution of the chemical would seem to
be a simple matter, but to judge by the ill-effects sometimes observed in
the dry rubber the simplicity of the operation appears to have been
overrated. Great care must be exercised in preparing the solution, and the
work should not be left to the few minutes preceding its actual
requirement; such has been found to be the case in several factories, so
that it is not surprising if the rubber is defective.
The powder should be added gradually to water with thorough stirring, which
should be continued for five minutes at least. Even then there may often be
seen at the bottom undissolved particles, sand, and other impurity. It is
necessary, therefore, in such cases to decant the solution through a piece
of cotton cloth before using. No solid particles should be allowed to enter
the latex.
ABUSE OF SODIUM BISULPHITE.--It is now generally recognised that the abuse
of sodium bisulphite, in the form of an excess, leads mainly to delay in
the period of drying and the production of an overpale rubber.[5] It is
probable that few estates, if any, now experience trouble due to this
non-observance of the rules and quantities laid down for use.
[5] "The Preparation of Plantation Rubber," Morgan, 1913, p. 74.
RESIDUAL TRACES OF SODIUM BISULPHITE.--The prolongation of the drying
period was attributed to the fact that traces of substances caused by the
decomposition of sodium bisulphite remained in the rubber if the rubber
were not sufficiently worked and washed on the rolls. These traces must
have been very minute, but they were sufficient to retard the progress of
drying. That much depended on the care exercised in washing is evident from
the fact that samples prepared with varying quantities of the chemical show
varying results on extraction. These samples were tested for the presence
of sulphates. Of the series tested that sample prepared with bisulphite in
the proportion of 1 part to 600 parts latex showed only a trace of sulphate
present; while the one prepared 1:2,400 gave an equal quantity.
Intermediate samples contained no trace of sulphate. On the whole,
therefore, the presence of sulphate in crepe rubber is adventitious, and
properly washed crepe prepared with moderate quantities of bisulphite may
be taken as free from any residual quantities. Meanwhile there cannot
possibly be any doubt of the advantages gained by the use of sodium
bisulphite, and it would not be very wide of the mark if the statement were
made that, in the event of this chemical being discarded, most contracts
for pale crepe could not be fulfilled.
SODIUM SULPHITE.--It would not be amiss to insist upon the point that while
the nature of sodium _bisulphite_, as employed in the preparation of
rubber, is anti-oxidant, sodium sulphite is employed chiefly for its
anti-coagulant property. It is not used, therefore, in the making of crepe
rubber, but is of service in the preparation of sheet rubber, where the aim
is to keep the latex in good fluid condition as long as is necessary, and
to retard coagulation slightly so that enclosed bubbles of gas or air may
escape. Formulae have been given for its use in the field when required. On
some estates this practice is not found necessary, but a quantity of
solution is always placed in the bottom of the reception vessels prior to
the straining of latex into them. Only a small quantity is used, and as a
working basis the following formula may be adopted:
_Sodium Sulphite: For Use in the Factory._
(_a_) Dissolve 2 ozs. of anhydrous sodium sulphite in a gallon of
water.
(_b_) The gallon of solution, placed in the bottom of the reception
jar or tank, is sufficient for the treatment of 40 gallons of
standardised latex (1-1/2 lbs. dry rubber per gallon).
The warning previously given regarding the necessity for thoroughness in
the preparation of solutions is here reiterated. Stirring should be
thorough, say for five minutes, and if there is any sediment or undissolved
matter the solution should be strained through cloth before using.
Where uniform jars or tanks are in use, the majority of which will contain
uniform quantities of latex daily, the practice of using the chemical can
be made almost fool-proof even in the hands of coolies. A calculation is
made of the quantity of powder required for each vessel daily. The
necessary number of lots is weighed out each morning and each placed in an
envelope. The process is thus simplified by the fact that the contents of
an envelope, neither more nor less, are required for each unit reception
vessel. Even the weighing can be done by a coolie if he is given a
counterpoise (of lead, for example) equivalent to the required weight.
It will not be found necessary to do any vigorous stirring of the solution
with the latex, as the latter is strained into the solution and the
continued addition of successive quantities is sufficient to give a good
mixture.
USE OF FORMALIN.--Few estates now use formalin (formaldehyde) as an
anti-coagulant. It must be acknowledged that when not abused there are
points in favour of its employment in preference to sodium sulphite, but
these are outbalanced by certain disadvantages. The argument may be stated
thus:
_Points for_: (1) If made up freshly it is an effective
anti-coagulant.
(2) Formalin being the solution of a gas in water, there is no
residual substance left in the rubber to delay drying.
(3) Its use gives a bright clear rubber.
_Points against_: (1) Its cost at all times is greater than that of
sodium sulphite.
(2) If the jar is not sealed there is loss by evaporation, thus
increasing the cost.
(3) Its effect upon the rubber is uncertain. Even in normal quantity
it is said to cause "brittleness" or "shortness."
Certain few estates, however, have continued its use, and no trouble is
claimed to ensue. The following formula is stated to give satisfactory
results in the preparation of sheet rubber, when applied as in the
preceding paragraphs bearing on the employment of sodium sulphite:
_Formula for Use of Formalin (Formaldehyde)._
(_a_) 1 pint of formalin is diluted with 5 gallons of water.
(_b_) Of this solution 1 gallon is required for 50 gallons of
standardised latex.
In noting this formula the writer gives no recommendation regarding its
use. Whatever may be the actual facts regarding the effect of formalin upon
the vulcanisation of rubber, when used in minimum proportions, there can be
no question concerning its injurious effect if used in excess. Beyond this
the factors of cost and loss militate against its wider employment.