which, possibly, is mangrove timber, locally known as 'bakau.'
Other suitable timbers are known, but as far as preliminary
experiments show mangrove timber gives the best yield. At present
this timber is in great demand as a fuel for steam plants, but
with the extension of the local coal industry the timber may
become cheaper.
"2. There would appear to be less valuable timber which would be
suitable for heating the retorts. Or, local coal might be used.
"3. Supplies of lime at reasonable rates are available, as the
limestone formation in the peninsula is quite considerable in
extent.
"4. Supplies of sulphuric acid are available from Japan,
Australia, Burma, etc., even at the present time, although
naturally rates are higher than normal. Under ordinary
conditions, supplies from England and parts of Europe would be
much cheaper than at current rates.
"For the benefit of many readers perhaps a brief and nontechnical
description of the preparation of acetic acid would not be amiss,
and would explain the necessity for the essentials indicated
above. In brief, the process is as follows:
"(_a_) A suitable timber is heated in a closed retort. This is
termed 'dry distillation,' and results eventually in the
carbonisation of the wood--_i.e._, charcoal is obtained in the
retort.
"(_b_) Tar, vapours and gases are distilled over during the
carbonisation of the wood. These liquors and gases pass through
condensers. The gases pass away, while the condensed liquors
separate out into (1) wood tar, (2) a watery liquor called
pyroligneous acid or crude wood vinegar.
"(_c_) The pyroligneous acid is separated from the tar, and again
distilled to obtain the acetic acid present.
"(_d_) This crude acid is steam-heated with milk of lime, which
fixes the acid, forming calcium acetate (or acetate of lime).
"(_e_) Eventually the calcium acetate is taken out in the form of
a thick paste, which is spread to dry. When dry this 'grey
acetate' is the main source of all glacial acetic acid now made.
"(_f_) The acetic acid is released from the 'acetate of lime' by
the action of sulphuric acid. It is then distilled several times,
and under various conditions, in order to increase its strength.
In the past copper tubes were used for this purpose, but owing to
the fact that traces of copper were found to be injurious to
rubber, some works instal tubes of glazed earthenware for the
distillation.
"Such is the process in outline, and it will be seen that no proposal
to manufacture _glacial acetic acid_ on an estate could be considered
feasible, although it would not present any great difficulty on a
large scale and under skilled direction. Furthermore, the cost of the
plant would be far too great for any estate."
Although it is clear that pure acetic acid is beyond the scope of an
estate, crude pyroligneous acid has been produced on a varying scale in
this country and in Ceylon. In the latter country some success was obtained
by the distillation of coconut shells with comparatively inexpensive plant.
In this country, wood-distillation was practised on a few estates, but
improved facilities for obtaining pure acetic led to a termination of the
experiments, although sufficient crude acid could then be made at a
reasonable cost.
The pyroligneous acid obtained, is generally clear, after nitration, and of
a dark brown colour. It has a peculiar odour reminiscent of smoked
sheet-rubber, or of creosotic substances in general.
Its acid content depends chiefly upon:
(_a_) The kind of timber heated in the retort.
(_b_) The efficiency of the apparatus.
(_c_) Condition of the timber as to moisture.
(_d_) The temperature employed, and rate of working.
(_e_) The point at which distillation ceases (_i.e._, the duration of
interval between commencement of heating and cessation of collection).
Samples received from estates for testing purposes were found to contain
equivalents varying from 2 per cent. to 10 per cent. of acetic acid.
They were all suitable coagulants when used in quantity calculated from the
discovered acidity, but produced rubber darker than ordinary when
air-dried. This effect was not of much importance in the preparation of
smoked sheets, but to produce a pale crepe it was necessary to employ
sodium bisulphite as an anti-oxidant.
This darkening in colour is to be ascribed to the presence of traces of
phenols,[24] which are stated to exert an effect upon the rubber during and
after vulcanisation.[25] This subject will be discussed in another section.
[24] Whitby, _Journal Soc. Chem. Industry_, vol. xxxv., No. 9, 1916.
[25] See also "Preparation and Vulcanisation of Plantation Rubber" (Eaton,
Grantham, and Day), Bulletin No. 27, F.M.S. Department of Agriculture,
April, 1918.
With this provision the crude pyroligneous acid which can be produced on
estates, could be employed as a coagulant until such time as the price of
glacial acetic acid was so low as to make the production of the crude acid
non-profitable. This point would be determined from a knowledge of the cost
of production per gallon, and the percentage of acetic acid per unit. For
example, if the cost of production (including cost of timber for
distillation, cost of fuel for heating the retort, cost of labour, etc.)
was 60 cents per gallon of crude acid containing 9 per cent. of acetic
acid, that would be equivalent approximately to buying glacial acetic acid
at $30 per demijohn of 44 lbs.
SMOKED WATER.--A weak solution of pyroligneous acid may also be obtained
by passing smoke through water. With this object in view, a machine was
designed by the Federated Engineering Company of Kuala Lumpur. In this the
principle of retorting was not employed. Smoke was produced by ordinary
combustion in a compartment of the apparatus, and was drawn through water
by the action of a high-speed fan worked by hand. A solution, equivalent in
effect to a 2 per cent. solution of acetic acid, could be obtained at a
comparatively cheaper cost than crude pyroligneous acid produced by dry
distillation as it was then being practised. This was chiefly because of
the wasteful methods of fuel combustion, in the latter process, in the
heating of the retort.
CHINESE VINEGAR.--This agent was found to be a satisfactory coagulant, and,
_a priori_, there is no reason why it should not be suitable, as it is
essentially a dilute solution of acetic acid.
The qualities sold were generally colourless, and were probably the result
of acetic fermentation of rice.
Samples tested showed a varying content of acetic acid, ranging roughly
from 3 per cent. to 8 per cent.; but on this basis of valuation it was
found generally that the price bore no relation to the degree of
efficiency.
It was advanced not only that the vinegar was an efficient substitute for
glacial acetic acid, but that it was also cheaper. This latter claim was
proved to have no foundation in fact, even at the high price of acetic acid
prevailing during the period of stress. It is not likely, therefore, that
vinegar can displace acetic acid, except as an expedient.
SULPHUROUS ACID.--The anti-oxidant effect of sodium bisulphite and sodium
sulphite is due to the liberation of the gas, sulphur dioxide. This gas
dissolves easily in water, forming an acid solution called sulphurous acid.
This acid solution is an effective coagulant in fairly small quantity. Not
only so, but it produces, in addition, the anti-oxidant effect noted in the
employment of sodium bisulphite. It is thus possible to produce rubber
varying in shade of paleness by means of a single solution.
In the event of sulphurous acid being used, it would be necessary to
import cylinders of sulphur dioxide from which the solution could be
prepared in factories each day. There would be no insurmountable difficulty
in this, as it is only necessary to pass the gas through a series of closed
vessels containing water. Enough solution could be prepared at one time for
three or four days, but preferably the solutions should be as fresh as
possible. Altogether there would seem to be possibilities in the use of
sulphurous acid for preparing pale crepe rubbers, providing the cost is
within comparable limits with the commoner coagulants at present in use,
and that no adverse effect on the rubber can be shown to result. If the
cost did not exceed the combined cost of acetic acid and sodium bisulphite,
the employment of sulphurous acid solution might be worthy of
consideration. There is one drawback to the use of sulphurous acid
solution, and that lies in the proximity of the limits of the quantities
necessary for coagulation and that which is in excess, and prevents
coagulation. Thus, with ordinary field latex having about 20 per cent. dry
rubber content, the minimum necessary for coagulation per 100 c.c. of latex
is about 8 c.c. of a 1 per cent. solution. The maximum quantity possible
for use is about 15 c.c. of a 1 per cent, solution, so that great care
would have to be exercised in avoiding an excess of coagulant, otherwise
coagulation would be effectually prevented.
It is believed that the preparation of rubber by this method is the subject
of a patent secured by Messrs. Boake, Roberts, and Co., London.
SUGARS.--Coagulation may be effected by the addition of small quantities of
sugars.[26] These are assumed to be effective by fermentative conversion
into lactic and acetic acids. The presence of lactic acid is supposed to
have a twofold effect:
[26] "Preparation and Vulcanisation of Plantation Para Rubber" (Eaton,
Grantham, and Day), Bulletin No. 27, F.M.S. Department of Agriculture;
Gorter and Swart, Bulletin No. 6, West Java Expt. Station.
(_a_) As a direct coagulant.
(_b_) In its action upon certain organisms which, in the ordinary
course of events, would delay or prevent coagulation. Although work on
an experimental scale has been done, as far as we know no practical
application has been made of the employment of sugars as coagulating
agents.
VARIOUS SALTS.--Of experimental interest only it may be recorded that
coagulation has been effected by means of various chemical "salts"--_e.g._,
calcium chloride, barium chloride, magnesium chloride, sodium chloride,
aluminium sulphate, magnesium sulphate, sodium sulphate, etc. None of these
has been found to have any practical application, except, perhaps, calcium
chloride, which is used in small quantity as an accelerating agent in a
special process of anaerobic coagulation, which will receive mention in the
following chapter.
At one period during the War and the dearth of acetic acid, it was found
that there were available in England large supplies of the acid sulphate of
sodium (sodium hydrogen sulphate), which proved to be an effective
coagulant. Experimental work gave satisfactory results, but no practical
application resulted when supplies of acetic acid were again obtainable.
VARIOUS PROPRIETARY COMPOUNDS.--We have seen many proprietary coagulants
advertised and pass into the limbo of forgotten things. They can generally
be divided into two classes. The first embraces those founded upon a
woefully incomplete knowledge of requirements. The second covers those
which meet requirements, but for which exaggerated claims are made and
excessive prices charged.
As as instance of a substance which fell under both classifications might
be mentioned the case of "Coagulatex." Pretentious claims were made, and it
was emphasised that the liquid contained no _vegetable acids_. Acetic and
formic acids might be quoted as examples of vegetable acids, and as these
have been shown to be the most satisfactory coagulants now employed one
fails to imagine where lay the value of the guarantee given by the
advertisers of "Coagulatex."
On analysis the liquid was found to consist mainly of sulphuric acid,
against the indiscriminate use of which warnings have been given. Thus it
was a dangerous substance for common use.
Furthermore, comparing the value with its sulphuric acid content, it was
found that the price required for "Coagulatex" was roughly four times the
contemporary cost of commercial sulphuric acid in the Federated Malay
States.
Those in charge of estates should realise, therefore, that no proprietary
coagulants should be adopted until a proper report of tests, and a
comparative valuation, has been obtained from one of the research
laboratories.
CARBONIC ACID GAS, CARBON DIOXIDE.--Now of only scientific interest, it may
be noted that some years ago great claims were made for the use of carbon
dioxide gas as a coagulant. In actual practice we were unable to effect
coagulation by passing the dry gas into latex. It was suggested that the
original investigators were misled by failure to secure a dry and clean
gas. It would appear that probably the gas was prepared by the action of
hydrochloric acid upon marble or limestone. Unless intervening "washers"
and "driers" were used, the liberated gas, when passed into latex, would
carry with it traces of hydrochloric acid, which would effect coagulation.
ALCOHOL.--In the cheap form of methylated spirit, alcohol has been employed
by us as a speedy coagulant for many years. Latex run slowly into alcohol
coagulates instantaneously. The method has been in common laboratory use.
The employment of alcohol has also been made the part-subject of a patent
process of coagulation, to which reference will be made in the succeeding
chapter.
VEGETABLE EXTRACTS.--At various times experimental work has been directed
towards the use of liquids of purely vegetable origin, such as the juices
of tropical fruits, and of a waste product of tropical industry--the
so-called "milk" (or water) of ripe coconuts.
In the former class there is usually a natural acidity, but in coconut
water the acidity is chiefly the result of fermentation of the carbohydrate
(sugar) constituents.
These substances were all found to effect a more or less satisfactory
coagulation, but it is unlikely that they would be suitable for practical
application on a large scale.
As being more directly related to the subject of coagulation in general
than to coagulants in particular, a discussion of several special processes
will be relegated to the ensuing chapter.
CHAPTER XX
_SPECIAL METHODS OF PREPARATION_
Every year appears to bring forth some new ideas in the mode of rubber
preparation. Some of them are based in principle upon the oldest known
method--_i.e._, the native Brazilian process of making "Hard Para." Others
strike a new note, and in a few cases the claims put forward are
substantially confirmed by results. In other instances the claims are too
pretentious, and discredit may be brought upon schemes which, although
lacking in comparative success, are yet commendable for the ingenuity
manifested.
To the present not one of these new methods has been able to compete to any
marked degree in general practice with the established methods of ordinary
preparation. A few continue to find local application, but most have either
been abandoned or are gradually falling into desuetude.
We do not propose to discuss in fine detail all the various claims made on
behalf of these special processes, or to enter into controversies. The aim
is to present to the reader an outline embodying the main principles and
advantages claimed.
DA COSTA PROCESS.--Briefly, this was a method by which coagulation was
effected with smoke. The smoke was generated by the combustion of wood in a
special compartment, and was forced into latex by means of a jet of steam.
It was really only applied to the preparation of coagulum intended for
crepe form. The exact degree of coagulation effected was uncertain, and the
final colour of the rubber precluded it from being classed as a modern No.
1 product.
"BYRNE CURING" PROCESS.--This is a process for treating coagulum obtained
by ordinary methods.
It was the subject of a patent obtained by Messrs. E. J. and F. A. Byrne,
and at one time had a considerable vogue on estates. The chief claim
advanced was that the rubber produced was in all respects equal to Fine
Hard Para, and could be shipped while still moist without detriment to the
physical qualities.
The principle of the process was the treatment of coagulum, in either sheet
or thick crepe form, with vapours produced by the volatilisation of two
special fluids. This treatment was undertaken in comparatively small wooden
sheds, in which the coagulum was placed. The "smoke" was conducted into the
curing sheds from furnaces outside the building. The sheds were covered
externally with "felt" material to prevent leakage of the vapours, and a
very dense smoke was obtained.
The furnaces were specially designed, and consisted essentially of a
"hot-plate" heated by a powerful kerosene blast-flame. On top of the
machine were two reservoirs controlled by taps. In these were placed the
special fluids which were released in definite proportion. The composition
of the fluids was not divulged, but it is assumed that the principal
ingredients were (_a_) wood tar products, (_b_) crude pyroligneous or
acetic acid. The mixture of these, dropping on the hot plate at the correct
temperature, spontaneously volatilised, to form dense whitish fumes, having
an intense and not disagreeable odour of wood combustion. A duct led from
the back of the machine into the curing-shed, where the vapours were
distributed through perforations in the pipe.
The coagulum usually remained under treatment in the shed for three to four
hours, and then was removed for ordinary air-drying. When taken from the
curing-shed it had a pinkish colour, which later developed into a dark
brown by a natural process of oxidation. The exterior of the rubber, on
shipment, resembled the appearance of smoked sheets; while the interior, on
cutting, was seen to be still white. As packed for shipping, the rubber
contained from 10 to 15 per cent. of original moisture, for the usual sheet
form, and even more when "slab" rubber was prepared.
Originally either crepe or sheet rubber was made, but later the preparation
of the crepe form was displaced largely by "slab" rubber. These "slabs"
were really very thick sheets, which had been subject to only slight
pressure.
Still later the preparation of the "slab" form was displaced by "loaf"
rubber. This form was built up by winding together ordinary thin sheets
which had been subject to the "cure." Only slight tension was needed,
during the operation of winding, to cause close adhesion of the component
wet layers, and the final result was a "loaf" or roll dark in colour, and
apparently dry when examined superficially. On being cut, even after an
interval of months, the middle portion was still so moist as to be quite
white.
In course of time it was discovered that all the claims made for the
process could not be substantiated, and for various reasons (which need not
be detailed) most of the estates which had adopted the scheme reverted to
ordinary methods of preparation. At the time of writing few, if any,
continue to work the process. It appears to be agreed, as the result of
investigations, that in no degree does the process yield advantage over
ordinary methods.
FREEZING PROCESS.--A patent was secured a few years ago to cover a process
whereby coagulation was effected by refrigeration.
Latex remained for several hours in the refrigerating chambers of an
ordinary ice-making plant. The resulting solid mass, on being thawed,
yielded a coagulum appearing in no way to differ from that obtained by
ordinary methods of coagulation.
Provided the process exerted no influence for good or evil upon the quality
of the resulting dry rubber, the value of it would appear to depend upon
the relative cost of working, plus considerations of capital expenditure
and depreciation on the plant. At the present time it would be difficult to
imagine that the cost of preparation alone would compare favourably with
that sustained by ordinary coagulative methods.
Furthermore, beyond the expensive refrigerating plant, the usual machinery
of a factory would still be required if the ordinary market demands are to
be met.
Finally, it has not been found[27] that any advantage in the final physical
qualities of the rubber is obtained by the employment of this process.
[27] "Preparation and Vulcanisation of Plantation Rubber" (Eaton, Grantham,
and Day), Bulletin No. 27, F.M.S. Department of Agriculture.
WICKHAM PROCESS.--This process, invented by Sir Henry Wickham, aimed at the
production of a rubber resembling Fine Hard Para. The principle employed
was that underlying the preparation of the best rubber in Brazil--viz.,
coagulation of superimposed thin layers of latex by the action of smoke and
heat.
In essential the machine employed consisted of a rotating drum into which
latex and smoke entered. The result was the formation of thin "skins" of
rubber which, coagulating _in situ_, formed a mass corresponding to "Fine
Hard."
That the rubber was fully satisfactory as to quality is acknowledged, but
economically and in practical utility the process was unsuccessful, the
rate of output being so low.
DERRY PROCESS.--The invention of Mr. R. Derry, late of the Singapore
Botanic Gardens, this in principle resembled the Wickham and other
processes. It aimed at a mechanical imitation of the native method of
producing Fine Hard Para.
In place of the rotating drum, an endless belt was used. This travelled
over pulleys, more or less horizontally placed. The upper of these could be
raised to varying height above the level of the other, and likewise could
be so adjusted as to tighten the belt.
The under layer of the belt impinged, in its travel, upon the surface of a
layer of latex contained in a shallow tray. The belt was operated by
hand-power, and the height of the latex trays was adjustable.
The trays of latex were situated at the lower end of the machine which lay
outside the smoking-chamber. It will be understood that the vastly major
part of the total length of belt was always within the chamber.
Smoke was generated by combustion of wood in an external structure, was
brought into the chamber by a wide duct, and was then distributed below
the belt by means of perforated pipes.
The thin film of latex picked up by the belt was coagulated partly by the
action of smoke constituents by evaporation due to heat. Assuming (1) that
the belt was of adequate length, (2) that the rate of travel was not
excessive, (3) that the latex was not too dilute, (4) that the temperature
of the smoke was sufficiently high, (5) that the smoke was sufficiently
dense and not too damp--then the process should be a continuous one.
It will be clear that success could only be obtained by a careful
adjustment of all these factors. The latex must, necessarily, be of a
fairly rich consistency (at least 2-1/2 lbs. dry rubber per gallon), but
unfortunately there is considerable difficulty in maintaining such latex in
a state of fluidity for the period demanded by this process, without loss
of latex. Naturally, the addition of an anti-coagulant would retard the
rate of output of the machine to a marked degree.
The layer of rubber thus formed on the belt was stripped off, and hung for
further air-drying, as it still contained a fair percentage of moisture.
As a really practicable method for treating plantation latex, the process
failed by reason of its low rate of output over a given interval. This
alone was sufficient to condemn it, apart from the facts (1) that it was
not shown to be a cheaper method than coagulation by acetic acid, (2) that
the resulting rubber was not proved to be of superior intrinsic value to
rubber prepared by ordinary methods.
SPONTANEOUS COAGULATION.--All readers will be aware of the phenomenon of
the curdling or souring of milk. The behaviour of _Hevea_ latex, under
certain conditions, may be taken to be analogous. Difficulty is experienced
in maintaining fluidity--a difficulty which appears to vary in great degree
according to locality, nature of soil, age of trees, the relative demand
made upon the trees by the system of tapping employed, etc.
It is sometimes found, before the latex reaches the store, that it may
exhibit one of various stages of premature (spontaneous) coagulation:
(_a_) To all appearances it may be quite fluid, but a close
examination shows it to consist mainly of a serum containing very
minute particles of rubber in suspension (microscopic coagulation).
(_b_) In a later stage these particles coalesce to form larger
"flocks" (macroscopic coagulation).
(_c_) The whole, or practically the whole, of the latex may have
coagulated, forming one mass of rubber with a milky residual serum.
Passing from this aspect of the question, it may be noted as peculiar facts
that:
(1) A shallow layer of latex is less likely to coagulate spontaneously
(_i.e._, without the addition of a coagulant) than a deeper volume.
(2) The shallow layer, and also the surface of the deeper volume
(where exposed to air), on standing will be found to develop a
superficial film of finely coagulated particles, yellowish in colour,
and having an offensive odour due to decomposition of protein matter.
(3) While this partial coagulation is confined only to the surface of
a shallow layer of latex, it will be found that below the surface film
of the deeper volume a much more definite coagulation has taken place.
The coagulation will be practically complete, and the coagulum, apart
from a spongy appearance, is normal in character. This coagulum is
free from the offensive odour noted above.
(4) On testing the surface film of both the shallow layer and the
deeper volume, it will be found to be _alkaline_ in character; whilst
the lower liquid surrounding the main portion of the coagulum in the
deeper volume of latex is of an _acid_ nature.
These observed facts are sufficient to indicate that there are apparently
_two distinct types of spontaneous coagulation_, and that the latter takes
place particularly where the latex is more or less out of contact with the
atmosphere. We may, therefore, differentiate thus:
(_a_) _In contact with air (aerobic)_: incomplete spontaneous
coagulation, accompanied by yellowish slime, offensive in odour and
alkaline in character.
(_b_) _Out of contact with air (Anaerobic)_: Practically or wholly
complete. There is no offensive odour under normal conditions and the
serum is acid in character.
It is concluded[28] that there are present in latex, on collection in the
field, two types of organisms. Those which work in contact with air
(aerobic) show a tendency to _prevent_ coagulation and to form an alkaline
yellow slime on the surface of the latex. The others, which work in the
absence of air (anaerobic), may, under favourable conditions, cause
complete coagulation unaccompanied by any decomposition or offensive odour
within a normal period. If air is rigidly excluded, the coagulum obtained
is quite satisfactory for all purposes.
[28] "Preparation and Vulcanisation of Plantation Rubber" (Eaton, Grantham,
and Day), Bulletin No. 27, F.M.S. Department of Agriculture, 1918; "De la
Coagulation naturelle du Latex d'Hevea Brasiliensis" (Denier and Vernet),
_Comptes Rendus l'Academie des Sciences_, No. 3, July, 1917.
This type of coagulation, without the employment of a chemical coagulant,
and under anaerobic conditions, was the subject of a patent granted in 1914
to Messrs. Maude, Crosse and others. The process has been in use on Cicely
Estate (Perak) for some years. With subsequent slight modifications the
apparatus consisted in essential of a tank with a loose cover. The flanges
of the cover were sufficiently long to dip into a water-seal surrounding
the tank. Thus the cover may rise and fall without an inrush of air.
Coagulation, in fact, can be effected thus in any kind of air-tight
receptacle; and experimentally the reader can obtain a satisfactory result
by filling completely with latex the bottle which has a loose stopper.
Under the patent held the coagulum may be prepared either for crepe-making,
or for sheets by a modification of the tank.
The crepe when dry does not have the bright appearance of the ordinary
"Fine Pale" standard prepared with the aid of the anti-oxidant sodium
bisulphite.
Unfortunately the addition of this substance to the latex in normal
proportions is not possible under anaerobic conditions, as it is found to
prevent coagulation, probably owing to its sterilising effect upon the
anaerobic organisms.
To prevent the oxidation of the rubber in actual practice, the freshly
prepared crepe is soaked in a solution of sodium bisulphite before hanging
to dry. The resulting colour of the rubber is quite good.
It was shown by Eaton and Grantham that anaerobic coagulation is slightly
uncertain in action. Owing probably to variations in the composition of the
latices, or to the extent of infection by organisms, coagulation may one
day be complete and on other days less satisfactory.
They found further that, by the addition of small quantities of sugars,
coagulation under both aerobic and anaerobic conditions was improved. The
conclusion formed was that the addition of sugars created a medium
favourable to the development of anaerobic organisms and unfavourable to
those which cause decomposition of the natural nitrogenous constituents of
latex.
This work was confirmed by Gorter and Swart,[29] who attributed the action
to the conversion of sugar to lactic, acetic, and succinic acids by
fermentation.
[29] Gorter and Swart, Bulletin No. 6, West Java Station.
Denier and Vernet, whose work has already been mentioned, studied the
presence of the organisms in latex, and succeeded in isolating one which,
under anaerobic conditions, effects coagulation within twenty-four hours.
Sometimes to produce complete coagulation it was found necessary to employ
small quantities of sugars--_e.g._, 1 gramme per litre of latex (1:1,000).
It is to be noted also that the addition of small quantities of soluble
calcium (lime) salts to latex has much the same effect as the employment of
sugars. Recent investigations[30] showed that the addition of 0.5 to 1
gramme of calcium chloride per litre of latex caused complete coagulation
in closed vessels within twenty-four hours, a result agreeing with the
findings of Barrowcliff.
[30] "Archief voor de Rubbercultuur," Nederlands Indies, 1920, 4, 273.
On page 308 of the same publication, experiments on the effect of sugars
are described, in connection with _aerobic_ coagulation. Observations from
a further set of experiments tended to indicate a direct connection between
the effects of tapping and spontaneous coagulation. It is suggested that
heavy tapping causes a diminution in the latex of those substances which
act in some way as accelerating agents in coagulation--_e.g._, sugars. The
smaller the proportion of these substances, the slower and less complete is
natural (spontaneous) coagulation.
ILCKEN-DOWN PROCESS.--This process is the subject of patents granted in
1915 to Messrs. Ilcken and Down. It has been in fair prominence, and has
been tried experimentally on several estates and in public demonstration.
It is a coagulating process, and, in the original specification, employed
as agents a mixture of alcohol (in the form of methylated spirit) and
benzene (petrol), or alcohol with petrol and coal-tar naphtha. The mixture
was injected in the form of a fine spray into the latex, contained in a
tank specially fitted with paddles.
Later modifications covered the addition of a small quantity of glycerine;
or, failing supplies of that substance, coconut oil.
Many advantages are claimed for the process, but most of them cannot be
substantiated. The two chief claims are:
1. The production of a uniform standard of rubber.
2. The obtainment from a unit volume of latex of a greater weight of
rubber than can be obtained from an equal volume of the same latex by
ordinary coagulation with acetic acid. It is to be inferred that the
agents employed have the power of adding to the coagulum some of the
substances which usually remain in solution in the clear serum.
Regarding the first of these claims, it has been shown[31] that the rubber
is not uniform in its behaviour on vulcanisation, and that its variability
is similar to that of rubber prepared by other processes.
[31] "Preparation and Vulcanisation of Plantation Rubber" (Eaton, Grantham,
and Day), Bulletin No. 27, F.M.S. Department of Agriculture, 1918.
The second claim has been the subject of much controversy. Experiments made
on estates under the supervision of, or in the absence of, the patentees
have given conflicting results. When varying factors have been eliminated,
the general conclusion was that no increase in weight of rubber was
obtained.
Private laboratory investigations led to a similar verdict, and Eaton[32]
records a confirmatory finding. More recently the claims made for the
process were investigated in Java[33] under varying conditions. Three
series of experiments were made:
[32] _Ibid._
[33] "Archief voor de Rubbercultuur" (De Vries and Spoon), Central Rubber
Station, Java, May, 1921.
(1) During the rainy monsoon and at a height of 1,800 feet.
(2) During the dry monsoon on a low-country estate.
(3) In the experimental gardens at Buitenzorg during bright sunny
weather and the most favourable conditions.
The agents used were (_a_) a mixture of alcohol and fusel oil, (_b_)
alcohol and petrol (benzene).
In these experiments no advantage in weight of rubber was obtained by the
Ilcken-Down process, and it would thus appear that the principal claim
fails to be substantiated.
The general composition of the rubber was approximately the same as
ordinary crepe obtained from undiluted latex. The rubber on vulcanisation
was found to be normal in behaviour, and was similar to the controls.
The coagulum ordinarily is affected by oxidation, and does not produce a
fine pale crepe. To remedy this defect the freshly prepared crepe is soaked
in a solution of sodium bisulphite and sulphuric acid.
It may be noted that in the recent experiments coagulation was effected in
vacuum in a specially designed wooden tank. From a study of the previous
section on "Spontaneous Coagulation," the reader will perceive that results
equal to those obtained by the Ilcken-Down process can be obtained
_without_ the necessity of using such agents as alcohol, petrol, or fusel
oil.
SLAB RUBBER.--This type of preparation has been the subject of much
discussion of recent years. There is nothing really special in the mode of
preparation, and in its original form "slab" rubber is only a thick sheet
which may be obtained by coagulation with acetic acid or other agents.
The coagulum, when removed from the serum, is subjected to comparatively
slight pressure, and the "slab" thus made is either placed to air-dry at
once, or may be subject to treatment in other liquids before drying.
The rubber is not allowed to remain until wholly dry, but is shipped while
still containing an appreciable percentage of enclosed moisture.
It is claimed[34] that the production of "slab" rubber by standardised
methods eliminates to a great degree the variability which at present
characterises plantation rubber, and that a fast-curing medium is obtained.
These claims will be discussed in later chapters dealing with the
vulcanisation of rubber, and demand no notice in this section.
[34] "Preparation and Vulcanisation of Plantation Rubber" (Eaton, Grantham,
and Day), Bulletin No. 27, F.M.S. Department of Agriculture, 1918.
From the producers' point of view, it may be noted that the preparation of
slab rubber is a simple process, but not altogether as pleasant probably as
might be desired, when undertaken in crude form.
The appearance of the partially dry slabs is unattractive, but that does
not signify if the quality of the vulcanised product satisfies
requirements.
For the average producer, the difficulty lies in having to meet the demands
of the general market. Even, therefore, if one assumes that the intrinsic
qualities of slab rubber are all that the claims advance, it would be
necessary for the producer to be assured of definite and regular sales.
At present it would probably be fair to state that practically all the
"slab" rubber being prepared is produced by those who are also consumers.
They are thus in the enviable position of being able to satisfy their
requirements as to the mode of preparation. Until such time, therefore, as
there exists a regular demand for "slab" rubber in the general market, the
vast majority of estates must proceed on ordinary lines of preparation.
PART VI
VULCANISATION
(BY DR. H. P. STEVENS)
CHAPTER XXI
_INTRODUCTORY DEALING WITH TREATMENT AND VULCANISATION_
In the foregoing chapters the methods of treating latex, coagulating,
rolling and curing, or drying, have been described in great detail. These
details will give the reader some idea of the precautions taken, and
procedure necessary to produce rubber which will be acceptable to the
market. The expressions "inferior rubber," "defective crepe," "poor quality
sheets," etc., are frequently met with, but these expressions must not be
taken to indicate any defect in the rubber for manufacturing purposes, but
merely that the rubber is defective for selling purposes--that is to say,
being unsightly, it will not fetch the full market price.
Raw rubber, as produced on the plantations, is almost invariably subjected
to the process of vulcanisation in the production of manufactured rubber
articles as we know them. Previous to the advent of plantation rubber, the
raw material was purchased by the manufacturer in a moist and impure
condition; frequently the rubber was adulterated with sand, dirt, and even
small stones. Consequently it was the invariable practice of the rubber
manufacturer to wash the raw rubber and convert it into crepe, which was
then hung and air-dried before use. The effect on the rubber, if of high
grade, was more severe than the washing and crepeing process on the
plantation, because the rubber was not a soft coagulum but generally dried
on the surface and semi-hard. The power required was considerable, and the
resulting crepe was consequently softer and more susceptible to heat than
plantation first latex crepe. Much of the "wild" rubber was soft and tacky
and inferior to "earth-scrap."
Vulcanising in its simplest aspect consists in mixing the rubber with
sulphur and heating the product under regulated conditions. The effect of
heat on the inferior grades of "wild" rubber is very marked. A soft,
sticky, and resinous material is transformed into a relatively tough and
elastic product. The effect of vulcanising on the better grades is less
marked, but immediately apparent. On the other hand, the effect of
vulcanising is least apparent on first latex plantation grades, because in
these we have a raw rubber prepared in a manner best suited to retain its
natural characteristics.
The need of vulcanising in the process of manufacturing rubber goods became
an axiom in pre-plantation days, and it is only quite recently that
attempts have been made to utilise raw rubber directly, without
vulcanisation, particularly for shoe soles. For this purpose a thick dense
crepe has been found satisfactory. Smoked sheet rubber is not generally
suitable, apparently owing to its microphysical structure. It is possible
that the process of rolling in the making of dense crepe compacts the
rubber particles, yielding a harder and more resilient product. The rolling
must not be carried too far, or the "working" of the rubber will
approximate to a preliminary mastication, and the product will be weakened.
The utilisation of crepe rubber directly has not yet been sufficiently
tested to enable a definite conclusion to be reached as to its future
scope, but it is obvious that for use in a raw state some modification in
preparation may be advantageous. The present method--_e.g._, coagulation
with acetic acid--does not yield the hardest and toughest rubber.
Hardness and toughness are actual drawbacks in the utilisation of rubber
which is required for vulcanising. When the output of plantation rubber
began to increase and to displace the inferior wild sorts, manufacturers
complained of the increased power consumption of their machines. The power
was required mainly to "break down" or "mill" the rubber preliminary to the
mixing with sulphur and other ingredients. It is obvious that a material
such as raw rubber cannot be mixed with powders such as sulphur with a
pestle and mortar, or in any simple form of mixing machine. This difficulty
was overcome by the earlier experimenters by immersing the rubber in a bath
of molten sulphur. The latter was gradually absorbed and "dissolved" in the
rubber, and the heat of the bath caused the dissolved sulphur to combine
with the rubber to produce vulcanised rubber. The limitations of such a
process are apparent. Thus the vulcanised rubber retains the form in which
it was originally shaped. Moreover, other ingredients, such as mineral
matters, cannot be dissolved or absorbed by the rubber in this manner. The
method eventually adopted consisted in "breaking down," "milling," or
"masticating" the rubber by passing it continuously between differentially
geared steam-heated rollers. By this means a high-grade rubber is converted
into a soft, plastic mass, which will "take up" sulphur, mineral matter,
and other ingredients as desired. The mixing operation may be carried
through on the same roller machine as was used for breaking down the
rubber, or separate machines of other designs may be adopted. Details of
the process will be found in books dealing with rubber manufacturing.[35]
It will suffice here to explain that when rubber is kneaded between two hot
rollers moving at different speeds the rubber forms a continuous band
around the slower moving roller, and if the distance between the rollers be
adjusted the excess of rubber held back by the nip of the rollers will form
a "bank" or moving wedge-shaped mass on the top of the nip. This closes the
space between the rollers, so that sulphur and powder placed on the rubber
pass round towards the nip, and are there driven into the rubber. In this
manner it is easy to mix, say, 10 per cent. of sulphur into the rubber
without a single particle falling through. In technical mixes where large
quantities of powders require to be mixed there is always some caking, and
part of the powder falls between the rollers into a tray underneath. This
is swept up with a broom and put back on to the rollers, the process being
repeated until the whole of the ingredients have been incorporated.
[35] For instance, "India-Rubber and its Manufacture," by H. L. Terry.
From this description it follows that, preliminary to mixing, it is
necessary to thoroughly masticate or "plasticise" the raw rubber. Much of
the "wild" rubber was of so inferior a quality that it very readily broke
down, and but little mastication was necessary. It was soft and resinous,
and readily took up the powders which were to be mixed with it. The better
grades of wild rubber, such as Fine Para, were more difficult to break
down, but not so difficult as most plantation rubber, because they had
already received a preliminary "working" in the process of washing and
crepeing, and we have already explained that such treatment takes more
power than the crepeing of the soft moist coagulum on the plantations. The
amount of "working" or "plasticising" produced in the rubber is connected
with the power expended; the greater the expenditure of power, _caeteris
paribus_, the greater the working effect on the rubber. Although the
manufacturers possessed a relatively soft rubber in the form of washed Fine
Para, it was customary in most cases to employ this rubber in conjunction
with washed lower grades to produce a soft plastic material for further
treatment. Now, however, the manufacturer has little else but plantation to
deal with, and most of it more difficult to break down than washed Para
crepe. This is the reason why a hard, tough rubber is no longer a
desideratum with manufacturers, although originally taken as an indication
of good quality. For the majority of purposes they want something which
will break down easily. Hence if a rubber could be produced answering to
these requirements, without loss of vulcanising quality, it would be
preferred.
Having incorporated sulphur and other ingredients, the plastic mass is
sheeted and run between layers of calico to prevent the superimposed sheets
from adhering. From this "calendered sheet" the article, whatever it may
be, is built up. The calender rollers are heated so as to keep the rubber
compound plastic. There is a limit to the thickness of the sheet which can
be produced. It is a difficult operation to perform satisfactorily so as
to yield a smooth surface and a sheet free from enclosed air. When cool the
rubber hardens and is readily handled. The object to be manufactured is
then built up from the calendered sheet. Thus in the manufacture of a motor
tyre the tread is built up on the casing or carcase by laying the sheets on
the canvas and rolling these with a hand or power operated roller, so that
they adhere firmly, the first layer to the canvas of the casing and
subsequent layers to one another. This rough description will suffice to
illustrate how important it is that the rubber when mixed should be plastic
enough to give a smooth sheet, and to allow the sheet to be manipulated in
building up the article in process of manufacture. The testing of rubber in
regard to its plasticity and power to absorb finely divided mineral matter
will be discussed in a later chapter. We may, however, point out here, that
the mineral matter is not generally added as an adulterant, but because of
certain specific properties it confers on the product.
To proceed with our outline of vulcanisation, we have now arrived at the
stage at which the goods are built up and ready for vulcanising. For this
purpose they are generally enclosed in some manner, either in metal moulds
bolted together, or tightly wrapped in cloth, as, _e.g._, in the
manufacture of inner tubes, hose, etc. In the latter case, you can detect
the cloth mark on the finished product. Sometimes the rubber is
spewed--that is, driven out of a barrel by means of an endless screw
revolving in it. In this way rubber tubing, perambulator tyres, and such
articles, may be made. More recently even tyre treads and the shaped rubber
for band tyres (heavy solid tyres) have been extruded in this manner, for
the process is much cheaper than building up a tyre from calendered sheet,
and then cutting the mass to shape by hand. But for spewing the rubber mass
must be very soft and plastic; this condition is not obtainable unless the
raw rubber originally used can be made thoroughly plastic without damage.
Nor can it be effected with a rubber mass containing much finely divided
mineral matter, as this hardens the mixture.
For other purposes the rubber is swollen in a solvent, such as coal-tar
naphtha, and subsequently masticated; the soft dough is then shaped or
spread on cloth, and vulcanised after allowing the solvent to evaporate.
Here, again, the properties of the raw rubber are of immense importance.
Thus, the more plastic the dough, the less solvent required, and the less
there is to drive off before vulcanising. The plasticity of the dough will
depend on the plasticity of the raw rubber, and so forth. It is evident
that the physical properties of the raw rubber are of great importance.
They directly affect the manufacturing operations up to the vulcanising
stage, and indirectly affect the results obtained on vulcanising.
The actual vulcanising consists of heating the mass of mixed rubber for
a definite time and at a definite temperature, each "heat" being chosen
to suit the particular mixture. These data are arrived at
empirically--that is, by trying a number of "heats" and choosing that
which appears the most suitable. The suitability will depend on the
nature of the article, the service to which it is to be put, and the
time it is intended to last. All vulcanised rubber goods, whatever the
process, have a limited life or period during which they can be relied
on to give useful service. After a time, vulcanised rubber tends to
harden, cracks appear on the surface when the article is bent or
stretched, and eventually the rubber becomes rotten and "perished." This
tendency varies with the quality of the original raw rubber and the
conditions of vulcanising. Before plantation rubber was available, the
manufacturers were dependent on inferior wild grades for a great part of
their output, and, consequently, the goods made from these inferior
rubbers never showed very good mechanical properties and soon
deteriorated. The severest critics of plantation rubber have admitted
the advantages to the manufacturers of the replacement of the lower wild
grades by plantation rubber.[36] But even the best grades give a
vulcanised product which rapidly deteriorates if the vulcanisation is
carried too far. This results from too long heating, or too high a
temperature, and the product is termed "overvulcanised" or
"overcured."[37] The appearance of the product is deceptive, as the
physical properties are remarkably good if the overvulcanising is not
more than 50 to 100 per cent. in excess of the normal cure. Only in the
case of very much overvulcanised rubber do we obtain a product which is
brittle from the beginning.
[36] See Williams, "The Rubber Industry," 1914, p. 284. It must also be
remembered that the inferior wild grades were derived from latices often
containing a large proportion of "resinous" matter, and which could not
yield a really high grade of vulcanised rubber whatever the care and skill
employed in preparation.
[37] The terms "curing" and "vulcanising" are generally employed as if
synonymous. Twiss has suggested that the former be applied in regard to a
change in physical properties, and the latter to the chemical change
whereby sulphur is combined with the rubber. The term "curing" is also
applied to the process of preparation of raw rubber. This must be kept in
mind so as to avoid confusion.
The degree of vulcanising will vary with the type of article to be
produced, and where a long life is desired, the tendency will be to
"undervulcanise"; but if the best mechanical properties are desired, the
tendency will be towards "overvulcanising," or, more correctly, "fully"
vulcanising. These considerations are aptly illustrated by reference to
pneumatic tyres. The inner tube need not possess high tensile strength,
provided that it is easily distensible, for the reason that, during use, it
is protected by the casing of the tyre proper, which confines and supports
it against the air-pressure applied. Inner tubes are therefore cured to
give a long life without developing the maximal physical properties. On the
other hand, the casing and tread of the tyre are required to withstand
severe mechanical conditions--particularly the constant flexing of the
cover, and the abrasion of the road surface. Tyres are not stored for any
long period, and, when put into service, have a limited period of useful
life. Consequently it is needful to develop maximal mechanical properties,
and vulcanisation is therefore carried further than in the manufacture of
inner tubes.
The rate of cure is controlled by a number of factors in addition to the
period and temperature of vulcanisation, in particular by the proportion
and nature of the other ingredients, especially sulphur and accelerators,
and also by the rubber itself. The main complaint as regards plantation
rubber is that it varies excessively in this respect. This matter will not
be discussed here, but is only introduced in order to explain the
importance of a constant rate of vulcanising to the manufacturer.
Plantation rubber should, therefore, be prepared so as to be as uniform as
possible in this respect, and the earlier part of this book gives full
details of the precautions advised, and in many cases adopted on the
plantations. Unfortunately, it is impossible to secure uniformity of
methods among all producers, even when they are Europeans, to say nothing
of the native producers, who account for perhaps one-third of the output.
Hence the importance of branding the rubber whenever possible, so that the
manufacturer may identify the rubber he purchases. If found satisfactory,
he can then secure further supplies from the same estate.
CHAPTER XXII
_TESTING OF PLANTATION RUBBER_
This subject may be subdivided into (_a_) Tests on the raw rubber; (_b_)
tests on the vulcanised rubber.
The tests on the raw rubber may be carried out (1) on the sample of sheet
and crepe as received. For this purpose the rubber is cut into a strip,
which is clamped between grips and gradually stretched to breaking-point.
The ring testing machine can be adapted for this purpose by replacing the
rollers with clamps. As the thickness of the samples to be tested will
vary, it is advisable to cut the strips of such a width that the
cross-sectional area of all test pieces is the same--say, 40 sq. mm. The
method is applicable to both sheet and crepe rubber. (2) Tests may be made
as to the behaviour of the rubber during milling or mastication. Small
batches are milled under uniform conditions, preferably in an enclosed
masticator such as Baker and Perkins supply. The power taken (as measured
by the current taken to drive the motor actuating the machine) and the time
are recorded. A further test may be applied to the milled or masticated
rubber, to ascertain the amount and the time taken to incorporate a finely
divided mineral matter, such as carbon black, zinc oxide, or one of the
refined clays.[38] The results are not very exact, and the difference in
plasticity and dryness noted are usually less than found when working with
full-sized machines in the factory. (3) The rubber, either raw or
masticated, may be "dissolved" in a "solvent," such as benzene, and the
viscosity of the "solution" measured. Generally speaking, the less viscous
the solution, the more plastic the rubber.
[38] Bulletin Rubber Growers' Association, January, 1921, p. 43; August,
1921, p. 340.
The testing of vulcanised rubber has been treated in such detail in the
recent works of Whitby[39] and De Vries[40] that a few special points only
will be dealt with here. The preparation of samples for testing involves
first the sheeting of the mixture of rubber, sulphur, and other
ingredients, if any. The sheets may be 1 to 2 mm. thick. They are soft and
adherent, and must be kept between layers of calico to prevent adhesion. A
sheet of rubber is then built up by laying three or four sheets evenly upon
one another, and pressing together to form a sheet 5 mm. thick. The thick
sheet is then roughly cut to shape and vulcanised in a mould by heating in
steam under pressure. From the vulcanised sheet so obtained the rings for
testing are cut (45 mm. internal diameter. 5 mm. face, and 4 mm. thick).
Rings obtained in this manner will not vary in diameter or thickness
(reckoned as sections of a tube), as these are controlled by the size of
the punch, but will vary a little in the face, as this is controlled by the
thickness of the sheet, which depends on the completeness with which the
mould is closed. More recently smaller moulds have been adopted, one mould
for each ring, and an annular space for moisture to develop a pressure
during vulcanising and prevent porosity. The moulds are vulcanised in an
oil bath, or oven of some description, in which a constant temperature is
maintained. I have adopted for some years a third method. The principle is
that used in the factory for making annular-shaped rubber articles, such as
washers, rings, elastic bands, etc. An aluminium mandrel, 45 mm. external
diameter, is taken, and the thin rubber sheet is wrapped round this, so as
to build up a tube about 4 mm. thick, the surplus rubber is cut off, and
the edge bevelled with a wet knife. The manipulation will vary somewhat
with the type of compound to be treated; thus, in some cases, it is
sufficient to well roll the tube with a hand roller to secure adhesion. In
other cases it is better to wipe the sheet of compound with a rubber
solvent previous to rolling. In the latter case time must be given for the
solvent to evaporate before vulcanising. The tube is next tightly wrapped
in wet cloth, and is then ready for the vulcaniser. Or the tube may be
enclosed in moulds which form an outer circular shell and take the place of
the cloth, but for most purposes, and in particular for the rubber-sulphur
mixing usually employed, it is sufficient to use cloth to obtain even and
regular tubes. The tube, after vulcanising, is slipped on to a wooden
mandrel and cut into rings on a lathe. Of these rings the internal diameter
is constant, for this is formed on the mandrel, also the face, which can be
cut accurately in the lathe, but the external diameter, and consequently
the thickness, may vary a little.
[39] "Plantation Rubber and the Testing of Rubber."
[40] "Estate Rubber."
It appears, therefore, that all methods result in rings of approximately
the correct size, and it is usual to check, and, if necessary, make an
allowance for variation in dimensions. It is not possible to do this, even
approximately, with soft rubbers, as the rubber gives under the pressure of
the micrometer. No doubt a photographic method would give more accurate
results, but would take too long. I have found that a very close
approximation is obtainable by weighing the rings as the specific gravity
of the standard rubber mix is known. It is not necessary to weigh each
ring, but the whole five or ten taken for testing may be weighed together.
The next point that arises is the choice of a formula for the test mix.
Practically all the work to date has been carried out on mixtures of rubber
with 7 to 10 per cent. of sulphur. For some purposes--_e.g._, detecting
variation in rate of cure--this mixing is satisfactory, but for other
purposes it is not. Nor is the behaviour of a rubber-sulphur mixing a sure
guide to the behaviour of one containing other ingredients, such as
litharge. Thus, two samples vulcanised satisfactorily when mixed with
sulphur only, but one of them gave unsatisfactory results in the presence
of litharge. It has long been recognised that mineral ingredients may
modify the product when vulcanised, but the modification is not necessarily
uniform. Consequently, tests should also be made, when practicable, with
vulcanised rubber containing other ingredients in addition to sulphur.
As regards physical tests on the vulcanised products, these usually involve
determination of breaking load and elongation at rupture (usually recorded
as final length--that is, including the original length reckoned either as
unity or as 100 units). Simultaneously a load-stretch curve is recorded on
an autographic attachment. The type of curve varies with (1) state of cure,
or degree to which the rubber is vulcanised; (2) proportion of sulphur
and/or other ingredients; (3) specific nature of the rubber used. The last
factor is almost negligible compared with the two former--at any rate for
average quality rubber. As (2) is kept constant for any batch of tests, or
even for every test, it follows that the load-stretch curve is mainly
dependent on the state of cure, and the degree of vulcanising may be
measured by comparing either the elongation produced at a given load or the
load produced at a given elongation. Either set of figures is readily
determined by measuring up the load-stretch diagram.
The peculiar type of the curves has long been a subject of comment and
speculation. Special properties have been attributed to the "slope" or
inclination of the upper and approximately straight portion of the curve.
According to the writer's investigations, the "slope" is largely dependent
on the degree of vulcanisation, so that it is difficult to "place" as an
index of the specific nature of a rubber.[41] Moreover, it has recently
been shown that the peculiar type of curve given by vulcanised rubber is
the result of plotting the load against the sectional area of the
unstretched test piece, whereas this area decreases progressively as the
test piece stretches. If this decrease be allowed for, the curve obtained
is an equilateral hyperbola.[42] Preliminary experiments with rubber
compounded with large proportions of finely divided mineral matter, such as
carbon black, show that the load-stretch curves obtained autographically
are likewise reducible to equilateral hyperbolae.
[41] Bulletin R.G.A., October, 1921, p. 397.
[42] _Hatschek Journal Soc. Chem. Ind._ 1921; _Trans._, p. 251.
CHAPTER XXIII
_THE PROPERTIES OF RUBBER_
This section, like the last, is divisible into two subsections. The first
deals with raw rubber, the second with vulcanised rubber.
We have already explained that, until recently, rubber was not used in the
unvulcanised condition, but that the excellent physical properties of
plantation rubber have made this possible. It is interesting to compare the
physical properties of raw rubber with that vulcanised with sulphur. A
compact sample of crepe as received from the East will give breaking strain
of over 30 kilos per sq. cm. and over 300 per cent. elongation. When mixed
with sulphur and vulcanised, a breaking strain of 150 kilos and elongation
of 1,000 per cent. are not unusual. It is possible that crepe rubber would
give higher figures if it could be prepared in the form of a compact ring,
as used for tests on vulcanised rubber. In any case, the figures for
vulcanised rubber are much in excess of those for raw crepe rubber. It must
also be remembered that a breaking strain of 150 kilos is not permanent
with vulcanised rubber, for reasons which will be explained later.[43] To
obtain a reasonably permanent vulcanised product, the vulcanisation would
not be carried further than to give a figure of 100 kilos. On the other
hand, raw rubber is remarkable on account of its great permanency, although
subject to some physical changes at ordinary atmospheric temperatures.
Tensile tests, although valuable, do not tell us all about the physical
properties of a sample of rubber. Abrasion tests, or tests designed to
measure resistance to wear and tear, would be more valuable, but,
unfortunately, these properties do not lend themselves to simple tests.
There are grounds for believing that raw rubber is superior in some
respects to fully vulcanised rubber, if prepared without the addition of
finely divided mineral substances which exert a toughening effect.
[43] _Journal Soc. Chem. Ind._, 1916, p. 872.
Sheet rubber gives results in some ways inferior to compact crepe rubber
when subjected to physical tests. Tensile strength seldom exceeds 15 kilos,
but the elongation is usually higher--up to 600 or 700 per cent. That is to
say, it stretches more, but breaks more easily. If, however, we take into
consideration the diminution in sectional area of the test piece during
stretching, it will be seen that crepe and sheet rubber have compensating
properties.
As this matter of sectional area reduction during stretching is important,
both for raw and vulcanised rubber, it may be briefly referred to here.
When rubber is stretched, the volume does not appreciably alter--at any
rate, as regards uncompounded rubber. Hence the reduction of sectional area
on stretching bears a simple relationship to the amount of stretching. If
we double the length of the test piece, we halve the sectional area; if we
treble the length, we reduce it to one-third, and so forth. Hence, if we
multiply the breaking strain by the final length (_i.e._, length at break,
taking the original length = 1), we obtain a figure, the "tensile product,"
which embodies both breaking strain and stretching capacity. In effect it
gives us the breaking strain calculated on the sectional area at the
_moment of rupture_ of the test piece. Adopting this formula, we obtain for
crepe--
_Tensile _Final Length--i.e., _Tensile
Strength._ Elongation + 1._ Product._
30 x 4 = 120
and for smoked sheet
15 x 8 = 120
The difference in properties between crepe and sheet may probably be
attributed to the heavier rolling of the crepe; which compacts the rubber.
But if the crepe is rolled too much, the tensile strength falls, and there
is no increased elongation to compensate. For the same reason, crepe which
has been rerolled in this country is inferior to crepe as received direct
from the plantation. At the most it is permissible to unite two or three
layers of thin crepe to a thicker one by a single passage through even
speed rollers, if the physical properties of the original rubber are to be
conserved.[44]
[44] Bulletin R.G.A., February, 1922, p. 64.
Attempts to prepare crepe for use in a raw state, by rerolling uneven or
irregular surfaced crepe in this country, only result in a rubber with
inferior physical properties. Nor can sheet be rerolled to give crepe of
good physical properties. The power required to break down the sheet and
the heat developed, even on cold rollers, are an indication of physical
properties destroyed. For subsequent vulcanisation this is not a matter of
importance, because the vulcanising process restores to the rubber the
properties which are lost in the process of rolling and milling or
mastication.
Raw rubber has been used to some extent for proofing purposes, as for the
manufacture of material for hoods of motor-cars. In this case no attempt is
made to preserve the physical properties. The rubber is masticated, mixed,
taken up with solvent and spread on the cloth exactly as if it were to be
vulcanised.
VULCANISED RUBBER.--We have already explained that the properties of
vulcanised rubber are dependent, to some extent, on the specific nature of
the raw rubber, or what De Vries terms the "inner qualities." That is to
say, differences appear on vulcanising which are not apparent from the
tests made on the raw rubber. Indeed, no investigation or analysis of the
raw rubber can enable one to foresee exactly how the rubber will behave on
vulcanisation. This illustrates the deficiency in our knowledge of
vulcanisation. When dealing with soft, resinous, or decomposed rubbers, it
is safe to anticipate a weak vulcanised product; but when we come to deal
with a number of samples of "standard" crepe or sheet--_i.e._, sheet or
crepe passing a certain standard of appearance--it is found that
differences in vulcanising properties cannot be foreseen. This matter is,
however, not so great a drawback as might be imagined, for reasonably well
prepared consignments of standard crepe or sheet differ but little from
one another, and the difference is mainly in the ease with which they break
down, or the rate or speed with which they vulcanise, and not with the
properties of the vulcanised product. Many of the plantation scrap grades
are equal to or nearly equal to "standard"; but some of these, as also the
rubber produced by native holders, show appreciable variation, and are the
source of most of the complaints which emanate from manufacturers. We shall
consider in turn the different grades and the effect of the usual surface
defects, such as mould, spots, etc.
CREPE RUBBER.--Oil marks and tackiness are the most serious defects from
the manufacturing standpoint. In the first part of this book we have shown
that damage caused by the so-called oil marks is not due to the oil, but to
traces of copper from the bearings of the machines. There are several
metallic compounds which cause deterioration of rubber both raw and
vulcanised, but copper is the most deadly, and rubber showing signs of
deterioration is rightly rejected by the manufacturers.
The only other defect of crepe rubber which has any bearing on its use in
manufacture is mould. Crepe rubber very seldom shows the ordinary surface
moulds not uncommon in sheet-rubber. There are, however, microscopic
growths which cause the development of coloured spots referred to in detail
in the earlier part of this book. The rubber hydrocarbon itself does not
appear to be affected by the moulds, but some of the serum constituents are
altered, with the result that the rubber vulcanises more slowly than it
otherwise would do. For this reason, crepe rubber with coloured spots may
give rise to trouble in the factory.
SHEET RUBBER.--The commonest defect is mould.[45] This is usually of a
light surface type, easily brushed off, and numbers of vulcanising tests
failed to trace any reduction in rate of vulcanising or other defect due to
this. In spite, however, of the harmlessness of light surface moulds, they
are looked upon with suspicion by the manufacturer. Occasionally samples of
smoked sheet are offered contaminated with a "heavy" type of mould. The
sheet feels damp and "heavy" or flabby, and contains an excess of moisture;
sometimes a moist exudation is noticeable on the surface, and "virgin"
patches are present. Such sheet vulcanises more slowly than F.A.Q. samples,
but does not necessarily show other defects after washing and drying.
[45] Bulletin R.G.A., February, 1921, p. 97; April, 1921, p. 190; June,
1921, p. 243; November, 1921, p. 472.
"Stretching rusty," as already explained, is due to a dry film on the
surface of the sheet, and according to a recent investigation, this film
consists, not of serum substances, but of a microscopic mould growth, which
presumably grows on the serum substances. A sample of sheet which stretches
rusty gives the rubber a "dry" appearance, and for a long time
manufacturers mistook the surface film for resin. On the assumption that
such rubber was "resinous" they rejected it, and to this day it is regarded
as a defect, although it has no influence on the vulcanising properties of
the rubber.
It is hardly necessary to point out that defective appearance, such as is
due to thickened edges, faint markings, bubbles, and so forth, have no
effect on the vulcanising properties of the rubber. They only point to some
irregularity or carelessness in preparation. The only justification for
distinguishing between rubber of good and bad appearance is that the former
bears the impress of careful preparation, and is therefore more likely to
be uniform in rate of vulcanising.
Similar considerations apply to the colour of smoked sheet, which may vary
from a pale yellow-brown, through various shades of red-brown to dark
brown. There are various factors affecting the colour, but the buyer can
see but one--viz., the "degree" of smoking--and the rubber, from his point
of view, may be undersmoked or oversmoked. No doubt the degree of smoking
affects the vulcanising properties, but to a less extent than was at one
time imagined. In a recent paper[46] it has been shown that the average
breaking strain and rate of cure of a number of samples of smoked sheets
were practically the same for light as for dark sheets.
[46] Bulletin R.G.A., December, 1921, p. 521.
VARIATION IN PHYSICAL PROPERTIES.--A very large number of tests on
vulcanised specimens of plantation rubber have been carried out. The
rubber was almost invariably mixed with 7 to 10 per cent. of sulphur, and
no other ingredient, and vulcanised to give the maximal breaking load.
Unfortunately, this determination is subject to a very appreciable
experimental error, so that a large number of determinations are necessary
to give a reliable figure. It is quite impracticable to make a large number
of determinations in routine testing, on account of the labour involved. It
is usual to make five, or possibly ten, determinations, although some
investigators have been content with two. It is generally conceded that any
exceptionally low figures should be ignored, as probably caused by some
flaw or irregularity in the test piece. On the other hand, a study of
actual determinations shows an occasional excessively high figure, and it
is questioned whether this also should be left out of account. Others
ignore all except the highest figure, and take this to represent the true
breaking strain. As a consequence, the figures published by different
workers show considerable variation. De Vries has analysed a large number
of the figures obtained in systematic examination of estate samples, and
has constructed curves to illustrate the results.[47] It is open to
question how far the variations shown are attributable to experimental
error. The figures show, however, that the variation in breaking strain is
relatively small, and not very different for crepe and sheet rubber. In our
opinion, undue importance should not be attached to very high or
exceptionally high figures for breaking strain, which are occasionally met
with. Provided the figure does not fall much below the average, the sample
may be regarded as satisfactory. It is very seldom that any sample of first
latex estate rubber does not show satisfactory figures.
[47] "Estate Rubber," p. 466.
THE RATE OF CURE OR RATE OF VULCANISATION is subject to more exact
measurement, whether this be based on the physical or the chemical
properties of the rubber. If the testing machine be provided, as is usual,
with an autographic attachment, the position of the curves traced on the
recording paper gives a measurement of the rate of cure. These load-stretch
curves, to which reference has already been made, take up a definite
position in accordance with the physical properties; it is only the length
of the curve, or the point where it terminates (which gives the breaking
strain and elongation at break), which is largely fortuitous.
As a measure of rate of cure we may take the actual measurements made on
the record.[48] It is convenient to measure the elongation produced by a
load of 130 kilos per sq. cm., as all fully vulcanised rings of soft rubber
should give higher breaking load figures. For less cured or weaker samples
a lower figure may be taken, such as 60 kilos. We have found that when
fully vulcanised to give the maximal breaking strain, the elongation at a
load of 130 kilos is in the neighbourhood of 850 per cent. (final length
950 per cent.). This applies to ordinary samples of estate rubber under the
conditions of testing indicated above. If, however, the proportion of
sulphur be considerably reduced, or mineral ingredients in a fine state of
division be added to the mixing, or accelerators, whether organic or
inorganic, be employed, the above relationship no longer holds. Nor does it
hold with regard to plantation rubber prepared in an exceptional manner,
as, for instance, matured coagulum or "slab."
[48] Bulletin R.G.A., June, 1921, p. 246.
There is a second method of determining the rate of cure--namely, by
analysing a vulcanisate produced under standard conditions, and determining
the amount of sulphur which has entered into chemical combination with the
rubber. For this purpose the weighed sample is cut thin or creped thin, and
exhaustively extracted with acetone to remove any "free" sulphur--that is,
sulphur not in combination with the rubber. The sulphur remaining is then
determined and calculated as a percentage of the raw rubber contained in
the sample taken. This gives the so-called coefficient of vulcanisation.
If we compare the coefficient with the time of cure at a constant
temperature for an ordinary sample of plantation rubber, they are found to
be approximately proportional, so long as the sulphur is in sufficient
excess. The amount of combined sulphur is, therefore, an index of the time
vulcanisation has been in progress (under standard conditions of
temperature, etc.), and, therefore, the coefficient is a measure of the
rate of cure.
The change in position of the load-stretch curve is not directly
proportional to the time of heating, and it therefore follows that it is
also not directly proportional to the coefficient. For ordinary samples of
crepe and sheet the relationship is, however, not very far removed from
proportionality. This applies particularly to sheet rubber. The
relationship is readily seen on plotting one against the other and tracing
the curves. For sheet we get an almost straight line; for crepe there is
some curvature.[49] For ordinary estate samples of sheet and crepe rubber
the maximal breaking strain is obtained when the coefficient reaches
approximately five units, so that this corresponds to the elongation of 850
per cent. at a load of 130 kilos.
[49] Bulletin R.G.A., June, 1921, p. 246, October, 1921, p. 398.
Either physical or chemical methods may, therefore, be used for determining
the rate of cure of ordinary sheet or crepe rubber, but great care must be
taken when interpreting the results obtained with rubber prepared in an
unusual manner. The rate of cure may be expressed in terms of the time
taken to vulcanise the rubber at a constant temperature (in our case 138 deg.
C.), so as to give an elongation of 850 per cent. at a load of 130 kilos,
or to give a coefficient of five units. The higher the figure so obtained,
the slower curing the rubber. To express the results more directly as rate
of cure, we have adopted the plan of taking an average crepe rubber,
calling the rate of cure 100 units, and expressing the rate of cure of
other samples in these terms. Thus, a sample which gave a coefficient of
four only, in the time taken by the standard to give a coefficient of five,
would have a rate of cure four-fifths of the standard, that is, 80; or if a
sample takes only two hours to give an elongation of 850 per cent., whereas
the standard takes three hours, the rate of cure of the sample will be 3/2
of standard or 150.[50]
[50] _Journal Soc. Chem. Ind._, 1918, p. 280.
As stated, the coefficient is approximately directly proportional to the
time of cure; it is also independent of the proportion of sulphur, if in
fair excess, and in the presence of inert ingredients. It is also
independent of the amount of mastication given to the original raw rubber,
however great. On the other hand, the position of the load-stretch curve is
variously modified by these factors--in some respects, therefore, the
coefficient is a more reliable index. However, the coefficient is
influenced by accelerators, so that here also great care must be exercised
when interpreting results. For the purpose of detecting variations in rate
of cure, it is best to choose a mixing which is particularly sensitive. In
the first place, there must be an ample excess of sulphur; and in the
second place, no ingredient should be added which will complicate the
load-stretch curves, and no accelerators should be present which may
possibly tend to obscure the vulcanising properties of the rubber itself.
It has been found, therefore, that the best mixing to use consists of
rubber with an excess of sulphur--say, in the proportion 9:1 without other
ingredients. The rate of cure of a specimen of plantation rubber is
attributed to the presence of certain natural vulcanising catalysts,
because it is found that carefully purified raw rubber (that is, with the
resinous and nitrogenous constituents removed) vulcanises very slowly or
hardly at all, but that on replacing the extracted matter the rate of
vulcanising is restored. The natural catalysts contained in the extracted
matter are influenced to a varying degree by some of the common ingredients
of manufactured rubber articles. This applies particularly to litharge
(oxide of lead), to which reference has already been made. Thus, acetone
extraction of raw rubber to remove resinous matter has but little effect on
the vulcanising properties of a mixture of rubber and sulphur. But if
litharge be a constituent, it is found that acetone-extracted rubber will
hardly vulcanise at all. From this, it follows that a rubber giving a low
acetone extract may be found to vulcanise exceptionally slowly in a mixing
containing litharge, whereas it shows no such defect when compounded with
sulphur only.[51] Litharge is used to a very large extent, as it has a
balancing effect in a rubber compound--that is to say, it allows of
appreciable variation in vulcanising conditions, without corresponding
alteration in the state of cure.[52]
[51] _Journal Soc. Chem. Ind._, 1916, p. 874.
[52] _Ibid._, 1915, p. 524.
INFLUENCE OF VARIOUS FACTORS IN RAW RUBBER PREPARATION ON THE "RATE OF
CURE," OR "RATE OF VULCANISATION."--As the capacity of a rubber for
vulcanisation depends on the presence of small quantities of accessory
substances in the serum which act as catalysts, the rate of vulcanisation
(or curing) will depend on the nature and quantity of such substances
present in the rubber. A very small quantity of these substances has a
considerable influence on rate of vulcanising, and as the substances are
difficult to isolate and identify, our knowledge of their formation and
chemical nature is not as definite as is desirable. Substances have been
isolated having the characteristics of "simpler bases." Bodies of this
class are formed by putrefaction of organic matter, and can be separated in
much larger quantity from coagulated latex, which has been allowed to
putrefy before working up than from such which has been worked up without
giving time for an appreciable amount of putrefaction to take place.
Further, rubber from putrefied coagulum vulcanised much faster than that
ordinarily prepared, so that we are justified in connecting the
putrefaction bases with the rate of vulcanisation. Moreover, it has been
shown that any treatment of the latex or coagulum which inhibits the
development of putrefactive organisms also prevents the rubber vulcanising
as fast as would otherwise have been the case.[53] Also, the crude bases
isolated from fast vulcanising rubber have the power of increasing the rate
of vulcanisation when added to ordinary slow vulcanising rubber.[54]
[53] Eaton and Co-workers: See Bulletin No. 27, F.M.S. Department of
Agriculture.
[54] _Journal Soc. Chem. Ind._, 1917, p. 365.
On the other hand, there are one or two facts which are difficult although
not impossible to fit in with theory. Thus, although the putrefaction bases
are very easily soluble in water and acetone, they cannot be removed by
washing on the creping rollers, or by acetone extraction. This may be due
to the power of colloidal substances to retain other crystalloidal
substances, such as the bases, which, in consequence, cannot be washed out.
A parallel case is the retention of small quantities of water soluble
substances in the soil. Also, the theory does not explain why rubber
obtained by evaporation of latex at relatively high temperatures is fast
vulcanising, although the possibility of putrefaction is excluded.
As regards practical results, it follows that the rate of vulcanisation (or
cure) of a sample of rubber will depend on the time allowed to elapse
between the collection of the latex and treatment till the rubber is dry,
as also on atmospheric conditions. Thus, slow drying will result in an
increased rate of cure, for it gives an opportunity for putrefactive
organisms to play a part. The results will, however, be influenced by the
extent to which the rubber was washed previous to hanging, and so forth.
Smoking is an antiseptic process and will, therefore, tend to inhibit the
action of micro-organisms and produce a slower vulcanising rubber. On the
other hand, sheet contains more serum than crepe, so that there is more
food material for growth of micro-organisms. The net result is to give a
rubber (sheet) which usually vulcanises a little faster than crepe.
Among other factors controlling the rate of cure, special mention should be
made of the nature and amount of coagulants. Weak "organic" acids, such as
acetic, lactic, tartaric, etc., used in the minimal proportions (1 to 1,200
of standardised latex in the case of acetic acid), give the fastest
vulcanising rubber; "strong" mineral acids, such as sulphuric acid, even
when used in the minimal proportions (1 to 2,000), yield slower vulcanising
rubber. Acid salts, such as alum, are intermediate in effect. Increased
proportions of coagulant cause a reduction in rate of vulcanising with all
coagulants, and the effect is least noticeable in crepe rubber,
intermediate in sheet rubber, and most pronounced in "slab" rubber
(discussed below).[55]
[55] Bulletin R.G.A., July, 1919, p. 39; September, 1920, p. 343; November,
1920, p. 433; October, 1921, p. 393; March, 1922, p. 134.
OTHER TYPES OF PLANTATION RUBBER.--We have up to now confined our attention
to ordinary thin air-dried crepe and smoked sheet, as almost all plantation
rubber is now marketed in one or other of these two forms. There are,
however, other types, to which reference has been made. Of these, the most
important is the thick blanket crepe, made chiefly in Ceylon by rolling
together thin crepe, which has been artificially dried (Colombo drier or
vacuum drier). The heat of the driers causes a surface stickiness, which is
got rid of by rolling several thin layers together to give one thick one.
This rubber vulcanises at about the same rate as ordinary thin crepe, for
the relatively high temperature of drying does not appear to influence the
rate of cure. The rubber is generally softer than air-dried crepe, and is
easily "let down" in naphtha; it is, therefore, suitable for some solution
work. Generally speaking, the properties of blanket crepe do not differ
materially from ordinary thin crepe. Another type of rubber seldom met with
is matured slab or crepe, prepared from it. This type of rubber is being
made in small quantities on one or two estates, who supply direct to the
manufacturer. The method of preparation has already been described. It is
unsuitable for sale in the open market, as it contains a variable amount of
moisture, has the various surface defects such as slime, mould, and "rust,"
and there is the additional disadvantage that it is not easy to judge of
its cleanliness or freedom from coarse impurities by inspection. If the
slab rubber be creped and air-dried on the spot, the product is of
satisfactory appearance, except that it is of low colour and may be
streaked. As the crepe so produced vulcanises almost as fast as the
original slab, the crepe embodies all the advantages of a fast curing
rubber with few of the disadvantages of the slab itself. We have made
experiments from time to time, and found that by a judicious use of sodium
bisulphite it is possible to produce a fast vulcanising crepe rubber
sufficiently even and light in colour to satisfy the Standards Committee.
A fast curing raw rubber is not necessarily a desirable type for all
manufacturing purposes. In the vulcanising of large masses of rubber, a
slower rather than a faster vulcanising rubber may be desirable, so as to
give ample time for the heat to penetrate and spread evenly throughout the
mass. But for many purposes a fast curing rubber enables a larger output to
be obtained, so that artificial organic accelerators are coming more and
more into use. The addition of such accelerators might be obviated, if a
suitable fast curing rubber were available, but it is essential that such
rubber should be uniform. It is just in this respect that slab rubber or
crepe made therefrom is found to be deficient.[56] The rate of cure depends
on the functions of wild bacteria, which are naturally sensitive to changes
of conditions, such as temperature, etc. The coagulated rubber depends on
chance circumstances for infection, and, as a natural result, the activity
of the bacteria and the nature and amounts of active vulcanising agent
produced will vary and be difficult to control. Consequently, the rate of
cure of slab rubber shows considerably greater variation than ordinary
crepe or sheet.[57] This, in our opinion, is the main difficulty of
utilising "slab," or crepe prepared from it. Experience in other
industries, using micro-organisms, has shown that the only method of
control has been to replace the wild growths by cultures of some particular
strain, as, for instance, in yeasts for brewing. To control the rate of
cure of slab, it might be possible to use a special culture for the
purpose.
[56] Bulletin R.G.A., January, 1920, p. 6; January, 1921, p. 47.
[57] _Ibid._, January, 1920, p. 68.
Other less usual methods of preparation, referred to in the earlier part of
this book, do not call for particular mention, as the properties of the
rubber do not differ much from ordinary sheet or crepe. It is mainly a
matter of variation in rate of cure.
This short account of the vulcanising properties of plantation rubber would
not be complete without a reference to Fine Hard Para, the premier rubber
of the Amazon. This rubber has come to be regarded as the standard
high-grade product with which plantation rubber may be compared, and many
manufacturers are still of the opinion that it is unsurpassed by any
plantation product. Yet, when subjected to the ordinary vulcanising tests,
we find that samples of Fine Hard Para give figures very similar to average
plantation rubber; indeed, it is not difficult to find specimens of
plantation rubber which give appreciably higher figures on testing. It is
claimed, however, that Fine Para is more uniform than plantation rubber,
and can be relied on always to give the same results. Yet tests on a series
of Fine Hard Para specimens gave variations in rate of cure similar to
those found for plantation. Some figures were published, which tended to
show that the variation was smaller for Fine Para, but it turned out that
each of the samples taken for examination consisted actually of a number of
slices cut from different balls, so that greater uniformity was not
unexpected.[58] The superiority of Fine Para is, therefore, somewhat of a
mystery. It is probable that some manufacturers prefer to use it because
they feel safer with it, and know actually how it will behave from long
experience. In one respect Fine Para is possibly superior to most
plantation rubber--that is, for the preparation of raw rubber solution for
sticking the seams of waterproof garments, and for similar purposes. The
method of preparation may well influence the strength of the raw rubber
when used for this purpose. Plantation rubber has been prepared in the same
manner as Brazilian Para, in particular on an estate in Java. The product
resembles Brazilian Para in appearance. Vulcanising tests gave satisfactory
figures, but, as already stated, this would not serve to show that the
rubber was equal to Brazilian Para from the manufacturer's standpoint.
[58] Bulletin R.G.A., September, 1920, p. 347.
INDEX
Acetic acid, 74, 279
Acid, acetic, 74, 279
--, carbonic, gas, 289
--, formic, 279
--, hydrochloric, 282
--, hydrofluoric, 282
--, mixing, with latex, 79
--, nitric, 282
--, oxalic, 279
--, pyroligneous, 282
--, quantity of, 76
--, sulphuric, 279, 286
Acids for coagulation, effect of, on rate of cure, 323
--, quantities necessary for modern requirements, 78
Air-drying, aids to normal, 143
--, of crepe, rate of, 138
--, progress of, 140
Alcohol, coagulation with, 289
Alum, coagulation with, 282
Anti-coagulant for transport, 61
Anti-coagulants, 46
Artificial driers, 133, 148
Ash on sheet, 276
Assembling cases for shipment, 156
Bags for packing, 154
Bakau, 147
Bales for packing, 154
Bark in crepe, 232
-- -- shavings, 56, 123
-- -- sheet, 276
Bases in vulcanised rubber, 322
Basket plants, 10
Blanket crepe, properties of, 324
Blemishes of surface, 252
Blister in sheet, 272
Block rubber, 129, 246
Breaking down of rubber, 304
-- load of test piece, 311
Bubbles in sheet, 269
Buildings, 159
Bulking latex, 69
Byrne curing process, 270
Calendered sheet, 304
Carbon dioxide, 289
Carbonic acid gas, 289
Cases, choice of, for packing, 153
Catalysts, natural, in rubber, 312
--, vulcanising, 312
Centralisation of factories, 221
Chinese vinegar, 286
Chinosol, 238
"Chula" drier, 148
Clippings, sheet, 275
Coagulant, 111
--, choice of, 74, 278
--, quantity of, 113
Coagulation, 74, 111
-- centres, 62
--, premature, 46
--, spontaneous, 294
-- with alcohol, 289
-- -- sugars, 287
-- -- various salts, 288
Coagulum, soft, 249
--, spongy undersurface of, 249
--, tearing of, 249
--, transport of, 59, 63
--, working of, 103
Coefficient of vulcanisation, 319
Collecting latex, 38
-- pails, 48
Collection, advantages of early, 60
Combustion, rate of, in smoke house, 191
Compound crepes, 126
-- -- No. 1, 151
-- -- No. 2, 151
Contents of cases, weight of, 156
Copper salts, cause of tackiness, 243
Cotton fibre in crepe, 230
Creosotic substances, 146
Crepe, air-drying of, 132
--, bark in, 232
--, bearing of defects in, on manufacture, 316
--, bisulphite streaks in, 235
--, colour of fine, 114
--, dirt in, 227
--, dirty edges of, 225
--, drying houses for, 178
--, fibre in, 230
--, general style of finish, 223
--, grades of, 150
--, greenish, tacky streaks in, 228
--, iron stains on, 225
Crepe, No. 1 fine pale, 110
--, oil marks on, 226
--, oxidation streaks in, 234
--, rate of air-drying of, 138
--, rust stains on, 226
--, smoked, 130
--, surface moulds on, 241
--, weight increased in drying house, 141
--, yellow latex streaks in, 234
-- rubber, defects in, 223
-- --, lower grades of, 120
-- --, preparation of, 110
-- --, tensile strength of, 313
Cups, cleaning, 40
--, water in, 44
Cure, rate of, 318
Curing, 307
Da Costa process, 290
Decentralisation of factories, 221
Defects of sheet, infrequent, 276
Derry process, 293
Designs and "layout" of tanks, 176
Dirt in sheet, 276
Discoloration of rubber, dark, 249
Drains for tanks, 176
Drier, Colombo Commercial Company's, 136
Driers, artificial--for crepe rubber, 133
-- --, for sheet rubber, 148
--, "Chula," 148
--, vacuum, 134
Drum furnaces, horizontal, 189
Drying chamber, floor of, 187
-- --, arrangements of, 186
-- houses for crepe, 178
-- --, hot air, 182
-- --, ventilation of, 185
-- --, windows of, 185
-- of rubber, 132
--, period of, 145
--, rate of, effect on rate of cure, 323
Earth scrap, 124
-- --, collection of, 58
Edges, thickened, after rolling, 251
Elongation of test piece, 311
Ends, thickened, after rolling, 251
Engines, 170
--, position of, 174
Factories, 172
--, centralisation of, 221
--, decentralisation of, 221
--, number of floors, 174, 178
--, ventilation of, 180
--, windows of, 181
Factory buildings, situation of, 216
--, choosing site for, 220
--, ideal arrangement of, 162
-- operation, 65
Fibre cotton, in crepe, 230
Field maintenance, 13
Fine hard Para properties of, 325
First latex and other grades, percentage of, 59
Floor of drying chamber, 187
-- factories, 173
-- furnace room, 196
Formalin, 87
Formic acid, 74, 279
Formula for test mix, 311
Freezing (coagulation) process, 293
Fuel, consumption of, 196
Fuels for smoking, 146
Furnace room, floor of, 196
-- --, Petaling type of, 192
Furnaces, horizontal drum, 189
--, "pot," 188
Germination, 6
Grades, number of, 151
Grading, 150
Grafting, 8
Grass squares, 14
Greasiness before smoking, 252
-- of surface, 258
Grit in crepe, 232
Hand rolling sheets, 104
Hevea Brasiliensis, 1
Hot air drying houses, 182
Hydrochloric acid, 282
Hydrofluoric acid, 282
Ilcken-Down process, 298
Instruments, method of using, 100
--, recording, 144
--, standardising, 98
Lallang, eradication of, 15
Latex, bulking, 69
-- cups, choice of, 40
--, decomposition of, in the field, 270
--, first and other grades, percentage of, 59
--, first quality, 150
--, mixing acid with, 79
--, mixing sodium bisulphite solution with, 117
--, preliminary treatment of, 65
--, reception of, at the store, 65
--, standard, 96
--, standardisation of, 69, 110
--, straining, 67
--, transport of, 59
Light, importance of, in factories, 172
Litharge, 312
Load stretch curve, 312, 319, 320, 321
Low grade rubbers, fibrous matter in, 124
Lower grade rubber, care in manufacture, 129
Lubrication of machines, 166
Lump rubber, naturally coagulated, 120
Machinery, 159
Machines, access to, 169
--, adequacy of, 160
--, arrangement of, 168
--, lubrication of, 166
--, position of, 173
--, sheeting, 166
--, speed of, 162
Mangrove, 147
Marking sheets, 105
Metrolac, 98, 100
Michie-Golledge system, 138
Mildew on surface, 260
Milky residue on serum, 249
Mixing acid with latex, 79
Moist glaze of surface, 258
Mould on surface, 260
Moulds, surface, on crepe, 241
Natural catalysts, 312
Nitric acid, 282
Nurseries, 9
Overcured, 307
Overvulcanised, 306
Oxalic acid, 279
Oxidation, prevention of, 56
--, variation due to, 254
Packing, 150
--, bags for, 154
--, bales for, 154
--, cases, choice of, 153
--, folding for, 155
--, methods of, 156
--, rooms, 211
Pale crepe, No. 1 fine, 110
--, rubber, former methods of making, 118
--, sheet, 89
Patches, 259, 260
--, virgin, 259
Payment by result, 53
Perished rubber, 306
Physical properties of rubber, variation of, 317
Pits for smoke houses, 188
Pitting of surface, 250
Plantation rubber, testing of, 309
Planting, 1
Plasticising of rubber, 304
Plasticity of plantation rubber, 309
"Pot" furnaces, 188
Power units, 170
Premature coagulation, 46
Preparation, special methods of, 290
Pyroligneous acid, 282
Racks, 186
Rate of cure, 307
Raw rubber, physical properties of, 313
-- --, tests on, 309
-- --, uses of, 315
Recommendations, Rubber Growers' Association, 152
Recording instruments, 144
Ribbing of sheet, 274
Rolling, 251
Rolls, grooving of, 164
-- running hot, 165
-- -- "free," 165
Roof of smoke house, 197
Rubber, drying of, 132
Rubber Growers' Association, Recommendations, 152
Rubber, properties of, 313
--, smoking, 109
Rust, cause of, 267
-- on sheet, 262
--, treatment to prevent, 265
Scrap washers, 57, 124
Screw plug, unsatisfactory, 73
Seed at stake, 10
Seeds, 2
Selection, 5
Senang folder, 156
Serum, milky residue on, 249
Sheet, ash on, 276
--, bark in, 276
--, bearing of defects on, in manufacture, 316
--, blisters in, 272
--, bubbles in, 269
--, clippings, 130, 275
--, creases in, 251
--, dirt in, 276
--, "dog ears," 251
--, grades of, 151
--, infrequent defects of, 276
--, pale, 89
--, ribbing of, 274
-- rubber, artificial driers for, 148
-- --, defects in, 249
-- --, preparation of, 89
Sheet rubber, rolling and marking of, 102
-- --, tensile strength of, 313
--, rust on, 262
--, splinters in, 276
--, stickiness in, 274
--, style of, 101
--, support marks on, 273
--, surface pattern of, 274
--, thick ends of, 275
Sheeting machines, 166
Sheets, mis-shapen, 251
--, thickened patches in, 251
--, torn, 251
--, unevenness of appearance, 253
Short weights, 157
Skimming, 100
Skimmings, 122
Slab rubber, 299
-- --, properties of, 324
Slope, 312
Smoke curing of sheet rubber, 143
-- --, temperature of, 144
--, houses, 183
-- --, Barker patent, 209
-- --, Devon type, 200
-- --, iron stoves for, 189
-- --, Jackson type, 200
-- -- of brick, 198
-- -- rate of combustion in, 192
-- --, roof of, 197
-- --, "Third Mile" type, 199
-- --, types of, 198
Smoked crepe, 130
-- sheets, colour of, 255
-- water for coagulation, 286
Smoking, effect on rate of cure, 323
--, greasiness before, 252
-- rubber, 109
Smooth rolling of sheets, 104
Sodium bisulphite, 80, 114
-- --, abuse of, 85
-- --, care of, 116
-- --, deterioration of, 115
-- --, evaluation of, 115
-- --, making a solution of, 85
-- --, quantity of, 84
-- --, residual traces of, 86
-- -- solution, mixing, with latex, 117
-- sulphite, 86
-- --, deterioration of, 115
-- --, evaluation of, 115
Sorting, 150, 152
-- rooms, 211
Spewing, 305
Splinters in sheet, 276
Spontaneous coagulation, 294
Spot disease, 235
-- -- in sheet rubber, 273
Spots, 259, 262
--, virgin, 259
Standard latex, 96
-- sheet, 102
Standardising instruments, 98
Stickiness in sheet, 274
Stock solution, method of making, 75
Storage of rubber, 212, 216
Stoves, iron, for smoke houses, 189
Straining latex, 67
Streaks, 262
Stumps, 9
Sugars, coagulation with, 287
Sulphuric acid, 280, 286
-- --, buying, 280
-- --, formula for use of, 280
-- --, storing, 280
Sun-drying sheet rubber, 147
Support marks on sheet, 273
Surface blotches, coloured, 249
--, dull or black, 258
-- pattern of sheet, 274
Tackiness, cause of, 243
--, copper salts cause of, 243
-- in rubber, 242
Tanks, 90
--, care of, 94
--, designs and "layout," 176
--, drains for, 176
--, installation of, 93
--, situation of, 175
--, water-supply for, 177
Tapping, 38
--, former systems of, 29
-- knives, 38
-- systems, 38
Tartaric acid, 279
Tensile product, 314
Test mix, formula for, 311
-- pieces, making of, 310
Testing of plantation rubber, 307
Thick ends of sheet, 275
Thinning, 19
Timber for smoking, 147
Tool sheds, 216
Transport, 60
-- by coolie, 62
-- of coagulum, 59, 63
-- of latex, 59
Trays, 167
Treatment of rubber in the factory, 301
-- to prevent rust, 265
Tree scrap, 55, 122
-- --, care of, 56
-- --, oxidation of, 56
Trees per acre, 26
Trenches, silt catchment, 11
Uniformity, 90
-- in colour, lack of, 246
Vacuum driers, 134
Variation due to oxidation, 254
Vegetable extracts, 289
Ventilation of drying houses, 185
-- factories, 180
Verandas, 175
Virgin spots, 258
Viscosity of rubber solution, 309
Vulcanisation, 301
--, rate of, 318
Vulcanised rubber, 318
-- --, tests on, 309
Vulcanising, 307
-- catalysts, 312
--, "heat," 306
Washers, scrap, 57, 124
Washings, 122
Water-supply for tanks, 177
Weeding, clean, 13
Weights, "short," 157
Wickham process, 293
Windows of drying houses, 185
-- factories, 181
Working of rubber, 304
Yields, 23, 25