_PREPARATION OF CREPE RUBBER_
NO. 1, OR FINE PALE CREPE.--Considering first the preparation of the
highest grade, fine pale crepe, it must be stated that the difficulties
attached to the process are generally not sufficiently appreciated. In this
pale rubber minor blemishes are so plainly apparent that their importance
is highly exaggerated, and what would worthily escape notice in smoked
rubber assumes disproportionate prominence in pale crepes. The very fact
that such a delicate material as colourless coagulum has to be manipulated
in coarse iron rollers, with the attendant oil and grease worries, should
be sufficient to deter one from criticising too harshly the occasional
lapses of an estate struggling to give of its best to the market. At the
same time there can be no doubt that if precautions are taken to attend to
all likely sources of contamination, defects in pale crepe may be avoided
to a wonderful extent; and on some estates the observance of elementary
rules enables the preparation of the finest pale crepe to be made almost
mechanically.
STANDARDISATION OF LATEX.--The question of the standardisation of latex has
been dealt with in a general way in Chapter VII., and the reader is now
familiar with the trend of the argument in its favour. It will be
recognised that the necessity for standardisation exists to the same degree
in the correct preparation of pale crepe as in the case of smoked sheet.
Unless the dry rubber content is invariable, and the quantities of
chemicals fixed, the colour of the crepe may vary appreciably.
It may be pointed out that it is not _essential_ to adopt the same standard
of dilution as for sheet preparation. Given that latices from all fields or
divisions are fairly uniform, and of high rubber content, the standard may
be taken at a figure equivalent, for example, to 2 lbs., or 2-1/2 lbs., or
even 3 lbs. per gallon. It is wise, nevertheless, to take a lower standard
for several reasons. For instance:
(_a_) The average dry rubber content varies with climatic conditions,
position of the cut on the tree, general health of the tree, etc. On a
rainy day the dry rubber content may be lowered too greatly by
adventitious circumstances.
(_b_) Recording instruments often fail to give even approximately
correct readings in rich latex. Errors may thus be made easily.
(_c_) A fairly soft coagulum means easier working on the machines,
less labour, and proportionately cheaper costs.
[Illustration: THREE GRADES OF CREPE RUBBER.
Left to right: fine pale crepe; second quality pale crepe; compound crepe.]
It is advised, therefore, that for general purposes the same standard as
that found suitable for sheet rubber should be taken--viz., 1-1/2 lbs. dry
rubber per gallon. At all events the standard should not exceed 2 lbs. per
gallon.
COAGULATION AND COAGULANT.--Coagulation may be undertaken without any
special arrangement of tanks, and is usually effected in the ordinary
"Shanghai" glazed earthenware jars containing about 45 gallons. Given
reasonable care, and a fairly fool-proof system of calculation for the
quantities of chemicals required, no difficulty need be experienced.
[Illustration: A WASHING SHED.
Sheets are soaked in hot water in tanks in the background, and then
scrubbed under a spray of cold water.]
On a larger scale it is advised that proper reception tanks, in which
standardisation can be effected, should be installed. Where both sheet
rubber and fine crepe are being prepared, the whole system of
sheet-coagulating tanks may be employed with considerable advantage, even
to the insertion of the partitions.
If ordinary jars are used, and the coagulum is left until the following
morning, the mass of rubber has to be cut up into pieces of a size suitable
for the machines. The knives or saws are sometimes rusty, and the colour of
the coagulum is affected. The coolies often feed into the machines lumps
which are too large, with the result that portions are thrust under the
cheek-blocks and become stained.
When a sheet-coagulating tank is used all labour of cutting the coagulum is
obviated. The long strips are handled and fed into the rolls easily. It may
be seen, likewise, that actual work is thus saved in machining.
QUANTITY OF COAGULANT.--For a general discussion on the coagulant and
quantities employed, the reader is referred to Chapter VIII. It is there
recommended that for latex standardised to a level of 1-1/2 lbs. per
gallon, the proportion of pure acetic acid should be in the ratio of
1:1,200. Directions are there given for the making of the solution, and the
calculation of the quantity required for any given volume of latex.
It is pointed out that for average _undiluted_ latex the basis of
calculation, for quantities of acetic acid required, should be taken on the
ratio 1:1,000. Where latex exceeds a dry rubber content of 3 lbs. per
gallon, it may be necessary to increase the quantity of acid to 1:800.
If a standard of 2 lbs. per gallon is adopted, the formula given for the
1-1/2 lbs. standard will not give full satisfaction, and the quantity of
acid solution must be increased slightly in order to obtain complete
coagulation. Assuming that the original solution is prepared in 1 per cent.
strength, the following difference would be noted:
(_a_) One part pure acetic acid to 100 parts water (theoretically 99
parts).
(_b_) _1-1/2 lbs. per gallon._ | _2 lbs. per gallon._
|
Of the above solution use 1 gallon | Of the above solution use 1
to every 12 gallons of standardised| gallon to every 10 or 11
latex. | gallons of standardised latex.
It is not possible to lay down an exact figure governing all cases, as so
much depends upon the treatment undergone by the latex before it reaches
the store.
Some estates continue to use solutions of greater strength, generally 5 per
cent., in crepe preparation. While such solutions may be effectively
stirred in when the latex is dilute, it is advised that for intimate
mixture the solution need not be stronger than 1 per cent.
In estimating the quantities of acetic acid required much depends upon the
interval which is to elapse between the addition of acid and the time of
working of the coagulum. If the rubber is to remain until next morning, the
average formulae will be found suitable; but if it is required to work the
coagulum with an interval of less than three hours, an excess of acid must
be employed. The excess need be comparatively small, unless the interval is
much reduced. For instance, it is the practice on some few estates to begin
the machining of the coagulum about half an hour after coagulation
commences; in which case it is usual to add from a quarter to a half of the
normal quantity in excess. It need scarcely be pointed out that unless this
procedure is strictly unavoidable it should be discouraged on account of
the waste of coagulant involved. Incidentally, the use of strong solutions
of acid under such circumstances may lead to increased deterioration of the
rolls.
COLOUR OF FINE CREPE.--We are sometimes assured that manufacturers do not
pay the attention to the question of colour which sale conditions would
lead one to believe. As far as we are concerned, and as long as there is no
direct traffic between producer and consumer, it must be recognised that in
the vast majority of cases we are forced to concern ourselves only with the
standards set up in the markets. This, in spite of the knowledge that, all
other things being equal, the arbitrary distinctions in colour afford no
indication of the intrinsic value of the rubber. Under present
circumstances it is plain that if paleness is demanded it has to be
supplied.
Probably without exception all estates employ sodium bisulphite as the
agent for the prevention of that darkening (oxidation) which is natural in
drying rubber.
SODIUM BISULPHITE.--A formula for use of this chemical is given in Chapter
VIII., and is applicable to latex standardised to 1-1/2 lbs. dry rubber per
gallon. If a higher standard is chosen the quantity calculated as in (_b_)
of that formula may be increased slightly, and the exact requirements found
by experience. The caution must again be given that the employment of an
excess of sodium bisulphite will lead to the production of an over-pale
rubber, and a prolongation of the drying period. If thick crepes are made,
an excess of the chemical is sometimes made visible by a greyish powder
deposited on the edges of the strips of dry rubber.
It must be emphasised that the formula in Chapter VIII. indicates the
_maximum_ quantities required for use with standard latex. Many estates
will find it expedient to use less of the chemical; and if it is found that
the desired result is not obtained from normal proportions, attention
should be directed to the points discussed in the following paragraph.
EVALUATION AND DETERIORATION OF SODIUM BISULPHITE AND SODIUM
SULPHITE.--Sodium bisulphite and sodium sulphite are both bought for our
purpose in the form of a fine crystalline powder, and on analysis good
specimens should contain over 90 per cent. pure substance, when packed in
well-sealed vessels.
It has often happened that shippers or local sellers, by inadvertence, have
supplied the one chemical in place of the other--to the detriment of the
rubber and the discomfiture of managers of estates. The error, as a rule,
has not been detected for some time, and then perhaps only as a result of
complaints or enquiries reaching the laboratories. To the layman, and
certainly to the native who usually has charge of these substances, it is
not a simple matter to distinguish between them without special knowledge.
There are certain elementary tests, however, which can be applied on all
estates serving to make the distinction, but affording no information
regarding the actual quality of the chemicals. They are given in a
comparative form on page 116. Samples of doubtful specimens may be sent to
the laboratories for analysis, but the bulk of the chemical should not be
used.
During the War some very poor shipments were received, and much trouble was
caused. Under normal conditions there can be no question that it is to the
interests of chemical manufacturers to supply the best article; and it is
anticipated that in future there should be no difficulty in procuring
shipments of a high degree of purity.
_Sodium Bisulphite._ | _Sodium Sulphite._
|
1. If in good condition it | 1. It has no perceptible
has a powerful odour of | odour.
sulphur dioxide.[9] |
|
2. In solution it should turn | 2. In solution it should turn
a blue litmus-paper red. | a red litmus-paper blue.
|
3. It exhibits a marked tendency | 3. The tendency to "cake"
to "cake" if the | is less marked than in
drum is allowed to | the case of the bisulphite.
remain open. |
[9] High-grade sodium bisulphite has very little odour, but by the time it
reaches the estate, and as a result of short exposure to the moist
atmosphere of the tropics, a little decomposition sets in and a strong
odour of sulphur dioxide gas is noticeable.
It will be evident that, as sodium bisulphite under normal conditions gives
off a gas when exposed to the atmosphere, it deteriorates in quality
continuously. It is the potential presence of this gas which makes the
powder effective as an anti-oxidant and disinfectant. It is within the
experience of all accustomed to the handling of this chemical, that in
addition to the loss of gas, the powder cakes into a hard mass on exposure.
If only the top layer is caked, the remainder may be in fair condition; but
no caked portions should be used, as they cannot be of good quality. They
may, however, be used for the treatment of scrap rubber, to be discussed
later.
CARE OF SODIUM BISULPHITE.--The ready tendency of sodium bisulphite to
deteriorate on exposure should give sufficient indication regarding its
treatment in storage. It should be bought only in drums (or other air-tight
containers), and should be stored in a dry place. No drum should be opened
until required, and the common practice of keeping an open drum on the
floor of the factory should be avoided.
Drums are of two sizes, generally containing 1/4 or 1/2 cwt. respectively.
It will be obvious that, although the prime cost may be cheaper with the
larger quantity, it would always be preferable to secure the smaller
drums, as the loss on exposure will be less.
Most commonly the 56 lb. drum is purchased. It should not be difficult to
calculate the period during which the contents will be consumed, on the
basis of a maximum of 1 lb. per 100 gallons of latex. A 56 lb. drum,
assuming no loss or waste, should be sufficient to treat _at least_ 5,600
gallons of latex (say, 8,500 lbs. of rubber)--if the bisulphite is of
first-class quality, and the use is applied only to the preparation of fine
pale crepe.
Where the quantity used per diem is small, it is advised that precautions
should be taken to preserve the quality of the chemical when a drum is
opened. It might be of advantage to place the contents of the drum in
smaller sealed tins, or to have made a special container (with a closely
fitting lid) into which the powder can be placed as soon as the drum has
been opened.
MIXING SOLUTION WITH LATEX.--Emphasis has been laid, in Chapter VIII., upon
the necessity for care in the preparation of the solution. Equal regard
must be given to the mixture of the solution with the latex.
On a few estates it used to be the practice to add the powder to the
solution of acid, with stirring. Obviously this led at least to a great
loss of efficiency, owing to the rapid escape of the gas which was evolved.
The solution of sodium bisulphite should be poured into the latex in as
uniform a distribution as possible. The mixture of solution and latex
should be thoroughly stirred, and if only natives are in charge a minimum
period of five minutes should be prescribed before the addition of the
coagulant. A thorough stirring should again follow the advent of the acid.
If these elementary rules are not observed faithfully, the deficiency will
most probably be manifested in the dry rubber in the shape of streaks of
varying shades of colour.
Finally it may be insisted upon that deteriorated sodium bisulphite should
not be used. In order to obtain an effect double the quantity may be
required, and the residual salts left in the rubber on evaporation of the
moisture will be responsible for prolonged drying, surface deposits, and
possibly "spot disease."
FORMER METHODS OF MAKING PALE RUBBER.--Merely as a matter of historic
interest it may be mentioned that previous to the introduction of sodium
bisulphite pale crepes were made in comparatively small quantity by various
devices, among which the following might be quoted:
(_a_) Use of excessive quantities of strong acetic acid.
(_b_) Extreme dilution of latex in conjunction with excessive
quantities of acid.
(_c_) Extreme dilution in conjunction with steaming and excess of
acid.
(_d_) Extreme dilution of latex in conjunction with excess of acid and
subsequent heating of the coagulum in hot water.
(_e_) The use of excess of a mineral acid such as sulphuric acid.
(_f_) The skimmings and very dilute latex, coagulated with excess of
acid.
WORKING THE COAGULUM.--Description of the details of necessary machinery
for crepe-making is relegated to Section III. of this book. Here we shall
treat only of the matter in general.
In the preparation of crepe rubber heavy machinery is necessary, and ample
engine-power must be available. The machines should comprise three types:
(_a_) With rolls cut in such fashion, and run at such different
speeds, as to have a macerating effect upon the coagulum. Such
machines or rolls will be referred to as "macerators."
(_b_) Intermediate rolls, grooved in varying designs and geared
differentially. These reduce the thick rough crepe obtained from the
macerators into a form suitable for passing to the rolls described in
(_c_). They are sometimes called "crepers," but as this term may be
applied equally to the macerating rolls, they will be termed the
"intermediate" rolls.
(_c_) Smooth rolls usually run at approximately even speeds and, as
their name denotes, devoid of any grooving. They are called "smooth"
rolls or "finishers."
Without such equipment it is not possible to prepare the grade which is
known as "fine pale crepe." In the common acceptation of this term crepe of
No. 1 quality generally connotes fineness and paleness with a thin crepe
which has a good, smooth, and fairly well-knit texture.
It is, of course, possible to make a thick pale crepe, using only the
macerators and intermediates, but the "finish" will be that typical of the
particular grooving of the intermediate rolls. For the preparation of crepe
ordinarily, the possession of smooth rolls is a _sine qua non_.
For reasons which will be explained more fully in the chapter dealing with
the defects of crepe rubber, the practice of preparing thick crepes direct
from the coagulum is now very uncommon. Thick crepes are generally made by
reworking dry rubber, either in the form of thin crepes or from air-dried
sheets. The market for the latter in Malaya is confined almost entirely to
Singapore, where factories buy native rubber and re-work it into thick
crepe.
The bulk of the output of No. 1 crepe from estates is in the form of thin
"fine pale crepe." The artificial standard set up by buyers and brokers
necessitates this thin crepe being of even texture and fairly free from
small holes ("looseness"). What difference the small holes are to make in
the vulcanising properties of the rubber is beyond our knowledge; but such
being the standard, it must be attained if the full price is to be
obtained.
In order to secure the desired effect the coagulum must be passed
consecutively through the three types of rolls, and undergoes a varying
degree of working in each.
Given the necessary equipment of machines, it is possible to make a good
specimen of thin pale crepe if the coagulum passes through all the rolls a
total of twelve times (or even less in exceptional cases). There is no
intention of suggesting that this is possible on all estates. Clearly the
number of times the rubber passes through the rolls will depend upon the
total efficiency of the machines. This in turn involves such factors as
(_a_) the size of the rolls, (_b_) the number of machines of each type,
(_c_) the gearing of the pinions, (_d_) the speed of the drive, etc. Again,
much depends upon the nature of the coagulum worked. A fairly soft coagulum
will offer less resistance, and conversely a dense coagulum will require
more machining.
It has been shown by the writers in previous publications that over-working
of the coagulum has an effect on the vulcanisation of the rubber; and this
has been confirmed by others.[10] Apart from this point, it should be
recognised that over-working, beyond that necessary to produce a thin crepe
of even texture, is to be deprecated, on the ground of economy, in working.
[10] Bulletin No. 27, Department of Agriculture F.M.S., April, 1918,
"Preparation and Vulcanisation of Plantation Para Rubber," Eaton, Grantham
and Day.
Owing to the existing differences in equipment and speed of drive, etc.,
the regular practice of any one estate may be unsuitable for another. It
remains, therefore, a matter of study for each estate to discover the
minimum number of times which rubber should pass through the machines,
consistent with the factors indicated above. In any case it may be assumed
that if any factory cannot prepare a good crepe by passing the rubber, say,
twelve times through the rolls, there is some deficiency in the machines,
or of speed; the coagulum may be too hard, or the rolls may be badly worn.
LOWER GRADES OF CREPE RUBBER.--Even a few years ago it was plain that the
lower grades of crepe (_i.e._, all grades lower than first latex rubber)
were not sufficiently appreciated in the market. There was often a marked
difference in price between a first-grade crepe and crepe made from
naturally coagulated lump. This arose chiefly from lack of knowledge. It
has since been recognised in some measure that no reason exists for such a
wide difference in price, and more recently the margin between even the
first-grade rubber and the lowest grade of scrap rubber has been a
gradually diminishing one. Providing sufficient care is exercised in the
preparation of the lower grades, one would expect to see but very small
difference in prices between any two grades. It is true that adequate
attention has been given to the preparation of the scrap grades only in
comparatively recent years, and it is acknowledged that when high prices
were ruling for first-grade rubbers sufficient attention was not generally
given to the subject of the preparation of the lower grades.
NATURALLY COAGULATED LUMP RUBBER.--The grade of rubber made from the
naturally coagulated lump which forms in buckets and carts is usually of a
mixed colour, due to the fact that the lumps oxidise very quickly. When
they are allowed to remain overnight before being machined, it can be
imagined that the colour of the dry crepe would be very dark, or would
contain very dark streaks. Such is ordinarily the case, unless special
precautions are taken.
Providing that the coagulated lump is free from bark, leaves, and
leaf-stems, and certain other precautions taken, the difference in price
between coagulated-lump crepes and first-grade crepes should be very
slight. Too often, however, not sufficient supervision is given to the
coagulated-lump rubber, and it is common to see it come into the factory
containing leaves and bark. These should be picked out before the latex is
strained, but obviously it would be better to ensure that they did not
enter the buckets in the first place.
It would seem reasonable to suppose that if some means could be employed
for preventing or checking the surface oxidation of naturally-coagulated
lump rubber, there would be a corresponding improvement in the colour of
the dry crepe. That such a method is practicable has been demonstrated on
many estates. The lump when lifted out of the latex is allowed to drain for
a few minutes, and is then (without squeezing) placed in a dilute solution
of sodium bisulphite. A 1 per cent. solution is sufficiently powerful. It
is not to be thought for a moment that by the use of sodium bisulphite any
previous oxidation will be counteracted; all that is claimed for the
treatment is that any further surface oxidation will be checked, and the
rubber may be allowed to remain until the next day, for working, if it is
so desired. It will probably be found that quite a quantity of latex has
been expressed from the lumps by contraction, and acid may be added to
obtain the rubber from this. On other estates the lump rubber is worked on
the machines as it is received, and the resulting crepe is submerged in a
weak solution of sodium bisulphite over-night. It is then rinsed in water
and hung to drip before weighing and placing in the drying house. Under
certain conditions some of the lump rubber darkens rapidly during transport
to the store, and any such oxidised portions must be rejected if a uniform
colour is to be expected in the crepe.
Following the procedure indicated above, some estates find it possible to
prepare from naturally coagulated lump rubber a crepe which can be classed
as No. 1 grade.
SKIMMINGS AND WASHINGS.--The skimmings of tanks, as already shown, may be
prepared sometimes as a second quality of smoked sheet; but generally they
are amalgamated with the rinsings of cups and buckets, treated with sodium
bisulphite and acid, and made into crepe form.
The cup-washings, as they arrive at the store, represent a very dilute
latex, the rubber from which is generally of a greyish colour.
Bucket-washings should yield a good type of pale rubber if they are
obtained properly. To obtain the maximum quantity of good rubber the
buckets should first be rinsed. A gang should be taken, a small quantity
(say a quart) of water poured into the first bucket, and this dilute latex
used progressively in all the buckets of that gang of tappers. The result
is a fair latex which can be added to the bulk of No. 1 latex, provided it
is free from dirt. Where sheet rubber is being prepared, carefully strained
cup-washings or bucket-washings may be utilised in the dilution of the
latex to the required standard, thus increasing slightly the percentage of
first-grade rubber.
TREE-SCRAP.--As tree-scrap is a naturally coagulated rubber, it should be
expected to show up well in quality. This is usually the case; but from
what has been said of the effect of sun-heat it will be understood that if
trees are not regularly "scrapped," there is a danger that the crepes may
be found to contain tacky streaks due to the inclusion of old scrap which
has been sun-baked. In hot dry weather, on widely planted areas tapped on
alternate days, it has been noticed that scrap remaining for two days often
exhibits a resinous appearance, and feels sticky to the touch.
If tree-scrap is to be made as a separate grade, as used to be the general
custom, care should be taken to see that it is free from bark and dirt. On
some estates where scrap-rubber is paid for per pound collected, it is
usually the rule to insist that scrap shall be washed free from dirt and
picked free of bark. This course is to be commended, but might probably
prove impracticable to the majority of estates. Theoretically, of course,
the operation of machining should rid the scrap of all traces of bark; but
in practice it does not do so.
Some proportion of the tree-scrap is usually found to be heavily oxidised,
and naturally if a crepe of uniform colour is to be obtained these dark
scraps must be rejected, otherwise dark streaks will be formed. Coolies
should be instructed to sort out the dark pieces before arriving at the
store.
BARK-SHAVINGS.--It has been intimated in a previous section that the method
of obtaining and collecting bark-shavings varies with the type of labour
employed.
Where the scrap is removed from the edge of the bark on each occasion
before tapping, the amount of rubber to be extracted from the dry shavings
is very small--so small, in fact, that when the price of rubber is low, it
is doubtful whether it pays to collect and work the material.
On the other hand, where trees are not "scrapped" before tapping, the
bark-shavings and tree-scrap are collected together, and the amount of
rubber derived from the mixture may be 30 to 40 per cent. upon the gross
weight--depending chiefly upon the quality of the tapping (_i.e._, in this
case, the thickness of the paring). Another factor influencing this figure
would be the effect of using an anti-coagulant on the cuts.
Bark-shavings entail such wear upon the ordinary machines during working,
especially if fairly free from rubber, that unless the factory is equipped
with a special "scrap-washer" it is advised that this material should be
sent for working to a factory having the necessary equipment. Whenever
possible, bark-shavings should receive treatment on the day of collection.
It used to be quite common to see heaps of bark-shavings accumulating on
the floor of a factory, and generating excessive heat. Yet these heaps were
allowed to stand about for a day or days. Is it any wonder then that
tackiness was found to develop when the rubber was dry? It is here
definitely laid down that no heaps of bark-shavings should be accumulated
even for half a day. Tanks should be provided in which the shavings should
be submerged in water.
EARTH-SCRAP.--Of all grades of crepe this is the one most liable to become
tacky in transit. This tackiness to a large extent cannot be avoided, as
old pieces of earth-scrap may be brought in amongst the bulk. Probably
these old pieces have been exposed to the sun for days, and have become
quite resinous. It would be practically impossible to go through all
earth-scrap in order to find these odd pieces, but unless this were done
one could not guarantee that the earth-rubber would always be free from
tackiness. The difficulty does not appear, however, on estates where
earth-rubber is collected systematically at very frequent intervals.
FIBROUS MATTER IN LOW-GRADE RUBBERS.--It is sometimes found in this and
other lower grade rubbers that pieces of cloth or cotton-waste are
concealed. Coolies may have used them for cleaning cups, or the store
coolies may have been at fault. Earth-scraps especially should be examined,
before working, for such extraneous matter.
SCRAP-WASHERS.--These are heavy machines specially devised for the
treatment of lower grade rubbers. In these the raw rubber is well
masticated and freed from impurities, if the machine functions efficiently.
There are several types of these machines, all of which are efficient. That
best known is the "Universal" washer, made by Joseph Baker, Sons, and
Perkins, Ltd. (formerly Perkins Engineering Company). Coming into local
favour during the War, the "U.E." scrap-washer, made by the United
Engineering Company (Singapore), gives very good service. The "C.C.C."
washer, made by the Colombo Commercial Company, is suitable for the
purposes of an average estate. There are others, less well known. Most of
these machines are made in varying sizes to meet the requirements of small,
medium, or large estates; and if funds are available, a scrap-washer should
be regarded as an essential item in the machinery of any estate employing
engine power.
The rate of output of scrap-washers will depend mainly upon the speed at
which they are driven, and when ordering the equipment it would be
advisable to state the ordinary speed of the back-shaft, length of drive,
etc. It does not follow that the larger the rate of output, the greater is
the efficiency of the washer. The point is not as to what quantity of
rubber can be taken out per hour, but what quantity is actually freed from
impurities.
It is advisable for the superintendent to obtain a thorough knowledge of
the general construction and principles of the particular scrap-washer
employed. In the past it was not uncommon to find superintendents innocent
of the fact that a certain type of washer possessed movable parts upon
which the efficiency of the cleansing largely depended. It was often found
that these parts, which were intended to be removed and cleaned at
intervals, had become firmly fixed and could not be removed for inspection.
It must be recognised also that the machines are liable to considerable
damage if extraneous substances are allowed to enter--for example,
tapping-knives, stones, pieces of iron, spouts, etc., which are sometimes
present in the loose scraps of rubber or shavings, owing to the
carelessness of coolies. Under the best regulated-system, such accidents
occasionally occur, but a great deal of trouble could be avoided by having
it understood that each charge must be sorted over before entering the
washer.
Again a deal of extra work, and much wear and tear, is caused by the
_abuse_ of the scrap-washer--_e.g._, in the cleansing of earth-scrap. As
this reaches the factory it often contains a quantity of internal or
adhering earth. Before entering the washer a good proportion of the
external soil could be removed if the scraps were thrown into a tank and
given a thorough soaking and stirring. In a similar manner dry
bark-shavings, which have been allowed to accumulate, could be softened.
In the actual working of scrap-washers instructions are generally given by
the makers. These sometimes advise the introduction of warm water (or of
steam into the cold water supply) for an interval during the working of
each charge. Where possible, such instructions should be followed, as by
this means the individual pieces of rubber are massed together, in the
final stage, into a "sausage" form which is easy to transport and to
manipulate in the ordinary crepeing battery.
COMPOUND CREPES.--The attitude of both buyers and sellers with regard to
the types of lower grade rubbers appears to be changing. In the past, from
any one estate there might be obtained as many as six grades of crepe below
No. I. These comprised:
(1) A pale rubber (often streaked) obtained from coagulation of cup
washings and bucket rinsings.
(2) A pale rubber (often streaked) obtained by coagulation of the
skimmings from the surface of the No. 1 latex.
(3) A streaked and dull rubber prepared from naturally-coagulated
clots found in cups, buckets, and latex carts.
(4) A streaked rubber prepared from scrap which had coagulated upon
the face of the cut bark.
(5) A brownish and streaked rubber made by maceration of bark-shavings
to which pieces of tree-scrap adhered.
(6) A dark rubber, often tacky, prepared from scrap found in or on the
ground near the base of the trees. As it is often a matter of weeks
between any two regular collections, it is easy to understand why the
dry rubber was more liable to be "tacky" than any other grade of
crepe.
It will have been evident to all who have acquaintance with these grades,
as shipped from many different estates, that the diversity in the various
shipments must have been rather bewildering. There appeared to be a
regrettable lack of uniformity, even in the appearance of, say, a bark
scrap rubber from any two estates. When, in addition to these variations,
the further complication of condition of cleanliness is introduced, one may
realise the difficulty attaching to the evaluation of these rubbers as they
appeared upon the market.
Although the foregoing paragraph is written in the past tense, it should be
pointed out that within certain limits the trouble continues to exist with
respect to the output of a great number of estates.
In the case of many, it has been realised that the manufacturer does not
want to buy a large number of "parcels," all differing in some respect. It
is probably correct to state that what a manufacturer requires is a big
"parcel" uniform in appearance and treatment, even though the colour may
not be so light as that of many smaller lots. This statement is modified
with the proviso that the rubber, no matter what its colour or appearance
may be, must be free from dirt, grit, and bark.
The difficulty of making a uniform product from several types of lower
grade rubbers has been successfully solved on several estates by the
preparation of a "compound" crepe composed of a mixture of the best lower
grades in approximately definite proportions daily. Naturally the shade of
colour of this compound crepe will depend largely upon the types of rubber
employed, but as a rule it is somewhat darker than the highest of the types
employed in the mixture. To the writers this seems immaterial as long as
the manufacturer is offered a larger and more uniform lot which can be
given uniform treatment in vulcanisation processes. Neither would it appear
that the seller suffers any monetary loss. In point of fact it will be
found probably that the reverse is the case. For instance, supposing it
were decided to mix for a compound crepe--
(_a_) Naturally coagulated lump rubber.
(_b_) Tree-scrap.
(_c_) Bark-shavings scrap.
The product would be darker in colour than (_a_) and slightly better than
(_b_). Let it be granted that there might be a monetary loss on (_a_), it
is probable that there would be a slight gain in comparison with the usual
prices obtained for (_b_) and (_c_). Now, as a general rule, the actual
percentage of crop made into (_b_) is appreciably less than that made into
(_c_) and still less than (_b_) and (_c_) together. Apparently, therefore,
there would be a margin of profit on the whole by making a compound crepe.
It may be pointed out, on the other hand, that there might be expended on
the manufacture of this crepe more time and labour, but as against this the
labour of sorting and grading would be simplified.
Unfortunately this process is not open to estates which do not possess a
scrap-washer. _It is essential that the rubber should be free from grit,
sand, and bark particles._ In the absence of a scrap-washer for the
cleansing of the bark-shavings, it would be futile to attempt to make a
compound crepe containing that type of rubber, as one would run the risk of
spoiling the whole. It seems certain that in course of time a scrap-washer
will be considered as necessary a piece of machinery as an ordinary
crepeing machine in the factories of estates having sufficient means. Until
that time the preparation of compound crepes must be the privilege only of
well-equipped estates, unless other estates can send their lower grade
rubbers for treatment in a scrap-washer to their more fortunate neighbours.
In previous publications a diminution in the number of grades of crepe
rubber has been advocated, and it is gratifying to find that in many cases
the amending grades suggested have been improved upon. Many estates now
make only three grades of crepe--viz.:
(_a_) No. 1. From latex coagulated in the store.
(_b_) No. 2. Compound.
(_c_) No. 3. Earth-rubber.
It will be seen that the compound crepe includes all types between fine
pale crepe and earth-rubber. Naturally one could not safely recommend the
inclusion of earth-rubber in any compound crepe, as the risk of possible
"tackiness" in the whole would be serious. In the case of the bark-shavings
rubber to be incorporated, it is first cleaned alone in the scrap-washer.
Then all types are mixed together again in the scrap-washer in proportions
ruled by the experience of the usual average percentages of each grade of
the crop.
Besides the estates having only three grades, there are others which make
four--viz.:
(_a_) No. 1. From latex coagulated in the store.
(_b_) No. 2. Compound, from cup washings, etc., skimmings, and
naturally coagulated lump.
(_c_) No. 3. Compound, from tree-scrap and bark-shavings rubber.
(_d_) No. 4. Earth-scrap.
Other variations are possible, but their number is limited, and they
all conduce to simplification of working, and a supply to the market
of rubber having greater uniformity.
NEED FOR INCREASED CARE WITH LOWER GRADE RUBBER.--In the ordinary procedure
of estate-working there appears to be an undesirable variety in the style
of lower grade crepes. On some estates an examination of these rubbers
would appear to suggest that there need be no expenditure of care in the
preparation or the form in which it is made. This is a great mistake. With
the exception of the lowest grade (earth-rubber), it would not be unfair to
state that the quality of the rubbers on testing should be very little
inferior to the No. 1 product. Often, as in the case of naturally
coagulated rubbers, they are superior in some respects to ordinary fine
pale crepe. Doubtless manufacturers are aware of these facts, but what
course is open to them if they find the rubber spoiled for their purpose by
the presence of particles of sand, grit, or bark? The possible injury
caused by these ingredients cannot be insisted upon too strongly, and it
must be evident that great care should be exercised in the preparation of
the lower grades of crepe.
As to the particular form of the lower grade crepe rubber, one may apply
the remarks made under the section dealing with the best grades. It is
common to find thin crepes, medium crepes, and "blanket" crepes. More often
than otherwise, the medium and thicker crepes are prepared direct in those
forms. It follows that they are liable to attacks of "spot" disease, which,
however, is not easily visible in the lowest grades, owing to the dark
colour of the rubber. Furthermore, it is not possible to cleanse the rubber
so thoroughly if thick crepes are made.
BLOCK RUBBER.--Few estates now prepare block rubber, which is essentially
crepe rubber pressed into blocks. In the ordinary process the fresh
coagulum is lightly rolled into thin crepe, which is then vacuum-dried.
There are slight variations in the subsequent procedure. Sometimes the
rubber as it comes from the vacuum drier is merely allowed to remain on
racks overnight before blocking. In other instances, the sticky rubber from
the vacuum drier is passed once or twice through wet, smooth rolls and hung
to dry for some days. The dry crepe is then folded into the pressing box or
cut to suit the size of the box. Pressure is applied for some time, and
finally the rubber is taken out in one homogeneous mass. Naturally the
appearance of the block depends upon the quality of the parent crepe. Some
block rubber is made up thick; other is made in slabs about 3 inches or 4
inches in thickness. With the latter, it should be possible, when held up
to the light, to see the shape of a hand held between it and the source of
light.
It is possible that an erroneous idea of the strength of block rubber has
been formed. It should only be necessary to point out that essentially
block rubber is merely pressed crepe rubber. It is inconceivable that the
mere action of pressing layers of crepe together would increase the
physical quality of the rubber.
The advantages which block rubber possesses are the compactness of the
output, its ease of packing, and a saving in freight; but there is the
disadvantage, from the consumer's point of view, that extra labour is
involved in the preparatory work of cutting up the blocks.
SMOKED CREPE AND SHEET CLIPPINGS.--There appears to be no certain demand
for any grade of smoked crepe, and probably all which is put into the
market is really comprised of (1) clippings obtained from the ends of
sheets, (2) sheets which have been malformed in machining, or (3) sheets
showing the presence of many "bubbles."
As to the first class it might be explained that through defective rolling,
thick ends or edges may be caused. These show signs of contained moisture
when the bulk of the sheet is perfectly dry, and as undue delay would
otherwise result these moist strips are trimmed and either returned to the
smoke-house, or machined to form crepe.
Similarly a torn or otherwise badly formed sheet, when brought from the
smoke-house, may be made into crepe, rather than it should prejudice the
selling price of the bulk under ruling conditions.
In the same manner, although "bubbles" have no influence upon the quality
of the rubber on vulcanisation, sheets thus affected are generally made
into crepe.
It cannot possibly be argued that rubber of this description would be in
any way inferior to the best smoked sheet for manufacturing purposes, but
owing to the prevailing system of evaluation for market purposes, it is
necessary to resort to the expedients indicated above.
On some estates the rubber specified in the three classes mentioned is not
made into crepe, but cut up into small pieces and shipped as "sheet
clippings" or "sheet trimmings"--a procedure which would appear to be
justified by a steady demand. In point of fact, the buyers are really
obtaining a first-class article (except in superficial appearance) at a
reduced price.