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trated in fig. 12. This is an exceedingly practical device, and the
release can be operated in any position, even with the foot, on all
sides of the rolls. This release acts by means of a friction-clutch,
which can be at once thrown in, without any difficulty, by a
sharp pull of the chain.



32



RUBBER MANUFACTURE.



Heywood and Bridge's friction-clutch (fig. 12a') consists of au
inner casting, comprising a boss B, which is keyed on to the shaft A,
and from which are two arms C and a rim D. The rim is divided
transversely, but retains its unity through its connection with the
arms, and through them with the boss. In the rim, at the trans-
verse divisions, sockets G G^ are cast, and in these are placed nuts
H, in which work the ends of right- and left-handed screws F.
These screws are actuated by means of levers LLj, attached to
a sliding sleeve M, on the shaft. When the sleeve is moved nearer
to the face of the clutch, the rim D is caused to expand, and binds
upon the internal surface of an external shell N, iigidly attached




to the other shaft-end. On moving the sliding-sleeve away from
the clutch-face, the rim contracts, and the machinery is thrown
out of gear. Coupling and uncoupling can be efiected without
any jarring by means of this contrivance.

In this department there are also centrifugal machines (fig. 13)
for whirling the rubber as free from water as possible before trans-
ferring it to the drying-room and vacuum-drying chambers. The
advantages and disadvantages of the vacuum-drying apparatus are
fully dealt with in the next section. .

(b) Loss on Waahing and Dryinc/.— The loss, or wastage, under-
gone by raw rubber in the course of purification and drying is some-
times very considerable, reaching in many cases as much as 65 or
68 per cent. It is very unequal in the different sorts, and frequently.



THE RAW MATERIAL. 33

also, different parcels of the same sort vary considerably, differences
of 14 to 18 per cent, being not uncommon.

Table showing the Loss on Washing of a Number of Raw

RUBJiBiB.l



Bolivian aud Peruvian I'aia
Upi'iver, line Pava, liavd cure .
Islands, fine Para, soft cure
Entrefine, medium Para .
Manaos negroheads .
Scrappy negroheads .
Cameta negroheads and scrap .
Cauclio or Peruvian ball .
Caucho or Peruvian slab .
Mattogrosso Para, genuine
Mattogrosso sheet .
Ceara or Manijoba scrap, I. a .

„ - ,, „ Il.a .

Columbian strip
Equador, Esmeralda, Guayaquil

and stri[>.
Centrals, West Indian, Columbian, Guate

mala, Nicaragua strips, scraj), sheet and

slab.
Rio, Santos, Mangabeira, Bahia, Pernam-

buco, Rlaranham sheet.
Guayule .....
Gambia, Senegal, Bissao ball, I.o
„ ll.a
Cape Coast, Benin, Accra lump, I

Sierra Leone, Massai, Conakry niggers or

twist.
Kassai, red and black, Lopori, Equateur

I a sorts.
Aruwimi, Mongalla, Sankuru, Lake Leopold

Thimbles

Batanga marbles aud clusteis .
Mozambique, Donde, Mohorro spindles
,, II. a marbles and bulls.

Madagascar pinky

,, black

„ niggers

Borneo La

„ ILb

.„ III.«



Dead Borneo, Pontianac, Jelutong,
Assam, Rangoon la
Il.re

Java I.o

,, Il.a, Peuang II. a
Ceylon Plantation Para .
Red Tonquin ....



Maximui
Loss.



Normal


Minimum


Loss.


Loss.


per cent.


per cent.


about


about


U


14




13


18


15


22


18


26


23


30


26


21


18 '


3.5


32


22


20


35


30


20


18


30


24


10


8


28


24


32


24


40


30


■ 28


20


35


25


40


35


38


32


45


40


22


20


7


4


12


10


45


38


38


30


16


10 .


25


22


30


28


32


30


- 60


50


43


38


45


43


liO


65


70


60


15


12


35


30


15


12


35


32


2





10


8



Cf. also the raw rubber tables j



in Herbst's Gummikakndcr, 1908, pp. 96-113-

3



34



RUBBER MANUFACTURE.



Drying Washed Rubber. — The method of drying the sheets of
washed rubber must depend entirely upon the properties of the
different kinds of rubber, and upon the uses to which they are to be
put. To state in general terms that such and such a method is the
best would hardly be fair, for each of the ordinary methods of
drying has its own special advantages and disadvantages. The
method of drying to be employed in any given case must therefore
be decided upon for that individual case.

Generally speaking, far too little attention is devoted to drying
washed rubber in a rational way. The drying is still frequently
carried out in a very unworkmanlike manner, clearly showing how




Fig. 13.

little intelligence is brought to bear upon this important subject.
Raw rubber dried in such an unsuitable way exhibits absolutely no
good properties ; the places in which it is dried are generally of
quite insufficient dimensions, so that it looks as if one were drying
rags instead of dealing with an expensive material like rubber,
sensitive to the most diverse influences. Very often no special
drying-rooms are provided at all, in order to economise space ; and it
is not an infrequent occurrence to find the boiler-house serving as a
drying-room. The value of raw rubber dried in such places is very
doubtful, the dust which settles on it being the least of the evils to
which it is exposed. The vacuum chambers introduced some years
ago have not proved to be of practical advantage in all cases.

The best apparatus for drying rubber is the drying-room,
designed and constructed throughout in the way best adapted to



THE RAW MATERIAL. 35

the properties of the washed rubber which is to be dried in it.
This is not so simple a matter as it would appear to be. It will be
well, therefore, to consider somewhat more fully the characteristics
of the washed raw rubber which is to be dried. The greater part of
the water which has to be removed is merely superficial,, and its
removal presents no difficulty. On the other hand, the water
actually occluded within the rubber can only be removed with
great difficulty, if one does not wish to fall into the error of
vacuum-drying. The mechanically held, superficial moisture can be
readily removed from the rubber sheet in a few days by means of
a gentle heat ; it is not advisable, however, at this stage to raise
the temperature above 50° to 60° C, since resinous rubber will not
stand that amount of heating. The temperature should therefore
not be raised to such a desfree as would be most favourable to
superficial drying, and the complete drying of the sheet of rubber
should be carried out at as low a temperature as possible. It is a
well-known fact that as soon as the superficial moisture is evapor-
ated the surface of the sheet contracts so that the water still
present gets completely enclosed, and can only be driven off", through
the rubber which surrounds it, with great difficulty, or not at all.
The difiusion-pressure depends very little on the temperature or
on the atmospheric pressure, but, on the other hand, the condition
of the warm air in the drying-room, especially its hygrometric
state, is a matter of essential importance. High temperatures are
therefore not necessary for complete drying, provided that proper
care is taken to see that the air is 'kept dry, and in this way
the detrimental efiect of high temperatures upon the rubber may
be avoided.

The conditions of the drying-room must, therefore, be such as to
(1) render possible safe and rapid drying of the washed rubber,
and (2) prevent further oxidation of the rubber. It must be re-
marked that goods made from rubber which has not been thoroughly
dried are subject to rapid deterioration, and that imperfectly dried
rubber may also, under certain circumstances, lead to the production
of porous goods. Then it should also be remembered that highly
oxidised rubber is always of less value, and, as is well known, the
oxidation of the washed rubber by atmospheric oxygen proceeds
very rapidly. The large surface-area of the washed sheet is very
favourable to this oxidation. These facts should be taken into
account in designing the drying chambers. Spacious drying-rooms
are disadvantageous and unsuitable, since they cannot be filled
or emptied in one operation. To fill them gradually with wet



36



RUBBER MANUFACTURE.



sheets of rubber often has disadvantages, for it tends to check
the drying of those batches which are already in the drying-
chamber and which are partly dried, and to prolong the time neces-
sary to dry them completely. A preliminary whirling of the moist
sheets in a centrifugal machine, after they leave the final washing
rolls, and before they are put into the drying chamber, may be
strongly recommended. The drying-rooms should be small chambers
which can be filled at one operation, and the drying should be carried
out in such a way that the moist air surrounding the rubber is being
continuously displaced by a fresh supply as dry as possible. A
good circulation of the air inside the chambers must therefore be




Air Sh3tc!>
Fig. 14.

maintained, and the air must be as dry as possible and heated to
a suitable temperature.

A modern arrangement for drying washed raw rubber is shown
in figs. 14 to 1*7. The size of the complete plant, as well as of the
individual drying chambers, depends upon the approximate weight
of rubber w^ashed per day. In the diagram fig. 14 the plant is
assumed to cover an area of 3 X 5 metres, and to reach a height of
4 metres, of which about 3 metres is useful space. Sufficient free
space is provided for the warm ascending air to collect, and from
there it is drawn away upwards. The chambers a are warmed and
aired from the main air trunk E by means of eight equidistant
adjustable air-nozzles, which force the warm air upwards. The
chambers are fitted with 20 to 25 drying-poles which can be hoisted



THE RAW MATERIAL.



Z7



up by means of wire rope; over these the sheets of rubber are
loosely hung, the current of warm air streaming past them. The
chambers B are fitted with racks, and the current of warm air

Section through portion of drying chamber fitted
with raclis.




Fig. 15.

enters at the side (fig. 15). The space C contains the heating and
ventilating plant ; in it are situated two large Sturtevant blowers,
with heating attachments D, which supply the necessary warm air.

Section through portion of drying chamber fitted
with poles.




Fig. 16.



The fan F, with trunk G, serves for the supply of cold air and can
be connected to the separate chambers as required for cooling-
purposes. The air-supply to the drying chambers is regulated from
the outside. The removal of the spent air is regulated by the
trunk H passing through the double top of the chamber ; the air is



38



RUBBER MANUFACTURE.



returned at once to the heating apparatus. On the other hand,
the steam is exhausted through the channels I, and led away into
the open air. All the drying chambers are without windows ; the
passage-way down the middle between them, which is provided
with rails, is lighted by means of a top-light.

A new mode of drjang which looks very promising may still
be mentioned. The drying is effected by means of special centri-
fugal machines, the washed rubber being first broken up into small
pieces and not run into the form of a sheet. The particles of watei
are frozen by means of a special arrangement, on the under frame
and outer jacket, and driven from the rubber by gently warming.

A great number of rubbers cannot be obtained in sheets suitable



Section 'hfough fan rhamber




Fig. 17.



for carrying about. The softer sorts, in particular, are so greatly
affected by the gentle heat of the drying chamber that even short
sheets break away from the drying-poles and fall to the ground.
Moreover, considerable quantities of rubber are required for a very
large number of rubber goods which present less difficulty in their
manufacture, or for such goods as must be pretty highly vulcanised,
and for these, as for soft kinds of rubber, drying in a vacuum-
chamber (fig. 18) is to be recommended. The sheets are put into
the chamber on perforated iron shelves, the door is closed, the
temperature raised from 50° to 60° C. or even higher under special
circumstances, and the chamber exhausted by means of an air-
pump. In the more perfect forms of apparatus, amongst which
that of Passburg in particular is in most general use, drying is
completed within an hour, provided one starts with sheets which
have been centrifugalised. It is strongly recommended to " batch



THE RAW MATERIAL.



39



up" vacuum-dried rubber immediately on its removal from the
chamber, in order to prevent oxidation of the hot dry material.

Air-dry in o- iiiay, under certain conditions, bring about slight
oxidation of the rubber in some cases, but never results in depoly-
merisation, in consequence of the heating. Vacuum-drying, on the
other hand, although not facilitating oxidation, results in quite an
appreciable depolymerisation as a result of the high temperature




Fig. 18.

to which the rubber is raised, and this effect is the more marked
in the softer sorts of rubber.^

The Further Working-up of the Dried Rubber. — The practice of
running the washed rubber into thin sheets greatly facilitates
drying by considerably increasing the surface area. When, how-
ever, the rubber has to be stored for any length of time, or when it
is to be used for the manufacture of cut-sheet, it is advisable — in
the latter case absolutely necessary— to reduce the rubber to some
more solid or compact forrii, in the first place in order to reduce
oxidation to a minimum by making the superficial area as small as
possible, and secondly, in order to produce homogeneous blocks, free
from pores, from which to prepare " cut-sheet." It was formerly
the general practice to work or knead all dried rubber into solid
masses, but it is nowadays the object of the manufacturer to avoid
these processes whenever it is at all possible, experience having
shown that it is wise to avoid all unnecessary treatment of rubber
on the rolls, not only on the score of expense, but on account of the

1 Cf. on this important sulject of the drying of washed raw rubber, articles
in the Gummi-Zeitung, 1906, xx. pp. 656, 681, 738, 789, 843, 1107.



40 RUBBER MANUFACTURE.

heating and depolymerisation wliich is brought about by such
treatment. In modern manufacture, on the large scale and by
rapid methods, there is generally no need for prolonged storage of
rubber after washing, and it may therefore be at once worked 'up
into the various mixings.

To effect this the mixing mill described below is set in motion,
steam being passed through the hollow rolls. It is advisable to
warm up the rolls to such an extent that mixing may proceed
rapidly, and prolonged working, which is so detrimental to the
rubber, be avoided. The rolls should not be allowed to get too hot,
or the rubber may also suffer from that cause; the temperature
must therefore sometimes be brought down by means of the cold
water supply through the rolls. During the process of mastication
between the rolls the rubber becomes spontaneously heated.

At a first glance, rubber-mixing appears to be a very simple
process, the mill itself automatically bringing about a complete
admixture even if. left unattended. Yet mixing is a process
that needs careful watching and intelligent manipulation on the
part of the workmen engaged in it, otherwise the rubber can be
very easily overworked ("killed"), the ultimate result of which
would probably be goods of poor quality and little durability.

As a rule there is one man to each mixing mill; only very
occasionally does it happen that one man has to look after two
mills at the same time ; in that case the mills should be placed
opposite one another, so that there is no need for the man to leave
his place, but only to turn round. The mixer generally receives
the mixed ingredients in galvanised iron boxes or tubs. His tools
consist of a scoop, a hand-broom, a spanner for opening out or
closing up the rolls, a short knife for cutting the rubber off the roll,
and a sieve fitted with a hinged lid, with which he sifts the
powdered "compounds" on to the rubber. The actual mixing is
carried out in the following way : — The mixer takes from the vessel
containing his material, first of all the raw rubber, passing the
separate sheets between the rolls and working them up into a
fairly smooth sheet surrounding the front roll, which is kept rather
hotter than the back one. When this stage is reached and the
rubber is sufficiently soft, the other ingredients are gradually added.
The mixer now passes his knife obliquely across the rubber sheet
on the front roll, lifts up a portion of the sheet and turns it over
across the roll, repeating this process a number of times. As the
rubber is meanwhile getting hotter, the rolls must be more and
more thoroughly cooled. Finally the mixer cuts off strips of the



THE RAW MATERIAL. 4 1

rubber, rolling them up and laying them aside until the rolls have
attained a temperature suited to the last stage of mixing, whereupon
the little rolls of rubber are again put through the mill and mixed
together to form an apparently homogeneous mass, which is cut
from the rolls without delay.

The above method of mixing is, however, by no means one
which can be applied generally. Mixings which contain much
paraffin, pitch, oil, or the like, must be treated differently from the
so-called dry mixings containing much waste and fibrous material.
Great damage may be done by unintelligent mixing. The manager
should, therefore, train only intelligent men, with keen powers
of observation, for the mixing mills, and should always keep a
watchful eye upon the mixing department. Moreover, he should
very carefully consider what mixing should be carried out on a
small pair of rolls, and for what mixings only the large rolls should
be used, for more depends upon this than many people imagine.
The fact that certain theorists are opposed to the use of large mills
because they are not economical, should not prevent anyone from
using large mills in order to treat his mixings with care and
produce first-class results. One can only utter a strong word of
warning against judging machines solely from the engineer's point
of view, or from the point of view of most rapid working and
lowest possible working cost ; the behaviour of the material under
the different conditions should be well considered in addition.

The way in which the different raw rubbers behave on the
mixing rolls is different in each individual case. The following
rules may be taken, in a general way, as a guide : —

Rubber from species of Hevea, SapiuTn, Micranda, and Gastilloa,
and from Manihot Glaziovii, requires rather more heat during
mixing than rubber from Hancornia, Ficus, Funtu^nia, and most
African LandoliMa species. "Mozambique spindles" and "red
Kassai " occupy an intermediate position. It will be seen that the
softer and more resinous African and Asiatic kinds must not be
treated in exactly the same way as American sorts, which are freer
from resin, and possess more "nerve." This is to be explained
partly by the solvent and depolymerising action on the rubber
of the rubber resins, and partly by the fact that the degree of
polymerisation is actually lower in some raw rubbers than in others.

It is very often desirable to thoroughly work the reclaimed
rubber in a pair of rolls running at a high speed before adding it
to the raw rubber on the mixing rolls. Substitute should be ground
up very fine on well-cooled rolls. Sulphur and mineral additions



42 RUBBER MANUFACTURE.

must be sifted as fine as possible before adding them to the rubber ;
they are often mixed together in the dry state in special mixers,
and are then dusted on to the rubber with the aid of a sieve, fitted
with a cover, or by means of a revolving sieve fixed over the mixing
rolls, in order to produce a mixing as devoid of granular structure
as possible. Such arrangements for the production of mixings
absolutely free from granules are to be found to-day in a state of
great perfection in nearly all rubber-shoe and pneumatic tyre
factories, as well as in rubber-thread factories. The additional cost
of such appliances is not great ; the improvement in quality brought
about by their use is frequently very noteworthy. The greatest
care must be taken during the progress of mixing to prevent
sulphur, resin, asphaltum, etc., from melting on one of the rolls and
getting baked into thick layers.

In the case of mixings which contain oil, vaseline, paraffin wax,
and similar substances, it is often advisable to incorporate these
materials either by first preparing the waste with them, or by
mixing them with the minerals to form a kind of thick dough and
then mixing these with the rubber. This method is not only more
cleanly, but also less detrimental to the rubber.

The Mixing and Weighing Room.— This room is used for weighing
out the separate constituents of the mixings to be subsequently
worked up in the mills. A particularly trustworthy man should
be placed in charge of this department. On the one hand, the
mixings with which this man is entrusted for the manufacture of
the different rubber goods are valuable, being the result of great
experience and often of costly experiments, and, on the other
hand, it is necessary, in. order to avoid subsequent troubles in the
course of manufacture or even later, that the various constituents
of the mixing be weighed out with the greatest possible care. Even
slight errors in weighing may have most unpleasant results, particu-
larly in the case of the sulphur for vulcanisation, the magnesia usta,
etc., or, in "floating" red goods, the amount of golden sulphide added.
The larger quantities of material should be weighed on a large, com-
paratively rough pair of scales, which when skilfully manipulated
will give perfectly satisfactory weighings. Sulphur and other im-
portant materials are, on the other hand, weighed out on a small
pair of more sensitive scales, the weights being, of course, accurate,
and kept scrupulously clean. Platform balances, although they are
to be obtained small, are generally rough, and are not to be recom-
mended for the finer weigliings. Moreover, they cannot be adjusted
by means of screws as can beam scales. In the case of mixings



THE RAW MATERIAL.



43



which the manager wishes to keep secret, a good plan is to tie up the
weights for the different items of the mixing in small bags, and to
hand these over to the weighing foreman each time a batch has to
be mixed. As soon as the ingredients are weighed out they are
carried straight away to the mixing rolls and mixed together.
Another method of keeping the mixings secret consists in using
counterpoises of various shapes which can be readily distinguished
from one another. This, however, is not the place to dilate upon
the outgrowths of the traffic in works-secrets, which is still carried
on here and there, for it is frequently quite possible nowadays to
fathom the secrets of the mixing in a perfectly legitimate way, by




m,u 1



Fig. 19.



carefully devised experiments based on chemical and physical know-
ledge, in spite of all attempts at preserving secrets.

The ingredients to be mixed with the rubber must be as dry and
as finely powdered as possible. The drying is often done simply
by heating on stoves. For whiting and other fillers which are used
in large quantities special drying plant, consisting of large heaters
or long troughs provided with arrangements for stirring, are often
used. In dealing with large quantities of material, indeed, the
workmen's wages form such an important item that the drying
and subsequent sifting are made as far as possible automatic. The
object of sifting is to remove all the coarser particles of the sub-
stance, as well as pieces of wood which may have got into the
material on opening the casks or cases. Sifting apparatus which
works continuously, like the cylindrical sieve shown in fig. 19, may
be used with advantage ; charges of 50 to 100 kilos. (1 to 2 cwt.)
at a time can be put through the hopper of this apparatus. The



44 RUBBER MANUFACTURE.

sifted material is stored in tightly- closed bins. For this purpose
it is usual to employ both large wooden zinc-lined bins and galvan-
ised iron ones, as well as small sheet-zinc boxes. It is convenient to
keep oils in so-called patent oil-holders, from which they can be
pumped out when required. The washed and dried raw rubber
is kept in pigeon-holes lined with sheet zinc, and these are best


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