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result to be avoided. Should the roller not run with sufficient ease
in its bearings, or should the centre of the roller not coincide with
the centre of the wooden block out of which it was turned, the cloth
will drag over the roll here and there, and in these places the rubber
will be less highly vulcanised. One then wonders why the proofed
material is good and bad in patches, the explanation being that
given above. If the roller is turned from a cross-grained wooden
block and has warped, the cloth will not make good contact with the
hollows, and will take up too much liquor. The same thing will
occur round the edges of the humps, too much solution being taken
up where the material is not in close contact with the roller. These
places become yellow, wrinkled, and hard. Now and again it
happens that the vapours given off in the cold-vulcanisation process
catch fire, owing to electric sparking between the cloth and some
conductor or other. The material is electrified by friction, and if
the potential difference is sufficiently great, the discharge takes place
in the form of electric sparks. To prevent this firing, the apparatus
already described is fitted up, a similar piece being placed behind
the vulcanising trough. Should a fire occur in spite of these pre-
cautions, the cloth should be lifted away from the roller by raising
the lever, and water should be slowl}^ poured into the trough. The
c'oth is meanwhile rapidly run on, and not cut off The flame
burns low with a blue colour, and a pungent smell of sulphur dioxide



THE VULCANISATION OP RUBBER.



95



is ^iven off. ^^[aterial which is to be proofed on one side only is
run over the vulcanising roller once ; material proofed on both
sides has to make two such journeys. Doubled stuff is generally
vulcanised and doubled on the calenders (fig. 46). It is not
nocessar}', however, to vulcanise both the cloths which are to be
doubled together, as is often done. When two cloths are proofed
for doubling, only one of them has a waterproof coat, this being the
smoother of the two, while the other has only just sufficient rubber
spread on it to make it adhere well. The first of these two cloths
is vulcanised, the second not ; for when they are carefully doubled




Fig. 46.

the lightly-proofed material is vulcanised when it comes in contact
with that having the thicker coating of rubber. Fig. 47 shows
diagramatically an arrangement for simultaneously vulcanising and
doubling the proofed material, a a are the insulated cast-steel
rolls, h is the vulcanising trough already described, c c the rollers
upon which tlie cloth to be vulcanised and the lightly-proofed cloth
respectively are rolled up, cl also a wooden roller upon which the
finished material is rolled up. After vulcanisation the material is
hung up in a well-ventilated room to dry oft'.

The injuriousness of the vapours of chloride of sulphur and of
carbon bisulphide is well known, and attempts have therefore been
made to completely enclose the machines, and then to carry away



96



RUBBER MANUFACTURE.



the vapours by means of fans. This arrangement is to be preferred.
Fig. 48 shows how the enclosing of the vulcanising machine in the
shop may be accomplished. The door and lid b are open for the




purpose of introducing the cloth ; while the machine is in operation
tliey are closed. cZ is a window to admit light into the chamber ;
through this the material can be watched, a condition which is
absolutely necessary, i is the steam pipe leading to the heating



=^s:



j!i|'if,ii|i




Fig. 48.

cliest e of fig. 42. Before starting, the trough is filled as usual,
and also the \essel e (fig. 49), the purpose of which is to replace the
liquor in the trough automatically as it is used up. If the cloth
gets creased, the lid c is opened, the hand is inserted through the
opening, and the creases can then be easily smoothed out. The
proofed cloth must obviously be braked in such a way as to cause



THE VULCANISATION OF RUBBER.



97



it to run under an even tension. In order to render this opera-
tion possible from outside the enclosure the brake g is provided.
Vulcanisation with this arrangement is a pleasant process, in so
far as that the poisonous vapours can scarcely be detected. The
door and the lid h are closed while the work is going on. Air
passes into the chamber through the opening e, and very little
vapour can therefore escape. In fig. 49 the automatic arrange-
ment for keeping up a constant level of liquor in the vulcanising
trough is shown in diagram, a is an iron vessel, lead-lined, closed at
h by means of an air-tight screw stopper. Tubes e and c, running
side by side, lead from the
vessel a into the vulcanis-
ing trough/, to a depth just
below the level at which
the liquor is to be kept.
Tube c. ends inside the
vessel right at the top,
whereas tube e only starts
from the bottom. Vessel
a is filled with vulcan-
ising liquor through the
opening h, which is closed
again immediately after.
On cock d being opened
the solution now runs into
the trough / until the level
of the liquid rises just above
the mouth of the tubes and
closes them. When in the
course of the vulcanising

operation the level falls below the mouth of the tubes, more liquor
flows from the vessel into the trough until the tubes are again
closed by the rise in level, when the flow of liquor is checked;
this goes on until, when work is finished, cock d is closed.

In addition to the room for vulcanising waterproof cloth, there
are generally others for curing cut-sheet goods.

The ventilating pipe is usually fixed on the opposite side of
the bench from that on which the workman stands ; along the top
of this runs a slot, through which the noxious vapours are drawn
by means of a fan connected with the pipe. From the back wall
of the vulcanising bench leading towards the rubber slot there rises
a sloping hood which covers the whole of the bench with the

7




Fig. 49.



98 RUBBER MANUFACTURE.

dipping vessels on it filled with carbon bisulphide ; as an additional
protection to the worker, a narrow ledge of wood runs along the
front upper edge of the cover. The dipper stands in front of the
bench and dips the small cut-sheet articles, by means of a fork
held in his left hand, into a vessel containing the chloride of
sulphur solution, whilst two others, sitting down, prepare the
articles for dipping by mounting each one upon a separate prong
of the fork. This arrangement for vulcanisation, although it is to
be preferred to the older measures adopted for the protection of the
workman, has nevertheless its own particular weak points. In
the first place, the continuous exhaust slot leads to an imperfect
removal of the vapours. Secondly, the sloping cover, and in par-
ticular the outwardly inclined ledge at the top, are in the way, and
the workers get into the habit of stooping down below them, and
so manage to inhale harmful quantities of the noxious vapours.
Thirdly, the vessels containing the liquor are merely stood upon the
bench, and the draught is only a very poor one, because of the great
diiference between the sectional area of the outlet and that of the
opening in front (which extends from the top of the bench to the
bottom of the protecting ledge on the sloping hood) ; the result is
that the vapours are not entirely prevented from diffiising out* into
the room. The strength of draught necessary must be arrived at
by experiment. Again, the arrangement is incomplete, since it
affords no protection to the workers sitting in front of the bench
preparing the articles for dipping, against the vapours arising from
the dipping forks as they are handed back to them. All these
imperfections can fortunately be remedied by adopting the following
solution of the problem. The dipping bench is on that side of the
room where the windows are, in such a position that the dippers
stand on both sides of it opposite one another. The female hands,
who fill the balloons, etc., have all their arrangements fitted up on
the opposite side of the room, along the back wall. The ventilating
apparatus consists of two entirely separate systems, viz. that for
the general ventilation of the room, which may supply warm air,
and that for local ventilation at the working places. The general
ventilating system, the main trunk of which is made of galvanised
iron, and suspended at a height of about 4 metres above the floor,
carries fresh air, drawn in by a large fan, through the six nozzles
to the upper parts of the workroom. This air, which is drawn by
the second localised system of ventilation direct to the w^orking
places, sinks down past the workers without being contaminated to
any appreciable extent by the vapours. The nose-pipes are 24 cm.



THE VULCANISATION OF RUBBER. 99

in diameter, and are arranged symmetrically in pairs ; they pass
the following quantities of air as determined by an anemometer : —

Per minute,
-r,. ^ . r left-hand, speed 300 metres, quantity 13-5 cub. metres ;
First l)air |,ight-hand, , 284 „ „ 12-8

a , . /left-hand, „ 280 ,, „ 12-6

Second pair I ^.g^^.1^^^'^^ „ 320 „



m, . -, . ("left-hand, ,, 280
Third pair {right-hand, „ 240



14-5
12-6
10-5



in all, a quantity of 4590 cubic metres of air per hour. Further
ventilation through the windows in the roof or top-lights is not
advisable, as it would interfere with the general circulation of the
system.

The draught on the working bench, which is quite independent
of the general ventilation, is induced by a fan through a flue
running along the floor the whole length of the shop. The fan
drives the air charged with bisulphide vapours through an under-
ground channel into the vertical air purifier outside the shop.
The dipping bench is provided with two series of dipping vessels,
placed alternately one on either side of the exhaust pipe, at intervals
of about 1'5 metres. Each vessel is sunk into a pocket in the
bench, its upper edge being flush with the bench top. The pocket
is a fixture, with the exception that the front of it is made in the
form of a flap, which can be opened. The small exhaust opening,
through which the pocket is connected with the exhaust pipe, is
situated underneath the dipping vessel, so that a protective draught
of air 4s set up round the vessel by the suction, and a favourable
means provided for the escape of the heavy vapours of carbon
bisulphide and chloride of sulphur from the lowest point of the
pocket. The top edge of the pocket is surrounded on three sides
by projecting, slightly-inclined ledges, an arrangement which
constitutes a distinct improvement, leading to efiicient working.
The remainder of the bench surface, which occupies the space
between two pockets, is filled up with a hollow inverted pyramidal
opening, covered with perforated metal, the apex of the pyramid
leading into the exhaust pipe. By this means the vapour given
off' and the drops of liquor which always get splashed about during
the dipping process, as well as the vapours coming from the forks
after they have been in the liquor, are drawn away direct through
the perforated plate to the exhaust pipe, and only traces of vapour
are left behind, to be carried by the air circulation into the exhaust
shaft.



CHAPTER III

THE MIXINGS.

The mixings used in rubber manufacture are in part the results of
rule-of -thumb experiments, and in part deductions from theoretical
considerations. No generally acknowledged theoretical basis for
mixing at present exists. The following are the principal points
to be taken into account when devising mixings: — The use to
which the article is to be put; the colour desired; the surface,
whether it is to be quite smooth, or to have a cloth impression, or
whether it is to be a matt or a highly polished surface ; the limits of
specific gravity, and of cost price. These points being fixed, one may
proceed to base upon them a suitable mixing. The first question
to decide is what kind of raw rubber will be most suitable. Out-
siders often imasrine that because Para rubber is the best it should
be used by preference for all kinds of rubber goods, and that all
other sorts of rubber are in a sense inferior for the purpose, or
merely substitutes for Para. This view cannot, however, be
altogether upheld. Para rubber is, in the first place, very high in
price, and to employ it without just reason in a case where an
equally good result could be obtained with a lower grade rubber
is, from the standpoint of the rubber specialist, a crime, for the
general adoption of this course of action would lead to an un-
necessary diminution of the quantity of Para (which, as it is,
is barely sufficient) available for purposes for which only Para,
or practically nothing but Para, can be used ; the demand for Para
would consequently increase — that is to say, prices would go up still
higher. For best quality air-tubes, elastic bands or rubber thread,
practically nothing but best Para comes into the question ; indeed,
under some circumstances the original lots as delivered have to be
sorted, and all the inferior pieces picked out and used for other
purposes. For hard-rubber goods which are to be highly polished
only such rubber can be employed as can be washed absolutely



THE MIXINGS. lOI

clean, for the slightest amount of impurity remaining in the rubber
would show itself in a very unpleasant way afterwards.

For " frictions " soft rubbers are the most generally used ; for
light-coloured, white and red goods, light-coloured rubbers are
the most useful ; for " floating " goods, naturally only such rubbers
can be used as, when mixed with a sufficient quantity of sulphur
and the necessary pigment, will still float. At this point we
must correct a very widespread notion, which is none the less
false, to the effect that every sort of raw rubber, if it be only
correctly vulcanised, will float. One can very easily be convinced
that most sorts of Aruwimi rubber, the lower grades being
naturally the greater offenders in this respect, will not float even
when vulcanised with only 10 per cent, of sulphur between the
plates of a press. This phenomenon depends in part upon the
contraction of the rubber during the process of vulcanisation, and
cannot be predicted straight away. The favourite plan of attri-
buting all faults to the wicked rubber resins is in this case also
unjust, for as a rule la Borneo, though showing, like Aruwimi,
a high resin content, exhibits a medium power of floating after
vulcanisation with 10 per cent, of sulphur; Guayule, too, when
vulcanised under similar conditions, often floats, in spite of its high
resin content. The contraction of rubber during vulcanisation
depends to a very great extent, if not exclusively, upon the
coefficient of vulcanisation. In some types of goods, which mus
possess considerable toughness, it is usual to work to a certain
degree of contraction, by the addition of suitable ingredients, chiefly
litharge, free from peroxide, in black goods, and, more frequently
still, magnesia usta in red or light-coloured goods. Magnesia can,
of course, be used in black goods, but not, conversely, litharge in red
or light goods. Litharge is, in fact, always partiallj^ converted,
during vulcanisation by heat, into the deep-black sulphide of lead.
Now, since no sulphuretted hydrogen is given off" during vulcanisa-
tion, as far as our knowledge at present extends, it may be that
either the temperature employed is too high, or that by-reactions
occur between sulphur and other substances present such as oils,
certain resins, etc., sulphuretted hydrogen being evolved, or it may
perhaps be assumed that on vulcanisation sulphur combines with
rubber to form a sulphide-like compound, which then interacts
directly with the litharge. The behaviour of mixings of this descrip-
tion when over-vulcanised also supports this view, for the coefficient
of vulcanisation decreases as the amount of lead sulphide formed
increases. ^ This fact is at the bottom of the comparatively favour-



I02 RUBBER MANUFACTURE.

able effect on regeneration processes, of the presence in the. waste
rubber of large quantities of litharge.

There are a number of brands of litharge on the market which
are more or less rich in peroxide of lead. Such litharge cannot be
recommended for use with rubber, the peroxide having a tendency,
under certain conditions which are not infrequent in practice (c/.
heating tubes for railways, etc.), to burn the rubber. By certain
authorities "dry heat" tests are prescribed, and in carrying out
this test it is frequently observed that the test pieces from samples
of rubber containing much litharge burn with a glow. It is not,
however, necessary that the oxidation should be accompanied by
visible combustion, for the so-called cold combustion is just as
destructive to the rubber as the more apparent kind. The use of
red-lead itself should, for these reasons, be entirely forbidden.

The use of litharge is limited by the facts, first of all, that it is
only possible in dark goods ; secondly, that it increases the specific
gravity very considerably; and thirdly, that in many classes of
goods it cannot be allowed, owing to its solubility in even dilute
acetic acid, combined with the fact that it is injurious to health.
It is true that in France and Switzerland wine-tubing is made from
mixings containing litharge (or that such tubing is imported from
Germany) ; still, it has not yet been shown that litharge is innocuous
in that connection. Besides, there are no real grounds for the use
of litharge for such a purpose, since the technical vulcanisation
effect which is necessary can be produced equally well, if not better,
by the appropriate use of magnesia usta.

Magnesia usta is commercially pure magnesium oxide, and is
made by calcining magnesium carbonate. As a rule, however,
about 2 to 3 per cent, of carbon dioxide remains obstinately fixed,
so that the best magnesia usta of commerce contains only about
96 to 98 per cent, of magnesium oxide (MgO). Besides genuine
magnesia usta there are on the market a number of products, also
called magnesia usta, which contain considerably less magnesium
oxide, together with a certain amount of carbonate or hydrate. The
value of a particular sample of magnesia is therefore conveniently
detei mined by re-calcining a weighed quantity and finding the loss
in weight. In storing magnesia usta it must be remembered that
this product tends to absorb carbon dioxide and moisture from
the air, and the most convenient vessel to use for the purpose is a
sheet-iron bin, with a closely-fitting lid, and provided with a slide
so that the magnesia can be taken out without opening the lid.

The use of lime for the same purpose as magnesia usta has



THE MIXINGS.



TO3



fallen off, in spite of the enormous difference in price between the
two products, in favour of a continually extending use of magnesia
usta; from, this it must be concluded that the results obtained with
magnesia are much more satisfactory. Magnesia usta possesses
over litharge the advantage of being innocuous — it is indeed
administered internally as a medicine — in addition to the further
advantages of not affecting the colour of the rubber, and of not
increasing its specific gravity to so great an extent as litharge.

Litharge and magnesia usta are specially suitable for obtaining
improved results from the vulcanisation of soft rubbers, which are
generally rich in resins. From the same standpoint they are
largely used in mixings which contain softening ingredients such
as oils, paraffin wax, pitch, etc.

These additions of oils, paraffin wax, pitch, etc., are made
principally with the object of helping to amalgamate such mixings
as contain considerable amounts of ingredients like barytes,
whiting, and especially carbon black, and which are, as a result,
rather "dry," and of reducing their micro-porosity by removing
from the mass the air which clings round the particles of the
powdery ingredients. The solid fats, paraffin wax, ceresin and
other waxes are liquefied before being added, and will then mix
rapidly and easily with the rubber and other ingredients.

The principal colourings for which there is a demand in rubber
goods are black, white, and red, and more seldom green. Other
colours are only called for in exceptional cases. Blacks are
produced by means of carbon black, lead sulphide, ferrous oxide,
and pitch. Asphaltum has only a low covering power; there is
practically no advantage in its use as against the use of pitch, and
it is therefore hardly ever employed now as a colouring agent.

In addition to these pigments there is litharge itself, which
blackens during the vulcanising process. The red pigments used
are vermilion, Knapp's golden sulphide of antimony (antimony
cinnabar), ordinary golden sulphide of antimony, and a number of
mineral colours the basis of which is oxide or hydrated oxide of
iron. For whites practically only oxide of zinc, sulphide of zinc,
and barium sulphate are used to any important extent, the last
two being generally used in conjunction in the form of lithopone.
Antimony oxide is only very seldom used. For green one has to
rely upon chrome greens, which are to be obtained in a variety
of shades. For grass-green pure anhydrous chromic oxide is
most suitable, while dark greens are best produced by chromium
hydrate green or Guignet's green.



I04 RUBBER MANUFACTURE.

Substances used as fillers are whiting, barytes, and china-clay,
in some special cases purified kieselguhr (atmido, atmoid, talite),
for eraser-rubber anhydrous silicic acid and ground-glass, ground
" galalith " (solidified milk albumen). The substitutes made from
fatty oils — white substitutes by the action of chloride of sulphur,
brown substitutes by the action of sulphur — which are addition
products of these oils, must also be reckoned amongst the fillers.

Finally, the rubber manufacturer has still at his disposal for
mixing purposes ground or prepared regenerated waste rubber.
For special purposes various fibrous materials are of service, the
chief amongst these being asbestos ; ground cedar- wood waste from
pencil-factories, hair, woollen- waste, graphite, etc., are also used.

In a very limited number of cases shellac and other resins
are used. In a few instances talc is also added to mixings — for
example, in rubber insulation for cables — but only good amorphous
qualities are used for this purpose. Whiting is in many cases
replaced by precipitated calcium carbonate, which, though more
expensive, is finer, or by magnesium carbonate. A little wax may
be added to hard-rubber mixings.

It is obvious that the substances to be added to a mixing must
in all cases be selected with reference to the purpose for which the
rubber is to be used.

Rubber articles which come into contact with food-stufis should
contain neither lead nor such zinc compounds as are soluble in
dilute acetic acid ; zinc oxide must therefore not be used in such
instances. In this connection it should be mentioned that lithopone
often contains considerable proportions of zinc compounds soluble
in dilute acetic acid, as a result of the method of manufacture
(Abschrecken), for only the zinc present as sulphide is insoluble in
acetic acid. Lithopone should, therefore, always be examined by a
competent analyst as to the amount of zinc soluble in acetic acid
present. And in general a constant analytical control of all raw
materials used in rubber manufacture cannot be too strongly
recommended. The quantity of material used by a rubber factory
in the course of a year amounts to such a goodly figure that it
pays well to entrust the supervision of deliveries of material, and
the accurate valuation of the supplies to be purchased, to a special
member of the staff".

In rubber goods which are to be used in contact with acids,
only those ingredients may be emploj^ed which are not attacked by
acids, viz. barytes, atmoid (etc.), and pumice stone. Whiting must
not be employed in tubing which is to be used for conveying wine,



THE MIXINGS.



105



beer, or acetic acid. In rubber which is to come into contact with
caustic alkalies, the use of substitutes and of saponifiable oils
should be avoided. Qualities to resist the action of oil are produced
by the use of pitch in conjunction with litharge or magnesia usta.
To minimise the action of bleach-liquor on rubber used in contact
with it, additions of earth wax, ceresine, or paraffin wax may be
made. Good electric insulation is obtained by the employment of


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Online LibraryAdolf HeilThe manufacture of rubber goods : a practical handbook for the use of manufacturers, chemists, and others → online text (page 9 of 21)