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114 RUBBER MANUFACTURE.

pitch has been added, are used, less in rubber goods than in
insulatinof masses for electric cables. The use of such substitutes
is restricted to dark-coloured mixings.

Mention must be made of certain kinds of coloured substitutes,
prepared by colouring the oil mixture before treating it with
chloride of sulphur. The use of these coloured substitutes is
limited to cold-cured goods, feince they are destroyed by hot
vulcanisation. As is well known, all chlorosulphide substitutes
are converted at the higher vulcanising temperatures into dark
viscous oils which, unlike those into which the brown sulphide
substitutes are converted by heat, do not set again on cooling.
All chlorosulphide substitutes do not decompose with equal facility.
If in the manufacture of light-coloured goods it is desired to use
white substitute, it is advisable to use as low a temperature as
possible for vulcanisation, this being rendered possible by the use
of suitable quantities of magnesia.

The colours used for colouring chlorosulphide substitutes must
be stable towards acids, and non-poisonous.

Many rubber factories are obliged by local conditions to get
their substitutes from special substitute factories. Such bought
substitute is generally examined as follows : — White substitute is
warmed with distilled water, to which is added a drop or two of a
solution of phenolphthalein, rendered faintly pink by addition of
a trace of alkali. If the pink colour remains — care being taken
to exclude air — the substitute is practically free from acid. If the
pink colour deepens, as often happens, it means that the manu-
facturer has added an excess of magnesia or other neutralising
agent, and so has made the substitute alkaline. Nothing is to be
said against this. The second test consists in comparing the carbon
bisulphide extract of the substitute with that of an approved
standard sample. Substitutes which are strongly alkaline should
be extracted with benzol in preference to carbon bisulphide, since
the latter is decomposed by alkalies, but in that case the comparison
should be made with the benzol extract of the standard sample.
The amount of ash or of magnesia present in a sample of substi-
tute is also occasionally of interest to the manufacturer. Substi-
tutes to be used for cold-cured goods must be perfectly neutral, and
must therefore not contain magnesia. Substitutes which do contain
magnesia are generally heavier than water, but for the same
reason they are ratlier more stable towards heat, as may be
determined by heating in a drying-oven at 150° C. In cases where
the chloride of sulphur, used in the manufacture of white substitute,



THE MIXINGS. 115

contained an excess of sulphur, a corresponding quantity of free
sulphur is found in the benzol extract. This, however, occurs very
rarely.

It scarcely ever happens that brown substitutes contain free
acid, since only crude oils are used — oils, that is, which have not
been refined by means of sulphuric acid — and since in the process
of manufacture only neutral sulphur is employed. But individual
samples of commercial products should be very carefully examined
as to the percentage of constituents present which are soluble in
hot acetone. This extraction is carried out in a Soxhlet or similar
apparatus. The acetone extract is evaporated to dryness and the
residue weighed.' The residue is then extracted several times with
a measured quantity of cold acetone, and the second residue dried
and weighed. The difference between the weights of the first and
second residues represents approximately the amount of unchanged
(non-sulphurised) oil present. The second residue may consist of
free sulphur and of paraffin wax, mineral oils, etc. It is treated
with strong alcoholic potash until all the sulphur is dissolved, the
alcohol is got rid of by evaporation, the residue thoroughly washed
with hot water, and any residue of paraffin wax, etc., is weighed.
It is true that only approximate numbers are obtained by this
method, since it is only a rough one, but still the numbers obtained
will give a sufficient indication of the nature of the substitute to
an expert. A suitable quality having been selected from amongst
the various commercial sorts, the application of this method of
examination, combined with determinations of melting-point and
specific gravity, will afford the necessary means of deciding whether
a new sample of substitute is equal to that which has been in use
hitherto. A brown substitute which is to be used in the manu-
facture of red-rubber goods should be examined as to its colour-
absorbing power, and the colour of the ground product should be
carefully taken into consideration. The best way of carrying out
these tests is by preparing and vulcanising test-mixings.

The Treatment of Besk for use in Rubber Mixings. — The well-
known unpleasant-smelling pseudo-rubber called Besk, Pontianac,
dead Borneo, Gutta-Jelutong (which is now almost indispensable),
always contains a great deal of sand and fragments of wood, even
after it has been carefully cleaned on the washing-rolls, so much
so that it is not often, or only in the lowest grade mixings,
that the washed product can be used without further purification.
Besk contains a small proportion of a useful rubber hydrocarbon,
the isolation of which may possibly come to be regarded in the



Il6 RUBBER MANUFACTURE.

future as a paying process, but the presence of an enormous pro-
portion of resins, together with the difficulty of removing mechani-
cal impurities, greatly detracts from its value. There are a number
of ways in which Besk can be prepared for use in rubber mixings.
One of the simplest consists in dissolving the Besk in, or melting
it down with, 25 to 40 per cent, of its weight of castor oil, and
filtering the thick solution so obtained through sieves which can be
heated. About 5 to 10 per cent, of magnesia usta is then added to
the filtered mass in order to bring about good results on vulcanisa-
tion. Tar, freed from naphthalene by evaporation, is also used
instead of castor-oil in other instances ; in that case, however, the
Besk mixture can only be used for dark goods. A more usual
method of purification than that which involves the solution,
melting, and filtration of the Besk, consists in forcing it through
very fine sieves by means of the tube machine. Besk mixtures are
generally used in order to remove the " dryness " from such
mixings as contain large proportions of ingredients like barytes or
whiting, and to make them easier to work up for a variety of
purposes. Quite -colossal quantities of Besk are also used for
proofing asbestos and other fabrics in the manufacture of packings.
In these cases it is merely good sticking properties that are neces-
sary, elasticity being of quite minor importance.

Asphaltum and Pitch as Ingredients of Mixings, — Asphaltum
and pitch are often added to dark mixings in order to produce
"density," and to render the goods less susceptible to oxidation.
These additions also facilitate the mixing of powdery ingredients,
and there is less risk of the rubber getting "killed" upon the
mixing-rolls. Moreover, the addition of pitch either brings about
only a very slight increase in the specific gravity of the mixing, or
it may even help to lower it. Natural asphaltum is not always
pure enough to add directly to the mixing without further trouble.
If it is very rich in mineral and, as in some instances, woody con-
stituents, it is advisable to dissolve it in a suitable solvent, filter the
solution, and then drive off* the solvent again. The usual forms of
extraction apparatus are most suitable for this purpose, the more
modern forms being so perfect in design and construction that only
a very small percentage of solvent is lost in the whole operation.
Generally speaking, the use of asphaltum has fallen oflf very much
for some considerable time, for the same effects can be produced by
means of pitch, which is cheaper. In the preparation of this pitch,
various kinds of tar, chiefly coal-tar, are thickened by evaporation
until all traces of naphthalene are removed, and the tar is converted



THE MIXINGS. II 7

into an extremely viscous, almost solid, pitch. Either hard, pure
asphalt or Carnauba wax is sometimes added to the pitch in order
to raise its softening point. Some makes of pitch are often met
with in commerce which are thickened by the addition of resin ;
such pitches are not to be recommended.

The addition of pitch considerably increases the gas-tightness
of rubber, and also diminishes its water-absorption; the latter
property is of especial importance in cable mixings.

Fats, Oils, Paraffin and other Waxes in Rubber Mixings. — At first
sight it seems absurd that, in spite of the well-known destruc-
tive action of fats and oils upon vulcanised rubber, not inconsider-
able quantities of such substances should often be added to the
mixings. But when it is realised that the destructive action of oils
must proceed from the surface inwards in order to bring about
complete destruction of the rubber mass, and that the extent of the
destructive action depends very much upon the amount of oil
present on the surface of the rubber, it will be seen that the oils
which are added to a mixing, in quantity which is never so great
as to cause the rubber mass to swell up to the extent necessary to
bring about its destruction, cannot in the ordinary way cause any
appreciable damage to the rubber goods in which they are used,
provided they are used with discretion. Moreover, the use of oils
is limited, on the whole, to those mixings which either contain a
great deal of waste rubber, or a large proportion of ingredients
tending to make the rubber porous. Leaving hard rubber out of
account, the materials mentioned in the heading to this section are
either not used at all in the finer mixings, or only in the smallest
quantities.

Naturally it makes all the difference in the world whether the
oils, etc., are added to the mixing before vulcanisation, or whether
they act locally on the surface of the rubber mass after vulcanisa-
tion. A whole series of oils, etc., are partially sulphurised when
suitably vulcanised, although it is only in the case of hard-rubber
goods that the combination of oil with sulphur goes on to such an
extent that one can talk about the formation of compounds similar
to substitutes. Exaggerated notions as to the formation of sub-
stitute from castor oil, rape oil, etc., during vulcanisation some-
times exist amongst rubber workers ; for it is very easy, by putting
through an acetone extraction of such rubber mixings, to show that
practically all the oil added remains, after vulcanisation, soluble in
acetone ; that is to say, that only a minimum quantity (or none at
all) of substitute insoluble in acetone has been formed, unless, of



I I 8 RUBBER MANUFACTURE.

course, very high temperatures, or ingredients which act quasi-
catalytically, have been employed.

In the use of saponifiable oils in conjunction with saponifying
agents such as lime, magnesia, litharge, zinc oxide, etc., the purpose
which the goods are destined to serve must have a decisive im-
portance attached to it. Such additions in notable quantity should
be used in steam packings only with great caution. The use of
saponifiable oils, of linseed oil in particular, in soft-rubber goods
should be as far as possible avoided. Rosin oil is often deceptive
in its action,v a strikingly good effect being first of all produced,
very soon followed, in the case of goods in the manufacture of
which appreciable quantities of the oil have been used, by a ruinous
hardening. Rosin oil, as is well known, is very susceptible to
chemical change, and should therefore only be used in small
quantities in practice. Opinions with regard to palm oil differ
very widely. Whereas by some it is regarded as a very valuable
addition, others consider it more dangerous than linseed oil. There
is much to be said for both opinions, for very much depends upon
the quantity used, and upon the many special points about the
mixings in which it is employed.

The pale, heavy mineral oils, as well as vaseline and paraffin
wax froin petroleum, are especially largely used. These bodies are
practically unaffected by the process of vulcanisation, and, provided
their bromide absorption coefficient is low, they are also practically
unaffected by other chemical influences, including that of the
atmosphere. These statements do not apply with absolute exacti-
tude to shale oil, or to analogous products of peat distillation. The
products from these two sources corresponding to the petroleum
products referred to above have generally an unpleasant smell
similar to that of solar oil, which serves to indicate their presence.
Some of them have an enormously high bromine absorption, which
points to the presence of unsaturated hydrocarbons of the olefine
series, and are therefore able to combine with sulphur when heated
with it, liberating considerable quantities of sulphuretted hydrogen
in the process. This reaction of the peat distillates finds applica-
tion in the preparation of ichthyol products ; they are of very little
use for rubber goods. Ceresine, obtained from ozokerite or earth-
wax, is used with advantage in rubber manufacture ; unfortunately
pure ceresines are rather expensive, and the wax is often adulterated.
There is nothing to be said against the use of the cheaper ceresines,
which are adulterated with good paraffin wax, provided they are
sold as such mixtures. But besides these true " mixed " qualities



THE MIXINGS. I 1 9

of ceresine, there are many kinds on the market which are adulter-
ated with rosin and other cheap filKng and colouring matters;
against tliese it is only necessary to utter a note of warning.

It is only in the manufacture of the various qualities of hard
rubber that the use of waxes is of any very great importance, and
the different kinds of wax are often used in conjunction with one
another. The most frequently used are Carnauba wax and Chinese
wax, although Japan wax, which belongs to another class, has
recently come into fairly extensive use.

In some factories stearic acid is used instead of paraffin wax.
In this connection the possibility of the goods in which it is present
coming in contact with metals must be taken into account, since
stearic acid, as is well known, acts vigorously upon a whole series
of metals.

There is one novelty which is worthy of note here : that is, the
highly-oxidised castor-oil manufactured by a South German firm ;
with the aid of this it is possible in some cases to dispense with
the use of the more expensive Besk mixtures and similar very
adhesive ingredients. Sometimes the oil in question is heated with
sulphur before being mixed with the rubber, but not to such an
extent as to lead to the formation of substitutes.

The foregoing account cannot, of course, lay any claim to
completeness, although the most important oily and waxy in-
gredients have been referred to. The authors are unfortunately
not in a position to report upon the results of experiments with
oils polymerised according to Axlerod's patent, which were stated
on his part to be suitable for use in rubber mixings. In this
process of polymerisation, as is well known, aluminium chloride is
employed as the catalytic agent for effecting polymerisation. It
may also be mentioned that Tung oil, polymerised by heat, has
been recommended as a kind of substitute with the properties of a
wax, but this product will not easily find general use, on account of
its unpleasant smell.



CHAPTER IV.
THE MANUFACTURE OF SOFT-RUBBER ARTICLES.

1. Manufacture of Hose. — (a) Vacuum and Pressure Hose, with and
without Insertion, for Conveying Purposes. — In the following pages
our intention is to describe the manufacture of hose as carried out
to-day in modern works. As may be gatliered from the plan of the
factory, the hose department is located in the largest shop, the
whole breadth of which is available for use, no obstructing columns
or pillars being present. This is a great advantage, because the
metal mandrels, 40 metres long, on which all large-size hose and
all tubing with insertion has to be made up, can then be carried
without hindrance from the working bench to the wrapping
machine, and from there to the trolleys on which it is run into the
vulcanising pipes.

The method of manufacture formerly adopted, and which is still
employed in many works, was to lay the calendered sheet of rubber
round the metal mandrel, which had previously been rubbed over
with French chalk, press the overlapping edges closely together, and
then cut along the tube from end to end. The cut edges were
moistened with benzine, and were then pressed together and kept
in position by a strip of rubber which was laid on top. This is the
method of manufacture still employed in the case of hose of large
dimensions and of qualities which do not lend themselves to being
dealt with on the machine. In the case of good qualities, where
the cut edges adhere together well, this method is also less risky ;
on the other hand, in the case of qualities in which the cut edges
do not adhere well together it is better to use the tube machine
method, which was first introduced in America, but which has since
been adopted almost universally.

The manufacture of hose is generally carried out in the follow-
ing three stages : —

1. Running the inside and outside coatings on the tube
machine.

120



MANUFACTURE OF SOFT-RUBBER ARTICLES. 121

2. Wrapping on the insertion.

3. Wrapping the hose prior to vulcanisation.

Thore still exist many factories in which the machining of the
inside coat of rubber is still done on the ordinary tube machines,
and the sleeve obtained then drawn on to the mandrel. But this
method has the following disadvantages: in the first place the
rubber gets contaminated with French chalk ; and secondly, thin
places are formed, owing to the uneven stretching which it receives
during the process of pulling it on to the mandrel ; small particles
of compounds, also, the presence of which is due to careless mixing,
form weak spots through which the liquid can find its way into
the insertion, perforating this and making the hose unfit for use.
The contamination with French chalk causes a bad join to be made,
unless the chalk is washed off quite clean with benzine. But if
the manufacture is to be carried out on thoroughly business-like
principles, tube machines must be used through which the mandrel
can be driven and the rubber squeezed out round it. Besides the
reduction in the amount of labour required to make hose by this
method, one has, in addition, the advantage of obtaining a uniform
thickness of wall, even in the case of the very thinnest- walled
tubing.

The construction of this machine has been so greatly improved
that it now forms a perfect type, as illustrated in fig. 50. The
mandrels are introduced through the hollow screw, and with their
coating of rubber are carried forward, more or less rapidly, immedi-
ately on leaving the machine, by an endless conveyor with grooves
on its working surface to take the mandrels. The speed of the con-
veyor is regulated by means of a friction driving-pulley fitted on
to the machine. In order to prevent the rubber from sticking to
the mandrel, the latter is finely powdered with French chalk after
entering the hollow screw, by means of a blowing apparatus
attached to the machine, and supplied with air from a small fan.
Instead of the usual feeding- hopper, this machine has a pair of
feeding-rolls, which force the rubber into the screw at a uniform
rate, and uniformly heated. Should the screw receive a little too
much rubber at any time, the rolls are raised by springs and do
not carry any more forward until the screw has worked ofi" the
excess. By thic,' arrangement the troublesome choking-up of the
machine is done away with and inequalities in the thickness of
the rubber tube are avoided. As the mandrel leaves the nozzle of
the machine it passes on to the conveyor in front of it, by which
it is carried along at a speed corresponding to that of the machine.



122



RUBBER MANUFACTURE.



As soon as the whole length of mandrel is covered with rubber
it is laid on a bench near at hand, and iti the case of hose with
insertion the second stage of manufacture is begun, namely, the
wrapping round it of the insertion.

The best way of doing this is still by hand, for all the insertion
wrapping-machines hitherto constructed have the disadvantage that
the layers of cloth must not overlap, for if they do the pressure of




Fig. 50.



the top roller will tend to cause the inside coating to be penetrated
by the cloth where the extra thickness, due to the overlap, occurs.
On the other hand, attempts to butt the edges of the canvas are
attended by the danger of leaving the hose liable to be easily burst.
The safest plan is, therefore, in tlie case of hose with hemp or
cotton insertion, to proceed as follows : — The proofed canvas is cut
on the bias into strips of such a width that they will go round the
hose the required number of times ; these strips are united by over-
lapping their ends to the extent of about 12-15 mm. and firmly



MANUFACTURE OF SOFT-RUBBER ARTICLES. 1 23

rolling them together, and the long strip thus produced is now laid
over the rubber sleeve (on the mandrel) and gradually rolled on to
it, by means of small hand-rollers, round the whole circumference,
until the beginning and end of the canvas are in superposition,
after the required number of turns have been made round the cir-
cumference. When this part of the process is completed the hose
is tightly stretched on the mandrel and bound on to it at both ends.
The wliole then passes through the hose machine again, when the
outer coat or cover is put on; during this process, however,
the chalking apparatus is not brought into action. As soon as the
outside coat has been put on, the hose is ready to be wrapped, a
necessary preliminary to vulcanisation.

Before going into the details of the third stage in the manufac-
ture of hose, it will be as well to get a clear idea of the object of
the wrapping process ; this is essentially as follows : — As made
upon the mandrel the hose is not yet suflficiently compactly bound
together as a whole, and it may therefore easily happen that
places are left between the insertion and the rubber coating where
these two do not adhere together well. For this reason, this kind
of hose is wrapped, before being vulcanised, with one or more
strips of cotton, wide or narrow according to circumstances. The
whole length of rubber being now tightly bandaged, it is shaped,
during vulcanisation, into a perfectly cylindrical tube. The cloth
impression visible on the surface is produced of course by the
wrapping-cloths, which are removed after the hose has been
cured. If the wrapping is badly and unevenly done, or if the
tube is not drawn on tightly enough, air blisters may be formed
during vulcanisation, which may rupture the cloth and make the
hose unsaleable, if the damage cannot be repaired by cutting a
piece out, making the place good, and curing once more. In inser-
tions spread with solution the presence of moisture is a factor
which may give rise to porosity or blistering, and ba^d mixings may
also cause the whole of the inside and outMde coats to blow and
become porous. As a rule the wrapping is done, in the case of a hose
40 metres long, by a gang of fifteen to eighteen hands, twelve to four-
teen of whom give to the hose, which is lying on a bench, a forward
rolling motion, while a man at the upper end pulls at the mandrel ;
three more hands are necessary to hold the wrapping-cloth and roll it
on. With the mechanical wrapping-machine (fig. 51) the method of
procedure is much simpler. The hose is supported on the two lower
rolls. A strip of cloth of the right width having been wrapped
round the end of the hose at a sharp angle, the upper pressure-



124



RUBBER MANUFACTURE.



is of just the same length as the under rollers (the
actual length of roller for &
40-metre leno^th of hose beinof
42 metres), is lowered, and the
machine set in motion. The
wrapping process is now accom-
plished automatically, the hose
rotating about its own axis on
the two rollers, while the upper
roller presses the cloth tightly
on to the hose ; meanwhile the
man in charge of the wrapping-
cloth has only to see that it is
kept tightly stretched.


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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 11 of 21)