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him of his method for the " metallisation " of rubber he gave
sufficient instructions for the technical preparation of perfect rubber
goods, and that these instructions have even up to the present time
lost no essential part of their technical importance; in every
instance they are still in the main far superior to any of the pro-
cesses devised by Goody ear's imitators. Thomas Hancock can only
be regarded, in relation to the vulcanisation of soft rubber, as
having confirmed Goody ear's results, though Hancock was actually
ahead of Goodyear in his application for a patent for his alternative
method. Without doubt he must be looked upon merely as the
inventor of another way of carrying out Goodyear 's invention (in
a strictly chemical sense), but, on the other hand, he has undoubted
rights to priority in the discovery of ebonite.

The discovery made by Alexander Parkes of Birmingham, in
1846, that soft rubber could be vulcanised by means of a solution of
chloride of sulphur in carbon bisulphide, is also, from the chemical



point of view, an original invention. This method of "cold"
vulcanisation is still as important as ever, whilst the numerous
other processes which were invented during the life of Goodyear 's
patents have almost entirely lost what importance they ever
possessed. Only Hancock's method, which consists in dipping the
articles to be vulcanised in a bath of molten sulphur, is still deserv-
ing of mention. The other processes are in many instances merely
modifications of Goodj^ear's or Hancock's, so disguised that it might
be difficult at first to recognise the circumventing of Goodyear's
patent. A favourite method selected for this purpose was that in
which vulcanisation is apparently eflPected by the action of metallic
sulphides, but in which it is really the free sulphur present in such
sulphides that is the vulcanising agent.

The preparation of soft rubber, according to Charles Goodyear's
process, consists in the mechanical admixture with the rubber of
from 3 to 15 per cent, of sulphur, the mixture being subse-
quently heated — in practice under pressure — at a temperature of
from 125° to 150° C. The time of " heating," " curing," or " vulcan-
ising " must be varied according to the percentage of sulphur used,
the nature of the rubber, the amount and the nature of the other
ingredients, the way in which the heat is applied, the form of the
article, and, last but not least, according to the technical eflfect which
it is desired to produce. The preparation of ebonite necessitates,
on the one hand, the addition of a much higher percentage of
sulphur, and, on the other hand, a longer time of curing and higher
temperatures. Although all this can be put so simply in a few
words on paper, it is just as difficult in practice to find the most
suitable conditions in any given case. As a matter of fact, in all
factories this particular part of the business is still carried out by
empirical methods, resulting from long experience, which are care-
fully preserved as valuable works-secrets. It is only quite recently
that a little attention has been given, side by side with this rule-of-
thumb, to theoretical considerations, and that this part of the
manufacture has been attacked with scientific assistance. This
can, however, scarcely result in a noteworthy revolution in the
methods of manufacture. We have certainly gained the knowledge
that rubber of any origin combines chemically with sulphur on
vulcanisation; that the process is one of simple addition without
substitution ; that the percentage of sulphur entering into chemical
combination with the rubber increases (a) with the time of vulcan-
isation ; (h) when certain bases, such as litharge, lime, and magnesia
are added to the mixing ; (c) when a good (metallic) conductor of


heat is substituted for a poor one for press plates and moulds ; {d)

when the articles cured are of thin instead of thick rubber, etc., etc.

We further know that, contrary to the statement of Clouth,^ Para

rubber vulcanises more rapidly and at lower temperatures than

African sorts.

In countries where the decimal system of weights and measures

is in use, it is customary to express the amount of sulphur to be

added for vulcanisation in percentages on the amount of rubber ;

e.g., for a soft-rubber mixing with African rubber, 10 to 15 per cent.

of the weight of the rubber, or for a hard-rubber mixing 30 to 50

per cent, of its weight may be specified. In order to arrive at

comparable figures for the degree of vulcanisation of a rubber, C. 0.

Weber's proposal has been adopted, according to which the sulphur

chemically combined with the rubber hydrocarbon is calculated as

a percentage upon the pure rubber substance present. Suppose, for

example, that analysis shows pure rubber 50 per cent., combined

sulphur 1*5 per cent. Then the so-called " coefficient of vulcanisa-

1'5 X 100
tion" is — — = 3 (per cent, of the pure rubber substance).

This vulcanisation coefficient has proved of considerable value in
judging samples of vulcanised rubber by analysis, and in controlling
the process of vulcanisation.

The properties of rubber undergo an essential alteration on
vulcanisation. The extent of the alteration varies, for the most
part, directly as the coefficient of vulcanisation. On vulcanisation
rubber loses its adhesiveness. As the coefficient of vulcanisation
increases, so does the hardness of the rubber, up to the extent of
becoming brittle ; its resistance to the action of chemicals and
solvents increases, its power of absorbing water diminishes, its
property of becoming distended by the action of organic solvents
becomes less marked. By selecting suitable conditions of vulcanisa-
tion, products possessing any degree of elasticity for any given
useful purpose can be obtained. Vulcanised rubber is not nearly
so sensitive to changes of temperature as is the unvulcanised sub-
stance. The colour of vulcanised rubber gradually deepens as the
coefficient of vulcanisation increases until the deep black of hard
rubber (ebonite) is arrived at. In goods vulcanised under pressure,
vulcanisation results in a marked contraction, or decrease in volume,
the extent of this increasing with the coefficient of vulcanisation.

Rubber acquires on vulcanisation a different smell from that
which it possesses in the raw state; and this is, in many cases
1 Gummi, Guttapercha^ und Balata, 1899, p. 123.


sufficiently characteristic to permit of conclusions being drawn as
to the kind of raw rubber used. In the case of rubbers which have
a bad smell in the raw state, the smell often becomes intensified on
vulcanisation ; as, for example, in the case of " Lumps " or " Guayule."

Soft rubber has the reputation of being perfectly waterproof.
It is not very well known that soft rubber undergoes a considerable
change when subjected to the prolonged action of hot water; it
absorbs an appreciable amount of water, becoming much lighter
in colour, and suffering a marked diminution in tensile strength.
At the same time its power of adhering to fabrics is greatly modified.
The thinner the coating of rubber, the more pronounced is the effect
of water. This property of soft rubber is utilised, to a limited
extent, in the working up of old rubber.

Vulcanisation by Goodyear's method by means of sulphur can
only occur at temperatures above that at which the mixture of
rubber and sulphur melts. Statements in a contrary sense,^ to the
effect that a thin sheet of unvulcanised rubber containing sulphur
can be vulcanised up to a certain point merely by the action of
light, have proved, on re-examining the facts, to be incorrect ; in no
case have such sheets been found to contain a trace of chemically
combined sulphur, and they therefore remain completely soluble in
benzole. The error has probably arisen from the fact that the
sheets in question had to be rendered plastic in the operation of
mixing-in the sulphur, but gradually regained, after being kept for
a long time, the original physical properties exhibited by the dry
unvulcanised rubber before mastication. If pieces of " young " Para
rubber about 1 mm. thick be removed from a ball, and allowed to
dry gradually, the slices attain in the course of time the appear-
ance and almost the physical properties of vulcanised rubber, and
when superficially examined might be judged to be vulcanised. In
the case of the unvulcanised sheet containing sulphur the illusion
is naturally still more likely to take place, because the addition of
sulphur partially destroys the adhesiveness of the rubber. A
rubber can only be considered vulcanised if it contain chemically
combined sulphur, or sulphur and chlorine. Up to the present no
method of reversing the vulcanisation process is known which does
not result in the breaking down of the rubber molecule. This point
will be more thoroughly dealt with when the subject of " Reclaim-
ing " is considered.

Vulcanisation by means of sulphur generally results in the
combination with the rubber of only a portion of the sulphur
1 Of. Fr. Clouth, Gummi, Guttapercha, und Balata, 1899, p. 131.


added, the other portion being merely held in a kind of solution in
the rubber, which is in a more or less viscous state at the vulcanis-
ing temperature, and crystallising out again on cooling. The
changes which occur during the process have been microscopically
examined by Pierre Breuil, who has published a number of
instructive photo-micrographs of them.^ The uncombined (" free ")
sulphur gives rise to the phenomenon of " blooming " or " sulphuring-
up," which imparts to the goods on which it occurs a greyish
appearance due to a coating of finely-divided sulphur, and which
may or may not be desirable from the purchaser's point of view.
Sulphuring-up can generally be prevented by using a mixing of
a suitable composition. Sometimes the goods are first of all allowed
to sulphur-up, the sulphur being afterwards removed by gently
heating with solutions of caustic alkalies, or by other means.

As already mentioned more than once, a mixture of plastic
rubber and sulphur becomes more or less viscous at the temperature
of melting sulphur. This property must be taken into account in
the manufacturing processes, where it is necessary to adopt some
means of preventing the articles, which have been made to a given
shape, from losing that shape when subjected to the vulcanising
temperature ; this is of especial importance in the case of so-called
" open " cured goods. Generally speaking, there is not much risk
of the goods actually melting down as a whole, since on the one
hand the softening only advances very slowly through the mass of
rubber, rubber being a bad conductor of heat, and, on the other
hand, the process of vulcanisation tends more and more as it goes
on to check anything of the kind; the fear is rather that when
the rubber gets soft at the high temperature, the goods will tend
to fall out of shape by their own weight if they are not carefully
supported in some way. In some kinds of tubing which is cured
in the open, this effect of the heat of vulcanisation may be re-
cognised by the fact that the tube has lost its circular cross-section,
and become more or less elliptical. This difficulty may be got over
by using rubbers which cure quickly, or by the addition, where
possible, of appreciable quantities of basic substances which facilitate
vulcanisation, such as litharge or magnesia usta, or by increasing
the proportion of sulphur in the mixing.

Nothing is yet known with absolute certainty as to how such

basic substances as those just mentioned act in facilitating

vulcanisation, and fantastic ideas on this subject, as well as on

the similar action of metallic sulphides, are therefore in existence

^ Le Caoutchouc et la Guttapercha, ii. 158 and 197.


amongst rubber specialists. We shall not occupy any space in
discussing these ideas.

The vulcanisation of thin-walled rubber goods and of lightly
proofed fabrics is best carried out by Alexander Parkes' process,
in which chloride of sulphur is employed as the vulcanising agent,
either in solution or in the form of vapour. In many instances
Parkes' cold vulcanisation process is competitive with Hancock's
process, in which vulcanisation is carried out in a molten stilphur
bath. The practical working of these two processes will be dealt
with later on.

It is a very remarkable fact that the processes of vulcanisation
which have just been referred to are still carried out in exactly
the same way as invented by Goodyear, Hancock, and Parkes re-
spectively. Even with reference to certain details, as, for example,
those relating to the use of magnesia usta in hard rubber {Goodyear)
or to the solvent which it is best to use for chloride of sulphur
{Parkes), manufacturers have, after making several departures
from it, again reverted in almost every instance to the original
practice of the inventor. Still it is true that considerable improve-
ments have been made in the course of time in the methods adopted
for preserving the form of goods during vulcanisation, and in the
methods of applying heat to them.

The Vulcanising Shop. — Having in the last section dealt fully
with the nature of vulcanisation, we shall in the present section
consider the details of the arrangement of a modern vulcanising

The vulcanising plant forms one of the most important items in
rubber-goods manufacture, and it is essential that the shop in which
the process is to be carried out should be provided, in every parti-
cular, with the most modern and most convenient arrangements
that have been devised in the course of the fifty years, roughly
speaking, that have elapsed since the rubber industry was founded.
As already stated in the introduction (Room D, fig. 1), it is advis-
able to place the vulcanising plant immediately at the back of the
boiler-house, so that the steam-pipes need not be unduly long, an
arrangement which tends to minimise condensation in the pipes.
Similarly, the shop should be close to the moulding shop, so that
the heavier moulds can be moved about with as little difficulty as
possible. These two points form, so to speak, the basis of the plan.
The building is provided with a span roof so that as much air and
light as possible can get in, and so that there are no pillars in the
shop to be in the way. It is advisable to have a ridge-turret the


whole length of the shop, provided on both sides with glass
jalousies which open very easily, in order to ensure efficient
ventilation. A fan driven by an electro-motor is employed to get
rid of the steam which escapes on opening the heaters. A plant
arranged for a large, well-organised works, and to be put down on
the lines of the accompanying plan, would be as follows : —

A mechanically driven traveller for raising and transporting the
loads which are to be stacked upon the various heater trolleys runs the
whole length and breadth of the vulcanising shop (A, fig. 30). In
the adjoining room B are the heaters, ranged along the long side of
the shop, with the fronts of the heater doors visible. The heaters
themselves are therefore in a separate compartment B in room A ;
B contains only the heaters, each of which is well insulated. This
compartment has a non-conducting top to still further protect the
heaters from cooling. We also see here against the wall the heater,
about one metre in diameter, which serves to receive the outlet
pipes on their way to the condensing tank F.

At a point just before the outlet pipes enter the tank non-
return valves are introduced. When steam is blown off from a
heater the steam enters the tank, which is filled and kept fed with
cold water, where it is condensed, the slight amount of steam pro-
duced being able to escape from the condensing tower, which is
carried up to 'a height of about 10 metres, while the condensed
steam runs back into the tank. All outlet pipes should be carried
into the water to a distance of about two metres. The outlet valves
are situated beneath the ends of the heaters in room A. By
arranging and building in the heaters in this way all possibility of
excessive radiation of heat from the heaters into room A is removed.

In the same room, in front of every heater, are rails on which
the vulcanising trolleys run, these being sunk into the ground to
a slight depth so that the top of the trolley is level with the main
transport rail, which runs the whole length of the vulcanising shop
and connects it with the other shops in the factory ; this arrange-
ment is such that the loaded trolleys can be run straight from the
main rail, over the turntables, on to the heater trolleys, thus doing
away with the heavy work of unloading and repacking in chalk.
There are four upright heaters in the shop, also of course well-lagged.
Two of these are provided with steps leading up to an iron plat-
form surrounding the heater breast-high. Under the main rail
track is a tank, used for cooling down the moulds, the whole
trolley load being lowered into and hoisted out of the tank by
means of a lifting tackle. In a smaller room the goods are packed




in chalk, the air being kept carefully free from dust by artificial
ventilation. In this room are also kept various kinds of iron boxes
or trays to take the goods, these being put straight on to the trolleys

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and not again unpacked before being cured. They are quite ready
to undergo vulcanisation for the necessary length of time, when
run into their respective heaters. Another room, D, is used for
drying and sifting the chalk used in the vulcanising process. In
this room the trolleys are emptied, after vulcanisation, of all goods


which have been cured in chalk. The chalk is then immediately
dried in the oven, put through a chain-pump into the revolving
sieve, and sifted ; it is then ready for further use.

A glance now at the heaters themselves. These should, if
possible, be constructed to stand a steam pressure of six atmos-
pheres, and the doors should be designed with a fairly wide flange
with a recess in order to give the packing a good firm hold, so that
it does not require to be renewed very often. The bolts for closing
up the heater door should be fairly strong and thick, so that as
few bolts as possible need be used, and the operations of opening
and closing the heaters be thus more rapidly carried out. The
steam supply to all the heaters is centralised; the steam pipe
from which all the supply pipes are taken, and on which are
also the inlet valves, manometers, thermometers and water-gauges
for every heater, is set up in the control-room G, quite by itself.
A similar steam pipe on the other side, fitted with similar apparatus,
is used for the press shop. In arranging for the supply of steam
to the heaters, and the outlets for condensed water and steam,
careful attention must be directed to the pipe system. Under
no circumstances should too small a pipe be used, and the outlet
pipe should always be larger than the supply pipe. To ensure
economical consumption of steam a Bourdon steam-trap may be
introduced at the outlet.

With reference to the control-room G, it may be said that it is
here that the correct conduct of the vulcanisation rests in the hands
of the man in charge. In order that vulcanisation may be success-
ful, this man must perform his duties with exactitude. The steam
passes from the boiler-house, through the superheater, where it is
dried, to the steam pipe H in the control-room. A steam trap is in-
terposed between the main steam supply pipe and the steam pipe H.
Projecting from this we see all the valves, the pipes from which
lead to the heaters corresponding to the numbers on the valves.
Manometer and thermometer tubes are brought into this room
from the heaters, the scales being placed directly above the corre-
sponding valves. In addition, there is an arrangement for intro-
ducing a mercury manometer for the purpose of checking the
others. Each manometer should be provided with a recording
apparatus so that the vulcanisation can be regulated at any moment,
and proper control be maintained over the " rise " as well as the
steady pressure. The heater having been closed, and the outlet
valve adjusted, the man in room A, who should have nothing else
to do than attend to the heaters, signals by means of an electric bell


to the foreman in charge of the vulcanisation, in room G, and he
sees by the indicator which heater is to be put on. He ascertains
the length of the " cure " from the ticket given to him, which shows
the quality and nature of the goods to be vulcanised, and by refer-
ring to his list of temperatures • and times for the various articles.
The times of vulcanisation are arranged alphabetically according
to qualities, kind of goods, and substance. After the charge of the
heater has been entered in the vulcanisation book, and the time of
putting on the heater also filled in, these particulars are also marked
up on the controlling table in the column corresponding to the
heater being used. This table is then hung up; on it is shown
which qualities are being vulcanised at the same time. As soon as
the steam pressure exceeds the number of atmospheres to which the
apparatus has been adjusted the attention of the foreman is called
to the fact by the ringing of a bell, if he has not already noticed it
on the manometer ; by this means the foreman is helped to keep a
strict watch, and no prolonged excess of pressure can escape his
notice. This whole business consists simply and solely in turning
steam on and off the heaters, and in seeing that the manometers and
valves are working accurately. When the time of vulcanisation is
completed, the valve is shut down, the electric alarm is taken out,
and the foreman signals to the man in room A, who sees by the
indicator which heater is referred to, to open the outlet valve. On
a further signal, when the pressure has dropped to zero, he opens
the heater door. The time at which the cure was completed is
entered by the foreman in his book, and the department concerned
in the further manipulation of the goods is rung up to fetch them
away from room A or D.

The above scheme provides for subdivision of labour, and does
away with the opportunities which otherwise occur for the man in
charge of putting on and regulating the heater to concern himself
with other jobs. In his separate room he has nothing to do but look
after the valves and manometers, while the man in room A has
only to see to the opening and closing of the heaters. The men
employed in these two rooms are, under this arrangement, not
troubled by dust, or by intense heat, which would tend to distract
their attention from their duties.

It has already been stated that the chief things to be sought
after in vulcanisation are the use of dry steam and the maintaining
of the proper temperature, the pressure of the steam not coming
into account as far as the cure itself is concerned. Pressure is,
however, by no means an unimportant factor.



The heater shown in fig. 31 is the ordinary type of simple
heater. As a rule the steam is not led in from a single opening,
but is distributed by means of a spray-pipe, which is laid along the
bottom of the heater ; this pipe is closed at the end and provided
with small holes at intervals along its walls. The pipe is covered
by a perforated iron plate. Considerable advantages follow from
the use of a jacketed heater for vulcanisation, not so much for hot-
air vulcanisation as for getting the outer jacket hot before letting
steam into the interior of the heater, and so to a large extent

Fig. 31.

preventing condensation and maintaining a uniform temperature.
To achieve this end the pressure in the outer jacket must be half
an atmosphere above that inside the heater itself.

The dimensions of single heaters are very various, and depend
upon those of the goods to be cured in them. Heaters from 80 cm.
to 3 metres in diameter are used ; for special purposes, indeed, such
as cable manufacture, the diameter is sometimes as great as 3J
metres ; the length or height of the heater varies from 1 metre to
5 metres. For hose, heaters are used which are as much as 51

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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 6 of 21)