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should also be avoided. The temperatures of the rolls, too, should
not differ, since apart from the so-called star-markings and blister-


formation, the rubber molecule also suffers, and this again reacts
upon the vulcanisation, and shows itself in the extensibility and
the tensile strength. The sheets, which show unequal constants
in other respects, would then be very liable to undergo partial
premature decomposition. The temperature to be employed must
be regulated according to the kind of Para used, the kind of treat-
ment it has already received, and the thickness of the sheet.

Sheets from which thread is to be cut are made from doubled-
sheet run on cloths sugh as are used in the spread-sheet manufacture,
and which will be described more fully later. The sheet is doubled
in the way described on p. 52, and is afterwards dealt with and
vulcanised, as described in the next sub-section.

(b) Spreadsheet — As mentioned at the beginning of this
section, spread-sheet is that most commonly made in Germany.
This kind of sheet has admittedly its advantages, but is a good
deal more costly to produce, on account of the enormous consumption
of benzine by which it is attended, and the greater amount of
labour involved. The homogeneity and purity of the sheet made
by this process can hardly be attained by any other method. The
statement made by Baur on p. 270 of his book EleJdrische Kabel,
that spread-sheet may be recognised by its lack of cleanliness, is
quite incorrect. In the manufacture of spread-sheet the place of
the calenders is taken by the spreading-machine shown in fig. 72.
For a daily output of 120 kilos, of sheet four spreading-machines
are generally considered necessary, and in addition to these, and
the washing- and mixing-rolls, a three - chambered masticator
720 mm. long and 750 mm. in diameter is required, together with
two solution-mills.

The process of manufacture is in this case as follows : — The
masticated rubber is dissolved in benzine in large tins, and then
worked up on the solution-rolls or mills until a clear solution, free
from, lumps, is obtained. This solution is diluted with benzine to
a degree dependent on the quality and thickness of the sheet.^

The rubber is spread on specially prepared cloths. The closely-
woven fabric used for this purpose is impregnated in a special
machine with shellac, and the rubber is then spread on the smooth
coat of shellac exactly as described in section 12. When the sheet
is finished it is stripped off the cloth on a stripping apparatus, and is
chalked and vulcanised. The shellac-coated fabric must be washed
out as soon as it has been used, and impregnated anew with shellac.

1 Cf. also the exhaustive article "Gummilosungeu" in Gummi-Zeitung^ 1906,
XX. p. 1204.


Spread-sheet is cured in the same way as calendered sheet.

Para-proofed twill is used as a wrapping-cloth for this material,
though, of course, it can only be used two or three times, after


which it is sold as water^^roof cloth. The vulcanisation is carried
out in a water-bath at 2 J to 2| atmospheres pressure, care being
taken to keep the temperature constant.

After vulcanisation the sheet is unwrapped and washed.

For .thread-cutting the automatic cutting-machine shown in
fig. 90 is emplo^'Cd, with drums 450 mm. in diameter and 700 mm.
long, on which the sheet, lightly coated with shellac, is rolled. On
an average six such machines are required to cut 120 kilos, of
thread per day, and three men are required to attend them. The
cutting is followed by boiling-out (to remove free sulphur), drying,
sorting, and tying into bundles, special care being taken with
the boiling-out process. The drying-room should be kept well
ventilated and dark.



Hancock is generally looked upon as the originator of the mamc-
facture of ebonite. He patented his process for the vulcanisation
of rubber by means of sulphur towards the beginning of the year
1843 without prejudice to the fact that Goodyear had already
obtained the substance " hard-rubber " ; Goodyear, however, had
not gone so far as to prepare useful articles from it on a manu-
facturing scale.

For a Long time, however, nothing more was heard of the process,
since the various experiments made did not turn out favourably
enough when they came to be put into practice. It was in 1851
that Goodyear, realising the importance of ebonite, took up his
experiments again and succeeded in preparing a substance which
was very suitable for articles in which, in addition to elasticity, the
qualities of durability, ease of working, and chemical stability are
essential. Goody ear's ebonite of that date consisted of a mixture
of Para, magnesia, whiting, shellac, lead or zinc salts, and sulphur,
these ingredients being intimately mixed together and run out into
strips which were vulcanised at a fairly high temperature. The
product obtained was dark in colour and hard, similar to jet or horn,
and lent itself to being worked quite well. Gradually Goodyear
omitted from his composition all the fillers except sulphur, his
mixing now consisting of rubber and sulphur only; at the same
time he raised the temperature of vulcanisation to about 165° C,
and so arrived at a perfect ebonite. The problem still remained of
finding a material with which to cover the plastic hard-rubber
composition before vulcanisation, and with the discovery by
L. Otto P. Meyer in 1854 that tinfoil was eminently suitable for
this purpose, ebonite manufacture took its place amongst the
industries in which manufacture in bulk can be successfully carried
out, and, advancing steadily, soon won for itself a large market.


For these reasons Charles Goodyear and L. Otto P. Meyer must be
regarded as the real founders of the hard-rubber industry.

Hard rubber, also called ebonite or vulcanite, is highly vulcanised
rubber containing a large proportion of sulphur. The best qualities
consist merely of rubber and sulphur. In the lower qualities, how-
ever, ebonite waste, • ground to dust, is also used, together with
mineral and other compounds and rubber substitutes, which are
enumerated below.

Ebonite, unless otherwise coloured by the addition of pigments
to the mixing, is of a rich deep black colour ; it is capable of taking
a high polish, and is easy to work mechanically ; it is powerfully
electrified by friction, and is a good insulator. It is diathermanous
(transparent to heat rays) in a considerable degree, and its re-
fractive power for radiation is high. Its refractive index is nearly
as high as that of flint glass, and reaches as high a value as 1*56 ;
its specific' heat is 0-331249; its mean coefficient of expansion
between 0° and 18° C. reaches 0-0000636, its coefficient of cubical
expansion being greater than that of mercury at increasing tem-
perature. The more important chemical behaviour of ebonite is in
line with these well-known physical properties. It is very stable
towards chemical reagents, more particularly when it contains no
other ingredients than rubber and sulphur. To this indifierence
towards the action of acids and alkalies hard rubber owes its great
importance and usefulness in chemical industry, especially in the
form of pipes for conveying acids, and of vessels of diflferent kinds.
In a compact mass it is very stable towards air, light, and changes
of temperature ; it can be softened in hot water, and in this condition
can be easily bent; on prolonged heating above 180° C. it loses its
valuable properties and slowly carbonises, -yielding no intermediate
products in the process. It swells up slightly in carbon bisulphide
and in coal-tar solvents, but does not dissolve appreciably in them ;
in general it is very stable towards most solvents of rubber.

The valuable property possessed by ebonite of becoming slightly
soft when heated and when in that condition of taking up slight
impressions, which become fixed on cooling, is made use of in the
manufacture of embossed sheet, which may be very varied in form
and design, and is a valuable manufacturing material for a number
of trades.

The actual technical manipulation of this article will be very
thoroughly dealt with in the following pages, for the manufacture
of ebonite is one of the most difficult branches of the rubber iiidustry.

The raw material for use in this branch must be purified with


the very greatest care, and in particular the raw rubber. The
reason for this is that when the ebonite comes to be polished later
on any small particles of sand or dust which may be present become
evident as projections from its surface. Hence it is necessary as a
prime condition for success that the shops in which ebonite goods
are made up should be absolutely dust-free.

The manufacturing process consists of the following stages : —

1. Mixing the rubber compound.

2. Rolling into the form of sheet.

3. Making up the various articles.

4. Vulcanisation.

And in addition there is the working up of the finished articles (on
the lathe, etc.), milling, etc., buffing and polishing or rubbing down
with oil. The process of mixing is carried out in exactly the same
way as in the case of soft-rubber mixings. The washed rubber
used, which must be absolutely free from sand, may be either Para,
Columbian, Congo-Lopori, Madagascar, Borneo, or good Niggers,
etc., according to the quality. Of these sorts, that which, after Para,
gives the best deep black polish, is Madagascar. The sulphur,
which is the most important ingredient added to the hard-rubber
mixing, should be mixed into the rubber as uniformly as possible.
The points of greatest importance in connection with the sulphur
itself are, first, its fineness, and secondly, its freedom from acid,
which has a most unpleasant effect upon hard-rubber mixings,
particularly those which do not contain magnesia usta or other
neutralising agent. Flowers of sulphur nearly always contain
traces of acid, and on storage will often become still more strongly
acid ; this conditions the presence of a certain amount of moisture
which leads to the production of porous, badly-marked aud faulty
goods. Freedom from grit is also a most essential quality in the
sulphur to be used, particularly in the case of goods which have
afterwards to be polished. In mixings of lower quality finely
ground hard-rubber waste — dust — is an important ingredient, and
the preparation of this material should be most carefully supervised.
On account of its importance the preparation is described sepai«ately
in the following section.

Preparation of Ebonite Dust. — The ebonite waste is sorted into
the following classes : —

1. Waste capable of taking a polish, consisting of rubber and
sulphur only.

2. Waste capable of taking a polish, containing rubber, sulphur,
and hard-rubber dust.



3. Waste not susceptible to polishing, containing mineral

This system of classification cannot, of course, be strictly carried
out; in class 1, for example, waste containing a certain amount of
oil or resin may also be included without disadvantage. Class 1
waste is used in mixings for polished goods, class 2 waste for lower
quality polished goods, and class 3 for " mechanical " ebonite
goods which are finished without a polish. It is in the nature of
things that, generally speaking, every factory works up its own
waste to the best advantage, and waste bought in is consigned on
sorting to a lower class, for safety's sake, albeit that owing to the
employment of trade marks it is possible to appraise waste branded

with certain marks at a higher
value, just as in the case of old
rubber shoes.

The sorted waste, after being
'. broken up into small pieces in
the stone crusher (fig. 91), is
introduced into a boiling dilute
solution of caustic soda, in order
to remove any dirt adhering to
it. The small fragments are
then washed and dried, and
passed on to the "Kaiser" mill
(fig. 92), which mills them into
very small granules about 1 mm.
across. This machine is. most
conveniently raised up on a
pedestal, which is closed in all round and into which the milled
waste passes, the " Kaiser " mill throwing out the granules at
the bottom through fixed, interchangeable steel grids. The hard-
rubber shot obtained in this way is now transferred first to the
simple but powerful milling-machine, with rolls 500 mm. in
diameter and 60 cm. long, shown in fig. 93. The rolls can be
stean>-heated or water-cooled. The main condition for obtaining
powerful friction between the rolls consists in correctly choosing
the speed ratios of the drive, otherwise the consumption of energy
may reach as high a figure as 50 h.p., which would render dust
manufacture a very costly business. A hopper to take the material
to be milled is fixed above the rolls. The hard-rubber granules are
milled until a product as fine as flour is obtained, the rolls being
closed up gradually during the process. This flour is, however,

Fig. 91.



Fio. 92.


FiQ. 93,


always mixed with coarser particles from which it must be separated
by means of a blast of air. or by centrifugal action. For this
purpose the plant employed consists of a ventilating shaft together
with means for producing the air-blast, and space for separating
out the coarser particles, allowing the fine dust to pass on to the
filtering shaft. The hard-rubber flour to be separated is drawn
by means of an exhaust-fan from the chest below the milling-rolls
through a trunk, and falls into a hopper placed close up to the
ventilating shaft. Two smooth wooden rollers close the mouth
of the hopper, and as they revolve allow only so much of the
ground ebonite to pass through as they can distribute. A special
fan blows a blast of air into the fan shaft, which can be regu-
lated by means of valves, and this passes through the valves to
the outlet, taking with it at this point the powder falling from
the hopper. The fine dust is carried forward some distance before
settling to the- ground, while the coarser dust drops in the
neighbourhood of the hopper, and is returned to the mills to be
re-ground. On the other hand, the fine dust is carried forward by
the air up to the filter, where it collects on the filter flannels, while
the air after passing through a " Cyclone " is led into the shaft and
so away. In this way two kinds of dust are obtained, one of which
the finer, is used in mixings of a corresponding quality, while the
other sort is used only in ordinary polished qualities. Instead of
this air-separation method, the centrifugal machine can be used
for separating the dust. This machine, illustrated in fig. 94, in
addition to the basket, has inserted a fine wire sieve and a flannel
filter. These centrifugal machines are specially made for the
purpose by the firm of Haubold. The details of procedure are of
a very simple nature, and need not be described. Before, however,
the dust is ready to be added to the mixing it is tested on a wire
sieve, and filtered through a magnetic sieve so as to remove every
particle of iron from it.

* 1. The Ebonite Mixing.— In the case of the higher qualities
the ebonite mixing consists roughly of the following ingredients : —

2,000 gms.
. 3,000 ,,
. 2,000 .,
200 ,,

500 ,,

In the case of Para the amount of sulphur is not allowed to
exceed the percentage given, since it has been found a suitable one
to transform Para into ebonite by vulcanising for six hours in the
water-bath, with steam at a temperature of 135° C, slowly rising,


. 10,000 gms.

B. Para

Sulphur .

. 3,000 .,


Linseed oil

200 ,,

Sulphur .


250 ,,

Linseed oil



during the last hour of heating, to 140° C. The results of a series
of experiments with different kinds of rubber, mixed with ebonite
dust and sulphur, are given in the tables which follow. From
these tables it will be seen that with' a constant percentage of
sulphur a longer time of cure did not give equally good results,
but that in order to effect this the amount of sulphur had to be
increased, a fact due in some measure to the resin-content of the
raw rubber. The experiments were carried out with sheets 2 mm.
thick, 5 cm. wide and 20 cm. long. Each sheet was tested in an
apparatus which slowly bends the sheet until it breaks. The
movement of the spindle is communicated to a pointer moving on a

Fig. 94.

graduated dial. The figures obtained show the amount of bending
at the breaking-point.

From the second table (p. 209) can be seen what influence certain
quantities of sulphur and other ingredients have on the strength and
structure of ebonite compounds made from different sorts of rubber.

Linseed oil, well boiled and free from water, or " white winter ■"
(cotton seed) oil, is added with the object of improving the polish ;
on the other hand, by adding a higher percentage of oil and adjust-
ing the amount of sulphur accordingly, that is to say, increasing
it, substitute is formed.

In order to produce a horny cut or fracture, and to facilitate
the further working up — turning, drilling, sawing — a certain amount
of white wax is added to the rubber mixing.



To the cheaper qualities, especially for mechanical purposes,
substitutes, asphaltum, pitch, and resins may be added, and in
order to be able to make better use of moulds, magnesia usta.

Mixing is best done in separate stages, the sulpliur and dust
being first worked in, and tlie oil added slowly afterwards. When
reclaimed rubber is used this should first be mixed with the raw
rubber, and this holds good also for substitute. Care must be taken
when mixing to avoid flaking, and the more finely the ingredients
are worked in the cleaner will be the product. Lime should always
be avoided, magnesia usta being used in preference where some
such agent is necessary.

The second stage in the manufacture consists in running the
sheet, starting with the mixed rubber, which has been quite
uniformly warmed up and kneaded. The sheet is, in most
instances, doubled on the doubling calenders until it has reached a
thickness when it can be dealt with on the doubling rolls, and it is
then laid in sheet form on racks and kept ready to be further
worked up. In running this sheet special care must be taken that
no bubbles of air are included between the successive layers of the
sheet, and that there are no markings on the surface, due to irregular
passage through the rolls ; if these defects exist in the calendered
sheet the result is the formation of blisters, and irregular expansion
and contraction of the sheet on curing. Once the sheets have been
laid out on the cloth racks great care must be taken to protect them
from dust and damp.






CO 03 aJ ,




We have now to consider the third step in the manufacture of
ebonite, namely, the making-up of the various articles, and as a
matter of fact ebonite sheet is, in the case of many articles, the
material with which the actual preparation of those articles begins.
In the following paragraphs, therefore, the process of manufacture
of such sheet is described in detail. The manufacture must be
carried out in shops absolutely free from dust, shops on the upper
floor being most suitable for the purpose.

In order to prepare ebonite sheet the following process, which is
found to give good results in every way, is employed. The sheet in
question is not rolled up from separate layers to the required thick-
ness on a heated bench, but is doubled on the doubling calenders
direct from sheet h mm. thick, a compact sheet free from blisters

being thus produced. The sheet is then taken for further manipu-
lation to the hot-bench, the ordinary form of which, shown in
fig. 95, has long been in use. This hot-bench consists of a cast-
iron plate, planed perfectly smooth and provided with interior
channels for the purpose of heating it uniformly by means of
steam. The chief use of the hot-plate is to warm up the rubber
sheet so as to cause it to shrink, and prevent its shrinking during
vulcanisation, and so to produce a uniform sheet. A second purpose
served by the plate is, however, that of a bench on which the sheet
is coated with tinfoil. This is done by means of a heavy iron
roller worked by two men, the roller making the sheet uniform in
thickness and homogeneous in texture, while at the same time
forcing the air out from between the tinfoil and the rubber. By
this means the tinfoil is hermetically sealed on to the rubber at
the edges, and thus the polished surface of the foil is communicated


to the sheet of ebonite, and the surface of the sheet is kept from
contact with water during the cure. Another advantage attending
the use of tinfoil is that the polish obtained on the vulcanised
sheet is very uniform and far superior in quality to any that could
be obtained by polishing processes subsequent to vulcanisation.
Well-trained hands are, however, necessary in order to successfully
carry through this method of manufacture ; the heavy rollers used
are likely, in particular, to contribute a great deal towards mishaps
and defective manufacture; their weight is sufficient, if they are
moved unevenly or in a jerky fashion, or crookedly across the sheet,
to produce a very uneven surface marked by wave-like depressions
and elevations, and by pittings or concavities due to the pressure
of imprisoned air. If the tinfoil is not put on evenly it will crease
and form blisters, these giving rise in their turn to spots on the
surface of the sheet, where the polish will vary in quality or degree.
In order to surmount these difficulties, and at the same time to
economise labour, a hot-plate has been constructed in which the
roller is moved mechanically (system of Haubold and Heil). The
manufacture in this case proceeds as follows : — The tinfoil in the form
of a sheet twice the length of the rubber sheet to be covered, and
folded in two, is lightly, evenly, and thinly coated with absolutely
anhydrous linseed or solar oil on the side which is to come in
contact with the sheet of rubber. The half -sheet as far as the fold
is now laid on the hot-plate, and slowly covered with the sheet of
rubber of the requisite thickness, by rolling the latter on to it ; the
sheet is then allowed to remain a few minutes for shrinking. The
other half of the sheet of foil, beyond the fold, which is to cover
the upper surface of the rubber sheet, is now laid smoothly over
the roller in such a way that as the roller mov^s forward the tin-
foil moves with it and is slowly laid on the sheet of rubber and
firmly rolled down by the roller. The whole of this manipulation
is carried out by the machine shown in fig. 96. The roller, about
300 mm. in diameter, is, as indicated in the diagram, moved forward
and back again automatically, and is then automatically cut out.
The twio side- edges, which have to be closed up air-tight by the tin-
foil, are pressed together by a small auxiliary roller, which can be
adjusted to any width and thickness of sheet.

In this way a sheet absolutely uniform in surface and thickness
can be prepared ; it is safest to cure such sheets in the water-bath.

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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 18 of 21)