Adolf Heil.

The manufacture of rubber goods : a practical handbook for the use of manufacturers, chemists, and others online

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The Physical and Chemical Properties of Rubber. — The physical
and the chemical properties of rubber are of almost equal importance,
so far as the rubber industry is concerned, though one particular
chemical property is of outstanding interest, the property, namely,
of combining with sulphur or chloride of sulphur to form vulcanised

Rubber, well-cleansed and dried, has a specific gravity of
from 0925 to 0967 in the case of samples which have been

1 Steiiikopff & Springer, Dresden.


carefully freed from enclosed air. Rubber is a bad conductor
of heat and electricity, and extensive use is made of its elec-
trical insulating power. By friction or pressure rubber becomes
electrified, this property being put to a practical use in the case
of certain electrical machines in which ebonite discs are used
instead of glass.

Good sorts of rubber are transparent in thin sheets, particu-
larly when stretched. Thicker sheets have a yellowish-white to
yellowish-brown colour. Rubber may be coloured by admixture
with pigments. The mineral pigments are specially suitable for
colouring purposes, the lakes of the aniline dyes, on a barium
sulphate or alumina base, being almost completely absorbed, even
when added in considerable quantity. The use of aniline lakes is
possible, however, Avhen it is a question of producing an ultra-
marine shade, or of colouring the surfaces of goods. The results
which are obtained by the admixture of pigments with rubber
may, in a sense, be compared with those obtained on mixing
the same pigments with oil, when one is dealing with colours
which in the dry state lack brilliancy. Further reference will
be made later to the changes of colour undergone by pigments
under the conditions of hot vulcanisation, a subject of great

At ordinary temperatures — say about 17° C. — un vulcanised
rubber has considerable elasticity and pliability. When subjected
to low temperatures its elasticity is temporarily destroyed, as is also
its pliability, though to a somewhat less degree. When, however,
the rubber once more attains the ordinary temperature, these two
properties are completely restored. If a strip of unvulcanised rubber
be stretched and while in that condition be moistened with water
and the latter caused to evaporate rapidly, the rubber remains in
its extended condition. When gently heated, unvulcanised rubber
softens, and can then to a certain extent be welded together. In a
similar manner rubber may be formed into compact masses by
powerful compression, especially at surfaces which have been
freshly cut. This property also belongs, though in a less degree,
to lightly vulcanised rubber. When unvulcanised rubber is
gently warmed and kneaded (masticated) it gradually attains a
more and more doughy, highly plastic condition, apparently under-
going a process of depolymerisation, in the course of which the
molecules, originally very complex, become increasingly simpli-
fied. Extensive use is made of this property of plasticity, in a
variety of ways, during the manufacture of rubber goods. Rubber


which has been rendered moderately plastic gradually reverts, on
cooling and storage, to its former elastic condition. When un-
vulcanised rubber is heated slightly above the melting-point of
sulphur, it becomes soft, and will then dissolve considerable quantities
of sulphur, at first without showing any signs of vulcanisation.
If, however, the molten sulphur be allowed to interact with the
softened rubber for a sufficient length of time, or at a somewhat
higher temperature, chemical combination between the sulphur and
the rubber gradually takes place. This process is termed " vulcan-
isation," and consists in the simple " addition " of sulphur to the
unsaturated rubber hydrocarbon, no substitution, with its conse-
quent elimination of hydrogen sulphide, taking place. If hydrogen
sulphide or analogous compounds be produced during vulcanisation,
their formation must be attributed either to the presence of some
resinous or other foreign substance, or to the correct time or tem-
perature of vulcanisation being considerably exceeded, resulting in
decomposition of the vulcanised rubber, accompanied by a breaking-
down of the rubber molecule. The vulcanisation of rubber is dealt
with more fully later on.

When rubber is melted it remains soft and sticky on cooling ;
when ignited, it burns with a smoky flame. In the dry distillation
of rubber a number of hydrocarbons are formed, and upon the
nature of some of these erroneous conclusions were formerly based
as to the chemical constitution of the rubber molecule, which was
regarded as a polymer of " isoprene." ^ Professor Tilden has, more-
over, stated that he obtained synthetic rubber from isoprene by
polymerisation, but attempts which have since been made to
repeat his experiments have not resulted in the production of
rubber. As already mentioned, rubber may be regarded with a
fair degree of certainty — according to Harries' investigations — as
a polymeride of cyclo-dimethyloctadiene.

In certain cases rubber is vulcanised by means of chloride of
sulphur, instead of molten sulphur, either in solution or in the form
of vapour. This so-called " cold " vulcanisation depends upon the
chemical addition of sulphur plus chlorine to the unsaturated
rubber hydrocarbon, also without substitution. An account of this
process also will be given in the chapter on vulcanisation. A
number of other addition products of the rubber hydrocarbon are
known ; for example, those with bromine, oxides of nitrogen, and,
in particular, ozone. A while ago it looked as though the nitrogen
oxide-addition products would prove to be of great service in
1 See Ditmar, Der pyrogene Zerfall des Kautschuks, Dresden, 1904.


rubber analysis, but the expectations formed in this direction have
so far remained unfulfilled. [The methods which involve the use
of the bromide would appear to be more hopeful.]

Eubber which has been vulcanised to the stage of soft rubber,
and which is therefore in reality only partially vulcanised rubber,
remains unsaturated to a greater or less extent, and can therefore
be vulcanised still further without any difficulty, until the stage
of hard rubber (ebonite) is reached. The activity of partially
vulcanised rubber due to its remaining double linkages, results
also in its ready oxidation by the oxygen of the air, and this
IS essentially the cause of the perishing of soft rubber. This
oxidation is favoured by the action of sunlight and high tem-
peratures, as well as by admixtures which increase the porosity
of the rubber. Further, rapid oxidation may be brought about by
the action of a solution of hydrogen peroxide in ether or acetone.
Contrary to a widely-spread notion, a comparatively higl:^ percent-
age of resins in a rubber does not affect the durability of the goods
made from it, provided that it has received correct treatment and
has been carefully vulcanised ; at any rate, the resin content per se
does not facilitate oxidation. There is just as little accuracy in the
oft-repeated assertion that high resin-content in a rubber brings
about porosity in the goods made from it. By using the right
proportion of sulphur, and selecting the temperature of vulcanisa-
tion with care, products free from pores can be made from rubbers
rich in resins.

Rubber is not appreciably affected either by alkalies, or by dilute
acids. Strong acids, which act as dehydrating agents, and particu-
larly sulphuric acid, char it. Strong nitric acid stains rubber a
deep yellow and dissolves it, on heating gently, with decomposition.
When left in contact with water for a long time rubber absorbs
notable quantities of it, becoming white in colour. In strong
ammonia it swells up. Most solvents for fats, such as carbon
disulphide, benzol, the various distillates from petroleum, ether, the
chlorides of carbon, turpentine oils, rosin oils, etc., dissolve rubber.
Solution is always preceded by a considerable swelling-up of the
rubber. Only carbon disulphide, benzol, coal-tar solvent naphtha,
and the petroleum benzines have, however, been used technically
up to now.

According to experiments made by Henriques there is scarcely
an organic liquid, especially amongst those of high boiling-
point, which does not act as a solvent of rubber to some extent
when heated with it for any length of time. This should always


be remembered in connection with the analysis of vulcanised or
un vulcanised rubber.

The Oxidation of Crude Rubber. — Crude rubber has, relatively,
little tendency to oxidation. What is usually called " oxidation " in
the case of crude rubber is a process which has actually nothing to
do with the oxidation of rubber, for what happens is that a portion
of the rubber acquires the consistency of bird-lime without any
corresponding increase in the amount of resin or in the oxygen -
content, as shown by analysis. This so-called oxidation is generally
most noticeable in rubbers rich in nitrogen, which still contain
latex, or of which the proteins are in a state of putrefactive
fermentation. The heat to which it is exposed during transport
has a softening action on rubber. Rubber rapidly absorbs oxygen
when exposed to its action, either in solution, or when merely dis-
tended by a solvent, or when kept stretched.

Crude Rubber Storage. — For this purpose cold, dark cellars are
suitable. The parcels of crude rubber are here loosely piled up,
separated by wooden partitions arranged like the shelves of an
oven, so that sufficient free space is left between the separate balls
and heating is thus prevented. Store-rooms which are in a warm
situation, or damp rooms in which moisture and water can collect
on the floor, should be absolutely avoided. The floor of the store-
room should, if possible, be cemented, and be inclined towards a

The Mechanical Purification of Crude Rubber. — Washing. — As
already stated when dealing with rubber-gathering, most rubbers
contain a quantity of impurities mechanically mixed with them,
such as sand, wood, stones, fragments of plant tissue, salts, etc.,
and in addition to these, as a rule, a quantity of water which is
quite small in good sorts, but in other kinds, especially the
cheaper ones, is much more considerable. Hence arises the need
for most careful purification of the crude material in order to free
it from these foreign substances before proceeding to work it
up further. The greatest care should be taken to obtain as pure
a washed rubber as possible, for a badly- washed material may
lead to very unpleasant consequences in the course of the later
stages of manufacture. For this reason the supervision of the
washing department is one of the most important duties in the
factory. Tlie purification is carried out in the following manner : —
The crude rubber is first put into the vessel shown in fig. 6, and
described in the section on the " washing shop," where it is softened
by hot water and then rinsed. The crude material as it reaches the



manufacturer is not in a suitable condition to be put straight through
the washing rolls, partly because it is too hard, and partly because
the separate pieces are too large. It is, however, not advisable to
soften the rubber unduly by too prolonged a heating, as this would
result in loss of " nerve " in the rubber, and would cause it to become
sticky. Similarly it would be a great mistake to leave the rubber,
once it has been softened, to lie about and get dry, since it is then
very susceptible to oxidation, especially if the latex happened to con-
tain much protein substance when coagulated. Generally speaking.

Fig. 6.

heating for from three to five hours is sufficient to soften the rubber ;
after this the surface is rinsed with water, to remove impurities
adhering to the lumps of rubber.

If the lumps are not too large they are now at once taken
to the preliminary washing rolls (crushing rolls), whereas the
larger pieces are first cut up on a hand-driven cutting machine
with circular knives, worked by means of crank-handles, before
being put through the crushing rolls (figs. 7 and 8). The cutting
machine referred to is of quite a new type, and is to be distinctly
preferred to the circular saw. At this stage the actual mechanical
" washing " of the raw rubber is begun. The material is brought on
to the washing rolls, a detailed description of which will be given
in the next section, At present it need only be said that the



crushing rolls break up the rubber into small pieces; during the
process a continuous stream of water is allowed to flow between
the rolls, with a two-fold object : first, to wash away a large pro-
portion of the dirt contained in the rubber, as well as the soluble
impurities; and, secondly, because the cold water prevents the
rubber from becoming too hot and consequently deteriorating in
quaHty, for in . any case the cleansing process itself does not
improve the "nerve" of the raw material. Still, this method

Fig. 7.

of purification is preferable to any otlier, and especially to the
chemical method, quite apart from the enormously liigh workino-
cost which would be associated with the latter, without any corre*^
sponding advantage to the quality of the rubber. When the
rubber has been through the crushing rolls, the small pieces are
thrown into a tank of water near at Jiand, and the material is
then passed on to the second pair of rolls (see fig. 9), which are
set up more closely than the first. Here it is broken up into
still smaller fragments, water being still allowed to flow freely
over the rubber the wliole time. In this second sta^e of the



process every fragment of the rubber is broken up, and the
water, penetrating all interstices, washes out every pai

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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 3 of 21)