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

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ently on the spreading-machine, and bearing in mind what a
multiplicity of such mixings exists, the need for great experience in
the management of a spreading-shop will be admitted without
hesitation. Difficulties are not at an end, however, when the variety
of the mixings has been dealt with; the fabrics used also differ
very Avidely in the raw material of which they are made, in thick-
ness of thread, and in the readiness with which they take up the
rubber dough. In order to realise the great differences that exist,
it is only necessary to compare a web material with a belting
cloth, and, on the other hand, a Para mixing with a mixing such
as is used for ordinary Tuck's packing. Mixings to be spread
must first of all be worked up with a suitable solvent. Petroleum
benzine is the one most frequently used, on account of the
comparatively slight smell attached to it ; solvent benzol is used
less frequently, because the persistent odour Avhich clings to articles
in the preparation of which it has been used is often an objection.
In works where the evaporated solvent is not recovered, the
injurious effect of the benzol vapours, which cannot be altogether
prevented, is against its use as a solvent. In most caises, also, when
everything is taken into account, there is a marked difference of
price in favour of benzine. The first and most important require-
ment for good spreading consists in the addition of the right
quantity of benzine to the rubber. The actual proportions used
are regulated by the kind of raw rubber employed in the mixing,
and by the percentage of rubber present, as well as by the kind of
fabric used. First of all, the proportion of 1 part of benzine to 1 part
of rubber may be taken as a general guiding rule, the proportion of
benzine increasing with the quality of the rubber . and decreasing
as the quality of the mixing becomes lower. A larger proportion
of benzine should also be added when a coarse fabric is to be
spread, so that the dough may be able easily to penetrate into the
meshes. The highest proportion of solvent is, however, necessary
when proofing very thin hat-cloths.

The fabric with its coating of rubber dough, after leaving the
adjustable spreading-knife fixed over the spreading-roller, jDasses
over a heating-table, wdiere the solvent is evaporated. The steam-
heated sheet-iron table is generally about 4 metres long by I'G



MANUFACTURE OF SOFT-RUBBER ARTICLES. 1 65

metres wide. Even if the solvent used boils practically between
the same limits, the rate of evaporation varies with the raw rubber
used, and is greater in the case of mixings containing little
rubber than in the case of those which contain more. Solvent
left in 'the rubber is detrimental to it, and iil it is not completely
removed before vulcanisation is effected, the durability of the
proofing is considerably lessened. The spread fabric should there-
fore pass so slowly over the heating-table that the last traces of
solvent are driven off. The actual speed will vary according to
the kind of proofing being dealt with; hence every machine is
arranged to work at three, sometimes at four, different speeds.
Petroleum benzine is preferable to solvent benzol also in tliis matter
of evaporation from the proofing, since benzol often contains an
appreciable quantity of constituents boiling above 130° C. which
can only be removed by unduly prolonged heating. Petroleum
benzine should, however, also be tested by fractional distillation,
and if it contain an appreciable amount of constituents boiling
above 130° C. it should be rejected as unsuitable. It is by no
means necessary that the benzine should consist chiefly of con-
stituents boiling below 100° C. ; on the contrary, a solvent boiling
between 90° and 130° C. is a much better solvent. But the benzine
should contain nothing boiling above 130° C, because its evaporation
would then be too slow, whereas a solvent boiling between 90° and
130° C. evaporates quite rapidly enough. It is well known that
temperatures considerably below the boiling-point suffice to bring
about the complete evaporation of benzines.

With a view to rapidity of working, the evaporation is often
forced. Such a method of procedure is to be entirely condemned,
for whenever rapid evaporation is resorted to blisters are formed,
and defective places in the article result from the loosening of the
rubber coating from the cloth. These blisters are productive of the
greatest injury in the case of the so-called balloon silk fabrics,
because this material has to withstand a high internal gas-pressure.

This defect also assumes far-reaching importance if the material
is to be used as a waterproof fabric.

It should be seen, therefore, that the proofing is absolutely
homogeneous, the number of coats varying, according to the
nature of the material and the purpose for which it is to be used,
up to as many as six, and each coat should be spread as thin as
possible.

Not only can fabrics be spread in one colour, but in different
colours, forming very beautiful patterns. This particular branch



66



RUBBER MANUFACTURE.



of manufacture was very thoroughly worked out by the late John
Minder, and the originality of his process justifies a careful
description of it. The general idea is as follows. If a piece of
stout paper be evenly folded together several times, and the
corners then cut off with the scissors, together with a few triangular
and curved pieces, a design is obtained, when the paper is unfolded,



«




*



Fig. 74.

somewhat similar to that shown in fig. 74. A piece of cotton
fabric, as generally used for bed-sheeting, is now spread with white
dough until an even coating is obtained. Then when the next coat
is put on, the perforated paper is put between the roller and the
fabric (see fig. 75). The rubber-covered roller a of the spreading-
machine carries the paper c through smoothly, and the perforations




Fig. 75

of the paper are shown in relief in the corresponding positions
on tlie proofed cloth. The thicker the paper and the stiffer the
consistency of the dough, the higher will be the relief of the
pattern. If a coloured dough— e.g., a pink rubber dough — be used
for tlie last coat instead of a white one, the perforated portions of
the paper will be represented by pink patterns on a white ground,
but only if the foUowing conditions are observed : — The spreading-
knife must rest firmly on the partly-j^roofed material so as actually



MANUFACTURE OF SOFT-RUBBER ARTICLES. 1 67

to press into the rubber roller a little. The spreading-machine
should then be run very slowly. By working in this way what
happens is that the coat of white rubber in front of the knife has
time to get softened, and that then the knife scrapes off a very
thin layer of white rubber instead of spreading on a coat of pink,
except in the places ovei' the perforations of the sheet of paper,
where the spreading-knife exerts no pressure; in these places a
coat of pink rubber is left on, and the design so produced appears
in slight relief. The pieces of paper which were originally cut out
of the sheet are now taken and stuck on to the same sheet of paper
in suitable positions, and the sheet so prepared is used in the same
way as before. The design now obtained is the same pink on white
arrangement as before, only in those places where the extra thick-
ness of paper has come a counter-sunk appearance will be observed ;
this is, of course, produced by the additional pressure of the knife
brought about by the extra thickness of paper. The sheet of
paper can now be employed in another way. Another piece of
material is put on the spreader, and spread with yellow dough,
about three coats being put on. On the yellow a white layer is
spread until a smooth coating, completely hiding the yellow, is
obtained. Pink dough is now put before the spreading-knife, when
on passing the material through, with the paper between it and the
roller, a design in three colours is obtained. The places correspond-
ing to the perforations appear in pink ; from the places over the
double thicknesses of paper the increased pressure of the knife
causes the white coating to be completely removed, showing a
pattern in yellow, and the groundwork of the whole remains
white. It will now be clear that in this manner, by putting on
several layers of rubber of different colours, and by building up or
lessening the thickness of the paper in varying degrees, patterns
can be produced on waterproof cloth in different colours and shades
of colour, which have a very fine appearance and a characteristic
effect. When it comes to proofing a piece of material with a
continuous pattern, the simplest way would be to imprint the
design on the rubber-covered roller. Such a roller would, however,
be very difficult, if not impossible, to make, for it would have to be
done with extreme accuracy, as an error of ^'^^ mm. in printing
the material, if one may so describe the process, would result in
faulty places. This can be got over, however, in the following
way : — A piece of cloth of a certain length is coated to a thickness
of 2 nun. with a rubber dough, not too soft. The design is then
impressed continuously in the rubber coating, and the material



i68



RUBBER MANUFACTURE.



is vulcanised by heat. Fig. 76 shows the arrangement of the
spreading-machine for producing the coloured design on the
proofed material, h is the wooden roller carrying the material




Fig. 76.

which has been already spread with tlie different-coloured layers of
rubber, c is a similar roller carrying the material on which the
^ design in relief has been im-

pressed. In " printing," the two
cloths run, one over the other,
between the roller a and the
spreading - knife e, the cloth
which is to be printed in colour
being above that which carries
the design in relief. After pass
ing over the heating-table each
cloth is rolled up on a separate
roller under the machine, in
the usual way. The "printed"
cloth exhibits fine eflfective de-
signs in a kind of relief. The
raised parts of the under-cloth
are subject to greater pressure
than the hollows, and the in-
termediate steps are gradual.
The copy reproduces all the
and stands out in the plastic




Fig. 77.



gradation of the origina4 design,
state from the ground-work.

There is yet a third method of covering the proofing with a
pattern. Figs. 77 and 77a illustrate the arrangement, a is the
rubber roller of the spreading-maching, h the roll of material to be
printed, d a wooden roller with its iron axle resting in bearings,



MANUFACTURE OF SOFT-RUBBER ARTICLES.



169



which can be raised or lowered by means of screws. Round the
wooden roller d and roller a runs a cloth c, on which is the im-
pressed design, in the form of an endless band. The ends of the
cloth must be joined together very neatly, otherwise, on printing, a
stripe will be produced in that particular place every time it passes
under the spreading-knife. Further, the joint should be made in
such a way that it can be easily undone, and the cloth removed
readily from the machine after use. In this method the piece of
proofed cloth is also printed continuously, but only a comparatively
short piece of printing-cloth is necessary. The rubber roller carries
the printing-roll — i.e. material c — continuously through under the




Fig. 77a.

knife. This cloth is alwaj^s kept taut by means of the adjusting
screws on roller d.

In order to get good results on the spreading-machine with
patterned cloths it is most important that the . roller should be
buiFed absolutely true. In the case of patterns consisting of simple
figures, such as stars, dots, flowers, etc., a comparatively cheap
printing-cloth can be made by punching the individual figures out
of a stiff" proofed material. When the design is a continuous one,
however, it will have to be impressed in the way already described.

One does not yet very often meet with spreading-machines
which are fitted with apparatus for recovery of the solvent, though
such machines are made in Germany. Their construction was first
described by Minder.^ In this arrangement heating-coils take the
place of the usual steam-chest, and these are contained in a long
shallow chamber with a false bottom which is kept cool. The cloth
passes through betw^een the steam pipes.

1 Gummi-Zeit, 1898, viii. No. 7.



I JO



RUBBER MANUFACTURE.



Only such colours should be used for the production of these
coloured patterns as are not affected by the vulcanising liquor.
The proofed cloths are treated in exactly the same wsij as others,
being powdered with French chalk, flour, or potato starch on
the special chalking-machine (fig. 78) in order to take away the
tackiness from the surface, and then vulcanised by means of
the cold cure.

Spreading-machines are now being partly displaced by calenders.
The special advantage of the latter over the former is that when
they are used no solvent is needed, and the loss occasioned by the




Fig.. 78.

evaporation of such solvent is therefore entirely avx)ided. But in
addition, and especially in the case of proofed material of medium
thickness, the work turned out is far more uniform than is the case
with spread material, and the work can be done very much quicker
with the calenders, since no time is taken up by the drying process.
The spreading-calenders are, however, not to be preferred to the
other type of machine unless the rolls are most fully suited to one
another, as regards both their circumference and their hardness, and
unless the motion of the rolls is perfectly even and capable of being
accurately regulated. Only solidly-built machines should be used
for this purpose, and they should be driven by an electric motor, so
that they can be run continuously at a uniforir. speed. The greatest



MANUFACTURE OF SOFT-RUBBER ARTICLES. I /I

difficulties are encountered in spreading very thin coats of rubber, for
the fabric is apt to get caught up and torn, the rolls being of necessity
very close together. But in any case, with well-trained hands, the
spreading-calenders can be used for most kinds of materials, and in
America this is almost exclusively the practice ; moreover, much
lower qualities can be used for spreading on the calenders, qualities
which could not be used at all in the form of solution at the
same price.

After being spread or calendered the cloth has still to be
vulcanised. This part of the manufacture has already been fully
dealt with.

13. The Manufacture of Imitation Leather Cloth. — In the following
pages this manufacture, which quite comes within the province of
rubber manufacture, will be described.

Formerly the fabric to be used as a basis — a good bleached
diagonal cotton is the most suitable — was proofed on the spreading-
machine, which has already been described in other sections, but
these cloths can no longer be produced by this method at a sufficiently
low price ; besides, the quality of the cloth prepared in this way is
not nearly as good, the cloth feeling considerably softer after
vulcanisation than cloth spread on the calenders, and so largely
losing its resemblance to leather cloth because of its lack of hard-
ness. The "friction process," which is to be briefly described, is
therefore decidedly to be preferred.

The cloth to be proofed must be dried, stretched, and glazed, in
order to free it from knots. It is then spread on the calenders with
an ordinary mixing as a foundation, weighing about 400 to 450
grams per square metre. On this the outer coat is frictioned, to a
weight of about 150 to 200 grams per square metre, and the coated
cloth, after the whole length has gone through the calenders, is passed
through the embossing-machine (fig. 79), provided with engraved
rolls which can be changed as desired. By this machine the pattern
is impressed on the cloth. The embossing calenders are of similar
construction to those used in paper manufacture, in which a bronze
or steel roller, on which the design is engraved, is mounted between
two large paper rollers which take up the pressure, generally pro-
duced by means of a weighted lever. This arrangement possesses the
advantage that a comparatively small engraved roller can be used.

On passing through tlie calenders the engraving on the roller
is sharply imprinted on the proofed cloth. Care must naturally be
taken to see that tlie diflerent qualities of mixing used have the
necessary relative plasticity, and tliat the outer coat of rubber is



172



RUBBER MANUFACTURE.



extremely firm and extensible, so that it is only very slightly
weakened in the hollows. If there is not sufficient substance in
the outer coat, it would easily be broken or torn at these places on
bending or folding. On the other hand, the rubber should not be
too hard, or the outer coat will be cut through by the engraved
roller, while the use of too plastic a mass will lead to blurred
impressions. After leaving the embossing-rolls the cloth passes





:-^ -^g^




^^T^^^i^^^^^^^^lBKK/^^^^


;.


1


IPI^IHJH^^^^^^^^^^^^Hf^^^^^^^^^^Bi^E.






'^^v^^B^^^^^^^^^^^bt^^'I' ' .^^nHH^IHilHIiBVsjiitaa


M

Ijl





Fig. 79.

over the varnishing machine, where it is slightly moistened with
the varnish, and from this direct to the vulcanising room, where
it is hung up and vulcanised. These two processes go hand in
hand, because the varnished cloth must be hung up immediately
on leaving the machine, and cannot be rolled up while the varnish
is still fresh. The best varnish is one mixed with linseed oil, varnish,
and hard resin, this being harder than shoe varnish, and j^ossessing
better drying qualities, which prevent it from afterwards sticking
together. The varnish should, when absolutely dry, give a highlj^
polished appearance to the cloth.



MANUFACTURE OF SOFT-RUBBER ARTICLES.



173



The stove already described (B.g. 40) is used for vulcanisation,
which is effected by means of hot air. After vulcanisation the
cloth is allowed to cool for twelve to fifteen hours, and is then hung
up in the air, so as partly to rid it of the smell of rubber, and to do
away with the tacky feel of the varnish.

14. Manufacture of " Cut-Sheet."— This product, invented by
Charles Macintosh, is a very fine article of commerce, chiefly used
in the manufacture of surgical goods. It has been produced in a
variety of ways from time to time, and it seems desirable to put
before the reader the different methods of manufacture from which
the rational methods at present in use have been evolved. In the
first place, pure Para rubber without any added sulphur or surrogates




Fig. 80.

was worked up into a soft mass by mastication between hot rolls;
this mass was forced into a cast-iron pot about 2 metres long and
35 cm. wide, and the process was repeated until the pot was filled,
when it was submitted to a high pressure. By this means a large
homogeneous block was obtained; this block was kept at the
temperature of a cool cellar for as long a period as four months,
whereby the original good mechanical properties of the rubber were
restored to it. Sheet was afterwards cut from these blocks, by
fixing them on a horizontal moving bed, wliich was then driven
against a sharp knife making about 700 to 800 oscillations per
minute, a copious flow of water being kept up over the knife. In
this way sheet of different thicknesses was cut by altering the
adjustment of the bed. The surface of cut-sheet is marked, as a
result of this mode of cutting, with ripple-like knife-cuts. The
finer these markings are in the finished sheet the more ra^^idly



174



ROBBER MANUFACTURE.



has the block been pushed against the knife, or the faster has
the machine been run. When a complete sheet had been cut off,
the block vv^as raised up on its supporting bed through a distance
equal to the thickness of the next sheet to be cut.

This method has been replaced by that about to be described,
because by^ its use it was only possible to prepare short lengths of
sheet of medium width. The method now in use is as follows : —
The sheets of washed rubber are worked in
masticators into the form of solid masses;
when these masses are quite homogeneous,
and while they are still plastic, they are forced
into vertical steel cylinders which have pre-
viously been heated (fig. 80). These cylinders
are 120 cm. long and 35 cm. inside diameter,
and have loose bottom-plates. The cylinders
are stood under a cross-bar through which
there passes a screw-spindle which is used to
force down the cover of the cylinder on to
the mass of rubber in it. A steel spindle runs
centrally through the cylinder, and forms when
withdrawn the hollow axis for the reception
of the cutting spindle. But this method has
again been improved upon by employing a
cylinder of about 43 cm. in diameter, com-
pletely filling it with the masticated rubber,
and only then forcing the steel spindle through
the whole mass of rubber, the compactness of
which is by this means increased (fig. 81).
The pressure necessary to drive the spindle
through is about 260,000 kilos. The moulds
are then subjected to a gradually increasing
pressure under the presses illustrated in fig. 80
until the whole has become quite cool and a
block free from pores is produced. The cylinders containing the
compressed rubber are no longer cooled down in a chamber by
means of ice or cold air, on account of the large amount of
expensive raw material which is kept lying idle during such a
process, but they are immersed in a freezing-tank containing salt
water cooled down to a temperature of — 5° C.

In fig. 82 is shown diagrammatically a modern freezing plant
for "cut-sheet" blocks (Haubold's system); with the aid of this
plant 320 kilos, of rubber can be completely frozen per diem.




Fig. 81,



MANUFACTUEE OF SOFT-RUBBER ARTICLES.



175



In this diagram we see the compressor a, built in one piece with
the condenser, the evaporation pipes 6 in the refrigerator (salt-
water reservoir) c, in which the freezing mixture (brine) is cooled
down till the temperature falls to — 12°C. in the evaporator. In
general, the scale on which it is advisable to design the freezing
plant is one which allows 30 to 35 square metres for the floor area
and 2 1 metres for the height of the cooling and storage space, and
provides for an artificial cooling capacity of 4000 calories at a salt-
water temperature of — 12°C. A small pump d is coupled to the
compressor in order to supply the cooling water necessary for the
condenser. If a supply of main water is available for this purpose
this pump is not necessary. The blocks are lifted in and out of the




Fig. 82.

tank by means of lifting tackle ; the steel spindle is forced out by
hydraulic pressure, and the cutting spindle having been inserted in
its place, the block is ready for cutting. The machine on which
these blocks are cut differs essentially in design from the earlier
form of machine ; in fig. 83 a cutting machine of the latest con-
struction is shown; in this machine the cone-drive is again used
instead of the friction-drive, because the desired results were not
achieved satisfactorily on all sides. The machine can also be used
for double blocks.

The block is cut sjjirally by the knife of the machine into the
form of a sheet, the speed of rotation of the block and the throw of
the knife being regulated and adjusted according to the thickness
of the sheet. The knife oscillates at the rate of up to 1200 times per
minute ; it is therefore most important to keep it well lubricated,
and to see that the apparatus for keeping the knife and block wet
with soap solution is in perfect working order. The knives are



176



RUBBER MANUFACTURE.



sharpened on a special emery-wheel machine, and the production of
a uniform sheet of rubber depends upon their having a true sharp
cutting- edge. Soap solution is used in order to prevent the sheets




from sticking together when they are laid on one another while
warm. The soap solution has no effect upon the later processes of
manufacture. The various thicknesses of cut-sheet are distinguished
by numbers as indicated in the following table : —



MANUFACTURE OF SOFT-RUBBER ARTICLES. 1 77



S^o.


mm.


No.


mm.


No.


mm.


No.


mm.


1 .


. 4-15


6


. 1-66


11 .


. 0-63


16 .


. 0-33


2 .


. 3-26


7


. 1-40


12 .


. 0-54


17 .


. 0-20


3 .


. 2-58


8


. 1-15


13 .


. 0-44


18 .


. 0-18


4 .


. 2-35


9


. 0-95


14 .


. 0-41


1 2 3 4 5 6 7 8 9 10 11 12 13 15 17 18 19 20 21

Online LibraryAdolf HeilThe manufacture of rubber goods : a practical handbook for the use of manufacturers, chemists, and others → online text (page 15 of 21)