Adolf Heil.

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

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good


hard


...


1-8


Para with substitute ....


...


tacky




good


4-4


Ceara ,, ,,


,soft"




pretty
good


firm


4-0


Congo . , , ,




sticky


soft


good


3-8


Para with 20 per cent, other in.urcdients


soft


good


hard


brittle


3-0


Ceara with 20 per cent, other in-


almost


firm


cracked


brittle


17


gredients.


good










Congo with 20 per cent, other in-


almost


good


bad


brittle


1-9


gredients.


good










Para with 10 per cent, substitute and


soft


too


almost


stiff


2-9


20 per cent, other ingredients.




flabby


good






Ceara with 10 per cent, substitute and


soft


good


firm


hard


2-0


20 per cent, otlier ingredients.












Congo with 10 per cent, substitute and


soft


good


brittle




2-1


20 per cent, otlier ingredients.












Congo Avith 50 per cent, other in-


soft-


good


firm to


cracked


3-5


gredients.






hard






Columbian with 50 per cent, other










3-9


ingredients.












Para with 50 per cent, other ingredients


good


brittle


hard




4-0


Cameroon with 50 per cent, other in-


good


luinl




...


17


gredients.












Congo with 10 per cent, of Pontianac


brittle










added.













84



RUBBER MANUFACTURE.



If too dilute a vulcanising liquor be employed tacky products
are obtained, which in course of time, often even in a few hours,
decompose.

Results of J, Minder's Experiments on Cold Vulcanisation. — Sample
pieces of rubber were vulcanised in liquor of the strengths indi-
cated in the following table, and were then heated in steam under a
pressure of three atmospheres for the length of time indicated.

The symbols used in the table have the following significance :
— good, O tacky, | soft, X brittle, and — hard.















Hours,












Strength of
Liquor.
























i


1


H


2


2i


3


H


4


44


5


54 -


6


1: 100





o


1


1


1








!


1


1


1


2: 100








o


1


1








1


1


1


1


3: 100











O


!








1


1


X


X


4: 100














o








1


X


X


X


5: 100














o








X


X


X


X


6: 100








— .








O




X


X


X ,


X


X


7: TOO











T-








O


X


X


X


X





8: 100

















o


X


X


X


X


— '





9: 100














o


X


X


X


X











10: 100











o


X


X


X


X














11 : 100





o


X


X


X


X


X

















12 : 100





o


X


X


X
























When we come to consider more closely the vulcanisation of
doubled fabrics, cold vulcanisation has to give way to the hot
process. As alread}^ mentioned, doubled waterproof cloth consists
of upper- and under-cloth. The latter takes the place of the lining.
Both cloths are proofed, the upper one so as to be quite waterproof,
while the under one is only coated to such a thickness as will
enable it to stick well. In carrying out the vulcanisation the
rubber coating of the upper-cloth is vulcanised, and then united
with the unvulcanised under-cloth on the doubling calenders. The
solvent has not undergone such complete evaporation at this stage
as to interfere with the adhesive properties of the rubber coating,
and, on the other hand, the process of vulcanisation is not complete
before the two cloths are doubled together, so that a perfect union
of them is effected between the rollers.

The vulcanising liquor is transferred to the under-cloth to a



THE VULCANISATION OF RUBBER. 85

sufficient -extent for both cloths to become thoroughly vulcanised.
It would not be correct to vulcanise both cloths first and then
double them together, since the under-cloth has not a waterproof
coating, and the liquor would penetrate through the thin adhesive
layer of rubber to the cloth, and might spoil the colour and give
rise to spots. One great drawback to cold-cured doubled cloths is
the " rustling " and the lack of suppleness in the feel of the finished
stufF.

It is not practicable to vulcanise coloured cloths in steam, since
they are considerably damaged in the process. The most satis-
factory method of vulcanisation for these materials is by means of
hot air in vulcanisinor stoves. Hot-air vulcanisation on drums in
jacketed heaters is not to be thought of, for it must be remembered
that the cloth is an exceedingly poor conductor of heat, and when
it is wrapped round a drum to a considerable thickness, it is only
the innermost layers, next the sheet-iron drum, and the outermost
layers, which become vulcanised, or these become more highly
vulcanised than the intermediate layers ; for as soon as equilibrium
of pressure is reached in these intermediate layers, all hot-air
circulation is at an end. To get over this difficulty it would be
necessary to interpose a sheet of tinfoil, equal in length to the
cloth between the layers, as is done in America, so as to conduct
the heat into the body of the stuff, and so bring about uniform
vulcanisation of the whole length. Little benefit would result
from the introduction of hot air under pressure into the stove, for
the cloth is a bad conductor, and once the air-pressure becomes
equalised throughout, things come to a standstill again.

The best method of vulcanisation involves the use of heating:
chambers, of which the internal arrangements are similar to those
illustrated in fig. 40 ; the proofed doubled cloth passes direct from
the spreading machine or calenders (fig. 41), through the heating
chamber, which is kept supplied with hot air, and in which the air
is kept in continual circulation. In this way an absolutely uniform
vulcanisation can be effected ; the chamber is completely enclosed,
and the cloth in passing through it makes a number of convolutions,
which can be increased or decreased at will according to the length
of cure required by the rubber ; the time of cure is the time which
elapses between the entry of the cloth into the chamber and its
exit, the temperature being that of the hot air.
^ Waterproof fabrics, coated on one side only, and on the inner
surface of whicli higher demands are made, are best cured by the
cold process, since it is not so 'easy, and indeed frequently almost



S6



RUBBER MANUFACTURE.



impossible, to ornament the proofed surface Avhen the goods are
cured by the hot-air process ; moreover, the number of ornamenting
ao-ents that can be used is smaller, and it may, indeed, be said that
a given process of ornamentation gives incomparably finer effects
with the cold cure than with the hot. The simplest method of
imparting to the rubber surface a pleasing appearance consists,
unquestionably, in spreading as thin a layer of rubber as possible,
and using perfectly transparent solution without, therefore, any
pigmentary substances. The pattern of the cloth, toned down by
the colour of the rubber, is then visible through the transparent




Fig. 40.



layer of rubber, to which the well-known " velvet " finish has been
given before vulcanisation, by means of starch. Very good results
are obtained by this process when cold vulcanisation is employed,
particularly on cloths with floral or fancy designs. By the use of
dyes soluble in benzole any desired colour can be given to the
rubber, modified, of course, by the colour of the cloth underneath.
The use of this process is, however, conditional on the employment
of cold vulcanisation, for apart from the difliculty of preserving the
transparency of the rubber-layer when it is vulcanised by heat, the
colourings produced would be destroyed, almost without exception,
by the hot process. One process for the ornamentation of proofings
must be regarded as particularly successful; this depends upon



THE VULCANISATION OF RUBBER.



87



the action of chloride of sulphur on the coating of starch covering
the tacky lay^r of rubber. The mode of operation is based upon
the observation that chloride of sulphur converts the powdery
coating of starch into a semi-transparent layer with a silky gloss,
and upon the further fact that whilst a double treatment of the
rubber coating to be vulcanised with a 3 per cent, or a 2 per cent.




Fig. 41.

solution of chloride of sulphur is equivalent to a single vulcanisation
with a 5-5 per cent, solution, the starch is in the former case much
more transparent than in the latter. In fact, the 5 per cent,
solution has not a much greater effect upon the starch than the 8
per cent, solution, and the result is practically the same if the first
solution is a 4 per cent, and the second a 1 J per cent., or conversely ;
this behaviour is utilised for purposes of ornamentation in the
following manner : —

The rubber-coated material is first treated with potato starch,



88 RUBBER MANUFACTURE.

that which does not adhere being carefully removed by means of a
brushing machine. The rubber is then vulcanised by means of a
3 per cent, solution of chloride of sulphur, passing on to a second
vulcanising machine in which a IJ per cent, solution is employed.
The vulcanising roller of this, however, is not smooth like that of
the first, but is turned down to a depth of about 1 cm. in those
places wliere it is desired that the liquor should not be carried
up by it. These grooves are arranged in regular groupings with
respect to breadth and axial distance, and the vulcanising liquor is
therefore taken up from the roller in the form of bands, in this
method of working. The result is that wherever the rubber has
been in contact with chloride of sulphur twice, the starch becomes
much more transparent than in those places which have been only
once wetted by the liquor. The surface is therefore much less
glossy in the latter places than in the former, which are almost
transparent and colourless, but have a fine silky gloss. The most
diverse effects can be produced in this way, and can be heightened
and modified by the use of rubbers coloured by suitable pigments.
The only difiiculty in connection with this method lies in the
circumstance that up to the present it has not been found possible
to produce other than striped patterns, since the vulcanising liquor
will not be carried up properly by engraved or elaborately turned
rollers. A very good effect is, however, produced by cutting the
bands on the vulcanising roller narrow, and very slightly waved
axially. This roller is then used for the first vulcanisation. For
the second vulcanisation, on the other hand, a roller is used which
is also provided with narrow bands, but slightly inclined to the
axis of the roller ; in this way a highly satisfactory " moir6 " effect
is produced.

The cold vulcanisation process is particularly well adapted to the
production of coloured effects, since delicate colours can be employed
which would be destroyed by hot vulcanisation. In particular, oil-
soluble aniline and resinate colours may be used. The lakes of
coal-tar dyes are particularly well adapted for this purpose. Great
circumspection is necessary in the selection of these, since many of
them are not fast to water. The lakes of the polysulphuretted azo-
dyestuffs in particular are defective in this respect, as are also
those of the croceins. The lakes of the basic dyes give good results
as regards fastness to water, as sulphonated colour bases ; also the
amido-azo colours. The most suitable base for these lakes is a
molecular mixture of aluminium hydrate and barium sulphate,
obtained by precipitating aluminium sulphate with soda and with



THE VULCANISATION OF RUBBER. 89

barium chloride. The basic colours are fixed upon this base with
tannin and tartar emetic ; the acid dyestufFs are mixed with soda
solution and precipitated at the" boil with the requisite amount of
barium chloride, a solution of aluminium sulphate being then added
until precipitation of the dyestuff is complete. The lakes must in
every instance be washed quite free from soluble salts. In prepar-
ing rubber mixings with these lakes special care must be taken
that all the materials used are absolutely dry, so that, as far as
possible, the formation of hydrochloric acid within the rubber on
cold vulcanisation may be prevented. The chances of damage to
the shade of colour as the result of such formation of hydrochloric
acid are certainly not very great, although this depends very much,
of course, upon the nature of the particular coal-tar dye which was
used in the preparation of the lake. But even the smallest pro-
duction of hydrochloric acid is sufficient to entirely destroy the
fastness of the colour towards water.

If it be desired to print in colour on the rubber surface, one
may adopt the method given in Frankenburg and Weber's English
patent, according to which the still tacky surface is dusted over
with finely-powdered oxycellulose, the printing being done on this
coating. Pigment colours (lakes) thickened with rubber solution
are used, and most beautiful effects can thus be produced.

In selecting colour-lakes for rubber printing, the same points
must be borne in mind as in selecting them for colouring the rubber
mass, and reference may therefore be made to what has been said
on that subject. But, in addition, it is of great importance that the
covering-power of the colours used for rubber printing should be
as great as possible. This condition is naturally best fulfilled by
the mineral pigments, whereas all lake pigments have a tendency
to transparency ; this depends, however, upon their method of
preparation, and especially upon the base used. If the printing is
to be done on a white rubber surface instead of a coloured one, the
case is, however, quite different, for then it is this transparency of
the pigment which is its desirable quality, since it imparts to the
print a brilliancy which cannot be attained by means of opaque
colours.

In the waterproof industry some attention has also been given
to the bronze-colour printing which has developed in recent years ;
indeed, more than in the calico-printing industry itself. The em-
ployment of this kind of printing is also conditional upon the use
of cold vulcanisation in the case of all colours other than silver-
white. Even the latter can only be cure^i by heat if it has been



QO RUBBER MANUFACTURE.

printed with aluminium, and then the silvery lustre suffers con-
siderably, doubtless on account of the presence of foreign metals
even in the purest commercial aluminium.

In printing with bronzes, however, one important fact must be
borne in mind ; the ordinary coloured bronzes consist actually of
alloys which always contain tin and frequently contain copper.
The destructive action of copper on cold-cured rubber is well known,
and the danger lying "therein is augmented by the unavoidable
action of the chloride of sulphur upon the copper during the
process of vulcanisation. Non-recognition of this fact will result
in considerable loss, for in the course of time the rubber in all the
places covered by the copper-containing bronze becomes completely
destroyed by oxidation, the waterproof material being perforated
in these places right through to the fabric. Even tin alloys, which
are frequently used, are not absolutely free from suspicion, for it is
a well-known fact that tin bronzes, free from copper, give rise to
an action quite similar to that occurring with copper. In every
case examined the rubber coating proved to be comparatively
highly vulcanised : in other words, too strong a solution of chloride
of sulphur was employed ; this appears to lead to the forination of
haloids of tin, which have an oxidising action on the rubber. These
difficulties can all be avoided by restricting oneself to the use of
pure or tinted aluminium bronzes, which, in addition .to their safety,
are possessed of a much greater brilliancy than tin bronzes.

In conclusion, we may refer to the influences to which the starch
is exposed if the chloride of sulphur is diluted. The starch adher-
ing to the surface of the rubber is exposed in a high degree to
the action of the vulcanising agent, and it is here that the property
of retaining water, possessed by starch, comes in, a property which
is a very undesirable one from the present point of view. Air-
dried potato starch contains, on an average, as much as 20 per cent,
of water, and it is evident that the presence of this must, result
in the decomposition of a considerable proportion of the sulphur
chloride taken up by the rubber, a circumstance which is, in fact,
responsible for the evil smell of the waterproof. This can be almost
entirely avoided by using benzole as a solvent, and by employing
only well-dried starch, still warm. The material must then be
vulcanised immediately the starch is put on, and it is best to carry
out the two operations consecutively on a combined machine. If,
on the other hand, carbon bisulphide be used as solvent, the advan-
tages gained by the use of well-dried starch, and of the combined
powdering and vulcanising machine, are nullifled by the deposition



THE VULCANISATION OF RUBBER.



91



of moisture on the surface, caused by the rapid evaporation of
the solvent and the consequent great decomposition of chloride
of sulphur which occurs on the moisture-laden starch. The hydro-
chloric acid set free in this decomposition often dextrinises the
starch to such an extent that the material acquires an unpleasant
black surface in place of the well-known velvety gloss.

The simplest method of getting rid of the unpleasant smell of
cold- vulcanised materials is to let them run for a few hours through
an airing apparatus in which air, heated to a temperature of about
70° C, is driven past them, removing the smell.

After-treatment of cold-cured waterproof with ammonia is of
doubtful value, since certain impurities in the ammonia tend to
make the smell worse. If it be possible, the cold-cured material




Fig. 42.

may be powdered with carbonate of magnesia, which absorbs the
residual acid, and takes away the unpleasant odour.

The Cold- Vulcanising Shop. — This shop is situated in a separ-
ate building, primarily on account of the great risk of fire. The
construction should be such that the building is provided with a
double floor ; the upper one, which is best formed of perforated iron-
plates, stands about 1 J metres above the lower one, which is bricked
or concreted, and on the ground-level. The space between the
floors can be completely opened by means of valves, so that a
current of air can pass through and carry away the vapours which
descend through the upper perforated floor. The side-walls should
have large windows all round, provided with glass jalousies, to
ensure a vigorous circulation of air.

The most suitable machine for the cold- vulcanisation of water-
proof cloth, reproduced in fig. 42, is very simple in its action. In
the lead-covered frame a rests the solid wooden trough 6, containing
the beechwood roller, which revolves easily in two conical bearings.



92



RUBBER MANUFACTURE.



Fig. 43 shows the wooden roller and trough in section. This
wooden roller is one of the details in the process which is most
worthy of attention ; it must be turned so as to be absolutely true
and perfectly smooth, and must under no circumstances become
warped while in use; to this end special care
must be exercised in the selection of a suitable
piece of wood from which to make it. If the
wooden roller gets warped, uneven vulcanisa-
tion will be the result, and spots and streaks
will often be found on the finished article.
The guide rollers over which the material is
led are seen {c c) in fig. 44. The rollers c' are
fitted into fixed bearings, while roller c runs in
two movable levers, which make it possible to set the roller to
any desired position, as shown in fig. 44. The object of this
piece of apparatus is as follows : — A length of cloth is solutioned,
on the spreading -machine, to each end of the material to be




Fig. 43.




Fig. 44.

proofed. After proofing, the material passes, together with these
end-cloths, on to the vulcanising machine, where it is rolled up
as at d, figs. 42 and 44. The first end-cloth is passed under
roller c, over the vulcanising roller, then under c in slotted lever i,
round c' over the heating-chest e and table / to roller g, on which
it is rolled up until the join between the proofed and unproofed



THE VULCANISATION OP RUBBER. 93

cloths comes over the vulcanising roller 1. Lever i is then de-
pressed by means of the handle a (fig. 44) ; this brings the proofed
cloth on to the roller, and the vulcanisation can be started. When
the other end of the proofed cloth, v^^here it joins the other end-
cloth, has passed the roller, lever i "fs raised. The final auxiliary
cloth thus passes right above roller 1 and is not moistened by the
vulcanising liquor, which would soon lead to its destruction. The
roller c slides along lever i when the handle is raised or lowered,
the lever being slotted. The two levers are connected by an iron
bar, on which the handle is fitted, e is a heating-chest designed to
hasten the evaporation of the solvent of the chloride of sulphur.
/ is an ordinary wooden table about 6 metres long, provided at
each end with a guide roller, to prevent, as far as possible, rubbing
of the material.

Just behind the front guide roller is an arrangement for dis-
charging the electricity. This consists of two strips
of wood (fig. 45), between which is clamped a copper-
strip on to which a number of needles, with their
points upwards, are soldered. The copper strip is
connected, by means of insulated wire soldered to
it, with a water tank outside. The needles are
set with their points just below the level of the
rounded edges of the wood, so as not to damage the
cloth passing over them. ' At g (fig. 42) the proofed
material is rolled up. The machine is belt-driven. An alternative
construction of the machine is that in which a heating drum, 1 metre
in diameter, is used instead of the heating chest. For the purpose
of powdering the surface of the material, either before or after vul-
canising, with potato-, rice- or maize-flour, a brushing apparatus is
fixed behind the machine a, or else by itself ; this is power-driven
also, and by means of it the material is powdered with the flour
and finely brushed. This method of powdering is employed in the
case of the so-called electric -finished goods and single-proofed
paletot-cloths.

To return to the vulcanising process. Particular care must be
taken to see that roller 1, in figs. 42 and 44, is clean. It should bo
rubbed dry with a cloth, because hydrochloric acid is formed upon
it and must be immediately wiped off". When dry the roller is
rubbed down with fine emery cloth, and then once again with a
dry cloth. Before starting operations the trough b should also be
carefully wiped out, the roller 1 being taken out for the purpose,
and afterwards replaced so that it will revolve easily in its bearings.




94 RUBBER MANUFACTURE.

At each end of the vulcanising roller a brass centre is let in, and
into this the pointed brass screw projects. The screws are adjusted
so that the roller runs easily : porcelain rollers and troughs have
latterly come into use in place of wooden ones. When everything
is in readiness the vulcanising liquor is poured into trough h, and
vulcanisation now proceeds automatically in the manner depicted
above. The material travels faster as it gets wound up on roller g,
since the circumference of the roll is continually increasing ; this
is, however, an unimportant point; the rubber cannot be over-
vulcanised by the quantity of solution taken up by the rapidly-
revolving roller 1, for though on the one hand the roller takes up
more solution the faster it revolves, yet, on the other hand, the
time of contact between rubber and roller becomes continually
shorter and shorter, and the net result is the same as at the
original speed. The proofed material is uniformly vulcanised, and
not, as is often thought, cured harder at the end. It may happen,
however, that on account of the material running too slowly over
the roller, the latter does not revolve fast enough to carry up the
requisite amount of solution ; the rubber is then under-cured, a


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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 8 of 21)