IMPROVE- MENT IN VULCANIZED SOFT RUBBER AND PROCESS OF MAKING THE SAME
Year: 1885

page_text: Making the same.
I, FRANZ WILHÖFT of the City and State of New York, United States of
America, Chemist, do hereby declare the nature of my invention for "IMPROVE-
MENT IN VULCANIZED SOFT RUBBER AND PROCESS OF MAKING THE SAME," and
in what manner the same is to be performed to be particularly described and
ascertained in and by the following statement :-
This invention has for its object to produce vulcanized soft rubber, in which all
the sulphur is chemically combined with the rubber. It consists essentially in
exposing the rubber with sulphur to a minimum heat of 330° F; also in the
product derived from such process, all as will be hereinafter more fully described.
Before entering upon an explanation of my new process of manufacturing
vulcanized soft rubber, may be well to outline first, the method now generally
in use by manufacturers, and to give a description of the goods produced by it.
This process consists in incorporating sulphur or some of its metallic combinations
with cleaned or purified rubber upon hot rolls, shaping the articles out of this mass,
and subjecting the same for a longer or shorter time to steam heat in.a vulcanizer
or to dry heat in presses. (Method of Charles Goodyear) In practice manufac-
turers at the present have only modified Goodyears plan as far as the quantity of
sulphur or sulphur combinations, the degree of heat, and the length of time. that
the goods are exposed to the heat, are concerned. These modifications or variations
of the old plan comprise the so-called "secrets" of our individual rubber manufac-
turers. The quantity of sulphur used at present ranges between 6 and 12 per cent
and in some cases even more of the weight of pure rubber and the heat varies
between 280° and 300° F. The duration of vulcanization is made to depend
upon the amount of sulphur or its metallic combinations contained in their com-
pounds. The more sulphur, the shorter the duration of the application of heat and
vice versa.
All vulcanized soft rubber goods as at present manufactured have two great
faults in common. First they readily loose their elasticity, and become sooner
or later stiff, brittle and worthless by after-vulcanization caused by the action of
the free sulphur they contain, and secondly they are continually covered by an
efflorescence of sulphur (bloom), which depends also upon free uncombined sulphur,

page_text: This blooming of vulcanized soft rubber is a very annoying feature in as much as
it destroys the appearance of soft rubber goods and makes the coloring of soft
rubber, which is otherwise easily accomplished, useless. In order to get rid of the
blooming of their goods, manufacturers generally resort to boiling their vulcanized
soft rubber goods in a solution of soda or potash to dissolve the efflorescence of
sulphur, although they are aware that while they, by this process, improve the
looks of their goods, they at the same time materially impair their quality.
The efflorescence of sulphur (bloom) which covers all vulcanized soft rubber,
which has not been boiled in soda or potash, is clear proof of the facts, either that
it contains more sulphur than can enter into a lasting combination with the rubber,
or that the combination, if effected at all, does not last any longer than the process
of vulcanization itself. On the other hand there cannot be the least doubt that by
vulcanization, a portion of the sulphur really combines with the rubber, because
the properties of both sulphur and rubber are changed by it. Rubber and sulphur
are both soluble in carbon bisulphide, but after having been mixed and vulcanized
the product cannot be dissolved by it. This proves conclusively that chemical
action has taken place.
In order to produce vulcanized soft rubber, in which the combination of India
Rubber and sulphur is so perfect that the mass, after vulcanization, does not part
with any of its sulphur, it requires a degree of heat much above that generally
used for vulcanizing soft rubber. It is well known that sulphur being an
allotropic body, changes its form, and with it its chemical properties under certain
circumstances.
Sulphur melts at about 235° F., into a lemon colored very liquid fluid. As the
heat is increased, this fluid becomes gradually darker and thicker, until it finally.
gets nearly black and so thick that it does not flow from the vessel containing it
when it is upset, and when poured into cold water becomes tough and elastic
(amorphous sulphur). This occurs under a heat of 330° to 390° F. When the
heat is raised to above 600° F., this thick mass is seen to become fluid again, but
of a black color until it finally boils and its heavy fumes condense on cold objects,
as flowers of sulphur.
Heat above 330° F., brings about the condition of sulphur, in which it will
combine with india rubber without having a tendency of separating from it.
In order to make vulcanized soft rubber which, after vulcanization does not
contain any free sulphur, it is necessary to use no more sulphur than is required
to vulcanize the rubber under the heat just mentioned.
This minimum of sulphur varies very much according to the quality of india
rubber to be used. For the best of para rubber 3 per cent is the minimum, but
for the lower grades the amount of sulphur may have to be increased to 4 or 5
per cent.
I have experimented with all degrees of heat between 330° and 362° F., and
have found that the following plan of manufacturing vulcanized soft rubber gives
the most perfect results.
I incorporate by means of hot rollers into a batch of well cleaned and perfectly
dry para rubber 31 per cent of flowers of sulphur, which should be free of sulphurous
acid, in such a thorough manner, that upon examination of the thinnest film of the
mass, no sign of sulphur is visible. Now the coloring and other matters are worked
in so as to produce a perfectly homogeneous mass, and this is worked up into such
articles as may be wanted. The goods are now placed within the vulcanizer, and
live steam from a boiler built for the purpose is admitted in the usual manner, until
the heat reaches about 350° F., equal to about 118 pounds of steam pressure to the
square inch. After continuing this heat for about 10 to 13 minutes, the vulcanization
is finished and the goods may be removed.
The result of this operation is vulcanized soft rubber which does not contain
any free sulphur, and therefore cannot bloom nor become hard and brittle by after
vulcanization.

page_text: By my improved process I am enabled to vulcanize the rubber, in about one
third the time it has heretofore taken.
Having now particularly described and ascertained the nature of my said
invention, and in what manner the same is to be performed, I declare that what
I claim is
First. As a new article of manufacture, vulcanized soft rubber in which all the
sulphur is chemically combined with the rubber, substantially as specified.
Second. The process of vulcanizing soft rubber, which consists in exposing it
with sulphur to a minimum heat of 330° F., substantially as specified.
Third. The process of vulcanizing soft rubber, which consists in mixing it with
a minimum of 3 per cent of sulphur, and subjecting it to a heat of 330° F., or
over, substantially as specified.