THE SOLUTION OF INDIA RUBBER.
India rubber presents some difficulties in its solution. If a piece
of pure gum just as received by the factory is placed in hot water
it will swell and whiten after a while, but will not dissolve. If a
similar piece is placed in benzole a similar but greatly exaggerated
action takes place. The piece if left to soak for a day or more swells
enormously, but very little solution is effected.
The swollen india rubber can be removed from the benzole in a single
piece. It will display all the layers and marks of the original piece
which was perhaps of not one hundredth part of its volume. Some parts
will be a perfect transparent jelly.
It has been found that masticated india rubber dissolves with
comparatively little difficulty. If the experimenter will place in a
porcelain mortar, the jelly-like mass obtained as above detailed, and
will rub it up thoroughly, it will be effectually masticated. This
requires a little patience, as the slippery material seems to elude
the pestle. Yet eventually it will all be reduced to a perfectly
homogeneous mass. Its action while being rubbed up is very peculiar.
At first no progress seems to be made. After a little the lumps yield
to the friction. The rubber then begins to attach itself to the pestle
and mortar, and begins to be drawn out into ever changing webs and
threads. As the operation approaches completion the material makes a
snapping, crackling noise familiar to all rubber workers. When complete
there will be no lump left, and the whole will be a uniform pulp.
If benzole or a volatile solvent has been used, the rubber will
easily be removed from the mortar with a spatula or palette knife. If
turpentine was the solvent it will be impossible to remove the last
traces except after long standing or by solution.
If replaced in the original solvent it will now come into nearly or
quite perfect solution. This is the best way of masticating on the
small scale. It is almost impossible to masticate untreated gum in an
ordinary mortar.
The dealers sell a special india rubber for the manufacture of cement
and solutions. This is so treated by mastication that it dissolves with
great readiness. It is also said that heating under pressure is used to
dissolve it in some factories.
Many solvents have been used and none work without some difficulty.
Benzole, coal tar naptha, petroleum naptha, carbon disulphide, ether
and chloroform, oil of turpentine and caoutchoucin are the best known.
The naptha best suited for its solution is termed solvent naptha. It
has a specific gravity of .850 at 60° F. (15½° C.); it boils at from
240° F. (115½° C.) to 250° F. (121° C.) and on evaporation should leave
no more than ten per cent. of residue at 320° F. (160° C.)
Payen recommends a mixture of 95 parts bisulphide of carbon with 5
parts of absolute alcohol.
Commercial chloroform is apt to be too impure to act as a good solvent.
It is apt to contain alcohol mixed with it as a preservative, which
impairs its effectiveness.
Some of these solutions are better suited than others for the
deposition of thin layers by evaporation. Turpentine gives a very
sticky and unmanageable solution, which dries very slowly. Payen’s
solution and the chloroform and the benzole solutions may be cited as
especially adapted for this purpose. Careful vulcanization by the cold
curing method can be applied to articles made by such deposition from
evaporation.
In the case of all of them some form of mastication for the india
rubber is needed. The simple mortar grinding of the gum swelled by the
solvent is the only practical treatment without special apparatus.
When it is remembered that fixed oils are destroyers of vulcanized or
unvulcanized india rubber it will be obvious how important it is to use
pure solvents. Too great care cannot be taken to preserve the liquids
pure and free from such matter.
A solid hydrocarbon may be used. Thus paraffin wax, such as candles
are made of, when melted acts as a solvent. The resulting liquid
solidifies when it cools, retaining an almost greasy feel.
Boiling oil of turpentine is recommended by some for the solution of
vulcanized india rubber. Phenyle sulphide, it is stated, will soften it
so as to render it workable. The latter discovery is credited to Dr.
Stenhouse.
It is stated that a solution or pasty mixture of one part of caoutchouc
in eleven parts of turpentine with one half part of a hot concentrated
solution of sulphur (potassium sulphide) gives on evaporation a film
neither tacky nor soft, a species of vulcanization taking place.
It is of much interest to note that an aqueous solution of india rubber
has been proposed in which the vehicle is a solution of borax in water.
This is well known to be a solvent for shellac and other resins. It
has been recommended often as a vehicle for rubbing up india ink.
The ink made by mixing lampblack with the shellac solution is nearly
waterproof. A shellac varnish is given by the plain solution.
The experiments upon india rubber were published in a recent trade
paper. One method of making the solution is as follows.
A solution of borax two fifths saturated is made by adding to two
volumes of saturated solution three volumes of water. To this is added
a solution of india rubber in benzole or other hydrocarbon of such
strength and in such quantity as to contain from three and one-half
to four and one-half per cent. of india rubber referred to the borax
solution. It is now vigorously shaken and heated to 120°-140° F.
(49°-60° C.) and the agitation, not too violent, is continued until it
cools. Ceara or Madagascar rubber answers best; Para is not so good for
this formula. This may be termed the indirect or emulsion method.
For direct solution from two to three volumes of water may be added to
three volumes of saturated borax solution. The india rubber is added in
extremely thin shavings and the solution is heated. For weak solutions
the boiling point need not be reached. For strong solutions the heating
should be done under pressure so as to bring up the pressure to one to
three atmospheres.
Such solutions may contain as much as eight per cent. of the gum. The
mixture is liable to coagulate or gelatinize just at the wrong time,
but it may be of value as a vehicle or as a waterproofing agent. It
deserves further investigation, which it is to be hoped it will duly
receive.
Great care is necessary in working with naptha, benzole, carbon
disulphide and similar liquids. Their vapor is given off at ordinary
temperatures and may travel some distance to a lamp or fire and become
ignited and carry the flame back to the vessel. Their vapors are also
anæsthetic and should be avoided as regards inhalation.