VARIOUS VULCANIZING AND CURING METHODS.
The regular methods of vulcanizing and curing can be departed from and
good results obtained. A few excellent methods differing essentially
from the ordinary ones are described which will be of service to
workers on the small scale, as they enable one to dispense with
vulcanizer and air bath entirely.
One type of curing process does away with the air or steam vulcanizer,
and substitutes, as the curing agency, a hot bath of liquid. For
this purpose a fluid is required that will not act injuriously upon
the india rubber, and which will give a curing temperature without
boiling away. One favorite liquid is glycerine. This can be heated
to the necessary degree and is an excellent substitute for the
expensive apparatus often used. For experimental work it is exceedingly
convenient.
In use it is placed in a vessel of proper size and a thermometer is
suspended so that its bulb dips into the liquid near one side and
does not touch the bottom of the vessel. The heat is applied by a gas
burner, alcohol lamp or oil stove. Of course the vessel may be placed
on an ordinary cooking stove or range, and the heat may be graduated
and adjusted by moving it about until it reaches a part of the stove
where the proper heat will be maintained.
The mould with its contents is immersed in the glycerine, care being
taken to see that it so placed as to assume the mean temperature of
the liquid and not to be heated too hot. This might happen if it stood
on the bottom of the vessel, so it is well to have it supported or
suspended a little above it.
It is easy to see that the whole may be so arranged that the screw
handle or pressure nuts of the mould will rise above the liquid. In
this case the press can be screwed down while the article is heating.
Instead of glycerine a strong solution of some salt in water has been
recommended. A solution of calcium chloride, or some other salt can be
substituted. Either are very cheap and will be quite satisfactory.
Another treatment which applies also to the mixing operation is by the
sulphur bath. Sulphur is melted in an iron vessel and brought to a
temperature of 248° F. (120° C.). A piece of unmixed pure caoutchouc
immersed in this bath will gradually absorb sulphur. The case is almost
parallel with the absorption of water or benzole by the gum. The piece
swells and thickens as it is acted on and eventually will contain
enough sulphur for vulcanization. It may absorb as much as fifty per
cent. The point of proper absorption must be settled more or less
empirically or by successive trials.
After enough has been taken up the piece is removed and dipped into
cold water, which cracks the adherent sulphur so that it can be brushed
or rubbed off. This gives a piece of mixed rubber ready for moulding
and curing. It can be heated and moulded and may be cured as desired,
in a liquid bath, hot press or vulcanizer.
It will be observed that this provides for the admixture of sulphur
only; no talc or other solid can be thus introduced. The addition of
these solids tends to make the rubber of a more attractive color and
their use is not to be deprecated in all cases. Hence the sulphur bath
process is not to be considered a perfect one.
In the sulphur bath the mixing and curing processes can be combined.
If the liquid sulphur is heated to the vulcanizing temperature, 284°
F. (140° C.), a thin strip of gum immersed in it will be vulcanized
completely in a few minutes. A heating of several hours at the lower
temperature will effect the same result.
The sulphur bath processes must be regarded as unsatisfactory. It is
not easy to feel that any dependence can be placed upon them as regards
reliability or constancy of product. The sulphur also will mostly
effect the surface. Thin pieces may be satisfactorily treated, but the
same confidence cannot be felt as is experienced when specific amounts
of ingredients have been mixed in with pure caoutchouc in a regular
mixing machine.
The sulphur bath is of value to the experimenter, enabling him to do
his own mixing without expensive apparatus.
Bromine, iodine, chlorine and nitric acid are vulcanizers. A piece of
sheet rubber dipped into liquid bromine is instantly vulcanized. Iodine
and nitric acid have also been used in commercial work.
Alkaline or alkaline earth sulphides can be employed in solution under
pressure for vulcanizing. At a vulcanizing temperature their solutions
will answer for thin sheet very well. Polysulphides of calcium have
thus been employed.
By simply lying embedded in finely divided sulphur at a temperature of
233° F. (112° C.) as much as ten per cent. of sulphur may be absorbed
by thin sheet rubber. This is one of the processes peculiarly suited
for work on the small scale. It may be used instead of the Parkes
process next to be described.
Chloride of sulphur is an orange red mobile liquid of a peculiar and
disagreeable odor. It boils at 276° F. (136° C.). It dissolves both
sulphur and chlorine so that it is not easy to obtain it in a pure
state. If unmixed india rubber is exposed to its action it will quickly
become vulcanized. At ordinary temperatures the mixing action takes
place, though it is much accelerated by a slight application of heat.
It is quite possible that this action may be of use to the reader
in his manipulation of india rubber. Thin sheet may be vulcanized
by being immersed in a solution of this substance in bisulphide of
carbon followed by slight heating. The thin layer of caoutchouc left
by evaporation of the chloroform solution of india rubber may thus be
vulcanized so as to become comparatively strong and elastic. Where the
same solution has been used as a cement or for patching overshoes and
finishing the patch, a vulcanization can thus be given to it.
The process is known as Parkes’ cold curing process.
A solution of one part of chloride of sulphur in forty parts of
bisulphide of carbon is of good strength for rapid work. A thin article
needs but an instant of immersion. It then is placed in a box or tray
upon some talc powder and is heated to about 104° F., (40° C.). One
minute of curing will suffice. It is advisable to wash off the articles
afterwards in water or in weak lye to remove any traces of acid.
Petroleum naptha can be used as the solvent instead of bisulphide of
carbon. The latter substance has an exceedingly disagreeable odor, and
its vapors must be considered rather injurious especially to those who
are not accustomed to them.
When thick articles are to be cured by this process a much more
diluted solution is used. One per cent. or less of the chloride of
sulphur is the proportion used. The object of this is to enable a
longer immersion to be employed so that the interior will be affected
before the outer layers become too much charged with the vulcanizing
material.
In this short description of the Parkes curing process hints for a
useful method may be found. The process is beyond doubt by far the
simplest known for treatment of india rubber. Exactly what reaction
takes place is unknown. Whether the sulphur or the chlorine is the
acting vulcanizer has not as yet been determined.
Its defect is that it produces surface action, analogous to
casehardening. One method of avoiding this is to remove the articles
from the sulphur chloride bath and at once to immerse them in water.
This prevents the rapid volatilization of the solvent and an equalizing
of the absorption ensues.