PROPERTIES OF UNVULCANIZED AND VULCANIZED INDIA RUBBER.
There are two broad divisions to which all varieties of india rubber
can be assigned--unvulcanized and vulcanized rubber. Speaking with
a certain amount of license it may be said that more properties
characterize the former than the latter. The vulcanized article is very
slightly affected by ordinary changes of temperature, cannot to any
considerable extent be changed by heat short of absolute destruction or
decomposition, cannot be united or moulded except in simple forms, is
highly elastic, and is insoluble in almost every solvent for ordinary
caoutchouc.
Unvulcanized caoutchouc possesses very interesting and peculiar
properties. The first part of the present chapter is devoted to this
substance. Those who have never seen the crude gum as imported are
familiar with the article almost pure in the form of sheet rubber and
black rubber articles generally. These are of nearly pure caoutchouc,
though recently the tendency is to vulcanize them to a considerable
degree.
A piece of pure gum containing no combined sulphur, iodine, or other
vulcanizing constituent will be found to exhibit a very striking
peculiarity. Two freshly cut surfaces when placed in contact will
adhere. This is not in consequence of any viscous or sticky coating.
When india rubber is cut the surface is perfectly dry and non-adherent
except to itself.
The writer once had this property of adhesion brought strongly to
his attention. In some analytical investigations of coal gas he had
proposed to use finely divided india rubber as an absorbent of sulphur.
This constituent it absorbs from gas, and it seemed that a basis for a
quantitative determination of sulphur might be found in such property.
Accordingly some raw india rubber was procured and with some trouble
was cut up into little pieces which were put into a bottle. A day or
two afterwards the pieces united wherever they were in contact, and an
irregular cavernous lump was the result. This involved no melting or
softening or change of shape. Each little piece was there intact and
distinct but firmly attached to its neighbors.
The analogy of this action is seen in lead. Two fresh surfaces brought
together, preferably with a twisting or wrenching pressure, adhere
quite firmly. The adherence of india rubber and of lead each to
itself is often exhibited by physical lecturers as an illustration of
cohesion. The cohesion of india rubber is however far more perfect than
that of lead, probably because of its comparatively great resistance
to oxidation, and because, owing to its elasticity larger areas can
be brought in contact. Comparatively great though this resistance to
oxidation is, oxygen, especially in the allotropic modification known
as ozone, may act quite powerfully on the gum. Sunlight also can affect
it injuriously.
A more familiar illustration of the uniting of two pieces of the
same material is seen in the welding of iron. The blacksmith heats
two pieces of iron until they are nearly white hot and are pasty in
consistency. On placing them in contact and hammering to force them
together they unite so firmly as to be practically one. It is necessary
that the surfaces of clean metal should be brought together. If the
pressure induced by the hammering is insufficient to bring this about,
a flux is added which dissolves the oxide and causes the metal to come
in contact with metal and to weld. The analogy with india rubber in its
cohesive action is evident. Surfaces long exposed or which are dusty do
not cohere. The relegation of ice is similar in effect.
The cohesion of india rubber is important and should be thoroughly
appreciated. It is not saying too much to assert that the entire
treatment of the raw gum depends upon this interesting property. The
great lumps of gum are torn to pieces and washed free from gravel and
dirt without going to powder, because owing to their elasticity they
yield and as fast as torn apart the pieces tend to reunite. Again
india rubber is mixed with pigments and vulcanizing reagents by a
method practically one of grinding or masticating, but the material
while it changes its shape, and by the admixture of the various
ingredients becomes less strong or easier torn, still remains intact,
as it welds together or coheres as fast as disintegrated.
As regards its chemical constitution the sap of a Para rubber tree has
been analyzed with the following general results: (Faraday).
Caoutchouc 30.70
Albuminous, extractive, and saline matter, etc. 12.93
Water 56.37
------
100.00
Its specific gravity is 1.012.
Caoutchouc itself or raw india rubber is a mixture of several
hydrocarbons of the following composition in general:
Carbon 87.5
Hydrogen 12.5
-----
100.0
Its specific gravity is from .912 to .942.
The hydrocarbons composing it are isomeric or polymeric with
turpentine. This fact brings it well within the range of familiar
vegetable products. As will be seen the products of its distillation
fall among the same polymers and isomers.
When pure it is nearly colorless, the dark color being due to
impurities. In thin sheets it is almost or quite transparent. It burns
readily, and with a very luminous, smoky flame, as might have been
anticipated from its composition. The action of heat and cold on it is
dependent on the degree of the temperature. At ordinary temperature
it is elastic and firm. It can be stretched and will return almost
to its original size when released from tension. Yet the return
to its shape is so liable to be incomplete, especially after long
sustained stretching, that pure unvulcanized india rubber is considered
imperfectly elastic.
Any elasticity it possesses is principally elasticity of shape as
distinguished from elasticity of volume. In other words when pressed or
stretched it may change shape to a great extent but hardly change its
volume at all. A cube of 2½ inches under a weight of 200 tons lost 1-10
of its volume only. This is largely due to the fact that it represents
an approximately solid body, or one destitute of considerable physical
pores. Solids and liquids are very slightly compressible. Whatever
degree of compressibility caoutchouc possesses is due principally to
its minute pores.
If the temperature is reduced to the freezing point of water a piece
of raw india rubber becomes rigid and stiff. On application of heat
it returns to its former pliable condition. The same return to
flexibility may be brought about by stretching it mechanically. This
may be rather a fallacy. Stretching india rubber warms it, so that in
this mechanically imparted rise of temperature we may find at least a
probable cause of the softening.
If the temperature is raised several effects are produced, according
to circumstances. A piece which has been stretched and held stretched,
has its tension increased by a degree of heat considerably less than
that of boiling water. Some offer the theory that it contains air
enclosed in its pores which, expanding, produces this effect. As the
boiling point is reached the material softens and becomes somewhat
plastic, so that it can be moulded into shape to a considerable extent
and stretched to threads of great fineness. Its elasticity also
disappears as the heat is maintained. These effects increase in extent
up to a heat of 248° F. (120° C.). The return to its original state is
not immediate however. Some time is required before the reduction of
temperature will have full effect.
If now a still higher degree of heat is applied, 392° F. (200° C.) the
india rubber softens to a viscous body, or melts. From this state it
cannot be restored. It remains permanently “burned” or melted whatever
is done to it. Some attempt at hardening may be made by the use of
vulcanizing chemicals, but the result will be very imperfect.
A further increase of heat brings about a destructive distillation.
India rubber treated in a retort to a heat exceeding 400° F. (204° C.)
evolves volatile hydrocarbons of oily consistency, and it distills
almost completely, a small residue of gummy matter or of coke if the
final heat has been pushed far enough being left. The distillate is
called caoutchoucin. According to Mr. Greville Williams it consists of
two polymeric hydrocarbons: one, caoutchin C_{10}H_{16}, boiling point
340° F. (171° C.); the other, isoprene C_{5}H_{8} (in formula equal to
one-half of caoutchin), boiling point 99° F. (37° C.). The mixture has
a strong naptha-like odor and has won considerable reputation as being
the best solvent for india rubber. How far it deserves its reputation
is a matter open to discussion.
The solution of india rubber like its fusion is a vexed point. There is
little question that it can be dissolved by proper treatment. Usually
naptha, carbon disulphide or benzole are used as solvents, the choice
being guided by motives of cheapness and efficiency.
It is worthy of remark that the formula given for caoutchoucin is
the same as that of the principal constituent of oil of turpentine,
and that the latter is often recommended as a solvent. Turpentine is
slightly more volatile than caoutchoucin, its boiling point being 322°
F. (161° C.) Other hydrocarbons have been recognized in the distillate
by Bouchardat, Himly and G. Williams, varying in boiling point from 32°
F. (0° C.) to 599° F. (315° C.), and in specific gravity from 0.630 to
0.921.
Although it has been spoken of as approximately solid it does possess
microscopic pores, to which its limited amount of elasticity of volume
is mostly due. Thus it is found to absorb water, in which it is quite
insoluble. As it does this it acts like a dry sponge and increases in
volume a little, owing to dilation of these minute pores. The water
absorbed may be as much as 18.7 to 26.4 per cent. with an increase of
volume of the gum of 15/1000 to 16/1000. When it has once absorbed
water it is very hard to get rid of it. Although the minute surface
orifices communicate with the entire system of capillary vessels and
pores, the surface pores on drying contract and seal up the absorbed
water within the mass. This is a clew to the impracticability of
the gatherer shipping dry rubber, and to the great difficulty the
manufacturer experiences in drying his washed and sheeted stock before
working it up by masticating or mixing and curing.
By proper manipulation caoutchouc may be made inelastic. This can be
done by the freezing process or by keeping it stretched for two or
three weeks. In this way threads can be made to extend and to remain
extended to seven or eight times their original length. They can then
be woven into a fabric. On gentle heating their original elasticity
reappears and they contract. In this way fluted braids can be made
which will have a high capacity for stretching.
The solution of caoutchouc is difficult often to bring about. We have
seen that in water it swells a little without dissolving. In benzole
it does the same, but swells to a greater extent, to 125 times its
original volume or even more. Some authorities (_Watts_) go so far
as to assert that no solvent completely dissolves it. Acting on it
repeatedly with benzole or other solvent and taking care not to break
up the swelled mass, from 49 to 60 per cent. of soluble matter can
be extracted. On evaporation this is deposited as a ductile adherent
film. The swelled up residue which remains undissolved is assumed to be
the constituent giving strength and elasticity, and is only sparingly
soluble. If the gum is masticated or kneaded at the temperature of
boiling water a change occurs not well understood, by which its
solubility is greatly increased. As solvents many liquids have been
named. Oil of turpentine, caoutchoucin, coal-tar, naptha, benzole,
petroleum-naptha, coal-tar-naptha, anhydrous ether, many essential
oils, chloroform, bisulphide of carbon, pure, or mixed with seven
or eight per cent. of alcohol, are among the solvents recommended.
A mixture of fifty parts of benzole and seventy parts of rectified
turpentine has been given as a solvent for twenty-six parts of the gum.
Mastication before or after immersion in the solvent is to be advised.
More will be said on this subject in a succeeding chapter.
Vulcanized india rubber is unaffected by changes of temperature within
ordinary range. It softens a little on heating. Even hard vulcanite
when heated can be bent and will retain the bend on cooling. It is
exceedingly elastic with elasticity of shape but far less compressible
as regards absolute change of volume than the raw gum. It melts at
392° F. (200° C.) It cannot be made to cohere, and no cement has yet
been discovered that will satisfactorily unite two surfaces. It is
unaffected by light, by ordinary acids and rubber solvents. In contact
with the latter solvents it swells sometimes to nine times its original
volume, but on heating returns to its original volume and shape. Of
water it will absorb no more than four per cent. and often much less.
If it is maintained at a high temperature 266° to 302° F. (130° to 150°
C.) for a long time it gradually loses its flexibility, especially
if in contact with metals. Often the escape of sulphuretted hydrogen
may be observed under these conditions. A small admixture of coal tar
operates to prevent this action.
Its composition and specific gravity vary widely as the most varied
mixtures are added by the manufacturer. Its relation of carbon to
hydrogen is unaffected by the mixtures added. While it may contain
twenty per cent. or more of sulphur it is believed that but a very
small quantity is combined with it, although the excess of sulphur
or some equivalent, such as sulphide of antimony is essential to
vulcanization. The combined sulphur is from one to two per cent. Some
or all of the excess of sulphur is mechanically retained, and as the
rubber in ordinary use is worked about, keeps escaping and forms a
whitish dust upon the surface. By treatment with alkali some of the
excess of sulphur can be removed when the rubber acquires the power of
absorbing a little more water, up to six and four-tenths per cent.
Boiling oil of turpentine is given as its solvent.