THE MANUFACTURE OF MASTICATED, MIXED SHEET, AND VULCANIZED INDIA RUBBER.
The manufacture of india rubber relates to the production of two
principal products. One is masticated unvulcanized sheet and thread
rubber; the other is unmasticated mixed and cured rubber, otherwise
vulcanized rubber. For the purposes of the rubber-stamp maker an
intermediate product is required, namely, unmasticated mixed sheet
which is uncured. This is really incompletely vulcanized india rubber.
It will be evident from the description to come that it is not
advisable for any one without considerable apparatus to attempt to
clean and to wash (“to sheet”), to masticate, or to mix india rubber.
These operations are best accomplished in the factories. The partially
vulcanized (“mixed sheet”) or the pure masticated article are regular
articles of commerce. Yet a full insight into the manipulation of india
rubber can only be obtained by understanding its treatment from the gum
up to the two separate lines of products we have indicated.
A third type of product is coated tissue, such as Mackintosh. This
really is a sequence of one of the other two processes and a few words
will be said of it in concluding the chapter.
As the caoutchouc is received by the manufacturer it appears an
utterly intractable mass. It occurs in lumps of every size, varying in
color and odor, and very tough but elastic. In virtue however of the
properties already described, its power of cohering when cut, and its
softening when heated, it becomes amenable to treatment.
It is to some extent received in such assorted condition as to secure
even grades, and then each grade may be washed by itself. It is
thrown into water which is in many cases kept at the boiling point by
steam-heat and left there for some hours. It absorbs some water and
also softens. Some gum is so soft that it will not stand hot water. For
such the water is kept cold. The purer gum floats; such pieces as have
stones, dirt, iron, etc. in them, perhaps placed there purposely from
fraudulent motives, sink and can be picked out for separate treatment.
The lumps are next cut up. A revolving circular knife driven by power
is often used, and sometimes an ordinary knife is adopted. At this part
of the operation there is frequently need for sorting, as the grades
received may have inferior pieces mixed with the good. The cutting is
mainly to secure good grading, and to remove concealed impurities. The
gum then goes to the washing rollers, called the washer and sheeter.
(See cut, p. 37.)
[Illustration: WASHER AND SHEETER.]
These are heavy corrugated rolls made very short, 9–18 inches in
length, to prevent springing. They are grooved or corrugated and have a
screw adjustment for regulating their distance apart. They are geared
together so as to work in corresponding directions, like a clothes
wringer or a rolling mill of any kind. The pieces of gum are fed into
the rolls and are drawn between and through them. The friction tends
to heat the gum. To prevent this and also to effect the washing, a
supply of water, either hot or cold, is kept playing upon the mass.
This dissolves out all soluble matter and washes away mechanically the
chips, dirt, etc. which may be present. The whole operation is one of
main force. The caoutchouc is torn and distended and delivered as a
rough perforated sheet. It is passed repeatedly through the machine,
the rollers being gradually brought closer together, or else different
sets of rolls are used, set to different degrees of fineness. The
wash water passes through a screen which catches any small detached
fragments of gum.
Other types of machines have been introduced; the above is a
representative form.
The rough sheets must now be perfectly dried, as water impairs the
final product. This is done in drying rooms by steam heat, generally,
at a temperature of about 90° F. (32° C.) The windows, if there are
any, are painted to exclude sunlight, which operates to deteriorate raw
gum. When absolutely dry the caoutchouc is removed and stacked away for
use.
[Illustration: MASTICATING MACHINE.]
To prepare pure gum for the manufacture of sheet rubber and as a
starting point for many other preparations, the india rubber is
“masticated” in special apparatus. The machine consists of a fixed
cylinder within which is a corrugated roller set eccentrically and
rotated by power. The perfectly dry sheets in the masticator are
pressed and rolled and ground and produce a mass of even consistency.
Here the welding or cohering action again appears in its fullest
development. The perfect dryness of the mass enables it to keep
reuniting as fast as divided. The action is assisted by the heat
generated, which is not inconsiderable. Sometimes the caoutchouc is
warmed before introduction, and sometimes the roller is heated by
passing steam through it.
[Illustration: MASTICATING MACHINE.]
The masticating machine the French picturesquely term the wolf (_loup_)
or devil (_diable_). It is given from sixty to one hundred turns a
minute, and a machine large enough to treat fifty pounds of gum in a
charge, requires five horse-power to drive it. In it the sheeted gum is
ultimately brought to the state of a perfectly homogeneous dark brown
translucent mass.
The masticated rubber is peculiarly amenable to mechanical and chemical
treatment. It can be shaped by heat and pressure, and it is the most
soluble form and is used for making cement and solution, and is moulded
into blocks for the manufacture of sheet and thread rubber. In the
process neutral body pigments, such as oxide of zinc, or soluble
transparent ones, such as alkanine may be introduced; easily decomposed
matter cannot be incorporated on account of the heat.
In all these machines special provision is made to prevent any oil from
getting into the gum. There is no greater enemy to india rubber than
oil or fats of any description. The flanges in the masticator that roll
just inside the bearing are for this purpose.
Sheet rubber is made from the blocks of masticated gum by slicing. A
machine is used for the purpose which carries a knife which works back
and forth in the direction of its length at high speed, making two
thousand cuts a minute. The knife is kept wet by a stream of water,
and about sixty cuts are made per inch. In many articles made from
this sheet the marks of the cuts can be seen as a fine ribbing. The
appearance is familiar to many readers.
The sheet is often cut from rectangular blocks, but cylindrical blocks
are also used. The latter are rotated in front of the knife edge and a
long, continuous sheet can thus be obtained.
The sheet rubber can be cut into threads on webbing and braid. Everyone
has noticed that these threads are usually square. The method of
preparation accounts for it. Vulcanized sheet is now almost universally
used for threads.
Round threads however can be made by forcing softened or partly
dissolved gum through a die.
It is from unvulcanized masticated sheet that toy balloons, tobacco
pouches, etc., are made. It is the starting point for india rubber
bands. For the usual form of the latter article the sheet is cemented
into a long tube which is afterwards cut transversely, giving bands
of any desired width. To make any of these articles satisfactory
vulcanization is imperative. Unvulcanized rubber for many years was
used, but it is now completely displaced by the vulcanized product.
Sheet rubber is made as above; is vulcanized by some of the absorption
processes described in the chapter on vulcanization.
We now come to the second product: regularly mixed and cured rubber.
Its starting point is the washed india rubber from the washer and
sheeter.
We have seen that the pure gum or caoutchouc is very sensitive to
changes of temperature. At the freezing point of water it is hard and
rigid, and at the boiling point is like putty in consistency. There
are several substances which can be made to combine with the gum and
which remove from it this susceptibility to change of temperature. The
process of effecting this combination is called vulcanization, and the
product is called vulcanized india rubber. Sulphur is the agent most
generally employed.
[Illustration: MAKING MIXED RUBBER.]
In the factory the normal vulcanization is carried out in two steps,
mixing and curing. The washed sheet india rubber which has not been
masticated and which must be perfectly dry is the starting point, and
the mixing rolls shown in the cuts are the mechanism for carrying out
the first step. These are a pair of powerful rollers which are geared
so as to work like ordinary rolls, except that one revolves about
three times as fast as the other. They are heated by steam, which is
introduced inside of them. The sheet is first passed through them a few
times to secure its softness, and then the operative begins to sprinkle
sulphur upon it as it enters the rolls. This is continued, the rubber
passing and repassing until perfect incorporation is secured. About ten
per cent. of sulphur is added, and a workman can take care of thirty
pounds at a time.
This material is incompletely vulcanized. It is in its present
condition very amenable to heat and is ready for any moulding process.
Generally it is rolled out or “calendered” into sheets of different
thickness from which articles are made in moulds by curing.
These sheets are of especial interest to the reader as they are the
material from which most small articles are made, including rubber
stamps.
This rolling of the mixed india rubber into sheets of definite
thickness is done by special calendering rolls. The product is termed
“mixed sheet.”
In the mixing rolls the incorporation of other material is often
brought about. Zinc white, lead sulphide, antimony sulphide, chalk,
clay, talc, barium sulphate, plaster of paris, zinc sulphide, lead
sulphate, white lead, oxides of lead, magnesia, silica, form a list
of ordinary mixing ingredients. These lower the cost of the finished
material and are often serious adulterants. For some cases the addition
if not carried too far is not injurious, or even may be beneficial. A
proper admixture renders the gum more easily moulded and treated in the
shaping processes.
[Illustration: MIXING ROLLS.]
The next step in the vulcanizing process is the heating of the mass,
which step is called “curing.” Up to a temperature in the neighborhood
of that of boiling water the mixed rubber can be heated without change
except as it is softened. But if the heat is increased it begins to
get a little more elastic and less doughy, and eventually becomes
“cured” or vulcanized. The temperature for vulcanization is about 284°
F. (140° C.). The word “about” is used advisedly, for it is not only a
question of heat but of time of exposure. After vulcanizing, including
the curing, india rubber cannot be moulded to any great extent. In the
manufacturing process, therefore, it is before curing placed in the
moulds, heated, shaped by pressure, and by exposure to a higher heat
in a steam oven called a vulcanizer, is at once cured.
To prevent adherence to the moulds they are dusted over with ground
soapstone, and the rubber itself is often thus coated.
The methods of vulcanization and curing, which may be of special use to
the reader, are given in the chapters devoted to that subject (chapter
XI.), and in the one devoted to rubber stamps.
Hard rubber, termed ebonite when black, and vulcanite when of other
colors, is simply vulcanized rubber containing a large percentage of
sulphur added in the mixing process.
The manufacture of coated tissues is effected in several ways. The
following is a typical process. A mixture of one part washed and
sheeted india rubber with one part zinc white, one fourth part sulphur,
and about one third part naptha is mixed into a dough-like mass and is
spread upon the cloth by machinery. The latter is simple. It consists
of a bare board arranged to move under a scraping bar. The cloth is
placed on the board and carried under the bar. The coating mixture is
fed on one side of the bar upon the surface of the cloth. As it passes
under, a regulated amount, according to the set of the bar, adheres. It
is then dried by steam heat and recoated, until ordinarily six coats,
each about one one-hundredth of an inch in thickness, have been given.
Three coats are given in each direction with intermediate drying. The
fabric is then cured by heat in vulcanizers.
Sometimes the sulphur is omitted from the mixture and cold curing, as
described later, is adopted. When the goods are made up the seams are
secured with rubber cement, a thick solution of masticated gum. Such
seams have to be vulcanized.
Sometimes two such fabrics before curing or vulcanization, are placed
face to face and allowed to adhere and are then cured or vulcanized.
Enough has been said in this outline of the manufacturer’s treatment
of india rubber to show that the first treatment requires machinery.
Very little can be done with mortar and pestle, although in making up
solution these simple instrumentalities are available. As a starting
point for making small articles masticated sheet rubber and mixed
sheet rubber are the staple materials. The preceding steps are best
accomplished in the factory.