Processing is a general term which includes
all the operations which are carried out on
the rubber and which alter its physical shape
or chemical composition.
The raw polymer, natural or synthetic can be
softened either by mechanical work, termed
action, by heat or by chemicals known as
peptisers. The increase of plasticity or
decrease of viscosity brought about by
mastication and peptisers is permanent; by
heat it may be permanent or temporary,
depending on the nature of the polymer. When
the rubber contains all the ingredients needed
it is known as a compound or preferably a mix.
If some ingredients have been withheld
deliberately, the partially completed compound
becomes a master batch. The master batch is
converted to the compound by the addition of
the withheld ingredients, which are usually
the vulcanising or curing ingredients.
Open Mixing Mill; The back roll is driven from
the gear box or back shaft and the front roll
from the back roll through roll end gears. The
gears will come out of mesh on a singly geared
mill if the nip ie. The distance between the
rolls is set too wide furthermore, there is
always a danger of stripping the teeth if the
nip is opened so that the gears are working on
the tips of the teeth.
The speeds of the two rolls are often
different, the friction ratio depending upon
the mills use. For natural rubber mixing a
ration of 1:1.25 for the front to back roll is
common. High friction ratios are used for
refining compounds and even speed rolls on
feed mills to calendars ie. When mills are
used other than for mixing. For mixing some if
the synthetic rubbers, a near even speed is
best or even an immense friction ratio, ie.
Less than 1.0. Other synthetic rubbers are
very difficult to mix on mills, so internal
mixers are often used for these.
Two alternative types of cooling are used. The
principal, one employees cored rolls ie. Water
is sprayed on to the outside of an auxiliary
drilled central core, with peripherally
drilled rolls the water is circulated through
a labyrinth of passages about 50 mm under the
Mills are usually fitted with various safety
devices, some to protect the operator, the
other to protect the mill.
Nills are fitted with a metal tray under the
rolls to collect droppings from the mill and
with guides or checks which are plates fitted
to the ends of rolls to prevent rubber being
contaminated with grease etc. Other
attachments which may be added, depending upon
the use of the mill are:
1. A mill apron, which replaces the mill tray
and consists a belt running under the nip
rolls up to the top of the back roll the belt
returns to the mill any powders powders which
fall from the nip.
2. A stock blender, which is a device for
blending the rubber mechanically instead of
depending upon the operator.
3. Cutting knives mounted against the roll to
produce strips far feeding other equipment or
for removing compound from the mill.
4. Scraper blades to remove soft stocks.
Because of its versatility, rapid mixing and
large throughout, is tending to replace the
mill as batch mixer. The details of internal
mixer are not iscussed here, as it has no
importance for small scale rubber industries.
The mastication stage in processing is, in
general only applied to Natural Rubber and the
internal mixer is an efficient machine for
this. The energy can be put into the rubber
quickly the shorter the masticating cycle; the
limitations to shortening the mastication
cycle are the time to load the machine and the
handling of the rubber after discharge from
The shape of the rubber being loaded and the
bulk density8 of the rubber affect the loading
of the machine. The modern 331/3 kg bale of
natural rubber is ideal for loading whole into
the type of mixer or alternatively rubber cut
from larger bales to the same shape. The low
bulk density of certain crepes means that to
obtain similar batch weights, larger volumes
must be loaded into the machine than for other
higher density rubbers. These low density
rubbers can take longer to load than to
masticate and hence are at a economic
disadvantage in processing.
The handling and storage of masticated rubber
in sheet form is not very satisfactory unless
the viscosity and dismension stability of the
rubbers can be assured. Mastication costs
money it reduces the viscosity, but at the
same time sults in a reduction in many
physical properties of the finished
vulcanisate. If the viscosity reduction is
obtained by using softeners then a saving in
compound volume cost results. The reduction in
physical properties of the vulcanisate is
probably no greater from using softeners than
from extra or separate mastication of the
With the availability of constant viscosity
and low viscosity natural rubber and of
synthetic rubbers of suitable viscosity for
direct mixing, the need for mastication may
well be eliminated in due course.
In operation, the ingoing side of the nip is
at the top of the rolls and rubber is added to
the nip. A band of rubber comes through the
nip and is formed around the front roll.
Varying degrees of difficulty arise in forming
these bends, depending upon the polymer.
Natural rubber if not formasticated is
difficult to band initially, but after a time,
depending upon mill nip and temperatures, a
band form. This starts with many holes and
after continued working, the band becomes
smooth. Powders process oils etc. are now
added into the nip and any which fall into the
tray under heath the rolls are returned to the
nip until they are absorbed by the rubber.
The compound is blended to homogenise the
additives and give adequate distribution of
the ingredients. Mills generally give good
Mills have a fairly high power in put. A 1.5-m
mill ie. With rolls 105 m long will handle
about 50 kg of rubber at sp.gr. 1.0-1.2 and
will require 45-75 kw A 2.1 m mill handle
100-150 kg of compound and needs a motor of
115-225 kw. The majority of electric power is
converted to heat so, to prevent compounds
vulcanising on the mill with the heat produced
to rolls are water cooled.
INTERNAL MACHINE MIXING
With internal mixers having low rotor speeds
and low ramthrust, a batch of compound can
take from say 5 min. to 10 min. to mix. An
error of ½ min. does not result in a marked
temperature increase in the compound when
dumped. However as mixing cycles are reduced
it becomes increasingly important to control
the time because this same time error on a
short cycle of say 2 min. Is far more
important than on a longer cycle of 10 min.
It is generally accepted that masterbatching
improves the physical properties of those
compounds where a high degree of carbon black
dispersion must be achieved. Where a master
batch can be carried from that used in the
compound eg: the masterbatch recipe may (a) be
the same as the compound recipe but without
curatives; (b) contain only rubber, filler and
some softeners, or (c) be richer in filler
than the final compound.
Masterbatches can be converted to compounds by
the addition of curatives using either a mill,
an internal mixer or a continuous mixer. A 200
LT internal mixer can use 20 rev/min. 30
rev/min or 40 rev/min for this operation,
depending upon the cooling and associated
handling facilities. The curing ingredients
themselves may be masterbatches, there is
considerable divergence of opinion in the
industry as to which system gives the best
distribution of ingredients.
Reference: rubber Technology & Manufacture by
- C.M. Blow
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