Note: Descriptions are shown in the official language in which they were submitted.
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,
This invention relates to methods and apparatus for
removing volatile substances from an extrudable polymeric
material.
In a method of manufacturing synthetic polymers
S which include synthetic rubber, the polymers are produced by
a polymerization process in a reactor. Upon leaving the
reactor, the polymeric material is subjected to a crumb
forming process to produce a slurry of the crumb in water.
It is then necessary to--separate the polymer from the aqueous
phase and finally produce dry polymer which may then be
suitably packaged for commercial use.
Removal of the water is effected in a two stage
water removal process. In a first stage, the slurry is
passed through a dewatering machine which operates at a
temperature up to 220F and removes water and other solvents
present until the water and other solvent content is normally
between 4% ànd 12% by weight of the total weight of the
material. In a second stage, the wet polymeric material may
be passed through a mechanical dryer. This is in the form of
an extruder commonly referred to a~ an extruder dryer or
expander dryer. While in the extruder dryer, the polymeric
matesial is pressurized as it i8 fed along the extruder
barrel by an extruder screw, back pressure to resist movement
along the barrel being a~ least psrtly provided by a multi-
orifice die plate at the outlet end of the dryer. Thetemperature of the polymeri.c material while in the extruder
dryer increases to temperatures in excess of 350-360F and
water and other volatile substances are removed to further
reduce the total volatile content to around 1% or less by
weight of the final extruded weight of extruded crumb.
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Some extruder dryers are provided with rotatable
die-face cutters mounted coaxially UpGn the die plates.
W~ile the die plates are of multi-orifice design for extru-
sion of crumb which is cut by the cutter into pieces of
convenient size for conveying and baling, the central region
of the die plate is devoid of orifices because of the area
required for mounting of the cutter. The use of this design
of die plate and cutter arrangement creates problems because
the polymeric material as it is fed along the extruder barrel
i~ forced by the screw against the whole of the facing sur-
face of the die plate and becomes substantially stationary
behind the orifice-devoid central region. Once material
becomes substantially stationary in thi~ position, its
residence ~ime in the dryer is increased as it is by-passed
by other material moving radially outside it in the extruder
barrel, which other material is capable of being extruded
directly through the orificec. Some polymeric materials,
such as bromobutyl, ha~e a low tem~erature degradation point
which is close to their extrusion temperature in an extruder
dryer. With such materials, the combination of the extrusion
temperature and the residence time of substantially sta~ionary
material behind the central region of the die plate is
sufficient to cause degradation of the substantially
stationary material by overheating. Hence the finally
extruded crumb may be contaminated with a small percentage
of degraded material which renders the use of this particular
design of extruder dryer unsatisfactory for heat sensitive
materials.
Similar disadvantages are found in other processes
performed upon polymeric materials in which volatile sub-
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stances are removed by volatile removal apparatus of thesame basic design as the above described extruder dryer.
The present invention provides a method for
removing volatile substances from a polymeric material
wherein the residence time for material is reduced in the
orifice-devoid central region behind the die plate. This is
made possible by causing the material to flow radially across
the end of the screw, between the screw and the mounting of
the cutter.
Accordingly, the invention provides a method of
removing volatile substances from an extrudable polymeric
material comprising:-
(a) entering the material into an inlet of an
extruder barrel;
(b) rotating an e~truder screw with a free down-
stream end within the barrel to feed the material along the
barrel towards an outlet end of the barrel while compressing
it and heating it to cause subsequent volatilization and
release of the ~olatile substance when the material is
subjected to a reduction in pressure and brought into contact
with ambient atmosphere; and
(c) by rotation of the extruder screw, continuously
forcing the material in an axia~ direction through orifices
provided in a die plate at the outlet end of the barrel and
severing the extrudate ~o formed into discrete portions by
the use of a cutter rotating upon a mounting upon the die
plate while continuously applying a force to cause movement
of material radially inwardly across the end of the screw
constantly to replace material situated axiaLly between the
3~ end of the screw and the mounting of the cutter.
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The continuous.ly applied force may be caused by an
abutment provided upon either the end of the screw or upon
the die plate and lying between screw end and die plate.
The invention also includes apparatus for removing
volatile substances from an extrudable polymeric material
comprising:
(a) an extruder barrel defining a barrel chamber
and formed with an inlet for the polymeric material and an
outlet spaced from the inlet;
(b) a multi-orifice die plate extending radially
across the chamber at the outlet for extrusion of polymeric
material flowing from the chamber;
(c) a cutter having a mounting and carried by the
mounting upon the die plate for cutting extruded polymeric
material as it i~ emitted from the extrusion orifices into
discrete portions;
(d) and an extrusion screw with a free downstream
end and rotatabLe within the chamber and having at least one
screw flight fox feeding polymeric material along the barrel
from the inlet to the outlet and through the die plate
orifices while subjecting the material to compression to
cause an increase in its temperature. for subsequent volatili-
zation and release of volatile substance from the polymeric
material, the extrusion screw having a downstream end towards
the die plate and an abutment extending radially of the
screw end, the screw flight having a downstream facing load-
applying surface and, when considered in the direction of
screw rota~ion, the abutment ha~ing a leading surface which
- extends axially from the end of the screw and forming an
extension of the load-applying surface from a position
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adjacent the wall of the chamber at least partly across the
end of the screw, said abutment sweeping around the axis of
rotation of the screw during screw rotation to direct
polymeric material to flow radially inwardly across said end
of the screw thereby to move polymeric material across the
end of the screw constantly to replace material situated
axially between the end of the screw and the mounting of the
cutter.
With this construction, polymeric material being
fed downstream by the screw flight is partly conveyed
directly against the leading surface and then by the leading
surface is directed to flow radially across the end of the
screw and behind the cutter mounting.
It is also to be preferred that the leading surface
extends to a position adjacent the circumference of the
chamber in such a manner that, when considered in the
direction of screw rotation and forwardly of the leading
surface, the outer end of the leading surface forms an obtuse
angle with a tangent to the circumferential point of the
chamber to which the outer end lies ad~acent. This then
enables polymeric material to be moved by the leading surface
across the screw end without a sudden change in direction of
motion of the material from i~s axial and circumferential
movement around the screw whereby resistance to such movement
across the screw is minimized. The obtuse angle is con-
veniently provided by a concave curva~ure of the leading
surface as it e~ends across the screw end. Conveniently,
the concavity may be provided by the leading surface following
the shape of an involute which may be based upon a diameter
substantially equal to the chamber diameter with the shape at
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.
the radially inner end of the surface corresponding to the
root end of the involute and with said inner end lying
substantially coincident with the axis of rotation of the
screw. It is found with this involute shape, a pumping
action takes place, as with the involute shaped vanes of a
pump, to assist in directing flow of the polymeric material
across the end of the screw.
Embodiments of the invention will now be described,
~ by way of example, with reference to the accompanying
drawings in which:-
FIGURE 1 is an axial cross-sectional view of part
of an apparatus according to a first
embodiment;
FIGURE 2 is a view in the direction of arrow II
in Figure 1 with a die plate partly cut
away to show an end of an extruder screw;
FIGURE 3 is an isometric view on an enlarged scale
of the end of the extruder screw removed
from the apparatus;
FIGURE 4 is a view similar to that of Figure 1 of
a second embodiment;
FIGURE 5 is an axial view on the end of a screw
used in the second embodiment, considered
in the direction of arrows VI in Figure 5
and on a larger scale;
FIGURE 6 is a cross-sectional view on line VI-VI
of Figure 5 of an end of the screw;
FIGURE 7 is a view slmilar to Figure 1 of a third
embodiment;
FIGURE 8 is an isometric view on an enlarged scale
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of the end of a screw used ln the third
embodiment; and
FIGURE 9 is a view of the end of the screw in the
direction of arrow IX in Figure 8.
In a first embodiment as shown in Figure 1,
apparatus for removing water and other volatile substances
from a polymeric material such as wet bromobutyl comprises
an extruder 1 ha~ing a barrel 2 formed with a cylindrical
barrel chamber 3. The barrel houses an extrusion screw 4
ha~ing a free downstream end and formed with a single screw
flight 5 having a downstream load-applying æurface 6 for ~`
feeding the wet bromobutyl from an inlet (not shown) for the
material to the outlet of the extruder at which position a
die plate 7 extends radially across the chamber. The die
pla~e is formed with a plurality of extrusion orifices 8
(see particularly Figure 2) for extrusion therethrough of
the polymer. It is intended that orifices towards the radial
center of the die plate are of larger cross-sectional area
than other orifices and for this purpose orifices 8a on an
inner pitch circle are of larger diameter as shown in
Figure 2. Mounted upon the die plate is a cutter (not shown)
which is rotatably held upon a mounting spindle 9 which
extends coaxially out from the die plate and from the extruder
and is held securely in position by a locking nut 10 disposed
in a central recess on the upætream face of the die plate.
The barrel, die plate, cutter and mounting for the cutter
together with the part of the extrusion screw 80 far des-
cribed are all of conventional design and will be described
in no greater detail. Suffice it to say that the extrusion
screw i of the type that, during rotation, compresses the
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wet polymeric material to cause an increase in temperature
of the material during its movement along the barrel by a
mechanical working process and then suddenly releases the
pressure conditions to result in volatilization of the
volatile substances which then escape from the material upon
exposure to ambient atmospheric conditions.
This embodiment differs from prior construc~ions
and according to the invention by the extrusion screw having
a downstream end towards the die plate and an abutment
extending radially of the screw end, the screw flight having
a downstream facing load-applying surface and, when considered
in the direction of screw rotation, the abutment having a
leading surface which extends axially from the end of the
screw and forming an extension of the load-applying surface
from a position adjacent the wall of the chamber at least
partly across the end of the screw, this abu~ment sweeping
around the axis of rotation of the screw during screw
rotation to direct polymeric material to flow radially
inwardly across said end of the screw thereby to move
polymeric material across the end of the screw constantly to
replace material situated axially between the end of the
screw end and the cutter mounting. This abutment i8 a pro-
jection 11 on the screw end and, in end view (F~gure 2), is
generally in the form of a sector of a circle with two
~5 generally radially extending edges 12 and 13 meeting at the
axis of rotation of ~he screw to make an included zngle
slightly less than 90. As may be seen from Figures 2 and
3, the edge 12 is not intended to extend along a radial line
of the screw. This edge 12 is that formed by a surface 14
(Figure 3) of the abutment and is a leading surface of the
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abutment when considered in the direction of rotation of
the screw and abutment around the rotational axis. The
rotational direction is anticlockwise as viewed in Figure 2.
Figure 2 shows that the leading surface is of concave
curvature as it extends radially outwards from the rotational
axis to the radiaLly outer edge 15 of the abutment which is
in the shape of an arc having a radius with a center
coincident with the rotatîonal axis and equal in length to
the outside radius of the screw fligh~. The concave curva-
ture of the surface 14 may be of any desired shape to urgethe wet polymerlc material to move radially across the
screw end face during screw rotation. However, in this
embodiment the leading surface has a shape which follows the
shape of an involute based upon a circle of diameter sub-
stantially equal to the chamber diameter with the shape atthe radially inner end of
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the surface corresponding to the root end of the involute,
i.e. the end commencing at the circum~erence of the circle
from which the involute is formed. The reason for this
involute shape will be given below. As may be seen from
Figures 2 and 3, the radially outer end o the leading sur-
face 14, considered forwardly of the surface, forms an
obtuse angle to the tangent to the circumferential point of
the chamber to which the outer end lies adjacent. Also, the
surface 14 is an extension (albeit at a different inclination)
of the downstream end of the load-applying surface 6 of the
screw flight 5.
The part of the end face 16 of the screw which is
devoid of the abutment is a planar face lying normal to the
rotational axis. The abutment has a maximum depth at the
edge 17 where the leading surface 14 meets the arcuate outer
edge 15 of the abutment and at this position has a slight
operating clearance from the upstream face of the die plate.
This depth is caused to decrease progressively across the
abutment by a curved end surfacè 18 of the abutment which
meets the end face 16 along the edge 13. Hence, rake is
provided upon the abutment rearwardly, in the sense of its
direction of rotation, from the edge 17.
The abutment 11 described above was added as a
modification to an ex~ruder of otherwise conventional design
and one which had previously had the problem of degradation
of polymeric material between the mounting spindle and the
end of the extrusion screw 4. This modification was provided
by replacing an existing end cap (not shown) with an end cap
19 (Figure 3) having the end surface 16 and a radially inner
region 20 of the abutment 11. A radially outer region 21 of
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the abutment, having an arcuate interface 22 with the inner
region, was formed by an extension to the screw flight S.
The curved end surface 18 enables the abutment to clear the
nut 10 as shown in Figure 1. The end cap 19 is retained on
the screw in conventional manner.
In use of the apparatus, wet bromobutyl is fed into
the extruder inlet and is mo~ed under compression by the screw
along the chamber 3 while increasing in temperature to at
least 320F by mechanical working. The wet material is moved
along the barrel by the surface 6 of the screw flight and
upon reaching the end of the flight, most of the material is
fed through the orifices 8 in the die plate where pressure
reduction results in volatilization and removal of volatile
substances, and is cut into smaLl pieces of manageable size
for conveying and baling by the cutter.
Some of the polymeric material, however, i.e. that
lying closest to the load-applying surface 6 at its downstream
end, is directed by the radially outer end of the leading
surface 14 of the abutment, to flow radially inwards across
the end of the screw before being e~truded. The obtuse angle
formed by the surface 14 with a tangent at any circumferential
point of the chamber wall to which it lies adjacent assists
in change in direction of flow of the polymeric material
across the end face 16 because it tends to blend that flow
direction with that around the chamber wall caused by the
surface 6. Upon meeting the leading surface 14, the polymeric
material is influenced to move radially inwards during screw
rotation by the concave shape of the surface which at its
outer end has a radially inwardly facing component. It is
also believed that the involute shape of the leading surface
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14 based upon a circle equal in diameter to the chamber
diameter further assists in the inward movement of the
material, as a pumping action is created by the surface in
the radial inward direction in a manner similar to that which
is understood to take place in pumps operating with
involutely-shaped radially positioned vanes.
The leading surface 14 thus directs the material
across the end face 16 of the screw so as constantly to force
out and replace material in regions at and around the axis
of rotation of the screw, i.e. directly behind the area
occupied by the nut 10 at which position the die plate is
devoid of extrusion orifices 8 or 8a. In addition, as the
surface 14 decreases in height towards its inner end because
of the inclined end surface 18, some of the material flows
over the surface 14 and across surface 18.
The orifices 8a, being of larger diameter than
orifices 8 in the die plate, encourage constant movement of
material across the regions at and around the rotational
axis of the screw as they are closest to this axis and their
larger diameters encourage the extrusion of polymeric
material through them.
In use of the apparatus of the above embodiment,
mo~ement of polymeric material across all regions of the end
face 16 and across the surface 18 is continuous and residence
time for the material is reduced significantly. Because of
this, bromobutyl dried by passage through this apparatus was
found to be completely free of degraded material although
normal extrusion pressures and temperatures were created
within the apparatus.
In second and third embodiments now to be described,
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similar advantages are obtained to those obtained in the
first embodiment.
In the second embodiment as shown in Figures 4, 5
and 6, apparatus for removing volatile substances from wet
bromobutyl comprises an extruder having a barrel 23 with
chamber 24, die plate 25 and mounting spindle 26 and locking
nut 27 of construction si~ilar to that described in the first
embodiment.
The construction of the second embodiment has a
screw 28 with a screw flight 29 and an end cap 30 having four
circumferentially spaced-apart axially extending abutments
31. These abutments are equally angularly spaced around the
screw rotational axis and are arcuately shaped in end view
(Figure 5) to present a convex leading surface 32 in the
sense of the direction of screw rotation which is anti-
clockwise in Figure 5. The whole of the end cap with the
abutments lies radially within the base diameter of the screw
flight. Each abutment is inclined from a peak 33 radially
outwardly to blend with a chamfered edge 34 of the end cap.
Each abutment also extends inwards from its peak while
tapering towards its inner end 35 to provide a clearance
from the nut 27 as shown by Figure 4.
In use of the apparatus of the second embcdiment,
as the polymeric material ifi forced along the ch~mber 28, it
passes the end cap 30 and a large proportion of it is
extruded directly through the extrusion orifices in the die
plate. Any wet material which is caused to flow radially
inwardly over the chamfered surface of the end cap is not
directed in that direction as in the first embodiment but is
merely urged in that direction by the screw flight. Upon
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moving radially within the vicinity of the sweep of the
abutments 31, however, the material is subjected to dis-
turbance caused by the rotation of the abutments. The
abutments effect movement of material across the end cap
merely by disturbance of the material so that the material
is constantly replaced particularly in the radially central
regions which lie immediately upstream of the locking nut 27.
In the third embodiment (Figures 7, 8 and 9),
apparatus for removing volatile substances from polymeric
material~ for instance wet bromobutyl, comprises an extruder
36 with a chamber 37 housing a rotatable screw 38. An inlet
(not shown) for polymeric material to be dried is provided
to the chamber and at an outlet end, a die plate 39 is
provided which is formed with extrusion orifices 40. A
mounting spindle 41 extends coaxially from the outside of the
die plate and is held in place by a locking nut 42. The die
plate differs from that in previous embodiments in that it
has a recess 43 in its rear face and which houses the locking
nut in a manner completely submerged below the rear face.
The extrusion screw has a screw flight 45 formed
with a downstream facing load-applying surface 46. An end
cap 47 (Figure 8) is held onto the end of the screw in normal
manner and comprises an end face 48 which is inclined relative
to the rota~ional axis of the screw so as to extend across the
screw and towards ~he die plate as it progresses away from the
downstream end of the screw flight.
The end cap is formed with an abu~ment 49 in the
form of a projection which is approximately semi-circular in
end ~iew as shown in Figure 9. When considered in the
direc~ion of screw rotation, i.e. anticlockwise in Figure 9,
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1 1 3~ ~ 1 2
the abutment has a concave leading surface 50 extending
towards the central radial regions of the end cap from a
position at the outside diameter of the screw flight on one
side of the end cap. The leading surface forms an obtuse
angle forwardly of itself with a tangent to an adjacent point
on the circumference of the chamber. In extending inwards
it extends to a position close to the rotational axis of the
screw while facing across the axis. The outer regions of the
abutment lie at the maximum diameter of the screw flight for
approximately three-quarters of their sectorial coverage and
then decrease to the base diameter. As shown in ~igure 8,
the leading surface extends axially along the end cap to
terminate close to the screw flight at edge 53.
As may be seen from Figures 7 and 8, an end surface
54 of the abutment lies in a plane normal to the rotational
axis and this together with the inclination of end face 48
causes a progressive reduction in depth of the surface 50.
The normal end surface 54 lies close to the upstream surface
of the die plate as shown by Figure 7.
In use of the apparatus, some of the wet pol~meric
material lying close to the load-applying surface 46 of the
screw flight comes close to the leading surface S0 of the
abutment upon reaching the end of the screw. The obtusely
oriented surface at its outer end assists in directin~
material across the end face 48 and across the radially
central regions so as to constantly replace material in these
regions and prevent any undesirable dwell time which could
result in degradation of the br~mobutyl. The inclined end
face 48 assists in urging the polymeric material on the face
towards the die plate during screw rotation.
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