U.S. patent application number 14/487823 was filed with the patent office on 2015-01-15 for extruder and process for extruding a polymer.
The applicant listed for this patent is Ineos Sales (UK) Limited. Invention is credited to Gary Lindsay Pitman.
Application Number | 20150017274 14/487823 |
Document ID | / |
Family ID | 39691041 |
Filed Date | 2015-01-15 |
United States Patent
Application |
20150017274 |
Kind Code |
A1 |
Pitman; Gary Lindsay |
January 15, 2015 |
EXTRUDER AND PROCESS FOR EXTRUDING A POLYMER
Abstract
The present invention relates to an extruder for a polymer, and
a process for extruding a polymer using said extruder. In
particular, the present invention provides an extruder comprising:
(i) two or more barrel sections (1a, 1b, 1c) arranged sequentially
from: (a) an upstream feed barrel section (1a) to which polymer
feedstock to be extruded is fed, the upstream feed barrel section
containing two or more screws each having a diameter of X; to (b) a
downstream barrel section (1c) from which melted polymer is passed
to a die and extruded; and (ii) one or more feed ports for feeding
polymer feedstock to be extruded into the extruder at its upstream
end, characterised in that the total area of the one or more feed
ports through which polymer feedstock is introduced is greater than
4*X, and a process for extrusion of polymer comprising feeding a
polymer feedstock to the feed port of such an extruder.
Inventors: |
Pitman; Gary Lindsay;
(Stirling, GB) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Ineos Sales (UK) Limited |
Hampshire |
|
GB |
|
|
Family ID: |
39691041 |
Appl. No.: |
14/487823 |
Filed: |
September 16, 2014 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
12736147 |
Sep 15, 2010 |
8865041 |
|
|
PCT/EP2009/053204 |
Mar 18, 2009 |
|
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14487823 |
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Current U.S.
Class: |
425/382.3 |
Current CPC
Class: |
B29C 48/285 20190201;
B29C 48/286 20190201; B29C 48/288 20190201; B29C 48/03 20190201;
B29C 48/40 20190201; B29C 48/68 20190201; B29C 48/397 20190201;
B29K 2101/12 20130101 |
Class at
Publication: |
425/382.3 |
International
Class: |
B29C 47/10 20060101
B29C047/10 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 31, 2008 |
EP |
08103227.8 |
Claims
1-8. (canceled)
9. An extruder comprising: (i) two or more barrel sections arranged
sequentially from: (a) an upstream feed barrel section to which
polymer feedstock to be extruded is fed, the upstream feed barrel
section containing two screws each having a diameter of X; to (b) a
downstream barrel section from which melted polymer is passed to a
die and extruded; and (ii) a feed port for feeding polymer
feedstock to be extruded into the extruder at its upstream end,
characterised in that the feed port area is in the shape of a
rectangle with rounded corners, the width of the feed port being
limited by the width of the upstream feed barrel section to up to
2*X, the feed port length being greater than 2.5*X and the total
area of the feed port through which polymer feedstock is introduced
being greater than 5*X.sup.2.
10. An extruder according to claim 9 which comprises three of more
barrel sections.
11. An extruder according to claim 9 which has a single feed port
which has an area greater than 5*X.sup.2.
12. An extruder according to claim 9 in which the total area of the
feed ports through which polymer is introduced is greater than
6*X.sup.2.
13. An extruder according to claim 9 in which the ratio of the
screw pitch to screw diameter within the upstream feed barrel
section is at least 1 and no greater than 2.5.
14. An extruder according to claim 9 in which the diameter of the
screws in the feed port barrel section is 200 mm or greater.
15. An extruder according to claim 9 wherein the polymer feedstock
is a polymer of ethylene or propylene and is in the form of a
powder having a flowability ratio of from at least 1 to less than
6, said flowability ratio of the polymer powder being measured
using the instantaneous shear test described in ASTM D 6773-02 at a
consolidation pressure of 2000 Pa and at a temperature that is
equal to the temperature of the polymer powder at the feed port
during the extrusion process.
16. An extruder according to claim 15 wherein the polymer feedstock
has a flowability ratio of less than 4.
17. An extruder according to claim 15 wherein the polymer feedstock
has a flowability ratio of less than 2.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of application Ser. No.
12/736,147 filed Sep. 15, 2010, pending, which is a 371 of
PCT/EP2009/053204, filed Mar. 18, 2009, which claims priority to
European Patent Application No. 08103227.8 filed Mar. 31, 2008, the
entire contents of each of which are hereby incorporated by
reference.
[0002] The present invention relates to an extruder for a polymer,
and a process for extruding a polymer using said extruder.
BACKGROUND OF THE INVENTION
[0003] The extrusion of polymer is a common processing step in
polymer production. In general, polymer powder may be metered at a
desired rate in to a feed hopper, then passed by gravity from said
feed hopper through what is known as a feed port and into an
extruder barrel in which there are rotating screws. The screws
rotate to move the polymer away from the feed port along the
barrel. Thermal and mechanical energy (from the screw) are provided
to the polymer powder in the barrel causing it to melt. Once melted
the polymer is passed into a suitable die and extruded to form
polymer in the desired extruded form, such as pellets or film.
[0004] In industrial scale extrusion processes typical extruders
are made up of barrel sections having a "standard" barrel length.
Typically the overall length of a standard barrel section is 2 to 3
times the diameter of the screw or rotor. Conventionally, at
industrial scale polymer is fed into the first barrel section in a
sequence, with at least a second barrel section being provided
"downstream". At least a third barrel section is also common to
ensure polymer is completely melted prior to extrusion. There are a
number of parameters which are important in the extrusion process,
including, for example, barrel dimensions, screw speed, screw pitch
and torque. The general direction in the art has been to increase
extrusion throughput, which has involved increasing all linear
dimensions of the barrel, the screw speed and the torque as much as
possible since these have, to date, been the limiting factors.
SUMMARY OF THE INVENTION
[0005] The principal requirement of the feeding of the polymer has
been to sufficiently fill the barrel as the polymer is moved along
it. The polymer is fed to the first barrel section, usually from a
hopper, through an area known as the feed port. It is typical that
the feed port width is maximised based on the barrel section width
i.e. up to the extremity of the screws, for example up to 2 times
the diameter of the screws or rotors in the case of twin screw or
twin rotor extruders.
[0006] Typically, the feed port also has an axial length of up to 2
times the diameter of the screw or rotor in this section of the
extruder, giving a feed port area with length and width both up to
2 times the screw diameter.
[0007] The feed rate of the polymer powder for twin-screw extruders
is generally controlled by the rate at which the powder is metered
into the hopper. The hopper is generally considered to run "empty"
by which is meant without significant volume of powder therein and
without "hold-up" of the powder at the base of the hopper. To date
the feed port area has not generally been limiting on the process,
and therefore has not caused a great deal of concern.
[0008] However, it has now been found that as capacity increases,
and even though the feed port area is generally increased (by
proportional increases in its width and length) as the screw and
barrel sizes are increased, the feed port area increases only in a
squared mathematical relationship whereas the throughput increases
in a cubic relationship, and there becomes an issue that the
polymer does not feed into the barrel through the feed port at a
rate fast enough to fill the barrel sufficiently. This issue is
especially significant for feedstocks with poor flowability.
[0009] Thus, in order to meet the continued requirements for higher
capacity it is necessary to design an extruder which has an
increased polymer feed capability compared to prior art
extruders.
[0010] Thus, in a first aspect the present invention provides an
extruder comprising: [0011] (i) two or more barrel sections
arranged sequentially from: [0012] (a) an upstream feed barrel
section to which polymer feedstock to be extruded is fed, the
upstream feed barrel section containing two or more screws each
having a diameter of X; to [0013] (b) a downstream barrel section
from which melted polymer is passed to a die and extruded; and
[0014] (ii) one or more feed ports for feeding polymer feedstock to
be extruded into the extruder at its upstream end, [0015]
characterised in that the total area of the one or more feed ports
through which polymer feedstock is introduced is greater than
4*X.sup.2.
[0016] The present invention relates to extruders having multiple
(two or more) screws. The screws are provided in parallel (or close
thereto) and may rotate in the same or opposite directions in use
(known respectively as co-rotating or counter-rotating). The screws
may be non-intermeshing, partially intermeshing, or fully
intermeshing.
[0017] The most common type of extruders having multiple screws,
and the preferred extruders of the present invention, are those
having two screws. Examples of such extruders are twin-screw
extruders and twin rotor mixer extruders. It is noted that the
terms "screw" and "rotor" are widely used in the extruder industry,
often apparently interchangeably. As used herein, the term "screw"
is meant to include both such screws and rotors, and where
reference to a "twin-screw extruder" is made, and unless otherwise
defined, it should also be read as also referring to twin rotor
extruders.
[0018] Preferably, the extruder comprises three of more barrel
sections.
[0019] For avoidance of doubt, although it is the diameter of the
two or more screws in the upstream feed barrel section that is
important in the process of the present invention, each barrel
section will contain a corresponding two or more screw sections in
communication with the two or more screws in the upstream feed
barrel section.
[0020] The present invention is characterised in that the total
area of the one or more feed ports through which polymer feedstock
is introduced is greater than 4*X.sup.2, where X is the diameter of
the screws in the upstream feed barrel section.
[0021] In one embodiment, the object of the present invention may
be achieved by adding a second feed port downstream of the first
feed port, or even multiple feed ports downstream of the first feed
port, such that the combined area of the ports is greater than
4*X.sup.2.
[0022] However, it is generally preferred that a single feed port
is used which has an area of greater than 4*X.sup.2.
[0023] Preferably, the total area of the one or more feed ports
through which polymer is introduced is greater than 5*X.sup.2, and
most preferably greater than 6*X.sup.2.
[0024] Although other shapes may in theory be used, for maximum
area per unit length and for simplicity of design and manufacture,
the feed port area will generally be a rectangular shape with width
limited by the width of the upstream feed barrel section i.e. 2*X.
The rectangular shape is preferably a rectangle with rounded
corners, since angled corners are more prone to powder sticking
therein.
[0025] It will be apparent that, to obtain the above areas for the
feed port, the length of the feed port may be greater than the
length of a so-called "standard" barrel.
[0026] Thus, the feed port length will be greater than 2*X,
preferably greater than 2.5*X and more preferably greater than
3*X.
[0027] It is possible to use an extended or "non-standard" barrel
having an increased length for the upstream feed barrel section.
Alternatively, the length/area of the feed port may extend the feed
port onto a second downstream barrel section such that at least
part of it feeds polymer directly into the second barrel section.
For example, the extruder may comprise three of more barrel
sections each having a length, L, and the feed port has a length of
greater than L.
[0028] Another important parameter in the process of the present
invention is the screw pitch. The screw pitch is the distance
between the adjacent points at which the screw helix returns to the
same radial position on the screw. For avoidance of doubt, where
the screw comprises more than one helix, the screw pitch as used
herein is the distance where an individual screw helix returns to
the same radial position, which will be greater than the distance
between adjacent helixes. The pitch is also shown schematically in
FIG. 2. The ratio of screw pitch to screw diameter within the
upstream feed barrel section is suitably at least 1 and no greater
than 2.5.
[0029] The use of a longer screw pitch can increase the extruder
throughput. However, without the use of an enlarged feed port
according to the process of the present invention it has been found
that as scale increases this results in the channels in the screw
being only partially filled, so that any further increase in pitch,
or an increase in speed, will not give an increase in throughput.
In addition in the case of long pitch screws the forward conveying
efficiency of the polymer feedstock will decrease.
[0030] In a second aspect, the present invention provides a process
for extrusion of polymer comprising feeding a polymer feedstock to
the feed port of an extruder as described herein.
[0031] The polymer feedstock is typically provided in the form of
powder or pellets. The polymer feedstock of the present invention
is preferably a polymer of ethylene or propylene, either
homopolymers thereof or copolymers thereof with another
comonomer.
[0032] The present invention applies particularly to relatively
high capacity extrusion processes and relatively large extruder
dimensions. Typically the present invention applies to extruders
with each screw having a screw diameter of 200 mm or greater in the
upstream feed barrel section.
[0033] Whilst the extruder and process of the present invention may
be used with any polymer powder, even those that are generally
considered as "free-flowing" or "easy-flowing", the present
invention applies particularly to polymer powders with relatively
poor flow properties.
[0034] Powder flow properties are typically characterised by
testing in a shear cell. An example of such a test is described in
ASTM D 6773-02 "Standard Shear Test Method for Bulk Solids Using
the Schulze Ring Shear Tester". As described in ASTM D 6773-02, the
flow function of a polymer powder is a plot of the unconfined yield
strength of the powder versus major consolidation stress. In
general, polymers are rated in their flowability in terms of the
flowability ratio, ff.sub.c, which is defined as the ratio of the
consolidation stress to the unconfined yield strength, as
follows:
TABLE-US-00001 ff.sub.c < 1 non-flowing 1 < ff.sub.c < 2
very cohesive (to non-flowing) 2 < ff.sub.c < 4 cohesive 4
< ff.sub.c < 10 easy-flowing 10 < ff.sub.c
free-flowing.
[0035] The present invention is particularly applicable to polymer
powders with a value of ffc less than 4, although it may also be
useful even for polymer powders with values above 4.
[0036] Preferably, therefore, the polymer powder to be extruded in
the process of the present invention has a flowability ratio of
less than 6. More preferably, the polymer powder to be extruded in
the process of the present invention has a flowability ratio of
less than 4, and most preferably has a flowability ratio of less
than 2. Preferably, the polymer powder to be extruded in the
process of the present invention has a flowability ratio of at
least 1. It should be noted that the flowability ratio of a polymer
powder may change as a function of the consolidation pressure and
also of temperature. As used herein, the flowability ratio of the
polymer powder is the value measured using the instantaneous shear
test described in ASTM D 6773-02 at a consolidation pressure of
2000 Pa and at a temperature that is equal to the temperature of
the polymer powder at the feed port during the extrusion process to
which it is to be fed. For example, for a polyethylene powder that
is to be extruded and wherein the temperature of the polyethylene
powder at the feed port of the extruder is 65.degree. C., the
flowability ratio of the polyethylene powder should be measured at
a temperature of 65.degree. C.
BRIEF DESCRIPTION OF THE DRAWINGS
[0037] The invention will now be described in more detail with
reference to the accompanying drawings, in which,
[0038] FIG. 1 shows an extruder of the invention having three
barrel sections; and
[0039] FIG. 2 shows in schematic form a top view of a section of
twin-screws of a non-intermeshing twin screw extruder.
DETAILED DESCRIPTION OF THE INVENTION
[0040] An extruder according to the present invention is shown
schematically in FIG. 1. In particular, FIG. 1 shows an extruder
having three barrel sections, denoted 1a (upstream feed barrel
section), 1b (second barrel section) and 1c (downstream barrel
section), each having a length, L. The barrels contain two
non-intermeshing screws of diameter X (not shown in FIG. 1, but
shown in FIG. 2). (The internal height of each barrel is a fraction
greater than X necessary to contain the screws and allow the
rotation of the screw during operation. For the same reason, the
internal width of each barrel is a fraction greater than 2*X.) In
the Figure the length L is approximately 2.5*X. To feed polymer
into the extruder there is provided a hopper, 2, with a feed port
the area of which is at least 4*X.sup.2. (As shown the feed port
has an axial length, Z, which is approximately 3*X, corresponding
to an area of approximately 6*X.sup.2). Also shown schematically in
FIG. 1 are a screw motor, 3, extruder gear box, 4, a transition
piece, 5, and a die, 6.
[0041] FIG. 2 shows in schematic form a top view of a section of
twin-screws of a non-intermeshing twin screw extruder. In this case
the width is 2*X. (Intermeshing or partially intermeshing would
have a lower width related to the extent of intermeshing.)
* * * * *