U.S. patent application number 11/988647 was filed with the patent office on 2009-08-27 for counter-rotating twin screw extruder.
Invention is credited to Jari Aarila, Carl-Gustaf Ek, Shigeki Inoue, Junichi Iwai, Yutaka Mizutani, Arne Syre, Takayuki Yamazawa.
Application Number | 20090213681 11/988647 |
Document ID | / |
Family ID | 35998791 |
Filed Date | 2009-08-27 |
United States Patent
Application |
20090213681 |
Kind Code |
A1 |
Ek; Carl-Gustaf ; et
al. |
August 27, 2009 |
Counter-rotating twin screw extruder
Abstract
A screw extruder having a body forming a chamber of two barrels
housing two counter-rotating axis-parallel rotors, a supply port
for the material to be mixed in the chamber at one end of the body,
a discharge port for discharging the mixed material at the other
end of the body, a conveying section with screws for feeding the
material from the supply port downstream to a mixing section which
comprises at least two mixing zones, each mixing zone having at
least one forward-conveying wing and at least one
backward-conveying wing downstream of the forward-conveying wing on
each rotor characterized in that a throttle valve is provided in
the chamber downstream of the mixing section, and downstream of the
throttle valve a second conveying section with screws and a second
mixing section are provided.
Inventors: |
Ek; Carl-Gustaf; (Vastra
Frolunda, SE) ; Mizutani; Yutaka; (Hiroshima City,
JP) ; Yamazawa; Takayuki; (Hiroshima City, JP)
; Iwai; Junichi; (Hiroshima City, JP) ; Inoue;
Shigeki; (Hiroshima, JP) ; Syre; Arne;
(Strathelle, NO) ; Aarila; Jari; (Porvoo,
FI) |
Correspondence
Address: |
Milbank Tweed Hadley & Mccloy
1850 K Street NW Suite 1100
Washington
DC
20006
US
|
Family ID: |
35998791 |
Appl. No.: |
11/988647 |
Filed: |
July 12, 2005 |
PCT Filed: |
July 12, 2005 |
PCT NO: |
PCT/EP2005/007555 |
371 Date: |
May 7, 2009 |
Current U.S.
Class: |
366/76.1 ;
366/84 |
Current CPC
Class: |
B29C 48/395 20190201;
B29C 48/2552 20190201; B29C 48/08 20190201; B29C 48/64 20190201;
B29B 7/465 20130101; B29C 48/268 20190201; B29B 7/90 20130101; B29C
48/05 20190201; B29C 48/09 20190201; B29C 48/404 20190201; B29B
7/488 20130101; B29C 48/575 20190201; B29C 48/40 20190201 |
Class at
Publication: |
366/76.1 ;
366/84 |
International
Class: |
B29B 7/46 20060101
B29B007/46; B29C 47/10 20060101 B29C047/10 |
Claims
1. Screw extruder having a body (1) forming a chamber (2) of two
barrels (3, 4) housing two counter-rotating axis-parallel rotors
(5, 6), a supply port (7) for the material to be mixed in the
chamber (2) at one end of the body (1), a discharge port (8) for
discharging the mixed material at the other end of the body (1), a
first conveying section (34) with screws (12) for feeding the
material from the supply port (7) downstream to a first mixing
section (44) which comprises at least one forward conveying wing
(14, 15; 21, 22) and at least one backward conveying wing (17, 18;
24, 25) downstream of the forward conveying wing (14, 15; 21, 22) a
throttle valve (40) downstream of the mixing zone (44), and
downstream of the throttle valve (40) a second conveying section
(47) with screws (27) and a second mixing section (48) on each
rotor (5, 6), characterized in that the first mixing section (44)
comprises at least two mixing zones (45, 46), each mixing zone (45,
46) having at least one forward conveying wing (14, 15; 21, 22) and
at least one backward conveying wing (17, 18; 24, 25) downstream of
the forward conveying wing (14, 15; 21, 22) on each rotor (5,
6).
2. The twin screw extruder according to claim 1 characterized in
that the downstream ends of the forward-conveying wings (14, 15;
21, 22) and the upstream ends of the backward-conveying wings (17,
18; 24, 15) of each mixing zone (45, 46) of the first mixing
section (44) are offset to form a passage for the material to be
mixed.
3. The twin screw extruder according to claim 1 characterized in
that the downstream ends of the backward-conveying wings (17, 18)
of the first mixing zone (45) on the side of the conveying section
(34) and the upstream ends of the forward-conveying wings (21, 22)
of the mixing zone (46) downstream of the first mixing zone (45)
are offset to form a passage for the material to be mixed.
4. The twin screw extruder according to claim 1 characterized in
that the length (L2) of the backward-conveying wings (17, 18) in
the first mixing zone (45) is shorter than the length (L4) of the
backward-conveying wings (24, 25) in the mixing zone (46)
downstream of the first mixing zone (45).
5. The twin screw extruder according to claim 1 characterized in
that the radial clearance (C) of the wings (11, 14; 17, 18) of the
first mixing zone (45) is greater than the radial clearance (C) of
the wings (21, 22; 24, 25) of the mixing zone (46) downstream of
the first mixing zone (45).
6. The twin screw extruder according to claim 1 characterized in
that the second mixing section (48) comprises at least one mixing
zone (49) consisting of at least one forward-conveying wing (30,
31, 32) and at least one backward-conveying wing (34, 35, 36)
downstream of the forward-conveying wing (30, 31, 32).
7. The twin screw extruder according to claim 1 characterized in
that the forward-conveying wings (30, 31, 32) and the
backward-conveying wings (34, 35, 36) of the mixing zone (49) of
the second mixing section (48) are positioned to form continous
flights in the shape of a "V".
8. The twin screw extruder according to claim 1 characterized in
that the number of the forward-conveying wings (14, 15; 21, 22; 30,
31, 32) and of the backward-conveying wings (17, 18; 24, 25; 34,
35, 36) of each mixing zone (45, 46, 49) corresponds to the number
of the flights of the screws (12, 27) of the conveying sections
(34, 47) upstream of the respective mixing zone (45, 46, 49).
9. The twin screw extruder according to claim 1 characterized in
that the length (L1, L3, L5) of the forward-conveying wings (14,
15; 21, 22; 30, 31, 32) of each mixing zone (45, 46, 49) is longer
than the length (L2, L4, L6) of the backward-conveying wings (17,
18; 24, 25; 34, 35, 36) of said mixing zone (45, 46, 49).
10. The twin screw extruder according to claim 1 characterized in
that the ratio of the length (L1 to L6) of the wings (14, 15; 17,
18; 21, 22; 24, 25; 30, 31, 32; 34, 35, 36) to the barrel diameter
(D) is 0.3 to 2.0.
11. The twin screw extruder according to claim 1 characterized in
that the lead of the wings (14, 15; 17, 18; 21, 22; 24, 25; 30, 31,
32; 34, 35, 36) of the first mixing section (44) and/or or the
second mixing section (48) is 2 to 6 times the barrel diameter
(D).
12. A method for compounding multimodal polymer compositions
comprising the steps of contacting the multimodal polymer
compositions with the twin screw extruder according to claim 1.
13. The method according to claim 12 characterized in that the
specific energy input for compounding a multimodal olefin polymer
is lower than 300 k Wh/t.
Description
[0001] The invention relates to a counter-rotating twin screw
extruder for compounding polymers.
[0002] When producing a polymer composition, the ingredients
thereof, as different polymers, fillers and additives, as
anti-oxidants, light stabilizers, etc. have to be mixed intimately
in order to obtain a composition as homogenous as possible. This is
done by compounding the ingredients in a compounding machine as a
counter-rotating twin-screw extruder.
[0003] While on one hand, the compounding should be carried out at
a high temperature and shearing rate in order to achieve a
homogenous composition, degradation of the polymers is caused by
too severe conditions.
[0004] Particular problems are encountered when compounding
multimodal polymers, as multimodal polyethylene materials.
Multimodal polymers are in many respects superior to corresponding
mono-modal materials. Multimodal polyethylene materials and, more
particularly, bimodal polyethylene materials have a widespread and
increasing use as materials for various applications as pipes,
wires and cables, films, blow-molded and injection-molded articles,
etc.
[0005] Multimodal polymer compositions such as bimodal polyethylene
materials consist of a low molecular weight polymer fraction and a
high molecular weight fraction. The high molecular weight molecules
are known to be most sensitive to the compounding conditions needed
to achieve the desired degree of homogenization.
[0006] For instance, undispersed domains of high molecular weight
molecules appear as white spots in colored materials. The white
spots may adversely affect the strength of the article. Further,
when compounding polymer compositions, e.g. for the production of a
film, gel particles appear as disfiguring spots in the finished
film which consist of high molecular weight polymer not adequately
compounded. Although compounding at higher temperatures and shear
rates may remove the white spots and gel particles, degradation of
the high molecular weight molecules may occur which negatively
effects the otherwise superior properties of the multimodal polymer
material.
[0007] Thus, the white spots and gel particles are a serious
problem in the polymer industry and a solution of the problem would
mean the removal of a serious obstacle to use otherwise superior
multimodal polymer compositions.
[0008] In EP-A-645 232 is described a way of reducing this problem
by adding liquid nitrogen or solid carbon dioxide to the polymer
feed. This is, however, a rather costly way. According to WO
98/15591, the problems may be tackled by compounding at a low shear
rate so that the temperature of the polymer increases slowly. This
requires a highly precise control of the process conditions of the
counter-rotating twin screw extruder, however, and the production
capacity is rather low.
[0009] U.S. Pat. No. 6,409,373 discloses a counter-rotating
twin-screw extruder having a mixing section upstream of and a
mixing section downstream of a throttle valve or gate plates, each
mixing section comprising forward-conveying wings and
backward-conveying wings downstream of the forward-conveying wings.
Although in a monomodal polymer material the number of gels may be
reduced, it is not possible to obtain multimodal polymer materials
of high homogeneity without adversely affecting the superior
quality of multimodal polymer materials with the known
extruder.
[0010] PATENT ABSTRACTS OF JAPAN vol. 0050. no. 85 (C-057); 3 Jun.
1981-06-03) & JP 56 031433 A discloses a twin screw extruder
having a throttle element between two supply sections with two
supply ports, each supply section having a conveying section with
screws and a mixing section.
[0011] PATENT ABSTRACTS OF JAPAN vol. 0060, no. 11 (C-088), 22 Jan.
1982 (1982-01-22) & JP 136633 A discloses a twin screw extruder
according to the pre-amble of claim 1.
[0012] U.S. Pat. No. 6,280,074 discloses a twin screw extruder
having screws with wings in the shape of "V" to form backward- and
forward-conveying movements.
[0013] It is an object of the invention to obtain multimodal
polymer materials of high homogeneity with a high production
capacity at low cost.
[0014] This object is attained with the counter-rotating twin-screw
extruder according to claim 1 for compounding.
[0015] The counter-rotating twin extruder according to the
invention comprises a mixing section having at least two mixing
zones, each mixing zone consisting of at least one, preferably at
least two forward-conveying wings and at least one, preferably at
least two backward-conveying wings downstream of the
forward-conveying wings on each rotor.
[0016] The number of forward-conveying wings and the number of
backward-conveying wings of each mixing zone preferably corresponds
to the number of flights of the screws of the conveying section
upstream of the mixing section. For instance, in each mixing zone
two, three or four forward-conveying wings and backward-conveying
wings, respectively, may be used.
[0017] To adjust the filling degree of the mixing section, a
throttle valve or gate is usually provided between the mixing
section and the discharge port of the extruder. Instead of a
throttle valve, a gear pump connected to the discharge valve may be
used to control the filling degree in the mixing section. As a
throttle valve, a rotary slot bar may be used as disclosed in
JP-A-3004647. In that solution two bars extend across the rotors
which have a convex side rotating in a concave depression in the
barrels, the throttle gap being defined by the distance between the
rotors and the throttle edge of the bars.
[0018] The twin screw extruder of the present invention has two
mixing sections, namely one upstream of the throttle valve and one
downstream thereof. Whereas the first mixing section upstream of
the throttle valve comprises at least two mixing zones, each mixing
zone having at least two forward-conveying wings and at least two
backward-conveying wings, the second mixing section downstream of
the throttle valve has preferably a lower number of mixing zones,
for instance only one mixing zone when the first mixing section has
two mixing zones.
[0019] In the first mixing section, mainly dispersive mixing takes
place, so that the particles of the powder material to be mixed are
broken up, whereas in the second mixing section, mainly
distributive mixing occurs. That is, in the first mixing zone of
the first mixing section next to the screws of the conveying
section, the polymer material starts to melt and continues to melt
in the second mixing zone of the first mixing section, and then the
polymer melt is charged through the throttle valve into the second
mixing section. There, the melt is kneaded further to distribute
the different polymers and optionally, fillers, additives and so
forth homogeneously in the melt. Due to its high dispersive mixing
efficiency of the at least two mixing zones of the first mixing
section, the twin screw extruder of the invention is particularly
effective for compounding multimodal polymer materials.
[0020] From the second mixing section, the polymer melt is
discharged through the discharge port into a gear pump or a
discharge extruder. From the gear pump or the discharge extruder,
the melt is passed through a die plate, after which it is cooled
and cut to pellets. Alternatively, the melt is discharged through a
discharge port directly after the first mixing section.
[0021] Whereas the filling degree of the first mixing section is
determined by the throttle valve, the filling of the second mixing
section downstream of the throttle valve may be adjusted by the
suction side pressure of the gear pump.
[0022] Preferably, the upstream ends of the forward-conveying wings
of the first mixing zone of the first mixing section are positioned
at the downstream end of the screws of the first conveying section,
and it is also preferred that the upstream ends of the
forward-conveying wings of the mixing zone of the second mixing
section are positioned at the downstream ends of the of the screws
of the second conveying section. That is, the screws of the first
and second conveying sections and the forward-conveying wings of
the first zone of the first mixing section and of the mixing zone
of the second mixing section, respectively, preferably are
positioned to form continuous closed flights.
[0023] In the first mixing section, the downstream end of the
forward-conveying wings and the upstream end of the
backward-conveying wings of each mixing zone are preferably offset
to form a passage for the material to be mixed. Due to these
passages, a mixing action in axial direction occurs, and it is
avoided that material, in particular unmelted material, is pressed
between the barrels and the wings on the rotors, which would cause
a deflection of the rotors resulting in a non-uniform mixing due to
a non-uniform gap between the wings and the barrels.
[0024] The offset between the forward-conveying wings and the
backward-conveying wings of each mixing zone usually depends on the
number of flights of the screw in the conveying section. In case of
two flights, the offset may be about 90.degree. in the
circumferential direction, and in case of three flights, about
60.degree..
[0025] In the first mixing section, the downstream ends of the
forward-conveying wings and the upstream ends of the
backward-conveying wings of one mixing zone and the downstream ends
of the backward-conveying wings of one mixing zone and the upstream
ends of the forward-conveying wings of the next mixing zone are
positioned in the same radial plane, respectively. However, an
axial shift of those ends is also possible, so that for instance in
one mixing zone the upward ends of the backward-conveying wings are
upstream of the downstream ends of the forward-conveying wings of
this zone. Forming those passages between the wings is particularly
preferred when the screws of the first conveying section have only
two flights. In case of three flights or more, the rotor is stiffer
so that the down-stream ends of the forward-conveying wings and the
upstream ends of the backward conveying wings of each mixing zone
may be connected to form a "V". Because the viscosity of the
polymer melt is lower in the second mixing section due to higher
temperature, the forward-conveying wings and the backward-conveying
wings of the mixing zone of the second mixing section form such Vs.
That is, the forward-conveying wings and the backward-conveying
wings of the second mixing section are preferably positioned to
form continuous closed flights having the shape of a "V".
[0026] The length of the forward-conveying wings is preferably
longer than that of the backward-conveying wings in each mixing
zone, and the length of the backward-conveying wings in the first
mixing zone of the first mixing section is preferably shorter than
the length of the backward-conveying wings in the mixing zone of
the first mixing section downstream of the first mixing zone.
[0027] The ratio of the length (L) of the wings to the inner
diameter (D) of the barrels is preferably between 0.3 and 2.0. For
instance the L/D ratio of the forward-conveying wings of both
mixing sections is about 1, the L/D ratio of the backward conveying
wings of the first mixing zone of the first mixing section and the
second mixing section about 1/2 and the L/D ratio of the
backward-conveying wings of the second mixing zone of the first
mixing section is about 3/4.
[0028] The lead or pitch of the wings of the mixing sections may
vary between 2 and 6D and usually all wings have the same lead. If
there is different lead along the screw, then normally in each
mixing zone there is the same lead.
[0029] The radial clearance of the wings of the first mixing zone
of the first mixing section is preferably greater than the radial
clearance of the wings of the second mixing zone of the first
mixing section, and the radial clearance of the wings of the second
mixing section is preferably smaller than the clearance of the
conveying wings of the second mixing zone of the first mixing
section. That is, the more the material melts and the lower its
viscosity, the smaller is the radial clearance. Based on the inner
diameter (D) of the barrels, the radial clearance may vary between
0.01 and 0.05 D. Usually, the radial clearance of the
forward-conveying wings and the backward-conveying wings of one
mixing zone are equal to obtain uniform balanced mixing. The tip
width of the conveying wings may vary between 0.01 and 0.05 D.
[0030] Whereas the two barrels form a common chamber in which the
rotors counter-rotate, no or only slight intermeshing takes place
between the two rotors.
[0031] When compounding polymer with a twin screw extruder of the
present invention, at a given specific energy input (SEI), the
homogeneity of the product is considerably improved. Thus, the
degradation of polymer chains is significantly reduced, which leads
to an improvement of the optical and mechanical properties of the
compounded product, especially long-term mechanical properties as
slow-crack growth resistance, i.e. resistance to internal pressure,
exemplified by pipe pressure testing.
[0032] In addition, the control of SEI by adjusting the throttle
valve is more convenient. While the prior art rotor design may give
large variation of SEI with a minor adjustment of the throttle
valve in some positions of the valve, the present design gives a
nearly uniform and controllable response of SEI over the whole
adjustment range of the throttle valve. This makes it easier to
balance between a desired homogeneity and SEI level to avoid
degradation of the polymer.
[0033] The twin screw extruder of the present invention is
particularly useful for compounding multimodal polymer
compositions, particularly compositions comprising a low molecular
weight ethylene polymer, for instance with a melt flow rate
MFR.sub.2 (D) of about 0.1 to 5,000 g/10 min, and a high molecular
weight polymer, for instance with a MFR.sub.21 (G) of 0.01 to 10.0
g/10 min, where MFR.sub.2 (D) and MFR.sub.21 (G) are determined
according to ISO 1133, conditions 4 and 7, respectively.
[0034] The compounded product is particularly useful for making
coloured bimodal polyethylene pipe materials comprising the bimodal
ethylene polymer as a base resin and the pigment, optionally as
compounded into a carrier polymer in the form of a master batch.
The coloured materials preferably have a high homogeneity rating of
5 or less, more preferably 3 or less, and still more preferably 2
or less, where the homogeneity is defined as the dispersion of
white spots measured according to the method ISO 11420: 1996 (E)
which method is normally used for evaluation of carbon black
agglomerates in polyolefin pipes. Suitable pigments are, among
others, carbon black, ultramarine blue and phtalocyanine blue. Of
these, carbon black is especially preferred. In addition, the pipes
have excellent mechanical properties and anti-sagging properties.
Also high-density polyethylene films with excellent optical
properties may be obtained, as well as large blow-molded articles
and wire and cable products such as the slotted cores of optical
cables. Said bimodal pipe materials are disclosed in WO 00/01765
and WO 00/22040, said bimodal film materials are disclosed in WO
99/51649, said bimodal blow-molding materials are disclosed in WO
01/14122 and said slotted core materials in WO 02/063345.
[0035] An embodiment of the inventive twin screw extruder will be
explained in more detail below with reference to the enclosed
drawing, in which:
[0036] FIG. 1 shows a longitudinal section through the
extruder;
[0037] FIG. 2 shows a cross-section along the line II-II in FIG. 1;
and
[0038] FIG. 3 shows a graph of the dispersion as a function of
specific energy input.
[0039] According to FIGS. 1 and 2, the extruder has a body 1
forming a chamber 2 comprising to cylindrical barrels 3 and 4 which
communicate with each other. Each barrel 3, 4 houses a rotor 5, 6
which rotate in opposite directions as shown by the arrows in FIG.
2 and are axis-parallel.
[0040] On the upper side at the left end or upstream side of the
extruder shown in FIG. 1 a supply port 7, shown with a dotted line,
is provided to supply the powder material to be mixed in chamber 2.
At the right hand or downstream side of the extruder in FIG. 1 a
discharge port 8 is provided for discharging the mixed molten
material into a gear pump from which it is passed through a die
plate, after which it is cooled and solidified and cut to pellets
(not shown).
[0041] The two rotors 5, 6 are supported rotatably at both axial
ends with bearings in end plates 10, 11 and are driven in opposite
direction by a drive not shown.
[0042] Disposed on each rotor 5, 6 are from the left to the right
side in FIG. 1 screw elements 12 with two flights, elements 13 each
with two forward-conveying wings 14, 15, elements 16 each with two
backward-conveying wings 17, 18, elements 19 each with two
forward-conveying wings 21, 22, elements 23 each with two
backward-conveying wings 24, 25, ring-like throttle elements 26,
screw elements 27 with three flights, elements 28 each with three
forward-conveying wings 30, 31, 32, elements 33 each with three
backward-conveying wings 34, 35, 36, and screw elements 37 with
three flights. The elements 12, 13, 16, 19, 23, 27, 28, 33 and 37
may be fixed to the rotor shafts by splining. However, it is also
possible to make the complete rotor in one piece, e.g. in forged
steel.
[0043] The throttle valve 40 is formed by rotary slot bars 41
extending across the rotors 5, 6 and rotating in a semi-circular
depression 42 in the barrels 3, 4.
[0044] The screw elements 12 form a first conveying section 34 for
feeding the material from the supply port 7 downward to a first
mixing section 44 which comprises a first mixing zone 45 and a
second mixing zone 46. Each mixing zone 45, 46 has two
forward-conveying wings 14, 15 and 21, 22, respectively, and two
backward-conveying wings 17, 18 and 24, 25, respectively, on each
rotor 5, 6.
[0045] In the first mixing section 44, mainly dispersive mixing
takes place, so that the particles of the powder material to be
mixed are broken up. In the first mixing section 44, the material
is molten mainly due to shear forces and partly by external
heating. In the first mixing zone 45, the polymer material starts
to melt and continues to melt in the second mixing zone 46.
[0046] From the throttle valve 40 the molten material flows to
screw elements 27 which form a second conveying section 47 and feed
the molten material to a second mixing section 48 which comprises
only one mixing zone 49 which has three forward-conveying wings 30,
31, 32 and three backward-conveying wings 34, 35, 36 on each rotor
5, 6. In the second mixing section 48, mainly distributive mixing
occurs, that is, the melt is kneaded further to distribute the
different polymers, fillers, additives and so forth homogeneously
in the melt. From the second mixing section 48, the polymer melt is
discharged through discharge port 8.
[0047] The upstream ends of the forward-conveying wings 14, 15 of
the first mixing zone 45 of the first mixing section 44 are
positioned at the downstream ends of the flights of the screws 12
of the first conveying section 34, and the upstream ends of the
forward-conveying wings 30, 31, 32 of the mixing zone 49 of the
second mixing section 48 are positioned at the downstream ends of
the flights of the screws 27 of the second conveying section
47.
[0048] In the first mixing section 44 in each mixing zone 45, 46,
the forward-conveying wings 14, 15 and 21, 22, respectively, and
the upstream ends of the backward-conveying wings 17, 18 and 24,
25, respectively, are offset with an angle of 90.degree. in the
circumferential direction, and the downstream ends of the
backward-conveying wings 17, 18 of the first mixing zone 45 and the
upstream ends of the forward-conveying wings 24, 25 of the second
mixing zone are also offset by an angle of 90.degree. in the
circumferential direction to form a passage for the material to be
mixed.
[0049] In contrast to this, the downstream ends of the
forward-conveying wings 30, 31, 32 and the backward-conveying wings
34, 35, 36 of the mixing zone 49 of the second mixing section 48
are positioned to form a "V".
[0050] The axial lengths L1, L3 and L5 of the forward-conveying
wings 14, 15; 21, 22; 30, 31, 32 is longer than the length L2, L4
and L6 of the backward-conveying wings 17, 18; 24, 25; 34, 35, 36.
Furthermore, the length L2 of the backward-conveying wings 17, 18
in the first mixing zone 45 is shorter than the length L4 of the
backward-conveying wings 24, 25 in the second mixing zone 46 of the
first mixing section 44. Thus, the ratio of the length of the wings
to the inner barrel diameter D is, for instance L1/D=L3/D=L5/D=1,
L2/D=0.5, L4/D=0.75 and L6/D=0.5.
[0051] The lead or pitch of the wings 14, 15; 17, 18; 21, 22; 24,
25; 30, 31, 32; 34, 35, 36 is for instance 4, and, usually, the
same for all wings.
[0052] In the first mixing section 44, the radial clearance C of
the wings 14, 15; 17, 18 of the first mixing zone 45 is greater
than the radial clearance C of the wings 21, 22; 24, 25 in the
second mixing zone 46 of the first mixing section 44. Based on the
diameter D of the barrels 3, 4, the radial clearance C may vary
between 0.01 and 0.05 D.
[0053] The tip width W of the wings 14, 15, 17, 18, 21, 22, 24, 25,
30, 31, 32, 34, 35, 36 may vary between 0.01 and 0.05 D.
EXAMPLE 1
[0054] The counter-rotating twin screw extruder as shown in FIG. 1
was used having a barrel diameter D of 90 mm, a length of the
chamber 2 before the throttle valve 40 of 7.5 and after the
throttle valve 40 of 3.5 based on the diameter D of the barrels,
and equipped with a gear pump at the discharge port and a
pelletizer.
[0055] The number of the wings 14, 15, 17, 18, 21, 22 and 24, 25 of
the first mixing section 44 was 2, the number of the wings 30, 31,
32 and 34, 35, 36 of the mixing zone 49 of the second mixing
section 48 was 3. Screws 12 had two flights and screws 27 three.
The radial clearance C was 3.9 mm for the forward-conveying wings
14, 15 and backward-conveying wings 17, 18 of the first mixing zone
45, 2.0 mm for the forward-conveying wings 21, 22 and
backward-conveying wings 24, 25 of the second mixing zone 46 and
1.5 mm for the forward-conveying wings 30, 31, 32 and
backward-conveying wings 34, 35, 36 of the second mixing section
48. The lead was 4 for all wings and the L/D ratios were
L1/D=L3/D=L5/D=1; L2/D=0.5; L4/D=0.75 and L6/D=0.5.
[0056] As polymer material bimodal polyethylene prepared according
to Inventive Material C of Example 3 of WO 00/22040 was used,
except that 1-butene was used as a comonomer instead of 1-hexene
and the hydrogen to ethylene ratio in the loop reactor was adjusted
so that the polymer produced in the loop reactor had an MFR.sub.2
of 400 g/10 min. Further, the 1-butene to ethylene ratio in the gas
phase reactor was adjusted so that the density of the resin was
0.951 g/cm.sup.3 and hydrogen to ethylene ratio in the gas phase
reactor was adjusted so that the bimodal polymer resin had a melt
index MFR.sub.5 of 0.25 g/10 min and MFR.sub.21 of 9.5 g/10 min,
where MFR.sub.5 and MFR.sub.21 were determined according to ISO
1133:1997, conditions T and G, respectively. Density was determined
according to ISO 1183-1987. Into the polymer material were added
about 2.3% by weight carbon black and about 0.35% of antioxidants
and stabilisers. The polymer material was fed with a throughput of
about 240 kg/h. The SEI was calculated on the basis of the power
consumption of the drive motor. The white spots of the product were
rated according to ISO 11420: 1996 (E).
COMPARATIVE EXAMPLE 2
[0057] Example 1 was repeated with an extruder having the same
barrel diameter, L/D ratios, throttle valve and other equipment.
However, it had only one mixing section upstream the throttle
valve. The mixing section consisted of two forward-conveying wing
elements followed by a backward-conveying wing element. All wing
elements had 2 flights, a radial clearance of 2 mm, a lead of 4 and
an L/D ratio of 1.
[0058] The same polymer material with the same throughput was used
as in Example 1.
[0059] The results of Example 1 in comparison to Example 2 are
shown in FIG. 3. It is evident from FIG. 3 that the inventive
concept makes it possible to reach a given homogeneity level of the
polymer with a lower specific energy input (SEI). This reduces the
risk of degradation of the polymer and thereby a combination of
good mechanical properties and good homogeneity can be
achieved.
EXAMPLE 3
[0060] Example 1 was repeated with a larger extruder having a
barrel diameter D of 380 mm. The throughput was 24 t/h. The polymer
material and all other parameters were the same as in Example 1.
The rating for the homogeneity according to the method ISO 11420:
1996 (E) was 5.2, the SEI 215 kWh/t.
COMPARATIVE EXAMPLE 4
[0061] Comparative Example 2 was repeated with a larger extruder
having a barrel diameter D of 380 mm as in Example 3. All other
parameters of the extruder were the same as in Comparative Example
3. The throughput and the polymer material were the same as in
Example 3. The rating for the homogeneity according to ISO 11420:
1996 (E) was 7.2, the SEI 245 kwh/t.
EXAMPLE 5
[0062] Example 3 was repeated, except that another polymer was
used. The polymer was prepared otherwise in a similar way than in
Example 1, but the gas phase reactor conditions were adjusted so
that the bimodal polymer had MFR.sub.5 of 0.25 g/10 min and
MFR.sub.21 of 6.7 g/10 min.
[0063] The rating according to the method ISO 11420: 1996 (E) was
4.4; the SEI was 210 kWh/t.
EXAMPLE 6
[0064] Example 5 was repeated, except that another polymer was
used. The polymer was prepared otherwise in a similar way than in
Example 1. but the gas phase reactor conditions were adjusted so
that the bimodal polymer had MFR.sub.5 of 0.30 g/10 min and
MFR.sub.21 of 11 g/10 min. The rating according to ISO 11420: 1996
(E) was 2.9, the SEI was 195 kWh/t.
EXAMPLE 7
[0065] Example 5 was repeated, except that another polymer was
used. The polymer was prepared otherwise in a similar way than in
Example 1, but the gas phase reactor conditions were adjusted so
that the bimodal polymer had MFR.sub.2 of 0.50 g/10 min and
MFR.sub.5 of 2.0 g/10 min and density of 0.942 g/cm.sup.3, where
MFR.sub.2 and MFR.sub.5 were determined according to ISO 1133:1997,
conditions D and T, respectively. The rating according to ISO
11420: 1996 (E) was 1.2, the SEI was 200 kWh/t.
COMPARATIVE EXAMPLE 8 TO 10
[0066] Examples 5 to 7 were repeated with the big extruder
according to Comparative Example 4.
[0067] The ratings according to ISO 11420: 1996 (E) were 7.2; 4.4
and 1.6, respectively, the SEI were 240, 230 and 245 kWh/t,
respectively.
* * * * *