U.S. patent application number 11/100452 was filed with the patent office on 2005-10-13 for method of producing pelletized polyolefin.
Invention is credited to Due, Magne, Fatnes, Anne-Maria, Hole, Olav, Knudsen, Karin.
Application Number | 20050228118 11/100452 |
Document ID | / |
Family ID | 34575778 |
Filed Date | 2005-10-13 |
United States Patent
Application |
20050228118 |
Kind Code |
A1 |
Knudsen, Karin ; et
al. |
October 13, 2005 |
Method of producing pelletized polyolefin
Abstract
Rotational moulded products are prepared by producing a mixture
of a polyolefin and optionally at least one additive, extruding
this mixture in melt form through orifices in a die, pelletizing
the mixture extruded through the orifices to form micropellets
having a particle size distribution D(v, 0.5) of 0.1 to 1 mm, and a
value of the ratio of D(v, 0.9)-D(v, 0.1) to D(v, 0.5) of no more
than 1, drying the micropellets to a residual water content of no
more than 1% weight, and if desired packaging the micropellets.
Inventors: |
Knudsen, Karin; (Stathelle,
NO) ; Fatnes, Anne-Maria; (Stathelle, NO) ;
Due, Magne; (Stathelle, NO) ; Hole, Olav;
(Stathelle, NO) |
Correspondence
Address: |
NIXON & VANDERHYE, PC
901 NORTH GLEBE ROAD, 11TH FLOOR
ARLINGTON
VA
22203
US
|
Family ID: |
34575778 |
Appl. No.: |
11/100452 |
Filed: |
April 7, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11100452 |
Apr 7, 2005 |
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10403004 |
Apr 1, 2003 |
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6894109 |
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10403004 |
Apr 1, 2003 |
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09831167 |
May 7, 2001 |
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6573314 |
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09831167 |
May 7, 2001 |
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PCT/GB99/04184 |
Dec 10, 1999 |
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Current U.S.
Class: |
524/570 |
Current CPC
Class: |
B29B 9/12 20130101; B29B
9/16 20130101; B29B 9/065 20130101; B29B 7/38 20130101; B29K
2023/06 20130101; B29B 9/14 20130101; B29B 7/603 20130101; B29B
7/90 20130101; B29B 2009/125 20130101; B29K 2023/12 20130101; B29B
7/88 20130101; B29C 41/04 20130101; B29C 48/00 20190201 |
Class at
Publication: |
524/570 |
International
Class: |
C08K 003/00 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 11, 1998 |
GB |
9827432.7 |
Claims
1.-20. (canceled)
21. A rotationally moulded polymer article having a pinhole density
on its surface of less than 1/cm.sup.2.
22. An article as claimed in claim 21 being free of pinholes.
Description
[0001] This invention relates to a method for the production of
pelletized polyolefin for rotational moulding.
[0002] Rotational moulding is a moulding process in which a
particulate polymer, the moulding powder, is filled into a mould
which is placed in an oven and rotated so that the polymer melts
and coats the inside surface of the mould. In order to ensure that
the moulded product is defect free, the moulding powder must have a
relatively small particle size and composition. Generally the
particle size is about 300 .mu.m. Where, as is normal, the moulding
powder has to contain colouring agents or other additives, e.g.
stabilizers, the moulding powder is conventionally produced by
grinding polymer pellets extruded from stabilized reactor grade
powder, ie. pellets normally of size 3 to 6 mm, to the correct
particle size for rotational moulding, usually with the colours or
other additives being added in with the polymer pellets or mixed
into the ground moulding powder. In addition to being costly, the
grinding process is noisy and dusty and generally the grinding
operation presents a worker unfriendly environment.
[0003] It is possible to mix polymer and additives using an
extruder and to extrude pellets containing polymer and additives.
The pellets however have not been found acceptable for rotational
moulding as the surface of the resulting moulded product is covered
with small holes, "pin holes".
[0004] We have now found that extruded micropellets may be used in
rotational moulding if their water content is reduced to less than
0.1% wt (10.sup.3 ppm), or more advantageously less than 200 ppm.
In this way the costly and worker-unfriendly grinding step of the
conventional process for production or moulding powder for
rotational moulding can be avoided. Furthermore the micropellets
are easier to handle than the ground powder and easier to transport
using conventional conveying systems, e.g. due to dry capability.
Moreover, having a higher bulk density, the micropellets have
advantage both in terms of storage and transportation volume and in
terms of the rotomoulding process itself.
[0005] Thus viewed from one aspect the invention provides a
rotational moulding composition comprising a pelletized olefin
polymer having a particle size distribution D(v, 0.5) of 0.1 to 1
mm, preferably 0.2 to 0.9 mm, especially 0.3 to 0.8 mm, a value of
the ratio of D(v, 0.9)-D(v, 0.1) to D(v, 0.5) of no more than 1,
preferably no more than 0.8, e.g. 0.45 to 0.70, and a water content
of less than 0.1% wt, preferably less than 200 ppm, more preferably
less than 150 ppm, especially less than 100 ppm, e.g. 10-200 ppm,
especially 30 to 140 ppm.
[0006] These very dry micropellets may be produced by a mixing,
pelletization and drying procedure and this forms a further aspect
of the invention.
[0007] Viewed from this aspect the present invention provides a
method of producing a rotational moulding micropellet composition,
said method comprising:
[0008] producing a mixture of a polyolefin and optionally but
preferably at least one additive, e.g. one or more selected from
colouring agents, stabilizers (e.g. heat or radiation stabilizers),
antioxidants, UV absorbers, antistatic agents, lubricants and
fillers (e.g. organic fillers;
[0009] extruding said mixture in melt form through orifices in a
die;
[0010] pelletizing the mixture extruded through said orifices to
form micropellets having a particle size distribution D(v, 0.5) of
0.1 to 1 mm preferably 0.2 to 0.9 mm, etc. and a value of the ratio
of D(V, 0.9)-D(v, 0.1) to D(v, 0.5) of no more than 1, preferably
no more than 0.8;
[0011] drying said micropellets to a residual water content of no
more than 0.1% wt. preferably less than 200 ppm, etc.;
[0012] and is desired packaging said micropellets, e.g. in
water-tight containers or in microperforated containers which
subsequently may be coated with a shrink wrap coating.
[0013] By virtue of the pelletization process, any additives (e.g.
stabilizers (for example heat or radiation stabilizers such as
UV-stabilizers, in particular HALS (Hindered amine light
stabilizers)), coloring agents, antistatics, antioxidants (e.g.
phenolic and phosphitic antioxidants), lubricants, etc) in the
mixture being pelletized are distributed very uniformly in the
resulting rotomoulding pellets. This results in a high degree of
homogeneity within and between the pellets. This is very important
for rotomoulding since the rotomoulding process itself does not
involve an extrusion step and so cannot itself cause additive
distribution to become uniform.
[0014] Typically additives such as antioxidants, lubricants and
UV-stabilizers will be used in quantities of about 100 to 5000 ppm,
e.g. 500 to 2500 ppm, relative to the overall polymer weight.
[0015] The method of the invention can advantageously be operated
on a continuous basis although the drying procedure may operate on
a batchwise basis, by operating on batches of micropellets from a
continuously operating pelletizer. In this way the operability (the
percentage of the time that the method is in operation) of the
method may be at least 95%. Operability of at least 95% is
desirable for any industrial full scale polymer moulding
composition production process.
[0016] Viewed from a still further aspect the invention provides
the use of the micropellet composition of or produced by the method
of the invention in rotational moulding. Viewed from an alternative
aspect, the invention also provides a rotational moulding process
in which a particulate polymer composition is transformed to
produce a moulded product, characterised in that as said
composition is used a micropellet composition of or produced by the
method of the invention.
[0017] Rotational moulding is a well established technique (see for
example SE-A-9203167) and the micropellets of the invention can be
used in conventional rotational moulding equipment.
[0018] Generally for the method of the invention the initial
feedstock will be a dry polyolefin (e.g. homo or copolymers of
C.sub.2-10 l-olefins, more particularly homo or copolymers of
ethylene or propylene, especially of ethylene) in pelletized or
unpelletized form, optionally reactor grade polymer or molten
polymer. The polyolefin may typically be produced by a
polymerization process catalysed by Ziegler-Natta, or chromium
based or metallocene or other single site catalysts. The polyolefin
may have a narrow or broad molecular weight distribution; however a
narrow molecular weight distribution, e.g. less than 4, is
preferred. The polyolefin conveniently has a MFR.sub.2.16 in the
range 2 to 10, especially 3 to 6. PE of density 950 to 920
kg/m.sup.3 is especially preferred.
[0019] Typically the initial feedstock will be at a temperature
between ambient and 30.degree. C.
[0020] Any colouring agent is preferably used in the form of a
master batch, ie. already mixed with a polymer, generally the same
or similar polymer as the initial feedstock. LDPE is convenient to
use in this regard. The colouring agent may be an inorganic or
organic material such as are conventionally used in moulded
polyolefin products. Carbon black is particularly preferred.
[0021] Initial feedstock, colouring agent and any other desired
additives, e.g. radiation stabilizers, antioxidants, antistatic
agents, etc., can be fed to an extruder, a mixer or a melt pump by
a control system that ensures the components are homogeneously
mixed in the desired ratio. Generally the initial feedstock will
make up at least 60% wt, more preferably at least 80% wt of the
resulting mixture and that mixture will be at least 85% wt.
prererably at least 90% wt. polyolefin. Thus for example 99-80% wt.
particulate HDPE and 1-20% wt. of a carbon black master batch
containing 40% wt. carbon black in LDPE may be fed to an extruder
using two loss-in-weight feeders.
[0022] Depending on the needs of mixing and whether the polymer is
already molten, a mixer, extruder or melt pump may be used mix the
components and build up the pressure necessary to ensure proper
flow through the orifices of the die. Generally the mixing will
involve feeding in additives from one or more storage tanks under
controlled flow conditions (e.g. using appropriate controlled
valves and pumps if necessary). The additives and polymer are fed
to a mixer/homogenizer to create a homogeneous feed stream for the
extruder. If desired, the additives may be mixed with a portion of
the polymer to create an additive masterbatch and this may be fed
to the extruder together with the remaining polymer. This may
involve feeding the masterbatch into the polymer through a
satellite extruder. The pressure at the extrusion die for the
overall mixture may be up to 550 bar for example; generally however
it will be between 100 and 300 bar. The temperature of the mixture
as it reaches the die plate will depend on the particular polymer
used but should be sufficiently high to permit the polymer to pass
through the die and should be kept as low as possible in order to
reduce or avoid polymer degradation.
[0023] The die plate should be of a form capable of withstanding
the pressures required for extrusion of the molten polymer and the
orifices should be of a diameter such that micropellets of the
desired size are formed. Generally orifice diameters will be in the
range 0.05 to 1.0 mm, more preferably 0.1 to 0.8 mm, still more
preferably 0.2 to 0.4 mm. For industrial, large scale operation,
the die-plate conveniently will contain a plurality of such
orifices, e.g. 1000-50000 and be capable of extruding at least 0.25
ton/hour more preferably at least 1 ton/hour.
[0024] The pelletizer may conveniently be an underwater pelletizer
operating by rotating a cutter across the downstream face or the
die plate in the presence of water which cools the melt causing it
to solidify quickly. The speed at which the pelletizer operates is
selected according to the die plate size and number of orifices and
to achieve the desired pellet size and shape. To produce
micropellets according to the method of the invention may require
the use of larger amounts of pelletizer water than is required for
the preparation of larger pellets and accordingly the composition
leaving the pelletizer generally has a very high water content,
e.g. 95 to 99% wt., more generally 97 to 98% wt.
[0025] This aqueous composition is preferably screened to remove
lumps and then subjected to a coarse dewatering operation, for
example by passage through a pre-thickener, a conical sieve at the
top of which the composition enters tangentially and the water
drains out through openings (e.g. 0.15.times.2 mm) small enough to
prevent the micropellets from passing through. The micropellets are
removed through a duct at the base of the sieve. Desirably this
should reduce the water content to 50-80% wt. The water removed can
be recycled to the pelletizer.
[0026] The still aqueous micropellet composition may then be
subjected to a second dewatering operation to reduce the water
content for example to 1 to 10% wt. This may typically be achieved
by using a centrifuge, e.g. a pusher centrifuge. Again the water
removed may be recycled to the pelletizer. The centrifuge rotation
rate and the residence time may be selected so as to achieve the
desired degree dewatering and will depend on the size of the
micropellets, the size of the centrifuge and the loading of the
centrifuge. Typically a g force of 300-800 g is required and the
residence time is of the order of minutes, e.g. <4 minutes.
[0027] In this way the water content can be reduced to a large
extent without using dryers which operate on a heating basis.
[0028] Reducing the water content of the resulting partially dried
composition down to the level desired for rotational moulding can
then be achieved by one or more further drying steps, e.g. by the
use of a fluid bed dryer in which heated gas (e.g. air) is passed
through a fluidized bed of the micropellets, or by the use of a
flash dryer.
[0029] Using a fluidized bed drier with an air input temperature of
about 95.degree. C. and an outlet temperature of about 75.degree.
C. the residence time required will generally again be of the order
of minutes, e.g. 7 to 13 minutes.
[0030] The water content of the pellets may be measured by the Karl
Fischer method. Thus 1 g of a sample is heated to 180.degree. C. in
an oven; the evaporated water is driven into a KF-solution; and
water is titrated and calculated as ppm.
[0031] After the micropellets are dried to the desired level, they
will desirably be screened to remove the coarse and if necessary
the fines fractions.
[0032] The dried and screened micropellets may then be conveyed for
example by a normal pneumatic conveying system to be packaged and
stored. After the final drying stage, e.g. during conveying,
storing and packing, the micropellets are preferably maintained
under dry conditions to prevent the moisture content from
increasing to an undesirable level. Moreover, it is preferable that
the piping used at this stage should be so selected and arranged as
to avoid electrostatic charging of the micropellets and the
disadvantages that might result therefrom, e.g. cross-contamination
between sequential product runs on the same equipment.
[0033] Viewed from a still further aspect the invention provides
apparatus for the production of rotational moulding polyolefin
pellets, said apparatus comprising:
[0034] (i) a mixer arranged to provide a mixture of a polyolefin
and at least one additive;
[0035] (ii) an extruder and pelletizer arranged to extrude and
pelletize said mixture;
[0036] (iii) an centrifuge arranged to dewater said mixture;
and
[0037] (iv) a fluidized bed drier arranged to dry said dewatered
mixture, e.g. to a water content of no more than 1% by weight,
preferably no more than 0.1% by weight, especially no more than 200
ppm, etc.
[0038] In this apparatus, the extruder and pelletizer is preferably
arranged to generate pellets having a particle size distribution
D(v, 0.5) of 0.1 to 1 mm, and a value of the ratio of D(v,
0.9)-D(v, 0.1) to D(v, 0.5) of no more than 1. Moreover, excess
water is preferably drained off the pelletized mixture prior to
centrifugation, e.g. in a pre-thickener as described above.
[0039] Since the micropellets according to the invention are so
dry, rotomoulded products can be made therewith without surface
pitting and irregularities which give rise to poor mechanical
properties and unfavorable appearance.
[0040] Comparison of rotomoulded products produced using such
rotomoulding pellets showed the numbers of visible "pinholes"
(holes visible on the product surface using light microscopy and of
at least 100 .mu.m diameter) to decrease as the water content of
the pellets is reduced so that at a water content of below about
150 ppm (wt.) pinholes were no longer visible by light microscopy.
Between 270 and 160 ppm, the number of pinholes visible by light
microscopy dropped by a factor of about 34. At 80 ppm, no pinholes
were visible on a 195 mm.times.195 mm test surface. Pinhole surface
density may readily be assessed by counting the number of pinholes
of diameter 100 .mu.m or more on an area (e.g. 10 cm.sup.2 to 50
cm.sup.2) of the rotomoulded product, for example using light
mircroscopy. Thus viewed from a further aspect the invention
provides a rotomoulded polymer (e.g. polyolefin) article having a
pinhole density on its surface of less than 10/cm.sup.2, preferably
less than 5/cm.sup.2, more preferably less than 1/cm.sup.2.
[0041] As mentioned above, the pellets of the invention have
improved dry flow and bulk density compared to the equivalent
polymers in ground form. Thus for example, for one polymer the
pellets according to the invention had dry flow and bulk density
values (according to ASTM-D 1895-89) of 14 s/100 g and 476
kg/m.sup.2 as compared to values of 22 s/100 g and 360 kg/m.sup.3
for the ground polymer.
[0042] Embodiments of the invention will now be described with
reference to the following non-limiting Examples and the
accompanying drawings in which:
[0043] FIG. 1 is a flow sheet illustrating the process stages or
one embodiment of the method of the invention;
[0044] FIGS. 2 to 4 are particle size distribution curves for
examples of the compositions of the invention; and
[0045] FIGS. 5 and 6 are light microscopy images of surfaces of two
rotomoulded products.
[0046] Referring to FIG. 1, polymer is fed from storage silos to
extruder feed tank 1 and carbon black master batch is fed from
storage tank 2 to extruder feed tank 3. From feed tank 1 the
polymer is fed to extruder 5 via standard loss-in-weight feeder 6.
The master batch is fed from tank 3 to a loss in weight feeder 4
and then to extruder 5. If desired, other additives, e.g. in master
batch form, may be blended in, optionally together with the carbon
black master batch or from independent parallel feeders (not
shown). The extruder 5 may be a standard extruder available from
Werner & Pfleiderer (e.g. ZSK70MC), Berstorff, or Kobe Steel
(e.g. Hyperktx 59 xht). In extruder 5, polymer and master batch are
mixed, brought into melt form and passed via gear pump 7 through a
die plate into pelletizer 8 which is fed with pelletizer water from
water tank 9. Suitable die plates are available from BKG or
Gala.
[0047] The aqueous micropellet containing product produced by
pelletizer 8 is passed through a lump catcher 15 into a
pre-thickener 10 as described above. Suitable pre-thickeners are
available from Krauss-Mafei (e.g. EC800). The separated water is
returned to tank 9 and the concentrated aqueous micropellet
composition is passed into pusher centrifuge 11. Suitable
centrifuges are available from Krauss-Mafei (e.g. SZ32). Water
removed by the centrifuge is returned to tank 9 and the centrifuged
micropellets are passed to fluidized bed drier 12. Suitable
fluidized bed driers are available from Buhler (e.g. OTWG160) or
Niro A/S (e.g. Vibro-Fluidizer). The dried micropellets from drier
12 are screened by screen 13 and conveyed by pneumatic conveyors 14
to be stored and/or packed.
EXAMPLE 1
[0048] A mixture of 94 parts by weight HDPE (Borecene ME8168
(MFR.sub.2=6, density=934 g/L and containing antioxidants,
UV-stabilizers and lubricants at about parts per thousand (by
weight) levels) from Borealis) and 6 parts by weight of a carbon
black master batch (containing 60% wt. LDPE and 40% wt. carbon
black) was extruded and pelletized as a melt at about 250.degree.
C. through a die plate which had 0.4 mm orifices and was heated to
about 290.degree. C. Samples of the resulting pellets were dried to
residual water contents of between 60 and 200 ppm.
[0049] The particle size distribution, shown in FIG. 2, was
measured using a Malvern Instruments particle size analyser. The
particle size distribution was: D(v, 0.9) 797 .mu.m, D(v, 0.1) 462
.mu.m and D(v, 0.5) 642 .mu.m, ie. (D(v, 0.9)-D(v, 0.1))/(D(v,
0.5))=0.52.
[0050] Two further samples of micropellets were produced
analogously, in the first case using a die plate with 0.3 mm
apertures and in the second case using Borecene ME 8166
(MFR.sub.2=3, density=940 g/L) in place of ME8168 and using a die
plate with 0.3 mm apertures. The particle size distributions, shown
in FIGS. 3 and 4 respectively were: D(v, 0.9) 701 .mu.m, D(v, 0.1)
410 .mu.m and D(v, 0.5) 510 .mu.m, (i.e. ((D(v, 0.9)-D(v,
0.1))/D(v, 0.5)=0.57) and D(v, 0.9) 740 .mu.m, D(v, 0.1) 410 .mu.m
and D(v, 0.5) 523 .mu.m (i.e. ((D(v, 0.9)-D(v, 0.1))/D(v,
0.5)=0.63).
EXAMPLE 2
[0051] Pinhole Density
[0052] Micropellets, prepared analogously to those of Example 1,
were rehydrated to water contents of 80 to 7000 ppm (by weight).
The water contents were measured by the Karl Fisher method (for
lower water contents) and by gravimetry (for higher water
contents).
[0053] Cubic boxes were prepared using the different samples using
a Rotospeed E-60 rotomoulding apparatus at an oven temperature of
270.degree. C. The same face of the box was cut off in each case
and cleaned. A mask with eight 14 mm.times.18 mm windows was placed
over the cut and cleaned face and, using a Wild Photo macroscope
M420 with ring lightening, a digital picture was taken of each
window and transferred to a computer. The images were processed to
determine the number of surface pinholes greater than 100 .mu.m in
diameter. FIGS. 5 and 6 show such images of surfaces produced using
270 ppm wt and 160 ppm wt water content micropellets. As can be
seen, at 160 ppm wt water content, virtually no pinholes can be
detected.
* * * * *