Note: Descriptions are shown in the official language in which they were submitted.
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ROTATIONAL MOLDING GRADE LLDPE RESIN
BACKGROUND OX THE INVENTION
The present invention relates to granular linear low
density polyethylene (LLDPE) which have been finished to
produce granular resins of desired bulk density and particle
size distribution, and having additives incorporated therein
for use in rotational molding of plastic products
The recent development of certain linear low density
polyethylene (LLDPE) has resulted in a new product which
presents certain superior properties over conventional
branched low density polyethylene (LOPE). These new resins
are manufactured by low pressure processes which produce the
resin as granular product large enough and dense enough that
conventional poulticing is unnecessary. This not only
saves cost but also avoids any resin degradation which might
result from the pellet forming operation during the
manufacturing process of the resin.
Since the polyethylene granules do not need to be
poulticed, conventional polyethylene compounding and
finishing operations are not suited for this product. For
example, in poulticing conventional LOPE, additives (e.g.
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antioxidant, thermal and US stabilizers, etc.) are
conveniently incorporated by melt blending in the mixer
and/or extrude used to form the pellets. (The term
"finishing" as used herein refers to a process for
converting virgin resin to usable form. Additives are
incorporated to impart the desired end-use properties to the
product, and particle shaping and size classification place
the product in a form suitable for the fabricators.)
Simply blending or tumbling the ingredients together
lo at ambient conditions is not satisfactory for several
reasons. Uniform additive dispersion is not only difficult
to obtain, but the "salt-and-pepper n mixture is not as
effective as additives actually contacting the individual
granules.
It has also been proposed to prepare a poulticed
master batch with additives, followed by grinding and
blending with the virgin resin.
- Intensive mixers have been proposed for treating
; resinous particulate materials. These intensive mixers
employ rotating blades to impart high energy to the system,
causing the mixing to take place at elevated temperatures.
Representative uses of intensive mixers in treating
particulate resins are discussed below.
U. S. Patent 3,229,002 discloses the use of an
intensive mixer for "polishing" thermoplastic pulverized
resin (e.g. polyolefins, nylon, etc.) to improve its
flyability and bulk density. The purpose of the treatment
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is to improve flyability and density for molding, coating,
and rug backing applications.
U. S. Patent 3,591,409 discloses treating resin
granules in an intensive mixer with wax and solid
particulate material to coat the resin with wax having the
solid embedded therein.
U. S. Patent 3,632,369 discloses the use of a high
intensive mixer for admixing pigment with ground resins.
The pigment addition is achieved by operating the intensive
lo mixer at conditions to produce abrasive adherence of the
pigment to the polymer particle.
U. S. Patent 3,736,173 discloses the use of a high
speed mixer to incorporate a curing agent into polyolefin
poulticed granules by penetration and diffusion.
; U. S. Patent 3,997,494 discloses the use of high
speed intensive mixer for incorporating filler material into
polymer pellets, then removing the blended materials from
the pellets until the filler material is used up.
U. S. Patent 4,230,615 discloses the use of a high
speed high intensity mixer to fully flux thermoplastic
resins.
Rotational molding resins should satisfy the
following criteria:
1. The granules must be free flowing in order to permit
charging to the mold and conforming to the mold
configuration.
2. The granules should be substantially spherical in shape
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and free of any tails or hairs which could interfere
with the flyability of the particles.
3. The particle size should be relatively small and the
particle size distribution of the granules should be
relatively narrow (less than 5.0 weight percent larger
than about 30 mesh) and only minor amounts (less than
15%) finer than 100 mesh.
4. The bulk density of the granules should be high to
provide good flyability, close compaction in the mold,
lo and reduce shipping costs.
5. The additives (e.g. antioxidant, US stabilizers,
etc.) should be thoroughly dispersed in the granules and
preferably in contact with all granules because no
mixing occurs during molding.
Rotational molding involves the following basic
steps:
1. The cavity of an unheated mold is charged with a
predetermined weight of the granules. (The free flowing
characteristic and high bulk density aid in this step).
2. The charged mold is placed in an oven and heated while
simultaneously rotating around two axes.
3. The double revolving motion results in formation of
hollow objects in the mold cavity, the powder being
evenly distributed to form walls of uniform thickness
when the resin fuses. The spherical granules free of
hairs and tails and having high bulk density aid in the
granules conforming to the mold. Also, the uniform
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dispersement of the additives is important since no
mixing occurs in the mold. The small amount of fines
(smaller than 100 mesh) is also important to fill the
interstices between the larger particles.
4. After all the resin particles have fused forming a
homogeneous layer on the mold walls, the mold is cooled
while still being rotated.
5. The mold is opened and the molded part removed.
SUMMARY OF THE INVENTION
lo Briefly, the present invention is a processed (e.g.,
finished or compounded) linear low density polyethylene
product for use in rotational molding comprising linear low
density polyethylene granules having additive material
incorporated into said granules or onto said granules and
further characterized as free flowing powder having a
particle size distribution of less than 5 weight percent
(preferably less than 2 weight percent) larger than 30 mesh
and less than 25 weight percent (preferably 15 White) finer
than 100 mesh, and bulk density, at least 20~ greater than
the corresponding unprocessessed linear low density
polyethylene. The processed LLDPE granules have the sharp
edges smoothed out and other irregular shapes of the
corresponding unprocessed LLDPE tend to be made to resemble
more rounded or spherical granules
The additive material may be any of those
conventionally used additives for rotational molding grade
resins including storage stabilizers, TV stabilizers,
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process stabilizers, pigment and the like.
Additives for rotational molding grade resins are
available in particulate form (normally smaller in particle
size than the resin granules, with ranges in particle size
between about 1 micron to about 1,000 microns). Liquid
additives and additive solutions may also be used. Most of
the particulate additives for rotomolding melt at intensive
mixing temperatures. These additives, being liquid and
plowable at intensive mixing conditions, coat the granules
lo during this step of the process The coating of the
individual resin granules provides additive on each granule
and ensures even additive dispersement throughout the bulk
material. This is extremely important in rotational molding
grade resins because the granules in the mold remain fixed
until fusion occurs. Granules without additives result in a
defect in the molded product.
The additives are preferably particulate materials
which fuse in the mixing step and at least partially coat
the resin granules.
The total amount of additives material is generally
between about 500 to 10,000 Pam concentration, preferably
1,000 to 5,000 Pam, based on the combined weight of the
granules and additive material.
The processed LLDPE of the present invention is
produced by the intensive mixing of granules of unprocessed
LLDPE and the desired additive material, cooling and sizing.
The unprocessed LLDPE will preferably have a particle size
I
distribution between about 5 and 200 mesh and bulk density
of between 20 and about 32 pounds per cubic foot. The
intensive mixing is continued until at least 80~ of the
LLDPE granules are smaller than about 30 mesh and the bulk
density has increased by at least 20% over the unprocessed
(unfinished) granules. After the completion of the mixing
the granules are withdrawn, cooled and preferably sized,
e.g. by sieve and screening to remove substantial amounts
(at least 95 White) of the particles larger than 30 mesh and
lo particles (at least 85 wit%) finer than 200 mesh.
DETAILED DESCRIPTION Ox TOE INVENTION
END PREFERRED EMBODIMENTS
LLDPE is made by polymerizing, in the presence of a
suitable catalyst, ethylene with an alpha-olefin comonomer
that contributes the side chain and hence lowers density.
Comonomer, either singly or in combination, such as
propylene, buttonhole, hexene-l, octene-l, 4-methylpentene-1
and pentene-l is used. Granular LLDPE may be made by gas
phase fluidized bed, or gas phase stirred bed. The low
Jo pressure gas phase processes produce a granular polyethylene
having a rather broad particle size distribution between
about 5 and 200 mesh and a bulk density of between about 20
to about 32 pounds per cubic foot, typically between 24 to
28 pounds per cubic foot. The granules of the present
invention have a particle size distribution suitable for use
in rotomolding applications and provide additives on
essentially all of the granules. These factors also combine
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to improve bulk density and flyability.
As used herein, the term "granules" means resin
particles in the form and size as discharged from the
reactor. (In polymerization operations which produce
granules, the particle size of the bulk of the granules fall
between about 5 and 200 mesh). Granules are to be
distinguished from (a) pellets which have been melt
processed into uniformly sized and shaped particles of
generally regular shape and (b) from "powder" or "fines"
lo which have a particle size smaller than 200 mesh. (All
mesh sizes" are expressed in terms of U. S. Sieve Series.)
The properties which must be improved by additives
include thermal and TV stabilization. Such additives
include organic and inorganic stabilizers, antioxidant,
pigments, etc. available in particulate and/or liquid form.
The additives which are added to LLDPE resin
granules typically include the following:
Additive Example_ Err Concentration
; Antioxidant Hindered Particulate 10-10,000 PAM
Jo and/or Phenol Or Liquid
TV Stab-
livers
Chloride Metal Particulate 10-3,000 PAM
Scavengers Stewart
Coloring Pigments Liquids Or 10-10,000 PAM
Agents Particulate
The conditions of such high speed stirring vary
depending on the individual high speed stirring apparatus
for instance, in the case of the high-intensity,
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vortical-action mixer (of. "Encyclopedia of Polymer Science
and Technology," vol. 4, pp. 124, Intrusions Pub., New
York, NAY. tl966)). e.g., Herschel mixer the conditions vary
depending on the revolving speed of stirring blades, the
peripheral speed of stirring blades, shape of the stirring
blades and the shape of the mixing tank, and other factors.
It is impossible to attain such a high speed stirring by
means of a low speed stirring apparatus such as a drum
tumbler which is driven at most at 60 rum Other
lo apparatus may be suitable for high speed stirring, e.g.,
centrifugal impact mixer, high speed dispersion mixer,
ribbon blender, conical dry blender, double arm mixer,
vertical action mixer, etc. as described in "the
Encyclopedia of Plastic Equipments" by Herbert R. Simon,
Reinhold Pub. Corp., New York, NAY. (1964), as it is or
after modification (for instance, increase of driving power)
to make it suitable for high speed stirring.
The incorporation of additives into or onto the
resin granules is accomplished in an intensive mixer
following two different mechanisms. If the additive in
question is liquid or has a melting point below that at
which the mixer is operating, the material will coat along
the surface of the virgin resin granule. Upon cooling, the
additive will encapsulate the granule. Highly volatile
additives may diffuse into the granule under these same
conditions. The second mechanism involves those additives
which do not melt at the polymer softening point. In this
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case, the granule surface softens and the mixing action
imparts enough kinetic energy into the additive and granule
that the collisions result in the additive being impinged
into the granule. The irregular surface and porosity of the
granules aids in the coating action by entrapping additives
particles. When the granule cools, the additives are
adhered to the surface. The presence of lower molting point
additives may improve the adherence of higher melting
additives as they may act as a bonding agent.
lo A variety of high intensive mixers may be used
including batch apparatus such as the Herschel mixer (U. S.
Pat. No. 2,945,634) and the Gelimat mixer manufactured by
Draiswerke GMBH and the horizontal continuous type with
rotating blades of the type manufactured by Wedco
International, Inc.
In the process of producing the present modified or
processed LLDPE granules, the LLDPE and additives may be fed
directly to the intensive mixer in the desired proportions
or the LLDPE or a portion thereof and the additives may be
premixed in a low speed blender then fed to the intensive
mixer.
In operation, resin LLDPE granules are delivered to
the intensive mixer in essentially the same form and shape
as discharged from the reactor. In the case of LLDPE, the
granules are irregularly shaped, generally rounded
agglomerations of smaller particles which exhibit
significant porosity.
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The temperature in the intensive mixer should be
sufficiently high to cause at least the outer surface of the
resin particles to soften but not so high as to cause melt
fluxing. The controlled temperature, of course, will depend
upon the material used. For LLDPE, temperature in the range
of 175 to 230F is satisfactory for most operations.
The additives may be introduced in particulate or
liquid form. However, in order to insure uniform
dispersement the particulate additives should be fusible at
lo the operating temperature of the intensive mixer.
The incorporation of additives into or onto the
resin granules is accomplished in an intensive mixer by
operating at a temperature above the resin softening
temperature and the additive fusion temperature of
particulate additives. These additives melt and coat along
the surface of the resin granules. Liquid additives
similarly coat the granules. Upon cooling, the additive
will encapsulate the granule. Highly volatile additives may
diffuse into the granule under these same conditions. The
irregular surface and porosity of the granules aids in the
coating action by entrapping additive material. The
collision of the granules plays a significant role in
additive transfer and dispersion.
The type of additives and final concentration will
depend upon the final product. Total additive levels for
rotational grade resins normally ranges between about 500
and 10,000 Pam, preferably 1,000 to 5,000 Pam based on the
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combined weight of the granules and additive material.
Other nonliquid additives may also be present. These
particles are also distributed and transferred from particle
to particle by particle collision and impregnation therein.
Rotation of mixer vanes mixes the resin and
additives. The granules collides with each other and with
the rotating vanes which (1) creates friction which
generates heat, (2) rounds the granules, (3) distributes the
additives among the resin granules and (4) breaks apart
agglomerates.
The particles upon leaving the mixer pass through an
agitation and cooling stage. This stage of the operation
may ye provided by a line having a heat exchanger. Air
introduced agitates and conveys the granules through a
cooling system such as a heat exchanger to storage. A
cyclone may be provided in the discharge line to separate
resin and air.
The final step in the process is to remove large
granules. A 30 or 35 mesh screen may be used for this
purpose. The large granules removed are recycled through
the intensive mixer. No accumulation of these large
particles has been observed due to the recycle indicating
the intensive mixer further reduces the particle size.
The increase in granular bulk density follows two
separate mechanisms. Bulk density in a material such as
granular LLDPE is dependent on two factors:
o Particle Size Distribution
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o Particle Shape
Particles exiting from the LLDPE fluid bed reactor
contain agglomerates of smaller particles as very
irregularly shaped particles. By subjecting the particles
to an intensive mixer, both the particle size distribution
and the particle shape are improved. The mixing action
breaks up the large agglomerates resulting in a downward
shift in the particle size distribution. (The average
particle size is reduced by at least 25% and preferably by
lo at least 50%). The heating of the granule surface aids in
the particle shape due to the mixing action and subsequent
polishing. The sharp edges are smoothed out, and other
irregular shapes may be brought to resemble more rounded or
spherical granules. The combination of breaking down large
agglomerate and rounding the particle results in better
packing and thus increased bulk densities. Moreover, the
polishing action avoids formation of any hairs or tails that
could impair flyability and decrease bulk density.
In rotomolding applications it is highly desirable
that the granules be substantially spherical and have a
narrow particle size distribution and small average particle
size. The residence time in the mixer affects all of these
properties.
The operating temperature is a function of residence
time that is, the longer the residence time, the more
kinetic energy is expended causing an increase in resin
temperature. It has been found that the best results are
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obtained with rotational molding grade LLDPE processed at
resin temperatures between 150F-250F, preferably 230F and
240F. (65-121C, 110-115C respectively).
Operating the intensive mixer to cause the resin to
reach this temperature produces an LLDPE granule having a
particle size distribution as follows:
larger than 30 mesh less than 20 wit%
smaller than 100 mesh less than 25 White,
preferably less than 15 wit%
lo After screening with a 30 mesh screen, the granules
- exhibit a flyability of greater than 3.6 g/sec. based on
ASTM D 1895-69 test method. The finished product is free
flowing and is ready for use in rotational molding
operations.
EXAMPLE 1
AMP A (Invention)
2,000 Pam Cyasorb UV531 (Trademark) (marketed by
American Cyanamid, fusion temperature 118-120F, i.e.,
48-49C), an organic I. V. stabilizer, and 500 Pam Irganox
1076 (Trademark) (marketed by Cuba Gerry Co., fusion
temperature 122-131F, i.e., 50-55~C), an organic
stabilizer, were placed into a one liter capacity ~Gelimat"
(Trademark) intensive mixer with LLDPE of 5 melt index and
0.935 density. The mixer was operated at a tip speed of 39
meters per second. The temperature in the mixer reached
230F (110C) after approximately 10 seconds then decreased
to 158F (70C) (one minute) the product was a free flowing
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granular powder.
SIMPLY (Comparison)
For comparison a sample of the same components in
the same proportions was prepared by dry blending.
Plaques were pressed from both products and placed
in a weatherometer to accelerate the weathering test.
Protection against TV light is assessed by measuring the %
elongation (ASTM D 638) after exposure in the weatherometer
for different set periods. A sample is "passed" if it
lo retains a minimum of 50% of its original elongation after
1,000 hours in the weatherometer.
The test results were:
Hours In Weatherometer: 0 500 1,000
% Retained Of Original Elongation:
Sample A (Intensive) 100 85 70
Sample B (Dry Blend) 100 5 --
Sample A product was not sieved to the preferred
particle sizes, however, the utilization of the entire
product demonstrates the superior character of the present
claimed product on the physical properties of a molded
product.
EXAMPLE
Sample C (Invention)
2,000 Pam Weston 619 (Trademark) (marketed by Borg
Warner Corp., fusion temperature 104-158F, i.e., ~0-70C)
an organic stabilizer, 1,500 Pam Cyasorb 513 (Trademark) and
500 Pam Irganox 1076 (Trademark) were mixed with the same
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LLDPE as in example 1 in a "Wedco" (Trademark) intensive
mixer. The product was a free flowing white granular
product with the particle size distribution shown below.
SAMPLE D (Commercial Ground)
For comparison, particle size distribution and dry
flow characteristics of a commercially available ground
product for rotomolding with unknown amounts and types of
additives was tested.
Dry Flow Time twined On (Mesh) _ _
lo ASTM D 1985 40 60 80 100 Pan
Sample C
(Intensive) 5 Seconds 8.0 41.0 17.0 10.0 24.0
Sample D
(Grenada Seconds 9.0 40.0 22.0 8.0 21.0
Resin) _
Both Sample C and Sample D were rotomolded into finished
articles.
Testing of both products yielded the following results:
_ _ . . . _ . . _
ESQUIRE Cold Temp. % Rutted Of Trig. Long.
ASTM Impact After Weathers~mç~-er Exp.
D 1693 ARM Test
to It 0 us S00 us lL000 Ho
Sample C
(Intensive) >1,000 Ho 59 100 60 60
Sample D
(Ground)
(Product) >1,000 Ho 54 100 100 90
It should be rooted that the particle size
distribution-of the present invention (Sample C) without
screening is very similar to the ground commercial sample.
The flyability of the present invention is far better than
16
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the ground product. The rotomolded products were very
similar in properties with both passing the weatherometer
test. It is possible that Sample D contained larger amounts
of US stabilizer.
