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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 ~L2311Z7~L

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 ~LZ3~27~

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 lZ39Z7~

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, ~3~2~

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 ~LZ30~

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, ~;23~Z7~

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 ~L~3~Z7~L

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 ~L230~

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 ~3(1 27~1L

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 ~23(~;~7~

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.

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 I I

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 ~23~2~

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 ~235~

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.