Method of making polymer powders and whiskers as well as particulate products of the method and atomizing apparatus

Method for making polymer particulates, such as spherical powder and whiskers, by melting a polymer material under conditions to avoid thermal degradation of the polymer material, atomizing the melt using gas jet means in a manner to form atomized droplets, and cooling the droplets to form polymer particulates, which are collected for further processing. Atomization parameters can be controlled to produce polymer particulates with controlled particle shape, particle size, and particle size distribution. For example, atomization parameters can be controlled to produce spherical polymer powders, polymer whiskers, and combinations of spherical powders and whiskers. Atomizing apparatus also is provided for atoomizing polymer and metallic materials.

FIELD OF THE INVENTION
 The present invention relates to a method for atomizing molten polymeric
 materials to produce fine polymer particulates and to the polymer
 particulates, such as polymer spherical powders, fibers and whiskers,
 formed by such atomization and to atomizing apparatus for polymers,
 metallic and other materials.
 BACKGROUND OF THE INVENTION
 Presently, commercial synthetic organic polymer powders are made by
 grinding of extruded polymer pellets often under cryogenic temperature
 conditions. Grinding is undesireable as a result of being highly
 energy-intensive and sensitive to contamination from the grinding
 equipment used and from environmental pollution. Due to the erratic nature
 of the grinding process, it is practically impossible to controlling
 quality, particle size, and distribution of polymer powders. Ball milling
 of extruded polymer pellets also suffers from similar problems.
 Some polymer materials, such as ultra-low molecular weight polyethylene,
 cannot even be ground or ball milled to form particulates as a result of
 their waxy nature.
 An object of the present invention is to provide a method for making
 polymer particulates from polymer materials in a manner that overcomes the
 aforementioned problems heretofore associated with grinding or ball
 milling to produce commercial polymer powders.
 Another object of the present invention is to provide a method for making
 polymer particulates from polymer materials that heretofore could not be
 ground or ball milled to particulate form.
 Still another object of the present invention is to provide a method for
 making polymer particulates by gas atomizing molten polymeric material in
 a manner to provide controlled particle shape, particle size and particle
 size distribution.
 Still a further object of the present invention is to provide a method for
 gas atomizing molten polymeric material to form polymer particulates
 without the need for the addition of flow modifiers, such as oils and
 greases or molybdenum disulfide, to the polymer material to lower
 viscosity.
 Still an additional object of the present invention is to provide atomized
 polymer particulates with particle shape, particle size and particle size
 distribution controlled as desired.
 Another object of the present invention is to provide atomized polymer
 particulates which heretofore could not be produced on a mass-production
 basis by grinding or ball milling of extruded material.
 Still a further object of the present invention is to provide atomized
 polymer particulates of improved quality with reduced contamination from
 flow modifiers and other additives heretofore used.
 A further object of the invention is to provide improved atomizing
 apparatus for gas atomizing fluid polymer materials, inorganic materials,
 metallic materials and others.
 SUMMARY OF THE INVENTION
 The present invention n involves a method for making polymer particulates
 by providing a fluid (e.g. molten) polymer material, which may include
 virgin polymer material and/or recycled polymer waste material, under
 conditions to avoid thermal degradation of the polymer material, atomizing
 the melt using gas jet means in a manner to form atomized droplets, and
 treating (e.g. cooling) the droplets to form solid polymer particulates,
 which are collected for further processing. Certain atomization parameters
 can be controlled for a given polymer material to produce polymer
 particulates with controlled particle shape, particle size, and particle
 size distribution. For example, atomization parameters can be controlled
 to produce polymer spheres, polymer fibers or whiskers, and combinations
 thereof.
 In an illustrative embodiment of the present invention, polymer material is
 heated in a melting vessel to an atomization temperature under an inert or
 other non-reactive gaseous atmosphere effective to increase the thermal
 degradation temperature of the polymer material. The gaseous atmosphere is
 used to raise the thermal degradation temperature of the polymer melt. The
 atomization temperature is selected to be between the polymer melting
 temperature and the raised thermal degradation temperature so as to avoid
 polymer thermal degradation at the atomization temperature.
 A stream of molten polymer is supplied from the melting vessel to an
 atomizing nozzle typically by providing gas pressure on the polymer melt
 in the melting vessel to force it through a melt supply or pour tube to
 the atomizing nozzle. The polymer stream is supplied to the atomizing
 nozzle where one or more gas jets are directed at the molten polymer
 stream to disintegrate it to form fine polymer droplets. The atomized
 polymer droplets fall by gravity and rapidly cool to form solidified
 polymer particulates that are collected in a collection chamber below the
 atomizing nozzle.
 The present invention also provides atomized polymer particulates that can
 comprise spherical polymer powders, polymer fibers or whiskers, and
 combinations thereof. Spherical polymer particulates can be atomized in
 the size range of about 200 microns or less mean particle size. Spherical
 particles in the size range of about 5 to about 200 microns diameter are
 especially useful as a product powder of the invention for a wide variety
 of commercial applications, such as paint additives, sprayable coating
 materials and the like. Fiber or whisker shaped polymer particulates
 having a diameter of 30 microns or less and length of about 1 millimeter
 to 1 centimeter also can be produced by the invention. Semicrystalline
 and/or amorphous polymer particulates can be made by the atomization
 method.
 The present invention is advantageous in that polymer particulates can be
 made from polymers which heretofore could not be ground or ball milled to
 this end due to their waxy nature. Moreover, polymer particulates can be
 made with controlled particle shape, particle size and particle size
 distribution without the need for the addition of flow modifiers, such as
 oils and greases or molybdenum disulfide, to the polymer material to lower
 viscosity and with reduced energy consumption compared to grinding and
 ball milling. As a result, atomized polymer particulates can be made
 having improved quality with reduced contamination. Polymer particulates
 can be made from virgin polymer materials and/or recycled polymer waste
 materials.
 The present invention also involves atomizing apparatus comprising means
 for discharging atomizing gas and a supply tube for molten material to be
 atomized by the atomizing gas wherein the supply tube includes a discharge
 end positioned proximate the atomizing gas and having a plurality of
 channels for improving distribution of the molten material toward the
 atomizing gas for atomization thereby in a manner to control particle
 size, particle shape and particle size distribution. The atomizing
 apparatus can be used to atomize polymer materials, inorganic materials,
 metals and alloys, and other molten or liquid materials.
 The above objects and advantages of the present invention will become more
 readily apparent from the following detailed description taken with the
 following drawings.

DETAILED DESCRIPTION OF THE INVENTION
 FIG. 1 schematically illustrates an embodiment of the present invention for
 making polymer particulates wherein apparatus is used including a melting
 zone M where the polymer is melted in a melting vessel or crucible 10, an
 atomization or disintegration zone or chamber A in the form a drop tube
 where the polymer melt is atomized as a spray of droplets and solidified,
 and a collection zone C having a hopper CH for directing the solidified
 atomized polymer particulates into a collection chamber CC. A support
 frame F having steel floor pads P is provided to support the apparatus on
 the floor.
 In stage 1 of a method embodiment illustrated in FIG. 1, the meltable
 organic polymer is initally placed in the crucible 10 in the form of solid
 polymer pellets or irregular chunks and heated to above the melting
 temperature of the polymer under an inert or other atmosphere that is
 selected to increase the polymer thermal degradation temperature. The
 polymer pellets and chunks can comprise virgin polymer material and/or
 recycled waste polymer material from a recycling faciltiy. Positive
 nitrogen (or other gas) gas pressure is used in stage 2 to transport the
 molten polymer to atomization stage 3 where the molten polymer is
 disintegrated into submicron size droplets by atomizing nozzle 30 of the
 type described in U.S. Pat. No. 5,228,620, the teachings of which are
 incorporated herein by reference. For melting and transport of high
 melting temperature aromatic polymers, such as ultra high molecular weight
 polyethylene (UHMWPE), polyphenylene oxide, polyphenylene sulfide, and
 polystyrene, that have high viscosities, an auger screw feed system (not
 shown) with controlled heating zones can be used to feed molten polymer to
 atomization stage 3. The atomization chamber A can be contructed of
 crystal clear polycarbonate (e.g. Lexan polycarbonate) to allow real-time
 observation of the disintegration process. If generally spherical powders
 are the desired particulates to be produced, high energy disintegration of
 the polymer melt is effected by using high atomizing gas pressures in the
 range of 800-1100 psi for polyethylene and like polymers. Lower atomizing
 gas pressures in the range of 300-800 psi can be used for making polymer
 fiber or whisker shaped particles by a lower energy disintegration
 mechanism. The atomized polymer droplets are allowed to drop under force
 of gravity of their own weight in chamber A and under the influence of the
 expanding high-pressure gas into the collection chamber CC. The solidified
 atomized polymer particulates can be collected from the collection chamber
 for further processing, which may include size classification by sieving,
 pneumatic size classification or hydraulic size classification, and
 packaging, densification/compaction.
 Referring to FIGS. 2, 3, and 4, the invention provides a high yield method
 for making polymer particulates that involves melting of the polymer in a
 crucible 10 under a controlled inert gas atmosphere that is selected to
 increase the polymer thermal degradation temperature. The crucible 10
 comprises a 3 inch in diameter by 10 inches long by 0.25 inch thick
 cylindrical pipe 10b and a 0.20-0.25 inch thick stainless steel base plate
 10a welded to the end of the pipe 10b. A 3/4 inch tapped hole is provided
 in the center of the base plate 10a. A stainless steel pour tube 12 is
 threaded into the tapped hole in the base plate 10a. The other end of the
 crucible 10 includes two 1/4 inch holes therein equipped with NPT hose
 barb fitting 10c for an inlet pressurizing gas (e.g. nitrogen) from a
 source S1, such as a standard laboratory gas cylinder, and similar fitting
 10d for outlet gases. The outlet fitting includes a needle shut-off valve
 10e that leads to an air vent when the needle valve is open. The inlet
 pressurizing gas is used initally for producing an inert atmosphere
 relative to the polymer to be melted by displacing oxygen (air) from the
 crucible 10. The outlet gas fitting 10d is connected to the air vent to
 allow for removal of the displaced oxygen (air) when the needle valve is
 open.
 To apply positive pressure on the molten contents or charge of polymer in
 the crucible 10, the crucible is fitted with a threaded stainless steel
 cap 10f equipped with a Viton o-ring seal 10g located between the cap and
 crucible end. The cap 10f is threaded onto an outer threaded surface of
 the upper end of the crucible 10 to effect an airtight seal using the
 o-ring seal 10g. The needle valve associated with the outlet 10d is closed
 when the crucible 10 is pressurized.
 Three pressure-tight brass feedthroughs 10k are silver soldered to the top
 of the cap 10f. As shown in FIG. 3, a feedthrough at the center of the cap
 10f receives a stainless steel stopper rod 15 that is connected to a
 conventional pneumatic actuator 15a located above the cap 10f on support
 frame F and controlled by a solenoid control SC. The other feedthroughs
 receive a thermocouple T1 and a conventional motorized stir bar 16 with
 two blades 16a oriented to pump polymer melt toward the bottom of the
 crucible 10. The stir bar 16 is connected to a conventional 0.13
 horsepower electrical motor (not shown) located above the cap 10f on the
 support frame F. The polymer is melted in the crucible 10 using an 800
 Watt/240 Volt band heater 20 attached by screw clamps, nuts/bolts or other
 fastening means on an outside of a lower base region of the crucible 10. A
 thermocouple T2 is provided between the band heater 20 and the crucible 10
 and connected by wires W2 to a conventional temperature controller (not
 shown). The thermocouple T1 is a temperature probe and also is connected
 by wires T1 to the temperature controller to provide a temperature control
 system for the band heater 20. A third thermocouple (not shown) can
 comprise thermocouple wire in the stopper rod 15 to monitor temperature of
 the melt exiting the crucible 10. The third thermocouple is connected by
 wires W3 to a separate temperature monitor.
 The motorized stir bar 16 having blades 16a is used to stir and homogenize
 the polymer melt in the crucible 10 and to remove temperature gradients
 prior to gas atomization. The stir bar 16 is inserted into the polymer
 melt through one of the brass feedthroughs located on the crucible cap or
 lid 10f as described above.
 Prior to atomization of the polymer melt, premature flow of the polymer
 melt through the pour tube 12 is prevented by the pneumatically actuated
 stopper rod 15 that is seated or sealed on a seat 12a of the pour tube 12.
 To this end, the pour tube 12 includes an enlarged upper end that includes
 the seat 12a for the stopper rod 15 to prevent the polymer melt from
 entering the pour tube. A melt supply passage 12b extends through the pour
 tube such that when the stopper rod 15 is lifted, the melt supply passage
 12b allows the polymer melt to flow from the crucible 10 to the atomizing
 nozzle 30, while keeping the melt at a near constant temperature as a
 result of the stainless steel pour tube being a relatively poor thermal
 conductor. This allows the polymer melt to remain at a near constant
 temperature without melt freeze-up in the pour tube 12 before it contacts
 the cold (-90 degrees C.) supersonic atomization gas discharged from the
 atomizing nozzle apparatus 30.
 The polymer melt exits from the passage 12b at the open bottom end 12c of
 the pour tube 12 and flows radialy outward and contacted by the atomizing
 gas from the nozzle apparatus 30. The melt supply tube 12b can have an
 inner diameter of 1/16, 1/8 or 3/16 inch for purposes of illustration and
 not limitation.
 The pour tube 12 can be provided with a flat annular bottom end 12c for
 radial polymer melt flow from the passage 12b toward the atomizing gas
 jets from the atomizing apparatus 30. The flat, annular open bottom end
 12c extends perpendicular to the longitudinal axis of the pour tube.
 Alternately, in accordance with another embodiment of the invention, the
 open bottom end 12c of the pour tube 12 can have a plurality of radially
 extending channels or gutters 12d machined therein using a Woodruff cutter
 for improving distribution of the molten polymer (or other molten or fluid
 material) as a uniformly distributed, segmented flow toward the atomizing
 gas jet discharge orifices 30c. In the pour tube embodiment illustrated in
 FIGS. 7A and 7B, the number of channels 12d (e.g. 20 channels) will be
 equal to the number of gas jet discharge orifices 30c (e.g. 20 discharge
 orifices 30c) of the atomizing nozzle apparatus 30. The channels or
 gutters 12d are rectangular shaped in plan view, FIG. 7B, and scallop
 shaped (undercut) in cross section view, FIG. 7A, and are spaced
 circumferentially around the open bottom end 12c to provide space and
 direction of the polymer melt to flow radially outward and form a thin
 film as the molten polymer leaves the edge of the bottom end 12c toward
 the atomizing gas jets from nozzle 30. For purposes of illustration only,
 the channels or gutters each can have a width dimension of 0.020 inch and
 a maximum depth dimension (in the axial direction of the pour tube) of
 0.078 inch by radial undercut centered on the longitudinal axis of the
 pour tube 12. Twenty (20) channels are provided about the bottom end of a
 0.125 inch inner diameter and 0.325 outer diameter pour tube. In the
 embodiment illustrated in FIGS. 7A and 7B, the width dimension of the
 channels 12d preferably should not exceed the diameter of the discharged
 orifices 30c in order to insure the liquid polymer is directed to the
 region of maximum gas velocity in the gas flow pattern when aligned with
 the position of the individual discrete gas jets exiting from orifices
 30c. The molten meterial distribution channels 12d thereby eliminate
 uncontrolled wandering of the molten material on the flat bottom surface
 of the bottom end 12c of the pour tube as the material travels radially
 toward the circumferential edge of the bottom end 12c, such wandering
 being discovered to result in non-uniform distribution of the molten
 material on the bottom end 12c where regions can be either starved or
 overloaded with molten material.
 By changing the alignment of the radial gutters 12d relative to the gas jet
 discharge orifices 30c of nozzle 30, the sphere to fiber ratio of the
 atomized polymer particulates can be varied, making it possible to
 optimize the output of micro fibers, if desired. The radial gutters 12d
 are considered aligned with the atomizing gas jets when the twenty (20)
 gutters 12d are aligned in a common vertical plane with the twenty (20)
 gas jet discharge orifices 30c of the nozzle 30. The radial gutters 12d
 are considered out of alignment with the atomizing gas jets when the
 twenty (20) gutters 12d are offset relative to the respective twenty (20)
 gas discharge orifices 30c of the nozzle 30 as a result, for example, of
 the pour tube 12 being rotated or displaced circumferentially relative to
 the nozzle orifices 30c. Rotation of the pour tube 12 can provide in situ
 control over the yields of fine and coarse powder and also particle sizes
 therebetween and can be effected by a suitable pour tube actuator to this
 end.
 In the embodiments of the atomizing nozzle apparatus 30 illustrated, the
 local ambient pressure on the bottom end 12c of the pour tube 12 is lower
 than that on the top of the melt in the crucible. The pressure
 differential ensures that the liquid melt will be drawn down to the
 atomizing zone of the nozzle 30. The low pressure zone at the bottom
 surface of end 12c encourages gas recirculation wherein the gas flowing
 from the orifices 30c reverses its downward flow direction to flow upwards
 along the center axis of the nozzle 30 toward the melt exit opening of
 passage 12b on the bottom surface of pour tube end 12c. The recirculating
 gas then turns and flows radially parallel to the bottom surface of the
 pour tube end 12c. The gas recirculation forces the liquid polymer to flow
 radially on the bottom surface or in channels 12d towards the
 circumferential edge of the end 12c in a manner to promote filming of the
 liquid melt, which filming facilitates the action of the high velocity gas
 flowing to cause film instabilites and atomization of the melt into
 droplets.
 Use of the pour tube channels 12d provides a uniform melt thickness for
 such atomization and provides spatial control over the local gas-to-melt
 ratio for uniform gas atomizaton of the melt. Thus, a uniform and narrow
 particle size distrbution of polymer powder (and also metallic powder) can
 be achieved by practice of this embodiment of the invention to this end.
 The pour tube 12 is disposed on a aluminum plate 11 that is positioned on
 support frame F and separates the melting zone M from the atomization zone
 or chamber A. Multiple ceramic washers 11a are disposed between the
 crucible and the plate 11 to reduce heat loss from the crucible.
 To charge the crucible, the solid polymer material to be atomized is placed
 in the crucible 10 after removal from the plate 11 and with the cap 10f
 removed from the crucible. The stopper rod 15 is engaged on the pour tube
 seat 12a and the pour tube 12 is screwed on the crucible at this time. The
 crucible with solid polymer charge therein then is positioned back on the
 plate 11, and the cap 10f is screwed thereon in provide an air-tight seal.
 The stopper rod 15 is connected to its pneumatic actuator, and the stir
 bar 16 is connected to its drive motor. The nitrogen inlet and outlet gas
 conduits are connected to the fittings 10c, 10d.
 In a typical exemplary heating sequence (stage 1), the band heater 20 is
 set to 150 degrees C. for polyethylene based polymers described below in
 the Examples (or other heater setting for other different polymers to be
 melted) and the temperature controller connected to thermocouples T1, T2
 is programed to heat the polymer material in the crucible 10 at a rate of
 10 degrees per minute to near the polymer melting temperature for the
 polyethylene based polymers (or other heating rate for other different
 polymers to be melted). The polymer melt is allowed to remain near the
 polymer melting temperature for about 30 minutes. The stir bar 16 then is
 turned on to aid in the melting of the polymer material. The crucible
 pressurizing gas is turned on and allowed to run from source S1 into the
 crucible for about 5 minutes in order to displace any oxygen (air) in the
 crucible, thereby creating an inert or non-reactive atmosphere therein.
 The band heater temperature then is increased to approximately 200 degrees
 and is allowed to remain at this temperature for 30 minutes for
 polyethylene based polymers described below (or other heating conditions
 depending on the particular polymer to be melted). After 30 minutes, the
 band heater temperature is increased to a temperature near 260 degrees C.
 (or other temperature depending on the particular polymer to be melted)
 and remains there until the polymer material melt in the crucible reaches
 the selected atomization temperature.
 Once the polymer melt reaches the selected atomization temperature, the
 stopper rod 15 is pressured by its pneumatic actuator to seal on the pour
 tube seat 12a. Then, the atomization gas is supplied from the 6000 psi
 nitrogen canister or other source S2 to the atomizing nozzle 30 via
 conventional dome regulator R1, pneumatic high pressure valve VV and high
 pressure stainless steel gas line L2. The dome regulator itself is
 pressurized from another high pressure source S3 to provide a dome
 diaphragm pressure that is adjusted via a high pressure hand regulator R2
 to correspond to the atomization gas pressure.
 The crucible 10 is pressurized (e.g. about 15 psi) with nitrogen or other
 pressurizing gas via a flexible rubber low pressure gas line L1 typically
 from the same source S1 used to initally displace oxygen from the
 crucible.
 Once the crucible is pressurized, the stopper rod 15 is lifted from the
 pour tube seat 12a, and the molten polymer is forced in stage 2 by the
 prevailing crucible gas pressure through the pour tube 12 for atomization
 in stage 3 by the atomizing nozzle 30 shown in FIG. 4. Ultra-high purity
 (99.99% purity) nitrogen, helium, or other atomizing gas is turned on via
 valve VV once initial the initial polymer melt flow starts from the
 atomizing nozzle 30, FIG. 5a.
 The atomizing nozzle 30 is of the close-coupled, discrete jet type
 described in U.S. Pat. No. 5,228,620 (e.g. Example 2 thereof), the
 teachings of which are incorporated herein by reference to this end. The
 atomizing nozzle 30 is attached to the plate 11 by multiple screws SS. The
 atomizing nozzle includes a nozzle body 30a having a bore 30b that
 receives the pour tube 12 and a plurality of gas jet discharge orifices
 30c (e.g. 20 gas jet discharge orifices with a diameter of 0.0292 inch)
 through which supersonic nitrogen atomizing gas is discharged at the flow
 of polymer melt from the pour tube 12. The nozzle body includes a gas
 inlet manifold 30d having dimensions described in Example 2 of U.S. Pat.
 No. 5,228,620 for receiving the atomizing gas from a line L2 leading to a
 atomizing gas source S2, such as the 6000 psi nitrogen canister, and
 supplying it to the orifces 30c.
 The present invention is not limited to the particular atomizing nozzle 30
 described hereabove and shown in the drawings and can be practiced using
 other types of gas atomizing nozzles such as discrete gas jet nozzles,
 converging/diverging gas jet nozzles, close-coupled annular slit nozzles,
 and the like.
 Upon contact with the atomization gas, the molten polymer is atomized in
 stage 3 into a spray of fine droplets, FIG. 5b. These droplets are
 subsequently cooled in air in the atomization chamber A, which is 2 feet
 by 2 feet in cross-section and 6 feet in length. The droplets solidify and
 fall of their own weight in chamber A in stage 5 and then fall into the
 collection chamber CC in stage 6 as fine polymer particles whose shape,
 size and size distribution depends on the nature of the polymer atomized
 and the atomizing conditions used. The collection chamber CC rests on the
 floor and is constructed of sheet metal with an outlet OT for the
 atomization gas to exit. The collection chamber CC includes an internal
 dividing wall W that acts as a baffle for the atomization gas and also a
 collection baffle for separation of the small polymer particles which are
 entrained in the gas stream.
 The temperature range in which a polymer material can be atomized pursuant
 to the invention is determined by the temperature at which the polymer
 thermally degrades. Degradation occurs when the chains of the polymer
 molecule break and begin to disassociate, thus resulting in possible
 hazardous material conditions. During degradation, fumes may be emitted
 along with possibility of the material ignition.
 Pursuant to the invention, heating of the polymer material in the crucible
 10 is conducted in an atmosphere inert or non-reactive to the polymer in
 order to increase the degradation temperature as compared to the thermal
 degradation temperature of the same polymer material when heated in
 ambient air where oxygen can react with the polymer. With respect to
 polyethylnene and other common polymers in widespread use, a nitrogen gas
 atmosphere can be used to increase the thermal degradation temperature in
 the crucible 10 and also to force the polymer melt through the pour tube
 12 for atomization. The invention is not limited to nitrogen gas for these
 purposes since other gases, such as inert gases (Ar/He), can be used to
 these ends.
 In practicing the invention, the melting temperature of the polymer
 material corresponds to the temperature at which a sample of the material
 undergoes an endothermic reaction in thermogravimetric/differential
 thermal (TG/DTA) analysis. The initial thermal degradation temperature is
 considered to be the temperature at which the polymer material first
 experiences a decrease in mass due to thermal degradation. The thermal
 degradation temperature is the temperature at which the polymer material
 shows a 50% decrease in mass in TG/DTA analysis.
 For some polymers, the thermal degradation temperature occurs shortly after
 the initial thermal degradation temperature (i.e. within 100 degrees C.),
 while for others it may be much later (e.g. 250 degrees C.). This
 information is useful to determine the temperature range in which a
 polymer material can be gas atomized pursuant to the invention. Typically,
 polymer materials are atomized pursuant to the invention at temperatures
 approaching the initial thermal degradation temperature, depending upon
 the type of particulate product desired, however.
 Knowledge of the thermal properties of the polymer material is useful in
 characterizing a polymer material for specific atomizing conditions
 pursuant to the invention. For example, it is known that linear
 polyethylene has a sharp melt point with 70% of the crystallinity within
 the polymer disappearing in a 3-4 degree C. interval, while branched
 polyethylene melts over a wide temperature range with 60% of its
 crystallinity disappearing over a 40 degree C. interval. An advantage of
 using the TG/DTA analysis to characterize the polymer to be atomized is
 that it has the capability of heating the material under both atmospheric
 and inert gas conditions that is helpful in determining temperature
 effects of oxidation on the particular heated polymer, since for many
 polymer materials, oxidation leads to degradation.
 Information obtained from differential scanning calorimetry (DSC) also is
 useful to determine the glass transition temperature T.sub.g, the melting
 temperature T.sub.m, the degradation temperature, and the crystallization
 temperature T.sub.c of the polymer material to be atomized.
 By knowing the heat of crystallization upon cooling (the area under the DSC
 curve for crystallization exotherm peak) and comparing the area to that of
 a 100% crystalline sample, the percent of crystallinity of a polymer
 sample can be determined. Knowledge of the crystallization temperature
 T.sub.c helps in practicing the invention to insure that the polymer melt
 does not solidify in the pour tube 12 prior to atomization and can be
 determined using a commercially available ParPhysica Rheometer by cooling
 a polymer sample at a temperature above the melting temperature T.sub.m at
 a constant shear rate to the crystallization temperature where viscosity
 of the sample sharply increases.
 The Table below sets forth T.sub.g, T.sub.m, T.sub.c, initial degradation
 temperature in inert gas and in compressed air, and degradation
 temperature determined for PE130 and PE520 polyethylene based polymers
 (available from Hoechst-Celanese) atomized in the Examples set forth
 below.
 TABLE
 The thermal data obtained from TG/DTA, DSC, and the ParPhysica Rheometer
 Initial Degradation Initial
 Degradation
 T.sub.c T.sub.c Temperature: Temperature:
 Degradation
 T.sub.g.sup.1 T.sub.m.sup.2 Physica DSC Inert Atmosphere.sup.3
 Compressed Air.sup.4 Temperature
 Material (.degree. C.) (.degree. C.) (.degree. C.) (.degree. C.) (.degree.
 C.) (.degree. C.) (.degree. C.)
 PE 130 -112 125 110 120 200 200 460
 PE 520 -109 120 105 115 220 200 460
 .sup.1 The glass transition temperature was found by DSC.
 .sup.2 The melt temperature was found by both TG/DTA and DSC.
 .sup.3 Nitrogen was used as the inert gas at a flow rate of 200 ml/min.
 .sup.4 Compressed air was used at a flow rate of 200 ml/min to determine
 the effects of oxidation.
 In general, in practicing the invention, atomization parameters can be
 controlled to produce desired atomization results. For example, the
 atomization gas pressure greatly affects the atomization results in that
 at low atomization gas pressure, the shear induced by the gas jets on the
 molten polymer material exiting from the pour tube 12 may not be enough to
 completely overcome the internal stresses within the polymer mateiral.
 Hence, elongated spheroids or fibers or whiskers can be formed, rather
 than spherical particles. The atomization gas pressure can be increased to
 increase the percentage of atomized spherical particles versus atomized
 fiber or whisker particles for example.
 The temperature of the polymer melt also exerts a large influence on the
 atomization results. For example, there exists a temperature window in
 which a particular polymer material can be atomized, but within this
 window the resulting particle size distribution of the atomized particles
 can vary greatly. Temperature of the polymer melt does not appear to
 affect the shape of the atomized particles, however, but use of the
 channels 12d on the pour tube 12 does.
 Since the viscosity of the polymer melt decreases with increasing
 temperature, the particle size distribution of the atomized polymer
 particles will vary with different atomization melt temperatures.
 Typically for a given polymer material, increasing the polymer melt
 temperature will produce a narrowing of or unimodal particle size
 distribution.
 The size (e.g. diameter) of the polymer melt stream is determined by the
 inner diameter of the pour tube 12 and can be controlled to have a direct
 effect on the particle size distribution of the atomized polymer
 particulates. Typically for a given polymer material, increasing the pour
 tube inner diameter will produce a broadening of or bimodal particle size
 distribution.
 In addition, the density, viscosity, molecular weight, elasticity, and
 surface tension of the polymer material affect the atomization results.
 The atomization parameters discussed hereabove can be controlled for
 particular polymer properties to produce desired atomization results. For
 purposes of illustration only, for a PE 130 polyethylene base polymer
 described in the Examples below, a maximum yield of atomized particulates
 was obtained using a 0.125 inch inner diameter pour tube at a polymer melt
 temperature of 185 to 205 degrees C. using a flat bottom pour tube 12. A
 maximum yield of particles less than 53 microns in size was achieved at
 205 degrees C. using a 0.125 inch inner diameter pour tube using a flat
 bottom pour tube. Particles less than 53 microns in diameter will find
 widespread commercial use as paint additives, sprayable coating materials,
 ink additives, cosmetic additives, paper fillers, self-reinforced
 composites and the like.
 For PE 520 polyethylene based polymer described in the Examples below, a
 maximum yield of atomized particulates was obtained using a 0.125 inch
 inner diameter pour tube at a polymer melt temperature of 205 to 215
 degrees C. using a flat bottom pour tube. A maximum yield of particles
 less than 53 microns in size was achieved at 215 degrees C. using a 0.0625
 inch inner diameter flat bottom pour tube. By using a 0.125 inch inner
 diameter gutter bottom pour tube and melt temperature of 220 degrees C.,
 the yield of particles less than 53 microns in size for PE 520 was
 increased.
 EXAMPLES
 As mentioned, the shape, size and size distribution of the polymer
 particulate product of the invention depends on the atomization or
 processing conditions and the nature of the polymer used. Spherical powder
 particulates, whiskers or fibers, and a mixture of both can be produced in
 practice of the invention. Specific examples are described below. Although
 three grades of commercial polyethylene polymers have been atomized and
 are described in Examples 1-3, this invention is applicable to other
 polymers that can be melt-processed at temperatures of 450 degrees C. and
 below and having suitable melt deformation (elasticity) and flow
 (viscosity) properties for atomization.
 Example 1
 The pressure crucible 10 in FIG. 3 was used to heat 125.4 grams of a
 polyethylene-based material (AC-6) produced by Allied Signal Inc. The AC-6
 is an ultra-low molecular weight low density polyethylene. The melt
 temperature for this material was approximately 103 degrees and the
 initial degradation temperature in an inert atmosphere, such as nitrogen
 was 250 degrees C. as determined by differential thermal analysis and
 thermogravitmetric analysis. The polymer material was heated from room
 temperature to 144 degrees C. over a 1 hour period. The crucible was
 pressurized to 20 psi using nitrogen gas to force the molten polymer
 through the flat bottomed pour tube of 1/8 inch inner diameter. Ultra-high
 purity (99.99% purity) nitrogen gas at 300 psi was applied to the molten
 polymer at the exit of the pour tube. The atomizing time was 10 seconds.
 The resulting atomized polymer particulates contained a mixture of
 elongated spheroids trapped in a web of fine fibers. The polymer fiber to
 polymer spheroid ratio was found to be 2:1. The atomization pressure was
 enough to form the polymer spheroids, but not high enough to break the
 elongated polymer spheres away from the polymer fibers. Due to the
 entanglement of the elongated spheres and fiber, size classification of
 the particulate product was not possible.
 Example 2
 The pressure crucible 10 in FIG. 3 was used to heat 110.2 grams of a
 polyethylene-based material (AC-6) produced by Allied Signal Inc. The
 polymer material was heated from room temperature to 195 degrees C. over a
 130 minute period. The crucible was pressurized at 15 psi using nitrogen
 gas to force the polymer through the flat bottomed pour tube of 1/8 inch
 inner diameter. Ultra-high purity (99.99% purity) nitrogen gas at 900 psi
 was applied to the molten polymer at the exit of the pour tube. The
 atomizing time was 20 seconds. Relative to the particulate product of
 Example 1, the resulting atomized polymer particulates contained more
 distinct spheres ranging in size from 10-150 microns diameter, and fewer
 fibers. The ratio of the fibers to the spheroids was found to be 1:1. Some
 of the spheroids were still entangled in the fibers, but there were fewer
 fibers than in Example 1, thus making optical classification with the use
 of a scanning electron microscope possible.
 Example 3
 The pressure crucible 10 in FIG. 3 was used to heat 297.7 grams of a
 polyethylene-based material Hoechst Wax PE 130 produced by
 Hoechst-Celanese Corporation. The Hoechst Wax PE 130 is an ultra-low
 molecular weight (2000 grams/mole) high density (0.98 grams/cubic
 centimeters) polyethylene. The polymer material was heated from room
 temperature to 197 degrees C. over a 65 minute period. The crucible was
 pressurized to 18 psi using nitrogen gas to force the molten polymer
 through the flat bottomed pour tube of 1/8 inch inner diameter. Ultra-high
 purity (99.99% purity) nitrogen gas at 900 psi was applied to the molten
 polymer at the exit of the pour tube. The atomizing time was 20 seconds.
 The resulting atomized polymer particulates contained mostly spheres with
 few fibers attached to larger spheres. The polymer fiber to polymer
 spheroid ratio was found to be 1:2. Many of the smaller spheres were
 either attached to larger spheres or were connected to other small spheres
 to form small agglomerates of spheres. Optical clssification using a
 scanning electron microscope showed a size distribution similar to that
 obtained in Example 2. The sizes of the spheres ranged from 10-150 microns
 diameter, FIG. 6a. The higher density or crystallinity of the PE 130
 polymer relative to AC-6 polymer used in examples 1-2 is more advantageous
 for the production of spheres.
 FIG. 6b is a photomicrograph of commercially ground low molecular weight
 polyethylene-based polymer particles (Acumist AC-18 from Allied Signal
 Inc.) having irregular faceted, angular surfaces for comparison to FIG. 6a
 of the low molecular weight Hoechst PE 130 particles pursuant to the
 invention.
 FIGS. 5a and 5b are photographs of molten polymer stream flowing from the
 atomization nozzle 30 before atomization, FIG. 5a, and during atomization
 at 1000 psi nitrogen atomizing gas, FIG. 5b, for the Hoeschst PE 130
 polymer to illustrate a typical atomization spray pattern.
 Example 4
 The pressure crucible 10 in FIG. 3 was used to heat 100 grams of a
 polyethylene-based material Hoechst Wax PE 520 produced by
 Hoechst-Celanese Corporation. The Hoechst Wax PE 520 is an ultra-low
 molecular weight (3000 grams/mole) low density (0.93 grams/cubic
 centimeters) polyethylene. The polymer material was heated from room
 temperature to 220 degrees C. at a heating rate of 10 degrees C. per
 minute. The crucible was pressurized to 15 psi using nitrogen gas to force
 the molten polymer through the pour tube with the 20 radial gutters in
 alignment with the 20 gas jets of the atomizing nozzle and of 1/8 inch
 tube inner diameter. Ultra-high purity (99.99% purity) nitrogen gas at
 1100 psi was applied to the molten polymer at the exit of the pour tube.
 The atomizing time was a few seconds. The resulting atomized polymer
 particulates contained 63% by volume spherical powders and 37% by volume
 microfiber particles, providing a sphere/fiber ratio of 1.7. The yield of
 0-53 micron particle size fraction was about 7% by weight, which was about
 0.63 times that obtained using a flat-bottomed pour tube of like inner
 diameter.
 Example 5
 The pressure crucible 10 in FIG. 3 was used to heat 100 grams of a
 polyethylene-based material Hoechst Wax PE 520. The polymer material was
 heated from room temperature to 220 degrees C. at a heating rate of 10
 degrees C. per minute. The crucible was pressurized to 15 psi using
 nitrogen gas to force the molten polymer through the pour tube with 20
 radial gutters set out of alignment with the 20 gas atomizing jets and of
 1/8 inch tube inner diameter. Ultra-high purity (99.99% purity) nitrogen
 gas at 1100 psi was applied to the molten polymer at the exit of the pour
 tube. The atomizing time was a few seconds. The resulting atomized polymer
 particulates contained 42% by volume spherical powders and 58% by volume
 microfiber particles, providing a sphere/fiber ratio of 0.72. The yield of
 0-53 micron particle size fraction was about 3.5% by weight, which was
 about 0.31 times that obtained using a flat-bottomed pour tube of like
 inner diameter.
 For PE 520 material, Examples 4 and 5 show that the alignment of the radial
 gutters 12d of the pour tube 12 relative to the gas jet orifices 30c of
 the atomizing nozzle 30 can be used to optimize the yield of the 0-53
 micron particle size fraction as well as maximizing either the spherical
 or fiber morphology of the particulates for specific end use applications.
 Example 6
 The pressure crucible 10 in FIG. 3 was used to heat 100 grams of a 50/50 by
 weight blend of PE 130/PE 520 and also separately a blend of PE
 130/ultra-low melting point phosphate glass (33.04% SF-37.28% SnO-5.47%
 PbF.sub.2 -24.22% P.sub.2 O.sub.3 in weight %). The atomizing conditions
 were similar to those described hereabove for atomizing the pure polymers
 (non-blended) with the exception that a pour tube having bottom gutters
 described hereabove was used in lieu of the flat bottomed pour tube (with
 a pour tube inner diameter of 1/8 inch for the PE 130/PE 520 blend) and
 (with pour tube inner diameter of 3/16 inch for the PE 130/glass blend at
 an atomization temperature of 205 degrees C.). The average yield of the PE
 130/PE 520 blended particulates was 75.7% of the initial pellet charge in
 the crucible compared to 82.1% and 64.8% for the pure PE 130 and PE 520,
 respectively, atomized under similar conditions using a similar pour tube
 with gutters. The e percent yield of the PE 130/phosphate glass blend was
 about 73% compared to 63% for the pure PE 130. As is apparent, blending of
 starting materials gives a unimodal particle size distribution.
 Example 7
 A graphite pressure crucible 10 (4.5 inch diameter by 8 inch long) in FIG.
 3 was used to heat 4.16 kilograms of a copper-8 atomic percent A1 alloy.
 The stopper rod 15 comprises a hard fired closed end alumina tube. The
 pour tube 12 comprised graphite fabricated with 20 radial slots or
 channels 12d on the bottom end 12c, each channel machined 0.020 inch in
 width and 0.085 inch deep with a 0.194 radial undercut centered on the
 longitudinal axis of the pour tube 12. Other components of the atomizing
 nozzle 30 are described in aforementioned U.S. Pat. No. 5,228,620
 incorporated herein by reference. A secondary cooling gas comprising high
 purity helium was discharged downstream of the atomizing location. The
 molten alloy was heated to an atomization pour temperature of 1300 degrees
 C., and then fed from the crucible without pressurization thereof through
 the pour tube with the 20 radial gutters in alignment with the 20 gas jets
 of the atomizing nozzle. Ultra-high purity (99.99% purity) nitrogen gas at
 1100 psig was applied to the molten molten alloy exiting the pour tube.
 The atomizing time was 43 seconds. As the liquid alloy exited the pour
 tube, a bright and stable ring of bright light radiating from the
 periphery of the pour tube was established, and a slender profile melt
 spray of atomized droplets was observed to produce fine atomized alloy
 powders of particle sizes of 1 to 100 microns.
 The present invention is not limited to the particular polymer materials
 and blends described hereabove and can be practiced to atomize a wide
 variety of polymer materials and blends thereof one with another or with
 other materials, including thermoplastics and some grades of thermosetting
 resins that are sold commercially in the fluid state. The present
 invention is advantageous in that polymer particulates can be made from
 polymers which heretofore could not be ground or ball milled to this end
 due to their waxy nature. Moreover, polymer particulates can be made with
 controlled particle shape, particle size and particle size distribution
 without the need for the addition of flow modifiers, such as oils and
 greases or molybdenum disulfide, to the polymer material to lower
 viscosity and with reduced energy consumption compared to grinding and
 ball milling. As a result, atomized polymer particulates can be made
 having improved quality with reduced contamination, high throughput and
 fast cycle time. Polymer particulates can be made from virgin polymer
 materials and/or polymer waste products.
 While the invention has been described with respect to certain embodiments
 thereof, those skilled in the art will understand that it is not intended
 to be limited thereto and that changes and modifications can be made
 therein within the scope of the appended claims.