Abstract:
Described herein are extrusion processes to produce hollow pellets. Also disclosed are pelletizer devices that can be used to produce the hollow pellets. The processes and devices make use of an extrusion die having a die orifice and an insert that is placed in the die orifice to produce the hollow pellets.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application claims the benefit of International Patent Application Serial Number PCT/US2009/044220, filed 15 May 2009, which claims the benefit of U.S. Provisional Patent Application Ser. No. 61/053,984, filed 16 May 2008, both of which are entitled “Method and Device for Extrusion of Hollow Pellets,” and are hereby incorporated by reference in their entirety as if fully set forth below. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to an extrusion process to produce hollow pellets, wherein an insert is placed in the die holes of an extrusion die, about which is extruded the molten material to form those hollow pellets. 
     2. Description of the Prior Art 
     Pelletization equipment and its use following extrusion processing has been introduced and/or utilized in applications by the assignee for many years as is exemplified by prior art disclosures including U.S. Pat. Nos. 4,123,207; 4,251,198; 4,500,271; 4,621,996; 4,728,176; 4,888,990; 5,059,103; 5,403,176; 5,624,688; 6,332,765; 6,551,087; 6,793,473; 6,824,371; 6,925,741; 7,033,152; 7,172,397; US Patent Application Publication Nos. 20050220920, 20060165834; German Patents and Applications including DE 32 43 332, DE 37 02 841, DE 87 01 490, DE 196 42 389, DE 196 51 354, DE 296 24 638; World Patent Application Publications WO2006/087179, WO2006/081140, WO2006/087179, and WO2007/064580; and European Patents including EP 1 218 156 and EP 1 582 327. These patents and applications are all owned by the assignee and are included herein by way of reference in their entirety. 
     These disclosures remain silent as to the use of inserts in the pelletization process. More specifically, these disclosures remain silent regarding the use of inserts in the extrusion die, wherein molten material flows about the extrusion die and insert such that a hollow pellet is generated. 
     The various embodiments of the present invention provide a cost effective method to prepare reproducible hollow pellets by use of a multiplicity of inserts in an equivalent multiplicity of die orifices through an extrusion die plate. 
     BRIEF SUMMARY OF THE INVENTION 
     Briefly described, in preferred form, the various embodiments of the present invention provide a process to extrude hollow pellets by use of at least one insert through the at least one die orifice in an extrusion die. Molten material passes to, and through, the die orifice containing the insert. The molten material is extruded, preferably with pressure, to give a hollow pellet on cooling, such that the hollow cavity formed can be at least one of continuously hollow throughout the pellet, completely and circumferentially enclosed within the pellet, and many combinations therebetween such that the enclosed hollow cavity is at least perforatedly connected in at least one locus to the outside of the pellet. 
     The hollow pellets are reproducible in structure and can be of any molten material, preferably polymeric, and any geometry both from the pellet shape as well as the hollow cavity shape. The hollow pellet obtained is dependent upon but not limited to the extrusion viscosity, die swell, material composition, temperature of the melt, rate of cooling, degree of crystallization, melt index, cutting speed of the pelletization process, and the like. 
     It is therefore an aspect of the present invention to provide an inexpensive and cost effective method to produce relatively consistent and reproducible hollow pellets utilizing at least one insert in the at least one orifice of an extrusion die about which the molten material is extruded such that the pellet shape, pellet diameter, cavity shape, cavity diameter, and penetration of that cavity or the lack thereof in and/or through the pellet is controlled. 
     In certain embodiments, an extrusion process for producing hollow pellets includes extruding molten material through an extrusion die comprising a die orifice and an insert disposed in the die orifice, and cooling the extruded molten material effective to produce a pellet having a hollow cavity. The extrusion die can be a single-body extrusion die, a removable extrusion die assembly, or other structure. In some cases, the extruding can be implemented using pressure. 
     The insert can include a mandrel, a plurality of fins, and a plurality of fins tapers. 
     The hollow cavity of the pellet can penetrate a first surface of the pellet and continuously extend through a second surface of the pellet. Alternatively, the hollow cavity can be encapsulated completely within the pellet. It is also possible for the hollow cavity to penetrate a first surface of the pellet and extend inwardly to an interior portion of a body of the pellet. If a pellet has more than one hollow cavity, any one or more of these types of hollow cavities can be incorporated into the pellet. 
     The molten material can be chosen from a polyolefin, a cross-linkable polyolefin, vinyl polymer, substituted vinyl polymer, polyester, polyamide, polyether, polythioether, polyurethane, polyimide, polycarbonate, polysulfide, polysulfone, wax, a copolymer thereof, or a formulation comprising at least two of the foregoing. 
     Another extrusion process for producing hollow pellets involves feeding a molten material into a pelletizer, extruding the molten material through an extrusion die of the pelletizer using pressure, and cooling the extruded molten material effective to produce a pellet having a hollow cavity. The pelletizer can be an underwater pelletizer. 
     The extrusion die of the pelletizer can have a die orifice and an insert disposed in the die orifice, where the insert comprises a mandrel, a plurality of fins, and a plurality of fins tapers. In some cases, the extrusion die includes a plurality of die orifices through which the molten material is extruded, such that each die orifice of the plurality of die orifices has an insert. 
     A pelletizer, according to some embodiments, can include an inlet for receiving a molten material, a die orifice that is downstream of the inlet and is for extruding the molten material, an insert disposed in the die orifice, where the insert comprises a mandrel, a plurality of fins, and a plurality of fins tapers, and an outlet for transporting the extruded molten material from the pelletizer. The pelletizer can be an underwater pelletizer. In some cases, the insert has at least four fins disposed about the mandrel such each of the at least four fins is disposed less than or equal to about 90 degrees apart from an adjacent fin. The extruded molten material includes a pellet having a hollow cavity. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic vertical sectional view of one embodiment of the single-body extrusion die assembly of the present invention in which the perforated is of single-body construction. 
         FIG. 2  is a schematic vertical sectional view of the removable insert extrusion die assembly of the present invention in which the perforated is of removable center construction. 
         FIG. 3  is a schematic vertical sectional view illustrating the association of the die orifice and insert. 
         FIG. 4  is a schematic view of the insert. 
         FIG. 5  is a cross-sectional view of the insert in the die hole. 
         FIG. 5   a  is a horizontal cross-sectional view of the insert in the die hole at line a. 
         FIG. 5   b  is a horizontal cross-sectional view of the insert in the die hole at line b. 
         FIG. 5   c  is a horizontal cross-sectional view of the insert in the die hole at line c. 
         FIG. 5   d  is a horizontal cross-sectional view of the insert in the die hole at line d. 
         FIG. 6  are illustrations of various pellet geometries in top view, cross-section, and side view, including  FIG. 6   a  that illustrates a top view of a cylindrical pellet through which the hollow completely penetrates. 
         FIG. 6   b  illustrates a cross-sectional view of the hollow approximately cylindrical pellet from  FIG. 6   a.    
         FIG. 6   c  illustrates a side view of the hollow approximately cylindrical pellet from  FIG. 6   a    
         FIG. 6   d  illustrates a top view of an approximately round pellet. 
         FIG. 6   e  illustrates the cross-section through the round pellet in  FIG. 6   d.    
         FIG. 6   f  illustrates a top view of an approximately rectangular pellet. 
         FIG. 6   g  illustrates a cross-sectional view through the pellet in  FIG. 6   f  showing a round hollow or cavity within that rectangular pellet. 
         FIG. 6   h  illustrates a top view of an approximately round pellet. 
         FIG. 6   i  illustrates a cross-sectional view through the pellet in  FIG. 6   h  wherein a cavity has perforations into and through the pellet wall. 
     
    
    
     DETAILED DESCRIPTION 
     Although only preferred embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its scope to the details of construction and arrangement of components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments and of being practiced or carried out in various ways. Also, in describing the preferred embodiments, specific terminology will be resorted to for the sake of clarity. It is to be understood that each specific term includes all technical equivalents that operate in a similar manner to accomplish a similar purpose. 
     Referring to the drawings,  FIG. 1  illustrates one embodiment of the present invention associated with components of a pelletizer. The pelletizer includes an inlet housing  12  from a melting and/or mixing apparatus (not shown). The inlet housing  12  includes a passageway  14  for molten material or other extrudate (hereinafter collectively referred to as “process melt”) that can include organic materials, oligomers, polymers, waxes, and combinations thereof without intending to be limited. Nose cone  16  directs the process melt to the upstream side of the single-body extrusion die  10  to which it is attachedly connected by a threaded rod (not shown). The threaded rod is screw threaded at one end into threaded bore  18  of nose cone  16  and at its distal end into threaded bore  20  of single-body extrusion die  10 . Alternately, the nose cone  16  can be continuous with the single-body extrusion die  10  and need not be attachedly connected as herein described. 
     The single-body extrusion die  10  contains at least one, and preferably a multiplicity of, die holes  22  concentrically arranged singly or in multiples thereof in at least one ring that extend from the upstream face  24  to the downstream face  26  of single-body extrusion die  10 . A plurality of knife blade assemblies  28  mounted on a rotatably driven cutter hub  30  in a cutting chamber (not shown) cut the extruded, cooled, and at least partially solidified process melt into pellets. The pellets thusly formed are transported mechanically, pneumatically, hydraulically, and in combinations thereof to downstream processing. 
     Areas of the downstream face  26  optionally can be cut out to provide at least one annular recess or cavity  32  peripherally adjacent to the die holes  22  such that the die holes  22  are contained in protrusions  34  that are continuous with the base plate  36  of single-body extrusion die  10 . Within die holes  22 , with or without protrusions  34 , are an equivalent number of inserts  50  detailed hereinbelow. Annular cover plate  38  overlays the annular recess or cavity  32  and is attachedly connected to base plate  36  and protrusions  34  by brazing, welding, or similar technique known to those skilled in the art. The cover plate  38  can be at least one of an abrasion and corrosion resistant metal, preferably nickel steel, a hard face material, preferably tungsten carbide, and many combinations thereof. Similarly, attachment of the cover plate  38  to the base plate  36  and/or protrusions  34  is preferably achieved by welding, brazing, and the like. The surface of the cover plate  38  and thus the downstream face  26  of single-body extrusion die  10  can optionally be coated with a chemical, abrasion, corrosion, and wear resistant coating as is known to those skilled in the art. 
       FIG. 2  illustrates a removable insert extrusion die assembly  100  in a second embodiment of the present invention. Removable insert extrusion die assembly  100  is comprised of base plate  105  and removable insert  110 . Similarly to  FIG. 1 , the removable insert extrusion die assembly  100  is attachedly connected to an inlet housing  12  from a melting and/or mixing apparatus (not shown). The inlet housing  12  includes a passageway  14  for process melt as heretofore described. Nose cone  16  directs the process melt to the upstream side of the removable insert  110  to which it is attachedly connected by threaded rod (not shown). The threaded rod is screw threaded at one end into threaded bore  118  of nose cone  16  and at its distal end into threaded bore  120  of removable insert  110 . 
     The removable insert  110  contains at least one and preferably a multiplicity of die holes  22  concentrically arranged singly or in multiples thereof in at least one ring that extend from the upstream face  124  to the downstream face  126  of removable insert  110 . A plurality of knife blade assemblies  28  mounted on a rotatably driven cutter hub  30  in a cutting chamber (not shown) cut the extruded, cooled, and at least partially solidified process melt into pellets. The pellets thusly formed are transported mechanically, pneumatically, hydraulically, and in combinations thereof to downstream processing as before. 
     Areas of the downstream face  126  optionally can be cut out to provide at least one annular recess or cavity  132  peripherally adjacent to the die holes  22  such that the die holes  22  are contained in protrusions  134  that are continuous with the removable center base plate  136  of removable insert  110 . Within die holes  22 , with or without protrusions  134 , are an equivalent number of inserts  50  detailed hereinbelow. Annular cover plate  138  overlays the annular recess or cavity  132  and is attachedly connected to removable center base plate  136  and protrusions  134  by brazing, welding, or similar technique known to those skilled in the art. The cover plate  138  can be at least one of an abrasion and corrosion resistant metal, preferably nickel steel, a hard face material, preferably tungsten carbide, and many combinations thereof. Similarly, attachment of the cover plate  138  to the removable center base plate  136  and/or protrusions  134  is preferably achieved by welding, brazing, and the like. The surface of the cover plate  138  and thus the downstream face  126  of removable insert  110  can optionally be coated with a chemical, abrasion, corrosion, and wear resistant coating as is known to those skilled in the art. 
     Heating and/or cooling processes can be provided by electrical resistance, induction, steam or heat transfer fluid as has been conventionally disclosed for the single-body extrusion die  10  as well as the removable insert extrusion die assembly  100 . The removable insert  110  and the base plate  105 ,  FIG. 2 , alternatively can be heated separately by similar or differing mechanisms. Preferably heating elements  46  are inserted into the single-body extrusion die  10  or the removable insert extrusion die assembly  100  as illustrated in  FIGS. 1 and 2 , respectively. Other designs as are known to those skilled in the art are included herein by way of reference without intending to be limited. 
     Turning now to  FIG. 3  for the single-body extrusion die  10 , the insert  50  is illustrated within die hole  22  that extends from upstream face  24  into and through optional protrusion  34  in base plate  36  to the downstream face  26  of cover plate  38 . Optional annular recess or cavity  32  is also shown for purposes of clarification. An analogous assembly follows for removable insert  110  and is not shown. 
       FIG. 4  illustrates the details of construction for insert  50  which comprises a mandrel  52 , a multiplicity of insert fin tapers  54 , and a multiplicity of fins  56 . The insert  50  can be made of any abrasion-resistant material and is preferably metal. The metal can be aluminum, brass, bronze, copper, steel, tool steel, carbon steel, vanadium steel, stainless steel, nickel steel, nickel, and the like without intending to be limited. More preferably the metal is a good heat conductor including brass, bronze, and copper. Without intending to be bound by any theory, it is believed that the thermally conductive metals maintain uniformity of temperature in the process melt propagating into and through the die hole  22 ,  FIGS. 1 ,  2 , and  3 . This is effective in minimizing loss of heat and/or variation in temperature as the material flows in the multiplicity of pathways formed by the multiplicity of fins  56 . 
     The dimensions of the insert  50  must be such that it does not exceed the dimensions of the die hole  22  at process temperature and must take into consideration the differential expansion wherein the metal of the insert  50  differs from that of the base plate  36 ,  FIG. 1 , or removable insert  110 ,  FIG. 2 . The fins  56  not only form a multiplicity of flow pathways for the process melt but further serve to maintain the position of insert  50  in die hole  22 . The minimum number of fins is at least two (2) and preferably at least three (3). More preferably there are four (4) or more fins  52  on insert  50 . The multiplicity of fins  56  can be oriented at any angle relative to the adjacent fins to form pathways through which flows the polymer melt. Preferably the fins are 180° apart or less. More preferably the fins are 120° or less apart. Most preferably the fins are 90° or less apart. 
       FIG. 5  shows the insert  50  within die hole  22  such that mandrel  52  is significantly contained within die land  60 , insert fin tapers  54  approximately correspond dimensionally to die hole taper  62 , and fins  56  are approximately contained within the pre-land tube  64 . The length of die land  60  typically ranges from at least approximately 0.38 millimeters (approximately 0.015 inch) to approximately 31.75 millimeters (approximately 1.25 inches) and is preferably at least approximately 0.64 millimeters (approximately 0.025 inch) to approximately 25 millimeters (approximately 1.00 inch). The mandrel  52  within die land  60  is preferably less than the length of the die land  60  and most preferably is at least approximately 0.50 millimeters (approximately 0.025 inch) less than the length of the die land such that the tip of the mandrel is very slightly recessed from the downstream face  26  of the die,  FIG. 1 , or downstream face  126 ,  FIG. 2 . The die land  60  and/or mandrel  52  can be cylindrical or tapered and can be round, oval, rectangular, and the like in geometry. Similarly, the die land  60  and mandrel  52  can be of similar or different geometry. The insert  50  can be press fit and preferably is slide fit into die hole  22 . 
     The insert fin tapers  54  are similar in angularity, at angle  66 , to the die hole taper  62  that can range from 0° to 90° as measured from the perpendicular cylinder imposed on the diameter of the pre-land tube  64  at the juncture with the die hole taper  62 . Preferably angle  66  ranges from 15° to 45° as described herein. The insert fin tapers  54  can be the same contour as, or different than, that of the die hole taper  62  and dimensionally must taper from the diameter of the fins  56  to the diameter of the mandrel  52 . Similarly the fins  56  can be similar to the geometry, cylindrical or tapered and combinations thereof for example, of the pre-land tube  64  or can be different in geometry. Preferably the pre-land tube  64  and the fins  56  are cylindrical. The length of the fins  56  can be the same as the length of the pre-land tube  64  and is preferably less than the length of the pre-land tube  64 . More preferably, the length of the fins is at least approximately 0.50 millimeters (approximately 0.020 inch) less than the length of the pre-land tube  64  such that the fins do not protrude outside the length of the pre-land tube  64 . 
       FIG. 5   a  illustrates an exemplary cross-sectional design of the fins  56  in pre-land tube  64  at line a.  FIG. 5   b  illustrates an exemplary cross-section design of the insert fin tapers  54  in the die hole taper  62  at line b.  FIG. 5   c  illustrates an exemplary cross-sectional design of the mandrel  52  at the attachment point to the insert fin tapers  54  in die land  60  at line c.  FIG. 5   d  illustrates an optional decreasingly tapered mandrel  52  in die land  60  at line d. 
       FIG. 6  illustrates the various geometries of the hollow pellets formed in accordance with the present invention.  FIG. 6   a  illustrates a top view of a cylindrical pellet through which the hollow cavity completely penetrates.  FIG. 6   b  illustrates a cross-sectional view of the hollow, approximately-cylindrical pellet from  FIG. 6   a ; and  FIG. 6   c  illustrates a side view of the same pellet.  FIG. 6   d  illustrates a top view of an approximately-round pellet with  FIG. 6   e  illustrating the cross-section through that pellet.  FIG. 6   f  illustrates a top view of an approximately-rectangular pellet with  FIG. 6   g  illustrating a cross-sectional view through that pellet, showing a round, hollow cavity within that rectangular pellet.  FIG. 6   h  illustrates a top view of an approximately round pellet with  FIG. 6   i  illustrating a cross-sectional view through that pellet, showing a cavity with perforations into, and through, the pellet wall. It is understood by those skilled in the art that many pellet shapes and cavity shapes can be achieved by methodologies of the present invention without intending to be limited. 
     Hollow pellet formation is significantly controlled by the melt rheology and particularly the melt viscosity. Fractional melt materials typically form collar or donut-shaped pellets as illustrated in  FIGS. 6   a, b , and  c  described hereinabove. As the melt viscosity decreases and thus the melt flow index increases, it was found that more closure of the pellet was achievable to form a completely enclosed cavity as illustrated in  FIGS. 6   d, e, f , and  g . As melt viscosity continues to drop, and thus melt flow index increases, less completely enclosed cavities were generated, perforations were introduced, and eventually the cavities were found to collapse or partially collapse leading to irregular cavity geometry. 
     Additionally, such factors as chemical composition, melting point range, and crystallinity are important as these affect the fluidity and temperature of the process melt. Crystallization is typically exothermic and thus adds to the melt process temperature thus lowering the viscosity. The narrower the melting point range the less cooling necessary to significantly increase the solidification and thus the more challenging to form a completely enclosed cavity as compared with a collar or donut-shaped pellet through which the cavity completely penetrates. The polarity, branching, and hydrophobic/hydrophilic interactions of polymers influence the properties in the melt phase as well as the processes leading to solidification. The ability of a material to swell on exiting the die is also an important factor in assessing the closure of the pellet as well as the necessary difference in diameter of the mandrel  52  and die land  60 ,  FIG. 5  to achieve a pellet of a desired diameter containing a cavity of a particular diameter. As the melt viscosity decreases the control of these variables decreases and the temperature influences of crystallization, if present, increases. 
     The moisture uptake was evaluated as a means of elucidating possible entrapment of moisture wherein pelletization was done in the preferred underwater pelletizing. It was anticipated that moisture would be proportionately high wherein entrapment of the transport fluid, preferably water, occurred in the hollow cavities generated. It was surprisingly found that moisture contents were significantly lower than expected after taking into account the difference in mass of a comparable diameter solid pellet to the reduced mass of the hollow pellet and even more surprising that moisture reduction increased as the polarity of the material increased. For example, both polyethylene and polypropylene hollow pellets were found to have comparable moisture content with solid pellets of comparable diameter whereas ethyl vinyl acetate hollow pellets were found to have approximately one-half to two-thirds the moisture of the solid pellet. 
     Examples of materials for use in making hollow pellets according to the instant invention include but are not limited to polyolefins, polyethylene, polypropylene, cross-linkable polyolefins, vinyl polymers and substituted vinyl polymers including aliphatic and aromatic, polyesters, polyamides, polyethers and polythioethers, polyurethanes, polyimides, polycarbonates, polysulfides, polysulfones, waxes, and copolymers and formulations thereof. 
     As was anticipated, back-pressure on the extrusion process increases with the use of the inserts and was found to be alleviated by at least one of increasing the number of holes through the die, increasing the temperature of the process melt, and increasing the temperature of the die. These factors, as is understood by one skilled in the art, are not surprising consequences.