Process for extruding polymers
BACKGROUND OF THE INVENTION 
     This invention relates to a process for extruding high viscosity polymeric materials. More specifically, it provides a process for making polymeric extrusions from high viscosity polymeric materials without generating surface roughness on the extrudate, and it permits extruding at higher rates and lower temperatures and reduced energy consumption. 
     Many polymeric materials are shaped into useful films, rods, tubes or are coated onto wire by extruding them through an orifice of relatively small dimensions. In such extrusion processes, the difficulties in obtaining a smooth extrudate increase as the viscosity of the polymeric material or output increases. The pressure required to force the material through the exit orifice also increases and an unstable flow phenomenon, often called melt fracture, occurs which results in a rough surface on the extrudate. Such rough surfaces can have adverse effects on extruded products, such as polyolefin rubbers, when, for example, the extrudate with a rough surface is pelletized under water. The interstices on the rough surface retain water and this makes drying the polymer difficult or impractical. 
     It is known that extrusion pressure can be reduced by forming the shaped die from a porous metal material and forcing a gaseous fluid through the pores. However, lubrication with a gas does not provide laminar flow or a smooth surface on the extrudate. It is also known to previously coat shaped extruded materials with various liquids using channels made of porous metal. Such methods have been typically utilized to lubricate the final shaping operation in manufacturing pipe or rods wherein the material is pulled through the die, rather than flowing through by upstream pressure. A method for reducing melt fracture of certain polymeric materials is known in which a small portion of the extruded material is separately heated to a higher temperature than the main mass (to reduce its viscosity) and then is applied as a surface layer in the forming die. This method is difficult to control, limited in viscosity difference, and is not as widely applicable as that of the present invention. 
     SUMMARY OF THE INVENTION 
     The present invention provides a process for making polymeric extrudates having a smooth surface which comprises feeding a molten polymeric core material, having a bulk viscosity of about 3×10 3  -10 6  Pa.s at processing temperatures of about 100°-250° C., under positive pressure through a die cavity wherein a portion of the wall of the die cavity is a rigid microporous structure, preferably metal, located upstream from a narrowed flow passage of the die cavity, said microporous structure having a substantially uniform pore size, simultaneously forcing under pressure through the microporous structure about 0.03-10%, based on the total weight of the extrudate, of a low viscosity liquid having a bulk viscosity of about 10 -1  -10 3  Pa.s at processing temperatures of about 100°-250° C. to form a sheath on the molten polymeric core material and substantially coat its surface and thus displacing the polymeric core material from contact with the die wall before the resulting sheath-core composite enters the narrowed flow passages of the die cavity, said process conducted under such conditions as to produce a pressure drop across the microporous structure of at least 0.10 MPa, usually within a range of about 0.2-12.0 MPa, more than the pressure applied on the polymeric core material, and the sheath-core composite flows through the narrowed flow passage of the die cavity and the die orifice. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 represents a cross sectional view of a single hole extrusion die assembly for use with the present invention in which a microporous cylinder, which forms part of the wall of the die cavity, is held between two metal die plates upstream from a narrowed flow passage in the die cavity. The space between the die plates is closed at its periphery and serves as a reservoir for the low viscosity liquid which is forced through the microporous wall of the cylinder. 
     FIG. 2 is a cross sectional view of a multiple hole die assembly. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The process of this invention can be best described by referring to the drawings. As shown, especially in FIG. 1, an assembly for conducting the process comprises a die assembly 13 comprising a top die plate 14, a microporous structure 12 in the form of a metal cylinder and a bottom die plate 17. The die plates have suitable seals 21 to contain the low viscosity liquid under pressure. The die assembly has an entrance port 10 and die plates that form die cavity 15 which serves as a flow passage for the molten polymeric core material being extruded. The die cavity has a narrowed flow passage 16 at its exit end. The narrowing of the flow passage of the die cavity begins at a point below the microporous structure 12 so that the molten polymeric core material is displaced from contact with the die wall by the low viscosity liquid that is forced through microporous structure 12 thereby forming a low viscosity boundry layer on the molten polymer before the resulting sheath-core composite passes through narrowed flow passage 16 of the die cavity. It is important that microporous structure 12 is located upstream from narrowed flow passage 16 since the boundary layer, or sheath, of low viscosity liquid reduces the effective viscosity before shear rate is increased. Maximum benefit is thereby obtained with a reduction in die pressures, reduced power input and the elimination of melt fracture. 
     The high viscosity polymeric material enters the die cavity through entrance port 10, under positive pressure, usually from about 0.1-20 MPa, supplied from any conventional means such as a plastics screw extruder 19, flows through the top die plate 14 in die cavity 15 and into the open end of the microporous cylinder 12. A low viscosity material that is liquid at processing temperatures is forced through passage 11 and through the walls of microporous structure 12 under pressure, usually from about 0.2-12 MPa.s, provided by a gear pump 20 or a conventional high pressure pump. The conditions used in the process require that one obtain a pressure drop across the microporous structure of at least 0.1 MPa, usually 0.2-12 MPa.s, more than the pressure applied on the polymeric core material. The difference between the pressure on the high viscosity molten polymeric material and the pressure on the lower viscosity liquid is a differential value identified as pressure drop. The lower viscosity liquid displaces the molten polymeric core material from the wall of the microporous structure, preferably a metal cylinder, due to its higher pressure, and continues to flow in an outer concentric cylinder as the molten polymeric core material and low viscosity liquid form a sheath-core composite and pass through the die cavity in the bottom die plate 17. After the low viscosity liquid forms a sheath around the polymeric core material, the resulting sheath-core composite flows through narrowed flow passage 16 of the die cavity and exits from die orifice 18. Surprisingly, a very uniform distribution of a small amount of low viscosity liquid on the molten polymeric core material can be achieved when conditions are adjusted for the required pressure drop across the microporous structure. The extrudate will have a smooth surface. Concurrently, the pressure required for forcing the molten polymeric core material through the die assembly is markedly reduced to as low as about one-fourth that required without application of the low viscosity liquid on the surface of the molten polymeric core material. 
     The microporous structure 12 of the extrusion die assembly is readily supplied by the insertion of a microporous structure between two metal die plates 14 and 17, as shown in the drawing. The die plates 14 and 17 are separated by the microporous structure 12. The space between the plates serves as a passageway and reservoir for the low viscosity liquid entering passage 11. The microporous structure 12 can be made of metal or ceramic material. Usually, and preferably, the microporous structure is made of metal and fabricated by molding metal powders of such metals as stainless steel, nickel or monel alloy. Metal microporous cylinders are commercially available in a variety of sizes and porosities. The most useful of these for the present invention are 15 to 35 mm long, have wall thickness of 0.7-7 mm and have a substantially uniform pore size (diameter) in the range of about 0.2-150 μm, usually about 0.5-90 μm. Such microporous cylinders are manufactured primarily for use as filter elements. The choice of a particular pore size of the microporous structure and the surface area of the cylinder exposed for penetration by the low viscosity liquid is influenced by the viscosity of the low viscosity liquid used in the process and is necessarily smaller as the viscosity decreases so that the required pressure drop across the microporous structure can be achieved without excessive liquid flow. 
     While the die cavity need include only one microporous structure, it is frequently most useful in commercial operations to include a multiplicity of microporous structures and die cavities in a single die assembly, all fed by one pump for each of the two liquids, i.e., the molten polymeric core material and the lower viscosity liquid, that are used in the process of this invention, as illustrated in FIG. 2. 
     The polymeric core material to be extruded can be supplied to the die assembly by a helical screw pump, i.e. extruder, which has a source of heat to help raise the temperature of the polymeric core material so that it is molten or liquefied and, also, to lower its viscosity. Other heating/pumping devices such as heated pressurized reservoirs (cylinders) or gear pumps can be used to pump the molten polymer. 
     The process of this invention is applicable to the extrusion of molten polymeric core materials having bulk viscosity of from about 3×10 3  -10 Pa.s, usually 10 4  -10 6  Pa.s at processing temperatures of about 100°-250° C. Prior to this invention, these polymeric materials often could be extruded only at restricted rates with high pressures and with high power consumption frequently exhibited the unstable flow phenomenon known as melt-fracture which leaves the extrudate with a rough surface. Processing temperatures for extruding the polymeric materials are well known to those skilled in the art and such temperatures are usually between about 100° C.-250° C. depending on the thermal stability and melting point of the polymers used. Polymeric materials that can be used in this process include polyolefin plastics e.g., polyethylene, polypropylene and their copolymers with high olefins e.g., those having 3-12 carbon atoms; poly(vinyl chloride) and copolymers thereof with vinylidene chloride, vinyl acetate and the like; uncured elastomers, such as the polyolefin elastomers, e.g. EPM and EPDM rubber; chlorinated polyethylene, (CM) and chlorosulfonated polyethylene (CSM) rubber, ethylene/vinyl acetate copolymers, ethylene/alkyl acrylate copolymers; acrylonitrile-butadiene copolymers; styrene-butadiene copolymers; natural rubber; fluoroelastomers, i.e. copolymers derived from units of vinylidene fluoride and hexafluoropropylene optionally containing tetrafluoroethylene, polychloroprene and polyester plastics such as polyethylene terephthalate and polybutylene terephthalate and polyester elastomers such as copolyetheresters derived from an aromatic acid, a poly(alkylene oxide) glycol, and an aliphatic alcohol. The polymeric materials used in the extrusion process can, and usually do, contain various conventional additives and insert fillers such as carbon black, silica gel, alumina and/or stabilizers, flame retardants, cross-linking agents, curing agents, blowing agents, etc. The high viscosity values of the polymeric material is often the result of the concentration of additives or inert fillers, for example, solids such as carbon black, especially in elastomeric compounds. The process is especially useful in the extrusion of temperature sensitive elastomers, since it permits operation at increased rates and low temperatures due to the lowered power input required. The process also eliminates a type of melt fracture termed die peel or die drool which can cause imperfections on the surface of the elastomeric extrudate. The process is also especially useful for the extrusion of temperature sensitive thermoplastics, such as poly(vinyl chloride) and its copolymers, e.g. with vinylidene chloride or vinyl acetate. The process of this invention permits extrusion of very high viscosity polymers at commercially feasible rates at reduced energy consumption. 
     The low viscosity liquid that forms the sheath on the resulting composite must always have a lower viscosity than the molten polymeric core material. The low viscosity sheath material can be supplied to the exterior wall of the microporous structure by a variable speed gear pump or a helical screw pump. The pump used for the low viscosity liquid must be capable of developing a fluid pressure at least 0.10 MPa greater than the pressure on the molten polymeric core material in the die cavity. A pressure drop of about 0.10 MPa-20 MPa across the wall of the microporous structure is generally obtained, preferably 0.2-12 MPa. The maximum pressure can be higher, but it is limited by the strength of the walls of the microporous structure. 
     Any low viscosity liquid material having a bulk viscosity of about, 10 -1  -10 3  Pa.s, usually 1-600 Pa.s, at processing temperatures of about 100°-250° C. can be used in the present invention. The low viscosity liquid does not become mixed with the polymeric core material under processing conditions. The viscosity of this liquid is low enough so that it can be pumped and will flow through the wall of the microporous structure in an amount of about 0.03-10%, preferably 1-5%, based on the total weight of the extrudate, to form a sheath on the molten polymeric material. The low viscosity liquid substantially coats the surface of the polymeric core material and thus displaces the polymeric core material from contact with the die wall, and this occurs before the polymeric sheath-core material enters the narrowed flow passage of the die cavity. Generally, the sheath formed by the low viscosity material is about 0.3-170 microns thick and in most cases only about 0.4-30 microns thick for small diameter extrusions. The low viscosity liquid should also be chemically stable at processing temperatures so that toxic fumes and smoking are avoided. Among the most useful materials are petroleum oils, mineral oils, hydrocarbon waxes, Chlorowaxes, silicone oils, glycerol, fatty acids, fatty acid metal salts, fatty acid esters, fatty acid amides, fatty alcohols, water soluble polymers neat and in solution, polymeric glycols, carboxy vinyl polymers, polymeric surfactants and polyolefins e.g. polyethylene, polypropylene or ethylene/propylene copolymers or terpolymers. The low viscosity liquid need not be a single material. It can be a mixture of two or more materials chosen to obtain the optimum viscosity and flow rate for the particular pore size of the microporous structure used or to impart particular surface properties to the extruded polymeric core material. For example, the viscosity of a low molecular weight liquid can be lowered further by mixing it with petroleum or mineral oils or waxes, i.e. 17.5% EPDM in paraffin wax, or 80% polyethylene/20% wax. 
     The choice of the low viscosity liquid is also dependent upon the desired effects on the properties of the extruded polymeric core material. For example, the present process has been found quite useful in providing a smooth thin coating of a crystalline polymer, such as polyethylene, on the surface of an amorphous or elastomeric core material, such as EPM or EPDM or chlorosulfonated polyethylene rubber to help prevent massing, i.e. sticking together, of the extruded strands or pellets made by chopping the strands. The low viscosity material forming the sheath can be removed from the extrudate or it can remain as a coating on the extrudate. The sheath can be removed by a variety of ways, for example, if the sheath is a water-soluble material, such as polyvinyl alcohol, it can be washed off. The sheath may remain on the extrudate, especially if the low viscosity material is substantially the same material as the high viscosity core material. 
     The successful operation of this process depends importantly on the pressure applied to the low viscosity liquid so that the required minimum pressure drop across the microporous structure is at least about 0.10 MPa, and a lower viscosity material is forced through the microporous structure to form a sheath on the polymeric material before the resulting liquid sheath-core composite enters the narrowed flow passage in the die cavity. The process permits a significant reduction of the die pressure on the extruding polymeric core material and one obtains a smooth polymeric extrudate at a higher rate of extrusion. 
     The following examples illustrate preferred embodiments of the invention. All amounts are by weight unless otherwise indicated. 
     EXAMPLE I 
     An EPDM rubber having an ethylene content (73 wt %) and a bulk viscosity of 10 5  Pa.s at 190° C. (ML4 at 120° C.=60) was melted and pumped by a 38 mm extruder at 200° C. to a four hole extrusion die of the type shown in FIG. 2, wherein the porous metal cylinders had a nominal pore size of 0.5 μm an exposed length between the top and bottom plates of 0.75 mm and located upstream from the narrowed flow passage of the die cavity. Various low viscosity liquids, as shown in Table I, were pumped into the space between the upper and lower die plates at the pressures indicated. The data in the table show that the presence of the low viscosity liquid reduced the extrusion pressure needed to force the EPDM rubber through the die and that when the pressure drop, or difference, between the low viscosity liquid and the core polymer being extruded was at least 0.26 Pa.s the extrudate had a smooth surface with no evidence of melt fracture. 
     This example also illustrates that smooth pellets made from smooth extrudates could be much more easily dried than pellets with rough surfaces when both were immersed in water during pelletization. The smooth pellets were quickly air dried during vacuum conveying, while the rough pellets could be dried only with prolonged treatment with hot air. 
     
                                           TABLE I                                 
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HIGH VISCOSITY POLYMER                                                    
                  LOW VISCOSITY LIQUID             PRES-                  
         PRES-                            PRES-    SURE                   
    RATE.sup.(1)                                                          
         SURE                                                             
             TEMP.        VISCOSITY                                       
                                  CONC..sup.(2)                           
                                          SURE                            
                                              RATE.sup.(1)                
                                                   DROP                   
                                                       EXTRUDATE          
TYPE                                                                      
    g/m/h                                                                 
         MPa °C.                                                   
                  TYPE    Pa s    wt % of EXTR.                           
                                          MPa g/m/h                       
                                                   MPa SURFACE            
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EPDM                                                                      
    50   8.18                                                             
             200  NONE    FOR COMPARISON               VERY               
                                                       ROUGH              
EPDM                                                                      
    73   4.92                                                             
             200  SILICONE                                                
                          1       0.02    5.0 0.015                       
                                                   0.08                   
                                                       ROUGH              
                  OIL                                                     
EPDM                                                                      
    89   4.64                                                             
             200  SILICONE                                                
                          12.5    0.03    4.9 0.027                       
                                                   0.26                   
                                                       SMOOTH             
                  OIL                                                     
EPDM                                                                      
    85   6.38                                                             
             200  SILICONE                                                
                          30      0.15    8.4 0.13 2.02                   
                                                       SMOOTH             
                  OIL                                                     
EPDM                                                                      
    88   8.26                                                             
             200  83% PAR-                                                
                          2.5     0.35    12.45                           
                                              0.31 4.19                   
                                                       SMOOTH             
                  AFFIN WAX                                               
                  +                                                       
                  17% EPDM                                                
                  RUBBER                                                  
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 .sup.(1) g/m/h = GRAM PER MINUTE PER DIE HOLE OPENING                    
 .sup.(2) CONC. = CONCENTRATION                                           
 
    
     EXAMPLE II 
     An EPDM rubber having an ethylene content (75 wt %) and a bulk viscosity of (5×10 4 ) Pa.s at 190° C. was melted and pumped by a 38 mm extruder at 200° C. to a six-hole die of the type shown in FIG. 2 wherein the porous metal cylinders, located upstream from the narrowed flow passage of the die cavity, had a nominal pore size of 40 μm for the first two cases shown in Table II and 90 μm for the last case. The exposed length between the top and bottom plates was 12.5 mm. 
     Low viscosity polyethylene or polypropylene was melted and pumped into the space between the upper and lower die plates at the pressure indicated. The extruded strands were smooth and were completely coated with polyethylene or polypropylene in the amount shown in Table II. 
     
                                           TABLE II                                
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HIGH VISCOSITY POLYMER                                                    
                  LOW VISCOSITY LIQUID             PRES-                  
         PRES-                            PRES-    SURE                   
    RATE.sup.(1)                                                          
         SURE                                                             
             TEMP.        VISCOSITY                                       
                                  CONC..sup.(2)                           
                                          SURE                            
                                              RATE.sup.(1)                
                                                   DROP                   
                                                       EXTRUDATE          
TYPE                                                                      
    g/m/h                                                                 
         MPa °C.                                                   
                  TYPE    Pa s    wt % of EXTR.                           
                                          MPa g/m/h                       
                                                   MPa SURFACE            
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EPDM                                                                      
    33   5.6 200  NONE    FOR COMPARISON               VERY               
                                                       ROUGH              
EPDM                                                                      
    40   5.5 200  POLY-   450     2.7     18.8                            
                                              1.1  13.3                   
                                                       SMOOTH             
                  ETHYLENE                                                
EPDM                                                                      
    53   2.2 200  POLYPRO-                                                
                          600     1.9     10  1.0  7.8 SMOOTH             
                  PYLENE                                                  
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 .sup.(1) g/m/h = GRAM PER MINUTE PER DIE HOLE OPENING                    
 .sup.(2) CONC. = CONCENTRATION                                           
 
    
     EXAMPLE III 
     High density polyethylene having a bulk viscosity of 6×10 3  Pa.s at 190° C. and chlorosulfonated polyethylene rubber having a bulk viscosity of 1×10 4  Pa.s at 121° C. were melted and pumped by a 18.75 mm extruder to a one hole die of the type shown in FIG. 1 wherein the porous metal cylinder, located upstream from the narrowed flow passage of the die cavity, had a nominal pore size of 40 μm and the exposed length between plates was 12.5 mm. 
     Silicone oil was pumped into the space between the die plates at the pressure indicated. The marked reduction in extrusion pressure and the improved extrudate surface are shown in Table III. 
     
                                           TABLE III                               
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HIGH VISCOSITY POLYMER                                                    
                    LOW VISCOSITY LIQUID           PRES-                  
           PRES-                          PRES-    SURE                   
      RATE.sup.(1)                                                        
           SURE                                                           
               TEMP.       VISCOSITY                                      
                                  CONC..sup.(2)                           
                                          SURE                            
                                              RATE.sup.(1)                
                                                   DROP                   
                                                       EXTRUDATE          
TYPE  g/m/h                                                               
           MPa °C.                                                 
                    TYPE   Pa s   wt % of EXTR.                           
                                          MPa g/m/h                       
                                                   MPa SURFACE            
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POLY- 23   14.8                                                           
               185  NONE   FOR COMPARISON              ROUGH              
ETHY-                                                                     
LENE                                                                      
POLY- 22   7.6 185  SILICONE                                              
                           12.5   6.8     11.7                            
                                              1.5  4.1 SMOOTH             
ETHY-               OIL                                                   
LENE                                                                      
CSM   44   18.1                                                           
               140  NONE   FOR COMPARISON              VERY               
RUBBER                                                 ROUGH              
CSM   45   7.9 140  SILICONE                                              
                           12.5   3.5     12.7                            
                                              1.6  4.8 SMOOTH             
RUBBER              OIL                                                   
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 .sup.(1) g/m/h = GRAM PER MINUTE PER DIE HOLE OPENING                    
 .sup.(2) CONC. = CONCENTRATION                                           
 
    
     EXAMPLE IV 
     A fluoroelastomer compound containing fillers and curing agents derived from units of vinylidene fluoride (60%) and hexafluoropropylene (40 %) having a bulk viscosity of 5×10 4  Pa.s at 121° C. was melted and pumped by a 38 mm extruder to a four hole die of the type shown in FIG. 2 wherein the porous metal cylinder, located upstream from the narrowed flow passage of the die cavity, had a nominal pore size of 10 μm and the exposed length between plates was 1.27 mm. 
     A polyethylene (49%), paraffin wax (49%) yellow dye (2%), PE/Wax Blend, having a bulk viscosity of 10 3  Pa.s was pumped into the space between the die plates at the pressure indicated. The marked reduction in extrusion pressure and the improved extrudate surface are shown in Table IV. No evidence of die peel was seen during this experiment. 
     
                                           TABLE IV                                
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HIGH VISCOSITY POLYMER                                                    
                   LOW VISCOSITY LIQUID            PRES-                  
          PRES-                           PRES-    SURE                   
     RATE.sup.(1)                                                         
          SURE                                                            
              TEMP.       VISCOSITY                                       
                                  CONC..sup.(2)                           
                                          SURE                            
                                              RATE.sup.(1)                
                                                   DROP                   
                                                       EXTRUDATE          
TYPE g/m/h                                                                
          MPa °C.                                                  
                   TYPE   Pa s    wt % of EXTR.                           
                                          MPa g/m/h                       
                                                   MPa SURFACE            
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Fluoro                                                                    
      45  12.4                                                            
              140  NONE   FOR COMPARISON               MELT               
elasto-                                                FRACTURE,          
mer.sup.(3)                                            DIE PEEL           
                                                       PRESENT            
Fluoro                                                                    
     112  8.3 130  PE/WAX 10.sup.3                                        
                                  3.8     13.3                            
                                              4.3  5.0 SMOOTH             
elasto-                                                STRANDS,           
mer.sup.(3)                                            NO EVI-            
                                                       DENCE OF           
                                                       DIE                
__________________________________________________________________________
                                                       PEEL               
 .sup.(1) g/m/h = GRAM PER MINUTE PER DIE HOLE OPENING                    
  .sup.(2) CONC. = CONCENTRATION                                          
 .sup.(3) VINYLIDENE FLUORIDE (60%) HEXAFLUOROPROPYLENE (40%)