Abstract:
This invention relates to a method of uniaxially expanding a fluoropolymer product including the steps of expanding a green fluoropolymer product in a first direction to create a first-expanded fluoropolymer product, and expanding the first-expanded fluoropolymer product in the same first direction. As a result with the subject invention, it has been found that ultra-high expansion and a variety of different porous/fibril structures can be achieved by using subsequent expansion steps in the same direction. Various considerations exist with such methodology, including the selection of rates of expansion of both steps, the amount of expansion of both steps, pre-heating and cutting the product between expansion steps. Although the present invention can be used with various fluoropolymer products, it is particularly well-suited for use with PTFE.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority of U.S. Provisional Appln. No. 60/533,096, filed Dec. 30, 2003, the entire contents of which are incorporated by reference herein. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to methods of expanding fluoropolymer products and, more particularly, to methods of expanding polytetrafluoroethylene products. 
     BACKGROUND OF THE INVENTION 
     Methods of forming fluoropolymer products are known in the art, including extruding polytetrafluoroethylene (PTFE) products. PTFE is commonly ram extruded after being treated with a lubricant and formed into a billet. With extrusion, a “green” PTFE product is formed. As is well known, the lubricant may be volatilized, and the “green” PTFE may be expanded into a fibrillated state (referred to as expanded PTFE or ePTFE) and, thereafter, heated above sintering temperature to coalesce the material into a stable state.  FIG. 1  is a micrograph showing a typical fibrillated microstructure of uniaxially expanded PTFE. The elongated dark portions which generally extend in a top-to-bottom direction in the plane of  FIG. 1  are nodes. Fibrils are thin hair-like structures which extend left-to-right in the plane of  FIG. 1  and interconnect the nodes. As is typical with uniaxially expanded PTFE, the fibrils are disposed generally along the expansion direction. 
     Techniques have been developed in the prior art to expand PTFE in multiple steps, but along different axes. For example, it has been known to expand PTFE in a first longitudinal direction and then separately in a second perpendicular transverse direction to form biaxially expanded PTFE. U.S. Pat. No. 5,476,589, which issued on Dec. 19, 1995 to Bacino, discloses a method including in sequence: 1. transverse expansion of a PTFE tape; 2. two separate longitudinal expansions of the tape; 3. transverse expansion of the tape once again; and, 4. sintering of the final product. 
     Additionally, U.S. Pat. No. 5,749,880, which issued on May 12, 1998 to Banas et al. discloses longitudinally expanding PTFE tubes; encasing a stent within the tubes; sintering the tubes; and causing radial expansion of the assembly, which results in nodal deformation in the tubes. The previous multiple expansion techniques, however, have never dealt with obtaining ultra-high uniaxially expanded fluoropolymer, more particularly, ultra-high uniaxially expanded PTFE. 
     SUMMARY OF THE INVENTION 
     This invention relates to a method of uniaxially expanding a fluoropolymer product including the steps of expanding a green fluoropolymer product in a first direction to create a first-expanded fluoropolymer product, and expanding the first-expanded fluoropolymer product in the same first direction. As a result with the subject invention, it has been found that ultra-high expansion and a variety of different porous/fibril structures can be achieved by using subsequent expansion steps in the same direction. Various considerations exist with such methodology, including the selection of rates of expansion of both steps, the amount of expansion of both steps, pre-heating and cutting the product between expansion steps. Although the present invention can be used with various fluoropolymer products, it is particularly well-suited for use with PTFE. 
     As used herein, a “green” product is an unexpanded or essentially unexpanded product. A “green” product may include minimal expansion that occurs unintentionally such as, for example, during take-up from extrusion which may generate a tensive force in the product. In any regard, no intentional expansion has occurred to a “green” product. 
     This invention will be better understood through a study of the following detailed description and accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a micrograph of a typical uniaxially expanded PTFE structure; 
         FIG. 2  is a flow chart of a process related to the subject invention; and, 
         FIGS. 3(   a )- 8 ( b ) are micrographs of various expanded PTFE structures formed in accordance with the subject invention. Each figure labeled as “a” is shown under a magnification of 100×, while the corresponding figure labeled “b” is the same structure as the “a” figure, but shown under a magnification of 500×. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The invention provides a process of uniaxially expanding a fluoropolymer product from a green state which includes subsequent steps of uniaxially expanding the fluoropolymer product. With reference to  FIG. 2 , a flow chart is provided setting forth process steps relating to the subject invention, including optional steps. 
     The discussion herein will be with reference to PTFE for illustrative purposes. It is to be understood that the process can be used with other fluoropolymers. 
     Box A of  FIG. 2  represents an initial forming process. Any method of preparing green PTFE may be utilized. By way of non-limiting example, steps  10 - 16  are described to provide an exemplary process of preparing green PTFE. As is well known in the art, raw PTFE resin may be blended with a lubricant to aid in extrusion (step  10 ); the blended PTFE may then be preformed into a billet (step  12 ); and, the billet ram extruded into a desired shape, such as a tube or sheet (step  14 ). The lubricant may then be volatilized to remove the lubricant (step  16 ), thus providing “dry” green PTFE. 
     The green PTFE now is prepared for a first expansion step as represented by box B. More specifically, as represented by step  18 , the green PTFE undergoes a first uniaxial expansion. It is preferred that the direction of expansion be coincident with the direction in which the PTFE product was formed or extruded. Although not required, it is further preferred that the green PTFE be heated prior to the first expansion as represented by step  20 . Heating may be performed during other steps of the process, such as during the expansion of step  18 . Expansion can be performed using any known technique, preferably using any known expansion oven. 
     The first-expanded PTFE product is then subjected to a second expansion step as represented by box C. Specifically, as set forth in step  22 , the first-expanded PTFE product is expanded a second time uniaxially, in the same direction as the first expansion was performed in step  18 . Again, although not required, it is preferred that preheating be performed prior to the second expansion (step  24 ). Heating may be continued during the expansion of step  22 , or, alternatively the first-expanded PTFE product may only be heated during expansion. If heating during steps  18  and/or  20  cause to heat the PTFE first-expanded product to a greater temperature than that desired in steps  22  and/or  24 , it is preferred that a sufficient cooling interval be provided before steps  22  and/or  24  to allow for effective preheating. With steps  22  and/or  24  calling for heating to a greater temperature than that used with any of the steps in box B, a time interval between the steps of box B and the steps of box C is optional. 
     Where there are equipment constraints or other constraints limiting the amount of expansion, the PTFE product may be cut in between expansion steps as represented by step  26 . As such, after initial expansion and cutting, a cut segment of the first-expanded PTFE product resulting from step  18  may be utilized and subsequently expanded in step  22 . 
     Although not shown, additional uniaxial expansion steps are possible. Once all uniaxial expansion has been completed, the product is sintered (step  28 ) and unloaded (step  30 ) from the expansion equipment, e.g. an expansion oven. As will be recognized by those skilled in the art, additional steps may be performed in the process. For example, the acted-upon PTFE may be internally pressurized during, before or between expansion steps  18  and/or  22  to promote radial expansion. Also, the PTFE product may be rotated during expansion (steps  18  and/or  22 ). Furthermore, the PTFE product may be heated at various stages of the process or heating may be continuous during all steps of the process (at the same or different temperatures). In addition, the PTFE product may be optionally subjected to various post-forming operations (step  32 ). For example, the PTFE product may be formed as a tube and slitted to form a sheet. The sheet may then be later acted upon, e.g., be uni-axially or multi-axially expanded. Sintering (step  28 ) may be performed before or after the post-forming operations. 
     With the process described above, various fibrillated PTFE structures can be achieved depending on specific process parameters being utilized. With reference to  FIGS. 3(   a )- 8 ( b ), six different expanded PTFE structures achieved by the subject invention are shown ranging across a spectrum of node and fibril shapes and densities. (Each pair of similarly numbered figures shows the same structure—the figure labeled “a” is shown under a 100× magnification, while the figure labeled “b” is shown under a 500× magnification.)  FIGS. 3(   a ) and ( b ) show a structure with relatively small nodes, large inter-nodal distances (IND) (the spacing between nodes) and branched fibrils. The structure of  FIGS. 4(   a ) and ( b ) is similar to that of  FIGS. 3(   a ) and ( b ), but with smaller nodes.  FIGS. 5(   a ) and ( b ) show a structure with small nodes and large IND, however, the fibrils are generally axially aligned and not branched.  FIGS. 6(   a ) and ( b ) show a structure with axially aligned fibrils and large IND; here, however, the nodes are larger in size.  FIGS. 7(   a )-( b ) and  8 ( a )-( b ) depict the densest structures with relatively small IND, small nodes and axially aligned fibrils. The structure of  FIGS. 8(   a ) and ( b ) is the densest with the  FIG. 7(   a )-( b ) structure having more porosity due to slightly longer fibril lengths (i.e., slightly larger IND). As will be appreciated by those skilled in the art, and as discussed below, various parameters in the process of the subject invention can be varied to obtain desired structural qualities, such as smaller or larger nodes, IND, fibril branching, and so forth. 
     The following table includes a listing of parameters that may be manipulated in the process of the subject invention to affect physical characteristics of the resulting structure: 
     
       
         
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Processing Variable 
                 Effect of the Processing Variable 
               
               
                   
               
             
             
               
                 1. Percentage of lubricant 
                 Percentage of lubricant may dictate the 
               
               
                 in raw PTFE resin used to 
                 homogeneity of the PTFE product 
               
               
                 make product 
               
               
                 2. Green tube wall thickness 
                 Wall thickness may limit the amount of 
               
               
                   
                 achievable expansion 
               
               
                 3. Design of extruder cone 
                 Different reduction ratios in raw 
               
               
                 on extruder 
                 extruding may effect how PTFE product 
               
               
                   
                 is worked 
               
               
                 4. Percentage of lubricant 
                 Overly dried tube may become fragile 
               
               
                 remaining in green tube 
                 and vulnerable to handling damage 
               
               
                 after volatilizing 
               
               
                 5. Preheating temperature 
                 Heat content of PTFE structure may 
               
               
                 before an expansion stage 
                 affect mobility of molecular structure 
               
               
                   
                 during expansion stage 
               
               
                 6. Preheating time interval 
                 Longer dwell time will allow PTFE to 
               
               
                 before an expansion stage 
                 approach preheating temperature during 
               
               
                   
                 an expansion stage 
               
               
                 7. Expansion percentage for 
                 Higher expansion for a stage may lead 
               
               
                 an expansion stage 
                 to a more open structure (i.e., more 
               
               
                   
                 porous structure) 
               
               
                 8. Expansion pull rate for 
                 Different pull rates may cause differ- 
               
               
                 an expansion stage 
                 ent node and fibril structures and 
               
               
                   
                 failure modes 
               
               
                 9. Stage expansion 
                 Heating during expansion may affect 
               
               
                 temperature 
                 heat content of PTFE; stage expansion 
               
               
                   
                 temperature may equal preheating 
               
               
                   
                 temperature 
               
               
                 10. Post expansion stage 
                 Most likely not desired, since exposure 
               
               
                 dwell temperature 
                 to heat between expansion stages may 
               
               
                   
                 inadvertently cause unwanted sintering 
               
               
                 11. Post expansion stage 
                 Most likely not desired, since exposure 
               
               
                 heating time interval 
                 to heat between expansion stages may 
               
               
                   
                 inadvertently cause unwanted sintering 
               
               
                 12. Cooling time between 
                 To maximize heating effect of sub- 
               
               
                 expansion stages 
                 sequent expansion stage, cooling below 
               
               
                   
                 subsequent heating temperature may be 
               
               
                   
                 desired 
               
               
                 13. Sintering between 
                 Low-level sintering may be utilized to 
               
               
                 expansion stages 
                 adjust node and fibril patterns for 
               
               
                   
                 subsequent expansion 
               
               
                   
               
             
          
         
       
     
     As will be recognized, Table 1 does not provide an exhaustive list of parameters that can be altered to affect the process of the subject invention. For example, the composition (e.g., the resin grade; manufacturer) of the raw PTFE resin itself may have an affect on obtainable PTFE structures. 
     Various tests were conducted utilizing the process of the subject invention. Examples 1-6 set forth below were all run utilizing extruded tubes with the subject invention and the following parameters: 
     Initial green tube wall thickness: 240 μm 
     Inner diameter of initial green tube: 6.32 mm 
     Standard deviation of initial green tube wall thickness: 9.3 μm 
     PTFE resin: ICI CD-123 
     Ram extruder: 0.248 mm mandrel and 137 reduction 
     Lubricant: Isopar G 
     Volatizing process: Green tubes dried at 125° F. for 120 minutes 
     Expansion oven: MM II oven 
     Examples 1-6 were run with various expansion percentages, expansion pull rates, preheating temperatures and preheating time intervals. Tables 2 and 3 set forth properties of the resulting structures. 
     Example 1 
     PTFE green tube was subjected to two expansion stages, the first expansion stage percentage being 300% and the second expansion stage percentage being 1500% for a collective expansion of 6300%. The first expansion stage included a preheating at 580° F. for 4 minutes and was conducted at a rate of 35 cm/s, while the second expansion stage included a preheating at 350° F. for 4 minutes and was conducted at a rate of 1 cm/s.  FIGS. 3(   a ) and ( b ) are micrographs taken from the outer surface of the resulting structure. The resulting structure has an IND of 80-100 μm and nodal lengths of approximately 40 μm. 
     Example 2 
     PTFE green tube was subjected to two expansion stages, the first expansion stage percentage being 620% and the second expansion stage percentage being 620% for a collective expansion of 5084%. The first expansion stage included a preheating at 350° F. for 20 minutes and was conducted at a rate of 35 cm/s, while the second expansion stage included a preheating at 580° F. for 4 minutes and was conducted at a rate of 1 cm/s.  FIGS. 4(   a ) and ( b ) are micrographs taken from the outer surface of the resulting structure. The resulting structure has an IND of 80-100 μm and nodal lengths of less than 40 μm. 
     Example 3 
     PTFE green tube was subjected to two expansion stages, the first expansion stage percentage being 300% and the second expansion stage percentage being 1500% for a collective expansion of 6300%. The first expansion stage included a preheating at 350° F. for 4 minutes and was conducted at a rate of 35 cm/s, while the second expansion stage included a preheating at 580° F. for 20 minutes and was conducted at a rate of 1 cm/s.  FIGS. 5(   a ) and ( b ) are micrographs taken from the outer surface of the resulting structure. The resulting structure has an IND of approximately 230 μm and nodal lengths of approximately 7.9 μm. 
     Example 4 
     PTFE green tube was subjected to two expansion stages, the first expansion stage percentage being 620% and the second expansion stage percentage being 620% for a collective expansion of 5084%. The first expansion stage included a preheating at 350° F. for 4 minutes and was conducted at a rate of 1 cm/s, while the second expansion stage included a preheating at 350° F. for 4 minutes and was conducted at a rate of 1 cm/s.  FIGS. 6(   a ) and ( b ) are micrographs taken from the outer surface of the resulting structure. The resulting structure has an IND of approximately 60 μm and nodal lengths of approximately 80 μm. 
     Example 5 
     PTFE tube was subjected to two expansion stages, the first expansion stage percentage being 300% and the second expansion stage percentage being 1500% for a collective expansion of 6300%. The first expansion stage included a preheating at 350° F. for 20 minutes and was conducted at a rate of 35 cm/s, while the second expansion stage included a preheating at 350° F. for 4 minutes and was conducted at a rate of 35 cm/s.  FIGS. 7(   a ) and ( b ) are micrographs taken from the outer surface of the resulting structure. The resulting structure has an IND of approximately 40 μm and nodal lengths of approximately 10 μm. 
     Example 6 
     PTFE green tube was subjected to two expansion stages, the first expansion stage percentage being 300% and the second expansion stage percentage being 1500% for a collective expansion of 6300%. The first expansion stage included a preheating at 350° F. for 4 minutes and was conducted at a rate of 1 cm/s, while the second expansion stage included a preheating at 580° F. for 4 minutes and was conducted at a rate of 35 cm/s.  FIGS. 8(   a ) and ( b ) and micrographs taken from the outer surface of the resulting structure. The resulting structure has an IND of approximately 15 μm and nodal lengths of approximately 3.9 μm. 
     
       
         
               
             
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Average Axial Properties 
               
             
          
           
               
                   
                 Final Wall 
                 Load at 
                 Stress 
                 % Strain at 
                 Stress at 
                   
                   
                 Young&#39;s 
               
               
                   
                 Thickness 
                 Peak 
                 at Peak 
                 Peak 
                 Break 
                 % Strain 
                 Toughness 
                 Modulus 
               
               
                 Example 
                 (micrometers) 
                 (kg) 
                 (kg) 
                 (%) 
                 (kg/mm2) 
                 at Break (%) 
                 (kg/mm2) 
                 (kg/mm2) 
               
               
                   
               
               
                 1 
                 38.20 
                 2.01 
                 2.77 
                 15.80 
                 2.40 
                 36.15 
                 0.86 
                 46.98 
               
               
                 2 
                 54.00 
                 2.25 
                 2.19 
                 14.78 
                 1.82 
                 32.29 
                 0.60 
                 32.25 
               
               
                 3 
                 41.80 
                 1.53 
                 1.92 
                 12.55 
                 1.70 
                 24.64 
                 0.38 
                 37.81 
               
               
                 4 
                 48.80 
                 1.63 
                 1.76 
                 14.22 
                 1.67 
                 20.18 
                 0.29 
                 28.93 
               
               
                 5 
                 48.00 
                 1.94 
                 2.13 
                 32.87 
                 1.41 
                 65.88 
                 1.19 
                 23.92 
               
               
                 6 
                 37.80 
                 2.59 
                 3.62 
                 18.86 
                 2.35 
                 48.11 
                 1.52 
                 59.72 
               
               
                   
               
             
          
         
       
     
     
       
         
               
             
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 3 
               
             
             
               
                   
               
               
                 Average Radial Properties 
               
             
          
           
               
                   
                   
                 Load at 
                 Stress at 
                 % Strain 
                 Load at 
                 Stress at 
                 Strain at 
                   
                 Young&#39;s 
               
               
                   
                 Wall Thickness 
                 Peak 
                 Peak 
                 at Peak 
                 Break 
                 Break 
                 Break 
                 Toughness 
                 Modulus 
               
               
                 Example 
                 (micrometers) 
                 (g) 
                 (g/mm2) 
                 (%) 
                 (g) 
                 (g/mm2) 
                 (%) 
                 (g/mm2) 
                 (g/mm2) 
               
               
                   
               
             
          
           
               
                 1 
                 38.20 
                 65.90 
                 69.01 
                 803.20 
                 65.02 
                 68.08 
                 834.53 
                 322.93 
                 13.77 
               
               
                 2 
                 54.00 
                 87.28 
                 64.65 
                 1127.00 
                 86.72 
                 64.24 
                 1136.64 
                 427.19 
                 9.92 
               
               
                 3 
                 41.80 
                 62.31 
                 59.63 
                 675.80 
                 61.77 
                 59.11 
                 675.51 
                 235.69 
                 15.54 
               
               
                 4 
                 48.80 
                 49.14 
                 40.28 
                 388.80 
                 47.48 
                 38.92 
                 406.09 
                 88.28 
                 15.86 
               
               
                 5 
                 48.00 
                 83.71 
                 69.76 
                 510.30 
                 83.32 
                 69.43 
                 516.67 
                 218.17 
                 13.15 
               
               
                 6 
                 37.80 
                 108.70 
                 115.03 
                 1200.00 
                 108.07 
                 114.36 
                 1199.67 
                 714.92 
                 12.32 
               
               
                   
               
             
          
         
       
     
     In reviewing the test results, it has been observed that changes in certain process parameters generally correlate to different final product characteristics. It has been found that the second expansion pull rate has the strongest correlative effect on the physical properties of the final expanded PTFE product. In particular, the radial strength and toughness of the final product is positively correlated to the second expansion pull rate, wherein the radial strength and axial toughness of a final PTFE product are greater where a higher second expansion pull rate is used with the inventive process. For example, with reference to Example 1 second expansion pull rate of 1 cm/s was used, resulting in an average radial load at peak of 65.90 g and an average axial toughness of 0.86 kg/mm 2 , while in Example 5, a second expansion pull rate of 35 cm/s was used, resulting in an average radial load at peak of 83.71 g and an average axial toughness of 1.19 kg/mm 2 . In addition, the IND of the final product correlatively decreases with (i.e., has a negative correlation to) an increase in second expansion pull rate. For example, in Example 1, the IND is 80-100 μm with a second expansion pull rate of 1 cm/s, while in Example 5, the IND is ˜40 μm with a second expansion pull rate of 35 cm/s. The following is a table of identified process parameters along with product characteristics to which the identified parameters are positively or negatively correlated: 
     
       
         
               
               
             
           
               
                 TABLE 4 
               
               
                   
               
               
                 Process Parameter 
                 Affected Final Product Characteristic 
               
               
                   
               
             
             
               
                 Second expansion pull rate 
                 Positively correlated to radial 
               
               
                   
                 strength and axial toughness 
               
               
                 Second expansion pull rate 
                 Negatively correlated to IND 
               
               
                 First preheating time interval 
                 Positively correlated to wall thickness 
               
               
                 Second preheating time interval 
                 Positively correlated to wall thickness 
               
               
                 Difference between expansion 
                 Negatively correlated to wall thickness 
               
               
                 stage percentages of first and 
                 (e.g., larger difference between expan- 
               
               
                 second stages 
                 sion stage percentages negatively 
               
               
                   
                 correlates to a thicker wall) 
               
               
                   
               
             
          
         
       
     
     Likewise, it has been found that varying certain process parameters together may have an interacting effect on the physical properties of the final expanded product. For example, increasing the difference between expansion stage percentages of first and second expansion stages along with the first or second preheating time interval positively may correlate to increased wall thickness. Such interacting process parameters are listed in Table 5. 
     
       
         
               
               
             
           
               
                 TABLE 5 
               
               
                   
               
               
                 Process Parameters 
                 Affected Final Product Characteristics 
               
               
                   
               
             
             
               
                 1. Difference between expansion 
                 Positively correlated to increased 
               
               
                 stage percentages of first and 
                 wall thickness 
               
               
                 second expansion stages, and 2. 
               
               
                 first preheating time interval 
               
               
                 1. Difference between expansion 
                 Positively correlated to increased 
               
               
                 stage percentages of first and 
                 wall thickness 
               
               
                 second expansion stages, and 2. 
               
               
                 second preheating time interval. 
               
               
                 1. First expansion pull rate, 
                 Positively correlated to observable 
               
               
                 and 2. first preheating time 
                 porosity of node and fibril structure 
               
               
                 interval 
                 (specifically, observable branching 
               
               
                   
                 of fibrils) 
               
               
                 1. Difference between expansion 
                 Positively correlated to observable 
               
               
                 stage percentages of first and 
                 porosity of node and fibril structure 
               
               
                 second expansion stages, and 2. 
                 (specifically, fibril packing density) 
               
               
                 second preheating time interval 
               
               
                   
               
             
          
         
       
     
     As is readily appreciated, various process parameters can be utilized, although certain ranges are preferred. With regards to overall expansion, it is desired to obtain an overall and final expansion of the PTFE product of 1,000%-10,000% collectively from the various expansion steps (i.e., the resulting PTFE structure had been expanded 1,000%-10,000% from its initial green state). It has been found, for example as indicated in Examples 1, 3, 5 and 6, that with performing the first expansion step (step  18 ) with a 300% expansion and the second expansion step (step  22 ) with a 1,500% expansion, a collective expansion of 6,300% can be achieved. Also, with performing the first expansion step (step  18 ) with a 620% expansion and the second expansion step (step  22 ) with a 620% expansion, as indicated in Examples 2 and 4, a collective expansion of 5,084% can be achieved. Each expansion step can expand a product an expansion percentage of 1%-800%. Advantageously, the collective expansion resulting from the various individual expansion steps can be significantly larger than the expansion provided by each of the individual stages. As will be appreciated by those skilled in the art, various combinations of expansions can be utilized. 
     As indicated above, the pull rates of expansions are critical factors in practicing the subject invention. Preferably, the pull rates of expansion of any of the expansion steps (steps  18  and  22 ) falls in the range of about 50 cm/s or less. The different expansion steps (steps  18  and  22 ) may be conducted at different rates, more preferably with the expansion rate of the first expansion step (step  18 ) being greater than the expansion rate of the second expansion step (step  22 ). The lower expansion rate of the second expansion step (step  22 ) allows for more uniform expansion than if the rate was not lowered from the first expansion step (step  18 ). By way of non-limiting example, the expansion rate of the first expansion step (step  18 ) can be about 35 cm/s with the expansion rate of the second expansion step (step  22 ) being about 1 cm/s. Also, the pull rates of expansion need not remain constant during an expansion step, but may be varied during an expansion step (e.g., from faster to slower, or vice versa). 
     With regards to heating, three primary factors are implicated: timing of heating; duration; and temperature. As indicated above, heating may occur at various points in the subject process, but is most preferred as preheating before expansion. Heating may occur for any duration, but it is preferred that the preheat steps (steps  20  and  24 ) each be conducted for between about 4 and 20 minutes prior to the corresponding expansion step. The most critical factor in determining duration of heating lies in the time needed to achieve a desired temperature. The longer a heating operation is conducted, the more likely it is that a desired temperature is achieved which can evenly affect the PTFE product. As for temperatures, it is also preferred that the preheat temperatures be maintained below the sintering temperature of the relevant material (e.g., 660° F. for PTFE) and, more preferably, that the preheat temperature of the first preheat step (step  20 ) be higher than the preheat temperature of the second preheat step (step  24 ). For example, with PTFE, the first preheat step may have a temperature of about 580° F., while the second preheat step (step  24 ) may have a temperature of about 350° F. Preheating can be conducted at any temperature, although using a temperature in the range of 350° F. to 580° F. generally avoids sintering effects. Some sintering may be desired to achieve a desired node and fibril structure and higher temperatures can be accordingly utilized. As for heating outside of the preheating steps (steps  20  and  24 ), the same criteria discussed above apply. 
     While the invention has been described in relation to the preferred embodiments with several examples, it will be understood by those skilled in the art that various changes may be made without deviating from the spirit and scope of the invention as defined in the appended claims.