Patent Publication Number: US-2012034118-A1

Title: Fluorinated Elastic Tube

Description:
TECHNICAL FIELD 
     The present invention relates to an elastic tube, and it relates to an elastic tube useful for controlling the circulation of fluid in the tube by means of the pressure in a tube diameter direction, such as an elastic tube preferably used in a pinch valve or a roller pump. 
     BACKGROUND ART 
     When a pinch valve is used, the circulation of fluid (e.g., liquid) is stopped by having pressure applied to an elastic tube in a diameter direction thereof, and the circulation of the fluid is started by having the pressure removed. Furthermore, when a roller pump is used, pressure is applied to an elastic tube in a diameter direction thereof by using a roller, and the roller is moved in an axial direction of the tube while maintaining this pressed state to send out fluid (e.g., liquid). When compared to ordinary valves and pumps, the pinch valve and the roller pump can have a simple flow path structure, and the possibility of contamination in the fluid is small. Therefore, they are often used in fields involving food, medical equipment, and others, and in recent years, they are used for sending photoresist liquids in the production of semiconductors. 
     In general, silicone rubbers are used for elastic tubes because the silicone rubbers are superior in sustaining elasticity. However, the silicone rubbers are inferior in chemical resistance when compared to fluororesins and others. Therefore, when a highly corrosive fluid (e.g., photoresist liquids, liquids for operating processing mechanical devices, and highly corrosive liquids used in fields such as pharmaceutical, food, medical services, and chemistry) is circulated through an elastic tube disclosed in Patent Document 1, the durability of the tube is greatly deteriorated. 
     To improve the durability of a tube, there have been proposed a technique of protecting the inner surface of a silicone elastomer tube with a fluororesin layer (e.g., Patent Document 1), and a technique of forming a tube with a fluoroelastomer (e.g., Patent Document 2). However, even if the inner surface of a silicone elastomer tube is protected with a fluororesin layer, the adhesion is not sufficient between the elastomer layer and the fluororesin layer on the inner surface thereof. In particular, when compressing and releasing are repeated in a diameter direction of the tube and stress is loaded therein, the stress is concentrated in a joining interface of the inner surface layer and the elastomer layer, resulting in an occurrence of an interlayer peeling particularly at a bent portion referred to as “cheek portion”, and thereby sufficient durability cannot be obtained. 
     On the other hand, when compared to silicone elastomers and others, a fluoroelastomer tube has a high tackiness. The “tackiness” refers to a property of easily sticking to a material identical to oneself or a different material, and when tackiness is high, for example, if an elastic tube is pressed by a roller of a roller pump and left at this state, inner surfaces thereof may stick to each other without being restored, resulting in a blockage of the tube. In addition, when the tube is repeatedly used, due to high tackiness, the inner surface of the tube can easily be damaged and the durability thereof becomes deteriorated. 
     To improve the durability of a fluoroelastomer tube, Patent Documents 3 and 4 propose covering the inner surface of the tube with a fluororesin layer having specific hardness. The thickness of the fluororesin layer is specified to be from 5 to 300 μm in Patent Document 3, and is specified to be from 0.5% to 70% (200 μm in Examples) of the thickness of a base material of the tube in Patent Document 4. In addition, as the fluororesin, non-amorphous fluororesins having high crystallinity are used, such as polytetrafluoroethylene (PTFE), tetrafluoroethylene-hexafluoropropylene copolymer, polyvinylidene fluoride (PVDF), tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer, tetrafluoroethylene-ethylene copolymer (ETFE), and vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene ternary copolymer. However, even with these examples, peeling between the fluoroelastomer and the fluororesin layer on the inner surface thereof occurs after repeated usage, and thereby sufficient durability cannot be obtained. 
     To solve both the peeling problem when the inner surface of a silicone elastomer tube is covered with a fluororesin layer, and the peeling problem when the inner surface of a fluoroelastomer tube is covered with a fluororesin layer; the present inventors have proposed to use a porous fluororesin, of which pores are filled with an elastomer, as an middle layer between the elastic tube and the fluororesin layer on the inner surface thereof (Patent Document 5). Although the durability of the tube was greatly improved with this technique, further improvement has been desired in durability. 
     PRIOR ART DOCUMENTS 
     Patent Documents 
     Patent Document 1: Japanese Utility Model Publication No. 4-47185 
     Patent Document 2: Japanese Utility Model Publication No. 59-41687 
     Patent Document 3: Japanese Patent Laid-open Publication No. 2001-193659 
     Patent Document 4: Japanese Patent Laid-open Publication No. 2004-1467 
     Patent Document 5: Japanese Patent Laid-open Publication No. 2008-30471 
     SUMMARY OF THE INVENTION 
     Problems to be Solved by the Invention 
     It is an object of the present invention to provide an elastic tube having further improved durability while ensuring solvent resistance. 
     Means of Solving the Problems 
     To solve the problems described above, the present inventors have intensively studied, and as a result, they have found that the durability of an elastic tube against repeated pressing can significantly be improved by selecting a fluoroelastomer as the elastomer, covering the inner surface of a hollow body made of this fluoroelastomer with an amorphous perfluororesin, and adjusting the thickness of the amorphous perfluororesin to not greater than 1 μm by utilizing its solvent solubility, thereby completing the present invention. 
     That is, the fluorinated elastic tube of the present invention is characterized in that it comprises an elastic hollow body made of a fluorinated polymer and an amorphous perfluororesin membrane having a thickness of not greater than 1 μm and being in direct close contact with the under surface of the hollow body. The amorphous perfluororesin membrane may have a critical surface tension of, for example, not higher than 20 mN/m. The amorphous perfluororesin may have a repeating unit of, for example, formula (1), (2a), or (2b): 
     
       
         
         
             
             
         
       
     
     wherein R 1 , R 2 , and R 3  are each independently a fluorine atom or a perfluoroalkyl group; p+q+r is an integer of from 1 to 6; p is an integer of from 0 to 5; q is an integer of from 0 to 4; and r is 0 or 1. 
     The above elastic hollow body made of a fluorinated polymer may be formed of, for example, a porous fluororesin (in particular, an expanded porous polytetrafluoroethylene) and a fluoroelastomer filling pores of the porous fluororesin. The above porous fluororesin may usually have a pore ratio of from 40% to 98% and a maximum pore diameter of from 0.01 to 20 μm. The preferred elastic hollow body is a roll-shaped product having a first layer made of a fluoroelastomer and a second layer made of a porous fluororesin, of which pores are filled with a fluoroelastomer. The thickness ratio of the first layer to the second layer (the first layer/the second layer) may be, for example, not greater than 6.5/1. The above elastic hollow body may have an inner diameter of, for example, from about 1 to 40 mm and a thickness of, for example, from about 0.5 to 25 mm. The above elastic hollow body may have a reinforcing layer and/or a low friction layer formed on the outer surface thereof. The reinforcing layer and/or low friction layer may be, for example, a tube-shaped product made of a fluororesin, a fluororesin coating layer, or a carbon-based coating layer. 
     The present invention further includes a pinch valve or a roller pump, using the above fluorinated elastic tube. 
     Effects of the Invention 
     In the present invention, since an amorphous perfluororesin membrane is used as a fluororesin membrane to be formed on the inner surface of an elastic hollow body, the fluororesin membrane on the inner surface can be formed to be extremely thin. When the fluororesin membrane (amorphous perfluororesin membrane) on the inner surface is formed to be extremely thin, rather than using an middle layer for adhesion (an adhesion layer), if the middle layer (an adhesion layer) is omitted, the fluororesin membrane (amorphous perfluororesin membrane) on the inner surface can be prevented from being peeled, making it possible to improve durability. Furthermore, in the present invention, since a fluoroelastomer is selected as the elastomer, solvent resistance can be ensured even though the inner surface is covered with an extremely thin fluororesin membrane (amorphous perfluororesin membrane) and a solvent can easily penetrate the coating film. 
    
    
     
       BRIEF EXPLANATION OF THE DRAWINGS 
         FIG. 1  is a schematic view showing one example of the fluorinated elastic tube of the present invention. 
         FIG. 2  is a schematic view showing one example of the elastic hollow body of the present invention. 
     
    
    
     MODE FOR CARRYING OUT THE INVENTION 
     The fluorinated elastic tube of the present invention comprises, for example, as shown in the cross-sectional view of  FIG. 1 , an elastic hollow body  10  made of a fluorinated polymer; and a fluororesin membrane  20  being in direct close contact with the inner surface of the hollow body  10 . The fluororesin membrane  20  is an amorphous perfluororesin membrane having a thickness of not greater than 1 μm. The formation of the amorphous perfluororesin membrane  20  on the inner surface makes it possible to prevent tacking that occurs when the elastic hollow body  10  is pressed. In addition, since the elastic hollow body  10  is made of a fluorinated polymer and the elastic hollow body itself has solvent resistance, the solvent resistance of the tube  1  can be ensured, even when the amorphous perfluororesin film  20  on the inner surface is formed to be as thin as not greater than 1 μm and when a liquid circulating the tube  1  penetrates the amorphous perfluororesin film  20 . Furthermore, when the amorphous perfluororesin film  20  on the inner surface is formed to be extremely thin, the amorphous perfluororesin film  20  becomes very difficult to be peeled, even when the tube  1  is repeatedly pressed, and the durability of the tube against repeated pressing can significantly be improved. This durability is beyond the durability obtained when the fluororesin film  20  is attached to the elastic hollow body  10  with an adhesive layer (an middle layer). 
     In general, fluororesins easily crystallize, and the crystallized portions thereof will not be dissolved in a solvent. Therefore, ordinary fluororesins are applied by, for example, melt coating; however, melt coating results in thick membranes. In contrast, the amorphous perfluororesin to be used as the fluororesin membrane  20  in the present invention can be dissolved in a solvent (in particular, a fluorinated solvent). Therefore, the thickness of the fluororesin film  20  after film formation can be made to be extremely thin by controlling a concentration thereof. In addition, the amorphous perfluororesin has lower critical surface tension, and therefore, the prevention of tacking is further improved. 
     The amorphous perfluororesin can be distinguished from ordinary fluororesins in that a ring structure (e.g., a five- or six-membered ring structure, and particularly, a five-membered ring structure) has been introduced in the main chain thereof. The perfluororesin having a ring structure introduced in the main chain thereof may include fluororesins having a repeating unit of formula (1), (2a), or (2b): 
     
       
         
         
             
             
         
       
     
     wherein R 1 , R 2 , and R 3  are each independently a fluorine atom or a perfluoroalkyl group; p+q+r is an integer of from 1 to 6; p is an integer of from 0 to 5; q is an integer of from 0 to 4; and r is 0 or 1. 
     A resin having a repeating unit (1) can be produced according to, for example, the method disclosed in Japanese Patent Laid-Open Publication No. 3-252474. In a preferred repeating unit (1), R 1  and R 2  are both a trifluoromethyl group. A polymer having this preferred repeating unit (1) is represented by, for example, formula (3) depicted below. The polymer of formula (3) is commercially available from DuPont as the product name “Teflon (registered trademark) AF.” 
     
       
         
         
             
             
         
       
     
     A polymer having a repeating unit (2a) or (2b) can be produced according to, for example, the methods disclosed in Japanese Patent Laid-Open Publication Nos. 2-129254 and 3-252475. A preferred repeating unit is represented by formula (4) depicted below. A polymer having a repeating unit (4) is commercially available from Asahi Glass Co., Ltd. as the product name “Cytop.” 
     
       
         
         
             
             
         
       
     
     wherein n is 1 or 2 (preferably, 2). 
     The amorphous perfluororesin film  20  may have a critical surface tension of, for example, not higher than 20 mN/m, preferably not higher than 18 mN/m, and more preferably not higher than 16 mN/m. If the amorphous perfluororesin film  20  has smaller critical surface tension, it becomes more difficult to cause tacking on the inner surface when the tube  1  is pressed. The lower limit of critical surface tension is not particularly limited, but it may be, for example, not lower than 6 mN/m, particularly not lower than 9 mN/m. 
     The amorphous perfluororesin film  20  has a thickness of not greater than 1 μm, and it may preferably have a thickness of not greater than 0.8 μm, more preferably not greater than 0.6 μm. The lower limit of thickness is not particularly limited, so long as the tacking of the elastic hollow body made of a fluororesin can be prevented, but it may be, for example, not smaller than 0.01 μm, particularly not smaller than 0.1 μm. 
     In this connection, the thickness of the amorphous perfluororesin film can be determined on the basis of a cross-sectional image obtained by an electron microscope. 
     On the other hand, so long as the elastic hollow body  10  is made of a fluorinated polymer, the elastic hollow body  10  may be a hollow body made of a fluoroelastomer alone, or may be a hollow body made of a fluoroelastomer and a porous fluororesin layer. The use of a porous fluororesin makes it possible to ensure the chemical resistance of the elastic hollow body  10  and reinforce the fluoroelastomer without deteriorating the elasticity thereof. 
     The fluoroelastomer may include: cross-linked products of a polymer having fluoromethylene in the main chain thereof; and fluorinated thermoplastic elastomers. The cross-linked precuts may include FKMs (binary FKMs, ternary FKMs, and perfluorovinyl ether-containing FKMs), FFKMs, TEF-Pr type fluoro-rubbers, TFE-Pr-VdF type fluoro-rubbers, fluorinated thermoplastic elastomers, and rubbers in which fluorinated polyether backbones are cross-linked with Si (e.g., liquid fluoro-rubbers) (see the formulae depicted below). In this connection, the liquid fluoro-rubbers are available from Shin-Etsu Chemical Co., Ltd. as “SIFEL” (product name). 
     
       
         
         
             
             
         
       
     
     Furthermore, the fluoroelastomer may be any of fluorosilicone rubbers such as cross-linked products of polysiloxane having fluoroalkyl groups bonded thereto (e.g., FMVQ; see the formula depicted below). 
     
       
         
         
             
             
         
       
     
     The fluoroelastomer does not necessarily have to be cured at the material stage, so long as the fluoroelastomer is finally cured (which is not limited to crosslinking, but broadly means to form a three-dimensional network structure; the same is true in the following unless indicated otherwise) by crosslinking of a crosslinkable elastomer or by interaction between the hard segments of a thermosetting elastomer. The fluoroelastomer raw material may be in either solid form (kneaded form) or liquid form. 
     The fluoroelastomer may form the elastic hollow body  10  by itself, but the fluoroelastomer may preferably be reinforced with a porous fluororesin (in particular, an expanded porous polytetrafluoroethylene). When using the porous fluororesin layer, for example, the pores of a hollow body made of a porous fluororesin may be filled with a fluoroelastomer, but the first layer  12  made of a fluoroelastomer and a porous fluororesin layer  13  (the second layer) having pores filled with a fluoroelastomer may preferably be layered to form a roll-shaped structure (hereinafter referred to as the roll-shaped elastic hollow body  10 ).  FIG. 2  is a schematic cross-sectional view showing one example of the roll-shaped elastic hollow body  10 . As disclosed in Japanese Patent Laid-Open Publication No. 2002-502735, the roll-shaped elastic hollow body  10  can be produced by winding a porous film, of which inside and surface are impregnated (coated) with a fluoroelastomer; and the mechanical strength of the elastic hollow body  10  can dramatically be improved. Furthermore, the roll-shaped elastic hollow body  10  exhibits remarkable excellent shape restorability when compression stress is released. 
     The thickness ratio (the first layer/the second layer) of the first layer  12  made of a fluoroelastomer and the second layer  13  made of a porous fluororesin layer having pores filled with a fluoroelastomer may be, for example, not higher than 6.5/1, preferably from 1/1 to 5/1. 
     The porous fluororesin may have a porosity of, for example, about from 40% to 98%, preferably about from 50% to 95%, and more preferably about from 60% to 90%. When the porosity is too small, the amount of the filling fluoroelastomer becomes small, so that pressing-force buffering function is deteriorated. In contrast, when the porosity is too great, the porous fluororesin has decreased mechanical strength. 
     The porosity is a value calculated, on the basis of the formula described below, from the apparent density p i  (in units of g/cm 3  as measured according to JIS K 6885) of a porous fluororesin, and the density (true density) ρ 2  (2.2 g/cm 3  in the case with PTFE) of an original fluororesin before being made porous. 
       Porosity (%)=(ρ 2 −ρ 1 )/ρ 2 ×100
 
     The maximum pore diameter of a porous fluororesin may appropriately be set from the viewpoint of the characteristics (ease to fill) of a fluoroelastomer to be used for filling, and may be, for example, not smaller than 0.01 μm, preferably not smaller than 0.1 μm, and not greater than 20 μm, preferably not greater than 10 μm. When the maximum pore diameter is too small, it is difficult to fill such a fluoroelastomer. In contrast, when the maximum pore diameter is too great, the mechanical strength may become insufficient. The maximum pore diameter can be measured according to the prescription (ethanol is used as a chemical agent) of ASTM F316-86. 
     The elastic hollow body  10  may have a thickness of, for example, about from 10% to 200%, preferably about from 20% to 150%, and more preferably about from 25% to 125%, relative to the inner diameter of the elastic hollow body  10 . Furthermore, the elastic hollow body  10  may have a thickness of, for example, about from 0.5 to 25 mm, preferably about from 0.7 to 10 mm, and more preferably about from 1.0 to 5 mm If the elastic hollow body  10  is too thin, when the tube  1  is used in a pinch valve or a roller pump, there is fear that the tube  1  cannot withstand the inner pressure of fluid in the tube  1  to cause rupture. Furthermore, the shape recoverability exhibited when compression stress (pressing force) is released becomes insufficient. In contrast, if the elastic hollow body  10  is too thick, it becomes difficult to block the tube  1  by pressing. 
     The elastic hollow body  10  may have an inner diameter of, for example, about from 1 to 40 mm, preferably about from 1 to 20 mm, and preferably about from 2 to 10 mm. The elastic hollow body  10  may have an outer diameter of, for example, about from 3 to 60 mm, preferably about from 4 to 40 mm, and more preferably about from 5 to 20 mm. 
     If necessary, a reinforcing layer and/or a low friction layer may further be formed on the outer surface of the elastic hollow body  10 . The formation of a reinforcing layer makes it possible both to prevent abrasion occurring when the outer surface of the fluorinated elastic tube  1  is subjected to pressing force, and to prevent the rupture of the fluorinated elastic tube  1  when the tube  1  is subjected to strong inner pressure. The formation of a low friction layer makes it possible both to disperse stress occurring when the fluorinated elastic tube  1  is subjected to pressing force associated with shearing, and to prevent rupture and abrasion on the outer surface of the fluorinated elastic tube  1 . 
     For the reinforcing layer, various materials can be used, including polymer materials such as vinyl chloride, polystyrene, polyesters (e.g., polyethylene terephthalate), polyolefins (e.g., polyethylene, polypropylene), polyamides, polyimides, and fluororesins; and inorganic materials such as glass fibers. 
     Furthermore, the reinforcing layer may have any of the shapes such as applied products; extruded tubes; extruded-stretched tubes; wound products of stretched films; wound products of solid films; wound products of porous films; products formed in a tube shape from threads; and wound products of woven fabrics, knitted fabrics, braided materials, laces, nets, or other materials. 
     The reinforcing layer may preferably be, although need not be, fixed on the elastic hollow body  10  from the viewpoints of further improvements in abrasion resistances and in rupture resistance. When the reinforcing layer is fixed on the elastic hollow body  10 , the fixing method is not particularly limited, and, for example, it may be fixed with an adhesive; however, the reinforcing layer  10  may preferably be fixed by making use of a liquid fluoroelastomer material, as an adhesive, which material is the same as the material of the elastic hollow body  10 . In addition, the reinforcing layer may also preferably be formed and fixed by making use of the contractive force of the reinforcing layer  10 . The use of the contractive force makes it possible not to deteriorate the elasticity of the tube  1 . 
     The reinforcing layer may preferably be any of the tubular products of fluororesins, particularly, tubular products of polytetrafluoroethylene (PTFE). The tubular products of fluororesins (particularly, tubular products of PTFE) have excellent abrasion resistance, chemical resistance, heat resistance, and other resistances. 
     When the reinforcing layer is formed from a fluororesin (particularly, PTFE), as the reinforcing layer, there may desirably be used any of wound products of porous fluororesin films; products formed in a tube shape from fluororesin threads; and wound products of woven fabrics, knitted fabrics, braided materials, laces, nets, or other materials, all of which are made of fluororesin threads. The use of these products makes it possible to firmly attach the reinforcing layer to the elastic hollow body  10  because an adhesive penetrates into the pores or between the fibers. 
     As the low friction layer, there can be used coating layers formed from low friction materials, for example, fluororesin-based coating layers such as PTFE and PFA, and carbon-based coating layers such as diamond, diamond-like carbon, and graphite. The preferred low friction reinforcing layer is a coating layer made of diamond-like carbon. The diamond-like carbon layer not only has low friction coefficient but also is less aggressive to a partner material, so that scratches and shavings are unlikely to occur on the partner material. The method of coating a low friction material is not particularly limited, but may include a method of applying and then solidifying a liquid product (e.g., a solution, a melt) containing the low friction material, and a method of vapor-depositing a low friction material (e.g., a method of transforming the low friction material into an ion or plasma, and then vapor-depositing it to a base material). In this connection, the physical vapor deposition (PVD) layer made of diamond-like carbon has been put into practical use by, for example, Nippon ITF Inc., as trademark “GENIUSCOAT F.” 
     The fluorinated elastic tube  1  of the present invention can be produced by appropriately employing any of the heretofore known methods. For example, when the elastic hollow body  10  is formed of a fluoroelastomer and a porous fluororesin layer, the porous fluororesin layer may be filled with the fluoroelastomer and then this filled product may be coated with the amorphous perfluororesin film  20 , or alternatively, the porous fluororesin layer may be coated with the amorphous perfluororesin film  20  and then the porous fluororesin layer may be filled with the fluoroelastomer. Furthermore, the elastic body (or porous fluororesin layer) may be formed in a hollow shape and then may be coated with the amorphous perfluororesin film  20 , or alternatively a planar elastic body (or porous fluororesin layer) may be coated with the amorphous perfluororesin film  20  and then this coated product may be wound. 
     In this connection, the elastic hollow body  10 , the planar elastic body, or the porous fluororesin layer may be subjected to etching or primer treatment for the purpose of improving their attachment to the amorphous perfluororesin membrane  20 ; however, such etching or primer treatment is not essential. 
     When the pores of a porous fluororesin layer is filled with a liquid fluoroelastomer, the viscosity (25° C.) of the liquid fluoroelastomer may be, for example, not higher than 1,000 Pa·s (10,000 poise), preferably not higher than 100 Pa·s, and more preferably not higher than 20 Pa·s. 
     When an elastic body (e.g., the elastic hollow body  10 , a planar elastic body) containing a fluoroelastomer is coated with the amorphous perfluororesin film  20 , for example, in the case when the porous fluororesin layer is filled with the fluoroelastomer and then coated with the amorphous perfluororesin film  20 , the amorphous perfluororesin film  20  may desirably be formed by the application of a liquid containing the amorphous perfluororesin dissolved therein. When a dispersion of a fluororesin has been applied, it is necessary to heat the applied liquid to a melting temperature range (about 300° C.) of the fluororesin for membrane formation. In contrast, when a liquid containing the amorphous perfluororesin dissolved therein has been applied, the applied liquid can be formed into a film by only volatilization to remove the solvent, so that there is no possibility that the fluoroelastomer is deteriorated. 
     As solvents for dissolving an amorphous perfluororesin, there are known, for example, fluorinated solvents. Such fluorinated solvents are commercially available as, for example, product name “Fluorinert,” product name “Novec,” from Sumitomo 3M Limited. 
     The fluorinated elastic tube  1  of the present invention can be used as a member for controlling the circulation of fluid by pressing, and can be used as, for example, an elastic tube in a pinch valve or a roller pump. The pinch valve is a device for controlling the circulation of fluid in a tube, by allowing the pinch valve, which operates by fluid pressure (e.g., air pressure, oil pressure), electricity, and other means, to press an elastic tube in the diameter direction from the side thereof, thereby flattening (particularly, blocking) the cross-section of the tube. Furthermore, the roller pump is a device for sending out fluid in the tube by allowing a pressing member such as a roller to press an elastic tube in the diameter direction, and by allowing the pressing member to move (particularly, to repeatedly move from the upstream side to the downstream side) in the axial direction of the elastic tube while maintaining this pressed state. 
     The type of fluid circulating in the tube  1  is not particularly limited, but it may be either gas or liquid, preferably liquid. 
     EXAMPLES 
     The present invention will hereinafter be described more specifically by reference to Examples, but the present invention is not limited to these Examples. The present invention can be put into practice after appropriate modifications or variations within a range meeting the gist described above and below, all of which are included in the technical scope of the present invention. 
     Example 1 
     A liquid fluoro-rubber (“SIFEL-617” available from Shin-Etsu Chemical Co., Ltd.) was applied to one side of a bidirectionally-expanded porous PTFE film (“ePTFE film” available from W.L. Gore &amp; Associates Co., Ltd.; having a width of 400 mm, a length (depth) of 816 mm, a porosity of 78%, a maximum pore diameter of 0.4 μm, and a thickness of 18 μm) to fill the pores of the porous PTFE film. This applied film was wound on a stainless steel bar material (having an outer diameter of 5 mm) while having the applied surface to be on the inner side and while preventing air from being wound inside. The liquid fluoro-rubber was cross-linked by heating at a temperature of 150° C. for 30 minutes. 
     After cooling, the wound product was twisted by hand to loosen the compression bonding to the stainless steel bar material, and the stainless steel bar material was pulled out to obtain an elastic hollow body  10  (having an inner diameter of 4.8 mm, an outer diameter of 9.6 mm, and a ratio of the thickness of the fluoro-rubber layer to the thickness of the porous PTFE layer filled with the fluoro-rubber of 3/1). 
     The elastic hollow body  10  was subjected to dip coating with an amorphous perfluororesin (product name “Teflon (registered trademark) AF-1600” available from DuPont), followed by solvent removal, to produce a fluorinated elastic tube  1 , on which inner surface an amorphous perfluororesin membrane  20  having a thickness of 0.5 μm was formed. 
     Example 2 
     A fluorinated elastic tube  1  was obtained in the same manner as described in Example 1, except that an amorphous perfluororesin membrane  20  having a thickness of 0.5 μm was formed on the inner surface using an amorphous perfluororesin available from Asahi Glass Co., Ltd. (product name “Cytop”). 
     Comparative Example 1 
     An elastic tube  1  was obtained in the same manner as described in Example 1, except that an amorphous partially-fluorinated resin membrane having a thickness of 10 μm was formed on the inner surface using an amorphous partially-fluorinated resin available from Asahi Glass Co., Ltd. (product name “Lumiflon”), instead of the amorphous perfluororesin film  20 . 
     Comparative Example 2 
     A fluorinated elastic tube  1  was obtained in the same manner as described in Example 1, except that the rubber to be filled in the pores of the bidirectionally-expanded porous PTFE film was changed to a liquid silicone rubber (“KE106” available from Shin-Etsu Chemical Co., Ltd.). 
     Comparative Example 3 
     The bidirectionally-expanded porous PTFE film (“ePTFE film” available from W.L. Gore &amp; Associates Co., Ltd.); having a width of 500 mm, a porosity of 90%, and a thickness of 20 μm) was compressed with a calender roll device having an outer diameter of 300 mm, a width of 600 mm, and an anti-rolling reaction force of 1 MN (maximum) under the conditions of a roll temperature of 70° C., a linear pressure of 8 N/mm 2 , and a feed rate of 6 m/min, to obtain a white opaque color film having a width of 500 mm, a length of 500 mm, a porosity of 5%, and a thickness of 2.1 μm. This white opaque film was sandwiched between two polyimide films (“UPILEX 20S” (product name) available from Ube Industries Ltd.); heat-pressed for 5 minutes using a hot press device having a press surface size of 750 mm×750 mm and a maximum welding pressure of 2 MN under the conditions of a press plate temperature of 400° C. and a surface pressure of 10 N/m 2 , and then, while maintaining the surface pressure, gradually cooled for 60 minutes to a press plate temperature of 25° C., to obtain a transparent PTFE film (dense PTFE film) having a width of 500 mm, a length of 500 mm, a porosity of 0%, and a thickness of 2 μm. 
     The dense PTFE film was cut in a size of a width of 400 mm and a length (depth) of 158 mm, and was wound ten times around a stainless steel bar material (having an outer diameter of 5 mm) so that the length (depth) direction thereof was corresponding to a wind-up direction (circumference direction), to form an inner layer having a thickness of about 20 μm. 
     A bidirectionally-expanded porous PTFE film (“ePTFE film” available from W.L. Gore &amp; Associates Co., Ltd.; having a width of 400 mm, a length (depth) of 81 mm, a porosity of 85%, a maximum pore diameter of 0.5 μm, and a thickness of 20 μm), which was to be used as an middle layer, was wound (the number of windings was 5) on the inner layer so that the length (depth) direction thereof was corresponding to the wind-up direction (circumference direction). This wound product was heated for 30 minutes at a temperature of 375° C. using a compulsive hot wind circulation/ventilation type thermo-hygrostat (“STPH-201” available from Espec Co., Ltd.) to respectively introduce heat seals between the films of the inner layer, between the films of the porous fluororesin film, and between the inner layer film and the porous fluororesin film, to obtain a cylindrical intermediate body having an outer diameter of 5.2 mm, which contained the stainless steel bar material having an outer diameter of 5 mm as a core material. Using a rubber spatula, 10 g of a heat-curable liquid fluoro-rubber (“SIFEL-8070A/B” available from Shin-Etsu Chemical Co., Ltd.) was applied to the porous fluororesin film surface of the cylindrical intermediate body to be impregnated in the pores. An excessive liquid fluoro-rubber was scraped off with the rubber spatula and a nonwoven fabric wiper to form a cylindrical intermediate body. 
     Separately, a heat-curable liquid fluoro-rubber (“SIFEL-617” available from Shin-Etsu Chemical Co., Ltd.) was applied to one side of a bidirectionally-expanded porous PTFE film (“ePTFE film” available from W.L. Gore &amp; Associates Co., Ltd.); 
     having a width of 400 mm, a length (depth) of 816 mm, a porosity of 78% a maximum pore diameter of 0.4 μm, and a thickness of 18 μm). This applied film was wound on the cylindrical intermediate body while having the applied surface to be on the inner side and while preventing air from being wound inside to form an outer layer (the number of windings was 35 times). 
     The liquid silicone rubber in the middle layer and the outer layer was cross-linked by heating at a temperature of 150° C. for 30 minutes. After cooling, the outer layer (elastic layer) was twisted by hand to loosen the compression bonding between the dense PTFE as the inner layer and the core material (stainless steel bar material), and the stainless steel bar material was pulled out to obtain a fluorinated elastic tube (having an inner diameter of 5 mm, an outer diameter of 9.6 mm, a length in the axial direction of 400 mm, a thickness of the inner layer of 20 μm, a thickness of the mid-layer of 100 μm, a thickness of the outer layer of 2.2 mm, a thickness of the fluoro-rubber layer as the outer layer (the first layer) of 1,550 μm; a thickness of the PTFE film layer filled with fluoro-rubber (the second layer) as the outer layer of 630 μm, and a ratio of the thickness of the first layer to the thickness of the second layer of 2.5/1). 
     The durability, chemical resistance, and obstructiveness of the fluorinated elastic tubes  1  obtained in Examples 1 to 2 and Comparative Examples 1 to 3 were evaluated in such a manner as described below. 
     1) Durability 
     Each of the fluorinated elastic tubes  1  was attached to a resin-made pinch valve for wet process (product name “Dymatrix AVPV3” available from Asahi Organic Chemicals Industry Co., Ltd.). This pinch valve has a 15 mm×10 mm prismatic piston (the tip peripheral portion is chamfered (having a curvature of 0.4)), and the pressing force of this piston can be controlled by pushing back the piston with compressed air while having the piston push, by means of a spring attached thereto, in a direction toward a flat plate. Each tube was inserted between the piston and the flat plate, and, without allowing any liquids to pass through the tube, the elastic tube was repeatedly pressed by the piston. The pressing conditions are as follows: 
     Pressing time: 1.5 sec for each pressing 
     Pressure release time: 1.5 sec for each pressure release 
     Maximum pressing force by spring: 1.3 MPa 
     Compressed air pressure: 0.4 MPa 
     The number of repetitions was counted until the fluororesin membrane on the inner surface of each tube was separated. 
     2) Chemical Resistance 
     This test was carried out in the same manner as described above in the section “1) Durability”, except that propylene glycol monomethyl ether acetate (PGMEA) was allowed to pass through each tube. 
     3) Obstructiveness 
     Each of the fluorinated elastic tubes  1  was attached to the above resin-made pinch valve for wet process (product name “Dymatrix AVPV3” available from Asahi Organic Chemicals Industry Co., Ltd.) so that the inlet side piping was connected to the outlet side piping, and each of the tubes  1  was pressed by the piston. The open end of the outlet side piping was submerged in water at a depth of 20 mm, pressure was applied to the open end of the inlet side piping for 30 seconds with compressed air (0.4 MPa), and the formation of air bubbles from the outlet side piping was visually observed. When air bubbles were not formed even by applying pressure for 30 seconds, it was determined as “excellent”, and when air bubbles were formed in applying pressure for 30 seconds, it was determined as “no good”. 
     The results are shown in Table 1 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 1 
               
             
            
               
                   
                   
               
               
                   
                 Inner layer 
                   
               
               
                   
                 (fluororesin membrane 20) 
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                   
                   
                 Critical 
                   
                 Middle 
                   
                   
                   
                   
               
               
                   
                   
                 surface 
                   
                 layer 
                 Outer layer 
               
               
                   
                   
                 tension 
                 Thick- 
                 (adhesive 
                 (elastic hollow 
                   
                 Chemical 
                 Obstructive- 
               
               
                   
                 Material 
                 (mN/m) 
                 ness 
                 layer) 
                 body 10) 
                 Durability 
                 resistance 
                 ness 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 Example 1 
                 Amorphous 
                 15 
                 0.5 μm 
                 Not formed 
                 Expanded 
                 ≧1,000,000 times   
                 ≧1,000,000 times   
                 Excellent 
               
               
                   
                 perfluororesin 
                   
                   
                   
                 porous PTFE/ 
               
               
                   
                 membrane 
                   
                   
                   
                 fluoro-rubber 
               
               
                   
                 (available from 
               
               
                   
                 DuPont, product 
               
               
                   
                 name “Teflon 
               
               
                   
                 AF-1600”) 
               
               
                 Example 2 
                 Amorphous 
                 19 
                 0.5 μm 
                 Not formed 
                 Expanded 
                 500,000 times 
                 500,000 times 
                 Excellent 
               
               
                   
                 perfluororesin 
                   
                   
                   
                 porous PTFE/ 
               
               
                   
                 membrane 
                   
                   
                   
                 fluoro-rubber 
               
               
                   
                 (available from 
               
               
                   
                 Asahi Grass Co, 
               
               
                   
                 Ltd., product 
               
               
                   
                 name “Cytop”) 
               
               
                 Comparative 
                 Amorphous 
                 33 
                  10 μm 
                 Not formed 
                 Expanded 
                  50,000 times 
                 Not evaluated 
                 Excellent 
               
               
                 Example 1 
                 partially- 
                   
                   
                   
                 porous PTFE/ 
               
               
                   
                 fluorinated resin 
                   
                   
                   
                 fluoro-rubber 
               
               
                   
                 membrane 
               
               
                   
                 (available from 
               
               
                   
                 Asahi Grass Co, 
               
               
                   
                 Ltd., product 
               
               
                   
                 name 
               
               
                   
                 “Lumiflon”) 
               
               
                 Comparative 
                 Amorphous 
                 15 
                 0.5 μm 
                 Not formed 
                 Expanded 
                 ≧1,000,000 times   
                  50,000 times 
                 Excellent 
               
               
                 Example 2 
                 perfluororesin 
                   
                   
                   
                 porous PTFE/ 
               
               
                   
                 membrane 
                   
                   
                   
                 silicone rubber 
               
               
                   
                 (available from 
               
               
                   
                 DuPont, product 
               
               
                   
                 name “Teflon 
               
               
                   
                 AF-1600”) 
               
               
                 Comparative 
                 Dense PTFE 
                 18 
                  20 μm 
                 Formed 
                 Expanded 
                 250,000 times 
                 250,000 times 
                 Excellent 
               
               
                 Example 3 
                 wound film 
                   
                   
                   
                 porous PTFE/ 
               
               
                   
                   
                   
                   
                   
                 fluoro-rubber 
               
               
                   
               
            
           
         
       
     
     As can be seen from Table 1, durability can be improved in the examples in which the amorphous perfluororesin membrane on the inner surface was formed to be extremely thin (Examples 1 and 2), as compared with the example in which the amorphous partially-fluorinated resin membrane had a thickness of 10 μm (Comparative Example 1) and the example in which the middle layer for adhesion was formed (Comparative Example 3). Furthermore, as can be seen from the comparison with Comparative Example 2, since the fluoro-rubber is used for the elastic hollow bodies  10  in Examples 1 and 2, there is no possibility that chemical resistance is deteriorated even when the amorphous perfluororesin membrane on the inner surface is formed to be thin. 
     INDUSTRIAL APPLICABILITY 
     The tube  1  of the present invention has excellent chemical resistance and therefore can be used for circulating fluids such as photoresist liquids, liquids for operating processing mechanical devices, and highly corrosive liquids used in fields including pharmaceutical, food, medical service, and chemistry. Furthermore, the tube  1  of the present invention has low tackiness and therefore can be used even for applications that cannot allow having a circulation component attached to the inner surface of the tube. 
     EXPLANATION OF NUMERALS 
       1  Fluorinated elastic tube 
       10  Elastic hollow body 
       20  Amorphous perfluororesin membrane (fluororesin membrane) 
       12  Layer made of fluoroelastomer 
       13  Layer made of porous fluororesin filled with fluoroelastomer