Foamed articles of fluorocarbon resins
This invention relates to foamed articles of fluorocarbon resins. 
     In recent years, foamed or expanded articles of fluorocarbon resins are used as insulations for communication cables, etc., taking advantage of their low dielectric constant, lightweightness and heat resistance. However, since fluorocarbon resins generally have a higher melting point than common synthetic resins, preparation of foamed articles from these resins invariably requires a higher molding temperature than common synthetic resins. Although extensive research has been conducted, blowing agents and foaming methods suited to fluorocarbon resins still remain to be developed. U.S. Pat. No. 3,072,583 discloses a process for producing such foamed articles. With this known process, fluoromethane gas is caused to penetrate into the resin to be extruded for a given period of time under a predetermined pressure, and the resin is then fed to an extruder to foam or expand the resin with the gas incorporated therein upon extrusion from the die. Nevertheless, this process encounter difficulty in giving a foamed article having uniform fine closed cells with a high degree of expansion because fluoromethane gas fails to penetrate into the resin sufficiently and further because the degree of penetration varies with lapse of time. Other known processes are disclosed in Unexamined Japanese Patent Publications Nos. SHO 52-53964, SHO 53-121864 and SHO 54-41969. These processes employ thermally decomposable organic or inorganic solid blowing agents to produce foamed articles, but the inorganic and organic blowing agents used have problems. The inorganic blowing agents, which start decomposition at relatively low temperatures, have difficulty in adjusting the rate of expansion and give molded products which are rough-surfaced or have a coarse cellular structure. The organic blowing agents rapidly decompose at high temperatures, making it difficult to afford foamed articles having uniform cells, while some kinds of blowing agents color the expanded product with the decomposition product. 
     An object of the present invention is to provide a process for producing a foamed or expanded article of fluorocarbon resin having a high degree of expansion and fine uniform closed cells. 
     Another object of the present invention is to provide a process for producing a foamed article of fluorocarbon resin in which the rate of expansion of resin is easily adjustable, overcoming the problem of rough surface and coarse cellular structure of the product. 
     Another object of the invention is to provide a process for producing a foamed article of fluorocarbon resin free of coloration due to the blowing agent. 
     Other objects and features of the present invention will become apparent from the following description. 
     The present invention provides a process for producing a foamed article of a fluorocarbon resin characterized in that the fluorocarbon resin is heated and molded in a molten state in the presence of: 
     (i) a depolymerizable polymer having a structural unit represented by the formula ##STR1## wherein R 1  is hydrogen or --COOR 4  group (wherein R 4  is hydrogen, a lower hydrocarbon group or fluorinated lower hydrocarbon group), R 2  is hydrogen, fluorine, a lower hydrocarbon group, fluorinated lower hydrocarbon group or --COOR 4  group (wherein R 4  is as defined above), and R 3  is (CH 2 ) k  COOR 4  group (wherein R 4  is as defined above, and k is 0 or 1) or ##STR2## group (wherein k is as defined above), or 
     (ii) a polyether compound represented by the formula 
     
         R.sup.5 (X.sup.1 O).sub.m R.sup.6                          ( 2) 
    
     wherein R 5  is hydrogen, halogen, hydroxyl group, a lower alkyl group or halogenated lower alkyl group, X 1  is a lower alkylene group or halogenated lower alkylene group, R 6  is hydrogen, a lower alkyl group, halogenated lower alkyl group, lower alkyloxy group or halogenated lower alkyloxy group, and m is an integer of from 2 to 30,000, or 
     (iii) a polycarbonyloxy compound having 2 to 20 carbon atoms. 
     In the specification and appended claims, the groups modified with the term &#34;lower&#34;, such as lower hydrocarbon group or lower alkyl group, are those having 1 to 6 carbon atoms. 
     Our research has revealed that when fluorocarbon resins are foamed in a molten state with heating in the presence of the compound (i), (ii) or (iii), the foregoing problems of the prior art can be substantially eliminated or greatly alleviated. The present invention has been accomplished based on this novel finding. 
     The fluorocarbon resins useful for the present invention are high polymeric compounds containing at least 50 wt.% of fluorine and having a number average molecular weight of about 20,000 to about 1,000,000 and a melt index (g/10 min) of about 0.5 to about 50 as measured according to ASTM D 1238-70. 
     Preferred examples of such fluorocarbon resins are: 
     (a) homopolymers of a monomer selected from the group consisting of tetrafluoroethylene, chlorotrifluoroethylene and 2,2-difluoroethylene, 
     (b) copolymers of at least two of the above monomers (a), and 
     (c) copolymers of at least one of the above monomers (a) with a monomer selected from the group consisting of hexafluoropropene, perfluoroalkyl vinyl ether, oxy-containing perfluoroalkyl vinyl ether and ethylene. 
     These fluorocarbon resins can be prepared by polymerizing the desired monomer or monomers by usual processes, such as emulsion, suspension, solution, block bulk and vapor-phase polymerization processes. A chain transfer agent such as methanol may be used in these processes. 
     When the fluorocarbon resin is a copolymer, the ratio of the monomers to be polymerized is not limited specifically. 
     The copolymer may be composed of at least three of the foregoing monomers, while the copolymer may contain other copolymerized ethylenically unsaturated compound insofar as the desired properties of the copolymer are not impaired. 
     Although not limitative, examples of oxy-containing perfluoroalkyl vinyl ethers are CF 3  CF 2  CF 2  OCF═CF 2 , ##STR3## 
     The depolymerizable polymer having a structural unit (1) is a homo- or co-polymer usually having a number average molecular weight of about 10,000 to about 5,000,000. It is a polymer of ethylenically unsaturated monomer(s) which is depolymerizable and vaporizable at a temperature of about 150° C. lower than the melting point of the fluorocarbon resin to the decomposition temperature thereof. The closer the depolymerization temperature to the melting point of the fluorocarbon resin, the better is the polymer. Preferred depolymerizable polymers are those wherein the hydrocarbon group is an aliphatic group, the substituent R 1  is H, R 2  is F or CH 3 , and R 3  is --COORf wherein Rf is H or a fluorinated lower aliphatic group. 
     More specific examples of preferred depolymerizable polymers are those having one of the following structural units. ##STR4## 
     The depolymerizable polymer having a structural unit of the formula (1) can be obtained by polymerizing a monomer represented by the formula ##STR5## wherein R 1 , R 2  and R 3  are as defined above, by the similar processes as used for preparing the fluorocarbon resin. 
     To obtain uniformly foamed articles, the depolymerizable polymer is used usually in an amount of about 0.01 to about 50 parts by weight, preferably about 0.05 to about 10 parts by weight, per 100 parts by weight of fluorocarbon resin. 
     When the depolymerizable polymer is used for practicing the invention, the composition of fluorocarbon resin and depolymerizable polymer usually has admixed therewith a heat-resistible nucleating agent in the form of fine particles of up to 50 μm in size, and serving as nuclei when the polymer is depolymerized. The expansion nucleating agent has the function of producing fine uniform cells. Examples of useful nucleating agents are talc, sericite, kieselguhr, boron nitride, silicon nitride, fine silica, alumina, zirconia, quartz powder, aluminum hydroxide, kaolin, bentonite, zinc sulfide, molybdenum disulfide, carbon black, graphite, titanium oxide and the like, although these examples are not limitative. The expansion nucleating agent is used usually in an amount of about 0.01 to about 10 parts by weight, preferably about 0.5 to about 5 parts by weight, per 100 parts by weight of the fluorocarbon resin. 
     The composition of fluorocarbon resin and depolymerizable polymer may further incorporate therein a metal compound which serves as a catalyst when the polymer is depolymerized. Examples of useful metal compounds are oxides or hydroxides of copper, cobalt, iron, nickel, chromium, vanadium, tantalum, cerium, thorium, manganese, bismuth, cadmium, gallium, iridium, rhodium, platinum, palladium, tin, tungsten, lithium, sodium, potassium, lead, ytterbium, molybdenum and silver, and compounds which are converted to oxides or hydroxides of such metals when heated to about 100° to about 500° C. More preferred metal compounds are oxides or hydroxides of chromium, cobalt, iron, nickel, cerium, thorium, manganese, bismuth, cadmium, lithium, sodium, potassium, lead and molybdenum. 
     Usually about 0.005 to about 5 parts by weight of the metal compound is used per 100 parts by weight of the fluorocarbon resin. 
     Other additives which can be admixed with the composition include fillers or reinforcing agents such as glass fiber, glass power, and asbestos fiber, stabilizers, lubricants, pigments and the like. 
     The polyether compounds to be used as blowing agents for the present invention and represented by the formula (2), i.e., R 5  (X 1  O) m  R 6  wherein R 5 , R 6  and X 1  are as defined above, are usually those having a number average molecular weight of 300 to 1,000,000 preferably 1,000 to 50,000, and containing at least two ether groups. More preferable examples of the compounds of the formula (2) are those represented by the formula 
     
         R.sup.7 (CY.sup.1.sub.2 CY.sup.2.sub.2 CY.sup.3.sub.2 O).sub.n CY.sup.4.sub.2 R.sup.8 
    
     wherein Y 1  to Y 4  are the same or different and are each hydrogen, halogen, a lower alkyl group or halogenated lower alkyl group, R 7  and R 8  are the same or different and are each hydrogen, halogen, hydroxyl group, a lower alkyl group, halogenated lower alkyl group, lower alkyl-oxy group, halogenated lower alkyl-oxy group, --OX 1  R 9  group (wherein X 1  is as defined above, R 9  is a lower alkyl-oxy group or halogenated lower alkyl-oxy group), --CY 1   2  COOH group (wherein Y 1  is as defined above) or --CY 1   2  COZ group (wherein Y 1  is as defined above, and Z is a halogen), and n is 1 to 10,000. 
     Examples of useful polyether compounds are: 
     F(CH 2  CF 2  CF 2  O) n  CH 2  CF 3  wherein n is 1 to 10,000. 
     F(CH 2  CF 2  CF 2  O) n  CH 2  CF 2  COF wherein n is 1 to 10,000. ##STR6##  wherein n is 1 to 10,000. C 3  F 7  O(CH 2  CF 2  CF 2  O) n  CH 2  CF 3  wherein n is 1 to 10,000. 
     F(CCl 2  CF 2  CF 2  O) n  CCl 2  CF 3  wherein n is 1 to 10,000. 
     In order to obtain uniformly foamed articles, the polyether compound is used usually in an amount of about 0.01 to about 50 parts by weight, preferably about 0.05 to about 10 parts by weight, per 100 parts by weight of the fluorocarbon resin. 
     When the polyether compound of the formula (2) is used as the blowing agent for fluorocarbon resins, a nucleating agent, metal compound and additives can also be admixed with the composition when required. As such nucleating agent, metal compound catalyst and additives, the same materials as used in combination with the depolymerizable polymer are usable in like proportions. 
     The polycarbonyl-oxy compounds useful as blowing agents for the present invention are aliphatic, alicyclic and aromatic carboxylic acids having at least two carbonyl-oxy groups in the molecule, and salts of such acids. These compounds have the function of foaming fluorocarbon resin on thermal decomposition when heated along with the resin. Examples of carboxylic acids having two carbonyl-oxy groups in the molecule are oxalic acid, itaconic acid, citraconic acid, acetonedicarboxylic acid, maleic acid, succinic acid, glutaric acid, phthalic acid, adipic acid and the like. Examples of carboxylic acids having three carbonyl-oxy groups in the molecule are citric acid, a conitic acid and the like. However, these examples are not limitative. Examples of salts of carboxylic acids having at least two carbonyl-oxy groups in the molecule are ammonium, sodium, potassium, magnesium, calcium and copper salts of above carboxylic acids. These examples are not limitative, either. 
     To obtain uniformly foamed articles, the polycarbonyl-oxy compound is used usually in an amount of about 0.01 to about 50 parts by weight, preferably about 0.05 to about parts by weight, per 100 parts by weight of fluorocarbon resin. 
     When the polycarbonyloxy compound is used as the blowing agent for fluorocarbon resins, a nucleating agent, metal compound and additives can be admixed with the composition when required, as in the foregoing cases. The same materials as already mentioned are usuable in similar proportions in this case. 
     According to the present invention, the composition containing as essential components a fluorocarbon resin and a blowing agent such as depolymerizable polymer is heated and molded in a molten state by extrusion expansion molding, injection expansion molding or die expansion molding. 
     When extrusion expansion molding is resorted to, the composition is made into fine particles or pellets, which are then continuously extruded by a screw extruder while being heated, to cover an electric wire or cable with the foamed resin or to give a foamed article in the form of a rod, pipe, plate, sheet, film, filament, strand or in other form. The foamed article is usable as a covering for electric wires or cables as above, heat insulator, sound-proof material, lightweight structural material, etc. To gradually depolymerize or decompose the depolymerizable polymer or like blowing agent and finally completely depolymerize or decompose the blowing agent when foamed articles are to be produced by extrusion expansion molding, the cylinder of the extruder is so heated that it has a progressively increasing temperature from the composition inlet toward the die portion, where the highest temperature is maintained. The extruder to be used usually has a cylinder diameter D of 20 to 50 mm and an L/D ratio of 15 to 25 wherein L is the length of the screw. However, these dimensions are not limitative. 
     The expansion ratio (%) of the foamed article is expressed by: 
     
         (A-B)/A×100 
    
     wherein A is the specific gravity of the fluorocarbon resin, and B is the specific gravity of the foamed article. 
     The expansion ratio of the product obtained by the extrusion expansion process is usually about 50 to about 75%. When the foamed article is used as a 3-mm-thick covering for electric wires, the dielectric constant of the resin is usually about 1.3 to about 1.6. 
     When injection expansion molding is resorted to, the composition is made into fine particles or pellents as in the case of the extrusion expansion process, and the material is fed to a screw in-line while being heated for injection expansion molding (injection pressure: about 300 to about 800 kg/cm 2 ). The foamed article obtained is usually about 0.65 to about 0.9 in specific gravity and about 60 to about 90% in expansion ratio. 
     In the case of die expansion molding, the components are usually pulverized into fine particles, about 20 to about 80 μm in size, and mixed together. The mixture is placed into a die, pressurized to about 20 to about 200 kg/cm 2  for the removal of air and then allowed to stand for about 1 to about 2 hours for foaming while being heated to the melting point of the fluorocarbon resin. The foamed article obtained is usually about 0.65 to about 0.9 in specific gravity and about 60 to about 90% in expansion ratio. 
    
    
     The present invention will be described below with reference to the following examples and comparative examples. 
     EXAMPLES 1-8 
     One part by weight of boron nitride (1 to 8 μm in particle size, product of Denki Kagaku Kogyo Co., Ltd., Japan) was added to 100 parts by weight of the fluorocarbon resin listed in Table 1, and the ingredients were uniformly mixed together by a V-type blender. The powdery mixture obtained was continuously extruded into a rod, 2 to 3 mm in diameter, by an extruder (cylinder diameter D: 25 mm, screw length L: 500 mm, L/D=20). At the same time, the rod was made into pellets, 3 to 4 mm in length, by a pelletizer. 
     With the pellets was admixed the depolymerizable polymer (up to 500 μm in particle size) listed in Table 1. Subsequently, the mixture was extruded into an expanded rod, 5 mm in diameter, by an extruder having the dimensions listed in Table 2 under the conditions given in the same table. Table 1 shows the expansion ratio, expanded state and degree of coloration of the rod. 
     
                                           TABLE 1                                 
__________________________________________________________________________
                Depolymerizable polymer                                   
                                  Expansion  Coloration                   
     Fluorocarbon            Amount                                       
                                  ratio (2)                               
                                        Foamed                            
                                             of foamed                    
Example                                                                   
     resin      Kind         (1)  (%)   state                             
                                             resin                        
__________________________________________________________________________
1    Copolymer of tetra-                                                  
                Methyl methacrylate                                       
                             0.1  40    Uniform                           
                                             None                         
     fluoroethylene/                                                      
                polymer, no. av.                                          
     hexafluoropropylene                                                  
                molecular weight = 30,000                                 
     (82/18, by wt.),                                                     
     M.I. = 6 g/10 min                                                    
2    Copolymer of tetra-                                                  
                Methyl methacrylate                                       
                             1.0  60    Uniform                           
                                             None                         
     fluoroethylene/                                                      
                polymer, no. av.                                          
     hexafluoropropylene                                                  
                molecular weight = 30,000                                 
     (82/18, by wt.),                                                     
     M.I. = 6 g/10 min                                                    
3    Copolymer of tetra-                                                  
                Methyl methacrylate                                       
                             5.0  70    Slightly                          
                                             None                         
     fluoroethylene/                                                      
                polymer, no. av.        uneven                            
     hexafluoropropylene                                                  
                molecular weight = 30,000                                 
     (82/18, by wt.),                                                     
     M.I. = 6 g/10 min                                                    
4    Copolymer of tetra-                                                  
                Copolymer of methyl                                       
                             0.1  50    Uniform                           
                                             None                         
     fluoroethylene/                                                      
                methacrylate/ethyl                                        
     hexafluoropropylene                                                  
                acrylate/methacrylic                                      
     (82/18, by wt.),                                                     
                acid (39/57/4, by wt.)                                    
     M.I. = 6 g/10 min                                                    
                no. av. molecular                                         
                weight = 30,000                                           
5    Copolymer of tetra-                                                  
                Copolymer of methyl                                       
                             1.0  60    Uniform                           
                                             None                         
     fluoroethylene/                                                      
                methacrylate/ethyl                                        
     hexafluoropropylene                                                  
                acrylate/methacrylic                                      
     (82/18, by wt.),                                                     
                acid (39/57/4, by wt.)                                    
     M.I. = 6 g/10 min                                                    
                no. av. molecular                                         
                weight = 30,000                                           
6    Copolymer of tetra-                                                  
                Copolymer of methyl                                       
                             5.0  70    Slightly                          
                                             None                         
     fluoroethylene/                                                      
                methacrylate/ethyl      uneven                            
     hexafluoropropylene                                                  
                acrylate/methacrylic                                      
     (82/18, by wt.),                                                     
                acid (39/57/4, by wt.)                                    
     M.I. = 6 g/10 min                                                    
                no. av. molecular                                         
                weight = 30,000                                           
7    Copolymer of tetra-                                                  
                Methyl methacrylate                                       
                             1.0  60    Uniform                           
                                             None                         
     fluoroethylene/                                                      
                polymer, no. av.                                          
     perflurobutyl vinyl                                                  
                molecular weight = 30,000                                 
     ether (98/2, by wt.),                                                
     M.I. = 10 g/10 min                                                   
8    Copolymer of tetra-                                                  
                Methyl methacrylate                                       
                             1.0  60    Uniform                           
                                             None                         
     fluoroethylene/                                                      
                polymer, no. av.                                          
     ethylene (50/50,                                                     
                molecular weight = 30,000                                 
     by wt.),                                                             
     M.I. = 15 g/10 min                                                   
__________________________________________________________________________
 Note                                                                     
 (1) In parts by weight per 100 parts by weight of fluorocarbon resin.    
 (2) As defined above.                                                    
 
    
     
                                           TABLE 2                                 
__________________________________________________________________________
                     Extrusion temperature (°C.)                   
Extruder             Cylinder                                             
     Cylinder Rotation of                                                 
                     Rear                                                 
                         Mid Front                                        
                                 Die                                      
Example                                                                   
     diam. (mm)                                                           
           L/D                                                            
              screw (rpm)                                                 
                     portion                                              
                         portion                                          
                             portion                                      
                                 head                                     
                                    Tip                                   
__________________________________________________________________________
1-7  30    22 15-30  360 380 380 340                                      
                                    320                                   
8    30    22 15-30  300 320 300 340                                      
                                    350                                   
__________________________________________________________________________
 L stands for the length of screw, and D for the diameter of cylinder.    
 
    
     EXAMPLES 9-13 
     One part by weight of the same boron nitride as used in Example 1 was admixed with 100 parts by weight of the fluorocarbon resin listed in Table 3, and the mixture was made into pellets by the same procedure as in Examples 1 to 8. 
     The depolymerizable polymer listed in Table 3 was added to the pellets, and the mixture was pulverized by an impact mill. The powdery mixture obtained was extruded by the extruder shown in Table 2 onto a tin-plated soft copper wire, 0.7 mm in outside diameter, to obtain an electric wire covered with a foamed resin with a thickness of 3 mm. Table 3 shows the expansion ratio, foamed state, degree of coloration and dielectric constant of the foamed resin. 
     
                                           TABLE 3                                 
__________________________________________________________________________
                           Expansion  Coloration                          
                                            Dielectric                    
Fluorocarbon                                                              
            Depolymerizable polymer                                       
                           ratio Foamed                                   
                                      of foamed                           
                                            constant of                   
Example                                                                   
     resin  Kind      Amount                                              
                           (%)   state                                    
                                      resin foamed resin                  
__________________________________________________________________________
 9   Same as in                                                           
            α-Methylstyrene                                         
                      1    60    Uniform                                  
                                      None  1.4                           
     Example 1                                                            
            polymer, no. av.                                              
            molecular weight =                                            
            50,000                                                        
10   Same as in                                                           
            α-Methylstyrene                                         
                      5    70    Uniform                                  
                                      None  1.3                           
     Example 1                                                            
            polymer, no. av.                                              
            molecular weight =                                            
            50,000                                                        
11   Same as in                                                           
            α-Methylstyrene                                         
                      10   70    Slightly                                 
                                      None  1.3                           
     Example 1                                                            
            polymer, no. av.     uneven                                   
            molecular weight =                                            
            50,000                                                        
12   Same as in                                                           
            α-Methylstyrene                                         
                      1    60    Uniform                                  
                                      None  1.4                           
     Example 7                                                            
            polymer, no. av.                                              
            molecular weight =                                            
            50,000                                                        
13   Same as in                                                           
            α-Methylstyrene                                         
                      5    70    Uniform                                  
                                      None  1.3                           
     Example 7                                                            
            polymer, no. av.                                              
            molecular weight =                                            
            50,000                                                        
__________________________________________________________________________
 Note:                                                                    
 The amount of depolymerizable polymer and the expansion ratio in Table 3 
 and Tables shown below are as defined above.                             
 
    
     EXAMPLES 14-18 
     The depolymerizable polymer (copolymer) listed in Table 4 and up to 500 μm in particle size was admixed with the same pelletized fluorocarbon resin as used in Example 1. A covered electric wire was prepared in the same manner as in Examples 9 to 13 using the resulting composition. Table 4 shows the expansion ratio, foamed state, degree of coloration and dielectric constant of the foamed resin. 
     
                                           TABLE 4                                 
__________________________________________________________________________
                                  Coloration                              
                                        Dielectric                        
Depolymerizable polymer                                                   
                       Expansion                                          
                             Foamed                                       
                                  of foamed                               
                                        constant of                       
Example                                                                   
     Kind         Amount                                                  
                       ratio (%)                                          
                             state                                        
                                  resin foamed resin                      
__________________________________________________________________________
14   αF5F/AFM (1/1 by wt.)                                          
                  1    60    Uniform                                      
                                  None  1.4                               
15   αF5F/AFM (1/1 by wt.)                                          
                  5    70    Uniform                                      
                                  None   1.35                             
16   4FM/MMA (9/1 by wt.)                                                 
                  1    60    Uniform                                      
                                  None  1.4                               
17   4FM/MMA (9/1 by wt.)                                                 
                  5    70    Uniform                                      
                                  None  1.3                               
18   5FM/MMA (4/1 by wt.)                                                 
                  1    60    Uniform                                      
                                  None  1.4                               
__________________________________________________________________________
 
    
     αF5F, αFM, 5FM, 4FM and MMA given in Table 4 represent the following monomers respectively. ##STR7## 
     COMPARATIVE EXAMPLE 1 
     With the same mixture as used in Example 1 of 100 parts by weight of fluorocarbon resin and 1 part by weight of boron nitride was admixed the known organic blowing agent given in Table 5, and the mixture was pulverized by a hammer mill. 
     The resulting mixture was extruded in the same manner as in Examples 1 to 8, giving a foamed rod having a diameter of 5 mm. Table 5 shows the expansion ratio, foamed state and coloration degree of the rod. 
     
                                           TABLE 5                                 
__________________________________________________________________________
                Conventional organic                                      
                               Expansion  Coloration                      
Fluorocarbon    blowing agent  ratio Foamed                               
                                          of foamed                       
Example                                                                   
     resin      Kind      Amount                                          
                               (%)   state                                
                                          resin                           
__________________________________________________________________________
Comp.                                                                     
     Same as in Example 1                                                 
                Dinitrosopenta-                                           
                          1    40    Uneven                               
                                          Colored                         
1               methylenetetramine        brown                           
__________________________________________________________________________
 
    
     EXAMPLE 19 
     A uniform mixture was prepared from 100 parts by weight of the same pelletized fluorocarbon resin composition as used in Example 1 and 0.5 part by weight of the same methyl methacrylate polymer as used in Example 1. 
     The mixture was fed to a screw in-line injection moding machine having a cylinder diameter of 42 mm and a nozzle orifice diameter of 3.5 mm at an injection pressure of 300 to 800 kg/cm 2  while rotating the screw at 70 to 180 rpm/min. The cylinder was heated to temperatures of 350° C., 370° C. and 380° C. at its rear portion, midportion and front portion, respectively, and the nozzle temperature was 380° C. The foamed resin obtained was 0.65 to 0.9 in specific gravity and 60 to 70% in expansion ratio. 
     EXAMPLE 20 
     A uniform mixture was prepared from 100 parts by weight of the same fluorocarbon resin power (20 to 80 μm in particle size and 0.2 to 0.7 g/cc in apparent density) as used in Example 1 and 0.5 part by weight of the same methyl methacrylate polymer as used in Example 1. 
     A 200 g quantity of the mixture was placed into a mold, 100 mm×100 mm×70 mm (height), which was then closed and thereafter pressurized to 20 to 200 kg/cm 2  for deaeration. Subsequently, the mixture was maintained at 280° to 380° C. for 2 hours, with the lid of the mold left open, giving a foamed resin which was 0.65 to 0.9 in specific gravity and 60 to 70% in expansion ratio. 
     EXAMPLES 21-27 
     One part by weight of boron nitride (1 to 8 μm in particle size, product of Denki Kagaku Kogyo Co., Ltd., Japan) was added to 100 parts by weight of the fluorocarbon resin listed in Table 6, and the ingredients were uniformly mixed together by a V-type blender. The powdery mixture obtained was continuously extruded into a rod, 2 to 3 mm in diameter, by an extruder (cylinder diameter D: 25 mm, screw length L: 500 mm, L/D=20). At the same time, the rod was made into pellets, 3 to 4 mm in length, by a pelletizer. 
     With the pellets was admixed the polyether compound listed in Table 6. Subsequently, the mixture was extruded into a foamed rod, 5 mm in diameter, by an extruder having the dimensions listed in Table 7 under the conditions given in the same table. Table 6 shows the expansion ratio, foamed state and degree of coloration of the rod. 
     
                                           TABLE 6                                 
__________________________________________________________________________
               Polyether compound      Expansion  Coloration              
     Fluorocarbon                 Amount                                  
                                       ratio (2)                          
                                             Foamed                       
                                                  of foamed               
Example                                                                   
     resin     Structural formula (1)  (%)   state                        
                                                  resin                   
__________________________________________________________________________
21   Copolymer of tetra-                                                  
               F(CH.sub.2 CF.sub.2 CF.sub.2 O).sub.n CH.sub.2 CF.sub.2    
               COOH               0.5  50    Uniform                      
                                                  None                    
     fluoroethylene/                                                      
               n = 5-100                                                  
     hexafluoropropylene                                                  
     (82/18, by wt.),                                                     
     M.I. = 6 g/10 min                                                    
22   Copolymer of tetra-                                                  
               F(CH.sub.2 CF.sub.2 CF.sub.2 O).sub.n CH.sub.2 CF.sub.2    
               COOH               1.0  60    Uniform                      
                                                  None                    
     fluoroethylene/                                                      
               n = 5-100                                                  
     hexafluoropropylene                                                  
     (82/18, by wt.),                                                     
     M.I. = 6 g/10 min                                                    
23   Copolymer of tetra-                                                  
               F(CH.sub.2 CF.sub.2 CF.sub.2 O).sub.n CH.sub.2 CF.sub.2    
               COOH               5.0  60    Uniform                      
                                                  None                    
     fluoroethylene/                                                      
               n = 5-100                                                  
     hexafluoropropylene                                                  
     (82/18, by wt.),                                                     
     M.I. = 6 g/10 min                                                    
24   Copolymer of tetra- fluoroethylene/ hexafluoropropylene (82/18, by   
     wt.), M.I. = 6 g/10 min                                              
                ##STR8##          0.5  50    Uniform                      
                                                  None                    
25   Copolymer of tetra- fluoroethylene/ hexafluoropropylene (82/18, by   
     wt.), M.I. = 6 g/10 min                                              
                ##STR9##          1.0  60    Uniform                      
                                                  None                    
26   Copolymer of tetra-                                                  
               F(CH.sub.2 CF.sub.2 CF.sub.2 O).sub.n CH.sub.2 CF.sub.2    
               COOH               1.0  60    Uniform                      
                                                  None                    
     fluoroethylene/                                                      
               n = 10-150                                                 
     perfluorobutyl                                                       
     vinyl ether                                                          
     (98/2, by wt.),                                                      
     M.I. = 10 g/10 min                                                   
27   Copolymer of tetra-                                                  
               F(CH.sub.2 CF.sub.2 CF.sub.2 O).sub.n CH.sub.2 CF.sub.2    
               COOH               1.0  60    Uniform                      
                                                  None                    
     fluoroethylene/                                                      
               n = 10-150                                                 
     ethylene                                                             
     (50/50, by wt.),                                                     
     M.I. = 15 g/10 min                                                   
__________________________________________________________________________
 Note                                                                     
 (1) In parts by weight per 100 parts by weight of fluorocarbon resin.    
 (2) As defined above.                                                    
 
    
     
                                           TABLE 7                                 
__________________________________________________________________________
                     Extrusion temperature (°C.)                   
Extruder             Cylinder                                             
     Cylinder Rotation of                                                 
                     Rear                                                 
                         Mid Front                                        
                                 Die                                      
Example                                                                   
     diam. (mm)                                                           
           L/D                                                            
              screw (rpm)                                                 
                     portion                                              
                         portion                                          
                             portion                                      
                                 head                                     
                                    Tip                                   
__________________________________________________________________________
21-26                                                                     
     30    22 15-30  360 380 380 340                                      
                                    320                                   
27   30    22 15-30  300 320 300 340                                      
                                    350                                   
__________________________________________________________________________
 L stands for the length of screw, and D for the diameter of cylinder.    
 
    
     EXAMPLES 28-34 
     One part by weight of boron nitride (1 to 8 μm in particle size, product of Denki Kagaku Kogyo Co., Ltd., Japan) was added to 100 parts by weight of the fluorocarbon resin listed in Table 8, and the ingredients were uniformly mixed together by a V-type blender. The powdery mixture obtained was continuously extruded into a rod, 2 to 3 mm in diameter, by an extruder (cylinder diameter D: 25 mm, screw length L: 500 mm, L/D=20). At the same time, the rod was made into pellets, 3 to 4 mm in length, by a pelletizer. 
     With the pellets was admixed the polycarbonyl-oxy compound listed in Table 8. Subsequently, the mixture was extruded into a foamed rod, 5 mm in diameter, by an extruder having the dimensions listed in Table 9 under the conditions given in the same table. Table 8 shows the expansion ratio, foamed state and degree of coloration of the rod. 
     
                                           TABLE 8                                 
__________________________________________________________________________
               Polycarbonyl-oxy Compound                                  
                             Expansion  Coloration                        
     Fluorocarbon       Amount                                            
                             ratio (2)                                    
                                   Foamed                                 
                                        of foamed                         
Example                                                                   
     resin     Kind     (1)  (%)   state                                  
                                        resin                             
__________________________________________________________________________
28   Copolymer of tetra-                                                  
               Oxyalic acid                                               
                        0.5  50    Uniform                                
                                        None                              
     fluoroethylene/                                                      
               (H.sub.2 C.sub.2 O.sub.4.2H.sub.2 O)                       
     hexafluoropropylene                                                  
     (82/18, by wt.),                                                     
     M.I. = 6 g/10 min                                                    
29   Copolymer of tetra-                                                  
               Oxyalic acid                                               
                        1.0  60    Uniform                                
                                        None                              
     fluoroethylene/                                                      
               (H.sub.2 C.sub.2 O.sub.4.2H.sub.2 O)                       
     hexafluoropropylene                                                  
     (82/18, by wt.),                                                     
     M.I. = 6 g/10 min                                                    
30   Copolymer of tetra-                                                  
               Oxyalic acid                                               
                        5.0  60    Uniform                                
                                        None                              
     fluoroethylene/                                                      
               (H.sub.2 C.sub.2 O.sub.4.2H.sub.2 O)                       
     hexafluoropropylene                                                  
     (82/18, by wt.),                                                     
     M.I. = 6 g/10 min                                                    
31   Copolymer of tetra-                                                  
               Citric acid                                                
                        0.5  50    Uniform                                
                                        None                              
     fluoroethylene/                                                      
     hexafluoropropylene                                                  
     (82/18, by wt.),                                                     
     M.I. = 6 g/10 min                                                    
32   Copolymer of tetra-                                                  
               Citric acid                                                
                        1.0  60    Uniform                                
                                        None                              
     fluoroethylene/                                                      
     hexafluoropropylene                                                  
     (82/18, by wt.),                                                     
     M.I. = 6 g/10 min                                                    
33   Copolymer of tetra-                                                  
               Oxyalic acid                                               
                        1.0  60    Uniform                                
                                        None                              
     fluoroethylene/                                                      
               (H.sub.2 C.sub.2 O.sub.4.2H.sub.2 O)                       
     perfluorobutyl                                                       
     vinyl ether                                                          
     (98/2, by wt.),                                                      
     M.I. = 10 g/10 min                                                   
34   Copolymer of tetra-                                                  
               Oxyalic acid                                               
                        1.0  60    Uniform                                
                                        None                              
     fluoroethylene/                                                      
               (H.sub.2 C.sub.2 O.sub.4.2H.sub.2 O)                       
     ethylene                                                             
     (50/50, by wt.),                                                     
     M.I. = 15 g/10 min                                                   
__________________________________________________________________________
 Note                                                                     
 (1) In parts by weight per 100 parts by weight of fluorocarbon resin.    
 (2) As defined above.                                                    
 
    
     
                                           TABLE 9                                 
__________________________________________________________________________
                     Extrusion temperature (°C.)                   
Extruder             Cylinder                                             
     Cylinder Rotation of                                                 
                     Rear                                                 
                         Mid Front                                        
                                 Die                                      
Example                                                                   
     diam. (mm)                                                           
           L/D                                                            
              screw (rpm)                                                 
                     portion                                              
                         portion                                          
                             portion                                      
                                 head                                     
                                    Tip                                   
__________________________________________________________________________
28-33                                                                     
     30    22 15-30  360 380 380 340                                      
                                    320                                   
34   30    22 15-30  300 320 300 340                                      
                                    350                                   
__________________________________________________________________________
 L stands for the length of screw, and D for the diameter of cylinder.    
 
    
     EXAMPLES 35-38 
     An expanded rod, 5 mm in diameter, was prepared in the same manner as in Example 28 with the exception of using 1.0 part by weight of oxalic acid per 100 parts by weight of the fluorocarbon resin listed in Table 10. Table 10 also shows the expansion ratio, foamed state and coloration degree of the rod. 
     
                                           TABLE 10                                
__________________________________________________________________________
Fluorocarbon resin***                                                     
                    Expansion                                             
                          Foamed                                          
                                Coloration                                
Ex.                                                                       
   Kind        Amount                                                     
                    ratio (%)                                             
                          state of rod                                    
__________________________________________________________________________
35 Thermoplastic                                                          
               1.0  50    Uniform                                         
                                None                                      
   (Daiel T-530,                                                          
   Product of Daikin                                                      
   Kogyo Co., Ltd., Japan)                                                
36 Thermoplastic                                                          
               1.0  50    Uniform                                         
                                None                                      
   (Daiel T-630,                                                          
   product of Daikin)                                                     
37 Thermoplastic*                                                         
               1.0  50    Uniform                                         
                                None                                      
38 Thermoplastic**                                                        
               1.0  50    Uniform                                         
                                None                                      
__________________________________________________________________________
 Note:                                                                    
 *Prepared by the process disclosed in Examined Japanese Patent Publicatio
 SHO 584728 and composed of 85 wt. % of rubber segment (vinylidene        
 fluoride/hexafluoropropene = 79/21 in mole ratio) and 15 wt. % of hard   
 segment (tetrafluoroethylene/ethylene /hexafluoropropene = 50/42/8 in mol
 ratio), M.I. 23 g/10 min.                                                
 **Copolymer of 85 wt. % of the above rubber segment and 15 wt. % of      
 vinylidene fluoride, M.I. 2 g/10 min.                                    
 ***After being foamed, thermoplastics used can be crosslinked by ionizing
 radiation.