Abstract:
The invention relates to the use of fluorinated copolymers based on trifluoroethylene (VF3) and tetrafluoroethylene (C 2  F 4 ), having melting points above 210° C. and soluble in the customary polar solvents, as protective coatings for a variety of substrates (plates, wires, cables, fibers, etc.) in the form of extruded films or sheaths or in the form of varnishes and paints.

Description:
FIELD OF THE INVENTION 
     This invention relates to fluorinated polymers having good heat stability, i.e. whose melting point is equal to or above 210° C. This invention relates also to the use of such polymers as protective coatings for a variety of substrates, for example in the form of varnishes and paints for metal substrates, glass, wood, and so on, or in the form of extruded or coextruded films or sheaths. 
     BACKGROUND OF THE INVENTION 
     Fluorinated homopolymers and copolymers are known for their good heat stability, their chemical resistance, especially to solvents, weather resistance and resistance to radiation (such as UV), their impermeability to gases and liquids, and their electrical insulating properties. Nevertheless, fluorinated polymers with melting points above about 170° C. have the disadvantage of generally being very sparingly soluble or even insoluble in customary polar organic solvents such as acetates and ketones. 
     Compared with others fluorinated polymers in the literature, those involved in the present invention have both a high melting point (cf. GAL&#39;PERIN et al., Polymer Science USSR, 1976, Vol. 18, p. 3073-3083, Pergamon Press Pub.) and a good solubility in polar organic solvents without their solution viscosity in this type of solvent being too high, especially in the case of solutions with a solids content of more than 10% by weight. Examples of polar organic solvents which may be mentioned in particular are methyl, isopropyl, and butyl acetates, acetone, methyl ethyl ketone, methyl isobutyl ketone, tetrahydrofuran, dimethylformamide, and dimethylacetamide. 
     Apart from the properties referred to above, the copolymers employed in the present invention also have a low refractive index, good flexibility (modulus of rigidity of between 150 and 350 MPa), and viscosity as well as transparency after stretching. 
     SUMMARY OF THE INVENTION 
     The present invention provides a method of protecting a substrate which comprises the step of coating the substrate with one or more heat stable fluorinated copolymers of trifluoroethylene and tetrafluoroethylene, said coplymer(s) preferably comprising from 96 to 70 mol % trifluoroethylene, and from 4 to 30 mol % tetrafluoroethylene. 
     In one embodiment, the coating step is preceded by a step which comprises mixing said one or more heat stable fluorinated copolymers of trifluoroethylene and tetrafluoroethylene with an organic or aqueous phase to form a varnish or paint. In this embodiment, the coating step comprises applying the varnish or paint to the substrate and subsequently removing the organic or aqueous phase. In another embodiment, the coating step is preceded by a step which comprises extruding or coextruding a monolayer or multilayer film, sheath, or sleeve comprising the heat stable fluorinated copolymer of trifluoroethylene and tetrafluoroethylene. In this embodiment, the coating step comprises applying the monolayer or multilayer film, sheath, or sleeve to the substrate. 
     The present invention likewise contemplates an article of manufacture comprising a substrate, for instance aluminum or an electrical circuit or an optical fiber, coated with one or more heat stable trifluoroethylene/tetrafluoroethylene copolymers as described above. In addition to coated substrates, the present invention contemplates articles of manufacture, for instance, sheaths, plates, ferrules, tips, nipples, terminals, and connectors, wherein the article is comprised of one or more heat stable fluorinated copolymers of trifluoroethylene and tetrafluoroethylene as described above. 
     This invention also includes a process for the preparation of a fluorinated copolymer of from 96 to 70 mol % of trifluoroethylene and from 4 to 30 mol % of tetrafluoroethylene. The process comprises the steps of forming an emulsion, suspension, or solution of about 96 to 70 mol % trifluoroethylene, about 4 to 30 mol % tetrafluoroethylene, and a catalytically effective amount of free-radical initiator and polymerizing the trifluoroethylene and tetrafluoroethylene to form the fluorinated copolymer. The free-radical initiator is preferably a percarbonate, perpivalate, or potassium persulphate. The polymerization step is conducted under reaction conditions described below to form a fluorinated copolymer having a number-average molecular weight between about 40,000 and 250,000 and having a polydispersity between about 1.7 and 2.9. 
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The polymers according to the invention are based on trifluoroethylene (VF3) and tetrafluoroethylene (C 2  F 4 ) and preferably consist of: 
     1. from 96 to 70 mol % of trifluoroethylene (VF3), 
     2. from 4 to 30 mol % of tetrafluoroethylene (C 2  F 4 ). 
     Their number-average molecular weight, Mn, is generally between 40,000 and 250,000 and their polydispersity, Mw/Mn, wherein Mw is their weight-average molecular weight, is generally between 1.7 and 2.9. 
     The polymers of the present invention are &#34;heat stable&#34;. That is, their melting point is at least 210° C. 
     They can be prepared by the polymerization techniques employed for the synthesis of fluorinated polyolefins. There may be mentioned emulsion, suspension, or solution polymerization in the presence of one or more polymerization initiators, preferably of the free-radical type, such as percarbonates, perpivalates, or potassium persulphate. 
     The polymerization is generally carried out under a pressure of between 10 and 90 bar and the polymerization temperature is generally between 40° and 120° C. and preferably between 50° and 90° C. 
     Emulsion polymerization requires the use of an emulsifier, which can be selected for example from the fluorinated emulsifiers of the formula C n  F 2n+1  COOM or C n  F 2n+1  C 2  H 4  SO 3  M, where M is the sodium, potassium, or ammonium cation. 
     Suspension polymerization requires the use of a protective colloid, which is generally selected from polyvinyl alcohols or cellulose derivatives. 
     To prepare the varnishes and paints based on copolymers described above which are intended especially for the protection of electrical circuits against corrosion, the copolymers are dissolved or emulsified in an organic solvent or, if appropriate, dispersed in an aqueous medium to produce paints in the aqueous phase. The copolymers can be used by themselves or in association with other fluorinated or nonfluorinated polymers such as polyvinylidene fluoride (PVDF), acrylic polymers, for example polymethyl methacrylate (PMMA), and so on. 
     The copolymers according to the invention can also be extruded or coextruded at temperatures of between 250° and 300° C., making it possible to produce protective coatings in the form of monolayer or multilayer sheaths or sleeves (coating of cylindrical objects, cables, wires, fibers) or in the form of monolayer or multilayer films (coating of flat objects). 
     By virtue of their low refractive index, one particularly advantageous application of the copolymers involves their use in the field of optical fibers, where they can be used either in the form of varnishes or paints or in the form of an extruded sheath as a protective coating for the core of the optical fibers. 
     Compared with the other fluorinated polymers used as protective coatings for optical fibers, the copolymers according to the invention have the additional advantage of a greatly improved heat stability. 
    
    
     EXAMPLES 
     For the polymers synthesized in the following Examples, the molecular weights were determined by gas chromatography in tetrahydrofuran (THF) at room temperature with a refractometric detector. The number-average and weight average molecular weights, Mn and Mw, are expressed in polystyrene equivalents. The proportions of the different monomers were confirmed by nuclear magnetic resonance (NMR) and are expressed in mol %. 
     Their Brookfield viscosity in 12 or 20% by weight solution in methyl acetate is measured; the results are indicated in Pa.s. 
     Their melting point, m.p., is measured by differential scanning calorimetry(DSC) with a temperature rise of 10° C. per minute. 
     The transparency of these polymers after 200% uniaxial stretching is also assessed qualitatively. Polymer strips 0.7 mm thick are molded and then stretched to twice their initial length. The transparency of the samples is assessed visually. 
     The results of the measurements described above for the polymers of Examples 1 to 8. are collated in Table 1. 
     Example 1 
     The following are introduced into a 3.3 liter autoclave equipped with a stirrer, a jacket, and a temperature regulating system: 
     
         ______________________________________deionized water       2      literspotassium persulphate 0.5    gramssodium acetate        0.3    gramsparaffin              8      gramsemulsifier (C.sub.6 F.sub.13 C.sub.2 H.sub.4 SO.sub.3 K)                 2.45   grams______________________________________ 
    
     After removal of the atmospheric oxygen, 150 grams of VF3 and 23 grams of C 2  F 4  are introduced and the temperature of the autoclave is increased to 80° C. The pressure is maintained at 24 bar by the introduction of a gaseous mixture of VF3 and C 2  F 4  of molar .composition 90/10. After the addition of 100 grams of VF3/C 2  F 4 mixture, 2.5 milliliters of ethyl acetate are added, followed by 1057 gramsof the above gaseous VF3/C 2  F 4  mixture, the temperature of the autoclave is then lowered to 30° C. and the autoclave is degassed. 
     A latex with a high solids content (36.8%) is formed. This is coagulated byfreezing and then washed 3 times with 3 liters of a water/methanol mixture of composition 75/25 by weight. After washing, the latex is dried at 90° C. and 1031 grams of VF3/C 2  F 4  copolymer are recovered in the form of a powder of molar composition 90/10. 
     The copolymer obtained is soluble in butyl acetate. Its flexural modulus ofelasticity, measured according to standard ISO 12086/2, is equal to 250 MPaand its viscosity, measured with a capillary rheometer at 300° C. for a shear of 100 s -1 , is equal to 900 Pa.s 
     Its refractive index is measured as follows: after cryogenic grinding of a polymer sample to a particle size of less than 10 μm (the edges of the ground particles must be well defined and not jagged), an optical microscope is used to compare its refractive index with those of solvents of known index in which it is immersed. Ethanol and isopropanol were used as reference solvents. The refractive index is equal to 1.372±0.003. 
     Example 2 
     The following are introduced into a 3.3 liter autoclave equipped with a stirrer, a jacket, and a temperature regulating system: 
     
         ______________________________________deionized water         2      lpotassium persuiphate   0.4    gparaffin                8      gemulsifier (C.sub.6 F.sub.13 C.sub.2 H.sub.4 SO.sub.3 K)                   2.45   gVF3                     1200   g______________________________________ 
    
     After the autoclave has been heated to 83° C., 0.08 g of potassium persulphate and 100 g of deionized water are introduced. The pressure is adjusted to 85 bar by the addition of 90 g of water. Further water is added to compensate the pressure drop due to the polymerization of the VF3. After the addition of 780 g of water and 0.08 g of potassium persulphate introduced in 4 portions at a rate of one injection per hour, the temperature of the autoclave is lowered to 40° C. and the autoclave is degassed. 
     A latex with a high solids content (29%) is formed. This is coagulated by freezing and then washed 3 times with 3 liters of a water/methanol mixtureof composition 75/25 by weight. After washing, the latex is dried at 90° C. and 975 g of VF3 homopolymer are recovered in the form of a powder. 
     The copolymer obtained is soluble in butyl acetate. 
     Example 5 
     This Example corresponds to a VF2/C 2  F 4  polymer marketed by Elf Atochem S.A. under the tradename KYNAR™SL. 
     Example 6 
     1400 g of deionized water and then 39.5 ml of a 1% by weight solution of hydroxyethyl cellulose are introduced into a 3.3 liter autoclave equipped with a stirrer, a jacket, and a temperature regulating system. After deaeration of the autoclave, 5 g of C 2  F 3  Cl, 1062 g of VF3, and12 ml of ethyl acetate are introduced. 
     After the autoclave has been heated to 50° C., 760 g of deionized water and 3 g of cyclohexyl perdicarbonate dissolved in 52 g of CCl 2  FCH 3  (FORANE™ 141 b) are introduced. The pressure inside the autoclave is then 85 bar. It is maintained at this value by the addition of water. C 2  F 3  Cl is also added, being introduced in 8 g portions, and 25 g of 
     
         ______________________________________sodium acetate          0.3    gparaffin                8      gemulsifier (C.sub.6 F.sub.13 C.sub.2 H.sub.4 SO.sub.3 K)                   2.45   g______________________________________ 
    
     After removal of the atmospheric oxygen, the temperature is increased to 82° C. and 150 g of a VF3/C 2  F 4  mixture of molar composition 80/20 are introduced so as to create a pressure of 24 bar. After the introduction of 99 g of gaseous VF3/C 2  F 4  mixture of molar composition 80/20, 1 ml of ethyl acetate is added, followed by a further 690 g of the above VF3/C 2  F 4  mixture. The temperature ofthe autoclave is lowered to 30° C. and the autoclave is degassed. 
     A latex with a high solids content (31%) is formed. This is coagulated by freezing and then washed 3 times with 3 liters of a water/methanol mixtureof composition 75/25 by weight. After washing, the latex is dried at 90° C. and 895 g of VF3/C 2  F 4  copolymer are recovered in the form of a powder of molar composition 80/20. 
     The copolymer obtained is soluble in butyl acetate. Its flexural modulus ofelasticity, measured according to standard ISO 12086/2, is equal to 250 MPaand its viscosity, measured with a capillary rheometer at 300° C. for a shear of 100 s -1 , is equal to 900 Pa.s. 
     EXAMPLES 3 to 8 (comparative) 
     Example 3 
     This example corresponds to a VF2 homopolymer marketed by Elf Atochem S.A. under the tradename KYNAR™ 1000. 
     Example 4 
     The following are introduced into a 3.3 liter autoclave equipped with a stirrer, a jacket, and a temperature regulating system: 
     
         ______________________________________deionied water        1.3    lethyl acetate         5      mlsodium acetate        0.073  g______________________________________ 
    
     water are added at the same time. 400 g of water and 112 g of C 2  F 3  Cl were added over a copolymerization time of 4 h. 
     After cooling of the autoclave, the residual monomers are degassed and the contents of the autoclave are then discharged, filtered, and dried. 
     814 g of VF3/C 2  F 3  Cl copolymer are recovered in the form of a powder of molar composition 90/10. 
     Example 7 
     The following are introduced into a 3.3 liter autoclave equipped with a stirrer, a jacket, and a temperature regulating system: 
     
         ______________________________________deionized water         1400   gsodium acetate          0.073  gparaffin                8      gemulsifier (C.sub.6 F.sub.13 C.sub.2 H.sub.4 SO.sub.3 K)                   2.45   gVF3                     1022   gC.sub.3 F.sub.6         208    gethyl acetate           28     ml______________________________________ 
    
     After the autoclave has been heated to 83° C., 16 ml of a 0.5% by weight solution of potassium persulphate are introduced. The pressure is adjusted to 85 bar by the addition of 90 g of water. Further water (700 g)is added to compensate the pressure drop due to the copolymerization. The autoclave is then degassed. 
     A latex with a high solids content (27%) is formed. This is coagulated by freezing and then washed 3 times with 3 liters of a water/methanol mixtureof composition 75/25 by weight. After washing, the latex is dried at 90° C. and 871 g of VF3/C 3  F 6 , copolymer are recovered inthe form of a powder of molar composition 99/1. 
     The copolymer obtained is soluble in butyl acetate. 
     Example 8 
     The following are introduced, after deaeration, into a 3.3 liter autoclave equipped with a stirrer, a jacket, and a temperature regulating system: 
     
         ______________________________________deionized water         1400   gethyl acetate           5      mlsodium acetate          0.073  gparaffin                8      gemulsifier (C.sub.6 F.sub.13 C.sub.2 H.sub.4 SO.sub.3 K)                   2.45   gVF3                     1132   gVF2                     98     g______________________________________ 
    
     After the autoclave has been heated to 83° C., 16 ml of a 0.5% by weight solution of potassium persulphate are introduced. The pressure is adjusted to 80 bar by the addition of 80 g of water. Further water (854 g)is added to compensate the pressure drop due to the copolymerization. The autoclave is then degassed. 
     A latex with a high solids content (27%) is formed. This is coagulated by freezing and then washed 3 times with 3 liters of a water/methanol mixtureof composition 75/25 by weight. After washing, the latex is dried at 90° C. and 963 g of VF3/VF2 copolymer are recovered in the form of a powder of molar composition 90/10. 
     Example 9 
     A powdery VF3/C 2  F 4  copolymer with a molar composition of 95/5 and a number-average molecular weight, Mn, of 170,000 is prepared under the operating conditions of Example 1. 
     Example 10 
     Preparation of a varnish 
     A solution with a solids content of 20% and a Brookfield viscosity of 0.8 Pa.s is prepared by dissolving the powdery copolymer obtained in Example 9in butyl acetate. 
     A varnish is then manufactured from this solution and a copolymer consisting of 70% by weight of methyl methacrylate and 30% of ethyl acrylate (Mn 27,000; Mw 93,000), marketed by Rohm &amp; Haas under the name B44, by mixing the following: 
     * 58.3 g of a solution of the VF3/C 2  F 4  copolymer of Example 9 inbutyl acetate; 
     * 25 g of a solution of B44 in butyl acetate (solids content 20%); and 
     * 25 g of butyl acetate. 
     The solids content of the varnish is 15.4%. 
     Use of VF3/C 2  F 4  copolymer in a varnish 
     The varnish described above is deposited on a chromed aluminum plate using a 150 μm spiral doctor blade. 
     To test its suitability for use as a retouching varnish, it is also deposited on an aluminum plate which has already been coated with white paint based on PVDF. 
     By way of comparison, a retouching varnish is used which contains the same proportions of fluorinated copolymer, B44, and solvent except that the fluorinated copolymer has been replaced with a terpolymer which comprises 72 mol % of VF2, 18% of C 2  F 4 , and 10% of C 3  F 6 , and melts at 90° C. 
     The coated plates are dried either for 30 minutes at 80° C. or for 1week at room temperature. 
     Table 2 lists the results of the measurements of such properties as adhesion, impact strength, gloss of the varnishes described above. 
     
                                           TABLE 1__________________________________________________________________________                     Brookfield                          % of Transparency after   Molar M.p.        viscosity                          polymer                               200% uniaxialEx.  Monomers   proportions         (°C.)            Mn   Mw/Mn                     (Pa · s)                          in solution                               stretching__________________________________________________________________________1 VF3/C.sub.2 F.sub.4   90/10 219            82,000                 1.9 2.65 20   breaks before 200%2 VF3/C.sub.2 F.sub.4   80/20 232            96,000                 2.4 7.30 12   transparent3 VF2   100   1704 VF3   100   204            83,000                 2.2 6.75 20   slightly                               transparent5 VF2/C.sub.2 F.sub.4   80/20 120                   transparent6 VF.sub.3 /C.sub.2 F.sub.3 Cl   90/10 1937 VF3/C.sub.3 F.sub.6   99/1  187            100,000                 2.1 10.40                          20   transparent8 VF3/VF2   96/10 191            146,000                 1.9 62.00                          20__________________________________________________________________________ 
    
     The gloss on a white background at 20° is measured according to standard ASTM D 523-85. 
     The adhesion is evaluated according to standard NFT 30-038. 
     The Persoz hardness is measured according to standard NFT 30-016. 
     The impact strength is expressed in meters for a weight of 1 kg. 
     The flexural strength is measured according to standard ISO 3270. 
     The gloss retention under QUV is measured according to standard ASTM 523-85. 
     
                       TABLE 2______________________________________            Varnish of  ComparativeTest/measurement Example 10  varnish______________________________________appearance in pot            excellent   excellentappearance of film            excellent   excellentthickness of dry film            14 μm ± 2                        9 μm ± 2gloss on white background            53.2% ± 1                        62.1% ± 1at 20° C.adhesion to chromed Al            excellent   excellentadhesion to white PVDF-            excellent   excellentcoated AlPersoz hardness  2.72        231impact strength  &gt;1 m        &gt;1 mflexural strength            excellent   excellentgloss retention under QUV            100%        100%after 1000 h______________________________________ 
    
     Use of VF3/C 2  F 4  copolymer as ultraflexible material 
     Ultraflexible materials can be used in particular for the production of primary or secondary sheaths, plates, and/or deformable ferrules, tips, nipples, terminals, and connectors. 
     The copolymer of Example 2 is molded at 250° C. into a 0.7 mm thick plate. Its tensile mechanical properties are measured according to standard ASTM D 1708: 
     
         ______________________________________stress at break        15.4 MPaelongation at break     513%modulus of elasticity   250 MPa______________________________________ 
    
     The cold (-30° C.) impact strength is measured for different fluorinated polymers according to the invention (Examples 1, 2 and 9), a PVDF (melting point 170° C.; Mw 285,000) marketed by Elf Atochem S.A. under the name KYNAR® 1000, and a PVF3 (melting point 199°C.; Mw 280,000). 
     The results are collated in Table 3. 
     
                       TABLE 3______________________________________Nature of the polymer          Izod impact strength (in J/m)______________________________________PVDF            40PVF3            60VF3/C.sub.2 F.sub.4 of example 9          280VF3/C.sub.2 F.sub.4 of example 1          200VF3/C.sub.2 F.sub.4 of example 2          180______________________________________