Abstract:
The present invention relates to compositions based on elastomeric polymers of vinylidene fluoride, hexafluoropropene and optionally tetrafluoroethylene suitable for providing microcellular foamed vulcanized articles by extrusion or compression molding. In the case of the production of extruded articles based on vinylidene fluoride and hexafluoropropene copolymers, the compositions comprise: the copolymer, bisphenol AF, a quaternary phosphonium or aminophosphonium salt, azodicarbonamide, MgO, Ca(OH) 2 , zinc stearate and a plasticizer, each of the components being contained in an exactly defined proportion.

Description:
This is a continuation of U.S. application Ser. No. 08/068,962, filed May 28, 1993 now abandoned. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to compositions based on elastomeric polymers of vinylidene fluoride, hexafluoropropene and, optionally, tetrafluoroethylene. 
     The above-identified polymers are being used more and more in fields where high chemical stability and exceptional resistance to temperatures exceeding 220° C. for continuous use are required. Because of these characteristics, these fluoroelastomers are used particularly for producing gaskets, O-rings, and seals of high technological value. 
     DESCRIPTION OF THE PRIOR ART 
     Because of the intrinsic characteristics of their chemical structure, the specific density of these fluoroelastomers varies from 1.78 to 2.01. This results, of course, in final articles comprising the compound having high specific densities. The cost of the articles is affected by this property. This, in turn, has retarded the use of these fluoroelastomers in fields is where the excessive weight of the final article has led to an unacceptable difference in cost which favors other rubbers having lesser chemico-physical properties. 
     The production of non-fluorinated elastomeric articles having a lower final weight through the use of formulations capable of providing microcellular foamed products is a generally known technology, applicable on an industrial scale. 
     This technology--utilized, for example, for natural and silicone rubbers--comprises the use of blends which, besides the usual curing systems, contain appropriate foaming agents. These foaming agents, as a consequence of their thermal decomposition during the processing step, are capable of generating gases, mainly nitrogen and carbon oxides, which swell the elastomeric material. 
     The main classes of such swelling agents are: 
     azodicarbonamide and its derivatives; 
     variously substituted hydrazides, for example p.toluene 
     sulphonyl hydrazide. 
     In order to control their thermal decomposition to meet processing requirements, such swelling agents can be associated with appropriate kickers, which modify the thermal decomposition kinetics thereof. 
     Examples of such kickers are: 
     organic acids (for example salicylic, glycolic, lactic, citric, succinic acids); 
     phosphoric acid; 
     organic bases (for example guanidines and ethanolamines); 
     inorganic bases (for example borax and potassium carbonate); 
     zinc, mercury, barium, cadmium, calcium, strontium, magnesium, lead, tin, silicium compounds. 
     In the case of the above-described vinylidene fluoride elastomeric polymers, the use of this technology has not yet permitted the production of foamed articles having acceptable appearance and physico-mechanical characteristics. In the best of circumstances, spongy materials with large bubbles and large inner cavities and a wrinkly surface appearance have been obtained. 
     SUMMARY OF THE INVENTION 
     It has now been found that the use of particular combinations of components usually used for the curing of the above-described fluoroelastomers, and of particular components known as swelling agents--each of these components being used in well-defined proportions--permits the production of microcellular foamed articles endowed with excellent characteristics as regards the dimensional and cell distribution homogeneity, the outer appearance, and their physico-mechanical properties. 
     Thus, it is an object of the present invention to provide compositions based on elastomeric polymers of vinylidene fluoride, hexafluoropropene and optionally tetrafluoroethylene suitable for providing microcellular foamed vulcanized articles exhibiting the above-described characteristics. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 shows a magnified (7.03 times) cross-sectional view of a product of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The compositions of the present invention vary slightly depending on whether the fluoroelastomer is a copolymer of vinylidene fluoride (78-83 mols %) with hexafluoropropene (17-22 mols %) or a terpolymer of vinylidene fluoride (50-79 mols %), hexafluoropropene (17-25 mols %) and tetrafluoroethylene (4-25 mol %). The compositions further vary depending on whether they are processed by extrusion or by compression molding. In all cases the Mooney viscosity of the polymer, measured according to ASTM standard 1646, M L  (1&#39;+10&#39;) at 121° C., ranges from 25 to 50 Mooney degrees. 
     Composition I 
     When they are processed by extrusion with a copolymer, the compositions comprise: 
     
         ______________________________________a)  the copolymer      100        parts by weightb)  bisphenol AF       about 0.5-1.0                             parts by weighthexafluoro-isopropylidene (bis-    4-hydroxybenzene!c)  a quaternary phosphonium or                  about 0.5-1.0                             parts by weight    aminophosphonium salt, as    further definedd)  azodicarbonamide   about 2.5-5.0                             parts by weighte)  MgO                about 1.0-2.0                             parts by weightf)  Ca(OH).sub.2       about 1.0-3.0                             parts by weightg)  zinc stearate      about 1.0-3.0                             parts by weighth)  polyoxyethyleneglycol having an                  about 0-1.0                             parts by weight    average molecular weight    ranging from about 3,000 to    5,000i)  a plasticizer, as further                  about 1.0-5.0                             parts by weight    defined______________________________________ 
    
     The quaternary phosphonium salt is a triphenylbenzyl-phosphonium salt. The aminophosphonium salt is a 1-chloro-N,N-diethyl-1,1-diphenyl-1-(phenylmethyl) phosphoranamine. 
     The former of these salts is sold by duPont de Nemours, in admixture with the copolymer, under the trade-mark Viton® C 20; the latter is sold by Ausimont, in admixture with the copolymer, under the trade-mark Tecnoflon® FOR M2. 
     The phosphonium or aminophosphonium salt can be used in the form of its adduct with bisphenol AF as, is described for example in U.S. Pat. No. 4,496,682 and in Italian patent application 41009/A 89. 
     Such adducts are sold by duPont de Nemours under the trade-mark Viton® C 50 and by Ausimont under the trade-mark Tecnoflin® XA 51. 
     The plasticizer cited in the preceding compositions is selected from the group consisting of esters of fatty acids and of adipic acid, polyethers derived from the polycondensation of ethylene oxide and/or propylene oxide, low molecular weight polyethylenes, dioctylphthalate, dibutylphthalate, di-(2-ethylhexyl)phthalate and other phthalic acid derivatives exhibiting plasticizing properties, 5,8,11,13,16,19-hexaoxa-tricosane, and the condensation products of derivatives of fatty acids and silicones on inorganic carriers. Among the fatty acid esters are, for example, the stearic acid esters and the sebacic acid esters. 
     Among the adipic acid esters it is possible to use, for example, di(butoxy-ethoxy-ethyl) adipate. 
     The above-mentioned polyethers have, preferably, an average molecular weight ranging from 600 to 1,000 and the polyethylenes an average molecular weight ranging from 1,100 to 1,500. 
     The condensation products of derivatives of fatty acids and silicones are available on the market, under the trade-mark Struktol® WS 280 of Schill und Seilacher, in the form of the above-described condensation products (75%) carried on SiO 2  (25%). 
     5,8,11,13,16,19-hexaoxa-tricosane is commercially available in admixture (at 94% by weight) with diethylene glycolmonobutylether (1.5%) and formaldehyde (0.1%), and it is sold by Thiokol Corp. under the trade-mark TP-90®. 
     Besides the above-described components, the compositions conforming to the present invention (including the ones discussed further on herein) can contain the usual fillers utilized in the fluoroelastomeric formulations, such as, for example, carbon black, barite, kaolin, TiO 2  and dyeing pigments; such fillers are added in the usual proportions with respect to the copolymer. 
     Composition II 
     When they are to be processed by subjecting the copolymers to compression molding, the compositions conforming to the present invention comprise the components from a) to h) as defined above, in the above-indicated proportions, and furthermore: 
     
         ______________________________________j)  a plasticizer, as defined               about 0-5.0 parts by weight    abovek)  ZnO or          about 3.5-7.0                           parts by weight.    dibasic lead phosphite               about 2.0-6.0                           parts by weight______________________________________ 
    
     In these compositions, the use of the plasticizer is not indispensable for the production of plates of little thickness. However, it is preferable to use it for plates of great thickness. 
     Composition III 
     When the terpolymers are processed by extrusion, the components from b) to i) are exactly the same as the ones of compositions I, but their proportions are slightly different: 
     
         ______________________________________a)  terpolymer       100         parts by weightb)  bisphenol AF     about 1.0-2.5                            parts by weightc)  quaternary phosphonium or                about 1.0-3.0                            parts by weight    aminophosphonium saltd)  azodicarbonamide about 2.5-5.0                            parts by weighte)  MgO              about 1.0-2.0                            parts by weightf)  Ca(OH).sub.2     about 1.0-3.0                            parts by weightg)  zinc stearate    about 1.0-3.0                            parts by weighth)  polyoxyethylene  about 0-1.0 parts by weight    glycol having    an average    molecular weight    from about 3,000    to 5,000i)  plasticizer      about 1-5.0 parts by weight______________________________________ 
    
     Composition IV 
     When the terpolymers are processed by compression molding, the compositions conforming to the present invention comprise the components from a) to h) defined in compositions III, in the proportions defined in said compositions, and furthermore: 
     
         ______________________________________j)    plasticizer as defined in                about 0-5.0 parts by weight compositions Ik)    ZnO or         about 2.0-7.0                            parts by weight dibasic lead phosphate                about 2.0-6.0                            parts by weight.______________________________________ 
    
     The copolymers and terpolymers are marketed by: 
     Ausimont, under the trademark Tecnoflon®; 
     Dupont de Nemours, under the trademark Viton®; 
     3M, under the trademark Fluorel®; 
     Asahi Chemicals, under the trademark Miraflon®. 
     The compositions according to the present invention can be prepared as follows: 
     a) preparation of the basic mix: in this step, reinforcing fillers, optional plasticizers and processing coadjuvants, cross-linking and swelling agents and kickers are added to the basic elastomer. The whole is homogenized in open or closed mixers according to the usual procedures, which are known to those skilled in the art; 
     b) the mix so obtained is then allowed to stand for at least 24 hours in accordance with the usual procedures; 
     c) after such period, the mix is taken up for the final homogenization in an open mixer. 
     Processing in an extruder is preferably conducted as follows: feeding occurs at 30°-50° C.; the temperature ranges from 60° to 80° C. in the extruder body and, at the extruder outlet, from 70° to 100° C. Vulcanization follows, which is usually carried out in an autoclave at 125°-150° C. 
     Processing in a press is usually conducted at 130°-160° C. 
     Post-curing is carried out in both cases in an oven, usually at 180°-260° C. for 20-35 hours, the raising time being 5-14 hours. 
     The microcellular foamed articles prepared according to the present invention exhibit excellent characteristics with regards to flexibility, resistance to abrasion, elasticity, resistance to fluids and to heat, and exhibit low densities. Also the mechanical properties are very good. In particular the compression set, especially at high temperatures, is excellent. 
     Such articles are useful in applications where they are subjected to particularly severe conditions, which are not tolerated by the microcellular foamed rubbers available at present on the market. Some of these applications are: industrial gaskets, electric cable coating, thermal and soundproofing insulation at high temperatures. 
     The following examples are given to better illustrate the inventive principle of the present invention and are not to be considered as limitative of the scope thereof. 
     EXAMPLE 1 
     Formulation A was prepared with the following composition: 
     
         ______________________________________Tecnoflon NM ®       100     parts(copolymer produced by Ausimont, containing80 mols % of vinylidene fluoride and 20 mols %of hexafluoropropene)Tecnoflon FOR Ml ®   2       phrproduced by Ausimont, containing 50% by weightof Tecnoflon NM and 50% of bisphenol AFTecnoflon FOR M2 ®   3       phrproduced by Ausimont, containing 30% by weightof the aminophosphonium salt (chloride) describedbefore and 70% of Tecnoflon NMGligogum 4000 ® (polyoxyethylene glycol                    0.3     phrhaving an average molecular weight of about4,000, marketed by Enichem Synthesiszinc stearate            2       phrTP 95 ® (di(butoxy-ethoxy-ethyl)adipate)                    2       phrproduced by Thiokol Corp.To 100 parts of composition A there were added:MgO-DE ® produced by Eigenman-Veronelli                    1       phrRhenofit CF ® (Ca(OH).sub.2) produced by Bayer                    2       phrTremin 283-600 EST ® produced by Quarzwerke                    10      phr(calcium silicate treated with aminosilane)Black MT                 5       phrPorofor ADC/K ® produced by Bayer                    3       phr(azodicarbonamide)TP 95 ®              2       phr______________________________________ 
    
     The composition was fed to an extruder Ital TG/60 15 D suited to manufacture a rod having a diameter of 6 mm. The temperatures were as follows: 
     feed section: 40° C. 
     body: 60° C. 
     head: 80°-90° C. 
     The screw rotated at a speed of 9 r.p.m. 
     Subsequently, the extrudate, coated with talc, was placed into a steam autoclave. The treatment was conducted at about 140° C. and at a pressure of about 6 atmospheres for a period of 90 minutes. 
     Post-curing occurred in an oven at 200° C. for 24 hours, with a raising time of 5 hours, or at 235° C. for 24 hours, with a raising time of 8 hours. 
     At the end of the treatment the rods had a diameter of 10 mm; they were free from large bubbles and wrinkles and exhibited an excellent dimensional and cell-distribution homogeneity. The photograph, enlarged by 7.03 times, of a rod section at the autoclave outlet is shown in FIG. 1. 
     EXAMPLE 2 
     To 100 parts of formulation A described in example 1 there were added: 
     
         ______________________________________MgO-DE               1         phrRhenofit CF          2         phrTremin 283-600 EST   10        phrGreen F 2G ®, an organic pigment                2         phrproduced by AuschemTiO.sub.2            2         phrTP 95                2         phrPorofor ADC/K        3         phr______________________________________ 
    
     The extrusion and the thermal treatments described in example 1 were carried out. 
     At the end of the treatment, the rod exhibited appearance characteristics like those of example 1. 
     EXAMPLE 3 
     The physico-mechanical characteristics of the rods prepared in examples 1 and 2 were determined. 
     The density and the results of the tensile tests according to standard ASTM D 412 (the rods were fixed by means of proper clamps) are reported in Table 1. 
     Table 2 shows the same results after different thermal post-treatments of the rods obtained in examples 1 and 2. 
     Tables 3 and 4 indicate the conditions and the results of various compression set tests (according to standard ASTM D 395) on the rod of example 1 (Table 3) and on the rod of example 2 (Table 4). The compression direction is normal to the rod axis. 
     
                       TABLE 1______________________________________DENSITY AND TENSILE TESTSFORMULATION    EXAMPLE 1    EXAMPLE 2______________________________________Maximum post-curing          200     235      200   235temperature (°C.)Density (g/cm.sup.3)          0.60    0.60     0.67  0.67Hardness, Shore A (points)          25      29       31    33Tensile strength (MPa)          1.04    1.30     1.20  1.41Elongation at break (%)          110     90       116   93______________________________________ 
    
     
                                           TABLE 2__________________________________________________________________________DENSITY AND TENSILE TESTS AFTER FURTHER THERMAL TREATMENTSFORMULATION EXAMPLE 1                EXAMPLE 2__________________________________________________________________________Maximum post-curing       200         235          200          235temperature (°C.)Further thermal (°C.)       220 220 220 250 250  250 220 220  220 250 250  250treatment (h)       24  48  72  24  48   72  24  48   72  24  48   72Density (g/cm.sup.3)       0.61           0.60               0.60                   0.60                       0.62 0.60                                0.68                                    0.66 0.67                                             0.67                                                 0.67 0.68Hardness, Shore A (points)       30  30  34  31  33   36  33  35   39  34  37   40Tensile strength (MPa)       1.15           1.20               1.20                   1.50                       1.52 1.60                                11.30                                    1.35 1.40                                             1.50                                                 1.58 1.65Elongation at break (%)       97  90  92  82  74   70  100 94   98  87  74   69__________________________________________________________________________ 
    
     
                                           TABLE 3__________________________________________________________________________COMPRESSION SET AS A FUNCTION OF THE OPERATIVE CONDITIONROD OF EXAMPLE 1__________________________________________________________________________Maximum post-crosslinking         200temperature (°C.)Test Temperature (°C.)         150             170     190Test temperature (h)         24      70      24      24Compression set (%)         25  50  25  50  25  50  25  50Reading after 30 minutes at         10.8             7.7 18.8                     17.9                         17.8                             16.0                                 41.0                                     31.0test temperature (%)Reading after 30 minutes at         14.2             7.5 22.9                     21.8                         23.1                             20.0                                 43.0                                     33.423° C. (%)__________________________________________________________________________Maximum post-crosslinking       235temperature (°C.)Test Temperature (°C.)       150         170   190        200Test temperature (h)       24    70    24    24    70   24Compression set (%)       25 50 25 50 25 50 25 50 25 50                                    25 50Reading after 30 minutes at       7.1          6.0             12.0                8.4                   11.0                      8.0                         22.5                            19.0                               41.0                                  --                                    43.0                                       23.0test temperature (%)Reading after 30 minutes at       8.9          6.0             16.0                10.2                   12.0                      9.0                         22.1                            20.0                               45.0                                  --                                    33.0                                       23.023° C. (%)__________________________________________________________________________ 
    
     
                                           TABLE 4__________________________________________________________________________COMPRESSION SET AS A FUNCTION OF THE OPERATIVE CONDITIONROD OF EXAMPLE 2__________________________________________________________________________Maximum post-crosslinking         200temperature (°C.)Test Temperature (°C.)         150            170     190Test time (h) 24     70      24      24Compression set (%)         25  50 25  50  25  50  25  50Reading after 30 minutes at         11.1             8.0                18.0                    17.5                        18.0                            15.0                                42.8                                    33.0test temperature (%)Reading after 30 minutes at         15.4             7.6                22.0                    21.3                        24.0                            20.0                                44.1                                    34.023° C. (%)__________________________________________________________________________Post-crosslinking       235temperature (°C.)Test temperature (°C.)       150       170   190      200Test time (h)       24   70   24    24    70 24  70Compression set (%)       25 50            25 50                 25 50 25 50 25 25                                  50                                    25                                      50Reading after 30 minutes at       7.5          4.6            10.0               8.0                 11.0                    8.0                       21.0                          14.0                             39.0                                30                                  23                                    58                                      52test temperature (%)Reading after 30 minutes at       10.0          5.1            16.0               9.0                 14.0                    11.0                       22.4                          18.0                             45.0                                31                                  25                                    43                                      4023° C. (%)__________________________________________________________________________ 
    
     EXAMPLE 4 
     To the final composition of example 1 there were added 3 phr of dibasic lead phosphite (Dyphos® produced by REAGENS), whereafter the press molding of 11 cm×11 cm×8 mm plates was effected. 
     It was operated in the press at 135° C. for 25 minutes with a pressure of 200 bar. Post-curing in an oven was effected at 200° C. for 24 hours, with a raising time of 8 hours. 
     The final dimensions of the plate were 13 cm×13 cm×12 mm. The appearance of the plate had characteristics like the ones of the rod of example 1. 
     EXAMPLE 5 
     To the final composition of example 2 there were added 2.5 phr of ZnO produced by CARBOCROM, whereafter the press molding according to the modalities of example 4 was carried out. 
     The appearance of the final plate had characteristics like the ones of the rod of example 1. 
     EXAMPLE 6 
     A rod having the same dimensions of the rod of example 1 was prepared by extrusion starting from a composition based on terpolymer Tecnoflon TN 80® produced by Ausimont, containing: 65 moles % of vinylidene fluoride, 20% of hexafluoroptopene and 15% of tetrafluoroethylene. 
     The composition was as follows: 
     
         ______________________________________Tecnoflon TN 80          100     partsTecnoflon FOR MI (having the same proportion of                    2.5     phrelastomer and of bisphenol AF as in the compositionof example 1)Tecnoflon FOR M2 (having the same propor-                    4.5     phrtion of elastomer and of aminophosphoniumsalt as in the composition of example 1)Porofor ADC/K            3       phrMgO-DE                   1       phrTremin 283-600 EST       10      phrCa(OH).sub.2             2       phrBlack MT                 5       phrZinc stearate            2       phrGlicogum 4000            0.3     phrTP 95                    4       phr______________________________________ 
    
     It was operated as in example 1. The appearance of the resulting rod had characteristics like those of the rod of example 1. 
     EXAMPLE 7 
     A microcellular foamed plate was prepared by compression molding starting from a composition based on the terpolymer of example 6. 
     The composition was as follows: 
     
         ______________________________________Tecnoflon TN 80  100         partsTecnoflon FOR MI 2.5         phrTecnoflon FOR M2 4.5         phrPorofor MDC/K    3           phrMgO-DE           1           phrCa(OH).sub.2     2           phrTremin 283-600 EST            10          phrBlack MT         5           phrZinc stearate    2           phrGlicogum 4000    0.3         phrTP 95            4           phrDyphos           3           phr______________________________________ 
    
     Molding was effected in accordance with the modalities of example 4. 
     The final plate appearance had characteristics like the one of the rod of example 1. 
     EXAMPLES 8-16 
     In Table 5 various compositions conforming to the present invention are shown, which are appropriate to obtain microcellular foamed cured articles based on vinylidene fluoride/hexafluoropropene copolymers by means of the extrusion technology. 
     All the rods, prepared following the procedure of example 1, had appearance characteristics like the one of example 1. 
     
                                           TABLE 5__________________________________________________________________________COMPOSITIONS BASED ON VDF - HFP COPOLYMERS USEFUL TO PREPAREFOAMED ARTICLES BY MEANS OF THE EXTRUSION TECHNOLOGYEXAMPLE NO. 8   9   10  11  12  13  14  15  16__________________________________________________________________________Tecnoflon NM       100 100 100 100 100 100 100 100 100Tecnoflon FOR M1       1   1   1   2   2   2   2   2   2Tecnoflon FOR M2       2   2   2   3   3   3   3   3   3Porofor ADC/K       4   4   4   3   3   3   3   3   3MgO-DE      1   1   1   1   1   1   1   1   1Ca(OH).sub.2       2   2   2   2   2   2   2   2   2Tremin 283-600 EST       --  --  10  10  10  10  10  10  10Black MT    10  10  --  5   5   5   5   5   5Zn stearate 2   2   2   2   2   2   2   2   2Bayferrox (1)       --  --  5   --  --  --  --  --  --Polyethylene AC 617/A       3   3   3   --  --  --  --  --  --Sumpar 2280 (2)       0.5 --  0.5 --  --  --  --  --  --Glicogum 4000       --  --  --  0.3 0.3 0.3 0.3 0.3 0.3ZnO         --  --  --  1   3   5   --  --  --TP 95       3 or 4           3 or 4               3 or 4                   3 or 4                       3 or 4                           3 or 4                               3 or 4                                   3 or 4                                       3 or 4Dyphos      --  --  --  --  --  --  1   3   3__________________________________________________________________________ (1) red iron oxide (2) dibutoxyethoxyethyl adipate produced by BP Chemicals 
    
     EXAMPLES 17-25 
     In Table 6 various compositions conforming to the present invention are shown, which are appropriate to prepare microcellular foamed cured articles based on vinylidene fluoride/hexafluoropropene copolymers via compression molding. 
     All the plates prepared according to the procedure of example 4 exhibit a final aspect having characteristics like the ones of the rod of example 1. 
     
                                           TABLE 6__________________________________________________________________________COMPOSITIONS BASED ON VDF - HFP COPOLYMERS SUITABLE FOR PREPARINGFOAMED ARTICLES BY MEANS OF THE COMPRESSION MOLDING TECHNOLOGYEXAMPLE NO.      17  18  19  20  21  22  23  24  25__________________________________________________________________________Tecnoflon NM      100 100 100 100 100 100 100 100 100Tecnoflon FOR M1      1   1   1   1   1   1   1   1   2Tecnoflon FOR M2      2   2   2   2   2   2   2   2   3Porofor ADC/K      --  --  4   --  --  4   4   4   3Porofor ADC/R (1)      --  --  --  4   --  --  --  --  --Celogen AZ (1)      --  --  --  --  4   --  --  --  --Cellmic CAP 250 (1)      --  5   --  --  --  --  --  --  --Cellmic CAP 500 (1)      5   --  --  --  --  --  --  --  --MgO-DE     1   1   1   1   1   1   1   1   1Ca(OH).sub.2      2   2   2   2   2   2   2   2   2Tremin 283-600 EST      --  --  --  --  --  --  --  10  10Black MT   10  10  10  10  10  10  10  --  5Zn stearate      2   2   2   2   2   2   2   2   2Bayferrox  --  --  --  --  --  --  --  5   --Polyethylene AC617/A      --  --  --  --  --  3   3   3   --Sumpar 2280      --  --  --  --  --  0.5 --  0.5 --Glicogum   --  --  --  --  --  --  --  --  0.3TP 95      --  --  --  --  --  3 or 4                              3 or 4                                  3 or 4                                      3 or 4__________________________________________________________________________ The aboveindicated compositions also contain 5 phr of ZnO or with 4 phr o Dyphos. (1) Azodicarbonamide 
    
     EXAMPLES 26-27 
     In Table 7 two compositions conforming to the present invention are shown, which are appropriate to prepare microcellular foamed cured articles based on vinylidene fluoride/hexafluoropropene/tetrafluoroethylene terpolymers by means of the extrusion technology. 
     All the rods prepared according to the procedure of example 1 exhibit a final aspect having characteristics like the ones of the rod of example 1. 
     
                       TABLE 7______________________________________COMPOSITIONS BASED ON VDF-HFP-TFE TERPOLYMERSSUITED TO PREPARE FOAMED ARTICLES BY MEANSOF THE EXTRUSION TECHNOLOGYEXAMPLE NO.        26     29______________________________________Tecnoflon TN 80    100    100Tecnoflon FOR M1   2.5    2.5Tecnoflon FOR M2   4.5    4.5Porofor ADC/K      3      3MgO-DE             1      1Ca(OH).sub.2       2      2Tremin 283-600 EST 10     10Black MT           5      --Green F2G          --     2TiO.sub.2          --     2Zn stearate        2      2Glicogum 4000      0.3    0.3TP 95              4      4______________________________________ 
    
     EXAMPLES 28-29 
     In Table 8 there are reported further two compositions conforming to the present invention, which are suitable for preparing microcellular foamed cured articles based on vinylidene fluoride/hexafluoropropene/tetrafluoroethylene terpolymers by means of the compression molding technology. 
     All the plates prepared according to the procedure of example 4 had a final aspect exhibiting characteristics equal to the ones of the rod of example 1. 
     
                       TABLE 8______________________________________COMPOSITIONS BASED ON VDF-HFP-TFE TERPOLYMERSSUITED TO PREPARE FOAMED ARTICLES BY MEANSOF THE COMPRESSION MOLDING TECHNOLOGYEXAMPLE NO.        28     29______________________________________Tecnoflon TN 80    100    100Tecnoflon FOR M1   2.5    2.5Tecnoflon FOR M2   4.5    4.5Porofor ADC/K      3      3MgO-DE             1      1Ca(OH).sub.2       2      2Tremin 283-600 ET  10     10Black MT           5      --Vulcol F2G         --     2TiO.sub.2          --     2Zn stearate        2      2Glicogum 4000      0.3    0.3TP 95              4      4______________________________________ The above compositions also contain 5 phr of ZnO or 3-5 phr of Dyphos. 
    
     EXAMPLES 30-35 (comparative) 
     These examples relate to a few compositions based on Tecnoflon NM copolymer and not conforming to the present invention. These compositions were press molded at 135° C. for 25 minutes following the procedure of example 4. 
     When leaving the press, the foamed plates were defective for one or more of the following characteristics: surface roughness, large bubbles, irregular dimension of the cells and non-homogeneous distribution of them. As a result it was not possible to effect post-curing in an oven. 
     The corresponding compositions are reported in Table 9. 
     
                       TABLE 9______________________________________EXAMPLE NO. 30      31     32    33   34    35______________________________________Tecnoflon NM       100     100    100   100  100   100Tecnoflon FOR M1       1.5     1      1     2    1     1Tecnoflon FOR M2       1.5     2      2     3    2     2Porofor ADC/K       --      --     --    3    4     4Porofor TSH/75 (1)       --      4      --    --   --    --Cellmic H (1)       5       --     5     --   --    --Polyethylene AC/617A       --      --     --    --   3     --MgO-DE      2       1      1     1    1     1Ca(OH).sub.2       3       2      2     2    2     2Bayferrox   --      --     --    --   5     --Tremin 600 EST       --      --     --    10   --    --Celite 350 (2)       --      --     --    --   --    5Black MT    10      10     10    5    10    5Zn stearate 2       2      2     2    2     2CaO         --      --     --    3    --    --Sumpar 2280 --      --     --    --   0.5   --TP 95       --      --     --    3    3     3Epon 828 (3)       --      --     --    --   1     --______________________________________ (1) (hydrazide) (2) (white filler) (3) (plasticizer)