Abstract:
The emission of the highly toxic volatile products, such as methyl bromides or iodides, or ethyl bromides or iodides, during the cure with peroxides of fluoroelastomers containing bromine or iodine atoms is substantially reduced by adding to the cure mix small amounts of substances capable of combining with the radicals which give rise to the aforesaid toxic products, thereby preventing them from forming. This procedure does not sensibly affect the cure trend and results.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a Continuation of application Ser. No. 08/026,165, filed Mar. 1, 1993, now abandoned, which in turn is a Continuation of application Ser. No. 07/798,176, filed Nov. 26, 1991, now abandoned, which in turn is a Rule 60 Continuation of application Ser. No. 07/450,466, filed Dec. 14, 1989, now abandoned. 
    
    
     SUMMARY OF THE INVENTION 
     1. Background of the Invention 
     The present invention is directed to reduction in the emission of highly toxic volatile products, such as methyl bromides or iodides or ethylbromoides or iodides during the cure of fluoroelastomers with peroxides. 
     2. Description of the Related Art 
     Thus, it is an object of the present invention to obviate the above-mentioned serious drawback by adding to the cure mix a substance capable of substantially reducing the emission of methyl or ethyl bromides or iodides during the cure. 
     In fact, it has surprisingly been found that certain substance types introduced into the mix in a moderate amount, generally ranging from 0.1 to 3 parts by weight for 100 parts by weight of fluoroelastomer, are capable of substantially reducing the emission of said toxic products without sensibly altering the cure trend or the characteristics of the resulting vulcanizate. The action mechanism of the additives according to the present invention is probably based on the fact that they are capable of combining with the methyl or ethyl radicals deriving from the organic peroxide, thereby forming new radicals capable of continuing the cross-linking reaction, so preventing any formation of methyl or ethyl bromides or iodides. However, on the basis of such assumption it is not possible to foresee which substances are actually suited as additives according to the invention: in fact, many substances which are surely capable of blocking the methyl and ethyl radicals and which drastically reduce the emission of the abovesaid bromides and iodides, negatively interfere with the cure process, causing it to become extremely slow and inefficacious and giving rise to vulcanizates of very bad quality. 
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The products suited as additives according to the present invention are the ones belonging to the following classes: 
     a) Benzothiazole and derivatives thereof of general formula: ##STR1## wherein: X represents H, --SH, ##STR2## in particular benzothiazole, 2-mercaptobenzothiazole, benzothiazole disulphide, morpholin-2-benzothiazole sulphenamide, zincmercapto-benzothiazole. 
     b) Maleamide derivatives of general formula: ##STR3## wherein A=H, T represents --OH, ##STR4## R represents H, alkyl with 1 to 6C, halogen, --NO 2  ##STR5## c) Thiurams derivatives of general formula: ##STR6## wherein Y represents ##STR7## R&#39; represents alkyl with 1 to 4C, in particular tetraalkylthiuram disulphide, zincdiethyldithiocarbamate. 
     d) Diphenylguanidine derivatives of general formula: ##STR8## wherein R 2  represents H, alkyl with 1 to 6C, in particular the compounds wherein R 2  represents H or CH 3 . Moreover other compounds such as maleic anhydride, diaryl-formamidines and the derivatives of diazo- and triazo-dicyclodecenes are resulted to be suited as additives. 
     The additives according to the present invention, when used in amounts generally ranging from 0.1 to 3 parts by weight for 100 parts by weight of elastomer (p.h.r.) and in particular from 0.2 to 2 p.h.r., beside sensibly reducing the emission of the toxic products methyl or ethyl bromide or iodide, in some cases also by 90%, impart to the curable composition the following characteristics: 
     increase of the time which is necessary for the scorching to occur (scorching time) and, by consequence, a higher processing safety; 
     decrease of the Mooney viscosity of the mix and therefore an improved processability; 
     moderate influence on the cross-linking rate and yield at 180° C. 
     All the additives indicated hereinbefore can be prepared according to known methods described in the technical literature. 
     The fluoroelastomers, to which the present invention is applicable, are all those which contain bromine or iodine atoms ascure sites. 
     Among the most known, the following are to be cited: copolymers of CH 2  =CF 2  with CF 3  --CF=CF 2  and optionally also with C 2  F 4  with perfluorovinylethers, containing little amounts of bromine or iodine which are introduced by copolymerizing a little amount of brominated or iodidated monomer such as brominated olefins, perfluorobromoalkylperfluorovinylethers, or also by using chain transferors consisting of bromo- or iodo-alkyl compounds. 
     The mixes based on fluoroelastomers curable with peroxides, which can be additioned according to the present invention comprise, beside the organic peroxide, conventional ingredients such as carbon black and other reinforcing fillers, peroxide curing co-agents (for example triallyl isocyanurate), metal oxides or hydroxides (for example PbO, ZnO), and processing aids. 
     The following examples are given merely to illustrate but not to limit the present invention. 
     The evaluation of the CH 3  Br emission under curing conditions was generally carried out according to the following method: 
     Modalities for Dosing the CH 3  Br Emission 
     20 g of a finely particled mix (polymer+fillers) are treated at 180° C. in a closed reactor, for 30 minutes at a pressure of 0.5 kg/cm 2  abs. in a nitrogen atmosphere. 
     Then the mix is cooled to 40°-50° C. and, as an interior standard, 1 cc of A114 (C 2  Cl 2  F 4 , b.p.=4° C.) is added. 
     The gas phase is mixed and subjected to gaschromatographic analysis. 
     Column: POROPAK Q 
     T=100° C. 
     In example 2, conversely, the actual emission occurring under practical curing conditions at 170° C. is determined by determining the CH 3  Br in air samples withdrawn above the mold. 
     Utilized elastomer: 
     P.1 terpolymer of CH 2  =CF 2  66.2% by moles, C 3  F 6  18.2%, C 2  F 4  15.2%, containing bromoperfluoroethylvinylether in an amount equal to 0.65% by weight of bromine. 
     P.2 terpolymer of CH 2  =CF 2  54.1% by moles, C 3  F 6  22.2%, C 2  F 4  22.9%, containing bromoperfluoroethylvinylether in an amount equal to 0.54% by weight of bromine. 
     The ingredients of the mixes, besides the additive according to the invention, are, in parts by weight for 100 parts by weight of terpolymer, the following: 
     2,5-dimethyl-2,5-di-ter.butyl-peroxy-hexane at 45% on inert filler (Luperco 101XL), 3 parts; 
     triallyl isocyanurate (TAIC) at 75% on inert filler, 4 parts; 
     PbO, 3 parts; 
     carbon black MT, 30 parts. 
     The evaluation of the cure trend is carried out by determining the Δ torque (MH-ML) by means of an ODR oscillating disc rheometer (Monsanto type) according to standard ASTM D 2084/81. 
     Measuring is effected at 180° C., the maximum torque MH is determined after 12 minutes, by &#34;arc 3&#34;. 
    
    
     EXAMPLE 1 
     The tests were carried out using terpolymer P.1. 
     The elastomer without the additive of the invention provided during curing a Δ torque equal to 71. 
     In some tests, the low reduction of the CH 3  Br emission was due to the insufficient amount of additive: by increasing said amount within the above-cited range, it is possible to improve the effect (see for example tests 7 and 8). 
     Test 21 is a comparison test and represents the case of products which drastically reduce the CH 3  Br emission, but at the same time they prevent or strongly reduce the cure. 
     
                                           TABLE 1__________________________________________________________________________                                          %                               Δ TORQUE                                          REDUCTIONTEST                         PHR    MH-ML      OF CH.sub.3 BrNo. ADDITIVE                 ADDITIVE                               (STANDARD = 71)                                          EMISSION__________________________________________________________________________ 1##STR9##                1.65   60         72% 2##STR10##               0.5    65         26% 3##STR11##               1.60   70         17% 4##STR12##               1.80   68         34% 5##STR13##               0.5    71         31% 6##STR14##               1.92   72         31% 7##STR15##               1.15   64         35% 8##STR16##               0.77   66         17% 9##STR17##               1.68   51         77%10##STR18##               0.9    59         57%11##STR19##               1.0    60         51%12##STR20##               1.8    52         84%13##STR21##               1.2    51         86%14##STR22##               0.5    65         28%15##STR23##               0.5    70         70-80%16##STR24##               0.5    71         70-90%17##STR25##               0.5    68         90%18##STR26##               0.5    67         87%19##STR27##               0.5    72         61%20##STR28##               0.5    80         79%    FOR COMPARISON21##STR29##               0.6     8         90%__________________________________________________________________________ 
    
     EXAMPLE 2 
     Test were carried out in order to check the correspondence between the CH 3  Br reduction determined by means of the laboratory test described for example 1 and the actual reduction achieved in the curing process in mold at 170° C. under actual conditions, determined by analysis of an air sample withdrawn above the molded article immediately after having drawn the vulcanizate from the mold. The additive utilized was mercaptobenzothiazole disulphide (MBTS); a test was carried out without the additive in order to have a comparison of the values of the Δ torque and of the CH 3  Br emission. 
     The elastomer utilized was of the type P.2 described hereinbefore. 
     The results are the average of analytical determinations on 5 air drawings. 
     
                                           TABLE 2__________________________________________________________________________           PERCENT REDUCTION                         PERCENT REDUCTION     Δ           OF CH.sub.3 Br EMISSION                         OF CH.sub.3 Br EMISSIONADDITIVE  PHR     MH--ML           LABORATORY TESTS                         MOLDING TESTS__________________________________________________________________________--     -- 62    --            --MBTS   0.25     64    79%           62%MBTS   0.5     66    85%           98%__________________________________________________________________________ 
    
     EXAMPLE 3 
     Characterization of the curable mixes and of the vulcanized product. 
     The characteristics compared with those of the non-additivated products are reported in the following tables. 
     In particular, from table 3 it is apparent that the mixes containing the additive according to the invention exhibit a remarkably higher scorching timen, a decrease of the Mooney scorch viscosity (at 135° C.) and a decrease of the Mooney viscosity at 121° C.; these characteristics are indicative of a better processability of the mix. 
     Table 4 contains some characteristic values which provide idications about the cure trend. 
     The characteristics of the cured material are reported in Table 5. 
     
                                           TABLE 3__________________________________________________________________________COMPOSITION OF THE MIX (ASTM D. 3182-82)                              ASTM D. 1646-82PARTS BY WEIGHT FOR 100 PARTS BY WEIGHT OF POLYMER                              121° C.                                      135° C.     additive            carbon                              MOONEY  MOONEY SCORCHMix No.polymer     (p. by wg.)            luperco                  TAIC                      PbO                         blackMT                              ML (1&#39; + 10&#39;)                                      MV   t 15__________________________________________________________________________1    P. 1 --     3     4   3  30   86      42   11&#39; 42&#34;2    P. 1 NPM (1,5)            3     4   3  30   84      40   24&#39; 18&#34;3    P. 1 MBT (0,5)            3     4   3  30   74      35   61&#39; 00&#34;4    P. 1 MBTS (0,5)            3     4   3  30   79      35   60&#39;5    P. 2 --     3     4   3  30   110     49   13&#39; 18&#34;6    P. 2 MBTS (0,25)            3     4   3  30   102     45   26&#39; 24&#34;7    P. 2 MBTS (0,5)            3     4   3  30   101     43   33&#39; 42&#34;__________________________________________________________________________ ##STR30## ##STR31## ##STR32## 
    
     
                                           TABLE 4__________________________________________________________________________       CURING CHARACTERISTICS  Mix No.       ASTM 02084-81 - ODR, 180° C.; arc +/-3  (from       ML   MH   ts 2                    t&#39;50                       t&#39;90                          V maxPOLYMER  tab. 3)       (lbf · in)            (lbf · in)                 (s)                    (s)                       (s)                          (lbf · in/s)                                MH--ML__________________________________________________________________________P.1    1    23   82   69 117                       243                          0.75  59P.1    2    20   95   75 129                       312                          1.01  75P.1    3    16   77   129                    219                       348                          0.41  61P.1    4    16   81   135                    195                       303                          0.7   65P.2    5    24   86   69 126                       360                          0.63  62P.2    6    19   83   90 144                       324                          0.73  64P.2    7    17   83   99 153                       330                          0.80  66__________________________________________________________________________ 
    
     
                                           TABLE 5__________________________________________________________________________       MECHANICAL PROPERTIES                                  after post-cure (250° C. +                                  24 h)       after press-cure (170° C. + 10&#39;)        ASTM                           ASTM                  ASTM D1414                   ASTM    D.624-81              D2240       78                   D.2240-8                           Tear-Strength       81          (*)       ASTM D.412-83                   Hardness                           170° C.                                  ASTM D.412-83  Hardness                                                      C.S.  Mix No.       Tensile propert.                      (points)                           (N/ (J/                                  Tensile properties                                                 (points)                                                      (%)  (from       (MPa)           (MPa)               (MPa)                   (%)                      H-shore                           /mm)                               /m)                                  (MPa)                                      (MPa)                                          (MPa)                                              (%)                                                 H-shore                                                      O-RPOLYMER  tab. 3)       M100           M200               T.S.                   E.B.                      A    F/s E/s                                  M100                                      M200                                          T.S.                                              E.B.                                                 A    214__________________________________________________________________________P.1    1    4.3 9.9 10.8                   234                      70   3.3 11.5                                  5.9 14.7                                          15.2                                              189                                                 73   30P.1    2    3.5 10.1               12.6                   240                      70   3.6 12.4                                  5.4 16.8                                          18.4                                              214                                                 73   33P.1    3    2   5   8   349                      67   3.4 14.2                                  4.6 13.2                                          14.9                                              219                                                 74   44P.1    4    2.9 8.7 10.8                   268                      68   3.6 13.2                                  5.0 14.5                                          16.1                                              216                                                 74   42P.2    5    3.7 8.3 11  306                      73   4.9 28.1                                  5.5 15.8                                          18.5                                              227                                                 74   33P.2    6    3.6 7.9 10.6                   338                      72   4.6 24 5.9 16.1                                          19.5                                              234                                                 76   35P.2    7    3.7 8.1 10.9                   319                      72   4.3 22.2                                  5.9 16  19  230                                                 76   34__________________________________________________________________________ (*) C.S. = compression set at 200° C. for 70 h. 
    
     EXAMPLE 4 
     The tests reported in Table 6 show that the reduction of the CH 3  Br emission and the cure trend vary on variation of the utilized polymer type and of the bromine content. 
     In all the tests reported in Table 6 the utilized additive was phenylmaleinimide: ##STR33## 
     Other components of the mix were: 
     
         ______________________________________Luperco        3 p.h.r.TAIC           4 p.h.r.PbO            3 p.h.r.carbon black   30 p.h.r.______________________________________ 
    
     
                       TABLE 6______________________________________                           % REDUCTION     ADDITIVE   Δ TORQUE                           OF CH.sub.3 BrPOLYMER   P.H.R.     MH--ML     EMITTED______________________________________P.1       --         71         --P.1       1,5        76         15%P.41      --         62         --P.41      1,5        55         29%P.2 + P.41(+)     --         65         --mixP.2 + P.41(+)     1,5        64         37%mix______________________________________ 
    
     Polymer P.41 contains: 
     CH 2  =CF 2  53,4%; C 2  F 4  23,9%; C 3  F 6  22,7%, and bromovinylether in an amount equal to 0.93% of Br. 
     (+) Bromine content of the polymeric mix: Br=0.72%.