Development of new nucleophile resistant vinylidene fluoride fluorocarbon elastomers.

The most common vinylidene fluoride (VF2) based fluorocarbon elastomers are copolymers of VF2 with hexafluoropropylene (HFP) and terpolymers of VF2, HFP and tetrafluoroethylene (TFE) as the third monomer.

These fluorocarbon elastomers are known to be highly susceptible to attack by nucleophiles such as amines. This "weakness" has been used as a positive property because it renders them easily vulcanizable in the presence of bisnucleophiles such as diamines or bisphenols. A reaction mechanism recently proposed by Arcella et al. (refs. 1 and 2) is reported in figure 1.

The following base sensitive sites have been identified to be responsible for the vulnerability of fluoroelastomers:

[Mathematical Expression Omitted]

These sites consist of VF2 unit flanked by HFP and/or TFE units.

From the cross-linking reaction mechanism of figure 1, it appears that the nucleophilic attack proceeds essentially in two steps, dehydrofluorination and subsequent addition. Furthermore it is known that dehydrofluorination proceeds beyond that strictly required for cross-linking and an excess of unsaturations remains in the vulcanized product (ref. 5). From this consideration, it appears that VF2 fluorocarbon elastomers suffer on contact with strong bases. This represents an important drawback of these high performance elastomers since it limits their use in important applications, for example the fabrication of sealing parts which will come in contact with mineral oils containing strong base additives. Nowadays, this is an important technical problem to be solved, since due to the trend towards lower drain interval oils, high basic oil formulations will become always more aggressive against elastomeric seal materials.

Several developed (refs. 6 and 7) peroxide curable VF2 based fluorocarbon elastomers show a better resistance against basic oils. The cross-linking reaction is typically effected by the presence in there macromolecular chain of a cure site monomer susceptible to free radical attack. The reaction mechanism is reported in figure 2

Vulcanized products from peroxide curable fluoroelastomers contain a much lower amount of residual double bonds and an improved resistance to nucleophiles is obtained. However, these materials still contain the monomer sequences sensitive to nucleophilic attack seen before and consequently satisfactory results cannot be obtained under very aggressive conditions.

Fluorocarbon elastomers which do not have the base sensitive monomer sequences have been described. An obvious solution to this problem is to eliminate the VF2 from the monomer composition as in the case of perfluoroelastomers of TFE, perfluoroalkylvinylethers (PAVE) (ref. 8) and TFE based fluorocarbon elastomers such as polymers of TFE, PAVE and hydrogenated olefin (ref. 9).

Recently reported, fluorocarbon elastomers (ref. 10) containing VF2 show improved resistance to nucleophiles. These products do not contain HFP and it is stated that the improved resistance is due to the absence of this monomer. In terms of base sensitive sites these systems do not contain the sequences (1) and (2) because of the absence of HFP. However, they probably still contain the sequence (3) which is know to be less susceptible to nucleophilic attack (ref. 3) probably because of the less pronounced electron attractive effect of the neighboring fluorinated chains than those in sequence (2).

Results and discussion

In this article a different approach is presented and used to develop a new family of fluorocarbon elastomers with improved nucleophile resistance, VF2 based and containing HFP. This is accomplished by adding a further monomer which acts as a "protecting" monomer that reduces the concentration for the based sensitive monomer triads (1), (2), and (3) in the macromolecular chains.

In order to reduce the base sensitive monomer sequences (1), (2) and (3) the monomer mixture VF2/-TFE/HFP is polymerized in the presence of an additional monomer to ideally generate new monomer sequences.

[Mathematical Expression Omitted]

This monomer "M" inserted between the fluorinated chain and the VF2 unit strongly reduces the acidic character of hydrogen atoms. The concentration of these is substantial so the entire polymer becomes less sensitive to nucleophiles. To reach this objective, it is necessary that the reactivity of the monomer "M" with fluorinated macroradicals is greater than that of the VF2 monomer.

The fluorine chemical literature indicates that polarity plays an important role on the rate of radical addition to olefins (ref. 11 ). Fluorinated radicals behave as electrophiles and their addition is retarded by electron withdrawing and accelerated by clectron donating substituents on the olefin. These radicals react more easily with an electron rich double bond such as hydrogenated olefins, than with electron poor ones.

From this consideration it appears that a hydrogenated olefin is a suitable monomer M to form the desired sequenccs since it will react with fluorinated macroradicals in preference to fluorinated electron poor monomers and in a particular VF2. However, standard techniques fail to convert a monomer mixture containing VF2, HFP, TFE and hydrogenated olefin (ethylene, E) into a commercially viable, high molecular weight, amorphous elastomer. To the best of our knowledge, this kind of fluoroelastomer has never been reported before.

To overcome difficulties associated with productivity on a commercial scale, a special micro-emulsion polymerization process has been designed and optimized. The starting microemulsion contains perfluoropolyether derivatives having customized molecular weight and functionality. Fast kinetics have been obtained also with very unfavorable monomer compositions giving high molecular weight polymers. We have verified that these results cannot be obtained using conventional emulsion polymerization techniques.

Figure 3 shows the 19F NMR spectra of a VF2/HFP/-TFE polymer before and after the introduction of E as the protecting monomer. In the spectrum of the VF2/TFE/-HFP/E tetrapolymer, close to the signal from the usual diad:

[Mathematical Expression Omitted]

a new peak appears due to the new diad:

[Mathematical Expression Omitted]

The ratio between the intensities of these two peaks is greater than the ratio between the molar content of E and VF2. This means that E is preferably linked to HFP and consequently, considerably reduces the base sensitive monomer triads (1) and (2).

These results have been confirmed by 13C NMR analysis as reported in figure 4, where spectra present two different signals for the CF of the two sequences (4) and (5). Furthermore, a selective proton irradiation experiment confirmed the identity of the new HFP-E signal.

According to these concepts a new base resistance (BR) peroxide curable fluorocarbon elastomer, Tecnoflon BR X715N, has been developed on a commercial scale. This product also contains a fluorinated processing aid to improve mold release, as in the case of P 819N7 (ref. 7). Data on this product are reported in tables 1-5 compared to those with standard peroxide curable fluoroelastomer Tecnoflon P819N. The latter is representative of the class of product having the minimum resistance to basic oils obtained from traditional monomers.

Table 1 - physical properties of raw polymer
Property                       P819N   XBR715N
Mooney viscosity
  ML (1+4) @ 1OO [degrees] C     76       76
  ML (1+4) @ 121 [degrees] C     38       38
D.S.C.
  Tg onset ([degrees] C)       -12.5    -11.3
  Tg midpoint ([degrees] C)    -8.3     -8.0
Table 2 - compound recipe
                   P819N   XBR715N
P819N 15007RS       100
XBR715N 17263/65               100
Luperco 101XL        3           3
Dry-mix TAIC         4           4
ZnO                  5           5
Carbon black MT     30          30
Luperco 101 XL: 2,5-dimethyl-2,5-di-(tert-butyl peroxy)
hexane
Dry-Mix TAIC: triallysocyanurate

[TABULAR DATA OMITTED]

Table 4 - typical physical properties
Press cure @ 170 [degrees] C x 10 min.       P819N   XBR715N
  100% modulus (MPa)                           4.3       4.8
  200% modulus (MPa)                          11.8      12.8
  Tensile strength (MPa)                      15.3        15
  Elongation at break (%)                      262       245
Shore A (points)                                70        68
Press cure @ 230 [degrees] C x 8 + 16 hrs.
  100% modulus (MPa)                           5.5        6.1
  200% modulus (MPa)                          17.7       18.5
  Tensile strength (MPa)                      19.5       20.1
  Elongation at break (%)                      216        214
  Shore A (points)                              72         70
Press cure @ 170 [degrees] C
  Tear strength (N/mm)                         4.2          3
  Tear energy (kJ/m)                          15.7       13.4
TR test
  TR 10% ([degrees] C)                          -7          6
Compression set 70 hrs @ 200 [degrees] C
  Disk (%)                                      30         36
Table 5 - fluid aging
ASTM 3 + 1% benzylamine                  P819N   XBR715N
  @ 160 [degrees] C x 3 days
Delta 100% modulus (%)                      -3        -5
Delta 200% modulus (5)                       /        -6
Delta tensile strength (%)                 -67       -11
Delta elongation at break (%)              -47        -4
Delta Shore A (points)                       3         2
Delta volume (%)                           4.9       3.9
Tensile stress relax. - applied strain
100%, relaxation time 24 hrs.
  Break                                     No        No
  Fissures                                 Yes        No

Table 1 shows properties of raw polymers. Rheometric and curing properties related to the compounds defined in table 2 are reported in table 3. Typical physical properties after press-cure and oven post-cure are reported in table 4. These data demonstrate that the new base-resistant product shows general properties comparable to the standard peroxide curable fluoroelastomer.

Improved resistance to basic oils is judged from the fluid resistance data reported in table 5. This test comprises thermally aging the elastomers in a 1% solution of benzylamine in ASTM #3 oil. Figure 5 also shows the appearance of the samples after aging. It is notable that the new product presents neither fissures nor breaks which supports inertness to the medium test fluid. A completely different behavior is observed from the comparative standard peroxide curable fluoroelastomer which appears to be highly damaged. Data on the properties of Tecnoflon BR X715N compounds as a function of the curative level are available from the authors; compound recipes include ZnO (5 phr) and MT N990 carbon black (30 phr).

Conclusion

A novel approach has been devised to develop a new class of nucleophile-resistant VF2 fluorocarbon elastomers. A product has been developed for the fabrication of shaft seals resistant to new motor oils containing basic additives. This material, Tecnoflon BR X715N, is now commercially available. It couples the typical properties of standard VF2 fluorocarbon elastomers such as excellent processing and chemical/thermal stability with a dramatically improved resistance to nucleophiles.

References

[1.] V. Arcella, G. Chiodini, N. De Fanti and M. Pianca, "Cross-linking chemistry of VF2 fluorocarbon elastomers by bis-nucleophiles, " presented at the ACS Rubber Division 151st technical meeting, Detroit (1991). [2.] V. Arcella, G. Chiodini, N. De Fanti and M. Pianca; to be published [3.] W.W. Schmiegel; 1 Angew. Makromol. Chem 76/77, 39 (1979). [4.] W.W. Schmiegel; Kautsch. Gummi Kunstst 31, 137, (1978). [5.] P. Venkateswarlu, R.A. Guenthner, R.E. Kolb and LA. Kestner; Paper presented at the ACS Rubber Division 136th technical meeting, Detroit (1989). [6.] D. Apotheker and P.J. Krustic; U.S. Patent 4,035,586 (1977). [7.] V. Arcella, R. Ferro, M. Albano, and A. Minutillo; Kautsch. Gummi Kunstst. 44, 833 (1991). [8.] D. Apotheker, P.J. Krusic; U.S. Patent 4,035,565 (1977). [9.] A.L. Moore; U.S. Patent 4,694,045 (1987). [10.] W.M.A. Grootaert and R.E. Kolb; EPA 0,335,705 (1989). [11.] J.M. Tedder; Angew. Chem. Int. Ed. Engl., 21, 401 (1982).