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Thermal degradation of N-nitrozodiphenylamine under curing conditions
Thermal degradation of N-nitrozodiphenylamine under curing conditions
1803
19. V. A. ZAKHAROV, Yu. I. YERMAKOV and E. G. KUSHNAREVA, Kinetika i kataliz 8: 1392, 1967 20. Yu. I. YERMAKOV, E. G. KUSHNAREVA and V. A. ZAI(HAROV, Materials at I V I n t e r n a t . Congress on catalysis, Moscow, 1968 21. V. A. ZAKHAROV, Yu. I. YERMAKOV and I. G. KUSHNAREVA, Kinetika i kataliz 8: 1390, 1967
THERMAL DEGRADATION OF N-NITROZODIPHENYLAMINE UNDER CURING CONDITIONS* B. A. DOOADKI~, A. V. DOBROMYSLOVA, S. P . LYAK~:~A a n d V. IV[. KAZAKOVA M. V. Lomonosov I n s t i t u t e of Fine Chemical Technology, Moscow
(Received 10 July 1968) N-NITROZODIPHENYLAMINE (N-NDPA) is one of the most efficient of the various agents introduced into rubber mixes to prevent scorching. We know that N-NDPA decomposes to radicals when heated [1]: Cell5
CsH5
\ /
C6H~
\
N - - N ~- O ~:~
/
N'-~ "NO
C6H5
These radicals m a y interact with those emerging in the curing of a rubber mix (i.e. radicals of sulphur, polymer, accelerator) with the result that the curing process is inhibited. Nitric oxide radicals ('NO) are capable of reacting with radicals formed in the curing of rubber mixes, and the inhibition of scorching was in fact attributed to the activity of these "NO radicals. I t was found that the rate of sulphur addition to polymer is greatly reduced when the curing was conducted in an atmosphere of pure nitric oxide, though this process cannot be completely suppressed [2]. While N-NDPA and products of N-:NDPA decomposition do not in themselves cause polymer degradation they m a y nevertheless react with polymer radicals and promote the degradation of polymer chains [3]. The diphenylnitric radical appearing in the thermal degradation of N-NDPA is bound to accelerate the vulcanization process, as tetraphenylhydrazine (TPH) which decomposes to yield two diphenylnitrie radicals greatly accelerates vulcanization; moreover, an S-shaped curve was obtained in [4] expressing the kinetics of sulphur addition.
I t will be seen from the above considerations t h a t the activity of :N-NDPA as a vulcanization retarder should be reduced with heating, as this eliminates * Vysokomol. soyed. A l l : No. 7, 1590-1594, 1969.
1804
B. A. DOGADKIN et a~.
NO. To verify this assumption dry N-NDPA was heated in a current of argon for different periods of time at 120 °, and the resulting degradation products were investigated as scorching inhibitors. The experiments were performed with rubber mixes based on natural rubber (smoked sheets) as follows (pts. by wt.): NR--100; Neozone D - - l ; Santocure--0.5; sulphur--2.5; zinc oxide --5; stearic acid--& 1 pt. (by weight) of N-NDPA and its thermal decomposition products was introduced. I n the course of heating N-NDPA not only retains its anti-scorching properties, but its ability to prevent scorching is enhanced (Fig. 1), At the same time the rate of sulphur addition is reduced, and the curve remains S-shaped. 7O
3
5O
oa ¢.
3o C;3
I
10
I
I
30 50 Time, rain
FIG. 1
I
70
~:
I
20 #0 Nme , min
FIG. 2
FIG. 1. Effect of degradation products of N-NDPA on the scorching resistance of rubber mixes: /--reference mix, 2--ditto+initial N-NDPA, 3-6--ditto+produc~ of heating N-NDPA for 2, 4, 8 and 12 hr respectively. FIG. 2. Effect of TPH and its degradation products on the scorching tendency of rubber mixes: /--reference mix, 2--ditto+initial TPH, 3--~tto~-product o f heating TPH for 2 hr.
N - N D P A was heated for 48 hr at 120 ° in an argon atmosphere in order to obtain more complete elimination of nitric oxide, but the decomposition of N-NDPA remained incomplete: the colour indicated continuous liberation of NO. A brown resin-like product was obtained with a nitrogen content close to t h a t of diphenylamine (DPA); this product also possessed anti-scorching properties, though in this respect it was inferior to the initial N-NDPA. The same was found on heating N-NDPA in a solvent and then introducing the resulting products into the rubber mix. I t was interesting to compare the thermal degradation products of N-NDPA and TPH, as a diphenylnitric radical is formed in both cases. The products of thermal degradation of T P H were obtained under conditions similar to tt~ose for N-NDPA; it was found t h a t these also are vulcanization retarders, unlike the initial T P H (Fig. 2).
Thermal degradation of N-nitrozodiphenylamine under ouring oonditions
1805
It is assumed therefore that either the diphenylnitric radicals emerging in the first moment of degradation, or products of the further interaction of these radicals, have an anti-scorching effect. The degradation products of T P H in boiling toluene were investigated by :~Iusso in reference [5]. It was found that the degradation of T P H resulted in 33-380//0 of diphenylamine and ~63°/o of products consisting of a mixture of polymer homologues of the diphenylnitric radical with a coefficient of polymerization up to n = 5 .
~N
HJ N
N
I
n-2
I ) P A accelerates the curing process; the retardation of scorching is therefore brought about by products of polymerization of the diphenyinitric radical. In the degradation of I~-NDPA it is possible that the dipheny]nitric radical undergeos conversions similar to those in the degradation of TPtt. A qualitative study of the products of degradation of N-NDPA was carried o u t by means of adsorption thin-layer chromatography with a loose layer of sorbent, the latter being aluminium I I I oxide (Brockman activity). In selecting the appropriate system of solvents we succeeded in separating products of the thermal degradation of N-NDPA from the unaltered initial N-NDPA. On increasing the heating time for N-NDPA from 30 min to 12 hr the intensity of the spot corresponding to the initial N-hVDPA is reduced, while that of the spot corresponding to the product of thermal degradation of N-NDPA is increased. Products of the thermal degradation of TPI-I were also separated chromatographically, and a comparative chromatogram was obtained for: the initial N-NDPA, N-NDPA heated for different periods of time; the initial DPA; the initial T P H and TPH heated for different periods of time at 120 ° (Fig. 3). Products of the degradation of N-NDPA and TPH have identical values of R/=0.65, and the shape and colour of the spots are similar for the solvent system hexane+ benzene (1 : 2). The value obtained for DPA under these conditions was RI=0.68. We may therefore say that the products of thermal degradation of T P H and N-NDPA are identical, and not unlike those of DPA. To study the qualitative changes occurring with N-NDPA when the latter is heated at 120° in a current of argon we recorded the IR spectra of the initial N-NDPA and products of heating the latter for 30 min, 2, 4, 8 and 12 hr.
1806
B. A. DOGADKIN ¢$ al.
A new band at 880 cm -1 was detected in the spectra of degradation products of N-NDPA, and this band is related in [6] to the out-of-plane deformation vibrations of the N - - H bond; on increasing the heating time for N-NDPA, the intensity of this band is increased. The intensity of the band at 1600 cm -1 increases, and a plateau appears up to 1640 cm-Z; these changes m a y be attributed to the appearance of para-substituted benzene rings. There is a rise in the intensity of bands at 1320 and 1290 cm -x corresponding to the symmetrical and asymmetrical vibrations of the C - - N bond [7]; this is probably due to the increased number of C - - N bonds owing to the appearance of nitrogen linked with three benzene rings. Moreover a new band was detected at 1390 cm -x, and this m a y be attributed / to the appearance of CeHs--N groups; the intensity of bands in the region \ of 400-500 cm -1 related to monosubstituted benzene rings is reduced [7]. The band at 1450 cm -1 related to the > N - - N O group is always present, though its intensity is reduced with the heating of N - N D P A [8]. In view of these changes in the I R spectra it appears probable that the diphenylnitric radicals appearing in the degradation of N - N D P A react with one another to yield structures similar to those developing in the thermal degradation of T P H [5].
11.5oe~ted
I
I
0 0 ~T-Fr~rrt
ooo
OO
f
f 2 3 /4 5 FIG. 3
FIo. 4
FIG. 3. Chromatogram of separation of thermal degradation products of N-NDPA and TPH on a layer of aluminJum oxide. Solvent system: mixture of hexane and benzene (I : 2): /--initial N-NDPA, 2--product of heating N-NDPA for 4 hr at 120°, 3--initial DPA, 4--initial TPI-I, 5--product of heating TPH for 4 hr at 120°. FIG. 4. E P R spectrum of products of heating N - N D P A at 120 °.
The degradation products of N - N D P A were studied b y the E P R method and it was seen that these produce characteristic singlet signals pertaining to polyerystalline free radicals. The degradation products retain the properties
Thermal degradation of N-nitrozodiphenylamine under curing conditions
1807
of stable radicals for a v e r y long time: t h e samples were checked after t h e y h a d been k e p t for one y e a r in air u n d e r o r d i n a r y l a b o r a t o r y conditions. The signal i n t e n s i t y increases as the heating time for the samples is e x t e n d e d up to 8 hr, indicating a n increased n u m b e r of free radicals. This is in good agreem e n t with the d a t a regarding t h e change in ~ o o n e y viscosity. To elucidate the s t r u c t u r e of the radical a s t u d y was m a d e of x y l e n e solutions of the p r o d u c t s o b t a i n e d on heating N - N D P A in an inert gas; the resulting E P R s p e c t r u m with five lines in the hyperfine s t r u c t u r e is depicted in Fig. 4. Analysis of this s p e c t r u m leads us to the conclusion t h a t the stable radical in question is not a diphenylnitric radical [9], as was previously supposed; it has two nitrogen a t o m s which m a y possibly be linked to a benzene ring in t h e para-position relative to one another. Similar E P R spectra were also o b t a i n e d for p r o d u c t s of d e g r a d a t i o n of T P H ; the l a t t e r are also stable radicals a n d remain stable for a long t i m e in air. I n the light of these e x p e r i m e n t s we assume t h a t the d e g r a d a t i o n of N - N D P A gives rise not only to NO, b u t also to polymeric radicals which are p r o d u c e d b y subsequent conversions o f diphenylnitrogen, a n d also h a v e the effect o f r e t a r d i n g vulcanization. Translated by R. J. A. HE,DRY REFERENCES
1. W. WATERS, Chemistry of Free Radicals, Foreign Lit. Pub. House, 1948 2. B. A. DOGADKIN, A. V. DOBROMYSLOVA and 0. N. BELYATSKAYA, Polymers, Moscow State Univ., 1965 3. O.N. BELYATSKAYA, B. A. DOGADKIN, A. V. DOBROMYSLOVA and L. A. TOMILINA, Vysokomol. soyed. 5: 164, 1963 4. B. A. DOGADKIN, A. V. DOBR0~SLOVA and 0. N. BELYATSKAYA, Vysokomol. soyed. 3: 1572, 1961 5. H. MUSSO, Chem. Ber. 92: 2881, 1959 6. M. A. SALIMOV and V. M. TATEVSKII, Dokl. AN SSSR, 112: 890, 1957 7. L. BELLAMY, Infrared Spectra of Complex Molecules, Foreign Lit. Pub. House, 1963 8. K. NAKANISI, Infrared Spectra and Structure of Organic Compounds, Izd. "Mir". 1965 9. M. K. DAS, A. V. PATANKAR and B. VENKATARAMAN, Arch. Sci. 13: 259, 1960
1803
19. V. A. ZAKHAROV, Yu. I. YERMAKOV and E. G. KUSHNAREVA, Kinetika i kataliz 8: 1392, 1967 20. Yu. I. YERMAKOV, E. G. KUSHNAREVA and V. A. ZAI(HAROV, Materials at I V I n t e r n a t . Congress on catalysis, Moscow, 1968 21. V. A. ZAKHAROV, Yu. I. YERMAKOV and I. G. KUSHNAREVA, Kinetika i kataliz 8: 1390, 1967
THERMAL DEGRADATION OF N-NITROZODIPHENYLAMINE UNDER CURING CONDITIONS* B. A. DOOADKI~, A. V. DOBROMYSLOVA, S. P . LYAK~:~A a n d V. IV[. KAZAKOVA M. V. Lomonosov I n s t i t u t e of Fine Chemical Technology, Moscow
(Received 10 July 1968) N-NITROZODIPHENYLAMINE (N-NDPA) is one of the most efficient of the various agents introduced into rubber mixes to prevent scorching. We know that N-NDPA decomposes to radicals when heated [1]: Cell5
CsH5
\ /
C6H~
\
N - - N ~- O ~:~
/
N'-~ "NO
C6H5
These radicals m a y interact with those emerging in the curing of a rubber mix (i.e. radicals of sulphur, polymer, accelerator) with the result that the curing process is inhibited. Nitric oxide radicals ('NO) are capable of reacting with radicals formed in the curing of rubber mixes, and the inhibition of scorching was in fact attributed to the activity of these "NO radicals. I t was found that the rate of sulphur addition to polymer is greatly reduced when the curing was conducted in an atmosphere of pure nitric oxide, though this process cannot be completely suppressed [2]. While N-NDPA and products of N-:NDPA decomposition do not in themselves cause polymer degradation they m a y nevertheless react with polymer radicals and promote the degradation of polymer chains [3]. The diphenylnitric radical appearing in the thermal degradation of N-NDPA is bound to accelerate the vulcanization process, as tetraphenylhydrazine (TPH) which decomposes to yield two diphenylnitrie radicals greatly accelerates vulcanization; moreover, an S-shaped curve was obtained in [4] expressing the kinetics of sulphur addition.
I t will be seen from the above considerations t h a t the activity of :N-NDPA as a vulcanization retarder should be reduced with heating, as this eliminates * Vysokomol. soyed. A l l : No. 7, 1590-1594, 1969.
1804
B. A. DOGADKIN et a~.
NO. To verify this assumption dry N-NDPA was heated in a current of argon for different periods of time at 120 °, and the resulting degradation products were investigated as scorching inhibitors. The experiments were performed with rubber mixes based on natural rubber (smoked sheets) as follows (pts. by wt.): NR--100; Neozone D - - l ; Santocure--0.5; sulphur--2.5; zinc oxide --5; stearic acid--& 1 pt. (by weight) of N-NDPA and its thermal decomposition products was introduced. I n the course of heating N-NDPA not only retains its anti-scorching properties, but its ability to prevent scorching is enhanced (Fig. 1), At the same time the rate of sulphur addition is reduced, and the curve remains S-shaped. 7O
3
5O
oa ¢.
3o C;3
I
10
I
I
30 50 Time, rain
FIG. 1
I
70
~:
I
20 #0 Nme , min
FIG. 2
FIG. 1. Effect of degradation products of N-NDPA on the scorching resistance of rubber mixes: /--reference mix, 2--ditto+initial N-NDPA, 3-6--ditto+produc~ of heating N-NDPA for 2, 4, 8 and 12 hr respectively. FIG. 2. Effect of TPH and its degradation products on the scorching tendency of rubber mixes: /--reference mix, 2--ditto+initial TPH, 3--~tto~-product o f heating TPH for 2 hr.
N - N D P A was heated for 48 hr at 120 ° in an argon atmosphere in order to obtain more complete elimination of nitric oxide, but the decomposition of N-NDPA remained incomplete: the colour indicated continuous liberation of NO. A brown resin-like product was obtained with a nitrogen content close to t h a t of diphenylamine (DPA); this product also possessed anti-scorching properties, though in this respect it was inferior to the initial N-NDPA. The same was found on heating N-NDPA in a solvent and then introducing the resulting products into the rubber mix. I t was interesting to compare the thermal degradation products of N-NDPA and TPH, as a diphenylnitric radical is formed in both cases. The products of thermal degradation of T P H were obtained under conditions similar to tt~ose for N-NDPA; it was found t h a t these also are vulcanization retarders, unlike the initial T P H (Fig. 2).
Thermal degradation of N-nitrozodiphenylamine under ouring oonditions
1805
It is assumed therefore that either the diphenylnitric radicals emerging in the first moment of degradation, or products of the further interaction of these radicals, have an anti-scorching effect. The degradation products of T P H in boiling toluene were investigated by :~Iusso in reference [5]. It was found that the degradation of T P H resulted in 33-380//0 of diphenylamine and ~63°/o of products consisting of a mixture of polymer homologues of the diphenylnitric radical with a coefficient of polymerization up to n = 5 .
~N
HJ N
N
I
n-2
I ) P A accelerates the curing process; the retardation of scorching is therefore brought about by products of polymerization of the diphenyinitric radical. In the degradation of I~-NDPA it is possible that the dipheny]nitric radical undergeos conversions similar to those in the degradation of TPtt. A qualitative study of the products of degradation of N-NDPA was carried o u t by means of adsorption thin-layer chromatography with a loose layer of sorbent, the latter being aluminium I I I oxide (Brockman activity). In selecting the appropriate system of solvents we succeeded in separating products of the thermal degradation of N-NDPA from the unaltered initial N-NDPA. On increasing the heating time for N-NDPA from 30 min to 12 hr the intensity of the spot corresponding to the initial N-hVDPA is reduced, while that of the spot corresponding to the product of thermal degradation of N-NDPA is increased. Products of the thermal degradation of TPI-I were also separated chromatographically, and a comparative chromatogram was obtained for: the initial N-NDPA, N-NDPA heated for different periods of time; the initial DPA; the initial T P H and TPH heated for different periods of time at 120 ° (Fig. 3). Products of the degradation of N-NDPA and TPH have identical values of R/=0.65, and the shape and colour of the spots are similar for the solvent system hexane+ benzene (1 : 2). The value obtained for DPA under these conditions was RI=0.68. We may therefore say that the products of thermal degradation of T P H and N-NDPA are identical, and not unlike those of DPA. To study the qualitative changes occurring with N-NDPA when the latter is heated at 120° in a current of argon we recorded the IR spectra of the initial N-NDPA and products of heating the latter for 30 min, 2, 4, 8 and 12 hr.
1806
B. A. DOGADKIN ¢$ al.
A new band at 880 cm -1 was detected in the spectra of degradation products of N-NDPA, and this band is related in [6] to the out-of-plane deformation vibrations of the N - - H bond; on increasing the heating time for N-NDPA, the intensity of this band is increased. The intensity of the band at 1600 cm -1 increases, and a plateau appears up to 1640 cm-Z; these changes m a y be attributed to the appearance of para-substituted benzene rings. There is a rise in the intensity of bands at 1320 and 1290 cm -x corresponding to the symmetrical and asymmetrical vibrations of the C - - N bond [7]; this is probably due to the increased number of C - - N bonds owing to the appearance of nitrogen linked with three benzene rings. Moreover a new band was detected at 1390 cm -x, and this m a y be attributed / to the appearance of CeHs--N groups; the intensity of bands in the region \ of 400-500 cm -1 related to monosubstituted benzene rings is reduced [7]. The band at 1450 cm -1 related to the > N - - N O group is always present, though its intensity is reduced with the heating of N - N D P A [8]. In view of these changes in the I R spectra it appears probable that the diphenylnitric radicals appearing in the degradation of N - N D P A react with one another to yield structures similar to those developing in the thermal degradation of T P H [5].
11.5oe~ted
I
I
0 0 ~T-Fr~rrt
ooo
OO
f
f 2 3 /4 5 FIG. 3
FIo. 4
FIG. 3. Chromatogram of separation of thermal degradation products of N-NDPA and TPH on a layer of aluminJum oxide. Solvent system: mixture of hexane and benzene (I : 2): /--initial N-NDPA, 2--product of heating N-NDPA for 4 hr at 120°, 3--initial DPA, 4--initial TPI-I, 5--product of heating TPH for 4 hr at 120°. FIG. 4. E P R spectrum of products of heating N - N D P A at 120 °.
The degradation products of N - N D P A were studied b y the E P R method and it was seen that these produce characteristic singlet signals pertaining to polyerystalline free radicals. The degradation products retain the properties
Thermal degradation of N-nitrozodiphenylamine under curing conditions
1807
of stable radicals for a v e r y long time: t h e samples were checked after t h e y h a d been k e p t for one y e a r in air u n d e r o r d i n a r y l a b o r a t o r y conditions. The signal i n t e n s i t y increases as the heating time for the samples is e x t e n d e d up to 8 hr, indicating a n increased n u m b e r of free radicals. This is in good agreem e n t with the d a t a regarding t h e change in ~ o o n e y viscosity. To elucidate the s t r u c t u r e of the radical a s t u d y was m a d e of x y l e n e solutions of the p r o d u c t s o b t a i n e d on heating N - N D P A in an inert gas; the resulting E P R s p e c t r u m with five lines in the hyperfine s t r u c t u r e is depicted in Fig. 4. Analysis of this s p e c t r u m leads us to the conclusion t h a t the stable radical in question is not a diphenylnitric radical [9], as was previously supposed; it has two nitrogen a t o m s which m a y possibly be linked to a benzene ring in t h e para-position relative to one another. Similar E P R spectra were also o b t a i n e d for p r o d u c t s of d e g r a d a t i o n of T P H ; the l a t t e r are also stable radicals a n d remain stable for a long t i m e in air. I n the light of these e x p e r i m e n t s we assume t h a t the d e g r a d a t i o n of N - N D P A gives rise not only to NO, b u t also to polymeric radicals which are p r o d u c e d b y subsequent conversions o f diphenylnitrogen, a n d also h a v e the effect o f r e t a r d i n g vulcanization. Translated by R. J. A. HE,DRY REFERENCES
1. W. WATERS, Chemistry of Free Radicals, Foreign Lit. Pub. House, 1948 2. B. A. DOGADKIN, A. V. DOBROMYSLOVA and 0. N. BELYATSKAYA, Polymers, Moscow State Univ., 1965 3. O.N. BELYATSKAYA, B. A. DOGADKIN, A. V. DOBROMYSLOVA and L. A. TOMILINA, Vysokomol. soyed. 5: 164, 1963 4. B. A. DOGADKIN, A. V. DOBR0~SLOVA and 0. N. BELYATSKAYA, Vysokomol. soyed. 3: 1572, 1961 5. H. MUSSO, Chem. Ber. 92: 2881, 1959 6. M. A. SALIMOV and V. M. TATEVSKII, Dokl. AN SSSR, 112: 890, 1957 7. L. BELLAMY, Infrared Spectra of Complex Molecules, Foreign Lit. Pub. House, 1963 8. K. NAKANISI, Infrared Spectra and Structure of Organic Compounds, Izd. "Mir". 1965 9. M. K. DAS, A. V. PATANKAR and B. VENKATARAMAN, Arch. Sci. 13: 259, 1960