- Email: [email protected]
Sorption and diffusion of water in ethylene-propylene diene rubber in presence of vulcanization accelerators
1348 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37.
A. YE. CHALYKHet aL
J. B. HENDRICKSON, J. Amer. Chem. Soc. 89: 7036, 1967 D. E. WILLIAMS, J. Chem. Phys. 47: 4680, 1967 A. ABE, R. L. JERNIGAN and P. J. FLORY, J. Amer. Chem. Soc. 88: 631, 1966 D. E. WILLIAMS, Acta Crystall. A28: Part 1, 84, 1972 K. V. MIRSKAYA, I. E. KOZLOVA and V. F. BEREZNITSKAYA, Phys. Stat. Sol. b. 62: 291, 1974; K, MIRSKY, Acta Crystall. A32: 199, 1976 D. E. WILLIAMS, Acta Crystall. A30: Part 1, 71, 1974 N . L . ALLINGER, J. Amer. Chem. Soc. 99: 8127, 1977 E. M. ENGLER, J. D. ANDOSE and P. yon R. SCHLEYER, Ibid. 95: 8005, 1973 A. L. RABINOVICH and V. G. DASHEVSKII, Vysokomol. soyed. A25: 537, 1983 (Translated in Polymer Sci. U.S.S.R. 25: 3, 629, 1983) G. PORED, Monatsh. Chem. 80: 251, 1949 O. KRATKY and G. PORED, Recueil. Trav. Chim. 68: 1106, 1949 L. D. LANDAU and Ye. M. L1FSHITS, Statisticheskaya fizika (Statistical Physics). Part 1, p. 431, lqauka, Moscow, 1976 T. M. BIRSHTEIN, Vysokomol. soyed. AI6: 54, 1974 (Translated in Polymer Sci. U.S.S.R. 16: 1, 60, 1974) Y. MIRAOKA, K. KAMIDE and H. SUZUKI, Brit. Polvmer J. 15: 107, 1983 V. G. DASHEVSKII, Zh. strukt, khim. 11: 912, 1970 P. M. HENRY, J. Polymer Sci. 36: 3, 1959 R. CHIANG, Ibid. 36: 91, 1959 L. H. TUNG, Ibid. 36: 287, 1959 W. R. KRIGBAUM, Ibid. 28: 213, 1958
Polymer ScienceU.S,S.R. Vol. 28, No. 6, pp. 1348-1355, 1986 Printed in Poland
0032-3950/86 $10.00+ .00 © Pergamon Journals Ltd.
SORPTION AND DIFFUSION O F WATER IN ETHYLENE-PROPYLENE DIENE RUBBER IN PRESENCE OF VULCANIZATION ACCELERATORS* A. YE. CHALYKH, A. A. I)ONTSOV, T. F. PETROVA a n d A. A. LAPSHOVA Institute of Physical Chemistry, U.S.S.R. Academy of Sciences Elastic Industry Research Institute
(Received 1 October 1984) The authors have studied the diffusion and sorption patterns of water vapour by the ethylene propylene diene elastomer SKEPT-40 and its composites with vulcanization accelerators in the activity interval of the vapour 0-1
Sorption and diffusion of water in ethylene-propylene diene rubber
1349
THE UNFLAGGING interest [1] in the problem of sorption and diffusion of water in polymer materials, in general, and hydrophobic elastomers, in particular, is due t o a number of factors and primarily practical requirements. It is known that most polymer materials are processed, used and stored in humid su:'roundings. They are also used as insulating coats, sheaths and seals protecting products from interaction with water [2] and act as active sorbents regulating the moisture content in a given volume [3]. The solution of the problems involved in the choice of polymer materials, determination o f the optimal conditions of their storage and use and calculation of the geometric dimensions of the composites [4] requires information, to a certain degree formal, on the coefficients of diffusion, permeability and solubility of water in the polymer materials, and the temperature and concentration functions of these parameters. On the other hand, this interest is due to the uncommon, at first sight, effects observed on interaction of water with hydrophobic polymers. Let us list some of them: firstly, the negative concentration function of the diffusion coefficient; secondly, the absence of sorption equilibrium in the elastomer-water system in the region of high ,~apour activity (p/ps>~0.9); thirdly, change in the phase state of the matrix: the appearance of turbidity and pore formation and, finally, the redistribution of the ingredients of the polymer composites over the volume of the material under the influence of water [5, 6]. This then calls for detailed study of the kinetics of sorption, the state of water in the polymer material and its influence on the solubility of the components. Unfortunately, such information is absent for m a n y polymer materials of practical importance so ruling out validated conclusions on the mechanism of the processes occurring in the matrices under the unfluence of the sorbed moisture. Since many of the above described effects are observed in systems with an ethylene-propylene elastomer the aim of the present work was to obtain systematic information on the solubility of water in a wide interval of vapour activity, determine the diffusion coefficients of water and study the influence on these parameters of vulcanization accelerators (V.a.) introduced into the elastomer. In the work we also try to evaluate the state of water in the polymer matrix. We used commercial ethylene-propylene diene rubber SKEPT-40 grade (SKEPT) with Mn = 57,500, Mw/M,, = 3' 7, freed of low molecular weight impurities by extraction with acetone for 24 hr in a Soxhlet apparatus. Technical v.a.s di(2-benzthiazyl) disulphide (DBTD), (benzthiazyl-2) morpholine disulphide (BMD) and N,N'-dithio dimorpholine (DTDM) were purified by recrystallization from a saturated solution in chloroform or alcohol. The powder-like crystalline v.a.s were mixed with polymer on rolls for 15 rain at 323 K. For the sorption measurements we used pressed films of pure SKEPT and its mixtures with a v.a. at a concentration of v.a. either below the equilibrium solubility (0' 5 wt. ~) or well exceeding it (2 wt. ~). The polymer films 0"3-0- 5 mm thick were pressed for 10 min at 426 K. The phase state of v. a. in the elastomer was determined by electron microscopy [7]. The water vapour was sorbed by the integral technique [8] with a vacuum sorption apparatus. The heat of sorption AH was calculated from the temperature dependence of the chemical potential [9]. Figure la presents the typical kinetic sorption curves of water vapour by the rubber studied. For S K E P T as with other elastomer sorbents described in reference [10] two regions of the activity of water vapour are observed characterized by different
1350
A. YE. CHALYKH
etal.
diffusion and sorption patterns associated with the kinetics of the establishment of sorption equilibrium and the phase state of water in the polymer matrices. For the first region (0.1
c~grov %
O~gr~v. 0"21
0
&o~-
O.3 ~
/i G
0.8
G
I
3O
AI
6O
h
I
9O 3
0.1
0.2
2
_
~
ii
I
/ I/i' /
I/II / I l/J~ //
0.I 0
30
60
90 V(,sec~s
FIo. 1
pip
oq
FIo. 2
FIG. I. Kinetic sorption curves of water vapour for SKEPT (a) and S K E P T + ~ 4 ~ (1), 318 (2) and 293 (3) K; and 0-85 (3); T=293 (1), 318 (2) and 308 (3) K.
a:p/p==0.69(1),0-73 (2) and 0-97 (3): T=308
BMD (b).
b:p/p==0.65(1),0.7 (2)
Fie. 2. Sorption isotherms of water vapour by SKEPT (1-4) and SKEPT+2~o BMD (5-7) at 293 (1, 2, 5), 308 O, 6) and 318 (4, 7) K. Curve 1 and the points in curve 5 are calculated from the data in reference [121.
Sorption and diffusionof water in ethylene-propylenediene rubber
1351
may be assumed that such a pattern for the systems containing a v.a. is linked with their heterogeneous structure [8]. For monophasic systems containing a v.a. in an amount below the solubility limit these effects are not observed. Figure 2 presents the isotherm of sorption of water vapour by SKEPT and its composites with a v.a. obtained experimentally at different temperatures. According to the Rodgers' classification they may be assigned to type III [1]. It was of some interest to compare.the experimentally found isotherms with the theoretically calculated by the group contribution method [12]. This method assumes the equal accessibility of all the functional groups of the macromolecules sorbing water in line with their hydrate numbers. The results of calculation are given in Fig. 2. The discrepancy between the calculated and experimental isotherms for SKEPT is evidently explained by the presence in the rubber of neglected impurities of the catalytic system, which could not be removed by extraction with organic solvents. In fact, spectral investigations showed that rubber purified by extraction with acetone contains compounds of the elements V, AI, Ca, CI and others leading to increase in the sorption capacity for water on av,:rage by one order. The sorption isotherms obtained for SKEPT with introduction o f 2 ~ BMD have a form similar to those of pure rubber but lie higher. The sorption capacity in this case was increased in proportion to the contribution of the groups of the accelerator molecules. Figure 3 presents the data obtained on the thermal effects of sorption of water for the elastomer composites studied. For all systems a single character of the concentration function • H - Cn2o is observed: as the content of the sorbate increases the magnitude AH falls. It should be noted that the mixing enthalpy on passing from pure rubber to its composite with a v.a. rises, which is linked with the energy interaction of the water molecules and active groups of accelerators. Foreign and Soviet researchers [13, 14] have shown the possibility of using the sorption isotherms and diffusion coefficients for structuro-morphological studies. It should be noted that such approaches must be based on specific theoretical models of the structural organization of the solution. Unfortunately, in some studies [15] in interpreting the experimental findings this point was overlooked and, therefore, the conclusions, as a rule, are essentially of a qualitative character. Since the hydrophobic elastomer studied by us contained impurities or chemical additives the molecules of which contain polar groupings treatment of the sorption isotherms was based on the assumption of the possibility of local sorption of the water molecules at the polar centres, clustering of the water molecules and their statistical distribution over the volume of the material. To identify the p/p, intervals within which the particular states of sorbed water may be realised, we used the traditional BET approaches [13] widely used in physicochemical research and those of Zimm and Landberg [16]. For SKEPT the BET equation allows one to describe only the initial portion of the isotherm to PIPs=0.3 (Fig. 4). In presence of a v.a. the region of concentrations in which the BET equation satisfactorily describes the sorption isotherm shifts to large p/p, =0.4. This allows us to state that in the above indicated activity regions of water vapour in the system studied the water molecules are preferentially sorbed at the polar centres of the impurities of the catalytic system
1352
A. YE. C~IALYKHe t aL
and v.a. The contribution of this process to the total sorption capacity is comparatively slight and amounts to 2.7 ~o (relative). As shown in reference [1, p. 4/0] for most polymer systems the dependence of the reciprocal of sorption @- 1 on the reciprocal value of the relative moisture content P - x is close to linear and is described by the semi-empirical equation
@-l=kl P - l - k 2
(1)
At k2 =0 equation (1) passes to the Henry equation, at k 2 < 0 it corresponds to the Langmuir equation and at k 2 > 0 it characterizes the process of formation of clusters. Figure 5 presents the isotherms of sorption of water by SKEPT and its composites with a v.a. in the coordinates of equation (1). Calculation of the constants kz showed that for SKEPT kz>>0 (16.7 x 10a), i.e. a tendency towards clustering is observed. In rubber containing a v.a. (2~o BMD) in the region 0.5
P/Ps c (1-p/p~) 28
AH, kE/mole
20
8
12 I
I
0.05
0.15
o,gpav.%
FIG. 3
I
I
0.g
0-#
P/Ps
FIG. 4
FIG. 3. Heat of sorption AH as a function of the content of water in SKEPT (1) and SKEPT+2% BMD (2). F1o. 4. Sorption isotherms of water vapour by SKEPT (1) and SKEPT+2% BMD (2) at 293 K in the coordinates of the BET equation.
It is of interest to evaluate the possibility of the process of clustering of the water molecules dissolved in the polymer matrix. For this purpose from the equation
Gl11~"1= - ( 1 - ~ol)(dull ~P"~ \
-- 1
(2)
63al ./P,T
(~'1 is the partial molar volume of water; q~l is the volumetric fraction of the sorbate; al is the activity of the water vapours).we calculated the clustering function, the concentration dependence of which is presented in Fig. 6. The form of the function varies for SKEPT and the composite with a v.a. which is probably connected with the influence of the polar groups of the v.a. on the distribution of the water molecules between the
Sorption and diffusion of water in ethylene-propylene diene rubber
1353
clusters and the phase of free water. With increase in the number of active centres introduced into the polymer the start of the process of clustering ( G l l / 1 " 1 > - 1 ) shifts to large p/p~ as compared with the pure polymer but at p/p~ = 0.5 (~0> 0.25 x 10- 3) fall in clustering is observed. Possibly there is enlargement (fusion) of the clusters which leads to fall in their number."
7
-5
20
3
0 10 -8 -16 0
2 FIG. 5
P/Ps
I
I
I
q
0.5
l.o
cp.lOa
FIG. 6
FIG. 5. Sorption isotherms of water vapour by SKEPT (1) and S K E P T + 2 ~ BMD (2) in the coordinates of equation (l). FIG. 6. Concentration dependence of the function of clustering G11/V1 on the volumetric fraction of sorbed water to for SKEPT (1) and S K E P T + 2 ~ BMD (2), GI1/Pl= - 1 (3). F r o m the Fick portions of the kinetic sorption curves we calculated for all the PIPs intervals the concentration functions of the relative diffusion coefficients Dx of water in SKEPT. The results of the calculations are given in Fig. 7. It may be seen that in the SKEPT-water system characterizing the partial diffusion mobility of the water molecules in the matrix of the elastomer the diffusion coefficient D1 is constant over the whole concentration interval. For the composites of the polymer with a v.a. (2 ~ BMD) in the region of concentrations of the sorbed water corresponding to pips> 0.5, a tendency for the diffusion coefficient to fall is observed; the total drop in D1 is, on average, 5 0 ~ . It is worth noting that in absolute terms Dt for water for lhe system with a v.a. is somewhat lower than for SKEPT which is probably connected with the influence of the phasic v.a. particles on the transfer processes. This effect is similar to the influence on the diffusion coefficient of the filler particles in the region of low concentrations. The character of the concentration dependence noted is connected, as shown above, with the thermodynamic non-ideality of the system. The apparent activation energy o f diffusion in S K E P T E = 2 2 . 6 kJ/mole is close to the activation energy of diffusion o f
1354
A. yt~. CHAt,VK~eta|.
gases in polyolefines [17]; for the rubber composite containing a v.a. its value is somewhat higher and amounts to 27.6 kJ/mole. The investigations of the diffusion and sorption of water by SKEPT in the activity interval 0.1
70 .
-[°gl/91~
n
7-4-
o__ o
1
N~ I
0.0
I
OB P/Ps
FIG. 7. Concentration dependence of the diffusion coefficient Dt for SKEPT (1) and SKEPT+2y. BMD (2). It should be noted that a special influence is exerted on the sorption capacity of the system by the impurities present in the commercial elastomer and not removed by extraction. The introduction into the polymer matrix of a v.a. leads to fall in D1, the appearance of concentration dependence of DI, increase in the total amount of sorbed water and thermal effects of sorption and the appearance of phase states in the kinetic sorption curves.
Translated by A. CRozY REFERENCES
1. Voda v polimerakh (Water in Polymers) (Edited by S. Rowland). 555 pp, Mir, Moscow, 1984 2. Z. A. KOGAN and G. D. RYBAKOV, Konservatsiya i upakovka (Preservation and Packaging). 264 pp, Mashinostroyeniye, Moscow, 1973 3. T. L CHALYKH and P. G. BABAYEVSKH, Sorbtsiya, gigroskopiehnost', pronitsayemost' (Sorption, Hygroscopicity and Permeability). p. 53, Ts/qHTLegprom, Moscow, 1981
Sorption and diffusion of water in ethylene-propylene diene rubber
1355
4. M . Z . AZARKH, A. N. KOVALEVA, A. S. KOSENKOVA and A. V. CHULYUKINA, Kauchuk i rezina, 5, 38, 1984 5. T. F. PETROVA, A. A. LAPSHOVA and A. A. DONTSOV, [bid, 8, 22, 1984 6. A. A. DONTSOV, G. I. TARASOVA and A. A. LAPSHOVA, Vysokomol. soyed. A24: 1895, 1982 (Translated in Polymer Sci. U.S.S.R. 24: 9, 2166, 1982) 7. A. Ye. CHALYKH, T. F. PETROVA, A. Ye. RUBTSOV and A. A. LAPSHOVA, Ibid. A28: 734, 1986 (Translated in Polmyer Sci. U.S.S.R. 28: 4, 817, 1986) 8. A. Ya. MALKIN and A. Ye. CHALYKH, Diffuziya i vyazkost' polimerov (Diffusion and Viscosity of Polymers). p. 259, Khimiya, Moscow, 1979 9. A. A. TAGER, Fizikokhimiya polimerov ( Polymer Physicochemistry). p. 319, Khimiya, Moscow, 1978 10. T. P. KOMAROVA, M. A. MARKELOV, S. A. NENAKHOV, E. I. SEMENENKO and A. Ye. CHALYKH, Vysokomol. soyed. AIS: 264, 1976 (Translated in Polymer Sci. U.S.S.R. 18: 2, 300, 1976) 11. K. RODGERS, Problemy fiziki i khimii tverdogo sostoyaniya organicheskikh soyedinenii (Problems of the Physics and Chemistry of the Solid State of Organic Compounds). p. 229, Mir, Moscow, 1968 12. D.V.van KREVELEN, Svoistva i khimicheskoye stroyeniye polimerov (Properties and Chemical Structure of Polymers). p. 304, Khimiya, Moscow, 1976 13. S. P. PAPKOV and E. Z. FAINBERG, Vzaimodeistviye tsellyulozy i tsellyuloznykh materialov s vodoi (Interaction of Cellulose and Cellulose Materials with Water). 230 pp, Khimiya, Moscow, 1970 14. T. P. GATOVSKAYA, V. A. KARGIN and A. A. TAGER, Zh. fiz. khim. 29: 883, 1955 15. M. SEFTON and K. CHANG, lqoveishiye instrumental'nye metody issledovaniya struktury polimerov (Latest Instrumental Methods for Studying the Structure of Polymers). p. 246, Mir, Moscow, 1982 16. B. H. ZIMM and J. L. LUNDBERG, J. Phys. Chem. 60: 425, 1956 17. S. A. REITLINGER, Pronitsayemost' polimernykh materialov (Permeability of Polymer Materials). 286 pp, Khimiya, Moscow, 1974
A. YE. CHALYKHet aL
J. B. HENDRICKSON, J. Amer. Chem. Soc. 89: 7036, 1967 D. E. WILLIAMS, J. Chem. Phys. 47: 4680, 1967 A. ABE, R. L. JERNIGAN and P. J. FLORY, J. Amer. Chem. Soc. 88: 631, 1966 D. E. WILLIAMS, Acta Crystall. A28: Part 1, 84, 1972 K. V. MIRSKAYA, I. E. KOZLOVA and V. F. BEREZNITSKAYA, Phys. Stat. Sol. b. 62: 291, 1974; K, MIRSKY, Acta Crystall. A32: 199, 1976 D. E. WILLIAMS, Acta Crystall. A30: Part 1, 71, 1974 N . L . ALLINGER, J. Amer. Chem. Soc. 99: 8127, 1977 E. M. ENGLER, J. D. ANDOSE and P. yon R. SCHLEYER, Ibid. 95: 8005, 1973 A. L. RABINOVICH and V. G. DASHEVSKII, Vysokomol. soyed. A25: 537, 1983 (Translated in Polymer Sci. U.S.S.R. 25: 3, 629, 1983) G. PORED, Monatsh. Chem. 80: 251, 1949 O. KRATKY and G. PORED, Recueil. Trav. Chim. 68: 1106, 1949 L. D. LANDAU and Ye. M. L1FSHITS, Statisticheskaya fizika (Statistical Physics). Part 1, p. 431, lqauka, Moscow, 1976 T. M. BIRSHTEIN, Vysokomol. soyed. AI6: 54, 1974 (Translated in Polymer Sci. U.S.S.R. 16: 1, 60, 1974) Y. MIRAOKA, K. KAMIDE and H. SUZUKI, Brit. Polvmer J. 15: 107, 1983 V. G. DASHEVSKII, Zh. strukt, khim. 11: 912, 1970 P. M. HENRY, J. Polymer Sci. 36: 3, 1959 R. CHIANG, Ibid. 36: 91, 1959 L. H. TUNG, Ibid. 36: 287, 1959 W. R. KRIGBAUM, Ibid. 28: 213, 1958
Polymer ScienceU.S,S.R. Vol. 28, No. 6, pp. 1348-1355, 1986 Printed in Poland
0032-3950/86 $10.00+ .00 © Pergamon Journals Ltd.
SORPTION AND DIFFUSION O F WATER IN ETHYLENE-PROPYLENE DIENE RUBBER IN PRESENCE OF VULCANIZATION ACCELERATORS* A. YE. CHALYKH, A. A. I)ONTSOV, T. F. PETROVA a n d A. A. LAPSHOVA Institute of Physical Chemistry, U.S.S.R. Academy of Sciences Elastic Industry Research Institute
(Received 1 October 1984) The authors have studied the diffusion and sorption patterns of water vapour by the ethylene propylene diene elastomer SKEPT-40 and its composites with vulcanization accelerators in the activity interval of the vapour 0-1
Sorption and diffusion of water in ethylene-propylene diene rubber
1349
THE UNFLAGGING interest [1] in the problem of sorption and diffusion of water in polymer materials, in general, and hydrophobic elastomers, in particular, is due t o a number of factors and primarily practical requirements. It is known that most polymer materials are processed, used and stored in humid su:'roundings. They are also used as insulating coats, sheaths and seals protecting products from interaction with water [2] and act as active sorbents regulating the moisture content in a given volume [3]. The solution of the problems involved in the choice of polymer materials, determination o f the optimal conditions of their storage and use and calculation of the geometric dimensions of the composites [4] requires information, to a certain degree formal, on the coefficients of diffusion, permeability and solubility of water in the polymer materials, and the temperature and concentration functions of these parameters. On the other hand, this interest is due to the uncommon, at first sight, effects observed on interaction of water with hydrophobic polymers. Let us list some of them: firstly, the negative concentration function of the diffusion coefficient; secondly, the absence of sorption equilibrium in the elastomer-water system in the region of high ,~apour activity (p/ps>~0.9); thirdly, change in the phase state of the matrix: the appearance of turbidity and pore formation and, finally, the redistribution of the ingredients of the polymer composites over the volume of the material under the influence of water [5, 6]. This then calls for detailed study of the kinetics of sorption, the state of water in the polymer material and its influence on the solubility of the components. Unfortunately, such information is absent for m a n y polymer materials of practical importance so ruling out validated conclusions on the mechanism of the processes occurring in the matrices under the unfluence of the sorbed moisture. Since many of the above described effects are observed in systems with an ethylene-propylene elastomer the aim of the present work was to obtain systematic information on the solubility of water in a wide interval of vapour activity, determine the diffusion coefficients of water and study the influence on these parameters of vulcanization accelerators (V.a.) introduced into the elastomer. In the work we also try to evaluate the state of water in the polymer matrix. We used commercial ethylene-propylene diene rubber SKEPT-40 grade (SKEPT) with Mn = 57,500, Mw/M,, = 3' 7, freed of low molecular weight impurities by extraction with acetone for 24 hr in a Soxhlet apparatus. Technical v.a.s di(2-benzthiazyl) disulphide (DBTD), (benzthiazyl-2) morpholine disulphide (BMD) and N,N'-dithio dimorpholine (DTDM) were purified by recrystallization from a saturated solution in chloroform or alcohol. The powder-like crystalline v.a.s were mixed with polymer on rolls for 15 rain at 323 K. For the sorption measurements we used pressed films of pure SKEPT and its mixtures with a v.a. at a concentration of v.a. either below the equilibrium solubility (0' 5 wt. ~) or well exceeding it (2 wt. ~). The polymer films 0"3-0- 5 mm thick were pressed for 10 min at 426 K. The phase state of v. a. in the elastomer was determined by electron microscopy [7]. The water vapour was sorbed by the integral technique [8] with a vacuum sorption apparatus. The heat of sorption AH was calculated from the temperature dependence of the chemical potential [9]. Figure la presents the typical kinetic sorption curves of water vapour by the rubber studied. For S K E P T as with other elastomer sorbents described in reference [10] two regions of the activity of water vapour are observed characterized by different
1350
A. YE. CHALYKH
etal.
diffusion and sorption patterns associated with the kinetics of the establishment of sorption equilibrium and the phase state of water in the polymer matrices. For the first region (0.1
~p/p=>~0"85) the absence of such. In the first region the system is monophasic and in the second heterogeneous. In the present work we studied the diffusion and sorption patterns of water vapour by SKEPT-40 rubber and composites based on it in the region of activity of water vapour 0.1
c~grov %
O~gr~v. 0"21
0
&o~-
O.3 ~
/i G
0.8
G
I
3O
AI
6O
h
I
9O 3
0.1
0.2
2
_
~
ii
I
/ I/i' /
I/II / I l/J~ //
0.I 0
30
60
90 V(,sec~s
FIo. 1
pip
oq
FIo. 2
FIG. I. Kinetic sorption curves of water vapour for SKEPT (a) and S K E P T + ~ 4 ~ (1), 318 (2) and 293 (3) K; and 0-85 (3); T=293 (1), 318 (2) and 308 (3) K.
a:p/p==0.69(1),0-73 (2) and 0-97 (3): T=308
BMD (b).
b:p/p==0.65(1),0.7 (2)
Fie. 2. Sorption isotherms of water vapour by SKEPT (1-4) and SKEPT+2~o BMD (5-7) at 293 (1, 2, 5), 308 O, 6) and 318 (4, 7) K. Curve 1 and the points in curve 5 are calculated from the data in reference [121.
Sorption and diffusionof water in ethylene-propylenediene rubber
1351
may be assumed that such a pattern for the systems containing a v.a. is linked with their heterogeneous structure [8]. For monophasic systems containing a v.a. in an amount below the solubility limit these effects are not observed. Figure 2 presents the isotherm of sorption of water vapour by SKEPT and its composites with a v.a. obtained experimentally at different temperatures. According to the Rodgers' classification they may be assigned to type III [1]. It was of some interest to compare.the experimentally found isotherms with the theoretically calculated by the group contribution method [12]. This method assumes the equal accessibility of all the functional groups of the macromolecules sorbing water in line with their hydrate numbers. The results of calculation are given in Fig. 2. The discrepancy between the calculated and experimental isotherms for SKEPT is evidently explained by the presence in the rubber of neglected impurities of the catalytic system, which could not be removed by extraction with organic solvents. In fact, spectral investigations showed that rubber purified by extraction with acetone contains compounds of the elements V, AI, Ca, CI and others leading to increase in the sorption capacity for water on av,:rage by one order. The sorption isotherms obtained for SKEPT with introduction o f 2 ~ BMD have a form similar to those of pure rubber but lie higher. The sorption capacity in this case was increased in proportion to the contribution of the groups of the accelerator molecules. Figure 3 presents the data obtained on the thermal effects of sorption of water for the elastomer composites studied. For all systems a single character of the concentration function • H - Cn2o is observed: as the content of the sorbate increases the magnitude AH falls. It should be noted that the mixing enthalpy on passing from pure rubber to its composite with a v.a. rises, which is linked with the energy interaction of the water molecules and active groups of accelerators. Foreign and Soviet researchers [13, 14] have shown the possibility of using the sorption isotherms and diffusion coefficients for structuro-morphological studies. It should be noted that such approaches must be based on specific theoretical models of the structural organization of the solution. Unfortunately, in some studies [15] in interpreting the experimental findings this point was overlooked and, therefore, the conclusions, as a rule, are essentially of a qualitative character. Since the hydrophobic elastomer studied by us contained impurities or chemical additives the molecules of which contain polar groupings treatment of the sorption isotherms was based on the assumption of the possibility of local sorption of the water molecules at the polar centres, clustering of the water molecules and their statistical distribution over the volume of the material. To identify the p/p, intervals within which the particular states of sorbed water may be realised, we used the traditional BET approaches [13] widely used in physicochemical research and those of Zimm and Landberg [16]. For SKEPT the BET equation allows one to describe only the initial portion of the isotherm to PIPs=0.3 (Fig. 4). In presence of a v.a. the region of concentrations in which the BET equation satisfactorily describes the sorption isotherm shifts to large p/p, =0.4. This allows us to state that in the above indicated activity regions of water vapour in the system studied the water molecules are preferentially sorbed at the polar centres of the impurities of the catalytic system
1352
A. YE. C~IALYKHe t aL
and v.a. The contribution of this process to the total sorption capacity is comparatively slight and amounts to 2.7 ~o (relative). As shown in reference [1, p. 4/0] for most polymer systems the dependence of the reciprocal of sorption @- 1 on the reciprocal value of the relative moisture content P - x is close to linear and is described by the semi-empirical equation
@-l=kl P - l - k 2
(1)
At k2 =0 equation (1) passes to the Henry equation, at k 2 < 0 it corresponds to the Langmuir equation and at k 2 > 0 it characterizes the process of formation of clusters. Figure 5 presents the isotherms of sorption of water by SKEPT and its composites with a v.a. in the coordinates of equation (1). Calculation of the constants kz showed that for SKEPT kz>>0 (16.7 x 10a), i.e. a tendency towards clustering is observed. In rubber containing a v.a. (2~o BMD) in the region 0.5
P/Ps c (1-p/p~) 28
AH, kE/mole
20
8
12 I
I
0.05
0.15
o,gpav.%
FIG. 3
I
I
0.g
0-#
P/Ps
FIG. 4
FIG. 3. Heat of sorption AH as a function of the content of water in SKEPT (1) and SKEPT+2% BMD (2). F1o. 4. Sorption isotherms of water vapour by SKEPT (1) and SKEPT+2% BMD (2) at 293 K in the coordinates of the BET equation.
It is of interest to evaluate the possibility of the process of clustering of the water molecules dissolved in the polymer matrix. For this purpose from the equation
Gl11~"1= - ( 1 - ~ol)(dull ~P"~ \
-- 1
(2)
63al ./P,T
(~'1 is the partial molar volume of water; q~l is the volumetric fraction of the sorbate; al is the activity of the water vapours).we calculated the clustering function, the concentration dependence of which is presented in Fig. 6. The form of the function varies for SKEPT and the composite with a v.a. which is probably connected with the influence of the polar groups of the v.a. on the distribution of the water molecules between the
Sorption and diffusion of water in ethylene-propylene diene rubber
1353
clusters and the phase of free water. With increase in the number of active centres introduced into the polymer the start of the process of clustering ( G l l / 1 " 1 > - 1 ) shifts to large p/p~ as compared with the pure polymer but at p/p~ = 0.5 (~0> 0.25 x 10- 3) fall in clustering is observed. Possibly there is enlargement (fusion) of the clusters which leads to fall in their number."
7
-5
20
3
0 10 -8 -16 0
2 FIG. 5
P/Ps
I
I
I
q
0.5
l.o
cp.lOa
FIG. 6
FIG. 5. Sorption isotherms of water vapour by SKEPT (1) and S K E P T + 2 ~ BMD (2) in the coordinates of equation (l). FIG. 6. Concentration dependence of the function of clustering G11/V1 on the volumetric fraction of sorbed water to for SKEPT (1) and S K E P T + 2 ~ BMD (2), GI1/Pl= - 1 (3). F r o m the Fick portions of the kinetic sorption curves we calculated for all the PIPs intervals the concentration functions of the relative diffusion coefficients Dx of water in SKEPT. The results of the calculations are given in Fig. 7. It may be seen that in the SKEPT-water system characterizing the partial diffusion mobility of the water molecules in the matrix of the elastomer the diffusion coefficient D1 is constant over the whole concentration interval. For the composites of the polymer with a v.a. (2 ~ BMD) in the region of concentrations of the sorbed water corresponding to pips> 0.5, a tendency for the diffusion coefficient to fall is observed; the total drop in D1 is, on average, 5 0 ~ . It is worth noting that in absolute terms Dt for water for lhe system with a v.a. is somewhat lower than for SKEPT which is probably connected with the influence of the phasic v.a. particles on the transfer processes. This effect is similar to the influence on the diffusion coefficient of the filler particles in the region of low concentrations. The character of the concentration dependence noted is connected, as shown above, with the thermodynamic non-ideality of the system. The apparent activation energy o f diffusion in S K E P T E = 2 2 . 6 kJ/mole is close to the activation energy of diffusion o f
1354
A. yt~. CHAt,VK~eta|.
gases in polyolefines [17]; for the rubber composite containing a v.a. its value is somewhat higher and amounts to 27.6 kJ/mole. The investigations of the diffusion and sorption of water by SKEPT in the activity interval 0.1
70 .
-[°gl/91~
n
7-4-
o__ o
1
N~ I
0.0
I
OB P/Ps
FIG. 7. Concentration dependence of the diffusion coefficient Dt for SKEPT (1) and SKEPT+2y. BMD (2). It should be noted that a special influence is exerted on the sorption capacity of the system by the impurities present in the commercial elastomer and not removed by extraction. The introduction into the polymer matrix of a v.a. leads to fall in D1, the appearance of concentration dependence of DI, increase in the total amount of sorbed water and thermal effects of sorption and the appearance of phase states in the kinetic sorption curves.
Translated by A. CRozY REFERENCES
1. Voda v polimerakh (Water in Polymers) (Edited by S. Rowland). 555 pp, Mir, Moscow, 1984 2. Z. A. KOGAN and G. D. RYBAKOV, Konservatsiya i upakovka (Preservation and Packaging). 264 pp, Mashinostroyeniye, Moscow, 1973 3. T. L CHALYKH and P. G. BABAYEVSKH, Sorbtsiya, gigroskopiehnost', pronitsayemost' (Sorption, Hygroscopicity and Permeability). p. 53, Ts/qHTLegprom, Moscow, 1981
Sorption and diffusion of water in ethylene-propylene diene rubber
1355
4. M . Z . AZARKH, A. N. KOVALEVA, A. S. KOSENKOVA and A. V. CHULYUKINA, Kauchuk i rezina, 5, 38, 1984 5. T. F. PETROVA, A. A. LAPSHOVA and A. A. DONTSOV, [bid, 8, 22, 1984 6. A. A. DONTSOV, G. I. TARASOVA and A. A. LAPSHOVA, Vysokomol. soyed. A24: 1895, 1982 (Translated in Polymer Sci. U.S.S.R. 24: 9, 2166, 1982) 7. A. Ye. CHALYKH, T. F. PETROVA, A. Ye. RUBTSOV and A. A. LAPSHOVA, Ibid. A28: 734, 1986 (Translated in Polmyer Sci. U.S.S.R. 28: 4, 817, 1986) 8. A. Ya. MALKIN and A. Ye. CHALYKH, Diffuziya i vyazkost' polimerov (Diffusion and Viscosity of Polymers). p. 259, Khimiya, Moscow, 1979 9. A. A. TAGER, Fizikokhimiya polimerov ( Polymer Physicochemistry). p. 319, Khimiya, Moscow, 1978 10. T. P. KOMAROVA, M. A. MARKELOV, S. A. NENAKHOV, E. I. SEMENENKO and A. Ye. CHALYKH, Vysokomol. soyed. AIS: 264, 1976 (Translated in Polymer Sci. U.S.S.R. 18: 2, 300, 1976) 11. K. RODGERS, Problemy fiziki i khimii tverdogo sostoyaniya organicheskikh soyedinenii (Problems of the Physics and Chemistry of the Solid State of Organic Compounds). p. 229, Mir, Moscow, 1968 12. D.V.van KREVELEN, Svoistva i khimicheskoye stroyeniye polimerov (Properties and Chemical Structure of Polymers). p. 304, Khimiya, Moscow, 1976 13. S. P. PAPKOV and E. Z. FAINBERG, Vzaimodeistviye tsellyulozy i tsellyuloznykh materialov s vodoi (Interaction of Cellulose and Cellulose Materials with Water). 230 pp, Khimiya, Moscow, 1970 14. T. P. GATOVSKAYA, V. A. KARGIN and A. A. TAGER, Zh. fiz. khim. 29: 883, 1955 15. M. SEFTON and K. CHANG, lqoveishiye instrumental'nye metody issledovaniya struktury polimerov (Latest Instrumental Methods for Studying the Structure of Polymers). p. 246, Mir, Moscow, 1982 16. B. H. ZIMM and J. L. LUNDBERG, J. Phys. Chem. 60: 425, 1956 17. S. A. REITLINGER, Pronitsayemost' polimernykh materialov (Permeability of Polymer Materials). 286 pp, Khimiya, Moscow, 1974