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Relaxation processes in crystallizing and non-crystallizing elastomers
Polymer Science U.S.S.R. Vol. 24, No. 1, pp. 67-78, 1982 Yrlnted iu Poland
0082-3950/82/010067-07507.50/0 O 1982 Pergamon Press Ltd.
RELAXATION PROCESSES IN CRYSTALLIZING AND NON-CRYSTALLIZING ELASTOMERS* L. A. AKOPYA~, M. V. ZOBr~A a n d G. M. B~a~T~.~.v Leningrad Branch, Research Institute of the Plastics Industry Physico-chemical Institute, U.S.S.R. Academy of Sciences
(Received 7 August 1980) The mutual relation between g-relaxation processes and tendency to crystalize of the ethylene-propylene-diene elastomer SKEPT-40 has been analysed, using data from various studies and those of the authors. It was shown that this tendency is due to the morphological type of structural microblocks, which are responsible for the multiplet g-transition. It may be evaluated by using relaxation spectrometry which showed that SKEPT-40 belongs to a group of non-crystallizing polymers. The relaxation times of g-processes are 1-2 orders higher compared with those of nitrile, styrene, isoprene and other rubbers. Besides chemical relaxation in SKEPT-40 with 126 kJ]mole activation energy a process with an activation energy of 97 kJ/mole is observed. It is suggested that these features of the relaxational properties of SKEPT should be taken into accotmt in predictions of the durability of materials az~dproducts. SYSTE~TIC studies o f t h e r e l a x a t i o n properties of the following elastomers h a v e a l r e a d y been r e p o r t e d : b u t a d i e n e - s t y r e n e SXS-30, butadiene-methy]sts=rene SKMS-10, isoprene S K I - 3 , N R (natural rubber), b u t a d i e n e S K D a n d b u t a d i e n e nitrile S Y ~ - I S , S K N - 2 6 a n d SKN-40. The results of these studies h a v e been r e p o r t e d in a m o n o g r a p h [1] a n d in our p a p e r s [2-4]. The objects of this w o r k were to o b t a i n general d a t a on the a b o v e m a t e r i a l s f r o m the v i e w p o i n t of their a b i l i t y to crystallize on deformation. I n a d d i t i o n to the above, we studied the r e l a x a t i o n properties of S X E P T - 4 0 , e t h y l e n e - p r o pylene-diene elastomer. Unfilled vulcanizates and model resins filled with 42% by weight commercial PM.75 carbon_were studied, based on SKEPT-40 elastomer. The vulcanizing mixture contained 3 wt. ~o dipentamethylene-thiuram tetrasulphide (Thiuram), 2 wt. % di-(benzthiazolyl)disulphide (Altax), 0.8 wt. ~o l~vN'-dithiodimorphol ine and 5~o tin oxide. Vulcanization t o o k 60 mizl at 150°. Relaxation was assessed from the results of relaxing tension at 20% deformation with uniaxial contraction in the 20-150 ° temperature range. The length of the experiment was determined by the condition that the proportion of relaxation processes remaining incomplete at raised temperatures was close to zero. This showed up all the slow physical and chemical relaxation processes. The data obtained were processed by relaxation spectrometric methods [1] using the coordinates of the relaxation model, E(t)-time t. Calculation of separate spectra was by graphical methods and EVI~. * Vysokomol. soyed. A24: No. 1, 58-62, 1982. 67
68
A x o r r x z , r et a/.
L.A.
It is known t h a t at high glass temperatures Tg in elastomers, a group of slow relaxation processes is observed. First of all, we have/-processes, characterizing supermolecular rebuilding of polymer structures and then d-processes, indicative of restructuring and cleavage of crosslinkages. In filled elastomers, (o processes were also observed, connected with migration of active filler particles.
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FzG. 1. Temperature dependence of log vt for various relaxation processes for samples of uufdled (a) and filled (b) erosslinked elastomers of SKEPT-40; 1--21, 2--22, 3--23, 4--6,, 5 - - 6 and 6--(p relaxational transitions.
In crosslinked SKEPT-40 copolymer, relaxation was characterized by five (Fig. la) and for the filled samples, by six discrete (Fig. lb) times. To explain the origins of the relaxation processes, we determined from the log ~/T ratio, the pre-exponential coefficients Ut and B~ in the activation energy equation. z~=B~e -Ul~T
(I)
The coefficient B~ is connected with the sizes of the kinetic units, participating in a given relaxation process (i----1, 2 .... n). The activation energy of the first three processes (11, As and An) for filled and unfilled elastomers has the same value (49 kJ/mole), indicating their common origin. The B~ coefficients have values of 10 -a to 10 -6 sec for unfilled and 10 -a to 10 -e sec for filled elastomers which are actually greater than the coefficient, B = 5 × 1 0 - n sec, for free segments. This indicates the participation of much larger structural units in the relaxation processes. I t is evident that, as with other elastomers, these processes are characteristic o f a slow physical relaxation step and are explained by cleavage and formation of structural microblocks, playing tile role of physical nodes in the molecular lattice of the elastomer. The elastomer under study contains 60-70~o ethylene groups in its chains, consequently it is close to PE in its relaxation behaviour a n d exactly like its amorphous phase. In this connection, it is interesting to note t h a t according to reference [5], PE is characterized by a trio of l-processes with the same value of the activation energy (49 kJ/mole). Evidently, molecular mobility in the amorphous phases of P E and SKEPT-40 have a similar origin.
Relaxation processes in crystallizing and non-crystaUizing elastomers
69
The slowest processes (6 relaxation) are typified b y a value B ~ 6 × 10 -1~ sec, ~vhich practically agrees with the vibration period of the - - S k S - - and - - C - - S ~ crossliI~ks and an activation energy of U ~ 137 kJ/mole. These values are the same for filled and unfilled elastomers. Judging from the activation energy, 6-relaxation processes are related to chemical relaxation and are explained b y migration of chemical crosslinks. ~l-Processes are slow and have B ~ I . I × 1 0 -9 sec for unfilled and 6.2 X 10 -g sec for filled elastomers and the same activation energy of U ~ 9 7 kJ/mole. The relatively large value of B in comparison with B ~ 6 × X 10 -is sec for chemical crosslinks indicates that a ~-process is connected with chemical nodes of colloidal dimensions, in volume 105 times greater than sulphur atoms, which correspond to linear dimensions of 10 nm. It is suggested that this 61-process is linked with the formation of bulky associations of vulcanizer ingredients, such as zinc oxide [6]. According to Aben's results, sulphur vulcanizates of S K E P T are characterized b y a trio of chemical relaxation processes. The first is concerned with the rupture of di- and polysulphide crosslinkages and has an activation energy of 97 kJ/mole, which accords with 61-processes. We did not observe the second, which had an activation energy of 84 kJ/mole. Aben connected this process with extraneous impurities in the vuleanizate. The third process, with 122 kJ/mole activation energy, is explained b y oxidation of the polymer chains in combination with chemical relaxation of the lattices from their monosulphide links. The value of U ~ 1 3 7 kJ/mole which we obtained, is somewhat larger than is probably due to the large concentration of monosulphide links in the vulcanizate under study. Experiments were carried out at substantially higher temperatures than Tg, consequently the ~-process of segmental mobility, absent beyond the glass point, was not observed. Calculation of the parameters of this process satisfies the known [8] relations
r,,=B,~e v'Jk~'
(9.)
r.]== U®I(1--To/T),
(3)
where v,, U, are relaxation time and activation energy of the ~-process for which the coefficient B = 5 . 0 × 10 -12 sec; U~o is the value of U at T-* ~ , assumed, according to [8], equal to 17 kJ/mo]e; T 0 = T g - - 5 0 ° (Tg is the standard glass point, equal for SKEPT-40, to 65°). At 20 °, U , = 3 6 kJ/mole, T , = 8 . 3 × 10 -s sec. The physical structural features of the discrete relaxation time spectra also allow one to specify the salient question of the crystallizability of ethylcne-pro~ pylene-diene rubbers and at. the same time to solve, it particularly for SKEPT-40. In [9-11], S K E P T is aligned with non-crystallizing rubbers b u t in [12-15] it is. remarked that S K E P T ' s ability to crystallize is determined by the o) value,, i.e. the content of propylene monomer residues in the chains and also [15] with the nature of their distribution. Crystallization occurs most clearly in the range 0 . 4 > w > 0 . 6 . With co=0-3-0.5 it is probably least b u t in the 50 ° region a decrease in recovery is noticed, analogous to that observed in crystallization of polymers.
L. A. AKOPYA_We t a / .
70
In addition, it was noted in [15] that there was an appreciable variance between the degrees of crystallization determined by DTA and X-ray diffraction methods.
log 8~ EsecJ
ua i , i¢3/mole 60g
2
6 3
1
-4
40
20
q 8 8
I
I 3
7~
I 5. Iog~A~eg
Fro. 2 l ~ o . 2. Ratio of activation energy to the logarithm for non-crystallizing (I) and crystallizing polymers 2--CKN-18, 3--SKN-26, 4--SK_~-40, 5 - - S K S - 3 0 PM-70), 7 - - S K D , 8 - - N K
2
4
6
log'r~ [secJ
Fro. 8 of relaxation time for various processes, (II). Here and in Fig. 3, 1 - - S K E P T - 4 0 , ARMK-15, 6--SKMS-10 (techl. carbon and 9 - - S K I - 3 .
FIG. 3. Ratio of log B a to the logarithm of relaxation time of various 2-processes for noncrystallizing (I) and crystallizing polymers (IX).
Figure 2 shows the ratio of U, to log vz and Fig. 3 that of log B, to log vx for crosslinked elastomers. An analysis of these shows that the values of activation energy and of size of structural microbloeks are grouped in two areas, determined by the ability or not of the polymers to crystallize. The first area incorporates non-crystallizing polymers. It is characterized by comparatively large values of activation energy of the ~-processes, Ux=4.9-55 kJ/mole and small dimensions of the microblocks (Bx~ 10-8-10 -5 sec) in comparison with the second area, which incorporates crystallizing polymers, for which Ux~25-34 kJ/mole, Ba=10-s-10 -1 sec. The large size of the microblocks and large values of the B~ coefficients are explained by the more regular chain structures of crystallizing polymers. This is evidently because chain segments entering into mierobl~cks of crystallizing polymers are more weakly joined than in non-crystallizing ones. Thus the ability of polymers to crystallize is determined by the morphology of the mieroblocks formed, causing multiplet 2-transitions, and may be evaluated by relaxation spectrometric methods, particularly from the physico-structural parameters of discrete spectra of relaxation times. The difference found in the parameters U~ and B~ for regions I and II (Figs. 2 and 3) indicate that at high temperatures, the greatest relaxation times v~ a r e characteristic for crystallizing polymers
Relaxation processes in crystallizing and non-crystallizing elastomers
71
and the smallest, for non-crystallizing ones. At low temperatures, the relationship for relaxation times is the reverse. This is due to the fact t h a t , according to formula (1), at high temperatures the relation of U/k,T~O and relaxation time is determined by coefficient B~. At low temperatures, U]kT--, ~ and z~ is determined by the value of Ux. The d a t a of Fig. 4 confirm the relationships of ~aa as dependent on temperature and ability of the polymer to crystallize. Curve I for S K E P T has the same slope as for non-crystallizing polymers but is located rather higher. This is explained by the fact t h a t T~, the relaxation time for X-processes with crosslinked and filled rubbers from SKEPT-40 (Fig. 2), is 1-2 orders greater t h a n for other elastomers. Consequently the change in the relations of ~a for SKEPT-40 occurs at 150-160 ° i.e. at much higher temperatures compared with other non-crystallizing polymers. This should be taken into account when forecasting the behaviour of rubbers and products, derived from SKEPT-40, over a broad temperature range.
I°9%3 E*ee3 E. IdN/m z 5-
0
! EO
I00 FIG. 4
150 T °
l 2
6
10 l o g t f ~ j
FIG. 5
FIe. 4. Calculated dependence of log v~son temperature for various polymers: 1--SKEPT-40, 2--SKI-3, 3--SK/), 4--SKI, 5--SKMS-10, 6--SKS-30, 7--crosslinked and 8--filled SKEPT-40 rubbers. Points are experimental data. FIe. 5. Prediction of relaxation models with compressed crosslinked (1) and filled elastomers {2, 3) from SKEPT-40 at 25°, taking account of all relaxation spectra (1, 2) and mean relaxation times, from GOST (3). Points are from continuous experiments. It was shown earlier [4] t h a t neglect of slow physical relaxation m a y reduce the precision of forecasting relaxation properties of rubbers and durability of commercial plastic products. This is due to the superimposition of physical processes on chemical ones and to using the activation energy according to GOST (U.S.S.R. standard [16]) calculated by taking a single relaxation time, as if it was an average. The result is t h a t the forecast curve is initially situated higher a n d then lower t h a n the experimental one. I t is evident t h a t an increase in
72
L . A . A x o P Y ~ et al.
T~ with S K E P T - 4 0 c o m p a r e d with other polymers raises the p r o b a b i l i t y of superimposing physical processes on chemical ones, which increases the need to t a k e a c c o u n t of k-processes in forecasting. Figure 5 gives curves for continuous forecasts of r e l a x a t i o n a l models of unfilled and filled elastomers, based on S K E P T - 4 0 using all the spectra (as in [14]) a n d according to GOST's [16] " m e a n " r e l a x a t i o n times. A comparison with the e x p e r i m e n t a l d a t a shows t h a t in the first case, the precision o f the forecast is r a t h e r higher. Thus the crystallizing ability of polymers is d e t e r m i n e d b y the morphological t y p e of the microblocks f o r m e d from their supermolecular structures, causing a m u l t i p l e t k-transition. This m a y be e s t i m a t e d b y r e l a x a t i o n s p e c t r o m e t r i c methods, p a r t i c u l a r l y b y the p h y s i e o - s t r u c t u r a l p a r a m e t e r s o f the diserete r e l a x a t i o n time spectra. S K E P T - 4 0 is related to the group of non-crystallizing polymers, where the r e l a x a t i o n times of the }.-processes are 1-2 orders g r e a t e r t h a n those of nitrile, styrene, isoprene a n d o t h e r polymers. Chemical r e l a x a t i o n of S K E P T - 4 0 is characterized b y two processes; the first (activation e n e r g y 97 kJ/mole) is linked with the f o r m a t i o n of b u l k y associations of vulcanizing groups; the second (activation e n e r g y 137 kJ/mole) with the m o b i l i t y of chemical crosslinks. I t is r e c o m m e n d e d to t a k e into a c c o u n t the rules developed here when predicting the d u r a b i l i t y o f materials and products.
Translated by C. W. C,~P
REFERENCES
l. G. M. BARTENEV, Structurai relaksatsionyye svoistva elastomerov (Structure and Relaxation Properties of Elastomers) p. 288, Khimiya, 1979. 2. G. M. BARTENEV, L. A. SHELKOVNIKOVA and L. A. AKONYAN, Mekhanika polimeroy (Mechanics of Polymers) p. 151, 1973 3. L. A. AKONYAN, N. A. OVRUTSKAYA and M. M. PLEKHOTINA, Kauchuk i rezina, No. 10, 36, 1979 4. L. h. AKONYAN, M. V. ZOBINA, h. I. BERDENIKOV and G. M. BARTENEV, Kauehuk i rezina, No. 1 22 1980 5. (l. M. BARTENEV, R. M. AL(lULIEV and D. M. KITEYEVA, Vysokomol. soyod. A23: 2003 1981. (Translated in Polymer Sei. U.S.S.R. 23: 9, 1981) 6. A. A. DONTSOV, Protsessy structuroobrazovaniya elastomerov (Structuring Proeesse~ hi Elastomers) p. 288 Khimiya 1978 7. W. ABEN, Rev. Ggn6rale Caoutehoues et Plastiques 51: 10, 727 1974 8. G. M. BARTENEV and N. M. LIALINA, Vysokomol. soyed. B18:350 1976 (Not translated m Polymer Sei. U.S.S.R.) 9. L. O. AMBER(I, In: Vulkanizatsiya elastomerov (Vulcanzation of Elastomers), p. 348, Khimiya, 1967 10. F. F. KOSHELEV, A. Ye. KORNEV and A. M. BUKANOV, Obshehaya tekhnologia rezini, (General Rubber Technology) 4th. edition, p. 527, Khimiya, 1978 11. E. TAKANO, Nikhon kikai tehakkaisi 80: 701, 320, 1977; ekspress-informatsia "Termostoikiye plastiki" (express information "Thermal Stability of Plastics") No. 21, 13, 1979 12. F. P. BALDWIN, Rubber Chem. and Teehnol. 45: 3, 709, 1972 13. M. F. BIYKHINA, Kristallizatsiya kauehukov i rezin (Crystallization of Rubbers and tlesins), p. 239, Khimiya, 1973
High temperature pyrolysis of PMMA
73
14. V. N. R E I K H a n d V. P. MIRONYUK, In: Spravotehnik rezinshehika (Rubber Technician's H a n d b o o k ) , p. 239, Khimiya, 1973 15. lYL GILBER, J. E. BRIGGS and W. OMANA, Brit. Polymer J. 11: 2, 81, 1979 16. Rezhli. Meted uskorennogo opredeleniya garantiinogo sroka khaueniya uplotnitelnikh detalei nepodvizhnykh soyedenenii. GOST 9.035-74 (Rubbers, Method for Rapid D e termination of Guaranteed Keeping Time of Compressed Articles of Fixed Compositions. U.S.S.R. standard 9.035-74)
Polymer Science U.S.S.R. Vol. 24, No. 1, pp. 73-77, 1982 Printed in Poland
0032-3950/82/010073-05507.50/0 © 1982 Pergamon Press Ltd.
THE EFFECT OF PHOSPHORUS ON THE HIGH TEMPERATURE PYROLYSIS OF POLYMETHYLMETHACRYLATE* A. G. GAL'CH:ENKO, N. A. KHALTI~IlVSKII, A. A. S~KHA~OVA, T. V. ])OPOVA, T. ~I. FI~U~ZE and AL. A~. BERLIN Institute of Chemical Physics, U.S.S.R. Academy of Sciences
(Received 13 August 1980) The processes of degradation of polymethylraethacrylate (PMMA), containing various phosphorus compounds, have been studied and the degassing rate and polymer surface temperature in relation to phosphorus content. The inhibiting action of phosphorus on the high temperature pyrolysis of PMMA was demonstrated. This is expressed by the reduction in the degassing rate when phosphorus is added, with the same temperature of the polymer surface.
O~E of the ways in which fire-resistence of polymeric materials has been studied is through the influence of the addition of various inhibitors on their thermal degradation. The effect of phosphorus and its compounds (as effective combustion inhibitors [1, 2]) on high temperature PMMA pyrolysis was the object of the present work. Thermal degradation of pure PMMA has been fairly well studied. There has been a series of studies connected both with the heating patterns on the surface of pyrolysing PMMA and with the kinetics and mechanism of degradation [5, 6]. The degradation of PMMA containing phosphorus as a combustion inhibiter received little attention. The reference [7] describes the effect of phosphorus, added as diallyl methylphosphonate, on the burning of polymer spheres. It was shown t h a t as phosphorus content was increased, the value of the oxygen index increased (from l0 to 15%) but the combustion velocity constant hardly * V y s o k o m o l . s o y e d . A24: N o . 1, 6 3 - 6 6 , 1982.
0082-3950/82/010067-07507.50/0 O 1982 Pergamon Press Ltd.
RELAXATION PROCESSES IN CRYSTALLIZING AND NON-CRYSTALLIZING ELASTOMERS* L. A. AKOPYA~, M. V. ZOBr~A a n d G. M. B~a~T~.~.v Leningrad Branch, Research Institute of the Plastics Industry Physico-chemical Institute, U.S.S.R. Academy of Sciences
(Received 7 August 1980) The mutual relation between g-relaxation processes and tendency to crystalize of the ethylene-propylene-diene elastomer SKEPT-40 has been analysed, using data from various studies and those of the authors. It was shown that this tendency is due to the morphological type of structural microblocks, which are responsible for the multiplet g-transition. It may be evaluated by using relaxation spectrometry which showed that SKEPT-40 belongs to a group of non-crystallizing polymers. The relaxation times of g-processes are 1-2 orders higher compared with those of nitrile, styrene, isoprene and other rubbers. Besides chemical relaxation in SKEPT-40 with 126 kJ]mole activation energy a process with an activation energy of 97 kJ/mole is observed. It is suggested that these features of the relaxational properties of SKEPT should be taken into accotmt in predictions of the durability of materials az~dproducts. SYSTE~TIC studies o f t h e r e l a x a t i o n properties of the following elastomers h a v e a l r e a d y been r e p o r t e d : b u t a d i e n e - s t y r e n e SXS-30, butadiene-methy]sts=rene SKMS-10, isoprene S K I - 3 , N R (natural rubber), b u t a d i e n e S K D a n d b u t a d i e n e nitrile S Y ~ - I S , S K N - 2 6 a n d SKN-40. The results of these studies h a v e been r e p o r t e d in a m o n o g r a p h [1] a n d in our p a p e r s [2-4]. The objects of this w o r k were to o b t a i n general d a t a on the a b o v e m a t e r i a l s f r o m the v i e w p o i n t of their a b i l i t y to crystallize on deformation. I n a d d i t i o n to the above, we studied the r e l a x a t i o n properties of S X E P T - 4 0 , e t h y l e n e - p r o pylene-diene elastomer. Unfilled vulcanizates and model resins filled with 42% by weight commercial PM.75 carbon_were studied, based on SKEPT-40 elastomer. The vulcanizing mixture contained 3 wt. ~o dipentamethylene-thiuram tetrasulphide (Thiuram), 2 wt. % di-(benzthiazolyl)disulphide (Altax), 0.8 wt. ~o l~vN'-dithiodimorphol ine and 5~o tin oxide. Vulcanization t o o k 60 mizl at 150°. Relaxation was assessed from the results of relaxing tension at 20% deformation with uniaxial contraction in the 20-150 ° temperature range. The length of the experiment was determined by the condition that the proportion of relaxation processes remaining incomplete at raised temperatures was close to zero. This showed up all the slow physical and chemical relaxation processes. The data obtained were processed by relaxation spectrometric methods [1] using the coordinates of the relaxation model, E(t)-time t. Calculation of separate spectra was by graphical methods and EVI~. * Vysokomol. soyed. A24: No. 1, 58-62, 1982. 67
68
A x o r r x z , r et a/.
L.A.
It is known t h a t at high glass temperatures Tg in elastomers, a group of slow relaxation processes is observed. First of all, we have/-processes, characterizing supermolecular rebuilding of polymer structures and then d-processes, indicative of restructuring and cleavage of crosslinkages. In filled elastomers, (o processes were also observed, connected with migration of active filler particles.
!o9~ £~c3
/5
/. 5
u
10-
,".
/
/
#
/ /
L-
/ /
s~
L
"~
_ ~......-" ~"
-'T 2.5
I
/
q
.-I~.-
6
.....-X ''~(
I
~
l
I
3"0
3"6
2'6
,,=0
fOal7;K"
FzG. 1. Temperature dependence of log vt for various relaxation processes for samples of uufdled (a) and filled (b) erosslinked elastomers of SKEPT-40; 1--21, 2--22, 3--23, 4--6,, 5 - - 6 and 6--(p relaxational transitions.
In crosslinked SKEPT-40 copolymer, relaxation was characterized by five (Fig. la) and for the filled samples, by six discrete (Fig. lb) times. To explain the origins of the relaxation processes, we determined from the log ~/T ratio, the pre-exponential coefficients Ut and B~ in the activation energy equation. z~=B~e -Ul~T
(I)
The coefficient B~ is connected with the sizes of the kinetic units, participating in a given relaxation process (i----1, 2 .... n). The activation energy of the first three processes (11, As and An) for filled and unfilled elastomers has the same value (49 kJ/mole), indicating their common origin. The B~ coefficients have values of 10 -a to 10 -6 sec for unfilled and 10 -a to 10 -e sec for filled elastomers which are actually greater than the coefficient, B = 5 × 1 0 - n sec, for free segments. This indicates the participation of much larger structural units in the relaxation processes. I t is evident that, as with other elastomers, these processes are characteristic o f a slow physical relaxation step and are explained by cleavage and formation of structural microblocks, playing tile role of physical nodes in the molecular lattice of the elastomer. The elastomer under study contains 60-70~o ethylene groups in its chains, consequently it is close to PE in its relaxation behaviour a n d exactly like its amorphous phase. In this connection, it is interesting to note t h a t according to reference [5], PE is characterized by a trio of l-processes with the same value of the activation energy (49 kJ/mole). Evidently, molecular mobility in the amorphous phases of P E and SKEPT-40 have a similar origin.
Relaxation processes in crystallizing and non-crystaUizing elastomers
69
The slowest processes (6 relaxation) are typified b y a value B ~ 6 × 10 -1~ sec, ~vhich practically agrees with the vibration period of the - - S k S - - and - - C - - S ~ crossliI~ks and an activation energy of U ~ 137 kJ/mole. These values are the same for filled and unfilled elastomers. Judging from the activation energy, 6-relaxation processes are related to chemical relaxation and are explained b y migration of chemical crosslinks. ~l-Processes are slow and have B ~ I . I × 1 0 -9 sec for unfilled and 6.2 X 10 -g sec for filled elastomers and the same activation energy of U ~ 9 7 kJ/mole. The relatively large value of B in comparison with B ~ 6 × X 10 -is sec for chemical crosslinks indicates that a ~-process is connected with chemical nodes of colloidal dimensions, in volume 105 times greater than sulphur atoms, which correspond to linear dimensions of 10 nm. It is suggested that this 61-process is linked with the formation of bulky associations of vulcanizer ingredients, such as zinc oxide [6]. According to Aben's results, sulphur vulcanizates of S K E P T are characterized b y a trio of chemical relaxation processes. The first is concerned with the rupture of di- and polysulphide crosslinkages and has an activation energy of 97 kJ/mole, which accords with 61-processes. We did not observe the second, which had an activation energy of 84 kJ/mole. Aben connected this process with extraneous impurities in the vuleanizate. The third process, with 122 kJ/mole activation energy, is explained b y oxidation of the polymer chains in combination with chemical relaxation of the lattices from their monosulphide links. The value of U ~ 1 3 7 kJ/mole which we obtained, is somewhat larger than is probably due to the large concentration of monosulphide links in the vulcanizate under study. Experiments were carried out at substantially higher temperatures than Tg, consequently the ~-process of segmental mobility, absent beyond the glass point, was not observed. Calculation of the parameters of this process satisfies the known [8] relations
r,,=B,~e v'Jk~'
(9.)
r.]== U®I(1--To/T),
(3)
where v,, U, are relaxation time and activation energy of the ~-process for which the coefficient B = 5 . 0 × 10 -12 sec; U~o is the value of U at T-* ~ , assumed, according to [8], equal to 17 kJ/mo]e; T 0 = T g - - 5 0 ° (Tg is the standard glass point, equal for SKEPT-40, to 65°). At 20 °, U , = 3 6 kJ/mole, T , = 8 . 3 × 10 -s sec. The physical structural features of the discrete relaxation time spectra also allow one to specify the salient question of the crystallizability of ethylcne-pro~ pylene-diene rubbers and at. the same time to solve, it particularly for SKEPT-40. In [9-11], S K E P T is aligned with non-crystallizing rubbers b u t in [12-15] it is. remarked that S K E P T ' s ability to crystallize is determined by the o) value,, i.e. the content of propylene monomer residues in the chains and also [15] with the nature of their distribution. Crystallization occurs most clearly in the range 0 . 4 > w > 0 . 6 . With co=0-3-0.5 it is probably least b u t in the 50 ° region a decrease in recovery is noticed, analogous to that observed in crystallization of polymers.
L. A. AKOPYA_We t a / .
70
In addition, it was noted in [15] that there was an appreciable variance between the degrees of crystallization determined by DTA and X-ray diffraction methods.
log 8~ EsecJ
ua i , i¢3/mole 60g
2
6 3
1
-4
40
20
q 8 8
I
I 3
7~
I 5. Iog~A~eg
Fro. 2 l ~ o . 2. Ratio of activation energy to the logarithm for non-crystallizing (I) and crystallizing polymers 2--CKN-18, 3--SKN-26, 4--SK_~-40, 5 - - S K S - 3 0 PM-70), 7 - - S K D , 8 - - N K
2
4
6
log'r~ [secJ
Fro. 8 of relaxation time for various processes, (II). Here and in Fig. 3, 1 - - S K E P T - 4 0 , ARMK-15, 6--SKMS-10 (techl. carbon and 9 - - S K I - 3 .
FIG. 3. Ratio of log B a to the logarithm of relaxation time of various 2-processes for noncrystallizing (I) and crystallizing polymers (IX).
Figure 2 shows the ratio of U, to log vz and Fig. 3 that of log B, to log vx for crosslinked elastomers. An analysis of these shows that the values of activation energy and of size of structural microbloeks are grouped in two areas, determined by the ability or not of the polymers to crystallize. The first area incorporates non-crystallizing polymers. It is characterized by comparatively large values of activation energy of the ~-processes, Ux=4.9-55 kJ/mole and small dimensions of the microblocks (Bx~ 10-8-10 -5 sec) in comparison with the second area, which incorporates crystallizing polymers, for which Ux~25-34 kJ/mole, Ba=10-s-10 -1 sec. The large size of the microblocks and large values of the B~ coefficients are explained by the more regular chain structures of crystallizing polymers. This is evidently because chain segments entering into mierobl~cks of crystallizing polymers are more weakly joined than in non-crystallizing ones. Thus the ability of polymers to crystallize is determined by the morphology of the mieroblocks formed, causing multiplet 2-transitions, and may be evaluated by relaxation spectrometric methods, particularly from the physico-structural parameters of discrete spectra of relaxation times. The difference found in the parameters U~ and B~ for regions I and II (Figs. 2 and 3) indicate that at high temperatures, the greatest relaxation times v~ a r e characteristic for crystallizing polymers
Relaxation processes in crystallizing and non-crystallizing elastomers
71
and the smallest, for non-crystallizing ones. At low temperatures, the relationship for relaxation times is the reverse. This is due to the fact t h a t , according to formula (1), at high temperatures the relation of U/k,T~O and relaxation time is determined by coefficient B~. At low temperatures, U]kT--, ~ and z~ is determined by the value of Ux. The d a t a of Fig. 4 confirm the relationships of ~aa as dependent on temperature and ability of the polymer to crystallize. Curve I for S K E P T has the same slope as for non-crystallizing polymers but is located rather higher. This is explained by the fact t h a t T~, the relaxation time for X-processes with crosslinked and filled rubbers from SKEPT-40 (Fig. 2), is 1-2 orders greater t h a n for other elastomers. Consequently the change in the relations of ~a for SKEPT-40 occurs at 150-160 ° i.e. at much higher temperatures compared with other non-crystallizing polymers. This should be taken into account when forecasting the behaviour of rubbers and products, derived from SKEPT-40, over a broad temperature range.
I°9%3 E*ee3 E. IdN/m z 5-
0
! EO
I00 FIG. 4
150 T °
l 2
6
10 l o g t f ~ j
FIG. 5
FIe. 4. Calculated dependence of log v~son temperature for various polymers: 1--SKEPT-40, 2--SKI-3, 3--SK/), 4--SKI, 5--SKMS-10, 6--SKS-30, 7--crosslinked and 8--filled SKEPT-40 rubbers. Points are experimental data. FIe. 5. Prediction of relaxation models with compressed crosslinked (1) and filled elastomers {2, 3) from SKEPT-40 at 25°, taking account of all relaxation spectra (1, 2) and mean relaxation times, from GOST (3). Points are from continuous experiments. It was shown earlier [4] t h a t neglect of slow physical relaxation m a y reduce the precision of forecasting relaxation properties of rubbers and durability of commercial plastic products. This is due to the superimposition of physical processes on chemical ones and to using the activation energy according to GOST (U.S.S.R. standard [16]) calculated by taking a single relaxation time, as if it was an average. The result is t h a t the forecast curve is initially situated higher a n d then lower t h a n the experimental one. I t is evident t h a t an increase in
72
L . A . A x o P Y ~ et al.
T~ with S K E P T - 4 0 c o m p a r e d with other polymers raises the p r o b a b i l i t y of superimposing physical processes on chemical ones, which increases the need to t a k e a c c o u n t of k-processes in forecasting. Figure 5 gives curves for continuous forecasts of r e l a x a t i o n a l models of unfilled and filled elastomers, based on S K E P T - 4 0 using all the spectra (as in [14]) a n d according to GOST's [16] " m e a n " r e l a x a t i o n times. A comparison with the e x p e r i m e n t a l d a t a shows t h a t in the first case, the precision o f the forecast is r a t h e r higher. Thus the crystallizing ability of polymers is d e t e r m i n e d b y the morphological t y p e of the microblocks f o r m e d from their supermolecular structures, causing a m u l t i p l e t k-transition. This m a y be e s t i m a t e d b y r e l a x a t i o n s p e c t r o m e t r i c methods, p a r t i c u l a r l y b y the p h y s i e o - s t r u c t u r a l p a r a m e t e r s o f the diserete r e l a x a t i o n time spectra. S K E P T - 4 0 is related to the group of non-crystallizing polymers, where the r e l a x a t i o n times of the }.-processes are 1-2 orders g r e a t e r t h a n those of nitrile, styrene, isoprene a n d o t h e r polymers. Chemical r e l a x a t i o n of S K E P T - 4 0 is characterized b y two processes; the first (activation e n e r g y 97 kJ/mole) is linked with the f o r m a t i o n of b u l k y associations of vulcanizing groups; the second (activation e n e r g y 137 kJ/mole) with the m o b i l i t y of chemical crosslinks. I t is r e c o m m e n d e d to t a k e into a c c o u n t the rules developed here when predicting the d u r a b i l i t y o f materials and products.
Translated by C. W. C,~P
REFERENCES
l. G. M. BARTENEV, Structurai relaksatsionyye svoistva elastomerov (Structure and Relaxation Properties of Elastomers) p. 288, Khimiya, 1979. 2. G. M. BARTENEV, L. A. SHELKOVNIKOVA and L. A. AKONYAN, Mekhanika polimeroy (Mechanics of Polymers) p. 151, 1973 3. L. A. AKONYAN, N. A. OVRUTSKAYA and M. M. PLEKHOTINA, Kauchuk i rezina, No. 10, 36, 1979 4. L. h. AKONYAN, M. V. ZOBINA, h. I. BERDENIKOV and G. M. BARTENEV, Kauehuk i rezina, No. 1 22 1980 5. (l. M. BARTENEV, R. M. AL(lULIEV and D. M. KITEYEVA, Vysokomol. soyod. A23: 2003 1981. (Translated in Polymer Sei. U.S.S.R. 23: 9, 1981) 6. A. A. DONTSOV, Protsessy structuroobrazovaniya elastomerov (Structuring Proeesse~ hi Elastomers) p. 288 Khimiya 1978 7. W. ABEN, Rev. Ggn6rale Caoutehoues et Plastiques 51: 10, 727 1974 8. G. M. BARTENEV and N. M. LIALINA, Vysokomol. soyed. B18:350 1976 (Not translated m Polymer Sei. U.S.S.R.) 9. L. O. AMBER(I, In: Vulkanizatsiya elastomerov (Vulcanzation of Elastomers), p. 348, Khimiya, 1967 10. F. F. KOSHELEV, A. Ye. KORNEV and A. M. BUKANOV, Obshehaya tekhnologia rezini, (General Rubber Technology) 4th. edition, p. 527, Khimiya, 1978 11. E. TAKANO, Nikhon kikai tehakkaisi 80: 701, 320, 1977; ekspress-informatsia "Termostoikiye plastiki" (express information "Thermal Stability of Plastics") No. 21, 13, 1979 12. F. P. BALDWIN, Rubber Chem. and Teehnol. 45: 3, 709, 1972 13. M. F. BIYKHINA, Kristallizatsiya kauehukov i rezin (Crystallization of Rubbers and tlesins), p. 239, Khimiya, 1973
High temperature pyrolysis of PMMA
73
14. V. N. R E I K H a n d V. P. MIRONYUK, In: Spravotehnik rezinshehika (Rubber Technician's H a n d b o o k ) , p. 239, Khimiya, 1973 15. lYL GILBER, J. E. BRIGGS and W. OMANA, Brit. Polymer J. 11: 2, 81, 1979 16. Rezhli. Meted uskorennogo opredeleniya garantiinogo sroka khaueniya uplotnitelnikh detalei nepodvizhnykh soyedenenii. GOST 9.035-74 (Rubbers, Method for Rapid D e termination of Guaranteed Keeping Time of Compressed Articles of Fixed Compositions. U.S.S.R. standard 9.035-74)
Polymer Science U.S.S.R. Vol. 24, No. 1, pp. 73-77, 1982 Printed in Poland
0032-3950/82/010073-05507.50/0 © 1982 Pergamon Press Ltd.
THE EFFECT OF PHOSPHORUS ON THE HIGH TEMPERATURE PYROLYSIS OF POLYMETHYLMETHACRYLATE* A. G. GAL'CH:ENKO, N. A. KHALTI~IlVSKII, A. A. S~KHA~OVA, T. V. ])OPOVA, T. ~I. FI~U~ZE and AL. A~. BERLIN Institute of Chemical Physics, U.S.S.R. Academy of Sciences
(Received 13 August 1980) The processes of degradation of polymethylraethacrylate (PMMA), containing various phosphorus compounds, have been studied and the degassing rate and polymer surface temperature in relation to phosphorus content. The inhibiting action of phosphorus on the high temperature pyrolysis of PMMA was demonstrated. This is expressed by the reduction in the degassing rate when phosphorus is added, with the same temperature of the polymer surface.
O~E of the ways in which fire-resistence of polymeric materials has been studied is through the influence of the addition of various inhibitors on their thermal degradation. The effect of phosphorus and its compounds (as effective combustion inhibitors [1, 2]) on high temperature PMMA pyrolysis was the object of the present work. Thermal degradation of pure PMMA has been fairly well studied. There has been a series of studies connected both with the heating patterns on the surface of pyrolysing PMMA and with the kinetics and mechanism of degradation [5, 6]. The degradation of PMMA containing phosphorus as a combustion inhibiter received little attention. The reference [7] describes the effect of phosphorus, added as diallyl methylphosphonate, on the burning of polymer spheres. It was shown t h a t as phosphorus content was increased, the value of the oxygen index increased (from l0 to 15%) but the combustion velocity constant hardly * V y s o k o m o l . s o y e d . A24: N o . 1, 6 3 - 6 6 , 1982.