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Kinetics of thermal decomposition of crosslinked polyether urethane elastomers
:~ol~,mer Science U.S.S.R. Vol. 20, pp. 230-238. Pergamon Press Ltd. 1978. Printed in Poland
0032-3950/78/0101-0230507.50/0
KINETICS OF T H E R M A L DECOMPOSITION OF CROSSLINKED POLYETHER U R E T H A N E ELASTOMERS* ~ . N . VOLKOVA, YU. A . OL'KHOV, S. 1~. BATURII~ a n d
L. P. SMIR~OV D e p a r t m e n t of the I n s t i t u t e od Chemical Physics, U.S.S.R. A c a d e m y of Sciences
(Received 19 April 1977) The use of tin dibutyl dilaurate for the preparation of crosslinked polyether urethanes changes the rules of thermal decomposition (reduces the activation energy a n d pre-exponential factor, changes the order of the reaction) and weakens the dependence of activation parameters of the reaction on crosslinking density, The a d d i t i o n of low molecular weight diol to polyurethane compositions increases the activation energy and pre-exponential factor of thermal decomposition.
POLYURETHANES have a valuable set of physical and chemical properties, which explain t h e wide range of practical application. These polymers, however, have a considerable shortcoming: t h e y break down comparatively easily at increased temperatures. I n view of the complexity of structure and the large variation in the composition of polyurethanes, the mechanism of thermal decomposition has not been explained finally. I t h a s been established that" the urethano group is normally the weakest link in these compounds [1-6]. I t is assumed [2, 3, 7] t h a t the urethane group breaks down b y the formation o f an intermediate six membered complex. There are results in the literature [8] about the ionic and radical [9-12] mechanisms of decomposition of polyurethanes. Although crosslinked polyurethanes have the most valuable practical properties, kinetic investigations of thermal decomposition were mainly carried out using linear polyurethanes [2, 7, 9, 12-16]; results of these investigations are conflicting. F o r example, kinetics of thermal decomposition of polyurethanes are described b y zero [12] a n d first [2, 15, 16] order equations and a first order auto-catalytic equation [9]. Effective activation energies o f thermal decomposition [7, 9, 12] show considerable variation. There is no systematic quantitative information available concerning the effect of crosslinking density and chain polarity on thermal stability of urethane elastomers. Main results were obtained b y TGA and I)TA and are qualitative data: the thermal stability of elastomers is related to the so-called initial temperature of decomposition and to temperat u r e s of typical points of TGA and T D A curves [17-23].
Results are given of investigating kinetics of thermal decomposition of crosslinked polyether urethane elastomers, in order to explain the effect of conditions o f synthesis, crosslinking density and chain polarity on thermal stability of these compounds. We note that, according to the kinetic theory of strength of ~olids [24], rules of thermal decomposition of polymers determine decomposi* Vysokomol. soyed. A20: No. 1, 199-205, 1978. 230
Crosslinked polyether urethane elastomers
23[
t i o n k i n e t i c s a n d t h e r e f o r e , t h e s t u d y o f l~inetics o f t h e r m a l d e c o m p o s i t i o n o f polyether urethanes, according to composition and structure, is also necessary t o c o n t r o l p h y s i c a l a n d m e c h a n i c a l p r o p e r t i e s , w h i c h a r e t h e m o s t ~i m p o r t a n t practical characteristics of polymers. Polyether urcthane clastomers obtained from a copolymer of tetrahydrofuran a n d propylene oxide ( T H F - P O ) , trimethylolpropane (TMP) as branching agent, 1,4-butanediol (BD) as polarity regulator of the polymer chain a n d 2,4-toluylenedi-isoeyanate (TDI) as crosslinking agent, were used. TABLE 1.
Series, :No.
II
III
IV
V
COMPOSITION, CONDITIONS OF SYNTHESIS AND
Sam.
pie, No.
Concentration, % of the weight of the copolymer TMP BD
1
0.5
2 3 4 4 5 6 7 7
1
2 3 3 0.5 2 3 3
8
3
9 10 11 12 13 14
5 7 10 0.6 0.6 0"6
15
16
0 0 0 0 0 0 0 0 0 0 0 0 0 1.4 5.7 10.7
v, × 10',
mole/ cm
0.02 0-48 2.32 3-23 3.23 0.17 2.46 3-11 3-11 7.40 14.00 18.00 36.60 0.04 0.05 0"36
T D I + B D w~hout TDBDL TDI+BD w~h TDBDL
log k0,
SOME
PROPERTIES
OF
SAMPLES
SeC -1
E, kcal/mole
Conditions of synthesis
6"92 5.22 5"02 5-30 5"50* 5"90 5"52 5-72 1"70~ 14.88 15"98 17"70 14"70 7.08 8.62 11.24
23-2 21"4 20.8 21"3 22-0* 23"2 22'0 22'5 11.55 45"5 48"5 52"7 45"8 25.8 29.5 35.5
Hardening at 30 ° with TDBDL The oligomer was freed f r o m BFs a n d POCT t
14.92
44"0
12.28
37.5
Hardening at 30 ° with TDBDL The oligomer was not freed from BFa Hardening at 60 ° without TDBDL The oligomer was not purified Hardening at 30 ° with TDBDL The oligomer was freed from BF3 and POCT
Hardening at 60 °
* Results were obtained by measuring the shear modulus. t Propylene oxide cyclic tetramer. $ Resultm were obtained by a ~ a v l m e t r i c method by decomposition in air.
A T H F copolymer containing 15 mole% PO was obtained b y cationic polymerization. The molecular weight of the copolymer, determined from hydroxyl group content in terms of bffunctionality of the oligomcr, was 1800+30 and found b y measuring condensation heat effects [25]--1700±50. TDI, TMP and BD were distilled i n vacuo. T h e moisture contrent of components containing h y d r o x y l was less t h a n 0-03%. The compositions contained OH- and NCO groups in a stoichiometric ratio. The compositions were hardened at 30 ° in
232
N . N . VoI~ovA e~ a/.]
the presence of tin dibutyl dilaurate (TDBDL) or at 60 ° (without a catalyst) for a period o f time sufficient for the completion of urethane formation [26]. Experimental values of crosslinking density of a three dimensional elastomer network vo were determined by the Class method [27] using T H F as solvent for swelling of the polymer. Conditions of synthesis, composition and properties of polyether urethanes are shown in Table 1. Kinetics of thermal decomposition were examined using automatic electronic vacuum balances designed by the Experimental Institute of Chemical Physics, U.S.S.R. Academy of Sciences. The initial weight of samples varied between 40 and 100 rag. Special experiments have shown that a variation of the sample in the interval indicated has no effect on process kinetics. The temperature of decomposition during the experiment was maintained with an accuracy of 4-0-5 °. Breakdown took place at a residual pressure of 1 × 10 -2 torr.
Dependence of the rate of thermal decomposition on crosslinking density. T h e d e n s i t y o f crosslinking o f p o l y m e r chains w a s c h a n g e d b y t h e a d d i t i o n t o t h e reaction mixture of various quantities of TMP during polymerization with a c o r r e s p o n d i n g v a r i a t i o n o f T D I c o n t e n t (Table 1, s a m p l e s 1-11). F i g u r e l a a n d b shows typical kinetic curves of the weight variation of samples during thermal
a f
20
b
~
j ~..~
.~ g
u
I/7
0
l 1
i 2
W O
1 2
1 0
I B
d
5o ~-
so
u
~ 1
,X
./I
z 3
I 2
I /4
I 6
200
600
ZOO0
T/me, hr, FIG. I. Kinetic curves of thermal decomposition of: a--sample I at 253 (1); 265 (2); 280.5 (3), 301.5 (4); 314 ° (6); b--samples 8 (2, ~), 9 (3, 6) and 10 (1, 5) at 241 (1-3) and 265° (4-6); c - - s a m p l e 15 at 231 (1) and sample 16 at 230!5° (2); d--sample 4 at 96 (1); 109 (2); 133 (3); 148 (g); 167 ° (5).
Crosslinked polyether urethane elastomers
233
decomposition of erosslinked polyurethanes. Kinetics of thermal decomposition for samples 1-7 are satisfactorily described by several (at least two) first order equations (Fig. 2), the rates of stages varying at least 6 to 10 fold to enable effective reaction rate constants to be determined using the method of constant time intervals [28]. In contrast to the method previously proposed [28], when analysing kinetic curves constant time interval was selected between more remote points and not between adjacent points, for example, between 1 and 6, 2 and 7, etc., which considerably increases the accuracy of calculation. The temperature dependence of rate constants of the most rapid stage of the reaction (Table 2) is satisfactorily described b y the Arrhenius equation. Activation energy E and pre-exponential factors determined as a result of analysing experimental results by the method of least squares, are shown in Table 1. These results confirm that both rate constants and activation parameters of thermal decomposition of polyether urethanes, which were hardened at 30 ° in the presence of T D B D L , are practically independent of the concentration of TMP, i.e. the number of crosslinks and the presence in the T H F - P O copolymer of traces of catalyst in polymerization of B F 3 and molecules free from functional groups. The extent of weight loss corresponding to the stage of reaction examined increases in proportion to the increase in the temperature of decomposition and urethane group contents in the polymer (Table 2). For samples obtained b y hardening the reaction mixture of T H F - P O , T M P and T D I without a catalyst (samples 8-11, Table 1), the initial stage of thermal decomposition is characterized b y a practically constant rate of gravimetric variation, which corresponds to linear sections of kinetic curves (Fig. lb). The effective activation energy of thermal decomposition of these samples, according 3
lg
20 JO l/0 GPavirne#,ic varia//on , ~"
FI0. 2. Relation between the rate of thermal decomposition and weight variation of samples 1-4 (numbers at the curves) at 301.5 (•); 298.5 (2); 300.5 (3) and 299° (4). to crosslintring density, varies in the concentration interval of TMP examined between 46 and 53 kcal/mole, i.e. it is much higher than the activation energy derived for the thermal decomposition of samples obtained in the presence o f
"~S~
N,
N, VOLKOVA £t al.
3?DBDL. A considerable increase (by 8-10 orders of magnitude) in the pre-exponential factor is observed at the same time. It should be noted that the activation energy of thermal decomposition of these samples shows an extremal depend•ence on crosslinking density. In order to verify whether TDBDL has a catalytic action on thermal decomposition, a series of experiments were carried out to examine the decomposition of linear polyether urethane prepared from a stoiehiometrie mixture of BD a n d T D I at 60 ° without TDBDL (samples 15, 16, Table 1). The initial stage of
~ABr.~ 2. RATE CONSTANTS oY THEB~rATJ DECO~POSrH01~ I
Sam-
pie, No.
T,°C
k X 103, SeC - 1
Am* --~-,%
Sample,
k × 103 T, °C
No. 253.0 265.0 280.5 301.5
0"17 O'3O 0"47 0.99
14 16 19 19
248.5 250.0 262.0 277.5 298.5 307-0 309.0
0"18 0.19 0"24 0-75 1-03 1-40 1"33
16 17 18 22 26 26 24
250.5 280.0 298.5 300.5 309-0 312.0
0"18 0'77 1.31 1'26 1.34 1.59
16 19 24 24 27 28
262-5 279.0 299.0 308-0
0.39 0-74 1.30 2.22
20 24 31 20
251-5 263.0 278.0 301.0 313.5
0.18 0.24 0.51 1-30 1.90
19 21 26 26 29
7t
Am
SeC-1
m
Sam-
m
11
241.0 253.5 265.0 270-5 281.5 287.0
0'01 0"05 0"14 0"16 0"58 0'60
18 21 2O 25 25 24
12
250.0 250.5 280.0 301.9
0"21 0-42 0'71 1'75
17 18 18 24
249.5 252-0 266-5 278.5 301.0
0'23 0"21 0"46 1"10 3"28
21 22 23 26 26
251.5 266"0 280.0 300-5 312"0
0"26 0"31 0"54 1 "45 2"31
20 25 26 34 27
228.5 241-0 253 "5 265'0 271.0 281.5
0"53 0'88 1 "03 1"67
18 19 15 18
1.91
19
218-0 230.0 241.0 253-5 265.0 270.5
0.006 0.010 0.035 0-140 0.270 0.415
20 20 2O 22 30 30
241-0 253-5 265-0 281.5
0.02 0"07 0.14 0.64
17 18 20 23
19 13
k × 103,
Jm
T, °C
see_ x
m
14
241.0 252-0 265-0 281.5 290-5
0.15 0.34 1.05 2.18 3.33
25 23 30 34 32
15
219.0 231.0 236-5 242.5 253.5 259.0
0.029 0.052 0.092 0-182 0.495 0.670
38 42 40 35 4O 42
16
217.0 224.0 230.5 231.0 234.5 242.5 248.0 253.5
0.040 0.068 0.092 0.144 0.157 0.320 0.420 0.510
50 55 60 50 50 60 60 60
ple,
No. 16 17 18 20 22 24
l0
* Gravimetric variation of the sample at a given stage of decomposition. t Resulte were obtained during the decomposition of the sample in air.
~o
0-02 0"03 0"07 0"16 0"55 0"76
20 22 25 24 27 30
Sam-
~m*
--,
241.0 247-0 254-0 265.0 276.0 282.0
0"2I 0"24 0"64 0"77 1 "30 1"80
i
s e c -1
No.
251"0 262"5 277.5 279.0 301"0 312.0
1.o5
k × 109, T, °C
ple,
,%
C~ ~D
~r f0
0
!
I
t~
N. N. V o ~ o v x
236
et at.
scribed by a first order equation. The temperature dependence of rate constants of thermal decomposition obtained by the methods indicated is described by a n Arrhenius relation (Fig. 3). 'Relation between the rate of thermal decom'position and chain "polarity. Chain polarity of polyether urethanes varied on adding different amounts of BD to! the reaction mixture. The addition of low molecular weight diol to polyurethanel compositions is a convenient method of changing the number of polar groups in the polymer chain and therefore, properties of crosslinked elastomers. Thus, i increasing the polarity of polyether urethanes increases both tensile strength at and breaking elongation eb [26]. It is therefore very interesting to observe the variation of heat stability of polyether urethane elastomers according to! 4 o ~/~
log
-5 -
3
~
-6-
l
!
-1' I
I
f.e FIG. 3
I
I
I0
20 JO Ot'avirne~ric zariah'o• , %
4o
FIG. 4
FXG. 3. Temperature dependence of rate constants of thermal decomposition of sample according to the variation of shear modulus (1) and according to weight variation (2).i FIG. 4. Relation between the rate of thermal decomposition and the weight variation o f samples 1 (1), 11 (2), 12 (3) and 13 (4) at 280 (1, 2, 4) and 278.5 ° (3).
the variation of the number of polar groups in the main chain, since results in the literature [18, 19, 23] are qualitative. As shown by Fig. 4, the rate of weigh~ variation in thermal decomposition of polyether urethane elastomers at high temperatures increases with an increase in the amount of BD added to the polymer chain (samples 1, 12-14, Table 1), however, the activation energy of decomposition of the samples studied increases in the same order, which corresponds to higher standard physical and mechanical characteristics, as noted previously. The increased activation energy on adding BD may be due to the formation of a more regular [26] and therefore, less stressed network structure~
Crosslinked polyether urethano olastomers
237
Thermel oxidative de4]radation. T h e h e a t stability o f p o l y e t h e r urethanes is m u c h lower d u r i n g decomposition in air t h a n d u r i n g d e c o m p o s i t i o n in v a c u u m . Kinetics o f weight r e d u c t i o n are satisfactorily described b y several (at least two) first order equations, the rates o f stages differing a t least 10 fold. R a t e c o n s t a n t s of the fastest stage of decomposition o f sample 7 in air are m u c h h i g h e r t h a n rate constants of the fastest stage of d e c o m p o s i t i o n o f t h e same sample in v a c u u m (Table 2). W e note t h a t the a c t i v a t i o n e n e r g y o b t a i n e d for the d e c o m position of sample 7 in air is h a l f the a c t i v a t i o n e n e r g y of t h e r m a l decomposition of this sample (Table 1). Translated by E. SEMERE REFERENCES
1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11.
12. 13. 14. 15.
16.
17. 18.
19. 20. 21. 22. 23.
T. MUKAIYAMA and J. HOSHINO, J. Amer. Chem. Soe. 78: 1946, 1956 E. DYER and J. WRIGHT, J. Amer. Chem. Soe. 81: 2138, 1959 A. F. MCKAY and G. R. VAVASOUR, Canad. ft. Chem. 31: 688, 1953 V. A. 0RLOV and O. G. TARAKANOV, Plast. massy, No. 5, 12, 1965 V. A. ORLOV and O. G. TARAKANOV, Plast. massy, No. 6, 11, 1965 S. A. STEPANYAN and A. V. KOZLOV, Vysokomol. soyed. B14: 246, 1972 E. DYER and E. R. READ, 7. Organ. Chem. 26: 4388, 1961 M. A. FLETCHER end M. V. LAKIN, J . Amer. Chem. Soe. 75: 3898, 1953 V. K. BELYAKOV, A. A. B ~ , I. I. BUKIN, V. A. ORLOV and O. G. TARAKANOV, Yysokomol. soyect. AIO: 599, 1968 (Translated in Polymer Sci. U.S.S.R. 10: 3, 700, 1968) C. J. PEDERSON, J. Organ. Chem. 23: 255, 1958 Ye. C. LUKOSHEVICHENE, M. M. GUTAUSKAS and E. E. TORNAU, Sb. Sintez i fizikokhimiya polimerov (Synthesis and Physico-Chemistry of Polymers). Izd. "Naukova 4umka", 1974 J. D. INGHAM and N. S. RAPP, 7. Polymer Sei. A2: 4941, 1964 J. D. INGHAM, Polymer Engng. Sci. 6: 36, 1966 N. S. RAPP and J. D. INGHAM, J. Polymer Sei. A2: 689, 1964 N. P. KIfRGAN, A. A. KACHAN, N. V. KULI:K, G. F. GONCHARENKO arid L. N. KORSAKOVA, Sintez i fiziko-khiraiya poliuretanov (Synthesis and Physieo-Chemistry of Polyurethanes). Izd. "Naukova dumka", 1967 Yu. Ye. MAL'KOV, A. I. BENIN end M. S. VILESOVA, Tezisy doklada na soveshehanii: Khimiya i tekhnologiya proizvodstva, pererabotki i primeneniya poliuretar~ov i iskhodnogo sur'ya dlya nikh (Chemistry and Technology of the Manufacture, Proceessing and Application of Polyurethanes and Raw Materials). Vladimir, 1973 N. P. APUKHTINA, Sb. Sintez i fiziko-khimiya polimerov (Synthesis and PhysicoChemistry of Polymers). Izd. "Naukova ctumka", 1970 D. Sh. KOROSHKINA, B. N. PANTELEYEVA, E. N. SOTNIKOVA, Ye. A. SIDOROVICH and N. P. APIfKHTINA, Sb. Sintez i fiziko-khimiya polimerov (Synthesis and PhysicoChemistry of Polymers). Izd. "Naukova dumka", 1974 A. F. M.ASLYIfK, V. V. MAGDINETS and N. K. IVCHENKO, Sb. Sintez i fiziko-khimiya polimerov, Izd. "Naukova dumka", 1972 L. L. CHERVYATSOVA, S. S. DEMCHENK0 and L. A. RED'KO, Sintez i fiziko-khimiya polimerov, Izd. "Naukowa 4umka", 1973 (~. B. GERMANOVA, Ye. V. KUVSHINSKH, L. V. MOZZHL~HINA and L B. BELOV, Sb. Sintez i fiziko-khimiya polimerov, Izd. "Naukowa dumka", 1973 N. P. SMETANKINA arid V. Ya. OPRYA, Sb. Sintez i fiziko-khimiya polimerov, Izd. "l~aukova dumka", 1975 J. FERGUSSON and Z. PETROVIC, Europ, Polymer J. 12: 177, 1976 e
23$
'
V . V . Gutt'YA~oVA et al.
24. V. R. REGEL', A. I. SLUTSKER and E. Ye. TOMASHEVSKII, Kineticheskaya priroda proehnosti tverdykh tel (Kinetics of the Strength of Solids). Izd. "l~auka", 1974 25. Ye. Yu. BEKHLI, D. D. NOVIKOV and S. G, ENTELIS, Vysokomol. soyed. A9: 2574~ 1967 (Translated in Polymer Sci. U.S.S.R. 9: 12, 2911, 1964) 26. Yu. A. OL'KHOV, S. M. BATURIN and S. G. ENTEI.IS, Vysokomol. soyed. A18: 150, 1976 (Translated in Polymer Sci. U.S.S.R. 18: 1, 174, 1976) 27. E. E. CLASS, E. K. GLADDING and S. J. PARISER, J. Polymer Sci., 45: 341, 1960 28. N. E. SHANK, Inter. J. Chem. Kinetics 5: 577, 1973 29. V. A . T O P O L K A R A Y E V , V. G. OSHMYAN, A1. AI. BERLIN, A . N. ZELENETSKII, E. V. PRUT and N. S. YENIKOLOPYAN, Dokl. AN SSSR 225: 1124, 1975 30. P. J. FLORY and J. J. REHNER, J. Chem. Phys. 11: 512, 1943 31. Yu. M. SIVERGIN, N. B. M I R E N S K A Y A , Ye. T. SHAIJIKOVA and A. A. BERLIN, Vysokomol. soyed. A l l : 1919,1969 (Translated in Polymer Sci .U.S.S.R. 11: 9, 2186, 1969)
Polymer Science U.S.S.R. Vol. 20, pp. 238-247 ~ ) Pergamon Press Ltd. 1978. Printed in Poland
0032-3950/78]0101-0238507.50/0
RULES GOVERNING HIGH TEMPERATURE STABILIZATION OF POLYHETEROARYLENES* V. V. GUR'YAI~OVA, N. G. AlCl~EI~KOVA,T. N. NOVOTORTSEVA, fix. B. BLYUMEI~FEL'D, V. A. BARAlqOVA, F. SH. I~/[ALYUKOVA, L. fix. SHESTERNII~A and B. iY[. KOVARSKAYA Scientific Industrial Association "Plastics"
(Received 28 April 1977) I t was shown that the addition of various compounds containing phosphorus s l o w s down thermal oxidative degradation of polyimides, polybenzoxazoles and their
model compounds, reduces the gravimetric loss of samples and the separation of volatile breakdown products. The amount of gel fraction formed during oxidation of stabilized polymers is higher than that formed during oxidation of unstabilized samples, which is evidence of the formation of a three dimensional structure in the polymer, more stable under conditions of thermal oxidation. During oxidation of stabilized polybenzoxazole practically the entire phosphorus remains in the solid polymer residue after breakdown. Some parameters of the three dimensional structure of polybenzoxazoles and polyimides were calculated from the ratio of gel and sol fractions using the Charlesby theory. A study was made of the reactivity of stabilizers containing phosphorus in relation to peroxide radical of model compounds.
I~ addition to the numerous studies carried out to find new polymer structures with maximum possible heat stability, investigations are being conducted to increase the heat stability of already existing heat resistant polymers by the introduction of stabilizing additives. It was proposed to add to the polymers * Vysokomol. soyed. A20: No. 1,207-214, 1978.
0032-3950/78/0101-0230507.50/0
KINETICS OF T H E R M A L DECOMPOSITION OF CROSSLINKED POLYETHER U R E T H A N E ELASTOMERS* ~ . N . VOLKOVA, YU. A . OL'KHOV, S. 1~. BATURII~ a n d
L. P. SMIR~OV D e p a r t m e n t of the I n s t i t u t e od Chemical Physics, U.S.S.R. A c a d e m y of Sciences
(Received 19 April 1977) The use of tin dibutyl dilaurate for the preparation of crosslinked polyether urethanes changes the rules of thermal decomposition (reduces the activation energy a n d pre-exponential factor, changes the order of the reaction) and weakens the dependence of activation parameters of the reaction on crosslinking density, The a d d i t i o n of low molecular weight diol to polyurethane compositions increases the activation energy and pre-exponential factor of thermal decomposition.
POLYURETHANES have a valuable set of physical and chemical properties, which explain t h e wide range of practical application. These polymers, however, have a considerable shortcoming: t h e y break down comparatively easily at increased temperatures. I n view of the complexity of structure and the large variation in the composition of polyurethanes, the mechanism of thermal decomposition has not been explained finally. I t h a s been established that" the urethano group is normally the weakest link in these compounds [1-6]. I t is assumed [2, 3, 7] t h a t the urethane group breaks down b y the formation o f an intermediate six membered complex. There are results in the literature [8] about the ionic and radical [9-12] mechanisms of decomposition of polyurethanes. Although crosslinked polyurethanes have the most valuable practical properties, kinetic investigations of thermal decomposition were mainly carried out using linear polyurethanes [2, 7, 9, 12-16]; results of these investigations are conflicting. F o r example, kinetics of thermal decomposition of polyurethanes are described b y zero [12] a n d first [2, 15, 16] order equations and a first order auto-catalytic equation [9]. Effective activation energies o f thermal decomposition [7, 9, 12] show considerable variation. There is no systematic quantitative information available concerning the effect of crosslinking density and chain polarity on thermal stability of urethane elastomers. Main results were obtained b y TGA and I)TA and are qualitative data: the thermal stability of elastomers is related to the so-called initial temperature of decomposition and to temperat u r e s of typical points of TGA and T D A curves [17-23].
Results are given of investigating kinetics of thermal decomposition of crosslinked polyether urethane elastomers, in order to explain the effect of conditions o f synthesis, crosslinking density and chain polarity on thermal stability of these compounds. We note that, according to the kinetic theory of strength of ~olids [24], rules of thermal decomposition of polymers determine decomposi* Vysokomol. soyed. A20: No. 1, 199-205, 1978. 230
Crosslinked polyether urethane elastomers
23[
t i o n k i n e t i c s a n d t h e r e f o r e , t h e s t u d y o f l~inetics o f t h e r m a l d e c o m p o s i t i o n o f polyether urethanes, according to composition and structure, is also necessary t o c o n t r o l p h y s i c a l a n d m e c h a n i c a l p r o p e r t i e s , w h i c h a r e t h e m o s t ~i m p o r t a n t practical characteristics of polymers. Polyether urcthane clastomers obtained from a copolymer of tetrahydrofuran a n d propylene oxide ( T H F - P O ) , trimethylolpropane (TMP) as branching agent, 1,4-butanediol (BD) as polarity regulator of the polymer chain a n d 2,4-toluylenedi-isoeyanate (TDI) as crosslinking agent, were used. TABLE 1.
Series, :No.
II
III
IV
V
COMPOSITION, CONDITIONS OF SYNTHESIS AND
Sam.
pie, No.
Concentration, % of the weight of the copolymer TMP BD
1
0.5
2 3 4 4 5 6 7 7
1
2 3 3 0.5 2 3 3
8
3
9 10 11 12 13 14
5 7 10 0.6 0.6 0"6
15
16
0 0 0 0 0 0 0 0 0 0 0 0 0 1.4 5.7 10.7
v, × 10',
mole/ cm
0.02 0-48 2.32 3-23 3.23 0.17 2.46 3-11 3-11 7.40 14.00 18.00 36.60 0.04 0.05 0"36
T D I + B D w~hout TDBDL TDI+BD w~h TDBDL
log k0,
SOME
PROPERTIES
OF
SAMPLES
SeC -1
E, kcal/mole
Conditions of synthesis
6"92 5.22 5"02 5-30 5"50* 5"90 5"52 5-72 1"70~ 14.88 15"98 17"70 14"70 7.08 8.62 11.24
23-2 21"4 20.8 21"3 22-0* 23"2 22'0 22'5 11.55 45"5 48"5 52"7 45"8 25.8 29.5 35.5
Hardening at 30 ° with TDBDL The oligomer was freed f r o m BFs a n d POCT t
14.92
44"0
12.28
37.5
Hardening at 30 ° with TDBDL The oligomer was not freed from BFa Hardening at 60 ° without TDBDL The oligomer was not purified Hardening at 30 ° with TDBDL The oligomer was freed from BF3 and POCT
Hardening at 60 °
* Results were obtained by measuring the shear modulus. t Propylene oxide cyclic tetramer. $ Resultm were obtained by a ~ a v l m e t r i c method by decomposition in air.
A T H F copolymer containing 15 mole% PO was obtained b y cationic polymerization. The molecular weight of the copolymer, determined from hydroxyl group content in terms of bffunctionality of the oligomcr, was 1800+30 and found b y measuring condensation heat effects [25]--1700±50. TDI, TMP and BD were distilled i n vacuo. T h e moisture contrent of components containing h y d r o x y l was less t h a n 0-03%. The compositions contained OH- and NCO groups in a stoichiometric ratio. The compositions were hardened at 30 ° in
232
N . N . VoI~ovA e~ a/.]
the presence of tin dibutyl dilaurate (TDBDL) or at 60 ° (without a catalyst) for a period o f time sufficient for the completion of urethane formation [26]. Experimental values of crosslinking density of a three dimensional elastomer network vo were determined by the Class method [27] using T H F as solvent for swelling of the polymer. Conditions of synthesis, composition and properties of polyether urethanes are shown in Table 1. Kinetics of thermal decomposition were examined using automatic electronic vacuum balances designed by the Experimental Institute of Chemical Physics, U.S.S.R. Academy of Sciences. The initial weight of samples varied between 40 and 100 rag. Special experiments have shown that a variation of the sample in the interval indicated has no effect on process kinetics. The temperature of decomposition during the experiment was maintained with an accuracy of 4-0-5 °. Breakdown took place at a residual pressure of 1 × 10 -2 torr.
Dependence of the rate of thermal decomposition on crosslinking density. T h e d e n s i t y o f crosslinking o f p o l y m e r chains w a s c h a n g e d b y t h e a d d i t i o n t o t h e reaction mixture of various quantities of TMP during polymerization with a c o r r e s p o n d i n g v a r i a t i o n o f T D I c o n t e n t (Table 1, s a m p l e s 1-11). F i g u r e l a a n d b shows typical kinetic curves of the weight variation of samples during thermal
a f
20
b
~
j ~..~
.~ g
u
I/7
0
l 1
i 2
W O
1 2
1 0
I B
d
5o ~-
so
u
~ 1
,X
./I
z 3
I 2
I /4
I 6
200
600
ZOO0
T/me, hr, FIG. I. Kinetic curves of thermal decomposition of: a--sample I at 253 (1); 265 (2); 280.5 (3), 301.5 (4); 314 ° (6); b--samples 8 (2, ~), 9 (3, 6) and 10 (1, 5) at 241 (1-3) and 265° (4-6); c - - s a m p l e 15 at 231 (1) and sample 16 at 230!5° (2); d--sample 4 at 96 (1); 109 (2); 133 (3); 148 (g); 167 ° (5).
Crosslinked polyether urethane elastomers
233
decomposition of erosslinked polyurethanes. Kinetics of thermal decomposition for samples 1-7 are satisfactorily described by several (at least two) first order equations (Fig. 2), the rates of stages varying at least 6 to 10 fold to enable effective reaction rate constants to be determined using the method of constant time intervals [28]. In contrast to the method previously proposed [28], when analysing kinetic curves constant time interval was selected between more remote points and not between adjacent points, for example, between 1 and 6, 2 and 7, etc., which considerably increases the accuracy of calculation. The temperature dependence of rate constants of the most rapid stage of the reaction (Table 2) is satisfactorily described b y the Arrhenius equation. Activation energy E and pre-exponential factors determined as a result of analysing experimental results by the method of least squares, are shown in Table 1. These results confirm that both rate constants and activation parameters of thermal decomposition of polyether urethanes, which were hardened at 30 ° in the presence of T D B D L , are practically independent of the concentration of TMP, i.e. the number of crosslinks and the presence in the T H F - P O copolymer of traces of catalyst in polymerization of B F 3 and molecules free from functional groups. The extent of weight loss corresponding to the stage of reaction examined increases in proportion to the increase in the temperature of decomposition and urethane group contents in the polymer (Table 2). For samples obtained b y hardening the reaction mixture of T H F - P O , T M P and T D I without a catalyst (samples 8-11, Table 1), the initial stage of thermal decomposition is characterized b y a practically constant rate of gravimetric variation, which corresponds to linear sections of kinetic curves (Fig. lb). The effective activation energy of thermal decomposition of these samples, according 3
lg
20 JO l/0 GPavirne#,ic varia//on , ~"
FI0. 2. Relation between the rate of thermal decomposition and weight variation of samples 1-4 (numbers at the curves) at 301.5 (•); 298.5 (2); 300.5 (3) and 299° (4). to crosslintring density, varies in the concentration interval of TMP examined between 46 and 53 kcal/mole, i.e. it is much higher than the activation energy derived for the thermal decomposition of samples obtained in the presence o f
"~S~
N,
N, VOLKOVA £t al.
3?DBDL. A considerable increase (by 8-10 orders of magnitude) in the pre-exponential factor is observed at the same time. It should be noted that the activation energy of thermal decomposition of these samples shows an extremal depend•ence on crosslinking density. In order to verify whether TDBDL has a catalytic action on thermal decomposition, a series of experiments were carried out to examine the decomposition of linear polyether urethane prepared from a stoiehiometrie mixture of BD a n d T D I at 60 ° without TDBDL (samples 15, 16, Table 1). The initial stage of
~ABr.~ 2. RATE CONSTANTS oY THEB~rATJ DECO~POSrH01~ I
Sam-
pie, No.
T,°C
k X 103, SeC - 1
Am* --~-,%
Sample,
k × 103 T, °C
No. 253.0 265.0 280.5 301.5
0"17 O'3O 0"47 0.99
14 16 19 19
248.5 250.0 262.0 277.5 298.5 307-0 309.0
0"18 0.19 0"24 0-75 1-03 1-40 1"33
16 17 18 22 26 26 24
250.5 280.0 298.5 300.5 309-0 312.0
0"18 0'77 1.31 1'26 1.34 1.59
16 19 24 24 27 28
262-5 279.0 299.0 308-0
0.39 0-74 1.30 2.22
20 24 31 20
251-5 263.0 278.0 301.0 313.5
0.18 0.24 0.51 1-30 1.90
19 21 26 26 29
7t
Am
SeC-1
m
Sam-
m
11
241.0 253.5 265.0 270-5 281.5 287.0
0'01 0"05 0"14 0"16 0"58 0'60
18 21 2O 25 25 24
12
250.0 250.5 280.0 301.9
0"21 0-42 0'71 1'75
17 18 18 24
249.5 252-0 266-5 278.5 301.0
0'23 0"21 0"46 1"10 3"28
21 22 23 26 26
251.5 266"0 280.0 300-5 312"0
0"26 0"31 0"54 1 "45 2"31
20 25 26 34 27
228.5 241-0 253 "5 265'0 271.0 281.5
0"53 0'88 1 "03 1"67
18 19 15 18
1.91
19
218-0 230.0 241.0 253-5 265.0 270.5
0.006 0.010 0.035 0-140 0.270 0.415
20 20 2O 22 30 30
241-0 253-5 265-0 281.5
0.02 0"07 0.14 0.64
17 18 20 23
19 13
k × 103,
Jm
T, °C
see_ x
m
14
241.0 252-0 265-0 281.5 290-5
0.15 0.34 1.05 2.18 3.33
25 23 30 34 32
15
219.0 231.0 236-5 242.5 253.5 259.0
0.029 0.052 0.092 0-182 0.495 0.670
38 42 40 35 4O 42
16
217.0 224.0 230.5 231.0 234.5 242.5 248.0 253.5
0.040 0.068 0.092 0.144 0.157 0.320 0.420 0.510
50 55 60 50 50 60 60 60
ple,
No. 16 17 18 20 22 24
l0
* Gravimetric variation of the sample at a given stage of decomposition. t Resulte were obtained during the decomposition of the sample in air.
~o
0-02 0"03 0"07 0"16 0"55 0"76
20 22 25 24 27 30
Sam-
~m*
--,
241.0 247-0 254-0 265.0 276.0 282.0
0"2I 0"24 0"64 0"77 1 "30 1"80
i
s e c -1
No.
251"0 262"5 277.5 279.0 301"0 312.0
1.o5
k × 109, T, °C
ple,
,%
C~ ~D
~r f0
0
!
I
t~
N. N. V o ~ o v x
236
et at.
scribed by a first order equation. The temperature dependence of rate constants of thermal decomposition obtained by the methods indicated is described by a n Arrhenius relation (Fig. 3). 'Relation between the rate of thermal decom'position and chain "polarity. Chain polarity of polyether urethanes varied on adding different amounts of BD to! the reaction mixture. The addition of low molecular weight diol to polyurethanel compositions is a convenient method of changing the number of polar groups in the polymer chain and therefore, properties of crosslinked elastomers. Thus, i increasing the polarity of polyether urethanes increases both tensile strength at and breaking elongation eb [26]. It is therefore very interesting to observe the variation of heat stability of polyether urethane elastomers according to! 4 o ~/~
log
-5 -
3
~
-6-
l
!
-1' I
I
f.e FIG. 3
I
I
I0
20 JO Ot'avirne~ric zariah'o• , %
4o
FIG. 4
FXG. 3. Temperature dependence of rate constants of thermal decomposition of sample according to the variation of shear modulus (1) and according to weight variation (2).i FIG. 4. Relation between the rate of thermal decomposition and the weight variation o f samples 1 (1), 11 (2), 12 (3) and 13 (4) at 280 (1, 2, 4) and 278.5 ° (3).
the variation of the number of polar groups in the main chain, since results in the literature [18, 19, 23] are qualitative. As shown by Fig. 4, the rate of weigh~ variation in thermal decomposition of polyether urethane elastomers at high temperatures increases with an increase in the amount of BD added to the polymer chain (samples 1, 12-14, Table 1), however, the activation energy of decomposition of the samples studied increases in the same order, which corresponds to higher standard physical and mechanical characteristics, as noted previously. The increased activation energy on adding BD may be due to the formation of a more regular [26] and therefore, less stressed network structure~
Crosslinked polyether urethano olastomers
237
Thermel oxidative de4]radation. T h e h e a t stability o f p o l y e t h e r urethanes is m u c h lower d u r i n g decomposition in air t h a n d u r i n g d e c o m p o s i t i o n in v a c u u m . Kinetics o f weight r e d u c t i o n are satisfactorily described b y several (at least two) first order equations, the rates o f stages differing a t least 10 fold. R a t e c o n s t a n t s of the fastest stage of decomposition o f sample 7 in air are m u c h h i g h e r t h a n rate constants of the fastest stage of d e c o m p o s i t i o n o f t h e same sample in v a c u u m (Table 2). W e note t h a t the a c t i v a t i o n e n e r g y o b t a i n e d for the d e c o m position of sample 7 in air is h a l f the a c t i v a t i o n e n e r g y of t h e r m a l decomposition of this sample (Table 1). Translated by E. SEMERE REFERENCES
1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11.
12. 13. 14. 15.
16.
17. 18.
19. 20. 21. 22. 23.
T. MUKAIYAMA and J. HOSHINO, J. Amer. Chem. Soe. 78: 1946, 1956 E. DYER and J. WRIGHT, J. Amer. Chem. Soe. 81: 2138, 1959 A. F. MCKAY and G. R. VAVASOUR, Canad. ft. Chem. 31: 688, 1953 V. A. 0RLOV and O. G. TARAKANOV, Plast. massy, No. 5, 12, 1965 V. A. ORLOV and O. G. TARAKANOV, Plast. massy, No. 6, 11, 1965 S. A. STEPANYAN and A. V. KOZLOV, Vysokomol. soyed. B14: 246, 1972 E. DYER and E. R. READ, 7. Organ. Chem. 26: 4388, 1961 M. A. FLETCHER end M. V. LAKIN, J . Amer. Chem. Soe. 75: 3898, 1953 V. K. BELYAKOV, A. A. B ~ , I. I. BUKIN, V. A. ORLOV and O. G. TARAKANOV, Yysokomol. soyect. AIO: 599, 1968 (Translated in Polymer Sci. U.S.S.R. 10: 3, 700, 1968) C. J. PEDERSON, J. Organ. Chem. 23: 255, 1958 Ye. C. LUKOSHEVICHENE, M. M. GUTAUSKAS and E. E. TORNAU, Sb. Sintez i fizikokhimiya polimerov (Synthesis and Physico-Chemistry of Polymers). Izd. "Naukova 4umka", 1974 J. D. INGHAM and N. S. RAPP, 7. Polymer Sei. A2: 4941, 1964 J. D. INGHAM, Polymer Engng. Sci. 6: 36, 1966 N. S. RAPP and J. D. INGHAM, J. Polymer Sei. A2: 689, 1964 N. P. KIfRGAN, A. A. KACHAN, N. V. KULI:K, G. F. GONCHARENKO arid L. N. KORSAKOVA, Sintez i fiziko-khiraiya poliuretanov (Synthesis and Physieo-Chemistry of Polyurethanes). Izd. "Naukova dumka", 1967 Yu. Ye. MAL'KOV, A. I. BENIN end M. S. VILESOVA, Tezisy doklada na soveshehanii: Khimiya i tekhnologiya proizvodstva, pererabotki i primeneniya poliuretar~ov i iskhodnogo sur'ya dlya nikh (Chemistry and Technology of the Manufacture, Proceessing and Application of Polyurethanes and Raw Materials). Vladimir, 1973 N. P. APUKHTINA, Sb. Sintez i fiziko-khimiya polimerov (Synthesis and PhysicoChemistry of Polymers). Izd. "Naukova ctumka", 1970 D. Sh. KOROSHKINA, B. N. PANTELEYEVA, E. N. SOTNIKOVA, Ye. A. SIDOROVICH and N. P. APIfKHTINA, Sb. Sintez i fiziko-khimiya polimerov (Synthesis and PhysicoChemistry of Polymers). Izd. "Naukova dumka", 1974 A. F. M.ASLYIfK, V. V. MAGDINETS and N. K. IVCHENKO, Sb. Sintez i fiziko-khimiya polimerov, Izd. "Naukova dumka", 1972 L. L. CHERVYATSOVA, S. S. DEMCHENK0 and L. A. RED'KO, Sintez i fiziko-khimiya polimerov, Izd. "Naukowa 4umka", 1973 (~. B. GERMANOVA, Ye. V. KUVSHINSKH, L. V. MOZZHL~HINA and L B. BELOV, Sb. Sintez i fiziko-khimiya polimerov, Izd. "Naukowa dumka", 1973 N. P. SMETANKINA arid V. Ya. OPRYA, Sb. Sintez i fiziko-khimiya polimerov, Izd. "l~aukova dumka", 1975 J. FERGUSSON and Z. PETROVIC, Europ, Polymer J. 12: 177, 1976 e
23$
'
V . V . Gutt'YA~oVA et al.
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Polymer Science U.S.S.R. Vol. 20, pp. 238-247 ~ ) Pergamon Press Ltd. 1978. Printed in Poland
0032-3950/78]0101-0238507.50/0
RULES GOVERNING HIGH TEMPERATURE STABILIZATION OF POLYHETEROARYLENES* V. V. GUR'YAI~OVA, N. G. AlCl~EI~KOVA,T. N. NOVOTORTSEVA, fix. B. BLYUMEI~FEL'D, V. A. BARAlqOVA, F. SH. I~/[ALYUKOVA, L. fix. SHESTERNII~A and B. iY[. KOVARSKAYA Scientific Industrial Association "Plastics"
(Received 28 April 1977) I t was shown that the addition of various compounds containing phosphorus s l o w s down thermal oxidative degradation of polyimides, polybenzoxazoles and their
model compounds, reduces the gravimetric loss of samples and the separation of volatile breakdown products. The amount of gel fraction formed during oxidation of stabilized polymers is higher than that formed during oxidation of unstabilized samples, which is evidence of the formation of a three dimensional structure in the polymer, more stable under conditions of thermal oxidation. During oxidation of stabilized polybenzoxazole practically the entire phosphorus remains in the solid polymer residue after breakdown. Some parameters of the three dimensional structure of polybenzoxazoles and polyimides were calculated from the ratio of gel and sol fractions using the Charlesby theory. A study was made of the reactivity of stabilizers containing phosphorus in relation to peroxide radical of model compounds.
I~ addition to the numerous studies carried out to find new polymer structures with maximum possible heat stability, investigations are being conducted to increase the heat stability of already existing heat resistant polymers by the introduction of stabilizing additives. It was proposed to add to the polymers * Vysokomol. soyed. A20: No. 1,207-214, 1978.