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Investigation of the hardening of unsaturated polyesters
INVESTIGATION OF THE HARDENING OF UNSATURATED POLYESTERS* YIy. I. KORZ~EVA, S. V. VINOGI~ADOVA,G. L. SLONIMSKII, V. V. KORSI~AKand A. A. ASKADSKII Instituto for Elemento-organic Compounds, U.S.S.R. Academy of Sciences (Received 15 February 1968)
THE structure of hardened polyester resins is the most important factor determining their properties. The amount of unsaturated monomer used in the copolymerization thus exerts a substantial effect [1-3]. An increase in its concentratior~ up to a certain limit raises the figures for the hardness, mechanical strength, heat and chemical resistance of copolymers and this is directly connected with an increase of the concentration of transverse bonds ill the copolymers. I t is known t h a t in the case of network elastomers the frequency of crosslinking m a y be approximately determined by measurement of stress-relaxation in specimens of these polymers. In this connection, it was of interest to carry out a similar investigation for hard network polymers. This investigation is clearly both of theoretical and also of practical value. It was of interest to find out if the experimental relaxation curves obtained under isothermal conditions with a constant uniaxial compression strain could be described by the Kohlrausch equation [4] and also to calculate the parameters in this equation which characterize the relaxation properties of the polymer. From the practical point of view, it would become possible by carrying out such an investigation to find the optimum amounts of the unsaturated monomer to be introduced into the resin. The use of this approach [5] made it possible to investigate the kinetics of copol)~merization between styrene and the unsaturated polyester based on 4,4'(fl,fl'-dihydroxyethoxyphenyl)-2,2-propane and fumaric acid. The stress-relaxation study was carried out under isothermal conditions at room temperature with a cor~stant compression strain, using a relaxometer based on Regel's system [6], the strain always being 4.8% and applied at a rate of 3 mm/min. Specimens 3 mm in diameter and 4.2 m m high were prepared for the tests. On completion of the development of the strain, the stress was measured at fixed intervals of time for one hour, and as a result of this a typical relaxation curve was obtained; this is shown schematically in Fig. 1, where all the conven* Vysokomol. soyed. All: No. 3, 519-524, 1969. 589
590
Yr. I. KORZE~VA et al.
tional symbols corresponding to the Koh]rausch equation are also shown: G(t)=a ~ -~-(70•e -ark -~G~ +~o e(-t/T)~,
(1)
where a(o is the stress at time t; a~ is the equilibrium stress; a 0 is the component of the stress which undergoes relaxation; a and k are parameters in the equation; is the relaxation time. The calculation of the parameters in the equation was carried out by the m e t h o d previously described [7]. EXPERIMENTAL AND DISCUSSION OF RESULTS
Copolymers of single and of mixed unsaturated polyesters with styrene were used as the experimental materials. The unsaturated polyesters were obtained by the polyeondensation of 4,4'-(fl,fl'-dihydroxyethoxyphenyl)-2,2-propane or 4,4'-(fl,fl'-dihydroxypropoxyphenyl)-2,2propane with fumaric acid or with a mixture of fumaric acid with adipic, sebacic or phthalie acid. Solutions of various concentrations of these polyesters in styrene were hardened in test-tubes in air in the-presence of 1% benzoyl peroxide for 2 hr at 60, 80, 100, 120 and 140°C, after which specimens for testing were machined from the castings obtained. As a result of the experiments carried out, a series of relaxation curves was obtained for specimens of all the copolymers studied, which are shown in Table 1 and in Fig. 2. The hardened resins based on the single and mixed polyesters shown in Table 1 differed both in the chemical structure of the polyester used and also in the styrene content, which was varied in the range from 25 to 75 weight %. I t turned out t h a t all the experimental relaxation curves could be described well by the Kohlrausch equation and this made it possible to calculate the parameters in this equation, the most important of which are shown in Table 1. The parameters of the Kohlransch eqaution m a y be divided into equilibrium and kinetic parameters [8]. The equilibrium stress a~ belongs to the first class: under given test conditions (a definite strain and temperature) this is an important characteristic of the usefulness of the materials obtained, and defines the stress range (from zero to a~) over which the plastic m a y be confidently used as a rigid construction material, for example, in glass-reinforced plastics. Of course, other things being equal, a higher value of a~ implies a higher hardness in the polymer system. In our case, for equal molar contents of styrene the highest value of the equilibrium stress a~ occurred for the mixed polyester of 4,4'-(fl,fl'-dihydroxyethoxyphenyl)-2,2-propane with fumaric and adipic acids, and the smallest value for the single propane polyester with fnmaric acid. This fact can clearly be explained by the fact t h a t a smaller number of double bonds participate i~ the copolymerization reactions in the case of the single polyester based on 4,4'-(fl,fl'-dihydroxypropoxyphenyl)-2,fl-propane, because of the screening effect of the bulky benzene nuclei and the side methyl substituents on the double bonds of the fmnar~te. In the case of the widely-used elastomers, the value of the equilibrium stress depends principally on the density of transverse bonds, e~nd it is precisely these which give rise to the final non-zero equilibrium stress. As the experiments
Investigation of hardening of unsaturated polyesters TABLE 1. C H A N G E I N T H E P A R A M E T E R S O F T H E K O H L R A U S C H THE
STRUCTURE
Polyester from
AND
C01VIPOSITI01~ O F T H E
Ratio polyester : : styrene
EQUATION, DEPENDING
HARDENED
ace,
591 ON
RESINS
~min
poise
1/B
weight
molar
kg/cm3
4,4'-(fl,fl'-Dihydroxyethoxyphenyl)-2,2-propane with f u m a r i c acid
40:60 50:50 55:45 60:40 70:30
1:5"7 1:3"7 1:3-1 1:2.5 1:1-6
209 428 401 195 114
30.5 13.0 37.0 6.0 16-0
0"599 0-300 0"217 0'280 0'611
1"5 × 7"0 × 9"9 × 6-1 × 1-7 ×
1013 1013 1013 10 la l013
1.8 2.3 3.5 1.6 1.3
4,4'-(B,B'-Dihydroxyethoxyphenyl)-2,2-propano with fumaric and terephtalic acids
30:70 40:60 45:55 50:50 55:45 60:40 65:35
1:8"9 1:5-8 1:4.7 1:3.8 1:3.2 1:2.6 1:2.3
112 230 231 329 304 253 252
90'0 6"5 4-5 2"0 1"2 7"2 4"0
0'281 0:198 0.188 0.150 0.133 0.236 0'253
8"7 X 1014 × × × × X ×
10 ~4 10 :a 1015 1015 1015 l014
1.8 1.8 1.9 2.0 2-0 1.8 1.7
8"6 1"1 7"1 2-6 1-0 8"4
l
4,4'-(fl,fl'-Dihydroxyethoxy-
30:70 40:60 50:50 60:40 70:30
1:8'9 1'5"8 1:3.8 1:2.6 1:1-7
364 425 439 292 232
15-0 1.8 22.0 14.0 9.0
0.869 0'274 0-284 0-442 0-408
2"1 × 9"1 × 1"2 × 9"8 • 6'1 ×
1012 1012 1014 1012 1012
2"8 3"3 3.0 2.0 1.6
30:70 35:65 40:60 45:55 55:45 65:35 70:30
1:9"0 1:7-2 1:5'9 1:4'8 1:3"2 1:2-3 1:1"7
473 517 465 464 580 800 450
13.0 6"5 75"0 70-0 9-0 42'0 i 5-5
0.741 0"338 0.382 0.261 0.159 0.395 0.470
2'0 9'4 6-9 4"1 1.2 7"7 3'7
× × × × × × ×
1012 10 r~ 1013 1014 1016 1013 1012
2.9 3.0 3.0 3.3 3.5 2.9 2.8
4,4'-(fl, f l ' - D i h y d r o x y e t h o x y phenyl)-2,2-propane with sebacic acid
30:70 40:60 50:50 60:40 70:30
1:9"4 1:6'1 1:4"0 1:2.7 1:1'7
83 134 170 170 ll0
1'7 4"2 70-0 4.5 33.0
0"277 0"357 0-305 0.198 0.318
1"3 1"5 2'7 4.7 2'3
× × × × ×
1013 10 la l014 1014 1014
1.3 1"4 2.1 1.7 1.6
4,4'-(fl,fl'-Dihydroxypropoxyphenyl)-2,2-propane with fumaric acid
45:55 50:50 55:45 60:40 65:35 70:30
1:5.0 1:4.0 1:3.3 1:2-7 1:2-2 1:1.7
129 231 128 110 72 70
6.0 18.0 6.0 90.0 2.0 13.0
0-255 0.230 0.166 0.215 0-256 0.370
3"6 × 2'0 × 2"9 × 2"4 × 1"2 × 6.6 ×
1013 1014 1012 1013 1013 1012
1.9 2.7 2.0 2.0 1.5 '1.5
phenyl)-2,2-propane with fumaric and phthalic acids
4,4'-(fl,fl'-Dihydroxyethoxyphenyl)-2,2-propane with fumaric a n d adipic acids
I
592
Yu. I.
KORZENEVA
et al.
carried out show, in the case of hard plastics the equilibrium stress is determined n o t only by the density of transverse bonds, but also depends to a considerable extent on the polymer structure. Although it is not possible to go into these TABLE 2. YIELD OF INSOLUBLE FRACTION OF HARDENED RESINS (WEIGHT ~o)
Ratio polyester : : styreno
Polyester from
50:50 60:40 70:30 4,4'-(fl,fl'-Dihydroxyethoxyphenyl)-2,2-propane 4,4"-(fl, fl'-Dihydroxycthoxyphenyl)-2,2-propane phthalic acids 4,4"-(B, fl'-Dihydroxyethoxyphenyl)-2,2-propane tercphthalic acids 4,4'-(p,B'-Dihydroxyethoxyphenyl)-2,2-propanc adipic acids 4,4'-(p,B'-Dihydroxyethoxyphenyl)-2,2-propane sebacic acids 4,4'-(p,B'-Dihydroxyethoxyphcnyl)-2,2-propane
with fumaric acid with fumaric and
98.~
9 8 - 3 96.1
}7.5
9 0 . 4 89.5
}0
99.6
~5.4
8 9 - 5 87.2
15.8 18.8
8 6 . 7 76-7 8 9 . 2 87.2
with fumaric and 99.6
with fumaric and with fumaric and with fumaric acid
interesting questions in more detail here, it should be noted t h a t the equilibrium stress a~ changes in a regular fashion with a change in the concentration of the unsaturated monomer for all the hardened systems investigated by us. With a low styrene concentration, the equilibrium stress is comparatively small, and this m a y be explained by the fact t h a t the amount of the "crosslinking agent" is inadequate. As the amount of styrene is increased, the equilibrium stress rises and reaches a maximum value at a styrene concentration which is equal, as a rule, to 3-4 moles to 1 mole of the polyester unit. With a further increase in the styrene concentration, the equilibrium stress once more decreases because of the plasticizing effect of the long styrene bridges between the polymer chains of the polyester. With an excess of styrene it becomes increasingly possible to form homopolystyrene, which m a y be a separate component in the hardened system and exert a considerable effect on its structure and mechanical properties. In this way, a m a x i m u m is observed on the curve relating the equilibrium stress to the styrene concentration (Fig. 3); the position of the m a x i m u m corresponds to the maximum hardness obtainable in the hardened resin and indicates the optimum styrene concentration required to achieve this hardness. The optimum polyester : styrene ratio was also determined by extracting the soluble fraction from the hardened resins. This method makes it possible for the copolymer formed to be separated from the styrene monomer, polystyrene and homopolyester which have not taken part in the reaction. I t is clear t h a t a smaller q u a n t i t y of extracted products implies t h a t copolymerization has proceeded more nearly to completion. To do this, a finely divided portion
Investigation of hardening of unsaturated polyesters
593
of the hardened resin was placed in a Soxhlet apparatus in which chloroform was used as the solvent. The extraction process was carried out for thirty hours, after which the insoluble fraction was dried to constant weight. Controlled experiments on artificially compounded mixtures established that extraction with chloroform for thirty hours made it possible to separate completely the styrene, polystyrene and homopolyester from the copolymer formed (Table 2). As m a y be seen from Table 2, the best results were obtained in the majority of cases when the p o l y e s t e r : m o n o m e r ratio taken in the reaction was 1 : l, in agreement with the results obtained from the investigation of the relaxation properties of these systems. Apart fi'om the equilibrium stress one m a y also assess the hardness of the resins from their viscosity, which in the case of a solid body, m a y be calculated by means of the equation [9] ~=E0~F (1 ~- l~c),
(2)
where ~/is the viscosity of the hardened specimen; T is the relaxation time; F is the gamma function; k is the parameter in equation (1). The calculations made have shown that the viscosity for all the resins investigated increases regularly as the styrene concentration is increased up to a certain limit corresponding to the optimum styrene concentration, and then the viscosity once more falls off (Table 1). A m a x i m u m thus exists in the relationship showing the dependence of viscosity on the concentration of crosslinking agent, similar to that shown in Fig. 3. Since the positions of the two maxima (for a~ and ~1) are approximately the same, both methods m a y be equally used to characferize the optimum composition of the resin to be hardened. Let us now go on to discuss the most important kinetic parameters which characterize the time for the body to reach a new equilibrium state after being displaced from its initial equilibrium. The calculations show (see Table l) that the relaxation time does not have any fixed dependence on the concentration of the unsaturated monomer. It is known from data in the literature [10] t h a t if a polymeric body is uniform in its structure, then the relaxation time exhibits certain clearly expressed relationships (for example, temperature dependence). If the body is non-uniform, the quantity T m a y take on very diverse values which do not conform to any regular relationships, i.e. this characteristic property is very sensitive to structural non-tmiformity. On the basis of these ideas, the preliminary conclusion m a y be drawn t h a t the hardened resins discussed have a non-uniform structure. I n conclusion, we shall discuss the possibility of approximately assessing the optimum properties of the hardened resin and its composition from the given measurements of stress relaxation under isothermal conditions at constant strain.
594
Yu. I. KORZENEVA et al.
F o r t h i s p u r p o s e w e shall i n t r o d u c e t h e f o l l o w i n g c o n v e n t i o n a l p a r a m e t e r representative of the relaxation processes:* 1
ai
-
(3)
p al-~l'
where 1/p is the reciprocal of the relative fall in stress after one hour's relaxation, a i is the initial stress which is developed in the specimen at the end of the "instantaneously" applied constant strain: ax is the stress after one hour's relaxation. Of course, the harder the polymer, the smaller is the value of the relative fall in stress and the larger is 1/p. Calculations indicate that the way in which 1/p depends on styrene concentration for products resulting from the eopoly-
Time
FIe.
1. Schematic diagram of a typical relaxation curve.
6",trg/em 2 /500'
I000
500
~
I
0
I 28
~
e-
~
~--~-
.,
~
?.
~
?.
,~__.~
I
40
60
Time, rain FIG, 2. Stress relaxation in hardened resins prepared from styrene and the u n s a t u r a t e d polyester based on 4,4'-(fl, fl'-dihydroxycthoxyphonyl)-2,2-propano and fumaric and adipie acids. P o l y e s t e r : styrerie ratio: 1 - - 6 5 : 35; 2 - - 5 5 : 45; 3 - - 4 5 : 55; 4--40:60; 5--35:65; 6--30:70; 7--70:30; 8--75:25. * T h i s parameter has been used previously by one of u s [11].
Investigation of hardening of unsaturated polyesters
595
merization of unsaturated polyesters with styrene passes through a maximum i n a s i m i l a r w a y t o t h e r e l a t i o n s h i p s f o r a ~ a n d g, t h e p o s i t i o n o f t h e s e m a x i m a c o i n c i d i n g o n t h e w h o l e ( F i g . 4). T h i s m e a n s t h a t a n a p p r o x i m a t e l y o p t i m u m a m o u n t o f t h e u u s a t u r a t e d m o n o m e r m a y b e c h o s e n f r o m t h e v a l u e o f 1/fl, w h i c h may be determined rapidly, thus avoiding the fairly long procedure of calculating t h e p a r a m e t e r s i n t h e K o h l r a u s c h e q u a t i o n . A t t h e s a m e t i m e , i t is c o m p l e t e l y
6"oo,/~l/c//72 800
3.0 I
\I
\
20 2 " 0
I.
25
[
I
I
I
f'O
J
I
I
I
6typene ~%
Fro. 3
Fro. 4
75
35
i
55 75 Polyester, %
55
FIG. 3. Dependence of the equilibrium stress ace on the styrene content of hardened resins prepared from: 1--4,4'-(fl,/~'-dihydroxyethoxyphenyl)-2,2-propane with fumarie and adipic acids; 2--4,4'-(fl, fl'-dihydroxyethoxyphenyl)-2,2-propane with fumarie and terephthalic acids; 3--4,4'-(fl,fl'-dihydroxypropoxyphenyl)-2,2-propano with fumarie acid. F~G. 4. Dependence of the reciprocal of the relative fall in stress 1/fl on the styrene content of hardened resins. The u n s a t u r a t e d polyesters are based on 4,4'-(fl,fl'-dihydroxyothoxyphenyl)-2,2-propane with: 1--fumaric and adipic acids; 2--fumaric and terophthalic acids; 3 - - f u m a r i c acid.
i m p o s s i b l e t o a s s e s s t h e p e r m i s s i b l e w o r k i n g regiorL f o r t h e h a r d e n e d r e s i n f r o m t h e v a l u e o f lift, w h e r e a s , k n o w i n g t h e v a l u e o f t h e e q u i l i b r i u m s t r e s s a ~ o n e may use the polymer with confidence over long periods of time in the stress range from 0 to a~.* Therefore, in order to characterize a polymeric material more * The value of a~ determined in this way is specific only for a single given constant strain e0. I n connection with this, in order to characterize completely the permissible working regions for a polymeric material it is necessary to carry out a series of stress relaxation experiments under isothermal conditions by applying different constant strains. Thus the value of a , will increase as the strain increases, b u t this is true only up to a certain limiting increase in strain, since if the strains are too large, stress relaxation will proceed fairly extensively because of the partial breakdown of the material and a~ will again begin to decrease and m a y even reach zero with excessively large strains. I t is therefore necessary at first to find the highest possible value of ace from a series of experiments at a constant temperature, and then to carry out the same experiments and calculations over the entire potential temperature range. The relationship thus obtained between the m a x i m u m equilibrium stress and the t e m p e r a t u r e will delineate a m a x i m u m bound for the mechanical working region of the polymeric material.
596
Yu. I. KO~ZENEVA et ed.
c o m p l e t e l y , it is still n e c e s s a r y to c a r r y o u t t h e calculations referred t o a b o v e in o r d e r to d e t e r m i n e t h e e q u i l i b r i u m elasticity, t h a t is, to derive a v a l u e for a®. CONCLUSIONS
(1) I s o t h e r m a l stress r e l a x a t i o n has b e e n i n v e s t i g a t e d in h a r d e n e d resins b a s e d on t h e u n s a t u r a t e d p o l y e s t e r s f r o m 4,4'-(fl,fl'-dihydroxyethoxyphenyl)2 , 2 - p r o p a n e a n d 4,4'-(fl, fl'-dihydroxypropoxyphenyl)-2,2-propane w i t h f u m a r i c , aclipic, sebacic, p h t h a l i c a n d t e r e p h t h a l i c acids a n d s t y r e n e , t h e c o n c e n t r a t i o n o f t h e u n s a t u r a t e d m o n o m e r being v a r i e d o v e r a wide range; t h e i n v e s t i g a t i o n s s h o w e d t h a t t h e r e l a x a t i o n curves could be well described b y t h e K o h l r a u s c h equation. (2) Calculation o f t h e p a r a m e t e r s in the K o h l r a u s c h e q u a t i o n m a k e s it possible t o assess t h e h a r d n e s s of t h e h a r d e n e d resins a n d to p o i n t o u t t h e o p t i m u m comp o s i t i o n for these resins.
Translated by G. F. MODLE~ REFERENCES
1. 2. 3. 4. 5.
6. 7. 8. 9. 10. 11.
P. J. FLORY, Amer. Chem. Soc. 62: 1057, 1940 J. BRYDSON and L. WELCH, Plastics 29: 323, 1956 M. M. COLLERDEAU, TOURNIAIRE and RUFFIER, Ind. Plast. Mod. 14: 17, 1962 F. KOI~LRAUSCH, Pogg. Ann. 119: 337, 1863 A. A. ASKADSKII, G. L. SLONIMSKII, V. V. KORSItAK, S. V. VINOGRADOVA and Yu. I. KORZENEVA, Vysokomol. soyed. B9: 16, 1967 (Not translated in Polymer Sci. U.S.S.R.) G. A. DUBOV and V. R. REGEL', Zh. tekh. fiziki 25: 2542, 1955 V. I. PAVL()V, A. A. ASKADSKII and G. L. SLONIMSKII, Mekhanika polimerov, No. 6, 15, 1965 L. Z. ROGOVINA, Disser4ation, 1965 G. L. SLONIMSKH, Zh. tekh. fiziki 9: 1719, 1939 V. I. PAVLOV, Dissertation, 1966 A. A. ASKADSKII, Vysokomol. soyed. 8: 1342, 1966 (Translated in Polymer Sci. U.S.S.I~. 8: 8, 1472, 1966)
THE structure of hardened polyester resins is the most important factor determining their properties. The amount of unsaturated monomer used in the copolymerization thus exerts a substantial effect [1-3]. An increase in its concentratior~ up to a certain limit raises the figures for the hardness, mechanical strength, heat and chemical resistance of copolymers and this is directly connected with an increase of the concentration of transverse bonds ill the copolymers. I t is known t h a t in the case of network elastomers the frequency of crosslinking m a y be approximately determined by measurement of stress-relaxation in specimens of these polymers. In this connection, it was of interest to carry out a similar investigation for hard network polymers. This investigation is clearly both of theoretical and also of practical value. It was of interest to find out if the experimental relaxation curves obtained under isothermal conditions with a constant uniaxial compression strain could be described by the Kohlrausch equation [4] and also to calculate the parameters in this equation which characterize the relaxation properties of the polymer. From the practical point of view, it would become possible by carrying out such an investigation to find the optimum amounts of the unsaturated monomer to be introduced into the resin. The use of this approach [5] made it possible to investigate the kinetics of copol)~merization between styrene and the unsaturated polyester based on 4,4'(fl,fl'-dihydroxyethoxyphenyl)-2,2-propane and fumaric acid. The stress-relaxation study was carried out under isothermal conditions at room temperature with a cor~stant compression strain, using a relaxometer based on Regel's system [6], the strain always being 4.8% and applied at a rate of 3 mm/min. Specimens 3 mm in diameter and 4.2 m m high were prepared for the tests. On completion of the development of the strain, the stress was measured at fixed intervals of time for one hour, and as a result of this a typical relaxation curve was obtained; this is shown schematically in Fig. 1, where all the conven* Vysokomol. soyed. All: No. 3, 519-524, 1969. 589
590
Yr. I. KORZE~VA et al.
tional symbols corresponding to the Koh]rausch equation are also shown: G(t)=a ~ -~-(70•e -ark -~G~ +~o e(-t/T)~,
(1)
where a(o is the stress at time t; a~ is the equilibrium stress; a 0 is the component of the stress which undergoes relaxation; a and k are parameters in the equation; is the relaxation time. The calculation of the parameters in the equation was carried out by the m e t h o d previously described [7]. EXPERIMENTAL AND DISCUSSION OF RESULTS
Copolymers of single and of mixed unsaturated polyesters with styrene were used as the experimental materials. The unsaturated polyesters were obtained by the polyeondensation of 4,4'-(fl,fl'-dihydroxyethoxyphenyl)-2,2-propane or 4,4'-(fl,fl'-dihydroxypropoxyphenyl)-2,2propane with fumaric acid or with a mixture of fumaric acid with adipic, sebacic or phthalie acid. Solutions of various concentrations of these polyesters in styrene were hardened in test-tubes in air in the-presence of 1% benzoyl peroxide for 2 hr at 60, 80, 100, 120 and 140°C, after which specimens for testing were machined from the castings obtained. As a result of the experiments carried out, a series of relaxation curves was obtained for specimens of all the copolymers studied, which are shown in Table 1 and in Fig. 2. The hardened resins based on the single and mixed polyesters shown in Table 1 differed both in the chemical structure of the polyester used and also in the styrene content, which was varied in the range from 25 to 75 weight %. I t turned out t h a t all the experimental relaxation curves could be described well by the Kohlrausch equation and this made it possible to calculate the parameters in this equation, the most important of which are shown in Table 1. The parameters of the Kohlransch eqaution m a y be divided into equilibrium and kinetic parameters [8]. The equilibrium stress a~ belongs to the first class: under given test conditions (a definite strain and temperature) this is an important characteristic of the usefulness of the materials obtained, and defines the stress range (from zero to a~) over which the plastic m a y be confidently used as a rigid construction material, for example, in glass-reinforced plastics. Of course, other things being equal, a higher value of a~ implies a higher hardness in the polymer system. In our case, for equal molar contents of styrene the highest value of the equilibrium stress a~ occurred for the mixed polyester of 4,4'-(fl,fl'-dihydroxyethoxyphenyl)-2,2-propane with fumaric and adipic acids, and the smallest value for the single propane polyester with fnmaric acid. This fact can clearly be explained by the fact t h a t a smaller number of double bonds participate i~ the copolymerization reactions in the case of the single polyester based on 4,4'-(fl,fl'-dihydroxypropoxyphenyl)-2,fl-propane, because of the screening effect of the bulky benzene nuclei and the side methyl substituents on the double bonds of the fmnar~te. In the case of the widely-used elastomers, the value of the equilibrium stress depends principally on the density of transverse bonds, e~nd it is precisely these which give rise to the final non-zero equilibrium stress. As the experiments
Investigation of hardening of unsaturated polyesters TABLE 1. C H A N G E I N T H E P A R A M E T E R S O F T H E K O H L R A U S C H THE
STRUCTURE
Polyester from
AND
C01VIPOSITI01~ O F T H E
Ratio polyester : : styrene
EQUATION, DEPENDING
HARDENED
ace,
591 ON
RESINS
~min
poise
1/B
weight
molar
kg/cm3
4,4'-(fl,fl'-Dihydroxyethoxyphenyl)-2,2-propane with f u m a r i c acid
40:60 50:50 55:45 60:40 70:30
1:5"7 1:3"7 1:3-1 1:2.5 1:1-6
209 428 401 195 114
30.5 13.0 37.0 6.0 16-0
0"599 0-300 0"217 0'280 0'611
1"5 × 7"0 × 9"9 × 6-1 × 1-7 ×
1013 1013 1013 10 la l013
1.8 2.3 3.5 1.6 1.3
4,4'-(B,B'-Dihydroxyethoxyphenyl)-2,2-propano with fumaric and terephtalic acids
30:70 40:60 45:55 50:50 55:45 60:40 65:35
1:8"9 1:5-8 1:4.7 1:3.8 1:3.2 1:2.6 1:2.3
112 230 231 329 304 253 252
90'0 6"5 4-5 2"0 1"2 7"2 4"0
0'281 0:198 0.188 0.150 0.133 0.236 0'253
8"7 X 1014 × × × × X ×
10 ~4 10 :a 1015 1015 1015 l014
1.8 1.8 1.9 2.0 2-0 1.8 1.7
8"6 1"1 7"1 2-6 1-0 8"4
l
4,4'-(fl,fl'-Dihydroxyethoxy-
30:70 40:60 50:50 60:40 70:30
1:8'9 1'5"8 1:3.8 1:2.6 1:1-7
364 425 439 292 232
15-0 1.8 22.0 14.0 9.0
0.869 0'274 0-284 0-442 0-408
2"1 × 9"1 × 1"2 × 9"8 • 6'1 ×
1012 1012 1014 1012 1012
2"8 3"3 3.0 2.0 1.6
30:70 35:65 40:60 45:55 55:45 65:35 70:30
1:9"0 1:7-2 1:5'9 1:4'8 1:3"2 1:2-3 1:1"7
473 517 465 464 580 800 450
13.0 6"5 75"0 70-0 9-0 42'0 i 5-5
0.741 0"338 0.382 0.261 0.159 0.395 0.470
2'0 9'4 6-9 4"1 1.2 7"7 3'7
× × × × × × ×
1012 10 r~ 1013 1014 1016 1013 1012
2.9 3.0 3.0 3.3 3.5 2.9 2.8
4,4'-(fl, f l ' - D i h y d r o x y e t h o x y phenyl)-2,2-propane with sebacic acid
30:70 40:60 50:50 60:40 70:30
1:9"4 1:6'1 1:4"0 1:2.7 1:1'7
83 134 170 170 ll0
1'7 4"2 70-0 4.5 33.0
0"277 0"357 0-305 0.198 0.318
1"3 1"5 2'7 4.7 2'3
× × × × ×
1013 10 la l014 1014 1014
1.3 1"4 2.1 1.7 1.6
4,4'-(fl,fl'-Dihydroxypropoxyphenyl)-2,2-propane with fumaric acid
45:55 50:50 55:45 60:40 65:35 70:30
1:5.0 1:4.0 1:3.3 1:2-7 1:2-2 1:1.7
129 231 128 110 72 70
6.0 18.0 6.0 90.0 2.0 13.0
0-255 0.230 0.166 0.215 0-256 0.370
3"6 × 2'0 × 2"9 × 2"4 × 1"2 × 6.6 ×
1013 1014 1012 1013 1013 1012
1.9 2.7 2.0 2.0 1.5 '1.5
phenyl)-2,2-propane with fumaric and phthalic acids
4,4'-(fl,fl'-Dihydroxyethoxyphenyl)-2,2-propane with fumaric a n d adipic acids
I
592
Yu. I.
KORZENEVA
et al.
carried out show, in the case of hard plastics the equilibrium stress is determined n o t only by the density of transverse bonds, but also depends to a considerable extent on the polymer structure. Although it is not possible to go into these TABLE 2. YIELD OF INSOLUBLE FRACTION OF HARDENED RESINS (WEIGHT ~o)
Ratio polyester : : styreno
Polyester from
50:50 60:40 70:30 4,4'-(fl,fl'-Dihydroxyethoxyphenyl)-2,2-propane 4,4"-(fl, fl'-Dihydroxycthoxyphenyl)-2,2-propane phthalic acids 4,4"-(B, fl'-Dihydroxyethoxyphenyl)-2,2-propane tercphthalic acids 4,4'-(p,B'-Dihydroxyethoxyphenyl)-2,2-propanc adipic acids 4,4'-(p,B'-Dihydroxyethoxyphenyl)-2,2-propane sebacic acids 4,4'-(p,B'-Dihydroxyethoxyphcnyl)-2,2-propane
with fumaric acid with fumaric and
98.~
9 8 - 3 96.1
}7.5
9 0 . 4 89.5
}0
99.6
~5.4
8 9 - 5 87.2
15.8 18.8
8 6 . 7 76-7 8 9 . 2 87.2
with fumaric and 99.6
with fumaric and with fumaric and with fumaric acid
interesting questions in more detail here, it should be noted t h a t the equilibrium stress a~ changes in a regular fashion with a change in the concentration of the unsaturated monomer for all the hardened systems investigated by us. With a low styrene concentration, the equilibrium stress is comparatively small, and this m a y be explained by the fact t h a t the amount of the "crosslinking agent" is inadequate. As the amount of styrene is increased, the equilibrium stress rises and reaches a maximum value at a styrene concentration which is equal, as a rule, to 3-4 moles to 1 mole of the polyester unit. With a further increase in the styrene concentration, the equilibrium stress once more decreases because of the plasticizing effect of the long styrene bridges between the polymer chains of the polyester. With an excess of styrene it becomes increasingly possible to form homopolystyrene, which m a y be a separate component in the hardened system and exert a considerable effect on its structure and mechanical properties. In this way, a m a x i m u m is observed on the curve relating the equilibrium stress to the styrene concentration (Fig. 3); the position of the m a x i m u m corresponds to the maximum hardness obtainable in the hardened resin and indicates the optimum styrene concentration required to achieve this hardness. The optimum polyester : styrene ratio was also determined by extracting the soluble fraction from the hardened resins. This method makes it possible for the copolymer formed to be separated from the styrene monomer, polystyrene and homopolyester which have not taken part in the reaction. I t is clear t h a t a smaller q u a n t i t y of extracted products implies t h a t copolymerization has proceeded more nearly to completion. To do this, a finely divided portion
Investigation of hardening of unsaturated polyesters
593
of the hardened resin was placed in a Soxhlet apparatus in which chloroform was used as the solvent. The extraction process was carried out for thirty hours, after which the insoluble fraction was dried to constant weight. Controlled experiments on artificially compounded mixtures established that extraction with chloroform for thirty hours made it possible to separate completely the styrene, polystyrene and homopolyester from the copolymer formed (Table 2). As m a y be seen from Table 2, the best results were obtained in the majority of cases when the p o l y e s t e r : m o n o m e r ratio taken in the reaction was 1 : l, in agreement with the results obtained from the investigation of the relaxation properties of these systems. Apart fi'om the equilibrium stress one m a y also assess the hardness of the resins from their viscosity, which in the case of a solid body, m a y be calculated by means of the equation [9] ~=E0~F (1 ~- l~c),
(2)
where ~/is the viscosity of the hardened specimen; T is the relaxation time; F is the gamma function; k is the parameter in equation (1). The calculations made have shown that the viscosity for all the resins investigated increases regularly as the styrene concentration is increased up to a certain limit corresponding to the optimum styrene concentration, and then the viscosity once more falls off (Table 1). A m a x i m u m thus exists in the relationship showing the dependence of viscosity on the concentration of crosslinking agent, similar to that shown in Fig. 3. Since the positions of the two maxima (for a~ and ~1) are approximately the same, both methods m a y be equally used to characferize the optimum composition of the resin to be hardened. Let us now go on to discuss the most important kinetic parameters which characterize the time for the body to reach a new equilibrium state after being displaced from its initial equilibrium. The calculations show (see Table l) that the relaxation time does not have any fixed dependence on the concentration of the unsaturated monomer. It is known from data in the literature [10] t h a t if a polymeric body is uniform in its structure, then the relaxation time exhibits certain clearly expressed relationships (for example, temperature dependence). If the body is non-uniform, the quantity T m a y take on very diverse values which do not conform to any regular relationships, i.e. this characteristic property is very sensitive to structural non-tmiformity. On the basis of these ideas, the preliminary conclusion m a y be drawn t h a t the hardened resins discussed have a non-uniform structure. I n conclusion, we shall discuss the possibility of approximately assessing the optimum properties of the hardened resin and its composition from the given measurements of stress relaxation under isothermal conditions at constant strain.
594
Yu. I. KORZENEVA et al.
F o r t h i s p u r p o s e w e shall i n t r o d u c e t h e f o l l o w i n g c o n v e n t i o n a l p a r a m e t e r representative of the relaxation processes:* 1
ai
-
(3)
p al-~l'
where 1/p is the reciprocal of the relative fall in stress after one hour's relaxation, a i is the initial stress which is developed in the specimen at the end of the "instantaneously" applied constant strain: ax is the stress after one hour's relaxation. Of course, the harder the polymer, the smaller is the value of the relative fall in stress and the larger is 1/p. Calculations indicate that the way in which 1/p depends on styrene concentration for products resulting from the eopoly-
Time
FIe.
1. Schematic diagram of a typical relaxation curve.
6",trg/em 2 /500'
I000
500
~
I
0
I 28
~
e-
~
~--~-
.,
~
?.
~
?.
,~__.~
I
40
60
Time, rain FIG, 2. Stress relaxation in hardened resins prepared from styrene and the u n s a t u r a t e d polyester based on 4,4'-(fl, fl'-dihydroxycthoxyphonyl)-2,2-propano and fumaric and adipie acids. P o l y e s t e r : styrerie ratio: 1 - - 6 5 : 35; 2 - - 5 5 : 45; 3 - - 4 5 : 55; 4--40:60; 5--35:65; 6--30:70; 7--70:30; 8--75:25. * T h i s parameter has been used previously by one of u s [11].
Investigation of hardening of unsaturated polyesters
595
merization of unsaturated polyesters with styrene passes through a maximum i n a s i m i l a r w a y t o t h e r e l a t i o n s h i p s f o r a ~ a n d g, t h e p o s i t i o n o f t h e s e m a x i m a c o i n c i d i n g o n t h e w h o l e ( F i g . 4). T h i s m e a n s t h a t a n a p p r o x i m a t e l y o p t i m u m a m o u n t o f t h e u u s a t u r a t e d m o n o m e r m a y b e c h o s e n f r o m t h e v a l u e o f 1/fl, w h i c h may be determined rapidly, thus avoiding the fairly long procedure of calculating t h e p a r a m e t e r s i n t h e K o h l r a u s c h e q u a t i o n . A t t h e s a m e t i m e , i t is c o m p l e t e l y
6"oo,/~l/c//72 800
3.0 I
\I
\
20 2 " 0
I.
25
[
I
I
I
f'O
J
I
I
I
6typene ~%
Fro. 3
Fro. 4
75
35
i
55 75 Polyester, %
55
FIG. 3. Dependence of the equilibrium stress ace on the styrene content of hardened resins prepared from: 1--4,4'-(fl,/~'-dihydroxyethoxyphenyl)-2,2-propane with fumarie and adipic acids; 2--4,4'-(fl, fl'-dihydroxyethoxyphenyl)-2,2-propane with fumarie and terephthalic acids; 3--4,4'-(fl,fl'-dihydroxypropoxyphenyl)-2,2-propano with fumarie acid. F~G. 4. Dependence of the reciprocal of the relative fall in stress 1/fl on the styrene content of hardened resins. The u n s a t u r a t e d polyesters are based on 4,4'-(fl,fl'-dihydroxyothoxyphenyl)-2,2-propane with: 1--fumaric and adipic acids; 2--fumaric and terophthalic acids; 3 - - f u m a r i c acid.
i m p o s s i b l e t o a s s e s s t h e p e r m i s s i b l e w o r k i n g regiorL f o r t h e h a r d e n e d r e s i n f r o m t h e v a l u e o f lift, w h e r e a s , k n o w i n g t h e v a l u e o f t h e e q u i l i b r i u m s t r e s s a ~ o n e may use the polymer with confidence over long periods of time in the stress range from 0 to a~.* Therefore, in order to characterize a polymeric material more * The value of a~ determined in this way is specific only for a single given constant strain e0. I n connection with this, in order to characterize completely the permissible working regions for a polymeric material it is necessary to carry out a series of stress relaxation experiments under isothermal conditions by applying different constant strains. Thus the value of a , will increase as the strain increases, b u t this is true only up to a certain limiting increase in strain, since if the strains are too large, stress relaxation will proceed fairly extensively because of the partial breakdown of the material and a~ will again begin to decrease and m a y even reach zero with excessively large strains. I t is therefore necessary at first to find the highest possible value of ace from a series of experiments at a constant temperature, and then to carry out the same experiments and calculations over the entire potential temperature range. The relationship thus obtained between the m a x i m u m equilibrium stress and the t e m p e r a t u r e will delineate a m a x i m u m bound for the mechanical working region of the polymeric material.
596
Yu. I. KO~ZENEVA et ed.
c o m p l e t e l y , it is still n e c e s s a r y to c a r r y o u t t h e calculations referred t o a b o v e in o r d e r to d e t e r m i n e t h e e q u i l i b r i u m elasticity, t h a t is, to derive a v a l u e for a®. CONCLUSIONS
(1) I s o t h e r m a l stress r e l a x a t i o n has b e e n i n v e s t i g a t e d in h a r d e n e d resins b a s e d on t h e u n s a t u r a t e d p o l y e s t e r s f r o m 4,4'-(fl,fl'-dihydroxyethoxyphenyl)2 , 2 - p r o p a n e a n d 4,4'-(fl, fl'-dihydroxypropoxyphenyl)-2,2-propane w i t h f u m a r i c , aclipic, sebacic, p h t h a l i c a n d t e r e p h t h a l i c acids a n d s t y r e n e , t h e c o n c e n t r a t i o n o f t h e u n s a t u r a t e d m o n o m e r being v a r i e d o v e r a wide range; t h e i n v e s t i g a t i o n s s h o w e d t h a t t h e r e l a x a t i o n curves could be well described b y t h e K o h l r a u s c h equation. (2) Calculation o f t h e p a r a m e t e r s in the K o h l r a u s c h e q u a t i o n m a k e s it possible t o assess t h e h a r d n e s s of t h e h a r d e n e d resins a n d to p o i n t o u t t h e o p t i m u m comp o s i t i o n for these resins.
Translated by G. F. MODLE~ REFERENCES
1. 2. 3. 4. 5.
6. 7. 8. 9. 10. 11.
P. J. FLORY, Amer. Chem. Soc. 62: 1057, 1940 J. BRYDSON and L. WELCH, Plastics 29: 323, 1956 M. M. COLLERDEAU, TOURNIAIRE and RUFFIER, Ind. Plast. Mod. 14: 17, 1962 F. KOI~LRAUSCH, Pogg. Ann. 119: 337, 1863 A. A. ASKADSKII, G. L. SLONIMSKII, V. V. KORSItAK, S. V. VINOGRADOVA and Yu. I. KORZENEVA, Vysokomol. soyed. B9: 16, 1967 (Not translated in Polymer Sci. U.S.S.R.) G. A. DUBOV and V. R. REGEL', Zh. tekh. fiziki 25: 2542, 1955 V. I. PAVL()V, A. A. ASKADSKII and G. L. SLONIMSKII, Mekhanika polimerov, No. 6, 15, 1965 L. Z. ROGOVINA, Disser4ation, 1965 G. L. SLONIMSKH, Zh. tekh. fiziki 9: 1719, 1939 V. I. PAVLOV, Dissertation, 1966 A. A. ASKADSKII, Vysokomol. soyed. 8: 1342, 1966 (Translated in Polymer Sci. U.S.S.I~. 8: 8, 1472, 1966)