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Kinetics of the stabilizing effect of calcium and zinc stearates in the thermal degradation of PVC: Part I
Polymer Degradation and Stability 25 (1989) 61-72
Kinetics of the Stabilizing Effect of Calcium and Zinc Stearates in the Thermal Degradation of PVC: Part I
Gy. L6vai, Gy. Ocskay, Zs. Nyitrai & G. Meszl6nyi Research and Development Company for the Organic Chemical Industry, H-1428 Budapest, POB 41, Hungary (Received 15 August, 1988; accepted 30 August 1988)
ABSTRACT A kinetic study of data obtained by infra-red spectroscopic measurements of carbonyl groups formed during the thermal degradation of P VC sheets, has shown that the replacement of the allyl-activated chlorine atoms by the more stable carboxylate groups originating from the Ca stearate/Zn stearate (CaStz/ZnSt2) system adequately explains the stabilizing effect of these stabilizers. Kinetic equations have been derived, suitable for calculation of the stabilizer consumption as well as the formation of stearic acid and ester groups during the degradation period. On comparing the carboxylate absorbances obtained in the infra-red spectra of sheets containing CaSt 2 or ZnSt z alone, with those containing mixtures of the stearates, the formation of the complex CaSt[ZnSt3] was inferred. The quantitative interpretation of the infra-red spectroscopic data has shown that this complexation is nearly complete.
INTRODUCTION The purpose of the present work was to clarify the chemistry of processes which inhibit the thermal degradation of PVC stabilized by Ca and Zn stearates, and to explain the synergistic effect observed when mixtures of these salts are used as heat stabilizers. The work was based on a kinetic analysis o f data obtained by infra-red spectral measurements o f thermally 61 Polymer Degradation and Stability 0141-3910/89/$03"50 © 1989 Elsevier Science Publishers Ltd, England. Printed in Great Britain
62
Gy. Lbvai, Gy. Ocskay, Zs. Nyitrai, G. Meszlbnyi
degraded PVC sheets containing Ca and/or Zn stearate. The degradation was carried out at 190°C in air.
EXPERIMENTAL Sheets were prepared by milling, at 180°C for 5min, blends containing suspension PVC Ongrovil S 5058, a Hungarian trade product having a K value of 58, and different quantities o f various mixtures o f CaSt 2 and ZnSt 2. Pentaerythritol (Pe) was usually used in a great molar excess to hinder the detrimental effect of ZnC12 formed during the degradation. The CaSt 2, ZnSt 2 and Pe used in these experiments were of technical grade quality. Compositions o f the stabilizer mixtures are given in Table 1. Samples of sheets, degraded at 190 + I°C in air, were withdrawn at intervals and were used directly for infra-red spectroscopic measurements. The infra-red spectra were evaluated by the base line m e t h o d and absorbances were corrected for differences in sheet thickness (0.04-0.06 cm); a Perkin-Elmer 577 spectrophotometer was used. Since the absorbances evaluated in this way were too large for quantitative treatment, in order to obtain smaller maxima, the sample holder of the spectrophotometer was equipped with a window having a cross section of 3 x 7 mm. Induction times, ~, were determined by measuring the time necessary for the blackening of the samples or by registering the time at which hydrogen chloride formation started. The hydrogen chloride formed was swept by a stream of air, into a 0.1N solution o f KC1 in water and the pH o f this solution was recorded continuously. The induction time for hydrogen chloride TABLE 1
Composition of Stabilizer Mixtures Sample
x° (mmol/mon a)
ZnStz/CaSt 2 (mol/mol)
Pe/ZnSt 2 (mol/mol)
CZ-1 CZP-6 CZP-15 CZP-19 CZP-37 CZP-49 CZP-50 CZP-53
2.30 2.83 3"11 3"36 2"37 2-01 1-65 2.88
1.23 0-72 0.18 0"27 1'24 1.44 0b 0'72
5.8 4"64 9"30 4.54 5"80 4'64 0 2.32
a mmol/mol monomer unit. b CaSt 2 only.
Effect o f cale&m and zinc stearates on PVC degradation
63
I
10
Fig. 1.
20 30 T i m e (rnin)
40
Graphic method for the determination of r by linear extrapolation of the hydrogen chloride evolution vs time curve.
formation was taken as the intercept on the time axis of the linear portion of the HC1 vs time plot (graphic method) as shown in Fig. 1.
I N F R A - R E D SPECTROSCOPIC M E A S U R E M E N T S A large number of publications deal with infra-red spectra of PVC stabilized with metal carboxylates. Only one, however, has been found in which the quantitative interpretation of the data of these spectra is discussed.~ This publication, in which the characteristic absorbances of carboxylate ions were used for the estimation of stabilizer consumption during the processing of PVC, gave unsatisfactory results. We have persisted with infra-red spectral work because we observed that the absorbances of mixtures of Caand Zn-stearates in PVC sheets did not correspond to the sum of the absorbances of the two components added separately. This may be seen clearly in Fig. 2, in which the infra-red spectra of PVC sheets containing Caor Zn-stearate (Fig. 2a) and their mixture (Fig. 2b) are presented. Comparison of the two spectra in Fig. 2b clearly shows that in the range of 1550-1630 cm-x, the measured absorbances are considerably greater than those obtained by calculation. Since this increase in the measured absorbances was observed just in that range in which Zn-stearate absorbs relatively weakly (1550-1630 c m - 1), we explained this increase by assuming that some of the stearate ions of the Ca-stearate were transferred as complex ligands to the Zn ions of zinc stearate: St- + ZnSt 2 ~-- ZnSt 3
(1)
The stearate anions (St-) are formed by the dissociation of the metal stearates. The CaSt* ion resulting from this dissociation may react further
64
Gy. L~vai, Gy. Ocskay, Zs. Nyitrai, G. MeszlOnyi i"
i
I
L
o/
(a)
g <
/
Complex St- ions
(b)
I 1600
1500
Wave n u m b e r ( c m "1)
Fig. 2. Representation of the complexation of CaSt 2 and Z n S t 2 by the comparison of measured and calculated infra-red spectra of their mixtures. (a) Spectra of CaSt 2 ( ) and Z n S t 2 ( - - - ) ; (b) spectra of CaSt 2 and Z n S t 2 mixtures: - - - , measured; . . . . , calculated. with the ZnSt 3 complex ion to give the molecular complex CaSt[ZnSt3]: CaSt + + ZnSt3 ~
CaSt[ZnSt3]
(2)
If the dissociation of the complex C a S t [ Z n S h ] is negligible, the complex exists practically completely in the molecular form. The idea o f the role o f complexation o f CaSt2-ZnSt2 and BaSt2-CdSt2 systems in the stabilization in PVC is not new. 2 However, the quantitative treatment o f the equilibrium of this complexation has not been reported. The equilibrium o f the formation o f CaSt[ZnSt3] m a y be written formally as follows:
Ca \/
stl,,o Stis7s
+ Zn /\
st,,,o
~
Stl6oo
Ca I StlsTs
[ st16oo 1 Zn~Stx6oo
(3)
"Stts,o
where indexes 1540, 1575 and 1600 denote the characteristic wave numbers of the corresponding stearate ions in the infra-red spectra. According to this scheme the St 1540 anion o f CaSt 2 is transferred to the Z n stearate and coordinated as a n 5 t 1 6 o o complex ligand to the central Zn ion.
Effect o f calcium and zinc stearates on PVC degradation
65
This could be the explanation of the observed increase of absorbance (the shaded area in Fig. 2b) in the 1550-1600cm -1 region of the infra-red spectrum o f the mixture. Assuming that the amounts of stearate ions, St154o and St1575 , in the CaSt 2, and St154o and St16oo in the Zn stearate are equal, we can calculate the absorbances corresponding to wave numbers 1540, 1575 and 1600 cm - i; in accordance with eqn (3): A1540 _ [CaStE] + [ZnSt2] + [C] /31540
(4)
A1575 --- [CaStE] + [C]
(5)
/31575 A16oo -
-
- [ZnSt2] + 2[C]
(6)
/31600
where [CaStE], [ZnStE] and [ C ] are the equilibrium concentrations of the two metal stearates and complex, respectively and/31540, e1575 and/3160o denote the corresponding extinction coefficients. Since, CaSt ° = [CaStE] + [C] (7) and
ZnSt2° = [ZnSt2] + [C]
(8)
eqns (4), (5) and (6) can be written in the form: A154o = CaSt o + ZnSt o _ [C] /31540
(9)
A 1575 _ CaSt02 /31575
(10)
Ax6°° - ZnSt ° + [ C ] /31600
(11)
where CaSt ° and ZnSt ° denote the initial concentrations of the two metal stearates. To make possible the quantitative interpretation of our infra-red spectroscopic data regarding the stearate groups, we derived equations expressing the concentrations of CaSt ° + ZnSt ° and [C], respectively, by employing eqns (9), (10) and (11). Thus we obtained: A1575 A 1575 A 1600 2(CaSt ° + ZnSt °) = - + + - (12) e15,0 /31575 /31600 A1575 A1600 A1545 --6 [C] = ~1575
/31600 2
/31545
(13)
66
Gy. l_~vai, Gy. Ocskay, Zs. Nyitrai, G. Meszl~,nyi
TABLE 2 Values o f x ° and [C] Obtained from Infra-red Spectroscopic Measurements, Compared with Weighed Amounts of x ° and with [CI, Calculated by Eqn (15)
Sample
x° (mmol/mon) (weighed)
x° (mmol/mon) (IR spectr.) eqn (12)
[C] (mmol/mon) (IR spectr.) eqn (13)
If] (mmol/mon) (calcd.) eqn (15)
CZ- 1 CZP-6 CZP-15 CZP-19 CZP-37 CZP-49 CZP-53 CZP-50
2"32 2"84 3"13 3"38 2"37 2.01 2-82 1"65
2"09 2-57 3-53 3"51 2'32 2"15 3"20 1"65
0-51 0.43 0-29 0"39 0"49 0.44 0.45 0
0"50 0"57 0'24 0"37 0"50 0'39 0"56 0
In Table 2 we have collected data for 2(CaSt2° + ZnSt °) and [C], obtained by substituting the appropriate infra-red absorbances into eqns (12) and (13). These data are compared in Table 2 with values o f 2(CaSt ° + ZnSt °) = x ° obtained by weighing, and with values o f [C-I, calculated assuming complexation to be controlled by the equilibrium: [CaSt2]. [ZnSt2] = Kd[C ]. On substituting [CaSt2] and [ZnSt2] by CaSt ° and ZnSt ° according to eqns (7) and (8) we have: (CaSt2° - [C]). (ZnSt ° - [C]) = Kd[C ]
(14)
where K a is the dissociation constant o f the complex. The solution o f eqn (14) is given by eqn (15) l-C] =
b - x/b 2 - 4. CaSt2° . ZnSt ° 2
(15)
where b = CaSt ° + ZnSt ° + K d. In these calculations, the extinction coefficient e 1575 obtained from eqn (10) was used. els4o and g156o were evaluated from the ratio AI575/Als4o, taken from the corresponding data o f the infra-red spectra o f Ca stearate, and employing eqn (12). The amounts o f stearic acid and ester groups formed during the degradation o f the PVC sheets were evaluated using the peaks which appear at wave numbers 1710cm -1 and 1745cm -1, respectively. These bands c o r r e s p o n d to the dimeric and m o n o m e r i c forms o f stearic acid respectively, 3 and ester groups in the samples. The separation o f peaks at
Effect o f calcium and zinc stearates on P V C degradation
10
20
67
30
Time (min)
Fig. 3.
Desorption curve of stearic acid in PVC at 190°C.
1745cm -1 was carried out by plotting the absorbances obtained at 1745cm -1 as a function of absorbances appearing at 1710cm -~, and comparing them with a corresponding curve obtained by using a sheet containing known amounts of stearic acid. This method was described in our previous paper. 4 The extinction coefficients for stearic acid and ester groups were determined from calibration curves obtained from the infra-red spectra of sheets containing known amounts of stearic acid and poly(vinyl acetate). We examined the material balance of carbonyl groups in PVC sheets degraded for various times, by comparing the initial carbonyl group content in the stabilizer systems with the sum of the quotients Ai/~ i of absorbances and extinction coefficients attributed to carboxylate ions, stearic acid and ester groups (eqns (4), (5) and (6)). It was found that the carbonyl group concentrations determined from the sum of carbonyl absorbances in the infra-red spectra were somewhat smaller than the measured values. Desorption experiments performed on PVC sheets containing only stearic acid have shown that the lack in carbonyl balance is due to the relatively high volatility of stearic acid. Figure 3 shows the desorption of stearic acid used alone in PVC at 190°C.
M E C H A N I S M A N D KINETICS OF S T A B I L I Z A T I O N The starting point for our kinetic analysis was the mechanism proposed first by Frye and Horst, 5'6 and widely accepted by others. 7'8 According to this, Ca and Zn stearates react with the allylic chlorine atoms in PVC, replacing them by stearate groups. Owing to the greater stability of these ester groups,
Gy. Lbvai, Gy. Ocskay, Zs. Nyitrai, G. Meszlbnyi
68
the chain propagation step of dehydrochlorination is believed to be hindered. __CH2(CH=CH).CHCH2__ heat ..~..CHz(CH=CH).+ICHCH2~ + HCI ester CaSC1
C1
formation [ ZnSt2 (16) ~CH2(CH=CH).CHCH2~ OCOR R=C17H35 The distribution of products (stearic acid, ester groups, unchanged stearate groups in the Ca and Zn stearates) formed from the stabilizers is expressed by the material balance equation: x ° = x+y+s
(17)
where x ° and x are the initial and actual concentrations, respectively, of CaSt 2 and/or ZnSt2, y denotes the concentration of ester groups, and s that of stearic acid. All concentrations are expressed in stearate equivalents. Based on this mechanism, the differential equations (18), (19) and (20) have been derived which describe the rates of formation of polyene sequences containing the allyl-activated chlorine atom (z*), as well as of ester groups and stearic acid, d2* dt
= w - kxz* - k3exz* I p
(18)
chain termination -~tt = k 3 o t x z *
ds dt -
dHC1 dt
- k2z*
(19) (20)
where HC1 is the concentration of the hydrogen chloride formed, w represents the rate of initiation, and kl, k2 and k3 are the rate constants of the termination and propagation steps and of ester formation, respectively. is x - ~ x , the molar ratio of stearate ions (x-), formed by the dissociation of Ca and Zn stearate, to the unreacted stearate groups (x) in the stabilizer system. The introduction of • was necessary for the calculation of the concentrations of stearate ions x-, assuming that the substitution of allylic chlorine atoms is of ionic character, having the rate k 3 x - z * (see eqn (19)).
Effect of calcium and zinc stearates on P VC degradation
69
Equations (18), (19) and (20) were reported previously. 9 The decay of ester formation by acidolysis, assumed previously, 9 however, was not confirmed by the data of our IR spectral measurements and was not therefore taken into account in eqn (19). Substituting z* from eqn (20) in eqn (18) and applying the steady state condition, dz*/dt .,~ O, leads, after integration, to eqn (21): s = HC1 =
k2
k2
--t-- ~y Wkl
(21)
Equation (21) describes the inhibiting action of the ester groups on stearic acid or hydrogen chloride formation. The relationship between y and s can be expressed by eqns (19), (20) and (17). After integration we obtain:
Y+S=(X°W~k3)[1-exp(-°~k3s~k2
~]A
(22,
The induction time for hydrogen chloride formation is widely used for the estimation of the activities of heat stabilizers. Since at time z, x = 0, and x ° = st + y~, z m a y be expressed by the alternative use of eqn (21) and eqn (22) as follows: z=
x° F k z / k l - 1 /3
v
y
x ° + ( K - 1) I x ° L z --
(23) k2
// k3 0
(24)
where K=k2/kl and v = wK denotes the rate of hydrogen chloride formation in unstabilized PVC (see eqn (21) if y = 0). Estimation of k2/kx, which is necessary for the evaluation of k3/k2, was described previouslyff using eqn (23), expressed by the reduced coordinates z/y~ and x°/y~. Values of aka/k 2 were estimated in two independent ways: (a)
F r o m data for y,, obtained from the infra-red spectroscopic measurements described in this paper. Applying eqn (22) for conditions t = v and x ° = y, + st we obtain: y = x°
1 - -
ctk3/k a
In [offka/k2)x ° + 1]
(25)
~ka/k 2 was evaluated from eqn (25) by using the 'trial and error' method.
Gy. IAvai, Gy. Ocskay, Zs. Nyitrai, G. Meszlbnyi
70
TABLE 3 Comparison of Values of ~tka/k2 Calculated from y, (Denoted by Ky) with Those Calculated from z (Denoted by K,)
Sample
CZ-1 CZP-6 CZP-15 CZP- 19 CZP-37 CZP-49 CZP-50 CZP-53
x°
y,
"C
(mmol/min)
(mmol/min)
(rain)
2-30 2-83 3.11 3-38 2-37 2.01 1.65 2-88
0.36 0.38 0-40 0"31 0-42 0.55 0.29 0'38
20 22 25 23 20 17 13 18
Ky
K,
(retool- 1 rain) (retool- 1 min) 0.17 0.11 0"10 0.06 0.19 0.40 0.26 0.15
0.31 0.150 0.16 0.06 0.26 0.31 0.26 0-02a
a The extraordinarily small value of K~ is due to the relatively small induction time of sample CZP-53, caused by the catalytic action ofZnC12 formed during the degradation. This sample contained a smaller quantity ofpentaerythritol than the others (see Table 1); Pe/ZnSt2 = 2.32, which was insufficient to hinder the deleterious action of ZnC12.
(b) From the data for induction time, z, by a method similar to that in (a), but using eqn (24). Values of v = 0.17 mmol/min and K = 2.9 were taken from our previous paper 4 for the evaluation of o~k3/k 2. Values of o~k3/k 2 evaluated from y, (denoted by Ky) and from -r (denoted by K0 are presented in Table 3. Values of Ky and K, in Table 3, however, seem not to correspond exactly. The deviations observed between these two sets of data, due to the non-isothermal initial conditions of heating of the samples, can be decreased by taking into account the time of warming up to the desired degradation temperature. This time, usually 2-3 min, increases the induction time and must be subtracted from the time data used, when measured values of x, y and s are compared with calculated values, resulting from eqns (17), (21) and (22). (The values o f s were obtained from infra-red data and the material balance equation (17).) Results of such calculations are shown in Fig. 4. The differences between the calculated and measured values in Fig. 4 do not exceed the normal error of infra-red spectroscopic measurements ( + 1.5 mol/mon 10-4). Thus we can conclude that our kinetic equations are suitable for the calculation of the consumption of stabilizers or the formation of stearic acid and ester groups during the degradation. Values of K r and K~ in Table 3 also show a considerable systematic dependence on the quantities and compositions of stabilizers. The explanation of this dependence requires some more knowledge about
Effect of calcium
and zinc stearates on P V C degradation
CZP-37 3
3 '
CZP- 6 ~X
\
~x
71
f
x x
/
\,x//o ~-
E 0
10
I
2O
I
30
0
10
20
30
[~,x CZP-19
I
x
y
2 /'¢
,
/ \ I
0
10
20
30
/ 0
Time (min)
~ 10
I 20
3O
Fig. 4. Comparison of measured and calculated values o f x, y, a n d s in thermally degraded P V C sheets, using the constants K r ( ) a n d K, ( . . . . . ) of ester formation, respectively. + , x; O,y;
C), s.
the molecular state of stabilizer systems at elevated temperatures. This problem, which is closely related to the synergism of stabilizer systems containing two metal stearates, will be discussed in a following paper.
CONCLUSION The application of kinetic equations for data relating to the time dependence of stabilizer consumption, formation of ester groups and stearic acid during the degradation of PVC, convincingly confirmed the mechanism proposed by Frye and Horst, according to which the stabilizing effect of CaSt 2 and ZnSt 2 is attributed to the replacement of the allyl-activated chlorine atoms by the more stable ester groups. Our kinetic equations, used for these calculations, contained only the rate constants of the individual steps of heat
72
Gy. I_~vai, Gy. Ocskay, Zs. Nyitrai, G. Meszl~nyi
degradation and stabilization, the rate of initiation, w, the rate constants k~ and k 2 of the termination and propagation steps of polyene formation, respectively, and the constant of ester group formation aka/k 2. The stabilizer system was represented in our calculations only by the initial stearate group content, x °, in the system. The ratio, ZnSt2/CaSt2, of the two stearates, however, was not taken into account; in consequence, a variation of the relative rate constant otka/k 2 was observed (Table 3). This variation was found to be caused not only by the variation of the ratio of the stabilizers, but also by the difference of the stearate group contents. We have tried to express these effects by introducing the degree of dissociation of the metal stearates, 0~,into the c o n s t a n t otk3/k2, assuming that the substitution of allylic chlorine atoms is of ionic nature. This approach seemed to be supported by our observation that CaSt 2 and ZnSt 2 form a considerably stable complex, having the composition CaSt[ZnSt3]. We assumed that this complexation would hinder the dissociation of the metal stearates, allowing only the metal stearate present in excess to dissociate. Accordingly, stabilization effected by stearate ions would be increased by increasing the excess of one of the metal stearates. However, this is contrary to experience, since molar ratios of ZnStE/CaSt2 close to 1 "1 were generally found to be the most effective in the stabilization of PVC. This contradiction will be resolved in a forthcoming paper, which will deal with the role and effects of ions on the rate of stabilizing processes in stabilized PVC systems.
REFERENCES 1. Mackenzie, M. W., Willis, H. A., Oven, R. C. & Michel, A., Eur. Polym. J., 19 (1983) 511. 2. Volka, K., Vymazal, Z., Starek, J. & Seidl, V., Eur. Polym. J., 18 (1982) 219. 3. Volka, K., Czako, L. & Vymazal, Z., Eur. Polym. J., 16 (1980) 149. 4. L6vai, Gy. & Ocskay, Gy., Die Angewandte Macromolekulare Chemie, 137 (1985) 37. 5. Frye, A. H. & Horst, R. W., J. Polym. Sci., 40 (1959) 419. 6. Frye, A. H. & Horst, R. W., J. Polym. Sci., 45 (1960) 1. 7. Braun, D. & Hepp, D., J. Polym. Sci., 33 (1971) 307. 8. Onozuka, M. & Asahina, M., J. Macromol. Sci. Rev. Macromol. Chem. 123 (1969) 235. 9. L6vai, Gy. & Ocskay, Gy., J. Makromol. Sci-Chem., A12 (1978) 467.
Kinetics of the Stabilizing Effect of Calcium and Zinc Stearates in the Thermal Degradation of PVC: Part I
Gy. L6vai, Gy. Ocskay, Zs. Nyitrai & G. Meszl6nyi Research and Development Company for the Organic Chemical Industry, H-1428 Budapest, POB 41, Hungary (Received 15 August, 1988; accepted 30 August 1988)
ABSTRACT A kinetic study of data obtained by infra-red spectroscopic measurements of carbonyl groups formed during the thermal degradation of P VC sheets, has shown that the replacement of the allyl-activated chlorine atoms by the more stable carboxylate groups originating from the Ca stearate/Zn stearate (CaStz/ZnSt2) system adequately explains the stabilizing effect of these stabilizers. Kinetic equations have been derived, suitable for calculation of the stabilizer consumption as well as the formation of stearic acid and ester groups during the degradation period. On comparing the carboxylate absorbances obtained in the infra-red spectra of sheets containing CaSt 2 or ZnSt z alone, with those containing mixtures of the stearates, the formation of the complex CaSt[ZnSt3] was inferred. The quantitative interpretation of the infra-red spectroscopic data has shown that this complexation is nearly complete.
INTRODUCTION The purpose of the present work was to clarify the chemistry of processes which inhibit the thermal degradation of PVC stabilized by Ca and Zn stearates, and to explain the synergistic effect observed when mixtures of these salts are used as heat stabilizers. The work was based on a kinetic analysis o f data obtained by infra-red spectral measurements o f thermally 61 Polymer Degradation and Stability 0141-3910/89/$03"50 © 1989 Elsevier Science Publishers Ltd, England. Printed in Great Britain
62
Gy. Lbvai, Gy. Ocskay, Zs. Nyitrai, G. Meszlbnyi
degraded PVC sheets containing Ca and/or Zn stearate. The degradation was carried out at 190°C in air.
EXPERIMENTAL Sheets were prepared by milling, at 180°C for 5min, blends containing suspension PVC Ongrovil S 5058, a Hungarian trade product having a K value of 58, and different quantities o f various mixtures o f CaSt 2 and ZnSt 2. Pentaerythritol (Pe) was usually used in a great molar excess to hinder the detrimental effect of ZnC12 formed during the degradation. The CaSt 2, ZnSt 2 and Pe used in these experiments were of technical grade quality. Compositions o f the stabilizer mixtures are given in Table 1. Samples of sheets, degraded at 190 + I°C in air, were withdrawn at intervals and were used directly for infra-red spectroscopic measurements. The infra-red spectra were evaluated by the base line m e t h o d and absorbances were corrected for differences in sheet thickness (0.04-0.06 cm); a Perkin-Elmer 577 spectrophotometer was used. Since the absorbances evaluated in this way were too large for quantitative treatment, in order to obtain smaller maxima, the sample holder of the spectrophotometer was equipped with a window having a cross section of 3 x 7 mm. Induction times, ~, were determined by measuring the time necessary for the blackening of the samples or by registering the time at which hydrogen chloride formation started. The hydrogen chloride formed was swept by a stream of air, into a 0.1N solution o f KC1 in water and the pH o f this solution was recorded continuously. The induction time for hydrogen chloride TABLE 1
Composition of Stabilizer Mixtures Sample
x° (mmol/mon a)
ZnStz/CaSt 2 (mol/mol)
Pe/ZnSt 2 (mol/mol)
CZ-1 CZP-6 CZP-15 CZP-19 CZP-37 CZP-49 CZP-50 CZP-53
2.30 2.83 3"11 3"36 2"37 2-01 1-65 2.88
1.23 0-72 0.18 0"27 1'24 1.44 0b 0'72
5.8 4"64 9"30 4.54 5"80 4'64 0 2.32
a mmol/mol monomer unit. b CaSt 2 only.
Effect o f cale&m and zinc stearates on PVC degradation
63
I
10
Fig. 1.
20 30 T i m e (rnin)
40
Graphic method for the determination of r by linear extrapolation of the hydrogen chloride evolution vs time curve.
formation was taken as the intercept on the time axis of the linear portion of the HC1 vs time plot (graphic method) as shown in Fig. 1.
I N F R A - R E D SPECTROSCOPIC M E A S U R E M E N T S A large number of publications deal with infra-red spectra of PVC stabilized with metal carboxylates. Only one, however, has been found in which the quantitative interpretation of the data of these spectra is discussed.~ This publication, in which the characteristic absorbances of carboxylate ions were used for the estimation of stabilizer consumption during the processing of PVC, gave unsatisfactory results. We have persisted with infra-red spectral work because we observed that the absorbances of mixtures of Caand Zn-stearates in PVC sheets did not correspond to the sum of the absorbances of the two components added separately. This may be seen clearly in Fig. 2, in which the infra-red spectra of PVC sheets containing Caor Zn-stearate (Fig. 2a) and their mixture (Fig. 2b) are presented. Comparison of the two spectra in Fig. 2b clearly shows that in the range of 1550-1630 cm-x, the measured absorbances are considerably greater than those obtained by calculation. Since this increase in the measured absorbances was observed just in that range in which Zn-stearate absorbs relatively weakly (1550-1630 c m - 1), we explained this increase by assuming that some of the stearate ions of the Ca-stearate were transferred as complex ligands to the Zn ions of zinc stearate: St- + ZnSt 2 ~-- ZnSt 3
(1)
The stearate anions (St-) are formed by the dissociation of the metal stearates. The CaSt* ion resulting from this dissociation may react further
64
Gy. L~vai, Gy. Ocskay, Zs. Nyitrai, G. MeszlOnyi i"
i
I
L
o/
(a)
g <
/
Complex St- ions
(b)
I 1600
1500
Wave n u m b e r ( c m "1)
Fig. 2. Representation of the complexation of CaSt 2 and Z n S t 2 by the comparison of measured and calculated infra-red spectra of their mixtures. (a) Spectra of CaSt 2 ( ) and Z n S t 2 ( - - - ) ; (b) spectra of CaSt 2 and Z n S t 2 mixtures: - - - , measured; . . . . , calculated. with the ZnSt 3 complex ion to give the molecular complex CaSt[ZnSt3]: CaSt + + ZnSt3 ~
CaSt[ZnSt3]
(2)
If the dissociation of the complex C a S t [ Z n S h ] is negligible, the complex exists practically completely in the molecular form. The idea o f the role o f complexation o f CaSt2-ZnSt2 and BaSt2-CdSt2 systems in the stabilization in PVC is not new. 2 However, the quantitative treatment o f the equilibrium of this complexation has not been reported. The equilibrium o f the formation o f CaSt[ZnSt3] m a y be written formally as follows:
Ca \/
stl,,o Stis7s
+ Zn /\
st,,,o
~
Stl6oo
Ca I StlsTs
[ st16oo 1 Zn~Stx6oo
(3)
"Stts,o
where indexes 1540, 1575 and 1600 denote the characteristic wave numbers of the corresponding stearate ions in the infra-red spectra. According to this scheme the St 1540 anion o f CaSt 2 is transferred to the Z n stearate and coordinated as a n 5 t 1 6 o o complex ligand to the central Zn ion.
Effect o f calcium and zinc stearates on PVC degradation
65
This could be the explanation of the observed increase of absorbance (the shaded area in Fig. 2b) in the 1550-1600cm -1 region of the infra-red spectrum o f the mixture. Assuming that the amounts of stearate ions, St154o and St1575 , in the CaSt 2, and St154o and St16oo in the Zn stearate are equal, we can calculate the absorbances corresponding to wave numbers 1540, 1575 and 1600 cm - i; in accordance with eqn (3): A1540 _ [CaStE] + [ZnSt2] + [C] /31540
(4)
A1575 --- [CaStE] + [C]
(5)
/31575 A16oo -
-
- [ZnSt2] + 2[C]
(6)
/31600
where [CaStE], [ZnStE] and [ C ] are the equilibrium concentrations of the two metal stearates and complex, respectively and/31540, e1575 and/3160o denote the corresponding extinction coefficients. Since, CaSt ° = [CaStE] + [C] (7) and
ZnSt2° = [ZnSt2] + [C]
(8)
eqns (4), (5) and (6) can be written in the form: A154o = CaSt o + ZnSt o _ [C] /31540
(9)
A 1575 _ CaSt02 /31575
(10)
Ax6°° - ZnSt ° + [ C ] /31600
(11)
where CaSt ° and ZnSt ° denote the initial concentrations of the two metal stearates. To make possible the quantitative interpretation of our infra-red spectroscopic data regarding the stearate groups, we derived equations expressing the concentrations of CaSt ° + ZnSt ° and [C], respectively, by employing eqns (9), (10) and (11). Thus we obtained: A1575 A 1575 A 1600 2(CaSt ° + ZnSt °) = - + + - (12) e15,0 /31575 /31600 A1575 A1600 A1545 --6 [C] = ~1575
/31600 2
/31545
(13)
66
Gy. l_~vai, Gy. Ocskay, Zs. Nyitrai, G. Meszl~,nyi
TABLE 2 Values o f x ° and [C] Obtained from Infra-red Spectroscopic Measurements, Compared with Weighed Amounts of x ° and with [CI, Calculated by Eqn (15)
Sample
x° (mmol/mon) (weighed)
x° (mmol/mon) (IR spectr.) eqn (12)
[C] (mmol/mon) (IR spectr.) eqn (13)
If] (mmol/mon) (calcd.) eqn (15)
CZ- 1 CZP-6 CZP-15 CZP-19 CZP-37 CZP-49 CZP-53 CZP-50
2"32 2"84 3"13 3"38 2"37 2.01 2-82 1"65
2"09 2-57 3-53 3"51 2'32 2"15 3"20 1"65
0-51 0.43 0-29 0"39 0"49 0.44 0.45 0
0"50 0"57 0'24 0"37 0"50 0'39 0"56 0
In Table 2 we have collected data for 2(CaSt2° + ZnSt °) and [C], obtained by substituting the appropriate infra-red absorbances into eqns (12) and (13). These data are compared in Table 2 with values o f 2(CaSt ° + ZnSt °) = x ° obtained by weighing, and with values o f [C-I, calculated assuming complexation to be controlled by the equilibrium: [CaSt2]. [ZnSt2] = Kd[C ]. On substituting [CaSt2] and [ZnSt2] by CaSt ° and ZnSt ° according to eqns (7) and (8) we have: (CaSt2° - [C]). (ZnSt ° - [C]) = Kd[C ]
(14)
where K a is the dissociation constant o f the complex. The solution o f eqn (14) is given by eqn (15) l-C] =
b - x/b 2 - 4. CaSt2° . ZnSt ° 2
(15)
where b = CaSt ° + ZnSt ° + K d. In these calculations, the extinction coefficient e 1575 obtained from eqn (10) was used. els4o and g156o were evaluated from the ratio AI575/Als4o, taken from the corresponding data o f the infra-red spectra o f Ca stearate, and employing eqn (12). The amounts o f stearic acid and ester groups formed during the degradation o f the PVC sheets were evaluated using the peaks which appear at wave numbers 1710cm -1 and 1745cm -1, respectively. These bands c o r r e s p o n d to the dimeric and m o n o m e r i c forms o f stearic acid respectively, 3 and ester groups in the samples. The separation o f peaks at
Effect o f calcium and zinc stearates on P V C degradation
10
20
67
30
Time (min)
Fig. 3.
Desorption curve of stearic acid in PVC at 190°C.
1745cm -1 was carried out by plotting the absorbances obtained at 1745cm -1 as a function of absorbances appearing at 1710cm -~, and comparing them with a corresponding curve obtained by using a sheet containing known amounts of stearic acid. This method was described in our previous paper. 4 The extinction coefficients for stearic acid and ester groups were determined from calibration curves obtained from the infra-red spectra of sheets containing known amounts of stearic acid and poly(vinyl acetate). We examined the material balance of carbonyl groups in PVC sheets degraded for various times, by comparing the initial carbonyl group content in the stabilizer systems with the sum of the quotients Ai/~ i of absorbances and extinction coefficients attributed to carboxylate ions, stearic acid and ester groups (eqns (4), (5) and (6)). It was found that the carbonyl group concentrations determined from the sum of carbonyl absorbances in the infra-red spectra were somewhat smaller than the measured values. Desorption experiments performed on PVC sheets containing only stearic acid have shown that the lack in carbonyl balance is due to the relatively high volatility of stearic acid. Figure 3 shows the desorption of stearic acid used alone in PVC at 190°C.
M E C H A N I S M A N D KINETICS OF S T A B I L I Z A T I O N The starting point for our kinetic analysis was the mechanism proposed first by Frye and Horst, 5'6 and widely accepted by others. 7'8 According to this, Ca and Zn stearates react with the allylic chlorine atoms in PVC, replacing them by stearate groups. Owing to the greater stability of these ester groups,
Gy. Lbvai, Gy. Ocskay, Zs. Nyitrai, G. Meszlbnyi
68
the chain propagation step of dehydrochlorination is believed to be hindered. __CH2(CH=CH).CHCH2__ heat ..~..CHz(CH=CH).+ICHCH2~ + HCI ester CaSC1
C1
formation [ ZnSt2 (16) ~CH2(CH=CH).CHCH2~ OCOR R=C17H35 The distribution of products (stearic acid, ester groups, unchanged stearate groups in the Ca and Zn stearates) formed from the stabilizers is expressed by the material balance equation: x ° = x+y+s
(17)
where x ° and x are the initial and actual concentrations, respectively, of CaSt 2 and/or ZnSt2, y denotes the concentration of ester groups, and s that of stearic acid. All concentrations are expressed in stearate equivalents. Based on this mechanism, the differential equations (18), (19) and (20) have been derived which describe the rates of formation of polyene sequences containing the allyl-activated chlorine atom (z*), as well as of ester groups and stearic acid, d2* dt
= w - kxz* - k3exz* I p
(18)
chain termination -~tt = k 3 o t x z *
ds dt -
dHC1 dt
- k2z*
(19) (20)
where HC1 is the concentration of the hydrogen chloride formed, w represents the rate of initiation, and kl, k2 and k3 are the rate constants of the termination and propagation steps and of ester formation, respectively. is x - ~ x , the molar ratio of stearate ions (x-), formed by the dissociation of Ca and Zn stearate, to the unreacted stearate groups (x) in the stabilizer system. The introduction of • was necessary for the calculation of the concentrations of stearate ions x-, assuming that the substitution of allylic chlorine atoms is of ionic character, having the rate k 3 x - z * (see eqn (19)).
Effect of calcium and zinc stearates on P VC degradation
69
Equations (18), (19) and (20) were reported previously. 9 The decay of ester formation by acidolysis, assumed previously, 9 however, was not confirmed by the data of our IR spectral measurements and was not therefore taken into account in eqn (19). Substituting z* from eqn (20) in eqn (18) and applying the steady state condition, dz*/dt .,~ O, leads, after integration, to eqn (21): s = HC1 =
k2
k2
--t-- ~y Wkl
(21)
Equation (21) describes the inhibiting action of the ester groups on stearic acid or hydrogen chloride formation. The relationship between y and s can be expressed by eqns (19), (20) and (17). After integration we obtain:
Y+S=(X°W~k3)[1-exp(-°~k3s~k2
~]A
(22,
The induction time for hydrogen chloride formation is widely used for the estimation of the activities of heat stabilizers. Since at time z, x = 0, and x ° = st + y~, z m a y be expressed by the alternative use of eqn (21) and eqn (22) as follows: z=
x° F k z / k l - 1 /3
v
y
x ° + ( K - 1) I x ° L z --
(23) k2
// k3 0
(24)
where K=k2/kl and v = wK denotes the rate of hydrogen chloride formation in unstabilized PVC (see eqn (21) if y = 0). Estimation of k2/kx, which is necessary for the evaluation of k3/k2, was described previouslyff using eqn (23), expressed by the reduced coordinates z/y~ and x°/y~. Values of aka/k 2 were estimated in two independent ways: (a)
F r o m data for y,, obtained from the infra-red spectroscopic measurements described in this paper. Applying eqn (22) for conditions t = v and x ° = y, + st we obtain: y = x°
1 - -
ctk3/k a
In [offka/k2)x ° + 1]
(25)
~ka/k 2 was evaluated from eqn (25) by using the 'trial and error' method.
Gy. IAvai, Gy. Ocskay, Zs. Nyitrai, G. Meszlbnyi
70
TABLE 3 Comparison of Values of ~tka/k2 Calculated from y, (Denoted by Ky) with Those Calculated from z (Denoted by K,)
Sample
CZ-1 CZP-6 CZP-15 CZP- 19 CZP-37 CZP-49 CZP-50 CZP-53
x°
y,
"C
(mmol/min)
(mmol/min)
(rain)
2-30 2-83 3.11 3-38 2-37 2.01 1.65 2-88
0.36 0.38 0-40 0"31 0-42 0.55 0.29 0'38
20 22 25 23 20 17 13 18
Ky
K,
(retool- 1 rain) (retool- 1 min) 0.17 0.11 0"10 0.06 0.19 0.40 0.26 0.15
0.31 0.150 0.16 0.06 0.26 0.31 0.26 0-02a
a The extraordinarily small value of K~ is due to the relatively small induction time of sample CZP-53, caused by the catalytic action ofZnC12 formed during the degradation. This sample contained a smaller quantity ofpentaerythritol than the others (see Table 1); Pe/ZnSt2 = 2.32, which was insufficient to hinder the deleterious action of ZnC12.
(b) From the data for induction time, z, by a method similar to that in (a), but using eqn (24). Values of v = 0.17 mmol/min and K = 2.9 were taken from our previous paper 4 for the evaluation of o~k3/k 2. Values of o~k3/k 2 evaluated from y, (denoted by Ky) and from -r (denoted by K0 are presented in Table 3. Values of Ky and K, in Table 3, however, seem not to correspond exactly. The deviations observed between these two sets of data, due to the non-isothermal initial conditions of heating of the samples, can be decreased by taking into account the time of warming up to the desired degradation temperature. This time, usually 2-3 min, increases the induction time and must be subtracted from the time data used, when measured values of x, y and s are compared with calculated values, resulting from eqns (17), (21) and (22). (The values o f s were obtained from infra-red data and the material balance equation (17).) Results of such calculations are shown in Fig. 4. The differences between the calculated and measured values in Fig. 4 do not exceed the normal error of infra-red spectroscopic measurements ( + 1.5 mol/mon 10-4). Thus we can conclude that our kinetic equations are suitable for the calculation of the consumption of stabilizers or the formation of stearic acid and ester groups during the degradation. Values of K r and K~ in Table 3 also show a considerable systematic dependence on the quantities and compositions of stabilizers. The explanation of this dependence requires some more knowledge about
Effect of calcium
and zinc stearates on P V C degradation
CZP-37 3
3 '
CZP- 6 ~X
\
~x
71
f
x x
/
\,x//o ~-
E 0
10
I
2O
I
30
0
10
20
30
[~,x CZP-19
I
x
y
2 /'¢
,
/ \ I
0
10
20
30
/ 0
Time (min)
~ 10
I 20
3O
Fig. 4. Comparison of measured and calculated values o f x, y, a n d s in thermally degraded P V C sheets, using the constants K r ( ) a n d K, ( . . . . . ) of ester formation, respectively. + , x; O,y;
C), s.
the molecular state of stabilizer systems at elevated temperatures. This problem, which is closely related to the synergism of stabilizer systems containing two metal stearates, will be discussed in a following paper.
CONCLUSION The application of kinetic equations for data relating to the time dependence of stabilizer consumption, formation of ester groups and stearic acid during the degradation of PVC, convincingly confirmed the mechanism proposed by Frye and Horst, according to which the stabilizing effect of CaSt 2 and ZnSt 2 is attributed to the replacement of the allyl-activated chlorine atoms by the more stable ester groups. Our kinetic equations, used for these calculations, contained only the rate constants of the individual steps of heat
72
Gy. I_~vai, Gy. Ocskay, Zs. Nyitrai, G. Meszl~nyi
degradation and stabilization, the rate of initiation, w, the rate constants k~ and k 2 of the termination and propagation steps of polyene formation, respectively, and the constant of ester group formation aka/k 2. The stabilizer system was represented in our calculations only by the initial stearate group content, x °, in the system. The ratio, ZnSt2/CaSt2, of the two stearates, however, was not taken into account; in consequence, a variation of the relative rate constant otka/k 2 was observed (Table 3). This variation was found to be caused not only by the variation of the ratio of the stabilizers, but also by the difference of the stearate group contents. We have tried to express these effects by introducing the degree of dissociation of the metal stearates, 0~,into the c o n s t a n t otk3/k2, assuming that the substitution of allylic chlorine atoms is of ionic nature. This approach seemed to be supported by our observation that CaSt 2 and ZnSt 2 form a considerably stable complex, having the composition CaSt[ZnSt3]. We assumed that this complexation would hinder the dissociation of the metal stearates, allowing only the metal stearate present in excess to dissociate. Accordingly, stabilization effected by stearate ions would be increased by increasing the excess of one of the metal stearates. However, this is contrary to experience, since molar ratios of ZnStE/CaSt2 close to 1 "1 were generally found to be the most effective in the stabilization of PVC. This contradiction will be resolved in a forthcoming paper, which will deal with the role and effects of ions on the rate of stabilizing processes in stabilized PVC systems.
REFERENCES 1. Mackenzie, M. W., Willis, H. A., Oven, R. C. & Michel, A., Eur. Polym. J., 19 (1983) 511. 2. Volka, K., Vymazal, Z., Starek, J. & Seidl, V., Eur. Polym. J., 18 (1982) 219. 3. Volka, K., Czako, L. & Vymazal, Z., Eur. Polym. J., 16 (1980) 149. 4. L6vai, Gy. & Ocskay, Gy., Die Angewandte Macromolekulare Chemie, 137 (1985) 37. 5. Frye, A. H. & Horst, R. W., J. Polym. Sci., 40 (1959) 419. 6. Frye, A. H. & Horst, R. W., J. Polym. Sci., 45 (1960) 1. 7. Braun, D. & Hepp, D., J. Polym. Sci., 33 (1971) 307. 8. Onozuka, M. & Asahina, M., J. Macromol. Sci. Rev. Macromol. Chem. 123 (1969) 235. 9. L6vai, Gy. & Ocskay, Gy., J. Makromol. Sci-Chem., A12 (1978) 467.