- Email: [email protected]
New assessment in thermal degradation of polymers
Polymer Testing 20 (2001) 3–6 www.elsevier.nl/locate/polytest
Material Behaviour
New assessment in thermal degradation of polymers Traian Zaharescu
*
R&D Institute for Electrical Engineering, 313 Splaiul Unirii, Bucharest 74204, Romania Received 15 February 1999; accepted 19 November 1999
Abstract The oxygen uptake method offers a reliable procedure to assess the thermal stability of polymers. The history of samples can be described by measurement of the amount of oxygen absorbed in tested material and by the estimation of thermal oxidation rate. The first order derivative of the analytical function relating the change in oxygen consumption with time presents a maximum value. Effects of material structure, formulation or ageing conditions can be explained by the sequence in the maximum values of oxidation rates. Mathematical equations that are fitted to rate data can be obtained and therefore stability characteristics, especially activation energy can be determined. This new derivative procedure is applied to the thermal characterization of synthetic elastomers containing fire retardants. 2000 Elsevier Science Ltd. All rights reserved. Keywords: Degradation; Polymers; Oxygen uptake
1. Introduction The stability of various polymeric systems can be assessed using different methods. The literature concerning the degradation of polymers offers a diversity of examples: IR spectroscopy [1,2], chemiluminescence [3,4], ESR [5,6], SEM [7,8], thermogravimetry [9,10], DSC [11,12] and so on. An alternative procedure used in many works on chemical stability of polymers is oxygen uptake that provides information on the oxygen amount involved in a certain oxidation process [13,14]. Fire retardants are the additives that increase the ignition resistance of polymers at elevated temperatures and prevent the propagation of flame. Some of them are halogenated compounds. The safety in service of polymeric products has demanded a detailed study of their thermal behaviour. The main goal of this paper is the application of the derivative procedure to the assessment of oxidation resistance by means of oxygen uptake. Similar results
* Tel.: +40-1-322-2813; fax: +40-1-321-3769. E-mail address: [email protected] (T. Zaharescu).
on the flammability characterization of elastomers mixed with metallic oxides were previously reported [14].
2. Experimental Ethylene–propylene elastomers (terpolymer — EPDM and copolymer — EPR) were supplied by ARPECHIM Pite ti (Romania). They presented the same ethylene/propylene ratio (3:2), but terpolymer contained additionally 3.5% ethylidene norbornene. Fire retardants, decabromdiphenyl oxide (DBDPO) and tertabrombisphenol A (TBBPA) provided by Merck , were added to the base materials in a concentration of 5% or 10%. Polymer materials were not previously purified in order to obtain reliable results for real applications. These systems were studied as unaged or in a thermally degraded state. Ageing temperatures of 100°C and 150°C for EPDM and EPR, respectively were selected to achieve significant modification in polymer structure over 3 h or 6 h of heating. Oxygen uptake measurements under isothermal and isobaric conditions were performed on ethylene–propylene elastomers in a laboratory equipment. The details of its functional features were presented in an earlier
0142-9418/00/$ - see front matter 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 1 4 2 - 9 4 1 8 ( 9 9 ) 0 0 0 7 1 - 9
4
T. Zaharescu / Polymer Testing 20 (2001) 3–6
paper [15]. Suitable testing temperatures were chosen to obtain minimal errors in oxidation rate.
3. Results and discussion Intensive thermal energy transfer is often accompanied by important changes in material structure. In this condition burning can be the final point of degradation [16]. Polyolefins behave like hydrocarbons providing radicals during thermal energy accumulation and the mechanism of thermal degradation proposed by Bolland and Gee [17] can be satisfactorily applied. In order to evaluate the chemical stability of ethylene– propylene elastomers with brominated compounds added to the specific thermal parameters of oxidation (induction time and process rate) were determined. The overall Table 1 Some kinetic characteristics on oxidation obtained for synthetic elastomer/brominated fire retardant systems System
Temperature Induction (K) time (min)
Oxidation rate (mol.g⫺1.min⫺1). 106
Unaged EPDM DBDPO 5%
180 190 200
34 23 17
21 36 58
Aged at 100°C EPDM DBDPO 5%
180 190 200
23 16 10
32 52 77
Unaged EPDM TBBPA 5%
130 150 170
29 17 9
23 54 83
Aged at 100°C EPDM TTBPA 5%
130 150 170
12 8 4
15 26 33
Unaged EPR DBDPO 5%
200 215 230
51 32 15
47 52 79
Aged at 150°C EPR DBDPO 5%
190 200 210
25 14 7
20 30 46
Unaged EPR TBBPA 5%
170 180 190
45 30 18
12 16 21
Aged at 150°C EPR TBBPA 5%
170 180 190
27 18 8
14 20 29
effects induced by the cleavage of weaker bonds can be estimated from the oxygen amount reacted with degrading polymer. The most important factor that affects the development of degradation is the ability of the additives to prevent or to slow down this process. The contribution of fire retardants could be proved by the measured changes in oxygen uptake. Table 1 lists some kinetic parameters determined on ethylene–propylene elastomer: 앫 the thermal stability of the two tested elastomers is not the same. The different ranges of temperature for degradation testing had to be selected because copolymer is more stable than terpolymer and thermal oxidation of EPR required higher temperatures; 앫 under similar ageing conditions synthetic elastomers are more stable in the formulation containing decabromdiphenyl oxide than the samples containing tetrabrombisphenol A; 앫 the oxidation rates and the oxidation induction times
Fig. 1. The Arrhenius graphs drown for the terpolymer samples containing DBDPO 5%. (a) virgin EPDM and (b) EPDM aged at 100°C for 6 h.
T. Zaharescu / Polymer Testing 20 (2001) 3–6
provide different values for activation energy corresponding to the oxidation process occurring in various stages of degradation. Fig. 1 is an example of this discordant behaviour. At the start of degradation the availability of polymer material for oxidation is predictable because of the low accumulation of oxygenated products. At an advanced damage level free energy (⌬G) reaches a minimum. It means that there is a fast increase in the number of oxygenated units from near zero up to a significant value which changes the surrounding elastomer molecules. Fig. 2a presents an example of the measured change in oxygen uptake for EPR mixed with DBDPO 10% at various temperatures. The effect of temperature on the thermal behaviour of elastomers is satisfactorily illustrated in Fig. 2b. The first
5
order relationship characterises the mathematical correlation between maximum oxidation rate and the corresponding time. Table 2 lists the relationships describing the action of oxygen on ethylene–propylene elastomers stabilized with fire retardants. Two main features must be emphasized. A positive sign of slopes signifies the increase in oxidation rate at elevated temperatures. Thus, the thermal stability of polymer samples on the action of energetic factors is diminished either due to the preexisting defects or due to the inefficiency of the additive. A negative sign of intercepts can be explained by the modification in reaction probability at lower temperatures. Thus, the deviation from linearity is justified by the simultaneous contribution of host polymer, the additive contained in the formulation, the diffusion rate of oxygen and the testing temperature. The large alkyl fragments created by the splitting of bonds gain different kinetic energies and their movement depends on their size. Molecular oxygen can easily diffuse into degrading material and transport to inner layers of the polymer which assures a constant concentration of O2. The diffusion coefficient is temperature-dependent so that the linearity of d[O2] = f(T) is not followed over the temperature range close to room temperature. In addition, the autocatalytic mechanism of oxidative degradation increases the final amount of degradation products, increasing the local disorder in the polymer. This derivative procedure offers the possibility to assess the oxidation rate at various medium or high temperatures. Assuming that the same reactions occur over these temperature ranges, it is possible to estimate the oxidation rate and, therefore, the safe service time. Table 2 displays the effect of increase in concentration of fire retardant. Although double quantity of additive was used, the effect did not increase in the same proportion. The slopes of relationships d[O2] = f(t) belonging to the same set are quite similar. The difference in the stabilization effectiveness of the two tested additives, DBDPO and TTBPA, can be evaluated by means of the slopes of d[O2] = f(t) functions. Ethylene–propylene copolymer (EPR) is hardly degraded compared to terpolymer (EPDM) and the brominated fire retardants studied in this paper preserved the same order in the thermal stability of synthetic rubbers [18].
4. Conclusions
Fig. 2. The time dependencies of (a) oxygen uptake and (b) oxidation rate for copolymer samples containing TBBPA 10%. (䊏,䊐) 220°C; (쎲,䊊) 210°C; (왖,왕) 200°C.
The oxygen uptake method and the associated derivative procedure may be successfully applied for thermal characterization of polymers. The degradation level can be predicted in relation to operation conditions and the life time of polymer materials may be estimated.
6
T. Zaharescu / Polymer Testing 20 (2001) 3–6
Table 2 Relationships describing the time dependency of oxidation rate for various synthetic elastomer/brominated fire retardant systems Polymer
Additive
Ageing conditions
Relationship d[O2] = f(T)
Correlation factor
EPDM
DBDPO
– 3 h at 100°C 6 h at 100°C
d[O2] = ⫺270.5+1.65 T d[O2] = ⫺152.3+1.52 T d[O2] = ⫺98.2+1.59 T
0.994 0.991 0.992
– 3 h at 100°C 6 h at 100°C
d[O2] = ⫺255.8+1.68 T d[O2] = ⫺107.6+1.45 T d[O2]= ⫺40.9+1.52 T
0.994 0.996 0.993
– 3 h at 100°C 6 h at 100°C
d[O2] = ⫺135.1+0.99 T d[O2] = ⫺112.3+0.85 T d[O2] = ⫺101.8+0.91 T
0.991 0.999 0.998
– 3 h at 100°C 6 h at 100°C
d[O2] = ⫺108.5+0.88 T d[O2] = ⫺98.3+0.85 T d[O2] = ⫺85.6+0.80 T
0.995 0.992 0.997
– 3 h at 150°C 6 h at 150°C
d[O2] = ⫺83.8+1.25 T d[O2] = ⫺70.4+1.33 T d[O2] = ⫺77.0+1.20 T
0.991 0.995 0.998
– 3 h at 150°C 6 h at 150°C
d[O2] = ⫺79.8+1.15 T d[O2] = ⫺54.7+1.10 T d[O2] = ⫺43.2+1.12 T
0.993 0.993 0.997
– 3 h at 150°C 6 h at 150°C
d[O2] = ⫺56.0+0.76 T d[O2] = ⫺48.9+0.71 T d[O2] = ⫺43.3+0.70 T
0.999 0.998 0.992
– 3 h at 150°C 6 h at 150°C
d[O2] = ⫺50.8+0.66 T d[O2] = ⫺46.6+0.64 T d[O2] = ⫺40.8+0.64 T
0.998 0.993 0.996
5% DBDPO 10% TTBPA 5% TTBPA 10% EPR
DBDPO 5% DBDPO 10% TTBPA 5% TTBPA 10%
References [1] D.J. Carlsson, S. C`hmela, J. Lacoste, ACS Series 475, Chapter 26. [2] S. Baccaro, U. Buontempo, Radiat. Phys. Chem., Part C 40 (1992) 175. [3] S. Jipa, T. Setnescu, R. Setnescu, C. Cazac, I. Mihalcea, Materiale Plastice (Bucharest) 44 (1993) 65. [4] N.C. Billingham, E.T.H. Then, A. Kron, Polym. Degrad. Stab. 55 (1997) 339. [5] B. Ra˚nby, Makromol. Chem., Macromol. Symp. 52 (1991) 1. [6] D.J.T. Hill, A.P. Lang, J.H. O’Donnell, P.J. Pomery, Polym. Degrad. Stab. 38 (1992) 205. [7] L. Costa, P. Goberti, G. Paganetto, G. Camino, P. Sgarzi, Polym. Degrad. Stab. 30 (1990) 13. [8] T. Zaharescu, I. Mihalcea, Polym. Degrad. Stab. 50 (1995) 39.
[9] I.C. McNeill, M.H. Mohammed, Polym. Degrad. Stab. 48 (1995) 175. [10] C. Vasile, L. Odochian, I. Agherghinei, Polym. J. Sci., Part A: Polym. Chem. 26 (1988) 1639. [11] R. Bharel, R.C. Anand, V. Choudhary, I.K. Varma, Polym. Degrad. Stab. 38 (1992) 107. [12] T. Zaharescu, V. Meltzer, R. Vıˆlcu, J. Mater. Sci. Lett. 15 (1996) 1212. [13] B. Ra˚nby, B. Rabek, in: B. Ra˚nby, B. Rabek (Eds.), Photodegradation, Photooxidation and Photostabilization, Hanser Verlag, Berlin, 1971. [14] T. Zaharescu, Materiale Plastice (Bucharest) 32 (1995) 138. [15] T. Zaharescu, R. Vıˆlcu, C. Podina˜, Polym. Testing 17 (1998) 587. [16] A. Bucsi, J. Rychly, Polym. Degrad. Stab. 38 (1992) 33. [17] J.L. Bolland, G. Gee, Trans. Faraday Soc. 42 (1946) 236. [18] T. Zaharescu, Rev. Roum. Chim. 41 (1996) 1017; 42 (1997) 921.
Material Behaviour
New assessment in thermal degradation of polymers Traian Zaharescu
*
R&D Institute for Electrical Engineering, 313 Splaiul Unirii, Bucharest 74204, Romania Received 15 February 1999; accepted 19 November 1999
Abstract The oxygen uptake method offers a reliable procedure to assess the thermal stability of polymers. The history of samples can be described by measurement of the amount of oxygen absorbed in tested material and by the estimation of thermal oxidation rate. The first order derivative of the analytical function relating the change in oxygen consumption with time presents a maximum value. Effects of material structure, formulation or ageing conditions can be explained by the sequence in the maximum values of oxidation rates. Mathematical equations that are fitted to rate data can be obtained and therefore stability characteristics, especially activation energy can be determined. This new derivative procedure is applied to the thermal characterization of synthetic elastomers containing fire retardants. 2000 Elsevier Science Ltd. All rights reserved. Keywords: Degradation; Polymers; Oxygen uptake
1. Introduction The stability of various polymeric systems can be assessed using different methods. The literature concerning the degradation of polymers offers a diversity of examples: IR spectroscopy [1,2], chemiluminescence [3,4], ESR [5,6], SEM [7,8], thermogravimetry [9,10], DSC [11,12] and so on. An alternative procedure used in many works on chemical stability of polymers is oxygen uptake that provides information on the oxygen amount involved in a certain oxidation process [13,14]. Fire retardants are the additives that increase the ignition resistance of polymers at elevated temperatures and prevent the propagation of flame. Some of them are halogenated compounds. The safety in service of polymeric products has demanded a detailed study of their thermal behaviour. The main goal of this paper is the application of the derivative procedure to the assessment of oxidation resistance by means of oxygen uptake. Similar results
* Tel.: +40-1-322-2813; fax: +40-1-321-3769. E-mail address: [email protected] (T. Zaharescu).
on the flammability characterization of elastomers mixed with metallic oxides were previously reported [14].
2. Experimental Ethylene–propylene elastomers (terpolymer — EPDM and copolymer — EPR) were supplied by ARPECHIM Pite ti (Romania). They presented the same ethylene/propylene ratio (3:2), but terpolymer contained additionally 3.5% ethylidene norbornene. Fire retardants, decabromdiphenyl oxide (DBDPO) and tertabrombisphenol A (TBBPA) provided by Merck , were added to the base materials in a concentration of 5% or 10%. Polymer materials were not previously purified in order to obtain reliable results for real applications. These systems were studied as unaged or in a thermally degraded state. Ageing temperatures of 100°C and 150°C for EPDM and EPR, respectively were selected to achieve significant modification in polymer structure over 3 h or 6 h of heating. Oxygen uptake measurements under isothermal and isobaric conditions were performed on ethylene–propylene elastomers in a laboratory equipment. The details of its functional features were presented in an earlier
0142-9418/00/$ - see front matter 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 1 4 2 - 9 4 1 8 ( 9 9 ) 0 0 0 7 1 - 9
4
T. Zaharescu / Polymer Testing 20 (2001) 3–6
paper [15]. Suitable testing temperatures were chosen to obtain minimal errors in oxidation rate.
3. Results and discussion Intensive thermal energy transfer is often accompanied by important changes in material structure. In this condition burning can be the final point of degradation [16]. Polyolefins behave like hydrocarbons providing radicals during thermal energy accumulation and the mechanism of thermal degradation proposed by Bolland and Gee [17] can be satisfactorily applied. In order to evaluate the chemical stability of ethylene– propylene elastomers with brominated compounds added to the specific thermal parameters of oxidation (induction time and process rate) were determined. The overall Table 1 Some kinetic characteristics on oxidation obtained for synthetic elastomer/brominated fire retardant systems System
Temperature Induction (K) time (min)
Oxidation rate (mol.g⫺1.min⫺1). 106
Unaged EPDM DBDPO 5%
180 190 200
34 23 17
21 36 58
Aged at 100°C EPDM DBDPO 5%
180 190 200
23 16 10
32 52 77
Unaged EPDM TBBPA 5%
130 150 170
29 17 9
23 54 83
Aged at 100°C EPDM TTBPA 5%
130 150 170
12 8 4
15 26 33
Unaged EPR DBDPO 5%
200 215 230
51 32 15
47 52 79
Aged at 150°C EPR DBDPO 5%
190 200 210
25 14 7
20 30 46
Unaged EPR TBBPA 5%
170 180 190
45 30 18
12 16 21
Aged at 150°C EPR TBBPA 5%
170 180 190
27 18 8
14 20 29
effects induced by the cleavage of weaker bonds can be estimated from the oxygen amount reacted with degrading polymer. The most important factor that affects the development of degradation is the ability of the additives to prevent or to slow down this process. The contribution of fire retardants could be proved by the measured changes in oxygen uptake. Table 1 lists some kinetic parameters determined on ethylene–propylene elastomer: 앫 the thermal stability of the two tested elastomers is not the same. The different ranges of temperature for degradation testing had to be selected because copolymer is more stable than terpolymer and thermal oxidation of EPR required higher temperatures; 앫 under similar ageing conditions synthetic elastomers are more stable in the formulation containing decabromdiphenyl oxide than the samples containing tetrabrombisphenol A; 앫 the oxidation rates and the oxidation induction times
Fig. 1. The Arrhenius graphs drown for the terpolymer samples containing DBDPO 5%. (a) virgin EPDM and (b) EPDM aged at 100°C for 6 h.
T. Zaharescu / Polymer Testing 20 (2001) 3–6
provide different values for activation energy corresponding to the oxidation process occurring in various stages of degradation. Fig. 1 is an example of this discordant behaviour. At the start of degradation the availability of polymer material for oxidation is predictable because of the low accumulation of oxygenated products. At an advanced damage level free energy (⌬G) reaches a minimum. It means that there is a fast increase in the number of oxygenated units from near zero up to a significant value which changes the surrounding elastomer molecules. Fig. 2a presents an example of the measured change in oxygen uptake for EPR mixed with DBDPO 10% at various temperatures. The effect of temperature on the thermal behaviour of elastomers is satisfactorily illustrated in Fig. 2b. The first
5
order relationship characterises the mathematical correlation between maximum oxidation rate and the corresponding time. Table 2 lists the relationships describing the action of oxygen on ethylene–propylene elastomers stabilized with fire retardants. Two main features must be emphasized. A positive sign of slopes signifies the increase in oxidation rate at elevated temperatures. Thus, the thermal stability of polymer samples on the action of energetic factors is diminished either due to the preexisting defects or due to the inefficiency of the additive. A negative sign of intercepts can be explained by the modification in reaction probability at lower temperatures. Thus, the deviation from linearity is justified by the simultaneous contribution of host polymer, the additive contained in the formulation, the diffusion rate of oxygen and the testing temperature. The large alkyl fragments created by the splitting of bonds gain different kinetic energies and their movement depends on their size. Molecular oxygen can easily diffuse into degrading material and transport to inner layers of the polymer which assures a constant concentration of O2. The diffusion coefficient is temperature-dependent so that the linearity of d[O2] = f(T) is not followed over the temperature range close to room temperature. In addition, the autocatalytic mechanism of oxidative degradation increases the final amount of degradation products, increasing the local disorder in the polymer. This derivative procedure offers the possibility to assess the oxidation rate at various medium or high temperatures. Assuming that the same reactions occur over these temperature ranges, it is possible to estimate the oxidation rate and, therefore, the safe service time. Table 2 displays the effect of increase in concentration of fire retardant. Although double quantity of additive was used, the effect did not increase in the same proportion. The slopes of relationships d[O2] = f(t) belonging to the same set are quite similar. The difference in the stabilization effectiveness of the two tested additives, DBDPO and TTBPA, can be evaluated by means of the slopes of d[O2] = f(t) functions. Ethylene–propylene copolymer (EPR) is hardly degraded compared to terpolymer (EPDM) and the brominated fire retardants studied in this paper preserved the same order in the thermal stability of synthetic rubbers [18].
4. Conclusions
Fig. 2. The time dependencies of (a) oxygen uptake and (b) oxidation rate for copolymer samples containing TBBPA 10%. (䊏,䊐) 220°C; (쎲,䊊) 210°C; (왖,왕) 200°C.
The oxygen uptake method and the associated derivative procedure may be successfully applied for thermal characterization of polymers. The degradation level can be predicted in relation to operation conditions and the life time of polymer materials may be estimated.
6
T. Zaharescu / Polymer Testing 20 (2001) 3–6
Table 2 Relationships describing the time dependency of oxidation rate for various synthetic elastomer/brominated fire retardant systems Polymer
Additive
Ageing conditions
Relationship d[O2] = f(T)
Correlation factor
EPDM
DBDPO
– 3 h at 100°C 6 h at 100°C
d[O2] = ⫺270.5+1.65 T d[O2] = ⫺152.3+1.52 T d[O2] = ⫺98.2+1.59 T
0.994 0.991 0.992
– 3 h at 100°C 6 h at 100°C
d[O2] = ⫺255.8+1.68 T d[O2] = ⫺107.6+1.45 T d[O2]= ⫺40.9+1.52 T
0.994 0.996 0.993
– 3 h at 100°C 6 h at 100°C
d[O2] = ⫺135.1+0.99 T d[O2] = ⫺112.3+0.85 T d[O2] = ⫺101.8+0.91 T
0.991 0.999 0.998
– 3 h at 100°C 6 h at 100°C
d[O2] = ⫺108.5+0.88 T d[O2] = ⫺98.3+0.85 T d[O2] = ⫺85.6+0.80 T
0.995 0.992 0.997
– 3 h at 150°C 6 h at 150°C
d[O2] = ⫺83.8+1.25 T d[O2] = ⫺70.4+1.33 T d[O2] = ⫺77.0+1.20 T
0.991 0.995 0.998
– 3 h at 150°C 6 h at 150°C
d[O2] = ⫺79.8+1.15 T d[O2] = ⫺54.7+1.10 T d[O2] = ⫺43.2+1.12 T
0.993 0.993 0.997
– 3 h at 150°C 6 h at 150°C
d[O2] = ⫺56.0+0.76 T d[O2] = ⫺48.9+0.71 T d[O2] = ⫺43.3+0.70 T
0.999 0.998 0.992
– 3 h at 150°C 6 h at 150°C
d[O2] = ⫺50.8+0.66 T d[O2] = ⫺46.6+0.64 T d[O2] = ⫺40.8+0.64 T
0.998 0.993 0.996
5% DBDPO 10% TTBPA 5% TTBPA 10% EPR
DBDPO 5% DBDPO 10% TTBPA 5% TTBPA 10%
References [1] D.J. Carlsson, S. C`hmela, J. Lacoste, ACS Series 475, Chapter 26. [2] S. Baccaro, U. Buontempo, Radiat. Phys. Chem., Part C 40 (1992) 175. [3] S. Jipa, T. Setnescu, R. Setnescu, C. Cazac, I. Mihalcea, Materiale Plastice (Bucharest) 44 (1993) 65. [4] N.C. Billingham, E.T.H. Then, A. Kron, Polym. Degrad. Stab. 55 (1997) 339. [5] B. Ra˚nby, Makromol. Chem., Macromol. Symp. 52 (1991) 1. [6] D.J.T. Hill, A.P. Lang, J.H. O’Donnell, P.J. Pomery, Polym. Degrad. Stab. 38 (1992) 205. [7] L. Costa, P. Goberti, G. Paganetto, G. Camino, P. Sgarzi, Polym. Degrad. Stab. 30 (1990) 13. [8] T. Zaharescu, I. Mihalcea, Polym. Degrad. Stab. 50 (1995) 39.
[9] I.C. McNeill, M.H. Mohammed, Polym. Degrad. Stab. 48 (1995) 175. [10] C. Vasile, L. Odochian, I. Agherghinei, Polym. J. Sci., Part A: Polym. Chem. 26 (1988) 1639. [11] R. Bharel, R.C. Anand, V. Choudhary, I.K. Varma, Polym. Degrad. Stab. 38 (1992) 107. [12] T. Zaharescu, V. Meltzer, R. Vıˆlcu, J. Mater. Sci. Lett. 15 (1996) 1212. [13] B. Ra˚nby, B. Rabek, in: B. Ra˚nby, B. Rabek (Eds.), Photodegradation, Photooxidation and Photostabilization, Hanser Verlag, Berlin, 1971. [14] T. Zaharescu, Materiale Plastice (Bucharest) 32 (1995) 138. [15] T. Zaharescu, R. Vıˆlcu, C. Podina˜, Polym. Testing 17 (1998) 587. [16] A. Bucsi, J. Rychly, Polym. Degrad. Stab. 38 (1992) 33. [17] J.L. Bolland, G. Gee, Trans. Faraday Soc. 42 (1946) 236. [18] T. Zaharescu, Rev. Roum. Chim. 41 (1996) 1017; 42 (1997) 921.