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Effect of gamma irradiation conditions on the radiation-induced degradation of isobutylene–isoprene rubber
Nuclear Instruments and Methods in Physics Research B 208 (2003) 480–484 www.elsevier.com/locate/nimb
Effect of gamma irradiation conditions on the radiation-induced degradation of isobutylene–isoprene rubber M. S ß en a
€ . Kanto , C. Uzun a, O glu b, S.M. Erdo gan a, V. Deniz c, O. G€ uven
a,*
a
Polymer Chemistry Division, Department of Chemistry, Hacettepe University, 06532 Beytepe, Ankara, Turkey b Food Irradiation and Sterilization Department, Ankara Nuclear Agriculture and Animal Research Center, Turkish Atomic Energy Authority, 06983 Ankara, Turkey c Department of Chemical Engineering, Kocaeli University, 41040 Kocaeli, Turkey
Abstract The effect of gamma irradiation conditions on the radiation-induced degradation of uncrosslinked, commercial isobutylene–isoprene rubbers has been investigated in this study. Influence of dose rate and irradiation atmosphere on the degradation of butyl rubber has been followed by viscosimetric and chromatographic analyses. Limiting viscosity number of all butyl rubbers decreased sharply up to 100 kGy and leveled off at around the same molecular weight, independent of dose rate. Slightly higher decrease in viscosity was observed for samples irradiated in air than in nitrogen especially at low dose rate irradiation. Cross-linking GðX Þ, and chain scission GðSÞ yields of butyl rubbers were calculated by using weight- and number-average molecular weights of irradiated rubber determined by Size Exclusion Chromatography analyses. G-value results showed that chain scission reactions in isobutylene–isoprene rubber in air atmosphere are more favorable than in nitrogen atmosphere, and that lower dose rate enhances chain scission over cross-linking. Ó 2003 Elsevier B.V. All rights reserved. Keywords: Isobutylene–isoprene rubbers; Gamma irradiation; Cross-linking; Degradation
1. Introduction Parallel to the increase in the number of vehicles throughout the world, waste tires pose important economical and ecological problems. Thermoset, polymeric materials, such as rubber tires are of great challenge concerning the environmental and ecological reasons in regard to their treatment methods. Ionizing radiation seems to offer unique
*
Corresponding author. Tel.: +90-312-2977989; fax: +90312-2977989. E-mail address: [email protected] (M. S ß en).
opportunities to tackle the problem of recycling of polymers and rubbers on account of its ability to cause cross-linking or/and chain scission of polymeric materials. There have been many works in the literature dealing with the radiation-induced breakdown of a variety of polymers. Some alteration of the molecular structure and/or morphology of the material may even be enhanced as reported by Burillo et al. [1] in reclaiming polymer and rubber wastes, by the use of ionizing radiation. Although literature is full of publications dealing with the recycling of waste tires [2–4], only a limited amount of work reported on the effect of
0168-583X/03/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0168-583X(03)01111-X
M. Sß en et al. / Nucl. Instr. and Meth. in Phys. Res. B 208 (2003) 480–484
irradiation conditions and additives on the radiation-induced degradation of rubbers. In this study we investigated the effect of dose rate and irradiation atmosphere on the degradation of uncrosslinked, commercial isobutylene–isoprene (IIR) rubber. The effect of radiation-induced degradation on the molecular weight has been investigated by chromatographic and viscosimetric techniques.
2. Experimental
481
2.2. Gamma irradiation IIP specimens of dimensions in 2 cm 2 cm with 5 mm thicknesses were irradiated at ambient temperature in air and nitrogen purged and sealed glass ampoules in Gamma cell 220 type Canadian made c-irradiator for low dose rate (0.18 kGy/h) irradiations and Isslodovately model Russian made c-irradiator for high dose rate (3.0 kGy/h) irradiations. 2.3. Viscosimetric studies
Limiting viscosity number [ ] (dl/g)
Three commercial isobutylene–isoprene (IIP) rubber samples were used in our experiments: Ex 165 (isoprene content: 0.8%) and Ex 268 (isoprene content: 1.7%) were obtained from Exxon company, and BK1685N (isoprene content: 1.7%) was obtained from the Nizhnekamsk company. All these butyl rubbers were used as received.
1.2
(a)
N2 Air
1.0 0.8 0.6 0.4 0.2 0
50
100 150 Dose (kGy)
Limiting viscosity number [ ] (dl/g)
0.0
1.2
200
The limiting viscosity number [g] of the rubbers were determined by using the Huggins equation [5]: 2
gsp =c ¼ ½g þ k 0 ½g c;
ð1Þ
where k 0 is constant for a given polymer at a given temperature in a given solvent, and c is the concentration of solution. The viscosities of solutions were measured by an Ubbelohde type viscometer,
Limiting viscosity number [ ] (dl/g)
2.1. Materials
1.2
(b)
N2 Air
1.0 0.8 0.6 0.4 0.2 0.0
0
50
100 150 Dose (kGy)
200
(c)
N2 Air
1.0 0.8 0.6 0.4 0.2 0.0
0
50
100 150 Dose (kGy)
200
Fig. 1. Variation of limiting viscosity numbers with irradiation dose, for low dose rate irradiation (0.18 kGy/h) in air and N2 ; (a) Ex 165, (b) Ex 268, (c) BK1685N.
M. Sß en et al. / Nucl. Instr. and Meth. in Phys. Res. B 208 (2003) 480–484
tetrahydrofuran (THF) being the solvent used throughout the experiments. The thermostat temperature was maintained at 25 0.02 °C for all measurements.
2.4. Size exclusion chromatography studies
1.2
(a)
N2 Air
1.0 0.8 0.6 0.4 0.2 0.0
0
50
100 150 Dose (kGy) Limiting viscosity number [ ] (dl/g)
Limiting viscosity number [ ] (dl/g)
Weight- and number-average molecular weights of unirradiated and irradiated rubber samples were determined by using a Waters 244 ALC/GPC chromatograph equipped with four Ultrastyragel columns with pore sizes 106 –105 – , and THF as the eluting solvent. The 104 –103 A flow rate was kept at 1 ml/min in all cases. A universal calibration curve based on the elution volume of polyethylene glycol (PEG) standards from Toyo Soda Company and using appropriate Mark Houwink–Sakurada constants, K ¼ 2:0 10 4 and a ¼ 0:67 for butyl rubber in THF [6] was constructed.
1.2
200
3. Results and discussion 3.1. Effect of gamma rays on the limiting viscosity number of IIR In order to see the effect of irradiation dose rate and atmosphere on the degradation of rubber samples, the limiting viscosity numbers were first determined. Fig. 1(a)–(c), show the effect of atmosphere on the limiting viscosity number of butyl rubber at low dose rate irradiation (0.18 kGy/h). Cross-linking reactions occur in most elastomers when treated with ionizing radiation, but only a few rubber varieties such as butyl rubber and butyl vulcanizates whose structural units contain quaternary carbon atoms undergo appreciable degradation reactions [7]. As shown in Fig. 1, limiting viscosity values of rubber samples used in this work decrease significantly up to 100 kGy irradiation dose but do not change appreciably beyond this dose value. Slightly higher values of
Limiting viscosity number [ ] (dl/g)
482
1.2
(b)
N2 Air
1.0 0.8 0.6 0.4 0.2 0.0
0
50
100 150 Dose (kGy)
200
(c)
N2 Air
1.0 0.8 0.6 0.4 0.2 0.0
0
50
100 150 Dose (kGy)
200
Fig. 2. Variation of limiting viscosity numbers with irradiation for high dose rate irradiation (3.0 kGy/h)) in air and N2 ; (a) Ex 165, (b) Ex 268, (c) BK1685N.
M. Sß en et al. / Nucl. Instr. and Meth. in Phys. Res. B 208 (2003) 480–484
483
weight- and number-average molecular weights were calculated. Representative SE chromatograms are given in Fig 3(a) and (b), for EX 268 and BK1685N respectively. We note that both size exclusion chromatograms continuously shift to higher elution volumes with increasing irradiation doses. The extent of shift decreases and peak maximum values converge for doses exceeding 100 kGy. The variation of Mw and Mn values with irradiation dose for Ex 268 are given in Fig. 4, from which we note that both Mw and Mn values decrease sharply up to 100 kGy, but remain almost unchanged beyond this dose value. Very similar decreases were observed for the other rubbers samples. To determine GðSÞ and GðX Þ values the following equations were used for irradiation doses in the range from 0 to 100 kGy [10]:
degradation was observed for low dose rate irradiation in air than in nitrogen for a given absorbed dose value. This is presumably due to the longer contact of rubber with oxygen in air and resulting in higher chain scission reactions [8,9]. On the other hand no significant dependence of limiting viscosity or average molecular weight on the irradiation atmosphere was observed for rubber samples following high dose rate irradiation (3.0 kGy/h), most probably due shorter contact time of the rubber with oxygen, as shown in Fig. 2. 3.2. Effect of irradiation conditions on the G(S) and G(X ) values of IIP For the investigation of the effect of dose rate and other irradiation conditions on the G values for cross-linking GðX Þ, and chain scission reactions GðSÞ on the IIP rubber samples, size exclusion chromatograms were taken, analyzed, and
1=Mw ¼ 1=Mw0 þ ½GðSÞ=2 2GðX ÞD 1:038 10 6 ; ð2Þ
(b)
(a) 0 kGy 0 kGy 21 kGy 21 kGy 31 kGy 31 kGy
62 kGy 125 kGy
62 kGy 94 kGy 125 kGy 157kGy 220 kGy
157kGy 220 kGy
20
25
30 35 40 Retention volume (ml)
45 20
25
30 35 40 Retention volume (ml)
45
Fig. 3. Size exclusion chromatograms of (a) Ex 265 and (b) BK1685N samples (see text).
500000
500000
M w LDR Air M w LDR N2 M n LDR Air M n LDR N2
Mol. Weight
400000 300000 200000 100000 0
(b)
Mw Mw Mn Mn
400000
Mol. Weight
(a)
300000
HDR Air HDR N 2 HDR Air HDR N 2
200000 100000
0
50
100 150 200 Dose (kGy)
250
0
0
50
100 150 200 Dose (kGy)
250
Fig. 4. Effect of dose rate and irradiation atmosphere on the weight and number average molecular weight of Ex 268 samples. Irradiation conditions are given in figures. HDR and LDR represent, high dose rate and low dose rate respectively.
484
M. Sß en et al. / Nucl. Instr. and Meth. in Phys. Res. B 208 (2003) 480–484
Table 1 Chain scission GðSÞ and cross-link GðX Þ yield of butyl rubbers at different irradiation conditions Rubber
Air
N2
GðSÞ
GðX Þ
GðSÞ=GðX Þ
GðSÞ
GðX Þ
GðSÞ=GðX Þ
Low dose rate (LDR) Ex 268 BK1675N
4.2 3.8
0.2 0.3
18.2 12.5
1.9 2.9
0.3 0.4
5.5 7.2
High dose rate (HDR) Ex 268 BK1675N
5.4 2.5
0.5 0.1
10.3 24.7
7.7 6.4
1.1 1.1
7.1 5.9
1=Mn ¼ 1=Mn0 þ ½GðSÞ GðX ÞD 1:038 10 6 ; ð3Þ where Mw0 and Mn0 are the weight- and numberaverage molecular weights of unirradiated samples. Mw and Mn are the corresponding values following exposure to irradiation dose D (kGy). Calculated GðSÞ and GðX Þ values of Ex 268 and BK1685N are listed in Table 1. The GðSÞ=GðX Þ > 4 values in this table show that chain scissions dominate over cross-linking reactions, for both air and nitrogen atmospheres. GðSÞ and GðX Þ values also indicate that irradiation of IIP rubbers in nitrogen atmosphere enhances slightly cross-linking reactions, especially at high dose rate irradiations. Due to the lower isoprene content of Ex 165 and its lower molecular weight compared with the other rubber samples its GðSÞ and GðX Þ values are not presented here.
4. Conclusion Viscosimetric and chromatographic studies show that, beside irradiation atmosphere, the irradiation dose rate is an important parameter for controlling of the degradation of butyl rubbers, especially at low dose rates. For doses exceeding 100
kGy, degradation of butyl rubbers becomes independent of atmosphere and dose rate, all rubber samples studied attain very similar average molecular weight values independent of initial molecular weight and isoprene content at this dose value.
References [1] G. Burillo, R. Clough, T. Czvikovszky, O. G€ uven, A. Le Moel, W. Liu, A. Singh, J. Yang, T. Zaharescu, Radiat. Phys. Chem. 64 (2001) 41. [2] B. Adhikari, D. De, S. Maiti, Prog. Polym. Sci. 25 (2000) 909. [3] V.K. Sharma, F. Fortuna, M. Mincarini, M. Berillo, G. Cornacchia, Appl. Energ. 65 (2000) 381. [4] Y. Fang, M. Zhan, Y. Wang, Mater. and Des. 22 (2001) 123. [5] J.F. Rabek, in: Experimental methods in polymer chemistry, John Wiley & Sons, New York, 1980, p. 127. [6] J.E. Puskas, R. Hutchinson, Rubber Technol. 66 (1993) 742. [7] J. Yang, in: R.D. Cooper, K.E. OÕShea (Eds.), Environmental application of ionizing radiation, Wiley, New York, 1998, p. 601. [8] S. Jipa, M. Giurginca, T. Setnescu, R. Setnescu, G. Ivan, I. Mihalcea, Polym. Degrad. Stab. 54 (1996) 1. [9] R. Chandra, V. Subhash, A.K. Verma, Polymer 23 (1982) 1457. [10] W. Schnabel, in: O. G€ uven (Ed.), Cross-linking and scission in polymer, NATO ASI Series, Kluwer Academic Publishers, The Netherlands, 1988, p. 15.
Effect of gamma irradiation conditions on the radiation-induced degradation of isobutylene–isoprene rubber M. S ß en a
€ . Kanto , C. Uzun a, O glu b, S.M. Erdo gan a, V. Deniz c, O. G€ uven
a,*
a
Polymer Chemistry Division, Department of Chemistry, Hacettepe University, 06532 Beytepe, Ankara, Turkey b Food Irradiation and Sterilization Department, Ankara Nuclear Agriculture and Animal Research Center, Turkish Atomic Energy Authority, 06983 Ankara, Turkey c Department of Chemical Engineering, Kocaeli University, 41040 Kocaeli, Turkey
Abstract The effect of gamma irradiation conditions on the radiation-induced degradation of uncrosslinked, commercial isobutylene–isoprene rubbers has been investigated in this study. Influence of dose rate and irradiation atmosphere on the degradation of butyl rubber has been followed by viscosimetric and chromatographic analyses. Limiting viscosity number of all butyl rubbers decreased sharply up to 100 kGy and leveled off at around the same molecular weight, independent of dose rate. Slightly higher decrease in viscosity was observed for samples irradiated in air than in nitrogen especially at low dose rate irradiation. Cross-linking GðX Þ, and chain scission GðSÞ yields of butyl rubbers were calculated by using weight- and number-average molecular weights of irradiated rubber determined by Size Exclusion Chromatography analyses. G-value results showed that chain scission reactions in isobutylene–isoprene rubber in air atmosphere are more favorable than in nitrogen atmosphere, and that lower dose rate enhances chain scission over cross-linking. Ó 2003 Elsevier B.V. All rights reserved. Keywords: Isobutylene–isoprene rubbers; Gamma irradiation; Cross-linking; Degradation
1. Introduction Parallel to the increase in the number of vehicles throughout the world, waste tires pose important economical and ecological problems. Thermoset, polymeric materials, such as rubber tires are of great challenge concerning the environmental and ecological reasons in regard to their treatment methods. Ionizing radiation seems to offer unique
*
Corresponding author. Tel.: +90-312-2977989; fax: +90312-2977989. E-mail address: [email protected] (M. S ß en).
opportunities to tackle the problem of recycling of polymers and rubbers on account of its ability to cause cross-linking or/and chain scission of polymeric materials. There have been many works in the literature dealing with the radiation-induced breakdown of a variety of polymers. Some alteration of the molecular structure and/or morphology of the material may even be enhanced as reported by Burillo et al. [1] in reclaiming polymer and rubber wastes, by the use of ionizing radiation. Although literature is full of publications dealing with the recycling of waste tires [2–4], only a limited amount of work reported on the effect of
0168-583X/03/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0168-583X(03)01111-X
M. Sß en et al. / Nucl. Instr. and Meth. in Phys. Res. B 208 (2003) 480–484
irradiation conditions and additives on the radiation-induced degradation of rubbers. In this study we investigated the effect of dose rate and irradiation atmosphere on the degradation of uncrosslinked, commercial isobutylene–isoprene (IIR) rubber. The effect of radiation-induced degradation on the molecular weight has been investigated by chromatographic and viscosimetric techniques.
2. Experimental
481
2.2. Gamma irradiation IIP specimens of dimensions in 2 cm 2 cm with 5 mm thicknesses were irradiated at ambient temperature in air and nitrogen purged and sealed glass ampoules in Gamma cell 220 type Canadian made c-irradiator for low dose rate (0.18 kGy/h) irradiations and Isslodovately model Russian made c-irradiator for high dose rate (3.0 kGy/h) irradiations. 2.3. Viscosimetric studies
Limiting viscosity number [ ] (dl/g)
Three commercial isobutylene–isoprene (IIP) rubber samples were used in our experiments: Ex 165 (isoprene content: 0.8%) and Ex 268 (isoprene content: 1.7%) were obtained from Exxon company, and BK1685N (isoprene content: 1.7%) was obtained from the Nizhnekamsk company. All these butyl rubbers were used as received.
1.2
(a)
N2 Air
1.0 0.8 0.6 0.4 0.2 0
50
100 150 Dose (kGy)
Limiting viscosity number [ ] (dl/g)
0.0
1.2
200
The limiting viscosity number [g] of the rubbers were determined by using the Huggins equation [5]: 2
gsp =c ¼ ½g þ k 0 ½g c;
ð1Þ
where k 0 is constant for a given polymer at a given temperature in a given solvent, and c is the concentration of solution. The viscosities of solutions were measured by an Ubbelohde type viscometer,
Limiting viscosity number [ ] (dl/g)
2.1. Materials
1.2
(b)
N2 Air
1.0 0.8 0.6 0.4 0.2 0.0
0
50
100 150 Dose (kGy)
200
(c)
N2 Air
1.0 0.8 0.6 0.4 0.2 0.0
0
50
100 150 Dose (kGy)
200
Fig. 1. Variation of limiting viscosity numbers with irradiation dose, for low dose rate irradiation (0.18 kGy/h) in air and N2 ; (a) Ex 165, (b) Ex 268, (c) BK1685N.
M. Sß en et al. / Nucl. Instr. and Meth. in Phys. Res. B 208 (2003) 480–484
tetrahydrofuran (THF) being the solvent used throughout the experiments. The thermostat temperature was maintained at 25 0.02 °C for all measurements.
2.4. Size exclusion chromatography studies
1.2
(a)
N2 Air
1.0 0.8 0.6 0.4 0.2 0.0
0
50
100 150 Dose (kGy) Limiting viscosity number [ ] (dl/g)
Limiting viscosity number [ ] (dl/g)
Weight- and number-average molecular weights of unirradiated and irradiated rubber samples were determined by using a Waters 244 ALC/GPC chromatograph equipped with four Ultrastyragel columns with pore sizes 106 –105 – , and THF as the eluting solvent. The 104 –103 A flow rate was kept at 1 ml/min in all cases. A universal calibration curve based on the elution volume of polyethylene glycol (PEG) standards from Toyo Soda Company and using appropriate Mark Houwink–Sakurada constants, K ¼ 2:0 10 4 and a ¼ 0:67 for butyl rubber in THF [6] was constructed.
1.2
200
3. Results and discussion 3.1. Effect of gamma rays on the limiting viscosity number of IIR In order to see the effect of irradiation dose rate and atmosphere on the degradation of rubber samples, the limiting viscosity numbers were first determined. Fig. 1(a)–(c), show the effect of atmosphere on the limiting viscosity number of butyl rubber at low dose rate irradiation (0.18 kGy/h). Cross-linking reactions occur in most elastomers when treated with ionizing radiation, but only a few rubber varieties such as butyl rubber and butyl vulcanizates whose structural units contain quaternary carbon atoms undergo appreciable degradation reactions [7]. As shown in Fig. 1, limiting viscosity values of rubber samples used in this work decrease significantly up to 100 kGy irradiation dose but do not change appreciably beyond this dose value. Slightly higher values of
Limiting viscosity number [ ] (dl/g)
482
1.2
(b)
N2 Air
1.0 0.8 0.6 0.4 0.2 0.0
0
50
100 150 Dose (kGy)
200
(c)
N2 Air
1.0 0.8 0.6 0.4 0.2 0.0
0
50
100 150 Dose (kGy)
200
Fig. 2. Variation of limiting viscosity numbers with irradiation for high dose rate irradiation (3.0 kGy/h)) in air and N2 ; (a) Ex 165, (b) Ex 268, (c) BK1685N.
M. Sß en et al. / Nucl. Instr. and Meth. in Phys. Res. B 208 (2003) 480–484
483
weight- and number-average molecular weights were calculated. Representative SE chromatograms are given in Fig 3(a) and (b), for EX 268 and BK1685N respectively. We note that both size exclusion chromatograms continuously shift to higher elution volumes with increasing irradiation doses. The extent of shift decreases and peak maximum values converge for doses exceeding 100 kGy. The variation of Mw and Mn values with irradiation dose for Ex 268 are given in Fig. 4, from which we note that both Mw and Mn values decrease sharply up to 100 kGy, but remain almost unchanged beyond this dose value. Very similar decreases were observed for the other rubbers samples. To determine GðSÞ and GðX Þ values the following equations were used for irradiation doses in the range from 0 to 100 kGy [10]:
degradation was observed for low dose rate irradiation in air than in nitrogen for a given absorbed dose value. This is presumably due to the longer contact of rubber with oxygen in air and resulting in higher chain scission reactions [8,9]. On the other hand no significant dependence of limiting viscosity or average molecular weight on the irradiation atmosphere was observed for rubber samples following high dose rate irradiation (3.0 kGy/h), most probably due shorter contact time of the rubber with oxygen, as shown in Fig. 2. 3.2. Effect of irradiation conditions on the G(S) and G(X ) values of IIP For the investigation of the effect of dose rate and other irradiation conditions on the G values for cross-linking GðX Þ, and chain scission reactions GðSÞ on the IIP rubber samples, size exclusion chromatograms were taken, analyzed, and
1=Mw ¼ 1=Mw0 þ ½GðSÞ=2 2GðX ÞD 1:038 10 6 ; ð2Þ
(b)
(a) 0 kGy 0 kGy 21 kGy 21 kGy 31 kGy 31 kGy
62 kGy 125 kGy
62 kGy 94 kGy 125 kGy 157kGy 220 kGy
157kGy 220 kGy
20
25
30 35 40 Retention volume (ml)
45 20
25
30 35 40 Retention volume (ml)
45
Fig. 3. Size exclusion chromatograms of (a) Ex 265 and (b) BK1685N samples (see text).
500000
500000
M w LDR Air M w LDR N2 M n LDR Air M n LDR N2
Mol. Weight
400000 300000 200000 100000 0
(b)
Mw Mw Mn Mn
400000
Mol. Weight
(a)
300000
HDR Air HDR N 2 HDR Air HDR N 2
200000 100000
0
50
100 150 200 Dose (kGy)
250
0
0
50
100 150 200 Dose (kGy)
250
Fig. 4. Effect of dose rate and irradiation atmosphere on the weight and number average molecular weight of Ex 268 samples. Irradiation conditions are given in figures. HDR and LDR represent, high dose rate and low dose rate respectively.
484
M. Sß en et al. / Nucl. Instr. and Meth. in Phys. Res. B 208 (2003) 480–484
Table 1 Chain scission GðSÞ and cross-link GðX Þ yield of butyl rubbers at different irradiation conditions Rubber
Air
N2
GðSÞ
GðX Þ
GðSÞ=GðX Þ
GðSÞ
GðX Þ
GðSÞ=GðX Þ
Low dose rate (LDR) Ex 268 BK1675N
4.2 3.8
0.2 0.3
18.2 12.5
1.9 2.9
0.3 0.4
5.5 7.2
High dose rate (HDR) Ex 268 BK1675N
5.4 2.5
0.5 0.1
10.3 24.7
7.7 6.4
1.1 1.1
7.1 5.9
1=Mn ¼ 1=Mn0 þ ½GðSÞ GðX ÞD 1:038 10 6 ; ð3Þ where Mw0 and Mn0 are the weight- and numberaverage molecular weights of unirradiated samples. Mw and Mn are the corresponding values following exposure to irradiation dose D (kGy). Calculated GðSÞ and GðX Þ values of Ex 268 and BK1685N are listed in Table 1. The GðSÞ=GðX Þ > 4 values in this table show that chain scissions dominate over cross-linking reactions, for both air and nitrogen atmospheres. GðSÞ and GðX Þ values also indicate that irradiation of IIP rubbers in nitrogen atmosphere enhances slightly cross-linking reactions, especially at high dose rate irradiations. Due to the lower isoprene content of Ex 165 and its lower molecular weight compared with the other rubber samples its GðSÞ and GðX Þ values are not presented here.
4. Conclusion Viscosimetric and chromatographic studies show that, beside irradiation atmosphere, the irradiation dose rate is an important parameter for controlling of the degradation of butyl rubbers, especially at low dose rates. For doses exceeding 100
kGy, degradation of butyl rubbers becomes independent of atmosphere and dose rate, all rubber samples studied attain very similar average molecular weight values independent of initial molecular weight and isoprene content at this dose value.
References [1] G. Burillo, R. Clough, T. Czvikovszky, O. G€ uven, A. Le Moel, W. Liu, A. Singh, J. Yang, T. Zaharescu, Radiat. Phys. Chem. 64 (2001) 41. [2] B. Adhikari, D. De, S. Maiti, Prog. Polym. Sci. 25 (2000) 909. [3] V.K. Sharma, F. Fortuna, M. Mincarini, M. Berillo, G. Cornacchia, Appl. Energ. 65 (2000) 381. [4] Y. Fang, M. Zhan, Y. Wang, Mater. and Des. 22 (2001) 123. [5] J.F. Rabek, in: Experimental methods in polymer chemistry, John Wiley & Sons, New York, 1980, p. 127. [6] J.E. Puskas, R. Hutchinson, Rubber Technol. 66 (1993) 742. [7] J. Yang, in: R.D. Cooper, K.E. OÕShea (Eds.), Environmental application of ionizing radiation, Wiley, New York, 1998, p. 601. [8] S. Jipa, M. Giurginca, T. Setnescu, R. Setnescu, G. Ivan, I. Mihalcea, Polym. Degrad. Stab. 54 (1996) 1. [9] R. Chandra, V. Subhash, A.K. Verma, Polymer 23 (1982) 1457. [10] W. Schnabel, in: O. G€ uven (Ed.), Cross-linking and scission in polymer, NATO ASI Series, Kluwer Academic Publishers, The Netherlands, 1988, p. 15.