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Effect of irradiation temperature and dose rate on the mechanical tensile behaviour of low density polyethylene
Eur. Polym. J. Vol. 29, No. 9, pp. 1247-1249, 1993
0014-3057/93$6.00+0.00 PergamonPress Ltd
Printed in Great Britain
EFFECT RATE
ON
OF
IRRADIATION
THE
TEMPERATURE
MECHANICAL
LOW
TENSILE
DENSITY
AND BEHAVIOUR
DOSE OF
POLYETHYLENE
G. SPADARO Dipartimento di Ingegneria Chimica dei Processi e dei Materiali, Universit~i di Palermo, viale delle Scienze, 90128 Palermo, Italy (Received 22 October 1992)
Abstract--The effect of ageing due to gamma radiation, at various dose rates and temperatures, on the mechanical tensile behaviour of low density polyethylene is studied. In order to detect synergisticeffects, tensile tests on samples subjected to thermal treatment corresponding to the same temperature for the same time as for the irradiation tests have also been performed. The results indicate a generalized decrease of the elongation at break, with brittleness of the material for the most severeageing treatments. For modulus values, an increase is observed for particular experimental conditions. These results are related to morphological and structural modifications induced in the polymer by this processing.
INTRODUCTION The effects of gamma radiation on polymers consist mainly of degradation and crosslinking phenomena, the extent of which depends on the chemical structure of the material and on the irradiation conditions such as the presence of gas, the irradiation temperature and the irradiation dose rate [1-3]. Furthermore, together with these main effects, other modifications must be taken into account. In particular the morphology, i.e. the degree of crystallinity and the size and the perfection of macromolecular crystals, can be modified [4]. All these modifications can affect the physical properties of the irradiated polymer and in particular its mechanical tensile behaviour. In fact, crosslinking can increase the modulus and the stress levels but decrease the elongation at break [5]. Degradation phenomena can dramatically modify the mechanical properties, inducing in some cases brittleness: in fact ductile materials significantly degraded may show brittle behaviour. Modifications in the crystallinity due to irradiation can affect mechanical properties and in particular the modulus and the ultimate strength. The situation is more complex when the polymer is irradiated in the presence of air at high temperatures. In this case, effects due to the annealing of the material and modifications in the diffusion coefficient of oxygen in the bulk of the material and in the mobility of free radicals must also be considered. In previous work [6, 7], the effects of gamma radiation, at various dose rates and temperatures, on the molecular structure and on the morphology of low density polyethylene were studied. Results indicated the presence of both degradation and crosslinking phenomena, the former prevailing at low dose rate and, generally, high temperature. These results were interpreted by considering that the kinetics of the radiation oxidative process are controlled by diffusion of atmospheric oxygen in the bulk of the
material and that, on increasing the temperature, both the diffusion coefficient of oxygen in the polymer and the mobility of free radicals produced by irradiation increase. In this work the effects of these molecular and morphological modifications on the mechanical tensile behaviour of the same polyethylene are discussed. EXPERIMENTAL PROCEDURES
The material used was low density polyethylene (LDPE) Riblene A11/SAK, manufactured by Enichem. Sheets I mm thick were obtained from pellets by compression moulding in a laboratory press at 473 K and 2.5 x 10-2 GPa for 5 rain, followed by rapid cooling to room temperature by means of running cold water through the press plates. From these sheets, samples were cut 10mm wide and 100mm long. Irradiation was conducted in air, at three temperatures (298, 323 and 373 K), and at four dose rates: 1 x 102, 3 x 102,9 x 102,5.4 x 103Gy/hr, by the IGS-3, a panoramic 3000 CP°Co irradiator [8]. Dose rates were measured by the Fricke dosimeter; a variance of 5% in the radiation absorption was accepted. The integrated dose was 1 x l0s Gy. Unirradiated samples were also subjected to thermal treatment corresponding to the same temperature for the same time as for irradiation tests. In the following discussion, samples exposed only to thermal treatment are indicated as annealed samples. Mechanical tensile tests were made by an Instron machine mod. I 115. The initial length between the crossheads was 4 cm and the initial deformation rate was 4.2 x 10-s sec-~. The data reported are averaged from at least eight tests. RESULTS AND DISCUSSION
The effects of annealing at 323 and 373 K on mechanical tensile behaviour of LDPE are shown in Figs 1-3, where modulus, elongation at break and ultimate tensile strength vs the ageing time are reported, respectively. Annealing at 323 K causes increase in the modulus and slight decrease in the elongation at break and ultimate tensile strength. At 373 K, more marked
1247
G. SPADARO
1248
13--
300 ~0
280 -
.'-,
~
~" 260 - ~ " ~ ~
r~ 323 K
!
f
~
t 373 K
ll
~.- 240
E 10
o
220 o
o 323 K
200
• 373 K
o "~
9
--
+
-
180 160
0
I
I
I
I
[
200
400
600
800
1000
I 1200
7
, 0
200
400
600
800
I
I
1000
1200
Time (hr)
Time (lit) Fig. 1. Modulus vs time for annealed LDPE.
Fig. 3. Ultimate tensile strength vs time for annealed LDPE.
effects are observed, with increase in the modulus and decrease in both the ultimate strength and elongation at break, In particular for long annealing times (318 and 1000 hr), LDPE changes its tensile behaviour from ductile to brittle with a dramatic decrease in the elongation at break. As already discussed [7], annealing at low temperature (323 K) induces few modifications in the molecular and structural properties of LDPE. This finding is consistent with the mechanical tensile behaviour of the material subjected to the same thermal treatment. At 373 K, gel fractions and intrinsic viscosity data indicated enhancement of oxidative degradation, together with some crosslinking, mainly for long annealing times. Both phenomena, oxidative degradation and crosslinking, contribute to the significant decrease in the elongation at break. Furthermore, calorimetric data showed an increase in the melting temperature, without modification in the melting enthalpy. The increase in the modulus can be related to the improvement in the crystal perfection, as seen by the increase in melting temperature, even if also the effect of annealing in the amorphous phase, such as a decrease in the free volume [9], should be taken into account. Finally, the dependence of ultimate tensile strength on annealing time can be explained by means of the two parameters: viz. increase in modulus, with increase in the stress level values, and decrease in the elongation at break.
The mechanical tensile behaviour vs dose rate for LDPE irradiated at various temperatures is reported in Figs 4-6. In particular the modulus of LDPE irradiated at room temperature (298K), see Fig. 4, is like the modulus of the unirradiated sample, except for the polymer irradiated at the lowest dose rate, for which there is a significant increase. At 323 K, the modulus increases on decreasing the irradiation dose rate, starting, at the highest dose rate (5 x 103Gy/hr), from a value of the same order of magnitude as the unirradiated polymer. Finally, for samples irradiated at 373 K, the modulus is higher than for the unirradiated sample and not significantly affected by irradiation dose rate. For the elongation at break, Er, see Fig. 5, a slight decrease is caused by irradiation at 298 K, with no effect of irradiation dose rate. For 323 K, Er of LDPE irradiated at 5.4 x 103 and 9 x 102 Gy/hr is of the same order of magnitude as for the corresponding samples irradiated at 298 K, whereas a strong decrease for low dose rates (3 x 102 and 1 x 102 Gy/hr) is observed. In all cases, however, the material has ductile behaviour. At the highest irradiation temperature (373 K), a more marked decrease in Er is seen. In particular, at low dose rates (3 x 102 and 1 x 102Gy/hr) the material changes its tensile behaviour from ductile to brittle. The ultimate tensile strength (Fig. 6) of the irradiated polymer is slightly lower than for the unirradiated one with a strong 260
600 ,.-,,
240
-ii
500 n 298 K
a. 220
400
* 323 K
200
• 373 K
300 o
o
200
180
323 K
-
t 373 K o
1oo --
*I N ~ , . , , I 200
400
I 600
Time
140 800
-
'
~
-
a
~
~
--
160
1000
1200
(hr)
Fig. 2. Elongation at break vs time for annealed LDPE.
0
L
I
I
I
I
1000
2000
3000
4000
5000
I 6000
Dose rate (Gy/hr) Fig, 4. Modulus vs irradiation dose rate for irradiated LDPE. Dotted line refers to unaged LDPE.
Mechanical tensile behaviour of LDPE 600
~- 500 400
300 0 •
200
373 K
•
o 100
I 1000
2000
3000
4000
5000
6000
Dose rate (Oy/hr) Fig. 5. Elongation at break vs irradiation dose rate for irradiated LDPE. Dotted line refers to unaged LDPE. strong decrease at the highest temperature and low dose rates, due to the brittleness of the material. Mechanical tensile behaviour of gamma-radiation aged LDPE can be interpreted considering molecular and structural modifications induced in the polymer by this processing, as discussed elsewhere [6, 7]. Irradiation at room temperature [6] causes both degradation and crosslinking and the ageing phenomenon is affected by the diffusion of atmospheric oxygen, which causes increasing degradation on decreasing dose rate. These molecular modifications occur essentially in the amorphous phase, as indicated by calorimetric tests. More complex is the situation for irradiation at high temperature. The increase in temperature increases the diffusion coefficient of oxygen inside the polymer I10] and favours the involvement of free radicals formed by irradiation in oxidative degradation. On the other hand, the tendency of free radicals towards crosslinking is favoured by increase in temperature. As already shown [7], both phenomena occur for our experimental conditions and a prevalence of degradation is usual, except for some extreme conditions. Furthermore also the effect of annealing already indicated must be taken into account. We observed that the melting temperature was not modified by irradiation 14
12 10 .o o
8
6 I
4 0
1000
I
I
I
I
2 0 0 0 3 0 0 0 4 0 0 0 5000 Dose rate (Gy/hr)
I 6000
Fig. 6. Ultimate tensile strength vs irradiation dose rate for irradiated LDPE. Dotted line refers to unaged LDPE. EPJ 2919--0
1249
at 323 K, as for the melting enthalpy, except at 1 x 102 Gy/hr where a significant increase was found. These results are in line with the increase in modulus under the same irradiation conditions. At 373 K, a simultaneous increase in the melting temperature and decrease in melting enthalpy were observed. The increase in the melting temperature was attributed to a reorganization of the crystals favoured by the high temperature and by the decrease in the molecular weight. The decrease in the melting enthalpy was also attributed to the oxidative degradation which caused the formation of "foreign" groups, such as carbonyl and hydroxy, as shown by the presence of additional melting peaks at lower temperature. However, these modifications in the crystallinity behaviour cause an increase in the modulus values, also if the effect of annealing and reduction of molecular weight in the amorphous regions are taken into account. In fact, these effects can modify the free volume in the amorphous phase, thus contributing to the increase in the modulus. Finally the dramatic decrease in the elongation at break of samples irradiated at 373 K and 3 × 102 and 1 x 102 Gy/hr, with brittle behaviour of specimens, is clearly due to the enhancement of oxidative degradation for these experimental conditions. CONCLUDING REMARKS
The irradiation of low density polyethylene at various temperatures and dose rates significantly modifies its mechanical tensile behaviour. A generalized decrease in the elongation at break is observed, with brittleness induced in the material for the most severe ageing conditions (high temperature and low dose rate). Always under these conditions, the modulus increases. These effects are due to the molecular modifications (crosslinking and oxidative degradation) and the consequent morphological effects with improvement in the crystal perfection, even if some modification in the amorphous phase with decrease in the free volume should be taken into account. REFERENCES
1. M. Dole. The Radiation Chemistry of Macromolecules. Academic Press, New York (1972). 2. A. Charlesby. Radiation effects in polymers. In Polymer Science (edited by A. D. Jenkins). North-Holland, Amsterdam (1972). 3. D. W. Clegg and A. A. Collier. Irradiation Effects on Polymers. Elsevier, London (1991). 4. H. Jenkins and A. Keller. J. Macromolec. Sci.-Phys. Bll, 155 (1983). 5. L. E. Nielsen. Mechanical Properties of Polymers and Composites. Marcel Dekker, New York (1974). 6. G. Spadaro, E. Calderaro and G. Rizzo. Acta Polym. 40, 702 (1989). 7. G. Spadaro. Eur. Polym. J. 29, 851 (1993). 8. E. Calderaro, E. 0lived and P. Tallarita. Quaderni dell'Istituto di Applicazioni ed Impianti Nucleari, Voi. 3. University of Palermo (1980). 9. C. E. Struik. Physical Ageing in Amorphous Polymers and Other Materials. Elsevier, Amsterdam (1978). 10. W. R. Vieth. Diff~ion In and ThroughPolymers. Oxford University Press, Oxford (1991).
0014-3057/93$6.00+0.00 PergamonPress Ltd
Printed in Great Britain
EFFECT RATE
ON
OF
IRRADIATION
THE
TEMPERATURE
MECHANICAL
LOW
TENSILE
DENSITY
AND BEHAVIOUR
DOSE OF
POLYETHYLENE
G. SPADARO Dipartimento di Ingegneria Chimica dei Processi e dei Materiali, Universit~i di Palermo, viale delle Scienze, 90128 Palermo, Italy (Received 22 October 1992)
Abstract--The effect of ageing due to gamma radiation, at various dose rates and temperatures, on the mechanical tensile behaviour of low density polyethylene is studied. In order to detect synergisticeffects, tensile tests on samples subjected to thermal treatment corresponding to the same temperature for the same time as for the irradiation tests have also been performed. The results indicate a generalized decrease of the elongation at break, with brittleness of the material for the most severeageing treatments. For modulus values, an increase is observed for particular experimental conditions. These results are related to morphological and structural modifications induced in the polymer by this processing.
INTRODUCTION The effects of gamma radiation on polymers consist mainly of degradation and crosslinking phenomena, the extent of which depends on the chemical structure of the material and on the irradiation conditions such as the presence of gas, the irradiation temperature and the irradiation dose rate [1-3]. Furthermore, together with these main effects, other modifications must be taken into account. In particular the morphology, i.e. the degree of crystallinity and the size and the perfection of macromolecular crystals, can be modified [4]. All these modifications can affect the physical properties of the irradiated polymer and in particular its mechanical tensile behaviour. In fact, crosslinking can increase the modulus and the stress levels but decrease the elongation at break [5]. Degradation phenomena can dramatically modify the mechanical properties, inducing in some cases brittleness: in fact ductile materials significantly degraded may show brittle behaviour. Modifications in the crystallinity due to irradiation can affect mechanical properties and in particular the modulus and the ultimate strength. The situation is more complex when the polymer is irradiated in the presence of air at high temperatures. In this case, effects due to the annealing of the material and modifications in the diffusion coefficient of oxygen in the bulk of the material and in the mobility of free radicals must also be considered. In previous work [6, 7], the effects of gamma radiation, at various dose rates and temperatures, on the molecular structure and on the morphology of low density polyethylene were studied. Results indicated the presence of both degradation and crosslinking phenomena, the former prevailing at low dose rate and, generally, high temperature. These results were interpreted by considering that the kinetics of the radiation oxidative process are controlled by diffusion of atmospheric oxygen in the bulk of the
material and that, on increasing the temperature, both the diffusion coefficient of oxygen in the polymer and the mobility of free radicals produced by irradiation increase. In this work the effects of these molecular and morphological modifications on the mechanical tensile behaviour of the same polyethylene are discussed. EXPERIMENTAL PROCEDURES
The material used was low density polyethylene (LDPE) Riblene A11/SAK, manufactured by Enichem. Sheets I mm thick were obtained from pellets by compression moulding in a laboratory press at 473 K and 2.5 x 10-2 GPa for 5 rain, followed by rapid cooling to room temperature by means of running cold water through the press plates. From these sheets, samples were cut 10mm wide and 100mm long. Irradiation was conducted in air, at three temperatures (298, 323 and 373 K), and at four dose rates: 1 x 102, 3 x 102,9 x 102,5.4 x 103Gy/hr, by the IGS-3, a panoramic 3000 CP°Co irradiator [8]. Dose rates were measured by the Fricke dosimeter; a variance of 5% in the radiation absorption was accepted. The integrated dose was 1 x l0s Gy. Unirradiated samples were also subjected to thermal treatment corresponding to the same temperature for the same time as for irradiation tests. In the following discussion, samples exposed only to thermal treatment are indicated as annealed samples. Mechanical tensile tests were made by an Instron machine mod. I 115. The initial length between the crossheads was 4 cm and the initial deformation rate was 4.2 x 10-s sec-~. The data reported are averaged from at least eight tests. RESULTS AND DISCUSSION
The effects of annealing at 323 and 373 K on mechanical tensile behaviour of LDPE are shown in Figs 1-3, where modulus, elongation at break and ultimate tensile strength vs the ageing time are reported, respectively. Annealing at 323 K causes increase in the modulus and slight decrease in the elongation at break and ultimate tensile strength. At 373 K, more marked
1247
G. SPADARO
1248
13--
300 ~0
280 -
.'-,
~
~" 260 - ~ " ~ ~
r~ 323 K
!
f
~
t 373 K
ll
~.- 240
E 10
o
220 o
o 323 K
200
• 373 K
o "~
9
--
+
-
180 160
0
I
I
I
I
[
200
400
600
800
1000
I 1200
7
, 0
200
400
600
800
I
I
1000
1200
Time (hr)
Time (lit) Fig. 1. Modulus vs time for annealed LDPE.
Fig. 3. Ultimate tensile strength vs time for annealed LDPE.
effects are observed, with increase in the modulus and decrease in both the ultimate strength and elongation at break, In particular for long annealing times (318 and 1000 hr), LDPE changes its tensile behaviour from ductile to brittle with a dramatic decrease in the elongation at break. As already discussed [7], annealing at low temperature (323 K) induces few modifications in the molecular and structural properties of LDPE. This finding is consistent with the mechanical tensile behaviour of the material subjected to the same thermal treatment. At 373 K, gel fractions and intrinsic viscosity data indicated enhancement of oxidative degradation, together with some crosslinking, mainly for long annealing times. Both phenomena, oxidative degradation and crosslinking, contribute to the significant decrease in the elongation at break. Furthermore, calorimetric data showed an increase in the melting temperature, without modification in the melting enthalpy. The increase in the modulus can be related to the improvement in the crystal perfection, as seen by the increase in melting temperature, even if also the effect of annealing in the amorphous phase, such as a decrease in the free volume [9], should be taken into account. Finally, the dependence of ultimate tensile strength on annealing time can be explained by means of the two parameters: viz. increase in modulus, with increase in the stress level values, and decrease in the elongation at break.
The mechanical tensile behaviour vs dose rate for LDPE irradiated at various temperatures is reported in Figs 4-6. In particular the modulus of LDPE irradiated at room temperature (298K), see Fig. 4, is like the modulus of the unirradiated sample, except for the polymer irradiated at the lowest dose rate, for which there is a significant increase. At 323 K, the modulus increases on decreasing the irradiation dose rate, starting, at the highest dose rate (5 x 103Gy/hr), from a value of the same order of magnitude as the unirradiated polymer. Finally, for samples irradiated at 373 K, the modulus is higher than for the unirradiated sample and not significantly affected by irradiation dose rate. For the elongation at break, Er, see Fig. 5, a slight decrease is caused by irradiation at 298 K, with no effect of irradiation dose rate. For 323 K, Er of LDPE irradiated at 5.4 x 103 and 9 x 102 Gy/hr is of the same order of magnitude as for the corresponding samples irradiated at 298 K, whereas a strong decrease for low dose rates (3 x 102 and 1 x 102 Gy/hr) is observed. In all cases, however, the material has ductile behaviour. At the highest irradiation temperature (373 K), a more marked decrease in Er is seen. In particular, at low dose rates (3 x 102 and 1 x 102Gy/hr) the material changes its tensile behaviour from ductile to brittle. The ultimate tensile strength (Fig. 6) of the irradiated polymer is slightly lower than for the unirradiated one with a strong 260
600 ,.-,,
240
-ii
500 n 298 K
a. 220
400
* 323 K
200
• 373 K
300 o
o
200
180
323 K
-
t 373 K o
1oo --
*I N ~ , . , , I 200
400
I 600
Time
140 800
-
'
~
-
a
~
~
--
160
1000
1200
(hr)
Fig. 2. Elongation at break vs time for annealed LDPE.
0
L
I
I
I
I
1000
2000
3000
4000
5000
I 6000
Dose rate (Gy/hr) Fig, 4. Modulus vs irradiation dose rate for irradiated LDPE. Dotted line refers to unaged LDPE.
Mechanical tensile behaviour of LDPE 600
~- 500 400
300 0 •
200
373 K
•
o 100
I 1000
2000
3000
4000
5000
6000
Dose rate (Oy/hr) Fig. 5. Elongation at break vs irradiation dose rate for irradiated LDPE. Dotted line refers to unaged LDPE. strong decrease at the highest temperature and low dose rates, due to the brittleness of the material. Mechanical tensile behaviour of gamma-radiation aged LDPE can be interpreted considering molecular and structural modifications induced in the polymer by this processing, as discussed elsewhere [6, 7]. Irradiation at room temperature [6] causes both degradation and crosslinking and the ageing phenomenon is affected by the diffusion of atmospheric oxygen, which causes increasing degradation on decreasing dose rate. These molecular modifications occur essentially in the amorphous phase, as indicated by calorimetric tests. More complex is the situation for irradiation at high temperature. The increase in temperature increases the diffusion coefficient of oxygen inside the polymer I10] and favours the involvement of free radicals formed by irradiation in oxidative degradation. On the other hand, the tendency of free radicals towards crosslinking is favoured by increase in temperature. As already shown [7], both phenomena occur for our experimental conditions and a prevalence of degradation is usual, except for some extreme conditions. Furthermore also the effect of annealing already indicated must be taken into account. We observed that the melting temperature was not modified by irradiation 14
12 10 .o o
8
6 I
4 0
1000
I
I
I
I
2 0 0 0 3 0 0 0 4 0 0 0 5000 Dose rate (Gy/hr)
I 6000
Fig. 6. Ultimate tensile strength vs irradiation dose rate for irradiated LDPE. Dotted line refers to unaged LDPE. EPJ 2919--0
1249
at 323 K, as for the melting enthalpy, except at 1 x 102 Gy/hr where a significant increase was found. These results are in line with the increase in modulus under the same irradiation conditions. At 373 K, a simultaneous increase in the melting temperature and decrease in melting enthalpy were observed. The increase in the melting temperature was attributed to a reorganization of the crystals favoured by the high temperature and by the decrease in the molecular weight. The decrease in the melting enthalpy was also attributed to the oxidative degradation which caused the formation of "foreign" groups, such as carbonyl and hydroxy, as shown by the presence of additional melting peaks at lower temperature. However, these modifications in the crystallinity behaviour cause an increase in the modulus values, also if the effect of annealing and reduction of molecular weight in the amorphous regions are taken into account. In fact, these effects can modify the free volume in the amorphous phase, thus contributing to the increase in the modulus. Finally the dramatic decrease in the elongation at break of samples irradiated at 373 K and 3 × 102 and 1 x 102 Gy/hr, with brittle behaviour of specimens, is clearly due to the enhancement of oxidative degradation for these experimental conditions. CONCLUDING REMARKS
The irradiation of low density polyethylene at various temperatures and dose rates significantly modifies its mechanical tensile behaviour. A generalized decrease in the elongation at break is observed, with brittleness induced in the material for the most severe ageing conditions (high temperature and low dose rate). Always under these conditions, the modulus increases. These effects are due to the molecular modifications (crosslinking and oxidative degradation) and the consequent morphological effects with improvement in the crystal perfection, even if some modification in the amorphous phase with decrease in the free volume should be taken into account. REFERENCES
1. M. Dole. The Radiation Chemistry of Macromolecules. Academic Press, New York (1972). 2. A. Charlesby. Radiation effects in polymers. In Polymer Science (edited by A. D. Jenkins). North-Holland, Amsterdam (1972). 3. D. W. Clegg and A. A. Collier. Irradiation Effects on Polymers. Elsevier, London (1991). 4. H. Jenkins and A. Keller. J. Macromolec. Sci.-Phys. Bll, 155 (1983). 5. L. E. Nielsen. Mechanical Properties of Polymers and Composites. Marcel Dekker, New York (1974). 6. G. Spadaro, E. Calderaro and G. Rizzo. Acta Polym. 40, 702 (1989). 7. G. Spadaro. Eur. Polym. J. 29, 851 (1993). 8. E. Calderaro, E. 0lived and P. Tallarita. Quaderni dell'Istituto di Applicazioni ed Impianti Nucleari, Voi. 3. University of Palermo (1980). 9. C. E. Struik. Physical Ageing in Amorphous Polymers and Other Materials. Elsevier, Amsterdam (1978). 10. W. R. Vieth. Diff~ion In and ThroughPolymers. Oxford University Press, Oxford (1991).