- Email: [email protected]
Effects of electron beam irradiations on the structure and mechanical properties of polycarbonate
ARTICLE IN PRESS
Radiation Physics and Chemistry 74 (2005) 31–35 www.elsevier.com/locate/radphyschem
Effects of electron beam irradiations on the structure and mechanical properties of polycarbonate J. Chena,, M. Czaykaa, Roberto M. Uribeb b
a School of Technology, Kent State University—Ashtabula, 43 Street, Ashtabula, OH 44001, USA Program on Electron Beam Technology, Kent State University, P. O. Box 1028 Middlefield, OH 44062, USA
Received 8 March 2004; accepted 13 December 2004
Abstract Structure and mechanical properties of polycarbonate (PC) were investigated with varied electron beam processing parameters such as dose, dose rate and dose fractionation. PC showed noticeable decrease in ductility after doses of 100 kGy; a decrease in tensile strength is relatively minor. Molecular weight degradation was also noticed after 100 kGy. At 150 kGy and high dose rate (21 kGy/s), the tensile strength and ductility were evaluated against dose fractionation. It was found that tensile strength and ductility decreased with increasing number of passes, which resulted from longer cooling off times. The same temperature effect was observed with lower dose rate (1.2 kGy/s) when mechanical properties of irradiated samples were decreased significantly compared with samples irradiated with higher dose rate. No significant variation in molecular weight change was noticed in both cases. r 2005 Elsevier Ltd. All rights reserved. Keywords: Polycarbonate (PC); Mechanical properties; Electron beam irradiation
1. Introduction Electron beams have been used to process polymers for enhancing mechanical, chemical and other properties (Singh and Silveman, 1992). Radiation-processed products that are now in wide use include wires and cables with cross-linked insulation, components of tires, and medical products. One of the most important process requirements is the absorbed dose in the irradiated material. The required dose is obtained by controlling dose rate (beam current) and conveyor speed. The recent developments in electron beam accelerators allow the machines to produce much higher beam current, and consequently beam heating of the processed material is Corresponding author. Tel.: +1 440 964 4311; fax: +1 440 964 4269. E-mail address: [email protected] (J. Chen).
inevitable. Some high-dose processes require continuous product cooling or multiple passes under the beam to provide time for heat dissipation. In this respect, very limited work has been done related to the effects of the combination of processing parameters on the mechanical properties of irradiated polymers. When the total dose needs to be applied through several passes, there is considerable time in between for the sample to cool down. Therefore using different conveyor speeds and number of passes to obtain the same total dose, might result in differences in structural changes or properties in the irradiated polymer since they are irradiated at different temperatures. Temperature dependence of radiation degradation or crosslinking was observed by earlier studies for polymers such as polytetrafluoroethylene (PTFE) and polystyrene (PS) (Oshima et al., 1995; Takashika et al., 1999). PTFE was found to have degradation via chain scission and
0969-806X/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.radphyschem.2004.12.004
ARTICLE IN PRESS 32
J. Chen et al. / Radiation Physics and Chemistry 74 (2005) 31–35
cross-links at high temperature above the melting temperature. Seguchi et al. (2002) have studied properties of polycarbonate (PC) and polysulfone (PSF) under different temperatures up to its glass transitional temperature. They reported that the hardness could be well controlled by the selection of dose and temperature. This work reports some results on the mechanical properties of PC when irradiated at different processing parameters such as dose, dose rate as well as number of passes under the beam. It has been reported by Acierno et al. (1980, 1981), that cross-linking predominates at small doses and main chain scission happens at higher doses. The radiation-induced chain scission was also reported by different authors (Rivaton et al., 1983, Kalkar et al., 1992). Kudon et al. (1996) compared high dose rate pulsed electron beam (4.2E10 Gy/s) results with data obtained from gamma irradiation at a low dose rate (0.51 Gy/s). They found no dose rate effects on scission probabilities of PC based on molecular weight measurements. In this study, we used electron beam operated under two different beam currents to achieve different dose rates. The range of dose rates in this study is much smaller than other studies, which used two different irradiation sources. Mechanical properties such as tensile strength and elongation at break as well as molecular weight changes were evaluated.
2. Experimental The sample material was lexanr PC manufactured by GE. Rectangular shape samples were cut with dimensions 17.5 cm 40.0 cm 3.2 mm thick. The panels were irradiated at the Kent State University’s NEO Beam facility. The facility houses a 150 kW electron beam accelerator and a radiation dosimetry laboratory. The samples were exposed to a 2 MeV electron beam. The beam length is 7.5 cm and the beam is scanned over a width of 1.2 m. Dosimetry was performed using Far West Technology, FWT-60-00 radiochromic dye film as described in ASTM E 1275-98. Three experiments were performed with different sample panels. In the first one, a group of samples was irradiated to different dose values by varying the number of passes that the samples were sent under the beam. The beam current was kept constant to a value of 36 mA beam current, yielding a dose rate of about 22 kGy/s. The conveyor speed was set to 9.1 m/min (30 ft/min). The samples were irradiated for 1, 2, 4, 6, and 8 passes under the beam corresponding to doses of 25, 50, 100, 150 and 200 kGy, respectively. In the second experiment, the effect of fractionated dose was studied. To do this, a group of samples was irradiated at the following conveyor speeds: 1.5, 3.0, 4.6, 6.1, 9.1 and 12.2 m/min (5, 10, 15, 20, 30, 40 ft/min), and the dose was kept constant at 150 kGy, by going through
the beam 1, 2, 3, 4, 6 and 8 times. The time between exposures varied from 2 to 10 min depending on the conveyor speed. In a third experiment, sample panels were irradiated to the same dose (150 kGy) but at two different dose rates, 22 and 1.2 kGy/s. This was accomplished by using two beam currents, 36 and 2 mA. The conveyor speed for the high dose rate was 1.5 m/min (5 ft/min) and the samples were irradiated for a single pass under the beam. In the lower dose rate case, the sample panels had to be irradiated for a total of 17 passes. However, samples were irradiated in a linear motion system (Korwin et al., 2000) which limited the time between exposures to about 20 s. After the irradiation, the samples were characterized using the following techniques. The first characterization technique consisted of the evaluation of tensile strength and elongation at break for each set of irradiation conditions. Irradiated panels from each group were machined using a CNC mill programmed to cut tensile samples according to ASTM 638. The tensile machine used in this study was an Instron/Saytex T Series with a 25 kN load cell. Samples were placed between the jaws and a load was applied at a rate of 2.54 cm/min until failure occurred. The strain was measured using the cross-head travel measured by the machine. Five samples were used and results were averaged for each data point. The second characterization technique consisted in the determination of the glass transition temperature (Tg) for each set of irradiation conditions. This was accomplished using differential scanning calorimetry (DSC). The instrument used to conduct these evaluations was a TA Q100 DSC. Samples were prepared by slicing sections of unstressed tensile samples used in the tensile tests. The samples measured approximately 3 mm 3 mm 1 mm thick. The mass of each sample was between 6 and 10 mg. Each sample was placed in a standard cup and covered with a lid and roll crimped. Samples were heated at a rate of 15 1C/min under nitrogen. The Tg for each sample was determined using the output data and TA’s built in software. The third method employed to characterize each irradiation condition was to measure the apparent viscosity of the samples at elevated temperature. The instrument used was a Monsanto MPT Processability Tester. Samples were prepared by shearing approximately 5 mm 5 mm pellets from unstressed material leftover from the two previous test methods. Ten grams of pellets were added to the heated cylinder of the MPT. Samples were held in the cylinder for 10 min to allow them to reach the cylinder temperature. The instrument was then activated to start the test. The instrument was programmed to push a heated piston through the cylinder at a rate of 2.54 cm/min. The material was extruded through a die with an L/D ratio of 20. The
ARTICLE IN PRESS J. Chen et al. / Radiation Physics and Chemistry 74 (2005) 31–35
3. Results and discussions
log Z ¼ 3:4log Z w þ k,
67
100 Strength Elongation
90
66
65
70 60
64
Elongation (%)
80 Strength (MPa)
100 95 90 85 80 70 65 60 0
50
100 150 Dose (kGy)
200
250
Fig. 2. Percentage of molecular weight with respect to a control sample as a function of dose for PC samples irradiated to doses in the interval from 25 to 200 kGy.
152
Tg (°C)
148
144
140
136
where Z is the melt viscosity, and Zw is the average molecular weight. Fig. 2 shows the relationship between the ratio of average molecular weight with respect to control and dose. It is easily noticed that decrease in molecular weight happens after a dose of about 100 kGy indicating that chain scission is the main mechanism at the higher dose levels. DSC was also used to characterize the variations in molecular weight. In amorphous polymers, Tg is found
50 63 40 62 50
105
75
Fig. 1 shows the tensile strength and elongation at break as a function of dose. Strength and elongation at break decrease as dose increases, with relatively insignificant changes before 100 kGy. The trend agrees well with previous reports (Arau´jo et al., 1998) with gamma irradiation. However, even with 200 kGy the change in tensile strength is within 5%, well below the percentage reported by the same authors. On the other hand, the change in ductility was significant, about 30% at a dose of 200 kGy. Melt viscosity analysis was used to characterize the change in molecular weight. The most important structural variable determining the flow properties of polymers is the molecular weight Z. It was well established (Fox et al., 1956) for essentially all polymers studied that for values of Z above a critical value Zc (which is about 600 for many polymers),
0
110
Zw/Zwo (%)
force on the piston was recorded by the instrument, which in turn calculated the apparent viscosity. Each condition set was evaluated at the same rate at a temperature of 275 1C. Three tests were run for each condition and the results were averaged.
33
100 150 Dose (kGy)
200
30 250
Fig. 1. Ultimate tensile strength and ductility as functions of dose for PC samples irradiated at doses between 25 and 200 kGy.
132 0
50
100 150 Dose (kGy)
200
250
Fig. 3. Glass transition temperature as a function of dose for PC samples.
to vary with the average molecular weight. Fig. 3 shows the DSC data at different doses. The trend agrees with the viscosity analysis. The effects of the number of passes on strength and ductility are shown in Fig. 4. In this group the beam current was kept the same at 36 mA (same dose rate). All samples were irradiated to the same total dose of 150 kGy. It is apparent from Fig. 4 that the mechanical properties decrease more with an increase in the number of passes. In other words, radiation caused more damage in mechanical properties when the same amount of dose was applied in more passes with shorter period of exposure per pass and longer period of cooling off time between passes than with the total dose in a single pass. Again, minor changes in strength and more significant variations in elongation at break are observed. The variations in mechanical properties noticed here could be a direct effect of temperature. No significant differences were observed in degradation of
ARTICLE IN PRESS J. Chen et al. / Radiation Physics and Chemistry 74 (2005) 31–35
34 68
100
67
90
148
70 65 60 64
144 140
50
136
63
40
62
30 0
2
4 6 Number of Passes
8
132
10
0
Fig. 4. Ultimate tensile strength and ductility as functions of number of passes for PC samples irradiated at total dose of 150 kGy. Dose/pass is 150, 75, 50, 37.5, 25 and 18.75 kGy corresponding to 1, 2, 3, 4, 6 and 8 passes.
110 105 100 Zw/Zwo (%)
Tg (°C)
80
66
Elongation (%)
Strength (MPa)
152
Strength Elongation
95 90 85 80 75 70 65 60 0
1
2
3 4 5 6 Number of Passes
7
8
9
Fig. 5. Percentage of molecular weight with respect to a control sample as a function of number of passes for a PC sample irradiated to 150 kGy. Dose/pass is 150, 75, 50, 37.5, 25 and 18.75 kGy corresponding to 1, 2, 3, 4, 6 and 8 passes.
molecular weight, which can be concluded from the results of viscosity and DSC analysis in Figs. 5 and 6. Although, samples irradiated with eight passes show a deviation from the rest of the group. Further study needs to be carried out to see if this is a trend after certain number of passes. With only one pass when there was no cooling off time, the tensile strength and ductility are comparable to the control sample. Seguchi et al. (2002) investigated the mechanical properties of PC irradiated at high temperatures. They found that the Rockwell hardness decreased with increasing doses with samples irradiated at room temperature. However, samples irradiated at higher temperatures showed an increase in hardness. The optimum hardness was achieved by irradiation at around the glass transition
1
2
3
4
5
6
7
8
9
Number of Passes Fig. 6. Glass transition temperature as a function of number of passes for a PC sample irradiated at 150 kGy. Dose/pass is 150, 75, 50, 37.5, 25 and 18.75 kGy corresponding to 1, 2, 3, 4, 6 and 8 passes.
temperature with very low dose (3 kGy) when radiation damage to materials is negligible. They attributed the improvement of mechanical properties to the denser molecular packing in the polymer matrix by rearrangement of molecules with synergistic effect of radiation and temperature. At low temperature, the molecular rearrangement might be difficult. In our study, intrinsic heating caused by the electron beam while using high beam current resulted in the same effect. The higher temperature may promote denser molecular packing as suggested by Seguchi et al. (2002). As a result, damage in mechanical properties is decreased. Table 1 summarizes the results for the samples irradiated to different dose rates in terms of their mechanical properties as well as viscosity and DSC measurements. The higher dose rate used here produced no change in tensile strength or ductility against the control samples as mentioned above. Both tensile strength and ductility decreased with radiation dose for samples irradiated at lower dose rate. The most significant change was the ductility, which dropped to 25% of the control. The same temperature effect is noticed here. Dose rate may not affect scission probabilities for PC as reported by Kudoh et al. (1996). They found no differences in degradation of the molecular weight for PC samples irradiated by high dose rate pulsed power e-beam and low dose rate gamma irradiation. Our analysis supports their conclusion, we did not notice any meaningful variations in molecular weight at different dose rates. However, mechanical properties can have noticeable differences as a result of different dose rates as observed in this study. Again, the temperature variations caused by the different dose rates produced by the electron beam is the main effect.
ARTICLE IN PRESS J. Chen et al. / Radiation Physics and Chemistry 74 (2005) 31–35
35
Table 1 Comparison of mechanical, glass transition temperatures, and apparent viscosity measurements of polycarbonate samples irradiated to the same dose but using two different dose rates Dose rate (kGy/s)
Tg (1C)
Zw/Zwo (%)
Tensile strength (MPa)
Elongation (%)
22.0 1.2
147.370.5 146.870.5
8570.35 8370.89
65.4470.55 63.7870.41
8074 2576
Uncertainties refer to one standard deviation from a set of measurements.
4. Conclusions Molecular weight degradation of PC may largely depend on dose. Mechanical properties such as tensile strength and ductility, on the other hand, are affected by temperature. The temperature variation can be a result of using different processing parameters such as beam current (dose rate effect) and a combination of conveyor speed and number of passes under the beam. The degradation of mechanical properties seems to lessen under higher temperature for PC. Similar temperature effects could exist for various polymers. For radiation processing of polymer materials, attention should be paid to other processing parameters besides dose, if mechanical properties are the key properties to be controlled in the irradiation process.
Acknowledgements The authors wish to acknowledge Mr. Edward Walton at Kent State University-Ashtabula for performing viscosity tests. The authors would also like to thank NEO Beam, the Alliance of Kent State University and Mercury Plastics Inc. for the use of the electron beam accelerator and thermal analysis facilities.
References Acierno, D., La Mantia, F.P., Titomanlio, G., 1980. Radiation effects on a polycarbonate. Radiat. Phys. Chem. 16, 95. Acierno, D., La Mantia, F.P., Spadaro, G., Titomanlio, G., 1981. Effect of radiation conditions on some properties of a polycarbonate. Radiat. Phys. Chem. 17, 31.
Arau´jo, E.S., Khoury, H.J., Silverira, S.V., 1998. Effects of gamma-irradiation on some properties of durolon polycarbonate. Radiat. Phys. Chem. 53, 79. Fox, T.G., Gratch, S., Loshaek, S., 1956. Viscosity relationships for polymers in bulk and concentrated solutions. In: Erich, F.R. (Ed.), Reology—Theory and Applications, Vol. 1. Academic Press, New York (Chapter 12). Kalkar, A.K., Shankar, K., Suresh, C., Subhas, C., 1992. Effect of gamma-irradiation on structural and electrical properties of poly (bisphenol-A carbonate) films. Radiat. Phys. Chem. 39, 435. Korwin, D.M., Twieg, R.J., Vargas-Aburto, C., Uribe, R.M., 2000. Progress report on cross-linked polyimides via electron beam irradiation, Progress Report to NASA Glenn Research Center Grant # NCC 3-721, KSU-PEBT, September–December, 2000. Kudoh, H., Celina, M., Malone, G.M., Kaye, R.J., Gillen, K.T., Clough, R.L., 1996. Pulsed e-beam irradiation of polymers—a comparision of dose rate effects and LET effects. Radiat. Phys. Chem. 48, 555. Oshima, A., Tabata, Y., Kudoh, H., Seguchi, T., 1995. Radiation induced crosslinking of polytetrafluoroethylene. Radiat. Phys. Chem. 45, 269. Rivaton, A., Daniel, S., Lemaire, J., 1983. The photochemistry of bisphenyl—a polycarbonate reconsidered. Polym. Photochem. 3, 463. Seguchi, T., Yagi, T., Ishikawa, S., Sano, Y., 2002. New material synthesis by radiation processing at high temperature—polymer modification with improved irradiation technology. Radiat. Phys. Chem. 63, 35. Singh, A., Silveman, J. (Eds.), 1992, Radiation Processing of Polymers. Hanser Publisher, New York pp. 2–6 and 231–247. Takashika, K., Oshima, A., Kuramoto, M., Seguchi, T., Tabata, Y., 1999. Temperature effects on radiation induced phenomena in polystyrene having atactic and syndiotactic structures. Radiat. Phys. Chem. 55, 399.
Radiation Physics and Chemistry 74 (2005) 31–35 www.elsevier.com/locate/radphyschem
Effects of electron beam irradiations on the structure and mechanical properties of polycarbonate J. Chena,, M. Czaykaa, Roberto M. Uribeb b
a School of Technology, Kent State University—Ashtabula, 43 Street, Ashtabula, OH 44001, USA Program on Electron Beam Technology, Kent State University, P. O. Box 1028 Middlefield, OH 44062, USA
Received 8 March 2004; accepted 13 December 2004
Abstract Structure and mechanical properties of polycarbonate (PC) were investigated with varied electron beam processing parameters such as dose, dose rate and dose fractionation. PC showed noticeable decrease in ductility after doses of 100 kGy; a decrease in tensile strength is relatively minor. Molecular weight degradation was also noticed after 100 kGy. At 150 kGy and high dose rate (21 kGy/s), the tensile strength and ductility were evaluated against dose fractionation. It was found that tensile strength and ductility decreased with increasing number of passes, which resulted from longer cooling off times. The same temperature effect was observed with lower dose rate (1.2 kGy/s) when mechanical properties of irradiated samples were decreased significantly compared with samples irradiated with higher dose rate. No significant variation in molecular weight change was noticed in both cases. r 2005 Elsevier Ltd. All rights reserved. Keywords: Polycarbonate (PC); Mechanical properties; Electron beam irradiation
1. Introduction Electron beams have been used to process polymers for enhancing mechanical, chemical and other properties (Singh and Silveman, 1992). Radiation-processed products that are now in wide use include wires and cables with cross-linked insulation, components of tires, and medical products. One of the most important process requirements is the absorbed dose in the irradiated material. The required dose is obtained by controlling dose rate (beam current) and conveyor speed. The recent developments in electron beam accelerators allow the machines to produce much higher beam current, and consequently beam heating of the processed material is Corresponding author. Tel.: +1 440 964 4311; fax: +1 440 964 4269. E-mail address: [email protected] (J. Chen).
inevitable. Some high-dose processes require continuous product cooling or multiple passes under the beam to provide time for heat dissipation. In this respect, very limited work has been done related to the effects of the combination of processing parameters on the mechanical properties of irradiated polymers. When the total dose needs to be applied through several passes, there is considerable time in between for the sample to cool down. Therefore using different conveyor speeds and number of passes to obtain the same total dose, might result in differences in structural changes or properties in the irradiated polymer since they are irradiated at different temperatures. Temperature dependence of radiation degradation or crosslinking was observed by earlier studies for polymers such as polytetrafluoroethylene (PTFE) and polystyrene (PS) (Oshima et al., 1995; Takashika et al., 1999). PTFE was found to have degradation via chain scission and
0969-806X/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.radphyschem.2004.12.004
ARTICLE IN PRESS 32
J. Chen et al. / Radiation Physics and Chemistry 74 (2005) 31–35
cross-links at high temperature above the melting temperature. Seguchi et al. (2002) have studied properties of polycarbonate (PC) and polysulfone (PSF) under different temperatures up to its glass transitional temperature. They reported that the hardness could be well controlled by the selection of dose and temperature. This work reports some results on the mechanical properties of PC when irradiated at different processing parameters such as dose, dose rate as well as number of passes under the beam. It has been reported by Acierno et al. (1980, 1981), that cross-linking predominates at small doses and main chain scission happens at higher doses. The radiation-induced chain scission was also reported by different authors (Rivaton et al., 1983, Kalkar et al., 1992). Kudon et al. (1996) compared high dose rate pulsed electron beam (4.2E10 Gy/s) results with data obtained from gamma irradiation at a low dose rate (0.51 Gy/s). They found no dose rate effects on scission probabilities of PC based on molecular weight measurements. In this study, we used electron beam operated under two different beam currents to achieve different dose rates. The range of dose rates in this study is much smaller than other studies, which used two different irradiation sources. Mechanical properties such as tensile strength and elongation at break as well as molecular weight changes were evaluated.
2. Experimental The sample material was lexanr PC manufactured by GE. Rectangular shape samples were cut with dimensions 17.5 cm 40.0 cm 3.2 mm thick. The panels were irradiated at the Kent State University’s NEO Beam facility. The facility houses a 150 kW electron beam accelerator and a radiation dosimetry laboratory. The samples were exposed to a 2 MeV electron beam. The beam length is 7.5 cm and the beam is scanned over a width of 1.2 m. Dosimetry was performed using Far West Technology, FWT-60-00 radiochromic dye film as described in ASTM E 1275-98. Three experiments were performed with different sample panels. In the first one, a group of samples was irradiated to different dose values by varying the number of passes that the samples were sent under the beam. The beam current was kept constant to a value of 36 mA beam current, yielding a dose rate of about 22 kGy/s. The conveyor speed was set to 9.1 m/min (30 ft/min). The samples were irradiated for 1, 2, 4, 6, and 8 passes under the beam corresponding to doses of 25, 50, 100, 150 and 200 kGy, respectively. In the second experiment, the effect of fractionated dose was studied. To do this, a group of samples was irradiated at the following conveyor speeds: 1.5, 3.0, 4.6, 6.1, 9.1 and 12.2 m/min (5, 10, 15, 20, 30, 40 ft/min), and the dose was kept constant at 150 kGy, by going through
the beam 1, 2, 3, 4, 6 and 8 times. The time between exposures varied from 2 to 10 min depending on the conveyor speed. In a third experiment, sample panels were irradiated to the same dose (150 kGy) but at two different dose rates, 22 and 1.2 kGy/s. This was accomplished by using two beam currents, 36 and 2 mA. The conveyor speed for the high dose rate was 1.5 m/min (5 ft/min) and the samples were irradiated for a single pass under the beam. In the lower dose rate case, the sample panels had to be irradiated for a total of 17 passes. However, samples were irradiated in a linear motion system (Korwin et al., 2000) which limited the time between exposures to about 20 s. After the irradiation, the samples were characterized using the following techniques. The first characterization technique consisted of the evaluation of tensile strength and elongation at break for each set of irradiation conditions. Irradiated panels from each group were machined using a CNC mill programmed to cut tensile samples according to ASTM 638. The tensile machine used in this study was an Instron/Saytex T Series with a 25 kN load cell. Samples were placed between the jaws and a load was applied at a rate of 2.54 cm/min until failure occurred. The strain was measured using the cross-head travel measured by the machine. Five samples were used and results were averaged for each data point. The second characterization technique consisted in the determination of the glass transition temperature (Tg) for each set of irradiation conditions. This was accomplished using differential scanning calorimetry (DSC). The instrument used to conduct these evaluations was a TA Q100 DSC. Samples were prepared by slicing sections of unstressed tensile samples used in the tensile tests. The samples measured approximately 3 mm 3 mm 1 mm thick. The mass of each sample was between 6 and 10 mg. Each sample was placed in a standard cup and covered with a lid and roll crimped. Samples were heated at a rate of 15 1C/min under nitrogen. The Tg for each sample was determined using the output data and TA’s built in software. The third method employed to characterize each irradiation condition was to measure the apparent viscosity of the samples at elevated temperature. The instrument used was a Monsanto MPT Processability Tester. Samples were prepared by shearing approximately 5 mm 5 mm pellets from unstressed material leftover from the two previous test methods. Ten grams of pellets were added to the heated cylinder of the MPT. Samples were held in the cylinder for 10 min to allow them to reach the cylinder temperature. The instrument was then activated to start the test. The instrument was programmed to push a heated piston through the cylinder at a rate of 2.54 cm/min. The material was extruded through a die with an L/D ratio of 20. The
ARTICLE IN PRESS J. Chen et al. / Radiation Physics and Chemistry 74 (2005) 31–35
3. Results and discussions
log Z ¼ 3:4log Z w þ k,
67
100 Strength Elongation
90
66
65
70 60
64
Elongation (%)
80 Strength (MPa)
100 95 90 85 80 70 65 60 0
50
100 150 Dose (kGy)
200
250
Fig. 2. Percentage of molecular weight with respect to a control sample as a function of dose for PC samples irradiated to doses in the interval from 25 to 200 kGy.
152
Tg (°C)
148
144
140
136
where Z is the melt viscosity, and Zw is the average molecular weight. Fig. 2 shows the relationship between the ratio of average molecular weight with respect to control and dose. It is easily noticed that decrease in molecular weight happens after a dose of about 100 kGy indicating that chain scission is the main mechanism at the higher dose levels. DSC was also used to characterize the variations in molecular weight. In amorphous polymers, Tg is found
50 63 40 62 50
105
75
Fig. 1 shows the tensile strength and elongation at break as a function of dose. Strength and elongation at break decrease as dose increases, with relatively insignificant changes before 100 kGy. The trend agrees well with previous reports (Arau´jo et al., 1998) with gamma irradiation. However, even with 200 kGy the change in tensile strength is within 5%, well below the percentage reported by the same authors. On the other hand, the change in ductility was significant, about 30% at a dose of 200 kGy. Melt viscosity analysis was used to characterize the change in molecular weight. The most important structural variable determining the flow properties of polymers is the molecular weight Z. It was well established (Fox et al., 1956) for essentially all polymers studied that for values of Z above a critical value Zc (which is about 600 for many polymers),
0
110
Zw/Zwo (%)
force on the piston was recorded by the instrument, which in turn calculated the apparent viscosity. Each condition set was evaluated at the same rate at a temperature of 275 1C. Three tests were run for each condition and the results were averaged.
33
100 150 Dose (kGy)
200
30 250
Fig. 1. Ultimate tensile strength and ductility as functions of dose for PC samples irradiated at doses between 25 and 200 kGy.
132 0
50
100 150 Dose (kGy)
200
250
Fig. 3. Glass transition temperature as a function of dose for PC samples.
to vary with the average molecular weight. Fig. 3 shows the DSC data at different doses. The trend agrees with the viscosity analysis. The effects of the number of passes on strength and ductility are shown in Fig. 4. In this group the beam current was kept the same at 36 mA (same dose rate). All samples were irradiated to the same total dose of 150 kGy. It is apparent from Fig. 4 that the mechanical properties decrease more with an increase in the number of passes. In other words, radiation caused more damage in mechanical properties when the same amount of dose was applied in more passes with shorter period of exposure per pass and longer period of cooling off time between passes than with the total dose in a single pass. Again, minor changes in strength and more significant variations in elongation at break are observed. The variations in mechanical properties noticed here could be a direct effect of temperature. No significant differences were observed in degradation of
ARTICLE IN PRESS J. Chen et al. / Radiation Physics and Chemistry 74 (2005) 31–35
34 68
100
67
90
148
70 65 60 64
144 140
50
136
63
40
62
30 0
2
4 6 Number of Passes
8
132
10
0
Fig. 4. Ultimate tensile strength and ductility as functions of number of passes for PC samples irradiated at total dose of 150 kGy. Dose/pass is 150, 75, 50, 37.5, 25 and 18.75 kGy corresponding to 1, 2, 3, 4, 6 and 8 passes.
110 105 100 Zw/Zwo (%)
Tg (°C)
80
66
Elongation (%)
Strength (MPa)
152
Strength Elongation
95 90 85 80 75 70 65 60 0
1
2
3 4 5 6 Number of Passes
7
8
9
Fig. 5. Percentage of molecular weight with respect to a control sample as a function of number of passes for a PC sample irradiated to 150 kGy. Dose/pass is 150, 75, 50, 37.5, 25 and 18.75 kGy corresponding to 1, 2, 3, 4, 6 and 8 passes.
molecular weight, which can be concluded from the results of viscosity and DSC analysis in Figs. 5 and 6. Although, samples irradiated with eight passes show a deviation from the rest of the group. Further study needs to be carried out to see if this is a trend after certain number of passes. With only one pass when there was no cooling off time, the tensile strength and ductility are comparable to the control sample. Seguchi et al. (2002) investigated the mechanical properties of PC irradiated at high temperatures. They found that the Rockwell hardness decreased with increasing doses with samples irradiated at room temperature. However, samples irradiated at higher temperatures showed an increase in hardness. The optimum hardness was achieved by irradiation at around the glass transition
1
2
3
4
5
6
7
8
9
Number of Passes Fig. 6. Glass transition temperature as a function of number of passes for a PC sample irradiated at 150 kGy. Dose/pass is 150, 75, 50, 37.5, 25 and 18.75 kGy corresponding to 1, 2, 3, 4, 6 and 8 passes.
temperature with very low dose (3 kGy) when radiation damage to materials is negligible. They attributed the improvement of mechanical properties to the denser molecular packing in the polymer matrix by rearrangement of molecules with synergistic effect of radiation and temperature. At low temperature, the molecular rearrangement might be difficult. In our study, intrinsic heating caused by the electron beam while using high beam current resulted in the same effect. The higher temperature may promote denser molecular packing as suggested by Seguchi et al. (2002). As a result, damage in mechanical properties is decreased. Table 1 summarizes the results for the samples irradiated to different dose rates in terms of their mechanical properties as well as viscosity and DSC measurements. The higher dose rate used here produced no change in tensile strength or ductility against the control samples as mentioned above. Both tensile strength and ductility decreased with radiation dose for samples irradiated at lower dose rate. The most significant change was the ductility, which dropped to 25% of the control. The same temperature effect is noticed here. Dose rate may not affect scission probabilities for PC as reported by Kudoh et al. (1996). They found no differences in degradation of the molecular weight for PC samples irradiated by high dose rate pulsed power e-beam and low dose rate gamma irradiation. Our analysis supports their conclusion, we did not notice any meaningful variations in molecular weight at different dose rates. However, mechanical properties can have noticeable differences as a result of different dose rates as observed in this study. Again, the temperature variations caused by the different dose rates produced by the electron beam is the main effect.
ARTICLE IN PRESS J. Chen et al. / Radiation Physics and Chemistry 74 (2005) 31–35
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Table 1 Comparison of mechanical, glass transition temperatures, and apparent viscosity measurements of polycarbonate samples irradiated to the same dose but using two different dose rates Dose rate (kGy/s)
Tg (1C)
Zw/Zwo (%)
Tensile strength (MPa)
Elongation (%)
22.0 1.2
147.370.5 146.870.5
8570.35 8370.89
65.4470.55 63.7870.41
8074 2576
Uncertainties refer to one standard deviation from a set of measurements.
4. Conclusions Molecular weight degradation of PC may largely depend on dose. Mechanical properties such as tensile strength and ductility, on the other hand, are affected by temperature. The temperature variation can be a result of using different processing parameters such as beam current (dose rate effect) and a combination of conveyor speed and number of passes under the beam. The degradation of mechanical properties seems to lessen under higher temperature for PC. Similar temperature effects could exist for various polymers. For radiation processing of polymer materials, attention should be paid to other processing parameters besides dose, if mechanical properties are the key properties to be controlled in the irradiation process.
Acknowledgements The authors wish to acknowledge Mr. Edward Walton at Kent State University-Ashtabula for performing viscosity tests. The authors would also like to thank NEO Beam, the Alliance of Kent State University and Mercury Plastics Inc. for the use of the electron beam accelerator and thermal analysis facilities.
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