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The effect of post-irradiation annealing on the crosslinking of high-density polyethylene induced by gamma-radiation
ARTICLE IN PRESS Radiation Physics and Chemistry 79 (2010) 710–717
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The effect of post-irradiation annealing on the crosslinking of high-density polyethylene induced by gamma-radiation C.J. Perez, E.M. Valle´s, M.D. Failla n Planta Piloto de Ingenierı´a Quı´mica- PLAPIQUI (UNS-CONICET), Camino ‘‘La Carrindanga’’ km 7, 8000 Bahı´a Blanca, Argentina
a r t i c l e in f o
a b s t r a c t
Article history: Received 25 November 2009 Accepted 4 January 2010
Samples of two high-density polyethylenes having different crystallinity levels were gamma irradiated under vacuum at doses ranging from 20 to 300 kGy. Subsequently, the vials containing the irradiated samples were exposed to different post-irradiation treatments. Parts of the specimens were annealed while still under vacuum. The annealing time was 4 h and the annealing temperatures 110 1C or 150 1C. Others were exposed directly to air opening the vials without any thermal treatment. It was verified that in all cases the dosage to produce an incipient gel increases with the crystallinity of the initial sample. The amount of gel produced after exposing specimens of the same polymer to a given dose increases with the annealing temperature. The largest increment in the amount of gel produced at the completion of the post-irradiation treatment was found on the samples with the highest initial crystallinity level. Evidence of oxidation was found in all irradiated samples. The extent of oxidation depends on the initial crystallinity of the sample, the irradiation dose and the type of post-irradiation treatment. The heat of fusion measured in the annealed samples decreases with the gel content while the fusion temperature was slightly affected. Ductile or brittle behaviors were observed after testing specimens under tensile stress. The yield stress increases proportionally to the crystallinity level that, in turn, depends on the total dosage applied to the samples. The extensibility of ductile samples is determined by the amount of gel produced regardless of the degree of initial crystallinity and the type of annealing process applied to each sample. & 2010 Elsevier Ltd. All rights reserved.
Keywords: Polyethylene Gamma irradiation Thermal treatment Mechanical properties
1. Introduction High-energy irradiation of polyethylene is widely used in industry for different purposes, including sterilization treatments and improvement of the thermo-mechanical stability of the polymer. The exposure of polyethylenes to high energy radiation results mostly in the formation of free radicals, which can subsequently react mainly by combination leading to chain linking. As a consequence of this process, several properties of the polymer change. The ductility decreases while the yield stress, breaking stress and softening point increase as it happens with the creep and impact resistance (Choda´k, 1995; Bhateja et al., 1983; Capaccio et al., 1978; Shultz, 1974; Lyons, 1973). When the irradiation process is performed at relatively low temperature, i.e. room temperature, the molecular structure, the crystallinity level, the environment, and the type of postirradiation treatment are among the most important factors affecting the reaction pathway that follow the free radicals. It is
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recognized that free radicals induced by the irradiation process are homogeneously distributed in the semicrystalline structure of the polyethylene. According to several authors (Lawton et al., 1958; Kang et al., 1967; Turner, 1971; Dole, 1979; Mallegol et al., 2001a, 2001b; Brunella et al., 2007), the free radicals located in the non-crystalline region disappear relatively fast, mainly by combination with other macroradicals or by reacting with oxygen if it is present in the irradiation environment. However, there are also macroradicals that remain trapped mainly in the crystalline phase. These radicals decay at a very slow rate, conducting to oxidation and changes in the properties of the material during storage or even throughout the serve life of the polymer (Lawton et al., 1958; Kang et al., 1967; Turner, 1971; Dole, 1979; Mallegol et al., 2001a, 2001b; Brunella et al., 2007; Suzuki et al., 1995; Ikada et al., 1999; O’Neill et al., 1999; Singh, 1999; Ebru et al., 2008). The convenience of irradiating the polymer in absence of oxygen and performing a thermal treatment after irradiation in order to speed up the radical decay minimizing the oxidative degradation has been recognized long ago (Lawton et al., 1958). It has been established that the extent of chain linking is higher in thermally treated samples than in the analogous irradiated but untreated material. The post-irradiation
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thermal treatment may consist in annealing the polymer at temperature above or below the melting point of the crystalline phase. This treatment favors radical decay enhancing crosslinking, but it can also produce changes in the semicrystalline structure that may affect the properties of the polymer. The present work is devoted to study the effect of postirradiation thermal treatment on the chain linking efficiency of two high-density polyethylenes irradiated using 60Co g-ray in vacuum. Sets of samples with different crystallinity levels were prepared and irradiated with doses ranging from 10 to 300 kGy. After irradiation, some of the specimens of the same set of samples were annealed while still under vacuum either at 110 1C or at 150 1C for 4 h, whereas others were exposed to air without any thermal treatment. All the materials were characterized in terms of gel content that was used as indicative of the chain linking efficiency. Additionally, thermal and tensile mechanical responses of some of the materials were studied. The thermal properties of interest were the enthalpy and the temperature of fusion. The tensile mechanical behavior was determined at room temperature. The yield stress and draw ratio after break were measured. These properties were analyzed as a function of the dose and gel content developed after the thermal treatment.
2. Experimental The polymers used in this study were two high-density linear polyethylenes supplied by Du Pont de Nemours and by Oxy Petrochemical. The polyethylenes have weight average molecular weight of 55,000 g/mol (PE5) and 81,000 g/mol (PE8) estimated from size exclusion chromatography. The polydispersity was 2.5 for PE5 and 3.1 for PE8. The polymers were used as received. It is known that they contain a small amount of hindered phenol antioxidant added as processing additive. Sheets of the materials were prepared by compression molding at 150 1C using a hydraulic press with thermostatically controlled platens. The samples were molded between 5-mm thick aluminum plates lined with aluminum film and held apart by 0.5 mm thick aluminum spacers. After molding, the samples were allowed to reach the semicrystalline state following two different procedures: (l) the first set of samples was obtained by quenching the samples to ice water temperature and (2) the second set was prepared by slowly cooling samples to ambient temperature keeping the samples between the press platens. The code used in this work for identification of the samples is #Q and #S for quenched and slow cooled samples, respectively. The symbol # can be either 5 or 8 for identifying the polyethylene PE5 or PE8, respectively. The heat of fusion (DHf) and the melting temperature (Tm) were measured on a Perkin-Elmer Pyris I differential scanning calorimeter. An indium standard was employed for the calibration using nitrogen as a purge gas. Specimens of about 9 mg were analyzed by heating from 30 to 150 1C at a rate of 10 1C/min. The heat of fusion was determined by measuring the area under the fusion endotherm and the melting temperature from the position of its maximum. Strip and dumb-bell shaped samples (gauge length = 4 mm, width =2 mm, and thickness 0.5 mm) were cut from molded sheets and placed inside Pyrex glass tubes. The tubes containing the polymer samples were first evacuated to 10-4 Torr for 2 days and then sealed off. Subsequently, these samples were irradiated by gamma rays in a 60Co radiation facility. The irradiation was performed at room temperature at a dose rate of 3.3 kGy/h as determined by dosimetry with a radiochromic thin-film dosimeter (Miller et al., 1988). Equal total doses of 20, 30, 50, 70, 100, 200 and 300 kGy were applied to each set of samples. The error in
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dose is estimated to be 710%. A week after the irradiation, some of the samples were exposed to air while others were annealed in vacuum inside the glass containers. The annealing was performed either at 110 1C or at 150 1C for a period of 4 h. After the thermal treatment, these samples were allowed to cool down slowly to room temperature and then exposed to air. The gel fraction of the irradiated samples was determined by extraction of the soluble portion using xylene at 125 1C. In order to minimize oxidation a 0.1 wt% of Irganox 1010 was added to the solvent. The extraction was performed by placing about 30 mg of each sample into a basket made from stainless steel mesh. Then the baskets were immersed in hot xylene for a period of 6 h under an inert atmosphere of nitrogen gas. After accomplishing various extraction periods, the samples were dried in a vacuum oven at 60 1C to constant weight. The extraction was considered complete when, after two consecutive periods of extraction, there was no detectable change of the dried gel weight. The total time of extraction varied between 36 and 48 h depending on the sample. The solvent was changed to fresh solvent between each consecutive extraction. The percentage of gel was taken as the ratio of the weight of the extracted to the non-extracted sample multiplied by 100. The values reported in this work correspond to an average of three samples and the standard deviation between them was about 3%. All the materials were studied by FTIR spectroscopy. The FTIR spectrum was recorded in transmission mode with a 4 cm 1 resolution employing a Nicolet 520 spectrometer. In order to detect the occurrence of oxidative degradation, the region of the spectrum between 2000 and 1600 cm 1 was analyzed. It corresponds to the portion of the spectrum where absorption bands associated to carbonyl groups appear (Silvertein et al., 1963). For comparative proposes, a carbonyl index was defined as the relationship between the carbonyl band centered at 1720 cm 1 and a reference band centered at 2020 cm 1 corresponding to methylene stretching. This method avoids measuring the sample thickness. The tensile mechanical behavior of dumb-bell shaped specimens was determined at room temperature using an Instron tester model 1022. All the tensile tests were performed at room temperature ( 24 1C) with a crosshead speed of 20 mm/min. The specimen cross-section was measured with a micrometer. The yield stress was obtained from the maximum in the engineering stress observed in the stress-elongation curves at low deformation levels. In order to measure the draw ratio after break, ink marks placed 1 mm apart were drawn on the deformation zone of the sample. The draw ratio after break was obtained from the relation between the final spacing of the ink marks and their initial spacing. The reported values of the mechanical properties result from an average of at least 7 tests for each sample. The experimental standard deviation of the data was around 6% for the yield stress and 9% for draw ratio after break.
3. Results and discussion Table 1 reports the enthalpy and the temperature of fusion of the initial samples used for irradiation. As expected, the slow cooled samples present higher enthalpy and slightly higher melting temperatures than the quenched ones. In addition, for equivalent thermal histories, the samples of PE5 showed higher enthalpy values than those corresponding to PE8. This is in accordance with the differences that exist between the molecular weights of the original polymers (Mandelkern, 1985, 2002). The level of crystallinity of the different samples was determined by comparing the enthalpy of fusion of the
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Table 1 Heat and temperature of fusion measured for the initial polyethylene samples and the irradiated material after annealing. Samples of PE5
Initial Dose (kGy) 0 30 50 70 100 200 300
5Q
5S
5Q
5S
T (1C) DH (J/g) 195 135 Annealed at 150 1C
DH (J/g) 230
T (1C) 136
T (1C) DH (J/g) 195 135 Annealed at 110 1C
DH (J/g) 230
T (1C) 136
223 222
135 137
223 225
135 138
198 191 177 180
133 133 131 130
220 216 209 206 207 204 201
245 244 244 244 239 237 235
137 137 138 137 137 138 138
201 188 174 173
133 131 130 129
134 134 133 132 133 134 132
Samples of PE8
Initial Dose (kGy) 0 30 50 70 100 200 300
8Q
8S
8Q
8S
T (1C) DH (J/g) 183 133 Annealed at 150 1C
DH (J/g) 221
T (1C) 137
T (1C) DH (J/g) 183 133 Annealed at 110 1C
DH (J/g) 221
T (1C) 137
204 200
135 135
204 191
135 133
172 169 160 158
131 131 129 129
200 197 196 193 192 188 187
225 226 225 227 224 222 222
138 137 137 138 137 139 139
178 166 167 148
133 130 131 127
133 133 133 132 133 133 133
Fig. 1. Gel fraction as a function of dose for samples of PE5. Code: 5Q-A (K) and 5S-A (’) samples exposed to air after irradiation; 5Q-110 1C (J) and 5S-110 1C (&) samples annealed at 110 1C after irradiation; 5Q-150 1C (Q) and 5S-150 1C (-) samples annealed at 150 1C after irradiation.
Fig. 2. Gel fraction as a function of dose for samples of PE8. Code: 8Q-A (K) and 8S-A (’) samples exposed to air after irradiation; 8Q-110 1C (J) and 8S-110 1C (&) samples annealed at 110 1C after irradiation; 8Q-150 1C (Q) and 8S-150 1C (-) samples annealed at 150 1C after irradiation.
samples with that corresponding to a 100% crystalline polyethylene (289 J/g) (Quinn and Mandelkern, 1958). Thus, the initial 5Q and 5S samples have an average crystallinity level of 65% and 80%, respectively, whereas the average crystallinity of the 8Q and 8S samples were 60% and 78%, respectively.
In order to study the combined effect of the initial degree of crystallinity and the type of annealing applied to the samples after irradiation, the gel data as a function of the dosage for PE5 and PE8 irradiated samples are plotted in Figs. 1 and 2. From Fig. 1 it can be concluded that the critical dosage for gelation of the quenched samples is lower than the one corresponding to the
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slow cooled specimens regardless of the treatment received by the samples. The gel point can be located below 50 kGy for the 5Q sample, while larger doses are needed to produce a measurable gel for the 5S samples. The amount of data points obtained from the PE8 samples is insufficient to locate the gel point in Fig. 2. However, from the general trend of those data points it can be expected that the 8Q sample should reach the critical point at a lower dosage than the 8S sample. In one of our previous works (Failla and Valle´s, 2003), the evolution of the molecular weight of PE8 samples irradiated with doses lower than the gel point also suggested that the quenched samples should arrive at the gel point at a lower dosage than the slowly cooled samples. The results reported in Figs. 1 and 2 also show that the annealed samples require lower critical dosage to reach the gel point than the samples exposed to air after irradiation, which are identified as A in the figures. In addition, samples annealed at 150 1C appear to need lower critical dosages than those annealed at 110 1C. For instance, no gel was detected in samples 5Q and 8Q irradiated with 30 kGy and annealed at 110 1C, while the gel amounts to 8 and 35 wt% for samples that received an equivalent dose but were annealed at 150 1C. The amount of gel increases with dosage for all the irradiated samples. Comparing the samples at a given dosage, the amount of gel generated in the samples exposed to air after irradiation are lower than those that were annealed. When compared at a given total dose, the data in Figs. 1 and 2 also reveal that the 5Q and 8Q annealed samples contain somewhat larger amounts of gel than the corresponding 5S and 8S annealed samples. For instance, the annealed sample 5Q irradiated with 200 kGy contains 70% gel while the 5S sample has about 55% gel. On the same line, the annealed 8Q specimen has a gel content of 85% while the 8S presents a gel content of only 75%. These results are in well agreement with similar observations performed by other authors (Lawton et al., 1958; Kang et al., 1967; Turner, 1971; Dole, 1979; Mallegol et al., 2001a, 2001b; Brunella et al., 2007). The difference in the amount of gel produced at a given dose for each set of samples can be due, at least in part, to the participation of trapped free radicals in chain linking reactions that are favored by the thermal treatment. It is known that radicals decay slowly after irradiation and they may persist for long time in polyethylenes (Naheed et al., 2003; Jahan et al., 2001). Annealing favors free radicals mobility facilitating combination reactions and thus increasing the amount of gel produced at a given dose. This effect is more notorious in the samples with higher crystallinity level, probably due to the fact that a larger fraction of macroradicals with limited mobility remains in the semicrystalline structure of those samples. Even though annealing favors radical combination, it may not produce the annihilation of all the trapped radicals. It is known from the work performed by Naheed et al. that the radicals can remain trapped after irradiation of polyethylene for more than 4.5 years after storage at 75 1C. Indirect evidence that some free radical may survive after the annealing performed in this work was obtained from analysis of IR spectra of the samples. A rather complex oxidation process can be initiated when free macroradicals react with oxygen leading to the generation of a great variety of chemical species, such as hydroperoxide, alcohols, acid, ketones, etc. (Lacoste and Carlsson, 1992). Among the oxidation products, those bearing carbonyl groups can be clearly detected by IR spectroscopy. As an illustrative example, Figs. 3 and 4 show the region of the infrared spectra where the carbonyl groups are easily identified for the samples of PE8 irradiated with a total dose of 200 kGy. The spectra were normalized with the absorption band at 2020 cm 1 that was used as reference to overcome the effect of the difference in the films thickness. The spectra were also slightly shifted along the absorbance axis for the purpose of
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clarity. A very weak band centered at 1740 cm 1 is discernible in the spectrum of the initial polymer that can be associated to the ester groups of the phenolic antioxidant. In addition a rather weak band centered at 1720 cm 1 is barely distinguished in the same spectrum indicating that slight oxidation takes place during the processing of the polymer. In the spectra corresponding to the
Fig. 3. Region of the FTIR spectra of 8Q samples of PE8. The legend above each curve identified the spectra for the original material, for the sample exposed to air (A), and for the samples annealed at 110 1C or 150 1C after irradiation.
Fig. 4. Region of the FTIR spectra of 8S samples of PE8. The legend above each curve identified the spectra for the original material, for the sample exposed to air (A), and for the samples annealed at 110 1C or 150 1C after irradiation.
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irradiated samples a set of overlapping peaks are clearly distinguished in the region between 1750 and 1650 cm 1. The bands centered at 1720 cm 1 can be ascribed to the stretching vibrations of carbonyl groups. This proves the occurrence of oxidation. The intensity of these bands is much larger in the samples exposed to air after irradiation than on those annealed at 110 and 150 1C. In addition, larger absorption bands centered at 1720 cm 1 are obtained from the irradiation of the specimens having higher initial crystallinity levels. These results indicate that some of the free radical survived the annealing treatment performed under vacuum. When exposed to air these macroradicals react with oxygen initiating the oxidation of the irradiated samples of PE8. A qualitatively similar process was observed in the case of the PE5 samples. We tried to compare qualitatively the samples in term of its oxidation level by defining a carbonyl index as described in the experimental section. Fig. 5(a) and (b) shows the calculated carbonyl index as a function of the irradiation dose for some of the samples. When analyzing these results it has to be considered that the accumulation of carbonyl species might be affected by the activity of the antioxidant, as well as by a small amount of dissolved oxygen that might have remained in the initial samples. This may occur even if high vacuum was applied before irradiation. The results reported in Fig. 5 show that the carbonyl index increases steadily with the dosage for the samples exposed to air after radiation and on the annealed ones. Larger levels of oxidation were measured on the samples exposed to air after
Fig. 5. Carbonyl index as a function of dose. Code for graph A: Code: 5Q-A (K) and 5S-A (’) samples exposed to air after irradiation; 5Q-110 1C (J) and 5S-110 1C (&) samples annealed at 110 1C after irradiation; 5Q-150 1C (Q) and 5S-150 1C (-) samples annealed at 150 1C after irradiation. Code for graph B: 8Q-A (K) and 8S-A (’) samples exposed to air after irradiation; 8Q-110 1C (J) and 8S-110 1C (&) samples annealed at 110 1C after irradiation; 8Q-150 1C (Q) and 8S-150 1C (-) samples annealed at 150 1C after irradiation.
irradiation. These results indicate that the annealing applied did not prevent from the occurrence of oxidation on the irradiated samples. Nevertheless, the annealing at 150 1C favors radical decay as the samples having this treatment showed the lowest carbonyl index when a comparison between samples is performed at a given dosage. The thermal behavior of some of the annealed samples was analyzed by calorimetry following the procedure presented in the experimental section. The thermograms obtained for all the samples were characterized by a well defined endotherm of fusion. Depending on the sample, the maximum of these endotherms were centered between 135 and 138 1C. The heat of fusion of a semicrystalline polyethylene depends on its crystallinity level, as well as on the perfection of the lamellar crystallites (Mandelkern, 1985, 2002). Table 1 displays the heat of fusion and melting temperature of the samples as a function of the total dose applied. The table includes the data corresponding to the initial samples as well as that resultant from samples of polyethylene subjected to a thermal annealing process entirely equivalent to that applied to the irradiated samples. The melting temperature and the heat of fusion for the irradiated samples exposed to air without annealing are not shown. Within the experimental uncertainty, they were entirely similar to those corresponding to the parent sample. This is in agreement with the finding of other authors that report no significant changes in the melting temperature and enthalpy of fusion of polyethylene after irradiation (Zoepfel et al., 1984; Khonakdar et al., 2006). Fig. 6(a) and (b) presents the heat of fusion as a function of the gel amount for some of the irradiated samples of PE5 and PE8, respectively. In the figure are also included the results from the initial set of polymer samples used for irradiating. These results are also reported in the rows identified as ‘‘Initial’’ in Table 1. The samples annealed at 150 1C display a clear decay in the heat of fusion with the gel amount. As it is reported in the correspondent columns in Table 1, this decay goes together with a slight reduction in the temperature of fusion. This trend in the evolution of the thermal properties with the gel amount is expected since the annealing process at 150 1C implies a melting and subsequent recrystallization of a crosslinked polyethylene (Mandelkern, 2002). The formation of chain linkages and crosslinked junctions as a consequence of the irradiation and the following annealing process affects the reorganization and chain folding of the polymer chains during the crystallization course. This results in a reduction in the crystallinity, and presumably the formation of crystal of smaller size and less perfection with the consequent reduction in the temperature and the heat of fusion. The enthalpy of fusion of the polymer samples annealed at 110 1C is higher than that of the corresponding parent material. This is probably due to a process of reorganization and perfection occurring mainly in the interfacial and crystalline regions of the polymer when it is annealed at temperatures close to the melting point. The enthalpy of fusion of the irradiated samples annealed at 110 1C decreases gradually with the gel amount. On the order hand, the fusion temperature remains almost constant with increasing doses, as it is evident from the results reported in Table 1. This effect can be associated to a progressive reduction in chain mobility imposed by molecular linkage that limits the increment in the crystallinity levels and prevent the thickening and perfection of the crystalline region. These results go along with the view that at low doses, as the ones applied here, chain linking proceeds preferentially in the non-crystalline phase (Choda´k, 1995; Dole, 1979), thereby making more difficult the movement and rearrangement of chain segments in that phase. It is well documented that the mechanical tensile response and the mechanical properties of polyethylene are mainly determined by the crystallinity level of the polymer (Shultz, 1974; Kennedy
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et al., 1994; Brooks et al., 1999; Jordens et al., 2001) as well as by the irradiation dosage (Bhateja et al., 1983; Capaccio et al., 1978; Failla and Valle´s, 2003; Dongyuan et al., 1987). Table 2 reports the yield stress results as a function of the dose for some of the irradiated samples. In addition, the variation of the yield stress with the heat of fusion, which is related to the crystallinity level, is presented in Fig. 7. Here a progressive increase of the yield stress with the heat of fusion is observed. The solid line in Fig. 7 reports the results extracted from our previous work (Failla and Valle´s, 2003). The arrows in Fig. 7 point out the data obtained from samples of the unirradiated material, these results fall
Fig. 6. Enthalpy of fusion as a function of gel. The graphs A and B correspond, respectively, to the sample annealed at 150 and 110 1C after irradiation. Sample code: 5Q, (’&); 5S, (~}); 8Q, (KJ); 8S, (mn).
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closely to the solid line. It has been shown previously that the yield stress data for high-density polyethylene can be represented by a linear plot of yield stress against crystallinity (Kennedy et al., 1994). The specimens obtained by annealing unirradiated PE5 display brittle behavior as expected from its molecular weight and crystallinity level (Failla and Valle´s, 2003; Kennedy et al., 1994; Jordens et al., 2001). The annealed samples obtained from 5S irradiated samples display also brittle behavior. All other samples exhibit ductile behavior. As the unirradiated 5S samples break without necking, the reported yield stress data represent an average of a large dispersion of values. This may be the reason for having obtained a yield stress value lower than that expected from the solid line. The results for the 5S samples irradiated with the lowest dose fall slightly above the solid line. When comparing the samples at about the same enthalpy of fusion, the ones that received the highest dosage present larger yield stress values. A separate dashed line has been drawn in Fig. 7 to signal the trend of the yield stress for the samples that received the highest total dose. These results agreed with those described in previous works that report that the yield stress of irradiated polyethylene increases
Fig. 7. Yield stress as a function of enthalpy of fusion. Sample code: 5Q, (’&); 5S, (~}); 8Q, (KJ); 8S, (mn). The filled and empty symbols identify, respectively, the data for the samples annealed at 150 and 110 1C, after irradiation. The arrow signal the data for the samples before being irradiated and the letter B stands for brittle behavior.
Table 2 Yield stress of samples.
Initial Dose (kGy)
5Q 5S 22 27 Annealed at 150 1C
Annealed at 110 1C
0 70 100 200 300
32 27 26 24 24
31 30 31 30 30
The value is expressed in MPa.
28 27 26 26
5Q
5S
33 36 37 37 38
8Q 8S 19 28 Annealed at 150 1C
Annealed at 110 1C
29 24 24 22 23
28 28 28 28 29
25 24 24 23
8Q
8S
33 33 33 34 34
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4. Conclusion
Fig. 8. Draw ratio after break against gel. Sample code: 5Q, (’&); 5S, (~}); 8Q, (KJ); 8S, (mn). The filled and empty symbols identify, respectively, the data for the samples annealed at 150 and 110 1C, after irradiation.
with dosage in samples having similar crystallinity values (Bhateja et al., 1983; Capaccio et al., 1978: Failla and Valle´s, 2003). Bhateja et al. (1983) found that the yield stress for irradiated polyethylene is related to the degree of crystallinity by a polynomial relationship, and for each total dose applied, the collected data delineated a separate curve. We tried to represent our data in a similar way by estimating the crystallinity from the enthalpy of fusion. However, we did not find the dependency of yield stress with the crystallinity as proposed by Bhateja et al. (1983). A plausible reason for the discrepancy may be related either to the annealing applied here that modifies the characteristics of the phase or to the way of estimating the crystallinity level as the mentioned authors used X-ray diffraction analysis. The deformability of the material is generally used as an indicator for the mechanical change produced by the irradiation on polyethylene (Wiesner, 1991). In this work, we determine the draw ratio after break as the appropriate parameter to compare the deformation capability of the samples. The draw ratio after break for the ductile samples is plotted against the gel amount in Fig. 8. The results of the 5S samples annealed at 110 1C are not included in this plot because they ruptured shortly after yielding. The annealed and non-irradiated PE8 and PE5 displayed ductile and brittle behavior, respectively. An arbitrary value of 1 was assigned to the draw ratio after break for the PE5 brittle samples. It can be observed that the draw ratio after break decreases gradually as the gel amount increases for both polyethylenes. The solid and dash line in Fig. 8 were drawn just to help the visualization of the dependency of the draw ratio at break with the amount of gel for the samples. The limiting extensibility as the gel increase can be associated to the increment in the concentration of chain links with dose. This restricts the mobility of the chains and reduces the average length of chain segments between crosslinks. The distinction in the draw ratio value between the samples of the polyethylenes at a given gel amount may be associated in part to differences that may exist in the concentration of chain linkages and its spatial distribution, as well. As was previously mentioned, it was necessary to apply higher irradiation doses to PE5 samples in order to produce gel amounts equivalent to those obtained on PE8. Thus, the samples of PE5 may have higher concentrations of crosslinking on the noncrystalline regions and probably shorter chain segments will be available for stretching and chain segment slippage. Thus, when comparing the draw ratio of the samples at equivalent gel content, the irradiated PE5 samples will show lower draw ratio at break than those corresponding to the PE8 samples.
In this work two high-density polyethylenes were irradiated with gamma rays under vacuum with doses ranging from 20 to 300 kGy. The crystallinity of the initial sample and the thermal treatment applied after irradiating the polymer affects the amount of gel generated at a given dosage. The samples having the lowest crystallinity levels display the largest amount of gel at a given dose. It was found that the kind of the thermal treatment applied after irradiating the polymer affects the amount of gel generated at a given dose. This effect was more significant on the samples having the highest initial crystallinity. Evidence of post-irradiation oxidation was found in all irradiated samples. The extent of oxidation depends on the initial crystallinity of the sample and on the type of post-irradiation treatment, being higher for the samples exposed to air without any post-irradiation annealing. The heat of fusion measured in the annealed samples decreases with the gel content while the fusion temperature was slightly affected. Ductile or brittle behaviors were observed after testing specimens under tensile stress. The yield stress increases proportionally to the crystallinity level that in turn depends of the total dosage applied to the sample. The extensibility of ductile samples is determined by the total dose and the amount of gel of the samples.
Acknowledgements The authors wish to thank to CONICET, UNS and ANPCyT for their financial support. References Bhateja, S., Andrews, E., Young, R., 1983. Radiation-induced crystallinity changes in linear polyethylene. J. Polym. Sci. Pt. B: Polym. Phys. 21, 523–536. Brooks, N.W., Ghazali, M., Duckett, R.A., Unwwin, A.P., Ward, I.M., 1999. Effects of morphology on the yield stress of polyethylene. Polymer 40, 821–825. Brunella, V., Bracco, P., Carpentieri, I., Paganini, M.C., Zanetti, M., Costa, L., 2007. Lifetime of alkyl macroradicals in irradiated ultra-high molecular weight polyethylene. Polym. Deg. Stab. 92, 1498–1503. Capaccio, G., Ward, I., Wilding, M., 1978. The plastic deformation of g-irradiated linear polyethylene. J. Polym. Sci. Pt. B: Polym. Phys. 16, 2083–2086. Choda´k, I., 1995. Properties of crosslinked polyolefin-based materials. Prog. Polym. Sci. 20, 1165–1199. Dole, M., 1979. Cross-linking and crystallinity in irradiated polyethylene. Polym. Plast. Technol. Eng. 13, 41–64. Dongyuan, L., Lianshui, Z., Yaqi, W., Wenxiu, C., 1987. The effect of radiation crosslinking on the mechanical properties of polyethylene sheets. Radiat. Phys. Chem. 29, 175–177. Ebru Oral, Godleski Beckos, C., Muratoglu, O.K., 2008. Free radical elimination in irradiated UHMWPE through crystal mobility in phase transition to the hexagonal phase. Polymer 49, 4733–4739. Failla, M.D., Valle´s, E.M., 2003. Effect of the temperature on the tensile mechanical behavior of irradiated linear polyethylene. J. Appl. Polym. Sci. 88, 1925–1935. Ikada, Y., Nakamura, K., Ogata, S., Makino, K., Tajima, K., Endoh, N., Hayashi, T., Fujita, S., Fujisawa, A., Masuda, S., Oonishi, H., 1999. Characterization of ultrahigh molecular weight polyethylene irradiated with g-rays and electron beams to high doses. J. Polym. Sci. Pt. A: Polym. Chem. 37, 159–168. Jahan, M.S., King, M.C., Haggard, W.O., Sevo, K.L., Parr, J.E., 2001. A study of longlived free radicals in gamma-irradiated medical grade polyethylene. Radiat. Phys. Chem. 62, 141–144. Jordens, K., Wilkes, G.L., Jazen, J., Rohlfing, D.C., Welch, M.B., 2001. The influence of molecular weight and thermal history on the thermal, rheological, and mechanical properties of metallocene-catalyzed linear polyethylenes. Polymer 41, 7175–7192. Kang, H.Y., Saito, O., Dole, M., 1967. The radiation chemistry of polyethylene. IX. Temperature coefficient of cross-linking and other effects. J. Am. Chem. Soc. 89, 1980–1986. Kennedy, M.A., Peacock, A.J., Mandelkern, L., 1994. Tensile properties of crystalline polymers: linear polyethylene. Macromolecules 27, 5297–5310. Khonakdar, H.A., Jafari, S.H., Wagenknecht, U., Jehnichen, D., 2006. Effect of electron-irradiation on cross-link density and crystalline structure of low- and high-density polyethylene. Radiat. Phys. Chem. 75, 78–86. Lacoste, J., Carlsson, D.J., 1992. Gamma-, photo-, and thermally-initiated oxidation of linear low density polyethylene: a quantitative comparison of oxidation products. J. Polym. Sci. Pt. A: Polym. Chem. 30, 493–500.
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Lawton, E., Powell, R., Balwit, J., 1958. Effect of physical state during the electron irradiation of hydrocarbon polymers. Part I. The influence of physical state on reactions occurring in polyethylene during and following the irradiation. J. Polym. Sci. 32, 257–275. Lyons, B.J., 1973. In: Dole, M. (Ed.), The Radiation Chemistry of Macromolecules, vol. II. Academic Press, New York, pp. 472 Chapter 14. Mallegol, J., Carlsson, D.J., Deschenes, L., 2001a. Post-g-irradiation in vitamin E stabilized and unstabilized HDPE. Nucl. Instrum. Methods Phys. Res. Sect. B 185, 283–293. Mallegol, J., Carlsson, D.J., Deschenes, L., 2001b. A comparison of phenolic antioxidant performance in HDPE at 32–80 1C. Polym. Degrad. Stab. 73, 259–267. Mandelkern, L., 1985. The relation between structure and properties of crystalline polymers. Polym. J. 17, 337–350. Mandelkern L., 2002. Crystallization of Polymer. 2nd ed., (Charp 7). Miller, A., Batsberg, W., Karman, W., 1988. A new radiochromic thin-film dosimeter system. Radiat. Phys. Chem. 31, 491–496. Naheed, N., Jahan, S., Ridley, M., 2003. Measurements of free radicals over a period of 4.5 years in gamma-irradiated ultra-high molecular weight polyethylene. Nucl. Instrum. Methods Phys. Res. Sect. B 208, 204–209.
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O’Neill, P., Birkinshaw, C., Leahy, J.J., Barklie, R., 1999. The role of long lived free radicals in the ageing of irradiated UHMWPE. Polym. Degrad. Stab. 63, 31–39. Quinn, F.A., Mandelkern, L., 1958. Thermodynamics of crystallization in high polymers: poly-(ethylene). J. Am. Chem. Soc. 80, 3178–3182. Shultz, J., 1974. Polymer Material Science. Prentice-Hall, New Jersey Chapter 11, p. 466. Silvertein, R.M., Bassler, G.C., Morrill, T.C., 1963. Spectrometric Identification of Organic Compounds fifth ed. J. Wiley Ed. Singh, A., 1999. Irradiation of polyethylene: some aspects of crosslinking and oxidative degradation. Radiat. Phys. Chem. 56, 375–380. Suzuki, T., Oki, Y., Numajiri, M., Miura, T., Kondo, K., Ito, Y., 1995. Structure dependence of gamma-ray irradiation effects on polyethylenes studied by positron anhihilation. Radiat. Phys. Chem. 45, 797–804. Turner, D.T., 1971. Role of free radicals in the radiation chemistry of polymers. J. Polym. Sci. Macromol. Rev. 5, 229–383. Wiesner, L., 1991. Effects of radiation on polyethylene and other polyolefins in the presence of oxygen. Radiat. Phys. Chem. 37, 77–81. Zoepfel, F.J., Markovic, V., Silverman, J., 1984. Differential scanning calorimetry studies of irradiated polyethylene: I. Melting temperatures and fusion endotherms. J. Polym. Sci. Pt. A: Polym. Chem. 22, 2017–2032.
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The effect of post-irradiation annealing on the crosslinking of high-density polyethylene induced by gamma-radiation C.J. Perez, E.M. Valle´s, M.D. Failla n Planta Piloto de Ingenierı´a Quı´mica- PLAPIQUI (UNS-CONICET), Camino ‘‘La Carrindanga’’ km 7, 8000 Bahı´a Blanca, Argentina
a r t i c l e in f o
a b s t r a c t
Article history: Received 25 November 2009 Accepted 4 January 2010
Samples of two high-density polyethylenes having different crystallinity levels were gamma irradiated under vacuum at doses ranging from 20 to 300 kGy. Subsequently, the vials containing the irradiated samples were exposed to different post-irradiation treatments. Parts of the specimens were annealed while still under vacuum. The annealing time was 4 h and the annealing temperatures 110 1C or 150 1C. Others were exposed directly to air opening the vials without any thermal treatment. It was verified that in all cases the dosage to produce an incipient gel increases with the crystallinity of the initial sample. The amount of gel produced after exposing specimens of the same polymer to a given dose increases with the annealing temperature. The largest increment in the amount of gel produced at the completion of the post-irradiation treatment was found on the samples with the highest initial crystallinity level. Evidence of oxidation was found in all irradiated samples. The extent of oxidation depends on the initial crystallinity of the sample, the irradiation dose and the type of post-irradiation treatment. The heat of fusion measured in the annealed samples decreases with the gel content while the fusion temperature was slightly affected. Ductile or brittle behaviors were observed after testing specimens under tensile stress. The yield stress increases proportionally to the crystallinity level that, in turn, depends on the total dosage applied to the samples. The extensibility of ductile samples is determined by the amount of gel produced regardless of the degree of initial crystallinity and the type of annealing process applied to each sample. & 2010 Elsevier Ltd. All rights reserved.
Keywords: Polyethylene Gamma irradiation Thermal treatment Mechanical properties
1. Introduction High-energy irradiation of polyethylene is widely used in industry for different purposes, including sterilization treatments and improvement of the thermo-mechanical stability of the polymer. The exposure of polyethylenes to high energy radiation results mostly in the formation of free radicals, which can subsequently react mainly by combination leading to chain linking. As a consequence of this process, several properties of the polymer change. The ductility decreases while the yield stress, breaking stress and softening point increase as it happens with the creep and impact resistance (Choda´k, 1995; Bhateja et al., 1983; Capaccio et al., 1978; Shultz, 1974; Lyons, 1973). When the irradiation process is performed at relatively low temperature, i.e. room temperature, the molecular structure, the crystallinity level, the environment, and the type of postirradiation treatment are among the most important factors affecting the reaction pathway that follow the free radicals. It is
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recognized that free radicals induced by the irradiation process are homogeneously distributed in the semicrystalline structure of the polyethylene. According to several authors (Lawton et al., 1958; Kang et al., 1967; Turner, 1971; Dole, 1979; Mallegol et al., 2001a, 2001b; Brunella et al., 2007), the free radicals located in the non-crystalline region disappear relatively fast, mainly by combination with other macroradicals or by reacting with oxygen if it is present in the irradiation environment. However, there are also macroradicals that remain trapped mainly in the crystalline phase. These radicals decay at a very slow rate, conducting to oxidation and changes in the properties of the material during storage or even throughout the serve life of the polymer (Lawton et al., 1958; Kang et al., 1967; Turner, 1971; Dole, 1979; Mallegol et al., 2001a, 2001b; Brunella et al., 2007; Suzuki et al., 1995; Ikada et al., 1999; O’Neill et al., 1999; Singh, 1999; Ebru et al., 2008). The convenience of irradiating the polymer in absence of oxygen and performing a thermal treatment after irradiation in order to speed up the radical decay minimizing the oxidative degradation has been recognized long ago (Lawton et al., 1958). It has been established that the extent of chain linking is higher in thermally treated samples than in the analogous irradiated but untreated material. The post-irradiation
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thermal treatment may consist in annealing the polymer at temperature above or below the melting point of the crystalline phase. This treatment favors radical decay enhancing crosslinking, but it can also produce changes in the semicrystalline structure that may affect the properties of the polymer. The present work is devoted to study the effect of postirradiation thermal treatment on the chain linking efficiency of two high-density polyethylenes irradiated using 60Co g-ray in vacuum. Sets of samples with different crystallinity levels were prepared and irradiated with doses ranging from 10 to 300 kGy. After irradiation, some of the specimens of the same set of samples were annealed while still under vacuum either at 110 1C or at 150 1C for 4 h, whereas others were exposed to air without any thermal treatment. All the materials were characterized in terms of gel content that was used as indicative of the chain linking efficiency. Additionally, thermal and tensile mechanical responses of some of the materials were studied. The thermal properties of interest were the enthalpy and the temperature of fusion. The tensile mechanical behavior was determined at room temperature. The yield stress and draw ratio after break were measured. These properties were analyzed as a function of the dose and gel content developed after the thermal treatment.
2. Experimental The polymers used in this study were two high-density linear polyethylenes supplied by Du Pont de Nemours and by Oxy Petrochemical. The polyethylenes have weight average molecular weight of 55,000 g/mol (PE5) and 81,000 g/mol (PE8) estimated from size exclusion chromatography. The polydispersity was 2.5 for PE5 and 3.1 for PE8. The polymers were used as received. It is known that they contain a small amount of hindered phenol antioxidant added as processing additive. Sheets of the materials were prepared by compression molding at 150 1C using a hydraulic press with thermostatically controlled platens. The samples were molded between 5-mm thick aluminum plates lined with aluminum film and held apart by 0.5 mm thick aluminum spacers. After molding, the samples were allowed to reach the semicrystalline state following two different procedures: (l) the first set of samples was obtained by quenching the samples to ice water temperature and (2) the second set was prepared by slowly cooling samples to ambient temperature keeping the samples between the press platens. The code used in this work for identification of the samples is #Q and #S for quenched and slow cooled samples, respectively. The symbol # can be either 5 or 8 for identifying the polyethylene PE5 or PE8, respectively. The heat of fusion (DHf) and the melting temperature (Tm) were measured on a Perkin-Elmer Pyris I differential scanning calorimeter. An indium standard was employed for the calibration using nitrogen as a purge gas. Specimens of about 9 mg were analyzed by heating from 30 to 150 1C at a rate of 10 1C/min. The heat of fusion was determined by measuring the area under the fusion endotherm and the melting temperature from the position of its maximum. Strip and dumb-bell shaped samples (gauge length = 4 mm, width =2 mm, and thickness 0.5 mm) were cut from molded sheets and placed inside Pyrex glass tubes. The tubes containing the polymer samples were first evacuated to 10-4 Torr for 2 days and then sealed off. Subsequently, these samples were irradiated by gamma rays in a 60Co radiation facility. The irradiation was performed at room temperature at a dose rate of 3.3 kGy/h as determined by dosimetry with a radiochromic thin-film dosimeter (Miller et al., 1988). Equal total doses of 20, 30, 50, 70, 100, 200 and 300 kGy were applied to each set of samples. The error in
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dose is estimated to be 710%. A week after the irradiation, some of the samples were exposed to air while others were annealed in vacuum inside the glass containers. The annealing was performed either at 110 1C or at 150 1C for a period of 4 h. After the thermal treatment, these samples were allowed to cool down slowly to room temperature and then exposed to air. The gel fraction of the irradiated samples was determined by extraction of the soluble portion using xylene at 125 1C. In order to minimize oxidation a 0.1 wt% of Irganox 1010 was added to the solvent. The extraction was performed by placing about 30 mg of each sample into a basket made from stainless steel mesh. Then the baskets were immersed in hot xylene for a period of 6 h under an inert atmosphere of nitrogen gas. After accomplishing various extraction periods, the samples were dried in a vacuum oven at 60 1C to constant weight. The extraction was considered complete when, after two consecutive periods of extraction, there was no detectable change of the dried gel weight. The total time of extraction varied between 36 and 48 h depending on the sample. The solvent was changed to fresh solvent between each consecutive extraction. The percentage of gel was taken as the ratio of the weight of the extracted to the non-extracted sample multiplied by 100. The values reported in this work correspond to an average of three samples and the standard deviation between them was about 3%. All the materials were studied by FTIR spectroscopy. The FTIR spectrum was recorded in transmission mode with a 4 cm 1 resolution employing a Nicolet 520 spectrometer. In order to detect the occurrence of oxidative degradation, the region of the spectrum between 2000 and 1600 cm 1 was analyzed. It corresponds to the portion of the spectrum where absorption bands associated to carbonyl groups appear (Silvertein et al., 1963). For comparative proposes, a carbonyl index was defined as the relationship between the carbonyl band centered at 1720 cm 1 and a reference band centered at 2020 cm 1 corresponding to methylene stretching. This method avoids measuring the sample thickness. The tensile mechanical behavior of dumb-bell shaped specimens was determined at room temperature using an Instron tester model 1022. All the tensile tests were performed at room temperature ( 24 1C) with a crosshead speed of 20 mm/min. The specimen cross-section was measured with a micrometer. The yield stress was obtained from the maximum in the engineering stress observed in the stress-elongation curves at low deformation levels. In order to measure the draw ratio after break, ink marks placed 1 mm apart were drawn on the deformation zone of the sample. The draw ratio after break was obtained from the relation between the final spacing of the ink marks and their initial spacing. The reported values of the mechanical properties result from an average of at least 7 tests for each sample. The experimental standard deviation of the data was around 6% for the yield stress and 9% for draw ratio after break.
3. Results and discussion Table 1 reports the enthalpy and the temperature of fusion of the initial samples used for irradiation. As expected, the slow cooled samples present higher enthalpy and slightly higher melting temperatures than the quenched ones. In addition, for equivalent thermal histories, the samples of PE5 showed higher enthalpy values than those corresponding to PE8. This is in accordance with the differences that exist between the molecular weights of the original polymers (Mandelkern, 1985, 2002). The level of crystallinity of the different samples was determined by comparing the enthalpy of fusion of the
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Table 1 Heat and temperature of fusion measured for the initial polyethylene samples and the irradiated material after annealing. Samples of PE5
Initial Dose (kGy) 0 30 50 70 100 200 300
5Q
5S
5Q
5S
T (1C) DH (J/g) 195 135 Annealed at 150 1C
DH (J/g) 230
T (1C) 136
T (1C) DH (J/g) 195 135 Annealed at 110 1C
DH (J/g) 230
T (1C) 136
223 222
135 137
223 225
135 138
198 191 177 180
133 133 131 130
220 216 209 206 207 204 201
245 244 244 244 239 237 235
137 137 138 137 137 138 138
201 188 174 173
133 131 130 129
134 134 133 132 133 134 132
Samples of PE8
Initial Dose (kGy) 0 30 50 70 100 200 300
8Q
8S
8Q
8S
T (1C) DH (J/g) 183 133 Annealed at 150 1C
DH (J/g) 221
T (1C) 137
T (1C) DH (J/g) 183 133 Annealed at 110 1C
DH (J/g) 221
T (1C) 137
204 200
135 135
204 191
135 133
172 169 160 158
131 131 129 129
200 197 196 193 192 188 187
225 226 225 227 224 222 222
138 137 137 138 137 139 139
178 166 167 148
133 130 131 127
133 133 133 132 133 133 133
Fig. 1. Gel fraction as a function of dose for samples of PE5. Code: 5Q-A (K) and 5S-A (’) samples exposed to air after irradiation; 5Q-110 1C (J) and 5S-110 1C (&) samples annealed at 110 1C after irradiation; 5Q-150 1C (Q) and 5S-150 1C (-) samples annealed at 150 1C after irradiation.
Fig. 2. Gel fraction as a function of dose for samples of PE8. Code: 8Q-A (K) and 8S-A (’) samples exposed to air after irradiation; 8Q-110 1C (J) and 8S-110 1C (&) samples annealed at 110 1C after irradiation; 8Q-150 1C (Q) and 8S-150 1C (-) samples annealed at 150 1C after irradiation.
samples with that corresponding to a 100% crystalline polyethylene (289 J/g) (Quinn and Mandelkern, 1958). Thus, the initial 5Q and 5S samples have an average crystallinity level of 65% and 80%, respectively, whereas the average crystallinity of the 8Q and 8S samples were 60% and 78%, respectively.
In order to study the combined effect of the initial degree of crystallinity and the type of annealing applied to the samples after irradiation, the gel data as a function of the dosage for PE5 and PE8 irradiated samples are plotted in Figs. 1 and 2. From Fig. 1 it can be concluded that the critical dosage for gelation of the quenched samples is lower than the one corresponding to the
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slow cooled specimens regardless of the treatment received by the samples. The gel point can be located below 50 kGy for the 5Q sample, while larger doses are needed to produce a measurable gel for the 5S samples. The amount of data points obtained from the PE8 samples is insufficient to locate the gel point in Fig. 2. However, from the general trend of those data points it can be expected that the 8Q sample should reach the critical point at a lower dosage than the 8S sample. In one of our previous works (Failla and Valle´s, 2003), the evolution of the molecular weight of PE8 samples irradiated with doses lower than the gel point also suggested that the quenched samples should arrive at the gel point at a lower dosage than the slowly cooled samples. The results reported in Figs. 1 and 2 also show that the annealed samples require lower critical dosage to reach the gel point than the samples exposed to air after irradiation, which are identified as A in the figures. In addition, samples annealed at 150 1C appear to need lower critical dosages than those annealed at 110 1C. For instance, no gel was detected in samples 5Q and 8Q irradiated with 30 kGy and annealed at 110 1C, while the gel amounts to 8 and 35 wt% for samples that received an equivalent dose but were annealed at 150 1C. The amount of gel increases with dosage for all the irradiated samples. Comparing the samples at a given dosage, the amount of gel generated in the samples exposed to air after irradiation are lower than those that were annealed. When compared at a given total dose, the data in Figs. 1 and 2 also reveal that the 5Q and 8Q annealed samples contain somewhat larger amounts of gel than the corresponding 5S and 8S annealed samples. For instance, the annealed sample 5Q irradiated with 200 kGy contains 70% gel while the 5S sample has about 55% gel. On the same line, the annealed 8Q specimen has a gel content of 85% while the 8S presents a gel content of only 75%. These results are in well agreement with similar observations performed by other authors (Lawton et al., 1958; Kang et al., 1967; Turner, 1971; Dole, 1979; Mallegol et al., 2001a, 2001b; Brunella et al., 2007). The difference in the amount of gel produced at a given dose for each set of samples can be due, at least in part, to the participation of trapped free radicals in chain linking reactions that are favored by the thermal treatment. It is known that radicals decay slowly after irradiation and they may persist for long time in polyethylenes (Naheed et al., 2003; Jahan et al., 2001). Annealing favors free radicals mobility facilitating combination reactions and thus increasing the amount of gel produced at a given dose. This effect is more notorious in the samples with higher crystallinity level, probably due to the fact that a larger fraction of macroradicals with limited mobility remains in the semicrystalline structure of those samples. Even though annealing favors radical combination, it may not produce the annihilation of all the trapped radicals. It is known from the work performed by Naheed et al. that the radicals can remain trapped after irradiation of polyethylene for more than 4.5 years after storage at 75 1C. Indirect evidence that some free radical may survive after the annealing performed in this work was obtained from analysis of IR spectra of the samples. A rather complex oxidation process can be initiated when free macroradicals react with oxygen leading to the generation of a great variety of chemical species, such as hydroperoxide, alcohols, acid, ketones, etc. (Lacoste and Carlsson, 1992). Among the oxidation products, those bearing carbonyl groups can be clearly detected by IR spectroscopy. As an illustrative example, Figs. 3 and 4 show the region of the infrared spectra where the carbonyl groups are easily identified for the samples of PE8 irradiated with a total dose of 200 kGy. The spectra were normalized with the absorption band at 2020 cm 1 that was used as reference to overcome the effect of the difference in the films thickness. The spectra were also slightly shifted along the absorbance axis for the purpose of
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clarity. A very weak band centered at 1740 cm 1 is discernible in the spectrum of the initial polymer that can be associated to the ester groups of the phenolic antioxidant. In addition a rather weak band centered at 1720 cm 1 is barely distinguished in the same spectrum indicating that slight oxidation takes place during the processing of the polymer. In the spectra corresponding to the
Fig. 3. Region of the FTIR spectra of 8Q samples of PE8. The legend above each curve identified the spectra for the original material, for the sample exposed to air (A), and for the samples annealed at 110 1C or 150 1C after irradiation.
Fig. 4. Region of the FTIR spectra of 8S samples of PE8. The legend above each curve identified the spectra for the original material, for the sample exposed to air (A), and for the samples annealed at 110 1C or 150 1C after irradiation.
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irradiated samples a set of overlapping peaks are clearly distinguished in the region between 1750 and 1650 cm 1. The bands centered at 1720 cm 1 can be ascribed to the stretching vibrations of carbonyl groups. This proves the occurrence of oxidation. The intensity of these bands is much larger in the samples exposed to air after irradiation than on those annealed at 110 and 150 1C. In addition, larger absorption bands centered at 1720 cm 1 are obtained from the irradiation of the specimens having higher initial crystallinity levels. These results indicate that some of the free radical survived the annealing treatment performed under vacuum. When exposed to air these macroradicals react with oxygen initiating the oxidation of the irradiated samples of PE8. A qualitatively similar process was observed in the case of the PE5 samples. We tried to compare qualitatively the samples in term of its oxidation level by defining a carbonyl index as described in the experimental section. Fig. 5(a) and (b) shows the calculated carbonyl index as a function of the irradiation dose for some of the samples. When analyzing these results it has to be considered that the accumulation of carbonyl species might be affected by the activity of the antioxidant, as well as by a small amount of dissolved oxygen that might have remained in the initial samples. This may occur even if high vacuum was applied before irradiation. The results reported in Fig. 5 show that the carbonyl index increases steadily with the dosage for the samples exposed to air after radiation and on the annealed ones. Larger levels of oxidation were measured on the samples exposed to air after
Fig. 5. Carbonyl index as a function of dose. Code for graph A: Code: 5Q-A (K) and 5S-A (’) samples exposed to air after irradiation; 5Q-110 1C (J) and 5S-110 1C (&) samples annealed at 110 1C after irradiation; 5Q-150 1C (Q) and 5S-150 1C (-) samples annealed at 150 1C after irradiation. Code for graph B: 8Q-A (K) and 8S-A (’) samples exposed to air after irradiation; 8Q-110 1C (J) and 8S-110 1C (&) samples annealed at 110 1C after irradiation; 8Q-150 1C (Q) and 8S-150 1C (-) samples annealed at 150 1C after irradiation.
irradiation. These results indicate that the annealing applied did not prevent from the occurrence of oxidation on the irradiated samples. Nevertheless, the annealing at 150 1C favors radical decay as the samples having this treatment showed the lowest carbonyl index when a comparison between samples is performed at a given dosage. The thermal behavior of some of the annealed samples was analyzed by calorimetry following the procedure presented in the experimental section. The thermograms obtained for all the samples were characterized by a well defined endotherm of fusion. Depending on the sample, the maximum of these endotherms were centered between 135 and 138 1C. The heat of fusion of a semicrystalline polyethylene depends on its crystallinity level, as well as on the perfection of the lamellar crystallites (Mandelkern, 1985, 2002). Table 1 displays the heat of fusion and melting temperature of the samples as a function of the total dose applied. The table includes the data corresponding to the initial samples as well as that resultant from samples of polyethylene subjected to a thermal annealing process entirely equivalent to that applied to the irradiated samples. The melting temperature and the heat of fusion for the irradiated samples exposed to air without annealing are not shown. Within the experimental uncertainty, they were entirely similar to those corresponding to the parent sample. This is in agreement with the finding of other authors that report no significant changes in the melting temperature and enthalpy of fusion of polyethylene after irradiation (Zoepfel et al., 1984; Khonakdar et al., 2006). Fig. 6(a) and (b) presents the heat of fusion as a function of the gel amount for some of the irradiated samples of PE5 and PE8, respectively. In the figure are also included the results from the initial set of polymer samples used for irradiating. These results are also reported in the rows identified as ‘‘Initial’’ in Table 1. The samples annealed at 150 1C display a clear decay in the heat of fusion with the gel amount. As it is reported in the correspondent columns in Table 1, this decay goes together with a slight reduction in the temperature of fusion. This trend in the evolution of the thermal properties with the gel amount is expected since the annealing process at 150 1C implies a melting and subsequent recrystallization of a crosslinked polyethylene (Mandelkern, 2002). The formation of chain linkages and crosslinked junctions as a consequence of the irradiation and the following annealing process affects the reorganization and chain folding of the polymer chains during the crystallization course. This results in a reduction in the crystallinity, and presumably the formation of crystal of smaller size and less perfection with the consequent reduction in the temperature and the heat of fusion. The enthalpy of fusion of the polymer samples annealed at 110 1C is higher than that of the corresponding parent material. This is probably due to a process of reorganization and perfection occurring mainly in the interfacial and crystalline regions of the polymer when it is annealed at temperatures close to the melting point. The enthalpy of fusion of the irradiated samples annealed at 110 1C decreases gradually with the gel amount. On the order hand, the fusion temperature remains almost constant with increasing doses, as it is evident from the results reported in Table 1. This effect can be associated to a progressive reduction in chain mobility imposed by molecular linkage that limits the increment in the crystallinity levels and prevent the thickening and perfection of the crystalline region. These results go along with the view that at low doses, as the ones applied here, chain linking proceeds preferentially in the non-crystalline phase (Choda´k, 1995; Dole, 1979), thereby making more difficult the movement and rearrangement of chain segments in that phase. It is well documented that the mechanical tensile response and the mechanical properties of polyethylene are mainly determined by the crystallinity level of the polymer (Shultz, 1974; Kennedy
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et al., 1994; Brooks et al., 1999; Jordens et al., 2001) as well as by the irradiation dosage (Bhateja et al., 1983; Capaccio et al., 1978; Failla and Valle´s, 2003; Dongyuan et al., 1987). Table 2 reports the yield stress results as a function of the dose for some of the irradiated samples. In addition, the variation of the yield stress with the heat of fusion, which is related to the crystallinity level, is presented in Fig. 7. Here a progressive increase of the yield stress with the heat of fusion is observed. The solid line in Fig. 7 reports the results extracted from our previous work (Failla and Valle´s, 2003). The arrows in Fig. 7 point out the data obtained from samples of the unirradiated material, these results fall
Fig. 6. Enthalpy of fusion as a function of gel. The graphs A and B correspond, respectively, to the sample annealed at 150 and 110 1C after irradiation. Sample code: 5Q, (’&); 5S, (~}); 8Q, (KJ); 8S, (mn).
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closely to the solid line. It has been shown previously that the yield stress data for high-density polyethylene can be represented by a linear plot of yield stress against crystallinity (Kennedy et al., 1994). The specimens obtained by annealing unirradiated PE5 display brittle behavior as expected from its molecular weight and crystallinity level (Failla and Valle´s, 2003; Kennedy et al., 1994; Jordens et al., 2001). The annealed samples obtained from 5S irradiated samples display also brittle behavior. All other samples exhibit ductile behavior. As the unirradiated 5S samples break without necking, the reported yield stress data represent an average of a large dispersion of values. This may be the reason for having obtained a yield stress value lower than that expected from the solid line. The results for the 5S samples irradiated with the lowest dose fall slightly above the solid line. When comparing the samples at about the same enthalpy of fusion, the ones that received the highest dosage present larger yield stress values. A separate dashed line has been drawn in Fig. 7 to signal the trend of the yield stress for the samples that received the highest total dose. These results agreed with those described in previous works that report that the yield stress of irradiated polyethylene increases
Fig. 7. Yield stress as a function of enthalpy of fusion. Sample code: 5Q, (’&); 5S, (~}); 8Q, (KJ); 8S, (mn). The filled and empty symbols identify, respectively, the data for the samples annealed at 150 and 110 1C, after irradiation. The arrow signal the data for the samples before being irradiated and the letter B stands for brittle behavior.
Table 2 Yield stress of samples.
Initial Dose (kGy)
5Q 5S 22 27 Annealed at 150 1C
Annealed at 110 1C
0 70 100 200 300
32 27 26 24 24
31 30 31 30 30
The value is expressed in MPa.
28 27 26 26
5Q
5S
33 36 37 37 38
8Q 8S 19 28 Annealed at 150 1C
Annealed at 110 1C
29 24 24 22 23
28 28 28 28 29
25 24 24 23
8Q
8S
33 33 33 34 34
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4. Conclusion
Fig. 8. Draw ratio after break against gel. Sample code: 5Q, (’&); 5S, (~}); 8Q, (KJ); 8S, (mn). The filled and empty symbols identify, respectively, the data for the samples annealed at 150 and 110 1C, after irradiation.
with dosage in samples having similar crystallinity values (Bhateja et al., 1983; Capaccio et al., 1978: Failla and Valle´s, 2003). Bhateja et al. (1983) found that the yield stress for irradiated polyethylene is related to the degree of crystallinity by a polynomial relationship, and for each total dose applied, the collected data delineated a separate curve. We tried to represent our data in a similar way by estimating the crystallinity from the enthalpy of fusion. However, we did not find the dependency of yield stress with the crystallinity as proposed by Bhateja et al. (1983). A plausible reason for the discrepancy may be related either to the annealing applied here that modifies the characteristics of the phase or to the way of estimating the crystallinity level as the mentioned authors used X-ray diffraction analysis. The deformability of the material is generally used as an indicator for the mechanical change produced by the irradiation on polyethylene (Wiesner, 1991). In this work, we determine the draw ratio after break as the appropriate parameter to compare the deformation capability of the samples. The draw ratio after break for the ductile samples is plotted against the gel amount in Fig. 8. The results of the 5S samples annealed at 110 1C are not included in this plot because they ruptured shortly after yielding. The annealed and non-irradiated PE8 and PE5 displayed ductile and brittle behavior, respectively. An arbitrary value of 1 was assigned to the draw ratio after break for the PE5 brittle samples. It can be observed that the draw ratio after break decreases gradually as the gel amount increases for both polyethylenes. The solid and dash line in Fig. 8 were drawn just to help the visualization of the dependency of the draw ratio at break with the amount of gel for the samples. The limiting extensibility as the gel increase can be associated to the increment in the concentration of chain links with dose. This restricts the mobility of the chains and reduces the average length of chain segments between crosslinks. The distinction in the draw ratio value between the samples of the polyethylenes at a given gel amount may be associated in part to differences that may exist in the concentration of chain linkages and its spatial distribution, as well. As was previously mentioned, it was necessary to apply higher irradiation doses to PE5 samples in order to produce gel amounts equivalent to those obtained on PE8. Thus, the samples of PE5 may have higher concentrations of crosslinking on the noncrystalline regions and probably shorter chain segments will be available for stretching and chain segment slippage. Thus, when comparing the draw ratio of the samples at equivalent gel content, the irradiated PE5 samples will show lower draw ratio at break than those corresponding to the PE8 samples.
In this work two high-density polyethylenes were irradiated with gamma rays under vacuum with doses ranging from 20 to 300 kGy. The crystallinity of the initial sample and the thermal treatment applied after irradiating the polymer affects the amount of gel generated at a given dosage. The samples having the lowest crystallinity levels display the largest amount of gel at a given dose. It was found that the kind of the thermal treatment applied after irradiating the polymer affects the amount of gel generated at a given dose. This effect was more significant on the samples having the highest initial crystallinity. Evidence of post-irradiation oxidation was found in all irradiated samples. The extent of oxidation depends on the initial crystallinity of the sample and on the type of post-irradiation treatment, being higher for the samples exposed to air without any post-irradiation annealing. The heat of fusion measured in the annealed samples decreases with the gel content while the fusion temperature was slightly affected. Ductile or brittle behaviors were observed after testing specimens under tensile stress. The yield stress increases proportionally to the crystallinity level that in turn depends of the total dosage applied to the sample. The extensibility of ductile samples is determined by the total dose and the amount of gel of the samples.
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