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Mechanical and thermal properties of gamma-ray irradiated polyethylene blends
Vacuum 70 (2003) 227–236
Mechanical and thermal properties of gamma-ray irradiated polyethylene blends Mariam Al-Alia, N.K Madia, Nora J. Al Thanib, M. El-Muraikhia, A. Turosc,d,* a
Physics Department, Faculty of Science, Qatar University, Doha P.O.B. 2713, Doha, Qatar b Scientific and Applied Research Center (SARC), University of Qatar, Doha, Qatar c Institute of Electronic Materials Technology, ul. Wolczynska 133, Warsaw PL-01-919, Poland d Soltan Institute for Nuclear Studies, Swierk, Poland
Abstract In this work, the results obtained by gamma-ray irradiation of polyethylene (PE) blends are presented. The blends were produced by injection molding at 2201C with different compositional ratios, i.e. 100/0, 15/85, 25/75, 50/50/, 75/25, 85/15, and 0/100, of low-density PE with high-density PE and linear low-density PE. The exposure of these blends to different doses of 60Co gamma radiation up to 1500 kGy improves the mechanical properties due to partial crosslinking. Diffraction scanning calorimetry, infrared spectroscopy and scanning electron microscopy have been used to investigate the effect of gamma irradiation on the thermal behavior of the polymer material. After the radiation treatment, the melting peak of the low-density PE component decreases in intensity as the absorbed dose increases. On the contrary, melting peak of the high-density component increases in intensity and shifts to lower temperatures as the absorbed dose increases. The correlation between the mechanical properties and the morphology of the blends is discussed. r 2003 Elsevier Science Ltd. All rights reserved. Keywords: Polymers; Polymer blends; Gamma-ray irradiation; Microstructure
1. Introduction Polyethylene (PE) is widely used in various fields because of its excellent properties such as softness, elasticity and insulation. They are also used in nuclear power plants and are exposed to ionizing radiation for a long time. Since aliphatic polymers are very sensitive to radiation, synthesis and development of radiation-resistant polymeric materials are strongly desired.
*Corresponding author. Tel.: 0603-092223; fax: 22-8349003. E-mail address: turos [email protected] (A. Turos).
Blending of synthetic and natural polymers is a well-known method of modifying properties of polymer. It is an economically viable and versatile way to prepare new engineering materials and to overcome deficiency in some material characteristics. However, most of the polymer blends are immiscible on the molecular scale and form heterogeneous systems that affect their physicomechanical behavior. There have been a few attempts to improve the miscibility by using radiation to modify one or both polymers [1]. Therefore, a study of the influence of radiation on the morphology, crystallization and melting behavior of the component polymers in the blend is
0042-207X/03/$ - see front matter r 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0042-207X(02)00648-6
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M. Al-Ali et al. / Vacuum 70 (2003) 227–236
essential for an understanding of how to optimize processing condition and properties. The mechanism responsible for these improvements should be also understood well. The effects of irradiation of PE have been studied extensively [2,3]. It is reported that oxidative degradation becomes a major process during gamma irradiation, particularly at lowdose-rate gamma irradiation. However, there have been suggestions in literature [4] that cross-linking may still be an important process during the lowdose-rate gamma irradiation and that peroxides can act as an initiator for radiation-induced crosslinking of PE. In this paper, we report on the role of LDPE admixtures and on gamma-ray irradiation in modifying the mechanical and thermal properties of HDPE. A variety of techniques have been used to identify and quantify the behavior of blends versus the blend ratio and/or irradiation dose, e.g. thermal analysis technique, FTIR, SEM, and tensile testing. Finally, we obtained good estimate of the proper blend ratio and the proper irradiation dose reasonable for desired properties.
2. Experimental
2.3. Tensile testing Tensile specimens of a dog-bone shape were produced with a gauge length of 65 mm, width of 1.25 mm and thickness of 3.1 mm. Tensile testing was carried out using the Lloyd instruments material testing machine connected to a remote microcomputer for data acquisition and analysis. The load was measured by a load cell 5 KN capacity, while the displacement was measured using an internal extensometer. The speed of testing was 100 mm/min. Three samples were tested under the same condition for each blend. 2.4. Differential scanning calorimeter (DSC) DSC (Perkin Elmer, model DSC7 Pyris) was used to observe melting and solidification behavior. To erase thermal history, first cooling and then heating thermograms were detected with the scanning rate of 101C/ min in booth. 2.5. Thermogravimetric analyzer (TGA) TGA (TGA& model with Pyris software) was used to observe the decomposition temperature (Td 1C) of PE blends. The scanning rate was 101C/ min.
2.1. Sample preparation 2.6. Scanning electron microscope (SEM) The investigated materials are native LDPE and HDPE (MFI: 6 g/10 min). Low density PE is of grade FE8000; it has a melt flow index 1.8–2.2, crystalline melting temperature 1111C and its density at 231C is 0.922–0.924 gm/cm3. The blend was produced by injection molding at 2201C with different compositional ratios, i.e., 0/100, 15/85, 25/75, 50/50/, 75/25, 85/15, and 100/0 of lowdensity PE with high-density PE. 2.2. Gamma-ray irradiation The samples were irradiated at room temperature in air using a gamma cell 220 manufactured by the atomic Energy Canada. The operating dose rate was 6 kGy/h, and a set of samples was produced spanning an integrated dose from 0 to 1500 kGy.
Philips Model XL30, SEM was used to study the morphology of the fracture surface. Before measurements, the fracture surfaces (obtained after immersing the sample in liquid nitrogen) were sputter coated with a thin layer of gold in a vacuum chamber. 2.7. FTIR analysis The infrared transmission spectra of the sample films were measured using an FTIR spectrophotometer model 670 (NEXUS) Nicolet. The wavelength of interest is at 1716 cm 1 which corresponds to the presence of carbonyl group, a by-product of the oxidation of PE due to gamma irradiation in air. The carbonyl index, which is defined here as the ratio of the area under the peak
M. Al-Ali et al. / Vacuum 70 (2003) 227–236
1716 cm 1 to the area under the peak at 1870 cm 1, is taken to eliminate the variation in the thickness of these samples.
3. Results and discussion 3.1. DSC and TG analysis The DSC reheating scans of the blends were used to determine two parameters signifying the melting behavior of LDPE/HDPE blends. The melting parameters include melting points (Tm1 for Table 1 Values obtained from DSC scan and TG for HDPE/LDPE blends HDPE (wt%)
Tm1 (1C)
0 15 25 50 75 85 100
112.5 111.5 111.8 110.8 111.5
Tm2 (1C)
%Crmelt
Td (1C)
125.1 128.1 127.2 129.6 133.0 132.1
36.0 47.0 45.5 47.6 54.0 60.5 65.5
487.2 500.9 500.0 496.0 500.0 501.50 500.7
Tm1 —melting point of LDPE; Tm2 —melting point of HDPE; %Crmelt —crystallinity; Td —decomposition temperature.
229
LDPE and Tm2 for HDPE) and crystallinity levels (%Crmelting). The results as a function of blend composition are presented in Table 1 and Fig. 1. The main observation is the presence of individual melting points with a small intermediate one in the case of the 15HDPE/85LDPE blend (c.f. Fig. 1). Consequently, some partial miscibility in the solid state may be suggested at this blend ratio but not in the other blends. There are a variety of results reported in the literature regarding the compatibility of LDPE/ HDPE blends. Rueda et al. [5] observed three endothermic peaks in DSC scan obtained from the HDPE/LDPE blends particularly at the HDPE fractions less than 50%. It was recognized as three different populations of lamella; one belongs to the LDPE component, the second belongs to the HDPE component and the third one is associated with the mixed phase. This mixed material is attributed to a process whereby the more defective chains from the HDPE constituent mix with the less defective one from the LDPE part. On the other hand, Alamo et al. [6] suggested co-crystallinity between blend components, where chains of one component crystallize into the lamella of the other component. The extent of the co-crystallization process will affect the thermal and physical
Fig. 1. DSC scan of PE blends.
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properties of the blend components. Such blends, consequently, exhibit intermediate crystallization temperatures, melting points, and crystallinity as compared with the individual blend components. Anyway, this verifies that there is some compatibility or partial miscibility that improves the thermal and mechanical properties. Morphology results obtained by the TEM study [7] showed that there is no apparent dispersed phase of HDPE observable in the matrix of LDPE. This verified that there is some compatibility or partial
miscibility that improves the thermal and mechanical behavior of the blend. Nevertheless, these blends do not have the necessary deformation strength requirements for some applications. This stimulated our interest to investigate the effectiveness of high-energy radiation to improve thermal stability and mechanical properties of such blends (discussed later). Referring to Table 1, it was observed that melting points and melting crystallinity peaks are observed for each component up to 75% HDPE.
Fig. 2. DSC scans of non-irradiated and irradiated 15HDPE/85LDPE blend.
M. Al-Ali et al. / Vacuum 70 (2003) 227–236
Beyond 75% HDPE, only one melting point is observed and the melting peak was narrow (not shown), indicating, also, that these particular PE grades are miscible. The sharp melting peak is attributed to thicker lamellae containing little or no branches. It should be noted that the HDPE does not have as much influence in shifting LDPE melting point, which could be due to limited miscibility of the LDPE component with linear chains of HDPE, whereas the presence of the LDPE has a more significant decreasing effect on the HDPE crystallinity levels resulting in melting point depression (Table 1). The explanation is that in the temperature region where the LDPE crystallizes, the HDPE component is already crystallized. Consequently, little changes are predicted in the crystallization or melting point of LDPE versus composition. In conclusion, when LDPE is used as a modifier for HDPE, the degree of branching would be expected to control the co-crystalization behavior of the blend and the point at which two discrete phases occur. Fig. 2 depicts the typical DSC curves obtained from the 15HDPE/85LDPE blend before and after irradiation. As mentioned before, the pristine samples, depending on the blend ratio, exhibit two endothermic peaks on heating indicating multiphase-system nature. The gamma irradiation promotes cross-linking simultaneously with scission and oxidation reactions [8], which cause the modification in the position and shape of the peaks. The influence of radiation dose on the thermal parameters, melting temperature and crystallinity of HDPE/LDPE blends is presented in Table 2. The two crystal melting endotherms Tm1 and Tm2 corresponding to those of the PE components are evident in the blends of lower composition ratio. Apparently, the melting temperatures of each blend components decrease as the radiation dose increases and finally flatten out at higher doses. All treated samples exhibit broadening in the melting peak (not shown) which increases with the increasing dose. At one and the same dose, we also noted such broadening in the melting peak when the LDPE component in the blends is increased. This is related to: (1) radiation induced thinner lamella; (2) increasing amount of branches incorporated in the polymer
231
Table 2 Values obtained from DSC scan and TG for the irradiated HDPE/LDPE blends DOSE (kGy)
Tm1 (1C)
Tm2 (1C)
% Crmelt
Td (1C)
15
100 500 1000 1500
109.5 104.9 102.8
123.0 120.3 117.4 122.7
38.3 39.0 36.0 45.0
475.9 448.9 463.9 470.7
25
100 500 1000 1500
110.5 105.2 102.8 102.7
124.7 122.4 118.3 117.4
41.0 33.0 36.0 32.0
466.7 406.9 472.3 466.1
50
100 500 1000 1500
109.4 104.3 — 103.6
125.0 122.5 121.3 119.3
40.0 49.0 42.0 36.0
455.8 459.1 474.2 472.5
75
100 500 1000 1500
— — — 104.0
128.3 124.5 118.7 113.0
54.2 47.0 29.0 31.0
464.1 460.0 474.2 470.8
85
100 500 1000 1500
— — — —
129.70 125.2 124.0 122.5
63.0 53.0 46.0 43.0
456.0 442.3 464.5 463.9
100
100 500 1000 1500
— — — —
129.8 125.8 123.6 122.8
63.0 58.0 51.0 49.0
478.5 464.7 471.1 472.3
HDPE (wt%)
Tm1 —melting point of LDPE; Tm2 —melting point of HDPE; %Crmelt —crystallinity; Td —decomposition temperature.
chain; and (3) most of these disturbances occur in the branched component of the blend (i.e. LDPE). On the other hand, at higher doses one sharp melting peak is frequently detected indicating probably very high chain scission followed by strong cross-linking. Referring again to Table 2, it is observed that the crystallinity of the PE blends exhibit lower values particularly at elevated radiation doses. The gamma radiation produces modification in PE, principally cross-links in the amorphous regions, which prevent the formation of new crystals. The blends irradiated with medium doses (100 and
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M. Al-Ali et al. / Vacuum 70 (2003) 227–236
500 kGy) exhibit a slight increase in the crystallinity. This is probably due to an improvement in the crystal perfection. Finally, the TG analysis shows that there is no appreciable change in the decomposition tempera-
ture, Td ; for all non-irradiated blends (Table 1) and irradiated blends (Table 2). However, there is an apparent change in the decomposition temperature for the same non-irradiated and irradiated blends.
Fig. 3. Maximum stress (N/mm2) versus the gamma irradiation dose (kGy) for 15HDPE/85LDPE and 25HDPE/75LDPE blends.
Fig. 4. Ductility (a.u.) versus the gamma irradiation dose (kGy) of 15HDPE/85LDPE and 25HDPE/ 75LDPE blends.
M. Al-Ali et al. / Vacuum 70 (2003) 227–236
3.2. Tensile test The results of the tensile tests for irradiated and non-irradiated 15HDPE/85LDPE blends, in comparison to the widely used 25HDPE/75LDPE, are presented in Figs 3–5 as stiffness, ductility and maximum stress respectively. As shown in Fig. 3, at a low exposure dose 100 kGy the stiffness increases and then falls rapidly with the increase of irradiation dose. Increased stiffness, at lower doses, means a higher degree of interchain interactions, which are introduced by covalent crosslinking or by non-covalent interactions such as those involved in the packing of crystalline domains. This can be easily established by observing the appreciable increase in the crystallinity of PE blends, particularly at low levels of irradiation doses (c.f. Table 2). The ductile tensile behavior of the two blends is shown in Fig. 4. The 500 kGy irradiated PE blends possess a worse ductility than the non-irradiated and the 100 kGy-irradiated materials. With the irradiation doses above 500 kGy, brittle behavior of the irradiated samples is observed but is of less importance. The 1500 kGy irradiated materials display nearly 20% ductility, so it is not brittle and
233
is still of great interest for some applications. In conclusion, radiation-induced cross-linking in PE blends is limited: cross-linking occurs for lower doses whereas at higher doses scission of molecular chains, resulting in a decrease of cross-linking, becomes predominant. As shown in Fig. 5 also, as the gamma irradiation proceeds up to 500 kGy there is expected an increase in the maximum stress indicating valuable elasticity for treated samples. The increase in the breaking strength can be attributed to the fact of cross-linking in addition to the HDPE content, as both present excess of tying molecules and intercrystal bonds. These types of links play an important role in the transmission of force between lamella. There is no steep change of all parameters investigated above in the case of 25HDPE/ 75LDPE composition in comparison to that evaluated for the 15HDPE/85LDPE ratio. The strong reduction detected in maximum stress at the 25% HDPE composition (Fig. 5) may be due to the formation of isolated domain in LDPE matrix particularly at high HDPE content indicating less miscibility than that detected for the 15% HDPE blend.
Fig. 5. Stiffness (N/mm2) versus the gamma irradiation dose (kGy) of 15HDPE/85LDPE and 25HDPE/75LDPE blends.
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M. Al-Ali et al. / Vacuum 70 (2003) 227–236
3.3. SEM Fig. 6a–c displays the fracture surface morphology of 15HDPE/85LDPE blend versus gamma irradiation. As shown in Fig. 6a, at 100 kGy the most prominent is the drawn nature of the fracture surface. Increases in the irradiation dose up to
500 kGy enhance the domain size of HDPE (Fig. 6b), which is responsible for the high toughness of the blends. A further increase of irradiation dose to 1500 kGy results in the domains of each component of the blend being deformed into irregular shapes and developing a continuous structure (Fig. 6c).
Fig. 6. SEM micrographas of fracture surfaces from the irradiated 15HDPE/85LDPE blend: (a) at 100 kGy, (b) at 500 kGy, and (c) at 1500 kGy.
M. Al-Ali et al. / Vacuum 70 (2003) 227–236
235
Fig. 7. Carbonyl index of irradiated PE blends.
3.4. IR analysis IR spectra of 15% HDPE and 25% HDPE were measured to study the structural changes in both blends after irradiation. Fig. 7 shows the changes observed in the carbonyl index of irradiated blends. Samples of 0% HDPE and 100% HDPE are also included to elucidate the effect of mixing process. Evidently, the appearance of this band originates from the main chain scission due to irradiation-induced oxidative degradation [9]. Moreover, the degradation of LDPE is rapidly obtained at 10 kGy and flattens out at elevated doses. On the other hand, for the HDPE-rich blends the main chain scission is shifted to the dose 500 kGy. Beyond 500 kGy, stabilization in the degradation process is detected. This is fairly in agreement with the tendency for stabilization obtained for the melting point and the melting crystallinity of irradiated blends (c.f. Table 2).
4. Conclusions 1. In general, polymers blends exhibit a phase inversion, depending on the compositional
ratio, and at that moment partial miscibility was noted. We found the threshold blend ratio at 15% of the HDPE content. 2. Irradiation-induced improvements in blend properties have been achieved through several different mechanisms, including scission, crosslinking, compatibilization, and morphology stabilization. 3. A competing process of oxidative chain scission has been problematic with the gamma-ray irradiation approach. Fortunately, the oxidative degradation of PE blends occurs in air at the gamma-ray doses above 500 kGy. This indicates that PE blends are suitable for long-term technical applications in radiation environment.
Acknowledgements This work was performed in the frame of the scientific cooperation between Scientific and Applied Research Center (SARC), University of Qatar and Institute of Electronic Material Technology (ITME), Warsaw, Poland. Financial support of SARC is gratefully acknowledged.
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References [1] Czvikovszky T, Hargitai H. Radiat Phys Chem 1999;55:727–30. [2] Feng Y, Ma Z-T. In: Singh A, Silverman J, editors. Radiation processing of polymers. Munich: Hanser Publishers, 1992. p. 1. [3] Osagawara M. Adv Polym Sci 1993;10a:37. [4] Singh A. Radiat Phys Chem 1999;56:375–80. [5] Rueda M, Viksne B-C. J Mater Sci 1996;31:3915–20.
[6] Alamo RG, Glaser RH, Mandelkern LJ. Polym Sci Polym Phys Ed 1988;26:2169. [7] Albano C, Snchez G, Ismayel A, Hernandez P. Int J Polym Anal Charact 1998;3:1–16. [8] Miguez JC, Mano EB, Pereira RA. Polym Degrad Stability 2000;69:217–22; Clampitt. J Polym Sci 1965;3A:671. [9] Yoshimasa H, Toshitaka O, Junichi U, Hidenori K, Kenji N, Fumio Y. Radiat Phys Chem 2001;62:133–9.
Mechanical and thermal properties of gamma-ray irradiated polyethylene blends Mariam Al-Alia, N.K Madia, Nora J. Al Thanib, M. El-Muraikhia, A. Turosc,d,* a
Physics Department, Faculty of Science, Qatar University, Doha P.O.B. 2713, Doha, Qatar b Scientific and Applied Research Center (SARC), University of Qatar, Doha, Qatar c Institute of Electronic Materials Technology, ul. Wolczynska 133, Warsaw PL-01-919, Poland d Soltan Institute for Nuclear Studies, Swierk, Poland
Abstract In this work, the results obtained by gamma-ray irradiation of polyethylene (PE) blends are presented. The blends were produced by injection molding at 2201C with different compositional ratios, i.e. 100/0, 15/85, 25/75, 50/50/, 75/25, 85/15, and 0/100, of low-density PE with high-density PE and linear low-density PE. The exposure of these blends to different doses of 60Co gamma radiation up to 1500 kGy improves the mechanical properties due to partial crosslinking. Diffraction scanning calorimetry, infrared spectroscopy and scanning electron microscopy have been used to investigate the effect of gamma irradiation on the thermal behavior of the polymer material. After the radiation treatment, the melting peak of the low-density PE component decreases in intensity as the absorbed dose increases. On the contrary, melting peak of the high-density component increases in intensity and shifts to lower temperatures as the absorbed dose increases. The correlation between the mechanical properties and the morphology of the blends is discussed. r 2003 Elsevier Science Ltd. All rights reserved. Keywords: Polymers; Polymer blends; Gamma-ray irradiation; Microstructure
1. Introduction Polyethylene (PE) is widely used in various fields because of its excellent properties such as softness, elasticity and insulation. They are also used in nuclear power plants and are exposed to ionizing radiation for a long time. Since aliphatic polymers are very sensitive to radiation, synthesis and development of radiation-resistant polymeric materials are strongly desired.
*Corresponding author. Tel.: 0603-092223; fax: 22-8349003. E-mail address: turos [email protected] (A. Turos).
Blending of synthetic and natural polymers is a well-known method of modifying properties of polymer. It is an economically viable and versatile way to prepare new engineering materials and to overcome deficiency in some material characteristics. However, most of the polymer blends are immiscible on the molecular scale and form heterogeneous systems that affect their physicomechanical behavior. There have been a few attempts to improve the miscibility by using radiation to modify one or both polymers [1]. Therefore, a study of the influence of radiation on the morphology, crystallization and melting behavior of the component polymers in the blend is
0042-207X/03/$ - see front matter r 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0042-207X(02)00648-6
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M. Al-Ali et al. / Vacuum 70 (2003) 227–236
essential for an understanding of how to optimize processing condition and properties. The mechanism responsible for these improvements should be also understood well. The effects of irradiation of PE have been studied extensively [2,3]. It is reported that oxidative degradation becomes a major process during gamma irradiation, particularly at lowdose-rate gamma irradiation. However, there have been suggestions in literature [4] that cross-linking may still be an important process during the lowdose-rate gamma irradiation and that peroxides can act as an initiator for radiation-induced crosslinking of PE. In this paper, we report on the role of LDPE admixtures and on gamma-ray irradiation in modifying the mechanical and thermal properties of HDPE. A variety of techniques have been used to identify and quantify the behavior of blends versus the blend ratio and/or irradiation dose, e.g. thermal analysis technique, FTIR, SEM, and tensile testing. Finally, we obtained good estimate of the proper blend ratio and the proper irradiation dose reasonable for desired properties.
2. Experimental
2.3. Tensile testing Tensile specimens of a dog-bone shape were produced with a gauge length of 65 mm, width of 1.25 mm and thickness of 3.1 mm. Tensile testing was carried out using the Lloyd instruments material testing machine connected to a remote microcomputer for data acquisition and analysis. The load was measured by a load cell 5 KN capacity, while the displacement was measured using an internal extensometer. The speed of testing was 100 mm/min. Three samples were tested under the same condition for each blend. 2.4. Differential scanning calorimeter (DSC) DSC (Perkin Elmer, model DSC7 Pyris) was used to observe melting and solidification behavior. To erase thermal history, first cooling and then heating thermograms were detected with the scanning rate of 101C/ min in booth. 2.5. Thermogravimetric analyzer (TGA) TGA (TGA& model with Pyris software) was used to observe the decomposition temperature (Td 1C) of PE blends. The scanning rate was 101C/ min.
2.1. Sample preparation 2.6. Scanning electron microscope (SEM) The investigated materials are native LDPE and HDPE (MFI: 6 g/10 min). Low density PE is of grade FE8000; it has a melt flow index 1.8–2.2, crystalline melting temperature 1111C and its density at 231C is 0.922–0.924 gm/cm3. The blend was produced by injection molding at 2201C with different compositional ratios, i.e., 0/100, 15/85, 25/75, 50/50/, 75/25, 85/15, and 100/0 of lowdensity PE with high-density PE. 2.2. Gamma-ray irradiation The samples were irradiated at room temperature in air using a gamma cell 220 manufactured by the atomic Energy Canada. The operating dose rate was 6 kGy/h, and a set of samples was produced spanning an integrated dose from 0 to 1500 kGy.
Philips Model XL30, SEM was used to study the morphology of the fracture surface. Before measurements, the fracture surfaces (obtained after immersing the sample in liquid nitrogen) were sputter coated with a thin layer of gold in a vacuum chamber. 2.7. FTIR analysis The infrared transmission spectra of the sample films were measured using an FTIR spectrophotometer model 670 (NEXUS) Nicolet. The wavelength of interest is at 1716 cm 1 which corresponds to the presence of carbonyl group, a by-product of the oxidation of PE due to gamma irradiation in air. The carbonyl index, which is defined here as the ratio of the area under the peak
M. Al-Ali et al. / Vacuum 70 (2003) 227–236
1716 cm 1 to the area under the peak at 1870 cm 1, is taken to eliminate the variation in the thickness of these samples.
3. Results and discussion 3.1. DSC and TG analysis The DSC reheating scans of the blends were used to determine two parameters signifying the melting behavior of LDPE/HDPE blends. The melting parameters include melting points (Tm1 for Table 1 Values obtained from DSC scan and TG for HDPE/LDPE blends HDPE (wt%)
Tm1 (1C)
0 15 25 50 75 85 100
112.5 111.5 111.8 110.8 111.5
Tm2 (1C)
%Crmelt
Td (1C)
125.1 128.1 127.2 129.6 133.0 132.1
36.0 47.0 45.5 47.6 54.0 60.5 65.5
487.2 500.9 500.0 496.0 500.0 501.50 500.7
Tm1 —melting point of LDPE; Tm2 —melting point of HDPE; %Crmelt —crystallinity; Td —decomposition temperature.
229
LDPE and Tm2 for HDPE) and crystallinity levels (%Crmelting). The results as a function of blend composition are presented in Table 1 and Fig. 1. The main observation is the presence of individual melting points with a small intermediate one in the case of the 15HDPE/85LDPE blend (c.f. Fig. 1). Consequently, some partial miscibility in the solid state may be suggested at this blend ratio but not in the other blends. There are a variety of results reported in the literature regarding the compatibility of LDPE/ HDPE blends. Rueda et al. [5] observed three endothermic peaks in DSC scan obtained from the HDPE/LDPE blends particularly at the HDPE fractions less than 50%. It was recognized as three different populations of lamella; one belongs to the LDPE component, the second belongs to the HDPE component and the third one is associated with the mixed phase. This mixed material is attributed to a process whereby the more defective chains from the HDPE constituent mix with the less defective one from the LDPE part. On the other hand, Alamo et al. [6] suggested co-crystallinity between blend components, where chains of one component crystallize into the lamella of the other component. The extent of the co-crystallization process will affect the thermal and physical
Fig. 1. DSC scan of PE blends.
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M. Al-Ali et al. / Vacuum 70 (2003) 227–236
properties of the blend components. Such blends, consequently, exhibit intermediate crystallization temperatures, melting points, and crystallinity as compared with the individual blend components. Anyway, this verifies that there is some compatibility or partial miscibility that improves the thermal and mechanical properties. Morphology results obtained by the TEM study [7] showed that there is no apparent dispersed phase of HDPE observable in the matrix of LDPE. This verified that there is some compatibility or partial
miscibility that improves the thermal and mechanical behavior of the blend. Nevertheless, these blends do not have the necessary deformation strength requirements for some applications. This stimulated our interest to investigate the effectiveness of high-energy radiation to improve thermal stability and mechanical properties of such blends (discussed later). Referring to Table 1, it was observed that melting points and melting crystallinity peaks are observed for each component up to 75% HDPE.
Fig. 2. DSC scans of non-irradiated and irradiated 15HDPE/85LDPE blend.
M. Al-Ali et al. / Vacuum 70 (2003) 227–236
Beyond 75% HDPE, only one melting point is observed and the melting peak was narrow (not shown), indicating, also, that these particular PE grades are miscible. The sharp melting peak is attributed to thicker lamellae containing little or no branches. It should be noted that the HDPE does not have as much influence in shifting LDPE melting point, which could be due to limited miscibility of the LDPE component with linear chains of HDPE, whereas the presence of the LDPE has a more significant decreasing effect on the HDPE crystallinity levels resulting in melting point depression (Table 1). The explanation is that in the temperature region where the LDPE crystallizes, the HDPE component is already crystallized. Consequently, little changes are predicted in the crystallization or melting point of LDPE versus composition. In conclusion, when LDPE is used as a modifier for HDPE, the degree of branching would be expected to control the co-crystalization behavior of the blend and the point at which two discrete phases occur. Fig. 2 depicts the typical DSC curves obtained from the 15HDPE/85LDPE blend before and after irradiation. As mentioned before, the pristine samples, depending on the blend ratio, exhibit two endothermic peaks on heating indicating multiphase-system nature. The gamma irradiation promotes cross-linking simultaneously with scission and oxidation reactions [8], which cause the modification in the position and shape of the peaks. The influence of radiation dose on the thermal parameters, melting temperature and crystallinity of HDPE/LDPE blends is presented in Table 2. The two crystal melting endotherms Tm1 and Tm2 corresponding to those of the PE components are evident in the blends of lower composition ratio. Apparently, the melting temperatures of each blend components decrease as the radiation dose increases and finally flatten out at higher doses. All treated samples exhibit broadening in the melting peak (not shown) which increases with the increasing dose. At one and the same dose, we also noted such broadening in the melting peak when the LDPE component in the blends is increased. This is related to: (1) radiation induced thinner lamella; (2) increasing amount of branches incorporated in the polymer
231
Table 2 Values obtained from DSC scan and TG for the irradiated HDPE/LDPE blends DOSE (kGy)
Tm1 (1C)
Tm2 (1C)
% Crmelt
Td (1C)
15
100 500 1000 1500
109.5 104.9 102.8
123.0 120.3 117.4 122.7
38.3 39.0 36.0 45.0
475.9 448.9 463.9 470.7
25
100 500 1000 1500
110.5 105.2 102.8 102.7
124.7 122.4 118.3 117.4
41.0 33.0 36.0 32.0
466.7 406.9 472.3 466.1
50
100 500 1000 1500
109.4 104.3 — 103.6
125.0 122.5 121.3 119.3
40.0 49.0 42.0 36.0
455.8 459.1 474.2 472.5
75
100 500 1000 1500
— — — 104.0
128.3 124.5 118.7 113.0
54.2 47.0 29.0 31.0
464.1 460.0 474.2 470.8
85
100 500 1000 1500
— — — —
129.70 125.2 124.0 122.5
63.0 53.0 46.0 43.0
456.0 442.3 464.5 463.9
100
100 500 1000 1500
— — — —
129.8 125.8 123.6 122.8
63.0 58.0 51.0 49.0
478.5 464.7 471.1 472.3
HDPE (wt%)
Tm1 —melting point of LDPE; Tm2 —melting point of HDPE; %Crmelt —crystallinity; Td —decomposition temperature.
chain; and (3) most of these disturbances occur in the branched component of the blend (i.e. LDPE). On the other hand, at higher doses one sharp melting peak is frequently detected indicating probably very high chain scission followed by strong cross-linking. Referring again to Table 2, it is observed that the crystallinity of the PE blends exhibit lower values particularly at elevated radiation doses. The gamma radiation produces modification in PE, principally cross-links in the amorphous regions, which prevent the formation of new crystals. The blends irradiated with medium doses (100 and
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500 kGy) exhibit a slight increase in the crystallinity. This is probably due to an improvement in the crystal perfection. Finally, the TG analysis shows that there is no appreciable change in the decomposition tempera-
ture, Td ; for all non-irradiated blends (Table 1) and irradiated blends (Table 2). However, there is an apparent change in the decomposition temperature for the same non-irradiated and irradiated blends.
Fig. 3. Maximum stress (N/mm2) versus the gamma irradiation dose (kGy) for 15HDPE/85LDPE and 25HDPE/75LDPE blends.
Fig. 4. Ductility (a.u.) versus the gamma irradiation dose (kGy) of 15HDPE/85LDPE and 25HDPE/ 75LDPE blends.
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3.2. Tensile test The results of the tensile tests for irradiated and non-irradiated 15HDPE/85LDPE blends, in comparison to the widely used 25HDPE/75LDPE, are presented in Figs 3–5 as stiffness, ductility and maximum stress respectively. As shown in Fig. 3, at a low exposure dose 100 kGy the stiffness increases and then falls rapidly with the increase of irradiation dose. Increased stiffness, at lower doses, means a higher degree of interchain interactions, which are introduced by covalent crosslinking or by non-covalent interactions such as those involved in the packing of crystalline domains. This can be easily established by observing the appreciable increase in the crystallinity of PE blends, particularly at low levels of irradiation doses (c.f. Table 2). The ductile tensile behavior of the two blends is shown in Fig. 4. The 500 kGy irradiated PE blends possess a worse ductility than the non-irradiated and the 100 kGy-irradiated materials. With the irradiation doses above 500 kGy, brittle behavior of the irradiated samples is observed but is of less importance. The 1500 kGy irradiated materials display nearly 20% ductility, so it is not brittle and
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is still of great interest for some applications. In conclusion, radiation-induced cross-linking in PE blends is limited: cross-linking occurs for lower doses whereas at higher doses scission of molecular chains, resulting in a decrease of cross-linking, becomes predominant. As shown in Fig. 5 also, as the gamma irradiation proceeds up to 500 kGy there is expected an increase in the maximum stress indicating valuable elasticity for treated samples. The increase in the breaking strength can be attributed to the fact of cross-linking in addition to the HDPE content, as both present excess of tying molecules and intercrystal bonds. These types of links play an important role in the transmission of force between lamella. There is no steep change of all parameters investigated above in the case of 25HDPE/ 75LDPE composition in comparison to that evaluated for the 15HDPE/85LDPE ratio. The strong reduction detected in maximum stress at the 25% HDPE composition (Fig. 5) may be due to the formation of isolated domain in LDPE matrix particularly at high HDPE content indicating less miscibility than that detected for the 15% HDPE blend.
Fig. 5. Stiffness (N/mm2) versus the gamma irradiation dose (kGy) of 15HDPE/85LDPE and 25HDPE/75LDPE blends.
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3.3. SEM Fig. 6a–c displays the fracture surface morphology of 15HDPE/85LDPE blend versus gamma irradiation. As shown in Fig. 6a, at 100 kGy the most prominent is the drawn nature of the fracture surface. Increases in the irradiation dose up to
500 kGy enhance the domain size of HDPE (Fig. 6b), which is responsible for the high toughness of the blends. A further increase of irradiation dose to 1500 kGy results in the domains of each component of the blend being deformed into irregular shapes and developing a continuous structure (Fig. 6c).
Fig. 6. SEM micrographas of fracture surfaces from the irradiated 15HDPE/85LDPE blend: (a) at 100 kGy, (b) at 500 kGy, and (c) at 1500 kGy.
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Fig. 7. Carbonyl index of irradiated PE blends.
3.4. IR analysis IR spectra of 15% HDPE and 25% HDPE were measured to study the structural changes in both blends after irradiation. Fig. 7 shows the changes observed in the carbonyl index of irradiated blends. Samples of 0% HDPE and 100% HDPE are also included to elucidate the effect of mixing process. Evidently, the appearance of this band originates from the main chain scission due to irradiation-induced oxidative degradation [9]. Moreover, the degradation of LDPE is rapidly obtained at 10 kGy and flattens out at elevated doses. On the other hand, for the HDPE-rich blends the main chain scission is shifted to the dose 500 kGy. Beyond 500 kGy, stabilization in the degradation process is detected. This is fairly in agreement with the tendency for stabilization obtained for the melting point and the melting crystallinity of irradiated blends (c.f. Table 2).
4. Conclusions 1. In general, polymers blends exhibit a phase inversion, depending on the compositional
ratio, and at that moment partial miscibility was noted. We found the threshold blend ratio at 15% of the HDPE content. 2. Irradiation-induced improvements in blend properties have been achieved through several different mechanisms, including scission, crosslinking, compatibilization, and morphology stabilization. 3. A competing process of oxidative chain scission has been problematic with the gamma-ray irradiation approach. Fortunately, the oxidative degradation of PE blends occurs in air at the gamma-ray doses above 500 kGy. This indicates that PE blends are suitable for long-term technical applications in radiation environment.
Acknowledgements This work was performed in the frame of the scientific cooperation between Scientific and Applied Research Center (SARC), University of Qatar and Institute of Electronic Material Technology (ITME), Warsaw, Poland. Financial support of SARC is gratefully acknowledged.
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