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polyethylene blends: Radiation stabilization of polypropylene
NIM B Beam Interactions with Materials & Atoms
Nuclear Instruments and Methods in Physics Research B 263 (2007) 451–457 www.elsevier.com/locate/nimb
Effect of electron beam radiation on the polypropylene/ polyethylene blends: Radiation stabilization of polypropylene C.V. Chaudhari a
a,*
, K.A. Dubey a, Y.K. Bhardwaj a, G. Naxane b, K.S.S. Sarma a, S. Sabharwal a
Radiation Technology Development Section, Bhabha Atomic Research Centre, Trombay, Mumbai 400 085, India b College of Engineering and Technology, Akola, Maharashtra, India Received 6 February 2007; received in revised form 27 June 2007 Available online 10 July 2007
Abstract The effect of incorporation of polyethylene in the polypropylene matrix, on the radiation sensitivity of polypropylene, has been investigated. The changes in the properties such as tensile strength, elongation at break, Shore D hardness, density and melt flow index were monitored as function of polyethylene content and electron beam radiation dose. A correlation between the mechanical properties and morphology of the irradiated polymeric blends has been observed, which has been explained on the basis of Fourier-transform infrared spectroscopy, scanning electron microscope and X-ray diffraction studies. Improvement in the mechanical properties of the polypropylene, irradiated to an optimum electron beam dose, could be achieved by blending it with polyethylene >20%. The optimum radiation dose was found to be dependent on blend composition and morphology, however, an absorbed dose of 250 kGy found to be effective enough to ensure good mechanical properties of the polypropylene/polyethylene blends. 2007 Elsevier B.V. All rights reserved. Keywords: Polyolefins; Free volume; Electron beam irradiation; Mechanical properties; Polymer blends
1. Introduction Polyolefins are widely used as structural materials because of their relatively low cost and general availability [1]. Radiation processing of polyolefins is an economically viable and versatile way to produce materials with enhanced chemical, mechanical, physical properties [2–5]. However, the radiation processing of polypropylene (PP) is of limited use as it undergoes predominantly chain scission when subjected to high-energy radiation. In addition to poor radiation resistance of PP, its poor impact resistance at low temperature further restricts its utilization in industrial domain [6]. The toughness and radiation resistance of PP is expected to increase via the addition of PE, as it undergoes predom-
inantly cross-linking on high-energy irradiation [7,8]. However, it is known that PP and polyethylene (PE) are immiscible and incompatible; consequently the mechanical properties of PP–PE blends are inferior to those of pure component. Several studies have been reported to enhance the compatibilization between PP and PE by the addition of interfacial agents or compatibilizers [9]. The present study presents the investigation done on effect of electron beam irradiation on PP–PE blends. The objective was to find suitable blend composition and radiation dose for significant improvement in the mechanical properties of the blends and improve upon radiation resistance of PP. 2. Experimental 2.1. Preparation of PP–PE blends
*
Corresponding author. Tel.: +91 022 25590175; fax: +91 022 25515050. E-mail address: [email protected] (C.V. Chaudhari). 0168-583X/$ - see front matter 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2007.06.027
Different proportions of LDPE (density 0.915 g/cm3 and melt flow index 1.5 g/10 min) and PP (density 0.904 g/cm3
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C.V. Chaudhari et al. / Nucl. Instr. and Meth. in Phys. Res. B 263 (2007) 451–457
resolution and averages of at least 16 scan in the standard wave number range 400–4000 cm1.
Table 1 Composition and designation of blends Sample designation
Polypropylene (%)
Polyethylene (%)
PE PE PE PE PE PE
100 80 60 40 20 00
00 20 40 60 80 100
00 20 40 60 80 100
and melt flow index 1.01 g/10 min) were mixed in melt condition by using a thermoplastic 2-roll mixing mill (12 · 6 in.). These blends were compression molded into sheets of dimension 12 · 12 · 0.2 cm using a compressionmolding machine under 150 kg/m2 pressure at 160 C. Composition and designation of blends employed in present study is given in Table 1. 2.2. EB irradiation Irradiation was carried out using linear electron beam accelerator (ILU-6) from Budker Institute of Nuclear Physics, Russia under conditions energy 1.8 MeV, current 10 mA and conveyor speed 13 mm s1. Under these parameters the electron beam delivered a dose of 10 kGy/ pass. Samples were irradiated in the dose range 50– 500 kGy. 2.3. Measurement of mechanical properties Tensile strength and percentage elongation at break were carried out for on dumbbell shaped specimens using universal testing machine with a crosshead speed of 200 mm/min (ASTM D2240). The notch impact strength (J/mm) was carried out on notch impact tester (ASTM D256). Shore D scale was used to determine the hardness of the blends (ASTM D2240). Melt flow index was determined according to the ASTM D 1238-79 standard at 190 C with a load of 2.16 kg. The density in g/cm3 was determined by using density gradient columns (ISO 1183:1987).
3. Results and discussion 3.1. FTIR analysis Compositional characterization of polymer blends was done by following their characteristic bands in the FTIR spectra as shown in Fig. 1. Since, the study of weak interactions in such a non-polar matrices cannot be made without a high level of accuracy, FTIR technique was used in this study to investigate the specific interaction between blend components. Our interest was only to ascertain the compositional characteristic of blends. Changes in the absorbance for different blend compositions at 720 cm1 were monitored with change in the PE fraction. The results from peak height ratio, showed a close agreement between calculated and expected value of PE weight fraction. 3.2. Scanning electron microscopy The scanning electron micrographs of fractured surfaces have been shown in Fig. 2. For all the compositions phase separation could be clearly figured. At higher PP fractions (PE < 20%), the PE matrix was found to be embedded (disperse phase) in the continuous PP phase, whereas, at lower PP weight fractions co-continuous morphology of PP and PE were observed. SEM of the blends also indicated the lower rigidity of PE 20% blend. 3.3. Density One of the most obvious effects brought about by blending was the change in density of the blends, since the density of the blends can be accurately predicted by additive rule [10]. Fig. 3 represents the variation in densities of PP–PE blends irradiated to different electron beam
0
20
2.4. Morphological studies
40
%T
Cryogenically fractured surfaces were examined by a scanning electron microscope (SEM). Acceleration voltages of 30 kV and magnification range from 200· to 10000· were used. The fractured surfaces were coated with a thin layer of gold prior to SEM examination.
PE 0% PE 20% PE 40% PE 60% PE 80% PE 100%
60
80
2.5. FTIR studies Fourier-transform infrared spectroscopy (FTIR, JASCO 660) was used for ascertaining compositional characteristics of the blends. Spectra was obtained at 4 cm1
100 700
710
720
730
Wave Number (cm)
740 -1
Fig. 1. FTIR spectrum of PP–PE blends.
750
C.V. Chaudhari et al. / Nucl. Instr. and Meth. in Phys. Res. B 263 (2007) 451–457
453
Fig. 2. Scanning electron micrographs (200· and 10000·) of PP–PE blends. (a) PE 20%, (b) PE 40%, (c) PE 60% and (d) PE 80%.
gases may be significant, which would lead to the significant reduction in the density on irradiation at higher doses.
0.93
Unirradited 50 kGy 150 kGy 250 kGy 350 kGy 500 kGy
Density (g/cc)
0.92
3.4. Melt flow index
0.91
0.90
0.89 0.0
0.2
0.4
0.6
0.8
1.0
Weight fraction of PE
Fig. 4 represents the effect of introduction of PE in PP on melt flow index (MFI). MFI of pure PP is higher (1.5 g/10 min) than that of pure PE (1.04 g/10 min) and MFI values for the blends are less then that of the pure PP and PE. It may be explained by considering the slower mobility of PE chain segments in a solution of PP with high viscosity, as a hindrance from PE to the diffusion of PP chain segment is greater; more energy is dissipated in the transport process [11]. Therefore, the diffusion speed of PP is slower in PP–PE blend with a lower value of MFI.
Fig. 3. Variation in density of PP–PE blends with electron beam radiation dose.
1.6
radiation doses. Densities of the blends were found to decrease with increase in radiation dose for the samples having higher content of PP, however, for the samples having higher contents of PE, density values were found to be slightly higher than the unirradiated samples. This behavior can be explained on the basis of free volume theory; the increase in cross-linking with increase in radiation dose for the samples having higher content of PE decreases the free volume and thus increases the density, on the other hand, irradiation of PP would result in the chain scission and would lead to increase in free volume and thus decrease in density. In case of the samples irradiated to higher doses, contribution from the trapped radiolytic
Melt Flow Index (g/10min)
1.4
1.2
1.0
0.8
0.6
0.4 0
20
40
60
80
100
Weight fraction of LDPE
Fig. 4. Dependence of the melt flow index on the composition of PP–PE blends.
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650 600
Elongation at break (%)
2
Tensile strength (kgf/c m )
PE 00 PE 20 PE 40 PE 60 PE 80 PE 100
550
250
200
150
100 PE 00 PE 100 PE 20 PE 40 PE 60 PE 80
50
0
500 450 400 350 300 250 200 150 100 50 0 0
0
100
200
300
400
100
500
300
400
500
Dose (kGy)
Dose (kGy)
Fig. 5. Variation in tensile strength of PP–PE blends.
200
Fig. 6. Variation in elongation at break for PP–PE blends.
0.38
3.5. Tensile strength
0.36
3.6. Notch impact strength Fig. 7 represents the variation of notch impact strength of blends with radiation dose. The observation for the PP and PE are in accordance with the observed trend of tensile
Notch impact strength (J/mm)
0.34
Fig. 5 represents the tensile strength (TS) as a function of radiation dose for the different content of PE in PP–PE blend. Tensile strength of PP was found to decrease with the increasing irradiation dose whereas; the TS of PE increased with increase in radiation dose. The difference between unirradiated and electron beam (EB) irradiated samples was due to the degrading and cross-linking nature of PP and PE, respectively. However, the presence of PE in the blends (PE > 20%) significantly improved the tensile strength on radiation exposure. It may be noticed that the nature of variation in TS with radiation dose is different for the samples having different content of PE indicating that not only the PE content influences the radiation response of the blends but also it strongly depends on the morphology of the blend. However, it can be safely concluded that PE alters the radiation response of PP–PE blends, and contributes positively to the mechanical properties on EB irradiation. Fig. 6 shows the elongation at break (%) as a function of dose and PP content in PP–PE blend. The elongation at break was found to decrease with the increase in radiation dose for all the samples as number of cross-links increased with the radiation dose for PE in the sample matrix, which prevents the structural organization during drawing [12]. This ever-increasing three-dimensional gel-like structure brings about a decrease in internal chain mobility and elongation. The decrease in percentage elongation at break was much sharper in pure PP in comparison to pure PE, which can be attributed to radiation-induced chain scission process in pure PP. However, no consistent trend was observed for PP–PE blends.
0.32 0.30 0.28 0.26 0.24 0.22
PE00 PE100 PE80 PE60 PE40 PE20
0.20 0.18 0.16 0.14 0.12 0.10 0.08 0.06 0
100
200
300
400
500
Dose (kGy)
Fig. 7. Variation of notch impact strength for PP–PE blends.
strength, i.e. impact strength of PP decreases with increase in radiation dose whereas impact strength of PE improves significantly with increase in radiation dose. From the profiles of different blend compositions, it could be inferred that the decrease in the impact strength of irradiated PP could be compensated with the incorporation of PE in PP matrix. The obvious reason for this is that the crosslinked network of LDPE imparts toughness in PP–PE blend matrix. However, unexpectedly low impact resistance of PP–PE blend containing 20% of PE was observed, which indicated poor interfacial adhesion coupled with the radiation degradation of PP phase at this composition.
4. Shore D hardness The radiation-induced cross-linking or degradation of polymer matrices is often reflected as the change in hardness of the sample as; hardness is generally referred to the resistance of material to the local deformation. The observations proved that the irradiated samples were more resistant towards local deformation consequently increase
C.V. Chaudhari et al. / Nucl. Instr. and Meth. in Phys. Res. B 263 (2007) 451–457
r ¼ a þ bD þ cD2 þ dD3 ;
70
PE 00 PE 20 PE 40 PE 60 PE 80 PE 100
68 66 64 62
Shore D
455
60
2
e ¼ a þ bD þ cD þ dD :
58 56 54 52 50 48 46 44 0
100
200
300
400
ð1Þ
3
500
Dose (kGy)
Fig. 8. Variation in Shore D hardness for the PP–PE blends.
in hardness reading. Fig. 8 shows the plot of hardness variation of samples with radiation dose. It was found that the hardness of the PP and PP–PE blend with lower PE content decreased with increase in absorbed dose whereas hardness of PE and blends with higher PE content (>20%) increased only marginally with increase in radiation dose. However, the trend observed was not in strict accordance with the trend observed in case of tensile strength and impact strength, it might be due to less sensitivity of Shore D hardness to cross-linking density of the matrix [13]. 4.1. Morphological dependence of radiation sensitivity of blends Simple kinetic adjustments were also carried out by means of mathematical equations to the mechanical properties analyzed namely tensile strength and elongation at break in order to theoretically predict the radiation dose up-to which improvement in these properties would be observed as reported by other workers for other blends [14,15]. The observed behavior of the variation of tensile strength and elongation at break for different compositions of PP–PE blends were fitted in a third order polynomial behavior, which is indicative of complex cross-linking and chain scission mechanism, that predominantly depend upon the radiation dose. General third order equation correlating tensile strength (r) and elongation at break (e) to radiation dose (D) can be written as follows:
ð2Þ
The mathematical expressions corresponding to the kinetic behavior of the blends showed a correlation index (r2) between 0.9 and 1. The values of fitting parameters a, b, c, d, a, b, c and d for different blend compositions are listed in Table 2. As can be inferred from the fitting data, in PP–PE blends the component most susceptible to cross-linking is PE. The variation in kinetic parameter values indicated that radiation sensitivity of the blends was not as per the expected weighted average of individual components. This could be due to differences in the morphology of macromolecular complexes formed at different blend compositions. Crystalline extent and content of polymers plays an important role in deciding the inter and intra molecular radical interactions, as cross-linking due to radical overlap is expected to restricted to amorphous domain of the polymer. The crystallinity of the sample can be calculated using Eq. (3) % Crystallinity ¼
qc ðqs qa Þ 100; qs ðqc qa Þ
ð3Þ
where qa is the density of completely amorphous polyethylene, qc the density of completely crystalline polyethylene and qs the density of the sample. For PP–PE blend system, Eq. (3) can be modified to q ðq qba Þ % Crystallinity ¼ bc s 100; ð4Þ qs ðqbc qba Þ where qbc is the weighted density of the completely crystalline phase of the PP–PE blends, and qba is the amorphous phase of the PP–PE blends. The density of the completely crystalline PP and completely amorphous PP are taken as 938 and 852 kg m3, respectively, and the density of the completely crystalline PE and completely amorphous PE are taken as 1000 and 855 kg m3 [16–18]. Fig. 9 represents the variation of crystalline index, as monitored by measured by density analysis. It is clear that, PP posses the highest crystalline structure, thereby reducing the extent of overlap between radiolytic species. However, the intermediate blend composition show negative deviation from additive values of crystallinity, indicating the disruption of crystalline structure of pure PP on introduction of PE in the blend. This will lead into higher amorphous content, and eventually would yield higher cross-linking density and
Table 2 Radiation sensitivity parameters for the PP–PE blends PE (%)
a
b
c
d
a
b
c
d
00 20 40 60 80 100
302.42 133.92 159.82 163.71 165.69 170.45
1.47 0.32 0.34 0.30 0.23 0.37
0.0027 0.0001 0.0013 0.0009 0.0005 0.0007
1.57E-6 4.08E-7 1.04E-6 7.57E-7 3.82E-7 3.65E-7
222.30 122.03 164.01 181.29 183.81 551.56
1.86 1.36 1.14 1.26 1.12 1.50
0.0058 0.0049 0.0032 0.0039 0.0036 0.0004
5.85E-6 5.04E-6 3.09E-6 3.90E-6 3.87E-6 1.83E-6
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In order to further explore the extent of homogeneity of PP–PE blends the heat of mixing of PP–PE blend system was calculated by using the following equation [22]
64 62 60
% Crystallinity
58
DH m (a)
56
2
¼ ð1 wb ÞM a qa ðda db Þ x
54
(b)
52
wb ; ð1 wb ÞM b qb þ wb M a qa ð5Þ
50 48 46 44 0.0
0.2
0.4
0.6
0.8
1.0
PE %
Fig. 9. Crystallinity of PP–PE blends: (a) experimental and (b) theoretical.
100
100
PP
PE
Intensity
80
80
PE40%
60 40 20
where M, w, d and q are the molecular weight of monomer unit, weight fraction, solubility parameter and the polymer density, respectively, and subscript a and b refers to PP and PE, respectively. The behavior of heat of mixing over the whole composition range is shown in Fig. 11, it was found to lie between 0.2 and 0.45 J. It is clear that DHm increases with increase in weight fraction of PE in the blend, attains maximum value at 50% and decreases afterwards. Scheneir has calculated the DHm value for many polymer pairs and showed for compatible polymer pairs, the value lies in the range 4 · 103–4 · 102 J [23]. Therefore, thermodynamics of the system predicts immiscibility of PP–PE
0 10
Intensity
60
20 2θ
30
PE 20% PE60%
0.5
PE80%
40
0.4 20
0.3
10
15
20
25
ΔHm
0
0.2
2θ 0.1
Fig. 10. WAXD 2h plot PP–PE blends (inset: pure PP and PE). 0.0 0.0
0.2
0.4
0.6
0.8
1.0
PE%
Fig. 11. Variation of heat of mixing for PP–PE blends.
0.920 0.918
Additive Experimental
0.916
3
Density (g/cm )
superior mechanical properties. XRD profiles of PP–PE blends are shown in Fig. 10, it shows the presence of all characteristic PP and PE peaks as reported earlier [19,20], with no shift in the peak position 2h, except for blends of PE content >20% composition. PE 20% composition shows shift in the 2h value as well as increase in full width at half maxima (FWHM). The crystallization behavior of a polymer in a blend is affected by many factors, such as composition, thermal history, interfacial interactions, size of dispersed particles and size distribution. It has been reported that PP is somewhat soluble in PE, however, the larger droplets would all crystallize and consume the remaining PP dissolved in the matrix, and thus prevents bridging growth, also as rheology is an important factor, the difference in the MFI values can lead to a practical inhomogenity [21]. The data obtained by mechanical property analysis (Table 2), however, are not supported by these inferences substantially. The most anomalous results were in case of blends with PE > 20% content, suggesting that morphological evaluations on the basis of bulk property analysis like density are inadequate to explain the observations.
0.914 0.912
(a)
0.910 0.908 0.906
(b) 0.904 0.902 0.0
0.2
0.4
0.6
0.8
1.0
PE%
Fig. 12. Density of unirradiated PP–PE blends: (a) experimental and (b) theoretical.
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457
was >20% in the blend. Reduction in elongation at break for PP, PE and all bends was observed with increase in the absorbed dose. The density measurements revealed significant cross-linking and chain scission upon irradiation in the PE and PP domains, respectively. However, at higher doses contributions from the trapped radiolytic products became significant. The optimum dose required for observable improvement in properties was different for the blends of different compositions. An absorbed dose of 250 kGy was found to be effective to ensure good mechanical properties of PP–PE blends with PE content >20%. Acknowledgements The authors sincerely thank Mr. Mohammad Assadullah and Mr. S.A. Khadir for their kind cooperation for electron beam irradiation during the studies. References Fig. 13. High resolution SEM of blend containing 20% PE.
blends over the entire composition range in the present study. As mentioned above, the deviation of observed density from calculated density (additive) of the blends can also provide an estimation of the miscibility of blends. Fig. 12 represents this variation in observed and calculated density. It is clear that all blend composition exhibit negative deviation i.e. there is increase in the free volume of the blends. The increased free volume would lead to the poor overlapping of free radicals, resulting in the poor crosslinking of the blend matrix. The increased free volume can be clearly seen in high resolution SEM profile of PE 20% blend (Fig. 13). The anomalous behavior of the mechanical and physical properties of PP–PE blends, is due to over all changes occurring in the crystallinity, free volume and intrinsic radiation response of blend component, hence a simplistic model to represent the radiation response of PP–PE system is difficult to device. 5. Conclusion The study showed that incorporation of PE in PP has a positive effect on the properties of irradiated PP–PE blend system. Tensile strength, impact strength and hardness improved for irradiated PP–PE blends when PE content
[1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17]
[18]
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[23]
L. Wang, B. Huang, J. Polym. Sci. Part B 28 (1990) 937. J. Sharif, S.H.S. Aziz, K. Hashim, Radiat. Phys. Chem. 58 (2000) 191. L. Spenadel, Radiat. Phys. Chem. 14 (1979) 683. S. Ahmed, A.A. Basfar, M.M.A. Aziz, Polym. Degrad. Stab. 67 (2000) 319. R.L. Clough, Nucl. Instr. and Meth. B 185 (2001) 8. S.C. Tjong, W.D. Li, R.K.Y. Li, Eur. Polym. J. 34 (1998) 755. Z. Hui, X. Jiufu, Radiat. Phys. Chem. 42 (1993) 117. A. Hassan, M.U. Wahit, C.Y. Chee, Polym. Test. 22 (2003) 281. H.M. Dahlan, M.D. KhairulZaman, A. Ibrahim, Radiat. Phys. Chem. 64 (2002) 429. J.W. Barlow, E. Nolley, D.R. Paul, Polym. Eng. Sci. 20 (1980) 364. J. Li, R.A. Shanks, Y. Long, Polymer 42 (2001) 1941. C.T. Ratnam, K. Zaman, Nucl. Instr. and Meth. B 152 (1999) 335. T. Yasin, S. Ahmed, F. Yoshii, K. Makuuchi, React. Funct. Polym. 57 (2003) 113. C. Albano, J. Reyes, M.N. Ichazo, J. Gonzalez, M. Rodriguez, Nucl. Instr. and Meth. B 208 (2003) 485. M.I. Chipara, Nucl. Instr. and Meth. B 131 (1997) 188. Y. Wong, F. Lam, Polymer Test. 21 (2002) 691. D. Campbell, R.A. Pethrick, J.R. White, Polymer Characterization: Physical Techniques, second ed., Stanley Thornes, Cheltenham, 2000, p. 472. L. Zhu, F.C. Chiu, Q. Fu, R.P. Quirk, S.Z.D. Cheng, in: J. Brandrup, E.H. Immergut, E.A. Grulke (Eds.), Polymer Handbook, fourth ed., Wiley-Interscience, New York, 1999, p. 19. S. Bertin, J.J. Robin, Eur. Polym. J. 38 (2002) 2255. J.J. Gunderson, S.M. Chilcote, Polym. Network. Blend. 4 (3–4) (1994) 173. X.M. Zhang, A. Ajji, Polymer 46 (2005) 3385. J.L. Gardon, Cohesive energy density, in: H.F. Mark, N.G. Gaylord, N.M. Bikales (Eds.), Encyclopedia of Polymer Science and Technology, Vol. 3, Interscience, New York, 1965. B. Schneier, J. Appl. Polym. Sci. 17 (1973) 3175.
Nuclear Instruments and Methods in Physics Research B 263 (2007) 451–457 www.elsevier.com/locate/nimb
Effect of electron beam radiation on the polypropylene/ polyethylene blends: Radiation stabilization of polypropylene C.V. Chaudhari a
a,*
, K.A. Dubey a, Y.K. Bhardwaj a, G. Naxane b, K.S.S. Sarma a, S. Sabharwal a
Radiation Technology Development Section, Bhabha Atomic Research Centre, Trombay, Mumbai 400 085, India b College of Engineering and Technology, Akola, Maharashtra, India Received 6 February 2007; received in revised form 27 June 2007 Available online 10 July 2007
Abstract The effect of incorporation of polyethylene in the polypropylene matrix, on the radiation sensitivity of polypropylene, has been investigated. The changes in the properties such as tensile strength, elongation at break, Shore D hardness, density and melt flow index were monitored as function of polyethylene content and electron beam radiation dose. A correlation between the mechanical properties and morphology of the irradiated polymeric blends has been observed, which has been explained on the basis of Fourier-transform infrared spectroscopy, scanning electron microscope and X-ray diffraction studies. Improvement in the mechanical properties of the polypropylene, irradiated to an optimum electron beam dose, could be achieved by blending it with polyethylene >20%. The optimum radiation dose was found to be dependent on blend composition and morphology, however, an absorbed dose of 250 kGy found to be effective enough to ensure good mechanical properties of the polypropylene/polyethylene blends. 2007 Elsevier B.V. All rights reserved. Keywords: Polyolefins; Free volume; Electron beam irradiation; Mechanical properties; Polymer blends
1. Introduction Polyolefins are widely used as structural materials because of their relatively low cost and general availability [1]. Radiation processing of polyolefins is an economically viable and versatile way to produce materials with enhanced chemical, mechanical, physical properties [2–5]. However, the radiation processing of polypropylene (PP) is of limited use as it undergoes predominantly chain scission when subjected to high-energy radiation. In addition to poor radiation resistance of PP, its poor impact resistance at low temperature further restricts its utilization in industrial domain [6]. The toughness and radiation resistance of PP is expected to increase via the addition of PE, as it undergoes predom-
inantly cross-linking on high-energy irradiation [7,8]. However, it is known that PP and polyethylene (PE) are immiscible and incompatible; consequently the mechanical properties of PP–PE blends are inferior to those of pure component. Several studies have been reported to enhance the compatibilization between PP and PE by the addition of interfacial agents or compatibilizers [9]. The present study presents the investigation done on effect of electron beam irradiation on PP–PE blends. The objective was to find suitable blend composition and radiation dose for significant improvement in the mechanical properties of the blends and improve upon radiation resistance of PP. 2. Experimental 2.1. Preparation of PP–PE blends
*
Corresponding author. Tel.: +91 022 25590175; fax: +91 022 25515050. E-mail address: [email protected] (C.V. Chaudhari). 0168-583X/$ - see front matter 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2007.06.027
Different proportions of LDPE (density 0.915 g/cm3 and melt flow index 1.5 g/10 min) and PP (density 0.904 g/cm3
452
C.V. Chaudhari et al. / Nucl. Instr. and Meth. in Phys. Res. B 263 (2007) 451–457
resolution and averages of at least 16 scan in the standard wave number range 400–4000 cm1.
Table 1 Composition and designation of blends Sample designation
Polypropylene (%)
Polyethylene (%)
PE PE PE PE PE PE
100 80 60 40 20 00
00 20 40 60 80 100
00 20 40 60 80 100
and melt flow index 1.01 g/10 min) were mixed in melt condition by using a thermoplastic 2-roll mixing mill (12 · 6 in.). These blends were compression molded into sheets of dimension 12 · 12 · 0.2 cm using a compressionmolding machine under 150 kg/m2 pressure at 160 C. Composition and designation of blends employed in present study is given in Table 1. 2.2. EB irradiation Irradiation was carried out using linear electron beam accelerator (ILU-6) from Budker Institute of Nuclear Physics, Russia under conditions energy 1.8 MeV, current 10 mA and conveyor speed 13 mm s1. Under these parameters the electron beam delivered a dose of 10 kGy/ pass. Samples were irradiated in the dose range 50– 500 kGy. 2.3. Measurement of mechanical properties Tensile strength and percentage elongation at break were carried out for on dumbbell shaped specimens using universal testing machine with a crosshead speed of 200 mm/min (ASTM D2240). The notch impact strength (J/mm) was carried out on notch impact tester (ASTM D256). Shore D scale was used to determine the hardness of the blends (ASTM D2240). Melt flow index was determined according to the ASTM D 1238-79 standard at 190 C with a load of 2.16 kg. The density in g/cm3 was determined by using density gradient columns (ISO 1183:1987).
3. Results and discussion 3.1. FTIR analysis Compositional characterization of polymer blends was done by following their characteristic bands in the FTIR spectra as shown in Fig. 1. Since, the study of weak interactions in such a non-polar matrices cannot be made without a high level of accuracy, FTIR technique was used in this study to investigate the specific interaction between blend components. Our interest was only to ascertain the compositional characteristic of blends. Changes in the absorbance for different blend compositions at 720 cm1 were monitored with change in the PE fraction. The results from peak height ratio, showed a close agreement between calculated and expected value of PE weight fraction. 3.2. Scanning electron microscopy The scanning electron micrographs of fractured surfaces have been shown in Fig. 2. For all the compositions phase separation could be clearly figured. At higher PP fractions (PE < 20%), the PE matrix was found to be embedded (disperse phase) in the continuous PP phase, whereas, at lower PP weight fractions co-continuous morphology of PP and PE were observed. SEM of the blends also indicated the lower rigidity of PE 20% blend. 3.3. Density One of the most obvious effects brought about by blending was the change in density of the blends, since the density of the blends can be accurately predicted by additive rule [10]. Fig. 3 represents the variation in densities of PP–PE blends irradiated to different electron beam
0
20
2.4. Morphological studies
40
%T
Cryogenically fractured surfaces were examined by a scanning electron microscope (SEM). Acceleration voltages of 30 kV and magnification range from 200· to 10000· were used. The fractured surfaces were coated with a thin layer of gold prior to SEM examination.
PE 0% PE 20% PE 40% PE 60% PE 80% PE 100%
60
80
2.5. FTIR studies Fourier-transform infrared spectroscopy (FTIR, JASCO 660) was used for ascertaining compositional characteristics of the blends. Spectra was obtained at 4 cm1
100 700
710
720
730
Wave Number (cm)
740 -1
Fig. 1. FTIR spectrum of PP–PE blends.
750
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453
Fig. 2. Scanning electron micrographs (200· and 10000·) of PP–PE blends. (a) PE 20%, (b) PE 40%, (c) PE 60% and (d) PE 80%.
gases may be significant, which would lead to the significant reduction in the density on irradiation at higher doses.
0.93
Unirradited 50 kGy 150 kGy 250 kGy 350 kGy 500 kGy
Density (g/cc)
0.92
3.4. Melt flow index
0.91
0.90
0.89 0.0
0.2
0.4
0.6
0.8
1.0
Weight fraction of PE
Fig. 4 represents the effect of introduction of PE in PP on melt flow index (MFI). MFI of pure PP is higher (1.5 g/10 min) than that of pure PE (1.04 g/10 min) and MFI values for the blends are less then that of the pure PP and PE. It may be explained by considering the slower mobility of PE chain segments in a solution of PP with high viscosity, as a hindrance from PE to the diffusion of PP chain segment is greater; more energy is dissipated in the transport process [11]. Therefore, the diffusion speed of PP is slower in PP–PE blend with a lower value of MFI.
Fig. 3. Variation in density of PP–PE blends with electron beam radiation dose.
1.6
radiation doses. Densities of the blends were found to decrease with increase in radiation dose for the samples having higher content of PP, however, for the samples having higher contents of PE, density values were found to be slightly higher than the unirradiated samples. This behavior can be explained on the basis of free volume theory; the increase in cross-linking with increase in radiation dose for the samples having higher content of PE decreases the free volume and thus increases the density, on the other hand, irradiation of PP would result in the chain scission and would lead to increase in free volume and thus decrease in density. In case of the samples irradiated to higher doses, contribution from the trapped radiolytic
Melt Flow Index (g/10min)
1.4
1.2
1.0
0.8
0.6
0.4 0
20
40
60
80
100
Weight fraction of LDPE
Fig. 4. Dependence of the melt flow index on the composition of PP–PE blends.
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650 600
Elongation at break (%)
2
Tensile strength (kgf/c m )
PE 00 PE 20 PE 40 PE 60 PE 80 PE 100
550
250
200
150
100 PE 00 PE 100 PE 20 PE 40 PE 60 PE 80
50
0
500 450 400 350 300 250 200 150 100 50 0 0
0
100
200
300
400
100
500
300
400
500
Dose (kGy)
Dose (kGy)
Fig. 5. Variation in tensile strength of PP–PE blends.
200
Fig. 6. Variation in elongation at break for PP–PE blends.
0.38
3.5. Tensile strength
0.36
3.6. Notch impact strength Fig. 7 represents the variation of notch impact strength of blends with radiation dose. The observation for the PP and PE are in accordance with the observed trend of tensile
Notch impact strength (J/mm)
0.34
Fig. 5 represents the tensile strength (TS) as a function of radiation dose for the different content of PE in PP–PE blend. Tensile strength of PP was found to decrease with the increasing irradiation dose whereas; the TS of PE increased with increase in radiation dose. The difference between unirradiated and electron beam (EB) irradiated samples was due to the degrading and cross-linking nature of PP and PE, respectively. However, the presence of PE in the blends (PE > 20%) significantly improved the tensile strength on radiation exposure. It may be noticed that the nature of variation in TS with radiation dose is different for the samples having different content of PE indicating that not only the PE content influences the radiation response of the blends but also it strongly depends on the morphology of the blend. However, it can be safely concluded that PE alters the radiation response of PP–PE blends, and contributes positively to the mechanical properties on EB irradiation. Fig. 6 shows the elongation at break (%) as a function of dose and PP content in PP–PE blend. The elongation at break was found to decrease with the increase in radiation dose for all the samples as number of cross-links increased with the radiation dose for PE in the sample matrix, which prevents the structural organization during drawing [12]. This ever-increasing three-dimensional gel-like structure brings about a decrease in internal chain mobility and elongation. The decrease in percentage elongation at break was much sharper in pure PP in comparison to pure PE, which can be attributed to radiation-induced chain scission process in pure PP. However, no consistent trend was observed for PP–PE blends.
0.32 0.30 0.28 0.26 0.24 0.22
PE00 PE100 PE80 PE60 PE40 PE20
0.20 0.18 0.16 0.14 0.12 0.10 0.08 0.06 0
100
200
300
400
500
Dose (kGy)
Fig. 7. Variation of notch impact strength for PP–PE blends.
strength, i.e. impact strength of PP decreases with increase in radiation dose whereas impact strength of PE improves significantly with increase in radiation dose. From the profiles of different blend compositions, it could be inferred that the decrease in the impact strength of irradiated PP could be compensated with the incorporation of PE in PP matrix. The obvious reason for this is that the crosslinked network of LDPE imparts toughness in PP–PE blend matrix. However, unexpectedly low impact resistance of PP–PE blend containing 20% of PE was observed, which indicated poor interfacial adhesion coupled with the radiation degradation of PP phase at this composition.
4. Shore D hardness The radiation-induced cross-linking or degradation of polymer matrices is often reflected as the change in hardness of the sample as; hardness is generally referred to the resistance of material to the local deformation. The observations proved that the irradiated samples were more resistant towards local deformation consequently increase
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r ¼ a þ bD þ cD2 þ dD3 ;
70
PE 00 PE 20 PE 40 PE 60 PE 80 PE 100
68 66 64 62
Shore D
455
60
2
e ¼ a þ bD þ cD þ dD :
58 56 54 52 50 48 46 44 0
100
200
300
400
ð1Þ
3
500
Dose (kGy)
Fig. 8. Variation in Shore D hardness for the PP–PE blends.
in hardness reading. Fig. 8 shows the plot of hardness variation of samples with radiation dose. It was found that the hardness of the PP and PP–PE blend with lower PE content decreased with increase in absorbed dose whereas hardness of PE and blends with higher PE content (>20%) increased only marginally with increase in radiation dose. However, the trend observed was not in strict accordance with the trend observed in case of tensile strength and impact strength, it might be due to less sensitivity of Shore D hardness to cross-linking density of the matrix [13]. 4.1. Morphological dependence of radiation sensitivity of blends Simple kinetic adjustments were also carried out by means of mathematical equations to the mechanical properties analyzed namely tensile strength and elongation at break in order to theoretically predict the radiation dose up-to which improvement in these properties would be observed as reported by other workers for other blends [14,15]. The observed behavior of the variation of tensile strength and elongation at break for different compositions of PP–PE blends were fitted in a third order polynomial behavior, which is indicative of complex cross-linking and chain scission mechanism, that predominantly depend upon the radiation dose. General third order equation correlating tensile strength (r) and elongation at break (e) to radiation dose (D) can be written as follows:
ð2Þ
The mathematical expressions corresponding to the kinetic behavior of the blends showed a correlation index (r2) between 0.9 and 1. The values of fitting parameters a, b, c, d, a, b, c and d for different blend compositions are listed in Table 2. As can be inferred from the fitting data, in PP–PE blends the component most susceptible to cross-linking is PE. The variation in kinetic parameter values indicated that radiation sensitivity of the blends was not as per the expected weighted average of individual components. This could be due to differences in the morphology of macromolecular complexes formed at different blend compositions. Crystalline extent and content of polymers plays an important role in deciding the inter and intra molecular radical interactions, as cross-linking due to radical overlap is expected to restricted to amorphous domain of the polymer. The crystallinity of the sample can be calculated using Eq. (3) % Crystallinity ¼
qc ðqs qa Þ 100; qs ðqc qa Þ
ð3Þ
where qa is the density of completely amorphous polyethylene, qc the density of completely crystalline polyethylene and qs the density of the sample. For PP–PE blend system, Eq. (3) can be modified to q ðq qba Þ % Crystallinity ¼ bc s 100; ð4Þ qs ðqbc qba Þ where qbc is the weighted density of the completely crystalline phase of the PP–PE blends, and qba is the amorphous phase of the PP–PE blends. The density of the completely crystalline PP and completely amorphous PP are taken as 938 and 852 kg m3, respectively, and the density of the completely crystalline PE and completely amorphous PE are taken as 1000 and 855 kg m3 [16–18]. Fig. 9 represents the variation of crystalline index, as monitored by measured by density analysis. It is clear that, PP posses the highest crystalline structure, thereby reducing the extent of overlap between radiolytic species. However, the intermediate blend composition show negative deviation from additive values of crystallinity, indicating the disruption of crystalline structure of pure PP on introduction of PE in the blend. This will lead into higher amorphous content, and eventually would yield higher cross-linking density and
Table 2 Radiation sensitivity parameters for the PP–PE blends PE (%)
a
b
c
d
a
b
c
d
00 20 40 60 80 100
302.42 133.92 159.82 163.71 165.69 170.45
1.47 0.32 0.34 0.30 0.23 0.37
0.0027 0.0001 0.0013 0.0009 0.0005 0.0007
1.57E-6 4.08E-7 1.04E-6 7.57E-7 3.82E-7 3.65E-7
222.30 122.03 164.01 181.29 183.81 551.56
1.86 1.36 1.14 1.26 1.12 1.50
0.0058 0.0049 0.0032 0.0039 0.0036 0.0004
5.85E-6 5.04E-6 3.09E-6 3.90E-6 3.87E-6 1.83E-6
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In order to further explore the extent of homogeneity of PP–PE blends the heat of mixing of PP–PE blend system was calculated by using the following equation [22]
64 62 60
% Crystallinity
58
DH m (a)
56
2
¼ ð1 wb ÞM a qa ðda db Þ x
54
(b)
52
wb ; ð1 wb ÞM b qb þ wb M a qa ð5Þ
50 48 46 44 0.0
0.2
0.4
0.6
0.8
1.0
PE %
Fig. 9. Crystallinity of PP–PE blends: (a) experimental and (b) theoretical.
100
100
PP
PE
Intensity
80
80
PE40%
60 40 20
where M, w, d and q are the molecular weight of monomer unit, weight fraction, solubility parameter and the polymer density, respectively, and subscript a and b refers to PP and PE, respectively. The behavior of heat of mixing over the whole composition range is shown in Fig. 11, it was found to lie between 0.2 and 0.45 J. It is clear that DHm increases with increase in weight fraction of PE in the blend, attains maximum value at 50% and decreases afterwards. Scheneir has calculated the DHm value for many polymer pairs and showed for compatible polymer pairs, the value lies in the range 4 · 103–4 · 102 J [23]. Therefore, thermodynamics of the system predicts immiscibility of PP–PE
0 10
Intensity
60
20 2θ
30
PE 20% PE60%
0.5
PE80%
40
0.4 20
0.3
10
15
20
25
ΔHm
0
0.2
2θ 0.1
Fig. 10. WAXD 2h plot PP–PE blends (inset: pure PP and PE). 0.0 0.0
0.2
0.4
0.6
0.8
1.0
PE%
Fig. 11. Variation of heat of mixing for PP–PE blends.
0.920 0.918
Additive Experimental
0.916
3
Density (g/cm )
superior mechanical properties. XRD profiles of PP–PE blends are shown in Fig. 10, it shows the presence of all characteristic PP and PE peaks as reported earlier [19,20], with no shift in the peak position 2h, except for blends of PE content >20% composition. PE 20% composition shows shift in the 2h value as well as increase in full width at half maxima (FWHM). The crystallization behavior of a polymer in a blend is affected by many factors, such as composition, thermal history, interfacial interactions, size of dispersed particles and size distribution. It has been reported that PP is somewhat soluble in PE, however, the larger droplets would all crystallize and consume the remaining PP dissolved in the matrix, and thus prevents bridging growth, also as rheology is an important factor, the difference in the MFI values can lead to a practical inhomogenity [21]. The data obtained by mechanical property analysis (Table 2), however, are not supported by these inferences substantially. The most anomalous results were in case of blends with PE > 20% content, suggesting that morphological evaluations on the basis of bulk property analysis like density are inadequate to explain the observations.
0.914 0.912
(a)
0.910 0.908 0.906
(b) 0.904 0.902 0.0
0.2
0.4
0.6
0.8
1.0
PE%
Fig. 12. Density of unirradiated PP–PE blends: (a) experimental and (b) theoretical.
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was >20% in the blend. Reduction in elongation at break for PP, PE and all bends was observed with increase in the absorbed dose. The density measurements revealed significant cross-linking and chain scission upon irradiation in the PE and PP domains, respectively. However, at higher doses contributions from the trapped radiolytic products became significant. The optimum dose required for observable improvement in properties was different for the blends of different compositions. An absorbed dose of 250 kGy was found to be effective to ensure good mechanical properties of PP–PE blends with PE content >20%. Acknowledgements The authors sincerely thank Mr. Mohammad Assadullah and Mr. S.A. Khadir for their kind cooperation for electron beam irradiation during the studies. References Fig. 13. High resolution SEM of blend containing 20% PE.
blends over the entire composition range in the present study. As mentioned above, the deviation of observed density from calculated density (additive) of the blends can also provide an estimation of the miscibility of blends. Fig. 12 represents this variation in observed and calculated density. It is clear that all blend composition exhibit negative deviation i.e. there is increase in the free volume of the blends. The increased free volume would lead to the poor overlapping of free radicals, resulting in the poor crosslinking of the blend matrix. The increased free volume can be clearly seen in high resolution SEM profile of PE 20% blend (Fig. 13). The anomalous behavior of the mechanical and physical properties of PP–PE blends, is due to over all changes occurring in the crystallinity, free volume and intrinsic radiation response of blend component, hence a simplistic model to represent the radiation response of PP–PE system is difficult to device. 5. Conclusion The study showed that incorporation of PE in PP has a positive effect on the properties of irradiated PP–PE blend system. Tensile strength, impact strength and hardness improved for irradiated PP–PE blends when PE content
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