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Thermal behavior of gamma-irradiated recycled polyethylene blends
Polymer Degradation and Stability 69 (2000) 217±222
Thermal behavior of gamma-irradiated recycled polyethylene blends JoaÄo Carlos Miguez Suarez a,*, Eloisa Biasotto Mano b, Romeu AbrahaÄo Pereira c,1 a Instituto Militar de Engenharia (IME), PracËa General TibuÂrcio, 80-22290-270 Rio de Janeiro, RJ, Brazil Instituto de MacromoleÂculas Professora Eloisa Mano, Universidade Federal do Rio de Janeiro (IMA/UFRJ), PO Box 68525-21945-970 Rio de Janeiro, RJ, Brazil c Centro Brasileiro de Pesquisas FõÂsicas, Conselho Nacional de Desenvolvimento Cientõ®co e TecnoloÂgico (CBPF/CNPq), Rua Xavier Sigaud, 150-22290-180, Rio de Janeiro, RJ, Brazil
b
Received 11 January 2000; accepted 24 January 2000
Abstract The interest on recycled materials from post-consumer polymers present in discarded commercial packaging has gained increasing attention. The development of new engineering materials based on degraded polymers is an interesting possibility. Blends of low cost plastics such as polyole®ns, polystyrene, poly(vinyl chloride), discarded in urban waste are of particular interest. The Instituto de MacromoleÂculas had developed a two-step process for recovering plastic residues which allows to obtain materials with good properties. One of these materials may be used as plastic lumber and consists of a recycled 75/25 LDPE/HDPE partially degraded blend. The exposure of this blend to low doses of 60Co gamma radiation improves its mechanical properties due to partial crosslinking. Dierential scanning calorimetry, gel permeation chromatography, infrared spectroscopy and X-ray diraction have been used to investigate the eects of gamma irradiation on the thermal behavior of polymer material. The melting and crystallization temperatures were found to decrease as the radiation dose increases, showing a tendency to stabilization at higher values. The correlation of the degradation process, melting heat and molecular weight distribution is discussed. # 2000 Elsevier Science Ltd. All rights reserved. Keywords: Recycling; Polyethylene blends; Gamma irradiation
1. Introduction During the last few years, the interest on recycled materials developed from post-consumer polymers present in discarded commercial packaging has gained increasing attention. In Brazil, several recycling programs are investigating a variety of alternatives for the disposal of municipal solid waste (MSW), the value per capita of which is growing at a higher rate (2±3%) every year as compared to the world average (1%) [1]. The possibility of the development of new engineering materials based on degraded polymers is a reality. Blends of low cost plastics such as polyole®ns, polystyrene, poly(vinyl chloride), discharged in urban solid waste, are of particular interest. The Instituto de MacromoleÂculas Professora Eloisa Mano developed a two-step process * Corresponding author. Tel.: +55-21-546-7248; fax: +55-21-546-7049. E-mail address: [email protected] (J.C. Miguez Suarez). 1 Present address: Faculdade F.C.L. Santa Marcelina-PracËa Annina Bisegna, 40-36880-000-MuriaeÂ, MG, Brazil.
for recovering plastic residues collected by the municipality of Rio de Janeiro city. The process allows to obtain materials with good properties due to its rather homogeneous characteristics [2,3]. One of these materials is a recycled 75/25 low density polyethylene (LDPE)/ high density polyethylene (HDPE) partially degraded blend, which ®nds use as plastic wood [4,5] and is a potential new material for lumber industry. This recycling process contributes for the preservation of natural products. Despite of its good properties, the recycled material does not ful®ll the deformation strength requirements for some applications. We found that the exposure of this blend to low doses of 60Co gamma radiation improves its mechanical properties [6±8]. The irradiation has the advantages of being a clean and continuous process with a very good control and is an important method in commercial use. The eects of the gamma radiation on polymers include crosslinking, chain scission and double bond formation, which change the molecular structure and properties.
0141-3910/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved. PII: S0141-3910(00)00065-3
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J.C. Miguez Suarez et al. / Polymer Degradation and Stability 69 (2000) 217±222
The present work is part of a general investigation on recycled polyole®n blends. This paper discusses the thermal behavior of recycled 75/25 LDPE/HDPE blend after exposure to 60Co gamma radiation in the air. Differential scanning calorimetry (DSC), gel permeation chromatography (GPC), multiple internal re¯ectance infra-red spectroscopy (MIR) and wide-angle X-ray diffraction (WAXS) have been used. The melting and crystallization temperatures were found to decrease as radiation dose increases, showing a tendency to stabilization at higher doses. The degradation process is discussed. 2. Experimental A recycled 75/25 LDPE/HDPE blend was prepared from post-consumer ¯exible plastic items (bags and other packaging ®lm residues) found in the municipal solid waste of the city of Rio de Janeiro, as described elsewhere [4,6]. PE-blend samples were irradiated with gamma rays in a 60Co industrial equipment at room temperature in air. The dose rate was 2.5 kGy/h and the radiation doses were 100, 250, 500, 1000, 1500 and 2000 kGy (10, 25, 50, 100, 150 and 200 Mrad). The mechanical properties are listed in Table 1 [6,8]. Thermal properties of the blend were studied by differential scanning calorimetry. A DSC7 Perkin-Elmer equipment with computer data system was calibrated with indium. Specimens weighing about 10 mg were cut with knife and heated at 10 C/min from ÿ10 to 170 C under nitrogen. The areas under the endotherms were measured in accordance to ASTM Standard D 3417 [9]. The degree of crystallinity was calculated by assuming the heating of fusion of perfectly crystalline polyethylene to be 289.3 J/g [10]. A Waters GPC model 600E, equipped with an infra-red detector refractometer and four Waters Ultrastyragel columns, and 1,2,4-trichloro-benzene at 135 C as the mobile phase (0.5 ml/min), was used to determine the molecular weight. Polystyrene was taken as the standard. Infra-red spectroscopy measurements on the 75/25 LDPE/HDPE blends were performed in a Perkin-Elmer model 1720 spectrometer in the 4000±400 cmÿ1 region, Table 1 Mechanical properties of recycled PE blend, before and after gamma irradiation [6,8] Dose (kGy)
Maximum stress (MPa)
Elongation at break (%)
Elastic modulus (MPa)
0 100 250 500 1000 1500 2000
12.4 13.3 12.9 13.2 13.9 14.2 14.4
176 163 117 67 54 44 33
501.3 524.2 ± 511.3 ± ± 661.4
using the multiple internal re¯ectance (MIR) technique and TGS detector. One hundred scans were signalaveraged. WAXS experiments were carried out at room temperature in a standard Seifert-FPM GmbH diractometer operating with Cu Ka (l=1.5418 AÊ) radiation in 40 kV and 30 mA, equipped with a graphite monochromator in the primary beam, using symmetrical transmission method with 2 scans at step intervals of 0.05 in the range of 5 to 80 . Lorentz, polarization and incoherent scattering corrections were made. Amorphous and crystalline re¯ection contributions were subsequently resolved with curve ®tting by a nonlinear least-squares method under the assumption that the intensity peak pro®le can be approximated by a Gaussian function [11]. The crystalline parameters of Bravais cell were determined by re®ning using the Rietveld method [12]. 3. Results and discussion Typical DSC curves obtained from the blend samples, before and after irradiation, are presented in Figs. 1 and 2, respectively. Interesting features may be pointed out from these data. The non-irradiated samples present a bimodal behavior with two endothermic peaks on heating and two exothermic peaks on cooling of dierent intensities. It is well known that mixtures of LDPE and HDPE, according to their proportions, may keep their individual melting points, what indicates multiphase-system nature, as shown by DSC data [13±15]. This suggests interfacial compatibility of the components in the polyole®n systems, what is con®rmed by the absence of rupture near the yield point in the stress±strain curves of polyole®n blends [8]. The lower peaks are attributed to melting and crystallization of cocrystals of low and medium density polyethylenes, whereas the higher peaks are associated to HDPE-rich crystals. The gamma irradiation promotes crosslinking simultaneously with scission and oxidation reactions [6,8], which cause the modi®cation in the position and shape of the peaks. The in¯uence of radiation dose in the thermal parameters, melting temperature, crystallization temperature and cristallinity, of the recycled PE-blend is shown in Fig. 3. For all non-irradiated and irradiated samples the melting peaks appear in the 100±130 C range. The melting temperature of the components decreases as the radiation dose increases, showing a tendency to rise and stabilization at higher doses. The depression in melting temperature suggests that the material underwent some deterioration, probably due to very high crosslinking and chain scission, induced by the increase in exposure dose. The melting endotherms of irradiated samples are wider as a consequence of the progressive decrease in
J.C. Miguez Suarez et al. / Polymer Degradation and Stability 69 (2000) 217±222
Fig. 1. DSC thermogram of non-irradiated recycled PE blend (0 kGy).
Fig. 2. DSC thermograms of irradiated recycled PE blend.
219
220
J.C. Miguez Suarez et al. / Polymer Degradation and Stability 69 (2000) 217±222
Fig. 3. Thermal properties of recycled PE blend, before and after gamma irradiation.
the molecular chain length when the exposition to gamma rays is extended for longer periods. Additionally the enlargement of the peaks may indicate that the distribution of crystal sizes is broader [16]. The existence of two exotherms in the non-irradiated blend is an evidence for the crystallization in two stages due to a cocrystallization phenomenon; each component crystallizes according to a characteristic distribution. In irradiated material the exothermic peaks broaden with increasing radiation dose and for higher doses (superior to 500 kGy), the DSC curves exhibit practically only one exotherm (Fig. 2). There is a de®nite tendency for the blend to present a decrease in the crystallization temperature of each component. The properties of a polymeric blend depend on the principal component, which behaves as the matrix. In the recycled 75/25 LDPE/HDPE blend the irradiation modi®es the characteristics of the principal component, LDPE, which has branched chains and lower crystallinity. Over time, the irradiation reduces the branching as chains are broken; the scission process is specially important for the structural integrity of the polymer. The resultanting shorter chains are able to pack together more easily, leading to a more similar macromolecular structure. It may be expected that in the irradiated recycled blend the cocrystallization occurs by the incorporation of HDPE segments in the linear methylene regions of LDPE allowing the most of the branched segments to crystallize separately at lower temperatures. The crystallinity of the recycled blend decreases with the gamma dose (Fig. 3). The gamma radiation produces modi®cations in poltethylene, principally crosslinks in the amorphous regions, that prevented the formation of new crystals [17]. The blend irradiated with the higher doses (1500 and 2000 kGy) presents a small increase in the crystallinity, as shown by the rise in the fusion heat.
This is due probably to an improvement in the crystal perfection as indicated by the small increase in the melting temperatures [18]. The number-average molecular weight (Mn ), the weight-average molecular weight (Mw ) and the polydispersity, calculated from GPC curves of the recycled PE blend, are presented in Table 2. The molecular weight values decrease with increasing radiation dose. This decrease, very intense up to 100 kGy, is also noticed in the polydispersity. These results could be explained by the occurrence of chain scission followed by a photo-oxidative degradation process on more sensitive sites of the molecular chain. The former is additionally supported by an observed decrease in gel content with the increase of gamma dose [8]. Essentially, as indicated by the polydispersity, the irradiation transforms the recycled polyethylene blend into a more similar lower molecular weight material with a structure predominantly linear. This evolution is further complicated by the occurrence of oxidative degradation, which can stien the molecular chains and lead to an embrittlement. This was con®rmed by the observed reduction in the elongation at break that leads to a ductile±brittle transition [8]. MIR spectra of recycled polyethylene blend were used for the interpretation of the structural changes in the material after irradiation. Fig. 4 shows the alterations detected in the intensity of the IR transmittance for the characteristic bands of the polyethylene chains at 720 and 1463 cmÿ1. The methylene group is modi®ed by the gamma rays and the initiation of crosslinking on the recycled blend starts with the breaking of C±H bonds. The high increase in the transmittance after irradiation with 100 kGy shows that the larger modi®cations occur at the initial, lower doses. At 500 kGy and higher doses a new absorption band at 1720 cmÿ1 was found. This absorption may be imputed to the carbonyl groups which resulted from the oxidative degradation of the chains. The degradative process seems to occur at a constant rate, as indicated by the carbonyl index (Table 3). This con®rms the tendency to stabilization showed by the crystallization temperature of the blend after irradiation with high doses. Table 2 Molecular weights and polydispersity of the recycled PE blend, before and after gamma irradiation Dose (kGy)
Average molecular weights Mn
Mw
0 100 250 500 1000 1500 2000
42 900 26 400 17 800 13 300 9600 7000 5300
267 400 112 300 45 300 27 900 19 600 14 800 11 000
Polydispersity Mw/Mn 6.2 4.2 2.5 2.1 2.1 2.1 2.1
J.C. Miguez Suarez et al. / Polymer Degradation and Stability 69 (2000) 217±222
221
Fig. 4. Transmittance in the characteristics absorption bands of recycled PE blend, before and after gamma irradiation.
Fig. 6. Average crystalline parameters of recycled PE blend, before and after gamma irradiation.
Table 3 Carbonyl index of recycled PE blend after gamma irradiation
may be identi®ed. There is no signi®cant dierence in Xray pro®les of non-irradiated and irradiated blend, but a general decrease in the intensity of both re¯ections as the radiation dose increases is observed. This shows that part of the crystalline regions were transformed in nonordered regions after the gamma exposure, con®rming the above discussed DSC results. The proximity of the peaks did not permit to conclude whether the crystalline phases are of cocrystalline nature or not. Fig. 6 shows the length of the average crystalline parameters of the recycled blend, before and after irradiation. It is interesting to observe that the lattice dimensions are unaltered as the radiation dose increases, although the crystal size distribution appears to increase as indicated by the widening of the melting endotherms. These data are coherent with our precedent results [6± 8]. Work proceeds in our laboratories for better understanding of the eect of 60Co gamma radiation on recycled polyole®n materials.
Absorbance (%) Dose (kGy)
1720 cmÿ1
1463 cmÿ1
Carbonyl index
500 1000 1500 2000
32.8 31.2 30.5 25.8
35.8 34.5 33.2 28.0
0.91 0.90 0.92 0.92
4. Conclusions The changes in the thermal properties of the recycled 75/25 LDPE/HDPE blend after exposing to 60Co gamma radiation up to 2000 kGy dose have been investigated. The gamma irradiation in air causes crosslinking and oxidative degradation and modi®es signi®cantly the thermal behavior of the recycled PE blend. The principal conclusions are listed below. Fig. 5. WAXS spectra of recycled PE blend, before and after gamma irradiation.
WAXS studies were carried out in an eort to obtain information on the recycled blend at the crystallite level. Fig. 5 shows X-ray spectra of the recycled blend, before and after irradiation. The (110) and (200) re¯ections from orthorhombic unit cell of polyethylene structure
1. The endotherms and exotherms of recycled PE blend samples, before and after irradiation, show bimodal peaks. Low doses of gamma rays are more eetive than high doses on the thermal characteristics of the recycled PE blend. 2. After the radiation treatment the melting peak of the LDPE component increases in intensity and shifts to lower temperatures as the absorbed dose
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increases. The same behavior is observed in the HDPE component but less markedly. 3. At the same time there is a decrease in the crystallization temperatures due to the slower crystallization rate of the LDPE branched segments. 4. The oxidative degradation of the recycled polyole®n occurs in the air at gamma rays doses equal or higher than 500 kGy. This suggests that the recycled material is suitable for a long-term technological application. Acknowledgements The authors are indebted for ®nancial support to the following Brazilian Agencies: CNPq, CAPES and CEPG/ UFRJ, and to EMBRARAD (Empresa Brasileira de RadiacËoÄes) for the gamma irradiation of the material. References [1] LeaÄo AL, Tan IH. Biomass and Bioenergy 1998;14:83. [2] Mano EB, Bonelli CMC. Revista de QuõÂmica Industrial (Brazil) 1994;62:18.
[3] Mano EB, Bonelli CMC, Guadagnini MA, Luiz SJM. PolõÂmeros: CieÃncia e Tecnologia (Brazil) 1994;4:19. [4] Mano EB, Dias ML, Bonelli CMC. Latin American Applied Research 1995;25:169. [5] Suarez JCM, Mano EB, Bonelli CMC. Proc. 2nd International Congress of Metallurgical and Materials Technology, SaÄo Paulo, SP, Brazil, 1997. p. 73. [6] Suarez JCM, Mano EB, Bonelli CMC. Polym Eng Sci 1999;39:1398. [7] Suarez JCM, Mano EB. Polym Testing 200;19:607. [8] Suarez JCM, Mano EB. J. Applied. Polym., accepted for publication. [9] Anon. Standard D 3417 Ð heats of fusion and crystallization of polymers by thermal analysis Ð standard test method. Philadelphia (PA): American Society for Testing and Materials, ASTM, 1982. [10] Wunderlich B, Cormier CMJ. Polym Sci 1967;5:987. [11] Alexander LE. X-ray diraction methods in polymer science. New York: Robert E. Krieger Pub Co, 1979. p. 41 [12] Young RA, Wiles DB. Advanced X-ray Diraction Methods in Polymer Science 1980;24:1. [13] Viksne A, Zicans J, Kalkis V, Bledzki AK. Angew Makromol Chem 1997;249:151. [14] Capaccio G, Ward IM, Wilding MAJ. Polym Sci Polym, Phys Ed 1978;16:2083. [15] Donatelli AAJ. Appl Polym Sci 1979;23:3071. [16] Klein PG, Gonzalez-Orozco JA, Ward IM. Polymer 1991;32:1732. [17] Aslanian VM, Vardanian VI, Avetisian MH, Felekian SS, Ayvasian SR. Polymer 1987;28:755. [18] Utracki LAM. Macromol Symp 1997;118:335.
Thermal behavior of gamma-irradiated recycled polyethylene blends JoaÄo Carlos Miguez Suarez a,*, Eloisa Biasotto Mano b, Romeu AbrahaÄo Pereira c,1 a Instituto Militar de Engenharia (IME), PracËa General TibuÂrcio, 80-22290-270 Rio de Janeiro, RJ, Brazil Instituto de MacromoleÂculas Professora Eloisa Mano, Universidade Federal do Rio de Janeiro (IMA/UFRJ), PO Box 68525-21945-970 Rio de Janeiro, RJ, Brazil c Centro Brasileiro de Pesquisas FõÂsicas, Conselho Nacional de Desenvolvimento Cientõ®co e TecnoloÂgico (CBPF/CNPq), Rua Xavier Sigaud, 150-22290-180, Rio de Janeiro, RJ, Brazil
b
Received 11 January 2000; accepted 24 January 2000
Abstract The interest on recycled materials from post-consumer polymers present in discarded commercial packaging has gained increasing attention. The development of new engineering materials based on degraded polymers is an interesting possibility. Blends of low cost plastics such as polyole®ns, polystyrene, poly(vinyl chloride), discarded in urban waste are of particular interest. The Instituto de MacromoleÂculas had developed a two-step process for recovering plastic residues which allows to obtain materials with good properties. One of these materials may be used as plastic lumber and consists of a recycled 75/25 LDPE/HDPE partially degraded blend. The exposure of this blend to low doses of 60Co gamma radiation improves its mechanical properties due to partial crosslinking. Dierential scanning calorimetry, gel permeation chromatography, infrared spectroscopy and X-ray diraction have been used to investigate the eects of gamma irradiation on the thermal behavior of polymer material. The melting and crystallization temperatures were found to decrease as the radiation dose increases, showing a tendency to stabilization at higher values. The correlation of the degradation process, melting heat and molecular weight distribution is discussed. # 2000 Elsevier Science Ltd. All rights reserved. Keywords: Recycling; Polyethylene blends; Gamma irradiation
1. Introduction During the last few years, the interest on recycled materials developed from post-consumer polymers present in discarded commercial packaging has gained increasing attention. In Brazil, several recycling programs are investigating a variety of alternatives for the disposal of municipal solid waste (MSW), the value per capita of which is growing at a higher rate (2±3%) every year as compared to the world average (1%) [1]. The possibility of the development of new engineering materials based on degraded polymers is a reality. Blends of low cost plastics such as polyole®ns, polystyrene, poly(vinyl chloride), discharged in urban solid waste, are of particular interest. The Instituto de MacromoleÂculas Professora Eloisa Mano developed a two-step process * Corresponding author. Tel.: +55-21-546-7248; fax: +55-21-546-7049. E-mail address: [email protected] (J.C. Miguez Suarez). 1 Present address: Faculdade F.C.L. Santa Marcelina-PracËa Annina Bisegna, 40-36880-000-MuriaeÂ, MG, Brazil.
for recovering plastic residues collected by the municipality of Rio de Janeiro city. The process allows to obtain materials with good properties due to its rather homogeneous characteristics [2,3]. One of these materials is a recycled 75/25 low density polyethylene (LDPE)/ high density polyethylene (HDPE) partially degraded blend, which ®nds use as plastic wood [4,5] and is a potential new material for lumber industry. This recycling process contributes for the preservation of natural products. Despite of its good properties, the recycled material does not ful®ll the deformation strength requirements for some applications. We found that the exposure of this blend to low doses of 60Co gamma radiation improves its mechanical properties [6±8]. The irradiation has the advantages of being a clean and continuous process with a very good control and is an important method in commercial use. The eects of the gamma radiation on polymers include crosslinking, chain scission and double bond formation, which change the molecular structure and properties.
0141-3910/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved. PII: S0141-3910(00)00065-3
218
J.C. Miguez Suarez et al. / Polymer Degradation and Stability 69 (2000) 217±222
The present work is part of a general investigation on recycled polyole®n blends. This paper discusses the thermal behavior of recycled 75/25 LDPE/HDPE blend after exposure to 60Co gamma radiation in the air. Differential scanning calorimetry (DSC), gel permeation chromatography (GPC), multiple internal re¯ectance infra-red spectroscopy (MIR) and wide-angle X-ray diffraction (WAXS) have been used. The melting and crystallization temperatures were found to decrease as radiation dose increases, showing a tendency to stabilization at higher doses. The degradation process is discussed. 2. Experimental A recycled 75/25 LDPE/HDPE blend was prepared from post-consumer ¯exible plastic items (bags and other packaging ®lm residues) found in the municipal solid waste of the city of Rio de Janeiro, as described elsewhere [4,6]. PE-blend samples were irradiated with gamma rays in a 60Co industrial equipment at room temperature in air. The dose rate was 2.5 kGy/h and the radiation doses were 100, 250, 500, 1000, 1500 and 2000 kGy (10, 25, 50, 100, 150 and 200 Mrad). The mechanical properties are listed in Table 1 [6,8]. Thermal properties of the blend were studied by differential scanning calorimetry. A DSC7 Perkin-Elmer equipment with computer data system was calibrated with indium. Specimens weighing about 10 mg were cut with knife and heated at 10 C/min from ÿ10 to 170 C under nitrogen. The areas under the endotherms were measured in accordance to ASTM Standard D 3417 [9]. The degree of crystallinity was calculated by assuming the heating of fusion of perfectly crystalline polyethylene to be 289.3 J/g [10]. A Waters GPC model 600E, equipped with an infra-red detector refractometer and four Waters Ultrastyragel columns, and 1,2,4-trichloro-benzene at 135 C as the mobile phase (0.5 ml/min), was used to determine the molecular weight. Polystyrene was taken as the standard. Infra-red spectroscopy measurements on the 75/25 LDPE/HDPE blends were performed in a Perkin-Elmer model 1720 spectrometer in the 4000±400 cmÿ1 region, Table 1 Mechanical properties of recycled PE blend, before and after gamma irradiation [6,8] Dose (kGy)
Maximum stress (MPa)
Elongation at break (%)
Elastic modulus (MPa)
0 100 250 500 1000 1500 2000
12.4 13.3 12.9 13.2 13.9 14.2 14.4
176 163 117 67 54 44 33
501.3 524.2 ± 511.3 ± ± 661.4
using the multiple internal re¯ectance (MIR) technique and TGS detector. One hundred scans were signalaveraged. WAXS experiments were carried out at room temperature in a standard Seifert-FPM GmbH diractometer operating with Cu Ka (l=1.5418 AÊ) radiation in 40 kV and 30 mA, equipped with a graphite monochromator in the primary beam, using symmetrical transmission method with 2 scans at step intervals of 0.05 in the range of 5 to 80 . Lorentz, polarization and incoherent scattering corrections were made. Amorphous and crystalline re¯ection contributions were subsequently resolved with curve ®tting by a nonlinear least-squares method under the assumption that the intensity peak pro®le can be approximated by a Gaussian function [11]. The crystalline parameters of Bravais cell were determined by re®ning using the Rietveld method [12]. 3. Results and discussion Typical DSC curves obtained from the blend samples, before and after irradiation, are presented in Figs. 1 and 2, respectively. Interesting features may be pointed out from these data. The non-irradiated samples present a bimodal behavior with two endothermic peaks on heating and two exothermic peaks on cooling of dierent intensities. It is well known that mixtures of LDPE and HDPE, according to their proportions, may keep their individual melting points, what indicates multiphase-system nature, as shown by DSC data [13±15]. This suggests interfacial compatibility of the components in the polyole®n systems, what is con®rmed by the absence of rupture near the yield point in the stress±strain curves of polyole®n blends [8]. The lower peaks are attributed to melting and crystallization of cocrystals of low and medium density polyethylenes, whereas the higher peaks are associated to HDPE-rich crystals. The gamma irradiation promotes crosslinking simultaneously with scission and oxidation reactions [6,8], which cause the modi®cation in the position and shape of the peaks. The in¯uence of radiation dose in the thermal parameters, melting temperature, crystallization temperature and cristallinity, of the recycled PE-blend is shown in Fig. 3. For all non-irradiated and irradiated samples the melting peaks appear in the 100±130 C range. The melting temperature of the components decreases as the radiation dose increases, showing a tendency to rise and stabilization at higher doses. The depression in melting temperature suggests that the material underwent some deterioration, probably due to very high crosslinking and chain scission, induced by the increase in exposure dose. The melting endotherms of irradiated samples are wider as a consequence of the progressive decrease in
J.C. Miguez Suarez et al. / Polymer Degradation and Stability 69 (2000) 217±222
Fig. 1. DSC thermogram of non-irradiated recycled PE blend (0 kGy).
Fig. 2. DSC thermograms of irradiated recycled PE blend.
219
220
J.C. Miguez Suarez et al. / Polymer Degradation and Stability 69 (2000) 217±222
Fig. 3. Thermal properties of recycled PE blend, before and after gamma irradiation.
the molecular chain length when the exposition to gamma rays is extended for longer periods. Additionally the enlargement of the peaks may indicate that the distribution of crystal sizes is broader [16]. The existence of two exotherms in the non-irradiated blend is an evidence for the crystallization in two stages due to a cocrystallization phenomenon; each component crystallizes according to a characteristic distribution. In irradiated material the exothermic peaks broaden with increasing radiation dose and for higher doses (superior to 500 kGy), the DSC curves exhibit practically only one exotherm (Fig. 2). There is a de®nite tendency for the blend to present a decrease in the crystallization temperature of each component. The properties of a polymeric blend depend on the principal component, which behaves as the matrix. In the recycled 75/25 LDPE/HDPE blend the irradiation modi®es the characteristics of the principal component, LDPE, which has branched chains and lower crystallinity. Over time, the irradiation reduces the branching as chains are broken; the scission process is specially important for the structural integrity of the polymer. The resultanting shorter chains are able to pack together more easily, leading to a more similar macromolecular structure. It may be expected that in the irradiated recycled blend the cocrystallization occurs by the incorporation of HDPE segments in the linear methylene regions of LDPE allowing the most of the branched segments to crystallize separately at lower temperatures. The crystallinity of the recycled blend decreases with the gamma dose (Fig. 3). The gamma radiation produces modi®cations in poltethylene, principally crosslinks in the amorphous regions, that prevented the formation of new crystals [17]. The blend irradiated with the higher doses (1500 and 2000 kGy) presents a small increase in the crystallinity, as shown by the rise in the fusion heat.
This is due probably to an improvement in the crystal perfection as indicated by the small increase in the melting temperatures [18]. The number-average molecular weight (Mn ), the weight-average molecular weight (Mw ) and the polydispersity, calculated from GPC curves of the recycled PE blend, are presented in Table 2. The molecular weight values decrease with increasing radiation dose. This decrease, very intense up to 100 kGy, is also noticed in the polydispersity. These results could be explained by the occurrence of chain scission followed by a photo-oxidative degradation process on more sensitive sites of the molecular chain. The former is additionally supported by an observed decrease in gel content with the increase of gamma dose [8]. Essentially, as indicated by the polydispersity, the irradiation transforms the recycled polyethylene blend into a more similar lower molecular weight material with a structure predominantly linear. This evolution is further complicated by the occurrence of oxidative degradation, which can stien the molecular chains and lead to an embrittlement. This was con®rmed by the observed reduction in the elongation at break that leads to a ductile±brittle transition [8]. MIR spectra of recycled polyethylene blend were used for the interpretation of the structural changes in the material after irradiation. Fig. 4 shows the alterations detected in the intensity of the IR transmittance for the characteristic bands of the polyethylene chains at 720 and 1463 cmÿ1. The methylene group is modi®ed by the gamma rays and the initiation of crosslinking on the recycled blend starts with the breaking of C±H bonds. The high increase in the transmittance after irradiation with 100 kGy shows that the larger modi®cations occur at the initial, lower doses. At 500 kGy and higher doses a new absorption band at 1720 cmÿ1 was found. This absorption may be imputed to the carbonyl groups which resulted from the oxidative degradation of the chains. The degradative process seems to occur at a constant rate, as indicated by the carbonyl index (Table 3). This con®rms the tendency to stabilization showed by the crystallization temperature of the blend after irradiation with high doses. Table 2 Molecular weights and polydispersity of the recycled PE blend, before and after gamma irradiation Dose (kGy)
Average molecular weights Mn
Mw
0 100 250 500 1000 1500 2000
42 900 26 400 17 800 13 300 9600 7000 5300
267 400 112 300 45 300 27 900 19 600 14 800 11 000
Polydispersity Mw/Mn 6.2 4.2 2.5 2.1 2.1 2.1 2.1
J.C. Miguez Suarez et al. / Polymer Degradation and Stability 69 (2000) 217±222
221
Fig. 4. Transmittance in the characteristics absorption bands of recycled PE blend, before and after gamma irradiation.
Fig. 6. Average crystalline parameters of recycled PE blend, before and after gamma irradiation.
Table 3 Carbonyl index of recycled PE blend after gamma irradiation
may be identi®ed. There is no signi®cant dierence in Xray pro®les of non-irradiated and irradiated blend, but a general decrease in the intensity of both re¯ections as the radiation dose increases is observed. This shows that part of the crystalline regions were transformed in nonordered regions after the gamma exposure, con®rming the above discussed DSC results. The proximity of the peaks did not permit to conclude whether the crystalline phases are of cocrystalline nature or not. Fig. 6 shows the length of the average crystalline parameters of the recycled blend, before and after irradiation. It is interesting to observe that the lattice dimensions are unaltered as the radiation dose increases, although the crystal size distribution appears to increase as indicated by the widening of the melting endotherms. These data are coherent with our precedent results [6± 8]. Work proceeds in our laboratories for better understanding of the eect of 60Co gamma radiation on recycled polyole®n materials.
Absorbance (%) Dose (kGy)
1720 cmÿ1
1463 cmÿ1
Carbonyl index
500 1000 1500 2000
32.8 31.2 30.5 25.8
35.8 34.5 33.2 28.0
0.91 0.90 0.92 0.92
4. Conclusions The changes in the thermal properties of the recycled 75/25 LDPE/HDPE blend after exposing to 60Co gamma radiation up to 2000 kGy dose have been investigated. The gamma irradiation in air causes crosslinking and oxidative degradation and modi®es signi®cantly the thermal behavior of the recycled PE blend. The principal conclusions are listed below. Fig. 5. WAXS spectra of recycled PE blend, before and after gamma irradiation.
WAXS studies were carried out in an eort to obtain information on the recycled blend at the crystallite level. Fig. 5 shows X-ray spectra of the recycled blend, before and after irradiation. The (110) and (200) re¯ections from orthorhombic unit cell of polyethylene structure
1. The endotherms and exotherms of recycled PE blend samples, before and after irradiation, show bimodal peaks. Low doses of gamma rays are more eetive than high doses on the thermal characteristics of the recycled PE blend. 2. After the radiation treatment the melting peak of the LDPE component increases in intensity and shifts to lower temperatures as the absorbed dose
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increases. The same behavior is observed in the HDPE component but less markedly. 3. At the same time there is a decrease in the crystallization temperatures due to the slower crystallization rate of the LDPE branched segments. 4. The oxidative degradation of the recycled polyole®n occurs in the air at gamma rays doses equal or higher than 500 kGy. This suggests that the recycled material is suitable for a long-term technological application. Acknowledgements The authors are indebted for ®nancial support to the following Brazilian Agencies: CNPq, CAPES and CEPG/ UFRJ, and to EMBRARAD (Empresa Brasileira de RadiacËoÄes) for the gamma irradiation of the material. References [1] LeaÄo AL, Tan IH. Biomass and Bioenergy 1998;14:83. [2] Mano EB, Bonelli CMC. Revista de QuõÂmica Industrial (Brazil) 1994;62:18.
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