Effect of Recycled HDPE with Prooxidant on the Photodegradation of HDPE Film

Transcription

1 Effect of Recycled HDPE with Pro-oxidant on the Photodegradation of HDPE Film Effect of Recycled HDPE with Prooxidant on the Photodegradation of HDPE Film M.S.F Samsudin a, M.A.A. Wahab, and Z. Ahamid Advanced Materials Department, PETRONAS Research Sdn Bhd, Malaysia Lot 3288&3289, Off Jalan Ayer Itam, Kawasan Institusi Bangi, Kajang, Selangor Darul Ehsan, Malaysia Received: 8 August 2011, Accepted: 2 July 2012 Summary Stability and photodegradation studies of HDPE film blended with recycled HDPE, with and without pro-oxidant, were carried out by subjecting the film to accelerated UV exposure up to 144 hours. Two types of recycled HDPE were used; one containing pro-oxidant (recycled degradable HDPE RDH) and the other without pro-oxidant (recycled standard HDPE RSH). The loading levels of recycled HDPE were varied from 10 to 30% by weight. The degradation behaviour of the film was then evaluated based on elongation at break, carbonyl index and oxidative induction time (OIT). The results obtained show that the degradation rate of HDPE films, with different levels and types of recycled HDPE (RSH and RDH), were directly correlated to UV exposure time. However, more pronounced changes in elongation at break and carbonyl index were observed, in particular the HDPE film blended with RDH. At 30% loading of RDH, the film degraded at a higher rate compared with film containing the same level of RSH. This suggests that HDPE blended with RDH even at a low loading of 10% could promote degradation of the film. The remaining pro-oxidants in RDH induced the formation of free radicals in the HDPE chain and accelerated the degradation process. Keywords: Degradation, Recycle, Degradable HDPE, Pro-oxidant, Accelerated UV Introduction Polyethylene is widely employed in the packaging and agricultural sectors including use in greenhouse and mulching films. Due to the non-biodegradable a Corresponding author ( Shamsul_farid@petronas.com.my) Smithers Rapra Technology, 2013 Progress in Rubber, Plastics and Recycling Technology, Vol. 29, No. 2,

2 M.S.F Samsudin, M.A.A. Wahab, and Z. Ahamid feature of this polymer its application leads to a large quantity of plastic waste causing serious environmental problems. The most common approach to make polyethylene oxo-biodegradable is to incorporate pro-oxidants [1] which induce abiotic (photo or thermal) oxidation, leading to the reduction of molecular mass to the level where the material becomes susceptible to microbial attack [2]. The most widely reported pro-oxidants currently in use are transition metal ions including iron, cobalt and manganese. Garthe and Kowal in their studies discovered some issues surrounding the degradation of polymers containing pro-oxidants in modern landfills, as the degradation period was not decreased [3]. This problem is due to the absence of air and water, as landfill areas are designed to block out these elements in order to prevent their contaminates entering the soil and drinking water; this prevents degradation and even with highly organic materials i.e. newspaper and food scraps, it may take years to fully degrade in landfills. Studies performed by ExcelPlas, Australia, reported concern from plastic recyclers as the degradable products containing pro-oxidant may contaminate batches of recycled resin and hence disrupt the stability and properties of the products [4]. A study performed by Eyenga et al. [5] reported that polyethylene containing prooxidant is still stable and did not exhibit much change in the melt flow index (MFI) after five extrusion cycles. However, the EPI bulletin [6] stated that the impact of EPI additives on the stability of recycled PE has not been properly studied to date. EPI claimed that PE products modified with TDPA (containing pro-oxidant), such as grocery bags and stretch films, did not have much effect on the recycling process since it contained very small concentrations of the pro-oxidant. The product would only degrade if the oxidation process occurs either by elevated temperature, UV light or mechanical stress. It also reported that the MFI of EPI products remains constant after three extrusion cycles, suggesting stability of the product against degradation. EPI recommended that batches of degradable bags should be blended at less than 50% with other plastics [6]. Thus in this present work, the effect of RSH and RDH (by controlling Fe as a pro-oxidant) on HDPE film degradation was carried out by subjecting the film to accelerated ageing using a UV weatherometer. The aim of this study is to observe the effect of RSH and RDH on the stability properties of HDPE films when exposed to UV. Changes in tensile elongation and carbonyl index (CI) were used as indicators of film degradation. No additional antioxidant or thermal stabilizer additives were added in this blending. 70 Progress in Rubber, Plastics and Recycling Technology, Vol. 29, No. 2, 2013

3 Effect of Recycled HDPE with Pro-oxidant on the Photodegradation of HDPE Film Experimental Material The high density polyethylene (HDPE) grade used in this study was ETILINAS HD5301AA with a melt flow index of 0.08 g/10 min and was supplied by PETRONAS. To simulate the recycling effect, both the RSH and RDH used were obtained from standard HDPE and degradable HDPE which had undergone three extrusion cycles. The initial degradable HDPE contained 50 ppm of Fe-based pro-oxidant. Compounding RSH and RDH were compounded with HDPE using a Brabender twin screw extruder, model DSK 42/7. Temperature was set at 170/180/200 C from feeding to die zone. The compounds were extruded at a screw speed of 85 rpm. Compounded samples were then blown into a film sheet with a thickness of 25 ± 5 µm. Testing and Analysis a) Melt Flow Index (MFI) Melt flow index was measured using a Ceast melt flow tester based on ASTM D1238, with a 21.6 kg load and a temperature of 190 C; a die diameter of mm was used. b) Ultraviolet (UV) The film samples were cut into rectangular specimens with an average thickness of 25 ± 3 µm and were placed in a QUV accelerated weathering tester (UVA 340 nm lamp). This weatherometer was programmed with a 20 h UV and 4 h condensation exposure cycle in accordance with ASTM D5208. The film samples were kept in the weatherometer from 48 to 144 hours in order to provide a range of UV exposure times. The film samples were then tested for tensile properties and carbonyl index. c) Carbonyl index Infrared absorbance spectra were analysed using a Perkin Elmer FTIR Spectrum 400 spectrometer. Six scans were collected from cm -1. The extent Progress in Rubber, Plastics and Recycling Technology, Vol. 29, No. 2,

4 M.S.F Samsudin, M.A.A. Wahab, and Z. Ahamid of oxidation was determined by measuring the level of the ketone carbonyl absorbance peak relative to the C-H stretching of the polyolefin, which remained essentially unchanged during oxidation. Typically, the absorbance peak for the ketone carbonyl formed during UV exposure can be observed at 1713 cm -1. The carbonyl index was calculated by taking the ratio of the carbonyl absorbance to the absorbance of the CH stretching at 1464 cm -1. This provides a means of quantifying the oxidative degradation over time. d) Tensile Tensile tests were conducted using an Instron 5565 universal testing machine (UTM) according to ASTM D882. The rectangular shape test pieces were cut from film samples to a size of 25.4 mm x 150 mm. Crosshead speed was set at 500 mm/min and the load cell used was 0.5 kn. e) Oxidative induction time (OIT) A TA Instruments differential scanning calorimetry (DSC) Q100 with an automatic gas switch was used for this analysis. The OIT was analysed at the isothermal temperature of 200 C in accordance with ASTM D3895. The oxygen gas flow was maintained at 50 ml/min. Results and discussion Preliminary Results MFI The MFI values for HDPE both with and without pro-oxidants are shown in Table 1. These values are based on an average of three samples. In general, there were slight increases in MFI value after the samples had undergone the recycling process, in this case, three cycles of extrusion. On average, the MFI of virgin HDPE increased at about 2%, while HDPE containing pro-oxidant increased at about 10%. Although the MFI increment in virgin HDPE was not that significant, it nevertheless shows the distinct effect of thermo-mechanical degradation such as chain scission and degradation [7]. In HDPE containing the pro-oxidant, the MFI increase is somewhat higher, indicating the prooxidant has reacted and behaved as a catalyst to initiate chain oxidation. It is worth noting that the type of pro-oxidant used in this work was iron-based (Fe) and the concentration was 0.2% by weight. 72 Progress in Rubber, Plastics and Recycling Technology, Vol. 29, No. 2, 2013

5 Effect of Recycled HDPE with Pro-oxidant on the Photodegradation of HDPE Film Table 1. MFI (g/10 min) of HDPE before and after recycling Description Before recycling (g/10 min) After recycling (g/10 min) Δ Change (%) HDPE (virgin) Degradable HDPE Carbonyl Index To confirm this hypothesis, a further characterisation was performed to measure the carbonyl index using FTIR. The results are summarized in Table 2. It is clear that the carbonyl index has increased in the same manner as with MFI, where HDPE with pro-oxidant produced a higher carbonyl index, indicating a greater magnitude of degradation. Table 2. Carbonyl index of HDPE before and after recycling Sample Before recycling (carbonyl index) After recycling (carbonyl index) Δ Change (%) HDPE (virgin) Degradable HDPE Experimental Results Carbonyl index The FTIR absorbance spectrum at 30% loading of RSH and RDH are shown in Figure 1. The spectrum produced by the HDPE film with 30% RDH is consistently higher than that of the HDPE with 30% RSH. The absorbance peak at 1713 cm -1 (carbonyl compound) is also more pronounced. The changes of the carbonyl index in HDPE blended with 30% RSH and 30% RDH are shown in Figure 2. All films were exposed to accelerated UV ranging from 48 to 144 hours. During the UV exposure, different oxygen containing groups were formed in the polyethylene during photodegradation [8]. The more amorphous the polymer material, the easier is the diffusion of oxygen. It was also reported [9] that oxygen attacking the polymeric chain can yield alkoxy and peroxy radicals. These radicals then abstract a hydrogen atom from the chain giving rise to β scission, leading to an accelerated reduction in molecular weight and the formation of carbonyl end groups. From Figure 2, it is clear that HDPE blended with 30% RDH degraded at a much faster rate. The degradation of HDPE blended with 30% RSH, although Progress in Rubber, Plastics and Recycling Technology, Vol. 29, No. 2,

6 M.S.F Samsudin, M.A.A. Wahab, and Z. Ahamid Figure 1. Carbonyl peak of HDPE film with 30% RSH and with 30% RDH after 144 h of UV exposure Figure 2. Comparison of carbonyl index occurring at a higher level than virgin HDPE, remained consistently gradual indicating no acceleration in molecular weight reduction. A more detailed study has shown that in the absence of a pro-oxidant, the rate of the degradation reaction is significantly lower. It was observed that even though the RSH was subjected to three cycles of extrusion, the changes in carbonyl index were minimal. This phenomenon was explained elsewhere [10, 11] where it was thought that pure aliphatic hydrocarbon polymers did not absorb UV radiation. However, a certain amount of UV can still be absorbed by impurities such as chromophores and carbonyl compounds, which cause a small amount of oxidation. In HDPE blended with 30% RDH, the presence of pro-oxidant 74 Progress in Rubber, Plastics and Recycling Technology, Vol. 29, No. 2, 2013

7 Effect of Recycled HDPE with Pro-oxidant on the Photodegradation of HDPE Film additives in RDH may have induced the formation of free radicals which rapidly combine with the available oxygen to form a peroxy radical and subsequently attracts hydrogen from a nearby polymer chain, regenerating the chain free radical and resulting in a hydro peroxide compound [12]. Elongation at Break In general, the elongation at break (E b ) response can be used as an indicator of molecular breakdown [13]. The strength and ductility of the polymer depends on the degree of intermolecular entanglements, which is usually enhanced as the length of the entangled polymer chain increases. Degradation affects the polymer chain and reduces the extent of entanglement, resulting in fracture at a lower degree of elongation. In Figure 3, incorporation of 30% RSH and 30% RDH in HDPE reduced the respective E b. However, the E b reduction in HDPE with 30% RDH is more pronounced. The elongation was slightly higher at the beginning, but fell dramatically after 48 h UV exposure to almost zero after 96 h UV exposure. The reduction of E b for HDPE with 30% RSH is rather gradual and the trend is similar to the manner in which virgin HDPE deteriorated. Figure 3. Comparison of elongation at break (E b ) To investigate this further, a more detailed study was conducted to look at the effect of different levels of RSH and RDH loading in HDPE. The results are shown in Figure 4 and Figure 5. The findings shown in Figure 4 confirm that due to the absence of pro-oxidant, the adverse effect on the elongation remains comparable between 10%, 20% and 30% loading. However, a different loading of RDH in HDPE presented an interesting finding where at 10%, the Progress in Rubber, Plastics and Recycling Technology, Vol. 29, No. 2,

8 M.S.F Samsudin, M.A.A. Wahab, and Z. Ahamid Figure 4. Elongation at break (E b ) for different loadings of RSH Figure 5. Elongation at break (E b ) for different loadings of RDH E b is almost higher than its RSH counterpart. It is thought that the presence of pro-oxidant in a small amount under a short period of UV exposure i.e. up to 48 h, produced a low level degradation, where the presence of a small amount of short chains allowed them to slide between one another causing the material to elongate further before it completely failed. Besides, as more RDH is added to HDPE and with prolonged UV exposure i.e. beyond 48 h, the degradation rate increased rapidly causing material to break at a very low percentage of elongation. At 96 h, HDPE with 30% RDH exhibited some brittleness suggesting the presence of a carbonyl compound in the existing RDH may easily have formed peroxy radicals in the HDPE when exposed to prolonged UV. This chain reaction then propagated rapidly and ultimately resulted in polymer chain scission which leads to a loss of E b and other properties. This is in agreement with work reported elsewhere [14]. 76 Progress in Rubber, Plastics and Recycling Technology, Vol. 29, No. 2, 2013

9 Effect of Recycled HDPE with Pro-oxidant on the Photodegradation of HDPE Film Oxidative Induction Time (OIT) Oxidation induction time gives an indication of the oxidative stability of the polymer as well as the residual concentration of efficient antioxidant. On the other hand OIT is one of the methods to measure the stability of polyolefin material [15]; higher oxidation times indicate stability of the material against oxidation. Figure 6 and Figure 7 show the OIT result of HDPE film at different loadings of RSH and RDH without UV exposure. In Figure 6, incorporation of RSH did not show significant changes in the OIT of HDPE film as the reduction of the OIT value is very marginal. It is thought that the RSH used in this study might contain sufficient antioxidant to prevent thermal chain oxidation Figure 6. Oxidative Induction Time (OIT) of HDPE film with different loadings of RSH Figure 7. Oxidative Induction Time (OIT) of HDPE film with different loadings of RDH Progress in Rubber, Plastics and Recycling Technology, Vol. 29, No. 2,

10 M.S.F Samsudin, M.A.A. Wahab, and Z. Ahamid and is able to withstand several processing stages such as extrusion and injection, which involve melting the HDPE in the presence of an antioxidant to minimise the degradation at each stage. However, a more severe degradation was observed in HDPE containing RDH. In Figure 7, it is clear that the OIT values decreased significantly in an almost linear trend as the loading of RDH increased. The presence of a pro-oxidant in RDH might consume a substantial amount of antioxidant during the recycling process, resulting in a decreased antioxidant level when it was mixed with the HDPE film. A comparison of OIT thermogram at 30% loading of RSH and RDH is shown in Figure 8. Figure 8. Comparison of Oxidative Induction Time (OIT) thermogram of HDPE film with 30% RSH and 30% RDH 4. Conclusion The study showed that the incorporation of RDH accelerated the degradation of HDPE films when it was exposed to UV. The degradation rate is much higher when the loading is at a maximum i.e. 30% of weight. However, the addition of RSH showed a minimal effect on the HDPE degradation. The degradation measured, based on carbonyl index, was found to be consistent with the deterioration in elongation at break and OIT. It is recommended that the use of RDH would require additional antioxidant or processing stabilizer additives in order to control or maintain the stability of HDPE film degradation. 78 Progress in Rubber, Plastics and Recycling Technology, Vol. 29, No. 2, 2013

11 Effect of Recycled HDPE with Pro-oxidant on the Photodegradation of HDPE Film References 1. Roy P.K., Surekha P., Rajagopal C. and Choudhary V., Effect of cobalt carboxylates on the photo-oxidative degradation of low-density polyethylene. Part-I. Polymer Degradation and Stability, 91 (2006) Ojeda T.F.M., Dalmolin E., Forte M.M.C., Jacques R.J.S., Bento F.M. and Camargo F.A.O., Abiotic and biotic degradation of oxo-biodegradable polyethylenes. Polymer Degradation and Stability, 94 (2009) Garthe J.W. and Kowal P.D., Degradable Plastics. extension/factsheets/c/c15.pdf (2002). 4. ExcelPlas Australia, The impact of degradable plastic bags in Australia. Final Report to Department of the Environmental and Heritage (2003). 5. Eyenga I.I., Focke W.W., Prinsloo L.C. and Tolmay A.T., Photodegradation: a solution for the shopping bag visual pollution problem. South African Journal of Science, 97 (2001) EPI TDPA Technology, EPI Environmental Plastics, Vancouver, BC, Canada (2002). 7. Rauwendaal C., Polymer extrusion. 2 nd ed. Munchen (1990). 8. Ranby B. and Rabek J.F., Photodegradation, photo-oxidation and photostabilization of polymers. London (1975). 9. Hinsken H., Moss S., Paiquet J.R. and Zweifel H., Degradation of polyolefin during melt processing. Polymer Degradation and Stability, 34 (1991) Rabek J.F., Polymer photodegradation: mechanisms and experimental methods. London (1995). 11. Wojtala A., The effects of properties of polyolefins and outdoor factors on the course of their degradation. International Polymer Science Technology, 28 (2001) Magugala B., Nhlapo N. and Focke N.N., Mn 2 Al-LDH and Co 2 Al-LDH-stearate as photo degradants for LDPE film. Polymer Degradation and Stability, 94 (2009) Myer E., Plastic failure guide, cause and prevention. New York (1996). 14. Hamid S.H. and Amin M.B., Lifetime prediction of polymer. Journal Applied Polymer Science, 55 (1995) Riga A.T. and Patterson G.H., Development of a standard test method for determining oxidative induction time of hydrocarbon by differential scanning calorimetry and pressure differential calorimetry. Oxidative Behaviour of Material by Thermal Analytical Techiques. ASTM STP EDS American Society for Testing and Material. (1997). Progress in Rubber, Plastics and Recycling Technology, Vol. 29, No. 2,

12 M.S.F Samsudin, M.A.A. Wahab, and Z. Ahamid 80 Progress in Rubber, Plastics and Recycling Technology, Vol. 29, No. 2, 2013