Study on the biodegradability of polyetylene in the presence of transition metal stearates (Mn, Fe, Co)

MINISTERY OF EDUCATION AND TRAINING VIETNAM ACADEMY OF SCIENCE AND TECHNOLOGY GRADUATE UNIVERSITY OF SCIENCE AND TECHNOLOGY ------------- PHAM THU TRANG STUDY ON THE BIODEGRADABILITY OF POLYETYLENE IN THE PRESENCE OF TRANSITION METAL STEARATES (Mn, Fe, Co) Scientific Field: Organic Chemistry Classification Code: 62 44 01 14 DISSERTATION SUMMARY HA NOI - 2018 The dissertation was completed at: Institute of Chemistry Vietnam Academy of Science and Technology Scientific Supervisors: 1. Prof. Dr. Nguyen Van Khoi Institute of Chemistry - Vietnam Academy of Science and Technology 2. Dr. Nguyen Thanh Tung Institute of Chemistry - Vietnam Academy of Science and Technology 1 st Reviewer: ........................................................................... ................................................................................. ................................................................................. 2 nd Reviewer: .......................................................................... ................................................................................. ................................................................................. 3 rd Reviewer: ........................................................................... ................................................................................. ................................................................................. The dissertation will be defended at Graduate University of Science And Technology, Vietnam Academy of Science and Technology, 18 Hoang Quoc Viet, Cau Giay District, Ha Noi City. At .. hour.. date.. month ..2018. The dissertation can be found in National Library of Vietnam and the library of Graduate University of Science And Technology, Vietnam Academy of Science and Technology. 1 INTRODUCTION 1. Background Plastics play an important role in the modern world. They have been found to be extremely versatile materials with many useful uses for human life since the 1950s. In 2015, 322 million tonnes of plastics were produced throughout the world. Average plastic consumption per capita in 2015 is 69.7 kg/person in the world, 48.5 kg/person in Asia, 155 kg/person in USA, 146 kg/person in Europe, 128 kg/person in Japan, 41 kg/person in Vietnam (a significant increase by 33 kg/person compared to 2010). Polyethylene is the most widely used thermoplastic in the world, consumed more than 76 million tons per year, accounting for 38% of total plastic consumption. Increased demand for plastics causes increase in waste and global environment pollution. In 2012, the amount of plastic waste dumped into the environment was 25.2 million tons in Europe, 29 million tons in the United States. According to environmental reports of the United Nations, around 22- 43% of the world's waste is buried in the landfill and 35% of waste in ocean. In Vietnam, the average annual volume of solid waste has increased by nearly 200% and will increase in the near future, estimated at 44 million tons per annum. According to the Marine Conservation Organization and the McKinsey Center for Business and Environment, plastic waste of Vietnam is the world's fourth largest by volume (0.73 million tons/year, representing 6% of the total in the world) in 2015. To solve this problem, in the past few decades, scientists have focused on the development of plastic materials which decompose easily. Adding pro-oxidant additives is the most interesting method. Prooxidant additves are usually transition metal ions introduced in the form of stearates or complexes with other organic compounds. Transition metals are used as prooxidant additves, including Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ca ..., the most effective of which are the stearate of Co, Mn and Fe. Under the influence of ultraviolet (UV) radiation, temperature or mechanical impacts, prooxidant additives promote the oxidation of polymer chains to form functional groups such as carbonyl, carboxyl, hydroxide, ester, etc... which can be consumed by microorganisms. In the presence of prooxidant additives, the degradation time of plastics from hundreds of years decreased to several years or even several months. For the above reasons, we propose the dissertation: “Study on the biodegradability of polyetylene in the presence of transition metal stearates (Mn, Fe, Co)”. 2 2. Objectives of the dissertation Studied and evaluated the biodegradability (including the degradation and the biodegradation in the soil environment) of polyethylene films containing prooxidant additives which is stearate salts of Fe (III), Co ( II) and Mn (II). 3. Main contents of the thesis - Research on the degradation process of PE films containing prooxidant additives under accelerated conditions (thermal oxidation and photo- oxidation) and natural weathering. - Research on the biodegradation process and level of oxidized PE films with prooxidant additives in soil. 4. Structure of the thesis The dissertation has 119 pages, including the Preface, Chapter 1: Overview, Chapter 2: Experiment, Chapter 3: Results and discussions, Chapter 4: Conclusions, Pubblications, with 62 images, 20 tables and 130 references. DISSERTATION CONTENTS CHAPTER 1. LITERATURE REVIEW The literature review provided an overview of plastic production and consumption, introduced polyolefins, the degradation of polyolefin, approaches to enhance the biodegradation of polyethylene (PE) and the degradation of PE containing prooxidant additives. Polyolefin especially polyethylene was widely used in plastic pakaging with 80%. However, polyolefins are very difficult to degrade in the natural emvironment so they causes global environment pollution. Combining polyethylene with prooxidant additives, which are organic salts of transition metals is the most effective and interesting method. In the presence of these additives the polyolefin will decompose in two stages: - The first stage: the reaction of oxygen in the air with the polymer. Under the influence of solar ultraviolet radiation (UV), heat, mechanical stresses, humidity... the polymer chains were cleaved into shorter chains to form functional groups such as carbonyl, carboxyl, ester, aldehyde, alcohol ... - The second stage: the biodegradation by microorganisms such as fungi, bacteria ..., which decompose the oligomer to form CO2 and H2O. The literature review showed that there were some research groups in the country to increase the degradability of polyethylene, but these studies focused on manufacture blend of polyethylene and starches. Thus enhancing the biodegradability of polyethylene with transition metal stearates is a promising new direction. 3 CHAPTER 2. EXPERIMENTS 2.1. Materials and equipments 2.1.1. Materials High density polyethylene (HDPE), linear low density polyethylene (LLDPE), low density polyethylene (LDPE), pro-oxidant additives Mn(II) stearate, Fe(III) stearate and Co(II) stearate, calcium carbonate filler (CaCO3). 2.1.2. Equipments Plastic SJ-35 Single Screw Extruder, twin screw extruder Bao Pin, INSTRON 5980 mechanical measuring device, UV-260 accelerated weathering tester, Thermo Nicolet Nexus 670 Fourier Transform Infrared Spectroscopy, differential scanning calorimeter (DSC 204 F1 Phoenix) and a thermogravimetry analysis system (TGA 209 F1 Libra), SM-6510LV and JEOL 6490 scanning electron microscope, thickness measuring íntrument Mitutoyo IP67, Scientech scales, readability 0,001 (g), oven and laboratory equipments. 2.2. Film preparation These films were made by extrusion blowing using a SJ-35 extruder with a 35 mm screw of L/D 28:1. The SJ-35 extruder is shown in Figure 2.2. Figure 2.2. Image of the SJ-35 extruder 2.3. Methods 2.3.1. Effect of ratio of prooxidant additives on the degradation of polyethylene films (PE) Fomulas of LLDPE films containing prooxidant additives were shown in Table 2.1. 4 Table 2.1. Fomulas of LLDPE films containing prooxidant additives (w/w) Samples LLDPE Prooxidant additives Ratio of prooxidant additives MnSt2: FeSt3: CoSt2 MnSt2 FeSt3 CoSt2 M1 99.7 0.0750 0.2250 0 1:3:0 M2 99.7 0.2455 0.0540 0 9:2:0 M3 99.7 0.2348 0.0522 0.0130 18:4:1 M4 99.7 0.2400 0.0533 0.0067 18:4:0.5 The LLDPE films with various pro-oxidant additive mixtures were made by extrusion blowing. Thermo- and photo-oxidative degradations were carried out to evaluate the degradability of LLDPE films. 2.3.2. Effect of prooxidant additive mixture content on the degradation of polyethylene films (PE) HDPE and LLDPE films with a thickness of 30 μm were blown. The pro-oxidant additves were incorporated into the film formulation at a concentration of 0.1, 0.2 and 0.3 %. The sample labeling of PE films were listed in Table 2.3. Table 2.3. Sample labeling of PE films PE resin Sample Pro-oxidant additives (%) PE resin Sample Pro-oxidant additives (%) HDPE HD0 0% LLDPE LLD0 0% HD1 0.1% LLD1 0.1% HD2 0.2% LLD2 0.2% HD3 0.3% LLD3 0.3% The PE films were carried out thermo- and photo-oxidatives and natural weathering process to evaluate the degradation degree. 2.3.3. The degradation of PE films containing CaCO3 and prooxidant additives HDPE films with a thickness of 30 μm containing 0,3% prooxidant additives (equivalent to 3% prooxidant masterbatch) and different CaCO3 filler contents (5, 10 and 20% - symbol HD53, HD103, HD203 respectively) were blown. The films were carried out photo-oxidative degradation. 2.3.4. The biodegradability of PE films in natural conditions - Buried in the soil - Determined the degree of mineralization 5 CHAPTER 3. RESULTS AND DISCUSSIONS 3.1. Effect of ratio of prooxidant additives on the degradation of polyethylene films (PE) 3.1.1. The mechanical properties of oxidized LLDPE films The mechanical properties of films after thermo- and photo-oxidative degradation are shown in Figures 3.1a and 3.1 b, respectively. M1 M2 M3 M4 0 9 18 27 § é b Òn k Ðo ® ø t (M P a) MÉu Ban ®Çu Sau 5 ngµy oxy hãa nhiÖt Sau 96 giê oxy hãa quang, nhiÖt, Èm M1 M2 M3 M4 0 200 400 600 800 1000 § é d · n d µ i k h i ® ø t (% ) MÉu Ban ®Çu Sau 5 ngµy oxy hãa nhiÖt Sau 96 giê oxy hãa quang, nhiÖt, Èm Figure 3.1 a. The tensile strength of oxidized LLDPE films with prooxidant additive mixtures Figure 3.1 b. The elongation at break of oxidized LLDPE films with prooxidant additive mixtures The results showed that the thermo-oxidative degradation of LLDPE films without CoSt2 increased with increasing MnSt2/FeSt3 ratio. The mechanical strength of the M2 sample decreased more than that of the M1 sample after 5 days of thermal oxidation. But photo-oxidative degradation of films decreased, the mechanical strength of the M1 sample decreased more than that of the M2 sample after 96 hours of photo-oxidation. The mechanical properties of oxidized LLDPE films with CoSt2 are lower than those of films without CoSt2 on both the thermo- and photo- oxidation. The results also showed that the higher CoSt2 content increase, the faster the deagradation is. 3.1.2. FTIR-spectroscopy of oxidized LLDPE films The changes in the peak intensity at 1700 cm -1 of LLDPE films after 96 hours of photo-oxidation are shown in Figure 3.2. Figure 3.2. Changes in the peak intensity at 1700 cm-1 of oxidized LLDPE films 6 The results showed that the peak at 1700 cm -1 of M3 film was the strongest intensity after photo-oxidation. The change in absorption intensity of carbonyl group is consistent with the change in mechanical properties as described in 3.1.1. Therefore, the additive mixture of MnSt2/FeSt3/CoSt2 with ratio 18:4:1 is used for further studies in this thesis . 3.2. . Effect of prooxidant additive mixture content on the degradation of polyethylene films (PE) 3.2.1. Thermo-oxidation of PE films 3.2.1.1. Mechanical properties of PE films after thermo-oxidation Elongation at break is commonly used to monitor degradation process rather than other mechanical properties. The film is considered to be capable of degradation when the elongation at break is ≤ 5% according to ASTM D5510 và ASTM D 3826 standard. Elongation at break of PE films with anh without prooxidation additives during thermal oxidation is shown in Figure 3.5 and 3.6. Figure 3.5. Changes in elongation at break of HDPE films after 12 days of thermal oxidation Figure 3.6. Changes in elongation at break of LLDPE films after 7 days of thermal oxidation As shown in Figure 1, the additive-free HDPE and LLDPE polymer films were slowly oxidized to a low extent. HD0, and LLD0 exhibit only about 9.4%, 20.1% loss while HD1, HD3 films lost about 48.4%, 52.8% of their elongation at break in 7 days, respectively. On the other hand LLD1, LLD3 experiences almost 100% loss in 7 days. Thus, HDPE films are oxidized more slowly than LLDPE films in both with and without prooxidant additives. These results show clearly that the pro-oxidant in PE has played a significant role in inducing oxidation in PE leading to their embrittlement. 3.2.1.2. FTIR-spectroscopy of PE films after thermal oxidation FTIR spectras of PE films before and after thermal treatment were shown in Figure 3.7 a and 3.7 b. 0 200 400 600 800 1000 0 3 6 9 12 E lo n g at io n a t b re ak ( % ) Time (days) HD0 HD1 HD2 HD3 0 200 400 600 800 1000 1200 0 1 2 3 4 5 6 7 E lo n g at io n a t b re ak ( % ) Time (days) LLD0 LLD1 LLD2 LLD3 7 Figure 3.7a. FTIR spectra of HDPE films after thermal oxidation Figure 3.7b. FTIR spectra of LLDPE films after thermal oxidation Figure 3.7 a and b showed that an increase in absorption in the carbonyl region was recorded with time in the samples thermally aged containing pro- oxidants. The plot of 1640 - 1850 cm -1 range of carbonyl groups, as determined by the overlapping bands corresponding to acids (1710 - 1715 cm -1 ), ketones (1714 cm -1 ), aldehydes (1725 cm -1 ), ethers (1735 cm -1 ) and lactones (1780 cm -1 ) was observed, thus indicating the presence of different oxidized products. The absorption maxima can be assigned to carboxylic acid and ketones as the major components followed by esters in agreement with the results obtained by Chiellini et al. 3.2.1.3. Carbonyl index (CI) of PE films after thermal oxidation Figure 3.10 and 3.11 show changes in the carbonyl index of HDPE and LLDPE films with and without pro-oxidant additives during thermal oxidation. Figure 3.10. Carbonyl index of HDPE films after 12 days of thermal oxidation Hình 3.11. Carbonyl index of LLDPE films after 7 days of thermal oxidation Oxidation of PE films leads to the accumulation of carbonyl groups. As the oxidation time increases, the oxygen absorption level and the rate of intermediate products formation increases resulting in rapidly increasing carbonyl group concentration. At the same time increasing the prooxidant additive content, the carbonyl index also increased. So the presence of prooxidant additive probably accelerated the oxidation degradation of films. 3.2.1.4. Different Scanning Calorimetry (DSC) of PE films after thermal oxidation Melting temperature (Tm), heat of fusion (ΔHf), degree of crystallinity 0 5 10 0 3 6 9 12 C ar b o n y l in d ex ( C I) Time (days) HD0 HD1 HD2 HD3 0 5 10 15 20 0 1 3 5 7 C ar b o n y l in d ex ( C I) Time (days) LLD0 LLD1 LLD2 LLD3 8 (IC) of HDPE and LLDPE films before and after 12 days of thermal oxidation were listed in Table 3.1. Table 3.1. Melting temperature (Tm), heat of fusion (ΔHf), degree of crystallinity (IC) of HDPE and LLDPE films before and after 12 days of thermal oxidation Samples Original 12 days of thermal oxidation Tm ( oC) ΔHf (J/g) IC (%) Tm ( oC) ΔHf (J/g) IC (%) HD0 135.3 172.3 58.8 135.1 175.0 59.7 HD1 134.8 170.3 58.1 133.7 186.3 63.6 HD2 134.9 170.7 58.3 133.5 190.9 65.2 HD3 134.6 170.5 58.2 133.0 195.2 66.6 LLD0 121.8 73.61 25.1 121.5 86.8 29.6 LLD1 121.5 73.67 25.1 120.6 124.5 42.5 LLD2 121.3 73.74 25.2 120.3 130.6 44.6 LLD3 121.0 73.86 25.2 120.0 139.6 47.7 The crystalline percentage (IC) which obtained from DSC scans shows that IC of films increases after thermal oxidation. The crystalline percentage of films containing prooxidant additives increases more strongly than that of control (HD0, LLD0). With the same prooxidant additive concentration, ΔIC of LLDPE films (17.4 – 22.4%) were significantly higher than that of HDPE (5.5 – 8.4%). This confirm that LLDPE films are oxidized more faster than HDPE films in both with and without prooxidant additives. 3.2.1.5. Thermal gravimetric analysis (TGA) of PE films after thermal oxidation Thermal gravimetric analysis (TGA) traces of PE films after thermal oxidation are shown in Figure 3.13. HD0 – 12 days LLD0 – 12 days HD3 – 12 days LLD3 – 12 days Figure 3.13. TGA traces of PE films after thermal oxidation 9 The results showed that the degradation of original and thermally degraded for 12 days PE films were only one stage. Degradation temperature of HD3, LLD3 films after 12 days thermal oxidation is lower than that of HD0 and LLD0. It is due to lower molecular weight products of chain scissions by thermal oxidation. 3.2.1.6. Surface morphology of PE films after thermal oxidation The changes in the surface morphology of thermally degraded for 12 days HDPE films and thermally degraded for 7 days LLDPE films are shown in Fig. 3.14 and 3.15. PE (origin) HD0 HD2 HD3 Figure 3.14. SEM micrographs of HDPE films after 12 days of thermal oxidation LLD0 LLD1 LLD2 LLD3 Figure 3.15. SEM micrographs of LLDPE films after 7 days of thermo-oxidation As seen from the figure 3.14 and 15, original HD0, LLD0 films and degraded these films present a smooth surface free of defects. In contrast, the surfaces of PE films with pro-oxidant after thermal aging showed a pronounced roughness with craters/grooves by effect of prooxidant additoves and thermal. 3.2.2. Photo-oxidation of PE films 3.2.2.1. Mechanical properties of PE films after photo-oxidation A decrease in elongation at break of PE films during photo-oxidative degradation is shown in Figure 3.18 and 3.19. Figure 3.18. Changes of elongation at break of HDPE films after 96 hours of photo-oxidation Hình 3.19. Changes of elongation at break of LLDPE films after 120 hours of photo-oxidation 0 200 400 600 800 0 24 48 72 96 Đ ộ d ãn d ài k h i đ ứ t (% ) Thời gian (giờ) HD0 HD1 HD2 HD3 0 200 400 600 800 1000 0 24 48 72 96 120 Đ ộ d ãn d ài k h i đ ứ t (% ) Thời gian (giờ) LLD0 LLD1 LLD2 LLD3 10 Elongation at break decreases with increasing time of photo-oxidative degradation and decreasing as UV radiation. The results showed that elongation at break of HD1, HD2, HD3 is 4.7 %, 2.5 %, and 0.2 %, respectively after 96 hours accelerated aging, while that of HD0 is 478.4%. Elongation at break of LLD1, LLD2, LLD3 is 3.2%, 2.1%, and 0.2%, that of LLD0 is 365.9%. Comparison of thermo-oxidative and photo-oxidative degradation of PE films showed that: - In both case, the HDPE films degraded more slowly than LLDPE films. This is due to the difference in the amorphous content, the chain scission occours only in the amorphous region. LLDPE is a low crystalline polymer (~25%) so oxygen easily penetrates the polymer matrix to