Dissertation Summary: Study on the biodegradability of Polyetylene in the presence of transition metal stearates (Mn, Fe, Co)

The thesis with the objective of researching to assess the biodegradability (including decomposition process and decomposition in soil environment) of polyethylene film containing additives promoting oxidation is Fe stearate salts (Fe). III), Co (II) and Mn (II).

AND TRAINING 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

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

1st Reviewer:

2nd Reviewer:

3rd 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

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 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

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

Table 2.1 Fomulas of LLDPE films containing prooxidant additives (w/w)

Samples LLDPE

Prooxidant additives Ratio of prooxidant

additives MnSt 2 : FeSt 3 : CoSt 2 MnSt 2 FeSt 3 CoSt 2

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

PE resin Sample

Pro-oxidant additives (%)

2.3.4 The biodegradability of PE films in natural conditions

- Buried in the soil

- Determined the degree of mineralization

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

0 200 400 600 800 1000

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

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 1200

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 15 20

(IC) of HDPE and LLDPE films before and after 12 days of thermal oxidation were listed in Table 3.1

crystallinity (IC) of HDPE and LLDPE films before and after 12 days of

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

Figure 3.13 TGA traces of PE films after thermal oxidation

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

Figure 3.14 SEM micrographs of HDPE films after 12 days of thermal oxidation

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

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

0 200 400 600 800 1000

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 oxidizing LLDPE chain to form oxidation products while HDPE is a higher crystalline polymer (~58%)

- The mechanical properties of both LLDPE and HDPE films of photo-oxidation decrease more rapidly than that of thermo-oxidation due to prooxidation additive mixture that used in this dissertation is Mn (II) stearate,

Co (II) stearate and Fe (III) stearate Co and Mn stearate promote thermal and photo oxidation while Fe only promotes photo oxidation so Fe stearate doesn’t promote thermo-oxidative degradation

- The difference in the mechanical properties of HDPE and LLDPE films of photo-oxidation is less than that of thermo-oxidation due to UV light

is the main agent that affects HDPE degradation

3.2.2.2 FTIR-spectroscopy of PE films after photo-oxidation

FTIR spectras of PE films before and after aging were shown in Figure 3.20 a and b

films after 96 hours of photo-oxidation

films after 96 hours of photo-oxidation

After photo-oxidation, FTIR spectras of PE films occur peak in the range of 1700 – 1800 cm-1 for carbonyl groups That shows the presence of various oxidation products such as aldehyde or ester (1733 cm-1), acid carboxylic (1700 cm-1), γ-lacton (1780 cm-1) Also, a slight increase in the

peak area of 3300 – 3500 cm-1 region which is attibuted to the hydroxyl group is observed

3.2.2.3 Carbonyl index (CI) of PE films after photo-oxidation

Carbonyl index is a parameter which used to evaluate the level of degradation CI values of original and oxidized PE films are shown in Figure 3.21 and 3.22

films after 96 hours of photo-oxidation

LLDPE films after 120 hours of

photo-oxidation

The results showed that carbonyl indexs increase with increasing the content of pro-oxidant additives at any time With the same amount of pro-oxidant additives, LLDPE films are oxidized strongly than HDPE films that

is similar to the decrease in mechanical properties

3.2.2.4 Different Scanning Calorimetry (DSC) of PE films after oxidation

photo-Melting temperature (Tm), heat of fusion (ΔHf), degree of crystallinity (IC) of HDPE films before and after photo-oxidation were listed in Table 3.3

crystallinity (IC) of HDPE films after 96 hours of photo-oxidation

Melting temperature (Tm), heat of fusion (ΔHf), degree of crystallinity (IC) of LLDPE films before and after photo-oxidation were listed in Table

0 5 10 15 20 25