Revolutionary Plastic Mechanical Recycling Process: Regeneration of Mechanical Properties and Lamellar Structures

Abstract

Plastic recycling is one method that can reduce the amount of waste plastics in the environment. Especially in the mechanical recycling process, the waste plastics are reprocessed by mechanical approaches and reproduced as recycled products. Based on the life cycle assessment studies, mechanical recycling shows lower emissions than other approaches which is the most suitable method for environments. However, the mechanical properties of recycled products are much more degraded than virgin plastics. In this chapter, revolutionary plastic mechanical recycling is introduced. The new type of twin-screw extruder with the addition of molten resin reservoir unit is applied with various extrusion conditions. As a result, the mechanical properties of recycled products were recovered than its original materials. The relationship between recycling process condition, mechanical properties, and mesoscale lamellar structure are also discussed. It can be found that steady shear which is a conventional process affected the degradation of mechanical properties. In another way, re-extrusion by using a new type of twin-screw extrusion with a suitable processing condition is related to the regeneration of mechanical properties and lamellar structures similar to virgin plastics. This chapter is expected to introduce the recent advances in plastic mechanical recycling process and propose the future prospects of plastic recycling technology.

1. Introduction

1.1 Plastic mechanical recycling process

The consumption of plastic has greatly increased all over the world due to being used in daily life. Single-use plastics have become the cause of environmental problems because they were disposed of after one-time use and gathered on land and in the marine environment [1]. The degradation of plastics in the environment takes a very long time, that is, decades to thousands of years [2]. For this reason, a suitable way for decreasing the amount of waste plastics is an attractive research subject in order to protect the future world environment.

Recycling is one of the processes that are able to decrease the amount of waste plastics. There were three main categories for plastic recycling; mechanical recycling, feedstock recycling, and energy recovery [3, 4]. Among them, mechanical recycling was attractive because it was evaluated to have a high environmental performance from the life cycle assessment (LCA) evaluation. The use of a mechanical recycling approach can reduce CO₂ emission, air emission with organic compounds, and waste production [5]. Mechanical recycling was carried out by the reprocessing of waste plastics with a mechanical method in order to produce new plastic products. The waste plastics were collection, cleaning, chipping, coloring or agglomeration, extrusion, and manufacturing of the end product [6]. However, the poor properties of mechanically recycled products were an obstruction to the usage of mechanical recycling [7].

In this chapter, firstly, the physical degradation of recycled plastic and its relationship with the changes in lamellar structure will be discussed. Then, with the importance of the changes in lamellar structure, the physical regeneration theory will be also introduced in order to improve the mechanical properties of mechanical recycling plastics. Then, based on the findings of physical degradation and physical regeneration theory, the revolutionary plastic mechanical recycling by using a new type of twin-screw extruder with the addition of new unit (Molten Resin Reservoir: MRR) will be introduced and showed some successful results which the mechanical properties of recycled plastics can be regenerated as similar to virgin plastics. This chapter is expected to combine the pain point of current techniques of plastic mechanical recycling process and introduce the new methods which will improve the quality of mechanical recycling process and mechanical properties of recycled plastic products.

2. Physical degradation of plastics, relationship with lamellar structures, and the regeneration of mechanical properties by suitable remolding conditions

Based on common sense, the properties of mechanically recycled plastics were inferior to virgin plastics in all aspects. It was not only poor physical properties such as dark color with strange odor but the mechanical properties were much degraded. In addition, the challenging works of gathering and separating to each type of plastic were one cause of the occurrence of contaminations and foreign matters. As a result, recycled plastics were brittle and became low strength and low stiffness compared to virgin plastics. From this evidence, recycled plastics had limited usability with narrow applications.

Regarding poor properties in mechanically recycled plastics, it was evidence that photodegradation, hydrolysis, and thermo-oxidation process affected the breakage of polymer chains into low molecular weight [8]. In addition, the UV-radiation and oxygen in the environment were also the most important factors for the degradation of the carbon–carbon backbone and the occurrence of chain scission in plastics [9]. As a result, the recycled plastics were become a brittle material which can be also leading to the formation of micro- and nano-plastic fragments.

In contradiction with earlier findings and a general common sense, in 2012, Yao et al. [10] investigated the molecular characteristics of virgin polypropylene (VPP) and compared with recycled polypropylene (RPP) which was obtained from byproducts of injection molding process. These byproducts were a part of VPP injection molding products namely “sprue” and “runners” (Figure 1) connected to the molded products which were used for tensile test and Izod impact strength test. The byproducts (sprue and runner parts) were grinding and mechanical recycling prior to remolding as RPP for characterization. It was generally found that the tensile properties of RPP were inferior to VPP due to shear deformation during mechanically recycling process. However, it is interesting to note that weight average molecular weight (Mw), number average molecular weight (Mn), and polydispersity index (PDI) of RPP were similar to VPP. This can be implied that there were no changes in chain length, chain scission, and polymer chemistry of RPP due to mechanically recycling process. In addition, the degradation of mechanical properties of RPP was strongly related to the morphological changes of inner structures. Figure 2 showed the SEM images of the cross-section of VPP and RPP test specimen broken under liquid nitrogen. The cross-sectional area of VPP (Figure 2(a)) was detected as a smooth surface; however, the rough surface was detected in RPP (Figure 2(b)). The distinction of fracture surface morphology can be related to the difference in inner structure such as crystalline and amorphous structure, thickness, and morphology caused by the mechanical recycling process. As a result, the mechanical properties of RPP were inferior to VPP [10].

Figure 1.

Part of injection molding plastic products. Sprue and Runners which were generally discarded were used as the raw material for mechanical recycling as RPP.

Figure 2.

SEM images of the fractured surface broken under liquid nitrogen: (a) VPP; (b) RPP [10].

In order to prove other physical properties compared between virgin and mechanical recycled plastics, Takatori et al. [11] studied the correlation of weight average molecular weight (Mw) with density, melt index, high load melt index, and mechanical properties such as yield stress and elongation at break of virgin high-density polyethylene (HDPE) and recycled HDPE originated from drinking bottle caps. It can be confirmed that main physical properties such as density, melt index, amorphous phase length, and crystal phase length of recycled HDPE were identical to virgin HDPE especially in case that they were similar in Mw [11]. From this study, it can be proved that the similarity between Mw of virgin and mechanical recycled plastics related to high possibility of comparable to other physical properties.

In order to improve the mechanical properties of recycled plastics, Tominaga et al. proposed a new promising way for improving the mechanical properties of RPP by variation of remolding conditions [12]. Their study also compared the mechanical properties between VPP and RPP which was originated from sprue and runners of injection molding products namely as “pre-RPP.” The main findings from this study were shown in Figure 3. The general molding condition at 210°C for 2 min with slow cooling to room temperature affected low elongation at break and toughness of pre-RPP samples as compared to VPP (Figure 3(a)). As compared to the changes of cooling conditions to the rapidly cooled ice water (Figure 3(b)), the averaged elongation at break and toughness of pre-RPP was increased as similar to VPP. Interestingly, the elongation at break and toughness of pre-RPP can be further increased at a longer time of molding condition from 2 to 6 min as shown in Figure 3(c). From these findings, it can be found that the suitable molding conditions; time and cooling method, affected the regeneration of mechanical properties especially in elongation at break and toughness of mechanical recycled RPP. In another way, this variation of molding condition has not significantly affected the changes in mechanical properties in VPP.

Figure 3.

Stress–strain curves of VPP and pre-RPP with different molding conditions. (a) 210°C, 2 min, slow cooling; (b) 210°C, 2 min, rapid cooling; (c) 210°C, 6 min, rapid cooling [12].

Tominaga et al. [13] further studied the variation of molding conditions of pre-RPP in 2015. With the variation of temperature, time, and cooling method, pre-RPP successfully improved mechanical properties, especially in tensile fracture stress, as higher than VPP. Figure 4 showed the tensile fracture stress of VPP and pre-RPP with different molding conditions. The molding condition was varied with temperature (210, 230, and 250°C), molding time (2, 6, and 10 min), and cooling condition (slow cooling; SC, quench cooling; Q). From these results, the cooling condition by rapid cooling in ice water or quench cooling (Q) affected the increase of tensile fracture stress, especially at high molding temperature and time. Interestingly, the molding condition at 250°C for 10 min with quench cooling affected the tensile fracture stress of pre-RPP much higher than VPP with the general molding condition at 210°C for 2 min with slow cooling (SC). Based on this study, it can be also confirmed that the mechanical properties of recycled plastics were strongly related to the molding conditions. The suitable molding condition was correlated to the regeneration of mechanical properties of mechanical recycled plastics.

Figure 4.

Tensile fracture stress of VPP and pre-RPP with different molding conditions [13].

These findings were applied to the study by using actual recycled materials. Takenaka et al. [14] used recycled polypropylene (RPP) originating from waste containers and packaging plastics from municipal waste in Japan. The result found that the suitable molding condition especially by quench cooling affected the elongation of RPP samples. In addition, the cross-sectional of the fracture surface was totally different between the brittle sample from the remolding by slow-cooling (SC); however, the ductile and elongation surface can be detected in quenched RPP (Figure 5). From this study, it was found that suitable molding condition was not only related to the regeneration of mechanical properties, but it was also related to the changes of inner structure morphology in RPP originated from waste container and packaging.

Figure 5.

(1) Elongated tensile test specimen; (2) SEM images of the fractured surface broken under liquid nitrogen; (a) RPP molding at 230°C, 10 min, slow cooling; (b) 230°C, 10 min, quench cooling [14].

For the detailed characterization of inner structure changes from suitable molding conditions in recycled plastic, a small angle X-ray scattering (SAXS) was used for characterization of thickness of long period (L), crystalline layer (Lc), amorphous layer (La), and intermediate layer (Li). Figure 6 showed the schematic image of the lamellar structure of semicrystalline plastics such as polyethylene.

Figure 6.

Lamellar structure of semicrystalline polymer; polyethylene [15, 16].

The relationship between the inner structure and mechanical properties of RPP with various molding conditions was investigated by Tominaga et al. [17]. RPP originated from byproducts of injection molding samples such as sprue and runner were molded (210 or 250°C for 2, 6, or 10 min and cooling by slow cooling or quenching) or annealed at 65°C for 2 or 8 h prior to characterization. It can be found that tensile fracture elongation showed a strong correlation with long period and thickness of amorphous layer of RPP. The shorten amorphous layer and long period with the larger number of tie molecules related to better elongation of RPP.

In the case of recycled materials from other sources; recycled plastics originated from unsorted waste containers and packaging from municipal waste in Japan, Yamasaki et al. [18] investigated the development of its mechanical properties by the optimization of molding conditions. With the complex of materials, it can be characterized that the unsorted recycled materials consisted of polyethylene (PE) and polypropylene (PP) in the ratio of 49:51. The molding conditions were compared up to 12 conditions with a variation of molding temperature (180, 210, 230, and 250°C), molding time (2, 6, and 10 min), and cooling condition (slow cooling, quench). It can be found that quench cooling affected the regeneration of toughness. Interestingly, the toughness had a strong relationship with a long period (L) as shown in Figure 7. Quench cooling affected the increasing of toughness with the decreasing of long period. In another way, slow cooling affected the decrease of toughness with the length of thickness over a long period. This can be also confirmed that the shortening of thickness over a long period by quench cooling affected the regeneration toughness in recycled PE and PP.

Figure 7.

Relationship between long period and toughness of unsorted recycled plastics originated from municipal waste containers and packaging with different molding conditions. Remolding by quenching affected the increasing of toughness with the shortening of a long period [18].

Based on the strong relationship between molding condition, mechanical properties, and lamellar structure of recycled plastics, Yao et al. [19] proposed an explanation of the physical degradation theory of semicrystalline plastics as shown in Figure 8.

Figure 8.

Schematic diagram of physical degradation mechanism. Blue color: crystalline structure; Red color: tie molecules [19].

Figure 8(a) shows the lamellar structure of virgin plastic which consists of crystalline as shown in blue color and tie molecules as shown in red color. There are plenty of tie molecules between crystalline lamellar. In this state, the force applied to the plastic specimen can be propagated from end to end. As a result, good tensile properties are demonstrated in virgin plastics. In contrast, as shown in Figure 8(b), heating during the remolding process caused the melting of lamellar crystals. As a result, the lamellar became thin in shape without any order. In this state, the polymer crystal had the “memory of structure” which is exhibited with a phenomenon where crystallization always occurs from the same location, unless it is when melted for a very long time [20]. Part of the retained lamellar structure shown in Figure 8(b) is account for this effect. The holding strength of tie molecules between lamellar crystals becomes weakened; hence, tensile deformation from the molding process results in the detachment of tie molecules from existing lamellar layers as shown in Figure 8(c). However, the lamellar structures can be reconstructed from this state by cooling, after molding. The detached tie molecules become dangling chains without returning to the original lamellar structure. Therefore, molded products consisted of a few tie molecules, as shown in Figure 8(d). This structure cannot sufficiently propagate force from one end of a specimen to the other, resulting in the severe degradation of mechanical properties. In short, physical degradation is caused by the transformation of lamellar structure with inferior mechanical properties from heat and shear accompanying the remolding process condition [19].

Based on the study on the relationship between suitable molding conditions and the regeneration of mechanical properties of semi-crystalline plastic such as RPP, then the studies were extended to amorphous plastics; polystyrene (PS). Oda et al. [21, 22] studied the development of mechanical properties of virgin and recycled PS by variation of molding temperature and holding time. As a result, molding conditions at 230°C, 40 MPa for 2 min affected the improvement of mechanical properties in PS. Interestingly, the new peaks can be detected in the wide-angle X-ray scattering (WAXS) profile which can be implied by the changes in the inner structures of the amorphous polymer due to the distinction of molding condition. In addition, it can be also concluded that the suitable molding condition for regeneration of mechanical properties was not only able to apply to semi-crystalline plastic but it can be also applied to the amorphous polymer such as PS.

3. Physical degradation of plastics from shear deformation: the current obstacle of plastic mechanical recycling

Extrusion process which was the main approach in plastic mechanical recycling consisted of shear deformation from screw in mixing zone and conveying zone. As a result, mechanical properties of recycled products were degraded which was caused by the memory of shear history which remained in the structure of plastics. Many studies have focused on the shear deformation and crystalline structure of plastics. Liu et al. [23] evaluated shear crystallization and changes in the crystalline structure of HDPE using synchrotron X-ray techniques. It can be found that shear accelerated the speed of crystallization and the formation of crystalline orientation along the flow direction. Abad et al. [24] evaluated the tensile properties of HDPE and low-density polyethylene (LDPE) after several extrusion cycles. The tensile properties were degraded with the greater number of extrusion cycles. Interestingly, the addition of antioxidation was able to reduce the effect of degradation.

Kaneyasu et al. [25] investigated the mechanism of steady shear deformation of HDPE. Mechanical recycling process was performed by steady shear treatment by using a cone and plate rheometer. The steady shear treatment was varied from 0 to 100 s⁻¹ at 180 °C for 10 min. Then, the product was remolded as a thin film with a thickness of 100 μm for further characterization. As a result, steady shear treatment at 100 s⁻¹ affected the degradation of elongation at break and Young’s modulus from those of virgin HDPE around 27.9% and 27.8%, respectively. Interestingly, it can be found that the degradation of elongation at break from steady shear had a strong correlation with the decreasing the thickness of the long period, the core crystalline layer, and the intermediate layer. In addition, the surface morphology of HDPE treated by 100 s⁻¹ showed the creation of crystalline orientation which is also related to the decreasing of elongation at break in HDPE. On the other hand, it can be confirmed that the degradation of elongation at the break of HDPE is caused by the physical degradation from the changes of lamellar structure and morphology. It was not caused from the changes of polymer chemistry due to there were no changes in molecular weight and molecular weight distribution of shear-treated HDPE as compared to its virgin HDPE. From this study, it can be evidence that steady shear treatment which was generally detected in the mechanical recycling process affected the degradation of mechanical properties which can be also related to the changes of thickness of lamellar structure and mesoscale surface morphology of HDPE (Figure 9).

Figure 9.

The comparison of tensile properties, morphology, and thickness of lamellar structure of virgin HDPE (VPE) and steady shear-treated HDPE at 100 s⁻¹. The elongation at break of steady shear-treated HDPE was decreased from its VPE with the creation of crystalline orientation at shear direction and the reduction of thickness of core crystalline layer and intermediate layer [25].

4. Physical regeneration theory and the revolutionary plastic mechanical recycling process

From the knowledge of physical degradation theory, it can be further thought about the development of mechanical properties of plastics by the relationship with the regeneration of lamellar structure. Yao et al. [26] have been also proposed the physical regeneration theory of semicrystalline plastics as shown in Figure 10.

Figure 10.

Schematic diagram of physical degradation and regeneration mechanism of semicrystalline plastics [26].

This physical degradation and regeneration mechanism can be theoretically explained by using Figure 10, which shows the energy levels and states of solid and molten polymers. First, in the crystalline state, the crystalline lamellar structure consisting of polymer chains is incomplete and there are many tie molecules, as shown in Figure 10(a) with a low energy level. Therefore, the polymer chains are constantly moving toward rearrangement in this state. The temperature at which the crystallization rate is the fastest in crystalline polymers is considered to be the temperature near the middle of glass transition temperature which molecular motion completely stops and the crystalline melting point. Therefore, the temperature at crystallization rate is the fastest is approximately 70°C for PP and approximately 0°C for PE. In other words, in many plastic products, which are recognized to be in a solid state when used, the inner polymer chains are in a metastable/non-equilibrium state, in which the inner polymer chains are constantly moving toward a stable state of energy. If the thermal history is sufficient for progression of crystallization, it is considered that the state in Figure 10(b) is infinitely approached. In this state, since the number of tie molecules is small, the transmission of force in the system is interrupted and the elongation property is greatly deteriorated. The proponents actually confirmed that the specimens in which the crystallization was accelerated by cooling and solidifying virgin PP very slowly became markedly brittle.

On the other hand, in the molten state, the energy level is lower in the compatible state where polymer chains are entangled with each other shown in Figure 10(d) than in the phase-separated state shown in Figure 10(c). However, when the lamellar structure in the state shown in Figure 10(d) is subjected to a strong shear deformation due to the extrusion and molding process, the entanglement between polymer chains was decreased and the system assumes the state shown in Figure 10(c). When the state shown in Figure 10(c) is cooled and solidified, the entanglement between the polymer chains has already decreased, so the state shown in Figure 10(b) is lower than the state shown in Figure 10(a), which has excellent mechanical properties. This is the reason why the physical properties of recycled plastics with a history of molding have deteriorated and shown in Figure 10(b). In addition, the system in which the molecules are separated from each other and melted as shown in Figure 10(c). However, it is possible to return to a more stable energy state shown in Figure 10(d) by leaving it in a molten state for a long time. In other words, physical properties of plastics can be “physically regenerated” by molding through a process that utilizes the self-regenerating ability of this polymer. However, once a stable state is established in a crystalline polymer, there is a phenomenon known as a “memory effect”, in which the polymer chain retains its structure for a long time [27]. Therefore, even if entanglement is formed by placing it in a molten state, it can be easily disentangled during the cooling process if the temperature degree is low. Conversely, if such measures are taken, there is a possibility that the physical properties can be stably reproduced.

Based on this physical regeneration theory, the creation of a new type of extrusion process was exhibited. Phanthong et al. [28] studied the development of mechanical properties and crystalline conformation of recycled polypropylene by re-extrusion using a new type twin-screw extruder. This was the establishment of a new unit which was a blank space added into the end of a twin-screw extruder, namely a “Molten Resin Reservoir: MRR”. This study compared the mechanical properties and inner structures of virgin polypropylene (VPP) and recycled polypropylene (RPP) extruded by using a twin-screw extruder with an additional MRR as shown in Figure 11. The tensile properties, crystalline type, inner structure conformation, and molecular weight distribution were also characterized.

Figure 11.

(a) New type of twin-screw extruder with additional MRR unit; (b) schematic image of MRR unit which was a blank space connected at the end of screw mixing zone of general extruder [28].

As a result, the tensile properties of RPP-extrusion were improved and were similar to those of VPP (Figure 12). In addition, the crystalline conformation of RPP-extrusion was similar to that of VPP by increasing the ratio between the helix and parallel band which could be attributed to the improvement of tensile properties. In addition, the molecular weight distribution of RPP-extrusion was similar to its original sample which can be implied to the fact that the re-extrusion process by the twin-screw extruder with the addition of MRR did not affect changes in the chain length and chain structure of RPP. This study succeeded in regenerating the tensile properties and inner structures of recycled PP.

Figure 12.

(a) Stress–strain curve of VPP, RPP-original sample without re-extrusion, and RPP-extrusion by using a new type twin-screw extruder with additional MRR unit; (b) photo images of the elongated tensile test specimen. RPP-extrusion had been more elongated than its original sample (RPP-original) and VPP [28].

The use of new type extrusion with MRR was also studied with the unsorted recycled plastics originating from waste container and packaging plastics in municipal waste in Japan. Yamasaki et al. [18] studied the re-extrusion process with various conditions of the existence of MRR, extrusion temperature (200, 230, and 250°C), screw speed (100 and 200 rpm), and cooling method (room temperature and ice water). As a result, the use of MRR affected the increase of elongation at the break of samples. In addition, the optimized condition for the unsorted recycled plastics derived from waste container and packaging was re-extrusion with the addition of MRR at 200°C, 100 rpm, and cooling in ice water (Figure 13).

Figure 13.

Elongation at break of recycled plastics derived from unsorted waste container and packaging and re-extrusion with various extrusion conditions [18].

The addition of MRR not only succeeded in the regeneration of mechanical properties in RPP, recycled polyethylene (RPE) was also tested in the re-extrusion process with MRR. Okubo et al. [29] studied the effect of new type twin-screw extruder with the addition of MRR for the regeneration of mechanical properties of RPE derived from plastic containers and packaging. As a result, the elongation at break of RPE re-extrusion by MRR was seven times higher than that of its original RPE (Figure 14).

Figure 14.

Photo images of the elongated tensile test specimen: (a) original RPE; (b) RPE re-extrusion with general extruder; (c) RPE re-extrusion by MRR; (d) VPE. RPE re-extrusion by using MRR was elongated higher than its original RPE around seven times and also similar to VPE [29].

For the consideration of the changes in lamellar structure from the re-extrusion by MRR, the lamellar shape of RPE was significantly changed from a distorted stripe-like lamellar structure to a stripe-like lamellar structure which was similar to VPE as shown in Figure 15.

Figure 15.

AFM phase images: (a) original RPE; (b) RPE re-extrusion with general extruder; (c) RPE re-extrusion by MRR; (d) VPE [29].

Based on the changes in lamellar morphology, the effects of MRR on microstructure of RPE was shown in Figure 16. It can be found that all samples exhibited different lamellar structures depending on the processing method. This indicated that the structure of the molten polymer is reflected in the structure of the molding as a result of melt-memory effects during polymer crystallization [30]. For the re-extrusion in general, extruder without MRR, the molten polymer was mixed in the kneading zone. As a result, the molten polymer had a dispersed structure as shown in Figure 16(a). In contrast, in the re-extrusion by using MRR, the flow of the molten polymer was laminar in MRR resulting in the alignment of the molten polymer chains in the flow direction as shown in Figure 16(b). As a result, re-extrusion of RPE by MRR can improve the distorted lamellar structure of RPE pellet yielding a virgin-like structure (Figure 16(c)). Consequently, the mechanical properties were also regenerated as virgin-like properties.

Figure 16.

Schematic diagram of the changes of polymer chain in the molten state: (a) general extruder; (b) new type extruder with the addition of MRR unit; (c) schematic images of the changes of lamellar morphology of virgin plastics by general recycling process. The lamellar structure was regenerated to be a virgin-like structure by re-extrusion using the addition of MRR unit [29].

5. Conclusion and future prospects

The degradation of mechanical properties of plastics was revealed that was caused by physical degradation. From the plastic processing by heat and shear from general extrusion or injection molding process affected the changes in lamellar structure, morphology, size, the amount of entanglement, and tie molecules. In another way, physical regeneration can be established by the suitable mechanical processing condition and the relaxation of memory from the molten state of polymer chains. The establishment of molten resin reservoir unit (MRR) which was a blank space connected at the end of the screw unit of the general extruder affected the relaxation and removable of the shear history of molten polymer. As a result, the lamellar structure and morphology can be regenerated as similar to its virgin materials which also affected to the regeneration of the elongation at break. These findings have already succeeded for recycle polyethylene and recycled polypropylene derived from plastic waste containers. From this study, it can be concluded that the mechanical recycling of plastics can be revolution which can be increased the quality of recycled plastic products which can be prolonged the lifetime used and decreased the amount of single used plastic waste in environment.

Future prospects of this work will be focused on the extension of plastic types and composite materials. In addition, the modification of extrusion condition and MRR will be also further studied in order to boost up the quality of the mechanically recycled plastics.

Acknowledgments

This study is based on results obtained from a project no. JPNP20012 commissioned by the New Energy and Industrial Technology Development Organization (NEDO) of Japan. This research was supported by the Environment Research and Technology Development Fund (3-1705) of Environmental Restoration and Conservation Agency of Japan.

Conflict of interest

The authors declare no conflict of interest.