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Article

Nihon Reoroji Gakkaishi Vol.45, No.3, 139~143(Journal of the Society of Rheology, Japan)©2017 The Society of Rheology, Japan

1. INTRODUCTION

The handling of waste plastics is a significant global issue. Japan produced 10.86 million tons of plastics and generated a total of 9.15 million tons of waste plastics in 2015. 7.63 million tons was effectively used, which was about 80 % of the total amount and about two times more than in 2000. However, more than half is typically thermally recycled, as shown in the breakdown of the treatment and disposal methods for 2015 (Fig. 1).1) This ratio has not changed much for more than 10 years, although the Containers and Packaging Recycling Law came into full effect in 2000.1,2) The ratio of material recycling does not increase because of a combination of factors: recycled plastic is chemically degraded; the inclusion of foreign polymers and inorganic contaminations makes refinement very difficult; the grey tone makes dyeing

impossible; and very bad material properties arise from a lack of toughness.3) Therefore, the applications of recycled plastic are limited (examples include imitation trees, buffer stops, and pallets), and there has not been applied to high-value added products. Among these factors, chemical degradation is caused by the breaking of polymer chains in various ways, including

Creation of Advanced Recycle Process to Waste Container and Packaging Plastic

— Polypropylene Sorted Recycle Plastic Case —

Nozomi Takenaka*, Aya Tominaga‡,*, Hiroshi Sekiguchi**, Ryoko Nakano**, Eiichi Takatori***, and Shigeru Yao†,*,**

* Faculty of Engineering, Graduate School, Fukuoka University ** Faculty of Engineering, Fukuoka University

*** Tosoh Analysis and Research Center(Received : January 20, 2017)

To address the dual problems of the depletion of natural resources and the environmental impact of a growing volume of wastes, effective recycling of various products and materials has become an important challenge. Especially, the amount of plastic waste is extremely large, and thus it is very important to establish effective process of recycling them as soon as possible. However, the mechanical properties and durability properties of the products made from waste plastics are inferior to those of products molded from virgin plastics, and thus their application area is presently limited. The reason of above phenomena has been thought to be caused by chemical degradation. However, we recently found that the main cause of this inferior property was reproducible physical degradation from the investigation of pre-consumer recycle polypropylene. In this study, we used recycled post-consumer container and container and packaging plastics (polypropylene component was sorted) and investigated that the physical properties of them were reproducible or not. And we found that the tensile mechanical properties of them were reproducible almost to the virgin revel by changing molding conditions. This results indicate that in the case of recycled post-consumer container and packaging plastics, they are not chemically degraded and can use to value-added fields.Key Words: Waste container and package plastic / Mechanical properties / Fracture surface properties /

Hot pressing condition / Cooling condition

† To whom correspondence should be addressed. Department of Chemical Engineering Fukuoka University, 8-19-1 Nanakuma, Jonan-ku, Fukuoka 814-0180, Japan, Tel : +81-92-871-6631(6420), Fax : +81-92-865-6031, E-mail: shyao@fukuoka-u.ac.jp ‡ E-mail: tommy.aya96@gmail.com Fig. 1. Treatment and disposal methods for waste plastic in 2015.1)

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heat, light, chemicals, and radiation, and is irreversible.4,5) As a result, recovering the material properties of recycled plastic has been considered impossible.

Despite this common belief, we carried out experiments using pre-consumer recycled propylene, which has molecular properties that are almost identical to virgin plastic, as a model plastic to investigate the reasons for the degradation of the material properties and methods to recover these properties. From the investigation, we found that pre-consumer materials show brittleness under standard molding conditions. Therefore, they have very unfavorable material properties, which implies that the material properties can worsen without chemical degradation. Thus, according to our results, the main reason for the degradation of material properties is not chemical degradation as was presumed, but rather a physical degradation from its molding history. Moreover, we found that the material properties of the recycled product could be improved by changing the heat treatment and cooling conditions during molding, and material properties close to the virgin material could be recovered.6-8) In short, the degradation of the material properties of recycled plastic can be regenerated by optimizing the molding conditions.

The mechanism of physical degradation and physical regeneration can be considered from the change of the number of tie molecules in a polymer. The number of tie molecules of recycled plastic is considerably smaller than in the virgin state. Consequently, recycled plastic that has been molded has significantly low mechanical properties and durability. This is the state of physical degradation. On the other hand, the polymer chains do not undergo chemical degradation. Thus, the material properties can be improved by changing the number of tie molecules by selecting the molding condition. This process involves the physical recovery of the material properties by optimizing the molding conditions of the recycled plastic.10-13)

In contrast, post-consumer recycled plastic is thought to be most probably chemically degraded product. Whether physical recovery, which improves the material properties by optimizing the molding conditions of pre-consumer recycled plastic, is also applicable to post-consumer plastic is a very intriguing problem. In particular, the physical recovery of container and packaging plastic would have a significant ripple effect because the amount of disposed container and packaging plastic is substantially large. Therefore, we used actually wasted, recycled container and packaging plastic to investigate the effect of the molding conditions on the mechanical properties.

2. EXPERIMENTAL CONDITIONS

2.1 Sample

We used recycled container and packaging plastic from Ecos Factory that was sorted to increase the polypropylene (PP) content (Fig. 2). The plastic was grey and included a large amount of inorganic contaminations, as is evident from the figure.

2.2 Melting Point Measurement

The melting point of sample was measured using differential scanning calorimetry (DSC) to analyze its composition. Thin films were used in the DCS, where each pellet was molded at 180 °C for 2 min and then slow cooled (SC). The sample mass was approximately 4–6 mg. A DSC3100 from Mac Science was employed for the melting point measurements, and a heating rate of 5 °C/min was used.

2.3 Molecular Weight Measurement

The molecular weight and its distribution were analyzed using high-temperature gel permeation chromatography (GPC) to investigate the extent of the chemical degradation in sample. The equipment used was an HLC®−8321GPC/HT.

2.4 Measurement of Mechanical Properties

Tensile tests were conducted to evaluate the molding condition dependence on the mechanical properties of the post-consumer container and packaging plastic. The molding conditions are summarized in Table I. The press machine used was an MH-10 from Imoto Machinery. The tensile specimens were punched out from molded thin films in the JIS K 7113 2(1/3) standard shape. We used five specimens for one

Fig. 2. Pellets of PP-selected recycled packaging plastics. (Obtained from ECOS FACTORY Co., Ltd.)

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measuring condition. The specimens prepared from pellets were subjected to tensile testing at 23.5 °C and 60 % relative humidity (RH), with a distance of 30 mm between the chucks and an elongation rate of 5 mm/min. The tensile tester used was a Little Senstar LSC-02/30-2 from Tokyo Koki Testing Machine.14,15)

2.5 Fracture Surface Observation

The fracture surface after the tensile test was observed using a scanning electron microscope (SEM). The specimens were cut into strips with a length of approximately 10 mm and a width of approximately 40 mm. Small notches were cut near both sides at the center, and the specimens were fractured in liquid nitrogen. The SEM used was a JSM-6060 from JEOL.

3. RESULTS AND DISCUSSION

3.1 Melting Point Measurement

Fig. 3 shows the endothermic curve from the DSC measurement at a heating rate of 5 °C/min. Peaks are found near 125 and 167 °C, which are the melting points of polyethylene (PE) and PP, respectively. Therefore, the sample contained PE and PP. Furthermore, the results indicated that most of the content was PP.

3.2 Molecular Weight Measurement

Fig. 4 shows the results of the GPC measurement. The table in Fig. 4 indicates that the weight average molecular weight was 560,000. The weight averaged molecular weight of virgin PP we used in the previous study was 170,000. Therefore, the molecular weight of this sample is guaranteed sufficient mechanical properties. The pellet was post-consumer. Thus, it was thought to be affected by chemical degradation. However, this result suggested that the recycled container and packaging plastic that is typically collected has not been subjected to significant chemical degradation, according to the molecular weight.

3.3 Measurement of Mechanical Properties

Fig. 5 shows the changes in the breaking strain under various pressing temperatures, pressing times, and quenching conditions. The figure shows that slowly cooled samples are brittle, while quenched samples are very ductile. Moreover, the ductility increases with the molding temperature. Fig. 5 indicates that the sample molded at 230 °C for 10 min and

Table I. Molding conditions of sample.

Fig. 3. Results of DSC measurement of container and recycled packaging resin.

Fig. 4. Molecular weight distribution and results of molecular weight measurement.

Fig. 5. Breaking strain properties of SC samples and quenched samples with various hot press temperatures.

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then quenched is the most ductile. Fig. 6 shows the tensile test results of samples that were pressed at 230 °C but under different cooling conditions. Curves represent the results of each specimen. The samples are brittle when slowly cooled, but become profoundly ductile when quenched. Moreover, while the breaking strain differs significantly among the slowly cooled samples, the breaking strain in quenched samples were almost same value, and this result shows that quenching improves equality. Fig. 7 shows photos of the specimens after the tensile tests shown in Fig. 6.

Table II lists the material properties other than the breaking strain. The fracture energy is higher in quenched samples than in slowly cooled samples, as with the breaking strain. The yield point does not differ much, and Young’s modulus decreases by quenching. One reason for this decrease in Young’s modulus is the decrease in crystallinity with quenching. However, the decrease in crystallinity is related

to softening but not related to ductile. This result indicate that the recycled sample has enough molecular weight to construct entanglement and tie molecule.

These results are similar to those obtained with our previous results, and demonstrate that the optimization of the molding conditions can also improve the mechanical properties of recycled plastic of PP sorted wasted container and packaging plastic.

3.4 Fracture Surface Observation

Fig. 8 shows SEM images of the fracture surfaces after the fracturing in liquid nitrogen of the samples shown in Fig. 6. The slowly cooled, sample in Fig. 8(a) shows a brittle fracture with a lack of viscosity. On the other hand, the quenched sample in Fig. 8(b) shows a very ductile fracture with elongation of the cross section. The results clearly show that quenching makes the sample ductile.

(a) (b)

Fig. 6. Stress–strain curves of thin film samples of molded recycled packaging plastic: (a) 230 ºC, 10 min, SC, (b) 230 ºC, 10 min, quenched.

Fig. 7. Macroscopic appearances of specimens after tensile tests: 230 ºC, 10 min, SC, (b) 230 ºC, 10 min, quenched.

Table II. Summary of results of tensile tests.

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4. CONCLUSIONS

An analysis was performed to determine whether the previously obtained insight on a model recycled plastic could be extended to the wasted recycled container and packaging plastic. First, molecular weight measurements showed that the post-consumer material had a sufficiently large molecular weight. However, the post-consumer material was extremely brittle under standard molding conditions. This indicated that the degradation of the material properties of the recycled plastic did not originate from chemical degradation. On the other hand, a significant improvement in ductility could be attained by quenching after press molding. These insights were the same as those we obtained with the model recycled plastic, which suggested that a similar mechanism exists.

Fracture surface observation results supported the above results and analysis. The samples used in this study were sorted to increase PP components, but DSC results suggested the inclusion of some PE. However, an inclusion rate of about 10 % had very little effect on the mechanical strength, and significantly good ductility could be obtained by improving the tensile properties. Past research suggested that the addition of a compatibilizer was necessary when there are multiple components; however, our results showed that the material properties of the matrix could be effectively improved without the compatiilizer. Moreover, we found that the inclusion of inorganic contaminations barely affected the mechanical properties.

ACKNOWLEDGMENT

This research was funded by the Environment Research and Technology Development Fund (3K143013) of the Ministry

of the Environment, Japan. We gratefully acknowledge their help with this work.

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Fig. 8. Fatigue fracture surface of molded recycled packaging plastic, parallel with resin flow: (a)230 ºC, 10 min, SC, (b) 230 ºC, 10 min, quenched.