J. Cent. South Univ. (2014) 21: 1725−1732 DOI: 10.1007/s11771­014­2116­z

Degradation behaviors, thermostability and mechanical properties of poly (ethylene terephthalate)/polylactic acid blends

XIA Xue­lian(夏学莲) 1 , LIU Wen­tao(刘文涛) 1, 2 , TANG Xin­ying(唐新颖) 1 , SHI Xiang­yang(史向阳) 3 , WANG Li­na(王丽娜) 1, 4 , HE Su­qin(何素芹) 1 , ZHU Cheng­shen(朱诚身) 1

1. School of Materials Science and Engineering, Zhengzhou University, Zhengzhou 450052, China; 2. Department of Nuclear and Quantum Engineering, Korea Advanced Institute of Science and

Technology, Daejeon 305­701, Korea; 3. Department of Quality Assurance, Citic Heavy Industries Co., LTD., Luoyang 471039, China;

4. Department of Materials and Chemical Engineering, Henan Institute of Engineering, Zhengzhou 450007, China

© Central South University Press and Springer­Verlag Berlin Heidelberg 2014

Abstract: It is difficult for polyethylene terephthalate (PET) to degrade, which caused severe pollution. In this work, polylactic acid (PLA) was introduced to improve the degradation of PET. PET/PLA was synthesized by extrusion blending. The thermal, crystalline and mechanical properties of blends were investigated with TGA, DSC, WAXD and universal testing machine. The degradation of the blends in soil, acid and alkaline buffer solutions was assessed, respectively. It was found that the introduction of a little PLA promoted crystallization of PET during injection molding process. The starting decomposition temperature lowered from 412.1 °C of pure PET to 330.4 °C at 50% PLA content, tensile and bending strength of blends gradually decreased with the PLA content increasing, while the degradation rate improved. Alkaline environment was most beneficial for blends to degrade. The degradation mechanism was discussed.

Key words: degradation; polyester; polylactic acid; mechanical property

1 Introduction

With a large number of polymer materials being used in various fields, white pollution has become a worldwide environmental problem. One of the most effective solutions to environmental problems is developing the biodegradable polymeric materials to replace non­degradable plastics. Applications of biodegradable polymeric materials range from packaging, consumer products and pharmaceuticals to agriculture [1−4]. Conventional polymers can last for many years after being discarded, such as polyethylene (PE), polypropylene (PP), polyvinyl chloride (PVC), polystyrene (PS) and acrylonitrile­butadine­styrene (ABS), which are improper to be used as short­term plastics, including wrapping, trash bags, diaper back sheets, fast­food tableware and mulch films. These short­term polymers are impractical and generally undesirable to recycle [5]. In contrast, biodegradable polymers chains can be split by water, oxygen, enzymes and living element, and be degraded to nontoxic compounds. The research of degradable polymer has

initiated with the increasing problems of white pollution during the last decade [6]. Great efforts have been made to obtain polymers with appropriate properties for biomedical and environmental­friendly materials. Many kinds of biodegradable polymers have been synthesized, such as polylactic acid (PLA) [7−9] polycaprolactone [10−11], polyglycolide [12−13], poly(α­amino acid)s [14−15], poly(orthoester)s [16−17] and polyanhydrides [18−20].

In contrast to most biodegradable aliphatic polyesters, aromatic polyester poly(ethylene terephthalate) (PET) is an important engineering thermoplastic with good thermal and mechanical properties, low permeability, good aesthetic appearance and chemical resistance. It is widely used in commercial products, for example, beverage containers, food packages, films, textile fibers, and engineering plastics in automobiles, electronics and blood vessel tissue engineering, etc. Although PET is innocuous for the human body, but it is considered as noxious material because it is less susceptible to attack of water and microbial, and hard to degrade even in damp environment. PET bottles can exist for 30 to 40 years in

Received date: 2013−01−08; Accepted date: 2013−06−19 Corresponding author: LIU Wen­tao, Associate Professor, PhD; Tel: +86–371−67763806; E­mail: wtliu@zzu.edu.cn

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humidity of 45%−100%, temperature of 20 °C, while its performances only lose 50%. In the same condition, the polyester films can exist for 90 to 100 years [1]. Of course, the stability of PET is mandatory for many of its uses. However, time­controlled degradation PET is in dire need for certain applications such as wrappings, trash bags, agricultural mulch films and biodegradable biomaterials. Additionally, the breakdown products of PET are ethylene glycol and terephthalic acid, which are known to have very low acute toxicity to bacteria, crustaceans, algae and mammals.

Unlike petroleum­based plastics, biodegradable green plastic PLA [21−24] is derived from renewable resource like corn, starch and sugar beets. PLA is degraded to nontoxic compounds by simple hydrolysis of the ester bond and does not require the presence of enzymes to catalyze this chemical reaction, so it is considered to be biodegradable and compostable [25−26]. It is one of the few polymers whose stereochemical structure can be easily modified by polymerizing a controlled mixture of the L­or D­isomers to yield high­molecular­mass amorphous or crystalline polymers, which can be used for food container and are generally recognized as safe (GRAS) [27]. Therefore, extensive attention has been paid to the practical use of PLA, which has excellent biodegradability as general­purpose polymers and has been used widely to alleviate environmental problems recent years [28−29]. However, PLA is not a good barrier for oxygen, has relatively high cost and belongs to the group of polymers with poor thermal stability and highly sensitivity to heat [30]. Moreover, its processability is not good for industrial processing operations in the case of a homopolymer structure [31], so the use of PLA for bottles is still limited.

Blending as a practical method has been paid considerable scientific and industrial attentions, it is one of the most simple and effective modified methods. It not only retains the advantages of the original polymers, but also provides some new performances to polymers by changing the aggregation structure. This method has emerged as an attractive way to preparing novel copolyesters with definite degrees of composition, randomness and crystallinity [32]. Recently, a good deal of research on PET blends has been reported in order to broaden the applications of the homopolymer [33−37].

Even though both PET and PLA belong to polyester, however, degradation of their blend has not been studied in detail previously. In this work, PET/PLA was synthesized by melt blending. The PLA is expected to function as a degradation accelerator and solve the inherent problem of non­degradation of PET. The thermal properties of the PET/PLA blends were studied by DSC and TGA, and the degradability and the

mechanical properties were also investigated.

2 Experimental

2.1 Materials and sample preparation PET, was purchased from Jiangsu Sanfangxiang

Group Co., Ltd., China, whose intrinsic viscosity is (0.85±0.02) dl/g. PLA, provided by Ningbo Universal Plastic Products Co., Ltd., China, with a molecular mass of 250000. Phenol and chloroform were bought from market, analytical pure. Prior to melt extrusion processing, PET was dried in a vacuum oven at 60 °C for 24 h, and PLA was dried in a vacuum oven at 40 °C for 48 h. Then the dry PET, PLA and appropriate amount of titanium butoxide were mixed 5 min in a high­speed mixer prior to extrusion. The melt blending of the dry PET and PLA was carried out by twin­screw extruder (TE­34 type, Chemical Industry, Chemical Machinery Institute). The barrel temperature ranged from 245 to 260 °C, and the screw speed was 150 r/min. The blends with PLA content 0%(pure PET), 10%, 20%, 30%, 40%, 50%, and 100%(PLA) were all extruded. The films of PET/PLA blends for degradation test were prepared by casting solution technique, solvent ratio of V (phenol): V (chloroform) was 1:1.

2.2 Properties characterizations Decomposition temperatures of PET/PLA blends

were measured by NETZSCH TG 209 (NETZSCH, Germany). The samples were heated from 30 °C to 650 °C at 10 °C/min in a nitrogen atmosphere.

Degradability test was performed with PET/PLA films which were square­shaped samples with 1 cm side length, 0.1 mm thickness, and 12.7 mg. Degradability of the blends was determined gravimetrically [32]. Typically, the films were placed respectively in the soil depth of 20 cm(water them every 10 d), in one bottle containing 250 mL of acetic acid buffer solution of pH 4.2, and in the other bottle containing 250 mL of phosphate buffer solution of pH 10.1. Both bottles were kept in a water bath at 25 °C for 80 d. Each kind of sample was taken out every 10 d, rinsed with distilled water, and dried under vacuum at 50 °C for 24 h. The results were given as mass loss percent.

The microscopic morphology of degraded film surfaces were observed by scanning electron microscopy (SEM), AUANTA 200, and the voltage was 20 kV. For the morphological measurements, the samples were coated with gold before the examination.

The mechanical properties were determined by computer controlled electronic universal testing machine CMT­5104 (Shenzhen Sansi Measuring Technology Co., Ltd.). Initial grip separation was set at 50 mm, and stretching speed was at 10 mm/min; Initial grip

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separation was set at 70 mm, cross­head speed at 2 mm/min. The results were obtained from the average measurements of at least five specimens.

The melt temperatures of PET/PLA were measured by NETZSCH DSC 242 (NETZSCH, Germany). The samples cut from injection molding products were heated from 30 °C to 300 °C at a heating rate of 10 °C/ min in a nitrogen atmosphere to reset previous thermal histories, after which all the samples were cooled from 300 °C to 30 °C at a cooling rate of 10 °C/min, and at last the samples were heated from 30 °C to 300 °C with the rate of 10 °C/min again. The melt crystallization and subsequent melting behavior of the samples were recorded in the cooling process and the second heating process.

WAXD measurements were carried, out by X­ray diffractometer (made by PANalytical), X’Pert PRO. The nickel­filtered Cu K X­ray beam with a pinhole graphite monochromater was used as the source. The wavelength of the X­ray beam is 0.1542 nm. The diffraction patterns of the blends were recorded in an interval of 2θ=5−35°.

3 Results and discussion

3.1 Effect of PLA content on thermostability of PET/PLA Thermostabilities of PET, PLA and PET/PLA were

measured at a heating rate of 10 °C/min. And the initial decomposition temperature and the fastest decomposition temperature are observed in Figs. 1 and 2.

With the PLA content increasing, both the starting decomposition temperature and the fastest decomposition temperature of the blends gradually decreased. The starting decomposition temperature lowered from 412.1 °C of pure PET to 330.4 °C at 50% PLA content. And the fastest decomposition temperature reduced from 435 °C of pure PET to 352.5 °C at 50% PLA content. Both the starting decomposition temperature and the fastest decomposition temperature of the PET/PLA were

Fig. 1 TG curves of PET/PLA blends with different PLA contents

Fig. 2 DTG curves of PET/PLA blends with different PLA contents

between those of pure PET and PLA. The more the content of PLA was, the more the negative influence was on the thermal properties of the blends because of the poor thermal stability of PLA. In another word, decomposition temperature of blends was higher than that of PLA, namely blending PLA with PET elevated the decomposition temperature and improved the thermal stability of PLA.

3.2 Effect of PLA content and environment on degradation of PET/PLA The degradation results of PET/PLA are shown in

Figs. 3, 4 and 5. With the time prolonging from 10 d to 80 d, the mass loss of the blends and the pure PLA gradually increases. For pure PLA, the mass loss increases from 0.24% of 10 d to 5.04% of 80 d in the soil. However, for pure PET, the mass does not change noticeably with time. The mass losses of the blends were between those of pure PET and PLA. At the same degradation time, the higher the PLA content in the blends, the higher the mass loss. The mass loss was lower than the hydrolytic degradation of PTT/PLA blends reported by ZOU et al [38]. From the figures, it

Fig. 3 Degradation of PET/PLA with different PLA contents in soil

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Fig. 4 Degradation of PET/PLA with different PLA contents in acetic acid buffer solution (pH 4.2)

Fig. 5 Degradation of PET/PLA with different PLA contents in phosphate buffer solution (pH 10.1)

could be found that, if the mass of PET was deducted, the mass loss ratio of PET/PLA would be higher than pure PLA. For example, the mass loss ratio of PET/PLA(50/50) buried in soil after 80 d was 3.23%. If the mass of pure PET was deducted, the mass loss ratio of remainder PLA would be 6.46%, which would be higher than that of pure PLA(4.72%) under the same conditions. It meant a part of PET degraded. Similar tendency can be observed in the acetic acid buffer solution of pH 4.2 (Fig. 4) and the phosphate buffer solution of pH 10.1 (Fig. 5). According these data, the PLA played a role of a degradation accelerator and improved the degradation of PET.

It can be checked that two different stages in the degradation curves. At the first stage, the hydrolytic degradation of polyesters involved the chemical scission of an ester linkage in the main chain by water, oxygen and enzymes. The decrease in the molecular mass changed the mechanical properties, but the initial mass did not change markedly. The damage of molecular microstructure could not be perceived by macroscopic view. The next degradation stage involved measurable mass loss in addition to further chain cleavage, and

fragments of polymer were phagocytosed by bacteria and microorganisms.

In addition, from Figs. 3, 4 and 5, it can also be found that the mass loss of the PET/PLA in the phosphate buffer solution of pH=10.1 was the most, and degradation in the acetic acid buffer solution of pH 4.2 was more than that in the soil.

The essence of polymer degradation is hydrolysis of ester bonds. After the films were placed 20 cm deep in the soil, even it was watered every 10 d, the soil became dry with the surface water evaporated easily or osmoses into deeper soil. As a result, the hydrolyzation was very slow in the poor water retention soil. While being placed in acetic acid and phosphate buffer solution, the films degraded easily. After enough water infiltrating into amorphous region and the border region of crystalline of the blends, degradation further happened with acid and alkaline catalysis. Meanwhile, some crystalline regions were disorganized and changed into amorphous which would be a new beginning site for degradation.

Scanning electron microscopy was used to investigate degradation morphological changes of blend with 50% PLA in different conditions as shown in Fig. 6. Blend film with 50% PLA in the air did not participate in the degradation reaction, whose surface was smooth although there were microvoids with regular shapes, which may be resulted from the solvent evaporation in the process of casting. Films degraded in soil, acetic acid and phosphate buffer solution, by contrast, whose surfaces were rough, which was a clear indication of degradation presence. It can be found from Figs. 6(b), (c) and (d) that the degradation rate was slow in soil, fast in acetic acid buffer solution and faster in phosphate buffer solution. The conclusion was consistent with that of the mass loss test.

As shown in Scheme 1, the dominant mechanism of the hydrolytic degradation in an acidic and neutral environment is different from that in an alkaline environment [1]. In unbuffered neutral water, the pH of the post­reaction mixture amounts to 3.5−4.0 due to the presence of the acid ends of the terephthalic acid (TPA) monoglycol ester resulting from hydrolysis. So, too much H + in acetic acid buffer solution prevents chemical equilibrium shifting to direction of depredation to some extent.

While, OH − in phosphate buffer solution could neutralize acid ends of the TPA monoglycol ester from hydrolysis, carrying degradation forward.

Figure 7 shows the micrographs of pure PET film placed in the solution of pH 10.1 for 80 d. Compared Fig. 7 with Fig. 6(d), a notable difference on the degradation rates in the same condition can be seen. Hydrolysis in the solid state turns to be a complex process highly dependent on chain mobility and

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Fig. 6 SEM images of PET/PLA (50/50) for 80 d in different environments: (a) Air; (b) Soil; (c) pH=4.2; (d) pH=10.1

Scheme 1 Hydrolytic cleavage of an ester bond under acidic or neutral conditions and alkaline conditions

Fig. 7 SEM image of pure PET placed in solution of pH=10.1 for 80 d

permeability. Pure PET is extremely stubborn, and it is very difficult to cleave the ester linkage in the main chain even though in an alkaline environment, let alone losing

in mass. By comparison, PLA of PET/PLA blends degrades rapidly, leaving behind PET segments which were riddled with thousands of gapping wounds and scars. It was easy for water, oxygen and microorganism to invade, PET segments were finally phagocytosed.

3.3 Effect of PLA content on mechanical properties of PET/PLA blends The tensile strength and flexural strength of

PET/PLA are shown in Figs. 8 and 9. With the PLA content increasing, the tensile strength and flexural strength of the PET/PLA decreased. In order to gain a better insight into the effect of structure on mechanical properties, the compatibility and crystallization of blends were investigated.

The melt temperatures (Tm) of PET and PET/PLA were measured at a heating rate of 10 °C/min in the first and second heating processes of DSC, and the results are

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Fig. 8 Tensile strength of PET/PLA with different PLA contents

Fig. 9 Flexural strength of PET/PLA with different PLA contents

shown in Figs. 10 and 11. The blends show a single melting temperature when the PLA content was below 30%, indicating the good compatibility of PET and PLA components [39]. However, the blends show two melting temperatures near 170 °C and 250 °C when the content of PLA exceeds 30%, indicating the PET and PLA formed crystallization respectively, and PET crystallization was not compatible well with PLA crystallization. The poor compatibility may result in the decrease in mechanical properties of PET/PLA blends.

PLA could be evenly dispersed in PET when the content of PLA was lower than 30%. In such a system, the continuous phase was PET, while the dispersed phase was PLA. Small quantity PLA chains can not gather together and form the crystallization during the cooling process. Therefore, on the thermograms of the heating process, there was no endothermal peak of PLA near 160 °C. In addition, it might be transesterification or ester exchange reaction [40] that occurred in polyesters at melt processing temperatures above 230 °C, which lead to the formation of PET­PLA copolymer. The copolymer primarily locates at the interface, whose role in interfacial tension reduction and miscibility enhancement was broadly recognized experimentally and

Fig. 10 First heating process DSC curves of PET/PLA with different PLA contents

Fig. 11 Second heating process DSC curves of PET/PLA with different PLA contents

theoretically in reactive processing [41−42]. With the PLA content increasing, large crowds PLA molecules gather together, which could accomplish the crystallization. Moreover, compatibility between PET crystal area and PLA crystal area was very poor. Therefore, two peaks near 170 °C and 250 °C appeared which were corresponding to the melting points of the pure PLA and PET, respectively.

The mechanical strength of the PET/PLA blends would be influenced by the crystalline state. WAXD patterns of PET and PET/PLA are shown in the Fig. 12. Preparation process of the samples completely imitated the injection molding process, the samples were melted at 260 °C, then were rapidly cooled to 80 °C, heat preservation for 13, after which the samples were cooled to room temperature in air. There are almost no peak that can be picked in PET’s curve. This implies that the pure PET cannot crystallize at this condition. Some incomplete crystallization appeared when the PLA content was lower than 30%, as shown in Fig. 12, the little sharp peaks situated at 23.2° and 26.1° correspond the PET crystalline planes of (110) and (100), respectively. Indicating that the introduction of a little

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Fig. 12 WAXD patterns for PET/PLA with different PLA contents

PLA promoted the crystallization of PET, but the crystallization was not complete just because the rate of cooling was rapid. It was incomplete crystallization that played the role of stress concentration, which could result in the decrease in mechanical properties of PET/PLA blends. There is no indication of PLA crystalline in all curves at 14.8°, 16.8°, 19.1° and 22.28°. These angles correspond to the planes of (011), (110) or (200), (113) or (203) and (210), respectively. When the content of PLA exceeded 30%, no more crystalline peak can be found, which heralded that neither PLA nor PET can complete the crystalline process in these blends during injection molding process. The similar conclusion can be observed from Fig. 10. Both PET and PET/PLA blends formed cold crystallization in the first heating process of DSC, indicating that PET and PET/PLA can not form perfect crystalline during injection molding process.

These results can explain the reasons of the reduction of mechanical properties with the content of PLA increasing. Both the incomplete crystal and amorphous would result in the reduction in mechanical properties of semi­crystal polymer. The tensile strength and flexural strength decrease sharply when the PLA content is low (<20%). It may be resulted from forming of the incomplete crystal in blends. These incomplete crystals, as grains, play a role of inclusions, and the stress concentrates around the grains, which causes the tensile strength and flexural strength of the blends decrease comparing with those of pure PET. Furthermore, the incompatibility of PET and PLA components lead to the reduction in mechanical properties of blends. The tensile strength and flexural strength become lower with the PLA content continually increasing. The more content of PLA, the more negative influence on the mechanical properties of the blends because of the poor

mechanical properties of PLA.

4 Conclusions

1) Biodegradable polyesters PET/PLAs with different PLA contents are synthesized by extrusion blending.

2) The PLA plays a role of degradation accelerator. The degradation rate of PET/PLA blends improves with the PLA content increasing. After PLA segments degrade rapidly, PET is riddled with thousands of wounds and scars. It is easy for water, oxygen and microorganism to invade PET segments. PET segments are finally phagocytosed. The degradation rate of the PET/PLA in alkaline environment is the fastest, and in acidic conditions is faster than that in the soil.

3) The PET improves the poor thermal properties of PLA, and the more PET content, the better thermal stability of PET/PLA.

4) The introduction of a little PLA promotes crystallization of PET.

5) The mechanical properties of PET/PLA gradually decrease with the PLA content increasing. The introduction of PLA changes aggregation structure of PET and disturbs the crystallization. Forming incomplete crystallization has negative effect on mechanical properties.

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(Edited by HE Yun­bin)