Biodegradabilities of various aliphatic polyesters in natural waters

PII:

s0141-3910(97)00155-9

Polymer Degradation and Stability 59 (1998) 321-332 0 1998 Elsevier Science Limited. All rights reserved Printed in Northern Ireland 0141-3910/98/$19.00

ELSEVIER

Biodegradabilities of various aliphatic polyesters in natural waters Ken-ichi Kasuya,” Ko-ichi Takagi,b Shin-ichi Ishiwatari, b Yasuhiko Yoshidab & Yoshibaru Doi”* “Polymer Chemistry Laboratory, The Institute of Physical and Chemical Research (RIKEN)) Saitama 351-01, Japan

Hirosawa 2-i. Wako-shi,

‘Department of Applied Chemistry, Faculty of Engineering, Toyo University, Kujirai, Kawagoe-shi, Saitama 350, Japan

(Accepted 12 July 1997)

Eight types of aliphatic polyesters were prepared by both biosynthetic and chemosynthetic methods, and their biodegradation tests were carried out at 25°C for 28days under aerobic conditions in different environmental natural waters. Biodegradabilities of melt-crystallized polyester films were evaluated in a temperature-controlled reactor by monitoring the time-dependent changes in the biochemical oxygen demand (BOD) and weight loss (erosion) of polyester film. The biosynthetic poly(3-hydroxybutyrate-co-14% 3-hydroxyvalerate) was degraded at a rapid rate in all natural waters used, and the weight-loss and BOD biodegradabilities of the films were 100% and 78 f 8% for 28 days, respectively. By contrast, the films of chemosynthetic poly(ethylene succinate) were eroded completely in freshwater within 10 days, whereas the films were hardly eroded after 28 days in seawater. 0 1998 Elsevier Science Limited. All rights reserved

1 INTRODUCTION

have been shown to be formed as a storage material in over 90 genera of bacteria, and 91 different constituents of PHA have been identified as various hydroxyalkanoic acids with three to 14 carbon atoms.3 At present, copolymers of 3-hydroxybutyrate and 3_hydroxyvalerate, P(3HB-co-3HV) are produced commercially by a fermentation process using glucose and propionic acid as the carbon sources for Ralstonia eutropha (Monsanto, USA).4 However, chemosynthetic aliphatic polyesters based on the polycondensation reaction of 1,4butanediol (or 1,2-ethanediol) with succinic acid (and adipic acid) are commercially available under the tradename Bionolle (Showa Highpolymer, Japan).5 In a previous paper,6 we evaluated the biodegradabilities of many biosynthetic and chemosynthetic aliphatic polyesters in a river water by monitoring the time-dependent changes in the biochemical oxygen demand (BOD) and weight loss (erosion) of polyester film at 25°C. This paper reports the influence of different natural waters on the biodegradabilities of these biosynthetic and chemosynthetic aliphatic polyesters.

Every year, several hundred thousand tons of nondegradable plastic products are discarded into marine environments via rivers, causing the death of numerous marine animals.’ Recently, there has been a growing demand for biodegradable plastics as a solution to problems concerning aquatic environments. In particular, the development of biodegradable fishing lines and nets is in worldwide demand to solve the problems caused by lost lines and nets. Aliphatic polyesters are one of the most promising materials for biodegradable fibers and films. Many types of aliphatic polyesters have been prepared by both biosynthetic and chemosynthetic methods. Polyhydroxyalkanoates (PHAs) are accumulated as intracellular granules by many bacteria.2 PHAs show thermoplastic properties and are biodegradable in the environment. The PHAs

*To whom correspondence should be addressed. Fax: + 8148(442)4467; e-mail: [email protected] 327

K.-i. Kasuya et al.

328

2 EXPERIMENTAL

niques in which melt samples were isothermally crystallized.‘O

2.1 Polyester samples 2.2 Biodegradation test Table 1 lists eight samples of aliphatic polyesters used in this study for the evaluation of biodegradability in different natural waters. The polyester samples l-3 were produced by a controlled fermentation. Sample 1 of poly(3-hydroxybutyrate) homopolymer, P(3HB), was produced from butyric acid by Ralstonia eutropha.’ Sample 2 of copolyester of 86mol% 3-hydroxybutyrate and 14mol% 3-hydroxyvalerate, P(3HB-co-14% 3HV), was produced by R. eutropha from butyric and pentanoic acids.7 Sample 3 of a copolyester of 90mol% 3hydroxybutyrate and 10 mol% 4_hydroxybutyrate, P(3HB-co-lo% 4HB), was produced by R. eutropha from butyric and 4-hydroxybutyric acids.8 Sample 4 of poly(.s-caprolactone), PCL, was prepared by the ring-opening polymerization of a-caprolactone in the presence of polymethylalminoxane catalyst in toluene at 60°C for 7 days. Samples 5-8 of chemosynthetic aliphatic polyesters were from Showa Highpolymer, Japan.5 The compositions of copolyesters were determined by analysis of the ‘H NMR spectra.g Molecular weight data were obtained at 40°C by using a Shimadzu 6A GPC system and a 6A refractive index detector with a Shodex 80M column.* Polystyrene standards with a low polydispersity were used to make a calibration curve. The melting temperatures (Tm) of polyesters were determined from the differential scanning calorimetry (DSC) endotherms. The percentages of crystallinity of polyester films were determined from the wideangle X-ray diffraction data.8 The films (about 0.1 mm in thickness) of polyester samples were prepared by conventional melt-crystallized tech-

The biodegradation test was conducted using a modified version of the MIT1 test611 All tests were performed under aerobic conditions in a temperaturecontrolled BOD reactor (Taitec BOD tester) at 25°C with stirring. A sample film (initial weight, about 10mg; initial thickness, about 0.1 mm) was placed in a 300-ml BOD reactor, and 200m1 of natural water were added into the reactor as an inoculum. Two freshwater samples from the river Arakawa (Saitama, Japan) and the lake Kasumigaura (Ibaragi, Japan) and two seawater samples from Tokyo Bay (Tokyo, Japan) and the Pacific Ocean (Ibaragi, Japan) were used for the biodegradation test. In addition, 0.2ml of mineral salts solution was added to the natural water. The mineral solution contained the following (per liter): 8SOg of KHzP04, 21.75g of K2HP04, 33.3Og of NazHP04.2H20, 1.70 g of NH&l, 22.5Og of MgS04..7H20, 27.5Og of CaCl* and 0.25 g of FeC13.6H20. The biodegradation test was carried out at 25°C for 28days, and the biochemical oxygen demand (BOD) was measured continuously with a BOD meter. The BOD biodegradability of polyester sample was calculated by subtracting the biochemical oxygen demand (BODb) of the control blank from that (BODt) of the test solution and by dividing the value (BOD,-BODb) by the theoretical oxygen demand (ThOD) of the test sample. 1’ The BODb of the control blank was determined from an average of four blank tests using 200ml of each natural water without the test sample. The weight-loss biodegradability of polyester films was calculated

Table 1. Molecular weights, thermal properties and X-ray crystakities

Molecular weights

Sample”

Melting temperature TM

1. 2. 3. 4. 5. 6. 7. 8.

Mnx lop3

Mw/Mn

350 186 223 110 30 40 33 30

2.1 2.6 2.5 1.7 2.9 3.6 2.4 2.7

P(3HB) P(3HB-~14% 3HV) P(3HB-~10% 4HB) Poly(e-caprolactone) Poly(ethylene succinate) Poly(ethylene adipate) Poly(butylene succinate) Poly(butylene adipate)

a3HB, 3-hydroxybutyrate;

3HV, 3-hydroxyvalerate;

4HB, 4-hydroxybutyrate.

of polyester samples

(“cl

178 155 159 62 106 49 117 60

X-ray crystalinity (%) 651!z5 57+5 46zk.5 63zt.5 61k5 74i5 63*5 7015

Biodegradabilities of various aliphatic polyesters

by dividing the weight of residual films after the test by the initial weight.

3

RESULTS

AND DISCUSSION

3.1 Biodegradation of biosynthetic polyesters In this study, we used three samples l-3 of biosynthetic polyesters, P(3HB), P(3HB-~-14% 3HV) and P(3HB-co-10% 4HB). The molecular weights, melting temperatures and X-ray crystallinities of the films are listed in Table 1. Figure 1 shows typical BOD biodegradation curves of P(3HB-~-14% 3HV) films of sample 2 in different natural waters at 25°C. The degree of BOD biodegradation of P(3HB-co-14% 3HV) film increased with time to reach around 80% within 28 days in both freshwater and seawater. Thus, the P(3HB-~-14% 3HV) film was found to be mineralized at a rapid rate in both freshwater and seawater. Table 2 lists the weight-loss- and BOD biodegradabilities of P(3HB-~-14% 3HV) films in different natural waters after 28 days. After 28 days of test, the films were completely degraded in all natural waters and the weight-loss biodegradabilities were 100%, indicating that P(3HB-~-14%

329

3HV) films were completely hydrolysed into watersoluble products by the function of microorganisms. In fact, no weight loss of P(3HB-co-14% 3HV) films was observed for 28 days at 25°C in the sterilized natural waters. In previous papers,12,13 we isolated several bacteria capable of hydrolysing P(3HB) and P(3HBco-3HV) from seawater and freshwater, purified some extracellular PHB depolymerases and demonstrated that P(3HB) film was hydrolysed into monomer and dimer of 3-hydroxybutyric acid. As Table 2 shows, however, a difference between weight-loss biodegradabilities and BOD biodegradabilities was in the range of 14-30% after 28days. As reported in a previous paper,6 we detected only a small amount of water-soluble intermediates after 28days of degradation test of P(3HB-co-3HV), suggesting that the majority (about 80%) of water-soluble products is utilized for energy generation by microorganisms in the test solution, whereas the minority (about 20%) of products is used for biomass formation. As shown in Table 2, P(3HB) film of sample 1 was also eroded at a relatively fast rate in freshwaters from the river and lake, and the weight-loss biodegradabilities were almost 100% after 28 days. By contrast, the rates of biodegradation of P(3HB)

b) Freshwater from lake (1

14 21 100 (d) Seawater from ocean

0 0

7

14

21

28

0

Time (day) Fig. 1. BOD biodegradation

curves of poly(3-hydroxybutyrate-co-14%3-hydroxyvalerate)

7

14

21

28

Time (day) films in different

natural

waters at 25°C.

330

K.-i. Kasuya

et al.

Table 2. Weight-loss biodegradabilities and BOD biodegradabilities of aliphatic polyester films in different natural waters for 28 days at 25°C Sample

Freshwater (river) WL biodeg.” (%)

1.

2. 3. 4. 5. 6. 7. 8.

P(3HB) P(3HB-co-14%3HV) P(3HB-co-10%4HB) Poly(s-caprolactone) Polytethylene succinate) Poly(ethylene adipate) ’ Poly(butylene succinate) Poly(butylene adipate)

100&O 100*0 100*0 100*0 100*0 100*0 2+1 24*7

BOD biodeg.b (%) 75& 16 7652 9Oztl 751t8 83+2 70&3 3*1 20*4

Freshwater (lake) WL biodeg.” (%) 93*7 100&O 74*26 100*0 lOOi0 95*5 22f 14 80* 13

BOD biodeg.’ (%) 52*7 71*1 55* 17 77* 1 77+2 68*8 12&8 50+ 10

Seawater (bay) WL biodeg.” (%) 41 f 16 1oozto 70&30 100*0 2It-1 100*0 2*2 34zt2

BOD biodeg.b (%) 27% 10 84&2 51*27 79*2 l&l 65*13 If1 20f2

Seawater (ocean) -.___ __-WL BOD biodeg.” biodeg.b (%) (%) 23*13 loo*0 59*15 67i21 5f2 57*14 2*3 11+10

14*10 78*5 43*14 56zt9 31t-2 461t 13 2+0 10+5

“Weight-loss biodegradability. bBOD biodegradability.

films in seawaters from both the bay and the ocean were slower than those in freshwaters. In addition, P(3HB-~-10% 4HB), films of sample 3 were completely degraded in freshwater from river for 28 days, and the weight-loss biodegradabilities were 100%. The biodegradabilities of P(3HB-co10% 4HB) films in freshwater from the lake and in seawaters from the bay and ocean were 70* 30% in 28days. Thus, all biosynthetic samples used in this study were hydrolysed and mineralized rapidly in all natural waters.i4 3.2 Biodegradation of chemosynthetic polyesters In this study, we used five samples 4-8 of chemosynthetic polyesters as listed in Table 1. Figure 2 shows typical BOD biodegradation curves of poly(s-caprolactone) (PCL) films of sample 4 in different natural waters at 25°C. The degree of BOD biodegradation of PCL film increased with time to reach around 80% within 28days in freshwaters from river and lake and in seawater from bay. As can be seen from Fig. 2, the rate of biodegradation in different natural waters decreased in the following order: in seawater from the bay> in freshwater from the river >in freshwater from the lake > in seawater from the Pacific Ocean. As listed in Table 2, PCL films in three natural waters, except for seawater from the ocean, were completely hydrolysed in 28days, biodegradabilities were and the weight-loss 100%. The difference between weight-loss biodegradabilities and BOD biodegradabilities for PCL samples was in the range of 17-33%. These results suggest that PCL films are completely hydrolysed into water-soluble products by extracellular

depolymerizing enzymes from PCL-degrading microorganisms,‘5~i6 and suggest that the products are utilized by microorganisms for energy generation and biomass formation. It has been concluded that PCL-degrading microorganisms are widely distributed in both freshwater and seawater. Figure 3 shows typical BOD biodegradation curves of poly(ethylene succinate) films of sample 5 in different natural waters at 25°C. The degree of BOD biodegradation of poly(ethylene succinate) films increased rapidly with time to reach around 80% within 7days in freshwaters from river and lake. The rates of biodegradation in freshwaters were as fast as those of P(3HB-co-14% 3HV) films in freshwaters. By contrast, poly(ethylene succinate) films were hardly eroded in seawaters from both bay and ocean. As shown in Table 2, the weight-loss biodegradabilities of poly(ethylene succinate) films were 100% after 28days in freshwater, whereas the values were as low as 4 & 3% in seawaters, suggesting that poly(ethylene succinate)degrading microorganisms are present in freshwater but rare in seawater. Table 2 lists the results of the biodegradation test for other chemosynthetic polyester samples 6-8 prepared by the polycondensation of alkanediol with alkanedicarboxylic acid. The rates of biodegradation of four aliphatic polyester samples 5-8 were strongly dependent on the source of natural water used. Poly(ethylene adipate) films of sample 6 were hydrolysed at a relatively high rate in all natural waters. In particular, the films were completely degraded for 28 days in freshwater and seawater from the bay, and the weight-loss biodegradabilities were almost 100%. By contrast,

Biodegradabilities of various aliphatic polyesters

Loo

331

loo (b) Freshwater from lake

80 60

Q !n

20 0 0

100

14

7

21

0

28

80 -

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60-

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40-

i

20-

o0

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7

14

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I

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80

0

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Time (day)

Fig. 2. BOD biodegradation

14

loo ((d) Seawater from ocean

(c) Seawater from bay

k2 x

I

14

21

28

Time (day)

curves of poly(e-caprolactone)

films in different

natural

waters at 25°C.

(a) Freshwater from river

0, 0

7

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0

28 100

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(d) Seawater from ocean

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0

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curves of poly(ethylene

7

14 Time (day)

succinate)

films in different

natural

waters at 25°C.

332

K.-i. Kasuya et al.

poly(butylene succinate) films of sample 7 were hardly eroded in natural waters expect for freshwater from lake, in which the weight-loss and BOD and 12&S% biodegradabilities were 22f14% after 28 days, respectively. The weight-loss- and BOD biodegradabilities of poly(butylene adipate) films of sample 8 in freshwater from lake were 80 f 13% and 50 f 10% after 28 days, respectively. The rates of biodegradation of poly(butylene adipate) films in freshwater from river and in seawater from bay and ocean were slower than that in freshwater from lake. The results from this study have demonstrated that the rate of biodegradation of chemosynthetic polyesters is dependent not only on the chemical structure of monomeric units but also on the source of natural water used.

ACKNOWLEDGEMENTS

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4. Byrom, D., Trends Biotechnol., 1987, 5, 246. 5. Takiyama, E. and Fujimaki, T., Biodegradable Plastics and Polymers, ed. Y. Doi and K. Fukuda. Elsevier, Amsterdam, 1994, p. 150. 6. Doi, Y., Kasuya, K., Abe, H., Koyama, N., Ishiwatari, S., Takagi, K. and Yoshida, Y., Polym. Degrad. Stab., 1996, 51, 281.

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13. Mukai, K., Yamada,

The authors thank Dr E. Takiyama of Showa Highpolymer Co., Ltd for his kindness in supplying some chemosynthetic polyester samples. This work was supported by Fisheries Agency of Japan.

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