Synthesis of Polylactic Acid from Fermentative Lactic

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In addition, the number average molecular weight (Mn) and molecular weight ... condensation polymerization and using p- toluenesulfonic acid as a catalyst.
Advanced Materials Research Vol. 626 (2013) pp 495-499 Online available since 2012/Dec/27 at www.scientific.net © (2013) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/AMR.626.495

Synthesis of Polylactic Acid from Fermentative Lactic Acid by Direct Polycondensation for Materials Application Lalita Ponmanee1,a, Chiravoot Pechyen2,3,b and Sarote Sirisansaneeyakul1,4,c* 1

Department of Biotechnology, Kasetsart University, Chatuchak, Bangkok, 10900, Thailand 2 Department of Packaging and Materials Technology, Kasetsart University, Chatuchak, Bangkok 10900, Thailand 3 Center for Advanced Studies in Agriculture and Food, KU Institute for Advanced Studies, Kasetsart University, Chatuchak, Bangkok 10900, Thailand (CASAF, NRU-KU, Thailand) 4 Center for Advanced Studies in Tropical Natural Resources, National Research UniversityKasetsart University, Kasetsart University, Chatuchak, Bangkok, 10900, Thailand (CASTNAR, NRU-KU, Thailand) a [email protected], [email protected], c*[email protected] *corresponding author Keywords: Lactic acid; Polylactic acid; Direct polycondensation

Abstract. Polylactic acid (PLA) is one of the major commercially available polymers which widely used in the food packing materials, fibers, agricultural films and biomaterials. PLA synthesis was carried out in a direct polycondensation with p-toluenesulfonic acid (PTSA) as a catalyst. The fermentative lactic acid prepared from Lactobacillus rhamnosus TISTR 108 was used as a monomer of polymerization. In addition, the number average molecular weight (Mn) and molecular weight (Mw) of PLA products were analyzed by end-group analysis and High Performance Liquid Chromatography (HPLC). The results indicated that only low molecular weight of PLA could be successfully produced from fermentative lactic acid by direct polycondensation under appropriate conditions; 0.25 wt% PTSA as catalyst, temperature at 170 °C and polymerizing time for 7 hr. The number average molecular weight (Mn) and molecular weight (Mw) of the PLA products were 2,627 and 232 Da, respectively. Furthermore, PLA products were formed as a film with blending of PLA synthesized from fermentative lactic acid and commercially available PLA by using solvent casting method. As a result, fermentative lactic acid is a new alternative substrate for PLA synthesis which contributes to the reduction of production cost, increasing the renewable resources value, and development of bioplastic environmentally friendly materials. Introduction From the environmental problems, many researches are focusing on the development of biodegradable polymers. The several types of biodegradable polymers have been studied such as polyhydroxyalkanoates (PHAs), poly-3-hydroxybutyrate (PHB) and polylactic acid (PLA) which are derived from renewable resources [1]. These polymers are degradable in soil, water or composed by enzyme or bacteria in natural. PLA is a well known biodegradable polymer that can be made from renewable resources and widely applications, especially in the food packing materials, fibers, agricultural films and biomaterials due to it good transparency, mechanical strength and safety [2]. The synthesis of PLA have three main routes; condensation and coupling agents, azeotrophic dehydrative and ring opening polymerization. Generally, PLA is synthesized via ringopening polymerization or ROP of lactide monomer. But this method is lengthy process, expensive and it is difficult to produce PLA because of the several steps involved in this reaction cycle. In contrast, the direct polycondensation of lactic acid is the simple reaction of PLA production and cheaper than ROP. However, the direct polycondensation is difficult to obtain high molecular weight of PLA polymer due to the resulting depolymerization reaction and the presence of water, impurities and low concentration of reactive end group. But the molecular weight of PLA polymer by this method can be increased by using catalyst [3,4,5]. Therefore, the PLA synthesis with All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP, www.ttp.net. (ID: 202.44.44.193-18/02/13,01:59:03)

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catalyst in polycondensation has been interested. Lactic acid is a natural organic acid with widely application in phamaceutical, chemical, food and health care industries. Besides, it is interesting for using as a starting material for production of biodegradable plastics due to it is environmental friendly material and can be manufactured either by carbohydrate fermentation and chemical synthesis. Therefore, PLA polymer, the biodegradable plastics from fermentative lactic acid was obtained more intention. The lactic acid fermentation from renewable resources such as starch derived glucose was considered as sustainable production process which is low production cost and low energy staring material [6,7]. The PLA polymer from fermentative lactic acid production composed of three processes as fermentation, separation of lactic acid and polymerization of PLA from lactic acid. In present work, the synthesis of PLA from commercial lactic acid and fermentative lactic acid prepared from Lb. rhamnosus TISTR 108 was studied by direct condensation polymerization and using p- toluenesulfonic acid as a catalyst. Finally, PLA products were increased their value added with forming as PLA film. Subjects and methods Microorganism. Lactobacillus rhamnosus TISTR 108, maintained in MRS medium at 4 °C, was used to produce lactic acid. The MRS medium consist of glucose 10 g, yeast extract 5 g, MgSO4 0.2 g, sodium citrate 0.5 g, MnSO4 0.03 g, FeSO4 0.03 g, KH2PO4 2.5 g and H2SO4 0.015 ml per liter. Subjects. Lactic acid 85 wt%, as a aqueous solution was purchased from Ajax Finechem, Australia. p-Toluenesulfonic acid monohydrate (PTSA) was purchased from Aldrich Chemical Co (Switzerland). Potassium hydroxide and Sodium hydroxide were provided by Ajax Finechem, Australia. Absolute ethanol was obtained from Merck, Germany and Isopropyl ether was obtained from CARLO ERBA, Italy. Methods. 1) Lactic acid fermentation. Lactic acid production was performed as fed batch fermentation under strictly anaerobic conditions carried out in a 2 l jar fermenter with an initial medium volume of 0.5 l, using 5 % (v/v) inoculum, temperature at 37°C, pH 5.7 agitation at 100 rpm for 141 h. After cell removal, lactic acid was recovered from fermentation broth and purified using Amberlite IRA-67 anion exchange resin. 2) Synthesis of PLA by direct condensation polymerization. The polymerization process was optimized using the Taguchi method. The experimental design for PLA synthesized by direct condensation polymerization was described in Table 1. Two steps direct condensation polymerization process were developed. The first step was to produce oligomer and then the oligomer of lactic acid was polymerized with p-Toluenesulfonic acid as a catalyst. The aqueous solution of commercial lactic acid (850 g l-1) was dehydrated to produce oligo (lactic acid) at 100±5 °C without catalysts under a nitrogen atmosphere for 5 h using 500 mL four-necked flask equipped with a mechanical stirrer. The second step was to make polymer, the solution of polymerization was carried out a 500 mL four-necked flask equipped with a mechanical stirrer and a reflux condenser with varying the polymerization temperature, concentration of catalyst, and polymerization time. 3) Synthesis of PLA from fermentative lactic acid. The development of the new alternative process for PLA synthesis was investigated by using the fermentative lactic acid prepared from Lb. rhamnosus TISTR 108 as the monomer (LA concentration; 180 g l-1) for PLA synthesis instead of a commercial lactic acid under the appropriate conditions of direct condensation polymerization. 4) Preparation of PLA films. To prepare PLA blend films with mixing PLA synthesized from fermentative and commercial lactic acids by using a solvent casting method. The weight ratio between both synthesized PLA from fermentative and commercial lactic acids was 50:50. Five grams of PLA was dissolved in 100 ml chloroform and stirring gently at room temperature. The dissolved solution was poured into glass plate and leaved to dry for 24 h at room temperature [8]. 5) Polymer characterization. The structure of PLA was characterized by ATR-FTIR for recording transmission spectra in the range of 4000–400cm−1. The number average molecular

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weight (Mn) was measured by titration against a standard alcoholic KOH concentration to obtain terminal functional groups and the number average molecular weight was calculated from the following equation (Eq. 1) [9,10].

=

× × ×

(1)

where w is the mass of the PLA sample, n is the number of reactive functional groups on the monomer (n = 2 for lactic acid), V is the volume of alkali consumed for neutralizing the functional groups and M is the molarity of the potassium hydroxide solution. The reported Mn values are an average of two measurements. The molecular weight of PLA was determined by a HPLC system. Size exclusion on PhenogelTM GPC/SEC column (300 × 7.8 mm; particle size 10 µm) using RI detector and tetrahydrofuran (THF) was used both as the solvent and the eluent. Eluetion was performed at a temperature of 23 °C and at a flow rate of 1 ml/min. Results and Discussion Lactic acid production by Lb. rhamnosus was performed with the fed batch fermentation under strictly anaerobic conditions carried out in a 2 l jar fermenter with an initial medium volume of 0.5 l, using 5 % (v/v) inoculum, temperature at 37 °C, pH 5.7 for 124 hr. The lactic acid production from glucose was shown in Fig. 1. The concentration of lactic acid was 117 g l-1, after 124 hr of fermentation. The yield and productivity of lactic acid were 92.7% and 2.10 g l-1 hr-1, respectively. For the synthesis of PLA performed under atmospheric pressure, the highest viscosity of polymer was obtained in the polycondensation. All syntheses were performed with drying solvent during the process and its circulation in the closed system. This is of essential significance for the progress of polycondensation, despite the equilibrium process of PLA formation. The conditions of PLA synthesis from the direct polycondensation of lactic acid in short reaction time and the molecular weight of PLA polymers were performed as shown in Table 1. The highest molecular weight and number average molecular weight of PLA were 423 Da and 42,288 Da, respectively, with using 0.25 wt% catalyst concentration of the oligomer, temperature at 170 °C and the reaction time of 7 hr (experiment 4). From the results, temperature and the concentration of catalyst gave more significant influence on Mw and Mn values of PLA polymer. However, the use of catalyst concentration higher than 0.25 wt% decreased the reaction rate and molecular weight of PLA, because an excess catalyst was not only accelerated the polycondensation, but also affected depolymerization of PLA. In contrast, an increase of temperature increased the Mw and Mn values of PLA. As a result, the optimal conditions of PLA synthesis were 0.25 wt% PTSA, temperature at 170 °C and polymerization time of 7 hr. The structure of PLA synthesized by direct polycondensation was characterized with ATR-FTIR. For the transmission spectra of PLA samples, the spectra of synthesized PLA were compared with standard PLA as shown in Fig. 2. The typical transmission spectra of PLA appeared in three main regions as follows: the C-H stretching, between 2800 and 3000 cm-1; the C=O stretching, between 1600 and 1800 cm-1; and the C-O stretching, between 1000 and 1200 cm-1 [11,12]. The peaks of standard PLA located at 2922, 1747, 1182, 1129, 1083 and 1042 cm-1. The PLA synthesized from commercial lactic acid (0.25% catalyst, 170 °C and 7 hr) showed the peaks of PLA at 2944, 1728, 1191, 1121, 1091 and 1041 cm-1. Moreover, the IR spectra of the PLA showed the same structure as the standard PLA. To develop an alternative process for PLA synthesis was investigated using the fermentative lactic acid prepared from Lb. rhamnosus TISTR 108 as the monomer substituting a commercial lactic acid. The synthesis of PLA was operated using the optimal conditions for direct condensation polymerization. These conditions for synthesizing PLA were 0.25 wt% PTSA as catalyst, temperature at 170 °C and polymerization time of 7 hr. The number average molecular weight (Mn) and molecular weight (Mw) of the synthesized PLA were shown in Table 3. The results showed that the molecular weight and number average molecular weight of PLA polymer derived from fermentative lactic acid were 232 and 2,627 Da, respectively. It indicated that at lower temperature for PLA synthesis was favorable to the formation of polymers with higher thermal stability and molar mass,

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and lowering tendency to create by-products. The fast growth observed in the molar mass, and then its fall during polycondensation in p-toluenesulfonic acid, may be connected with the formation of molar mass stabilizer as a result of the thermal rearrangement of PLA. One possibility to thermal degradation of PLA is the thermal break-up of the ester bond upon formation of the acidic stabilizer of molar mass. Furthermore, PLA products were formed as a film by blending of PLA synthesized from fermentative and commercial lactic acids by using solvent casting method. PLA blend film showed clear appearance and quite brittle.

Fig. 1 Fed batch fermentation of lactic acid by Lb. rhamnosus using glucose as substrate under strictly anaerobic condition (5% v/v inoculum, 37 °C, pH 5.7, 124 hr).

Fig. 2 The FTIR spectrum of PLA from commercial lactic acid.

Table 1 The molecular weight of PLA synthesis from commercial lactic acid by direct polycondensation. Catalyst concentration Time Experiment Temperature (°C) Mn Mw (wt% of the oligomer) (hr) 1 0.25 140 4 16,409 259 2

0.25

150

5

17,908 317

3

0.25

160

6

26,453 350

4

0.25

170

7

42,288 423

5

0.50

140

5

16,802 285

6

0.50

150

4

17,634 323

7

0.50

160

7

32,297 286

8

0.50

170

6

25,965 333

9

0.75

140

6

18,719 270

10

0.75

150

7

18,849 277

11

0.75

160

4

20,747 307

12

0.75

170

5

30,032 344

13

1.00

140

7

19,557 270

14

1.00

150

6

23,518 307

15

1.00

160

5

25,873 323

16

1.00

170

4

25,533 317

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Table 2 The molecular weight of PLA synthesized from fermentative lactic acid by direct polycondensation (0.25 wt% PTSA as catalyst, 170 °C, 7 hr). Sample Monomer Mn Mw PLA1

Commercial LA

42,288

423

PLA2

Fermentative LA

2,627

232

Conclusions Both commercial lactic acid and that prepared from Lb. rhamnosus TISTR 108 could be used as monomer for PLA synthesis. Although, the molecular weight of PLA polymer from fermentative lactic acid was lesser than that produced from commercial lactic acid, but available lactic acid from fermentation is a promising substrate for the synthesis of PLA, which are capable to reduce the production cost and to use less energy consumed starting material for the production of PLA. Moreover, this enhances value added renewable resource and develops an environmental friendly bio-plastic material. However, higher molecular weight PLA found its difficulty to be produced by the direct condensation polymerization. That is water contamination in aqueous solution was hard to remove between free acids at equilibrium. This resulted in water and polyester formed as a byproduct in the process instead. To solve this, the monomer, lactic acid was firstly polymerized to oligomeric PLA, simultaneously depolymerized into lactide, and finally re-polymerized to attain PLA by ROP mechanism. This method obtained higher molecular weight of PLA. Furthermore, blending PLA synthesized from fermentative and commercial lactic acids could be casting as film with using a solvent casting method. Acknowledgement This research was supported by the Department of Biotechnology, the Department of Packaging and Materials Technology and the Graduate School, Kasetsart University. Also, the Higher Education Research Promotion and National Research University Project of Thailand, Office of the Higher Education Commission is acknowledged. References [1] K. Woong and S. I. Woo. Macromol Chem Phys. Vol. 203 (2002), p. 2245-2250. [2] H.R. Kricheldorf. Chemophere (2001), p. 43-49. [3] I. Pillin, N. Montrelay, A. Bourmaud and Y. Grohens. Polym Degrad Stab Vol. 93(2) (2008), p. 321-8. [4] F.P. Carrasco, P. J. Gamez, O.O. Santana and M. L. Maspoch. Polym Degrad Stab Vol. 95 (2010), p. 116-125. [5] A. N. Vaidya, R. A. Pandey, S. Mudliar, M. S. Kumar, T. Chakrabarti and S. Devotta. Environ Sci Technol. Vol. 35 (2005), p. 429-467. [6] J. Qin, X. Wang, Z. C. M Zheng, H. Tang and P. Xu. Bioresour Technol. Vol. 101 (2010), p. 7570-7576. [7] M.T. Gac, M. Kaneko, M. Hirata, E. Toorisaka and T. Hano. Bioresour Technol. Vol. 99 (2008), p. 3659-3664. [8] J. W. Rhim, S. I. Hong and C. S. Ha. Food Sci Technol. Vol. 42 (2009), p. 612-617. [9] V. Lassalle, G. B. Galland and M. L. Ferreira. Bioprocess Biosyst Eng. Vol. 31 (2008), p. 499–508. [10] R. K. Kondabagil and S. Divakar. World J Microb Biot. Vol. 19 (2003), p. 859–865. [11] M. Hans, H. Keul and M. Moeller. Macromol Biosci. Vol. 9 (2009), p. 239-247. [12] F. Codari, D. Moscatelli, G. Storti and M. Morbidelli. Macromol Mater Eng. Vol. 295 (2010), p. 58-66.

Advanced Materials Engineering and Technology 10.4028/www.scientific.net/AMR.626

Synthesis of Polylactic Acid from Fermentative Lactic Acid by Direct Polycondensation for Materials Application 10.4028/www.scientific.net/AMR.626.495