NEW SYNTHETIC ROUTES TO LACTIC ACID POLYMERS

Transcription

1 NEW SYNTHETIC ROUTES TO LACTIC ACID POLYMERS Lilian Tamie Matsura Shu Hui Wang Departamento de Engenharia Metalúrgica e de Materiais, Escola Politécnica, Universidade de São Paulo, São Paulo, SP, , Brasil. Abstract New biodegradable polymers are becoming subject of intense research effort due to the rejection to the increasing disposal volume of post-consume plastic products, mainly packaging products. Poly(lactic acid) and copolymers are completely biodegradable in the presence of water and microorganisms, however their wide application is still restricted due to the high cost involved in the polymer synthesis using the cyclic monomer lactide. Because of its high price poly(lactide) has been used mainly in the medical materials as suture and implants. We have produced poly(lactic acid) by direct polyesterification of lactic acid in solution using catalytic amount of sulfuric acid or stannous oxide. Molecular structure and molar mass of poly(lactic acid)s were characterized by FTIR, acid-base titration and dilute solution viscosimetry. The influence of the catalyst in the reaction kinetics and in the molecular mass of the resultant poly(lactic acid) was analyzed. The synthesized polymers have proven to be biodegradable as expected. INTRODUCTION The traditional applications of synthetic polymers are based on their relative inertness to biodegradation in contrast with the natural polymers, as cellulose and proteins. In these days, due to the high increasing disposal volume of synthetic polymers products, in majority bio-resistant, the research and development of biodegradable polymers products has become an important question of strategy, due to their environmental implications. Besides, biodegradable polymers are useful in applications such as suture, chirurgical implants, controlled drug delivery implants, agricultural chemicals delivery systems among others. CONGRESSO BRASILEIRO DE ENGENHARIA E CIÊNCIA DOS MATERIAIS, 14., 2000, São Pedro - SP. Anais 58501

2 There is a great expectation on the fast increase of biodegradable polymers demand. A study by Freedonia Group in 1995 forecast a demand over tons in USA by the year 2000, and a study made by BASF (1995) expect a potential demand around tons in Europe. There are quantities well significant for a future very close e need development in quality products production technology to satisfy the consumer. The poly(lactic acid) or poly(lactides) is completely biodegradable in natural systems when in presence of water and microorganisms. In despite of the good mechanical properties of poly(lactic acid) as a thermoplastic, the relative cost of the intermediate lactide has difficulted the possibility of its commercialization, an intense purification between various recrystallization steps is described in the literature as a basic requirement to obtain a polymer with high molecular mass. Besides, pure poly(lactide)s usually present visible thermal degradation during the process in melt with significant reduction of molecular mass, due to its high degree of crystallinity. basically to: Then, a development project screening the production in scale must be oriented Production of the polymer from direct polymerization of lactic acid; Determination of the correlations between the polymers s molecular mass derived of lactic acid and the technological properties of the material. Preparation of new copolymer structures that brings benefits in terms of improvement and broaden of the chemical, physical and mechanical properties in function of the degree of polymerization. Poly(lactic acid) and its copolymers with others hydroxyacids are used in many applications, such as sutures [1], bone fixation, bone replacement [2] and deposits for longterm drug releases [3]. The main advantages of these aliphatic polyesters are biocompatibility and biodegradability. We have high confidence in the technological potential of this Project in terms of competitiveness of the polymers derived from lactic acid in the biodegradable polymers market. CONGRESSO BRASILEIRO DE ENGENHARIA E CIÊNCIA DOS MATERIAIS, 14., 2000, São Pedro - SP. Anais 58502

3 In this paper we present the initial results acquired by a direct condensation reaction of lactic acid catalyzed by a strong acid, sulfuric acid, and by a basic oxide, stannous oxide, and the physicochemical characterization of the poly(lactic acid)s thus prepared. M H H O C COOH CH 3 Catalyst Solvent ( O CH CH 3 CO)M Fig. 1 - Polymerization of lactic acid. EXPERIMENTAL Materials L-Lactic acid (88% in water) with 99% optical purity was kindly donated by Purac- Sintese and was used after drying. Solvents were PA grade and were used without further purification. Concentrated sulfuric acid PA from Nuclear was used without further purification and stannous oxide PA from CAAL was used without further purification. Polymerization reaction Toluene and THF were used as solvent for the solution polymerization of lactic acid. Solutions containing 20% of monomer and 0,3 to 1% of catalyst were heated to reflux in round botton flask under stirring. After 24 to 48 hours the reaction was stopped and the polymeric solution was allowed to cool prior to precipitation in methanol. Characterization The FTIR spectra were obtained by deposition of a thin film of sample over a KBr plate. The molecular weights were determined by capilar viscosimetry at 25ºC of diluted solution in chloroform. The thermal transitions were observed by DSC using a heating rate of 20 ºC/min, from 20 to 190 º C, and sample sizes of 10 to 15 mg. Each sample was submitted to a second heating cycle after quenching to ambient temperature to ensure the same thermal history. CONGRESSO BRASILEIRO DE ENGENHARIA E CIÊNCIA DOS MATERIAIS, 14., 2000, São Pedro - SP. Anais 58503

4 RESULTS AND DISCUSSION The polymerization reaction was performed in a solution mixture of toluene and THF. Prior to reaction the commercial lactic acid (88% in water) was carefully dried in a vacuum oven at 80 C for 8 h. The polymerization process is shown schematically in Figure 2. The reaction was allowed to proceed for 24 h and after that the poly(lactic acid) obtained was recovered by precipitation in methanol. Lactic Acid Toluene + THF Catalyst Polymerization Drying Polymer Solution Precipitation Fig.2 Scheme of the synthetic process. The recovered polymer was dissolved in chloroform and reprecipitaded twice in methanol. The solid polymer was dried in vacuum oven at 80 C for 4h and stored in dissecator. Viscosity measurement at 25 ºC in dilute chloroform solutions allowed the estimation of inherent viscosity of 22.3 ml/g at 0,5 %, for poly(lactic acid) by H 2 SO 4, and a viscometric molar mass, M v, of 104,000, using constants suggested by Migliaresi and Fambri [4]. The FTIR spectra of dried lactic acid is shown in Figure 3 and characteristic absorption bands can be assigned to (1) alcohol O-H group stretch at 3400 cm -1, (2) carboxylic acid group C=O stretch at 1760 cm -1, O-H at 3000 cm -1, and C-O stretch at 1100 cm -1 and 1220 cm -1, and hydrocarbon group (3) C-H stretch at 2950 cm -1 and 2970 cm -1.[5] CONGRESSO BRASILEIRO DE ENGENHARIA E CIÊNCIA DOS MATERIAIS, 14., 2000, São Pedro - SP. Anais 58504

5 Figures 4 and 5 show the spectra of obtained poly(lactic acid)s, and it can be in both spectra the disappearance of the broad bands due to the alcohol hydroxyl group and acid hydroxyl group, although small band due to terminal groups can still be observed. The polymers obtained by using different catalysts still have differences in their FTIR spectra at the finger print region, and it can be attributed to differences in microstructure and crystallinity (Figure 5)[6]. The precipitated polymers are very crystalline, as it will be shown further on also by DSC analysis. The specific gravity of samples were 1,25 g/cm 3 and 1,28 g/cm 3 for the Poly(lactic acid)s by H 2 SO 4 and SnO, respectively, compared to 1,25 g/cm 3 in the literature.[7] Fig. 3 FTIR Spectra of Lactic Acid. Fig. 4 FTIR Spectra of Poly(lactic acid) polymerized by H 2 SO 4. CONGRESSO BRASILEIRO DE ENGENHARIA E CIÊNCIA DOS MATERIAIS, 14., 2000, São Pedro - SP. Anais 58505

6 SnO H 2 SO 4 Fig. 6 FTIR Spectra of Poly(lactic acid) polymerized by H 2 SO 4 and polymerized by SnO. The DSC analysis (Figures 7and 9) have shown in the first heating cycle of poly(lactic acid)s a broad range of melting temperature and polymodal curves characteristic of wide range of crystallite size with varied perfection, usually observed in semi-crystalline polymers with wide molar mass distribution. The peak range for melting was 60 C and 50 ºC for the poly(lactic acid)s by H 2 SO 4 and SnO, respectively. The glass transition, Tg, was not observed in the first heating cycle and its absence can be attributed to the high crystallinity of the poly(lactic acid)s. After quenching to ambient temperature and in a second heating cycle (Figures 8 and 10) the Tg s of the poly(lactic acid)s were observed along with a crystallization peak. The calculated Tg s were 46 ºC and 42 C for the poly(lactic acid)s by H 2 SO 4 and SnO, respectively, however the quenched samples presented this transition over 10 degree range. mw PLA 1-1o. heating Temperature ( C) Fig. 07 DSC of the first heating of Poly(lactic acid) polymerized by H 2 SO 4. CONGRESSO BRASILEIRO DE ENGENHARIA E CIÊNCIA DOS MATERIAIS, 14., 2000, São Pedro - SP. Anais 58506

7 mw PLA 1-2o. heating Temperature ( C) Fig. 08 DSC graph of the second heating of Poly(lactic acid) polymerized by H 2 SO 4. In these second heating cycles, also a cold crystallization peak was also observed around 108 ºC and 107 ºC for the poly(lactic acid)s by H 2 SO 4 and SnO, respectively. The areas observed for heat emission from cold crystallization and the subsequent melting peak are of comparable size and are a strong indication of the amorphous nature of the sample after quenching. Samples were also submitted to a third heating cycle, however results are in accordance to those observed in the previous heating. The observed melting temperatures at the peak were 133 and 130 for the poly(lactic acid)s by H 2 SO 4 and SnO, respectively. PLA 2-1st heating 5 0 mw Temperature (oc) Fig. 09 DSC graph of the first heating of Poly(lactic acid) polymerized by SnO. CONGRESSO BRASILEIRO DE ENGENHARIA E CIÊNCIA DOS MATERIAIS, 14., 2000, São Pedro - SP. Anais 58507

8 PLA 2-2nd heating mw Temperature (oc) Fig. 10 DSC graph of the second heating of Poly(lactic acid) polymerized by SnO. CONCLUSION The direct solution polymerization condensation of lactic acid under controlled condition of humidity and appropriate catalysts is a feasible low cost polymerization process for large scale preparation of the biodegradable poly(lactic acid), as demonstrated by the presented results. Further characterization and thermo-mechanical properties analysis are in progress. Improvements of polymerization conditions and also preparation of copolymers are under way in order to reach biodegradable polymers with a broader range of properties thermal and mechanical properties. ACKNOWLEGMENT Authors thank PURAC-SINTESE for donation of lactic acid and FAPESP (99/ ) for financial support. REFERENCES 1. Schneider, A.K., U.S.Patent to Ethicon Inc 2. Vert, M; Chabot, F; Leray, J.L.; Christel, P.; Macromolec. Chem.Supp. v.5, p. 30, Pitt, C.G.;Martsz, T. A.; Schindler, A., Controlled Release of Bioactive Materials, R.W. Baker, Editor, Academic Press, N. Y., p. 19, Migliaresi, C.; Fambri, L.; Macromol. Symp., v.123, p. 155, 1997 CONGRESSO BRASILEIRO DE ENGENHARIA E CIÊNCIA DOS MATERIAIS, 14., 2000, São Pedro - SP. Anais 58508

9 5. Silverstein,R. M.; Bassler, G. C.; Morril, T. C.; Spectrometric Identification of Organic Compounds, 4 th ed., John Wiley and Sons, NY, p , Urbanski, J.; Czerwinski, W.; Jamicka, K.; Majewska, F.; Zowall, H.; Handbook of Analysis of Synthetic Polymers and Plastics, Ellis Horwood Limited, Div. Of John Wiley and Sons; Warsaw, Poland, p , Brandrup, Ed.; Polymer Handbook, 3 rd Ed.; John Wiley and Sons, NY, p. VI/62, 1989 CONGRESSO BRASILEIRO DE ENGENHARIA E CIÊNCIA DOS MATERIAIS, 14., 2000, São Pedro - SP. Anais 58509