Polylactic acid fiber

Abstract: A polylactic acid fiber comprises a blend of poly-­ L-lactic acid and poly-D-lactic acid and is improved by spinning and then drawing.

Description: The present invention relates to a polylactic acid fiber having a high strength and a high thermal resistance, and more specifically to a novel polylactic acid complex fiber comprising a blend of 30 to 70 % by weight of poly-L-lactic acid and 70 to 30 % by weight of poly-D-lactic acid, which has physical properties incomparably superior to those of a conventional polylactic acid fiber. Polyglycolic acid and polylactic acid, which are aliphatic polyesters, are interesting in vivo degradable and absorbable polymers which undergo non-enzymatic hydrolysis in vivo to form glycolic acid and lactic acid, respectively, as degradation products which undergo metabolism in vivo. Polyglycolic acid is widely used clinically as an absorbable suture. Since it shows a high degradation and absorption rate in vivo, however, it cannot be used in a part where it is required to maintain its strength for more than several months. Meanwhile the formation of a fiber from polylactic acid and application thereof as an absorbable suture are also-under investigations [see B. Eling, S. Gogolewski, and A. J. Pennings, Polymer, 23 , 1587 (1982); US-A-3 531 561 and US-A-3 636 956. However, a polylactic acid fiber is unsatisfactory with respect to mechanical properties and thermal properties [see S. H. Hyon, K. Jamshidi, and Y. Ikada, "Polymers as Biomaterials", edited by Shalaby W. Shalaby, Allan S. Hoffman, Buddy D. Ratner, and Thomas A. Horbett, Plenum, N. Y., (1985)]. A blend of poly-L-lactic acid and poly-D-lactic acid useful as monofilaments of surgical sutures is disclosed in JP-A-61-36321. An object of the present invention is to provide a polylactic acid fiber having a high strength and a high melting point which are well over the mechanical properties (tensile strength: 686465,5 KPa (70 kg/mm 2 )or lower) and thermal properties (melting point: 180°C or lower) of the conventional polylactic acid. Under these circumstances the inventors of the present invention have made intensive investigations with a view to improving the physical properties of a polylactic acid fiber. As a result, they have completed the present invention. The above-mentioned object of the present invention has been accomplished by providing a polylactic acid fiber which comprises a blend of 70 to 30 % by weight of poly-L-lactic acid and 30 to 70 % by weight of poly-D-lactic acid, characterized in that said fiber has a tensile strength of at least 1827960 kPa (186.4 kg/mm 2 ) and a melting point of 243°C or more, said polylactic acids having a weight-average molecular weight of 20 000 to 1 000 000 and an optical purity of at least 90%. Poly-L-lactic acid and poly-D-lactic acid each of which is polylactic acid in its entity only differ from each other in optical activity. The invention provides a fibrous article for the medical use which is composed of the polylactic acid fiber as defined above. Moreover the invention provides a process for preparing a polylactic acid fiber, in which a blend of 30 to 70% by weight of poly-L-lactic acid and 70 to 30 % by weight of poly-D-lactic acid is spun to a fiber by a dry method and subsequently the spun fiber is drawn at a temperature of from 100 to 220°C at a draw ratio of at least 21, said polylactic acids having a weight-average molecular weight of 20 000 to 1 000 000 and an optical purity of at least 90%. The spinning may be conducted from a solution of the blend in a solvent. The spun fiber is drawn for improvement of its physical properties such as tensile strength. The weight-average molecular weights of poly-L-lactic acid and poly-D-lactic acid are determined by measurement of solution viscosities thereof. Those having a weight-average molecular weight of 20,000 to 1,000,000 are suitable. Where high mechanical properties are required, a polymer having a high weight-average molecular weight of 100,000 to 1,000,000 is preferably used. Where high degradation and absorption rates are required while giving priority to the degradation and absorption rates rather than the mechanical properties, poly-L-lactic acid or poly-D-lactic acid having a comparatively low weight-average molecular weight of 20,000 to 100,000 is preferably used and the use of poly-L-lactic acid and poly-D-lactic acid both having a weight-average molecular weight of 20,000 to 100,000 is more preferred. With respect to the optical purities of poly-L-lactic acid and poly-D-lactic acid, the higher, the better. However, an optical purity of 90 % or higher will suffice. A commercially available 90 % aqueous solution of poly-L-lactic acid was used as a starting material to be used in the present invention, while poly-D-lactic acid prepared by a fermentation method was used as another starting material. However, they are not limitative in working of the present invention. L-Lactide and D-lactide, which are monomers for obtaining polylactic acid, were synthesized in accordance with the method of Lowe (US-A-2,668,162). The specific rotatory power [a] (in dioxane at 25°C and 578 nm) of the obtained L-lactide was -260° while that of the obtained D-lactide was +260°. Polymerization of the lactide was carried out by the bulk ring-opening polymerization method. A series of commercially available ring-opening polymerization catalysts can be used in the polymerization. The inventors of the present invention used tin octanoate (0.03 wt.% based on the lactide) and lauryl alcohol (0.01 wt.% based on the lactide) as an example of the catalyst. The polymerization was conducted in a temperature range of 130 to 220°C. The specific rotatory powers of the obtained poly-L-lactic acid and poly-D-lactic acid were -147° and +147°, respectively, irrespective of the molecular weight. A specific example of production of a polylactic acid fiber according to the present invention will now be described. Poly-L-lactic acid and poly-D-lactic acid each having a weight-average molecular weight of 20,000 or higher is dissolved in a solvent. Poly-L-lactic acid and poly-D-lactic may be separately dissolved or simultaneously dissolved in the same vessel. However, it is preferred to respectively dissolve them in separate vessels and mix them just before spinning. This is because isomeric polymers having a comparatively low molecular weight of 20,000 to 100,000 are liable to form a complex with each other in a state of a solution so that the viscosity of a solution containing both of them increases in a short time after dissolution of them, resulting in gelation. The concentration of a solution may be adjusted according to the molecular weight of a polymers and the desired fineness of a fiber. It is preferably 1 to 50 wt.%, more preferably 5 to 20 wt.%. The blending ratio of poly-L-lactic acid to poly-D-lactic acid can be arbitrarily chosen according to the purpose, and is preferably 30 wt.%: 70 wt.% to 70 wt.%: 30 wt.%. A blending ratio of 1:1 is most preferred for forming a good polylactic acid complex fiber. In blending poly-L-lactic acid and poly-D-lactic acid, it is preferred to use polymers having the same molecular weights. However, a complex is formed even if polymers having different molecular weights are blended. The spinning method for producing a polylactic acid fiber is a dry process The polylactic acid. concentration of a spinning solution is suitably 1 to 50 wt.%. In the dry process, the temperature around a nozzle is preferably set in a range of 20 to 100°C according to the kind of solvent used, and the temperature in a drying cylinder is desirably set in a range of 40 to 120°C. Examples of organic solvents which can be used in dry spinning of a blend include chloroform, methylene chloride, trichloromethane, dioxane, dimethyl sulfoxide, benzene, toluene, xylene, and acetonitrile. The fiber thus obtained is drawn by a dry or wet hot drawing method. The drawing temperature is 100 to 220°C, preferably 120 to 200°C. In such a method, the fiber may be drawn by single or multiple stage drawing. In the present invention, however, multiple stage drawing is preferred. In the present invention, there can be obtained a polylactic acid fiber having a high tensile strength of 1827960 kPa (186.4 kg/mm 2 ) or higher. Thus, the fiber of the present invention is by far superior in mechanical properties to the conventional fiber. A polylactic acid complex is formed in the polylactic acid fiber of the present invention. Since an undrawn fiber and a fiber having a low draw ratio according to the present invention have a porous structure, application of them as a fiber for separation of a gas or a liquid is conceivable when they are used in the form of hollow fiber. It is also conceivable to use the fiber of the present invention as a medical fiber such as an absorbable suture, an artificial tendon, an artificial ligament, an artificial blood vessel, or a reinforcing material for bone plate or screw, which is used in vivo. Further, application of the fiber of the present invention as an industrial rope or fiber is conceivable. The polylactic acid complex fiber of the present invention can provide a fibrous material having improved physical properties in all fields of applications where the use of a homopolymer of poly-L-lactic acid or poly-D-lactic acid has heretofore be considered. [Examples] The following Examples will illustrate the polylactic acid complex fiber of the present invention. Examples 1 to 3 Spinning dopes were prepared by combinations of six kinds of poly-L-lactic acids and poly-D-lactic acids having different weight average molecular weights as shown in Table 1 at a blending ratio of 1:1 using chloroform as a solvent. Wet spinning and dry spinning were conducted by ejecting these dopes from a nozzle having an orifice diameter of 0.5 mm and a number of orifices of 10. Wet spinning was conducted by using a mixture of ethanol and chloroform (100:30 V/V) as a coagulating liquid at 50°C. Dry spinning was conducted by drying spun fibers using a drying cylinder having a length of 50 cm at 50°C at a spinning rate of 0.2 ml/min at a take-off rate of 1 m/min. Fibers spun by these methods were drawn in a silicone oil bath having a temperature of 120 to 200°C at various draw ratios. With respect to the obtained fibers, the tensile strength, elastic modulus, melting point, and heat of fusion were measured under the following measurement conditions. The results in the case of wet spinning are shown in Table 2, while those in the case of dry spinning are shown in Table 3. Tensile Strength and Elastic Modulus The measurement was made using Tensilon/UTM-4-100 manufactured by Toyo Baldwin K.K. at a pulling rate of 100 %/min at a temperature of 25°C and a relative humidity of 65 %. Meltina Point and Heat of Fusion They were measured by conducting thermometry in an atmosphere of a nitrogen gas using a Perkin-Elmer Model DSCI-B. The measurement was made using about 3 to 4 mg of a sample. The calibration of the temperature and the heat of fusion was made using indium having a high purity of 99.99 %. Comparative Examples 1 and 2 Spinning dopes were prepared from a 5 % chloroform solution of poly-L-lactic acid (weight-average molecular weight: 40.0 x 10 4 ) and a 5 % chloroform solution of poly-D-lactic acid (weight-average molecular weight: 36 x 10 4 ). Dry spinning was conducted under the same conditions as those of Examples without blending. Drawing of the obtained fibers was attempted in a silicone oil bath having a temperature of 170°C. The fibers were molten and could not be drawn. Accordingly, drawing was conducted at 160°C. The results of tests of the physical properties of the obtained fibers are shown in Table 4.

Claims: A polylactic acid fiber which comprises a blend of 30 to 70% by weight of poly-L-lactic acid and 70 to 30% by weight of poly-D-lactic acid, said polylactic acids having a weight-average molecular weight of 20000 to 1000000 and an optical purity of at least 90%, characterized in that said fiber has a tensile strength of at least 1827960 kPa (186.4 kg/mm 2 ) and a melting point of at least 243°C. A fibrous article for medical use, which is composed of the polylactic acid fiber as defined in claim 1. A process for preparing a polylactic acid fiber according to claim 1, characterised in that a blend of 30 to 70% by weight of poly-L-lactic acid and 70 to 30% by weight of poly-D-lactic acid, said polylactic acids having a weight-average molecular weight of 20000 to 1000000 and an optical purity of at least 90%, is spun to a fiber by a dry method and subsequently the spun fiber is drawn at a temperature of from 100 to 220°C at a draw ratio of at least 21.

Inventors: GEN SHOKYO, IKADA YOSHITO, GEN, SHOKYO, IKADA, YOSHITO
Applicants: DAICEL CHEM, DAICEL CHEMICAL INDUSTRIES, LTD.
Application year: 1988
Main classification: D01F6
IPC codes: D01F6, A61L17