Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Treating polymer containing material or treating a solid...
Reexamination Certificate
2001-06-25
2003-03-04
Boykin, Terressa M. (Department: 1711)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Treating polymer containing material or treating a solid...
C528S481000
Reexamination Certificate
active
06528617
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a process for producing an aliphatic polyester excellent in stability having at least 50% of aliphatic hydroxycarboxylic acid units, which is a biodegradable polymer useful as a medical material or a substitute for general-purpose resins.
More precisely, the invention relates to a process for producing an aliphatic polyester excellent in stability, which comprises:
subjecting an aliphatic polyester containing at least 50% of an aliphatic hydroxycarboxylic acid unit, obtainable by solid-phase polymerization under a flowing gas in the presence of a volatile catalyst so as to have a desired weight average molecular weight within the range of 50,000 to 1,000,000, to a heat treatment at a temperature equal to or higher than the reaction temperature of the solid-phase polymerization under a flowing gas with maintaining the aliphatic polyester in a solid state.
BACKGROUND OF THE INVENTION
Associated with the environmental protection, waste disposal becomes a problem in these days. In particular, wastes of moldings and worked goods of general-purpose polymer materials are problematic in that, when buried for land reclamation, they will remain semi-permanently in the land as impurities, because they are lacking in ability to degrade or disintegrate by the action of microorganisms. In addition, additives such as plasticizer and others will be released out from them to pollute the environment.
Further, when the wastes are incinerated, there arise serious problems that the high combustion heat generated therefrom will damage furnaces and the exhaust fumes and gas from the combustion will cause air pollution, ozone layer destruction, global warming, acid rain, etc.
In that situation, there is an increasing demand for polymer materials which are tough but degradable when buried for land reclamation as wastes as well as produce low combustion heat that will not damage furnaces even when incinerated. However, polymer materials that satisfy the demand are not always available.
Polylactic acid which is one of aliphatic polyhydroxycarboxylic acids is highly transparent and tough, and has a characteristic that it is easily hydrolyzed in the presence of water. Therefore, when used as a general-purpose resin, it is friendly to the environment, since it is easily degraded without polluting the environment. In addition, when left in living bodies as a medical material, it is degraded :and absorbed in the bodies without harming the bodies after having accomplished its object as the medical material, and thus is gentle to living bodies. These excellent properties of polylactic acid have already been noticed prior to the present application.
Heretofore, the following technologies are disclosed as methods for modifying biodegradable polymers represented by polylactic acid and a copolymer of lactic acid and glycolic acid.
WO90/15629 discloses a technology of subjecting medical materials of a lactic acid polymer to a heat treatment at a temperature of 100° C. or more which is equal to or lower than the melting point of the lactic acid polymer with continuously discharging the gas in the system for at least 10 minutes. In that invention, the medical materials of the lactic acid polymer to be subjected to the heat treatment are in various forms such as filaments, strings, knittings, non-woven cloths, woven cloths, and moldings. The heat treatment affords medical materials of the lactic acid polymer having an improved stability in living bodies and a sufficient strength that endures for a long period of time. However, in that invention, although the stability of the shaped products is improved in the bodies, there is no disclosure of the technology for improving the stability of the polymer before the molding process. Actually in Examples, the molecular weight of the polymer remarkably decreased after its spinning as compared to that before the spinning.
Moreover, Japanese Patent Laid-Open No. 231688/1996 discloses a process for producing polylactic acid which comprises a first step of obtaining polylactic acid by melt-polymerization using lactide as main starting material, a second step of pelletizing the polylactic acid polymerized and formed in the first step, a third step of solid-phase polymerization of the polylactic acid pellets obtained in the second step at a temperature lower than the melting point, and a fourth step of sublimating the monomer which remained in the polymerization of the third step.
There are, descriptions in the publication that “lactide and decomposed products thereof remarkably decrease the glass transition temperature and melt viscosity of the polymer, and also remarkably deteriorate the ability of molding and working as well as thermal stability thereof” and “a polylactic acid having a glass transition temperature of 55° C. or higher is obtained by removal of the unreacted monomers (lactide, lactic acid oligomers) by sublimation”. Therefore, the improvement of thermal stability in the invention means the improvement of the glass transition temperature of polylactic acid, and this improvement is achieved by removal of lactide and lactic acid oligomers by sublimation. In Examples, there is disclosed polylactic acid wherein the content of lactide is reduced up to 5000 ppm by conducting the fourth step.
However, in the aliphatic polyester of the present invention, the content of lactide is 1000 ppm or less at the time before the heat treatment, and therefore the improvement of thermal stability resulted in by the heat treatment according to the invention is not achieved by removal of lactide.
Furthermore, when an extruded film is produced using polylactic acid containing much lactide, i.e., 5000 ppm or more, a lot of lactide vaporized is generated from the lip of the extruder to contaminate the film roll. This means that the roll should be cleaned regularly at the continuous production of the film. The inevitable cleaning shortens the continuous production period of time and renders the production inefficient. To the contrary, since polylactic acid before and after the heat treatment according to the present invention contains 1000 ppm or less of lactide, there is almost no possibility of contamination of the roll.
On the other hand, EP-953589A2 invented by the present inventors discloses a process for producing aliphatic polyesters having a weight average molecular weight of 50,000 to 1,000,000 and having at least 50% of an aliphatic hydroxycarboxylic acid unit, which comprises subjecting a crystallized, aliphatic polyester prepolymer having a weight average molecular weight of 2,000 to 100,000 and having at least 50% of an aliphatic hydroxycarboxylic acid unit, to solid-phase polymerization in the presence of a catalyst. It also describes the use of a volatile catalyst as the catalyst.
For obtaining an aliphatic polyester having a desired weight average molecular weight by solid-phase polymerization in the presence of a volatile catalyst, it is important to control the vaporization of the catalyst by the method, for example, controlling the flow rate of a flowing gas. However, in the case of using an organic sulfonic acid as the volatile catalyst, the aliphatic polyester containing the organic sulfonic acid in an amount of 50 to 300 ppm calculated as sulfur does not increase in weight average molecular weight through solid-phase polymerization, but has low stability.
Furthermore, EP-953589A2 also discloses a process of solid-phase polymerization with changing the reaction temperature, i.e., at two differentj temperatures raised stepwise. The polyactic acid obtained using an organic sulfonic acid as the catalyst causes decompositon when the reaction temperature is raised in the presence of the catalyst, whereby decomposed products are formed. Therefore, when polylactic acid containing such large amount of the organic sulfonic acid that the polymerization proceeds is obtained by solid-phase polymerization with the temperature raised, the yield of polyactic acid might be decreased through the acceleration of the decomposition.
Ajioka Masanobu
Hiraoka Shoji
Kotaki Yasushi
Ogawa Shinji
Sakai Masayuki
Boykin Terressa M.
Mitsui Chemicals Inc.
LandOfFree
Process for producing aliphatic polyester excellent in... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Process for producing aliphatic polyester excellent in..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Process for producing aliphatic polyester excellent in... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3078830