Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Mixing of two or more solid polymers; mixing of solid...
Reexamination Certificate
2000-03-28
2002-04-16
Mullis, Jeffrey (Department: 1711)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Mixing of two or more solid polymers; mixing of solid...
C525S09200D, C525S09200D, C525S09200D
Reexamination Certificate
active
06372844
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a novel resin composition wherein a resin additive itself has degradability. The resin composition of the present invention is a novel resin composition having improved wettability, and is especially useful as a degradable plastic or a bioabsorbable plastic which can be degraded under a natural environment or in a living body. Furthermore, the present invention relates to a method for accelerating the hydrolysis of a resin such as an aliphatic polyester, and a method for inhibiting its heat deterioration.
2. Description of the Related Art
Polyhydroxycarboxylic acids typified by polylactic acid (PLA), polyglycolic acid (PGA), polycaprolactone (PCL) and the like are utilized as biodegradable plastics which can be degraded by water, an enzyme and the like under a natural environment of in a living body.
Moreover, Japanese Application Laid-Open No. 64824/1987, for example, discloses a method for obtaining a lactic acid-glycolic acid copolymer (PLGA) useful as a base material for a slowly releasable agent and having a low molecular weight and a polydispersity by the ring-opening polymerization of glycolide (GLD) which is a cyclic dimer of glycolic acid and lactide (LTD) which is a cyclic dimer of lactic acid.
In recent years, as global environment is getting worse, much attention is increasingly paid to the recycling of resins and the use of additives which are safe for living bodies and less harmful to the global environment. In addition, with the diversification of consumer's needs, demands for biodegradable plastics such as polyhydroxycarboxylic acids also increase. For example, a phthalic ester-based plasticizer which is one of resin plasticizers is considered as an endocrine disrupting substance (an environmental hormone), and thus its safety is insufficient. Accordingly, it is proposed to restrict the use of the phthalic ester-based plasticizers, and for the replacement of these plasticizers, research has been conducted for safer resin additives.
For example, PLA is utilized as a material for disposable containers, packaging materials and the like owing to its good processability and the excellent mechanical strength of its molded products. However, PLA has a disadvantage that its degradation velocity is relatively slow under conditions (e.g., in sea water, soil or the like) other than in compost, and hence PLA can scarcely be used in a field where it is desired that PLA degrades and vanishes within several months.
Accordingly, in order to accelerate the hydrolysis of PLA, mixing a hydrophilic additive such as polyethylene glycol can also be conceived. However, PLA is less hydrophilic, and for this reason, it is scarcely compatible to a hydrophilic substance such as polyethylene glycol. Therefore, the additive tends to bleed out during molding or after molding, the mechanical strength of molded articles decreases, and appearance such as transparency is impaired. In consequence, such a conception is not practical.
So far as the inventors know, there has not been found yet, for example, a method which comprises adding an additive to an aliphatic polyester such as PLA to effectively accelerate degradability without remarkably impairing the properties of the aliphatic polyester (mechanical strength, appearance and the like).
On the other hand, also in the field of the slowly releasable agents, there is highly required a product that can slowly release an agent within a relatively short period of time. With regard to PLA, its degradation in a body is too slow, so that it remains in the boy for a long time after the agent has been released, which is not preferable. Therefore, a product having a high degradation velocity has been investigated, for example, by using PLGA of a low molecular weight instead of PLA. However, PLGA still is not all-purpose, and some problems are indicated, for example, as follows.
(1) PLGA is amorphous and has a glass transition temperature (Tg) of around 30 to 40° C., and therefore, a product containing it softens and adhesively melts in summer season.
(2) In manufacturing a product by adding a hydrophobic agent, an incorporation ratio of the agent (the content of the agent in the product) does not increase.
(3) Since PLGA is a copolymer, the scattering of quality occurs between lots in a polymer production process.
Accordingly, it would be an important contribution in the field of the slowly releasable agent to realize the acceleration of the biodegradation of a crystalline and hydrophobic homopolymer such as PLA. Namely, a method for accelerating the degradation of an aliphatic polyester is desired also in an application where it is used in a living body.
Furthermore, in the case of an aliphatic polyester such as PLA, especially a high-molecular weight polymer, it is known that the remarkable decrease of the molecular weight occurs by heating during molding. However, an effective method for inhibiting the heat deterioration of the aliphatic polyester has not been developed yet.
SUMMARY OF THE INVENTION
An objective of the present invention is to provide a novel resin composition wherein hydrolysis is accelerated and heat deterioration is inhibited.
Another objective of the present invention is to provide a method capable of blending an additive with a resin without separation and also capable of accelerating the hydrolysis of a resin such as an aliphatic polyester or inhibiting the heat deterioration of the same by the function of the additive.
As a result of intensive studies for achieving the above objectives, the present inventors have found that when a copolymer having a hydrophilic segment and a hydrophobic segment is mixed with a resin such as a polyester, they can be mixed without separation and also the wettability of the resin can be enhanced, whereby hydrolysis can be accelerated. Furthermore, they have also found that when the copolymer is mixed with a resin such as, especially, a polyhydroxycarboxylic acid, remarkable effects can be exerted. That is to say, a degradation rate can be increased and heat deterioration during heating can be inhibited without impairing the properties of the polyhydroxycarboxylic acid.
Namely, a first aspect of the present invention is directed to a resin composition comprising a block or a graft copolymer (A) having a polyamino acid as a hydrophilic segment (a-
1
) and a degradable polymer as a hydrophobic segment (a-
2
), and a resin (B).
Furthermore, a second aspect of the present invention is directed to a method for accelerating the hydrolysis of a resin which comprises the step of mixing 1 to 50% by weight of a copolymer (A) having a weight-average molecular weight of 1000 to 100000 with a resin (B) having a weight-average molecular weight of 3000 to 500000.
A third aspect of the present invention is directed to a method for inhibiting the heat deterioration of a resin.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
A resin composition of the present invention comprises a block or a graft copolymer (A) and a resin (B).
Typical examples of the resin (B) for use in the present invention includes, but are not limited to, the following resins including degradable resins.
1. Polyolefin-based resins
High density polyethylene, low density polyethylene, linear low density polyethylene, polypropylene, polyisopropylene, polyisobutylene, polybutadiene, and the like; homopolymers and copolymers synthesized from one or more of olefin monomers such as ethylene, propylene and butylene; copolymers with any other monomers; and mixtures thereof.
2. Polystyrene-based resins
Polystyrene, acrylonitrile-butadiene-styrene copolymer and the like; homopolymers and copolymers synthesized from one or more of styrene-based monomers; copolymers with any other monomers; and mixtures thereof.
3. Polycarbonates
Homopolymers and copolymers synthesized from one or more of polyoxymethylene, polybutylene terephthalate, polyethylene terephthalate, polyphenylene oxide and the like; copolymers with the any other monomers; and mixtures thereof.
4. Degra
Asou Yukiko
Shinoda Hosei
Tamatani Hiroaki
Burns Doane Swecker & Mathis L.L.P.
Mitsui Chemicals Inc.
Mullis Jeffrey
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