Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – From reactant having at least one -n=c=x group as well as...
Reexamination Certificate
2000-04-28
2001-10-30
Cooney, Jr., John M. (Department: 1711)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
From reactant having at least one -n=c=x group as well as...
C528S080000, C528S084000, C528S361000
Reexamination Certificate
active
06310171
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a resin composition for foam, which has biodegradability and is made from a principal raw material, lactic acid capable of contributing to the protection of global environment, said resin composition being used as a packing cushioning material.
BACKGROUND ART
A large amount of plastic foams have been used as packing and packaging materials based on lightweight properties, cushioning property and formability thereof, and are made of raw materials, for example, chemical products from petroleum, such as polystyrene (PS), polyolefin and the like. However, it is difficult to effect disposal of the plastic foams after use to cause the following serious social problems. That is, a burning furnace is damaged by high combustion calorie generated on incineration and, furthermore, the plastic foams are not degraded when buried under the ground and also occupy the space of the place for disposal due to large volume.
A harmful influence of the untreated waste foams to be exerted on the natural ecosystem, for example, contamination of rivers, seas, etc. has becoming more serious. Therefore, there has been developed a biodegradable resin, which is degraded in the ecosystem and exerts little influence on the global environment. For example, a polyhydroxybutyrate resin synthesized in the microorganism, or a polyester of an aliphatic glycol and an aliphatic carboxylic acid, or a polyester resin containing caprolactone as a principal component has been suggested. However, the former is inferior in purity because it is synthesized by microorganism, and is also drastically inferior in productivity and use thereof is limited.
The latter is surely superior in productivity because it is prepared from those, which are easily available in large amount at cheap price, such as petroleum natural gas. However, it is not suited for practical use as a biodegradable foamed resin because it is a crystalline resin and has a low grass transition temperature. Furthermore, since it is prepared from the petroleum natural gas and a carbon dioxide gas is newly added to a carbon dioxide gas system that is present on the surface of the globe when it is degraded. Therefore, it does not contribute to the inhibition of an increase in carbon dioxide gas. From a long-term point of view, the availability is lowered in future because its raw material source is limited, thereby making it impossible to substantially contribute to the protection of the global environment.
Furthermore, a polymer of glycolic or lactic acid as a biodegradable raw material has been obtained by ring-opening polymerization of glycolide or lactide and employed as medical fibers. However, since the crystallizability as a constituent feature for fiber-forming capability is imparted to a resin, the resin polymer has never been used widely as a packaging container or a cushioning material if it is not formed into a foam.
An object of the present invention is to provide an expandable resin composition which has not only biodegradability but also excellent productivity, i.e. an expandable resin composition which is degradable due to microorganism and causes less burden to the global environment on disposal after use, and which has high productivity and is suited for practical use.
To attain the object, the present inventors have studied intensively about essential conditions for a biodegradable resin having high expandability with respect to base polymers, additives for increasing the molecular weight, and additives used for foaming. As a result, they have found a biodegradable resin composition having enough productivity to be suited for practical use, thus completing the present invention.
DISCLOSURE OF THE INVENTION
That is, the present invention provides a resin composition having biodegradability and expandability, comprising a high-molecular weight polylactic acid having a melt viscosity of 0.01 to 50 in terms of a melt index value (MI), characterized in that said resin composition is prepared by compounding 0.1 to 5% by weight of at least one compound, which is selected from a polyisocyanate having an isocyanate group of not less than 2.0 equivalents/mol, an epoxy compound having an epoxy group of more than 2.0 equivalents/mol and an acid anhydride having an anhydrous carboxyl group of more than 2.0 equivalents/mol, with a prepolymer of lactic acid wherein a molar ratio of a L-isomer to a D-isomer is within a range from 95:5 to 60:40 or from 40:60 to 5:95, and reacting the resulting mixture.
BEST MODE FOR CARRYING OUT THE INVENTION
In view of the biodegradability as a first constituent feature, possible inhibition of an increase in carbon dioxide gas in the natural world, and enough productivity and cost to be suited for practical use, a polylactic acid resin prepared from lactic acid, as a raw material derived from starch of cereals such as corn, is preferred. However, since the crystallizability is required to those which are usually used as fibers, those composed exclusively of an L-isomer of optical isomers are used. To the contrary, the crystallizability must be reduced as possible to form a foam. The reason is that crystallization of a crystalline resin proceeds in the process of impregnating with a blowing agent, thereby to inhibit the expandability.
Accordingly, the polymer in the present invention is a substantially amorphous polylactic acid resin obtained by ring-opening polymerization of lactide, which is prepared by using lactic acid wherein a molar ratio of an L-isomer to a D-isomer is within a range from 95:5 to 60:40 or from 40:60 to 5:95. Those wherein the molar ratio of the L-isomer to the D-isomer exceeds 95:5 or less than 5:95 can not be used because of high crystallizability, low expansion ratio and un-uniform foaming. The molar ratio is preferably within a range from 90:10 to 60:40 or from 40:60 to 10:90.
On the other hand, the glass transition temperature (Tg) gradually decreases in accordance with the ratio of the L-isomer to the D-isomer, and reaches a minimum value when the ratio is 50:50. When the glass transition point decreases, the expandability and the heat resistance of the foam are lowered, which is not preferred. That is, the glass transition temperature is preferably not less than 50° C. To adjust the glass transition temperature within the above range, it is necessary to adjust the proportion of the D-isomer within a range of not more than 40 mol % or not less than 60 mol %, and preferably not more than 30 mol % or not less than 70 mol %.
With respect to increase of the molecular weight as the second constituent feature, the melt viscosity of a general ester biodegradable resin is from about 30 to 100 in terms of a melt index value (MI), and is from about 100,000 to 250,000 in terms of a molecular weight. Even in case of an amorphous resin, the melt viscosity is not enough to form a foam with a high degree of foaming, which has a high expansion ratio of not less than ten times.
As a means for increasing the molecular weight, a polymer resin can be produced directly from lactide by using a multifunctional alcohol having three or more functional groups. However, it becomes difficult to remove the resulting resin from the reaction vessel and the productivity is lowered. Accordingly, it is preferred that a resin is once removed at the stage where the molecular weight is slightly low and then the molecular weight is increased by the following procedure.
To increase the molecular weight, a method of mixing with a crosslinking agent (coupling agent) capable of reacting with a terminal carboxyl group, a hydroxy group and an ester bond in a molecular chain in a molten state and reacting the resulting mixture is generally used, and a compound having an isocyanate group, an epoxy group or an anhydrous carboxyl group is useful.
Among these compounds (compounds having an isocyanate group, an epoxy group or an anhydrous carboxyl group), a compound having a functional group, other than the isocyanate group, of more than 2.0 eq./mol is required to react more effectively. The compound
Kubo Takahiro
Naito Hiroshi
Nakae Tsunahiro
Onishi Katsumi
Yama Masahiro
Cooney Jr. John M.
Kanebo Limited
Morgan & Finnegan , LLP
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