Organic compounds -- part of the class 532-570 series – Organic compounds – Carbohydrates or derivatives
Reexamination Certificate
2000-10-16
2002-12-17
Nutter, Nathan M. (Department: 1711)
Organic compounds -- part of the class 532-570 series
Organic compounds
Carbohydrates or derivatives
Reexamination Certificate
active
06495679
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a starch ester wherein reactive hydroxyl groups in the same starch molecule have been replaced by acyl groups and groups derived therefrom (hereinafter referred to collectively as “acyl groups”). Further, the present invention relates to a starch ester preferable as a base polymer in biodegradable starch-based plastic processed articles.
As used herein, the plastic processed articles include molded processed articles and modified processed articles molding-processed or modified as a whole or partially with a plastic composition, and the molding processing includes injection molding, extrusion molding, inflation, T-die extrusion, calendering, compression molding (press molding), transfer molding, casting, laminating, vacuum molding, blow molding, foam molding, coating, flow casting, heat fusion, stretching etc. (see 5th Handbook of Chemistry, Applied Chemistry I, p. 773, Table 10.22, Mar. 15, 1995, compiled by the Japanese Society of Chemistry and published by Maruzen). Accordingly, the molded processed articles include not only molded articles having a three-dimensional form but also films, sheets, coated papers, etc. Further, the modified processed articles include not only papers, processed papers and non-woven fabrics, but also articles produced by adding starch-substituted derivatives as modifiers to papers, non-woven fabrics, etc., made of natural materials.
BACKGROUND ART
The basic method of modifying starch, associated with the present invention, is esterification (acylation), and the starch ester produced by this reaction has been known as low-substituted starch (starch ester) esterified in an aqueous reaction system (“Starch Science Handbook”, K. K. Asakura Shoten, p. 550).
With respect to high-substituted starch ester (esterified starch), a method of reacting an acid anhydride in pyridine by use of dimethyl amino pyridine or an alkali metal as a catalyst (“Starch Chemistry & Technology” authored by Whistler, published by Academic Press, pp. 332-336), a method of reacting an acid anhydride at a high temperature of 100° C. or more by use of an aqueous solution of an alkali metal hydroxide as a catalyst (Japanese National Publication No. 508,185/1993, and p. 73 in the March issue of Die Starke, 1972), and a method of reaction in a non-aqueous organic solvent (Japanese Patent Laid-Open No. 188,601/1996) are known.
With an increasing awareness of environmental problems in recent years, starch esters produced by the methods described above have been used in various biodegradable plastic materials. However, these materials, whether used alone for forming molded articles or films or in combination with various synthetic resins, require a general-purpose plasticizer (phthalate type or fatty ester type) in order to achieve workability (for example, injection workability, extrusion workability, stretchability, etc.) at the same levels as ordinary thermoplastic plastics (thermoplastic resin).
Even if produced using the plasticizer, products such as injection-molded articles hardly achieve impact strength at the same levels as with impact strength polystyrene (high impact polystyrene). It has also been difficult to achieve molded articles having an impact resistance of 1.8 kgf·cm/cm (17.64 J/m) or more in terms of Izod impact strength (ASTM D256:−23° C.).
Further, products such as inflation films have hardly achieved stretchability (tensile elongation) as good as that of polyethylene.
In particular, these tendencies become significant as the ratio of the starch ester in the plastic composition (plastic material) to be molded is increased.
Even if a biodegradable resin (biodegradable polymer) other than the starch ester is mixed in an attempt to improve the impact strength or tensile elongation of the starch ester, the desired improvement effects cannot be attained unless the content of the biodegradable resin is made to be higher than the content of the starch ester. As a result, such products cannot truly be said to be biodegradable plastics that are based on a starch ester.
Further, the phthalate or fatty ester type plasticizer described above is suspected of being a physiologically disturbing substance, which adversely affects vegetables, foods, and the growth of animals. Accordingly, one should avoid adding the plasticizer described above to biodegradable plastics that are to be disposed of in landfills, etc.
In view of the foregoing, an object of the present invention is to provide a starch ester which can be used as a thermoplastic material capable of being thermo-plasticized in the absence of a plasticizer or by using a small amount of a plasticizer.
Another object of the present invention is to provide a starch ester from which a thermoplastic plastic material having superior impact strength and tensile elongation can be easily prepared.
DISCLOSURE OF INVENTION
The present inventors made extensive study regarding the development of safe biodegradable plastics in the absence of a plasticizer or by using a small amount of a plasticizer, by use of starch which is an abundant raw material produced every year. The result of these studies is the novel starch ester having the constitution described below.
The present invention relates to a starch ester wherein reactive hydroxyl groups in the same starch molecule have been replaced by a C
2-4
acyl group (referred to hereinafter as “short-chain acyl group”) and a C
6-18
acyl group (referred to hereinafter as “long-chain acyl group”), and the extent of substitution by the short- and long-chain acyl groups are regulated so as to make the starch ester thermo-plasticized and moldable even in the absence of a plasticizer.
From the viewpoint of workability, the starch ester as used herein is preferably one having a glass transition point by differential thermal analysis (JIS K 7121: referred to hereinafter as “glass transition point”) of 140° C. or less, preferably 130° C. or less. The lower limit of the glass transition point shall be usually 80° C., preferably 100° C.
To easily attain each characteristic, a starch ester having the workability or showing the glass transition point as described above is preferably one wherein the degree of substitution by the long-chain acyl group is from 0.06 to 2.0, the degree of substitution by the short-chain acyl group is from 0.9 to 2.7, and the degree of substitution by the total acyl groups is from 1.5 to 2.95, more preferably one wherein the degree of substitution by the long-chain acyl group is from 0.1 to 1.6, the degree of substitution by the short-chain acyl group is from 1.2 to 2.1, and the degree of substitution by the total acyl groups is from 1.7 to 2.9.
The starch ester of the present invention can also be used in a starch ester-based polymer alloy by incorporating the starch ester with a biodegradable resin. Polycaprolactone, polylactic acid or cellulose acetate can be used particularly preferably as the biodegradable resin.
Further, the starch ester of the present invention can be formed into a molded processed article which has been molded and processed as a whole or partially with said starch ester or a polymer alloy having said starch ester incorporated with a biodegradable resin.
The molded processed article can be formed into an injection-molded article showing a degree of water absorption (after immersion in tap water at 23° C. for 24 hours) of 0.5% or less and an Izod impact strength of 1.8 kgf·cm/cm, or into a film having a film thickness of 100 &mgr;m or less and a tensile elongation (JIS K 6301) of 200% or more.
Further, the starch ester of the present invention can be formed into a plastic processed article which has been molded and processed, or modified, as a whole or partially with a plastic composition comprising an organic or inorganic reinforcing filler added to said starch ester or to a polymer alloy which is an admixture of the starch ester and a biodegradable resin.
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, the means of the present invention is described in detail. The blend unit is expressed on
Japan Corn Starch Co. Ltd.
Nutter Nathan M.
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