| PATENT NUMBER | This data is not available for free |
| PATENT GRANT DATE | February 3, 1998 |
| PATENT TITLE |
Aliphatic polyester resin and method for producing same |
| PATENT ABSTRACT |
An aliphatic polyester resin having a .lambda. value representing the magnitude of non-linearity of elongational viscosity of 1.5 to 8.0, said .lambda. value being defined by the following formula (1): .lambda.=.lambda..sub.1 /.lambda..sub.0 ( 1) wherein .lambda..sub.0 denotes the elongational viscosity at the transition point and .lambda..sub.1 denotes the elongational viscosity when strain become twice that of a transition point, said transition point meaning a point between a linear region, i.e., the infinitesimal-deformation region, and a non-linear region, i.e., the large-deformation region. According to the present invention, an aliphatic polyester resin having excellent formability and forming stability as well as good biodegradability is provided. |
| PATENT INVENTORS | This data is not available for free |
| PATENT ASSIGNEE | This data is not available for free |
| PATENT FILE DATE | August 21, 1996 |
| PATENT CT FILE DATE | December 20, 1995 |
| PATENT CT NUMBER | This data is not available for free |
| PATENT CT PUB NUMBER | This data is not available for free |
| PATENT CT PUB DATE | June 27, 1996 |
| PATENT FOREIGN APPLICATION PRIORITY DATA | This data is not available for free |
| PATENT CLAIMS |
What is claimed is: 1. An aliphatic polyester resin having a .lambda. value representing the magnitude of non-linearity of elongational viscosity of 1.5 to 8.0, said .lambda. value being defined by the following formula (1): .lambda.=.lambda..sub.1 /.lambda..sub.0 ( 1) wherein .lambda..sub.0 denotes the elongational viscosity at a transition point and .lambda..sub.1 denotes the elongational viscosity when strain becomes twice that of the strain at the transition point, wherein the transition point means the point between a linear region (infinitesimal-deformation region) and a non-linear region (large-deformation region), wherein the aliphatic polyester resin is mainly constituted by an aliphatic glycol and an aliphatic dicarboxylic acid or anhydride, the resin being obtained by reacting a prepolymer having a weight-average molecular weight (Mw) of at least 20,000 with polyisocyanate, wherein the resin has a weight-average molecular weight (Mw) of at least 60,000 and is provided with long chain branched formation. 2. The aliphatic polyester resin of claim 1 wherein at least one part of the polyisocyanate is trimethylol propane.hexamethylene diisocyanate.adduct, cyclic hexamethylene diisocyanate trimer or hexamethylene diisocyanate.water.adduct. 3. The aliphatic polyester resin of claim 1 wherein the resin has a unit selected from the group consisting of ethylene glycol, propylene glycol, butanediol, 1,6-hexanediol, decamethylene glycol, neopentyl glycol and 1,4-cyclohexanedimethanol as an aliphatic glycol unit, and has a unit selected from the group consisting of oxalic acid, succinic acid, adipic acid, suberic acid, sebasic acid, dodecanoic acid, succinic anhydride and adipic anhydride as an aliphatic dicarboxylic acid unit. 4. The aliphatic polyester resin of claim 1 wherein the resin has a weight-average molecular weight (Mw) of at least 100,000. 5. An aliphatic polyester resin having a .lambda. value representing the magnitude of non-linearity of elongational viscosity of 1.5 to 8.0, said .lambda. value being defined by the following formula (1): .lambda.=.lambda..sub.1 /.lambda..sub.0 ( 1) wherein .lambda..sub.0 denotes the elongational viscosity at a transition point and .lambda..sub.1 denotes the elongational viscosity when strain becomes twice that of the strain at the transition point, wherein the transition point means the point between a linear region (infinitesimal-deformation region) and a non-linear region (large-deformation region), wherein the aliphatic polyester resin is mainly constituted by an aliphatic glycol and an aliphatic dicarboxylic acid or anhydride, the resin being obtained by reacting the glycol and the dicarboxylic acid with at least one polyfunctional compound selected from the group consisting of a tri- or more polyol, a tri- or more oxycarboxylic acid or anhydride and a tri- or more polycarboxylic acid or anhydride, wherein the resin has a weight-average molecular weight (Mw) of at least 20,000 and is provided with long chain branched formation. 6. The aliphatic polyester resin of claim 5 wherein the resin contains at least one member selected from the group consisting of trimethylol propane, glycerine and pentaerythritol. 7. The aliphatic polyester resin of claim 5 wherein the resin contains at least one member selected from the group consisting of trimesic acid, propane tricarboxylic acid, trimellitic anhydride, pyromellitic anhydride, benzophenone tetracarboxylic anhydride, cyclopentane tetracarboxylic acid, malic acid, citric acid and tartaric acid. 8. The aliphatic polyester resin of claim 5 wherein the resin has a unit selected from the group consisting of ethylene glycol, propylene glycol, butanediol, 1,6-hexanediol, decamethylene glycol, neopentyl glycol and 1,4-cyclohexanedimethanol as an aliphatic glycol unit, and has a unit selected from the group consisting of oxalic acid, succinic acid, adipic acid, suberic acid, sebasic acid, dodecanoic acid, succinic anhydride and adipic anhydride as an aliphatic dicarboxylic acid unit. 9. The aliphatic polyester resin of claim 1 wherein the resin is obtained from reactants consisting of the prepolymer and the polyisocyanate. 10. The aliphatic polyester resin of claim 5 wherein the resin is obtained from reactants consisting of the glycol, the dicarboxylic acid and the at least one polyfunctional compound selected from the group consisting of a tri- or more polyol, a tri- or more oxycarboxylic acid or anhydride and a tri- or more polycarboxylic acid or anhydride. 11. A method for producing an aliphatic polyester resin comprising blending 3 to 500 parts by weight of the aliphatic polyester resin in any one of claims 1 to 8 with 100 parts by weight of an aliphatic polyester resin having a .lambda. value indicating the magnitude of non-linearity of the elongational viscosity of 1.5 or less so that the .lambda. value becomes a specific value in a range of 1.5 to 8.0. 12. A method for producing an aliphatic polyester resin comprising blending 3 to 500 parts by weight of the aliphatic polyester resin in any one of claims 1 to 8 with 100 parts by weight of an aliphatic polyester resin having swell measured at 190.degree. C. of 40% or less so that the swell becomes a specific value in a range of 40 to 200%. -------------------------------------------------------------------------------- |
| PATENT DESCRIPTION |
TECHNICAL FIELD The present invention relates to an aliphatic polyester resin and a method for producing same having sufficient molecular weight for practical use and specific melt properties (excellent in melt tension and remarkable non-linear elongational viscosity characteristics). Particularly, the present invention relates to the aliphatic polyester resin and the method for producing it, with the resin having improved melt properties relative to prior resins as well as excellent forming stability, thickness uniformity and the like in any forming procedure. BACKGROUND ART Conventionally, although plastics are used in various industries, large amounts of plastic waste have the possibility of polluting rivers, oceans, and soil to become a great social problem. To prevent such pollution the development of biodegradable plastics has been desired; for example, poly(3-hydroxybutylate) produced by fermentation methods using microorganisms, blends of general-purpose plastics and starch, a naturally occurring polymer, and the like are already known. The former polymer has a drawback in that it is poor in molding properties because the polymer has a heat decomposition temperature close to its melting point and raw material efficiency is very bad because it is produced by microorganisms. On the other hand, since the naturally occurring polymer of the latter does not by itself have thermoplasticity, the polymer has defects in molding properties, and is greatly limited in its range of application. On the other hand, although it is known that aliphatic polyesters are biodegradable, they have hardly been used because molecular weight high enough to achieve a practical molded product cannot be obtained. Recently, it has been found that a ring-opening polymerization of .epsilon.-caprolactone produces a higher molecular weight polymer, and it has been proposed to use the polymer as a biodegradable resin. However, the resulting polymer is limited to only special applications because of a low melting point of 62.degree. C. and the high cost thereof. Therefore, some of the inventors of the present invention proposed high-molecular weight aliphatic polyesters having sufficient physical properties for practical use, for instance, in Japanese Patent Laid-Open Nos. 4-189823, 5-70579, 5-179016, and Japanese Patent Application No. 6-246445. These aliphatic polyester resins exhibit excellent biodegradability and physical properties having a high utility value. However, it has become apparent that, in comparison with popularly used resins such as polyethylene resins and polypropylene resins, the above-mentioned polyester resins still have the following problems that need to be improved: (i) moldability of the resins is sometimes impaired by drawdown in blow molding and in sheet thermoforming; (ii) the product loss sometimes increases in laminate molding and film casting because of an increase in neck-in; (iii) the stability of cells is occasionally impaired (due to open cells or open bubbles) to some extent in extrusion foam molding and bead foam molding using a chemical foaming agent and/or a volatile foaming agent; (iv) the stability of film-forming is sometimes impaired to some extent in inflation film forming, resulting in poor appearance of the rolled film; and (v) in stretch blow molding, molding becomes sometimes impossible because the thickness of products becomes uneven so that some holes occur during the stretching process. Further, the thickness uniformity and form stability of the aliphatic polyester resins should be improved. The following means may be used to overcome the foregoing problems: controlling the molecular weight of aliphatic polyester resins to an optimum value; elevating the temperature controlling level of molding machines; and improvement of devices for molding machines, such as improvement of air rings or stabilizing plates used for inflation film forming. However, by using these means it is difficult to obtain a large increase, e.g., 20 to 30%, in the molding rate. Conventional low density polyethylene is advantageously blended in polypropylene to improve the moldability thereof. However since the above kind of polyethylene is a non-biodegradable component, it is not preferable to mix such component with an aliphatic polyester resin even if the moldability of the resin is elevated. An object of the present invention is therefore to provide an aliphatic polyester resin and a method of producing same, which can solve the afore-mentioned problems and exhibit the following advantages: excellent melting characteristics, such as excellent moldability for various kinds of molding methods; sufficient physical properties for practical use; a small combustion calorific value in the case of disposal after use; and biodegradable by microorganisms, which means easy disposal. DISCLOSURE OF INVENTION The inventors of the present invention have investigated various kinds of polymerizing and manufacturing conditions to obtain a high molecular aliphatic polyester having sufficient physical properties for practical use and excellent melt properties, for instance, superior melt tension and large non-linearity in elongational viscosity. As a result, an aliphatic polyester resin having specific melt properties and a specific molecular range was obtained, which exhibits excellent characteristics for each of various kinds of molding methods while maintaining biodegradability. It was also found that the moldability of the resin was largely elevated in blow molding (including direct blow and stretching blow), sheet thermoforming, extrusion foam molding, bead foam molding, inflation film forming, laminate molding, casting film, and the like. From the above, the present invention was achieved. In other words, the present invention provides an aliphatic polyester resin having a .lambda. value representing the magnitude of non-linearity of elongational viscosity of 1.5 to 8.0, said .lambda. value being defined by the following formula (1): .lambda.=.lambda..sub.1 /.lambda..sub.0 ( 1) wherein .lambda..sub.0 denotes the elongational viscosity at a transition point and .lambda..sub.1 denotes the elongational viscosity when strain becomes twice that of a transition point, transition point meaning the point between a linear region (infinitesimal-deformation region) and a non-linear region (large-deformation region). Further, the present invention provides the aliphatic polyester resin in which swell measured at 190.degree. C. is 40 to 200%. Furthermore, the present invention provides the aliphatic polyester resin having a melt viscosity of 1.0.times.10.sup.3 -1.0.times.10.sup.6 poises at a temperature of 190.degree. C. and a shear rate of 100 sec.sup.-1, and having a melting point of 70.degree.-160.degree. C. Still further, the present invention provides the aliphatic polyester resin having a weight-average molecular weight of at least 20,000. Yet further, the present invention provides the aliphatic polyester resin consisting of an aliphatic glycol (including cyclic ring) and an aliphatic dicarboxylic acid and having a weight-average molecular weight (Mw) of at least 20,000. Further, the present invention provides the aliphatic polyester resin mainly constituted by the aliphatic glycol and the aliphatic dicarboxylic acid, the resin being obtained by reacting the glycol and the dicarboxylic acid with at least one polyfunctional compound selected from the group consisting of tri- or more polyol (or anhydride), tri- or more oxycarboxylic acid (or anhydride) and tri- or more polycarboxylic acid (or anhydride), and a resin having a weight-average molecular weight (Mw) of at least 20,000 provided with long chain branched formation. Furthermore, the present invention provides a aliphatic polyester resin containing an urethane bond of 0.03 to 3.0% by weight. Still further, the present invention provides the aliphatic polyester resin obtained by reacting 100 parts by weight of an aliphatic polyester prepolymer having a weight-average molecular weight of at least 20,000 and having a melting point of at least 60.degree. C. with 0.1 to 5 parts by weight of diisocyanate. Yet further, the present invention provides the aliphatic polyester resin having a repeated chain structure of the prepolymer through the urethane bond. Further, the present invention provides the aliphatic polyester resin in which one part or all of the aliphatic polyester resin has a repeated chain structure of the prepolymer through the urethane bond mainly constituted by the aliphatic glycol and the aliphatic dicarboxylic acid, the prepolymer being obtained by reacting the glycol and the dicarboxylic acid with at least one polyfunctional compound selected from the group consisting of tri- or more polyol (or anhydride), tri- or more oxycarboxylic acid (or anhydride) and tri- or more polycarboxylic acid (or anhydride), and the prepolymer having a weight-average molecular weight (Mw) of at least 20,000 provided with long chain branched formation. Furthermore, the present invention provides the aliphatic polyester resin having a unit selected from the group consisting of ethylene glycol, propylene glycol, butanediol, 1,6-hexanediol, decamethylene glycol, neopentyl glycol and 1,4-cyclohexanedimethanol as an aliphatic glycol unit, and having a unit selected from the group consisting of oxalic acid, succinic acid, adipic acid, suberic acid, sebasic acid, dodecanoic acid, succinic anhydride and adipic anhydride as an aliphatic dicarboxylic acid unit. Still further, the present invention provides the aliphatic polyester resin containing as tri- or tetra-functional polyol as the third component at least one selected from the group consisting of trimethylol propane, glycerine and pentaerythritol. Yet further, the present invention provides the aliphatic polyester resin containing as tri- or tetra-functional oxycarboxylic acid and/or tri- or tetra-functional polycarboxylic acid at least one selected from the group consisting of trimesic acid, propane tricarboxylic acid, trimellitic anhydride, pyromellitic anhydride, benzophenone tetracarboxylic anhydride, cyclopentane tetracarboxylic acid, malic acid, citric acid and tartaric acid. Further, the present invention provides the aliphatic polyester resin in which one part or all of the aliphatic polyester resin is mainly constituted by the aliphatic glycol and the aliphatic dicarboxylic acid, the resin being obtained by reacting the prepolymer having a weight-average molecular weight (Mw) of at least 20,000 with polyisocyanate, and the resin having a weight-average molecular weight (Mw) of at least 100,000 provided with longer chain branched formation. Furthermore, the present invention provides the aliphatic polyester resin in which the polyisocyanate is trimethylol propane.hexamethylene diisocyanate.adduct, cyclic hexamethylene diisocyanate trimer or hexamethylene diisocyanate.water.adduct. Still further, the present invention provides a method for producing the aliphatic polyester resin comprising blending 3 to 500 parts by weight of the aliphatic polyester resin with 100 parts by weight of an aliphatic polyester resin having .lambda. value indicating the magnitude of non-linearity of the elongational viscosity of 1.5 or less so that the .lambda. value becomes the specific value in a range of 1.5 to 8.0. Yet further, the present invention provides a method for producing the aliphatic polyester resin comprising blending 3 to 500 parts by weight of the aliphatic polyester resin with 100 parts by weight of an aliphatic polyester resin having swell measured at 190.degree. C. of 40% or less so that the swell becomes a specific value in a range of 40 to 200%. The present invention will be described below in further detail. The aliphatic polyester resin according to the present invention must have a .lambda. value indicating the magnitude of non-linearity of the elongational viscosity of 1.5 to 8.0. This aliphatic polyester resin will be further illustrated below. The aliphatic polyester resin of the present invention mainly consists of a polyester obtained by reacting two components of glycols and dicarboxylic acid (or acid anhydrides thereof), and if necessary as a third component, with at least one polyfunctional component selected from the group consisting of trifunctional or tetrafunctional polyols, oxycarboxylic acids, and polybasic carboxylic acids (or acid anhydrides thereof). The aliphatic polyester resin may be one which has hydroxyl groups at ends and which may be highly polymerized. Further, the aliphatic polyester may be reacted with a coupling agent so as to make it even higher molecular weight, which is preferable for enhanced toughness. In the present specification, the term "aliphatic polyester" sometimes means the absence of urethane bonds. It has been known to obtain polyurethane by reacting a low molecular weight polyester prepolymer having a number-average molecular weight of 2,000-2,500, which has hydroxyl groups as the terminal groups, with diisocyanate as a coupling agent in the preparation of rubber, foam, coatings and adhesives. However, the polyester prepolymers used in these polyurethane foams, coatings and adhesives are prepolymers having a low molecular weight and a number-average molecular weight of 2,000-2,500 which is the maximum that can be prepared by non-catalytic reaction. To obtain practical physical properties as the polyurethane, it is necessary that the content of diisocyanate should be as much as 10-20 parts by weight in relation to 100 parts by weight of this low molecular weight prepolymer. When such a large amount of diisocyanate is added to the low molecular weight polyester melted at 150.degree. C. or higher, gelation occurs so that normal resins which can be molded in the form of a melt cannot be obtained. Therefore, polyesters which are obtained by the reaction of a large amount of diisocyanate as a raw material for such low molecular weight polyester prepolymers cannot be used as the raw material for the various molding materials of the present invention. Also, as shown in the case of polyurethane rubber, although a method is conceivable in which hydroxyl groups are converted into isocyanate groups by the addition of diisocyanate, and then the number-average molecular weight thereof is further increased by using glycols, the same problem as mentioned above arises because 10 parts by weight of diisocyanate relative to 100 parts by weight of the prepolymer should be used in order to obtain practical physical properties. When a relatively high molecular weight polyester prepolymer is to be used, heavy metal catalysts required to prepare the prepolymer would promote the reactivity of the above-mentioned isocyanate groups, undesirably causing poor preservativity, and the generation of crosslinking and branching; hence a number-average molecular weight of not more than around 2,500 (corresponding to a weight-average molecular weight of about 5,000) of polyester prepolymers would be the limit if they were to be prepared without catalysts. The polyester prepolymers to obtain the aliphatic polyester resin used in the present invention are relatively high molecular weight saturated aliphatic polyesters having substantially hydroxyl groups at the ends thereof, weight-average molecular weights of at least 20,000, preferably at least 40,000, and melting points of 60.degree. C. or higher, which are obtained by reacting glycols and polybasic carboxylic acids (or acid anhydrides thereof) in the presence of catalysts. When a prepolymer having a weight-average molecular weight of lower than 20,000 is used, the small amounts, i.e. 0.1-5 parts by weight, of coupling agents used in the present invention cannot provide polyesters having good physical properties. When a polyester prepolymer having a weight-average molecular weight of 20,000 or higher is used, the use of small amounts of coupling agents even under severe conditions such as a molten state and the like can produce polymeric polyesters, without gelation. That is, the aliphatic polyester resin of the present invention may be a linear polymer, as one embodiment, in which the prepolymer consisting of the aliphatic glycol and aliphatic dicarboxylic acid, which has a weight-average molecular weight of 20,000 or more, preferably 40,000 or more is combined through the urethane bond derived from, for example, diisocyanate as a coupling agent. Further, in the present invention, the above prepolymer may have an extremely wide molecular weight distribution and/or branched long chains due to the polyfunctional components. This prepolymer may be reacted with, for example, a polyisocyanate as a coupling agent to obtain an aliphatic polyester resin having branched long chains combined through the urethane bonds. When oxazoline, epoxy compounds, and acid anhydrides are used as a coupling agent, the polyester prepolymer has a chain structure through the ester bond rather than urethane bonds. In addition, the aliphatic polyester of the present invention may be obtained by a catalytic reaction between glycol and polybasic acids (or anhydrides thereof) proceeding for long hours, such as 10 to 40 hours, more preferably, for 12 to 24 hours, and most preferably, for 16 to 20 hours. The thus-obtained aliphatic polyester has terminal groups substantially comprising hydroxyl group and is relatively high-molecular. The weight-average molecular weight of the aliphatic polyester is 20,000 or more and, more preferably, 40,000 or more. The resin may be a saturated aliphatic polyester having a melting point of 60.degree. C. or more (the foregoing reaction being carried out without using coupling agents). This aliphatic polyester has a significantly broad molecular-weight distribution and/or can possess a long-chain branch due to a polyfunctional component. The aliphatic polyester can be used alone. Further, a mixture of the aliphatic polyesters having different components may also be used. Furthermore, the aliphatic polyester may be used as one component of a composition including linear polymers. The molecular weight of the aliphatic polyester having the above long-chain branched structure may be further raised by the foregoing coupling reaction, if required. The components used in the present invention will be discussed below in further detail. Examples of aliphatic glycols which can be used to produce the aliphatic polyester resin of the present invention include ethylene glycol, propylene glycol, butanediol, 1,6-hexanediol, decamethylene glycol, neopentyl glycol, 1,4-cyclohexanedimethanol and the like. Ethylene oxides may also be used. These glycols can be used in combination. Examples of aliphatic dicarboxylic acids or anhydrides thereof include oxalic acid, succinic acid, adipic acid, suberic acid, sebasic acid, dodecanoic acid, succinic anhydride, adipic anhydride and the like. Generally, these aliphatic dicarboxylic acids are commercially available and they are useful for the present invention. The aliphatic dicarboxylic acids (or anhydrides thereof) can be used in combination. (Third component) To these aliphatic glycols and aliphatic dicarboxylic acids, a third component comprising at least one polyfunctional component selected from the group consisting of trifunctional or tetrafunctional polyols, oxycarboxylic acid, and polybasic carboxylic acids (or acid anhydrides thereof) may be added if necessary. The addition of this third component, which causes the branching of long chains and which, in its elongational viscosity behavior, shows a non-linear range following the linear range, can impart desirable properties including moldability because the ratio of weight-average molecular weight (MW)/number-average molecular weight (Mn), i.e., the molecular weight distribution, increases with increases in its molecular weight. In terms of the amount of polyfunctional components to be added without fear of gelation, a trifunctional component of 0.1 to 2 mol %, preferably 0.1 to 1.8 mol %, more preferably 0.1 to 1.5 mol % or a tetrafunctional component of 0.1 to 1 mol %, preferably 0.1 to 0.8 mol % is added relative to 100 mole % of the total of aliphatic dicarboxylic acid (or acid anhydride thereof) components. With less than 0.1 mol %, effects represented by the moldability do not appear. Further, with more than 2 mol %, gelation components increase and practicability is remarkably reduced. The added amount is dependent upon desirable molding methods in the above ranges. (Polyfunctional components) Examples of polyfunctional components as the third component include trifunctional or tetrafunctional polyols, oxycarboxylic acids, and polybasic-carboxylic acids. The trifunctional polyols representatively include trimethylol propane, glycerin or anhydrides thereof. The tetrafunctional polyols representatively include pentaerythritol. The trifunctional oxycarboxylic acid components are divided into the two types of (i) a component which has two carboxyl groups and one hydroxyl group in one molecule, and (ii) another component which has one carboxyl group and two hydroxyl groups in one molecule. Malic acid which has two carboxyl groups and one hydroxyl group in one molecule becomes practical and sufficient for the purposes of the present invention. The tetrafunctional oxycarboxylic acid components are the following three types of components: (i) A component which has three carboxyl groups and one hydroxyl group in one molecule; (ii) Another component which has two carboxyl groups and two hydroxyl group in one molecule; and (iii) The remaining component which has three hydroxyl groups and one carboxyl group in one molecule. Any type can be used, though in view of commercial availability at low cost, citric acid and tartaric acid are practical and sufficient for the purposes of the present invention. As a trifunctional polybasic carboxylic acid (or acid anhydride thereof) component trimesic acid, propane tricarboxylic acid and the like can be used. Among them, trimesic anhydride is practical for the purposes of the present invention. As a tetrafunctional polybasic carboxylic acid (or anhydride thereof) various types of aliphatic compounds, cycloaliphatic compounds, aromatic compounds and the like, described in certain publications, can be used. In terms of commercial availability, pyromellitic anhydride, benzophenone tetracarboxylic anhydride and cyclopentane tetracarboxylic anhydride for example are practical and sufficient for the purposes of the present invention. These glycols and dibasic acids mainly consist of aliphatic series, while small amounts of other components, for example, aromatic series may be concomitantly used. These other components may be blended or copolymerized in amounts up to 20% by weight, preferably up to 10% by weight, and more preferably up to 5% by weight because using these compounds degrades biodegradability. The polyester prepolymer or the aliphatic polyester used in the present invention has hydroxyl groups at the terminals. To introduce the hydroxyl groups, it is necessary that glycols be used somewhat excessively. For preparation of the polyester prepolymer or the aliphatic polyester having a relatively high molecular weight, it is necessary to use deglycol-reaction catalysts in the deglycol reaction subsequent to the esterification. The deglycol reaction may be conducted under highly reduced pressure at 5 mmHg or less, preferably 1 mmHg or less in the presence of catalysts. Examples of the deglycol-reaction catalysts include titanium compounds such as acetoacetoyl type titanium chelate compounds and organic alkoxy titanium compounds and the like. These titanium compounds can be used in combination. Examples of compounds used in combination include diacetoacetoxy oxytitanium (Nippon Chemical Industry Co., Ltd.; Nursem Titanium) tetraethoxy titanium, tetrapropoxy titanium, tetrabutoxy titanium and the like. The amount of the titanium compound used is 0.001-1 part by weight, and preferably 0.01-0.1 part by weight relative to 100 parts by weight of the polyester prepolymer. These titanium compounds may be blended before the esterification, or may be blended immediately before the deglycol reaction. To the polyester prepolymer are added coupling agents in order to increase its number-average molecular weight. Examples of the coupling agents include polyisocyanate, oxazoline, diepoxy compounds, acid anhydrides and the like. Di- or tri-isocyanate is particularly preferred because little gelation occurs. In the cases of oxazoline and diepoxy compounds, it is necessary that the terminal hydroxyl groups are reacted with acid anhydrides and the like to convert them into carboxyl groups, then coupling agents are used. Although not limited, examples of diisocyanate include 2,4-tolylene diisocyanate, a mixture of 2,4-tolylene diisocyanate and 2,6-tolylene diisocyanate, diphenylmethane diisocyanate, 1,5-naphthylene diisocyanate, xylylene diisocyanate, hydrogenated xylylene diisocyanate, hexamethylene diisocyanate, isophorone diisocyanate and the like. Particularly, hexamethylene diisocyanate is preferably used in terms of prepared resin hue, reactivity at the time of blending polyesters, and the like. The adding amounts of these coupling agents are 0.1-5 parts by weight, and preferably 0.5-3 parts by weight relative to 100 parts by weight of the prepolymer. Addition of less than 0.1 part by weight causes insufficient coupling reaction, whereas with more than 5 parts by weight gelation tends to occur. Further, in order to elevate the number-average molecular weight and the weight-average molecular weight, and introduce relative long chain branches (hereinafter referred to as LCB) into the polymer, the following polyfunctional coupling agents may be used as part of the foregoing coupling agents or in substitute thereof. Preferably, tri-functional isocyanate and tetra-functional isocyanate, in particular tri-functional isocyanate, may be used as a polyfunctional coupling agent. Although there is no particular restriction to the kind of tri-functional isocyanate, for instance, trimethylolpropane.hexamethylenediisocyanate.adduct, cyclic hexamethylenediisocyanate trimer or hexamethylene diisocyanate.water.adduct are exemplified. In addition, methyl acetate, hexane, heptane, toluene, xylene and the like may be employed as a diluent where these polyfunctional coupling agents are used. For instance, an aliphatic polyester resin of the LCB type can be obtained as follows: 1 mole of succinic acid and 1 to 1.1 moles of 1.4-butanediol are esterified and deglycolized to produce prepolymers having a weight-average molecular weight of 20,000 or more, and preferably, 40,000 or more; 100 parts by weight of the thus-obtained prepolymers are reacted with 0.1 to 1 part by weight of diisocyanate to elevate the weight-average molecular weight to 50,000 or more, and then, further reacted with 0.1 to 4 parts by weight of tri-functional isocyanate to increase the weight-average molecular weight to 100,000 or more. If the amount of the tri-functional isocyanate is less than 0.1 part by weight, the effect thereof is small. Whereas, if the amount is more than 4 parts by weight, gels can undesirably mix readily in the resultant aliphatic polyester resin. The addition is preferably performed when the prepolymer is in a uniformly melted state under easily stirrable conditions. Although it is not impossible for the coupling agents to be added to the prepolymer in the solid state and melted and mixed through an extruder, adding the agents in a polyester preparation unit, or adding them to the prepolymer in a melted state (for example, in a kneader) is more practical. The thus-obtained aliphatic polyester resin of the present invention is required to have a .lambda. value in a range of from 1.5 to 8.0, which .lambda. value represents non-linearity in elongational viscosity. By setting the .lambda. value in the foregoing range, the aliphatic polyester resin of the present invention can be preferably applied to various kinds of molding machines generally used for thermoplastic resin. By investigating the appropriate melt properties specific for each of the molding methods it becomes apparent that problems occurring in conventional techniques are avoidable if the .lambda. value is maintained in a range of from 1.5 to 8.0. Thus, the .lambda. value can be set in a wide range such as above, a surprising and unexpected fact, considering the prior art. The .lambda. value is represented by the following formula (1): .lambda.=.lambda..sub.1 /.lambda..sub.0 ( 1) wherein .lambda..sub.0 denotes an elongational viscosity at the transition point and .lambda..sub.1 denotes an elongational viscosity when strain becomes twice that of at the transition point. In the above meaning, the transition point is a point between a linear region, i.e., the infinitesimal-deformation region, and a non-linear region, i.e., the large-deformation region. The following elongational viscosity was measured by a monoaxial tensile stress meter (Melten Rheometer) manufactured by Toyo Seiki Co., Ltd. such that uniform strands, as samples, with a diameter of 2.0 to 6.0 mm were prepared at a setting temperature of 190.degree. C. by using a capillary rheometer manufactured by Toyo Seiki Co., Ltd., and then, subjected to measurement at a strain rate of 0.1 sec.sup.-1 and at a temperature 30.degree. C. higher than the melting point of the samples estimated by DSC. Practically, the transition point was determined as a separation point of the linear region and the non-linear region by comparison with a line obtained from measurement at a strain rate of 0.03 sec.sup.-1, as is shown in FIG. 1. Further before the measurement, the samples were sufficiently preheated in a silicone oil so as to remove residual strain. After setting the samples to a roller, the roller was slightly rotated to remove surface waviness of the samples before starting measurment. The preferable .lambda. value is in a range of from 1.5 to 8.0 and, in particular, from 1.8 to 7.5, as described above. When the .lambda. value is less than 1.5, sufficient strain hardening cannot be obtained, thus the resultant resin exhibits inferior uniformity in thickness and insufficient molding stability. Meanwhile, if the value exceeds 8.0, gelation, fish-eyes or the like frequently appear and, in the worst case, the melt flow of the resin is reduced so that molding becomes impossible. However, the optimum range varies depending on the kinds of molding methods, as shown below. The following examples are the preferable .lambda. value for each of the molding methods: For gas foam molding of which the expansion ratio is medium to high, i e., 4 to 80, the preferable .lambda. value is in a range of from 3.0 to 8.0 and, in particular, from 4.5 to 8.0. When the .lambda. value is less than 3.0, the stability and uniformity of cells may become inferior, particularly in the case of a high expansion ratio of more than 10. Meanwhile, if the .lambda. value exceeds 8.0, the extrusion characteristics worsen, thus impairing the molding stability. For foam molding in which the expansion ratio is small, i.e., 1.1 to 5, and in which a chemical foaming agent is mainly employed, the preferable .lambda. value is in a range of from 2.5 to 8.0 and, in particular, from 2.5 to 7.0. When the .lambda. value is less than 2.5, the stability and uniformity of cells may become inferior. Meanwhile, if the .lambda. value exceeds 8.0, the extrusion characteristics may become worse, thus impairing the molding stability and cost. In bead foam molding, the preferable .lambda. value is largely affected by the final expansion ratio and is in a range of from 1.5 to 8.0 and, in particular, from 2.0 to 7.0. When the .lambda. value is less than 1.5, the stability and uniformity of cells become inferior. Meanwhile, if the .lambda. value exceeds 8.0, cell-cracks occur due to gelation, etc., and further, the fusing characteristics tend to be inferior at the time of mold-foaming. In extrusion laminate molding, the preferable .lambda. value is in a range of from 3.0 to 7.0 and, in particular, from 4.5 to 6.5. When the .lambda. value is less than 3.0, neck-in may increase or both sides of the laminated film may become thicker. Meanwhile, if the .lambda. value exceeds 7.0, gelation and fish-eyes may occur frequently, impairing appearance and printability. In T-dye film forming, the preferable .lambda. value is in a range of from 2.0 to 7.0 and, in particular, from 2.5 to 6.5. When the .lambda. value is less than 2.0, neck-in may increase. Meanwhile, if the .lambda. value exceeds 7.0, gelation and fish-eyes occur easily, impairing the appearance and printability. In inflation film forming, the preferable .lambda. value is in a range of from 1.5 to 7.0 and, in particular, from 1.7 to 5.0. When the .lambda. value is less than 1.5, the thickness distribution becomes larger, and further, the film-forming stability is impaired, resulting in lumps, surface waviness, thick portions and the like. Meanwhile, if the .lambda. value exceeds 7.0, gelation and fish-eyes may occur, impairing appearance and printability. In addition, the cost thereof becomes somewhat undesirable. For blow molding, the preferable .lambda. value is in a range of from 1.5 to 7.0 and, in particular, from 2.5 to 7.0, though the value varies to some extent depending on the size of the articles. When the .lambda. value is less than 1.5, drawdown readily occurs due to a shortage of melt tension. Further the thickness uniformity after blowing is also impaired. Meanwhile, if the .lambda. value exceeds 7.0, gelation and fish-eyes may occur, impairing the appearance and printability. In addition,-the cost thereof becomes somewhat undesirable. Resins used for medium or large articles preferably have a larger .lambda. value within the foregoing range, as compared with the small articles. For stretch blow molding, the preferable .lambda. value is in a range of from 1.5 to 6.0 and, in particular, from 1.5 to 5.0. When the .lambda. value is less than 1.5, thick portions are readily produced during longitudinal stretching by a rod and blowing up, and in the worst case, holes occur making blowing up impossible, thus providing no articles. Meanwhile, if the .lambda. value exceeds 6.0, cost becomes somewhat undesirable. For sheet forming (vacuum forming), the preferable .lambda. value is in a range of from 2.0 to 7.0 and, in particular, from 2.5 to 7.0. When the .lambda. value is less than 2.0, noticeable sagging occurs during vacuum (thermo) forming, and further, if vacuum forming machines with a width of 1040 mm, as commonly used in Japan, are employed, unevenness in the finished articles become so large that excellent products cannot be obtained in some cases, depending on the shape thereof. Meanwhile, ii the .lambda. value exceeds 7.0, gelation and fish-eyes occur, thus sometimes impairing the appearance and, in the worst case, causing holes. Further, a method of increasing molecular weight may be employed at the same time to decrease sagging, as long as the extrusion characteristics are not impaired. For manufacturing a biaxially oriented film by a tenter and inflation method, the preferable .lambda. value is in a range of from 2.0 to 7.0 and, in particular, from 2.5 to 6.0. When the .lambda. value is less than 2.0, film-cracks may readily occur due to nonuniform extension during stretching. Meanwhile, if the .lambda. value exceeds 7.0, gelation and fish-eyes may occur, sometimes causing film-cracks and impairing extension. In addition, this can cause inferior printability. The aliphatic polyester resin of the present invention is mainly characterized in that the .lambda. value thereof is within a specific range as above. However, if the swell value thereof is considered as the second melt property, a resin with another level of improved molding properties can be attained. The swell value described below is determined as follows: a 2.0 cm sample of flow from a melt indexer for the MFR measurement specified by JIS K6760 at 190.degree. C. under a load of 2.16 kgf is cut and the diameter measured at a point 5.0 mm from the bottom end. Swell is calculated from the following equation: Swell={(diameter of sample-2.095)/2.095}.times.100 The swell range is preferably 40 to 200%, and more preferably 45 to 150% in the aliphatic polyester resins in accordance with the present invention. A swell of less than 40% sometimes causes poor thickness uniformity or poor molding stability, whereas a swell exceeding 200% causes gelation and fish eye formation, resulting in difficult molding due to decreased melt flowability in severer cases, thus resulting in economical disadvantages. However, there is an optimum range for most molding methods just like with the .lambda. value. Examples of preferable swell values in various molding methods are as follows: The swell value range is preferably 50 to 200%, and more preferably 60 to 180%, in gas foaming molding. A swell of less than 50% causes unstable and nonuniform cells. Further, when a foamed board article having a thickness over 5 mm is produced, a resin having a relatively high swell value within the range is preferably used. When a swell exceeds 200%, stable molding cannot be achieved due to poor extruding characteristics. In low expansion ratio foam molding (expansion ratio: 1.1 to 5.0 times) mainly using chemical foaming agents, the swell is preferably 40 to 120%, and more preferably 40 to 100%. A swell of less than 40% causes unstable and nonuniform cells, whereas a swell exceeding 120% sometimes causes economical disadvantages, as well as poor extrusion characteristics and unstable molding although depending on the screw shape of the molding machine. In bead foam molding, the moldability greatly depends on its final expansion ratio, and the swell preferably ranges from 40 to 150%, and more preferably from 40 to 120%. A swell of less than 40% causes unstable and nonuniform cells, whereas a swell over 150% may cause open bubbles probably due to gelation. In extruding laminate molding, the swell preferably ranges from 40 to 200%, and more preferably from 60 to 150%. When the swell is less than 40%, neck-in is noticeable, i.e., both ends of the laminate film become thick, occasionally resulting in increased production loss. On the other hand, since a swell over 200% causes large amounts gelation and fish eye formation in the film appearance and printability are deteriorated. In T-die film forming, the swell preferably ranges from 40 to 150%, and more preferably from 40 to 100%. When the swell is less than 40%, neck-in is noticeable, i.e., both ends of the laminate film become thick. On the other hand, since a swell over 150% causes gelation and fish eye formation in large amounts, the film appearance and printability are deteriorated. In inflation film forming, the swell ranges from 40 to 100%, and preferably from 40 to 80%. When the swell is less than 40%, the thickness uniformity is deteriorated and the film cannot be stably formed. Thus, the molding speed cannot be increased. On the other hand, a swell over 100% impairs physical properties, such as tear strength, as well as increases cost. In blow molding, the swell preferably ranges from 40 to 200%, and more preferably from 50 to 150%, although it depends on the size of the molded article. When the swell is less than 40%, some drawdown may occur probably due to its decreased melt tension. Also thickness uniformity after blow up may not be secured in some cases. On the other hand, since the swell over 200% causes large amounts gelation and fish eye formation film appearance and printability are deteriorated, with economical disadvantages. When an article having a larger size is molded, a resin having a larger swell value is preferably used. In stretching blow molding, the swell preferably ranges from 40 to 120%, and more preferably from 40 to 100%. A swell of less than 40% may form holes and nonuniform thickness during the longitudinal stretching from a rod and blow up. A swell over 100% is somewhat uneconomic. When a biaxially stretched film is produced using a tenter or inflation method, the swell preferably ranges from 40 to 100% and more preferably from 45 to 80%. When the swell is less than 40%, the cells are readily ruptured during stretching probably due to nonuniform stretching, resulting in decreased productivity. On the other hand, swell over 100% may cause economical disadvantages, increased gelation and fish eyes, cell rupture, and deteriorated appearance and printability. When the melt viscosity is taken account of as the third melt property in the present invention, an aliphatic polyester resin having a further improved moldability can be provided. Although the melt viscosity varies in accordance with desired molding method and applications, it is preferable to have a melt viscosity of 1.0.times.10.sup.3 -1.0.times.10.sup.6 poise, preferably 5.0.times.10.sup.3 -5.0.times.10.sup.5 poise, and more preferably 7.0.times.10.sup.3 -1.0.times.10.sup.5 poise at a temperature of 190.degree. C. at a shear rate of 100 sec.sup.-1. If the melt viscosity is less than 1.0.times.10.sup.3 poise, it is difficult to applied to some types of molding methods due to its low viscosity. Further, with more than 1.0.times.10.sup.6 poise, extrudability is lowered due to the high viscosity which might cause poor moldability in practical. The melt viscosity at a shear rate of 100 sec.sup.-1 was calculated from a graph which shows the relationship between the apparent viscosities and the shear rates measured by a capillary rheometer using a nozzle having a diameter of 1.0 mm and L/D of 10 at a resin temperature of 190.degree. C. The melting point of the aliphatic polyester resin used in the present invention is preferably 70.degree. to 160.degree. C., more preferably 80.degree. to 150.degree. C., especially 80.degree. to 140.degree. C. A melting point lower than 70.degree. C. will give the resin poor heat resistance, whereas at higher than 160.degree. C. it is difficult to carry out molding and biodegradability becomes poor. To achieve a melting point higher than 70.degree. C. the prepolymer needs to have a melting point of at least 60.degree. C. When urethane bonds are contained in the aliphatic polyester resin used in the present invention, the amount of urethane bonds is preferably 0.03 to 3.0% by weight, more preferably 0.1 to 2.0% by weight, and most preferably 0.5 to 1.5% by weight. The amount of urethane bonds is measured by .sup.13 C NMR, showing good correlation with the charged amount. Less than 0.03% by weight of urethane bonds has little effect on polymerization and leads to poor molding properties, whereas more than 3% by weight causes gelation. Further, the combustion heat generated from the aliphatic polyester resin of the present invention and molded articles thereof is 6,000 cal/kg or less, which is lower than those of polyethylene and polypropylene, thus facilitating incineration thereof where the resins are not processed by biodegradation. It is needless to say that when the above-mentioned aliphatic polyester resin is used, antioxidants, thermal stabilizers, UV absorbers as well as lubricants, waxes, coloring agents, crystallizing promoters and the like can be used concomitantly if necessary. That is, antioxidants include hindered phenol antioxidants such as p-tert-butyl hydroxytoluene and p-tert-butyl hydroxyanisole, sulfur antioxidants such as distearyl thiodipropionate and dilauryl thiodipropionate, and the like; heat stabilizers include triphenyl phosphite, trilauryl phosphite, tris-nonylphenyl phosphite and the like; UV absorbers include p-tert-butyl phenyl salicylate, 2-hydroxy-4-methoxybenzophenone, 2-hydroxy-4-methoxy-2'-carboxybenzophenone, 2,4,5-trihydroxybutylophenone and the like; lubricants include calcium stearate, zinc stearate, barium stearate, sodium palmitate and the like; antistatic agents include N,N-bis(hydroxyethyl) alkyl amine, alkyl amine, alkyl allyl sulfonate, alkyl sulfonate and the like; flame retarders include hexabromocyclododecane, tris-(2,3-dichloropropyl) phosphate, pentabromophenyl allyl ether and the like; anti-blocking agents include the combination of inorganic fillers such as silica and oleamide and the like; inorganic fillers or nucleating agents include calcium carbonate, silica, titanium oxide, talc, mica, barium sulfate, alumina, mixture of NaHCO.sub.3 and citric acid and the like; crystallizing promoters include polyethylene terephthalate, poly-transcyclohexane dimethanol terephthalate and the like; organic fillers include wood powder, rice hull, waste-paper such as newspaper, starches (including modified materials such as alpha-starch), cellulose and the like. Prior art aliphatic polyester resins having a .lambda. value of less than 1.5 also can be used in the present invention by blending with aliphatic polyester resins having a .lambda. value of 1.5 to 8.0 in accordance with the present invention in a predetermined mixing ratio. Such blending is one of the characteristic features in the process of the present invention. In such a case, 3 to 500 parts by weight of the aliphatic polyester resin in accordance with the present invention may be blended to 100 parts by weight of the aliphatic polyester resin having a .lambda. value of less than 1.5. Although not limited thereto, blending methods include dry blending and/or melt kneading. Another example of blending methods is as follows: Both polymers are pre-blended with a tumbling mixer or Henschel mixer then blended at a temperature higher than the melting point of the resin by 30.degree. to 120.degree. C., preferably 40.degree. to 100.degree. C., and more preferably 50.degree. to 90.degree. C., in a uniaxial or biaxial extruder. When the kneading temperature is not 30.degree. C. higher than the melting point, the extrusion load is too large. On the other hand, temperatures exceeding the melting point by more than 120.degree. C. cause the deterioration of the aliphatic polyester resin. Additional kneading step may be preferably incorporated, for example, additives and the aliphatic polyester resin are preliminarily dried before kneading, or a vacuum vent extruder is used. In order to reduce the degradation of the resin, the water content during kneading may be controlled to 0.1 weight % or less, suitably 0.05 weight % or less, preferably 0.02 weight % or less, and more preferably 0.005 weight % or less. Dry blending will also be effective in many cases, although the effect depends on the screw shape of the molding machine. BRIEF DESCRIPTION OF DRAWINGS FIG. 1 is a graph illustrating elongational viscosity behaviors of resins used in Example 1 and Comparative Example 1, wherein A-1 represents the result at a strain rate of 0.1 sec.sup.-1, A-2 represents the result at a strain rate of 0.03 sec.sup.-1, B-1 represents the result at a strain rate of 0.1 sec.sup.-1, and C represents a transition point; and FIG. 2 is a set of schematic views illustrating structural models of a linear polyester (B1), a long chain-branched polyester (A1), a longer chain-branched polyester (F1), and a non-thickened long chain-branched polyester (H1). BEST MODE FOR CARRYING OUT OF THE INVENTION The present invention will now be explained with reference to Examples and Comparative Examples. Instruments and conditions for measuring physical properties are as follows: The molecular weight was determined by GPC as follows: Instrument: Showa Denko SYSTEM-11 Columns: Shodex GPC K-801+two K-80M+k-800P (both sample and reference sides) Solvent: Chloroform Column Temperature: 40.degree. C. Flow Rate: 1.0 ml/min. Polymer Concentration: 0.1% by weight Detector: Shodex RI Standard for Molecular Weight Determination: PMMA (Shodex M-75) Injection Volume 0.8 ml/min. Melt flow index (MFR) was determined at 190.degree. C. under a load of 2.16 kgf, according to JIS K6760. Swell was determined as follows: 2 cm sample of flow from a melt indexer for the MFR measurement at 190.degree. C. under a load of 2.16 kgf is cut and the diameter measured at a point 5.0 mm from the bottom end. Then, the swell was calculated from the equation below: ##EQU1## wherein the average diameter means the average of several measurements, and the figure "2.095" means the nozzle diameter of the melt indexer. The melt viscosity at a shear rate of 100 sec.sup.-1 was calculated from a graph which shows the relationship between the apparent viscosities and the shear rates measured by a capillary rheometer made by Toyo Seiki Co., Ltd. using a nozzle having a diameter of 1.0 mm and L/D of 10 at a resin temperature of 190.degree. C. The elongational viscosity was determined as follows: Using the capillary rheometer made by Toyo Seiki Co., Ltd., a uniform strand having a 2.0 to 6.0 mm diameter was prepared at 190.degree. C. The elongational viscosity of the strand was measured using a uniaxial tensile viscometer made by Toyo Seiki at a temperature 30.degree. C. higher than the melting point determined with DSC and at a strain rate of 0.1 sec.sup.-1. Other conditions are as described above. The melting point was determined by DSC in a nitrogen atmosphere as follows: Using a Perkin Elmer DSC-7, ca. 5 mg of sample was accurately weighed. The sample was heated up from room temperature to 200.degree. C. at a heating rate of 10.degree. C./min., held at 200.degree. C. for 5 min., cooled to -60.degree. C. at a cooling rate of 10.degree. C., held at -60.degree. C. for 5 min., then heated up again to 200.degree. C. at a heating rate of 10.degree. C./min. Gelation and fish eyes were evaluated with numbers and the size of gelation and fish eyes present in a 20-cm-by-20-cm film having a thickness of ca. 50 microns produced using a biaxial drawing test machine made by Toyo Seiki Co., Ltd. from a 0.5-mm thick sheet prepared by a pressing machine at 190.degree. C. The heat of combustion was determined according to the calorimetric method in JIS M 8814. |
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