| PATENT NUMBER | This data is not available for free |
| PATENT GRANT DATE | September 20, 1994 |
| PATENT TITLE |
Polyester flat and split yarn |
| PATENT ABSTRACT |
Flat and split yarn formed by extrusion molding, comprising as a main component an aliphatic polyester having a melt viscosity of 2.0.times.10.sup.3 -4.0.times.10.sup.4 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.-190.degree. C. The present invention provides flat and split yarn having excellent in heat stability and mechanical strength as well as having biodegradability. |
| PATENT INVENTORS | This data is not available for free |
| PATENT ASSIGNEE | This data is not available for free |
| PATENT FILE DATE | April 20, 1993 |
| PATENT FOREIGN APPLICATION PRIORITY DATA | This data is not available for free |
| PATENT CLAIMS |
What is claimed is: 1. A polyester flat and split yarn comprising an aliphatic polyester having a melt viscosity of 2.0.times.10.sup.3 -4.0.times.10.sup.4 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.-190.degree. C., wherein said aliphatic polyester is obtained from ingredients consisting essentially of diisocyanate and a prepolymer in a molten state by adding from 0.1 to 5 parts by weight of diisocyanate to 100 parts by weight of a prepolymer in a molten state, wherein said prepolymer is obtained from a reaction of at least an aliphatic glycol and an aliphatic dicarboxylic acid, wherein the reaction includes (a) succinic acid, or its anhydride, reacted with 1,4-butanediol, (b) succinic acid, adipic acid, or their anhydride, reacted with 1,4-butanediol, or (c) succinic acid, or its anhydride, reacted with ethylene glycol, and has a number-average molecular weight of at least 10,000. 2. The flat and split yarn as claimed in claim 1 wherein the aliphatic polyester has a repeated chain structure in which a polyester prepolymer having a number-average molecular weight (Mn) of 10,000 or more and obtained by reacting an aliphatic glycol, aliphatic dicarboxylic acid and, as the third component, 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, is combined through urethane bonds. 3. The flat and split yarn as claimed in claim 2 wherein the polyester prepolymer contains one or more compounds selected from the group consisting of trimethylol propane, glycerin and pentaerythritol as the trifunctional or tetrafunctional polyol of the third component. 4. The flat and split yarn as claimed in claim 2 wherein the polyester prepolymer contains one or more compounds selected from the group consisting of malic acid, citric acid and tartaric acid as the trifunctional or tetrafunctional oxycarboxylic acid of the third component. 5. The flat and split yarn as claimed in claim 2 wherein the polyester prepolymer contains one or more compounds selected from the group consisting of trimesic acid, propane tricarboxylic acid, trimellitic anhydride, pyromellitic anhydride, benzophenone tetracarboxylic anhydride and cyclopentane tetracarboxylic anhydride as the trifunctional or tetrafunctional polybasic carboxylic acid of the third component. 6. A method for producing flat yarn comprising: melt extruding as a main component an aliphatic polyester having a melt viscosity of 2.0.times.10.sup.3 -4.0.times.10.sup.4 poises at a temperature of 190.degree. C. and shear rate of 100 sec.sup.-1, and having a melting point of 70.degree.-190.degree. C, wherein said aliphatic polyester is obtained from ingredients consisting essentially of diisocyanate and a prepolymer in a molten state by adding from 0.1 to 5 parts by weight of diisocyanate to 100 parts by weight of a prepolymer in a molten state, wherein said prepolymer is obtained from a reaction of at least an aliphatic glycol and an aliphatic dicarboxylic acid, wherein the reaction includes (a) succinic acid, or its anhydride, reacted with 1,4-butanediol, (b) succinic acid, adipic acid, or their anhydride, reacted with 1,4-butanediol, or (c) succinic acid, or its anhydride, reacted with ethylene glycol, and has a number-average molecular weight of at least 10,000, to form a film; then stretching the film. 7. A method for producing flat yarn comprising: melt extruding as a main component an aliphatic polyester having a melt viscosity of 2.0.times.10.sup.3 -4.0.times.10.sup.4 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.-190.degree. C., wherein said aliphatic polyester is obtained from ingredients consisting essentially of diisocyanate and a prepolymer in a molten state by adding from 0.1 to 5 parts by weight of diisocyanate to 100 parts by weight of a prepolymer in a molten state, wherein said prepolymer is obtained from a reaction of at least an aliphatic glycol and an aliphatic dicarboxylic acid, wherein the reaction includes (a) succinic acid, or its anhydride, reacted with 1,4-butanediol, (b) succinic acid, adipic acid, or their anhydride, reacted with 1,4-butanediol, or (c) succinic acid, or its anhydride, reacted with ethylene glycol, and has a number-average molecular weight of at least 10,000, to form a film; stretching the film; then splitting the stretched film. -------------------------------------------------------------------------------- |
| PATENT DESCRIPTION |
BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to flat and split yarn having excellent heat stability and mechanical strength, such as tensile strength, knot strength which are prepared by using aliphatic polyesters with biodegradability and sufficiently high molecular weights and specific melt properties for practical use. 2. Discussion of the Background In recent years, plastics have been increasingly used as packing materials, and as agricultural, fishery and forestry industrial materials and the like, all of which are required to have corrosion resistance, weathering resistance, wear resistance, high strength and the like. On the other hand, the waste from the large amounts of plastics used as the above materials has the possibility of polluting rivers, oceans and soil, and cause a great social problem. The appearance of plastics having biodegradability is thus eagerly awaited in order %o prevent such pollution. 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 a 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 polymeric material sufficient enough to obtain 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 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 a high cost thereof. Further, although glycolic acid, lactic acid and the like are polymerized by a ring-opening polymerization of glycolide and lactide thereof to obtain polymers with higher molecular weights so as to be sometimes used as medical fibers and the like, the polymers are not used in great amounts as industrial parts, automotive members, domestic articles and the like because their decomposition temperatures are close to their melting point and they have defects in their molding properties. Flat yarns and fabrics thereof as one of packaging materials and agricultural materials required to have corrosion resistance, high strength and the like mainly consist of a high density polyethylene resin and polypropylene resin from among the polyolefins. The flat yarn is used as tying string and bands and cloth formed by weaving the yarn is useful as a curing sheet, civil engineering sheet, rice and wheat bags, cement bags, packing material such as flexible containers or the like, and as primary base cloth for carpet. Polyethylene terephthalate, polypropylene terephthalate and the like are not used for producing the flat yarn because of unstable winding and yarn separation when forming, and large deterioration rate thereof. Further, it is no exaggeration to say that polymeric polyesters (referring to polymeric polyesters having number-average molecular weights of at least 10,000) generally used for forming of the yarn which is used one of the packing materials and agricultural, fishery and forestry industrial materials are limited to polyethylene terephthalate, a condensate of terephthalic acid (including dimethyl terephthalate) and ethylene glycol. Although there are cases of 2,6-naphthalenedicarboxylic acid being used instead of terephthalic acid, there are no reports of trials which resulted in providing the obtained polymers with biodegradability. Therefore, it is safe to say that there has been no concept of trying to make the molding of flat and split yarn using biodegradable aliphatic polyesters in which aliphatic dicarboxylic acid was used practical. One of the reasons why this application concept has not been thought of is felt to be that in spite of the required special molding conditions and physical properties for the above flat and split yarn, most of the above-mentioned aliphatic polyesters have melting points of 100.degree. C. or lower even if they are crystalline, and have poor heat stability when melted above that. Of further importance is that the properties, particularly mechanical properties such as tensile strength, of these aliphatic polyesters show markedly poor values even when they are the same level of number-average molecular weight as the above-mentioned polyethylene terephthalete, so just conceiving that the molded articles having required strength and the like would be obtained was difficult. Another reason seems to be that studies for improving the physical properties of the aliphatic polyesters by increasing their number-average molecular weights were not make sufficiently advanced because of their poor heat stability. The object of the present invention is to provide flat and split yarn prepared by using the above-mentioned aliphatic polyesters as their components which have sufficient high molecular weights for practical use, have excellent mechanical properties represented by heat stability and tensile strength, and which may be decomposed by microorganisms and the like, i.e., are biodegradable as one means of waste disposal so that they may be easily disposed of after the use thereof. SUMMARY OF THE INVENTION As a result of various studies of the reaction conditions for obtaining polyesters having sufficient high molecular weight for practical use and molding properties suitable for flat and split yarn, the present inventors have obtained specific aliphatic polyesters that maintain biodegradability while having sufficient high molecular weights for practical use, then have found that flat yarn produced from the polyester has superior tensile strength, knot strength and resistivity to yarn separation, and split yarn produced from the polyester has heat stability and mechanical strength, both of them having the above-mentioned biodegradability to achieve the present invention. That is, the present invention provides: (A) flat and split yarn mainly consisting of an aliphatic polyester having a melt viscosity of 2.0.times.10.sup.3 -4.0.times.10.sup.4 poises at a temperature of 190.degree. C. and a shear rate of 100 sec.sup.-1, and having a melt temperature of 70.degree.-190.degree. C.; (B) flat and split yarn according to (A) in which the number-average molecular weight of the aliphatic polyester is at least 10,000 and includes 0.03-3% by weight of urethane bonds; (C) flat and split yarn according to (A) or (B) consisting of an aliphatic polyester obtained by reacting 0.1-5 parts by weight of diisocyanate with 100 parts by weight of an aliphatic polyester prepolymer having a number-average molecular weight of at least 5,000 and a melting point of at least 60.degree. C. Further, the present invention provides a method for producing a flat yarn by extruding an aliphatic polyester to make film and stretching it, wherein the polyester having a melt viscosity of 2.0.times.10.sup.3 -4.0.times.10.sup.4 poises at 190.degree. C. and under a shear rate of 100 sec.sup.-1 and melting point of 70.degree.-90.degree. C. Still further, the present invention provides a method for producing split yarn by extruding an aliphatic polyester to make film, stretching it and splitting it, wherein the polyester having a melt viscosity of 2.0.times.10.sup.3 -4.0 .times.10.sup.4 poises at 190.degree. C. and under a shear rate of 100 sec.sup.-1 and melting point of 70.degree.-90.degree. C. DETAILED DESCRIPTION OF THE INVENTION The present invention will be described below in further detail. The aliphatic polyester 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 polyesters are prepared by reacting relatively high molecular weight polyester prepolymers which have hydroxyl groups at ends with a coupling agent so as to make them even higher molecular weight polymer. 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 have hydroxyl groups as the terminal groups, with diisocyanate as a coupling agent in the preparation of rubbers, foams, 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 no normal resins which can be molded in the form of a melt can be obtained. Therefore, polyesters which are obtained by reacting a large amount of diisocyanate with a low molecular weight polyester prepolymers as a raw material cannot be used as the raw material for the flat and split yarn of the present invention. Also, as shown in the case of polyurethane rubbers, 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 to undesirably cause poor preservativity, generation of crosslinking and branching; hence a number-average molecular weight of not more than around 2,500 of polyester prepolymers would be the limit if they were to be prepared without catalysts. The polyester prepolymers to obtain the aliphatic polyesters used in the present invention are relatively high molecular weight saturated aliphatic polyesters having substantially hydroxyl groups at the ends thereof, number-average molecular weights of at least 5,000, preferably at least 10,000, and melting point of 60 .degree. C. or higher, which are obtained by reacting glycols and dibasic carboxylic acids (or acid anhydrides thereof) in the presence of catalysts. When a prepolymer having a number-average molecular weight of lower than 5000 is used, the small amounts of 0.1-5 parts by weight of coupling agents used in the present invention cannot provide flat and split yarn polyesters having good physical properties. When polyester prepolymers having a number-average molecular weight of 5,000 or higher is used, with a hydroxyl value of 30 or less, the use of small amounts of coupling agents even under severe conditions such as a molten state and the like can produce high molecular weight polyesters, without gelation as the reaction is not affected by remaining catalysts. Therefore, the polymer for the flat and split yarn of the present invention has a repeated chain structure in which a polyester prepolymer having a number-average molecular weight of 5,000 or more, preferably 10,000 or more and consisting of an aliphatic glycol and aliphatic dicarboxylic acid, is combined through the urethane bond derived from, for example, diisocyanate as a coupling agent. Further, the polymer for the flat and split yarn of the present invention has a repeated chain structure in which the above-mentioned polyester prepolymer provided with branched long chains derived from polyfunctional components is combined through the urethane bond derived from, for example, diisocyanate as a coupling agent. When oxazoline, epoxy compounds, and acid anhydrides are used as a coupling agent, the polyester prepolymer has a repeated chain structure through the ester bond. The flat and split yarn of the present invention consisting of an aliphatic polyester having a melt viscosity of 2.0.times.10.sup.3 -4.0.times.10.sup.4 poises at a temperature of 190.degree. C. at a shear rate of 100 sec.sup.-1 and a melt temperature of 70.degree.-190.degree. C., particularly the flat and split yarn of the present invention which essentially consists of an aliphatic polyester obtained by reacting 0.1-5 parts by weight of diisocyanate with 100 parts by weight of an aliphatic polyester prepolymer having a number-average molecular weight of 5,000 or higher and a melting point of 60.degree. C. or higher has biodegradability when buried in the earth; and generates lower heat of combustion than polyethylene and polypropyrene even when thermally disposed of. The flat yarn of the present invention has excellent tensile strength, knot strength and resistance to yarn separation. That is, the tensile strength is at least 2.0 g/d and the knot strength is at least 1 g/d. The flat yarn can be used as tying strings and bands, and cloth formed by weaving the yarn is useful as a curing sheet, civil engineering sheets, rice and wheat bags, cement bags, packing materials such as flexible containers or the like, and as primary base cloth for carpet. A net formed by knitting is also useful as frost shield nets or light shield nets. Further, the split yarn of the present invention has excellent thermal stability and mechanical strength, and is thus useful as a split yarn for ropes, nets and the like for packing, agriculture, fishery and forestry. Examples of glycols which can be used as a reaction component include aliphatic glycols. Among them those having a straight chain alkylene group with even number carbon atoms of 2, 4, 6, 8 and 10 such as: ethylene glycol, 1,4-butanediol, 1,6-hexanediol, 1,8-octanediol, 1,10-decanediol, and mixtures thereof are preferable. Of these glycols, those having a smaller number of carbon atoms, such as ethylene glycol, 1,4-butanediol and 1,6-hexanediol, are preferable because they can produce an aliphatic polyester having a high crystalinity and a high melting point. In particular, ethylene glycol and 1,4-butanediol are most suitable because they produce good results. Examples of aliphatic dicarboxylic acids or anhydrides thereof which provide aliphatic polyester by reacting with glycols include aliphatic dicarboxylic acids. Among them those having a straight chain alkylene group with even number carbon atoms of 2, 4, 6, 8 and 10 such as: succinic acid, adipic acid, suberic acid, sebacic acid, 1,10-decanedicarboxylic acid, succinic anhydride and mixtures thereof are preferable. Of these dicarboxylic acids, those having a smaller number of carbon atoms, such as succinic acid, adipic acid and succinic anhydride, are preferable because they can produce an aliphatic polyester having high crystalinity and high melting points. In particular, succinic acid, succinic anhydride and an acid mixture of succinic acid or succinic anhydride and another dicarboxylic acid such as adipic acid, suberic acid, sebacic acid or 1,10-decanedicarboxylic acid are preferable. In the system of an acid mixture containing two or more acid components, for example, succinic acid and other dicarboxylic acids, the mixing ratio of succinic acid is at least 70 mol %, preferably at least 90 mol %, and the mixing ratio of the other carboxylic acids is 30 mol % or less, preferably 10 mol % or less. A combination of 1,4-butanediol and succinic acid or succinic anhydride and a combination of ethylene glycol and succinic acid or succinic anhydride are particularly preferable for the present invention because the combinations exhibit melting points close to that of polyethylene. Third component To these glycols and dicarboxylic acid, if necessary, may be added as a third component 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). The addition of this third component, which causes the branching of long chains, can impart desirable properties in molten state to the polyester prepolymer, 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 0.1-5 mole %, or a tetrafunctional component of 0.1-3 mole % is added relative to 100 mole % of the total of aliphatic dicarboxylic acid (or acid anhydride thereof) components. 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 to the purposes of the present invention in view of commercial availability at low cost. 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 to 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 literatures, can be used. In view of commercial availability, for example, pyromellitic anhydride, benzophenone tetracarboxylic anhydride and cyclopentane tetracarboxylic anhydride are practical and sufficient to the purposes of the present invention. These glycols and dibasic acids are mainly consisted 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 for aliphatic polyesters to be used in the present invention has hydroxyl groups at the terminals. To introduce the hydroxyl groups, it is necessary that glycols are used somewhat excessively. For preparation of the polyester prepolymer having a relatively high molecular weight, it is necessary to use deglycol-reaction catalysts in the deglycol reaction subsequent to the esterification. 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 which has a number-average molecular weight of at least 5,000, preferably at least 10,000, and whose terminal groups are substantially hydroxyl groups are added coupling agents in order to increase its number-average molecular weight. Examples of the coupling agents include diisocyanate, oxazoline, diepoxy compounds, acid anhydrides and the like. Diisocyanate is particularly preferred. 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 hue of prepared resins, 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 polyester 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. The addition is preferably performed when the polyester is in a uniformly melted state under easily stirtable conditions. Although it is not impossible for the coupling agents to be added to the polyester prepolymer in the solid sate and melted and mixed through an extruder, adding the agents in a polyester preparation unit, or adding them to polyester prepolymer in a melt state (for example, in a kneader) is more practical. The aliphatic polyester to be used in the present invention is required to have selected melt properties for the flat and split yarn formed by extrusion molding. That is, the aliphatic polyester to be used in the present invention needs to have a melt viscosity of 2.0.times.10.sup.3 -4.0.times.10.sup.4 poises, preferably 3.0.times.10.sup.3 -3.0.times.10.sup.4 poises and more preferably 5.0 .times.10.sup.3 -2.0.times.10.sup.4 poises and most preferably 1.0.times.10.sup.4 -2.0.times.10.sup.4 poises at a temperature of 190.degree. C. at a shear rate of 100 sec.sup.-1. With less than 2.0.times.10.sup.3 poise, since a base film become greatly uneven in thickness during melt extrusion, it is difficult to wind the film stably, and satisfactory physical properties cannot be obtained even if the film can be wound. With more than 4.0.times.10.sup.4 poises, since melt fracture occurs, the film becomes uneven, or the film cannot be easily stretched during melt extrusion, or the stretch ratio obtained is low. The melt viscosity at a shear rate of 100 sec.sup.-1 was calculated from a graph which shows the relation between the apparent viscosities and shear rates measured 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 to be used in the present invention needs to be 70.degree.-190.degree. C. preferably 70.degree.-150.degree. C., and more preferably 80.degree.-135.degree. C. A melting point of less than 70.degree. C. will give flat and split yarn poor heat resistance to distort them, whereas with higher than 190.degree. C. make it is difficult to produce flat and split yarn. To achieve a melting point higher than 70.degree. C. the polyester prepolymer need to have a melting point of at least 60.degree. C. The amount of urethane bonds contained in the aliphatic polyester of the present invention is 0.03-3.0% by weight, preferably 0.05-2.0% by weight, and more preferably 0.1-1.0% 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 a little effect on polymerization and leads to poor molding properties, whereas more than 3% by weight causes gelation. It is needless to say that when the above-mentioned aliphatic polyester is used to obtain the flat and split yarn according to the present invention, if necessary, lubricants, waxes and coloring agents crystallizing promoters as well as antioxidants, heat stabilizers, UV absorbers and the like can be used concomitantly. 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, trisnonylphenyl 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 palmirate 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-dicyclopropyl) phosphate, pentabromophenyl allyl ether and the like; inorganic fillers include calcium carbonate, silica, titanium oxide, talc, mica, barium sulfate, alumina and the like; crystallizing promoters include polyethylene terephthalate, poly-trans-cyclohexane dimethanol terephthalate and the like. (The method for producing the flat yarn) The raw material used in the present invention comprising as a main component an aliphatic polyester can be melted and extruded by a known molding machine using a circular die, a T-die or the like. The extrusion temperature is generally 120.degree.-260.degree. C., preferably 170.degree. C. to 230.degree. C., more preferably 180.degree. C. to 210.degree. C. At higher than 260.degree. C., film swings largely to make the forming unstable. And at higher than 230.degree. C., gelation takes place and causes non-stretching and generation of fish eyes. The raw material is melted and extruded to form a film, and the resultant film is hardened by cooling, slit in a ribbon-like shape, stretched and then subjected to relaxation heat treatment. In melt extrusion, a multi-layer flat yarn can be formed by laminating co-extrusion of two or three layers of the polyester and other resins in order to improve the physical properties of the flat yarn. A ribbed flat yarn can also be formed by using ribbed die lips. A conventional wet or dry extrusion process, i.e., an in-bath, steam heated roll or oven heating plate process, can be employed for stretching at a high temperature with a total stretch ratio of 3 to 9 times, preferably 4 to 8 times. Since the raw material comprising as a main component an aliphatic polyester according to the present invention has high dependency of its strength to the stretch ratio, the strength can easily be adjusted by the stretch ratio. Two-step stretching is more preferred than one-step stretching. The polymer used in the present invention can be cold-stretched. In the first step of two-step stretching, the temperature is 40.degree. C. to 110.degree. C., preferably 60.degree. C. to 90.degree. C., and the stretch ratio is within the range of from 30% to 90%, preferably 60% to 85%, of the total ratio. In the second step, the temperature is 80.degree. C. to 120.degree. C., preferably 90.degree. C. to 100.degree. C., and the stretch ratio is within the range of from 70% to 10%, preferably 40% to 15%, of the total ratio. If the temperature in the first step is 40.degree. C. or less, although stretching can be made, resistivity to yarn separation deteriorates, and the yarn is easily split and broken. In other words , the textile formed significantly deteriorates. At higher than 110.degree. C., the film is not oriented. The relaxation heat treatment is preferably effected at 90.degree. C. to 140.degree. C., more preferably 110.degree. C. to 150.degree. C. The relaxation rate is preferably 5% to 30%, more preferably 10% to 20%. I f the relaxation heat treatment temperature is less than 90.degree. C. or if the relaxation rate is less than 5%, the rate of aging contraction of the flat yarn increases. Conversely, if the relaxation heat treatment temperature exceeds 140.degree. C. or if the relaxation rate exceeds 30% , the yarn has a wavy shape and thus shows poor winding appearance. The method for producing the split yarn The split yarn according to the present invention may be produced by subjecting the above flat yarn to a split roll. A razor blade, a needle blade, a round saw tooth blade or the like may be used as a blade for the split roll. In the present invention, the method of production of the split yarn from the flat yarn is not limited. If the aliphatic polyester used in the present invention has a number-average molecular weight of at least 10,000, preferably at least 20,000, and contains very small amounts of bonds formed by a coupling agent. If the aliphatic polyester has a melting point of 70.degree. to 190.degree. C. and crystallinity, strong flat and split yarns can be formed. The flat yarn obtained can be used for packing materials such as tying string and the like, agricultural materials and various fabrics. Further, the split yarn obtained can be used as twisted yarns (twine) for ropes , nets and the like for packing, agriculture, fishery and forestry use. |
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