High-strength ultra-fine fiber construction and method for producing the same

Abstract: A high-strength ultra-fine fiber construction includes a high-strength multi-filament yarn (a) including ultra-fine filaments of a single filament fineness less than 0.8 denier, (b) having a tenacity of 6.5 g/d or more, and (c) having an elongation at break of 15% or more. In order to obtain such a fiber construction, islands-in-sea-type or divisional type conjugate spinning is initially carried out to obtain conjugate multi-filament yarns, which are subjected to weaving to produce a fabric, and the island components are then separated by a sea removal or island dividing treatment in order to obtain a fiber construction containing high-strength ultra-fine fibers.

Description: The present invention relates to a fiber construction suitable for various industrial uses. More particularly, the present invention relates to a fiber construction of a high tenacity as well as a high softness, which is capable of being melt spun, as well as woven, without accompanying operational difficulties such as generation of naps or filament breakages. Further, more particularly, the present invention relates to a fiber construction of reduced air permeability, which is practically useful as a basic fabric for an air bag and the like. A fiber construction made of synthetic fibers is widely used for materials in various fields of industry: such as rubber reinforcing materials for items such as tires, belts such as V-belts or conveyor belts, and hoses; woven or knitted fabrics such as a canvas, tent, tarpaulin, cover sheet, seat belt, and air bag fabrics; fish nets; ropes; and sewing threads. It has heretofore been known that a thread used for producing the fiber construction for the above items must have a single filament denier of 5 or more. Such a limitation to the fineness of the single filament is derived, on one hand, from requirements as to thread performances, such as obtaining a high-strength thread, and reducing the specific surface area to prevent the fiber construction from being degraded at its surface and, on the other hand, is derived from a productivity requirement such as that of preventing operational difficulties, such as naps and filament breakage, from increasing during melt spinning. However, due to the recent industrial use of a fiber construction in various textile related products, such as canvas, tent, tarpaulin, and air bag fabrics, it has been frequently required that the fiber construction provides a high-strength, reduced weight and increased softness. In order to meet the above-mentioned requirements in various field of use, various improvements have been attempted including, for example, reducing the total denier of the filaments, or the denier of a single filament, in the fabric. When an application to a basic fabric for an air bag is considered, a first requirement is that the air bag has a low air permeability as well as a high mechanical strength, which allows the bag to be quickly expanded when a shock is generated. Additionally, it is also a requirement that a driver or a passenger is prevented from being abrasively wounded especially on his or her face when the air bag is inflated, that the air bag is in a compact condition when it is stored, and that any substantial dimensional change does not occur during a prolonged storage in an automobile body. It is, however, difficult to meet all of the above requirements. For example, when a basic fabric for an air bag is produced so as to meet the requirement for reducing the air permeability, the basic fabric becomes too thick, which, on the one hand, causes the fabric to be lacking in a compactness in a folded state and, on the other hand, causes the impact force to be increased upon the contact between the air bag and a human body, thereby causing a driver or passenger to be abrasively wounded upon inflation of the air bag. In short, a construction of the basic fabric for the air bag must be such that a plurality of requirements, which contradict each other, are met. Various basic fabrics have been proposed previously for utilization in an air bag. For example, JP-A-03-243442 proposes a basic fabric made of nylon yarns with no coating, with a single filament denier of 4 to 7 and with a total denier of 400 to 1,000 (below; non-coated type), as well as a basic fabric of the same construction with a coating of resin such as chloroprene or silicon (below; coated type). Despite an advantage of low gas permeability, this type of basic fabric for an air bag is defective in that both the total denier of the filaments constituting the yarn as well as the denier of the single filaments are large, so that the fabric is lacking in softness, thereby, on the one hand, reducing its compactness and, on the other hand, increasing the impact force upon contact of the inflated air bag with a human body. Furthermore, insofar as a coated basic fabric may be required, the process for producing this is complicated and it is difficult to obtain a uniform thickness of the coating of the resin on the surface of the basic fabric. In order to obviate the above-mentioned drawbacks, various improvements have been proposed. For example, JP-A-01-104848 proposes a basic fabric made of multi-filament yarn with a reduced value of the total denier, in order to reduce the thickness of the fabric, thereby on the one hand making the obtained basic fabric more soft and, on the other hand, more compact in a folded state. However, more reduction in the value of the total denier alone causes the gas permeability to be excessively increased, which inevitably results in a requirement for coating by a resin and which in turn reduces the compactness of the basic fabric. In order to obtain both of the low gas permeability and an improved compactness, a method is conceivable in which the total denier is reduced, while increasing the density of woven fabric. However, the usual range of single filament deniers between 4 and 7 (d) provides a limitation in improvement in the softness of the fabric which can be achieved by the reduction of the value of the total denier, which makes it difficult to improve the softness to a desired value. Furthermore, in order to obtain a neatly foldable structure in the coated type product, JP-B-64-041438 has proposed a basic fabric for an air bag, wherein it is produced from a multi-filament yarn of a tenacity of 8.5 g/d or more and of a single filament denier of 3 (d) or less. Furthermore, JP-C-04-214437 proposes a non-coated type of basic fabric for an air bag made of polyethylene terephthalate multi-filament yarns with a single filament denier of 4 dtex or less and of a total denier in a range between 250 to 400 dtex. However, in these prior art proposals, as described in the disclosed embodiments, the reduction in single filament denier is, at best, limited to about 2 denier. As a result, only a partial improvement in the basic fabric as to its softness as well as neatness when folded can be obtained, which does not meet the above-mentioned various requirements for the basic fabric for an air bag. Furthermore, the yarns in these publications are obtained by a direct spinning method as disclosed therein. Such a direct spinning method is defective in that the finer the denier of the single filament, the more frequent the occurrence of naps and breakage of single filaments during spinning as well as weaving. In addition, the weaving process of a fabric for an industrial use such as a basic fabric for an air bag is usually done by using non-twisted yarns in a non-sized condition, which makes it easy for the yarns to generate naps as well as for the single filaments to break. Furthermore, in the case of an extremely fine yarn with a fineness of a single filament of 1.0 denier or less, even if no nap is generated in the yarns during the preceding spinning and winding process, the following weaving process may easily cause the yarns to produce naps and to break. As a result, it is considered that the prior art methods disclosed in the above-mentioned publications make it difficult to produce, in industry, a basic fabric for an air bag from ultra-fine yarns with a single filament denier smaller than 0.8 (d), from the view points of reduced operational performance during the spinning process (a low spinnability) as well as the weaving process (a low weavability). Furthermore, JP-A-01-122752 discloses a method wherein a high density woven fabric is subjected to a shrinkage process, which is followed by a thermal fixing process and then by a calendering process, to obtain a basic fabric for an air bag with a high dimensional stability. However, since, in this prior art, the thread used in the prior art has a single filament denier of 1 (d) or more, the calendering process is insufficient to improve the softness to a desired level. As further prior art, JP-A-04-002835 discloses a non-coated type of basic fabric for an air bag made from a polyethylene terephthalate of low weight and a small thickness, which fabric has an air permeability of 0.5 cc/sec/cm 2 or less, an elongation at break of 650 psi or more, a tenacity of 300 lb or more and a tearing strength of a trapezoidal shaped test piece of 40 lb or more. In this prior art, the fineness of a single fiber is, at best, limited to about 1 denier, and the above-mentioned calendering process is essential for suppressing the gas permeability, so that the above-mentioned problems have remained unsolved. In still further prior art, a fabric of ultra-fine fiber with a fineness of 1 denier or less has been proposed for use in clothes; see JP-A-05-213131. However, the ultra-fine fibers used for the clothes in the prior art are produced using a polymer of an intrinsic viscosity of a value in a range between about 0.6 to 0.7, so that the tenacity of the yarn is as low as a value in a range between 2.5 to 4.5 g/denier, which makes it difficult to obtain an air bag of a desired strength. In view of this, it has also been proposed to combine the ultra-fine yarns with conventional yarn of a high tenacity and usual fineness, thereby preventing the tenacity from being too low. However, such a solution of combining the yarns causes the advantage of using the ultra-fine fibers, i.e., the high softness, to be lost. As can be seen form the above, there is no disclosure in the above prior art of the production and use of a fabric having ultra-fine fibers, of a single filament denier of 0.8 or less, while maintaining a desired high tenacity. As far as the use of ultra-fine fibers is concerned, JP-U-56-056500 proposes the use of such an ultra-fine fiber for production of an extendable fabric for a parachute. However, in this publication, only the degree of fineness of the single fiber is mentioned, and no mention is made of the tenacity. In other words, nothing is mentioned in this prior art as to ultra-fine fibers capable of providing a high tenacity. JP-A-05-106117 discloses composite fibers consisting of polyester and polyamide fibers in which the polyester fibers are removable to provide ultra-fine polyamide fibers. There has been a long felt need, not yet realized, to produce a fiber construction for an industrial use, capable of providing a desired mechanical strength and softness, as well as a low air permeability, if it is needed, while suppressing the occurrence of naps as well as maintaining a desired quality. The present invention addresses the problem of providing a fiber construction for use in industry, capable of overcoming the above-mentioned various difficulties of the prior arts. Another problem addressed by the present invention is that of providing a fiber construction for use in a material for an industry, capable of providing desired mechanical properties as well as a desired degree of softness, while reducing the number of naps and obtaining a reduced air permeability, if needed, in the case of a woven fabric as the fiber construction. Still another problem addressed by the present invention is that of providing a method of producing such a fiber construction, capable of reducing the number of naps and breakage of single fibers which occur during processing to obtain the fiber construction, including spinning and weaving. In order to solve these problems, according to the present invention, a high-strength ultra-fine fiber construction is provided, formed as a high-strength multi-filament yarn, including ultra-fine filaments with the fineness of a single filament being less than 0.8 denier, the tenacity of the yarn being 6.5 g/d or more, and the elongation at a break being 15% or more. Accordingly, the present invention provides a high strength ultra-fine fiber construction as defined in claim 1. In the high-strength ultra-fine fiber construction according to the present invention, the high-strength multi-filament yarn is preferably made from polyethylene terephthalate fiber with an intrinsic viscosity (IV) of 0.8 or more. According to a second aspect of the present invention, there is provided a method for producing a high strength ultra-fine fabric as defined in claim 16. The high-strength ultra-fine fiber construction according to the present invention can be produced by simultaneously extruding two components of polymer from a spineret so as to create conjugated filaments of an islands-in-sea-type or a divisional type cross-sectional shape and of a single filament fineness of 3.0 denier or more, creating a multi-filament yarn having a number of filaments of 120 or less, forming a fabric from the multi-filament yarns under substantially non-twisted and non-sized conditions, and reducing the fineness of the single filament in the multi-filament yarns constructing the fabric by removing the polymer constituting the sea portions in the cross-section of the filament (in the case of the islands-in-sea-type) or by subjecting the filaments to a dividing treatment (in the case of the divisional type of a conjugate fiber) so as to provide individual fibers having a single fiber fineness of 0.8 denier or less. In the islands-in-sea-type conjugate fibers, the islands are of a polyester polymer and have a single island denier of 0.8d or less. The conjugate fiber may have a tenacity of 7.0g/d or more. Likewise, the divisional type cross-section conjugate fiber is divisible to provide polyester fibers. The most important process feature of the present invention resides in that the filament constituting the fiber construction is formed as high-strength ultra-fine fiber which is easily obtained by spinning of conjugate fiber which is followed by dissolving, or by spinning of conjugate fiber which is followed by dividing. As a result, a high quality of fiber construction of an increased mechanical performance as well as an increased softness is obtained, while reducing the number of naps as well as the generation of breakage of filaments. Furthermore, in the case of a woven fabric, a particular value of the number of the picks of weft yarns per unit of length and/or warp yarns per unit of width is desirable in that the above-mentioned performances are maintained, while obtaining a reduced air permeability. According to the present invention, the fiber construction, herein, refers to any object, of which a part or an entire part is constructed from fibrous material. The fibrous material, herein, refers to any such a material, which, when approximated to a circular cylindrical shape, has a ratio of its height to its diameter (below: aspect ratio) of 10 or more. Typical examples of such a fiber construction include a fabric such as a woven fabric, a knitted fabric and a non-woven fabric, and a strand such as a rope and a swing thread. In the fiber construction, the fibrous material can be in a continuous state or a discontinuous state. Furthermore, in addition to the fabric and the rope, a combined structure thereof with coated or impregnated resin is within the scope of the fiber construction according to the present invention. Furthermore, the fiber construction may be three dimensional, including a structure made from a resin which is reinforced by a reinforcing material within the structure or on its outer surface. In short, the high-strength ultra-fine fiber construction refers to a fiber construction wherein a high-strength ultra-fine fiber disclosed in this specification is used therein at least in part. The fiber construction using the high-strength ultra-fine fibers according to the present invention can be used as various constructions of a limitless shape, including a one dimensional construction such as a rope; a two dimensional construction including a fabric such as a woven fabric, a knitted fabric and a non-woven fabric; and a three dimensional construction including combined construction made from a resin to which a strengthening material made of a different resin is added. The fabric as the two dimensional construction is, however, preferable, since the ultra-fine fiber in this structure is the most effective to provide a desired softness, lightness and hand. In the multi-filament yarn for constructing the high-strength ultra-fine fiber construction, it is essential that the fineness of the single filament component of the multi-filament yarn is less than 0.8 denier. When the fineness of the single filament component of the multi-filament yarn is 0.8 denier or more, a desired improved softness cannot be obtained even if the total denier of the yarn is reduced. However, an excessively reduced fineness of the single filament requires an increased number of filaments in the yarn in order to obtain the desired strength which is necessary for a material for industrial use, which makes it necessary to combine a plurality of yarns after the conjugate spinning, which on the one hand renders the system industrially inapplicable and, on the other hand, results in a poor quality product. In view of this, a preferred range of single filament deniers is 0.1 denier or more and less than 0.8 denier, more preferably 0.1 denier or more and 0.5 denier or less. The multi-filament yarn for constructing the high-strength ultra-fine fiber construction according to the present invention must have a tenacity of 6.5 g/d or more, more preferably, of 7.5 g/d or more. This strength characteristic is essential in order to satisfy the usual requirements for the high-strength ultra-fine fiber construction. In other words, if the tenacity is lower than the above range, it is difficult to satisfy the mechanical strength which is required in practice. Due to the fact that the high-strength ultra-fine fiber construction according to the present invention provides the fiber construction, a desired mechanical performance is obtained without using yarns including increased fineness of a single filament. Furthermore, it is necessary that the multi-filament yarn constituting the high-strength ultra-fine fiber construction according to the present invention has a value of elongation at break which is 15% or more, more preferably 18% or more. A value of the elongation at break smaller than 15% causes the softness to become inferior, i.e., the fabric to be harder, and naps and filament breakage to be easily generated during spinning or weaving. Contrary to this, elongation at break which is excessively high causes the elongation of the fiber construction itself to be excessively large, thereby causing the dimensional and shape stability to be worsened. Thus, it is usually desirable that the value of the elongation at break is 40% or less. In addition, in the case of a basic fabric or an air bag as the fiber construction according to the present invention, an elongation at break larger than 40% is not preferable from the view point of restriction of gas permeability. It should be noted that the above values of the tenacity and the elongation at break are of the yarn as a constituent of the fiber construction and not those of the yarn just when it is produced, i.e., before the fiber construction is formed. In other words, the above ranges of the values of tenacity and elongation are those after the ultra-fine fiber is constructed not those before the sea component is removed or the conjugate fiber is divided. The sea component removal or dividing treatment to produce the ultra-fine fiber is, from the view point of efficiency, preferably done, after the formation of the fiber construction, by any suitable method, including a solvent treatment process or a dissolving process. During such a treatment to produce fine fibres, the fiber is, itself, shrunk, so that a small increase in elongation of the fiber is obtained before the ultra-fine fiber is actually obtained. The multi-filament yarn for constructing the high-strength ultra-fine fiber according to the present invention is made from a polyester and may be an aromatic polyester, such as a polyethylene terephthalate, polybutylene terephthalate, and polyethylene naphthalate. It is possible that, in order to improve any desired characteristic, such as spinnability, the polymer used may include additional copolymer components or additives so long as the original characteristics are not impaired. Furthermore, in order to improve the performance of the fabric, it is possible to add additives such as antioxidants and pigments, so long as the spinnability is not impaired. The yarn made from polyester is chosen in view of the ease of production of the ultra-fine fiber by means of the conjugate spinning process as well as the dimensional stability and the mechanical characteristic of the fiber construction obtained. In particular, a polyethylene terephthalate fiber including units of polyethylene terephthalate in an amount of 85% by weight or more is preferable. In the case where the ultra-fine fiber is constructed as a polyethylene terephthalate fiber, a polymer of a higher viscosity enables a desired strength as well as elongation easily to be obtained. Thus, it is desirable that the intrinsic viscosity of the finally obtained ultra-fine fiber is 0.8 or more. An intrinsic viscosity of the finally obtained ultra-fine fiber lower than 0.8 makes it difficult to obtain the desired strength, on one hand, and reduces the heat resistance as well as the dimensional stability, on the other hand. Furthermore, in the high-strength ultra-fine fiber construction according to the present invention, when the ultra-fine fiber constructing the fiber construction is polyethylene terephthalate fiber, it preferably has a crystallite volume (C) measured by wide-angle X-ray diffraction of 8 × 10 4 Å 3 and/or a long period in the direction of the meridian (Dm) measured by small-angle X-ray diffraction of 120Å or longer, and/or a crystallite orientation (Fc) measured by wide-angle X-ray diffraction of 0.90 or more. When the crystallite volume (C) is lower than 8 × 10 4 Å 3 , or the long period (Dm) is shorter than 120Å or the crystallite orientation (Fc) is smaller than 0.90, it becomes difficult to obtain a high-strength, on one hand, and the durability is apt to be reduced since hydrolysis easily occurs, on the other hand. Furthermore, in the high-strength ultra-fine fiber construction, when the ultra-fine fiber constituting the fiber construction is polyethylene terephthalate fiber, it is preferable that the amorphous orientation (Fb) is 0.975 or more. When the amorphous orientation (Fb) is lower than 0.975, the dimensional stability and heat resistance are reduced, which makes it difficult for the fiber construction to be used in an industrial application. In the high-strength ultra-fine fiber construction according to the present invention, the ultra-fine filament constituting the high-strength multi-filament yarn is entirely made from substantially the same polymer component, or from two or more polymer components. So that the high-strength fiber is easily obtained and spinning becomes easy, it is desirable that the ultra-fine filament constituting the high-strength multi-filament yarn is entirely and substantially of the same polymer component. In order to obtain the ultra-fine filament constituting the high-strength multi-filament made entirely and substantially from a single polymer component, in addition to a direct spinning process, a conjugated spinning process including the islands-in-sea-type conjugate spinning and a dividing type conjugate spinning and a blend spinning process can be employed. In the conjugate spinning or blend spinning process, only a single kind of component is finally left in order to obtain the ultra-fine fiber. From the view point of ease of production, it is desirable that the islands-in-sea-type conjugate spinning is employed, wherein, after the conjugate spinning, the yarn is subjected to a process for removing the sea portions, thereby obtaining the ultra-fine fiber. Such an islands-in-sea-type conjugate spinning is especially preferable in the situation that the total denier of the high-strength multi-filament yarn to be obtained is large, and the denier of the single filament of the ultra-fine fiber is small. Furthermore, in the case of a fabric as the fiber construction using the high-strength ultra-fine fiber according to the present invention, in order to make it possible to obtain an advantage of the softness, lightness and touch resulting from the ultra-fine fiber, the total denier of the multi-filament is 1000 (d) or less. For a desired strength level for industrial use, it is a further requirement that the total denier is 100 (d) or more and 1000 (d) or less. When a fabric is made of a yarn of a total denier larger than 1000 (d), even if the single filament is an ultra-fine fiber having a fineness smaller than 0.8 (d), the thickness of the fabric is increased, thereby causing the softness to become insufficient. The woven fabric as produced above can have improved characteristics, depending on weaving density, namely a tensile strength of 100 kg/3 cm (length of sample) or more, preferably, 150 kg/3 cm or more, and, more preferably, 170 kg/3 cm or more, and a softness evaluation value measured by means of a cantilever method, of 60 mm or less, more preferably 45 mm or less. when the tensile strength is smaller than 100 kg/3 cm, a case may arise that, for industrial use, the mechanical strength is too low. Furthermore, it is not desirable that the softness evaluation value is larger than 60 mm, because the softness increasing effect of the present invention is not provided. A fabric using the high-strength ultra-fine fiber according to the present invention can be developed for various industrial uses. Among others, due to the fact that softness as well as lightness are high and that a low gas permeability can be easily obtained by suitably designing the construction of the fabric, a preferred application is as a basic fabric for an air bag. The various characteristics required of a basic fabric for an air bag include, in addition to mechanical characteristics such as tensile strength and tearing strength, softness, lightness, compactness (neatness when folded) and low gas permeability. In order to allow the basic fabric for an air bag to meet these requirements, it is desirable that the total denier of the high-strength ultra-fine multi-filament yarn is in a range between 180 to 450 (d). When the total denier is larger than 450 denier, an increased density of the fabric, which is enough to obtain a sufficiently low gas permeability without any coating of a resin, causes the thickness of the fabric to be excessively increased, thereby reducing the softness and lightness. Furthermore, the degree of the compactness is likely to be worsened when the air bag is to be stored. Contrary to this, when the total denier is smaller than 180 (d), the mechanical strength of the fabric is likely to be insufficient, even if the fabric is woven at a density as high as possible, which is not preferable, since the bag is apt to be broken when it is inflated. Thus, a more desirable range of the total denier is 200 (d) or more and 375 (d) or less. Another variant of a multi-filament yarn for constructing a basic fabric for an air bag, which satisfies the above condition, is as follows. Namely, when the high-strength ultra-fine multi-filament yarn is woven into a fabric, a cover factor K of a fabric preferably of 1,900 or more, more preferably 2,000 or more, makes it possible to obtain a desired characteristic of the basic fabric for the air bag even when uncoated. A cover factor K smaller than 1,900 may make the gas permeability too high in the uncoated condition, such that inflation becomes insufficient, on one hand, and the mechanical strength of the fabric may be low, on the other hand, which makes the fabric unsuitable. According to the present invention, the basic fabric for an air bag has improved characteristics when the tensile strength of the basic fabric is 100 kg/3 cm or more, the softness evaluation value by the cantilever method is 60 mm or less, and the amount of air passing through the fabric under a pressure drop of 1.27 cm is 5.0 cm 3 /sec/cm 2 or less. In other words, the basic fabric for air bag according to the present invention can have a low gas permeability as well as an increased softness. The fabric which satisfies the above conditions can have a tearing strength as high as 8 kgf or more and a burst strength of 40 kg/cm 2 or more when measured by the method stipulated in JIS (Japanese Industrial Standard)-L-1018A (Mullen method). By using the high-strength ultra-fine fiber according to the present invention, i.e., by using the fiber as weft and warp yarns for constructing the basic fabric for an air bag, a high-strength ultra-fine fiber yarn which can be produced by conjugate spinning followed by dissolution removal process or from a conjugate spinning followed by a dividing process and which has particular values of tenacity, total denier and single filament denier, a high quality basic fabric for an air bag can be obtained, wherein, in addition to the improved mechanical properties, a low gas permeability as well as a small number of naps and filament breakage during the weaving process are obtained. It should be noted that the basic fabric for an air bag using the high-strength ultra-fine filament according to the present invention can be used as a non-coated type fabric where the fabric is used without any resin coating, although such a resin coating can, if necessary, be employed. The high-strength ultra-fine fiber construction according to the present invention is constructed from a high-strength ultra-fine fiber obtained by using the conjugate spinning method including, as mentioned above, the islands-in-sea-type or the dividing type. In detail, the such a high-strength ultra-fine fiber can be easily made by the following method. In case where the islands-in-sea-type is employed, conventional islands-in-sea-type conjugate spinning may be initially executed, where two kinds of polymers are simultaneously extruded from a spineret, so as to obtain a multi-filament yarn in which the fineness of the conjugate single filament is preferably 3.0 denier or more. In this case, for the island portion, a polymer which finally constitutes the fabric is used, while, for the sea portion, a polymer with a solubility in a particular solvent higher than that of the first polymer for constructing the island portion is used. It is necessary that the first polymer for constructing the island component must be a high-strength and a high elongation type, in order to allow the filament to have sufficient mechanical strength, which makes it preferable for the first polymer to have a high degree of polymerization. In particular, as mentioned previously, it is preferable that, in case of the polyethylene terephthalate, the intrinsic viscosity for the ultra-fine fiber is 0.8 or more. Therefore, as for a viscosity of a polymer as chips for use in the melt spinning, it is preferable that, in case of the polyethylene terephthalate, the intrinsic viscosity is 1.0 or more, in order to obtain the above-mentioned high-strength and large elongation characteristic. When the ratio of the sea portion to be removed is reduced, the efficiency for producing the fiber is naturally increased. A reduction in the ratio of the sea to island component causes the density of the fabric to be increased, and the gas permeability to be reduced. In view of this, it is, generally, preferable that the ratio of the sea to island portion is as small as possible. Namely, the proportion of the sea portion is preferably 20% or less, and more preferably 10% or less. A low ratio of the sea to island portion may cause the island portions to connect with each other, which makes smooth drawing of the conjugate filament, in particular drawing at a higher drawing ratio, more difficult, and which therefore makes it difficult to obtain a desired mechanical strength and elongation characteristic. Thus, it desirable that the ratio of the sea portion to the island portion is at least 5%. During a conventional melt spinning process, the polymers of the island and sea portions form, in the spineret, a combined flow, which is, in a transverse cross-section, in an island-in-sea form, and is extruded from the spineret to create the multi-filament yarn. In order to prevent the polymers from being thermally degraded during the melt spinning, it is desirable that the total duration of the stay of the polymers in the spinning machine is as short as possible. Namely, the stay time is preferably shorter than 10 minutes, or more preferably in a range between 1 to 5 minutes. When the two kinds of polymer are extruded from the spineret, the number of filaments of the conjugated yarn is preferably 120 or below, and more preferably 20 or more and 90 or less. If the number of filaments is larger than 120, the spacing between spinning holes in the spineret is reduced due to the limited area of the spineret, which may cause the filaments to contact each other after the extrusion of the filament, which makes it difficult to obtain a stable state of spinning. In the conjugated fiber according to the present invention, due to the complicated cross sectional shape at the spineret, the polymers as extruded therefrom are likely to be bent, which makes it easy for the filaments to contact each other. In the islands-in-sea-type conjugate spinning, the number of conjugate filaments (number of the holes in the spineret) and the number of the islands in a single conjugate filament is determined in accordance with the total denier of the multi-filament yarn to be obtained from the spineret and the single filament denier in the high-strength ultra-fine fiber construction. When the number of the filaments in the conjugate yarn is small, the number of islands must be correspondingly increased, while the fineness of the single conjugate filament is increased, so that the cooling of the yarn is likely to be non-uniform, so that the stability of the spinning operation is impaired. Contrary to this, when the number of filaments in the conjugate yarn is excessively large, the number of islands in a single conjugate filament (a hole of the spineret) becomes small, which causes the denier of the single conjugate filament to become small. In this latter case, during cooling of the spun yarn, contact between the single filaments may occur, thereby generating naps and breakage of the filaments, which negate the advantages of using the conjugate yarn. A balanced determination should be carried out between the number of filaments in the conjugate multi-filament yarn from a spineret and the number of islands in a filament in such a manner that the denier of the conjugate filament after being subjected to the drawing process is preferably in a range between 3 (d) to 10 (d). It is preferable that a heating zone with a length of 10 to 100 cm is arranged just below the spineret, and is temperature controlled in a range between 200°C to 350°C. The length and the temperature of a heating tube containing the heating zone can be optimally determined in accordance with various spinning factors such as the characteristic of the two kinds of polymer, the denier of the conjugate filament and the number of conjugate filaments in a multi-filament yarn. The provision of the heating tube is effective for delaying the solidification of the melt polymer, which is effective for generating a high-strength. Furthermore, in order to prevent thermal degradation at a high temperature, the atmosphere in the heating tube may be sealed by means of a high temperature inert gas. The yarn as melt spun is, after passing through the high temperature atmosphere area, subjected to cooling by a cooling air flow, then, to finishing so that a finishing agent is supplied to the yarn, and finally to winding by a take-up roll for controlling the spinning speed. It is preferable that the un-drawn yarn taken up by the roller is, without being wound, subjected, directly, to a drawing process. However, it is also possible that the undrawn yarn is wound once and is then subjected to a drawing process at a later separate stage. The spinning speed is usually 1500 m/min or less. The drawing process is carried out by a conventional thermal drawing, and a two stage drawing is preferable. The drawing ratio is optimally determined in dependence upon the kind of polymer used as well as the spinning speed. The drawing ratio is determined by various factors such as birefringence in an undrawn condition, the drawing temperature, and an allocation of drawing ratios when a multi stage drawing process is employed, and is usually 3.0 or more and preferably 3.0 to 6.5 and more preferably 4.0 to 6.0. The conjugate yarn thus obtained after the drawing process preferably has a single filament denier of 3 (d) or more and preferably of 5 (d) or more. When the single filament denier is less than 3 (d), an unevenness in the fineness is likely to be generated due to the sway of the yarn in the heating tube during the spinning. Such an unevenness in the fineness makes it difficult to obtain a uniform state of drawing, on one hand, and naps and filament breakage are likely to be generated, which makes it difficult to produce a yarn having the desired properties, on the other hand. Following the drawing, the drawn yarn is preferably subjected to a thermal setting treatment. Such a thermal setting treatment may be done by a conventional method wherein the yarn is contacted with a hot roller or hot plate, or the yarn is passed through a gas at a high temperature. By changing the tension as well as the temperature during the thermal setting treatment, it is possible to control the value of the dry heat shrinkage. For example, when the fabric is of such a type that requires a low air permeability, it is especially preferable that the dry heat shrinkage by heat treatment at 150°C and for 30 minutes is 1% or more and 10% or less, more preferably, 3% or more and 8% or less, since a suitable shrinkage of the fabric is obtained when the sea component is removed from the fabric, so that a low gas permeability is obtained after the removal of the sea component. The desired level of actual shrinkage is determined in accordance with the warp and weft yarn density during weaving and the desired final air permeability and mechanical strength. The high-strength ultra-fine fiber obtained by the island-in-sea-type conjugate spinning according to the present invention can be additionally subjected to a filament interlacing process during the drawing and heat setting process. In order to effect the interlacing, a known method such as an air interlacing can be employed. For example, in case of the air interlacing, air pressure in accordance with the denier of the yarn and the tension is suitably changed in order to obtain a desired level of the interlacing. In this case, the degree of the interlacing is preferably 20 or more and more preferably 50 or more. As a result of the drawing process, conjugate multi-filament yarn having at least one of, and preferably each of, a single filament fineness of 3 to 8 denier, a tenacity of 7.0 g/d or more, preferably 8.0 g/d or more, and an elongation at break of 13.5% or more can be obtained. In order to obtain the high-strength ultra-fine fiber construction using the above-mentioned multi-filament yarn, a fiber construction is created using the conjugate yarns. For example, in the case of a woven fabric as the fiber construction, the obtained conjugate filament yarns are used for weft and warp yarns, so that a conventional weaving process is carried out. The weaving is usually carried out with non-twisted and non-sizing conditions. However, twisting or sizing can be performed when necessary. A suitable weaving construction such as a plain weave or twill can be selected to match the object. A weaving density is also suitably determined. In particular, in the case of a basic fabric for an air bag, in order to obtain a desired value of the cover factor K which determines the air permeability of the final woven fabric, the weaving density is determined while considering filament denier as well as degree of the shrinkage which is determined by the amount of the sea component to be removed during the removing treatment and the heat applied during the removing treatment. In the conjugate spinning method according to the present invention, in order to obtain a fiber construction constructed from ultra-fine fiber of single filament fineness of 0.8d or less, a large denier filament yarn including single filaments of a fineness of 3.0 denier or more is at a first stage formed to the desired shape of the fiber construction, and then a process for obtaining the ultra-fine fiber is carried out. As a result, it is possible to avoid the generation of filament breakage as well naps during the spinning process as well as naps and filament breakage during the formation of the fiber construction which would otherwise occur if a yarn of such a fineness is produced from the beginning of the spinning process and is used for producing the fabric. In other words, the use of a large denier filament yarn including single filaments of a fineness of 3.0 denier or more for weaving according to the present invention can eliminate any necessity of sizing as well as twisting, while carrying out an effective weaving operation, on the one hand, and, on the other hand, obtaining a fabric of increased softness and lightness. Thus produced woven fabric as the fiber construction is, thereafter, subjected to a treatment for removing the sea components so that the filaments constructing the fabric become ultra-fine fibers. The treatment for removing the sea components is, depending from the characteristic of the polymer in the sea component, selected from dissolving by water, decomposition by various solution such as an acid solution or an alkali solution, or dissolution by an organic solvent. Furthermore, as disclosed in JP-A-56-118961, prior to the removing treatment, a preliminary treatment for rendering brittle the polymer to be removed is possible. These treatments can be done under conditions of heating or pressurizing so long as the characteristic of the island components which constitute the fiber construction is not deteriorated. When it is necessary to limit the air permeability as is the case for the basic fabric for an air bag, it is desirable that the space occupied by the sea components prior to the removing treatment is simultaneously reduced by shrinkage of the fabric. In view of this, it is desirable that the removal of the sea components is carried out by passage through a solvent while being heated. Furthermore, heating to 70°C or more is preferable for making shrinkage take place. The polymer for use as the sea component can be any conventional polymer. For example, as for a type capable of being dissolved by a solvent, polystyrene can be used. As for a type capable of being dissolved by a water or decomposable by an aqueous solution, a polyester copolymerized with 5 sodium isophthalic acid can be used. Furthermore, another type of water soluble polymer can be used. As an example of a polymer capable of elution by water, JP-A-04-361659 discloses a water soluble polyester, including, as a main acid component, a terephthalic acid, including 5-sodium-sulfo-isophthalic acid in an amount of 8 to 16 mol% and isophthalic acid in an amount of 5 to 40 mol%, and, as a main diol component, ethylene glycol, including a copolymerized poly-ethylene glycol of a molecular weight of 5000 or less, at a ratio of copolymerization of 10 weight% or less. Such a polymer capable of elution by water is more preferable from the view point that damage to the island components during the removing process is reduced. By using the above process, it is possible to obtain, after the sea portion removing treatment, a fiber construction in which the multi-filament yarn (a) is only constructed from a high-strength ultra-fine polyester fiber with a single filament fineness of 0.8 denier or less, (b) has a tenacity of 6.5 g/d or more, (c) has an elongation at break of the multi-filament yarn of 15% or more, and (d) has a total yarn fineness of between 100 d and 1000 d When the method of conjugate spinning of islands-in-sea-type is employed, even if the degree of elongation of the yarn obtained at the end of the conjugate spinning process is as low as about 13%, the following sea component removing treatment at the time of fiber construction causes the elongation of the filament constructed therefrom to be increased so as to attain a degree of elongation of 15% or more. From the view point of obtaining stable the islands-in-sea-type conjugate spinning for producing the high-strength ultra-fine fiber, it is preferable that the island component finally constituting the ultra-fine fiber is a filament yarn constructed from a single kind of polymer. In the fiber construction according to the present invention, it is necessary to harmonize the total denier of the ultra-fine fiber and the single filament denier in accordance with the fiber construction to be made. In a case where the island-in-sea-type conjugate spinning method is employed, it is possible, from the construction of the spineret, to obtain a number of ultra-fine filaments (number of islands) from a conjugate yarn that is larger than the divided number obtained by the divisional type conjugate spinning. In other words, in the divisional type conjugate spinning, there exists a limit to the number of divisions when the single conjugate filament denier is reduced. Thus, due to the existence of a limitation on the number of the divisions in the divisional type conjugate spinning, the denier of a single conjugate filament should be reduced in order to obtain the desired fineness after dividing treatment. Such a reduction in the denier of the single conjugate filament causes the stability of the spinning operation to be impaired. In short, the islands-in-sea-type conjugate spinning method is preferable over the divisional type conjugate spinning in executing stable spinning, while obtaining ultra-fine fiber as desired. Finally, the fiber construction according to the present invention is possibly subjected to a heat set process so long as the characteristics according to the present invention are not substantially impaired. EXAMPLES Now, examples of the present invention will be explained in detail. The various properties referred to in this specification are measured as follows. a. Intrinsic Viscosity (IV) of Polyester Using an Ostwald viscometer, the relative viscosity η rp of a solution, to 25 ml of ortho-chlorophenol, of a sample of 2 gram is measured at a temperature of 25°C, and the intrinsic viscosity (IV) is calculated from the following approximate equation. IV = 0.0242η rp + 0.2634, where, η rp = t × d t 0 × d 0 , and t: time for drop of the solution (sec) t 0 : time for drop of ortho-chlorophenol (sec) d: density of the solution (g/cc), and d 0 : density of the ortho-chlorophenol (g/cc) b. Sulfuric Acid Relative Viscosity (η rn ) of Polyamide A sample of 1 wt% is dissolved in a 98% sulfuric acid, and the relative viscosity is measured using an Ostwald viscometer at a temperature of 25°C. c. Tenacity and Elongation at Break of Conjugate Yarn Measurement is carried out using a method stipulated by JIS-L-1017. d. Tenacity and Elongation at Break of Ultra-Fine Fiber Measurement is carried out after removing the yarn from the fiber construction. In other words, in the case of a woven fabric, yarn samples are disassembled from the fabric. In the case of a knitted fabric or non-woven fabric, filaments as samples are taken out therefrom while preventing the yarn from being damaged.The disassembled filaments are cut to a length of 25 cm and, while applying a load which corresponds to the denier of the filament as obtained by spinning, multiplied by 1/3. Then, the weight of the sample of the above length is measured, from which the denier of the dismantled fiber as a weight of a length of 9000m is calculated. Then, by using a tensile tester, model RTM-100 produced by Orientic corporation, a tensile test is executed on a sample length of 15 cm, at a speed of 30 mm/min, and the strength of the maximum strength point is read out. The tenacity is calculated from the strength as read out divided by the denier of the dismantled filament, and an average value of five samples is calculated. e. Crystallite Volume (C) of Fiber A wide angle X-ray generating device produced by Rigaku Denki K.K. is used. As X-ray source, CuKα with an Ni filter is used, under conditions of output power of 35 KV and 15 mA, and of slit size of 2 mm(). The camera condition is such that the direction of the equator (2) is 10 to 35° in steps of 0.05°, the direction of meridian (2) is 10 to 35° in steps of 0.05°, and the direction of the circumference (2) is 90 to 270° for steps of 0.5°. The measurement is carried out for an accumulated time of 2 seconds.The preparation of the samples for measurement is carried out as follows.The conjugate yarns are cut to a length of 20 cm, and 10 to 20 are collected in a parallel condition. The yarns are then wrapped in gauze to prevent the yarns from being separated. Then, the island removing treatment or dividing treatment is carried out to obtain ultra-fine fibers while preventing the yarns from being folded or crimped. The crystallite size is calculated from a peak width at half height of plane orders (010), (100) and (-105), respectively, obtained by a transmission method using the following Scherrer equation, L(hkl) = Kλ β 0 cos  3 , where L(hkl): the size in a direction transverse to the (hkl) plane of the crystallite, K: 1.0 λ: wavelength of X-ray β 0 : (β E 2 - β 1 2 ) 1/2 β E : peak width at half height as observed (measured value) β 1 : 1.05 × 10 -2 rad. β B : Bragg angle Using the value L as obtained from the above equation, a crystallite volume (C) is calculated from the following equation. C = L (010) × L (100) × L (-105) (Å 3 ) f. Long Period in Meridian Direction (Dm) of Fiber A small angle X-ray generating device (model RU200) produced by Rigaku Denki K.K. is used. As X-ray source, CuKα with an Ni filter is used under conditions of output power of 50 KV and 200 mA, and of slit width of 1 mm(). The camera condition is such that the radius of the camera is 400 mm, Kodak DEF-5 films are used, and the exposure time is 120 minutes.From a distance (r) on the small angle X-ray scattering photograph, the long period (J) can be calculated from the following Bragg equation. J = (λ/2) sin((tan -1 (r/R)) R: camera radius λ: wavelength of X-ray J: long period The high-strength ultra-fine fiber according to the present invention provides a scattering pattern at 4 spots of laminer shape. Thus, from the distance between the scattered points corresponding to the axis of the fiber, a long period Dm (Å) can be calculated from a definition as explained in "KOBUNSHI NO X-SEN (X-ray for High Polymer)", part 2, section 5, by L.E. Alexander, translated by Hamada and Kajii in supervision by Sakurada, published by Kagaku Dojin (publisher), 1973. g. Crystallite Orientation (Fc) of Fiber From a peak width H° at the half height of the strength distribution curve along a Debye ring in an equator line interference at the (010) plane obtained by wide angle X-ray measurement the crystallite orientation Fc can be calculated from the following equation. Fc = (180° - H°)/180° h. Amorphous Orientation (Fb) of Fiber A polarization fluorescence method is employed. A model FOM-1 device produced by Nippon Bunko Kogyo K.K., is used, while employing a transmission light method, where the exciter light wavelength is 365 nm, and the fluorescence wavelength is 420 nm. The result is expressed by a mean value of 5 samples.The samples of ultra-fine fibers or normal fibers obtained as explained in the section (e) are, under non-folded and non-crimped condition, immersed in a solution including fluorescing agent at about 0.2% at 55°C for 4 hours and then washed with water and air-dried to obtain a sample to be tested. i. Tensile Strength of Fabric In accordance with JIS (Japanese Industrial Standard)-K-6328 (strip method), measurement was carried out on a 3 cm sample. The results are expressed as a mean value of the measured values in directions of warp and weft. j. Tearing Strength of Fabric The measurement is carried out in accordance with JIS-L-1096 (trapezoid method). The results are expressed as a mean value of the measured values in directions of warp and weft. k. Softness of Fabric The result is expressed by a degree of softness and stiffness measured in accordance with JIS-L-1096 (45 degree cantilever method). l. Permeability of Fabric This is expressed by the amount of air passing through a fabric when the pressure drop across the fabric is 1.27 cm of water. This measurement is carried out in accordance with the A -method in JIS-L-1096. The result (rate of air pass) is expressed in units of cm 3 /sec/cm 2 . m. Degree of Ease of Making Yarn The degree of ease of operation during treatments for producing the fiber construction according to the present invention is relatively expressed as the number of naps and fiber breaks generated. n. Cover Factor The cover factor K of a fabric is calculated by the following equation. K = N W × D W 1/2 + N F × D F 1/2 , where N W is the warp density (number of warp yarns per one inch), D W is the weft density (number of weft yarns per one inch), N F is the denier of the warp yarn, and D F is the denier of the weft yarn. Example 1 A polyethylene terephthalate (PET) of an intrinsic viscosity (IV) of 1.20 as an island component and a polyethylene terephthalate copolymerized with 5.0 mol% of 5-sodium-sulfo-isophthalic acid (Co-PET), of an intrinsic viscosity (IV) of 0.70 as a sea component were employed. These two polymer components were subjected to a two component conjugate melt spinning by a conventional islands-in-sea method. The number of holes in the spineret was 60. The number of islands in a single conjugate filament was 16. The ratio between island and sea components was 91 (island)/9 (sea). The spinning temperature was 290°C. A heating tube of length 300 mm at a temperature of 320°C was arranged just below the spineret. The spinning speed was 600 m/min. The thus melt spun yarns were, without being wound, continuously subjected to a two stage drawing, where drawing in a heated condition is carried out at a total drawing ratio of 5.5 and a temperature of 215°C at the final drawing roll, and then to a relaxing process at a relaxation ratio of 3.0%, so that a conjugate filament yarn of the islands-in-sea-type of a total denier of 335 (d) and of number of filaments of 60 was obtained. Stable spinning as well as drawing can be carried out without generating a remarkable number of filament breakages or naps. In the obtained filament yarn, the fineness of a single filament as a conjugate filament was 5.58 denier, the tenacity was 8.6 g/d, and the elongation was 14.8%. The sea component of the yarn was dyed by a cation dye and then the cross section of the yarn was observed by a scanning electron microscope. As a result of the observation, it was confirmed that a high quality of the islands-in-sea construction was created. Then, by using the filament yarns as weft and warp yarns, a fabric of a plain weave construction was woven at a warp density of 62 warp yarns per one inch and a weft density of 61 weft yarns per one inch. Although the weaving was carried out under non-sizing condition and non-twisted condition, naps and filament breakages were not generated during the weaving process. Then, the woven fabric was treated, in a relaxed state, in a boiling aqueous solution of 1% sulfuric acid for 60 minutes. The fabric was then passed through an aqueous solution of sodium hydroxide at a temperature of 90°C, thereby removing the sea component (polyethylene terephthalate copolymerized with 5-sodium-sulfoisophthalic acid). The fabric was then subjected to the usual drying and thermal set treatments. The filament constituting the fabric is one in which the original conjugate filament is reduced in fineness. The yarn dismantled from the fabric had a total denier of 310 (d), a number of filaments equal to 960 (= 60 (number of holes) × 16 (number of islands)), a single filament denier of 0.32 (d), a tenacity of 7.6 g/d, and an elongation at break of 19.5%. The weaving density after the heat set treatment was 63 warp yarns per one inch and 61 weft yarns per one inch, and the cover factor of the fabric was 2183. Example 2 As in Example 1, but changing the amount of total extrusion from that in the first embodiment and additionally subjecting the yarn to air-interlacing before winding, a drawn conjugate multi-filament yarn, of the islands-in-sea-type, of a total denier of 235 (d) and of 60 filaments was obtained. The island/sea ratio was 90 (island)/10 (sea). The properties of the obtained filament were such that a single filament denier was 3.92 (d), the tenacity was 8.5 g/d, and the elongation at break was 16.7%, and the degree of the interlacing was 60. Using the yarns as weft and warp yarns, a fabric of a plain weave was woven with a weaving density of 71 warp yarns per one inch and 70 weft yarns per one inch. The weaving was carried out under non-sizing and non-twisting conditions, and nap and filament breakage did not occur. As in Example 1, the fabric was treated, in a relaxed condition, in a boiled aqueous solution of 1% sulfuric acid, and was then passed through an aqueous solution of sodium hydroxide at a temperature of 80°C, thereby removing the sea components. The fabric was then subjected to the usual drying and heat set treatments. The filament constituting the fabric was that obtained by reducing the fineness of the conjugate fiber. In the yarn dismantled from the fabric, the total denier was 222 (d), the number of filaments was 960, the single filament denier was 0.23, the tenacity was 7.4 g/d, and the elongation at break was 20.0%. The weaving density of the fabric after the heat set treatment was 73 warp yarns per inch and 72 weft yarns per inch. The cover factor of the fabric was 2160. Example 3 As in Example 1, but using a spineret having 90 holes and 12 islands in a single conjugate filament and changing the total amount extruded from that in Example 1, a drawn conjugate multi-filament yarn of the islands-in-sea-type with a total denier of 855 (d) and containing 90 filaments was obtained. The island/sea ratio was 87 (island)/13 (sea). The properties of the obtained filament were such that a single filament denier was 9.50 (d), the tenacity was 8.9 g/d, and the elongation at break was 16.5%. Using the yarns as weft and warp yarns, a fabric of a plain weave was woven with a weaving density of 28 warp yarns per one inch and 28 weft yarns per one inch. The weaving was carried out under non-sizing and non-twisting conditions, and nap and filament breakage did not occur. As in Example 1, the fabric was treated, in a relaxed condition, in a boiling aqueous solution of 1% sulfic acid, and then was passed through an aqueous solution of sodium hydroxide at a temperature of 80°C, to thereby remove the sea components. The fabric was subjected to the usual drying and heat set treatments. The filament constituting the fabric was that obtained by reducing the fineness of the conjugate fiber. In the yarn dismantled from the fabric, the total denier was 774 (d), the number of filaments was 1080, the single filament denier was 0.73, the tenacity was 7.4 g/d, and the elongation at break was 21.2%. The weaving density of the fabric after the heat set treatment was 30 warp yarns per inch and 29 weft yarns per inch. The cover factor of the fabric was 1641. Comparative Example 1 As in Example 1, using a polyethylene terephthalate of an intrinsic viscosity (IV) of 1.20 as an island component and a polyethylene terephthalate copolymerized with 5.0% of 5-sodium-sulfo-isophthalic acid of an intrinsic viscosity (IV) of 0.70 as a sea component, two component conjugate melt spinning was carried out using a conventional islands-in-sea-type conjugation spinning. The number of holes in the spineret was 60. However the number of islands in a single hole (conjugate filament) was only 6. The ratio between island and sea was 91 (island)/9 (sea). The spinning temperature was 290°C. A heating tube of length 300 mm of temperature of 320°C was arranged just below the spineret. The spinning speed was 600 m/min. The thus melt spun yarns were, without being wound, continuously subjected to a two stage drawing, where drawing in a heated condition was carried out at a total drawing ratio of 5.5 and a temperature of 215°C at the final drawing roll, and then to a relaxing process at a relaxation ratio of 3.0%, so that an islands-in-sea-type conjugate filament yarn of a total denier of 340 (d) and 60 filaments was obtained. A stable spinning as well as drawing could be done without generating a remarkable number of filament breakages or naps. In the obtained filament yarn, the fineness of a single filament of a conjugate filament was 5.67 denier, the tenacity was 8.6 g/d, and the elongation was 17.5%. Then, by using the filament yarns as weft and warp yarns, a fabric of a plain weave was woven with a warp density of 57 warp yarns per one inch and a weft density of 56 weft yarns per one inch. Although the weaving was carried out under a non-sizing condition and a non-twisted condition of the yarns, naps and filament breakage were not generated during the waving process. Then, the removal of the sea component was carried out in the same way as that in the Example 1. Then, the fabric was subjected to the usual drying and thermal set treatments. The filament constituting the fabric was that in which the original conjugate filament was reduced in fineness. The yarn dismantled from the fabric had a total denier of 320 (d), 360 (= 60 (number of hole) x 6 (number of islands)) filaments, a single filament denier of 0.89 (d), a tenacity of 7.6 g/d, and an elongation at break of 19.4%. The weaving density after the heat set treatment was 58 warp yarns per one inch and 57 weft yarns per one inch, and the cover factor of the fabric was 2058. Comparative Example 2 Except the chips of polyethylene terephthalate used had an intrinsic viscosity (IV) of 0.70, the same islands-in-sea-type conjugate spinning process as that of Example 1 was carried out. The number of holes in the spineret was 60. The number of islands in a single hole (conjugate filament) was 16. The ratio between island and sea was 91 (island)/9 (sea). The spinning temperature was 280°C. No heating tube was used. The spinning speed was 600 m/min. Then, the melt spun yarns were, without being wound, continuously directed to a one stage drawing device with a drawing ratio of 3.3, and then subjected to a relaxing process at a relaxation ratio of 3.0%, so that a conjugate filament yarn of the islands-in-sea-type of a total denier of 235 (d) and with 60 filaments was obtained. A stable spinning as well as drawing can be done without generating a remarkable number of filament breakages or naps. In the obtained filament yarn, the fineness of a single filament as a conjugate fiber was 3.92 denier, the tenacity was 3.9 g/d, and the elongation was 24.0%. The sea component of the yarn was dyed by a cation dye and the cross section of the yarn was observed using a scanning electron microscope. As a result of the observation, it was confirmed that a high quality islands-in-sea construction was created. Then, by using the filament yarns as weft and warp yarns, a fabric of a plain weave construction was woven with a warp density of 71 warp yarns per one inch and a weft density of 70 weft yarns per one inch. Although the weaving was carried out under non-sizing condition and non-twisted condition, naps and filament breakage were not generated during the weaving process. Then, the woven fabric was passed, under a relaxed state, through a boiling aqueous solution of sulfuric acid. Then, the fabric is passed through an aqueous solution of sodium hydroxide at a temperature 80°C, thereby removing the sea component (polyethylene terephthalate copolymerized with 5-sodium-sulfo-isophthalic acid). Then, the fabric was subjected to the usual drying and thermal set treatments. The filament constituting the fabric was that in which the original conjugate filament was reduced in fineness. The yarn dismantled from the fabric had of total denier of 222 (d), 960 filaments, a single filament denier of 0.23 (d), a tenacity of 3.4 g/d, and an elongation at break of 27.2%. The weaving density after the heat set treatment was 73 warp yarns per one inch and 73 weft yarns per one inch, and the cover factor of the fabric was 2175. Comparative Example 3 A fabric was woven as in Comparative Example 2 but the weaving density was changed to 52 warp yarns and 52 weft yarns. The weaving density after the heat set treatment was 53 warp yarns per one inch and 53 weft yarns per one inch, and the cover factor of the fabric was 1579. Comparative Example 4 Using chips of polyethylene terephthalate of an intrinsic viscosity (IV) of 1.20, and using a spineret having 60 holes, melt spinning was carried out using a conventional direct spinning method. The spinning temperature was 300°C. A heating tube of length 300 mm at a temperature of 300°C was arranged just below the spineret. The spinning speed was 500 m/min. The thus melt spun yarns were, without being wound, continuously subjected to a drawing process where drawing in a heated condition is carried out at a total drawing ratio of 5.9 and a temperature of 220°C at the final drawing roll, and then to a relaxing process at a relaxation ratio of 3.0%, while being subjected to an air interlacing treatment, so that a drawn yarn of a total denier of 420 (d) and 60 filaments was obtained. In this case, a stable spinning as well as drawing can be done without generating a remarkable number of filament breakage or naps. In the wound filament yarn, the fineness of a single filament was 7.00 denier, the tenacity was 9.5 g/d, and the elongation was 17.2%. Then, using the filament yarns as weft and warp yarns, a fabric of a plain weave was woven with a warp density of 54 warp yarns per one inch and a weft density of 54 weft yarns per one inch. Although the weaving was carried out under non-sizing condition and non-twisted condition of the yarns, naps and filament breakage were not generated during the weaving process. The weaving density after the heat set treatment was 56 warp yarns per one inch and 55 weft yarns per one inch, and the cover factor of the fabric was 2275. Comparative Example 5 Using chips of polyethylene terephthalate of an intrinsic viscosity (IV) of 1.20, and using a spineret having 240 holes, melt spinning was carried out using a conventional direct melt spinning method. The spinning temperature was 300°C. A heating tube of length 300 mm at a temperature of 300°C was arranged just below the spineret. The spinning speed was 600 m/min. The thus spun yarns were, without being wound, continuously subjected to a drawing process, where drawing in a heated condition was carried out at a total drawing ratio of 5.4 and at a temperature of 220°C at the final drawing roll, and then to a relaxing process at a relaxation ratio of 3.0% while being subjected to an interlacing treatment by air flow, so that a drawn yarn of a total denier of 420 (d) and of number of filaments of 288 was obtained. In this case, due to a sway of the yarn in the heating tube, contact between filaments occurred, causing filament breakages to be frequently generated during the drawing process. In the wound filament yarn, the fineness of a single filament was 1.46 denier, the tenacity was 8.6 g/d, and the elongation was 15.2%. Weaving under the same conditions as specified in the Example 1 was tried. However, a stable operation was impossible due to the frequent occurrence of naps. Comparative Example 6 As in Example 1, but increasing the amount extruded and setting the drawing ratio to a value of 6.1, a conjugate multi-filament yarn of a total denier of 335 (d), with 60 filaments, a single filament denier of 5.58 (d) as a conjugate filament, a tenacity of 10.2 g/d, and an elongation at break of 10.5% was obtained. A small number of naps were generated during the spinning. Then, using the filament yarns as weft and warp yarns, a fabric of a plain weave was woven with a warp density of 62 warp yarns per one inch and a weft density of 61 weft yarns per one inch. The weaving under non-sizing and non-twisted conditions caused a large number of naps to be generated during the waving process. Then, the woven fabric was treated, in a relaxed state, in a boiled aqueous solution of 1% sulfuric acid for 60 minutes. Then, the fabric was passed through an aqueous solution of sodium hydroxide at a temperature 90°C, thereby removing the sea component (polyester copolymerized with 5-sodium-sulfo-isophthalic acid). Then, the fabric was subjected to the usual drying and thermal set treatments. The filament constituting the fabric was that in which the original conjugate filament was reduced in fineness. The yarn dismantled from the fabric had a total denier of 310 (d), 960 filaments, a single filament denier of 0.32 (d), a tenacity of 9.1 g/d, and an elongation at break of 12.1%. The weaving density after the heat set treatment was 63 warp yarns per one inch and 61 weft yarns per one inch, and the cover factor of the fabric as calculated was 2183. However, the quality of the fabric was poor due to the occurrence of naps. As shown in Tables I and II, according to the present invention (Examples 1 to 5), the fiber construction can provide improved properties of mechanical strength, softness, as well as the allowing the yarns to pass the weaving process without generating deficiencies such as naps and filament breakage. Furthermore, in the Examples 1, 2 and 5, where the weaving density is increased to obtain a cover factor of 1900 or more, in addition to the improvement of the mechanical strength, softness and an operational improvement in the weaving, it is also possible to obtain a low gas permeability, which renders the fiber construction qualities suitably balanced for use as a basic fabric for an air bag. Contrary to this, as far as Comparative Example 1 is concerned, after reducing the fineness of the single filament by the removal process, the single filament denier is still excessively large, resulting in a reduction in softness. In Comparative Example 2, the strength is low not only in the state of the conjugate yarn but also in the state after the removal or divisional process for obtaining the ultra-fine fiber. Thus, in comparison with Example 2 according to the present invention, the mechanical property is impaired, which renders the obtained fiber construction unsuitable for industrial use. An increased mechanical strength using such a yarn can be obtained if the total denier is increased by combining a plurality of yarns or if the weaving density is highly increased. However, such a solution necessarily and inevitably causes the softness to be impaired. In Comparative Example 3, the weaving density is further reduced over Comparative Example 2, and therefore the mechanical properties are further impaired. In Comparative Example 4, the finally obtained single filament denier is as large as 7.0 (d), which causes the softness to be impaired. In Comparative Example 5, a yarn with a single filament denier as low as 1.46 (d) is produced by the direct spinning method, thereby producing a large number of filament breaks and naps. In other words, the yarn cannot be spun without generating operational difficulties. In Comparative Example 6, the drawing ratio is excessively increased during the melt spinning process, which causes the elongation of the yarn to be reduced, which causes naps to be generated often during the weaving process, which makes the operational quality poor. The following Table III indicates, for Examples 1 and 3 and Comparative Example 2, the crystallite volume V, a long period (Dm) in the meridian direction, a crystallite orientation (Fc), and an amorphous orientation (Fb). Crystallite Volume (C) (×10 4 ) Å 3 Long Period in Meridian Direction Å Crystallite Volume (Fc) Amorphous Orientation (Fb) Example 1 12.2 150 0.926 0.985 Example 3 12.5 151 0.928 0.987 Comparative Example 2 5.7 99 0.868 0.972 As will be clear from the Table III, according to the present invention, improved values are obtained as to all of the crystallite volume (V), the long period (Dm) in the meridian direction, the crystallite orientation (Fc), and the amorphous orientation (Fb). This result shows, from the X-ray micro structure view point, that the fiber construction according to the present invention is improved in mechanical strength as well as durability. Contrary to this, in Comparative Example 2, all of the listed properties are lower than those in the values of Examples 1 and 3 in the present invention, which can testify that the construction is not suitable for industrial use. In short, the ultra-fine fiber construction formed from a high-strength multi-filament yarn according to the present invention is constructed from ultra-fine filaments of a single filament denier in particular range of values and an increased strength of a value in a particular range of values, which allows the fiber construction to provide desired mechanical properties, which allows the ultra-fine fibers to fully display their designated functions. A fabric as the fiber construction can attain all of the required characteristics such as an increased mechanical strength, a high softness, an increased neatness when folded, as well as a low air permeability. Furthermore, the method for producing the ultra-fine fiber construction according to the present invention can suppress any occurrence of naps and fiber breakage during a melt spinning process as well as a process for formation of the fiber construction. In other words, such a product can be produced without generating any operational problems, which otherwise may lower the productivity.

Claims: A high strength ultra-fine fiber construction including a high strength multi-filament yarn, which said multi-filament yarn has a tenacity of 6.5g/d or more and an elongation at break of 15% or more and consists of ultra-fine filaments, which are polyester fibers having a single fiber fineness less than 0.8d, and wherein the multi-filament yarn has a total yarn fineness of between 100 d and 1000 d. A high-strength ultra-fine fiber construction according to claim 1, wherein the said polyester fibers include polyethylene terephthalate fibers having an intrinsic viscosity (IV) of 0.8 or more. A high-strength ultra-fine fiber construction according to claim 1 or 2, wherein the said polyester fibers have a crystallite volume (C) with a value of 8 X 10 4 Å 3 or more. A high-strength ultra-fine fiber construction according to any preceding claim, wherein the said polyester fibers have a long period in a meridian direction with a value of 120Å or more. A high-strength ultra-fine fiber construction according to any preceding claim, wherein the said polyester fibers have a crystallite orientation (Fc) with a value of 0.90 or more. A high-strength ultra-fine fiber construction according to any preceding claim, wherein the said polyester fibers have an amorphous orientation (Fb) with a value of 0.975 or more. A high-strength ultra-fine fiber construction according to any preceding claim, wherein a single fiber has a fineness of 0.1 denier or more and 0.8 denier or less. A high-strength ultra-fine fiber construction according to any preceding claim, wherein all of the ultra-fine fibers of the high-strength multi-filament yarns are composed of substantially the identical polymer composition. A high-strength ultra-fine fiber construction according to any preceding claim, wherein the said high-strength multi-filament yarn includes ultra-fine fibers obtained by removing a sea portion from an islands-in-sea-type conjugate filament. A high-strength ultra-fine fiber construction according to any preceding claim, wherein the fiber construction is a fabric. A high-strength ultra-fine fiber construction according to claim 10, wherein the fabric is a fabric woven or a knitted from the multi-filament yarn. A high-strength ultra-fine fiber construction according to claim 10 or 11, wherein the fabric has a tensile strength of 100kgf/3 cm, and has a softness, measured by means of a cantilever method, of 60mm or less. A high-strength ultra-fine fiber construction according to claim 10, 11 or 12, wherein the fabric is a basic fabric for an air bag and includes a multi-filament yarn with a total denier between 180 and 450 denier. A high-strength ultra-yarn fiber construction according to claim 13, wherein the basic fabric for the air bag has a cover factor K value of 1900 or more, while no resin coating exists on surfaces of the fabric. A high-strength ultra-fine fiber construction according to claim 13 or 14, wherein the basic fabric for the air bag allows air to pass at the rate of 5.0 cm 3 /sec/cm 2 at a pressure drop of 1.27 cm of water across the fabric, while no resin coating exists in surfaces of the fabric. A method for producing a high-strength ultra-fine fabric comprising the steps of: extruding simultaneously melted polymers of island and sea components, of which the island component polymer is a polyester using a spineret for obtaining a conjugate multi-filament yarn having filaments in an islands-in-sea configuration, with 120 filaments or less, and of a single conjugate filament denier of 3.0 (d) or more; forming a fabric using conjugate multi-filament yarns while the yarns are in a substantially non-twisted and non-sized condition, and; removing the sea component polymer so that the multi-filament yarn from which the fabric is constructed becomes a high strength multi-filament yarn of polyester fibers with a single fiber denier of less the 0.8 (d), a tenacity of 6.5 g/d or more, an elongation at break of 15% or more, and a total yarn fineness of between 100 d and 1000 d. A method for producing a high-strength ultra-fine fabric structure according to claim 16, wherein the polymer of the island component is a meltspun high viscosity polyethylene terephthalate polymer with an intrinsic viscosity (IV) of 1.0 or more. A method for producing a high-strength ultra-fine fabric structure according to claim 16 or 17, which further comprises a step for providing an atmosphere at a temperature between 200 and 350°C just below the spinneret for obtaining a gradual cooling of the yarn, and a step of drawing at a drawing ratio of 3.0 or more, thereby obtaining the conjugate filament yarn of the islands-in-sea-type. A method for producing a high-strength ultra-fine fabric structure according to claim 16, 17 or 18 wherein all island components are constructed from the identical polymer.

Inventors: MIZUKI TATSURO, TAHARA AKIO, TAKAHASHI HIROSHI, MIZUKI, TATSURO TORAY, TAHARA, AKIO, TAKAHASHI, HIROSHI, MIZUKI, TATSURO TORAY - SHATAKU D-303,
Applicants: TORAY INDUSTRIES, TORAY INDUSTRIES, INC.
Application year: 1995
Main classification: D01F8
IPC codes: D01F8, B60R21