Description WOVEN FABRIC FOR AIR BAGS, AIR BAGS AND PROCESS FOR PRODUCTION OF THE WOVEN FABRIC FOR AIR BAGS [TECHNICAL FIELD] [0001] The present invention relates to woven fabric for airbags. [BACKGROUND ART] [0002] Recently along with increasing concern about traffic safety, various airbags have been developed for assured safety of passengers on car accident and commercialized rapidly because of recognition of the effectiveness thereof. [0003] An airbag protects a driver or a passenger by expanding itself in a quite short period of time after vehicle collision and thus, capturing the passenger thrown away in reaction of the collision and absorbing the impact then. For that purpose, the fabric for the bag desirably has low air permeability. The fabric should also have a strength higher than a particular value for endurance to the impact during inflation of the airbag. It is further needed to reduce the yarn displacement, i.e., to improve yarn displacement resistance, in the sewn region as much as possible, to keep the internal pressure in the bag at a particular value or higher for inflation of the airbag and protection of passengers. In addition, compactness in packing is desirable from the points of appearance in the car and relationship with other parts, and there is increasing demand for cost reduction. [0004] Coated fabrics, i.e., airbag fabrics coated with a resin or bonded to a film, have been proposed as means for reducing the air permeability of fabric. [0005] However, resin coating or film bonding is unfavorable for airbag fabrics, because it leads to increase in fabric thickness and decrease in compactness in packing. In addition, addition of such a resin coating step or a film-bonding step also causes a problem of increase in production cost. [0006] To solve the problems above, recently proposed were uncoated fabrics having low air permeability that were prepared by weaving synthetic filament yarns such as of polyamide or polyester fiber at high density without resin processing. For example, disclosed as a means for improving air permeability is a method of using a fabric in a symmetrical fabric structure that was prepared by using a synthetic filament yarns having a yarn fineness of 300 to 400 dtex (see, for example, Patent Document l). If prepared by the method, a fabric consisting of warp and weft yarns of synthetic fiber yarn having a fiber fineness of 300 to 400 dtex at a yarn counts of 23 to 28 yarns/cm in a symmetric fabric structure, wherein the warp yarns and the weft yarns are woven substantially symmetrically in yarn density, has an air permeability of 10 L/dm2-min or less at a test pressure difference AP of 500Pa and shows mechanical properties isotropic in the warp and weft yarn directions. [0007] However, the method demands production of a fabric containing the warp and weft yarns respectively at a density of 23 to 28 yarns/cm, for obtaining favorable air permeability and particular mechanical performances. For that reason, if the edgecomb resistance, an indicator of the degree of airbag yarn displacement in the sewn region by inflation of the airbag for passenger protection, is examined, there is a problem of ill-balance thereof in the fabric warp and weft directions. [0008] On the other hand, known for improvement in productivity while making the base cloth isotropic in strength and flexibility, is a method of specifying the so-called density coefficient rate defined by Formula^ {weft yarn density (yarns/cm)x(weft yarn fiber fineness (denier))1'2} -s- {warp yarn density (yarns/cm)x(warp yarn fiber fineness (denier))1'2}, (see, for example, Patent Document 2). [0009] The method allows reduction the difference in bending resistance between in the fabric warp and weft directions if the density coefficient rate is adjusted to 0.92 or less, and thus, allows improvement in airbag comnactness in nackine. [0010] However, the document only focuses on isotropy in bending resistance. Reduction of air permeability, the most important property for airbag, was not even mentioned there. In addition, when the edgecomb resistance is examined, the fabric edgecomb resistance in the weft direction is extremely low, compared to that in the warp direction, causing a problem of extremely ill-balance between the edgecomb resistances in the warp and weft directions. [0011] In addition, disclosed is an airbag fabric wherein the weaving density of warp yarns and the weaving density of weft yarns are made different from each other for improvement in productivity while keeping the airbag isotropic during inflation (see, for example, Patent Document 3). [0012] However, the document discloses fabrics wherein the weaving density of warp yarns is higher than the weaving density of weft yarns. In the fabrics prepared by the method, the fabric edgecomb resistance in the weft direction is extremely lower than that in warp direction, causing a problem that the balance in the edgecomb resistances in the warp and weft directions is very low. [0013] Alternatively, an airbag base cloth woven at ultrahigh weaving density was disclosed as a base cloth superior in the yarn displacement resistance in the sewn region (see, for example, Patent Document 4). [0014] However, the invention described in the document uses a fabric having high weaving density as the means of raising the edgecomb resistance. Thus, the fabric is inferior in the compactness in packing needed for airbag, and thus, is not a base cloth superior both in edgecomb resistance and compactness in packing. [0015] As described above, there is no airbag fabric produced by conventional technology that is superior all in low air permeability, high strength and compactness in packing that are needed for airbag fabrics and smaller in yarn displacement in the airbag sewn region when the airbag is inflated for protection of passenger. Patent Document 1: Japanese Unexamined Patent Publication No. 3-137245 (Claim 1) Patent Document 2' Japanese Unexamined Patent Publication No. 2001-200447 (Claim 1, Paragraph 0013) Patent Document 3: Japanese Unexamined Patent Publication No. 2000-303303 (Claims 3 and 7, Paragraph 0038) Patent Document 4- Japanese Unexamined Patent Publication No. 2006-16707 (Claim 1) [SUMMARY OF THE INVENTION] [Problems to be Solved by the invention] [0016] An object of the present invention is to provide an airbag fabric and an airbag that eliminates the problems associated with the conventional technology and is favorable in low air permeability and compactness in packing that are needed for airbag fabric and resistance to the yarn displacement in the airbag sewn region when the airbag is inflated for protection of passenger. [Means to Solve the Problems] [0017] Accordingly, the present invention relates to an airbag fabric consisting of warp and weft yarns of the synthetic fiber yarn, characterized by satisfying the following requirements^ (l) the total fineness of the synthetic fiber yarn is 100 to 700 dtex; (2)CF2/CF1>1.10 wherein, CF1 represents the cover factor of the warp yarns-' CF1 = (Dwx0.9)i/2*Nw, CF2 represents the cover factor of the weft yarns: CF2 = (Dfx0.9)1/2*Nf, Dw represents the total fineness of the warp yarn (dtex), Df represents the total fineness of the weft yarn (dtex), Nw represents the weaving density of warp yarns (yarns/2.54 cm), and N£ the weaving density of weft yarns (yarns/2.54 cm); (3) EC1 > 400N and EC2 > 400N, wherein, ECl represents the edgecomb resistance (N) in the warp direction, as determined according to ASTM D6479-02, and EC2 represents the edgecomb resistance (N) in the weft direction, as determined according to ASTM D6479-02; (4) 0.85 < EC2/EC1 < 1.15; and (5) the air permeability, as determined according to the Frajour type method specified in JIS L1096 at a test pressure difference of 19.6 kPa, is 1.0 L/cm2-min or less. [0018] The present invention also relates to an airbag characterized by being produced from the airbag fabric by sewing. [0019] The present invention also relates to a method of producing the airbag fabric, characterized in that the fabric is woven while the warp yarn tension is adjusted to 75 to 230 cN/yarn. [0020] The present invention also relates to the method of producing the airbag fabric according to the present invention, characterized by weaving the fabric while making a difference of 10 to 90% between the tensions of the top and bottom yarns applied during warp yarn shedding. [Advantageous Effects of the Invention] [0021] The present invention provides a fabric superior in low air permeability and packability and also in yarn displacement resistance in the airbag sewn region. [DESCRIPTION OF THE PREFERRED EMBODIMENTS] [0022] The airbag fabric according to the present invention has warp and weft yarns of the synthetic fiber filament satisfying the following requirements-' [Synthetic fiber yarn] Examples of the synthetic fibers for use in the airbag fabric according to the present invention include various synthetic fibers such as polyamide-based fibers, polyesterbased fibers, aramide-based fibers, rayon-based fibers, polysulfone-based fibers, and ultrahigh-molecular weight polyethylene-based fibers. Among them, polyamide and polyester-based fibers, which are produced in great amount and thus cost-effective, are preferable. [0023] Examples of the polyamide-based fibers include fibers of nylon 6, nylon 66, nylon 12, nylon 46, copolymerized polyamide of nylon 6 and nylon 66, copolyamides of nylon 6 with a polyalkylene glycol, a dicarboxylic acid, an amine or the like, and the like. Nylon 6 and nylon 66 fibers are particularly superior in impact resistance and thus preferable. [0024] Examples of the polyester-based fibers include homopolyester such as polyethylene terephthalate or polybutylene terephthalate, or any copolymer thereof additionally containing units derived from another acid component of a polyester, such as an aliphatic dicarboxylic acid, for example, isophthalic acid, 5-sodiumsulfoisophthalic acid or adipic acid. [0025] These synthetic fibers may contain additives such as heat stabilizer, antioxidant, photostabilizer, lubricant, antistatic agent, plasticizer, thickener, pigment, and flame retardant, for improvement in productivity in spinning-drawing and processing steps and in the properties of the fiber. [0026] The crosssectional shape of the synthetic fiber monofilament for use is preferably circular. The fiber may have a flat crosssectional shape instead of the circular crosssectional shape. Use of a flat crosssectioned fiber leads to increase in packing efficiency of the yarn when woven into fabric and reduction in volume of the voids among the fabric monofilaments, and thus, gives a fabric smaller in air permeability, •t compared to the fabric made with a circular crosssectioned fiber having the same fiber fineness in the same fabric structure. As for the flat crosssectional shape, when the monofilament crosssectional shape is approximated to ellipse, a flatness ratio, which is defined as the ratio of the major diameter (Dl) to minor diameter (D2), (D1/D2), is preferably 1.5 to 4, more preferably 2.0 to 3.5. The flat crosssectional shape may be truly geometrically elliptical or, for example, rectangular, rhomboid or cocoon-shaped, and it may also be bilaterally symmetrical or bilaterally unsymmetrical. Alternatively, the shape may be in combination of these shapes. Yet alternatively, it may be a shape based on that above, with some irregularity thereon or voids inside. [0027] In the airbag fabric according to the present invention, yarns of the same synthetic fibers among the synthetic fibers described above are preferably used as the warp and weft yarns. The warp and weft yarns of the same synthetic fiber yarn mean that the warp yarns and the weft yarns are made of a polymer of the same kind, the warp and weft yarns have the same monofilament fineness, and the warp yarn and weft yarns have the same total fineness. [0028] The same kind of polymers means, for example, polymers having a common main recurring unit of polymer such as combination of nylon 66 resins or polyethylene terephthalate resins. For example, a combination of a homopolymer and its copolymer is also included in the same kind of polymers according to the present invention. Further, a combination of copolymers identical in presence or absence, kind and amount of the copolymerization components is preferably for production control, because there is no need for differentiation of the warp and weft yarns. [0029] The phrase "same monofilament fineness or total fineness" means that the variance in the monofilament fineness or total fineness is not larger than 5% of the lowest fiber fineness of the warp and weft yarns. [0030] The synthetic fiber yarn for use in the present invention preferably consists of relatively lowmonofilament-fineness synthetic fiber filaments having a monofilament fineness of 1 to 7 dtex. When the monofilament fineness is 1 dtex or more, it is possible to produce the synthetic fiber filament without any additional device. On the other hand, it is possible to improve the flexibility of the synthetic fiber filament, by making the monofilament fineness 7 dtex or less. The monofilament fineness is more preferably 1.5 to 4.0 dtex, more preferably 2.0 to 3.0 dtex. When the monofilament fineness is in the range specified above, the volume of the voids among the fabric monofilaments becomes smaller, leading to further increase in packing efficiency of the fiber. It may be reduce the rigidity of the synthetic filament and thus improve the packability of the airbag by adjusting the monofilament fineness into the low range above. It is also possible to reduce air permeability. As will be described below, it is also possible to improve the stability of the fabric structure made with the warp and weft yarns drastically and to improve yarn displacement resistance significantly, by producing the fabric under a certain weaving condition, for example in the state where the warp yarn tension is raised. [0031] The tenacity of the synthetic fiber yarn constituting the airbag fabric according to the present invention is preferably 8.0 to 9.0 cN/dtex, more preferably 8.3 to 8.7 cN/dtex both for warp and weft yarns, for satisfying the mechanical properties required for airbag fabric and from the point of spinning operation. [0032] The airbag fabric according to the present invention satisfies the following requirements (l) to (5). [0033] (l) Total fineness of synthetic fiber yarn The total fineness of the synthetic fiber yarn should be 100 to 700 dtex. A total fineness of 100 dtex or more allows preservation of favorable fabric strength. When the total fineness is less than 100 dtex, the weft yarn may be bent because of its low rigidity during formation of the bent warp-yarn structure described below. As a result, the warp yarn does not have a significant bent structure, leading to insufficient increase in the contact length between the warp and weft yarns. In other words, the edgecomb resistance is not increased in the warp direction. In addition, low air permeability is not obtained. Alternatively when the total fineness is 700 dtex or less, it may assure the compact packability and low air permeability. The total fineness is more preferably 200 to 500 dtex, still more preferably 300 to 400 dtex. It is possible, by controlling the total fineness in the range above, to improve the strength, edgecomb resistance, low-air permeability, flexibility, compact packability of the fabric balancedly. [0034] (2) Relationship between the warp yarn cover factor (CFl) and the weft yarn cover factor (CF2) of fabric In the airbag fabric according to the present invention, it is important for the warp yarn cover factor (CFl) and the weft yarn cover factor (CF2) to satisfy the relationship of: CF2/CF1>1.10, preferably the relationship of CF2/CF1>1.12. [0035] The warp yarn cover factor (CF l) or the weft yarn cover factor (CF2) of fiber is a value calculated from the total fineness and the weaving density of the warp or weft yarn. When the total fineness of warp yarn is designated as Dw (dtex), the total fineness of weft yarn as Df (dtex), the weaving density of warp yarns as Nw (yarns/2.54 cm), and the weaving density of weft yarns as Nf (yarns/2.54 cm), they are represented by the following Formulae'-CFl = (Dwx0.9)1/2xNw CF2 = (Dfx0.9)i'2xNf [0036] It is essential to make the weaving density of warp yarns not identical with the weaving density of weft yarns and adjust the ratio of CF2/CF1 to 1.10 or more, for improvement in edgecomb resistance of the fabric balancedly both in the warp and weft directions. [0037] In addition, the airbag fabric according to the present invention preferably has the warp and weft yarns of the same synthetic fiber filaments and also a weaving density of warp yarns Nw and a weaving density of weft yarns Nf that satisfy the relationship of Nf/Nw > 1.10. The weaving density of weft yarns is the number of multifilament yarns woven per 1 inch (2.54cm) of the fabric in the warp direction, as defined in JIS L1096 8.6.1. Alternatively, the weaving density of warp yarns is the number of multifilament yarns woven per 1 inch (2.54cm) of the fabric in the weft direction. A Nf/Nw ratio of 1.10 or more is preferable, for balanced improvement in edgecomb resistance both in the warp and weft directions. Preferably, Nf/Nw > 1.12. [0038] (3) Edgecomb resistance in the warp and weft directions For improvement in balance of the edgecomb resistance in the warp and weft directions, the inventors had studies intensively on the relationship of the cover factor or the weaving density of warp yarns and the cover factor or the weaving density of weft yarns with the edgecomb resistance in both directions. The edgecomb resistance was determined, as the cover factor or the weaving density of warp yarns was kept constant and the cover factor or the weaving density of weft yarns was altered. Results are summarized in Table 1. Table 1 is a table showing the results obtained by measuring the change in the cover factors of warp and weft yarns (CF1 and CF2), the change in the ratio CF2/CF1, the change in the edgecomb resistances in the warp and weft directions (ECl and EC2) and change in the ratio EC2/EC1, while the weaving density of warp yarns was kept constant at 56 (yarns/2.54 cm) and the weaving density of weft yarns was changed to 52, 56, 62, or 64 (yarns/2.54 cm), each at level 1, 2, 3 or 4. [0039] As shown in Table 1, increase in the cover factor or the weaving density of weft yarns relative to the cover factor or the weaving density of warp yarns leads to improvement in the edgecomb resistance of fabric in the weft direction. In addition, the edgecomb resistance in the warp direction is also found to be improved, and thus, the edgecomb resistances are improved both in the warp and weft directions balancedly. The edgecomb resistance is a value, as determined according to ASTM D6479-02; and specifically, the edgecomb resistance in the machine direction is the maximum load applied to the fabric that is measured when a pin is inserted along a weft yarn and the weft yarn is pulled in the direction of warp yarn with the pin, while the edgecomb resistance in the weft direction is the maximum load applied to the fabric that is measured when a pin is inserted along a warp yarn and the warp yarn is pulled in the direction of weft yarn with the pin. [0040] The mechanism for the improvement in the edgecomb resistances in the warp and weft directions of a fabric having a cover factor or a weaving density of weft yarns relatively higher than the cover factor or the weaving density of warp yarns (hereinafter, referred to as "weft-rich fabric") over the edgecomb resistance of a fabric having warp yarns and weft yarns having the same cover factor or weaving density (hereinafter, referred to as, "symmetrical fabric") seems to be as follows: [0041] During weaving in a loom, the weft yarn is inserted between the threads of warp yarns moving upward and downward by shedding movement at a tension greater than that of the warp yarns, and thus, the resulting fabric has a structure in which the weft yarns are strained relatively in the fabric, and in which the warp yarns relatively bent as shuttle in the inside and outside of the fabric. [0042] In the structure of the weft yarn strained and the warp yarns bent, the resistance of the weft yarn to movement in the warp direction is considered to be mainly governed by the contact length with the bent warp yarns, while the resistance of the warp yarn to movement in the weft direction is considered to be mainly governed by the number of contact points. [0043] Specifically, because a weft-rich fabric can have structure in which the warp yarns are bent more significantly than that of the symmetrical fabric and thus the contact length between the warp and weft yarns is more, the resistance to the movement of the weft yarns by the pin, i.e., the edgecomb resistance thereof in the warp direction, seems to be increased. In addition, the edgecomb resistance of warp yarn to movement by a pin and thus, the edgecomb resistance in the weft direction are greater, probably because the number of the contact points between the warp and weft yarns is greater in weft-rich fabric than in symmetrical fabric. [0044] On the other hand, the fabric having a cover factor or a weaving density of warp yarns relatively higher than the cover factor or the weaving density of weft yarns (hereinafter referred to as, "warp rich fabric ") has a smaller number of weft yarns inserted between the treads of warp yarns, and thus, has a warp yarn-bent structure smaller than that of the symmetrical fabric and a smaller contact length between the warp and weft yarns, and consequently, the resistance of the weft yarn to movement by a pin and the edgecomb resistance in the warp direction are smaller. The warp rich fabric has a smaller number of contact points between the warp and weft yarns than symmetrical fabric, and thus, the resistance of the warp yarn to movement in the weft direction by a pin is smaller and thus, the edgecomb resistance thereof in the weft direction is also smaller. [0045] The warp yarn cover factor (CFl) and the weft yarn cover factor (CF2) of fabric are both, preferably 950 to 1250. Control of the cover factor in the range above leads to desirable balanced improvement in compact packability, low-air permeability and edgecomb resistance of the fabric. A cover factor of 950 or more in each case is effective in reducing the air permeability and increasing the edgecomb resistance. A cover factor of 1250 or less in each case is effective in improving the compact packability. [0046] The sum of CFl and CF2 is preferably 2000 or more and less than 2300 for balanced improvement all in compact packability, low-air permeability and edgecomb resistance. A sum of CFl and CF2 at 2000 or more is effective in producing a fabric having low-air permeability and improved edgecomb resistance. Alternatively, the compact packability remains unimpaired when the sum of CFl and CF2 is less than 2300. [0047] The airbag fabric according to the present invention desirably has an edgecomb resistance in the warp direction and an edgecomb resistance in the weft direction respectively of 400N or more, preferably of 450N or more and more preferably of 500N or more. When the edgecomb resistances in both directions are 400N or more, it is possible to prevent yarn displacement in the sewn region as much as possible and retain the internal pressure in the airbag when the airbag is inflated for passenger protection. [0048] (4) Relationship between the edgecomb resistances in the warp and weft directions It is particularly important that the airbag fabric according to the present invention preferably has a ratio of the edgecomb resistance in the warp direction (ECl) to the edgecomb resistance in the weft direction (EC2) satisfying the following Formula^ 0.85 1.10 wherein, CFl represents the cover factor of the warp yarns: CFl = (Dwx0.9)1/2xNw, CF2 represents the cover factor of the weft yarns: CF2 = (Df^0.9)1/2xNf, Dw represents the total fineness of the warp yarn (dtex), Df represents the total fineness of the weft yarn (dtex), Nw represents the weaving density of warp yarns (yarns/2.54 cm), and Nf represents the weaving density of weft yarns (yarns/2.54 cm); (3) ECl > 400N, and EC2 > 400N wherein, ECl represents the edgecomb resistance (N) in the warp direction, as determined according to ASTM D6479-02, and EC2 represents the edgecomb resistance (N) in the weft direction, as determined according to ASTM D6479-02; (4) 0.85 < EC2/EC1 < 1.15; and (5) the air permeability, as determined according to the Frajour type method specified in JIS L1096 at a test pressure difference of 19.6 kPa, is 1.0 L/cm^-min or less. 2. The airbag fabric according to Claim 1, wherein the warp and weft yarns are made of the same synthetic fiber yarn. 3. The airbag fabric according to Claim 1 or 2, wherein the fineness of the monofilament fibers for the warp and weft yarns is 1 to 7 dtex. 4. The airbag fabric according to Claim 1 or 2, wherein the cover factor of the warp yarns CFl and the cover factor of the weft yarns CF2 are both 950 to 1250. 5. The airbag fabric according to Claim 1 or 2, wherein the sum of the cover factors of the warp yarns CFl and the cover factor of the weft yarns CF2 is 2000 or more and less than 2300. 6. An airbag, characterized by being prepared by sewing the airbag fabric according to any one of Claims 1 to 5. 7. A method of producing the airbag fabric according to any one of Claims 1 to 5, characterized by weaving the fabric at a warp yarn tension adjusted to 75 to 230 cN/yarn. 8. A method of producing the airbag fabric according to any one of Claims 1 to 5, characterized by weaving the fabric while making a difference of 10 to 90% between the tensions of the top and bottom yarns applied during warp yarn shedding. 9. The method of producing an airbag fabric according to Claim 7 or 8, wherein a bar temple is used as the temple during weaving.