Airbag fabric possessing improved packed volume and stable air permeability

Abstract: A fabric for use in an air bag is provided. The fabric of the invention is produced by mechanically compressing a preliminary fabric constructed substantially of synthetic yarn such that the packed volume per unit area of the compressed fabric is less than the packed volume per unit area of the preliminary fabric. Air permeability is not adversely affected.

Description: The invention relates generally to a process to provide a plain weave air bag fabric, for use in the manufacture of an air bag, and more particularly one which can be packed into small volumes without unduly affecting air permeability. US 5 073 418 A forms the closest prior art according to the preamble of claim 1. Fabrics used for manufacturing air bags are required in general to possess a limited and controlled air permeability. As will be appreciated, such fabrics are generally woven structures formed from synthetic yarns made up of a plurality of individual filaments. Formation of such fabrics may be carried out on weaving machines using air-jet, water-jet or mechanical means for insertion of filling yarns between a plurality of warp yarns in a manner well known to those of skill in the art. Such woven textile materials are disclosed for example in U.S. Patents 5,704,402 to Bowen et al. (WO 97/57068), wherein the weave structure of such a fabric is disclosed, but not a corresponding process to achieve said structure. Further examples in this regard are shown in 5,566,434 to Beasley; 5,508,073 to Krummheuer et al.; 5,503,197 to Bower et al.; 5,356,680 to Krummheuer et al.; 5,421,378 to Bower et al.; 5,277,230 to Sollars, Jr.; 5, 259, 645 to Hirabayashi et al; 5,110,666 to Menzel, et al.; 5,093,163 to Krummheuer et al.; 5,011,183 to Thornton et al.; 4,977,016 to Thornton et al.; 4,921,735 to Bloch and 3,814,141 to Iribe et al., to which the skilled reader is referred for details. As will be appreciated, very low controlled air permeabilities may be achieved through the use of coatings applied to the fabric construction. The primary coatings of use have been chloroprene (neoprene), silicone and other elastomeric resins. However, the use of such coatings presents a disadvantage from both an economic as well as a functional standpoint. Specifically, the use of coatings may add substantial cost while at the same time adding bulk to the finished product which translates to a greater folded volume of the final configuration thereby requiring a greater allocation of space within the vehicle deployment system. In the attempt to avoid the use of coatings while at the same time achieving low and controlled air permeabilities, a number of approaches have been taken. The patents to Thornton et al. and Bloch propose the achievement of low permeability through the use of calendering to close the voids at the interstices between overlapping yarns in the fabric. While such calendering operations may reduce permeability, such operations also generally stiffen the fabric thereby increasing the volume requirements for a packed bag formed of such calendered material. Fabrics have also been produced using extremely tight weave constructions thereby packing the yarns so tightly together as to achieve the desired low air permeability. One such known construction is a 420 denier nylon 6,6 fabric having 22.5 threads per cm (57 threads/inch) in the warp, and 21 threads per cm (53 threads/per inch) in the fill, and sold under the trade designation MICROPERM™ by Milliken & Company in LaGrange, Georgia. A problem associated with this practice is once again the fact that the fabric produced may have relatively poor foldability due to the very high number of threads per inch within the woven construction which increases the stiffness and hence the packed volume requirement. Packed volume (i.e. foldability) is becoming an increasingly important feature of air bag fabrics. Specifically, good foldability is crucial if the air bag is to be accommodated in the steering wheel of motor vehicles in the least amount of space. In addition, good foldability also makes possible the trouble-free inflation of the air bag for protecting a vehicle occupant in the event of a collision. Further, these issues of packing and trouble-free inflation become even more important as complex folding patterns are utilized to control initial impact in instances where an occupant may be directly facing the deploying cushion. The difficulty in improving foldability is that processes which are recognized to generally improve the drape of a fabric and thereby its foldability such as for example, physical, pneumatic or hydraulic impingement practices also tend to dramatically increase the air permeability of the fabric to levels outside the range generally considered to be desirable. In U.S. Patent 5,508,073 to Krummheuer et al., it has been proposed that improved foldability of air bag fabric can be achieved without sacrificing air permeability so long as yarns having very low filament linear densities are utilized in the construction. In light of the above, a need exists for a fabric for use in an air bag which can be produced with improved foldability without sacrificing physical properties and without being restricted to the use of low DPF yarns. The present invention provides a process for providing such a fabric and therefore represents a useful advancement over the state of the art. The process has the features recited in claim 1 below. Further embodiments are defined in the dependent claims. The present invention can provide an air bag fabric of improved foldability which may be constructed from a broad range of yarn types. The improved foldability is achieved by means of inexpensive mechanical treatment processes without substantially increasing air permeability characteristics of the fabric, which mechanical treatment processes additionally reduce variations in physical properties across the width of the fabric as may be introduced during weaving. The present invention can provide a woven air bag fabric which does not exhibit substantial increases in permeability when subjected to biaxial forces in the warp and fill directions. Surprisingly, it has been found that the improved foldability as measured by packed volume under compressive loading and elimination of substantial increases in permeability can be achieved by mechanically compressing the fabric. Accordingly, in one embodiment of the present invention a woven fabric constructed substantially of synthetic yarn is provided which has undergone processing by mechanical compression. The compressed fabric has a packed volume per unit area of fabric which is less than the packed volume per unit area of the fabric prior to mechanical compression. In another embodiment of the present invention, a woven fabric constructed substantially of synthetic yarn is provided which has undergone processing such that the compressed fabric has a packed volume per unit area of fabric which is less than the packed volume per unit area of the fabric prior to processing. In addition, the static air permeability of the fabric does not substantially increase beyond the value measured in the untensioned state when the fabric is subjected to biaxial tensions in the range up to about 27 kg per cm (150 pounds force per linear inch). In another embodiment of the present invention, these permeability characteristics are achieved for fabric formed using highly efficient plain and basket weave constructions. Other objects, features and aspects of the present invention will be apparent through reference to the description of preferred embodiments and accompanying figures as set forth below. A full and enabling disclosure of the present invention, including the best mode thereof, directed to one of ordinary skill in the art is set forth more particularly in the remainder of the specification including reference to the accompanying figures in which: FIG. 1 is a cross-section view of a typical air bag installation mounted in a steering wheel of an automobile; FIG. 2 is a view illustrating the air bag of FIG. 1, in expanded condition; FIG. 3 is a cross-sectional schematic view of a potentially preferred processing technique for the fabric, according to the present invention; and FIG. 4A and 4B illustrate a folded construction for an air bag fabric useful in the testing of packed volume characteristics. FIG. 5 illustrates a device for measuring packed volume of the fabric according to the present invention; FIG. 6 is a permeability curve for a woven airbag fabric obtainable by the present invention when subjected to tandem biaxial force in both the warp and fill directions; FIG. 7 illustrates the percentage change in permeability in a prior art close packed plain woven air bag fabric and an airbag fabric obtainable by the present invention. Repeat use of reference characters in the present specification and drawings is intended to represent to same or analogous features or elements of the invention. It is to be understood that the illustrated and described exemplary and potentially preferred embodiments are in no way intended as limiting the broader aspects of the present invention to such illustrated and described embodiments. On the contrary, it is intended to cover all alternatives, modifications and equivalents as may be included within the scope of the appended claims. Referring now to FIG. 1, there is illustrated a schematic cross-sectional view of a typical driver's side air bag system 10. As shown, a typical system includes a folded inflatable air bag 12 disposed within a confined module 14 housed within the steering wheel 16 or dash panel if used for passenger side restraint. The air bag 12 is typically formed from a fabric 18 and is fixed to an inflator 20 which in turn is linked to a collision detection sensor (not shown). When the deceleration of the vehicle exceeds a certain level, the collision detection sensor sends a signal to the inflator 20 thereby inducing a chemical reaction of a gas-generating agent to inflate the air bag 12 from the stowed position of Fig. 1 to the inflated position illustrated in Fig. 2 in opposed relation to a vehicle occupant 22. While in the illustrated embodiment, the module 14 is disposed within the steering wheel, it is to be appreciated that the module may also be positioned in any other location opposing a vehicle occupant including the dash panel, door panel, or forward seat as may be desired. As will be appreciated, the material of construction for the air bag 12 typically includes at least a portion of woven fabric. Such fabric is generally woven from synthetic yarns which yarns are, in turn, formed from a plurality of filaments twisted together in known configurations. Filaments formed of polyester or nylon are generally preferred and filaments formed of nylon 6,6 may be most preferred. It is contemplated that suitable linear densities for the yarn used in the fabric according to the present invention may range from about 4.44 tex (40 denier) to about 133.33 tex (1200 denier) while the denier of the individual filaments therein may range from between 0.11 tex (1 denier) to about 1.11 tex (10 denier). The fabrics according to the present invention are preferably manufactured in a relatively tight construction, using either a plain or Panama weave. However, twill weaves may also be utilized if desired. By way of illustration only, and not limitation, some typical constructions for the fabric according to the present invention are set forth in Table I below. Nylon 6,6 Air bag Fabric Fabric Reference Yarn (Denier) tex Filament (Denier) tex Weave Type (Threads per Inch) Threads per cm Warp Fill 1 (210) 23.33 (6) 0.37 Plain (71) 28 (71) 28 2 (210) 23.33 (3) 0.33 Plain (68) 26.8 (70) 27.6 3 (315) 35 (6) 0.67 Plain (60) 23.6 (60) 23.6 4 (315) 35 (3) 0.33 Plain (58) 22.8 (60) 23.6 5 (315) 35 (3) 0.33 Plain (56) 22 (57) 22.5 6 (420) 46.66 (3) 0.33 Plain (49) 19.3 (49) 19.3 7 (420) 46.66 (6) 0.67 Plain (52) 20.5 (56) 22 8 (420) 46.66 (3) 0.33 Plain (50) 19.7 (50) 19.7 9 (630) 70 (6) 0.67 Plain (41) 16.1 (41) 16.1 10 (630) 70 (6) 0.67 Plain (42) 16.5 (42) 16.5 11 (630) 70 (3) 0.33 Plain (39) 15.4 (41) 16.1 12 (840) 13.33 (6) 0.67 Plain (37) 14.6 (37) 14.6 In looking to the data of Table I, it is to be understood that filament deniers of about 3 (0.33 tex) are believed to be representative of low denier per filament constructions while deniers of about 6 (0.67 tex) are believed to be representative of regular denier per filament constructions. The designation of threads per inch is in the state to which the fabric may be finished. That is, the thread density may be achieved either on the loom or through finishing (i.e. scouring and drying). In any event, it is to be understood that these constructions are in no way intended to be limiting to the scope of the invention herein but are provided only as illustrative of air bag fabric types which may benefit from further processing to improve foldability (i.e. reduce packed volume characteristics) through further processing in accordance with the present invention. Testing was carried out on each of the fabric constructions listed in Table I to evaluate both air permeability and packed volume characteristics before and after being subjected to further processing in accordance with the preferred practices of the present invention. Specifically, following formation and any finishing which may have been desired to achieve the constructions as listed in Table I, the fabric was thereafter subjected to compressive forces so as to force the yarns of the fabric closer together thereby tending to increase the density (mass per unit area) of the resulting fabric by about 4 - 10 % or more in comparison to that of the fabric before compression. In the inventive practice illustrated in Fig. 3, the fabric 18 made up of warp yarns 24 and fill yarns 26 is passed adjacent to and in intimate contact with a rubber belt 28 into a nip between a nip roll 30 and a heatable drum 32. At this nip, the rubber belt is elongated due to the curvature around the nip roll and the force exerted by the drum. As the rubber belt 28 exits the nip, it recoils, thereby compacting the fabric which adheres to the rubber belt 28 and slips against the surface of the drum 32. The fabric is preferably held between the belt and the drum for approximately 180° of revolution around the drum 32 so as to permit complete recoil. One potentially preferred piece of equipment for use in practicing such mechanical compression of the fabric 18 is believed to be available from Morrison Textile Machinery Corporation having a place of business at Fort Lawn, South Carolina. While the particular operating parameters utilized in practice of the process as illustrated in Fig. 3 may be varied in a manner as may be desired by those of skill in the art to achieve optimum results, in general, it is believed that in order to reduce the packed volume characteristics of the fabric 18 without unduly increasing the permeability thereof or otherwise degrading the fabric, the temperature of the drum 32 should be maintained between about room temperature and about 163°C (325°F). Lower temperatures of about 120°C (250°F) or less may be preferred. As previously indicated, it has surprisingly been found that air bag fabrics which undergo such compression actually demonstrate improved foldability on a per area basis compared to that demonstrated prior to undergoing such treatment despite the fact that the post treatment fabric is denser on an area basis. At the same time, the air permeability of the fabric is not adversely affected and, in fact, in many instances actually decreases. The fabric produced thus exhibits unexpectedly good properties for use in a folded air bag configuration wherein packed volume and air permeability represent critical parameters. The dynamic air permeability measurements for each of the fabrics is set forth in Table II below as is the packed volume measurements for such fabric for a fixed area of fabric both before and after processing. The fabric reference designations correspond to those set forth in Table I. Nylon 6,6 Air Bag Fabric PRE-TREATMENT POST-TREATMENT Fabric Reference Packed Volume (at 0.4 Pounds Force per in 2) at 2.8 kPa Dynamic Air Permeability at 50kPa Packed Volume (at 0.4 Pounds Force per in 2) at 2.8 kPa Dynamic Air Permeability at 50kPa 1 (25 in 3 ) 410 cm 3 1300 mm/sec (21 in 3 ) 344 cm 3 1200 mm/sec 2 (24 in 3 ) 393 cm 3 850 mm/sec (21 in 3 ) 344 cm 3 900 mm/sec 3 (31 in 3 ) 508 cm 3 1620 mm/sec (23 in 3 ) 377 cm 3 1440 mm/sec 4 (28 in 3 ) 459 cm 3 1210 mm/sec (25 in 3 ) 410 cm 3 1430 mm/sec 5 (33 in 3 ) 541 cm 3 614 mm/sec (27 in 3 ) 442 cm 3 611 mm/sec 6 (30 in 3 ) 492 cm 3 1910 mm/sec (27 in 3 ) 442 cm 3 1580 mm/sec 7 (42 in 3 ) 688 cm 3 520 mm/sec (32 in 3 ) 525 cm 3 628 mm/sec 8 (36 in 3 ) 590 cm 3 753 mm/sec (30 in 3 ) 492 cm 3 672 mm/sec 9 (38 in 3 ) 623 cm 3 1490 mm/sec (34 in 3 ) 557 cm 3 1480 mm/sec 10 (55 in 3 ) 901 cm 3 570 mm/sec (49 in 3 ) 803 cm 3 703 mm/sec. 11 (35 in 3 ) 573 cm 3 644 mm/sec (31 in 3 ) 508 cm 3 722 mm/sec 12 (44 in 3 ) 721 cm 3 1200 mm/sec (36 in 3 ) 590 cm 3 1050 mm/sec As indicated, the air permeability measurements set forth in Table II above are for dynamic air permeability which represents the performance of the fabric under instantaneous application of a differential pressure. Such dynamic testing is believed to provide a more realistic portrayal of fabric performance in an air bag during a collision event wherein the bag is inflated within a few milliseconds. In actually carrying out the testing procedures, the equipment is set at a particular differential pressure desired. The set pressure is then built up within a cylinder and exhausted quickly across the fabric. The measurement in millimeters per second represents the flow of a volume of gas (mm 3 ) through a given area of fabric (mm 2 ) within a short length of time (sec) upon application of a defined differential pressure drop across the fabric. In reference to the data of Table II, values are provided for packed volume of both the pre-treatment and post-treatment fabric at an applied pressure of 2.8 kPa (0.4 pounds force per in 2 ). While performance parameters at a specific pressure have been listed, it is to be understood that such measurements are only for purposes of comparative evaluation between fabric which has undergone treatment to enhance foldability and fabric which has not undergone such treatment evaluated under comparable conditions. In FIG. 6 there is illustrated a permeability curve for a 70 tex (630 denier) fabric using multifilament nylon yarn with a filament linear density of 0.33 tex (3 denier) per filament and having a weave density of 15.7 threads/cm (40 threads per inch) in the warp direction by 15.7 threads/cm (40 threads per inch) in the fill direction which has undergone mechanical compression as taught herein. Static permeability values were taken over a range of biaxial tensions applied across the fabric to replicate the stresses encountered during a collision event. As illustrated, this plain weave fabric is characterized by an air permeability of slightly greater than 27 l/min (1 cfm) in the untensioned state when measured with a differential pressure of 124 Pascal (0.5 inches of water) across the fabric. As the fabric is subjected to biaxial loading in both the warp and the fill direction, the measured air permeability initially decreases and thereafter recovers to a level just above 27 l/min (1 cfm). Importantly, the permeability of the fabric according to the present invention does not spike upwardly as biaxial tensions are applied. This is in dramatic contrast to prior art plain weave fabrics which are known to exhibit permeability increases of several hundred percent when subjected to biaxial tension in the range of interest. In FIG. 7, representative curves illustrating percentage change in permeability taken at a pressure of 500 Pascal (2 inches of water) across the fabric are provided for a high weave density prior art fabric and for a fabric according to the present invention. An important feature of the fabric according to the present invention resides in the fact that although its initial untensioned air permeability is greater than 0.3 m/min (1 cfm per square foot) of fabric at 124 Pa (half an inch of water), as is generally advocated for good permeability performance, the fact that the permeability does not substantially increase when tension is applied indicates that this fabric will actually perform as well as a high weave density, low permeability fabric with a starting untensioned air permeability of substantially less than 27 l/min (1 cfm). Aside for the benefits associated with non-increasing permeabilities, the fabric according to the present invention also exhibits excellent performance character when measured according to other recognized standards. By way of example only and not limitation, the grab tensile strength of the 70 tex (630 denier) fabric according to the present invention has been measured to be in excess of 295 kg (650 pounds) pursuant to ASTM D-5034; the stiffness of the 70 tex (630 denier) fabric according to the present invention has been measured to be less than 0.91 kg (2 pounds) pursuant to ASTM D-4032; the trapezoidal tear of the 70 tex (630 denier) fabric has been measured to be about 68 kg (150 pounds); the tongue tear of the 70 tex (630 denier) fabric according to the present invention has been measured to be about 7 kg (60 pounds) pursuant to ASTM D-2661; and seam strength of the 630 denier fabric as measured pursuant to ASTM D-1683 is about 154 kg (340 pounds) force in the warp direction, about 145 kg (320 pounds) force in the fill direction and about 268 kg (590 pounds) force in the bias direction. The weight of this 70 tex (630 denier) fabric is about 34 g/m 2 . (7.1 ounces/square yard) The comparative evaluation of packed volume characteristics for treated and untreated fabric as set forth in Table II was carried out using a testing technique and apparatus substantially as illustrated in FIGS. 4A, 4B and 5. Specifically, two square fabric panels 34, 36 having a length of 71 cm (28 inches) on each side as illustrated in FIG. 4A were placed in overlying relation to one another so as to simulate the face and back of a simple air bag configuration, after which seams 37 were applied as shown. The seams were formed of 138 nylon thread at 8 - 12 stitches per 2.54 cm (inch). The resulting square double layered fabric configuration was then folded in a fan configuration along fold lines 38 on either side of the double layered fabric configuration to yield a substantially rectangular configuration with fan folds along either elongate boundary edge as shown in FIG. 4B. The layered fabric configuration was thereafter folded in a fan configuration along fold lines 40 at either end so as to yield a substantially square final folded configuration. For evaluation, the fabric structure folded in the manner described was placed in a test confinement chamber 42 having internal dimensions of 12.7 cm x 12.7 cm (5 inches X 5 inches). A platen 44 machined to conform with the internal dimensions of the test confinement chamber 42 is thereafter lowered into the test confinement chamber attached to an In-stron tester such as will be well known to those of skill in the art. The force applied by the platen 44 is monitored by a display 46. As will be readily appreciated, the volume occupied by the fabric within the test confinement chamber for any given applied force can thus be determined by simply monitoring the displacement of the platen 44 within the confinement chamber at such applied force. Moreover, by starting with fabric samples of equal surface areas which are folded in the same manner, a true comparative evaluation of pre-treatment and post-treatment performance is possible. It has been found that the application of 4,5 kg force (10 pounds force) applied by the platen 36 across the 161 cm (25 in 2 ) opening of the test confinement chamber (i.e. 28 kPa (0.4 pounds force per in 2 )) provides good reproducibility in the evaluation. As can be seen through reference to Table II, the packed volume of the pre-treatment fabric was in each case greater than the packed volume of the post-treatment fabric when measured under the same applied pressure. In addition, this beneficial result was achieved without substantially increasing air permeability of the fabric. In addition to the above-identified advantages of improved foldability with retained air permeability character, the processed fabric according to the present invention is believed to provide the further benefit of reducing any variation in physical properties such as air permeability which may exist across the width of a woven fabric. These variations are generally understood to be due to different levels of residual stress induced during the weaving process. Such stresses may differ from yarn to yarn and machine to machine due to slight differences in gripping mechanisms and yarn beat-up. Such residual stresses introduced during the weaving operation can be reduced by balancing the uneven yarn crimp as may exist across the fabric width. This may be achieved by subjecting the fabric to mechanical compression in accordance with the preferred practice of the present invention. The advantages of the fabric according to the present invention can thus be seen to result in a more compact air bag system which does not sacrifice air permeability thereby providing designers with additional flexibility in choices regarding the use of such systems. The air bag system comprises the air bag itself, the accommodation for the air bag in the vehicle, and the control system for releasing the air bag function. Other embodiments of the invention will, of course, be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. However, it is intended that the specification and example contained herein be considered as exemplary only.

Claims: A process to provide a plain weave air bag fabric comprising the steps of: providing an uncoated plain weave fabric (18), characterized in comprising the further step of supplying said plain weave fabric (18) into and around the nip of a continuous rubber belt (28) and a heated drum (32), so as to compact the woven fabric (18) therebetween, and allowing the compacted fabric to recoil as it exits the nip of the belt (28) and the heated drum (32), so as to provide a fabric having a permeability greater than 0.5 cubic centimeter of air per second per square centimeter of fabric (1 cubic foot of air per minute per square foot of fabric) at a pressure drop of 124 Pascal (0.5 inches of water) across the fabric in its untensioned state and having a permeability of less than 2.5 cubic centimeter of air per second per square centimeter of fabric (5 cubic foot of air per minute per square foot of fabric) at a pressure drop of 124 Pascal (0.5 inches of water) across the fabric when the fabric is subjected to substantially equivalent biaxial tension in both the warp and fill direction equal to or less than 27 kilogram force per linear centimeter (150 pounds force per linear inch). The process according to claim 1, wherein the permeability of the fabric (18) never exceeds about 2, preferably about 1.5, and preferred about 1 cubic centimeter of air per second per square centimeter of fabric (4, preferably 3, and preferred 2 cubic foot of air per minute per square foot of fabric) at a pressure drop of 124 Pascal (0.5 inches of water) across the fabric when the fabric is subjected to substantially equivalent biaxial tension forces in both the warp and the fill direction up to about 27 kilogram force per linear centimeter (about 150 pounds force per linear inch). The process according to claim 1, wherein the yarn (24,26) in said fabric is substantially all nylon 6.6. The process according to claim 3, wherein said heated drum (32) operates at a temperature of between room temperature and 163°C (325°F). The process according to claim 4, wherein said drum temperature is below 121°C (250°F). The process according to any of the preceding claims, wherein the fabric (18) is formed using plain and basket weave constructions. The process according to any of the preceding claims, wherein the fabric (18) is a 630 denier fabric. The process according to claim 7, wherein the fabric (18) has a filament density of 3 denier per filament. The process according to claim 8, wherein the fabric (18) has a weave density of 15,7 threads per cm (40 threads per inch) in the warp direction and 15,7 threads per cm (40 threads per inch) in the fill direction. The process according to any of the preceding claims, wherein the air permeability does not increase substantially as biaxial tensions are applied.

Inventors: HURST MICHAEL D, KESHAVARAJ RAMESH, HURST, MICHAEL D., KESHAVARAJ, RAMESH
Applicants: MILLIKEN & CO, MILLIKEN & COMPANY
Application year: 1999
Main classification: B60R21
IPC codes: D03D1, B60R21