Patent Publication Number: US-2005116451-A1

Title: Head-protecting airbag

Description:
The present application claims priorities from Japanese Patent Application No. 2003-394210 of Kino et al., filed on Nov. 25, 2003, Japanese Patent Application No. 2003-399400 of Kino et al., filed on Nov. 28, 2003, Japanese Patent Application No. 2004-062632 of Kino et al., filed on Mar. 5, 2004, and Japanese Patent Application No. 2004-069366 of Kino et al., filed on Mar. 11, 2004, the disclosures of which are hereby incorporated into the present application by reference.  
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to a head-protecting airbag which is folded and housed in upper edge of windows, and is deployable to cover interior side of windows upon inflow of inflation gas.  
      2. Description of Related Art  
      Japanese Patent Laid-Open No. 2001-233156 discloses a head-protecting airbag which is made by hollow-weaving method from polyamide, polyester yarns or the like. Outer surface of the airbag is coated with coating agent such as silicone for keeping internal pressure of the airbag completely inflated.  
      However, since this conventional airbag is coated with coating agent all over its outer surface to enhance air-tightness, it is difficult to suppress a rise of internal pressure when the completely inflated airbag engage an occupant&#39;s head. Therefore, the conventional head-protecting airbag has a room for improvement in suppressing a rise of internal pressure, and increasing energy absorption to assure protection of occupants&#39; heads.  
     SUMMARY OF THE INVENTION  
      An object of the present invention is to provide a head-protecting air bag capable of suppressing a rise of internal pressure, and having improved energy absorbing property to assure protection of occupants&#39; heads.  
      The object of the present invention is achieved by a head-protecting airbag having following constructions:  
      The head-protecting airbag is folded and housed in upper edge of windows inside a vehicle, and is deployable to cover interior side of windows upon inflow of inflation gas. The airbag includes a protection portion inflatable for protecting occupants&#39; heads upon airbag deployment, by separating a vehicle&#39;s inner wall and a vehicle&#39;s outer wall. Both of the inner wall and outer wall are made of fabric, and at least one of the walls is made of uncoated fabric which is not coated by coating agent.  
      In the airbag of the present invention, at least either one of the inner wall or outer wall as part of the protection portion is made of uncoated fabric. Accordingly, if an occupant&#39;s head impacts on the protection portion of the completely inflated airbag, inflation gas leaks from either the inner wall or outer wall made of uncoated fabric, which helps suppress a rise of internal pressure of the protection portion. In comparison with a conventional head-protecting airbag coated by coating agent such as silicone substantially all over outer surface, consequently, the airbag of the present invention contributes to suppress the rise of internal pressure upon engagement of occupant&#39;s head. Moreover, in the airbag of the present invention, since inflation gas leaks uniformly from a substantially entire area of either the inner wall or the outer wall made of uncoated fabric, it is prevented that the internal pressure of the completely inflated airbag rises partially, so that energy generated upon engagement of occupant&#39;s head is uniformly absorbed.  
      Therefore, the head-protecting airbag according to the present invention suppresses a rise of internal pressure, and has improved energy absorbing property for assuring protection of occupants&#39; heads.  
      The remaining wall out of the inner wall and outer wall is desirably made of coated fabric that has a coating layer for preventing gas leakage thereon.  
      When the inner wall is made of coated fabric which has a coating layer on outer surface, the coating layer helps increase coefficient of friction of surface of the inner wall in comparison with a case employing uncoated fabric for the inner wall. Consequently, the occupant&#39;s head becomes unslippery against the inner wall, and therefore, restraint performance is improved. Contrarily, when the outer wall is made of coated fabric which has a coating layer on outer surface, even if a window pane located outward of the outer wall is broken, the outer wall protected by the coating layer is not easily damaged. In addition, since the inner wall is made of uncoated fabric, coefficient of friction of a surface of the inner wall is lower than a case having a coating layer, so that the airbag smoothly deploys in a gap between the occupant&#39;s head and window, even if the gap is narrow. Accordingly, the airbag is desirably employed in a compact car which is limited in space.  
      More specifically, it is desired that air permeability H of the uncoated fabric is in a range of 5.0 cm 3 /cm 2 ·s·H 25.0 cm 3 /cm 2 ·s. Furthermore, it is desired that thickness t of the protection portion at complete inflation of the airbag is predetermined in a range of 100 mm·t·280 mm.  
      In the above airbag, moreover, it is desired that yarn density of the coated fabric is smaller than yarn density of the uncoated fabric.  
      An airbag with this arrangement is lighter in weight than an airbag having the same yarn density in both the inner wall and outer wall, for a difference of the yarn density between the outer wall and inner wall. Moreover, since a wall made of coated fabric is thinner than a wall made of uncoated fabric, the airbag having this arrangement is folded into a compacter shape compared to an airbag having the same yarn density for both the inner wall and outer wall.  
      It is also appreciated that both of the inner wall and outer wall are made of uncoated fabric whose air permeability H is in a range of 5.0 cm 3 /cm 2 ·s·H·25.0 cm 3 /cm 2 ·s.  
      With this arrangement, too, when an occupant&#39;s head impacts on the protection portion of the completely inflated airbag, inflation gas leaks uniformly from an entire area of the inner wall and outer wall of the protection portion, which contributes to suppress a rise of internal pressure of the protection portion. Accordingly, the completely inflated airbag properly protects an occupant&#39;s head by high energy absorbing property of the protection portion.  
      In a head-protecting airbag like this, too, it is desired that thickness t of the protection portion at complete inflation of the airbag is predetermined in a range of 100 mm·t·280 mm. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
       FIG. 1  is a front view of a head-protecting airbag device employing a first embodiment of an airbag according to the present invention, as viewed from vehicle&#39;s interior;  
       FIG. 2  is a front view of an airbag of the first embodiment flatly developed;  
       FIG. 3  is an enlarged section taken along line III-III of  FIG. 2 ;  
       FIG. 4  is a schematic enlarged section taken along line IV-IV of  FIG. 1 ;  
       FIG. 5  shows a graph of results from impacter tests conducted on airbags of the first embodiment;  
       FIG. 6  is a schematic section of a modification of the airbag of the first embodiment;  
       FIG. 7  is a schematic section of another modification of the airbag of the first embodiment;  
       FIG. 8  is a front view of an airbag of the second embodiment flatly developed;  
       FIG. 9  is an enlarged section taken along line IX-IX of  FIG. 8 ; and  
       FIG. 10  shows a graph of results from impacter tests conducted on airbags of the second embodiment. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS  
      Preferred embodiments of the present invention are described below with reference to the accompanying drawings. However, the invention is not limited to the embodiments disclosed herein. All modifications within the appended claims and equivalents relative thereto are intended to be encompassed in the scope of the claims.  
      Referring to  FIG. 1 , it is seen that an airbag  18  being a first embodiment of the present invention is employed in a head-protecting airbag device M mountable on a vehicle V. The head-protecting airbag  18  is folded and housed in a front pillar FP and a roof side rail RR in upper edges of doors and windows W 1 , W 2  and a rear pillar RP. The vehicle V includes a center pillar CP between the front pillar FP and the rear pillar RP, which is arranged substantially vertically.  
      The head-protecting airbag device M includes an inflator  8 , mounting brackets  9  and  13 , mounting bolts  10  and  14 , and the airbag  18 , and is housed while being covered by an airbag cover  16  at vehicle&#39;s interior side. The airbag cover  16  is constituted by lower edges of a front pillar garnish  3  covering vehicle&#39;s interior side of the front pillar FP and a roof head lining  4  covering vehicle&#39;s interior side of the roof side rail RR.  
      The front pillar garnish  3  and the roof head lining  4  are made from synthetic resin, and are attached to vehicle&#39;s interior side of an inner panel  2  as part of vehicle body  1  in the front pillar FP and the roof side rail RR by not-shown mounting means. The lower edges of the front pillar garnish  3  and the roof head lining  4  are adapted to open inward at their lower ends to allow the deploying airbag  18  to protrude therefrom.  
      The inflator  8  has a substantially cylindrical shape, and is provided toward its leading (front) end with not-shown gas discharge ports for discharging inflation gas. The leading end part of the inflator  8  including the vicinity of the gas discharge ports is inserted into a later-described gas inlet port  22  of the airbag  18 . Thus and by means of a clamp  11  mounted around a rear end of the inlet port  22 , the inflator  8  is connected to the airbag  18 . The inflator  8  is attached to the inner panel  2  of the vehicle body  1  by a mounting bracket  9  holding the inflator  8  and mounting bolts  10  for securing the mounting bracket  9  to the inner panel  2 .  
      An output ratio X (KPa/L) of the inflator  8  used in the foregoing embodiment to capacity of the airbag  18  (inflator output/airbag capacity) is desirably in a range of 8·X·18 (more desirably, 10·X·15). If the output ratio X is less than 8 KPa/L, it is difficult for the airbag to retain enough internal pressure when completely inflated. To the contrary, if the output ratio X exceeds 18 KPa/L, the internal pressure of the completely inflated airbag is too high. Both cases have problems in proper protection of occupants&#39; heads. The output ratio X of the inflator  8  used in the foregoing embodiment is predetermined to be 10.4 KPa/L. Output of the inflator  8  is 260 KPa (28.3 L tank), and capacity of later-described gas admissive portion  19  of the airbag  18  is 25 L.  
      Here, the inflator  8  is mounted on the vehicle V as part of an airbag module that is composed of the inflator  8  and the airbag  18  assembled together.  
      The airbag  18  is housed in a folded state from the front pillar FP extending obliquely upward to a position above the rear pillar RP in the roof side rail RR, passing over the center pillar CP. As indicated by double-dotted lines in  FIG. 1 , the airbag  18  upon deployment is adapted to cover vehicle&#39;s interior side of each of the windows W 1 , W 2 , the center pillar CP and the rear pillar RP.  
      The airbag  18  is formed by hollow-weaving method of polyester, polyamide yarns or the like. The airbag  18  includes a gas admissive portion  19  which admits inflation gas G inside to separate its vehicle&#39;s inner wall  19   a  and vehicle&#39;s outer wall  19   b , and a non-admissive portion  29  which admits no inflation gas G. As shown in  FIG. 3 , the airbag  18  has a coating layer  37  all over the outer surface of the outer wall  19   b . That is, in the airbag  18 , the outer wall  19   b  is made of coated fabric with the coating layer  37 , while the inner wall  19   a  is made of uncoated fabric without a coating layer.  
      The gas admissive portion  19  includes a gas feed passage  21 , a gas inlet port  22 , and a protection portion  23 . In a head-protecting airbag, a gas admissive portion usually has a capacity of 10 to 40 L. The gas admissive portion  19  of the airbag  18  has a capacity of 25 L.  
      The gas feed passage  21  is arranged in front-rear direction of vehicle V along an upper edge  18   a  of the airbag  18 . In the illustrated embodiment, the gas feed passage  21  joins an upper end of a later-described vertical chamber  27 B of a front protection portion  24  and an upper end of a later-described vertical chamber  27 E of a rear protection portion  25 . The gas feed passage  21  introduces inflation gas G discharged from the inflator  8  into the protection portion  23  located below the gas feed passage  21 . The gas inlet port  22  to be joined with the inflator  8  is communicated with the gas feed passage  21 , and projects upward from a longitudinally middle position of the gas feed passage  21 . In the illustrated embodiment, the inlet port  22  is located above a vertical chamber  27 C of the front protection portion  24 , and is opened rearward.  
      The protection portion  23  is adapted to cover interior sides of windows W 1  and W 2  for protecting occupants&#39; heads upon deployment of the airbag  18 . The protection portion  23  includes a front protection portion  24  for covering an interior side of window W 1  at side of front seat, and a rear protection portion  25  for covering an interior side of window W 2  at side of rear seat.  
      Each of the front and rear protection portions  24  and  25  are partitioned by later-described partitioning portions  32  into a plurality of vertical chambers  27  each of which extends vertically. The vertical chambers  27  line up in front-rear direction in each area of the front and rear protection portions  24  and  25 . In the illustrated embodiment, the front protection portion  24  includes three vertical chambers  27 A,  27 B and  27 C, and the rear protection portion  25  includes two vertical chambers  27 D and  27 E. The vertical chambers  27 B and  27 E are communicated with the gas feed passage  21  at upper ends. Remaining vertical chambers  27 A,  27 C and  27 D are closed at upper ends. The vertical chambers  27 A and  27 C are communicated with the vertical chamber  27 B at lower end, and the vertical chamber  27 D is communicated with the vertical chamber  27 E at lower end. In other words, the vertical chambers  27 A,  27 C and  27 D admit inflation gas G via the vertical chambers  27 B and  27 E.  
      In the foregoing embodiment, thickness t of the individual vertical chambers  27  at complete inflation is predetermined in a range of 100 mm·t·280 mm (desirably in a range of 120 mm ·t·250 mm, and further desirably, in a range of 140 mm·t·200 mm). If the thickness at complete inflation is less than 100 mm, the airbag  18  becomes too thin to protect an occupant&#39;s head when engaging the head. To the contrary, if it exceeds 280 mm, the airbag  18  becomes too thick upon deployment to go in between a gap between the occupant&#39;s head and window when the gap is narrow, which hinders smooth deployment of the airbag  18 . In the illustrated embodiment, thicknesses t 1 , t 2  and t 5  of the vertical chambers  27 A,  27 B and  27 E at complete inflation are 150 mm, as shown in  FIG. 3 . Thickness t 3  of the vertical chamber  27 C at complete inflation is 100 mm, and thickness t 4  of the vertical chamber  27 D is 130 mm.  
      The thickness t 3  of the vertical chamber  27 C at complete inflation is predetermined smaller than the thicknesses t 1 , t 2 , t 4  and t 5  of other vertical chambers  27 A,  27 B,  27 D and  27 E. The vertical chamber  27 C is adapted to cover interior side of the center pillar CP upon deployment of the airbag  18 , and needs to deploy in a narrower gap between the center pillar garnish  5  and a seat located inward of the garnish  5 . Accordingly, it is desired that the vertical chamber  27 C is deployed in a thin state not having admitted much gas. Moreover, since the center pillar garnish  5  is projected inward than adjacent windows W 1  and W 2 , the vertical chamber  27 C for covering interior side of the center pillar garnish  5  is preferably thinner than other vertical chambers  27  upon air bag deployment. For these reasons, the thickness t 3  of the vertical chamber  27 C is predetermined smaller than those of other vertical chambers  27 A,  27 B,  27 D and  27 E by being partitioned in substantially middle position in its front-to-rear dimension in the upper end with a later-described extension  32   a  of a partitioning portion  32 A.  
      The non-admissive portion  29  has a configuration in which the vehicle&#39;s inner wall  19   a  and the outer wall  19   b  are joined together. The non-admissive portion  29  includes mounting portions  30 , a peripheral portion  31 , partitioning portions  32 , and a panel portion  33 . The peripheral portion  31  is located to encircle the gas admissive portion  19  in outer periphery of the airbag  18 . A joint cloth  35  is joined at front end of the peripheral portion  31 .  
      The mounting portions  30  project upward from upper edges of the peripheral portion  31  and the joint cloth  35  in the upper edge  18   a  of the airbag  18 . The mounting portions  30  are provided in plurality ( 6 , in the illustrated embodiment). As shown in  FIG. 4 , a mounting bracket  13  for mounting the airbag  18  to the inner panel  2  is attached to each of the mounting portions  30 . Each of the mounting portions  30  is secured to the inner panel  2  together with the mounting bracket  13  by a bolt  14 .  
      In the airbag  18 , a mounting portion  30 A located in the vicinity of front end of the joint cloth  35  is attached to vehicle body  1  in a lower part of the front pillar FP. Upon deployment of the airbag  18 , accordingly, a tension in front-rear direction is exerted between the mounting portion  30 A and a mounting portion  30 B located in the vicinity of rear end of the airbag  18 . In the airbag  18  having a plurality of vertical chambers  27  lined up in front-rear direction, especially, each of the vertical chambers  27  inflates in shrinking manner in front-rear direction, and therefore, a great tension is exerted in front-rear direction upon deployment of the airbag  18 . Therefore, even if an occupant&#39;s head is in a position of the joint cloth  35  upon airbag deployment, the head is prevented from sliding out of the joint cloth  35 .  
      The panel portion  33  has a rectangular sheet shape, and is located between the front and rear protection portions  24  and  25  below the gas feed passage  21 . The panel portion  33  serves to define an entire shape of the airbag  18 , and to minimize the time to complete inflation of the airbag  18  by reducing a volume of the gas admissive portion  19 .  
      Each of the partitioning portions  32  extends from upper edges of the peripheral portion  31  or the panel portion  33  into an area of the front or rear protection portion  24 / 25 . The partitioning portions  32  serve to regulate thickness of the airbag  18  at complete inflation by partitioning the front and rear protection portions  24  and  25  into the vertical chambers  27 . A partitioning portion  32 A located in the vertical chamber  27 C has an extension  32   a  extending downward in middle position of front-to-rear dimension in the upper end of the vertical chamber  27 C. The extension  32   a  partitions the vertical chamber  27 C in middle position of front-to-rear dimension in the upper end part for regulating the thickness of the vertical chamber  27 C which is located inward of the center pillar CP at complete inflation.  
      The coating layer  37  provided on outer surface of the vehicle&#39;s outer wall  19   b  is formed by coating agent such as silicone for preventing gas leakage. As shown in  FIG. 3 , the coating layer  37  is formed on outer side of the outer wall  19   b  so as to cover an exterior side  0  of the airbag  18  substantially all over.  
      Air permeability H of the uncoated vehicle&#39;s inner wall  19   a  is predetermined in a range of 5.0 cm 3 /cm 2 ·s·H·25.0 cm 3 /cm 2 ·s (desirably, in a range of 8.0 cm 3 /cm 2  ·s·H·20.0 cm 3 /cm 2 ·s). If the air permeability H is less than 5.0 cm 3 /cm 2 ·s, a fabric forming the inner wall  19   a  becomes so airtight that internal pressure of the completely inflated airbag  18  is hardly suppressed from rising at engaging an occupant&#39;s head, which annuls a meaning of employing uncoated fabric. On the other hand, if the air permeability exceeds 25.0 cm 3 /cm 2 ·s, gas leaks from the completely inflated airbag  18  so much that the airbag  18  cannot restrain an occupant&#39;s head with sufficient cushioning property when the head moving outward of the vehicle has high kinetic energy. Here, air permeability H in this specification is measured according to JIS L 1096 8.27.1 A method (Frazier method).  
      In the airbag  18  in the foregoing embodiment, air permeability H of the vehicle&#39;s inner wall  19   a  is predetermined at 16.67 cm 3 /cm 2 ·s (refer to Table 1). The inner wall  19   a  and outer wall  19   b  are woven by 6,6 Nylon yarn, and silicone is used as a coating agent forming the coating layer  37 .  
      How the airbag  18  is mounted on vehicle V is now described. The airbag  18  is manufactured by hollow-weaving method except the joint cloth  35 , and coating agent is applied all over the exterior side  0  of the outer wall  19   b  to form the coating layer  37 . Then the joint cloth  35  is joined thereto. Subsequently, the airbag  18  is folded-up. More specifically, the airbag  18  is bellows-folded, from flat expanded state, on subsequent crest and valley folds C extending in front-rear direction, as indicated by double-dotted lines in  FIG. 2 , so that a lower edge  18   b  of the airbag  18  is brought closer to the upper edge  18   a.    
      After the folding work, a not-shown breakable wrapping member is wound around the airbag  18  for keeping the folded-up configuration, and the mounting bracket  13  is attached to each of the mounting portions  30 . In the meantime, the inflator  8  is joined with the gas inlet port  22  utilizing the clamp  11 , and the mounting bracket  9  is mounted therearound. Thus the inflator  8  is assembled with the airbag  18  to form an airbag module.  
      By locating the individual mounting brackets  9  and  13  at predetermined positions of the inner panel  2 , and fastening them with the bolts  9  and  14 , thereafter, the airbag module is mounted on the vehicle body  1 . Then, a not-shown lead wire extending from a predetermined control device for actuating the inflator is connected to the inflator  8 . If the front pillar garnish  3 , the roof head lining  4 , and further the center pillar garnish  5  and the rear pillar garnish  6  are attached to the vehicle body  1 , the airbag  18  is mounted on the vehicle V together with the airbag device M.  
      When the inflator  8  is actuated after the airbag device M is mounted on the vehicle V, inflation gas G is discharged from the inflator  8  and flows through the gas feed passage  21  from the gas inlet port  22 , as indicated by double-dotted lines in  FIG. 2 . Then gas G flows into the protection portion  23  from the gas feed passage  21 , and the protection portion  23  starts to inflate while unfolding. The airbag  18  then breaks the wrapping member, protrudes downward by pushing and opening the airbag cover  16  in the lower edges of the front pillar garnish  3  and the roof head lining  4 , and inflates to cover interior sides of the windows W 1  and W 2 , the center pillar CP, and the rear pillar RP, as indicated by double-dotted lines in  FIG. 1 .  
      In the airbag  18 , the inner wall  19   a  as part of the protection portion  23  is made of uncoated fabric having no coating layer. Accordingly, if an occupant&#39;s head engages the protection portion  23  (the front/rear protection portion  24 / 25 ) of the completely inflated airbag  18 , inflation gas G leaks from the inner wall  19   a , which helps suppress a rise of internal pressure of the protection portion  23 . In comparison with a conventional head-protecting airbag coated by coating agent such as silicone substantially all over outer surface, consequently, the airbag  18  in the foregoing embodiment contributes to suppress the rise of internal pressure at engaging the occupant&#39;s head. Moreover, in the airbag  18 , since inflation gas leaks uniformly from a substantially entire area of the inner wall  19   a , it is prevented that the internal pressure of the completely inflated airbag  18  rises partially, so that energy generated upon engagement of the occupant&#39;s head is uniformly absorbed.  
      Therefore, the head-protecting airbag  18  in the foregoing embodiment suppresses a rise of internal pressure, and has improved energy absorbing property to assure protection of an occupant&#39;s head.  
      In the airbag  18 , the outer wall  19   b  as part of the protection portion  23  is made of coated fabric having a coating layer  37  of silicone or the like for preventing gas leakage on outer surface. Accordingly, even if a window pane located outward of the outer wall  19   b  is broken upon completion of inflation of the airbag  18 , the outer wall  19   b  protected by the coating layer  37  is hard to damage. In addition, since the inner wall  19   a  is made of uncoated fabric, coefficient of friction of a surface of the inner wall  19   a  is lower in comparison with a case a coating layer is applied on surface of the inner wall, so that the airbag  18  smoothly deploys in a gap between the occupant&#39;s head and window, even if the gap is narrow. The airbag  18  is desirably employed in a compact car which is limited in space.  
      Furthermore, the inner wall  19   a  of the airbag  18  is made of uncoated fabric whose air permeability H is predetermined in a range of 5.0 cm 3 /cm 2 ·s·H·25.0 cm 3 /cm 2 , so that a thickness t of the protection portion  23  upon complete inflation of the airbag  18  is in a range of 100 mm ·t·280 mm. Therefore, the occupant&#39;s head is properly protected by the protection portion  23  of the completely inflated airbag  18 .  
       FIG. 5  shows a graph showing results of impacter test testing some airbags that meet requirements of the first embodiment. Airbags of Examples 1 and 2 are airbags that meet the requirements of the first embodiment. An airbag of Example 1 is the airbag  18 . An airbag of Example 2 has the same construction as that of Example 1 except in that its thickness t is 180 mm.  
      The impacter test was also conducted for airbags of Comparative Examples 1 and 2. An airbag of Comparative Example 1 is made from 6,6 Nylon yarn, and has the same shape as that of Example 1. This airbag is coated substantially all over outer surface by silicone or the like such that both inner wall and outer wall have coating layers on outer sides, as shown in Table 1. An airbag of Comparative Example 2 has the same shape as that of Example 1, but has no coating layer.  
      As shown in Table 1, the airbag of Example 1 and the airbag of Comparative Example 1 have substantially the same values in yarn density, tensile strength and tear strength. In other words, the airbag of Example 1 and the airbag of Comparative Example 1 differ from each other in existence of coating layers, i.e., the airbag of Example 1 has a coating layer only on outer surface of the vehicle&#39;s outer wall, while the airbag of Comparative Example 1 has coating layers both on outer wall and inner wall. On the other hand, the airbag of Comparative Example 2 differs from the airbag of Example 1 in not having any coating layers on the inner wall or on the outer wall. Here, air permeability H of the outer walls of Examples 1 and 2, and of the inner and outer walls of Comparative Example 1 are almost 0 since these walls have coating layers thereon. Actually, air permeability H of these walls were unmeasurable.  
      The impacter test was conducted by moving a hammer head having 6.8 kg weight toward protection portions of completely inflated airbags at 7.6 m/s velocity, substantially horizontally to be perpendicular to the protection portions, and measured deceleration and moving amount of the hammer head upon impact on the protection portions. In the graph of  FIG. 5 , each area of portions encircled by traces drawn by changes of deceleration and moving amount of hammer heads represents absorbing amount of energy of the hammer head by the protection portion of the airbag.  
      In the tests, the airbag of Comparative Example 1 is highly airtight since it is provided on both the inner and outer walls with coating layers, so that inflation gas does not easily leak from the inner wall or outer wall. Accordingly, when the hammer head impacts on the protection portion, the protection portion decelerates the hammer head suddenly in restraining it. When the movement of the hammer head toward the protection portion then stops completely, the protection portion pushes the hammer head back to a position before impact suddenly by a reaction force produced by a rise of internal pressure of the protection portion caused by engagement of the hammer head.  
      Contrarily in the airbags of Examples 1 and 2, when the hammer head impacts on the protection portions, the protection portions restrain the hammer head while leaking inflation gas from the inner wall. Although moving amounts of the hammer head toward the protection portions are greater than in Comparative Example 1, accordingly, the protection portions decelerate the hammer head gradually in restraining it. Since gas leakage from the inner wall helps suppress the rise of internal pressure of the protection portions, when the movement of the hammer head toward the protection portions stops completely, the hammer head is pushed back to a position before impact gradually. At this time, deceleration (acceleration in being pushed back) of the hammer head is even lower than the deceleration when moving toward the protection portions. As shown in  FIG. 5 , consequently, traces of test results of the airbags of the first embodiment have greater areas than that of a test result of the airbag of Comparative Example 1. That is, the airbags in Examples 1 and 2 are superior in energy absorbing property to the airbag of Comparative Example 1.  
      On the other hand, the airbag of Comparative Example 2 leaks inflation gas from both outer wall and inner wall of the protection portion when the hammer head impacts on the protection portion. Accordingly, the hammer head bottomed out because of leakage of great deal of inflation gas, and was not restrained by the protection portion.  
      The test results show that the airbag  18  is capable of suppressing a rise of internal pressure of the protection portion  23  when an occupant&#39;s head engages the protection portion  23 , and has improved energy absorbing property for assuring protection of an occupant&#39;s head, as shown in  FIG. 5 , on condition that the inner wall  19   a  is made of uncoated fabric while the outer wall  19   b  is made of coated fabric having a coating layer  37  on outer surface.  
      Table 1 also discloses a result of impacter test conducted on Comparative Example 3 employing a hollow-woven airbag in which air permeability H of an uncoated inner wall is 33.33 cm 3 /cm 2 ·s, and thickness t at completion of inflation is 180 mm. In this airbag of Comparative Example 3, when the hammer head impacts on the protection portion, a great deal of inflation gas leaks from the inner wall because of too high permeability H of the inner wall, so that the hammer head was not restrained by the protection portion. If a load (weight or speed) of the hammer head is lower than in the impacter test, the airbag of Comparative Example 3 would be able to restrain the hammer head properly, too. However, if the uncoated inner wall  19   a  is made of fabric whose air permeability H is predetermined in a range of 5.0 cm 3 /cm 2 ·s·H·25.0 cm 3 /cm 2 ·s, the occupant&#39;s head is properly protected even if the head having an increased kinetic energy impacts on the protection portion  23 , which is more preferable.  
      Although the airbag  18  has the coating layer  37  on the outer wall  19   b , it will also be appreciated, as in an airbag  18 A shown in  FIG. 6 , that an inner wall  19   a  located at interior side I is made of coated fabric having a coating layer  37  on outer surface, while an outer wall  19   b  located at exterior side O is made of uncoated fabric. If the inner wall  19   a  is made of coated fabric like this, coefficient of friction of a surface of the inner wall  19   a  is increased in comparison with a case where the inner wall  19   a  is made of uncoated fabric. This enhances restraint performance of occupant&#39;s heads by making the head unslippery against the inner wall.  
      An airbag  18 B shown in  FIG. 7  may be adopted, too. In the airbag  18 B, yarn density of an outer wall  19   d  having a coating layer  37  is lower than that of an inner wall  19   c  having no coating layer, e.g., the yarn density MD 67.5 yarn/24.5 mm and CD 60.5 yarn/24.5 mm of the inner wall  19   c , against the yarn density MD 45.0 yarn/24.5 mm and CD 45.0 yarn/24.5 mm of the outer wall  19   d . The coating weight of the coating agent forming the coating layer is 57.2 g/m 2  in the airbag  18 B, too, as in the airbag  18  in the first embodiment. The airbag  18 B with this arrangement is lighter in weight than the airbag  18  which has the same yarn density in both the inner wall  19   a  and outer wall  19   b , for a difference of the yarn density of the outer wall  19   d . Moreover, since the outer wall  19   d  is thinner than the inner wall  19   c , the airbag  18 B is folded into a compacter shape compared to the airbag  18 . The yarn density of fabric used for a wall having a coating layer is desirably 45.0 yarn/24.5 mm or more each for MD/CD, because thinner fabric will require more coating agent.  
      Although the head-protecting airbags  18  and  18 A in the foregoing embodiments are described as manufactured by hollow-weaving method, the airbag according to the present invention should not be limited thereby. The present invention may be applied to an airbag manufactured by stitching up fabric cloth members cut in predetermined shapes. In this case, coating layers may be arranged in inner surfaces of the airbag.  
      The second embodiment of the present invention is now described. An airbag  118  in the second embodiment is formed by hollow-weaving method of polyester, polyamide yarns or the like as the airbag  18 . As shown in  FIGS. 8 and 9 , the airbag  118  has a similar construction to the airbag  18  in the first embodiment except in that a vehicle&#39;s inner wall  119   a  and outer wall  119   b  forming the gas admissive portion  119  are both made of uncoated fabric having no coating layers of silicone or the like on outer surfaces. Therefore, descriptions of the same members will be omitted by giving the same reference numerals to those members.  
      The vehicle&#39;s inner wall  119   a  and outer wall  119   b  of the gas admissive portion  119 , or a protection portion  23 , of the airbag  118  are made of uncoated fabric. Air permeability H of the inner wall  119   a  and outer wall  119   b  is predetermined in a range of 5.0 cm 3 /cm 2 ·s·H·25.0 cm 3 /cm 2 ·s (desirably, in a range of 8.0 cm 3 /cm 2  ·s·H·20.0 cm 3 /cm 2  ·s). If the air permeability H is less than 5.0 cm 3 /cm 2 ·S, a fabric forming the inner wall  119   a  and outer wall  119   b  becomes so airtight that internal pressure of the completely inflated airbag  118  is hardly suppressed from rising at engaging an occupant&#39;s head. Contrarily, if the air permeability H exceeds 25.0 cm 3 /cm 2 ·s, gas leaks from the completely inflated airbag  118  so much that the airbag  118  cannot restrain an occupant&#39;s head with sufficient cushioning property.  
      The airbag  118  is woven by 6,6 Nylon yarn, and air permeability H of the inner wall  119   a  and outer wall  119   b  is predetermined at 16.67 cm 3 /cm 2 ·s (refer to Table 2).  
      In the second embodiment, the inner wall  119   a  and outer wall  119   b  of the protection portion  23  for protecting occupants&#39; heads are made of uncoated fabric whose air permeability H is 16.67 cm 3 /cm 2 ·s. Accordingly, when an occupant&#39;s head impacts on the protection portion  23  (front/rear protection portion  24 / 25 ) of the airbag  118 , inflation gas G leaks uniformly from an entire area of the inner wall  119   a  and outer wall  119   b  forming the protection portion  23 , which contributes to suppress a rise of internal pressure of the protection portion  23 . Of course, the protection portion  23  retains cushioning property sufficient for protecting occupants&#39; heads, even in a condition where certain amount of inflation gas G has leaked therefrom. Accordingly, the protection portion  23  of the completely inflated airbag  118  has high energy absorbing property.  
      Therefore, the airbag  118  in the second embodiment also suppresses a rise of internal pressure when an occupant&#39;s head impacts thereon, and has improved energy absorbing property to assure protection of occupants&#39; heads.  
      In the airbag  118 , too, moreover, thickness t of the protection portion  23  at complete inflation is predetermined in a range of 100 mm·t·280 mm. Accordingly, an occupant&#39;s head is properly protected by the protection portion of the completely inflated airbag.  
      Although the head-protecting airbag  118  in the second embodiment is described as manufactured by hollow-weaving method, manufacturing method of the airbag should not be limited thereby. The present invention may be applied to an airbag manufactured by stitching up cloth members cut in predetermined shapes, on condition that the airbag meets a requirement of air permeability H, as in later-described Examples 5 and 6.  
       FIG. 10  shows a graph showing results of impacter test testing some airbags that meet requirements of the second embodiment. Airbags of Examples 3 to 6 are airbags that meet the requirements of the second embodiment. An airbag of Example 3 is the airbag  118 . An airbag of Example 4 has the same construction as that of Example 3 except in that its thickness t is 180 mm. An airbag of Example 5 is made by sewing work of fabric members cut in predetermined shapes whose air permeability H is 10.67 cm 3 /cm 2 ·s. Sealing is applied to the sewn portion for preventing gas leakage, and thickness t of the airbag of Example 5 at complete inflation is 150 mm. An airbag of Example 6 has the same construction as that of Example 5 except in that its thickness t is 180 mm.  
      The impacter test was also conducted for airbags of Comparative Examples 4 and 5 under the same conditions as conducted on Examples 3 to 6. An airbag of Comparative Example 4 is made from 6,6 Nylon yarn, and has the same shape as that of Example 3. This airbag is coated by silicone on outer surfaces of an inner wall and outer wall forming a protection portion, as shown in Table 2. An airbag of Comparative Example 5 is made of fabric manufactured by hollow-weaving method. Its air permeability H is 33.33 cm 3 /cm 2 ·s, and its thickness t at complete inflation is 180 mm.  
      As shown in Table 2, the airbag of Example 3 and the airbag of Comparative Example 4 have substantially the same values in yarn density, tensile strength and tear strength. In other words, the airbag of Example 3 and the airbag of Comparative Example 4 differ from each other in existence of a coating layer. Since the airbag of Comparative Example 4 has coating layers on outer surfaces of both the inner an outer walls, air permeability H of those walls are almost 0. Actually, air permeability H of Comparative Example 4 was unmeasurable. In a line of coating weight of coating agent in Table 2, “Face” represents an inner wall side, and “Back” represents an outer wall side.  
      The impacter test for the second embodiment was conducted by moving a hammer head having 6.8 kg weight toward protection portions of completely inflated airbags at 6.5 m/s velocity, from a direction perpendicular to the protection portions, and measured deceleration and moving amount of the hammer head upon impact on the protection portions. That is, the impacter test for the second embodiment was conducted at later speed of hammer head than in the test for the first embodiment. In the graph of  FIG. 10 , each area of portions encircled by traces drawn by changes of deceleration and moving amount of hammer head represents absorbing amount of energy of the hammer head by the protection portion of the airbag.  
      In the tests, the airbag of Comparative Example 4 is highly airtight since it is coated by coating agent on the outer surface, so that inflation gas does not easily leak from the protection portion. Accordingly, when the hammer head impacts on the protection portion, the protection portion decelerates the hammer head suddenly in restraining it. When the movement of the hammer head toward the protection portion then stops completely, the protection portion pushes the hammer head back to a position before impact suddenly by a reaction force produced by a rise of internal pressure of the protection portion caused by engagement of the hammer head.  
      Contrarily in the airbags of Examples 3 to 6, when the hammer head impacts on each of the protection portions, the protection portion restrains the hammer head while leaking inflation gas. Although moving amounts of the hammer head toward the protection portions are greater than in Comparative Example 4, accordingly, the protection portions decelerate the hammer head gradually. Since gas leakage helps suppress a rise of internal pressure of the protection portions, when the movement of the hammer head toward the protection portions stops completely, the hammer head is pushed back to a position before impact gradually. At this time, deceleration (acceleration in being pushed back) of the hammer head is even lower than the deceleration when moving toward the protection portions. As shown in  FIG. 10 , consequently, traces of test results of the airbags of Examples 3 to 6 have greater areas than that of a test result of the airbag of Comparative Example 4. That is, the airbags in Examples 3 to 6 are superior in energy absorbing property to the airbag of Comparative Example 4.  
      In an airbag of Comparative Example 5, too high air permeability H allowed great deal of inflation gas to leak from the protection portion, so that the hammer head bottomed out and was not restrained by the protection portion.  
      The test results show that the airbag  118  is capable of suppressing a rise of internal pressure of the protection portion  23  when an occupant&#39;s head impacts on the protection portion  23 , and has improved energy absorbing property for assuring protection of occupants&#39; heads, as shown in  FIG. 10 , on condition that the inner wall  119   a  and outer wall  119   b  of the protection portion  23  are made of uncoated fabric whose air permeability H is in a range of 5.0 cm 3 /cm 2 ·s·H·25.0 cm 3 /cm 2 ·s.  
      Here, Example 3 in the impacter test for the second embodiment employs the same airbag as that used in Comparative Example 2 in the impacter test for the first embodiment. That is, the test for the second embodiment was conducted under condition that speed of hammer head is slower than in the test for the first embodiment, and therefore, it is proved that an airbag in Example 3, or in Comparative Example 2, is capable of protecting a hammer head properly with sufficient energy absorbing property in a case where the speed of hammer head is slow, or where the load of hammer head is low. As a result, airbags that meet requirements of the first embodiment are more suitable for vehicles in which higher load is likely to be generated by occupants&#39; heads upon impact than airbags that meet requirements of the second embodiment. For example, the airbags that meet requirements of the first embodiment are more suitable for such vehicles as have third-row seating, while the airbags that meet requirements of the second embodiment are more suitable for vehicles such as hatchback type compact cars.  
                                       TABLE 1                                           Comparative   Comparative   Comparative           Example 1   Example 2   Example 1   Example 2   Example 3                                                                Yarn Density   MD     67.3           67.3     67.3     60.0       (Yarn/25.4 mm)   CD     60.1           60.7     60.1     60.0       Coating Weight   Outer wall     57.2               —     57.2       (g/m 2 )   Inner wall   —   —     60.4   —   —       Tensile Strength   MD   700         693   700   560       (N/cm)   CD   621         631   621   561       Tear Strength   MD   177         177   177   106       (N)   CD   165         179   165   106       Air Permeability H   Outer wall   —   —   —      16.67   —       (cm 3 /cm 2  · s)   Inner wall      16.67         —      16.67      33.33                                     Thickness t at Inflation (mm)   150   180   150         180       Inflator Output Ratio X (KPa/L)     10.4                               Restraint Performance   ∘   ∘   ∘   x   x       Deceleration   ∘   ∘   x   —   —                  
 
     
       
         
           
               
               
               
               
               
               
               
             
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                   
               
               
                   
                   
                   
                   
                   
                 Comparative 
                 Comparative 
               
               
                   
                 Example 3 
                 Example 4 
                 Example 5 
                 Example 6 
                 Example 4 
                 Example 5 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 Yarn Density 
                 MD 
                   67.3 
                 
                   
                 
                   59.0 
                 
                   
                 
                   67.3 
                   60.0 
               
               
                 (Yarn/25.4 mm) 
                 CD 
                   60.1 
                 
                   
                 
                   59.0 
                 
                   
                 
                   60.7 
                   60.0 
               
               
                 Coating Weight 
                 Face 
                 — 
                 — 
                 — 
                 — 
                   65.2 
                 — 
               
               
                 (g/m 2 ) 
                 Back 
                 — 
                 — 
                 — 
                 — 
                   62.9 
                 — 
               
               
                 Tensile Strength (N/cm) 
                 MD 
                 700 
                 
                   
                 
                 669 
                 
                   
                 
                 693 
                 560 
               
               
                   
                 CD 
                 621 
                 
                   
                 
                 667 
                 
                   
                 
                 631 
                 561 
               
               
                 Tear Strength(N)  
                 MD 
                 177 
                 
                   
                 
                 135 
                 
                   
                 
                 177 
                 106 
               
               
                   
                 CD 
                 165 
                 
                   
                 
                 142 
                 
                   
                 
                 179 
                 106 
               
            
           
           
               
               
               
               
               
               
               
            
               
                 Air Permeability H (cm 3 /cm 2  · s) 
                    16.67 
                 
                   
                 
                    10.67 
                 
                   
                 
                 ※ 
                    33.33 
               
               
                 Thickness t at Inflation (mm) 
                 150 
                 180 
                 150 
                 180 
                 150 
                 180 
               
               
                 Inflator Output Ratio X (KPa/L) 
                   10.4 
                 
                   
                 
                 
                   
                 
                 
                   
                 
                 
                   
                 
                 
                   
                 
               
               
                 Restraint Performance 
                 ∘ 
                 ∘ 
                 ∘ 
                 ∘ 
                 ∘ 
                 x 
               
               
                 Deceleration 
                 ∘ 
                 ∘ 
                 ∘ 
                 ∘ 
                 x 
                 — 
               
               
                   
               
               
                   ※ Air Permeability of an airbag of Comparative Example 4 is unmeasurable, and is almost 0.