Abstract:
An inflatable automotive protective cushion and method of formation. The protective cushion includes a multilayer film coating with an adhesive layer and an overlying barrier layer. The adhesive layer is adapted to bond to a textile substrate and the barrier layer is adapted to block air permeability.

Description:
TECHNICAL FIELD 
     This invention relates to automotive protective devices such as side air curtains. More particularly, this invention relates to multilayer film coatings applied to the inside and/or outside of airbag cushion fabric to reduce gas permeability of the automotive protective device. This multilayer film coating is applied by dry lamination melt bonding. Specifically, the multilayer film coatings have adhesive and sealing properties and include a first thermoplastic adhesive layer and a second barrier layer wherein the melting point of the adhesive layer is lower than the melting point of the barrier layer. 
     BACKGROUND 
     Airbags for motor vehicles are known and have been used for a substantial period of time. These devices are installed on the driver and passenger side of automobiles and, in the event of a collision, are rapidly inflated with gas, to act as an energy absorbing barrier between the driver or passenger and the steering wheel or dashboard of the automobile. 
     More recently, airbag safety restraints in the form of an inflatable restraining curtain disposed along the side of a vehicle between an occupant and window or door openings have played a well recognized role in preventing injury to the occupant during a collision event. Typically, such curtains are inflated rapidly by the pressure of a reaction gas released from an inflator at the outset of generating agent in the inflator induces a chemical reaction activated by a collision signal from a collision detecting sensor when deceleration of the vehicle exceeds a certain level. The gas which is generated by the inflator is then conveyed to the airbag curtain. Inflatable curtains are typically deployed downwardly from a storage position along the roof rail so as to at least partially cover window and/or door openings across the side of the vehicle. The deployed curtain thus provides both a degree of cushioning restraint as well as a barrier preventing the occupant from being ejected from the vehicle. Due to the extended duration of a roll-over collision event where the vehicle may turn over several times it is desirable for the curtain-type airbags to remain inflated for an extended period of time so as to maintain a degree of head protection and barrier restraint until the entire event is concluded. 
     Curtain-type airbags are typically formed by sewing together panels of fabric to form a plurality of pillows when inflated or by weaving two layers of fabric that are interconnected at certain areas in a Jacquard loom. Such pillowed fabrics typically utilize seams which control the shape and size of the inflated cushion. Upon inflation of these airbag cushions in response to a collision event, pressure is applied in great force, particularly on the seams and can result in seam combing. In this regard it is to be understood that the term “seam combing” refers to the phenomenon wherein applied pressure causes the seaming threads to spread apart and thereby release additional gas pressure. 
     Performance can be improved by applying substantial quantities of permeability blocking coating materials. In the past, coatings have been applied to fabrics intended for use in automotive airbags in order to resist the unwanted permeation of gas through the fabric and, to a lesser extent, to protect the fabric from detriment by the hot gases used to inflate the bags. Polychloroprene was utilized in the early development of airbags. More recently, silicone (polydimethylsiloxane or similar materials) has gained increasing acceptance. 
     Since the airbag must retain its integrity during a collision event, in order to sufficiently protect the driver or passenger, there is a great need to provide coatings which provide both effective permeability characteristics and sufficient restriction of seam combing for the airbag to function optimally, if and when necessary. Thus, there exists a need for providing good adhesion and a strong bond between the individual yarns in order to effectuate long-term rigidity of the fibers to prevent unraveling at cut edges or at seams while simultaneously providing aging stability and excellent low air permeability characteristics. 
     One approach to decreasing coating weights while maintaining low permeability performance of the airbag fabrics has been to use a two layered coating system, as disclosed for example in commonly assigned U.S. Pat. Nos. 6,177,365 and 6,177,366 to Li (all hereby incorporated by reference in their entirety). Alternative coating compositions have been disclosed based on polyurethanes, such as in U.S. Pat. No. 5,110,666 or on polyurethane/polyacrylate dispersions as found in commonly assigned U.S. Pat. No. 6,169,043 (all hereby incorporated by reference in their entirety). 
     Current industry practice typically utilizes conventional coating methods employing solvent or water based coatings for airbag fabrics. Consequently, the solvent or the water needs to be removed by drying and the organic polymer needs to be cured by subsequent exposure to heat. The coating method is predominantly some form of knife over air/gap or transfer coating. 
     An alternative approach is to laminate polymeric films to the airbag fabric. Traditional lamination methodology employs a tie coat of an adhesive layer on the fabric before film lamination. This type of approach is taught in U.S. Pat. Nos. 6,770,578, 6,908,528, and 6,753,275 (all incorporated by reference in their entirety), all of which teach application of an adhesive layer as an aqueous or solvent containing dispersion which necessitates the removal of water or solvent at a subsequent point in the process. 
     SUMMARY 
     This invention is believed to provide advantages and alternatives over prior practices by providing a method of forming a multilayer coating on airbag fabrics using lamination melt bonding of multi-component films. 
     According to one potentially preferred aspect, the invention provides multi-component films having a barrier layer and an adhesive layer which is the layer in direct overlying relation to the textile substrate. 
     According to a potentially preferred feature, the adhesive layer has a melting point at least 30° C. lower than the barrier layer in order to facilitate adhesion of the film to the textile substrate. 
     According to another potentially preferred feature, the glass transition temperature of this adhesive layer is preferably less than 20° C. and the softening temperature is preferably greater than 110° C. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings which are incorporated in and which constitute a portion of this specification illustrate an exemplary embodiment of the invention which, together with the detailed description set forth below will serve to explain the principles of the invention wherein; 
         FIG. 1  illustrates an exemplary side curtain airbag; 
         FIG. 2  is a cross section of a textile substrate with a multi-component film laminate disposed across one side of the textile substrate; 
         FIG. 3  is a cross section of a textile substrate with a multi-component film laminate disposed across both sides of the textile substrate; 
         FIG. 4  illustrates a lamination process in which a multi-component film is laminated to both sides of a textile substrate; 
         FIG. 4   a  illustrates a lamination process in which a multi-component film is laminated to one side of a textile substrate; 
         FIG. 5  is a graph of the leak rate of a bi-component film laminated one piece woven curtain airbag; 
         FIG. 6  is a graph of the seam combing resistance of a bi-component film dry laminated fabric compared to film laminated fabrics with solvent based adhesive layers; 
         FIG. 7  is a graph of the seam combing resistance of a bi-component film dry laminated weft insertion warp knit fabric compared to the same fabric film laminated utilizing a solvent based adhesive layer; and 
         FIG. 8  is a graph of the seam combing resistance of a bi-component film dry laminated loose construction fabric compared to the same fabric uncoated. 
     
    
    
     DETAILED DESCRIPTION 
     Reference will now be made to the drawings wherein like reference numerals are utilized to designate like components in the various views. In  FIG. 1  there is shown an interior of a vehicle  110  after inflation of an exemplary side curtain airbag  126 . While the present invention will be described in relation to a side curtain airbag  126 , it is to be understood that the present invention is in no way limited to such a construction. To the contrary, it is contemplated that the present invention may also find application in any number of other airbag constructions including driver side, passenger side and side impact airbags as may be desired. 
       FIG. 2  depicts a cross section of coated airbag fabric  128  as may be used to form at least part of the side curtain airbag  126  or other airbag as may be desired. As illustrated, the coated airbag fabric  128  preferably includes a textile substrate  130  laminated with a multi-component film  136 . The multi-component film  136  includes a barrier layer  134  and an integral melt bonding thermoplastic adhesive layer  132  which is in direct overlying relation to the textile substrate  130 . Of course, additional layers may also be used if desired. 
     The multi-component film  136  is preferably bonded to the textile substrate  130  by dry lamination melt bonding. In this regard, it is to be understood that the adhesive layer  132  is not solvent or water based as in traditional wet lamination methods. The dry lamination melt bonding occurs when the temperature is raised above the melting point of the adhesive layer resulting in a molten adhesive layer yielding a bonding between the textile substrate  130  and the barrier layer  134  when the adhesive layer resolidifies. Ideally, the adhesive layer  132  has a melting point at least 30° C. lower than the barrier layer  134 . 
     According to the potentially preferred practice, the tensile strength of the barrier layer  134  is preferably greater than 5 N/mm 2  measured by EN ISO 527-3/95 test method. The elongation is preferably greater than 200% at break as measured by the above method and the modulus is preferably greater than 3 N/mm 2  as measured per EN ISO 527-3/95 test method at elongation greater than 100%. Tear resistance of the barrier layer is preferably greater than 35 N/mm 2  based on DIN 53515/77 test method. The above described properties were measured on a 20 μm thick film. 
     The multi-component film  136  can be either co-extruded or blown depending on the polymer system of each layer The barrier layer  134  preferably has a thickness of at least 15 μm and the adhesive layer  132  preferably has a thickness in the range of 20 to 50 μm. 
     By way of example only, the textile substrate  130  can be a single layer or multi layer construction such as woven fabric, knit fabric, weft insertion warp knit fabric, Rachel knit fabric and/or non-woven fabric. This includes multi-layered woven textiles which have preconfigured air holding cavities, also known as one piece woven (OPW) in the industry. 
     By way of example only, each layers can comprise additives, such as antioxidants, lubricants, anti-blocking agents such as silica, clay based materials, talc, etc., flame retardants that can be either halogen based or halogen-free components. Also, one layer might comprise of a coloring component to distinguish the adhesive layer from the barrier layer. 
     In the case of the multilayer fabrics like the one piece woven jacquard commonly utilized for air curtains, both sides of the textile substrate  130  can be laminated with the inventive multi-component film  136  sequentially in a single process. By way of example only and not limitation, an exemplary double sided lamination process is shown in  FIG. 4 . As shown, in this exemplary practice multi-component films  136  and  136 ′ are laminated to opposite outer sides of the textile substrate  130 . Multi-component film  136  is laminated under heat and pressure to the top side of the textile substrate  130  by contact with hot roll  138 , while multi-component film  136 ′, which is in contact with the bottom side of the textile substrate  130 , away from hot roll  138 , is not laminated at this point in the operation. When the film-textile-film composite is pressed against hot roll  140 , multi-component film  136 ′ is then laminated to the bottom side of the textile substrate  130 . The resultant laminated product is shown in  FIG. 3 . 
       FIG. 4   a  shows another potential lamination process wherein components corresponding to those previously described are designated by like reference numerals in a 200 series. As shown, in this exemplary process a multi-component film  236  is laminated to one side of a textile substrate  230  via a belt calender  245 . By way of example and not limitation, the multi-component film  236  is joined to the outer side of textile substrate  230  by nip rolls to form a composite structure  228  and this composite structure  228  is subjected to heat and pressure at the belt calender  245  to produce the laminated product. 
     As will be appreciated, the belt calender  245  utilizes of a series of small rolls which move a compression belt  252  in a continuous fashion around the large calender roll  250 . The composite structure  228  is oriented so as to be pressed between the belt  252  and the calender roll  250  with the textile substrate  230  is in contact with the belt  252  and the composite structure  228  in contact with the large calendar roll  250 . Heat and pressure at the large belt calendar roll  252  results in the laminated product such as that illustrated in  FIG. 2 . 
     In one potentially preferred practice, film lamination takes place at hot roll temperatures greater than the melting point of the adhesive layer  132  component of the multi-component film  136 . Above said melting point, the adhesive layer  132  is preferably in a substantially flowable condition which serves to promote good bonding by causing the thermoplastic material forming the adhesive layer  132  to flow around the fiber elements forming the textile substrate  130  and into the interstices between the fiber elements. Thus, the surface fiber elements preferably become partially embedded into the adhesive layer  132 . Film lamination preferably takes place at pressures from about 1 bar to about 10 bar, with pressures of about 4 bar to about 7 bar being most preferred. It is to be understood that the sequence of film laminating steps described above is exemplary only, and variations thereof are possible. 
     In order to further describe the present invention, the following non-limiting examples are set forth. These examples are provided for the sole purpose of illustrating some preferred embodiments of the invention and are not to be construed as limiting the scope of the invention in any manner. 
     EXAMPLES 
     A bi-component film comprising a barrier layer made form a polyether or polycarbonate based polyurethane and an adhesive layer made from a co-polyamide based resin was dry laminated to a woven jacquard fabric which was subsequently formed into a side curtain airbag. The adhesive layer was in direct overlying relation to the fabric substrate. A belt calender similar to that illustrated in  FIG. 4A  was used to form the laminate. The thickness of the bi-component film was 45 micron with the adhesive layer thickness being 30 μm and the barrier layer thickness being 15 μm. The woven curtain airbags were leak tested for gas retention in a rollover type of application. The curtain was inflated to a peak pressure of 137.9 kPa (20 psi) and the time taken for the bleed down to 50% of the peak pressure was recorded. The airbags were also tested for gas retention after heat aging (105° C. for 400 hours) and humidity aging (83° C. &amp; 95% RH for 400 hours). These results are also summarized in  FIG. 5 . 
       FIG. 6  shows the seam combing resistance of dry laminated fabric versus wet laminated fabric. The graph illustrates that the adhesive layer of the dry laminated bi-component film provides adhesion as measured by the seam combing resistance test comparable to solvent based adhesive polyurethane coatings and better than solvent based silicone adhesive coatings. 
     Bi-component film dry lamination was also tested on weft insertion warp knit (WIWK) fabrics and lower construction woven fabrics. These types of substrates cannot be coated by traditional knife over air/gap process coatings because of the openness of the fabrics. Moreover, in order to accomplish traditional wet lamination methods utilizing solvent or water based adhesive layers, the film must be coated with the solvent based adhesive first and then laminated rather than utilizing the classical wet lamination process of applying the adhesive coating to the fabric substrate.  FIG. 7  shows that the bi-component film dry lamination of a loosely constructed weft-insertion fabric substantially increases the seam combing resistance compared to wet lamination utilizing a solvent based silicone adhesive layer. Furthermore,  FIG. 8  demonstrates the marked effect that the bi-component dry lamination has on seam combing resistance compared to uncoated airbag fabrics. 
     While the present invention has been illustrated and described in relation to certain potentially preferred embodiments and practices, it is to be understood that the illustrated and described embodiments and practices are illustrative only and that the present invention is in no event to be limited thereto. Rather, it is fully contemplated that modifications and variations to the present invention will no doubt occur to those of skill in the art upon reading the above description and/or through practice of the invention. Therefore, it is intended that the present invention shall extend to all such modifications and variations as may incorporate the broad aspects of the invention in the full spirit and scope thereof.