FIELD OF THE INVENTION
This invention relates to novel airbag coating compositions comprising at least two separate and distinct layers. The first layer (base coat), being in contact with the airbag surface, comprises a composition of at least one coating material which may comprise up to 30% by parts of the total amount of material in the first layer of a silicone resin which provides excellent adhesion, excellent tensile strength, and overall lower cost than standard silicone airbag coating materials. The second layer, being a coating for the first layer, provides excellent reinforcement and blocking characteristics to permit effective potential long-term storage and optimum use upon the occurrence of a collision. Such a second layer (topcoat) is preferably a silicone material but may also be selected from the group consisting of homopolymer and coplymer resins based on ethylene, propylene, acrylates, methacrylates, vinyl esters, acrylic acid, methacrylic acid, polyurethanes, polyamides, and inorganic materials such as talc, silica, silicate, calcium carbonate, alumina, and the like. This two-layer system permits excellent strength and blocking properties to prevent undesired adhesion between portions of the target airbag when stored as well as to prevent seam combing at relatively low cost due to the inexpensive basecoat materials and the relatively low amount required for the topcoat. An airbag fabric coated with this inventive multi-layer system is also contemplated within this invention.
BACKGROUND OF THE INVENTION
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.
With the advent of new curtain-type airbags comprising a plurality of pillows formed from stitched areas of the bag article (to protect passengers during roll-over and side impact collision events), there is now a greater emphasis on providing such curtain-type airbags which will retain their inflation pressure for extended period after deployment, and will perform if and when necessary even upon storage of a long duration (years for example). Such pillowed fabrics thus comprise seams which control the shape and size of the inflated cushion. Upon inflation of such specific airbag cushions, pressure may be applied in great force, particularly on the seams, during an inflation in response to a collision event. These stitched areas or seams thus must retain their strength upon inflation, thereby setting forth the requirement that the individual yarns at such seams will not slip (i.e., “comb-out”) easily (which would result in the leakage air at too great a rate to afford sufficient protection), particularly upon inflation of the airbag. With such an expansion in stitching/seam requirements as compared with traditional driver-side and passenger-side airbags, the areas for potential air leakage have also increased dramatically.
In the past, coatings have been applied to fabrics, intended for use in automotive airbags, to resist the unwanted permeation of air 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 the polymer of choice in the early development of this product, but the desire to decrease the folded size of the completed airbag, and the tendency of polychloroprene to degrade, with exposure to heat, to release the components of hydrochloric acid (thereby potentially degrading the fabric component as well as releasing hazardous chemicals), has led to the almost universal acceptance of silicone (polydimethylsiloxane or similar materials) as a more suitable coating. In the quest for the most compact folded size possible, coating levels of polymer have dropped from around 2.5 ounces per square yard of fabric, to levels approaching 0.5 ounces per square yard.
New developments in airbag technology, particularly newer designs being placed in the sides of the passenger compartment (as noted above), have introduced the requirement that the bags hold pressure longer under use. This, and the evolution of the lower coating levels of silicone polymer, have begun to highlight the effect that, when a sewn seam is put under stress, a naturally lubricating silicone coating may allow the yarns from which the fabric is constructed to shift. This shifting can lead to leakage of the inflating gas through the new pores formed from the shifting yarns, or, in drastic cases, cause the seam to fail. 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 yarn shifting for the airbag to function properly, if and when necessary. Furthermore, such airbags must also exhibit excellent blocking characteristics, as noted above, to permit full, instantaneous inflation of the airbag itself upon the occurrence of a collision. In recent years, silicone coatings have been utilized to provide such desired permeability and strength characteristics. However, the relative cost of such coating materials (such as polydimethylsiloxane) is sufficiently high that new, more inexpensive alternatives are being sought. 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. Such a necessary improvement has not been afforded the airbag industry within the prior art. However, the inventive two-layer coating system does provide the necessary strength, durability, permeability, and reliability to the airbag industry, particularly for large-scale production of heavily stitched/seamed curtain-type airbags.
DESCRIPTION OF THE INVENTION
Although silicones and chloroprene have been the predominant coatings utilized in the airbag industry traditionally, as noted above, it has been determined that these coatings exhibit certain shortcomings which actually make them undesirable as the sole coatings present on target fabrics. For example, silicones are very expensive and act as natural fiber lubricants. Silicone elastomers are very flexible which permits facilitation of folding of coated airbags for long-term storage in airbag modules. However, compared with other elastomers, silicones possess poor tensile strength and poor tear resistance. As such, these compounds do not provide the best overall strength to prevent shifting of seam yarns, and certainly are not cost-effective selections for this purpose. Chloroprene (Neoprene) degrades very easily and thus does not exhibit sufficient aging stability. Furthermore, thicker coatings of such rubber compounds are required to reduce the air permeability to an acceptable level which can result in higher costs, although neoprene is not as expensive as the aforementioned silicones. Also, neither of these two traditional coating materials permits a single application over the entire airbag fabric in order to provide both air permeability characteristics over the non-stitched (and thus plain fabric) portion with effective adhesion for the individual fibers within the seamed areas. Lastly, most polyurethanes, etc., which provide the desired high tensile strength characteristics in the base coat are also highly tacky at elevated temperatures and thus do not provide beneficial anti-blocking properties. Thus, it is imperative to coat such excellent base coat components with a second layer comprising compounds exhibiting excellent blocking characteristics.
Thus, an object of the invention has been to provide excellent strength and adhesion to the seam yarns (or yarns at cut edges of the target fabric) while simultaneously allowing for low adhesion (anti-blocking) as well as desirable low air permeability over the target airbag fabric, all at a relatively low cost. Another object of the invention has been to provide a coating system or composition which is easy to apply to target airbag fabrics as well.
Accordingly, this invention provides a coating system (i.e., composition) for airbag fabrics comprising at least two layers wherein the first layer is in contact with at least a portion of the airbag fabric and is at least one coating material comprising no more than about 30% by total parts of said first layer material of a silicone resin and the second layer is a coating for at least a portion of said first layer and is at least one material providing sufficient blocking characteristics for the first layer material selected from the group consisting of at least one non-silicone compound and at least one silicone compound. In particular, the first layer material must possess certain properties critical to properly seal a woven seam at high pressure. Thus, this layer must be comprised of a coating material exhibiting a tensile strength of greater than 600 psi, preferably greater than 800 psi, and most preferably greater than 1,000 psi, with an elongation at break of between about 100 and 600% of its original length. The first layer thus may be comprised of one or more of polyurethanes, polyacrylates, polyamides, butyl rubbers, hydrogenated nitrile rubbers, ethylene-vinyl acetate copolymers, and the like. Potentially preferred are polyamides, polyurethanes, polyacrylates, and mixtures thereof. Furthermore, these first layer materials also are generally available at lower cost than the standard silicone resins and rubbers which possess the desired tensile strength and elongation characteristics noted above. The add-on weight of this first layer over the target fabric is from about 0.1 to about 2.0 ounces per square yard, preferably this add-on weight is from about 0.5 to about 1.2, more preferably from about 0.6 to about 0.9. The first layer materials may be present in water dispersions/emulsions or in organic solvent solutions in order to provide better seam combing resistance to the resultant treated airbag fabric. Such emulsions and solutions are also easy to handle and apply to the target fabric surface. This first layer surprisingly works synergistically with the second layer to provide the desired properties.
As noted above, the first layer may also comprise up to about 30% by weight of the total amount, in parts, of the first layer, of a silicone resin. Such a component may be utilized to modify conventional rubbers to improve their weatherability (aging stability). For example, such resins are known to be added to ethylene-propylene-diene monomer (EPDM) rubber to provide such weatherability improvements. Preferably, if this silicone resin is utilized in the first layer, it is present in an amount of from about 5 to about 12% by total parts of the first layer material. An amount in excess of 30% by total parts deleteriously affects the ability of the first layer to perform as needed. Any well known silicone resin may be utilized within the first layer, including those listed below for the second layer material. One non-limiting example of a potentially preferred silicone resin for use in this first layer is a silicone resin known and marketed under the name Dow Corning® Fabric Coating 61.
The second layer must possess, at the very least, good blocking properties to permit full, instantaneous inflation of the airbag when necessary after long duration storage within an airbag module. Furthermore, said second layer may also preferably provide reinforcement for the first layer. Silicones (such as polydimethylsiloxane and other silicone resins which are to be avoided in large quantities within the first layer) provide particularly good performance regarding this requirement. Since the add-on weight of the second layer necessary to effectuate proper blocking properties for the first layer (from about 0.1 to about 2.5 ounces per square yard as well, preferably from about 0.2 to about 1.5) is much lower than the standard add-on weight for such silicone compounds on curtain-type airbag fabrics, the costs associated with this two-layer system (when a silicone is used as the topcoat) is dramatically lower than for past coating compositions. Furthermore, the costs associated with the first layer non-silicone components are also much lower than for the silicone resins, rubbers, and the like, traditionally used in airbag coating applications. Thus, the overall costs of the inventive coating system is, again, much lower than for silicones used alone to provide any similar strength and air permeability characteristics to the plain fabric. Also, a non-silicone compound or polymer may be used as the topcoat (second layer) for this inventive coating system as long as that non-silicone provides an effective blocking characteristic benefit for the first layer. Potentially, any resins or compounds with melting/softening point above 100° C. can be used as the topcoat material to provide anti-blocking characteristics. Polyacrylates, ethylene-vinyl acetates, ethylene-methyl acrylates, polyurethane and the like, with high melting/softening points or crosslinking structures, provide such beneficial antiblocking stability, which, again, would, upon utilization, reduce the overall costs of the inventive coating system by eliminating the need for expensive silicones. Other materials that may be used for the second layers, for examples, are talc, calcium stearate, silica, calcium carbonate, polypropylenes and high density polyethylene. In such an event, the non-silicone second layer would be added in an amount of from about 0.1 to about 2.5 ounces per square yard, preferably, from about 0.2 to about 1.5.
With regard to the first layer, potentially preferred materials include a polyurethane, available from Stahl USA, Peabody Mass., under the tradename Ru 40-350 (40% solids); polyacrylates, (a) available from Rohm & Haas, under the tradename Rhoplex® E-358 (60% solids), and (b) available from Para-chem Southern, Inc., Greenville, S.C., under the tradename Pyropoly AC 2000®; a polyamide dispersion marketed under the trade designation MICROMID® 632 hpl by Union Camp Corporation which is believed to have a place of business in Wayne, N.J.; other polyurethane resins, Witcobond® 253 (35% solids), from Witco, and Sancure, from BFGoodrich, Cleveland, Ohio; hydrogenated NBR, such as Chemisat® LCD-7335X (40% solids), from Goodyear Chemical, Akron, Ohio; and butyl rubber, such as Butyl rubber latex BL-100, from Lord Corporation. As noted above, mixtures or combinations of non-silicone materials may also be utilized such as a dispersion of polyurethane and polyacrylate, as merely an example. Potentially preferred compositions are noted below including dispersions comprising polyurethane and polyacrylate. Preferably, in such an instance, the ratio of polyurethane to polyacrylate should be in an amount of from about 0.1:1 to about 10:1; preferably from about 1:1 to about 8:1; more preferably from about 2:1 to about 5:1; and most preferably from about 2:1 to about 2.5:1.
The substrate across which the elastomeric resin coatings are applied to form the airbag base fabric in accordance with the present invention is preferably a plain woven fabric formed from yarns comprising polyamide or polyester fibers. Such yarn preferably has a linear density of about 100 denier to about 630 denier. Such yarns are preferably formed from multiple filaments wherein the filaments have linear densities of about 6 denier per filaments or less and most preferably about 4 denier per filament or less. Such substrate fabrics are preferably woven using jacquard looms or possibly through the utilization of fluid jet weaving machines as disclosed in U.S. Pat. Nos. 5,503,197 and 5,421,378 to Bower et al. (incorporated herein by reference). The fabric substrate with applied coating system will hereinafter be referred to as an airbag base fabric. Other possible components present within either of the two layers (or both) composition are thickeners, antioxidants, flame retardants, curing agents, coalescent agents, adhesion promoters, and colorants. Any well known thickener for polyurethanes and/or polyacrylates may be utilized in this invention. One potentially preferred thickener is marketed under the trade designation NATROSOL® 250HHXR by the Aqualon division of Hercules Corporation which is believed to have a place of business at Wilmington, Del. Also, in order to meet Federal Motor Vehicle Safety Standard 302 flame retardant requirements for the automotive industry, a flame retardant is also preferably added to the compounded mix. Any well known airbag flame retardant may be used (including aluminum trihydrate, as merely one example). One potentially preferred flame retardant is DE-83R marketed by Great Lakes Chemical.
One other potentially preferred compound to be added to the first layer material is a fluoroelastomer. Such a fluoroelastomer is selected from the group consisting of fluoropolymers and fluoromonomer-containing copolymers. Such compounds have shown improved aging performance for the first layer materials (such as with polyurethanes). Particularly preferred fluoropolymers and/or fluoromonomer-containing copolymers include those with excellent aging stability such as, without limitation, polytetrafluoroethylene (Teflon® from DuPOnt), polyvinylidenefluoride (Kynar® from Elf Atochem, Inc.), polychloro-trifluoroethylene (Aclar® from Allied Signal), copolymer of vinylidenefluoride and hexafluoropropylene, copolymer of vinylidenefluoride and methylmethacrylate, copolymer of tetrafluoroethylene and vinylidenefluoride, and the like. Such a compound may be present in the first layer material in an amount of from 0 to about 30% of the total amount of parts in the first layer; preferably from about 1 to about 20%; more preferably from about 5 to about 15%.
Once compounding is complete (and the preferably resultant dispersion possess a viscosity of about 8,000 centipoise), the first layer is coated across the fabric substrate and dried to form a thin coating film(at a temperature preferably at about 320° F. for about 2 minutes; lower or higher temperatures, as well as correspondingly shorter or greater times may also be used). The second layer (possessing a similar viscosity to facilitate application in a large-scale procedure) is then applied in the same manner over at least a portion (preferably all) of the second layer. The second layer is then dried and cured as well (at a temperature of about 380° F. for about 2 minutes for the preferred silicone materials; again, differing temperatures and times may be followed). This discrepancy in temperatures required for drying and curing is yet another improvement over the utilization of silicone materials alone since lower temperatures are required (translating into safety improvements and lower energy costs) for the first layer at least.
The coating applications are performed through any standard coating procedures such as, and not limited to, scrape coating, transfer roll coating, cast coating and fixed-gap coating. These terms includes, and are not limited to, knife-over-gap table, floating knife, knife-over-foam pad, knife-over-roll, slot die methods, to name a few different method types. The resultant airbag base fabric is substantially impermeable to air when measured according to ASTM Test D737, “Air Permeability of Textile Fabrics,” standards.
Such airbag fabrics must pass certain tests in order to be utilized within restraint systems. One such test is called, as alluded to above, a blocking test which indicates the force required to separate two portions of coated fabric from one another after prolonged storage in contact with each other (such as an airbag is stored). Laboratory analysis for blocking entails pressing together coated sides of two 2 inch by 2 inch swatches of airbag fabric at 5 psi at 100° C. for 7 days. If the force required to pull the two swatches apart after this time is greater than 50 grams per square yard, or the time required to separate the fabrics utilizing a 50 gram weight suspended from the bottom fabric layer is greater than 10 seconds, the coating fails the blocking test. Clearly, the lower the required separating shear force, the more favorable the coating.
Another test which the specific coated fabric must pass is the oven aging test. Such a test also simulates the storage of an airbag fabric over a long period of time upon exposure at high temperatures and actually is used to analyze alterations of various different fabric properties after such a prolonged storage in a hot ventilated oven (>100° C.) for 2 or more weeks. For the purposes of this invention, this test was used basically to analyze the air permeability of the coated fabric after storage under a pressure of about 125 Pascals. Airbag fabrics generally should exhibit an air permeability level of less than about 0.2 cfm at 125 Pa. Again, the lower the air permeability, the better the coating.
Surprisingly, it has been discovered that the multi-layer coating system of this invention provides a coated fabric which passes both the blocking test and oven aging test with very low air permeability. This unexpectedly beneficial multi-layer system thus provides an airbag fabric which will easily inflate after prolonged storage and will remain inflated for a sufficient amount of time to ensure an optimum level of safety within a restraint system. Furthermore, it goes without saying that the less coating composition required, the less expensive the final product. Additionally, the less coating composition required will translate into a decrease in the packaging volume of the airbag fabric within an airbag device. This benefit thus improves the packability for the airbag fabric.
As previously indicated, the substrate fabric is preferably a woven nylon material. In the most preferred embodiment such substrate fabric will be formed from fibers of nylon 6,6 woven on a jacquard loom. It has been found that such polyamide materials exhibit particularly good adhesion and maintenance of resistance to hydrolysis when used in combination with the coating according to the present invention.
DETAILED DESCRIPTION OF THE INVENTION AND THE PREFERRED EMBODIMENTS
In order to further describe the present invention the following nonlimiting 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.