Silicone-coated air-bags and their manufacture

Abstract: An air bag coating composition is obtained by emulsifying (A) an organopolysiloxane containing at least two alkenyl groups each attached to a silicon atom, (B) an organo-hydrogenpolysiloxane containing at least three hydrogen atoms each attached to a silicon atom, and (C) a curing catalyst in water in the presence of an emulsifier to form an aqueous silicone emulsion. The composition cures into a coating which firmly adheres to an air bag base fabric and provides a tack-free surface.

Description: This invention relates to making air bags by coating air bag fabric with an adherent silicone composition, and to air bags made thereby. For safety, advanced automobiles are now fitted with air bags which are normally folded and received in the steering wheel, but upon detection of collision shocks, can be instantaneously inflated between the wheel and the driver to help protect the driver from injury. The air bags are generally formed from nylon fabric by coating it with chloroprene rubber. These air bags have a short life, since chloroprene rubber is poorly resistant to heating and weathering. The air bags are also required to be flame retardant so that they are resistant against any fire or explosion which may break out upon collision of the car. Since chloroprene rubber is not very flame retardant, prior art air bags formed from chloroprene rubber-coated fabric are further coated with a flame retardant silicone at locations where they might be exposed to a blast. Air bags formed from silicone rubber-coated fabric have the advantage of eliminating need for a flame retardant coating, because well-known flame retardants can be blended in the silicone rubber to render the rubber itself flame retardant. Air bag manufacturers are becoming more interested in silicone rubber coating compositions, for that reason and because of their heat resistance and weather resistance. A typical air bag using silicone rubber-coated fabric has a design whereby it is normally folded within the automotive steering wheel, but instantaneously inflated when explosive gas is injected upon collision. On inflation, the coating applied to the air bag fabric is instantaneously stretched along with the air bag fabric. Thus not only the base fabric but also the coating should be mechanically strong. To this end, the currently available silicone coating compositions use base polymers of relatively high molecular weight, as well as containing curing agents, flame retardants, adhesion aids, reinforcements and other additives. The coating compositions using high molecular weight base polymers, however, are quite difficult to apply to the coating thickness of 40 to 100 µm normally required for air bags using knife coaters, offset roll coaters, gravure coaters or the like. The coating compositions must therefore be diluted with organic solvents to a viscosity low enough to allow easy coating onto the base fabric. See for example EP-A-431 881. However, the use of organic solvents has serious problems including the risk of electrostatic ignition and potential hazard to operators' health since organic solvents can be taken into the body by way of inhalation or skin contact. Cured coatings are formed on base fabric by applying an organic solvent-diluted liquid of the coating composition and causing the solvent to evaporate off. The evaporated organic solvent must be recovered at any expense since otherwise it causes air pollution. Among means for changing a coating composition to a solventless system, the simplest way would be to reduce the degree of polymerization of the basic polymer until the viscosity of the composition is reduced to a sufficient level to allow for coating by means of a knife coater or the like. However, a coating composition based on a polymer having a lower degree of polymerization forms a cured coating which tends to be low in mechanical strength, so that such a coating would crack upon air bag inflation and allow hot explosive gas to bleed out. If the viscosity of a coating composition is reduced too far, the composition will strike through base fabric, typically plain weave fabric of nylon fibers, resulting in a less smooth surface. If the composition on the fabric rear surface is cured as struck-through and then wound up, the coated fabric gives rise to blocking. Additionally, the struck-through composition will adhere to rolls of the coating machine, adversely affecting operating efficiency and outer appearance. It is thus desired to have a silicone rubber coating composition having an adequate viscosity to allow coating without the need for organic solvent for dilution. Furthermore, conventional silicone rubber coating compositions tend to form cured coatings which are tacky on their surface, also contributing to a blocking problem. The general problem addressed herein is to provide novel air bags coated with silicone-based compositions, and methods of making these. In particular, preferred aims include the use of an air bag coating composition which (i) is free of organic solvent, (ii) cures to a coating having firm adhesion to an air bag base fabric, and (iii) has a tack-free surface. The achieving of a good working viscosity is a particularly preferred aim. In one aspect the present invention provides a method of making an air bag, comprising applying a coating composition to an air bag fabric and curing the composition to form a cured coating adhering to the fabric, characterised in that the coating composition is in the form of an aqueous silicone emulsion comprising (A) organopolysiloxane containing per molecule at least two alkenyl groups bonded to silicon atom, and having a viscosity of at least 3 x 10 -4 m 2 s -1 (300 centistokes) at 25°C; (B) organohydrogenpolysiloxane containing per molecule at least three hydrogen atoms bonded to silicon atom, and (C) curing catalyst. We have found that silicone emulsions as described can be applied to an air bag base fabric, such as a plain weave fabric of polyamide or polyester, and then cured to obtain cured coatings which may have better adhesion to the base fabric than coatings obtained by diluting the same components (A), (B) and (C) with an organic solvent. We have also found that these cured coatings can present a tack-free surface. In addition they have good rubbery properties so that they resist cracking when the air bag is inflated. The aqueous silicone emulsion has a suitable viscosity for application to the base fabric without dilution using organic solvent. Eliminating the use of organic solvent ensures a good working environment, and the emulsion is not hazardous to workers' health. The emulsion is readily curable at room temperature, e.g. by simply removing water although it cures much faster if heated. In either case it forms a cured coating having good rubbery properties including strength, elongation and hardness. Another aspect of the invention is an air bag comprising a base air bag fabric with a cured coating which has been formed thereon in the manner described above. It may be noted that aqueous silicone emulsions as described above are known as such from our JP-A-63/314275 where however they were used for making release paper. BRIEF DESCRIPTION OF THE DRAWING FIG. 1 schematically illustrates a surface tack test employed in the Example and Comparative Example. FIG. 2 schematically illustrates a test for examining cured properties in the Example and Comparative Example. DETAILED DESCRIPTION Component (A) used herein is an organopolysiloxane containing at least two alkenyl groups each attached to a silicon atom in a molecule. The alkenyl groups include vinyl and allyl groups and mixtures thereof. No limits are imposed on organic groups other than the alkenyl groups attached to a silicon atom, molecular structure, degree of polymerization and the like. Preferably, component (A) is an organopolysiloxane of the following general formula: R 1 / aSiO (4-a)/2 wherein R 1 , which may be identical or different, is a substituted or unsubstituted monovalent hydrocarbon group, preferably having 1 to 12 carbon atoms, especially 1 to 6 carbon atoms, and a is a positive number of 1.98 to 2.01. Examples of the monovalent hydrocarbon group include alkyl groups such as methyl, ethyl, propyl, butyl and hexyl groups; alkenyl groups such as vinyl and allyl groups; aryl groups such as phenyl and tolyl groups; and substituted ones of these groups wherein some or all of the hydrogen atoms are replaced by halogen atoms, cyano groups or the like, such as chloromethyl, 3,3,3 - trifluoropropyl and cyanoethyl groups. Preferred groups represented by R 1 are methyl, ethyl, phenyl, γ- trifluoropropyl, cyanoethyl, vinyl and allyl groups. The organopolysiloxane in one preferred aspect contains two or more alkenyl groups as defined above and at the same time at least 50 mol%, especially at least 80 mol% of a methyl group. With a methyl content less than 50 mol%, the properties inherent to methylpolysiloxane tend to be undesirably lost and the material cost would become higher. The organopolysiloxane should have a viscosity of at least 3 × 10 -4 m 2 s -1 (at least 300 centistokes (cs)) at 25°C, especially in the range 1 × 10 -2 to 1 × 10 2 m 2 s -1 (1 × 10 4 to 1 × 10 8 cs) at 25°C. With a viscosity of less than 3 × 10 -4 m 2 s -1 the coating composition tends to form a coating having mechanical strength below the practically acceptable level. These organopolysiloxanes should preferably have a substantially linear molecular structure. The organopolysiloxanes are often blocked with a silanol, methyl or vinyl group, preferably with a trivinylsilyl group at the end of their molecular chain. The organopolysiloxanes may be prepared by conventional well-known methods, for example, by subjecting cyclic polysiloxanes which are well - known oligomers to ring-opening polymerization in the presence of acid or alkali. Component (B) is an organohydrogenpolysiloxane containing at least three hydrogen atoms each attached to a silicon atom (≡SiH) in a molecule. It preferably represented by the following general formula: R 2 c H d Si (4-c-d)/2 wherein R 2 is an alkyl group e.g. having 1 to 10 carbon atoms such as methyl, and c and d are positive numbers with the sum of c and d being 1 to 3. Some illustrative, non - limiting examples of the organohydrogenpolysiloxane are represented by the following structural formulae. In the formulae, ℓ , m and n are integers. Component (B) is preferably blended in an amount to provide 0.5 to 4 ≡SiH bonds per alkenyl group of component (A) and at the same time, in an amount of 0.5 to 30 parts by weight per 100 parts by weight of component (A) in order to provide improved adhesion of the coating composition to base fabric. If the amount of component (B) exceeds 30 parts on this basis, the resulting cured coating tends to remain somewhat tacky on the surface and be insufficiently strong to prevent cracking. Component (C) is a curing catalyst which may be selected from conventional platinum series catalysts. It is effective for promoting addition reaction between the alkenyl group in the organopolysiloxane as component (A) and the ≡ SiH bond in the organohydrogenpolysiloxane as component (B). Exemplary catalysts are platinum black, chloroplatinic acid, alcohol modified chloroplatinic acid, and complex salts of chloroplatinic acid with vinyl siloxanes. The catalyst is added in a catalytic amount, preferably in an amount of about 10 to 5,000 ppm, more preferably about 50 to 1,000 ppm based on the weight of component (A). When platinum or platinum compounds as mentioned above are added to the composition, there is the risk that the composition undergoes scorching during storage. To prevent this undesirable phenomenon, a reaction retarder is preferably used in combination with the platinum catalyst. Exemplary retarders are methylvinylsiloxane, acetylene alcohol, and acetylene alcohol whose hydroxyl group is blocked with a trimethylsilyl group. The retarder, when used, is added in an amount of about 0.01 to 10 parts, preferably about 0.05 to 5 parts by weight per 100 parts by weight of component (A). In order to enhance the adhesion of the coating composition to base fabric, any of well - known adhesive aids such as organo - functional silanes and amino - functional silanes may be added to the coating composition. Also reinforcements such as colloidal silica may be added for reinforcing purposes. The coating composition is in the form of an aqueous silicone emulsion which is prepared by emulsifying a blend containing components (A) to (C) in water in the presence of an emulsifier. Suitable emulsifiers for use herein include anionic and nonionic surfactants, for example, sulfonic acids, sulfuric acids, phosphoric acids and salts thereof capable of surface activation, more illustratively, alkylsulfates such as sodium laurylsulfate, aliphatic hydrocarbon group - substituted benzenesulfonates such as sodium dodecylbenzenesulfonate, aliphatic hydrocarbon group - substituted naphthalene - sulfonates, polyethylene glycol sulfates, and lauryl phosphates. Preferably the emulsifier is used in an amount of 0.3 to 20 parts, more preferably about 0.5 to 5 parts by weight per 100 parts by weight of the organopolysiloxane as component (A). Less than 0.3 parts of the emulsifier on this basis would be less effective for helping emulsification, often failing to form an aqueous emulsion. More than 20 parts of the emulsifier would eventually yield an elastomer having low rubbery properties including strength, elongation and hardness. Water is used in any desired amount to ensure formation of an aqueous emulsion suitable as an air bag coating composition. Often 25 to 600 parts, especially about 50 to 200 parts by weight of water is used per 100 parts by weight of the organopolysiloxane as component (A). The aqueous emulsion may be prepared by mixing components (A), (B) and (C) and thereafter mixing them with an emulsifier and water. Alternatively, components (A), (B) and (C) are respectively mixed with an emulsifier and water to form aqueous emulsions, which are then mixed together. If it is desired to increase the viscosity of the coating composition at application, polyvinyl alcohol or similar thickeners may be added to the composition. The base fabrics to which the coating composition is cured may be e.g. fibers of nylon, Tetron, Vinylon, polyester and polyurethane and woven fabrics thereof, with nylon and polyester being preferred. A plain weave is the most useful weave. The coating composition may be applied to base fabric by any desired coating technique, for example, calendering, knife coating, brush coating, dipping, or spraying. Thereafter, the coating is cured simply by allowing to stand at room temperature while removing water, but preferably by hot air vulcanization at 100 to 180°C for from several seconds to 20 minutes. Cured products having a varying crosslinking density covering silicone rubber, silicone resin and silicone gel are obtained depending on the particular components used. EXAMPLE Examples are given below (by way of illustration and not by way of limitation). All parts are by weight. Example and Comparative Example A composition designated A was prepared by mixing 100 parts of a methylvinylsiloxane consisting of 99.85 mol% of dimethylsiloxane unit and 0.15 mol% of methylvinylsiloxane unit, end -- blocked with a dimethylvinylsiloxy group and having an average degree of polymerization of about 8,000 with 1.3 parts of a methylhydrogenpolysiloxane of the following formula (3), 0.05 parts of ethyl cyclohexanol as an addition reaction controlling agent, 100 ppm of platinum, and 3 parts of epoxy carbon functional silane of the following compositional formula (4). The blend was then mixed with 100 parts of water and 10 parts of dodecylbenzenesulfonate by means of a combination mixer to form an aqueous silicone emulsion, and polyvinyl alcohol was added to the emulsion for adjustment to a viscosity of about 10 Pa.s (100 poise). For comparison purposes, another composition designated B was prepared as above except that water, dodecylbenzenesulfonate and polyvinyl alcohol were omitted and instead, the blend was diluted with toluene to a viscosity of about 10 Pa.s (100 poise). Each of the coating compositions A and B was applied to one surface of plain weave fabric of 420-denier nylon filaments (46 filaments/ inch in warp and weft) to a dry coating thickness of about 100 µm. The coating was cured by allowing it to stand at a temperature of 25°C and a relative humidity of 60% for 48 hours. A coated fabric was obtained. These samples were examined by the tests shown in FIGS. 1 and 2 wherein the coated fabric is shown with a nylon fiber fabric 1 and a cured silicone coating 2. For examining tackiness, a tensile test was carried out as shown in FIG. 1. Two pieces of the coated fabric were mated together such that the coatings 2 faced each other. They were compression bonded at 25°C under a pressure of 20 kgf/cm 2 for 12 hours and cut into strips 2.5 cm wide. Using a tensile tester, a 180° peel test was carried out as shown in FIG. 1. With one piece fixed, the free end of the other piec was pulled back at a rate of 10 mm/min. to measure the load required to peel the other piece from the one piece. The load indicates the surface tack of the cured silicone coatings. Also, the samples were examined for cured property as shown in FIG. 2. The sample was scratched on the cured silicone coating 2 surface by moving back and forth a stainless steel tool 3 with a weight 4 of 200 grams resting thereon. The number of strokes of the scratching tool taken before the cured silicone coating 2 was torn from the nylon fabric 1 indicates the adhesion of the silicone coating to the nylon fabric. The results are shown in Table 1. Example Comparative Example Composition A B Type aqueous emulsion solvent type Viscosity (Pa.s/poise) 10/100 10/100 Tack, g/2.5 cm wide 0 200 Adhesion, strokes > 100 50 There has been described a coating composition adapted to apply to air bag base fabric which forms a cured coating having improved adhesion to the base fabric and a tack-free surface. The composition can be adjusted to an adequate viscosity to apply without the need for organic solvent. Due to the eliminated use of organic solvent, the composition is fully safe during application and not harmful at all to the operator.

Claims: A method of making an air bag, comprising applying a coating composition to an air bag fabric and curing the composition to form a cured coating adhering to the fabric, characterised in that the coating composition is in the form of an aqueous silicone emulsion comprising (A) organopolysiloxane containing per molecule at least two alkenyl groups bonded to silicon atom, and having a viscosity of at least 3 x 10 -4 m 2 s -1 (300 centistokes) at 25°C; (B) organohydrogenpolysiloxane containing per molecule at least three hydrogen atoms bonded to silicon atom, and (C) curing catalyst. A method according to claim 1 in which the organopolysiloxane (A) is of the general formula: R 1 a SiO (4-a)/2 in which groups R 1 are selected independently from substituted and unsubstituted monovalent hydrocarbon groups and a is from 1.98 to 2.01. A method according to claim 2 in which at least 80% of the groups R 1 are methyl groups. A method according to any one of the preceding claims in which the organopolysiloxane (A) has a substantially linear structure end-blocked with silanol, methyl or vinyl group. A method according to any one of the preceding claims in which the organopolysiloxane (A) has a viscosity at 25°C in the range 1 x 10 -2 to 1 x 10 2 m 2 s -1 (1 x 10 4 to 1 x 10 8 centistokes). A method according to any one of the preceding claims in which the organohydrogenpolysiloxane (B) is of the general formula: R 2 c H d Si (4-c-d)/2 in which each R 2 is an alkyl group and c and d are positive numbers whose sum is from 1 to 3. A method according to any one of the preceding claims in which the organohydrogenpolysiloxane (B) is provided in an amount from 0.5 to 30 parts by weight per 100 parts by weight of organopolysiloxane (A). A method according to claim 7 in which the organohydrogenpolysiloxane (B) is included in an amount to provide from 0.5 to 4 Si-H bonds for each alkenyl group of the organopolysiloxane (A). A method according to any one of the preceding claims in which the curing catalyst (C) is a platinum series catalyst. A method according to any one of the preceding claims in which the silicone emulsion has anionic or nonionic surfactant as emulsifier. A method according to any one of the preceding claims in which the silicone emulsion contains from 0.3 to 20 parts by weight of emulsifier per 100 parts of the organopolysiloxane (A) . A method according to any one of the preceding claims in which the silicone emulsion contains from 25 to 600 parts by weight of water per 100 parts of organopolysiloxane (A) . A method according to any one of the preceding claims in which the composition is cured by drying using hot air vulcanisation at from 100 to 180°C for up to 20 minutes. An air bag comprising a base air bag fabric with a cured coating thereon formed in accordance with any one of claims 1 to 13.

Inventors: INOUE YOSHIO, MOMII KAZUMA, INOUE, YOSHIO, MOMII, KAZUMA
Applicants: SHINETSU CHEMICAL CO, SHIN-ETSU CHEMICAL CO., LTD.
Application year: 1993
Main classification: C09D183
IPC codes: B32B25, B60R21, C09D183, C08L83, C08J3