Abstract:
An apparatus ( 10 ) for inflating an inflatable vehicle occupant protection device comprises a container ( 12 ) for storing a supply of gas. A gas ( 26 ) is stored in the container ( 12 ) at an elevated pressure. The gas comprises an oxygen rich oxidizer gas. A gas generating material ( 84 ) is stored in the container ( 12 ) and is exposed to the oxidizer gas at the elevated pressure. The gas generating material ( 84 ) comprises a cellulose based binder blended with an anti-oxidant material. An igniter ( 52 ) is provided for igniting the gas generating material ( 84 ).

Description:
FIELD OF THE INVENTION 
     The present invention relates to a hybrid inflator for inflating an inflatable vehicle occupant protection device, and particularly relates to a gas generating material for inflating an inflatable vehicle occupant protection device. 
     BACKGROUND OF THE INVENTION 
     A hybrid inflator for inflating a vehicle occupant protection device includes a quantity of a stored gas and a gas generating material. The stored gas typically comprises an inert gas and an oxidizer gas. The oxidizer gas helps to support the combustion of the gas generating material. An igniter is actuatable to ignite the gas generating material. As the gas generating material burns, it generates heat and a volume of combustion gas. The heat and combustion gas increase the pressure of the inert gas. The heated inert gas and combustion gas form the inflation fluid. The inflation fluid is directed into the air bag to inflate the air bag. When the air bag is inflated, it expands into the vehicle occupant compartment and helps to protect the vehicle occupant. 
     U.S. Pat. No. 5,125,684 discloses a gas generating material for use in a vehicle occupant restraint system. The gas generating material comprises cyclotetramethylenetetranitramine (HMX) or cyclotrimethylenetrinitramine (RDX), an oxidizer salt, and a cellulose based binder. The advantage of using the cellulose based binder in the gas generating material formulation is that the cellulose based binder produces a low-level of carbon monoxide upon combustion compared to conventional polymeric binders. 
     Cellulose based binders are generally resistant to oxidation and degradation at atmospheric pressure. It has been discovered, however, that cellulose based binders oxidize and degrade, over time, when stored in a high pressure oxygen rich atmosphere (e.g., an atmosphere with a pressure greater than 1,000 psi and a concentration of oxygen greater than 10% by weight). Free radicals of oxygen in a high pressure oxygen rich atmosphere oxidize the chemical double bonds of the cellulose based binder. The oxidized bonds cleave and cause the polymer chain of the cellulose based binder to fragment. 
     SUMMARY OF THE INVENTION 
     The present invention is an apparatus for inflating an inflatable vehicle occupant protection device. The apparatus comprises a container for storing a supply of gas. A gas is stored in the container at an elevated pressure. The gas comprises an oxygen rich oxidizer gas. A gas generating material is stored in the container and is exposed to the oxidizer gas at the elevated pressure. The gas generating material comprises a cellulose based binder blended with an anti-oxidant material. An igniter is provided for igniting the gas generating material. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and other features of the invention will become apparent to one skilled in the art upon consideration of the following description of the invention and the accompanying drawing in which the figure is a sectional view of an inflator which is constructed in accordance with the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     An inflator  10  provides inflation fluid for inflating a vehicle occupant protection device, such as an air bag (not shown). The inflator  10  includes a generally cylindrical container  12 , a generally cylindrical diffuser  14 , and a manifold assembly  16 . 
     The container  12  includes a generally cylindrical one-piece steel side wall  20  that defines a chamber  22 . The side wall  20  has a longitudinal central axis  24 . The chamber  22  is filled with a gas  26  under pressure, which is introduced into the chamber  22  through end cap  30 . The end cap  30  extends through an opening  34  at the right end (as shown in the Figure) of the container and is connected to the container  12  by an annular weld  36 . The end cap  30  includes a passage (not shown) through which the gas  26  is conducted into the chamber  22 . Once the chamber  22  has been filled with gas  26  at a desired pressure, the passage is closed by suitable means such as a steel ball (not shown) welded in place. 
     The gas  26  is stored in the container  12  at a pressure of about 1000 psi to about 5,000 psi. The gas  26  is preferably stored in the container  12  at a pressure of about 2,000 psi to about 3,500 psi. The end cap  30  may also include a conventional pressure switch (not shown) from which the gas pressure in the chamber  22  can be monitored if the gas pressure in the chamber  22  drops below a set pressure. 
     The gas  26  stored in the container  12  comprises a mixture of at least one inert gas and at least one oxygen rich oxidizer gas. Preferred inert gases are helium (He) and argon (Ar). Preferably, the inert gases comprise a mixture of argon and helium, with helium being present in an amount sufficient to act as a leak detector. Preferred oxygen rich oxidizer gases include oxygen and nitrous oxide. The oxygen rich oxidizer gas is preferably the only gas other than the inert gases present in the gas  26  stored in the container  12 . 
     Preferably, the gas  26  stored in the container  12  comprises, on a weight basis, about 10% to about 25% oxygen, and about 1% to about 5% helium, with the balance being argon. A preferred composition of the stored gas  26  is 75% argon, 20% oxygen, and 5% helium. 
     The manifold assembly  16  is secured to the container  12  by a friction weld  38  at the left end (as viewed in the Figure) of the container  12 . The manifold assembly  16  is disposed in coaxial relationship with the end cap  30  and the side wall  20  of the container  12 . The manifold assembly  16  projects both axially into and axially away from the container  12 . The manifold assembly  16  includes a generally cylindrical metal manifold plug  40  that is disposed partially outside of the container  12 . The manifold plug  40  includes a generally cylindrical side wall  42 , which defines a generally cylindrical interior cavity  44 . A plurality of circular outlet openings  46  are disposed in a circular array in the manifold side wall  42 . The outer end  48  of the manifold plug  40  is closed by a circular end wall  50 . An actuator assembly  52  is disposed in the manifold end wall  50  and extends into the manifold cavity  44 . 
     A burst disk  58  extends across a circular opening at the interior end  60  of the manifold plug  40 . The burst disk  58  blocks gas flow from the chamber  22  of the container  12  into the manifold cavity  44  until the burst disk  58  is ruptured by the actuator assembly  52 . 
     The manifold assembly  16  also includes a cylindrical holder  62 , which is coaxial with the manifold plug  40  and is disposed within the container  12 . The holder  62  includes a generally cylindrical side wall  64 , which defines a generally cylindrical cavity  66 . The holder  62  is welded to the periphery of the burst disk  58 , which is in turn welded to the interior end  60  of the manifold plug  40 . The manifold plug  40 , holder  62 , and the burst disk  58  are thus all welded together to form the unitary manifold assembly  16 . 
     A plurality of circular inlet openings  70  are arranged in a circular array in the holder side wall  64 . The openings  70  provide fluid communication between the chamber  22  of the container  12  and the holder cavity  66 . 
     A booster charge  72  is disposed in a cylindrical chamber  74  formed in the end of the manifold holder  62  removed from the burst disk  58 . The booster chamber  74  is connected in fluid communication with the holder cavity  66  through a generally cylindrical opening  76 . The booster chamber  74  and opening  76  are coaxial with the burst disk  58  and the actuator assembly  52 . 
     The booster charge  72  is readily ignited to ignite a gas generating material  84 . The booster charge  72  is securely held in the chamber  74  and is enclosed by a thin covering of polymeric material (not shown), which is destroyed upon burning of the booster charge  72 . The ignitable material forming the booster charge  72  is preferably boron potassium nitrate (BKNO 3 ), but could have a different composition well known to those skilled in the art, if desired. 
     A generally cylindrical metal housing  80 , having a chamber  82 , encloses the gas generating material  84 , which is disposed within the chamber  82 . One end of the housing  80  is disposed adjacent the manifold holder  62  and has a threaded, interior circumferential surface  86 . The threaded surface  86  of the housing  80  engages a threaded, exterior circumferential surface  88  on the manifold holder  62  so that the housing  80  is mounted on the manifold holder  62 . The housing  80  is coaxial with the holder  62  and the booster charge  72  in the booster chamber  74 . 
     In a preferred embodiment, the gas generating material  84  comprises a plurality of randomly oriented cylindrical grains  89  disposed within the chamber  82 . The grains  89  may be similar or identical in configuration. 
     Although the gas generating material  84  has been illustrated as a plurality of randomly oriented cylindrical grains  89 , it is contemplated that the gas generating material  84  could be formed with a different configuration if desired. For instance, the gas generating material  84  may have a multi-lobe cross-sectional configuration or may comprise a plurality of stacked cylinders. 
     At its end opposite from the manifold assembly  16 , the housing  80  is substantially closed except for a circular orifice  90 . The housing orifice  90  is disposed in a coaxial relationship with the housing chamber  82 . The inside of the housing chamber  82  is in fluid communication with the chamber  22  in the container  12  through the housing orifice  90 . The orifice  90  is continuously open so that the gas  26  stored in the chamber  22  can flow into the housing chamber  82  around the gas generating material  84 . 
     Disposed between the gas generating material  84  and the orifice  90  is a flat baffle plate  92  and a flat circular orifice plate  94  through which an orifice (not shown) extends. These plates  92  and  94  help retain the gas generating material  84  within the chamber  82 . During burning of the gas generating material  84 , combustion products from the burning gas generating material impinge against the baffle plate  92 . After passing the baffle plate  92 , the combustion products enter into the chamber  22  through the orifice plate  94  and the housing orifice  90 . 
     The actuator assembly  52  includes a cylindrical housing  100  having a cylindrical chamber  102  in which a piston  104  and a pyrotechnic charge  106  of ignitable material are disposed in coaxial relationship. The actuator housing  100  is secured to the manifold end wall  50  and is disposed in a coaxial relationship with the burst disk  58 , the booster charge  72 , and the gas generating material  84 . The diameter and length of the actuator assembly  52  are sufficiently smaller than the diameter and length of the manifold cavity  44  so that the stored gas  26  can flow from the chamber  22  and the holder cavity  66  through the manifold cavity  44  to the manifold outlet openings  46  when the burst disk  58  is ruptured. 
     The piston  104  is formed from a single piece of metal and has a cylindrical head end portion  110 . A smaller diameter cylindrical piston rod  112  extends axially away from the head end portion  110 . A cylindrical central passage  114  in the piston rod  112  is coaxial with and extends through the head end portion  110  and piston rod  112  of the piston  104 . The cylindrical piston rod  112  has a tip  116  at its outer end portion. 
     The pyrotechnic charge  106  is disposed in the actuator chamber  102  in a position that is adjacent to the head end portion  110  of the piston  104 . A squib  120  is located adjacent the pyrotechnic charge  106 . Two electrically conductive pins  122  and  124  are connected with the squib  120 . The pins  122  and  124  extend through an opening in the manifold assembly  16 . The pins  122  and  124  provide a path for electrical current to actuate the squib  120 . 
     The squib  120  and pins  122  and  124  are included in an electrical circuit  130 . The electrical circuit  130  further includes a power source  132 , which preferably is the vehicle battery and/or a capacitor, and a normally open switch  134 . The switch  134  is part of a sensor  136  that senses a condition indicating the occurrence of a vehicle collision. The collision indicating condition may comprise, for example, sudden vehicle deceleration caused by a collision. If the collision indicating condition is above a predetermined threshold, it indicates the occurrence of a collision for which inflation of the inflatable vehicle occupant protection device is desired to help protect an occupant of the vehicle. 
     The diffuser  14  is larger in diameter than the container  12  and is mounted on the outside of the container  12  to encircle both the container  12  and the manifold assembly  16 . The diffuser  14  also extends substantially the entire length of the manifold assembly  16  and a significant portion of the length of the container  12 . 
     The diffuser  14  includes a cylindrical diffuser tube  140  having an annular, radially inwardly directed in-turned lip  142  at one end. The in-turned lip  142  tightly engages a cylindrical outer side surface of the container wall  20 . An end cap  144  is welded to the end of the diffuser tube  140  opposite from the in-turned lip  142 . The end cap  144  is connected to an outer end portion of the manifold assembly  16 . A mounting stud  146  is connected with the diffuser tube  140  adjacent the end cap  144 . The mounting stud  146  is used to mount the inflator assembly to a reaction can (not shown), which can be mounted at a desired location in the vehicle. The diffuser  14  defines a diffuser chamber  150  around the manifold assembly  16  and the container  12 . The diffuser  14  has openings  152 , which provide fluid communication from the diffuser chamber  150  to the inflatable vehicle occupant protection device. 
     Upon the occurrence of sudden vehicle deceleration indicative of a collision for which inflation of the vehicle occupant protection device is desired, the crash sensor  136  closes the normally open switch  134 . Closure of the normally open switch  134  causes electric current to be transmitted from the power source  132  to the squib  120 . This in turn causes the squib  120  to ignite the pyrotechnic charge  106 . Burning of the pyrotechnic charge  106  forces the piston rod  104  to move axially and penetrate the burst disk  58 . Burning gases from the pyrotechnic charge  106  flow through the passage  114  and ignite the booster charge  72 . The burning booster charge, in turn, ignites the gas generating material  84  to produce initial combustion products such as carbon monoxide, carbon dioxide, water, hydrogen cyanide and nitrogen, and a first quantity of heat. 
     As the gas generating material  84  burns, the hot combustion products flow through the orifice  90  to mix with and heat the stored gas  26  in the chamber  22  of the container  12 . Any partially combusted initial combustion products (i.e., carbon monoxide, hydrogen cyanide, etc.) of the gas generating material  84  further combust in the presence of the oxygen rich oxidizer gas to form an essentially non-toxic subsequent combustion product and second quantity of heat. The first quantity of heat and the second quantity of heat increase the temperature and hence the pressure of the stored gases  26  in the chamber  22  including the inert gases. 
     The stored gas  26 , and the combustion products provide an inflation fluid that flows from the chamber  22  through the manifold inlet openings  70  into the manifold assembly  16 . The inflation fluid flows through the manifold assembly  16  into the manifold cavity  66 , and then through the manifold outlet openings  46  into the diffuser chamber  150 . The inflation fluid then flows from the diffuser  14  through openings  152  into the vehicle occupant protection device. 
     In accordance with the present invention, the gas generating material  84  comprises a fuel. The fuel of the gas generating material can be any non-azide nitrogen containing fuel commonly used in a gas generating material for inflating a vehicle occupant protection device. The non-azide nitrogen containing fuel is a material capable of undergoing rapid and substantially complete oxidation upon combustion of the gas generating material. In a preferred embodiment of the present invention, the non-azide nitrogen containing fuel is a nitramine. Preferred nitramines are selected from the group consisting of cyclotrimethylenetrinitramine (RDX), cyclotetramethylenetetranitramine (HMX), and mixtures of cyclotetramethylenetetranitramine and cyclotrimethylenetrinitramine. 
     The non-azide nitrogen containing fuel can also be other non-azide nitrogen containing organic fuels typically used in a gas generating material for inflating a vehicle occupant protection device including: cyanamides such as dicyanamide and salts of cyanamides; tetrazoles such as 5-aminotetrazole and derivatives and salts of tetrazoles; carbonamides such as azo-bis-dicarbonamide and salts of carbonamide; triazoles such as 3-nitro-1,2,4-triazole-5-one (NTO) and salts of triazoles; guanidine and other derivatives of guanidine such as nitroguanidine (NQ) and other salts of guanidine and guanidine derivatives; tetramethyl ammonium nitrate; urea and salts of urea; and mixtures thereof. 
     The fuel is incorporated in the gas generating material in the form of particles. The average particle size of the fuel is from about 1 μm to about 100 μm. Preferably, the average particle size of the fuel is from about 1 μm to about 20 μm. 
     The amount of fuel in the gas generating material  84  is that amount necessary to achieve sustained combustion of the gas generating material. The amount can vary depending upon the particular fuel involved and other reactants. A preferred amount of fuel is in the range of about 20% to about 80% by weight of the gas generating material. More preferably, the amount of fuel in the gas generating material is from about 40% to about 70% by weight of the gas generating material. 
     The gas generating material  84  further includes an oxidizer. The oxidizer can be any oxidizer commonly used in a gas generating material for inflating a vehicle occupant protection device. A preferred oxidizer is an inorganic salt oxidizer. Examples of inorganic salt oxidizers that can be used in a gas generating material for inflating a vehicle occupant protection device are alkali metal nitrates such as sodium nitrate and potassium nitrate, alkaline earth metal nitrates such as strontium nitrate and barium nitrate, alkali metal perchlorates such as sodium perchlorate, potassium perchlorate, and lithium perchlorate, alkaline earth metal perchlorates, alkali metal chlorates such as sodium chlorate, lithium chlorate and potassium chlorate, alkaline earth metal chlorates such as magnesium chlorate and calcium chlorate, ammonium perchlorate, ammonium nitrate, and mixtures thereof. 
     When ammonium nitrate is used as the oxidizer, the ammonium nitrate is preferably phase stabilized. The phase stabilization of ammonium nitrate is well known. In one method, the ammonium nitrate is doped with a metal cation in an amount that is effective to minimize the volumetric and structural changes associated with phase transitions to pure ammonium nitrate. A preferred phase stabilizer is potassium nitrate. Other useful phase stabilizers include potassium salts such as potassium dichromate, potassium oxalate, and mixtures of potassium dichromate and potassium oxalate. Ammonium nitrate can also be stabilized by doping with copper and zinc ions. Other compounds, modifiers, and methods that are effective to phase stabilize ammonium nitrate are well known and suitable in the present invention. 
     Ammonium perchlorate, although a good oxidizer, is preferably combined with a non-halogen alkali metal or alkaline earth metal salt. Preferred mixtures of ammonium perchlorate and a non-halogen alkali metal or alkaline earth metal salt are ammonium perchlorate and sodium nitrate, ammonium perchlorate and potassium nitrate, and ammonium perchlorate and lithium carbonate. Ammonium perchlorate produces upon combustion hydrogen chloride. Non-halogen alkali metal or alkaline earth metal salts react with hydrogen chloride produced upon combustion to form alkali metal or alkaline earth metal chloride. Preferably, the non-halogen alkali metal or alkaline earth metal salt is present in an amount sufficient to produce a combustion product that is substantially free (i.e., less than 2% by weight of the combustion product) of hydrogen chloride. 
     The oxidizer is incorporated in the gas generating material in the form of particles. The average particle size of the oxidizer is from about 1 μm to about 100 μm. Preferably, the average particle size of the oxidizer is from about 1 μm to about 20 μm. 
     The amount of oxidizer in the gas generating material  84  is that amount necessary to achieve sustained combustion of the gas generating material. The amount of inorganic salt oxidizer necessary to achieve sustained combustion of the gas generating composition is from preferably about 20% to about 60% by weight of the gas generating material. 
     The gas generating material  84  also includes a binder that is mixed with the fuel and oxidizer to provide an intimate mixture of the oxidizer and the fuel. The binder of the present invention is a cellulose based binder. By cellulose based, it is meant that the binder is a polymer that is a chemical derivative of cellulose. Preferred cellulose based binders are esters of cellulose such as cellulose acetate, cellulose acetate propionate, cellulose acetate butyrate, cellulose propionate, cellulose propionate-butyrate, and combinations thereof. Cellulose esters are preferred because these cellulose based binders when combined with solvents are readily extruded and molded. Upon removal of the solvent, the binders form highly resilient solids that are neither brittle at a temperature of about −40° C. nor capable of losing their shape or configuration at a temperature of about 125° C. Examples of other cellulose based binders that can be used in the gas generating material of the present invention are ethers of cellulose such as ethyl cellulose and triethylacetylcellulose and nitrates of cellulose such as nitrocellulose. 
     A preferred amount of binder is from about 1% to about 20% by weight of the gas generating material  84 . More preferably, the amount of binder is from about 2.5% to about 15% by weight of the gas generating material. 
     In accordance with the present invention, the gas generating material  84  further includes an antioxidant. The antioxidant inhibits oxidation of the cellulose based binder when the gas generating material is stored in the high pressure oxygen rich gas in the chamber  22 . The antioxidant inhibits oxidation of the cellulose based binder by preferentially reacting with free radicals of oxygen in the high pressure oxygen rich gas in the chamber  22 . The rate at which the antioxidant reacts with free radicals of oxygen is several orders of magnitude greater than the rate at which the cellulose based binder reacts with the free radicals of oxygen in the hybrid inflator. Moreover, the antioxidant reacts with and terminates free radical chain reactions in any cellulose based binder that is oxidized and therefore could degrade. 
     A preferred antioxidant of the present invention is 2,2-methylene bis(4-methyl)6-t-butylphenol. 2,2-methylene bis(4-methyl)6-t-butylphenol is commercially available from Cyanamid Corporation under the tradename AO2246. 2,2-methylene bis(4-methyl)6-t-butylphenol is preferred as the antioxidant because 2,2-methylene bis(4-methyl)6-t-butylphenol is readily dissolved in solvents utilized for processing the gas generating material  84 . 
     Examples of other antioxidants that can be used in the gas generating material  84  of the present invention are substituted phenolic compounds such as phenyl-betanaphthylamine, which is commercially available from Uniroyal Co. under the tradename PBNA, polymerized trimethyl dihydroquinoline, which is commercially available from Uniroyal Co. under the trade name NAUGARDQ, diphenylamine-diisobutylene reaction product, which is commercially available from Uniroyal Co. under the tradename OCTAMINE, N-phenyl-N′-(1,3-dimethylbutyl)-p-phenylene diamine, which is commercially available from Uniroyal Co. under the trade name FLEXZONE 7L, N-phenyl-N′-cyclohexyl-phenylene diamine, which is commercially available from Uniroyal Co. under the trade name FLEXZONE 6H, N-phenyl-N′-cyclohexyl-p-phenylene diamine, which is commercially available from Universal Oil Products under the trade name UOP-36, and di-tert-butylhydroquinone, which is commercially available from Eastman Chemicals Co. under the trade name DTBHQ. The antioxidant of the present invention may also include mixtures of these antioxidants. 
     The amount of antioxidant is that amount effective to retard oxidation of the cellulose based binder by the high pressure oxygen rich atmosphere in the hybrid inflator. A preferred amount is from about 0.1% to about 1% by weight of the gas generating material. At an amount less than 1% by weight of the gas generating material, the antioxidant does not impair ignition of the gas generating material  84 . A more preferred amount is about 0.5% by weight of the gas generating material. 
     The present invention may also comprise other ingredients commonly added to a gas generating material  84  for providing inflation gas for inflating an inflatable vehicle occupant protection device, such as plasticizers, burn rate modifiers, coolants, and ignition aids, all in relatively small amounts. 
     Preferably, the components of the gas generating material  84  are present in a weight ratio adjusted to produce upon combustion a gas product that is essentially free of carbon monoxide. By essentially free of carbon monoxide, it is meant that the amount of carbon monoxide in the combustion gas product is less than 4% by volume of the gas product. 
     The gas generating material is prepared by adding, to a conventional mixer, the cellulose based binder, the antioxidant, and a solvent. The solvent readily dissolves the cellulose based binder and can be removed by evaporation. A preferred solvent is an organic solvent such as ethyl alcohol, ethyl acetate, acetone, or mixtures thereof. 
     The cellulose based binder, antioxidant, and solvent are mixed until a viscous, yet still fluid solution is formed. The solution of cellulose based binder and antioxidant is poured into an extruder such as a heat jacketed twin screw extruder. The fuel, oxidizer and other ingredients such as plasticizer, burn rate modifier, and coolant, if utilized, are added to and mixed with the solution of cellulose based binder and antioxidant. Alternatively, the cellulose based binder, antioxidant, solvent may be mixed with the fuel, oxidizer, and other ingredients, if utilized, before being mixed in the extruder. The oxidizer and fuel form a viscous slurry, having a dough like consistency, with the solution of cellulose based binder and antioxidant. 
     The viscous slurry is advanced from the extruder, through a shaping device or die with a predetermined diameter, and cut to desired length. Preferably, the gas generating material has the shape of the plurality of cylindrical grains  89 . 
     The solvent is evaporated from the gas generating material by heating the gas generating material at an elevated temperature (i.e., about 50° C. to about 60° C.) The gas generating material so formed is generally a resilient solid, like a hard rubber, capable of withstanding shock without permanent deformation at 85° C. and not brittle at −40° C. 
     From the above description of the invention, those skilled in the art will perceive improvements, changes and modifications. Such improvements, changes and modifications within the skill of the art are intended to be covered by the appended claims.