The present invention relates to gas generating composition pellets adapted to supply gas components to inflate an air bag system, and also relates to a gas generating system for an air bag which uses the gas generating pellets. More specifically, the present invention is concerned with novel pellets formed of a gas generating composition, which are adapted to generate operating gases in an air bag system provided in an automobile, airplane or the like, for protecting passengers from impacts, and a gas generating system for an air bag which uses these gas generating pellets.
When a motor vehicle, such as an automobile, collides with something at a high speed, a passenger of the vehicle may crash into a hard structure, such as a steering wheel or windscreen, inside the vehicle, due to inertia. To prevent the passenger from being injured or killed in such a crash, an air bag system has been developed wherein an air bag is rapidly inflated by means of a gas, thereby preventing the passenger from colliding with the dangerous part of the vehicle. A gas generating agent used in this air bag system is required to satisfy very strict conditions. For example, the bag must be inflated in a considerably short period of time, normally within 40 to 50 milliseconds. Further, the atmosphere in the bag should be harmless to human bodies, namely, its composition should be close to that of the air inside the vehicle.
At present, the gas generating agent generally used in the air bag system contains, as a base, an inorganic azide compound, in particular, a sodium azide, for example. Although the sodium azide has excellent combustion characteristics, an alkaline component produced by this compound as a by-product upon generation of a gas shows toxicity, which means that this compound does not satisfy the above-described requirement for the safety of passengers. Since the sodium azide itself shows toxicity, there is a concern about an influence of this compound on environments when the air bag system is disposed of as a waste.
To overcome the above shortcomings, several kinds of non-azide gas generating agents, may replace those containing sodium azide, have been developed. For example, Japanese laid-open Patent Publication No. 3-208878 discloses compositions containing tetrazole, triazole, or metallic salts thereof, and an oxygen containing oxidizing agent, such as alkali metal nitrate, as major components. Japanese Patent Nos. 64-6156 and 64-6157 disclose gas generating agents containing metallic salts of bitetrazole compounds containing no hydrogen, as major components.
In addition, Japanese Patent No. 6-57629 discloses gas generating agents containing a transition metal complex of tetrazole or triazole. Japanese laid-open Patent Publication No. 5-254977 discloses a gas generating agent containing triaminoguanidine nitrate, and Japanese laid-open Patent Publication No. 6-239683 discloses a gas generating agent containing carbohydrazide, while Japanese laid-open Patent Publication No. 7-61885 discloses a gas generating agent containing a nitrogen containing non-metallic compound, such as cellulose acetate and nitroguanidine. Further, U.S. Pat. No. 5,125,684 discloses the use of nitroguanidine as an energy substance that coexists with 15-30xc2x0 of a cellulose-based binder. Also, Japanese laid-open Patent Publication No. 4-265292 discloses a gas generating composition obtained by combining derivatives of tetrazole and triazole, oxidizing agent and a slag-forming agent.
With respect to gas generating behaviors and bag inflating behaviors in the conventional air bag systems as described above, it has been believed that the air bag system on the side of a driver seat (hereinafter abbreviated as a xe2x80x9cD seatxe2x80x9d), in particular, operates with sufficiently safe inflating behaviors, since the passenger in the driver seat is seated in a relatively fixed position. As air bag systems are being widely used, and becoming normally installed on recent vehicles, however, it has been desired to develop even safer air bag systems to deal with various situations, namely, those suitable for various types of drivers whose sitting heights vary from person to person, or who may drive while holding the steering wheel close to his/her body. The safer air bag systems have been also desired when a child is seated in a cabin seat, such as a passenger seat (hereinafter abbreviated as a xe2x80x9cP seatxe2x80x9d).
Although the air bag systems having the conventional inflating behavior characteristics may be used, technologies for providing safer air bag systems have been desired which can reduce the rate of increase in the initial inflating speed, e.g., reducing the inflating speed of an air bag on the side of the D seat for a period of 10 milliseconds from the start of gas generation, to reduce the possibility of injuries due to rapid inflation of the air bag in the initial period, while maintaining sufficient passenger restricting capability after 30 to 50 milliseconds from the start of the gas generation. The similar technologies have been also desired to control gas generating behaviors on the side of the P seat.
Japanese laid-open Patent Publication No. 8-207696 discloses an example of such technologies in which the gas is generated in two steps such that the air bag is relatively slowly inflated in an initial operating stage and the gas is rapidly generated in the second stage. In this type of technology, however, the structure inside the gas generator becomes complicated, resulting in an increased size of its container, and increased manufacturing cost.
Thus, there has not yet been disclosed a technology to control the inflating speed of the air bag by controlling gas generating behavior only by gas generating composition pellets. It has been thus desired to develop a technology for controlling the gas generating behaviors by the gas generating composition pellets alone, with a simple structure and a low cost.
As a result of diligent studies in an attempt to solve the above problems, the inventors have reached the present invention based on a finding that the above problems may be solved by controlling the physical shape of the gas generating pellets into a suitably determined shape.
According to the present invention, there is provided gas generating composition pellets for an air bag system, characterized in that the pellets are controlled such that in a tank test conducted with respect to a gas generator using the pellets, where a desired maximum tank pressure is P (kPa), and a period of time from the start of rising of the tank pressure to the time when the maximum tank pressure P (kPa) is reached is T milliseconds, the tank pressure measured after 0.25xc3x97T milliseconds is not higher than 0.20xc3x97P (kPa), and the tank pressure measured after 0.80xc3x97T milliseconds is not lower than 0.70xc3x97P (kPa). In particular, the gas generating pellets may be formed of a non-azide gas generating composition and have holes, and each pellet may be formed with a length of L(mm) and a hole having an inside diameter d(mm) of 0.2 to 1.5 (mm), such that the value of L/d is 3.0 or larger.
The present invention also provides a gas generator for an air bag system, characterized in that in a tank combustion test conducted with respect to the gas generator using the above-described, pellets, where a desired maximum tank pressure is P (kPa), and a period of time from the start of rising of the tank pressure to the time when the maximum tank pressure P (kPa) is reached is T milliseconds, the tank pressure measured after 0.25xc3x97T milliseconds is not higher than 0.20xc3x97P (kPa), and the tank pressure measured after 0.80xc3x97T milliseconds is not lower than 0.70xc3x97P (kPa).
In the present invention, the tank test is conducted in the following manner. (Tank Combustion Test)
An inflator filled with gas generating pellets is fixed to the inside of a tank made of stainless steel and having a content volume of 60 liters. After the tank is air-tightly closed at room temperature, the inflator is connected to an external ignition circuit. By using a pressure transducer installed in the tank, pressure increases or changes in the tank are measured from time 0 to 200 milliseconds where the time 0 indicates a point of the time when the switch of the ignition circuit is turned on. Measurement data are processed by a computer, and finally represented as a tank pressure/time curve from which characteristics of the gas generating pellets can be evaluated. After the combustion, a portion of the gas in the tank is sampled out, to be analyzed in respect of its CO and NOx components, for example.