The present invention relates to a gas generating composition which is suitable for an air bag restraining system of automobiles or the like, its molded article and an inflator for an air bag using the same.
As a gas generating agent for an air bag as a passenger-protecting device in automobiles, a composition using sodium azide has been often used so far. However, a toxicity [LD50 (oral-rat) = 27 mg/kg] to human bodies or hazard in handling of sodium azide has been regarded as a serious problem. Therefore, as safe non-azide based gas generating compositions, gas generating compositions containing various nitrogen-containing organic compounds have been developed to replace the sodium-azide based composition.
For example, US-A 4,909,549 discloses a composition comprising hydrogen-containing tetrazole and triazole compounds and an oxygen-containing oxidizing agent. US-A 4,370,181 discloses a gas generating composition comprising a hydrogen-free bitetrazole metal salt and an oxygen-free oxidizing agent. US-A 4,369,079 discloses a gas generating composition comprising a hydrogen-free bitetrazole metal salt and an alkali metal nitrate, an alkali metal nitrite, an alkaline earth metal nitrate, an alkaline earth metal nitrite or a mixture thereof. US-A 5, 542, 999 discloses a gas generating agent comprising a fuel such as GZT, TAGN, NG (nitroguanidine), NTO or the like, a basic copper nitrate, a catalyst for reducing toxic gases and a coolant. JP-A 10-72,273 discloses a gas generating composition comprising a bitetrazole metal salt, a bitetrazole ammonium salt or aminotetrazole and ammonium nitrate.
However, the non-azide based gas generating composition is problematic in a combustion temperature, a burning rate, phase transfer, amounts of generated carbon monoxide and nitrogen oxides, a gas output and the like. For example, the gas generating composition of US-A 4,369,079 has a high combustion temperature, and requires a large amount of a coolant in actual use. The composition of US-A 5,542,999 has a low burning rate, and might not be completely burned in a short time. In the gas generating agent of JP-A 10-72, 273, a shape is changed due to phase transfer of ammonium nitrate in the range of the use temperature, which damages a molded article of the gas generating agent, and in result, stable combustion cannot be conducted.
Accordingly, an object of the invention is to provide a gas generating composition which has a low combustion temperature, a high burning rate, small amounts of generated carbon monoxide and nitrogen oxides and an excellent combustion stability, its molded article and an inflator for an air bag using the same.
The invention provides a gas generating composition comprising (a) a bitetrazole compound as a fuel and (b) a basic metal nitrate as an oxidizing agent.
Further, the invention provides a gas generating composition comprising (a) a bitetrazole compound as a fuel, (b) a basic metal nitrate as an oxidizing agent and (c) a binder and/or a slag-forming agent.
Still further, the invention provides a molded article in the form of a single-perforated cylinder, a perforated (porous) cylinder or pellets, the molded article being obtained from the gas generating composition.
Furthermore, the invention provides an inflator for an air bag using the gas generating composition and the molded article.
The gas generating composition and its molded article of the invention are easy to handle because of a low toxicity and less hazard, and have a high burning rate and a low combustion temperature. Further, amounts of generated carbon monoxide and nitrogen oxides are reduced in the combustion.
The bitetrazole compound being a fuel as component (a) which is used in the invention is preferable because the content of the nitrogen atom in a molecule is high, the toxicity is low and the burning rate is increased in combination with component (b).
As the bitetrazole compound, at least one selected from metal salts of bitetrazole such as a potassium salt of bitetrazole, a sodium salt of bitetrazole, a magnesium salt of bitetrazole, a calcium salt of bitetrazole and a copper salt of bitetrazole, an ammonium salt of bitetrazole and a melamine salt of bitetrazole is proposed. Of these, an ammonium salt of bitetrazole is preferable because the content of the nitrogen atom is 81.4 % by weight, LD50 (oral-rat) is 2,000 mg/kg and the combustion efficiency is good. The bitetrazole compound here includes 5-5' compounds and 1-5' compounds relating to two tetrazole rings, and 5-5' compounds are preferable because of the cost and the easy procurement.
As the basic metal nitrate being an oxidizing agent as component (b) which is used in the invention, at least one selected from a basic copper nitrate, a basic cobalt nitrate, a basic zinc nitrate, a basic manganese nitrate, a basic iron nitrate, a basic molybdenum nitrate, a basic bismuth nitrate and a basic cerium nitrate is proposed. Of these, a basic copper nitrate is preferable.
The basic copper nitrate is excellent in thermal stability, as compared with ammonium nitrate as an oxidizing agent, because no phase transfer occurs in the range of the use temperature and the melting point is high. Further, since the basic copper nitrate acts to decrease a combustion temperature of a gas generating agent, amounts of generated nitrogen oxides can be decreased.
When the gas generating composition of the invention is a two-component-type composition comprising components (a) and (b), the contents of the components are as follows.
The content of component (a) is preferably 5 to 60 % by weight, more preferably 15 to 55 % by weight. The content of component (b) is preferably 40 to 95 % by weight, more preferably 45 to 85 % by weight.
Further, a preferable example of the two-component-type gas generating composition is a composition comprising 15 to 35 % by weight of ammonium bitetrazole as component (a) and 65 to 85 % by weight of a basic copper nitrate as component (b).
As the binder and/or the slag-forming agent as component (c) used in the invention, at least one selected from carboxymethylcellulose (CMC), sodium carboxymethylcellulose (CMCNa), a potassium salt of carboxymethylcellulose, an ammonium salt of carboxymethylcellulose, cellulose acetate, cellulose acetatebutyrate (CAB), methyl cellulose (MC), ethyl cellulose (EC), hydroxyethyl cellulose (HEC), ethylhydroxyethyl cellulose (EHEC), hydroxypropyl cellulose (HPC), carboxymethylethyl cellulose (CMEC), fine crystalline cellulose, polyacrylic amide, amine products of polyacrylic amide, polyacrylic hydrazide, a copolymer of an acrylic amide and a metal salt of acrylic acid, a copolymer of polyacrylic amide and polyacrylic ester, polyvinyl alcohol, acrylic rubber, guar gum, starch, silicone, molybdenum disulfide, Japanese acid clay, talc, bentonite, diatomaceous earth, kaolin, calcium stearate, silica, alumina, sodium silicate, silicon nitride, silicon carbide, hydrotalcite, mica, a metal oxide, a metal hydroxide, a metal carbonate, a basic metal carbonate and a salt of molybdic acid is proposed.
As a metal oxide being component (c), at least one selected from copper oxide, iron oxide, zinc oxide, cobalt oxide, manganese oxide, molybdenum oxide, nickel oxide and bismuth oxide is proposed. As a metal hydroxide, at least one selected from cobalt hydroxide and aluminum hydroxide is proposed. As a metal carbonate and a basic metal carbonate, at least one selected from calcium carbonate, cobalt carbonate, a basic zinc carbonate, a basic copper carbonate, a basic cobalt carbonate, a basic iron carbonate, a basic bismuth carbonate and a basic magnesium carbonate is proposed. As a molybdate, at least one selected from cobalt molybdate and ammonium molybdate is proposed. These compounds as component (c) can act as the slag-forming agent and/or the binder.
In case of increasing the ignitability of the gas generating composition, a sodium salt of carboxymethylcellulose and a potassium salt of carboxymethylcellulose are preferable. Of these, a sodium salt of carboxymethylcellulose is more preferable.
When the gas generating composition of the invention is a three-component-type composition comprising components (a) to (c), the contents of the components are as follows.
The content of component (a) is preferably 5 to 60 % by weight, more preferably 15 to 55 % by weight. The content of component (b) is preferably 40 to 95 % by weight, more preferably 45 to 85 % by weight. The content of component (c) is preferably 0.1 to 15 parts by weight, more preferably 0.5 to 10 parts by weight per 100 parts by weight in total of components (a) and (b).
A preferable example of the three-component-type gas generating composition is a composition comprising 15 to 35 % by weight of an ammonium salt of bitetrazole as component (a), 65 to 85 % by weight of a basic copper nitrate as component (b) and 0.5 to 10 parts by weight, per 100 parts by weight in total of components (a) and (b), of a sodium salt of carboxymethylcellulose as component (c).
The gas generating composition of the invention can be molded in a desired shape, and formed into a molded article in the shape of a single-perforated cylinder, a perforated (porous) cylinder or a pellet. These molded articles can be produced by a method in which the gas generating composition is added to and mixed with water or an organic solvent and the mixture is extrusion-molded (molded articles in the form of a single-perforated cylinder, a hollow tube having one through-hole in the longitudinal direction, a perforated (porous) cylinder and a hollow tube having plural through-holes in the longitudinal direction) or by a compression-molding method using a pelletizer (molded article in the form of a pellet) .
The gas generating composition or the molded article obtained therefrom in the invention can be used in, for example, an inflator for an air bag of a driver side, an inflator for an air bag of a passenger side, an inflator for a side air bag, an inflator for an inflatable curtain, an inflator for a knee bolster, an inflator for an inflatable seat belt, an inflator for a tubular system and an inflator for a pretensioner in various vehicles.
Further, the inflator using the gas generating composition or the molded article obtained therefrom in the invention may be a pyrotechnic type in which a gas is supplied only from a gas generating agent or a hybrid type in which a gas is supplied from both of a pressurized gas such as argon or the like and a gas generating agent. In the hybrid inflator, a section having a function to generate a gas by burning a gas generating agent is "a gas generator".
Moreover, the gas generating composition or the molded article obtained therefrom in the invention can also be used as an igniting agent called an enhancer (or a booster) for transferring energy of a detonator or a squib to a gas generating agent.
The invention is illustrated more specifically below by referring to Examples. However, the invention is not limited thereto. By the way, in Tables 1 to 4 below, BHTNH3 indicates an ammonium salt of bitetrazole, BHTK a potassium salt of bitetrazole, BCN a basic copper nitrate and CMCNa a sodium salt of carboxymethylcellulose respectively.
Gas generating compositions each having a formulation shown in Table 1 were produced. A combustion temperature and a gas outlet (unit "mol/100 g" indicates the number of mols of a generated gas per 100 g of the composition) and amounts of generated CO and NO of these compositions according to theoretical calculations are shown in Table 1.
It indicates that any combustion temperatures in Examples 1 to 7 are quite lower than those in Comparative Examples 1 and 2 and not more than 1,900 K. The amount of generated NO is effectively decreased. Further, the amounts of generated CO and NO are not permitted in practice unless the amount of generated CO being 2 x 10-3 mol/100 g or less and the amount of generated NO being 2 x 10-4 mol/100 g or less are attained at the same time. It is found that Examples 1 to 7 satisfy these conditions.
Gas generating compositions each having a formulation shown in Table 2 were produced. These compositions were tested for a friction sensitivity and a drop hammer sensitivity according to the explosives performance test method of JIS K 4810-1979. The results are shown in Table 2.
Gas generating compositions each having a formulation shown in Table 3 were produced. With respect to these compositions, a melting temperature, a temperature at which to start heat generation and a temperature at which to start TG weight loss were measured with a TAS-type differential thermal analyzer manufactured by Rigaku K.K. A rate of temperature rise in the measurement was 20°C/min, a measurement atmosphere was a nitrogen gas, and an amount of a sample in the measurement was 1 to 2 mg. The results are shown in Table 3.
Gas generating compositions each having a formulation shown in Table 3 were produced. Each of these compositions was molded into a strand. A burning rate was measured in a nitrogen atmosphere at a pressure of 50, 70 or 90 kg/cm2. A burning rate at 70 kg/cm2 and a pressure index of 50 to 90 kg/cm2 are shown in Table 4.
As stated above, the respective values shown in Examples 8 to 15 reveal that the compositions in these Examples meet the practical conditions as the gas generating composition for the inflator gas.