FIELD OF THE INVENTION
The invention relates to a propellant for use in a micro-gas generator and a method of manufacturing such a propellant.
BACKGROUND OF THE INVENTION
In the prior art gas generants are known for various safety device purposes, specifically for use as safety devices applications such as seat belt retractors, seatbelt pre-tensioners, buckle pre-tensioners, air bag inflators, head rest actuators, seat interlocks, hood lifters and other pedestrian protection devices that require high reliability gas generation devices. For example, for seatbelt pre-tensioners, the gas produced is designed to actuate a rack and pinion style device to better position the vehicle occupants, prior to airbag deployment, in the event of a crash. A known example of such propellants, as for instance described in the U.S. Pat. No. 6,964,715, is manufactured by Special Devices, Inc. and known as “Green Global Gas Generant” containing 19%±1% 5-aminotetrazole, 17%±1% azodicarbonamide 1%±0.2% aluminum powder, 60%±1% potassium perchlorate, and 3%±0.5% ethyl cellulose. It has been discovered that these prior art gas generants do not tolerate extended exposure to high temperatures in the range of 120° C. without changing its gas generation properties such as its ballistic performance, including specifically how much gas is generated over a certain time period.
In particular, it was discovered that propellants like the “Green Global Gas Generant” exhibit a change in ballistic performance after these propellants were exposed to high temperatures over a certain time span. Such high temperature aging resulted in altering the ballistic performance towards a steep pressure increase over a certain time period like 2 milliseconds (ms) that may not be desirable since a more gradual pressure increase is desired.
Therefore, for some applications, specifically installing the micro-gas generator in areas where high temperatures are to be expected like for instance in the engine compartment of a vehicle, it is desirable to create a micro-gas generator tolerating such high temperatures over a long time period without changing its ballistic properties. A particular parameter of interest for the ballistic performance is the “quickness”, i.e. the rate at which gas is generated, or put in other words, how much gas is generated during a certain time period such as for example over a time period of 8 milliseconds (8 ms).
SUMMARY OF THE INVENTION
According to one aspect of the invention a propellant for a micro-gas generator is provided, the propellant comprising 5-aminotetrazole, aluminum, a binder and an oxidizer and is substantially free from azodicarbonamide.
According to another aspect of the invention, a method for manufacturing a propellant for a micro-gas generator is provided, comprising: providing a propellant mixture of 5-aminotetrazole, aluminum powder, potassium perchlorate, ethyl cellulose and fluoropolymers; adding acetone as a solvent to solvate ethyl cellulose and fluoropolymers; evaporating the acetone using a low level of vacuum until the mixture is a damp cake; and granulating and drying the damp cake forming dried propellant granules.
DETAILED DESCRIPTION OF THE INVENTION
According to a preferred embodiment, the oxidizer comprises potassium perchlorate.
According to another preferred embodiment, the binder comprises ethyl cellulose as a binder constituent.
According to another preferred embodiment, the binder comprises fluoropolymers as a binder constituent.
According to a preferred embodiment, the fluoropolymers of the binder are terpolymers of at least one of a group consisting of copolymers of hexafluoropropylene (HFP), vinylidene fluoride (VF2), and tetrafluoroethylene (TFE).
According to another preferred embodiment, the propellant comprises 30-34% 5-aminotetrazole.
According to another preferred embodiment, the propellant comprises 0.6-1.4% aluminum powder.
According to another preferred embodiment, the binder comprises 0.6-1.4% terpolymers of the at least one of the group consisting of copolymers of hexafluoropropylene (HFP), vinylidene fluoride (VF2), and tetrafluoroethylene (TFE).
According to another preferred embodiment, the propellant comprises 60-68% potassium perchlorate.
According to another preferred embodiment, the binder comprises 1-3% ethyl cellulose and 0.6-1.4% terpolymers of the at least one of the group consisting of copolymers of hexafluoropropylene (HFP), vinylidene fluoride (VF2), and tetrafluoroethylene (TFE).
According to another preferred embodiment, the propellant consists of 30-34% 5-aminotetrazole, 0.6-1.4% aluminum powder, 60-68% potassium perchlorate, 1-3% ethyl cellulose and 0.6-1.4% terpolymers of the at least one of the group consisting of copolymers of hexafluoropropylene (HFP), vinylidene fluoride (VF2), and tetrafluoroethylene (TFE).
According to another preferred embodiment of the propellant manufacturing method according to the invention, after granulating and drying the damp cake, the resulting intermediate product is further processed by densifying or tableting the dried propellant granules to form dry propellant tablets and re-granulating said dry propellant tablets to generate re-granulated dry propellant granules and then passing the re-granulated dry propellant granules through a sieve in order to filter for re-granulated dry propellant granules of a desired granule diameter. This process allows modifying the sieve cut and therefore allows tailoring of the ballistic properties of the propellant to meet customer requirements without requiring a change of the formulation. Depending on the sieve cut and therefore the size of the densified granules the propellant generates gas faster or slower.
BRIEF DESCRIPTION OF THE DRAWINGS
In the drawings show:
FIG. 1 the ballistic properties of a propellant according to the invention in a diagram showing the pressure in psi after firing over the time in milliseconds for a variety of propellants exposed to different high-temperature 120° C. exposure time spans;
FIG. 2 the ballistic properties of a propellant according to the prior art in a diagram showing the pressure in psi after firing over the time in milliseconds for a variety of propellants exposed to different high-temperature 120° C. exposure time spans; and
FIG. 3 an example of a typical micro-gas generator as known in the prior art for which the propellant according to FIG. 1 may be used.
DETAILED DESCRIPTION OF THE DRAWINGS
FIG. 3 shows an example of a typical micro-gas generator as known in the prior art for which the propellant according to FIGS. 1 and 2 may be used. An initiator 3, including contact pins or lead wires 1, and captured within a molded body 2, is capable of conducting electric current from an external source such as a control circuit that responds to rapid deceleration, when the micro gas generator is used in an automobile seat belt pre-tensioner to a metallic bridge wire or similar. When electrically energized with an appropriate signal, the initiator 3 produces a high temperature arc or spark to initiate the explosion of an initiation charge 4 surrounding a bridge wire. The molded body 2, formed within a retainer 5, is fastened to a propellant can 6 containing the output propellant 7. The gas pressure will finally rupture the propellant can 6 when enough pressure has built up in the output can and release the gas to its intended destination, for instance an engine compartment actuator assembly for lifting the hood in case of pedestrian impact or in case of a seatbelt pretension to a rack and pinion device or a piston and steel cable device tightening the seatbelt.
In FIG. 2 the ballistic properties of a prior art propellant are shown in a diagram providing the pressure in psi after firing over the time in milliseconds for a propellant for a variety of high-temperature 120° C. exposure times, namely i) no exposure, denoted by reference numeral 10, ii) exposure for 48 hours denoted by reference numeral 11; and iii) exposure over 1000 hours denoted by reference numeral 12. For the purpose of this diagram, an experimental setting was used capturing and sensing the pressure of the firing. As FIG. 2 reveals, the graphs 11 and 12 are close together, i.e. have a ballistic performance that is similar and distinguishes for both graphs 11 and 12 significantly from the graph 10 demonstrating the ballistic properties of the propellant at no exposure to high temperature. The graphs 11 and 12 reveal that the effect of significantly altering the ballistic performance of the propellant is already for the most part completed after only 48 hours of exposure to 120° C., i.e. an exposure beyond 48 hours, for instance 1000 hours as shown in the graph 12, does not make any significant further difference. As these graphs reflect, the 120° C. exposed propellant reaches the maximum pressure at point 13 at about 3 ms while the non-temperature-exposed propellant reaches the maximum pressure at about 8 ms at point 14. While the maximum pressures both for the temperature-exposed propellants at point 13 and of the non-temperature-exposed at point 14 are about the same, the negligible difference may be explainable for the most part by dynamic effects resulting from the very fast gas generation for the temperature-aged propellant. As a conclusion, the prior art non-temperature-exposed propellant builds up pressure more gradually, reaching peak pressure over a period of approximately 8 ms, as compared to the temperature-exposed propellant of the same formulation which reaches this same peak pressure in a period of only about 2 ms. The formulation of the prior art propellant shown in FIG. 2 contains 19%±1% 5-aminotetrazole, 17%±1% azodicarbonamide 1%±0.2% aluminum powder, 60%±1% potassium perchlorate, and 3%±0.5% ethyl cellulose.
FIG. 1 shows a diagram similar to FIG. 2 but in contrast to FIG. 2 demonstrates the ballistic performance of a propellant according to the incident invention in a diagram providing the pressure in psi after firing over the time in milliseconds for the propellant for a variety of high-temperature 120° C. exposure times, namely i) no exposure, denoted by reference numeral 16, ii) exposure for 48 hours denoted by reference numeral 17; and iii) exposure over 2000 hours denoted by reference numeral 18. As the diagram of FIG. 1 reveals, the maximum pressure is reached for all levels of temperature exposure at about the same point denoted 19 at about 8 ms like for the non-temperature exposure aged prior art propellant demonstrated by graph 10 in FIG. 2. This makes the application of the propellant according to the incident invention more versatile, namely temperature exposure does not have any influence on the ballistic performance of the propellant according to the incident invention that would go beyond negligible variations. Put in other words, the propellant according to the incident invention generates gas more gradually in comparison with temperature-exposure aged propellant according to the prior art. The specific example demonstrated in FIG. 1 has a formulation consisting of 32%±1% 5-aminotetrazole, 1%±0.2% aluminum powder, 64%±1% potassium perchlorate, 2%±0.5% ethyl cellulose and 1%±0.2% terpolymers of at least one of the group consisting of copolymers of hexafluoropropylene (HFP), vinylidene fluoride (VF2), and tetrafluoroethylene (TFE). The terpolymers of at least one of the group consisting of copolymers of hexafluoropropylene (HFP), vinylidene fluoride (VF2), and tetrafluoroethylene (TFE) are commercially available as “Viton® B”, a composition manufactured and distributed by the DuPont Performance Elastomers L.L.C. it is noted that the propellant according to the incident invention is not limited to these exact aforementioned ranges of the propellant used in FIG. 1, meaning that a propellant with, to some extent altered ranges, provides a similar ballistic performance irrespective of temperature-exposure aging.
The propellant with the ballistic performance shown in FIG. 1 was manufactured by adding acetone as a solvent to solvate ethyl cellulose and Viton® B=terpolymers of at least one of the group consisting of copolymers of hexafluoropropylene (HFP), vinylidene fluoride (VF2), and tetrafluoroethylene (TFE); evaporating the acetone using a low level of vacuum until the mixture is a damp cake; and granulating and drying the damp cake forming dried propellant granules. The dried propellant granules were then densified/tableted to form dry propellant tablets and the dry propellant tablets were then re-granulated to generate re-granulated dry propellant granules that were then passed through a sieve in order to filter for re-granulated dry propellant granules of a desired granule diameter. In the alternative to densified granules, a number of different applications are possible like pressed pellets, powder, extrusion, cast grains etc.
A wide variation of applications is possible for the propellant according to the incident invention. In context with a vehicle, specifically installing the micro-gas generator in the engine compartment or close to other heat sources in a vehicle like the transmission is possible, but also in connection with other safety device applications such as seat belt retractors, buckle pre-tensioners, airbag inflators, head rest actuators, seat interlocks, hood lifters, and other pedestrian protection devices that require high reliability gas generation devices. Other applications are envisaged such as propellants for use in automotive inflator systems in a pressed or extruded tablet or grain form. Also applications in aerospace and defense are envisaged such as thrusters, actuators, canopies and seat ejection motor applications.
Patent Application
20170174580
