Plastic and nonmetallic article shaping or treating: processes – Explosive or propellant article shaping or treating
Reexamination Certificate
1999-09-24
2001-11-13
Silbaugh, Jan H. (Department: 1732)
Plastic and nonmetallic article shaping or treating: processes
Explosive or propellant article shaping or treating
C264S003200, C264S003300, C149S019700, C149S046000, C149S060000
Reexamination Certificate
active
06315930
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a propellant composition, particularly a propellant composition having a low burn rate exponent related to burning stability.
BACKGROUND OF THE INVENTION
Propellants have numerous and widespread applications including in vehicles with air bag modules. Among other parameters, propellants can be characterized based on their stability when burning in an air bag inflator. Preferred stability relates to a constant and/or uniform burning of the propellant, along its length, from its outer surface inwardly to its center. A quantitative representation related to propellant combustion stability is in the form of a burn rate exponent n, which is a material parameter defined by [ln(R
2
/R
1
)]/[ln(P
2
/P
1
)], where R
2
is burn rate at pressure P
2
and R
1
is a burn rate at pressure P
1
, (not equal to P
2
). The burn rate exponent of most propellants has a value between zero and one. The closer the burn rate exponent value is to one the greater is the instability of the inflator. The operating pressure of the inflator is related to the propellant and inflator hardware properties through the relation:
P
=
C
(
A
b
A
c
)
1
1
-
n
P=instantaneous operating pressure of the inflator.
C=constant
A
b
=instantaneous burning surface area of the propellant.
A
c
=outlet orifice area of the inflator.
It is well known that it is difficult to achieve adequate stability of inflators using propellants with burn rate exponents above about 0.7. Conversely, desired burning stability of the inflator is present when the burn rate exponent of the propellant is no greater than about 0.7.
Certain materials are known to have low and desirable burn rate exponents. Ammonium nitrate based propellants typically have a burn rate exponent of about 0.5. Consequently, ammonium nitrate has a desirable high stability during its combustion. However, ammonium nitrate suffers from certain drawbacks as part of a propellant composition. Especially in comparison with other useful propellant compositions, ammonium nitrate propellants are typically difficult to ignite and have a low burning rate once ignited. It is known to combine ammonium nitrate with another material, such as a secondary explosive, to enhance or achieve the necessary burning rate and/or ease of ignition. Such known propellant compositions have also increased the burn rate exponent of the resulting propellant composition to a burn rate exponent value of greater than 0.7.
It would be beneficial to provide a propellant composition, for use in vehicle inflators or other applications, that includes ammonium nitrate so that the propellant is stable during its burning, while achieving a necessary or desirable propellent burning rate and ignition thereof.
SUMMARY OF THE INVENTION
In accordance with the present invention, a propellant is provided that has a relatively low burn rate exponent.
Preferably, the burn rate exponent is no greater than 0.7. As the value of the burn rate exponent increases above 0.7, the operation of the gas generator becomes increasingly unstable and impractical. Conversely, a propellant having a burn rate exponent of about 0.7 and less is desirably stable, particularly when used with an inflator to inflate an air bag or other inflatable in a vehicle. Stability refers to a propellant property by which there is a predictable and uniform output of inflator gas from the inflator. In one embodiment, the propellant composition includes phase-stabilized ammonium nitrate (PSAN). The phase-stabilized ammonium nitrate constitutes at least a majority, by weight, and preferably a substantial majority, of the propellant composition. The PSAN has the property that, after being subject to temperature cycling in the range of −40° C. through 90° C. through a substantial number of cycles of arbitrary shape and duration, the propellant composition having the PSAN is functional in an inflator. In one embodiment, in addition to the ammonium nitrate, the PSAN includes potassium nitrate in the range, by weight, of about 8%-14%, or any other one or more phase-stabilizing components could be utilized in addition to, or as a substitute for, the potassium nitrate.
In the preferred embodiment, in connection with achieving the desired relatively lower burn rate exponent, the propellant composition also includes fibrous cellulose, such as available from pulp board or wood pulp. The fibrous cellulose is different from non-fibrous cellulosic material such as nitrate cellulose, cellulose acetate, and cellulose acetate butyrate. The fibrous cellulose is useful in achieving a lower burn rate exponent, instead of a secondary explosive, inasmuch as the secondary explosive with the PSAN typically increases the burn rate exponent of the propellant composition to a much greater value. Consequently, it is most preferred that there be no secondary explosive, or substantially none, in the propellant composition. However, some secondary explosive could be included but, in no event, should the amount by weight of secondary explosive be greater than 20% of the propellant composition.
The propellant composition may also include is hydroxypropylcellulose (HPC) or other cellulose ethers or derivatives, which is the soluble part of the binder system of the propellant composition. The fibrous cellulose can also act as a contributor to the binder system. Small amounts of other materials can be included in the propellant composition including stabilizers, plasticizers and a ph control component.
In one embodiment, the burning rate of the propellant is controllable, based on controlling the thickness of the propellant and concomitantly the density of the propellant. That is to say, as a finished product, the propellant is at least 85% of theoretical density. With such density, the number and sizes of pores are reduced so that the porosity of the propellant is limited. Accordingly, the burning or combustion of the propellant from its outer surface inwardly towards its center is substantially uniform and constant. In one embodiment, the propellant composition includes PSAN and the fibrous cellulose.
When making the propellant composition, the combination of materials that includes PSAN and fibrous cellulose has a first or lower density after being formed. Later, the density of this composition is increased further to a greater percentage of theoretical density, namely, 85% or greater. In one embodiment, the increase in density (i.e. the reduction in the porosity) of the propellant composition having PSAN and fibrous cellulose is accomplished by essentially flattening the propellant to a ribbon-like structure using appropriate pressing or flattening machinery.
Based on the foregoing summary a number of salient aspects of the present invention are recognized. The propellant composition of the present invention generates a predictable gas output based on a pyrotechnic inflator having greater stability, particularly where the propellant is utilized in an inflator for an air bag. This stability is preferably achieved with a propellant composition that includes phase-stabilized ammonium nitrate and fibrous cellulose. This combination results in a relatively low burn rate exponent, such as no greater than 0.7. Preferably, the greater stability is achievable using a highly densified propellant composition, namely, at least 85% of theoretical density. In one embodiment, the propellant composition has a first density, which is increased to a second density, preferably, using a roller mill or other functionally comparable machinery. Preferably, the fibrous cellulose is utilized to achieve such density. In making the propellant, its thickness can be controlled resulting in a desired burning rate.
Additional advantages of the present invention will become readily apparent from the following discussion, particularly when taken together with the accompanying drawing.
REFERENCES:
patent: 3173817 (1965-03-01), Wright
patent: 3959044 (1976-05-01), Young
patent: 4102953 (1978-07-01), Johnson et al
Autoliv ASP Inc.
Poe Michael I.
Sheridan & Ross P.C.
Silbaugh Jan H.
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