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Description  |
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This invention is concerned with composite propellants, that is explosives
comprising a solid oxidising agent and an organic binder which both acts
as a fuel and gives adequate mechanical strength to the mixture. Composite
propellants may contain a number of optional ingredients, such as
combustion regulators and combustion accelerators, the latter usually
being metals, such as aluminium.
Conventional composite propellants usually contain about 25% by weight of
binder, and the charges are shaped solely by casting them in moulds.
However, when the proportion of binder is substantially reduced, the
viscosity of the mixture obtained on mixing the various constituents of
the composite propellant increases and the mechanical properties of the
charges produced are inferior and many charges show cohesion defects.
Moreover, the use of conventional composite propellants as gas generators
in inflatable cushion protection devices for high speed vehicles, such as
automobiles, cannot be considered because these propellants do not fulfil
the condition that the gases they produce should be non-toxic, and because
they possess poor mechanical properties which make it impossible to
produce charges of small thickness. These disadvantages apply especially
to composite propellants which have, as the main constituents, an
oxidising agent of the metal perchlorate or chlorate type, a binder which
contains oxygen but not nitrogen, and a rather ineffective inert
combustion regulator.
The composition of the combustion gases of a known solvent-free powder (not
of the composite type), which is at present commonly used as a gas
generator, is given below:
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Composition of the combustion gases
Main constituents:
CO.sub.2 10.8 mols/kg
N.sub.2 4.84 mols/kg
H.sub.2 O 4.11 mols/kg
H.sub.2 7.78 mols/kg
CO 8.75 mols/kg, corresponding
to 24.5% by weight.
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The combustion products also contain other gases in small proportions and
solid residues. These products are produced by the combustion of a
solvent-free powder composition comprising:
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Nitrocellulose (nitrogen content 11.7%)
55.8 parts
by weight
Nitroglycerine 37.2 parts
Various ballistic additives
7.5 parts
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The reaction takes place at a pressure of 200 bars and a temperature of
about 2,640.degree. K. The addition of additives, such as copper oxide,
potassium dichromate and manganese dioxide, to such solvent-free powder
compositions does not enable the carbon monoxide content to be reduced to
below 8% by weight and, physiologically, a carbon monoxide content greater
than 0.05% is dangerous. Restriction of the nitrogen oxide content is even
more necessary and the total of these oxides must not exceed a few parts
per million (ppm).
We have now developed composite propellant compositions which have good
mechanical strength and which yield combustion gases which are
substantially free of toxic gases and which are, therefore, suitable for
use as gas generators for inflatable cushion protection devices.
According to the present invention, there is provided a composite
propellant which comprises:
a. from 78 to 92% by weight of an oxidising agent which is an alkali metal,
alkaline earth metal or ammonium chlorate or perchlorate or a mixture of
two or more thereof,
b. from 7.9 to 17.2% by weight of an organic binder which contains oxygen
but not nitrogen,
c. from 0.1 to 0.8% by weight of a carbon-containing combustion regulator
and, optionally, a second combustion regulator,
d. optionally, up to 5% by weight of a metal combustion accelerator, and
e. optionally, up to 4% by weight of a plasticiser.
The preferred oxidising agent is potassium perchlorate, used alone or
together with up to 6% by weight of ammonium perchlorate; other preferred
oxidising agents are sodium perchlorate and potassium and/or sodium
chlorates, individually or as mixtures.
The preferred binders are cellulose acetates, particularly cellulose
triacetate, and silicone rubbers, particularly silicone rubbers with a
carbon content less than 33%. The preferred proportion of cellulose
triacetate is from 8 to 17.2% by weight, and that of silicone rubber is
from 8 to 14.6% by weight. Below 8% by weight, the binder does not coat
the grains of oxidising agent perfectly. The upper limit for the
proportion of binder is determined by the necessity of obtaining a carbon
monoxide content of not more than about 500 ppm on combustion.
Suitable carbon-containing combustion regulators are, for example,
acetylene black and graphite. The preferred proportion of
carbon-containing combustion regulator is 0.15 to 0.5% by weight. The
second combustion regulator which may optionally be present is preferably
copper dichromite in a proportion of from 0.5 to 5% by weight.
The preferred combustion accelerator is aluminium which preferably has a
specific surface area of from 3400 to 3800 cm.sup.2 per cm.sup.3.
Many plasticisers may be employed, the preferred being tricresyl phosphate,
diethyl phthalate and triacetin. The best results with respect to
mechanical strength and toxicity of the gases produced, are obtained with
triacetin which, for the same weight of plasticiser, introduces the least
carbon into the composition. The role of the plasticiser is to provide
good homogenisation during the mixing of the composite powder, to improve
the ease with which it can be manufactured and, for the same binder
content, to improve the mechanical properties of the charges produced.
Preferred composite compositions according to the invention are as follows:
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Composition A
Cellulose triacetate
8.5 to 17 parts by weight
Potassium perchlorate
80 to 92 parts
Plasticiser 1 to 3 parts
Acetylene black (combustion
0.15 to 0.5 part
regulator)
Aluminium 0.5 to 2 parts
Composition B
Silicone resin with a carbon
8.5 to 14 parts by weight
content less than 33%
Catalyst for the resin
0.8 to 1.5 parts
Potassium perchlorate
80 to 92 parts
Acetylene black 0.15 to 0.5 part
Aluminium 0.5 to 2.5 parts
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In order that the invention may be more fully understood, the following
Examples are given by way of illustration only.
EXAMPLE 1
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Composition of the composite propellant
Cellulose triacetate (with an
10 parts by weight
acetyl content of 65%)
Potassium perchlorate (particle
size of 16 .mu.) 88 parts
Triacetin (plasticiser)
3 parts
Acetylene black (combustion
regulator) 0.2 part
Aluminium 1 part
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Method of working
The triacetate granules were passed through a mill and introduced into a
malaxator with 3 parts of the plasticiser and 50 parts of cyclohexanone as
solvent.
A mixture of the potassium perchlorate, acetylene black and aluminium had
been homogenised separately in a mixer, and this mixer was then introduced
into the malaxator in three stages. Mixing in the malaxator was carried
out for 21/2 hours, and after the malaxator had been opened, the paste
obtained, which tended to dry quite quickly, was immediately poured into
and packed into moulds having the dimensions of the charges to be
produced.
Satisfactory mechanical properties (impact resistance tests and vibration
resistance tests) were obtained with the following particle sizes: 16.mu.
for KClO.sub.4 (material of one particle size only) or 20.mu. and 8.mu.
(material consisting of mixture of two particle sizes) for KClO.sub.4 ;
3.mu. for acetylene black; the aluminium had a specific surface area of
between 3,400 and 3,800 cm.sup.2 /cm.sup.3.
EXAMPLE 2
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Composition of the composite propellant
Silicone resin RTV 121 (or RTV 502
13 parts by weight
or RTV 141)
Catalyst for the resin
1.3 parts
Potassium perchlorate (mixture of
87 parts (75 parts at 20.mu.
two particle sizes, 20.mu.and 8.mu.)
and 12 parts at 8.mu.)
Acetylene black 0.3 part
Aluminium 2 parts
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The silicone resins RTV 121, RTV 502 and RTV 141 are sold by RHONE POULENC
and are silicone rubbers with a carbon content less than 33% by weight.
For example, the composition in % by weight of RTV 141 resin was as
follows: C : 29.6; H : 8; O : 22.6; Si : 39.8.
Method of working
The oxidising agent and the additives were introduced into a mixer and,
after being homogenised for two hours, were transferred to a malaxator in
which the silicone resin had been dissolved in 50 parts of
trichloroethylene. After malaxating for two hours, the catalyst was
introduced and, since the viscosity increased very rapidly, moulding
preferably by casting, had to be carried out during the following 15
minutes. Evaporation of all of the solvent took place in 24 hours at
20.degree. C.
In order to obtain a carbon monoxide content of not more than about 0.05%
in the combustion gases (under normal conditions of pressure and
temperature), the amount of binder in the compositions was restricted to
17% in the case of cellulose triacetate and 14% in the case of the
silicone resins.
Since the mechanical properties increase with the percentage of binder
used, and as this percentage cannot, in any case, be less than 8%, it is
of value to use a proportion of binder less than the preferred limiting
proportions of 17% and 14% mentioned above and to use additives which
improve the mechanical properties of the composite propellant powder. The
aluminium present in the compositions examplified above improves the
mechanical strength of the composition, particularly the impact resistance
and vibration resistance; its influence on the rate of combustion is also
valuable when the composite propellant is used as a gas generator. The
composition described in Example 1 but omitting the aluminium burns at a
pressure of 70 bars at 26 mm/second, while the same composition containing
the maximum percentage of aluminium, 5%, burns at 44 mm/second. The
maximum aluminium content is determined by the rise in the reaction
temperature due to the exothermic properties of aluminium and which, in
turn, leads to an increase in the carbon monoxide content of the
combustion gases as is shown in the following table:
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Proportion of
CO at the neck of the pipe (ppm)
triacetate
0% Al 2% Al 3% Al 4% Al
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8 9 32.2 80
10 42 103 261
12 17 126 288 514
14 75 317 600 1,020
16 210 690 1,360 3,450
18 700 1,635
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Since gas generators used in inflatable cushion protection devices for high
speed vehicles must liberate all of their combustion products within
periods of time which are generally less than 20 milliseconds, the rates
of combustion needed dictate the use of charges of low thickness. It has
been found that the rate of combustion varies greatly as a function of the
thickness of the wall of the charge. For a composition based on silicone
resin (Example 2), the rate of combustion of 36 mm/second which can be
observed on a block of substantial thickness can be increased to 50
mm/second and even higher with low thicknesses. This increase in the rate
is due, in particular, to heat exchange by radiation between the two
opposite walls, but in the case of a translucent propellant, this
phenomenon is accompanied by local variations in the rate of combustion
which manifest themselves by an unevenness in combustion which disturbs
the properties of the gas generator, and blisters and craters appear on
the combustion surface. Carbon-containing combustion regulators, and
especially acetylene black, make it possible to overcome these
disadvantages. A suitable percentage of one of these carbon-containing
combustion regulators, such as those indicated in the Examples, leads to a
higher rate of combustion being maintained for charges of low thickness,
whilst reducing the unevenness in combustion and hardly increasing the
production of carbon monoxide at all. We have found that combustion
regulators which do not contain carbon, such as talc or chalk, have no
effect on charges of low thickness and that combustion regulators of the
metal salt type, such as copper dichromate, had to be used in large
proportions, which could be as much as 9%, and that such a content was
prejudicial to the mechanical properties of the charge since it was
necessary to use a low content of binder so as not to increase the
production of carbon monoxide. We have found, moreover, that by limiting
the combustion temperature by adding aluminium in an amount of not more
than 5%, it is possible to use carbon-containing combustion regulators
whilst being able to keep the carbon monoxide content below 500 ppm.
It should be noted that the compositions with a low binder content which
can be used as gas generators have the characteristic that the rate of
combustion as a function of pressure is substantially linear and parallel
for temperatures of 60.degree., +20.degree. and -30.degree. C.
When used as a gas generator, the composition of Example 2 has a potential
of 1,468 cal/g and burns at a temperature of 2,184.degree. C under a
pressure of 70 bars. The solid residues correspond to 43% of the original
mass and, under normal conditions of temperature and pressure, 0.307 litre
of gas per gram of propellant is obtained, corresponding to 13.7 mols/kg.
These gases have the following composition under the conditions of use in
inflatable cushions (1 bar, 100.degree. C):
h.sub.2 o = 44%
co.sub.2 = 28%
o.sub.2 = 28%
co .apprxeq. 0.05%
when used as a gas generator, the composition of Example 1 has a potential
of about 1,400 cal/g and burns at a temperature of 1,730.degree. C under a
pressure of 70 bars. The solid residues correspond to 6.75% of the
original mass and, under normal conditions of temperature and pressure,
0.357 liter of gas per gram of powder is obtained, corresponding to 16
mols/kg. These gases have the following composition under the conditions
of use in inflatable cushions.
H.sub.2 O = 20.6%
co.sub.2 = 31.2%
o.sub.2 = 48.2%
co .ltoreq. 0.05%
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