Note: Descriptions are shown in the official language in which they were submitted.
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PRODUCTION OF A PYROTECHNIC DELAY COMPOSITION
THIS INVENTION relates to the production of a pyrotechnic delay composition of
the
type used, for example, in delay elements employed for the delayed initiation
of
explosives. More particularly, the invention relates to a process for the
production of
such composition, and to a pyrotechnic delay composition.
According to a first aspect of the invention, there is provided a process for
producing
a pyrotechnic delay composition, the process including
admixing an oxidizer, a fuel, a surfactant and a liquid, to form a paste or
slurry;
drying the paste or slurry to remove the liquid and to obtain a solid product;
if necessary, rendering the solid product into particulate form; and
optionally, classifying the solid particulate product, to obtain a pyrotechnic
delay composition in particulate form.
The pyrotechnic delay composition, when used in the manufacture of delay
elements
employed for the delayed initiation of explosives, provides the delay elements
with a
desired burning rate.
The liquid may be an organic liquid such as a solvent. Instead the liquid may
be
water.
Typically, in the manufacture of pyrotechnic delay compositions of the kind in
question by a process of the type to which the present invention relates, the
exact
oxidizer and fuel constituents, their particle sizes and their proportions in
the paste or
slurry mixture, are selected in accordance with practical and economic
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considerations, bearing in mind safety and the intended use of the delay
compositions.
The oxidizer may be in solid particulate form. The oxidizer may comprise red
lead,
barium sulphate and/or potassium perchlorate. The oxidizer may comprise 40-90%
by mass of the composition.
The fuel may also be in solid particulate form. The fuel may comprise silicon,
zinc
and/or magnesium. The fuel may comprise 5-60% by mass of the composition.
The surfactant or surface active agent may be in particulate form, and may be
a
wetting agent and/or a rheology modifier and/or a binder (binding agent). The
surfactant may be selected from the so-called non-ionic surfactants, anionic
surfactants and cationic surfactants. The surfactant may thus comprise an
acrylic
ester, a styrene polymer, and/or an acylic copolymer, which are all wetting
agents;
and/or a polyethelene glycol, a powdered smectite clay, carboxymethyl
cellulose,
polyvinyl alcohol and/or polyvinyl pyrrolidone which are rheology modifiers or
thickeners. The surfactant may be in the form of an aqueous dispersion when
admixed with the oxidizer and the fuel. The surfactant may comprise 0.25% to
4%,
by mass, of the paste or slurry before drying, i.e. on a wet basis. Typically
the
surfactant may comprise 0.1 % to 2%, by mass, of the paste or slurry.
In particular, at least one of the surfactants used may function, in the paste
or slurry,
as a rheology modifier such as a thickening agent. Such rheology modifiers
will
typically be selected for their ability, not only to alter or modify the
burning rate of
delay elements made from the delay compositions in question, but also to
resist so-
called sedimentation or separation of the constituents of the paste or slurry,
after
formulation thereof and before the drying.
The surfactant may comprise 0.1-2% by mass of the composition. Thus, the
composition may comprise
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oxidizer 40-90%
fuel 5-60%
surfactant 0.1-2%
It is expected that the mixing of the constituents to formulate the paste or
slurry will
be carried out in a more or less conventional manner, using conventional plant
or
equipment such as Z blade or high shear mixers, the surfactant or surfactants
being
added at a convenient time and in the required proportions, to become
homogeneously dispersed in the paste or slurry. It is contemplated that two or
more
surfactants may be employed together, suitable surfactant mixtures thus being
used
when desired.
Likewise, drying of the paste or slurry, rendering of the solid product into
particulate
form, and classifying the solid particulate product, may be carried out in
more-or-less
conventional manner. For example, to obtain the particles, drying of the paste
or
slurry in an oven, and hand granulation (eg forcing the material through a
screen)
may be employed. However, in another embodiment of the invention, spray drying
of the slurry, whereby the slurry is pumped through an orifice in a two fluid
nozzle
while simultaneously passing compressed air through the nozzle, to atomize the
slurry into droplets, with the resultant droplets being dried by means of hot
air,
thereby to obtain more-or-less spherical product particles, may be employed.
Traditionally, burning rates of delay elements made from delay compositions
are
controlled or modified by altering the particle size and/or the oxidizer:fuel
mass ratio
in the mixture. However, there is a lower particle size limit, below which
control of
burning rate by changing particle size becomes problematic and unreliable;
and,
similarly, the oxidizer:fuel ratio has upper and lower limits, beyond which
the
compositions can no longer reliably sustain combustion. It is thus a feature
of the
present invention that its technique for controlling burning rates avoids or
reduces
these problems.
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The invention extends also to a pyrotechnic delay composition when produced in
accordance with the first aspect of the invention.
According to a second aspect there is provided a pyrotechnic delay
composition,
which is in solid, particulate form, with the particles comprising an
oxidizer, a fuel and
a surfactant.
The oxidizer, fuel and surfactant may be as hereinbefore described.
As hereinbefore described, the delay composition may comprise, on a mass
basis,
oxidizer 40-90%;
fuel 5-60%; and
surfactant 0.1-2%.
The invention will now be described, by way of non-limiting illustrative
example, with
reference to the following examples.
EXAMPLE 1
Pyrotechnic delay compositions, in accordance with the invention, were
formulated
having the following compositions (proportions expressed as % by mass):
Constituent A B
barium sulphate particles (d50 approx. 3pm) (oxidizer) 38 38
red lead particles (d50 approx. 3pm) (oxidizer) 54 53
silicon particles (d50 approx. 3pm) (fuel) 7 7
Solsperse 20000 (100% (ie undiluted) active polymeric 1 2
dispersant) (surfactant)
100% 100%
The pyrotechnic delay compositions were produced as follows: The oxidizers,
fuel
and surfactant were mixed by hand or by using a high shear mixer, together
with
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sufficient water, to obtain a slurry; the slurry was then oven dried to a
consistency
that allowed hand granulation by pushing the dried product through a 1 mm
screen;
thereafter, the resultant granules were further oven dried to reduce the water
content
to less than 1%, by mass. Solsperse 20000 is manufactured by Avecia, and
5 distributed in South Africa by Lubrisol.
The composition comprising 1% Solsperse 20000 and 54% red lead had a burning
rate of 19 milliseconds/mm in 3.6 mm inner diameter aluminium tubes, whereas,
for
the composition comprising 2% Solsperse 20000 and 53% red lead the burning
rate
decreased to a value of 80 milliseconds/mm.
EXAMPLE 2
Pyrotechnic delay compositions, in accordance with the invention, were
formulated
having the following compositions (proportions expressed as % by mass):
Constituent A B
barium sulphate particles (d50 approx. 3pm) (oxidizer) 38 38
red lead particles (d50 approx. 3pm) (oxidizer) 54 51
silicon particles (d50 approx. 3pm) (fuel) 7 7
Acrinol 296D (Aqueous dispersion of acrylic ester/ styrene 1 4
polymer) (surfactant)
100% 100%
The pyrotechnic delay compositions were produced in the same manner as in
Example 1. The Acrinol 296D was obtained from BASF SA (Pty) Limited of 852
16tn
Road, Midrand, Gauteng, South Africa.
The composition comprising 1% Acrinol 296D and 54% red lead had a burning rate
of
22 milliseconds/mm in 3.6 mm inner diameter aluminium tubes, whereas, for the
composition comprising 4% Acrinol 296D and 51 % red lead the burning rate
decreased to a value of 55 milliseconds/mm.
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EXAMPLE 3
A pyrotechnic time delay composition was prepared (as hereinafter described)
having the following composition in terms of solids on a dry basis
(proportions
expressed as % by mass):
Constituent %
red lead particles (d50 approx. 3pm) (oxidizer) 38.25
barium sulphate particles (d50 approx. 3pm) (oxidizer) 54.25
silicon particles (d50 approx. 3pm) (fuel) 7
smectite clay particles (BENTONEOEW) (rheology modifier/ 0.5%
thickener)
100%
All four the dry particulate constituents were homogeneously mixed with water
to
form a slurry in which the water formed 50% by mass, with the solids thus
forming
50%. The BENTONEOEW was obtained from Carst & Walker (Pty) Limited of Zenith
House, 12 Sherborne Road, Parktown, Johannesburg, South Africa. The slurry was
pumped, at a low pressure of 10-100kPa, along a feed line and through a 1.5mm
or
2mm diameter orifice of a centrally positioned (in a spray-drying chamber)
upwardly
directed two fluid spray nozzle (together with compressed air), thereby being
atomized and thus formed into droplets, while low pressure air at a
temperature of
210 C was fed into the chamber via filters and a heater, by a fan, to dry the
droplets.
Spray-drying thus took place in the chamber to form more or less spherical
dried
particles of more-or-less homogeneous composition. These particles had a
moisture
content of about 0.1% by mass and remained in the chamber for a period of 1-40
seconds. The dried particles were collected through a solids outlet of the
camber.
The drying air, which issued from the chamber at 800C via an air outlet, was
cleaned
by passing it through a cyclone, a primary bag filter, and two secondary
filters. Dried
particles were withdrawn from an outlet of the cyclone. Dried fines were
removed
from the bag filter.
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The dried product was found to comprise acceptably low proportions of both
oversize
and undersize particles which could be used, without additional classifying,
as a
pyrotechnic time delay composition in the manufacture of pyrotechnic time
delay
elements.
EXAMPLE 4
A pyrotechnic time delay composition was prepared (as hereinafter described)
having the following composition in terms of solids on a dry basis
(proportions
expressed as % by mass):
Constituent %
barium sulphate particles (d50 approx. 3pm) (oxidizer) 54.75
silicon particles (d50 approx. 3pm) (fuel) 44.75
smectite clay particles (BENTONE EW) (rheology 0.5
modifier/thickener)
100%
The pyrotechnic time delay composition was produced in the same manner as that
of
Example 3.
As was the case in Example 3, the dried product was found to comprise
acceptably
low proportions of both oversize and undersize particles which could be used,
without additional classifying, as a pyrotechnic delay composition in the
manufacture
of pyrotechnic time delay elements.
Conventionally, in pyrotechnic time delay compositions, an oxidizer such as
red lead
is used to import sensitivity into the composition, particularly for
compositions having
a slow burning rate, e.g. about 210ms/mm. It has thus unexpectedly been found
that,
by employing a surfactant in accordance with the invention in the production
of a
pyrotechnic time delay composition, it is possible to eliminate the use of red
lead,
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which is desirable due to the hazardous nature of red lead, while still
obtaining
acceptable burning rates.
Furthermore, it is important that the surfactant used is such that little or
no gas is
generated by the surfactant when the composition burns. Gas generated by the
burning surfactant could lead to malfunctioning of a delay element
incorporating the
composition.
