Note: Descriptions are shown in the official language in which they were submitted.
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LOW ENERGY FUSE
This invention relates to non-electric low-energy
fuses, that is to say, transmission devices in the form of
elongate plastics tubing housing reactive or detonable
particulate substances at a core loading sufficiently low
for there to be no cross-initiation of a similar tube placed
alongside (or lateral direct initiation of a surrounding
commercial emulsion blasting explosive) when such a device
is fired. Ordinarily the core material detonates but in
some types rapid deflagration or pyrotechnic reaction
suffices as when the tubing i5 connected to a detonator
within which a deflagration to detonation transaction
occurs. The signal transmission tubing is itself initiated
by an electric cap, a non-electric detonator, an electric
discharge device or indeed by any other means capable of
initiating the required self-sustaining reaction or
detonation of the core material. A favoured type of low
energy fuse is the so-called shock tube or signal tube as
described in, and cross-referenced in, European Patent
Specification No 327219 (ICI). Another distinct class of
low-energy fuse is that described in US Patent Specification
No 4290366 (Atlas Powder Company). The contents of these
prior Specifications and their references are incorporated
by reference herein, in their entirety.
The mining, quarrying and construction industries who
are the principal users of commercial explosives and
accessories and are continually extending the frontiers of
their operations into new situations that challenge the
reliability of current accessories. Of present relevance is
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the trend towards increasing use of emulsion explosives and
ANF0 and heavy ANFO blasting agents, the deployment of non-
electric low-energy fuse initiation down-hole as well as on
the surface as inter-hole link-ups, coupled with long sleep
times (that is the periods of time when the fuse is in
contact with the explosive before firing). Commonly the
hydrocarbon fuel phase of such explosives is an oil or a
petroleum fraction such as diesel, and invariably the
plastics from which transmission tubes have been formed have
been wholly or mainly of polyethylene (e.g. LLDPE) or a
related (co)polymer in which the back-bone chain is a
polyethylene and the chain carries side substituents which
may be hydrocarbyl or functional groups such as carboxyl and
its salt and ester derivatives (e.g, 'Surlyns'). All such
pol~mers are prone to ingress of hydrocarbons of the
explosive's fuel oil phase when in prolonged contact
therewith. This is so to a greater or lesser extent
depending upon the nature of those hydrocarbons, the
_ chemical and physical structure of the polymer of the
transmission tubing, and the temperature of the fuel phase
(as when an emulsion explosive is loaded hot). Even surface
transmission tubing may be in prolonged contact with oil
where there is spillage of emulsion explosive or engine
oils, and this too may become hot in many of the
inhospitable environments in which blasting operations take
place.
The Applicants have contrived mis-fires of non-electric
transmission devices of the types above-described
attributable to penetration of deleterious amounts of
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hydrocarbons into the interior core of the transmission
tubing after prolonged contact.
This invention provides an improved plastics
transmission tubing for use as a low-energy fuse wherein the
starting plastics material is equivalent to any of the
currently used plastics which are susceptible to oil
penetration over an extended period of time of being in
contact therewith e.g. wholly or predominantly made from
addition polymers such as a polyolefin or derivatised
polyolefin of the kinds hereinbefore described or another
oil absorbing plastics material e.g a condensation polymer
such as polyamide or polyester, and which contains in its
central core a detonable signal transmitting particulate
material (such a~ loose, consolidated, bound and/or
thread/filament carried material) wherein the plastics
tubing is an extruded tubing but is modified such that it
has dispersed therein discrete flow orientated units of a
melt incompatible material in sufficient concentration and
aspect ratio to present a barrier to the ingress of hot
fuel, such as diesel, and thereby effectively enhance the
oil resistance of the fuse.
The dispersed units are preferably in the form of
platelets or flakes to act as an effective barrier and
mitigate the effect of penetrating hot fuel to thereby
extend the sleep time of the transmission tubing.
Although flow orientated as the tube is formed, the
barrier units will be randomly dispersed and will be
advantageously up to about 1 mm across. The optimum
dispersed proportion of barrier units is determined by
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experimentation having regard to the sleep time required.
To date, 1 to 5% w/w of PTFE flakes have been found to give
suitable results. These results also suggest that other
melt incompatible polymer or copolymer flakes or platelets
wi]l also be useful. Thus the invention also provides a
method of manufacturing a signal transmission tubing for use
as a low energy fuse, the method comprising extruding a
plastics tubing from a melt wherein discrete units of a melt
incompatible material are dispersed to provide tubing having
a random distribution of flow-oriented units therein to
thereby enhance the oil resistance of the fuse.
According to a further aspect of the invention there is
provided a method of extending the operational life of a
transmission tubing for use as a low energy fuse which will
be in contact with hot fuel oil such as diesel, the method
comprising forming a transmission tubing of which the
plastics material is wholly or predominantly a polyolefin or
a derivatised polyolefin of the kinds above described (but
may also be another oil absorbing plastic such as polyamide
or polyester) and which contains in its central core a
reactive signal transmitting particulate substance (such as
loose, consolidated, bound or thread/filament carried
material), wherein discrete units of a melt incompatible
material are dispersed within the tubing during forming
thereof to present a barrier to the ingress of hot fuel.
This modification has been shown to be capable of
giving a substantially extended operational life to the
transmission tube.
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In an example of the invention, a polyethylene
transmission tube was constructed as follows.
A blend of 85% linear low density polyethylene (LLDPE)
and 15% low functionality (2%) ethylene-vinyl acetate (EVA)
to which about 1% w/w of polytetrafluoroethylene (PTFE)
flakes of about 1 mm across were added was extruded by a
Battenfelder extruder (5.0 cm diameter, 24:1 l/d metering
screw), throuqh a 3.0 cm outer die and a 1.4 cm inner
mandrel to form a transmission tubing having the flakes of
PTFE randomly dispersed throughout. The melt was subjected
to a 15:1 drawdown over 25 cm through a 7.6 mm diameter
sizing die and processed as known per se in the art. The
large tube dimensions were about 7.6 mm outer diameter
(O.D.) extruded at a rate of about 5 m per minute. After
stretching, the tube size was about 3 mm O.D. and produced
at a rate of 45 m per minute. A reactive/detonable core
mixture comprising explosive powder (HMX/A1) was added to
the large tube at a rate sufficient to give a final core
load of about 20 mgLm (4 4 g/m2 of internal area). The
tensile strength of this tube was about 140 N/m2. A break
load of 80 kg was required at an extension of 160%. The
finished tubing was then immersed in hot diesel at 50C and
after more than 100 hours the transmission tube was
successfully detonated.
The mixed particles which are reactive or detonable to
provide for signal transmission may be selected from a
variety of reagents known per se in the field of
pyrotechnics and would include oxidisers such as
perchlorates, permanganates and peroxides; secondary high
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explosives such as PETN, RDX, HMX, TNT, dinitroethylurea;
and tetryl and metal/quasi metal fuels such as aluminium and
silicon.
It will be appreciated that the core loading will be
variable depending on the sleep time field conditions, and
strength required but typically it will be in the range of
15 to 25 mgm~1.
Of course the temperature (and therefore penetration)
of the fuel used in the field will vary considerably (from
say 25C to 70C) and therefore this should be borne in mind
when constructing a low energy fuse of the invention which
must have a specified minimum sleep time.
The invention also extends to low-energy fuse
assemblies comprising delay elements and/or detonators
connected to one or both ends of the transmission tubing as
described hereinbefore.
