Note: Descriptions are shown in the official language in which they were submitted.
N 36312 2 ~ 0
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 i9 fired. Ordinarily the core material
detonates but in some types rapid deflagration or
pyrotechnic reaction suffices as when the tubing is
con~ected 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 di~charge 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, ~uropean 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
are the principal users of commercial explosives and
accessories and are continually extending the frontiers
of their operations into new situations that challenge
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the ~eliability of current accessories. Of present
relevance is the trend towards increasing use of emulsion
explosives and AN~0 and heavy ANF0 blasting agents, the
deployment of non-electric low-energy fuse initiation
do~n-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 i8 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 functlonal groups such as
carboxyl and its salt and ester derivatives (e.g.
'Surlyns'). All such polymers are prone to ingress of
hydrocarbons of the explosive's fuel oil phase when in
prolonged contact therewith. This is 80 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.
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The Applicants have contrived mis-fires of non-
electric transmission devices of the types above-
described attributable to penetration of deleterious
amounts of hydrocarbons into the interior core of the
transmission tubing after prolonged contact.
This invention provides a plastics transmission
tubing for a low-energy fuse of which the plastic is
wholly or predominantly a polyolefine or derivitised
polyolefine 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 or
reactive signal transmitting particulate substance (such
as loose/ consolidated, bound and/or thread/filament
carried material) characterised in that the oil permeable
plaRtic tubing i8 coated with a skin of shellac resin as
a barrier to penetration of hydrocarbon fuels of the
kinds used as the components of emulsion explosives.
Preferably ~uch a low energy fuse would be formed
from an extrudable blend of about 80~ linear low density
polyethylene, about 10~ ethylene-acrylic acid copolymer
and about 10~ ethylene vinyl acetate copolymer into a
tube in which there is provided a core loading of from 15
to 25 mgm~l, more preferably about 20 mgm~l of a
reactive/detonable mixture comprising HMX explosive and
aluminium particles, and having an outer skin of de-waxed
shellac resin to enhance the oil resistance of the fuse.
Shellac i8 the only known commercial resin of animal
origin (in fact from the insect Kerria lacca). The
4 2 ~ 5 ~
forms, components (so far as known) and properties of
shellac are described in Kirk Othmer 3rd Edition, Volume
20 at pages 737 to 747; this disclosure is incorporated
herein by reference.
Shellac resin may be the sole resin component of the
barrier skin (applied, say, as a concentrated solution of
a de-waxed shellac in, for example, alcohol). However
the presence of other resins or polymers, either as
mixtures with shellac resin or coupled to the shellac
resin species by chemical "cross-linking" is allowed
especially when adequate barrier properties are retained
with enhancement in skin adherence, toughness, or
abrasion resistance. The Kirk Othmer article contains
references to chemically modified shellac resins.
Barrier skin~ ba~ed on, containing or derived from de-
waxed shellac re~in are preferred.
Adherence of the shellac resin skin to the
underlying tube surface may be enhanced by chemical,
thermal, flame or plasma treatment~ of the tube surface
or by application of a suitable priming coating or a
combination of pre-treatment and priming coating.
Applicant has found that chromic acid cleaning of shock
tube formed with a polyethylene based tubing of the kind
exemplified in EP-A-327 219 followed by rinsing, drying,
appllcation of a de-waxed shellac resin coating and a
final warm air drying to remove alcohol solvent resulted
in ~he shock tube still reliably firing after immersion
in hot (50C) emulsion fuel phase (Shell RTM Derv fuel
oil) after more than 300 hours, very much longer than was
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the case with unprotected shock tube from the same
stock. Of course, one may optionally overcoat the
she:Llac-protected tube to provide further protection
aga:inst abrasion and de-lamination etc.
In the attached single figure a transverse section
through a non-electric low energy fuse of the invention
shows a plastics tubing 1 carrying a thinly distributed
inner deposit of reactive or energetic material and an
outer coating 3 of shellac resin.
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 aforesaid.
