Note: Descriptions are shown in the official language in which they were submitted.
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Background of the Invention
Field of the Invention
The present invention relate~ to an 1mproved
explosive connecting cord for use ln transmitting a
detonation wave to an explosive charge, and more parti-
cularl~ to an explos~ve connecting cord of the type known
as "low-energy detonating cord". $he invention relates
also to a method and apparatus for manufacturing detonating
cord.
Description of the Prior Art
The hazards a~sociated with the use o~ electrical
; lnltiation systems for detonating explosive charges in
mining operations, i.e., the hazarde of premature initiation
by stray or extraneous electricity from such 60urces as
llghtning, statlc, galvanic action, stray currents, radio
transmltters, and transmission lines, are well-recognized.
For this reason, non-electric lnitiation through the use
of ~ ~uitable detonating ~use or cord has been looked
upon a8 a widely respected alternative. A typical high-
energy detonating cord has a unlform detonation velocity
of about 6000 meters per second and comprl~es a core of
30-50 grain~ per foot (6 to 10 grams per meter) Or
pentaerythritol tetranitrate (PETN) covered with variou~
combination~ of materials, such as textiles, waterprooflng
materials, pla6tics, etc. However, the ma~nltude of the
noise produced when a cord having such PETN core loadings
is detonated on the 6urface Or the earth~ as ln trunklines,
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1:~14675 .
often is unacceptable in blasting operations in developed
areas. A1BO, the brisance (shattering power) of such a
; cord may be sufficiently high that the detonation impulse
can be transmitted laterally to an adjacent section of the
cord or to a mass of explosive which, for example, the cord
contacts along its length. In the latter situation, the
cord cannot be used to initiate an explosive charge in a
borehole at the bottom (the "bottomhole priming" technique),
as is sometimes desired.
Low-energy detonating cord (LEDC) was developed
to overcome the problems of noise and high brisance
a3sociated with the above-described 30-50 grains per
foot cord. LEDC has an explosive core loading of only
about 0.1 to 10 grains per linear foot (0.02 to 2 grams
pe~ meter) of cord length, and often only about 2 grains
per foot (0.4 gram per meter). This cord is characterized
by low brisance and the production of little noise, and
therefore can be used as a trunkline in cases where noise
has to be kept to a minimum, and as a downline for the
bottomhole priming of an explosive charge.
U.S. Patent 2,982,210 describes a low-energy
detonating cord comprised of a continuous core of a
granulax cap-sensitive high explosive such as PETN of
~uch diameter as to contain from 0.1 to 2 grains per
foot (0.02 to 0.4 g/m) of explosive, encased in a metal
sheath, which may be covered with a fabric countering or
a coating of plastic. The metal sheath is reported to
be essential for the propagation of detonation in explosive
cores of æuch low loadings.
Because LEDC having a metal sheath is not
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amenable to continuous manufacture in unlimited length,
and because this cord is electrically conductive along
its length owing to the conductivity of the metal sheath,
attempts have been made to eliminate the metal sheath by
; resorting to other expedients to overcome the effect of
its absence. Such attempts have not always met with complete
success especially with core loadings of about 2 grains per
foot (0.4 g/m) or less. For example, it is stated in
U.S. Patent 3,125,024 that a uniform detonation velocity -~
can be obtained even without a metal sheath with a
granular PETN core in loadings of 1.5 to 10 grains per
foot (0.32 to 2 grams per meter~ of length provided
that the specific surface of the PETN is from about 900
to 3400 square centimeters per gram and the granular core
is confined within a woven textile sheath surrounded by
a protective and reinforcing covering, i.e., a thermo-
plastic layer or a series of waterproofing and reinforc-
ing materials including a second textile sheath. However,
woven or wound sheathing is relatively expensive to
apply, both in terms of the type of equipment required
and limitations thereby imposed on cord production rates.
Furthermore, even with the high PETN specific surface
and the confinement afforded by the woven textile sheath
and thermoplastic covering, reliable, high-velocity
detonation is not achieved when the PETN core loading
is at the lower end of the LEDC range.
British Patent 815,534 and U.S. Patent 3,311,056
describe low-energy detonating cords having an explosive
core confined within a polymeric sheath. The British
patent describes a cord having a granular core of finely
11~46~S
divided explosive in loadings from 2 to 15 grains per foot
(0.4 to 3 g/m) confined in a flexible sheath of a thermo-
plastic polymer, which may be wrapped in woven fabric and
wire for strength and abrasion resistance. The detonating
cord described in U.S. Patent 3,311,056 is a non-rupturing
type of cord by virtue of a thick expandable sheath of
elastomeric polyurethane which surrounds the explosive core,
the ratio of the amount of explosive in grains per foot to
sheath thickness in inches to prevent rupturing being less
than 130/1, and preferably from about 10/1 to 100/1 (ratio of
the amount of explosive in grams per meter to sheath thickness
in centimeters less than 11/1, and preferably from about
0.8/1 to 8/1)~ Explosive core loadings of 1 to 400, preferably
2 to 100, grains per foot (0.2 to 80, preferably 0.4 to 20,
grams per meter) are described, and thus the cord encompasses
high-energy as well as low-energy detonating cords. The 2-20
grains per foot (0.4-4 grams per meter) cord claimed has a
PETN core confined in a lead sheath. Moreover, although
explosive cores made of self-supporting compositions of the
type used in sheet explosives, e.g., those shown in U.S.
Patents 2,992,087 and 2,999,743, are disclosed, the low-
energy de~onating cords having loadings of 5 and 10 grains
per foot ~1 and 2 grams per meter) have granular explosive
cores, confining lead sheaths, and low ratios of explosive
loading to polyurethane sheath thickness (48 and 20 grains
per foot per inch, 4 and 1.7 grams per meter per centimeter,
of sheath thickness).
U.S. Patent 3,384,688 describes the preparation
of a textile-sheathed cord having enhanced sensitivity
to side initiation and the ability to propagate detonation
~;
at lower loading densities by the use of a special finely
divided granular PETN core in a loading of 10 grains per
, .
linear foot (2 g/m). U.S. Patent 3,382,802 prescribes a
maximum particle size of 100 microns, with at least half the
particles smaller than 50 microns, for a core of granular
primary explosive in low loading, e.g., 5-10 grains per
." . .
-~ foot (1-2 grams per meter), encased in a sheath of spiral-
wound thread-like elements made of metal or thermoplastic,
spiral-wound fibrous sheaths, and a thermoplastic outer shell.
10As can be seen from the above-discussed patents,
heretofore granular explosive cores have been used in
detonating cords having core loadings of 10 grains per foot
~2 grams per meter) or less. Moreover, metal or heavy
textile sheathing generally has been indicated, especially
when the loading drops to below 2 grains per foot (0.4 grams
per meter). Self-supporting explosive compositions in which
a crystalline high explosive compound is admixed with a
binding agent can be extruded rapidly in the form of cords
and would enable higher cord production rates to be attained
as contrasted to production rates attainable with cords
having granular cores. Also, bonded explosive compositions
have high density and can detonate at a higher velocity for
a given diameter when contrasted to lower-density explosives.
However, since the common bonded explosive compositions con-
tain less-sensitive materials, such compositions are less
sensitive to initiation than totally explosive granular
compositions and would not be expected to detonate under all
of the same conditions as such granular compositions. Thus,
while U.S. Patent 3,311,056 describes certain detonating
cords having bonded explosive cores,the low-loading cores
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~,
therein are granular PETN and lead azide/aluminum, and
even these are lead-jacketed. Also, it is known that
the cord diameter and explosive loading have to be sufficiently
large if self-supporting sheet explosive compositions are to
; propagate a detonation at uniformly high velocity. The afore-
mentioned U.S. Patent 2,992,087 discloses that a cord made by
extruding a nitrocellulose-based PETN sheet explosive to a
PETN loading of 20 grains per foot (4 grams per meter)
detonates at a velocity of greater than 6400 meters per
second; and the aforementioned U.S. Patent 3,311,056 discloses
bonded-explosive cores in PETN loadings of 17.5 and 20.0 grains
per foot (3.7 and 4 grams per meter). However, cords having
bonded-explosive cores in explosive loadings of 10 grains
per foot (2 grams per meter) or less have been avoided
despite the fact that such loadings have been found operable
with granular PETN explosives. U.S. Patents 3,338,764,
3,401,215, 3,407,731, and 3,428,502 describe the preparation
of detonating cord having an explosive loading of 50 to 200
grains per foot (10 to 40 grams per meter) by the extrusion
of a flexible elastomer-bonded explosive composition, pre-
ferably around an axially positioned reinforcing yarn or
thread. The wrapping of reinforcing yarns or threads around
the extruded cord, e.g., as in a braided structure, and the
bonding of the yarns to the cord with a latex or liquid
polymer is reported to be less desirable than an internally
placed reinforcing means.
In the art of manufacturing detonating cord,
threads have been used also for the purpose of facilitating
the sheathing of powdered explosive cores. For example,
3~ U.S. Patent 3,683,742 describes circularly guiding one or
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more roughened threads through a funnel which feeds dust-
- like explosive into a sheath continuously manufactured
at the lower end of the funnel, the threadts) being
deflected from the funnel's vertical axis and introduced
into the sheath together with the explosive. The thread(s)
entrain the dust-like explosive and conduct it into the
sheath, whereby a granular explosive core is formed
around internal thread(s).
British Patent 1,416,128 and Belgian Patent
815,257 describe enclosing a column of dry, pulverulent
explosive within a boundary of joined-together axial
threads, and drawing the column/thread assembly through
a compressing die under a tension exerted on the threads
so as to form the core of a detonating fuse. The thus-
formed core, in which the threads enwrap and form a
sleeve around the explosive, is shown enwrapped with a
reinforcing layer of wound textile material, which is
coated with plastic for waterproofing.
U.S. Patent 2,687,553 describes the use of
longitudinal threads in cord manufacture for the purpose
of reinforcing a thermoplastic coating to overcome the
latter's elasticity. The resulting cord has an explosive
core enclosed in a sheath of thermoplastic material in
which strong threads are embedded in a longitudinal
direction. The entire periphery of the explosive core is
in direct contact with the thermoplastic sheath, and the
threads are surrounded by the thermoplastic.
Summary of the Invention
The present invention provides an improved
low-energy detonating cord comprising
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.
(a) a continuous solid core of a deformable
bonded detonating explosive composition comprising at least
about 55 percent by weight of a cap-sensitive crystalline
- high explosive compound selected from the group consisting
of organic polynitrates and polynitramines admixed with
a binding agent, the particles of crystalline high explosive
compound in the composition having their maximum dimension
in the range of about from 0.1 to 50 microns, the average
maximum dimension generally being no greater than about 20 :
microns, and the core containing about from 0.5 to 10 grains
of crystalline high explosive compound per foot (0.1 to 2
grams per meter) of length; and
(b) enclosing the core, protective sheathing
consisting solely of one or more layers of plastic material
preferably having a total thickness of about from 0.005 to
0.075 inch (0.127 to l.90i mm), the plastic material being
one which is capable of flowing at a temperature not exceed-
ing the melting point of the crystalline high explosive
compound by more than about 75~C, e.g., a material which
flows at a temperature not exceeding about 200C when the
high e~plosive compound is PETN.
In a preferred cord of the invention, the bonded
explosive core is reinforced peripherally by at least one
strand of yarn between the core and the plastic sheath, or
in or around the sheath, and most preferably the cord
contains core-reinforcement means consisting essentially of
at least one continuous strand of yarn on the periphery of
the core and running substantially parallel to the core's
longitudinal axis, the strand(s) having sufficient tensile
~0 strength as to prevent the core from necking down to a
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- failure point under forces normally encountered in borehole ;;
loading, e.g., to provide the reinforced core with a tensile
strength of at least about 10 pounds (4.5 kilograms), and -
preferably at least about 20 pounds (9 kilograms), to enable
it to withstand more unusual forces.
A particularly preferred cord of the invention
is one in which the crystalline high explosive compound in
the bonded composition is pentaerythritol tetranitrate (PETN),
the PETN loading in the core is about from 2 to 10 grains
per foot (0.4 to 2 grams per meter) of length, the plastic
material is a polyolefin extrudable at a temperature of
about 175C, and at least about four reinforcing strands
of a polyamide or polyester yarn are substantially
uniformly distributed on the periphery of the core.
The present invention also provides a method
of producing a detonating cord comprising
(a) forming a mixture of a cap-sensitive
crystalline high explosive compound and a binding agent
therefor into a continuous solid core, e.g., by extrusion;
(b) drawing strands of yarn under tension
sufficient to form a moving cage of substantially
parallel longitudinal strands:
(c) allowing the moving cage to entrain the
core within it, whereby the cage becomes a conveyor for
the core;
(d) applying a layer of soft plastic material
around the moving cage, usually after the entrainment of
the core therein, while effecting substantially no change
in the diameter of the core after its entrainment within
the cage; and
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(e) hardening the plastic material.
In one preferred embodiment of the method, the
core is extruded into the continuously moving yarn cage,
and the caged core unit, with or without peripheral
support, subsequently moves into and through a plastic
coating extrusion die wherein the plastic material is
formed into a sheath over the caged core unit. In
another, the yarn strands and the core are moved separately
into a plastic coating extrusion die, and the formation
of the cage, entrainment of the core, and formation of
the sheath all occur within the confines of the die, either
simultaneously or with sheathing following entrainment.
In each case, substantially no reduction in the diameter
of the core as a result of compression occurs.
Also provided by this invention is an apparatus
for producing a detonating cord comprising
(a) a first extrusion means, for forming a mass
of a deformable bonded detonating explosive composition
into a continuous solid core;
(b) means for orienting strands of yarn sub-
stantially parallel to one another in annular array;
(c~ means for drawing the substantially parallel
strands under tension sufficient to form them into a
moving cage, the strand-orienting means being 50 positioned
with respect to the first extrusion means that the cage
internally entrains and conveys the core emanating from
the first extrusion means;
(d) a second extrusion means, for applying a soft
plastic material in the form of a sheath to a substrate
moving therethrough, the second extrusion means being so
~
positioned with respect to the first extrusion means and
- the strand-orienting means that the caged core moves through
the second extrusion means as the substrate for the sheath
application with substantially no previous, coincident,
or subsequent reduction in core diameter; and
(e) means adapted to receive the passage of the
sheathed, caged core for hardening the plastic material.
Preferably, the first extrusion means is associated
with an extruder chamber which contains an opening for
drawing a vacuum, and particle-screening means for excluding
oversize foreign particles from the core. The strand-
orienting means can be a separate guide plate or a component
of the second extrusion means.
Brief Description of th:e Drawing
-
In the accompanying drawing, which illustrates
specific embodiments of the explosive connecting cord,
method, and apparatus of the invention,
FIGS. 1 and 5 are perspective views in partial
longitudinal cross-section of sections of different embodi-
mentg of the connecting cord of the invention;
FIG. 2 is a schematic representation of the
apparatus of the invention: and
FIGS. 3 and 4 are cross-sectional views of
different embodiments of portions of the apparatus shown
in FIG. 2.
_tailed Description
Referring to the section of low-energy detonating
cord 1 shown in FIG. 5, the cross-sectioned portion shows
a continuous solid core 2 of a deformable bonded detonating
explosive composition, e.g., superfine PETN admixed with
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a binding agent such as plasticized nitrocellulose, the
diameter and explosive content o~ the core being such that
about from 0.5 to 10 grains of explosive are present therein
per foot (0.1 to 2 grams per meter) of length; and a
protective plastic sheath 4, e.g., about from 0.005 to
0.075 inch (0.127 to 1.905 mm) thick, which encloses core 2.
In the section of cord shown in FIG. 1, core-reinforcement
means 3 consisting of a mass of filaments derived from
multi-filament yarns around and in contact with the periphery
of core 2 runs parallel to the longitudinal axis of core 2,
and sheath 4 encloses core 2 and core-reinforcing filaments
3. In another portion of FIG. 5 sheath 4 has been removed
to reveal the peripheral appearance of core 2, and in other
portions of FIG. 1, sheath 4 has been removed to reveal the
peripheral appearance of filaments 3 on core 2, and filaments
3 removed to reveal the peripheral appearance of core 2.
- The low-energy detonating cord of this invention
combines the features of a continuous solid (i.e., non-
hollow) core of a deformable bonded detonating explosive
composition having a low-loading, i.e., 0.5 to 10 grains
per foot (0.1 to 2 grams per meter) of length, of crystalline
high explosive in a binder therefor, and only a light
protective plastic sheath enclosing the core. An additional
feature, which is preferred, is longitudinal fiber reinforce-
ment of the core external thereto. It has been found that,
contrary to the teachings of the prior art on low-energy
detcnating cords, a deformable bonded detonating explosive
in the form of a cord can be made to propagate a detonation
reliably ev~n in loadings below 5-10 grains per foot, at a
rate that is useful in blasting operations, e.g., above
:~
about 4000meters per second, without confinement in a metal or
woven textile sheath, spirally wound textile, plastic, or ~-
metal strands or filaments, or a thick plastic sleeve. It has
been found that the just-mentioned confinement is unnecessary
if the core is a continuous solid rod of bonded explosive,
e.g., a plastic-bonded explosive, containing at least about 55
percent of explosive by weight, and a "superfine" crystalline
high explosive component (as will be described later), and re-
inforcement means for the core is external thereto. In the
cord of this invention the explosive particles in the core are ;~
held together with a binding agent, 8. g., an organic polymeric
composition, and this has been found to have a beneficial
effect in assuring a uniform, high core density and consequent-
ly reliability of detonation, high density being an important
consideration particularly in small-diameter, low-loading cords
of low brisance. Relative to the bonded core it has been found
that, despite the fact that central, internal reinforcement has
been reported to be preferred in high-loading cords made of
self-supporting explosives (U.S. Patent 3,338,764), external
reinforcement filaments are important for the proper function-
ing of low-loading cords made with this type of core. Further-
more, external reinforcing textile yarns are longitudinal,
running substantially parallel to the cord axis. Such a cord
is readily adapted to be made by high-speed continuous manu-
facturing techniques in contrast to those low-energy detonating
cords of the prior art which employ woven or wound textile re-
inforcement.
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The bonded explosive composition which constitutes
the explosive-core in the cord contains at least one finely
divided cap-sensitive crystalline high explosive compound,
which can be an organic polynitrate such as PETN or mannitol
hexanitrate, or polynitramine such as cyclotrimethylenetri-
nitramine (RDX) or cyclotetramethylenetetranitramine (HMX).
PETN is the most readily available of these compounds and is
satisfactory for use under conditions most commonly encountered
in blasting, and for these reasons is the preferred crystalline
explosive in the bonded explosive core. The crystalline high
explosive compound is admixed with a binding agent, which can
be a natural or synthetic organic polymer, e.g., the soluble
nitrocellulose described in U.S. Patent 2,992,087, or the mix- ,
ture of an organic rubber and a thermoplastic terpene hydro-
carborl resin described in U.S. Patent 2,999,743. The composi-
tions described in these patents can be used for the core of
the present cord. Other ingredients may be present in the
composition, such as additives used for plasticizing the
binder or densifying the composition. Other compositions
which can be used are those described in U.S. Patents
3,338,764 and 3,428,502.
The detonating cord of this invention is a "low-
energy" cord, i.e., one which, when detonated, produces rela-
tively little noise and exhibits relatively low brisance.
Therefore, for a given core composition, the core diameter is
such that a~out from 0.5 to 10, preferably at least about 2,
grains of t~e crystalline high explosive compound are present
per foot of core length (0.1 to 2, preferably at least about
0.4, grams per meter~. With trunklines containing cores of
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higher loadings, the noise level is likely to be a problem in
certain areas. Below about 0.5 grain per foot (0.1 gram per
meter), the reliability of complete propagation of detonation
is low unless a high-energy binding agent and/or plasticizer
is included in the ccre composition. With such a composition,
e.g., with a composition having a high-viscosity nitrocellulose
binder plasticized with trimethylolethane trinitrate, as is
described in U.S. Patent 3,943,017, loadings of particulate ;
high explosive in the core as low as about 0.1 grain per foot
(0.02 gram per meter) may be feasible. Loadings of about from
2 to 10 grains per foot (0.4 to 2 grams per meter) have been
found to be particularly advantageous for downline and trunk-
line cords. With explosive cores of low loading, as in the
present cord, it is important that the crystalline high explo-
sive component be in the "superfine" particle size range, i.e.,
the maximum dimension of the particles should be in the range
of about from 0.1 to 50 microns, and generally the average
maximum dimension should be no greater than about 20 microns.
Larger explosive particles, extreme variations in particle
size, and particulate foreign matter are undesirable inasmuch
as they interfere with the uniform propagation of detonation
`~ in the core. A preferred explosive for use in the core is one
having micro-holes, as made by the process described in U.S.
Patent 3,754,061.
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The explosive loading of the core is a function
of the crystalline high explosive content of the
bonded composition and the core diameter. The
crystalline high explosive content can vary, e.g., from
about 55 percent up to about 90 percent by weight of
the core composition. Although a low explosive content
can to some extent be compensated for by a large core
diameter, it is more efficient and the propagation of
detonation more reliable if, for any given loading,
the explosive content is as high as possible, preferably
at least about 70 percent by weight of the core composi-
tion. For explosive contents in the range of about from
55 to 90 percent, core diameters of about from 0.010 inch
~0.025 cm) to 0.060 inch (0.152 cm) will be used to achieve
core loadings of 0.5 to 10 grains per foot (0.1 to 2 grams
per meter) of core length. A diameter of about 0.027 inch
(0.069 cm) is used to achieve the preferred core loading of
2 grains per foot (0.4 gram per meter). The explosive
composition also contains about 1 to 10 percent, preferably
2 to 5 percent, by weight of a binding agent, and in addition
a plasticizer if needed, to make the composition extrudable,
and to provide cohesiveness in the core.
The density of the core varies with the
specific particulate explosive and binding agent used
and their content, and the nature and amount of other
additives, if present. Generally, cores based on the
compositions described in the aforementioned U.S. Patents
2,992,087 and 2,999,743 will have a density of about 1.5
grams per cubic centimeter. A core density of this
magnitude, in contrast to densities of only about 1.2
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grams per cubic centimeter attained with particulate
cores, has the advantage of affording a better transmission
of the detonation wave and hence a higher detonation
velocity for a given diameter. Although the shape of
the core's cross-section is not critical to the proper
functioning of the cord, it is usually preferred to use
a core of substantially circular cross-section to
facilitate the production of cords having the circular
configuration in common use.
The bonded explosive core is enclosed in sheathing
as a means of protecting it against abrasion or othPr damage
that can occur during handling and preparations for blasting.
Inasmuch as the sheath is primarily protective, it is
relatively thin, i.e., in the range of about from 0.005
to 0.075 inch (0.013 to 0.191 cm), except that a sheath
up to about 0.125 inch (0.318 cm) thick may be used if the
cord i3 to be subjected to extremely stressful conditions,
as are encountered in surface quarry operations. Uniform
protection is difficult to provide with sheaths which are
thinner than about 0.005 inch (0.013 cm). A sheath which
is thicker than about 0.125 inch (0.318 cm) is not reguired
in the present cord, and, in any event, adds unnecessarily
to the thickness and cost of the cord, limits its flexibility,
and may be difficult to load into small-diameter boreholes.
A sheath thickness of about from 0.020 to 0.050 inch
(0.051 to 0.127 cm) is preferred from the point of view
of ease of applicability to the core and degree of protection
afforded. Thu~, with preferred core loadings of 2 to 10
grains per foot and preferred sheath thicknesses of 0.020
to 0.050 inch, the ratio of the core loading (gr/ft) to
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sheath thickness (in) is 40/1 to 500/1 (3/1 to 39/1 for core
loadings of 0.4 to 2 grams per meter and sheath thicknesses
of 0.051 to 0.127 cm).
Within the useful sheath thickness range, it
often is advisable to use a thicker sheath when the
explosive loading in the core is near the low end of the- ~ 3
core loading range, inasmuch as in such cases this may
assure reliable initiation and propagation of the
detonation. Also, as the core explosive loading ~-
10 increases, increasing the sheath thickness may assure ~-
a continuity of detonation through knots and half-hitches.
The sheath consists solely of one or more plastic
layers. This means that any layer of which the sheath is
constructed consists essentially of plastic and that no
confining metal or woven textile layer is present in the
sheath, either adjacent to or separated from the core.
The sheath is made of a plastic, i.e., deformable,
substance that is capable of flowing, e.g., is extrudable,
at a temperature not greatly in excess of the melting point
of the explosive in the core, i.e., not more than about 75-C
above the explosive's melting point. This allows the
plastic sheath to be applied to the core, e.g., by
extrusion or other conventional coating procedure,
without causing a deleterious transformation of~the
explosive. The plastic should be flexible and tough
when hardened. Although the temperature of the plastic
which can ~e used during the application of the sheath
to the coxe will vary depending on the time of contact
between the core and the overlying soft plastic, on
the rate of heat exchange between the core and plastic,
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and on the stability of the binding agent in the core,
with a PETN-containing core the plastic should be fluent
at a temperature not exceeding about 200C. The plastic
substance can be a thermosetting material such as a rubber
or other elastomer, or a thermoplastic material such as
wax, asphalt, or one or more polyolefins, e.g., polyethylene
or polypropylene; polyesters, e.g., polyethylene terephthalate;
polyamides, e.g., nylon; polyvinyl chloride; ionomeric resins,
e.g., ethylene/methacrylic acid copolymer metallic salts; ~`
etc. Thermoplastic sheaths are preferred, and most preferably
polyethylene, on the basis of availability, ease of applica-
tion, etc.
To enable the cord to retain its structure and
dimensions under field use, reinforcement means is used to
enhance the tensile strength of the cord and prevent the core
from necking down to a failure point under forces normally
encountered in borehole loading. While such reinforcement
can be provided by a material suspended in the plastic layer(s)
of the protective sheath, e.g., by fragments or strands of
yarn held therein, for example, in the manner shown in U.S.
Patent 2,687,553, or on the outer periphery of the sheath, it
is preferred that the core be reinforced by at least one, and
usually pre~erably four or more, continuous strands of yarn
which are substantially in contact with the periphery of the
core and run substantially parallel to the core's longitudinal
axis.
The presence of the yarn strands between the core
and the sheath is preferred to yarn strands within the plastic
layer of the sheath because heat is less readily
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1~467~i
transferred from the plastic to the core when hot plastic is
extruded onto the core. The term "yarn" is used herein in
the sense given in Standard Defi;ni*ions of Terms Relating to
Textile Materials, ASTM Designation D 123-74a, where "yarn"
is defined as a generic term for a continuous strand of
textile fibers, filaments, or material occurring as a
number of fibers twisted together, a number of filaments
laid together without twist, a number of filaments laid
together with more or less twist, a single filament (mono-
filament) with or without twist, or one or more strips madeby the lengthwise division of a sheet of material such as a
natural or synthetic polymer with or without twist.
Varieties of yarn included in this definition are single
yarn, plied yarn, cabled yarn, cord, thread, fancy yarn,
etc. The strandts) of yarn are held in place around the
core by the plastic sheath, which encloses the core and
peripheral strand(s). Any yarn can be used which has a high
enough tensile strength as to prevent the core from necking
down under forces normally encountered in borehole loading
to such a degree that it fails to propagate a detonation.
This usually requires that the core be provided with a ten-
sile strength of at least about 10 pounds. For added
assurance that the cord will withstand more extreme forces,
a tensile strength of at least about 20 pounds in the
reinforced core is preferred. The yarn material, filament
count, and denier, and the number of yarns, will be selected
QO a~ to pxovide the required tensile strength.
Multifilament yarns may be preferred inasmuch as these, in
contrast to monofilaments, tend to spread out around the
core, providing an insulating effect in the coating
~ ~ ~.46'7;~ ~ :
operation and a more widespread caging effect. Also,
fewer strands and lower deniers can be employed with
stronger fibers. Yarns larger than 2000 denier are ;
not preferred, as these add unduly to the thickness
of the cord. While any natural fiber can be used in
the yarn, synthetic fibers of the polyester, polyamide,
and polyacrylic types are preferred on the basis of
their superior strength. Especially preferred are nylon,
polyethylene terephthalate, and the all-aromatic
polyamide made by the condensation of terephthalic acid
and para-phenylenediamine. These fibers in deniers of
800 or higher have tensile strengths of at least about 10
pounds and thus a single strand or yarn thereof in the present
cord i8 adequate. Multiple yarns give added strength,
however, and therefore are preferred. Also, they can
be used in lower deniers, e.g., down to about 400 denier.
In the preferred cord, at least four multifilament
yarns are spaced substantially uniformly around the
core's periphery, resulting in a uniform distribution
of reinforcement about the core. There is no significant
advantage to be gained by placing multifilament yarns adjacent
to one another in the cage prior to the application thereto
of the plastic sheath, the cage-drawing and plastic
coating operations in any case causing a spreading-out
or diffusion of the filaments in multifilament yarns
which may cause the yarns to blend around the core. For
this reason, and in consideration of the circumference of
the core and the denier of the yarns, the use of more than
about twelve yarns is superfluous. Usually the layer of
filaments will be no thicker than about 0.010 inch (0.025 cm).
-22-
1~146~
Tex~ured and multiplex yarns (as described
in U.S. Patent 3,338,764~ are especially effective core
reinforcing means inasmuch as they can become firmly
bonded to the plastic sheath which surrounds them.
Application of an adhesive coating, e.g., a soft wax, -
to the strands also improves the bonding between the
~trands and plastic sheath, decreasing yarn mobility and
possible resulting interference with the core, as well as
increasing the peel strength of the sheath.
The process and apparatus of the invention will
now be explained with reference to FIGS. 2-4. In FIGS. 2
and 3, 5 is a ram or piston-type extruder having a ram or
piston 6, and a cylindrical chamber or barrel 29, which is
surroundéd by heating coils 7. Extruder chamber 29 is
provided with vacuum port 25, and screen 26, which is
mounted on one side of a multi-apertured support plate 27.
A mass 28 of a deformable bonded detonating explosive
composition is shown in extruder chamber ~9 and in
the apertures of plate 27. Plate 27 has its other side
adjacen~ to the reduced-diameter die portion of chamber 29
into which explosive mass 28 is forced by the action of
ram 6 and i9 shaped into a solid rod or core 2.
Adjacent to the die portion of extruder 5 is strand-
orienting plate 8, which i8 a means for orienting strands of
yarn including 9 and lO into a substantially parallel annular
array. Plate 8 has an axial channel, and strand-receiving
radial grooves in one surface communicating with the axial
channel, the grooved surface of the plate curving as it meets
the axial channel. Plate 8 is supported in such a position
~0 that its grooved surface meets the surface of extruder 5
~$~46'7~
so that the plate's axial channel is coaxial with the
core 2 emanating from the die portion of extruder 5 by
action of ram 6. Strands 9 and 10 are drawn off respective
spools 11 and 12 by capstan 13, which constitutes a méans
for drawing or pulling strands under tension sufficient
to form them into a moving cage 14. Core 2 emanating from
extruder 5 is entrained within cage 14 and becomes conveyed
thereby. Capstan 13 draws-cage 14 (containing core 2) ~
through extrusion die 15 of a second extruder, whereby a ;;
plastic material is applied around the cage in the form of
a sheath 4. Extrusion die 15 has an annular outer portion
17 and an inner tubular member 16, so positioned that a
soft plastic material 30 delivered to die 15 through the
wall of 17 by known means (not shown) is formed into a tube
between facing surfaces of outer portion 17 and inner
tubular member 16, and cage 14 moves through the axial
channel in tubular member 16. Vacuum port 18 passes through
the wall of tubular member 16 and opens into the latter's
axial channel. Tubular member 16 and strand-orienting
plate 8 are maintained in spaced-apart, coaxial relation-
ship, and are joined together by connecting tube 19, which
surrounds cage 14 in the space between plate 8 and tubular
member 16.
The sheathed core-containing cage (cord 1)
formed at the exit of die 15 moves through vessel 20,
e.g., a water tank, which is a means for hardening the
plastic sheath material. The cord, after passing over
capstan 13, subsequently is collected on windup 22,
the winding of the cord being facilitated by its passage
over tension-control means 21, e.g., an air dancer.
-24-
6'7~
Extruder piston 6 is connected to sensing means 23, which
sense~ the speed of the piston and emits a signal in
accordance thexewith to signal processor 24, which is
connected to the drive means for capstan 13 and to the
drive means for windup 22 and adjusts their speeds in
accordance with the signal received from sensing means 23. ;
FIG. 4 shows an alternative extrusion die 15 which
can be used in the present apparatus in conjunction with
an extruder for forming the explosive core. This parti-
10 cular die includes a means for orienting yarn strands -
into a substantially parallel annular array and thus can
be used in the apparatus shown in FIG. 2 without strand-
orienting plate 8. In this embodiment an axial channel in
extrusion die 15 has a cylindrical portion 31 and a conical
portion 32. A hollow conical insert 33 is positioned so
that its apex portion is nested within conical die portion
32 with a small spacing between facing surfaces. Capstan
13 draws yarn strands 9 and 10 through apertures in the
yarn-~uide ring 34, and thence along the inner surface of
adjacent conical insert 33. The strands converge in the
conical portion of insert 33 and thereafter become oriented
sub~tantially parallel to one another and formed into a
cage by passage through a cylindrical portion of insert 33.
Explosive core 2 moves into the cylindrical
portion of insert 33, where it is entrained by the yarn
cage formed therein. Plastic material 30 is introduced
~, _
into an annulus formed between the walls of conical insert
33 and extrusion die 15. This annulus communicates
with cylindrical die portion 31 via the space between
conical die portion 32 and the apex portion of
-25-
1~1467~
- insert 33. The cylindrical portion of insert 33 communi-
cates coaxially with cylindrical die portion 31. The
core-containing cage 14 formed in the cylindrical portion
of insert 33 is drawn through a stream of plastic material
30 which flows through cylindrical die portion 31 having
entered there from the space between conical die portion 32
and the apex portion of insert 33. Plastic material 30
is formed into a sheath around caged core 14 to form cord 1.
The preparation of a preferred cord of the
invention is illustrated by the following example.
Example 1
A. Referring to FIG. 3, mass 28 in extruder
chamber 29 is a l-lb (455-g) slug of a deformable
bonded explosive composition consisting of a mixture
of 76.5% superfine PETN, 20.2% acetyl tributyl citrate,
and 3.3% nitrocellulose prepared by the procedure described
in U.S. Patent 2,992,087. The supe,rfine PETN is of the
type which contains dispersed microholes prepared by the
method de~cribed in U.S. Patent 3,754,061, and has an
average particle size of less than 15 microns, with all
particle~ smaller than 44 microns. The temperature of
chamber 2g is maintained at 63C by heating coils 7 to
a~sist in maintaining the extrudability of the explosive com-
position therein. After the slug of explosive has been
placed in chamber 29, ram 6 i~ advanced to seal off
chamber 29, and a vacuum is drawn through port 25. A
vacuum level of -29.2 inches of mercury is maintained for
1 minute. This is done to prevent the entrapment of
air in the explosive composition, a condition which can
cause discontinuities in the extruded core, deleteriously
-26-
~L1467~ ~ ~
affecting its ability to propagate a detonation. Ram 6 ~-
is then advanced fur~her un~il explosive mass 28 is -~
compressed but not yet to such a degree as to cause
extrusion to occur.
Strands 9 and 10, and ~our additional strands
(not shown), are threaded into the radial grooves of
plate 8, and are drawn through the axial channels of
plate 8 and tubular member 16 by actuating the drive on
capstan 13. Each of the six strands is a 1000-denier
strand of polyethylene terephthalate yarn, and their
tension is controlled at four ounces (each) by tension-
control means 21. At the same time the drive on windup
, ,
22 and the means for moving plastic material 30 are
actuated. Plastic material 30 is low-density polyethylene
at a temperature of 150C. Vessel 20 is a two-compartment
trough containing water at 81C in the first compartment
through which the cord passes, and water at 21C in the
second compartment. This two-zone cooling assists in
providing a more uniform cooling of the plastic sheath and
promoting a tighter fit of the sheath on the cage. The
diameter of the portion of extruder 5 wherein core 2 is
formed is 0.030 in (0.076 cm). The spacing between the
facing surfaces of outer portion 17 and inner tubular
member 16 of die 15 is such as to produce a polyethylene
sheath 4 having a thickness of 0.035 in (0.089 cm).
After capstan 13, tension-control means 21,
windup 22, and vessel 20 are operating, ram 6 is advanced
at a rate of 0.500 in (1.270 cm) per minute. Explosive
mass 28 is forced through screen 26, which screens out
particles larger than 0.010 inch, and through the apertures
-27-
:
1~467S
in plate 27, and is formed into solid core 2 having a
0.030 in (0.076 cm) diameter. The core moves out of
extruder S at a rate of 248 feet per minute, and the speed
of the cage being advanced by capstan 13 and wound on
windup 22 is matched to the core extrusion rate by signals
received from signal processor 24. A vacuum is drawn
through port 18 to assist in collapsing the plastic
sheath onto the core-containing cage 14 passing through
tubular member 16. A vacuum level of -5.9 inches of
mercury is maintained in tubular member 16.
Cord 1 accumulated on windup 22 has an outer
diameter of 0.100-in (0.254 cm), a 0.930-in (0.076-cm)
diameter core, and a 0.035-in (0.089-cm)-thick polyethylene
coating. The PETN loading in the core is 2.5 grains per
foot (0.533 g/m) (gr/ft PETN per in. coating=71/1; g/m PETN
per cm coating=6/1), and the core density is l.S grams per
cubic centimeter. The filaments of the yarns surround the
core substantially completely as shown in FIG. 1. The cord is
flexible and light, and has a tensile strength of 100 pounds
(45 kilograms).
The cord, initiated by a No. 6 blasting cap,
the end of which is in coaxial abutment with the exposed
end of the cord, detonates at a velocity of 6900 meters
per second. The cord does not initiate itself from one
section spliced together side-by side with another. Detona-
tion of a continuous length of the cord is propagated through
knots of various types. Also, the cord is difficult to
initiate if the cap-to-cord abutment is not coaxial.
B. The same cord is made by the procedure
described in Section A above, except that extrusion die 15
-28-
~ 467~
shown in FIG. 4 replaces die 15 and strand-orienting
plate 8 shown in FIG. 3. In this procedure, capstan 13
draws four strands of yarn through the cylindrical
portion of insert 33 under tension sufficient to form
them into a moving cage of longitudinal substantially
parallel strands; the cage entrains the core; and the
core-containing cage is drawn through the stream of poly
ethylene flowing through the cylindrical portion of the
die's axial channel whereby a sheath of soft polyethylene
is applied around the cage. As in the procedure described
in Section A, substantially no reduction in the diameter
of the core occurs during the operation.
Cords having different core diameters, sheath
thicknesses, and numbers of reinforcing yarn strands can be
produced by the procedures described above by suitable
modification of die sizes and extrusion rates.
The use of the low-energy detonating cord of the
invention, and the effects of various parameters such as
core loading and diameter, sheath thickness and composition,
and number and type of reinforcing yarns, are shown by the
following examples.
Example 2
Four grains (0.26 g) of the superfine PETN described
in Example 1 is placed in a 0.003-in (0.08-mm)-thick coined
bottom aluminum shell, the end of which is butted against
the side of a 10-foot (3-meter~ length of the cord described
in Example lA, with the exception that the cord in this case
has a 0.050-in-(0.127-cm) diameter core having a PETN load-
ing of 7 gr/ft (1.49 g/m). This cord serves as a trunkline.
One end of a 5-foot (1.5-meter~ length of the cord described
-29-
g6'7~ ' -
in Example lA (downline) is inserted into the aluminum
3hell ~the booster) 80 ac to contact the ~ETN. The other
end of the downline is butted with its side against the
percussion-sensitive element of a percussion-type delay
cap. The trunkline is detonated by means of a No. 6
blasting cap having its end in coaxial abutment with the
exposed end of the cord. The detonation is transmitted
from the trunkline to the booster, from the booster to ~-
the downline, and from the downline to the percussion-
10 type delay cap.
The same results are obtained with trunkline
cords having 10 gr/ft (2.13 g/m) and 4.4 gr/ft (0.938 g/m), .
i.e., 0.060-in- (0.152-cm) and 0.040-in- (0.102-cm)
diameter, cores; and with downline cords having 3.0 gr/ft r
(0.638 g/m) and 2.2 gr/ft (0.469 g/m), i.e., 0.033-in-
(0.084-cm) and 0.028-in- (0.07-cm) diameter, cores.
- Example 3
The following tests show the kinds of abusive
treatment with respect to knotting, tension, and abrasion
20 the cord of this invention is capable of withstanding.
A. One end of a 60-foot (18-meters)-long down-
line o~ the cord described in Example lA is butted with
its fii~e against the percussion-sensitive element of a
percussion-type delay cap. The cap is embedded in a
2-pound (0.9 kg) chub cartridge ~flexible film tube having
constricted sealed ends), 2 inches ~5 cm) in diameter
and l& inches ~41 cm) in length, containing a nonexplosive
composition simulating a water gel explosive. The cap and
cord are secured in position in the film cartridge by two
30 half hitches. The cartridge is lowered into a simulated
-30-
~ .4~Y~
50-foot (15-meter)-deep borehole under various loading
conditions which could be encountered in field use, the
simulated hole being the inside of a vertical 5-inch
(13-cm)-diameter steel pipe. The other end of the 2.5 gr/ft
downline is connected to the booster and 7 gr/ft trunkline
as described in Example 2. After the pipe has been loaded
under the described conditions, the trunkline is detonated
as described in Example 2. The downline detonates
completely, and the percussion-sensitive delay cap
detonates within its designed timing, after
the downline-cap-cartridge assembly has been subjected
to the following loading conditions:
I. The cartridge is allowed to fall freely
for the entire length of the downline.
II. The free fall of the cartridge is stopped
1 suddenly every 15 feet (4.6 m).
III. ~he cord moves against the rough edge of
the steel pipe as the assembly is lowered into the pipe.
; IV. Conditions II and III are combined.
V. A 7-pound (3.2 kg) sand bag is dropped into
the pipe, removed and dropped again for a total of five
times, onto the assembly positioned in the pipe in each
of case~ I, II, III, and IV, the sand bag scraping
against the cord in its fall.
B. A knot is tied in the cord described in
Example lA, and a 7-pound (3.2 kg) weight is suspended
from the end of the cord. The weight is dropped into
the 50-foot pipe described in Part A above, while the
free fall of the weight is stopped 5 times, thereby
e~erting increased tension on the knot. Five cords
4~i7S : ~:
'-'~
handled in this manner subsequently detonate completely, ;~
with no cut-offs at the knots.
Example 4 -
-
The use of the cords described in Examples 1
and 2 to transmit detonation waves to the bottom charge
of a column of blasting explosive charges in boreholes
is as follows:
Six 25-foot (7.6-meter)-deep, 3-inch (7.6-cm)-
diameter boreholes spaced 8 feet (2.4 meters) apart are ` ~
each loaded with three aligned 2 x 16 inch (5 x 41 cm) ~-
chub cartridges of a water gel explosive described in U.S.
Patent 3,431,155, wrapped in polyethylene terephthalate
film. Embedded in the bottom cartridge in each hole is a
percussion-type delay cap, connected to the cord (downline)
described in Example lB in the manner described in Example
2. The other end of each downline is connected to the
trunkline cord described in Example 2 (except having four
yarn strands) in the manner described in Example 2. No
stemming is used. Detonation of the trunkline results in
sequential detonation of the charges in the holes starting
with the bottom charge, as concluded from the delay timing
of the caps used. There is no evidence of column
disruption.
Examples 5-10
Cords are made as described in Example 1. The
core explosive composition is 76.1% superfine PETN, 20.3%
acetyl tributyl citrate, and 3.6% nitrocellulose (by weight).
Four strands of the same yarn as that described in
Example 1 are used. The same plastic coating composition
as that described in Example 1 is used. The core is
1~14~
extruded to different diameters, and coatings of different
thicknesses are applied. The detonation properties of
the cords (initiated as described in Example 1) are
summarized in the following table:
Cor ~TN D ton-tlon V loclty ~ c.) t Cord
Dl~. gr./~t. ~ r ~ -lJ~5Y~- ~lr-rJ~r 0.1~5
~ ln. ~c~.) ~g-/ .) ~0.170) (0.203~ ~0.229) ~O.~S~) ~0.315)
S 0.013~0.033~-) 0.5~0.107) 6600
6 0.020~0.051) 1.0~0.213) 6700 6600 6600
0 7 0,030~0.076) 2.~0.533) 6800 6800 6600 6700 6700 ~`
8 o. o~0 ~0.102) ~. ~ 10.938) 6B00 6000 6000 7200
~ 0.050~0.127) 7.0~ 9) 7000
10 0.0~0~ 2) ~0.~2.13J 7000
..
`~ (a) This cord is initiated and propagates a detonation
,'~ in 50~ of the trials made; all other cords detonate
reliably.
~Outer diameter in inches ~cm).
These examples show that the detonation
velocity of the cords tested is within the 6900 meters
per second + 5% range regardless of the PETN loading and
the thickness of the plastic coating. With this particular
core composition and a coating thickness of 0.044 inch
(0.112 cm), however, reliability of detonation becomes
compromised somewhat at the minimum PETN loading and core
diameter.
Examples 11-14
The explosive cord described in Examples 5-10
is tested in three different core loadings and diameters
for reliability of initiation and continued propagation
with minimum coating thicknesses.
-33-
::
` ~1467~ ~`
llo. of ~ton~t~on- Out
of lD TriDl-- ~e coAting
PETN . Thldcne~s ~ Specified ::
Ex. gr /ftj Core D ~m. O (0 025) 0.015 (0 064)
11 0.5~0.10O 0.013(0.033) 0 10 -
12 1.0(0.2131 0.020(0.051 4 10 ,
13 2.5(0.533~ 0.030(0.076) ~ 10
1~ 7.0(1.~ 0.050(0.127) 10 .
*in. (cm.)
These examples show that as the core diameter
and PETN loading increase, the plastic coating has a
diminishing effect on the cord's ability to be initiated
and propagate a detonation.
Example 15
The cord described in Examples 5-10 having a 0.030
in. (0.076 cm) core is made with different coating materials
` and thicknesses. All samples (at least 150 feet (46 m)
in length) of the cord with 0.020-in. (0.051-cm), 0.028-in.-
(0.071-cm), and 0.033-in. (0.084-cm)-thick coatings
of low-density polyethylene, high-density polyethylene,
and a metallic salt of a copolymer of ethylene and
methacrylic acid (an ionomeric resin) detonate reliably at
a velocity of about 7200 meters per second with four strands
as well as eight strands of the yarn. The extrusion die
temperature i9 175C for applying the high-density poly-
ethylene, and 135C for applying the ionomeric resin.
The minimum tensile strength is 70 lbs (32 kg) for
all samples made with 4 strands of the yarn, and 140 lbs
(64 kg) for all samples made with 8 strands of the yarn.
All samp~e~, regardless of coating material thickness and
type, detonate after the following treatment: A 6-lb
~2.7 kg) weight is tied to one end of the cord. The weight
-34-
` 1~14~;`7~
is allowed to drag the cord by gravity over the edge of a
concrete block, and then the cord is dragged back to its
starting point. This procedure is repeated five times.
Examples 16-19
The effect of core loading and sheath coating
thickness on the behavior of the cord described in
Examples 5-10 when knotted, as may occur in field use,
is shown in the following table: ~ -
Ti~ - D ton~-
tion Prop g~t~-
., 10 r~N C~,.tlAg ~!hrough l~not- `~
gr./~t. Cor- Dl~--. Thlclcn--- Half-
Xx ~-/m.) ln. ~c~.)ln. ~c~.) ltch I~not
~,6 2.5(0.533) 0.030~0.076)0-035(0-Oa9) 15(~) 5~b)
17 3.0~0.63B) 0.033(0.0a~)0.03~(0.086) 15(~) 5tb)
1~ 3.~0.723) 0.035(0.0~9)0.033(0.08~) 13(-) 2~b)
19 ~ 0.~53) 0.0~0~0.102)0.0~3~0.109) 1~) ~b)
(a) Out of 15 trials
(b) Knot tied under 10-lbs. tension; out of 5 trials
These examples show that the specified cords
propagate a detonation through knots rather than cut-off
at the knots owing to excessive brisance. They also show
that as the explosive loading is increased an increase in
the sheath thickness will assure the propagation of
detonation through knots.
Examples 20-24
The cord described in Examples 5-10 having a
0.030 in. (0.076 cm.)-diameter core is made with different
n~mbers of mul'i-filament strands of polyethylene
terephthaLate (PET) yarn and an aramide yarn made from
the condensation polymer of terephthalic acid and
para-phenylenediamine (all 1000 denier per strand). The
effect of these variables on cord strength and the cord's
-35-
~ ~14~
ability to propagate a detonation thro~gh knots is shown - ;
in the followin~ table:
Times Detonation ;~
Tensile Propagates
PET Aramide Strength Through Knots
Yarn Yarn of Cord Half-
Ex. No. o Strands lb.(kg.) Hitch Knot
2 43(20) 4(a) 2,0,0,0(b)
- 21 4 82(37)1O(a) 3,0,0,0(b)
22 8 150(68)1O(a) 3,3,3,0(b)
23 2105(48)g(a) 2,1,0,0(b)
24 4198(90)1O(a) 3,3,3,3(b)
'
(a) Out of 10 trials
(b) Knots tied under 10, 20, 30, and 40 lbs.
~4.5, 9.1, 13.6, and 18.2 kg) tension, respectively;
out of 3 trials each`
These examples show that the tensile strength
of the cord for a given number of yarn strands of the
same denier varies with the tensile strength of the yarn.
In this case, the aramide provides a higher tensile
strength cord with fewer strands than the polyester.
The examples also show that a larger number of strands
of a given fiber, or a stronger fiber, will increase
the cord's ability to propagate a detonation through
tighter knots.
Example 25
A continuous solid core of a bonded explosive
composition consisting of ~by weight) 75% superfine PETN
and 25% of a binding agent consisting of a butadiene,
acrylonitrile, methacrylic acid copolymer (described
in the aforementioned U.S. Patent 3,338,764) is attached to a
--36--
67S
single fitrand of aramide yarn made from the condensation
polymer of terephthalic acid and para-phenylenediamine.
The core and supporting strand are dragged together through
a tubular coating die, which applies a 0.025-in. s
(0.064-cm.)-thick sheath of low-density polyethylene
around them. The resulting cord, which has a PETN ~ -
loading of 7 grains per foot, detonates at about 7000
meters per second when initiated by the procedure des-
cribed in Example 1, and has a tensile strength of
10 about 75 pounds (34 kg.).
Example 26
The deformable bonded explosive composition
described in Example 1 (except that the superfine PETN
content is 76 percent, acetyl tributyl citrate twenty
percent, and nitrocellulose 4 percent) is extruded so as
to form ten 4-foot (1.2-meter)-long cords, five having a
0.030-inch (0.076-cm) diameter (2.5 gr/ft or 0.533 g/m
PETN) and five having a 0.050-inch (0.127-cm) diameter
(7.0 gr/ft or 1.49 g/m PETN). The extruded cords are
20 slipped into low-density polyethylene tubing having an
inner diameter of 0.060 in. (0.152 cm) and an outer diameter
of 0.080 in. (0.20 cm). The ratios of explosive loading
to wall thickness of these cords are 250/1 and 700/1,
respectively, in gr/ft loadings and thicknesses in inches
(18/1 and 50/1, respectively, in g/m loadings and thicknesses
in cm). All of the cords have tensile strengths of about
10 pounds (4.5 kilograms).
The cords are initiated by a No. 6 blasting cap,
the end of the cord being in coaxial abutment with the
30 exposed end of the cord. All of the cords detonate without
~4~
cut-offs, consuming all of the plastic coating. The average
detonation velocity for all ten cords is 7300 meters per - -
second.
- In the process of this inv~ntion substantially no
reduction is effected in the core diameter after the core
has been formed. The process produces a high-density core
without requiring a reduction in the diameter of the core
as is required, for example, in processes for making cords
having a particulate explosive core. Elimination of a
change of core diameter during the process simplifies
process control with respect to achieving a required final
core explosive loading and avoids the possible penetration
of the core by the surrounding yarn strands.
In detonating cords having small-diameter, low-
loading cores, the presence of particles of foreign matter,
e.g., sand, metal, etc., may interfere with the detonation
of the cord if the particles are large enough. For this
reason, an important feature of the present process is the
provision of a core explosive composition exempt of such
particles by virtue of the procedures and conditions
employed in its preparation, or the presence of particle-
screenin~ means in the extruder used for making the core.
For core~ having diameters of about 0.030 in. (0.076 cm)
and larsex, particles larger than about 33% of the core
diameter ~Aould be excluded. For smaller-diameter cores,
particles larger than about 0.005 in. (0.013 cm) should
be excluded.
In the present method when strands of yarn and
the explosive core move separately into a plastic coating
extrusion die, the cage formed therein usually will entrain
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the core and the sheath will subsequently be formed on the ~; r
caged core unit. However, the cage-formation, core-
entrainment, and sheathing may occur substantially simul-
taneously. Also, the two extrusion means of the apparatus, ~;
i.e., the core-forming die and the sheath-forming die, may
be components of separate extruders, or may be positioned
together in a single co-extrusion unit.
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