Note: Descriptions are shown in the official language in which they were submitted.
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FTELI~ OI~' THE INVENTION
This invention relates to shock tubing and, more
particularly, to a method of producing coloured shock tubing
that facilitates measurement of core loading.
DESCRTPTIOPT Oh' THE RELATED A~2T
Persson, in U.S. Fatent No: 3,590,739, first described
hollow tubes containing an inner coating of a reactive
material, such as a pyrotechnic or. explosive composition,
which could be used td support the propagation of a gaseous
percussion wave throughout the length of the tube. These
hollow tubes, commonly known as shocDc tulbes, are widely used
by the explosives industry as a non-electric means to cause
the initiation of non-electric detonating caps, and thus to
cause the ignition of a main explosive charge.
I5 Shock tubes are typically produced by the continuous
extrusion of a polymeric resin int~ a flexibly, hollow tube.
The inside surface of the tube is coated with a suitable
reactive material, which reactive material adheres to the
surface of the tube. The tube may be subsequently stretched
2P in order to increase its length.
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The production of shock tubes was initially restricted
to the use of a limited number of polymers in order to
obtain the physical properties which were sought for the
product. These properties included the ability to withstand
the conditions typically found in blasting environments,
while maintaining a sufficient degree of reactive material
adherence to the polymer material to ensure the propagation
of the gaseous percussion wave throughout the length of the
shock tube.
In order to ensure propagation of the gaseous
percussion wave, it is essential that a minimum core loading
of reactive material is maintained throughout the length of
the tube. This minimum core loading varies depending on the
type of reactive material used, and on the inner tube
5.5 diameter, but is generally in the order of 20 mg of reactive
material per running meter of 1.3 mm inside diameter shock
tube, or about 4.4 g/mz of internal tube area.
Measurement of the core loading of shock tubing is
conveniently performed during production by exposing the
shock tube produced to a radiation source, generally an
infrared (IR) light source, which radiation is absorbed by
the reactive material on the inner surface of the shock
tube.
The radiation used for measuring the core loading is
2~ essentially not absorbed by the polymer of the hollow tube
since absorbance by the polymer would either prevent or
interfere with the determination of the core loading. Thus,
the core loading of the reactive material in the shock
tubing can be determined by measuring the level of
absorption of the radiation which passes through the shock
tube since the absorbance of the radiation is related to the
core loading of the shack tubing.
~ne method to improve the physical characteristics of
shock tubing has been to provide a laminated product made of
at least two layers of different polymeric resins. The
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inner layer is a polymer having sufficient adherence
properties to maintain the minimum core loading of the
reactive material, and allows core loading to be measured
since the polymer is sufficiently transparent to the
radiation used for measuring core loading. An outer layer
of polymeric material is extruded over the inner tube layer
and has the necessary physical properties to withstand the
conditions encountered during use.
The properties of the laminated, or two layer, shock
tubes can be further enhanced by the addition of strands or
cords of reinforcing materials between the inner and the
outer layers of polymeric materials in order to reduce
stretching of the tube on site.
A further improved feature of commercial shock tubing
has been to colour the outer layer of polymeric material in
order to make the shock tubing more visible on-site, and to
colour code the shock tube according to use, length, or
shock tube propagation velocity. This outer layer of
coloured polymeric material is generally visually opaque and
blocks common sources of radiation from passing through the
shock tube.
While blocking of radiation is desirable in order to
reduce ultra-violet (UV) light from passing thraugh the tube
and causing the potential degradation of the polymeric
material and the potential W induced desensitization of the
reactive material, the outer layer of coloured polymeric
material also blocks the radiation frequencies used to
measure the core loading of the shock tube.
In commercial practice, it is, therefore, necessary to
measure the core loading of the shock tube prior to
extruding the outer layer of coloured polymeric material
over the inner layer of polymeric material, since it has not
been possible to measure core loading through the outer
layer of coloured polymeric material.
While the two layer shock tubes of the prior art are
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commercially viable, it would be desirable to reduce the cost
of the two layer shock tube of the prior art.
In GB Patent No. 2,215,441A, a single layer shock tube is
described which is produced from an extruded blend of polymeric
materials, which material blend provides a shock tube with
suitable physical characteristics. Unfortunately, the presence
of typical colouring materials in the material blend used to
extrude the single layer shock tube would result in the
inability to measure the core loading of the shock tube using
conventional radiation absorption equipment.
SIJN~1ARY OF THE INVENTION
It has now been found that coloured shock tubing can be
produced by using a coloured compound to effect colouration of
the shock tube, which coloured compound is sufficiently
transparent to radiation to allow the core loading of the
coloured shock tube to be measured.
It is an object of the present invention to provide a
coloured shock tube which permits core loading of the reactive
material to be measured, after the shock tubing has been
coloured, using radiation absorption equipment.
Accordingly, the present invention provides a method of
producing shock tubing, which shock tubing has a hollow tube
with an inner core loading of a reactive material for the
propagation of a shock wave within said tube, and which
reactive material absorbs radiation of a selected frequency,
which process comprises:
forming a visually coloured hollow tube comprising a
coloured compound and having an inner surface and an outer
surface; and
coating the inner surface of said tube with a core loading
of said reactive material,
characterized in that said visually coloured hollow tube
is essentially transparent to infrared or near infrared
radiation.
The coloured compound can be selected from the group
consisting of fillers, pigments, or dyes, and may be blended
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into, and form part of the tube or may be a coating on the
surface of the tube.
The coloured shock tubing is, preferably, produced by
the addition of suitable coloured fillers, pigments, or,
preferably, dyes to the polymeric resin mixture used in the
manufacture of the hollow tube. Suitable colouring
materials can be discrete organic dyes, or may be, for
example, pigments prepared by a "Lake" process. For
example, materials, such as diazo, disazo, or Lake based
l0 pigments may be used. In this manner, shock tubing can be
produced which can be, for example, yellow, red, orange,
blue, green, or violet.
The resultant visually coloured, hollow tube, when
viewed in the absence of reactive material, may be partially
transparent, translucent or opaque to visible light.
The polymeric resin of the shock tube of the present
invention must be sufficiently transparent to the radiation
frequency used so that a sufficient amount of radiation
passes through the polymeric resin to allow the core loading
to be measured. The polymeric resin is, preferably, a
polyethylene based material such as, for exaanple, linear low
density polyethylene, ultra low density polyethylene, or low
density polyethylene and can include blends or copolymers of
the above resins with other resins or monomers such as
~5 ethylene/vinyl acetate, vinyl acetate, or ethylene/acrylic
acid.
The reactive material can be any suitable material for
the propagation of the gaseous percussion wave, but must
absorb the radiation at the frequency used for measuring
core loading. If necessary, suitable fillers which absorb
at a desired radiation frequency, can be added to the
reactive material which fillers will absorb radiation at the
selected frequency used.
Preferably, the radiation frequency used is a near
infrared radiation frequency, and, more preferably, is a
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broad band peaking at 900 nm.
In a further preferred feature, it is desirable to
provide a method to produce a coloured shock tube according
to the present invention, as hereinabove described, wherein
the coloured shock tube absorbs W radiation so that UV
degradation of the polymer or the reactive material is
avoided. This can be accomplished by the addition of a UV
absarbing material to the polymeric resin used to produce
the hollow tube.
The present invention, thus, provides a method of
producing a shock tubing which comprises:
mixing a coloured compound with a polymeric resin to
produce a coloured polymeric resina
extruding said coloured polymeric resin to form a
visually coloured hollow tube having an inner surface and an
outer surface; and
coating the inner surface of said tube with a core
loading of a reactive material, which reactive material
absorbs radiation of a selected frequency.
While it is preferable, in the present invention, to
have a single extrusion process to provide a single walled
shock tube, as described hereinabove, it is also possible to
use the present invention to provide a coloured
over-extruded shock tube, wherein a suitable colouring
~5 compound is included in any one of the polymeric resin
layers of a mufti-layer shock tube.
Further, it is also possible to coat the inner, or,
more preferably, the outer surface of the hollow tube of the
shock tubing with a coloured coating material, which coating
material comprises the coloured compound and will adhere to
the hollow tube to provide a thin coating on the surface of
the tube, and which coating material is essentially
transparent to the radiation frequency used.
Thus, the present invention also provides a method of
producing a shock tubing which comprises:
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extruding a hollow tube having an inner surface and an
outer surface;
coating the inner surface of said tube with a core loading
of a reactive material, which reactive material absorbs
radiation of a selected frequency; and
coating the outer surface of said tube with a coating
material, which coating material comprises a coloured compound,
wherein said coating material and said hollow tube are
essentially transparent to said radiation.
In a further aspect, the present invention also provides a
shock tubing produced according to a process as hereinbefore
defined.
In a still further aspect, the present invention also
provides a method of determining the core loading of shock
tubing comprising:
preparing shock tube according to the methods defined
hereinabove;
exposing said shock tube to a radiation source having a
frequency range which is absorbed by said reactive material,
and which is essentially not absorbed by said hollow tube or
said coloured compound; and
measuring the absorption of said radiation.
In a yet still further aspect, the invention also provides
shock tubing comprising a hollow tube having an inner surface
and an outer surface, said inner surface being coated with a
core loading of reactive material, and wherein the shock tubing
is characterized by the hollow tube being visually coloured
and being essentially transparent to infrared or near infrared
radiation
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will now be described, by way of example
only, with reference to the following figures, wherein:
Figure 1 is a cross-sectional drawing of a single layer
shock tubing according to the present invention;
Figure 2 is a schematic drawing of a production facility
to produce the shock tubing described in Figure 1; and
Figure 3 is a cross-sectional drawing of a two layer shock
tubing according to the present invention.
In Figure 1, a shock tube 10 is shown having a single wall
hollow tube 11 having an inner coating of a reactive
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material 12.
Tube 11 is coloured and comprises a mixture of 80%
linear low density polyethylene (LLDPE), 10% of a low
functionality ethylene-vinyl acetate resin (EVA) having 2%
vinyl acetate, and 10% ethylene-acrylic acid. The tube is
coloured by the addition of a yellow disazo dye, as a
coloured compound, to produce a yellow hollow tube.
The reactive material 12 comprises a mixture of HMX
(cyclotetramethylene tetranitramine) and aluminum. Reactive
material 12 absorbs IR radiation from an IR radiation
absorption means having a frequency peaking at 900 nm while
coloured tube 11 does not absorb sufficient IR radiation at
the stated frequency to substantially interfere with the
measurement of the IR absorption of the reactive mat~:rial.
Other resin blends, such as for example, those resin
blends described in GB Patent Ido. 2,215,441A, may be
utilized provided that acceptable shock tubing properties
are obtained.
The coloured shock tube of Figure 1 is produced by the
following process described with reference to Figure 2.
In Figure 2, an extruder 20 is shown having supply
hoppers 25 and 26. The product exiting extruder 20 is fed
in series through a stretching means 21, an IR absorption
means 22, and a packaging means 23:
In extruder 20, a mixture of polymeric resin pellets
and coloured compound are fed from hopper 25 to extruder 20
and is extruded at a temperature of about 205°C into a
continuous hollow yellow tube 11. Reactive material 12 is
fed from hopper 26 into a mandrel located within the centre
of the extrusion die.
The coloured compound may be added separately to hopper
25, but is, preferably, added as pellets of a pre-blended
concentrated "masterbatch'° mixture of coloured compound and
polymeric resin. Masterbatches of suitable materials of use
in the present invention are commercially available from
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Korlin Concentrates as Korlin Colour Numbers YE 9571 or RD
9575 which are, respectively a disazo and a Lake red
pigment. The pigments have been blended into a linear low
density polyethylene resin. Additional property enhancing
materials, such as for example, benzotriazole U.V. absorbers
may also be added to the masterbatch.
Material 12 is allowed to fall into tube 11 at a
controlled rate, as tube 11 is forming, and adheres to the
inner walls of the tube 11 produced. The resultant shock
tube 10 thus produced, is allowed to cool and is passed
through stretching means 21 which stretches shock tube l0 to
provide a six-fold increase in the length of the tube.
After stretching, shock tube 10 has a 3 mm outside diameter,
a 1:3 mm inside diameter, and a core loading of 18 mg/m.
Stretched shock tube 10 is fed to IR absorption means
22 wherein TR radiation having a frequency peaking at 900 nm
is directed through shock tube 10. The absorption of the
radiation which passes through shock tube 10 is measured and
compared to a calibrated standard level of absorption.
Thus, the core loading level of shock tube 10 is determined
b~ comparison of the IR absorption of the shock tube
produced to the IR absorption of known standards. Shock
tube 10 is finally fed to packaging means 23 wherein the
shock tube 10 is wound onto cylindrical drums.
Additional shock tubing production details are more
fully described in GB Patent No. 2,215,441A.
The resultant shock tube is coloured and has acceptable
properties far explosive industry use. Verification of the
core loading of the tube produced can be accamplished at any
time by passing the tube through. an IR absorption means
similar to the means used during production.
A second embodiment of the present invention is shown
in Figure 3 wherein a cross-sectional view of a multiple
layer shock tube 15 is shown. Shock tube 15 has an inner
hollow tube 16 over which an outer layer 17 of polymeric
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material has been over extruded. The inner hollow tube 16
has an inner coating of a reactive material 12.
Inner tube 16 is a colourless tube which has been
prepared by extrusion of Surlyn~, or in general, a polymeric
material which suitable adhesive properties for the reactive
material 12 to remain on tube 15. Reactive xaaterial 12 is
the same HMX/aluminum mixture described in Figure 1.
After inner tube 16 has been formed, outer layer 17
which comprises a linear low density polyethylene (LLDPE)
and a coloured compound as described with respect to Figure
1, is over extruded.
Measurement of the core loading of shock tubs 15 can
still be determined by measuring the absorption of IR
radiation projected through shock tube 15.
* Trade Mark
