Note: Descriptions are shown in the official language in which they were submitted.
CIL 603
`- 113~;41~ :
This invention relates to an explosive booster used
for the detonation of explosive compositions which are insensi-
tive to detonation by ordinary blasting caps. Typical of such
cap-insensitive explosives are, for example, ammonium nitrate/
fuel oil (ANFO) mixtures and aqueous slurry blasting agents.
A large variety of booster types are known in the ~ -
explosives art. Most boosters employed with cap-insensitive
blasting agents comprise a cast or pressed cap-sensitive high -~
explosive charge usually having a recess or well adapted to
receive a blasting cap or other initiator such as a length of
detonating cord. Sometimes such cast or pressed boosters com-
prise a core of explosive which is sensitive to detonating cord
or cap initiation, which core is itself surrounded by a sheath
of non-cap-sensitive explosive. Such a core-and-sheath booster
is disclosed, for example, in United States patent No. 3,037,452.
Other typical cast or pressed boosters are disclosed in United --
States patent Nos. 3,037,453, 3,341,382, 3,371,606, 3,491,688, ;
3,437,038, 3,359,902, 3,604,353, 3,604,354 and 4,009,060. In
.
general, all of the aforedisclosed boosters are water-resistant
and are generally insensitive to initiation from impact. They
are, however, not without some limitations and disadvantages.
In substantially all cases the explosive of choice ~or cast and
pressed boosters is trinitrotoluene (TNT) because of its
relatively cheap cost and ready availability. However, since
TNT ls, except under unusual conditions, insensitive to
blasting cap or detonating cord initiation, it has been neces-
sary to combine with the TNT an amount of another cap-sensitive
explosive in order to insure initiation of the TNT. This
. ,,~ .
;~ combination may be in the nature of a sensitive core material
. . .
~ 30 surrounded by the TNT sheath or it may be made by placing
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sensitive explosive in side-by-side abutment with the TNT.
Alternatively the TNT may be made more sensitive by mixing a
sensitive material with the TNT to form a cap-sensitive com-
bination. Typical of the cap-sensitive materials which may be
, combined or mixed with TNT are, for example, pentaerythritol
tetranitrate (PETN) and cyclonite (RDX).
~ ~ It will be obvious that the manufacture of two-
;?: component boosters, of say, a PETN core surrounded by a TNT
sheath, will be substantially more costly than a single cast
; 10 or single press booster of, say, TNT alone. Not only does the
two~component booster require additional manufacturing steps
but also the cost of the cap-sensitive ingredient, for example,
PETN, is substantially greater than the cost of an equ valent
volume of TNT. In addition, because of the presence of cap-
~ sensitive PETN or the like, the manufacture and the use of the -
¦ multi-component booster carries with it an additional risk or
hazard factor.
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A method has now been found for manufacturing a cast
~- booster comprising TNT alone as the only self-explosive material
which booster is sensitive to blasting cap and detonating cord
initiation, is insensitive to accidental initiation by impact
and is cheap to manufacture.
It has been surprisingly discovered that by control-
ling or maintaining the density of cast TNT to a value not
~- greater than 1 Mg/m by the inclusion therein of discrete voids
or gaseous pockets, a booster charge may be provided which is
sensitive to blasting cap initiation~ It has been noted, how-
ever, that the detonation wave generated by the initiation of
low density TNT of 1 Mg/m3 or less is of a low intensity and is
not adequate for the initiation of regular or normal density
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cast TNT in contact with it or for the initiation of an adjacent
insensitive blasting agent. It is postulated that the density
difference at the interface between the low density TNT and an
adjacent insensitive explosive, creates an impedance discontinu-
ity which tends to reflect away a substantial amount of the
energy of the detonation wave generated by the low density TNT
thus preventing boostering. To overcome this difficulty, the
booster of the present invention comprises an elongated TNT
casting of gradually increasing density from end to end. The
means by which the gradually increasing density may be conveni-
ently created during the period of solidification and cooling
of the casting of the void-containing TNT is by the incorpora-
tion in the molten TNT of particulate sodium nitrate and particulate
~,
expanded vermiculite and gas bubbles. The booster of the invention
i~ is preferably adapted for the convenient attachment or insertion
,~ ~ of the initiating cap or detonating cord against or within the
most sensitive low density portion of the casting. The addition
"~ of particulate sodium nitrate, preferably having a grit size of
- less than 20 mesh-Tyler and in an amount of up to 20~ by weight
of the total booster, provides for an increase in the overall
density of the cast booster without reducing its sensitivity or
its energy output.
From computed values of detonation pressure obtained
from TNT containing up to 20% by weight of particulate sodium
nitrate, it can be observed that no substantial reduction in
detonation pressure takes place from that observed with unadul-
terated TNT. It is, therefore, possible to prepare practical
and useful primers of the type described herein without the
need of any separately cast TNT portion. The addition of
, 30 particulate expanded vermiculite (filter grade) to a castable
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TNT/sodium nitrate mixture tends to slow the settling of the
sodium nitrate and any solid void-containing material during
the period of cooling and solidification of the TNT blend.
The incorporation of the discrete voids within the
TNT casting is most easily accomplished by adding to the molten
TNT sufficient glass microspheres to achieve the desired low
density and the resulting sensitization. Such glass micro-
spheres, especially in the presence of particulate expanded
vermiculite, tend to settle only very slowly as the TNT soli-
difies thus producing a gradual decreasing density from top to
bottom in the final cast product. Discrete voids may also be
incorporated in the TNT casting by the mechanical "whipping-in"
of air or by the addition of particulate gas-generating materials
such as, for example, sodium borohydride. While the addition
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of whipped-in air or the generation of gas in situ have the ~
,~ obvious advantage of economy over the use of glass microspheres, ~ -
t will be appreciated that difficulties may be encountered in
providing a controlled distribution and sizing of whipped-in
, ~
or generated gas bubbles.
A further aspect of the present invention provides
for a combination booster comprising a mixture of TNT and one
.:
other cap-sensitive solid castable explosive wherein the quantity
of the cap-sensitive castable explosive can be substantially
reduced by the incorporation in the mixture of discrete voids.
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The following examples taken in conjunction with the
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attached drawings which show in Fig. l a solid cast TNT primer
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of the invention and in Fig. 2 a TNT cast primer of the invention
having an axial tunnel or cap well, illustrate the invention.
~- The percentage of each constituent is expressed by weight of
dry material.
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i
~ Example 1
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~` With particular reference to Fig. 1, into a two inch
'~ diameter x five and one-half inch long paper or plastic cyclin- -
der 1 was poured a three inch column of full density TNT 2 which
was allowed to solidify but was maintained at a temperature of
75C. The remaining space 3 in the cylinder was filled with a
,
mixture of TNT (77%), glass microspheres (3.8%), filter grade
expanded vermiculite (3.8%) and particulate (passing 20 mesh)
sodium nitrate (15.4%). The initial pouring temperature of the
mixture was from 85-97C. A length of reinforced detonating
cord 4, containing 10 g~m of PETN was set in place at the upper
;;".;
surface of the molten mixture in space 3 and the entire casting
was allowed to cool and solidify by natural convection in air
at 20-25C. ,
,
` On initiation of the detonating cord 4 by means of an
electric blasting cap,~the whole assembly detonated. From
chronometer readings it was evident that the full density por-
tion of the TNT was detonated at its normal high velocity.
Further evidence of complete detonation was obtained through obser-
vation of a normal size o crater formed in a mild steel plate
placed under the booster. A similar cylindrical casting of TN~
devoid of glass microspheres, vermiculite and sodium nitrate
failed to initiate in contact with 10 g/m detonating cord.
Example 2
With partiaular reference to Fig. 2, a cardboard
cylinder 10, two inches in diameter x seven inches long and
containing a removable central coaxial pin of about five-
sixteenths inch diameter (not shown) was filled in three stages.
,A base portion comprising a two and one-half inch column of
; 30 unadulterated full density TNT was cast. On top of this, a
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three inch columni 12 of gradually increasing density TNT was
,,;~ cast with the greater density portion in contact with the
,' previously cast unadulterated ~NT. The gradually increasing
,' density TNT, which tends towards segregation upon cooling, was
~' comprised of the same mixture as described in Example 1. On top
of this segregating portion was cast a one and one-half inch
column 13 of TNT containing 10.5% by weight of glass micro-
spheres of grade C15/250 manufactured by 3M. Cooling of the
'~ upper TNT/microspheres portion was effected quickly to prevent
segregation and to maintain a constant low density throughout.
After cooling, the coaxial pin was removed and the booster was
~-~ detonated by means of an electric blasting cap 14 placed within
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the co-axial channel or tunnel 15 and close to the interface
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of the low density andi the increasing density portions of the
casting. On detonation upon a one inch thick mild steel plate
a crater was produced indicating complete detonation at high
' ` velocity. A booster of similar dimensions but co~prised of
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~;~ ,unadulterated TNT could not be~initiated with a similar electric
blasting cap.
It will be obvious to those skilled in the art that
'- graduated density initiator charges of the type described in
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the present invention can be prepared from any castable explo-
~i
~, sive mixture. W~hile no particular advantage will accr~e in
' the use of castable explosive material in a cap sensitive
range at natural full density (e.g. composition B or Pentolite),
the use of voids introduced in the manner described herein will
' allow reductions in the quantity of sensitive component
employed and thus in cost of booster.
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