The present invention relates to a method of manufacturing permitted
detonators. These detonators are useful in mining operations.
Detonators used for initiating high explosives for blasting in the
underground coal mines have to undergo certain statutory tests before they
qualify for usage. The tests ensure that during blasting operations, the
flammable atmosphere of coal dust/methane does not catch fire causing
undesirable accidents. Such detonators are termed permitted detonators and
the tests they are subjected to are called Incendivity Tests.
Blasting operations in underground coal mines are carried out by
detonating high explosive cartridges which generate enormous heat and
pressure due to exothermic reaction of its chemical ingredients. The
detonation temperatures could exceed the auto ignition temperature of the
coal/methane air mixture which might be present around the blast area
immediately upon blasting. The explosives and detonators to be used in such
mining conditions have therefore to be designed (called nonincendive or
permitted explosives and detonators) such that there is no danger of any
untoward incident of ignition of methane or coal dust mixtures with air. Tests
designed to evaluate this property of non-incendivity are called gallery tests.
Detonators used for initiating detonation in the underground coal mines too

have to undergo such tests which are essentially simulation
tests in chambers of specified dimensions containing most
easily ignitable admixture of methane and air.
Conventional detonators used for general purpose usage
are made of aluminium shells which do not pass the tests
meant for permitted detonators due to reasons well known' to
those conversant in the art for producing permitted
detonators. Conventional detonators made of aluminum,
when fired, lead to oxidation of aluminum of the shell
with the atmospheric oxygen. This heat of oxidation is so high
that detonators made of aluminum shells almost always produce
an ignition in the test chamber, thereby disqualifying them*
as non-incendive or permitted detonators. Alternative shell
materials are copper and steel. Oxidation of copper or
iron is far less exothermic. Whereas the choice of copper
as shell material is age-old, the usage of steel has been
in vogue for nearly two decades and is the subject matter
of several Indian patents, the latest being Indian
patent No. 148437.
Permitted detonators made out of steel, in spite of
having the necessary rust preventive coating in the form of a
layer of zinc, copper etc., as described in the earlier
patents, are likely to corrode under extreme atmospheric
conditions during transportation, storage and usage.

Superior corrosion resistance using an outer coat of polymeric resin are known
but these resins used in the experiments failed the incendivity tests and hence
the idea of modified coating to render it more non incendive using additives
which further facilitate in passing the incendivity tests was thought of. Such
modified coatings may be available off the shelf but the judgment of choosing
the right coating material is the key to satisfactory passing of the incendivity
tests.
Further, lead azide is the conventional primary explosive used. When
used in copper or copper plated shells it can react with the shell material under
certain conditions to form copper azide which is highly sensitive material and its
formation has to be avoided. This was done in two ways, namely, (a) Using a
chemical inhibitor along with the lead azide. (b) Using a substitute for lead azide.
Both these solutions have their associated problems.
Hence there exists a need to provide a modified polymeric coating, using
additives to render it more non-incendive to pass the incendivity test and there
also exists a need to eliminate the formation of dangerous copper azide inside
the detonator.
The present invention seeks to provide an improved method of
manufacturing a permitted detonator with superior characteristics not only in
respect of corrosion resistance but also with respect to the safety characteristics
in incendivity tests.

Accordingly, the present invention provides a method of manufacturing a
permitted detonator useful for mining, said method comprising the steps of:
deep-drawing steel and electro-galvanizing the steel to produce a shell of
required length ;
coating an outer surface of the shell with a polymeric resin as herein described,
said coating optionally containing an additive as herein described ;
filling the shell with a secondary explosive and a primary explosive ; and
consolidating the explosives in said shell.
Preferably, the step of deep-drawing steel and electro-galvanizing the
steel comprises making a cup out of a steel strip having a thickness of 0.5 to 0.8
mm and hardness of 95 -100 Vickers number ; annealing the cup by known
methods to restore original hardness ; electroplating the annealed cup with zinc
to achieve a thickness of 5 to 10 microns ; and deep drawing said cup into shells
of required lengths of 35 to 70 mm.
Preferably, the polymeric resin is selected from polyester, epoxy and
combinations thereof. The coating of polymeric resin may be applied by
electrostatic technique, electro-phoretic technique, electro-deposition technique,
etc. Preferably, the polymeric resin is applied by an electrostatic technique.
The polymeric resin provides corrosion resistance to the shells. The
coating could be a pure or mixed resin. The range of percentages is governed by
ease of application, ease of curing, etc, and would be evident to a skilled person.
As an example it could be between 40-50% for a binary mixture.

Preferably, the additive is selected from the group consisting of flame
retardants such as chlorinated or brominated bis phenols, phosphates of
inorganic or organic nature which improve non-incendive characteristics, and
pigments such as copper powder, bronze powder, chrome yellow and the like
which impart color to the shells.
Preferably, after the secondary and primary explosives are filled in the
shell, a pyrotechnic delay element is pressed over the primary explosive in the
case of a delay detonator. In the case of an instantaneous detonator, an
electrical squib is fitted and crimped on the top of the shell.
Preferably, the secondary explosive is selected from the group consisting
of pentaerythritol tetranitrate (PETN), cyclonite (RDX), 2,4,6 - trinitrotoluene
(TNT) and mixtures thereof.
Preferably, the primary explosive is selected from the group consisting of
lead azide, lead styphnate, DDNP (diazodinitro phenol) and admixtures thereof.
The amount of secondary explosive is preferably 0.2 to 1.0 g., and the
amount of primary explosive is preferably 0.1 to 0.25 g.
More preferably, the amount of secondary explosive is 0.3 to 0.6 g and the
amount of primary explosive is 0.15 to 0.25 g.
The invention will now be described in detail.
In the drawing accompanying the Provisional specification,
Fig. 1 shows a shell for a detonator produced by the method according to the
invention. The shell 1 is made of steel and has a polymeric resin coating 2 on its
outer surface and a zinc coating 3 on its interior surface. The polymeric resin

coating 2 has a thickness of at least 10 microns and the zinc coating 3 has a
thickness of 5 to 10 microns. The shell 1 has an internal diameter of 6.72 mm,
external diameter of 7.61 mm and a length of 35 -70 mm.
The shell is filled first with a secondary explosive charge, and then a
primary explolsive charge and the explosives are consolidated inside the shell.
The filling is done by volumetric dosing with the desired explosives and
mechanically consolidating the material by means of a press.
In the case of a delay detonator, a time delay element is pressed over the
primary charge. In the case of an instantaneous detonator, an electrical squib is
fitted and crimped on the top of the shell.
The method of producing detonators according to this invention, includes
producing shells out of deep drawing quality electro-galvanized steel. The
electro-galvanizing is imparted at a convenient stage of deep drawing. For
example cups are made out of steel strip of thickness 0.5mm - 0.8mm and
hardness 95 -100 Vickers number. The cups are annealed to restore the original
hardness and electroplated with zinc to achieve a thickness of 5 -10 microns.
In a preferred embodiment, the cups are then drawn in deep drawing
machines to the desired lengths ranging between 35 mm - 70 mm depending
upon whether it is an instantaneous or a delay permitted detonator. The shells
are then coated on their outside with a polymeric resin of polyester, epoxy or a
combination thereof by an electrostatic technique generally referred to as powder
coating to a thickness of about 10 microns or more. Alternatively the coating can

be provided by electro-deposition, electro-phorectic technique or other known
methods.
To the aforesaid polymeric coating material, which is in the form of a fine
powder, can be added a number of additives, which impart different
characteristics desirable for a permitted detonator. Whereas the polymeric
material provides the desired corrosion resistance, it is the additives, which when
judiciously used, enhance the non-incendive characteristics of the detonators so
made. Examples of such additives are Flame Retardants such as chlorinated or
brominated Bis phenols, Phosphates of inorganic or organic nature, which will
improve the non-incendive characteristics. Other additives are pigments, which
will impart to the shells a distinct color so as to be differentiated from non-
permitted detonators made out of aluminium shells, which are white. When the
additives are phosphates, it is preferred to use 1 to 2 gm of the phosphates.
The shells so made are then filled and consolidated with explosive
charges of known quantities of secondary and primary explosives, in that order.
An electrical squib may be fitted and crimped on the top of the shell in the case
of an instantaneous detonator. In the case of a delay detonator, a pyrotechnic
delay element of the desired delay timing ranging between 25 msec to 150 msec
is pressed over the primary explosive charge. The details of the delay element
and squib are not described herein as they are not the subject of this invention.
The secondary explosive charge is generally PETN (penta erytheretoltetra
nitrate) in the range of 0.3 - 0.6 gms. The primary charge is of the order of 0.15-

0.25 gms and comprises an admixture of lead azide (LA) and lead styphnate
(LS) in a ratio such that lead azide is in larger proportion.
The permitted detonators made by this invention possess another superior
safety property when compared with the conventional copper or copper coated
detonators. When the primary charge used is based on lead azide it is always a

matter of suspicion that, in contact with a copper surface as
it exists inside such detonators, a more reactive copper
azide is formed. The copper azide if formed can render the
detonator unsafe to physical handling as copper azide is
highly sensitive to friction and'impact. 'In the case of the
present invention the shells are made of steel and not copper
and the inside surface of the shell is only zinc-coated. So,
there is no possibility of the formation of more sensitive
copper azide.
EXAMPLES:
EXAMPLE 1
Shells made of electrogalvanized steel cups having
dimensions 38.1 mm length and nominal diameter 7 mm were
coated with a specially prepared polyester resin powder with
bronze pigment and zinc phosphate as additives in a standard
powder coating equipment. The shells were mounted on a jig
such that the coating is imparted only on the outside. The
shells were then heated to a temperature of 180°C to cure the
polymer. The shells so formed were subjected to accelerated
tests by coating a thin layer of saturated solution of
ammonium nitrate in viscous gum medium and inserting them in
a 100% humidity chamber.
As control, a few shells of zinc and copper coated steel
with the same corrosive ammonium nitrate layer were also
introduced in the same 10 0% humidity chamber.

Result : The control samples showed extensive corrosion in less than 24 hours
whereas the shells produced in accordance with the present invention did not
show any sign of corrosion even after two weeks
EXAMPLE 2 : Instantaneous Detonators
Detonators were made using shells described in Example (1) using 0.55
gms of PETN and 0.15 gms of lead azide and lead styphnate mixture in the ratio
of 70:27 with 3% aluminium powder. 50 numbers were fired in the Buxton
apparatus for incendivity characteristics in an 8.5% methane air atmosphere.
Result: No ignitions
As control, a powder coating of a standard commercially available epoxy
polyester with bronze pigment powder (without the flame retardant phosphate
additive) was given on shells and assembled as detonators as above. 50
numbers were fired in the Buxton apparatus for incendivity.
Result: 4 ignitions were observed.
EXAMPLE 3 : Delay detonators
Detonators were first pressed using shells described in Example
2 using 0.55 g of PETN. Then 0.15 g of lead azide and aluminium
powder mixture in the ratio of 97:3 was pressed along with a
pyrotechnic delay element of desired delay timing ranging from
25 millisecond to 150 millisecond pressed over the primary charge. 50

numbers were fired in the Buxton apparatus for incendivity
characteristics in an 8.5% methane air atmosphere.
Result: No ignition.
The aforesaid examples demonstrate the improved
characteristics of detonators produced by the method of the'
present invention.
The present invention has been described with reference
to preferred embodiments and therefore is not limited to
these embodiments. The invention extends to all embodiments
of the invention coming within the scope of the invention as
herein claimed.

WE CLAIM :
1. A method of manufacturing a permitted detonator useful for mining, said
method comprising the steps of:
deep-drawing steel and electro-galvanizing the steel to produce a shell of
required length ;
coating an outer surface of the shell with a polymeric resin as herein
described, said coating optionally containing an additive as herein described ;
filling the shell with a secondary explosive and a primary explosive ; and
consolidating the explosives in said shell.
2. Method as claimed in claim 1, wherein said step of deep-drawing and
electro-galvanizing comprises :
making a cup out of a steel strip having a thickness of 0.5 to 0.8 mm and
hardness of 95 -100 Vickers numbers ;
annealing the cup by known methods to restore original hardness ;
electroplating the annealed cup with zinc to achieve a thickness of 5 to 10
microns ; and
deep-drawing said cup into shells of required lengths of 35 to 70 mm.
3. Method as claimed in claim 1 or 2, wherein said polymeric resin is
selected from the group consisting of polyester, epoxy and combinations thereof.

4. Method as claimed in any one of claims 1 to 3, wherein said coating of
polymeric resin is applied by an electrostatic technique.
5. Method as claimed in any one of claims 1 to 4, wherein said additive is
selected from the group consisting of flame retardants, phosphates of inorganic
or organic nature which improve non-incendive characteristics and pigments
which impart colour to the shells.
6. Method as claimed in claim 5, wherein the flame retardant is a chlorinated
or brominated bis phenol.
7. Method as claimed in claim 5 or 6, wherein the pigment is copper powder,
bronze powder or chrome yellow.
8. Method as claimed in any one of claims 5 to 7, wherein the additive
comprises 1 to 2 g of phosphates.
9. Method as claimed in any one of claims 1 to 8, wherein after the shell is
filled with secondary explosive and primary explosive, a pyrotechnic delay
element is pressed over the primary explosive charge to produce a delay
detonator.

10. Method as claimed in any one of claims 1 to 8, wherein after the shell is
filled with secondary explosive and primary explosive, an electrical squib is fitted
and crimped on the top of the shell to produce an instantaneous detonator.
11. Method as claimed in any one of claims 1 to 8, wherein said secondary
explosive is selected from the group consisting of pentaerythritol tetranitrate
(PETN), cyclonite (RDX), 2, 4, 6- trinitrotoluene (TNT), and mixtures thereof.
12. Method as claimed in any one of claims 1 to 11, wherein said primary
explosive is selected from the group consisting of lead azide, lead styphnate,
diazo dinitrol phenol (DDNP), and mixtures thereof.
13. Method as claimed in any one of claims 1 to 12, wherein the amount of
secondary explosive is 0.2 to 1.0 g and the amount of primary explosive is 0.1 to
0.25 g.
14. Method as claimed in claim 13, wherein the amount of secondary
explosive is 0.3 to 0.6 g and the amount of primary explosive is 0.15 to 0.25 g.
15. A method of manufacturing a permitted detonator, substantially as herein
described with reference to the examples and the drawing accompanying the
Provisional Specification.

16. A permitted detonator when manufactured by a method as claimed in
any one of claims 1 to 15.

There is disclosed a method of manufacturing permitted
detonators useful for mining, comprising the steps of:
deep-drawing steel and electrogalvanizing the steel
to produce shells of required lengths; coating outer
surface of the shells so produced with a polymeric
resin; filling the shells with a secondary explosive
and a primary explosive; and consolidating the explosives.