50
Percussion primer
2.
Electric Match (Squib) : A metal wire is coated with a dab Zirconium
Zr
50
of heat-sensitive composition.
An electric current is
passed through the wire, and the heat that is produced ignites the match composition. A burst of flame occurs a Reference 1.
that ignites a section of fuse or a charge of pyrotechnic composition.
Squib compositions usually contain potas-
sium chlorate (low ignition temperatures! ). Lead mononitroresorcinate (LMNR) is also included in many squib mixtures.
Several squib formulas are listed in Table 6.1.
3.
First Fire:
An easily-ignited composition is placed in lim-5.
Primer:
A term for the device used to ignite smokeless ited quantity on top of the main pyrotechnic mixture. The powder in small arms ammunition. An impact-sensitive first fire is reliably ignited by a fuse or squib, and the composition is used.
When struck by a metal firing pin,
flame and hot residue that is produced then ignites the a primer emits a burst of flame capable of igniting the main charge. Black powder moistened with water contain-propellant charge. Several typical primer mixtures are ing a binder such as dextrine is used in the fireworks in-given in Table 6.2.
dustry as a first fire, and also secures the fuse to the 6.
Friction Igniter:
A truly "self-contained" device should item.
First fires are often referred to as "primes" - a be ignitible without the need for a safety match or other term similar to another with a distinct meaning (see #5, type of external ignition source. Highway flares (fusees), below).
other types of distress signals, and some military devices 4.
Delay Composition: A general term for a mixture that use a friction ignition system. The fusee uses a two-part burns at a selected, reproducible rate, providing a time igniter; when the two surfaces are rubbed together, a delay between activation and production of the main effect.
flame is produced and the main composition is ignited.
A fuse containing a core of black powder is an example of Typically, the scratcher portion of these devices contains a delay. Highly-reproducible delay mixtures are needed red phosphorus and the matchhead mixture contains po-for military applications, and much research effort has tassium chlorate (KC1O 3 ) and a good fuel. Several fric-been put into developing reliable compositions.
tion igniter systems are given in Table 6.3.
12 8
Chemistry of Pyrotechnics
Heat and Delay Compositions
129
a black powder core significantly improved the safety record of TABLE 6. 3
Friction Igniter Mixtures
the blasting industry.
However, the development of modern,
% by
Refer-
long-range, high-altitude projectiles created a requirement for a new generation of delay mixtures. Black powder, under speci-Component
Formula
weight
ence
fied conditions, gives reproducible burning rates at ground level.
However, it produces a considerable quantity of gas upon ignition 1.
Main composition
Potassium chlorate
KC1O
(approximately 50%of the reaction products are gaseous), and its 3
60
3
burning rate will therefore show a significant dependence on ex-Antimony sulfide
Sb 2S 3
30
ternal pressure (faster burning as external pressure increases).
Resin
10
To overcome this pressure dependence, researchers set out to Striker
develop "gasless" delays - mixtures that evolve heat and burn Red phosphorus
P
56
at reproducible rates with the formation of only solid and liquid Ground glass
Si0 2
24
products.
Such mixtures show little, if any, variation of burn-Phenol /formaldehyde
(C13H1202)7
20
ing rate with pressure.
resin
One could begin such a project by setting down the requirements for an "ideal" delay mixture [4] : 2.
Main composition
Shellac
-
40
2
Strontium nitrate
Sr(N0 3 ) 2
3
1.
The mixture should be stable during preparation and stor-Quartz
Si0 2
6
age.
Materials of low hygroscopicity must be used.
Charcoal
C
2
2.
The mixture should be readily ignitible from a modest ig-Potassium perchlorate
KC1O,,
14
nition stimulus.
Potassium chlorate
KCIO 3
28
3.
There must be minimum variation in the burning rate of Wood flour
5
the composition with changes in external temperature and Marble dust
CaCO 3
2
pressure.
The mixture must readily ignite and reliably burn at low temperature and pressure.
Striker
4.
There should be a minimum change in the burning rate with Lacquer
61
small percentage changes in the various ingredients.
Pumice
2.2
Red phosphorus
P
26
5.
There must be reproducible burning rates, both within a Butyl acetate
C
batch and between batches.
6 H 12 0 2
10.8
The newer "gasless" delays are usually a combination of a metal oxide or chromate with an elemental fuel. The fuels are metals or high-heat nonmetallic elements such as silicon or boron. If an organic binder (e.g. , nitrocellulose) is used, the resulting mixture DELAY COMPOSITIONS