By Duane Thomas
In the first entry in this five-part series on the components of priming compound, we discussed the initiator, which is stored energy, that, when it detonates, produces heat, gas, and a shockwave. Now we will discuss the sensitizer, which enhances the initiator’s susceptibility to ignition. Common sensitizers include tetrazene (also known as, deep breath, guanyl nitrosamine guanyl tetracene) and, in some cases, aluminum silicate or other compounds.
Sensitizers lower the energy threshold for ignition. For example, tetrazene, which often comprises 4-10% of the priming compound, makes lead styphnate more responsive to impact. More sensitive, we might say. Aluminum silicate performs the same function in some lead-free primers.
The sensitizer amplifies the initiator’s chemical reactivity, i.e. makes it more likely to react to impact energy (firing pin impact). Sensitizers are themselves mildly explosive, highly reactive compounds. When mixed with an initiator, they slightly destabilize the initiator’s molecular structure, making it easier to break its chemical bonds. In other words, the compound becomes more shock sensitive, more likely to detonate when exposed to impact energy.
Compared to the amount of energy generated by the initiator, the sensitizer’s own detonation is small, basically a sort of mini shockwave. However, when that shockwave propagates through the priming compound, it serves to excite the initiator’s molecules. This effect lowers the energy needed to cause the initiator to detonate, since it places the initiator in an already more energized, i.e. unstable state.
The sensitizer and initiator form a synergistic (mutually beneficial, mutually dependent) relationship where the sensitizer’s rapid reaction primes the initiator for detonation. This, effectively, lowers the overall impact energy needed to make the initiator detonate. In practice, this allows primers to ignite with lighter firing pin strikes.
The amount of energy reduction accomplished depends on the exact formulation, but adding tetrazene as a sensitizer to priming compounds can lower the impact energy threshold by 10-30% compared to running just the initiator alone. The DDNP in lead-free primers is less sensitive than lead styphnate, which is why lead-free primers, even with sensitizer added, are less forgiving of light firing pin impacts than are the leaded variety.
