Minol (pronounced mine-ol) is a military explosive developed by the Admiralty early in the Second World War to augment supplies of trinitrotoluene (TNT) and RDX, which were in short supply. The aluminium component in Minol significantly prolongs the explosive pulse, making it ideal for use in underwater naval weapons (e.g. naval mines, for which it was developed, depth charges and torpedoes) where munitions with a longer explosive pulse are more destructive than those with high brisance.[1]: M139 Minol cannot be used in weapons fired from gun barrels (e.g. artillery shells) because there is a risk of detonation when subjected to over 250 gs of acceleration.[citation needed] Initially, three Minol formulas were used. All percentages shown are by weight:
These three Minols suffered from expansion, spewing and gassing due to the reaction of fine aluminium powder with moisture and structural phase transitions in ammonium nitrate. To improve stability of Minol and increase production, more coarse aluminium powder was introduced. Later it was found that aluminium chips, such as filings, flakes and shavings, also gave good performance and improved stability.[1]: M136–M137
To solve the problem with dimensional instability, pure ammonium nitrate was replaced by a solid solution of 10% of potassium nitrate in ammonium nitrate. Thus, a new formula was adopted:
The addition of potassium nitrate minimized expansion of Minol, making it more stable to temperature changes than TNT, but didn't solve the expansion problem. Minol IV could still expand and develop cracks after prolonged thermal cycling. A new composition, with 20% of potassium nitrate in solid solution, was developed. It didn't expand or crack even when cycled for months, but wasn't adopted for production and service.[1]: M137
Since the 1950s, Minol has been superseded by more modern PBX compositions, due to their superior explosive yield and stability when being stored; Minol is regarded as obsolete. Generally, any Minol-filled munitions encountered will be in the form of legacy munitions or unexploded ordnance dating from before the 1960s.
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