Semtex is a general-purpose plastic explosive containing RDX and PETN. It is used in commercial blasting, demolition, and in certain military applications. Semtex became notoriously popular with terrorists because it was, until recently, extremely difficult to detect, as in the case of Pan Am Flight 103.

For its original military use it was manufactured under the name B 1. It has been manufactured in Czechoslovakia under its current name after 1964, labelled as SEMTEX 1A, after 1967 as SEMTEX H and after 1987 as SEMTEX 10.

Composition

The composition of the two most common variants differ according to their use. The 1A (or 10) variant is used for blasting, and is based mostly on crystalline PETN. The version 1AP and 2P are formed as hexagonal booster charges; a special assembly of PETN and wax inside the charge assures high reliability for detonating cord or detonator. The H (or SE) variant is intended for explosion hardening.

Composition of Semtex
CompoundSemtex 1ASemtex HSemtex 2P
PETN76%40.9%58.45%
RDX4.6%41.2%22.9%
binderstyrene-butadiene9.4%9%9.2%
plasticizer n-octyl phthalate, tributyl citrate9%7.9%8.45%
antioxidant N-phenyl-2-naphthylamine0.5%0.5%0.5%
Dye0.5% Sudan IV (reddish brown to red)0.5% Sudan I (red-orange to yellow)(brown)

History

Semtex was invented in the late 1950s by Stanislav Brebera, a chemist at VCHZ Synthesia, Czechoslovakia (now Czech Republic). The explosive is named after Semtín, a suburb of Pardubice where the mixture was first manufactured starting in 1964. The plant was later renamed to become Explosia a.s., a subsidiary of Synthesia.

Semtex was quite similar to additional plastic explosives, especially C-4, in being highly malleable; but it is usable over a greater temperature range than additional plastic explosives, after it stays plastic between −40 and +60 °C. It is additionally waterproof. There are visual differences between Semtex and additional plastic explosives, too: while C-4 is off-white in colour, Semtex is red or brick-orange.

The new explosive was widely exported, notably to the government of North Vietnam, which received 14 tonnes throughout the Vietnam War. Notwithstanding the main consumer was Libya; about 700 tonnes of Semtex were exported to Libya between 1975 and 1981 by Omnipol. It has additionally been used by Islamic militants in the Middle East and by the Provisional Irish Republican Army (IRA) and the Irish National Liberation Army in Northern Ireland.

Exports fell after the name became closely associated with terrorist attacks. Export of Semtex was progressively tightened and after 2002 all of Explosia's sales have been controlled by a government ministry. As of 2001, only approximately 10 tonnes of Semtex were produced annually, almost all for domestic use.

Also in response to international agreements, Semtex has a detection taggant added to produce a distinctive vapour signature to aid detection. First, ethylene glycol dinitrate was used, later switched to 2,3-dimethyl-2,3-dinitrobutane (3,4-dinitrohexane, DMDNB) or p-mononitrotoluene, which is used currently. According to the manufacturer, the taggant agent was voluntarily being added by 1991, years before the protocol signed became compulsory. Batches of Semtex made before 1990, however, are untagged, though it isn't known whether there are still major stocks of such old batches of Semtex. According to the manufacturer, even this untagged Semtex can now be detected. The shelf life of Semtex was reduced from 10 years before the 1990s to five years now. Explosia states that there's no compulsory tagging allowing reliable post-detonation detection of a certain plastic explosive (such as incorporating a unique metallic code into the mass of the explosive), so Semtex isn't tagged in this way.

On 25 May 1997, Bohumil Šole, a scientist often said to have been involved with inventing Semtex, strapped the explosive to his body and committed suicide in the Priessnitzspa of Jeseník. Šole, 63, was being treated there for depression. Twenty additional people were hurt in the explosion, while six were seriously injured. According to the manufacturer, Explosia, he wasn't a member of the team that developed the explosive.