Semtex is a plastic explosive that was first produced in Czechoslovakia. Since it's production, Semtex has changed its name many times and has been improved greatly from the original product. Although first used and intended for mining, Semtex has expanded its uses to military application, explosion hardening, and boosting. Because of its military uses it has grown popular with terrorists.
Czechoslovakia created the first Semtex plastic explosive in the late 1950's and started large scale production in the 1960's. Semtex changed its name three times in the next twenty three years, from b1 to Semtex 1A, Semtex H, and then to Semtex 10. Semtex was originally produced as a mining tool to help clear mines, boost explosions, and even commercial blasting. Semtex 1A was used mainly in the Czech Republic whereas Semtex H was exported abroad. Semtex was mostly used in Vietnam for mine clearance and pyrotechnic works around 1967. At that time the Czech republic was a chief arms exporter and the addition of Semtex attracted many more customers. In 1981, the export into risky territories was banned due to recent terrorist interest and activity with Semtex. Semtex was allowed to be exported only into the Warsaw Treaty countries. In 1989 all exportation of this explosive was temporarily banned. The exportation began again after negotiations in Montreal, Canada on March 1, 1991. Since then, production has slowed drastically and the majority of the work done with Semtex is marking, tagging the Semtex so that it can be recognized and reworking it. 
Semtex is made from a combination of PETN and RDX, two high grade military explosives. Semtex requires binders, plasticisers and antioxidants to become the finished product that is used very little today. 
First developed as a mining explosive, Semtex later proved more useful for a variety of demolitions including commercial blasting and military purposes. Despite the Czech Republic's attempt to label their exports, some Semtex has been lost and ended up being used in terrorist bombings.  Libyan terrorists specifically used Semtex because it would not be detected by the airports security in the bombing of Pan Am flight 103. In 1988, Libyan terrorists decided to use Semtex because it was pliable enough to be molded into a radio/cassette player. It was strong enough that the little amount hidden in a briefcase in the cargo hold was enough to take down the plane. 
Semtex 1A is composed of 76% PETN and 4.6% RDX with some other ingredients like binder's, plasticizers and antioxidants that give it its physical properties. It is used mostly for blasting because it is mostly composed of PETN. Semtex H is composed of the same compounds and mixtures as Semtex 1A but it is only 40% PETN and 41% RDX. The version 1AP and 2P act as booster charges. A special assembly of PETN and wax inside these charge provides a high reliability for detonating cord or detonator. Semtex H is intended for explosion hardening.