An Assessment of Printing Methods for Producing Two‐Dimensional Lead‐Free Functional Pyrotechnic Delay‐Lines for Mining Applications

Abstract

AbstractShock‐tube detonators, used in the mining industry to initiate charges of blasting explosives, often contain pyrotechnic delay‐lines for controlling the sequence of blasts. Traditionally, delay‐lines consist of pyrotechnic compounds of toxic metals such as lead which are pressed into the metallic tubing. Since lead is well‐known to be a hazard both to people and the environment, lead‐free alternatives were investigated to create a delay‐line composition. Further, to create delay‐lines cheaply and reproducibly, a composition was developed that could be printed onto a substrate. Screen printing was investigated for this purpose. The pyrotechnic ink that we developed consists of two inert inks (a fuel and an oxidiser) so that it can be stored and transported safely and conveniently without being classified as an energetic. After being transported to its destination, the two inks can be mixed to form an energetic (wet) ink, printed, and used as a delay‐line when dry. A silicon‐bismuth trioxide composition bonded together using nitrocellulose was found to be a suitable ink for screen printing. The burning rate was measured as 52±7 mm s−1 for a thickness of 100 μm. This thickness reduces the raw delay‐line material by over 90 % compared to traditional 3D delay‐lines. The target burning rate was 24 mm/s, but the measured rate is sufficiently close to this that changes in geometry could meet this requirement. The critical thickness for a self‐sustainable burn was measured as 40±5 μm. The thermal output and viscosity of the ink were determined as functions of the particle size. The viscosity of the ink was determined as 7.04±0.05 Pa s, which is suitable for screen printing. Milling was used to reduce the particle size below 1 μm. Ignition of the printed delay‐line using shock tubing was achieved through overprinting with a shock‐sensitive ink. The burning rate of a delay‐line is one of its most defining properties when used in mining applications. Thus the effect on the delay‐line's burning rate of the pyrotechnic composition printed thickness, and the ink solvent was investigated. We found that the burning rate could be modified by changing the silicon content in the pyrotechnic, and to some extent, by adding aluminum. Adding a third, slowly evaporating solvent, to the ink vehicle was found to reduce the critical thickness to less than 40 μm, decrease the burning rate to 46±7 mm s−1, and enhance the ink's flow properties. We found that the width of printed lines did not affect the burning rate for widths between 1–5 mm. This is an interesting result and could be used to determine the most cost‐effective shape to produce printed delay‐lines. Ignition of the printed delay‐line using shock tubing was achieved through overprinting with a shock‐sensitive ink.