Name: Sensitive Bridgeless E-match
Source: Alan Yates
| percent | parts | component |
|---|---|---|
| 77.78% | 7 | Potassium Chlorate |
| 11.11% | 1 | Antimony Trisulfide |
| 11.11% | 1 | Lampblack (conductive) |
Using 1 part to 3 grams makes a good quantity which can be kept in a small (~75 ml) polypropylene jar ready for dipping countless e-match heads.
Pre-mill and screen the Potassium Chlorate and Antimony Trisulfide separately. There will be no need to screen the Lampblack, and doing so will just make a huge mess.
Place Lampblack and Antimony Trisulfide in a jar and add sufficient Acetone to make into a thin paste. Add the Potassium Chlorate and mix well. Add one part syrupy nitrocellulose lacquer and mix until smooth.
Use a hot water bath or allow to stand and evaporate if the mixture is too thin, add Acetone if it is too thick. Tuning the viscosity for dipping is a matter trial and error.
This is a sensitive composition and must be treated with respect. It is relatively friction and shock sensitive. Don't mix it dry, while the composition could be diapered there is no need, the final product is a paste.
Typical dipped-wire e-match geometry is 1 mm spacing, 5 mm long. Resistance is around 3 Ohms. It will fire instantly from a 9V battery and take 50 mA continuously without ignition.
This sensitivity is generally on-par with or slightly less than conventional bridged e-matches. "High current" e-matches for use with mains power or beefy capacitive discharge circuits are better made using my insensitive bridgeless pyrogen. It offers a larger safety margin and won't explode violently when initiated with a CD unit as these e-matches sometimes do.
Coating over the e-match with a buffer layer and a final thick NC lacquer dip is highly recommended for production use. Even ping-pong ball "NC lacquer" is OK to use for binding and sealing, although it does desensitise the e-match somewhat. Aqueous binding works just as well but it takes much longer to dry. 5% Dextrin is more than sufficient, less than 3% CMC or SGRS should work too, but I've only tried Dextrin.
Sulfur can substitute the Antimony Trisulfide with a noticeable increase in sensitivity (of all kinds, especially shock). The Sulfur used should be acid-free, do not use "Flowers of Sulfur". A 3% Calcium Carbonate buffer is highly recommended if you make this modification.
Iron Sulfide might be a better substitution if you have no Antimony Trisulfide, prepare some by igniting Iron powder and Sulfur (about 56:32) in a crucible. It has about double the melting point which will make it less effective, Bismuth Trisulfide might be worth a try instead (81:19), or simply get elemental Antimony and prepare the Trisulfide from that (72:28). Using excess sulfur (e.g. 50:50 mixes) in these preparations and burning it off (outside!) is an simple and expedient production method.
A pinch of Manganese Dioxide can be added to further sensitise the composition. About 1% by mass is plenty, it lowers the ignition temperature by as much as 100 K. The shock sensitivity seems more enhanced than the friction sensitivity by this modification, it is only required if you experience problems or want a very sensitive e-match for use with weak firing systems.