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Overview[]
The Contex was a filter that, due to fear of arsenical gases, began to supplement existing GC (general civilian) and CD (civilian duty) filters. Contex was attached to an existing filter via adhesive tape. What separates the Contex from its asbestos-wool predecessors was that the filtering matrix comprised a resinous-wool mixture that functioned by means of an electric charge.
Precursory work[]
In 1919*, Japanese scientist Mototaro Eguchi noted that combinations of resins and waxes, when solidified in an electrical field, produced a material known as an 'Electret' - a material that possesses quasi-permanent electrical charge. Such 'Electrets' were already known to science, but little work had been done.
In the context of respiratory protection, some early work on resin based filters can be seen from a 1927 patent** filed in Germany. This patent regarded the usage of Shellac (a resin produced by lac insects) granules to increase filtration abilities of breathing apparatus. Akin to Eguchi, the patent claims curing resinous material over an electric field produces an Electret that can be used as a filtering medium. This particular filter can be considered a forerunner to filters such as the Contex, but it should be noted that this filter was conceptualised on the usage of resin granules by themselves, rather than a resin-wool mix as seen with Contex.
Perhaps the earliest resin-wool filter can be attributed to Danish inventor Nicolai Hansen, who in 1932, filed a British patent for a smoke filter. Hansen's filter consisted of a resin (Hansens patent refers to several such as Fir resin, Colophonium, or Frankincense) which was subjected to dissolvement (ethyl-aleohol). Once the resin had been suitably dissolved, a 'carrier' (wool) was soaked with the resinous solution and was dried. The resin-wool matrix was characterised by extremely efficient filtering of particulate matter, including arsenical smokes which were considered difficult to satisfactorily filter out. Hansens patent even specifically refers to arsenical smokes, noting that ''the concentration at which these substances are intolerable to inhale are as low as 0.25 to 2 milligrams per cubic meter of air. Therefore the filters intended to clean the air from these substances must be extremely efficient'' Hansens filter differs in that his resin was not subject to any electrical influence and consisted only of dissolved resin dried onto wool fibres.
References[]
* Conflicting sources as to when Eguchi actually made the Electret, sources claim 1919 (www.sciencedirect.com) or 1922 (Scientific America, 1960 issue)
**It should be noted this patent was machine-translated, thus some information may not be accurate due to translator error.
Method for the manufacture of smoke filters (N. Hansen, 1931, 1932)
Verfahren zum Auffangen schwebender fluessiger oder fester Stoffteilchen aus bewegten Gasen (Platen-Munters, 1927, 1931)
Asbestos in World War II Respirator Containers (Porton Down, 1989)