SMOKE FILTERS
The first types of the Standard Box Respirator contained cotton pads,
which sufficed to remove the ordinary smoke of the battlefield and even
that from the earlier toxic materials. Improved methods of producing
toxic smokes, by means of which smaller particles were obtained, led,
early in 1918, to the recognition of the need of improved protection
against these smokes. The first attempts to meet this need consisted
in improving the filtering qualities of these pads. It was soon found,
however, that to make better filter pads would greatly increase the
total resistance of the canister. This was highly undesirable, since
the resistance of the ordinary canister was already so high as to be
very uncomfortable. To overcome this objection, some of the early
designs of filter canisters were provided with a mechanical valve,
which could be operated by hand, to by-pass the air around the filter
when the canister was used against gas alone, or so set as to make the
air pass through the filter when smoke was feared. This introduced a
factor of uncertainty among the men during a gas attack, since each man
must decide for himself whether smoke was present. This reason alone
was sufficient for discarding this design.
A preliminary study of the situation indicated that any filter for fine
smoke particles must have a high resistance per unit of area, but that
the total resistance must be comparatively low. In order to secure
the large area necessary to bring the total resistance within reason,
the experimental work was developed along three lines: The formation
of a filter into a bag, cup, or jacket to surround the outside of
the canister; the use of an arrangement sufficiently compact to go
inside the canister; and the use of a filter as a separate unit, to be
attached to the canister by an air connection.
A survey of the possible filtering materials indicated that only two
offered promise, namely, paper and felt.
PAPER FILTERS
Reports that the British had developed thin, creped, sulfite-cellulose
wood pulp paper for filters led to an intensive study of this material
by the Chemical Warfare Service.
[Illustration: FIG. 104.—Crepe Paper Doughnut Filter Canister.]
In general we may say that the development of paper filters (in sheet
form) met with little success. Papers affording the required protection
did not live up to the resistance specifications. The reason for this
probably is in the method of making paper. The pulp is fed onto the
screen of a Fourdrinier machine under conditions that do not permit of
uniformity in the distribution of the fibers and consequently there is
no uniformity in the size of pores. In order to eliminate the large
holes, which allow the smoke to pass readily, the paper must be pressed
to reduce these pores to the proper magnitude. This naturally results
in an approximately equal decrease in the size of the small pores, with
a consequent increase in the final resistance out of all proportion to
the protection gained. A very satisfactory paper was finally produced,
but the resistance was too high and it was necessary to increase the
total available filtering area, which resulted in the accordion type
of filter. This filter was incapable of development on a large scale
because of the large amount of hand work required in assembling. The
lack of uniformity in a single sheet has been overcome with some
success by making up a filter from 40 to 80 layers of tissue or crepe
paper, trusting that the law of chance would bring the large pores in
some successive layer. Such a filter was adopted by the British, but
since it did not give protection comparable with that afforded by felt
filters, it was rejected in the United States.
In the so-called “doughnut” filter use was made of tissue paper.
Instead of seeking for uniformity in a vertical direction through a
block of tissues, it was sought along the axis horizontal with the
sheet. The effectiveness of such a filter was less than that of felt.
In addition, serious difficulty was met in cutting the pile of tissue
paper into the proper shape so that eventually it was abandoned as a
production possibility.
FELT FILTERS
Work on the felt filters started about June, 1918. Great difficulties
were met in the beginning, as a felt satisfactory for this purpose must
be made under carefully controlled conditions and production conditions
during the war did not readily lend themselves to such control.
However, the opportunities afforded in felt making for uniform packing
and arranging of the fibers (the whole process of making a felt, is
one of gradual packing of fibers into a relatively small volume) are
such as to assure a greater degree of success than is the case in paper
making.
Very successful filters have been obtained with the use of felt. There
are two serious objections to its use, however. The first is the
great cost of the filter (this was above one dollar per filter at the
close of the War); the second is that felt is a valuable industrial
commodity. It is thus very desirable that a cheaper and a less
important industrial material be found.
THE 1919 CANISTER
Just before the Armistice, the Gas Defense Long Island Laboratory
brought out the so-called “1919 Canister,” which consisted of an oval
section, perforated metal, war gas material container with a central,
flat, perforated breathing tube connected to a nozzle at one end. (See
also page 228.) After this inner container is packed with the war gas
chemicals, a filter jacket is slipped over it and the top edge sealed
to the inner container.
[Illustration: FIG. 105.—1919 Felt Filter Canister.]
Attempts were made to put paper filters on this canister by wrapping
it with layers of paper. In some cases, layers of coarse burlap or
mosquito netting were applied between the layers of paper to give
mechanical strength and air space. The fact that many filters gave good
protection showed that a filter of this type and material is possible,
but the operations of wrapping and sealing require careful work in
production and inspection and even with the greatest skill and care,
imperfections are almost impossible to avoid. This chance of defects,
together with the labor involved, makes the process undesirable.
A THEORY OF SMOKE FILTERS
Tolman, Wells and Gerke, during the course of their work on toxic
smokes, developed the following theory of smoke filters.
The phenomena occurring in the filtration of smoke are exceedingly
complicated, but the general nature of the process may be simply
described in terms of the kinetic properties of the small particles
comprising the smoke.
A filter may be regarded as a series of minute capillaries through
which the smoke slowly flows. In order that filtration may take place,
it is not necessary to assume that the capillaries of the filter are
smaller than the particle, for the particles may diffuse to the walls
of the capillaries and it is believed that with typical filters this is
the actual method of smoke removal for particles less than 10⁻⁴ cm. in
diameter.
In accordance with this view as to the nature of smoke filtration,
the important factors involved are (1) the Brownian motion of the
smoke particles, (2) the area and arrangement of the internal surface
presented by the filter, (3) the flow of the smoke as a whole, and
(4) the attractive forces between the filter surfaces and the smoke
particles. The first three of these factors determine how many
particles come within the range of the mutual forces of the particle
and filter surface, and the fourth factor determines the chance or
expectation that the particle will permanently adhere to the surface of
the filter.
TESTING SMOKE FILTERS
All the early tests made on smoke filters used diphenylchloroarsine,
because it was felt that the filter must be tested against a toxic
smoke. A man test was developed as representative as possible of actual
conditions in the field, and the time necessary for a man to detect
diphenylchloroarsine smoke in the effluent stream when breathing at
a normal rate, using a carefully controlled concentration of smoke
produced by detonation, was used as the criterion of the protection
offered by the canister. This test was subject to extensive individual
variations, due to the varying physiological resistances of different
men to diphenylchloroarsine smoke. Further, it was quite inadequate for
rapid testing on a large scale. A testing machine was then developed,
which gave results comparable with those obtained in the man test. The
method used in detecting the gas was physiological, that is, by smell
or by its irritating action towards the membranes of the eye. While
these are purely qualitative tests, they are much more sensitive than
any possible chemical tests.
Because of the desirability of having a method which could be
controlled chemically, other methods were developed.
Ammonium chloride is a solid smoke, consisting of particles of quite
variable sizes. It is sensitive to dilution and clogs the pores of the
filtering medium quite rapidly. For this reason it was used in the
study of the rate of plugging or clogging of the filter (the closing of
the pores of the fabric or other material to the passage of air).
The smoke is produced by the reaction of ammonia and hydrogen
chloride-air streams. The smoke thus generated is passed from the
mixing chamber to a larger distribution box and from there through
the filter, at a standard rate. The concentration of the smoke may
be accurately determined by chemical means or photometrically, using
a Hess-Ives Tint Photometer, the Marten Photometer, or a special
photometer developed by the Chemical Warfare Service.
A comparison of a large number of tests with those of other smokes
would indicate that ammonium chloride smoke offers accurate information
as to protection sought, but is hardly a desirable smoke for testing on
a large scale.
The third method developed was the sulfuric acid smoke. This smoke was
produced by passing dry air through a tower of solid pieces of sulfur
trioxide and then mixing the vapor with a large volume of air at 50 per
cent relative humidity. It is not a clogging smoke and the filtering
efficiency does not change materially in the time of exposure required
for a test. The smoke lends itself easily to chemical analysis and
offers data as to exact particulate cloud concentrations which will
penetrate canisters; photometric measurements are also applicable.
[Illustration: FIG. 106.—Tobacco Smoke Apparatus for Testing Canisters.]
The fourth method consists in the use of tobacco smoke. This is
generated by passing air over ignited sticks of a mixture of tobacco
(63 per cent), rosin (30 per cent) and potassium nitrate (7 per cent).
This smoke is composed of particles of extreme uniformity in size;
chemically it is relatively inert. It is not a clogging smoke and is
not sensitive to moisture and dilution. The density of the effluent
smoke is compared with that of the entering smoke in a Tyndall beam,
and the filtering capacity of the material determined in terms of
the amount of air necessary to dilute the entering air to the same
concentration of the effluent air. The method is simple in manipulation
and the test is a rapid one (50 canisters per day). Because of
the apparent superiority of tobacco smoke as a testing smoke, the
accompanying disadvantages are possibly outweighed.
From the standpoint of inherent chemical properties, the general
desirability of a suitable testing smoke would decrease in the
following order: tobacco, sulfuric acid, ammonium chloride.