Activated Charcoal Filter Layer for Gas Masks

All conventional gas mask filters consist of a replaceable filter cartridge which contains at least one activated charcoal layer. Activated charcoal for gas mask filters usually has a specific or "internal" surface of 500 to more than 2000 m.sup.2 /g, determined according to the BET method. It is a particular feature of activated charcoal that it can permanently and very unspecifically adsorb a large number of substances in its micropores, which can comprise up to 50% of the total volume. Toxic gases, e.g. HCN, which are only weakly bound by the normal physical adsorption, can be bound using metal compounds, e.g. silver, copper or chromium compounds, which are applied, providing superimposed chemical sorption. The activated charcoal filter layer of gas mask filters is usually formed as a bulk filter, in which the medium to be purified flows through a fixed bed of the activated charcoal particles. In order to guarantee a sufficient period of functioning of the filter, a sufficient amount, i.e. mass of the adsorber material, must be present. At the same time, however, the adsorption kinetics are proportional to the available "external" surface of the particles, so that small particles are advantageous in this connection. In addition, larger activated charcoal particles can often be fully utilized only in their outer areas. These are usually already saturated--requiring an interruption and replacement of the filter cartridge--while the charcoal is only slightly charged on the inside. The use of the smallest possible particles in a bulk filter, however, necessarily leads to a high pressure drop. For practical purposes, the particle size is limited in a downward direction by the pressure drop related to it. A further disadvantage of bulk filters is that abrasion phenomena occur as a result of the activated charcoal particles rubbing against each other, and that the charcoal in powder form increases the flow resistance even more.

In general, the opinion is that good filter performance necessarily requires high flow-through resistance, because only then can there be good contact between the gas to be purified and the adsorber grains. In order to also preclude break-throughs across cavities which form when particles settle, the packing must be firmly compressed. In this way, high flow-through resistance of the activated charcoal bulk filters for gas masks is pre-programmed. But this not only results in the effect of physiological stress on the gas mask wearer, but also increases the feeling of constriction.

It is therefore the object of the present invention to create an activated charcoal filter layer for gas masks, i.e. a gas mask filter, with low flow-through resistance and high adsorption performance.

The solution according to the invention is an activated charcoal filter layer for gas masks with a highly airpermeable, three-dimensional carrier framework essentially stable in shape, formed of wires, monofilaments or stays, to which a layer of granular, particularly spherical activated charcoal particles with a diameter of 0.1 to 1 mm is affixed. The distance between the wires, monofilaments or stays is at least twice as great as the diameter of the activated charcoal particles. Preferably, it is approximately three to ten times as great. Accordingly, the openings or pores of the carrier framework have a diameter of 1 to 5 mm, preferably 1.5 to 2.5 mm. The diameter of the wires of which the carrier framework can be structured is preferably 0.1 to 0.8 mm. If the highly air-permeable, three-dimensional carrier framework, essentially stable in shape, is composed of monofilaments or threads of plastic, glass or liquefied minerals, the diameter is preferably 0.2 to 1 mm.

The invention will be further described with reference to the accompanying drawings, wherein

FIGS. 1 to 5 are views of different carrier frameworks in accordance with the invention, before and after application thereto of adsorber particles.

More specifically, the carrier framework can consist of wire netting or wire mesh with a mesh width of 1 to 5 mm, preferably 1.5 to 2 mm, superimposed in layers, between which spacers with approximately the same length as the diameters indicated are located. Such a carrier framework is shown in FIG. 1, magnified approximately five times, without and with bead charcoal adhering to it.

Another carrier framework which is very useful for the purposes of the invention consists of plastic or wire spirals with a length equal to and up to approximately ten times their diameter. For example, monofilaments of thermoplastic synthetics with a diameter of 0.2 to 1 mm can easily be produced using usual means, e.g. by wrapping them around a mandrel with the desired inclination. The elasticity is sufficient to strip such a spiral from the mandrel by compressing it, after it has solidified and been cut to the desired length. Such carrier structures in the shape of a spiral generally have a diameter of 0.3 to 10 mm and a length equal to and up to approximately ten times that. Such a carrier structure, without and with charcoal particles affixed to it, is shown in FIG. 2, magnified approximately three times.

Wires or monofilaments with a length of 2 to 10 mm, which are held together by twisted metal wires or plastic monofilaments and then project perpendicular from this axis in the shape of a spiral staircase, are also very useful carrier structures for the purposes of the invention. Such structures are produced in various sizes as test tube or bottle brushes. For the purposes of the invention, they are then cut to a length of several centimeters, e.g. 1 to 10 cm. Filter elements made of this carrier structure are shown in FIG. 3.

Furthermore, tree-shaped fiber brushes are commercially available, whose fibers, with a length of 1 to 10 mm, radiate out from a center point and are generally attached on a yarn or monofilament, from which they can be removed after being charged with the activated charcoal particles, e.g. with a suede cutting machine, for the purposes of the invention. Such carrier structures are shown in FIG. 4.

A preferred embodiment of the carrier framework is a large-pore reticulated PUR foam with a liter weight of 20 to 60 g and pores with a diameter of 1.5 to 4 mm. Such foams are produced in known manner by first evacuating a large open-pore foam block located in a closed chamber. Then an explosive gas mixture is allowed to flow in and is then ignited. The cell walls are destroyed by the explosion and melt to form stays. Therefore reticulated foams do not have walls, but rather consist of a grid of stays, which form cages with a diameter of about 1 to 5 mm. These "foams" are elastic, can be easily compressed by hand, even at a thickness of several centimeters, and then resume their original shape. When they are completely covered with activated charcoal beads, as shown in FIG. 5, they are relatively rigid and can no longer be compressed with the same amount of force.

The reticulated foams can already possess the optimal thickness required for production or for use as an activated charcoal filter layer. In spite of the thickness required, i.e. several centimeters, the open-pore structure permits them to be coated with activated charcoal beads or grains also on the inside, and they can have the desired shape of the activated charcoal filter layer of a gas mask directly, or this can be punched out of a larger plate. It is also possible to subsequently cut up these plates to form elementary filters in the form of strips or chips, with a size of approximately a few cubic centimeters. This results in complete independence from the shape of the objects to be filled, as is also the case for the spirals, fiber brushes or "bottle brushes" described as carrier structures. The various elementary filters can be placed in the cavities to be filled, together with heterophilic fibers or threads of hot-melt glue. The entire assembly can be solidified after filling, so that even under great mechanical stress, there is no risk of settling or abrasion, as is the case with bulk filters.

The possibilities of carrier structures for gas mask filters as described are not to be viewed as limiting. A person skilled in the art can easily think of other carrier frameworks which can be produced with the known methods of metal or plastics processing, in a cost-effective manner, and which might even occur as waste products. The highly air-permeable three-dimensional carrier frameworks are supposed to be essentially stable in shape on the basis of the thickness or the strength of the wires, monofilaments or threads of which they are composed, i.e. they should not simply collapse, but can certainly be elastic, so that they can be compressed to a certain extent, but then resume their original shape. When the granular, especially spherical activated charcoal particles are affixed to them and these carrier frameworks are preferably completely covered with the activated charcoal particles, the rigidity is increased and the filter elements, i.e. the entire activated charcoal filter layers, are then relatively rigid, pressure-resistant structures.

Depending on the material of which the carrier framework is formed, the activated charcoal particles can be affixed to it directly, or an adhesive mass is required. Plastic materials, particularly fiber materials, are commercially available, which have the property of first becoming sticky on the surface at an elevated temperature, within a certain temperature interval, without melting. This property, which could be designated as a built-in hotmelt glue, can be utilized to affix the activated charcoal particles to them. Such fibers can be heterophilic fibers made of two coaxially arranged components, the outer one of which demonstrates a lower melting point. Unstretched amorphous polyester fibers which become soft and sticky at approximately 80 to 85.C, without melting, and subsequently crystallize at higher temperatures and then have the thermal stability of a normal polyester fiber, are also suitable. Such fibers with adsorbents affixed to them are described for textile surface filters, for example in DE-AS-32 00 959.

Another possibility preferred for the purposes of the invention is to affix the activated charcoal particles to the carrier framework with an adhesive mass. With this alternative, a person skilled in the art has a greater choice with regard to the material of which the carrier framework is made, as well as with regard to the adhesive mass.

With both possibilities, the diameter of the wires, monofilaments or stays of the surface structure alone or with the adhesive mass is dimensioned in such a way that complete coverage of the carrier structure with the activated charcoal particles is possible, in order to produce a carrier framework, i.e. a filter element completely covered with the activated charcoal particles, in a preferred embodiment of the invention.

An activated charcoal filter layer according to the invention can also be produced in the following manner: The activated charcoal particles are preferably affixed continuously on the fibers or monofilaments which become sticky on the surface when heated as described, or on continuous wires or monofilaments which have been coated with an adhesive mass, and these carriers charged with bead charcoal are cut up into small pieces with a length of 0.5 to 3 cm. These carriers can, in addition to being straight, also be sine-shaped or zigzag-shaped, in order to result in an even more open structure. When cut up, or later, and possibly in combination with heterophilic fibers or threads of hot-melt glue, an activated charcoal filter layer with the desired thickness can be formed as an irregular structure of the wires or monofilaments coated with the activated charcoal, and preferably be compacted or welded in a mold or in the filter cartridge, possibly by being heated.

In order to fix the activated charcoal particles on the carrier, both inorganic and organic adhesive systems can be used. The latter include polymers, particularly acrylic acid derivatives, polyurethanes, polystyrenes, polyvinyl acetates as well as hot-melt glues. Those masses which consist of polymers which can be cross-linked, which pass through a viscosity minimum before being cross-linked, are preferred. Such adhesive systems, such as IMPRANIL(.RTM.)-High-Solid-PUR reactive products from BAYER(.RTM.), are highly viscous at first, i.e. they offer good initial adhesion when the carrier framework is being covered with the activated charcoal particles. With an increase in temperature, they demonstrate a great decrease in viscosity, which results in better wetting of the activated charcoal particles and therefore especially good adhesion after hardening, due to cross-linking. When the viscosity minimum is reached, small constrictions form at the contact sites between the carrier framework and the activated charcoal particles, due to capillary forces. Because the activated charcoal beads are practically attached at only one point, almost their entire surface is accessible to the gas to be cleaned after hardening.

When using metallic or ceramic carriers, adhesive masses of enamel or glazes can be used; in this case, the work has to be carried out in an inert atmosphere, due to the high temperatures required to melt these coatings, so that the effectiveness of the activated charcoal particles is not impaired or destroyed by oxidation.

The activated charcoal particles must be pourable and abrasion-resistant. It is most practical if their diameter is three to five times smaller than the diameter of the pores or openings of the highly air-permeable carrier framework. Commercially available activated charcoal beads with a diameter of 0.1 to 1 mm are not only the most easily pourable form, but also withstand the greatest stress, due to their symmetry. Granular activated charcoal particles are also suitable, however, as long as they are not too angular or too irregular in their shape, because it is important that the activated charcoal particles can still penetrate into structures with a thickness of several centimeters when they are affixed on their surface structure.

It is certainly possible to produce activated charcoal filter layers directly in the thickness usual for gas masks.

Activated charcoal particles suitable for gas mask filters should have an internal surface of 600 to 2000 m.sup.2 /g, preferably 1000 to 1600 m.sup.2 /g, determined according to the BET method. The activated charcoal particles should be very pressure-resistant and preferably highly resistant to moisture. A very abrasion-resistant spherical activated charcoal can be produced, for example, from coal tar pitch or petroleum distillation residues. Additional hardening of the surface as well as noteworthy moisture resistance can be achieved with special post-treatment. The production of suitable activated charcoal beads is described, for example, in EP-B-118 618, DE-B-29 32 571 and DE-A-30 41 115.

In order to increase the abrasion resistance, the activated charcoal can also be impregnated at its surface in a plastic dispersion or a coal tar pitch solution or bitumen solution, and subjected to slight post-activation. The sensitivity with regard to steam can be significantly reduced by adding ammonia gas during post-activation and cooling to 100.degree. C. with exclusion of air.

The activated charcoal particles can contain metal compounds, particularly compounds of the metals silver, copper and chromium. In addition, encapsulated enzymes which decompose poisons, as described in EP-B-118 618, can also be present.

With the filters described, excellent separation effects for pollutants and gases were achieved at extremely low pressure drops. It was surprisingly shown that it is not necessary for flow to go through the activated charcoal grains, but rather only past them, in order to achieve high effectiveness with a low pressure drop. The Brownian motion of the gas molecules is sufficient to achieve a high adsorption velocity. The pressure drop of an, activated charcoal filter layer according to the invention is less than 10 mm, preferably less than 5, and especially less than 2 mm water column at a thickness of 4 cm with a circular cross-section of 100 cm.sup.2 when an air flow of one liter per second flows through it. A loosened activated charcoal filter layer according to the invention has a greater volume than a bulk filter, with the same performance, but significantly lower flow-through resistance. The amount of activated charcoal, 100 g, which is usual for a gas mask today, can be contained in a volume of approximately 350 ml with the carrier structures according to the invention.

Because of the varied possibilities of structuring the filter material according to the invention, the shape of the gas mask filter can also be adapted to the most varied needs. For example, the filter can certainly be housed in a hood mask, e.g. around the head or at the neck, and then serves as additional head or neck protection against impacts. In this case, the filtered air should flow past the eyes, in order to prevent fogging of the visor window. A plate-shaped filter can be worn on the chest or back and connected with the mask element by way of a flexible hose. Cylindrical filter elements with a diameter of several centimeters can also be housed directly in a flexible hose, or be coupled together to form a hose, using suitable means. Such replaceable filter elements can also have various functions. It is most practical if the inlet opening of the hoses containing the filter elements or comprised of them is located on the inside of a protective suit.

Ten wire grids, 50.times.60 cm, mesh width 2 mm, wire thickness 0.30 mm, were formed into a package with a height of approximately 4 cm using steel tubes with a thickness of 2 mm as spacers, and dipped into a mixture consisting of a "masked" pre-polymer polyurethane (Bayer HS 62), an emulsifier (Bayer HS DW), a cross-linker (Bayer HS C) and water. The viscosity (approximately 2000 mP.s) was adjusted in such a way that it was easy to shake off the excess adhesive and the three-dimensional framework was coated with 440 g adhesive. This was placed in a large bowl and activated charcoal in spherical form (diameter 0.3 -0.6 mm) was poured over it. Approximately 3 kg adsorber remained in the framework. The adhesive layer was crosslinked by increasing the temperature to 170.degree. C. The activated charcoal beads were fixed very firmly, but only at one point, on the carrier framework. In order to measure the air permeability, an area of approximately 100 cm.sup.2, sealed laterally, was subjected to an air flow of 1 1/sec. The pressure drop was less than 2 mm water column. 120 mg CCl.sub.4 per liter was also added to the air flow. After passing through the filter material, the CCl.sub.4 concentration was less than 1 mg/l. Only after adsorption of almost 50 g CCl.sub.4 did break-throughs start to be imminent.

The carrier framework used was a large-pore, reticulated polyurethane foam, 4 cm thick, with a weight of 80 g/l and a pore diameter of 2.5-3 mm. This foam was squeezed with the same adhesive mass as in Example 1 (adhesive application, dry 55 g/l), charged with activated charcoal beads as in Example 1 (approximately 260 g/l) and finally heated to 170.degree. C. for a short time. This resulted in a filter material with a very slight pressure drop (approximately 1 mm water column at 1 1/sec on 100 cm.sup.2). The adsorptive properties approximately corresponded to those of Example 1.

The same carrier framework as in Example 1 was coated with the slip of an enamel with a low melting point, and activated charcoal beads were made to adhere on this. After drying, the filter was subjected to hardening at 650.degree. C., in a nitrogen atmosphere. Such a filter is especially suitable for high operating temperatures and aggressive substances.

It will be understood that the specification and examples are illustrative but not limitative of the present invention and that other embodiments within the spirit and scope of the invention will suggest themselves to those skilled in the art.