Air Filter of Polyurethane Mesh Containing Carbon Adsorbent

This invention relates to an air-cleaning filter element prepared by bonding an adsorbent material to and between a plurality of elastic-flexible webs with an adhesive agent, which has the inherent adsorptive performance of said adsorbent.

Heretofore, as filter materials for air cleaner and other uses, adsorbent materials, e.g. activated carbon have been commonly used as packed in plate-shaped or sigzag-shaped filter housings but the adsorbents tend to give dust on vibration or friction of the housings in the course of transportation or use, thereby soiling the housings. There also are generated gaps in the carbon layer and the resultant "short-cuts" of the fluid to be treated resulted in the failure of the filter to display its expected cleaning performance.

To overcome the above disadvantages, various methods have heretofore been proposed for molding an adsorbent with a binding agent to provide a dimensionally stable adsorbent product. Recently, it has been proposed to place a granular adsorbent in a nonwoven fabric case and seal the case to provide a sealed filter element, or to retain a side of an adsorbent-packed filter element with a spring member so as to prevent generation of gaps.

However, the binding agents herefore used are disadvantageous in that the solvents contained in such binders tend to drastically reduce the adsorptive capacity of the adsorbent or cause a considerable pressure drop on passage of the fluid. Or the filter housing is of necessity complicated in structure. Thus, the prior art devices have not provided fully satisfactory results.

In view of the foregoing disadvantages, the present inventors conducted intensive studies on the web to be used, the type of binder, molding conditions, etc. and have perfected a method of producing an air-cleaning filter element which is free from a deterioration of the adsorbent material and any significant fluid pressure drop on passage of the fluid.

This invention is therefore concerned with an air-cleaning filter element prepared by applying an adhesive agent to and between at least a pair of three-dimensionally mesh-structured elastic-flexible webs, disposing an adsorbent material, such as activated carbon, in a size range of 0.5 to 10 mm, preferably 1 to 5 mm in diameter, between said elastic-flexible webs and pressing the webs together as superimposed.

The mesh-structured elastic-flexible web to be employed has a three-dimensional skeletal structure which can be attained by selective hydrolysis of a polyurethane foam and which contains 9 to 31 air spaces (hereinafter referred to as cells) per 25 linear millimeters, the number or density of such cells being selected according to the grain size of the activated carbon or other absorbent employed. Thus, for example, a web having 9 cells/25 linear mm is preferred for the adsorbent having a grain size of 4 to 5 mm and a web having 31 cells/25 mm for non-uniform irregularly shaped adsorbent grains of the order of 1 mm.

As regards the thickness of said mesh-structured elastic-flexible web, a web material of the described type may be sliced or otherwise processed to a thickness suitable for the grain size of carbon to be bonded thereto and the intended application. An appropriate thickness can be selected within the range of 3 to 10 mm.

The adsorbent employed according to this invention may for example be activated carbon, active clay, acid clay, silica, alumina or any of molded or crushed metal oxide catalysts, or a mixture of two or more of such materials, although activated carbon is particularly advantageous. The activated carbon may be one based on coal, petroleum oil distillation residue, wood, fruit shell or the like, provided only that it has a BET specific surface area of 500 to 2000 m.sup.2 /gram. The adsorbent is normally used as cylindrical, spherical or irregular-shaped grains from 0.5 mm to 10 mm, preferably from 1 mm to 5 mm in diameter.

The adhesive agent employed according to this invention may be of any type only if it is able to secure said adsorbent in position on said mesh-structured elastic and flexible web. Thus, solvent-sensitive adhesives, heat-sensitive adhesives and reaction-sensitive adhesives may be mentioned as examples, although solvent-sensitive adhesives are particularly desirable. As examples of solvent sensitive adhesives which are preferably used in the present invention there are mentioned various synthetic rubber latices which are aqueous emulsion of synthetic rubber of 0.03.mu. to 1.5.mu. in particle size. The examples of the solid body component of the latices are described below.

(1) Butadiene polymers, or copolymers from butadiene and styrene, styrene derivatives, acrylonitrile, methacrylonitrile, isoprene, isobutylene, etc.

(2) Copolymers from isoprene and styrene, styrene derivatives, etc.

(3) Chloroprene polymers, or copolymers from chloroprene and styrene, styrene derivatives, acrylonitrile, isoprene, etc.

(4) Copolymers from acrylates and styrene, styrene derivatives, vinyl chloride, vinyl acetate, acrylonitrile and methacrylate.

(5) Methacrylonitrile polymers, and copolymers from methacrylonitrile and styrene, etc.

(6) Vinyl acetate polymers and vinyl chloride polymers.

Also usable are the above latices having been carboxy-modified or suitably modified in other ways.

These latices may be an aqueous emulsion obtained mainly from emulsion polymerization, and are used, with the resin solid content adjusted to 20 to 50% and preferably, 35 to 45%.

The present air-cleaning filter element can be prepared by the steps of applying said adhesive agent to mesh-structured elastic-flexible webs, disposing the adsorbent over the coated surfaces of said webs and bonding the webs together by the applicaation of pressure.

The application of the adhesive agent to elastic-flexible webs can be accomplished by direct brushing, roller or calender-coating, spray-coating or any other suitable procldure. It is, however, more desirable to take the steps of coating a release paper with the adhesive by brushing, roller-coating, spray-coating or the like, pressing the paper against the elastic-flexible web in face-to-face relationship and, then, peeling off the release paper to let the adhesive be transferred from the release paper to the surface of the elastic-flexible web.

While the amount of adhesive should vary with its type, the preferred amount as the solid matter thereof is 5 g. to 550 g. per square meter of web, preferably, 100 g. to 500 g. or, for still better results, 250 g. to 500 g. per m.sup.2.

With a smaller amount of adhesive, the carbon cannot be effectively locked in position on the web, and with an excess of adhesive, the open cells of webs are covered with the adhesive to cause an undesirably great pressure drop on passage of fluid.

When the adhesive contains a solvent, the solvent should be evaporated off prior to deposition of the adsorbent, for otherwise the solvent will be adsorbed on the adhesive to interfere with the latter's adsorptive ability. When the application of adhesive to the elastic-flexible web is performed through a release paper, the evaporation of solvent may take place after the application of adhesive to the release paper and before the pressing of the paper against the web, or after joining, i.e. before peeling-off or thereafter. The solvent evaporation may be accomplished either by allowng the material to stand at ambient temperature and pressure of, if necessary, at elevated temperature and/or pressure. The deposition of the adsorbent on the elastic-flexible web may be accomplished by a suitable procedure, e.g. by hand or by mechnical vibrations. While the deposition amount of adsorbent is optional according to the intended application, it is normally 200 to 2500 grams per square meter of the web.

The bonding of carbon to the adhesive-coated webs can be accomplished by supporting the webs between wood, plastic, metal or other plates and applying an external load of 0.01 to 0.02 kg/cm.sup.2.

There are cases in which the edges of thus-formed filter element are heat-sealed.

The air-cleaning filter element according to the present invention may be one prepared by using not less than 3 sheets of elastic-flexible webs. For example, in case of using 5 sheets of webs (A), (B), (C), (D) and (E) are superimposed in the order mentioned, adsorbent is fixed in each space between webs, which are neighboring each other, i.e. the space between webs (A) and (B), the space between webs (B) and (C), the space between webs (C) and (D), and the space between webs (D) and (E). The adsorbent in each space of the webs is bonded to two webs existing both sides of the adsorbent.

The folowing examples are further illustrative of this invention.

An elastic-flexible web was applied by brushes with an adhesive agent at an application rate as resin solids of 350 g/m.sup.2, followed by scattering uniformly activated carbon on the surface at a rate of 3 l/m.sup.2, then overlapped by the elastic-flexible web applied with an adhesive agent in the similar manner, and put between plates to be allowed to stand at ambient temperature for 10 hours under load of 0.01 kg/cm.sup.2 applied. Among the materials hereby used, elastic-flexible web was one having a size [15 cm.times.30 cm.times.5 mm (thickness)] and a standard cell number of 9 pieces/25 mm, while the activated carbon was cylindrical-formed one showing 1180 m.sup.2 /g in BET specific surface area, 4 to 6 mesh in grain size (4 mm in average particle size), and 450 g/l in packing density, with the adhesive agent as indicated in Table 1.

With the filter materials produced in this way, performance testing was carried out in accordance with the following methods, along with the obtained results shown in Table 1:

1. Pressure loss

The filter element was out into a disc 66 mm across and placed in a column of the same inside diameter. Dry air at room temperature was introduced into the column at a linear flow rate of 0.5 to 2 m/sec. and the linear flow rate of the effluent air was measured with an electronic anemometer for pressure drop determination.

2. Acetone adsorption rate

The filter element was cut into a disc 66 mm across and fitted with an adsorption column. A gas containing about 1.0 vol. % of acetone in a gas holder was circulated to the adsorption column at a linear flow rate of 15 cm/sec. and the adsorption rate was determined from the time when the acetone concentration had reached one-tenth of its initial concentration.

3. Acetone equilibrial adsorption

The filter element was cut into a disc-shaped piece 66 mm across and, in accordance with JIS K-1474, the equilibrial adsorption of acetone with an initial concentration of 1000 ppm was determined.

By varying the application amount of a synthetic rubber latex (carboxy-modified SBR latex), in the same procedure as described in Example 1, filter materials were produced to carry out the performance testing as mentioned in Example 1, to obtain the results shown in Table 3.

In accordance with the same procedure as described in Example 1, and by crushing the cylindrical-formed activated carbon as used in Example 1 to a uniform grain size of 24 to 48 mesh (average particle size of 0.42 mm), by the use of a carboxy-modified SBR latex as an adhesive agent and an elastic-flexible web of 50 pieces/25 mm in standard cell number as a mesh-structured elastic-flexible one, with the application amount of the adhesive agent of 400 g (solids)/m.sup.2, a filter was produced to determine the pressure drop and equilibrium adsorption quantity for acetone, as being indicated in the following:

Pressure loss;

(at a flow rate of 1 m/sec. as a linear flow rate) 75.6 mm aq.

Acetone equilibrial adsorption; (2.0 g/100 cm.sup.2 -filter)

The drawings show the embodiments of the air filter of the present invention. FIG. 1 is a partially exploded perspective view of the three-dimensionally mesh-structured elastic-flexible web. FIG. 2 and FIG. 3 are partially exploded perspective views of the air filter of the present invention, respectively. FIG. 2 is directed to the present air filter comprising two sheets of the three-dimensionally mesh-structured elastic-flexible web and one layer of activated carbon existing between the two sheets of web. FIG. 3 is directed to the present air filter comprising five sheets of web and four layers of activated carbon, each layer existing between one sheet of web and another sheet of web.

1: three-dimensionally mesh-structured elastic-flexible web.

2: activated carbon

3: the present air filter