Field

This application relates to air purification, especially in respirators, more particularly to monolithic carbonaceous adsorbents and to a process for preparing the adsorbents. Background

Exposure of civilians or military personnel and first responders to toxic gases and vapors without proper respiratory protection can be harmful, incapacitating and/or potentially lethal depending on the type and amount of substance(s) and duration of exposure. Efficient respiratory protection against such hazards is a problem which requires equipment such as air-purifying respirators designed to reduce or eliminate the inhalation exposure to harmful agents. Important factors in the performance of such air-purifying respirators include efficiency of removal of toxic agents, capacity to retain agents after removal, effective performance against a broad range of toxic agents, or design specific to particular agents, breathing resistance, size, weight and shape. All the preceding factors can influence the effectiveness of respiratory protection against toxic agents, and the ability of the wearer to perform useful or necessary functions while wearing the equipment.

Adsorbent materials housed in a filtering device attached to a close-fitting face mask provide protection from a variety of gases and vapors for a limited time. The adsorbents presently used in respiratory protection primarily consist of granular activated carbon packed at maximum packing density into a container forming a bed of adsorbent. The adsorbent bed must be immobilized in its final form by mechanical means, such as fixing a top and bottom plate in place under compression, with side walls providing the support for the top and bottom plates.

The granular adsorbents are characterized by a large internal surface and small pores. Manufacture involves the carbonization of an organic raw material, which in the case of respirator adsorbents is commonly derived from coal or natural cellulosic materials such as coconut shell. Following carbonization, the material is crushed and sized to select a distribution of granule sizes. The carbonized material is then activated, which is a process of modifying the material by application of heat, steam, and other chemicals or gases to increase the internal surface area. Some adsorbent materials are modified by post-activation addition of inorganic and organic materials called impregnants, intended to augment the adsorptive capacity of the carbon. This is done to improve the adsorption of toxic substances or initiate the chemical decomposition of some highly toxic gases to less toxic products. Physical adsorption of these products and other harmful gases is accomplished by the adsorbent material. The current activated carbons used in respiratory protection are produced by carbonization of raw materials such as wood, coal and coconut shells at temperatures of at least 800°C in the absence of oxygen, followed by chemical or physical activation of the carbonized product. These materials show good adsorbent properties attributed mostly to their high specific surface area of 800-1500 ^' m ² /g which is contained predominantly within micropores. However, they are characterized by some major disadvantages. The adsorbent bed must be made in a housing under compression to maintain the packing density and shape and the granules in the housing must be immobile with the aid of compressing the housing. In other words, significant change to the housing such as rough handling, or a temperature increase resulting in expansion, would reduce the performances of the adsorbent bed due to air penetration through mobile granules resulting in less adsorbing interaction with micropores. The possible friction between granules would also give fine powdery carbon causing malfunction of the respirator. It is difficult to economically design an adsorbent structure which could be integrated into a respiratory mask instead of protruding in one direction. It is also of concern that the consistency of the end product depends on the natural source of the raw materials. The adsorbing capacity would vary due to the difference of raw materials such as coal and coconut shell in terms of texture or ingredients.

Carbon structures have been prepared differently in prior art. Generally, the methods fall into two kinds. The first kind is to bind a carbonized porous material using binding materials such as polymer. A drawback is that the binder could block the pores which are essential to all applications. If the binding is followed by a second carbonization, the whole process is multistep, which is usually costly. As well, some adsorbing sites are not able to be accessed due to packing and molding. The second method involves mechanical forces such as pressure or extrusion to make a carbon structure. The voids between beads or granules are reduced due to compression. This is required for gas storage or other applications that need compact packing to increase the adsorbing capacity per unit volume. For respiratory protective application, a low breathing resistance or pressure drop is critical while maintaining the adsorbing capability by having a reasonably high specific surface area and bulk density. A new generation of respirator design, which relies on the properties and structure of the adsorbent material, is needed to improve the general performance of both the respirator and users. Additionally, a cost effective and "green" preparation method is needed.

Summary There is provided a process for preparing a self-supporting monolithic porous carbonaceous adsorbent structure, the process comprising: drying a mixture of non-gelling polymeric carbon precursor particles and an organic latex binder at a temperature of 100°C or less to form a solid monolith of polymeric carbon precursor particles bound by an organic polymer matrix in a pre-determined shape; carbonizing the solid monolith in the pre-determined shape at a temperature of 800°C or less to form a self-supporting monolithic porous carbonaceous adsorbent structure.

There is further provided a self-supporting monolithic porous carbonaceous adsorbent structure produced by the process.

There is further provided a self-supporting monolithic porous carbonaceous adsorbent structure comprising a free-standing monolith of carbon particles having a specific surface area of 800 m ² /g or more.

The process involving organic latex binder allows binding of the precursor particles at low temperature, thus the process is cost effective because little or no heat is required at the binding stage, and the process is environmentally friendly because no decomposition occurs during binding which means that there is little or no generation of harmful and corrosive chemicals during binding. In addition, the organic latex binder, which is polymeric, is also carbonized during carbonization/pyrolysis to produce porous carbon. Further, organic latex binder forms a thin layer of continuous film binding the precursor particles into a single piece, thus the mechanical strength of the adsorbent structure is enhanced both before and after carbonization/pyrolysis.

Because no compression is required to bind the precursor particles together, the resulting adsorbent structure retains voids between precursor particles, thus maintaining good air permeability resulting in low breathing resistance and/or pressure drop in the adsorbent structure. Further, gas flow channels throughout the adsorbent structure are formed naturally by gases formed during carbonization. The gas flow channels start from active sites in micropores within the adsorbent structure and lead to a surface of the adsorbent structure. Voids between carbon particles are integrated into the channels. Therefore, the gas flow channels ensure that air has access to active adsorbing sites within the monolithic porous carbonaceous adsorbent structure, especially when used for respiratory protection. The process comprises a single carbonization step producing a self-supporting adsorbent structure with desired shape. The adsorbent structure is formed before carbonization, so both the precursor particles and the latex binder are carbonized during the same pyrolysis process. The adsorbent structure forming process is simplified. There is little or no binder blocking pores or active adsorption sites. As a self-supporting structure, the adsorbent structure can be loaded into an air purification device without compression avoiding crushing of carbon particles during fabrication of the air purification device (e.g. a respirator). Ergonomic shapes or designs of adsorbent structures can be practically attained. The binding step can be performed without the addition of any heat if speed is not essential.

The present process produces a self-supporting monolithic porous carbonaceous adsorbent structure of any desired shape useful for air purification, especially for respiratory protective applications. The adsorbent structure may be one integral piece that does not need compression to maintain the structure's integrity. The process is energy efficient and environment friendly. The process involves a pyrolysis/carbonizing step that creates efficient tortuous gas flow path in the whole monolith, low breathing resistance (pressure drop), and strong mechanical strength of the adsorbent structure.

Further features will be described or will become apparent in the course of the following detailed description. It should be understood that each feature described herein may be utilized in any combination with any one or more of the other described features, and that each feature does not necessarily rely on the presence of another feature except where evident to one of skill in the art.

Brief Description of the Drawings

For clearer understanding, preferred embodiments will now be described in detail by way of example, with reference to the accompanying drawings, in which: Fig. 1 depicts a flow diagram illustrating steps in one embodiment of a process for producing a self-supporting monolithic porous carbonaceous adsorbent bed in accordance with the present invention;

Fig. 2 depicts a a solid monolith of polymeric carbon precursor particles bound by an organic polymer matrix, where the organic polymer matrix is best seen in exploded area A;

Fig. 3 depicts a self-supporting monolithic porous carbonaceous adsorbent bed comprising gas flow channels therein, the carbonaceous adsorbent bed having maintained the same shape after carbonization as the monolith of Fig. 2 had before carbonization;

Fig. 4 depicts air flow in the adsorbent bed of Fig. 3 where air flows between and through carbon particles formed from carbonization of the polymeric precursor particles; and,

Fig. 5 depicts a graph of equivalent flow rate (SLPM) vs. pressure drop (mml-bO/cm) for a self-supporting monolithic porous carbonaceous adsorbent bed of the present invention compared to a porous carbonaceous adsorbent bed having carbon particles not bound by an organic latex binder. Detailed Description

One embodiment of a process for producing a self-supporting monolithic porous carbonaceous adsorbent bed is depicted in Fig. 1.

In a first step, type and size distribution of the polymeric carbon precursor particles are selected. The size distribution of the precursor particles is particularly important for providing a good balance between adsorption capacity of the carbon and pressure drop across the adsorbent bed. This balance is a result of a balance between accessible particle surface area and void size between particles. Larger voids provides for lower pressure drop and faster air flow through the bed at the expense of adsorption capacity. Smaller particles provide for larger accessible surface area and better adsorption capacity at the expense of faster air flow and lower pressure drop. Low breathing resistance is enhanced by a properly chosen particle size distribution and resulting packing of the precursor particles. Pressure drop through a monolith of the same bed depth with properly chosen particle sizes can be significantly lower than the National Institute for Occupational Safety and Health (NIOSH) standard at a flow rate of 85 liter per minute (SLPM). The particle size distribution is preferably in a range of about 10-30 mesh US sieve size, which is equivalent to average particle diameters in a range of about 0.595-2.00 mm. A mixture of particles having different particle sizes within the range is preferred. The precursor particles are preferably beads or granules or a mixture thereof. Preferably the precursor particles have low aspect ratios, where aspect ratio is a ratio of a longest dimension (e.g. length) to a shortest dimension (e.g. width) of the particle. Aspect ratios of about 1 :2 or less, or about 1 :1.5 or less, or about 1 :1.25 or less are preferred. An aspect ratio of about 1 :1 is most preferred. Spherical or nearly spherical particles are most preferred. Packing density of the precursor particles and tortuosity of the path through the adsorbent bed is determined by a combination of the particle size distribution and the aspect ratio and can be chosen to give the best combination of low breathing resistance and high tortuosity, leading to a longer residence time through the bed and better gas removal capacity.

The carbon precursor particles comprise an organic polymer that forms porous carbon when carbonized. The carbon precursor particles preferably yield when carbonized micropores having diameters of about 2 nm or less and mesopores having diameters of about 2-50 nm. The relative yield of these pores is dependent on the carbonization conditions and the presence of any promoting agents. The porous carbon derived from these precursor particles has a specific surface area greater than about 800 m ² /g, which arises mainly from the micropores. The high specific surface area is desirable because higher surface area leads to greater efficiency of air purification as toxic gases and other pollutants have more surface area on which to adhere. The carbon precursor particles may be synthetic or from a natural source. Some examples of organic polymers include poly(vinylidene chloride) (PVDC), PVDC copolymer, polystyrene (PS), PS copolymer, phenolic resin, cellulosic polymer (e.g. cellulose) or any mixture thereof. Poly(vinylidene chloride) (PVDC) is particularly preferred. The use of a non-gelling polymeric carbon precursor is of particular note as gelling can occlude channels in the adsorbent bed thereby reducing effective surface area and reducing air permeability resulting in high breathing resistance and/or pressure drop in the adsorbent bed.

A latex is a stable dispersion (e.g. emulsion) of polymer microparticles in an aqueous medium. In a second step, a latex binder is selected and used to coat the precursor particles by mixing the latex with the precursor particles. The polymer in the latex binds the precursor particles together in a polymer matrix once the coated precursor particles are dried. The polymer in the latex preferably comprises a synthetic organic polymer. The latex polymer is also chosen to form porous carbon upon carbonization. Latex polymers may comprise, for example, poly(vinylidene chloride) (PVDC) latex which is an emulsion of PVDC and/or PVDC copolymer in aqueous medium, polystyrene (PS) latex which is an emulsion of PS and/or PS copolymer in aqueous medium, or mixtures thereof. Poly(vinylidene chloride) (PVDC) latexes are particularly preferred.

Solid content of the latex should be properly selected to provide sufficient polymer binder to effectively coat the precursor particles so that a monolith of sufficient structural integrity is formed upon drying, and so that the monolithic porous carbonaceous adsorbent bed formed after carbonization is self-supporting, while minimizing the amount of water that would need to be evaporated during drying. The solid content of the latex is preferably in a range of about 15-65% by weight of the latex, more preferably about 40-55% by weight. Relative amount of precursor particles and latex also contributes to forming monoliths of sufficient structural integrity. The precursor particles are preferably coated with latex in a latex to precursor ratio in a range of about 1 :4 to 1 :20 by weight, more preferably about 1 :4 to 1 :6 by weight, where the weight of the latex includes the weight of the polymer binder and the water in the latex.

In a third step, the mixture of latex-coated precursor particles is molded into a desired shape, preferably in a mold. The presence of water in the latex provides a relatively fluid mixture so that that the latex-coated precursor particles may be poured and shaped in the mold.

In a fourth step, the molded latex-coated precursor particles are dried, preferably in the mold. Drying is performed at a temperature of about 100°C or less, no higher than the boiling point of water. Drying may be performed at a temperature of 90°C or less, 80°C or less, 70°C or less, 60°C or less, 50°C or less, 40°C or less, or 30°C or less. Drying may be performed at ambient temperature or higher, for example 20°C or higher, or 25°C or higher. Drying temperatures at the lower end require less energy input but result in longer drying times. The latex polymer binds the precursor particles together at a low temperature in a range of from about ambient temperature to about 100°C. The latex polymer has a melting point greater than the drying temperature, for example greater than about 100°C, so that the latex polymer remains substantially in the solid state to reduce the chance of melted polymer flowing away from the precursor particles leaving the precursor particles uncoated with binder. Further, within this temperature range there is little or no decomposition of either the precursor particles or the latex polymer. In the case of PVDC-based precursor particles and/or PVDC-containing latex, if decomposition were to occur during binding of the precursor particles, hydrogen chloride (HCI) gas would be produced, which would cause a number of problems. Further, since the latex is an aqueous dispersion, the binding step comprises evaporation of water rather than softening of dry polymer. Therefore, no pressure is needed during molding and drying because the precursor particles are bound by a generally continuous film formed by latex polymer on the precursor particles' surfaces. Not having to apply pressure during drying is useful for maintaining spaces between precursor particles thereby maintaining good air permeability in the resulting adsorbent bed. With reference to Fig. 2, after the mixture is dried, an adsorbent bed 1 comprising a solid monolith of polymeric carbon precursor particles 5 (two labeled) bound by an organic polymer matrix 7 is demolded.

In a fifth step, the demolded solid monolith 1 of desired shape is pyrolyzed to carbonize the now dry precursor particles and latex polymer. Both the precursor particles and the latex polymer are carbonized to form porous carbon. The porous carbon created by carbonization of the latex polymer preferably has a specific surface area of no less than about 800 m ² /g. Carbonizing the solid monolith is performed at a temperature of 800°C or less to form a self- supporting monolithic porous carbonaceous adsorbent bed. The carbonization is preferably performed under a controlled stepwise temperature profile. Carbonization is preferably performed slowly, with a number of temperature plateaus and a slow ramp rate between temperature plateaus. The specific profile used depends to some extent on the size and/or weight of the monolith being carbonized. Time and ramp rate for each step are generally different for samples with different sizes and/or weights, but sample size and/or weight is not necessarily proportional to time and ramp rate. In one embodiment, temperature plateaus may be at about 180°C, 200°C, 350°C and 700°C with ramp rates in a range of about 0.2-3°C/minute. Temperature plateaus at about 200°C and about 700°C are particularly useful for obtaining good results. Soaking times at a given temperature plateau may vary, for example soaking times may be in a range of from about 0.5 hours to about 4 hours.

In a specific embodiment, a monolith 20 mm in diameter and 40 mm in length) may be carbonized using the following temperature profile:

25°C to 180°C, ramp rate of 2°C/min;

180°C, soaking for 0.5 h;

180°C to 200°C, ramp rate of 0.5°C/min;

200°C, soaking for 3 h; 200°C to 350°C, ramp rate of "TC/min;

350°C, soaking for 2 h,

350°C to 700°C, ramp rate of 1°C/min

700°C, soaking for 3 h; and,

700°C down to 25°C, ramp rate of -3°C/min.

Carbonization is preferably performed in an inert atmosphere, e.g. under Ar or 2 or in vacuo, to reduce oxidation of carbon, for example to reduce oxidation of carbon to carbon dioxide and/or carbon monoxide. Because the demolded solid monolith already possesses the desired shape and already possesses structural integrity, the solid monolith may be carbonized without the need for a container thereby permitting process gases to flow through and around the monolith during carbonization resulting in a more homogeneous product. If PVDC-based precursor particles and/or PVDC-containing latex are used, hydrogen chloride (HCI) gas is produced on decomposition during carbonization, which must be scrubbed from the product gas stream during carbonization. With reference to Fig. 3, a self-supporting monolithic porous carbonaceous adsorbent bed 10 produced after carbonization is a whole piece and is ready to be assembled into an air purification device, such as a respirator. Because the adsorbent bed is self-supporting, respirator design is simplified and more rugged. Gas evolution during carbonization encourages formation of tortuous gas flow paths 15 (one labeled) from one end of the adsorbent bed 10 to the other end. The tortuous gas flow paths 15 become air flow paths through which air may flow after the monolith is carbonized. The air flow paths are the paths through which air flows in the adsorbent bed during an air filtering application such as in a respirator. The created gas flow paths 15 ensure efficient adsorbing interaction by directing the air to microporous adsorbing sites in or on carbon particles 20 (two labeled). The carbon particles 20 are formed from carbonization of the polymeric precursor particles. Adsorbing sites arise from the micropores in the precursor particles or on the surfaces of carbon particles formed by carbonization of the latex polymer. As seen in Fig. 4, air (depicted by arrows) may flow between the carbon particles 20 (only one labeled) formed from carbonization of the polymeric precursor particles or through internal channels 21 (only one labeled) in the carbon particles 20. The adsorbent structure preferably comprises carbon particles having a specific surface area of 800 m ² /g or more. Performance of a self-supporting monolithic porous carbonaceous adsorbent bed of the present invention was compared to performance of a prior art carbonaceous adsorbent bed. The prior art bed was formed with carbon particles of similar particle size distribution as those in the bed of the present invention, but the carbon particles were not bound together with an organic latex binder. The size distribution of the carbon particles is given in Table 1.

Table 1

Performance was determined by measuring pressure drop across the adsorbent bed at different equivalent flow rates. Fig. 5 depicts a graph of equivalent flow rate (SLPM) vs. pressure drop (mml-bO/cm) illustrating the results. The results clearly show that the pressure drop is significantly reduced in an adsorbent bed of the present invention in comparison to the prior art bed, especially at a high flow rate. The pressure drop may be reduced by up to 75%.

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