The present invention relates to an active air filter layer and a filter construct comprising such a layer. The present invention relates also to a method for improving an air filter’s capacity of capturing particles.

Background

A mechanical air filter is a device composed of porous, often fibrous, materials which remove solid particulates such as dust, mold, and bacteria from the air. Particles larger than the pores of the filter layer remain in the pores. Also electrostatic filter layer result is retention of particles without deactivating those. Filters with an adsorbent or catalyst such as charcoal (carbon) may also remove odors and gaseous pollutants. Air filters are used in applications where air quality is important, e.g. in building ventilation systems such as offices, clean rooms, hospitals, nuclear power stations but also in engines. Various filters are used also for personal protection. Some of the most recent filter materials are based on various pore sized PTFE applicable also within HEPA (High Efficiency Particulate Air) and ULPA (UltraLow Particulate Air). Conventional face masks comprise a minimum three-ply layer, the active material being made from of a static charge carrying melt-blown polymer, most commonly polypropylene, placed between non-woven fabric. One of the functions of melt-blown polypropylene material is to prevent the flow of microbes through the mask.

Most of the current active filtration solutions are based on electrocharged polymer layer. The mechanism is used in respirator filters that meet the stringent NIOSH filter efficiency and breathing resistance requirements because it enhances particle collection without increasing breathing resistance. There are known means on how to produce the specific, charge carrying filtration layer (electrostatic layer). In addition to melt-blowing processes, many of those are relaying on cumbersome and slow electrospinning processing or materials providing only a moderate charge or said charge lifetime. Such electrostatic charge decays over a period of time.

The N99 and N95 masks present the only available design capable of stopping the COVID-19 virion, because it contains an electrocharged layer or layers capable of attracting and capturing aerosol droplets down to micrometer dimensions, virions, and bacteria in the 100s of nanometer dimensions. An N95 mask or N95 respirator is a particulate-filtering facepiece respirator that meets the U.S. National Institute for Occupational Safety and Health (NIOSH) N95 classification of air filtration, meaning that it filters at least 95% of airborne particles. An N99 mask or N99 respirator is a particulate-filtering facepiece respirator that meets the U.S. National Institute for Occupational Safety and Health (NIOSH) N95 classification of air filtration, meaning that it filters at least 99% of airborne particles. N95 respirators are considered functionally equivalent to certain respirators regulated under non-U. S. jurisdictions, such as FFP2 respirators of the European Union and KN95 respirators of China. N99 respirators are considered functionally equivalent to certain respirators regulated under non-U. S. jurisdictions, such as FFP3 respirators of the European Union and KN99 respirators of China.

HEPA filters are composed of a mat of randomly arranged fibers typically composed of fiberglass with diameters between 0.5 and 2.0 micrometers. The air space between HEPA filter fibers is typically much greater than 0.3 pm. HEPA filters are used in the prevention of the spread of airborne bacterial and viral organisms (infections).

Also metal impregnated filters and face masks are available in the marked. Metal ions such as Silver and Copper are known to have antimicrobial activity. The PuraWard fiber (Purafil Filtration Group) product bulletin teaches that copper ions weaken the amino acids of the cell wall, allowing silver to invade the cell resulting in elimination of 99.96% of tested bacteria and 99.98% of tested virus.

WO 2004/024278 discloses an active agent incorporated in the porous dielectric carrier. The active agent may be an antimicrobial or an antitoxin, such as metals including silver and copper.

Nanodiamond (ND) also referred to as ultrananocrystalline diamond or ultradispersed diamond (UDD) is a unique nanomaterial which can be produced by detonation synthesis. The detonation nanodiamonds comprise sp ³ carbon and the non-diamond carbon mainly comprising sp ² carbon species. Sp ² carbon provides and active surface to the diamonds. Functionalization of detonation nanodiamonds is discussed in e.g. WO 2014/174150, WO 2014/191633 and WO 2015/092142. The use of nanodiamond particle on various fibrous materials has already been demonstrated in other industrial applications.

Sato K. et al. (Advanced Powder Technology 29 (2018) 972-976) discloses cellulose nanofiller/nanodiamond composite films. Positively charged nanodiamonds (ND) and negatively charged cellulose nano fibrils (CNF) are combined in an aqueous solution. Such films are very dense and coating alike in structure. There is a need for providing improved active layers with high filtration efficiency and long lifetime. Preferably said layer would be flexible as well as has moderate manufacturing costs. Preferably said layer is not only able to capture particulates but also various pathogens such as bacteria and viruses. Said solutions are desirably available in industrial quantities and able to be scaled up said in short time frame. Moreover, any effective filtration solution with simultaneously reduced pressure drop would result in energy savings within HEPA/UPLA filtration as well as by lower airflow resistance easier breathing when applied in a mask construction.

Summary

The present disclosure generally relates to removal of particles from air by filtration.

The first aspect of the present invention is an active filter layer comprising a filter material. Characteristic features of such filter layer are depicted in claim 1 .

The second aspect of the present invention is a filter construct. Characteristic features of such construct are depicted in claim 8.

The third aspect of the present invention is a method for a method for improving a filter’s capacity of capturing particles. Characteristic features of such method are depicted in claim 15.

The invention enhances efficient removal of particles, including bacteria and viruses, with a reasonable or even lowered airflow resistance.

Detailed description

Nanodiamond particles are commercially available both in their positively charged and negatively charged forms which makes them effective and species for making electrostatic layers. The inventor has now surprisingly found that they can be applied also for various high-performance filter layers including those used in face masks and HEPA filters. Nanodiamonds having a high surface charge (zeta potential are commercially available at brand names uDiamond Hydrogen D (highly zeta positive, hydrogen terminated nanodiamonds in dispersion), uDiamond Vox D (highly zeta negative, carboxylated nanodiamonds in dispersion) or uDiamond Amine D (highly zeta positive nanodiamonds in dispersion).

The small and spherical, highly charged nanodiamond particles applied onto/into the porous substrate can form a three-dimensional structure on nanoscale, effective capturing not only the, particles and bacteria but the smallest of the viruses. The nanodiamond particles provide a non-decaying charge which is not affected for example by moisture. The present disclosure allows manufacturing electrostatic layers capturing either negatively or positively charged species, including particles, bacteria and viruses. A filter construct can be designed to comprise either one filter layer capturing oppositely charged species or two oppositely charged filter layers capturing both negatively and positively charged species. Further, the performance of a filter construct can be tailored by modifying the electrostatic layer nanodiamond concentration and/or the thickness of the filter layer containing the nanodiamond particles. The nanodiamond particles can be applied, for example, by drying them from their dispersion form directly onto a fibrous carrier material, allowing them located throughout said fibrous layer material surfaces. The use of 4-6 nm nanodiamond particles in their dispersion form allows also their even spread and adhesion throughout the complex, fine-sized porous structure.

The broadest embodiment of the invention is an active filter layer comprising filter material characterized by said filter material layer having detonation nanodiamonds (NDs) incorporated therein at amount of 0.001 to 10 g/m ² . The amount of incorporated nanodiamonds may be 0.001 to 5 g/m ² , 0.001 to 2 g/m ² ; 0.01 to 0.1 g/m ² , such as 0.075 g/m ² ). The amount of nanodiamonds may be adapted to the properties of the filter material and the needs of the application.

In this connection the term “filter material” means the filter material before incorporating the nanodiamonds. The material may be a mechanical filter without electrostatic charge, it may be charged, it may be functionalized, or it may comprise incorporated active agents such as metal ions or charcoal. The filter material can be natural polymer based or a synthetic material such as a polymer or a mixture of polymers. The filter material can also be ceramic material or metal based.

In this connection the expression “an outer surface of the filter material” means the outer surface of the sheet excluding the surface of the pores. The term “surface of the filter material” means the whole open surface of the material layer and includes also the (inner) surface of the pores within the material.

The filter material has a continuous porosity. In this context the term continuous porosity means open porosity allowing gas flow through the porous layers. The pores or series of pores form a tubular open-ended structure elongating from the first surface of the filter layer to the second surface of the filter material layer. In this connection the free space within the filter material are referred as “pores”. The pore size can vary from 100 nm to 30 micrometers and the diameter of a pore can vary throughout the pore structure. Filter material layers (also those formed from a filter media) may be composed of a mat of randomly arranged fibres or any other porous material such as ceramic of metallic structure. The fibrous material may comprise a synthetic or natural polymer such as nylon, polyethylene, polypropylene, polyester, glass fibre or any cellulosic fibre. The fibrous material is typically a mixture of various polymers, one such representative being marketed as “ES Fiber Cotton”. The microporous structure can also be created by stretching a selected polymer material, for example PTFE, to arrive into desired microporous structure. In PTFE layers typically applied within HEPA and LILPA filters, the micropore diameter varies typically between 0.1 to 10 micrometers.

The pores can also be manufactured by needle punching, a typical method producing “needle punched nylon” applied widely for example within safety masks.

The metal and ceramic based filter structures are typically manufactured by sintering selected sized metal or ceramic particles to form a filter structure. If applying nanodiamond particles on either metallic or ceramic filter structure, such filters can be re-activated by washing away the trapped species and nanodiamonds, followed by re-introducing a new load of nanodiamond particles to filter pores surfaces.

Nonwoven fiber layers or mats may have a very high proportion of void volume. Non-woven fibrous filter media are formed by the random distribution of fibers in a specific space exhibit a complicated pore size structure. The material may be electrostatically charged. Such static charge decays over a period of time.

The air space between the HEPA filter fibers (comprised of glass-fibre) is typically much greater than 0.3 pm. The filter's minimal resistance to airflow, or pressure drop, is usually specified around 300 pascals (0.044 psi) at its nominal volumetric flow rate.

Due to nanoscale size of the detonation nanodiamonds the pressure loss caused by the filter material will not remarkably be increased when nanodiamonds are incorporated. If adhering the nanodiamond particles to a filter material with larger pore size, it is possible to reduce the air flow resistance without compromising on the filter particulate capturing performance. Such a feature allows lower pressure drop in HEPA/ULPA filters and thus, lower overall energy consumption. Such a feature also allows lower air flow resistance within safety masks, making such a mask more user friendly due to easier breathing. Nanodiamonds create active surface to the filter layer or increase the active surface. They have an ability to capture particles, viruses, bacteria, pollen, allergens and fungus and possibly also at least partially deactivate those. The nanodiamonds may adhered on to the surface of the filter material. The nanodiamonds can also be inside, completely or in part, the filter body material matrix. This allows a direct interaction with the foreign particles, viruses and bacteria, and enhances capturing those. The distribution and thus the particle to particle distance of the nanodiamond particles may be adjusted by selecting suitable application method.

Low amount of nanodiamonds does not affect on filter layer weight or bendability properties. It does not remarkable increase the production cost or increase air flow resistance. If choosing a cheaper filter material without but applying nanodiamonds thereto, the cost can be even lowered.

The amount of nanodiamonds may be 0.05 to 1 .0 wt.-% of the total weight of said layer.

The nanodiamonds may have charge of at least +40 mV or less than -40 mV; preferably at least +50 mV or less than -50 mV. The nanodiamonds may have charge of at least +55 Mv or less than -55 mV. When defining less than - 55 mV, it is meant a value like - 56 mV or less.

The nanodiamond surface charges may be measured of nanodiamonds diluted to 0.1 wt.-% aqueous dispersion using Malvern Zetasizer NanoZS according to manufacturer’s instructions. The nanodiamond particle size distributions may be measured of samples diluted to 0.5 wt.-%.

A generally recognised method for defining zeta potential and/or particle size distribution for a nanodiamond dispersion with high concentration is not available, thus water dilutions are often used.

Measuring particle size distribution (PSD) as well as the particles zeta potential has to be carried out with a tool designed for measuring the PSD with a tool designed for defined primary particle size measured as a water dispersion by dynamic light scattering microscopy. For example, Malvern Zetasizer Nano ZS is commonly applied for particles sizing less than 10 nm and is thus favored by many working with 4-6 nm sized detonation nanodiamond particles.

When manufacturing novel nanodiamond dispersions comprised from those 4-6 nm non-agglomerated particles there are qualitative needs to secure that said manufactured dispersions hold specified quality both in terms of said dispersion particle size distribution and contained particles zeta potential. Whilst working with completely new nanodiamond dispersions, the produced data can only be accepted if said data complies with tool manufacturer quality reports. When it comes qualifying an acceptable measurement with Malvern Zetasizer Nano ZS tool, the following quality measures have to be passed:

• Z-average size, the best value to report when used in quality control setting as defined in ISO 13321 and more recently in ISO 22412

• Cumulants analysis, as defined in ISO 13321 and more recently in ISO 22412

• Y-intercept or Intercept

• Polydispersity Index, as defined in ISO standard document 13321 :1996 E and ISO 22412:2008

If any of the above measures is not meeting the tool quality requirement, the tool will report “Refer to quality report”. Tailoring the applied dispersion concentration per studied property (PSD or zeta potential) has to be continued until the tool will report “Result quality good”. When it comes to carboxylated nanodiamond dispersion analysed with Malvern Zetasizer Nano ZS tool, 0.1 wt.% concentration for measuring the dispersion zeta potential and 0.5 wt.% concentration for measuring the dispersion particle size dispersion (PSD) were found to be delivering scientifically and technically acceptable zeta potential and PSD values, respectively.

In one embodiment the zeta potential value of the nanodiamond is measured of samples diluted to 0.1 wt-% and particle size distribution is measured of samples diluted to 0.5 wt-% using Malvern Zetasizer NanoZS.

Nanodiamonds are not in form of salts but they may form complexes with for example metal ions added for improving antibacterial characteristics.

Starting from an active filter layer (active filter media), it is possible to recover the nanodiamonds by extraction and followed by filtration. Then the charge can be measured in concentrated aqueous solution. Solvent change, if necessary, can be done using knowns methods such as evaporation and then adding an aqueous solvent. Alternatively, the filter material can be burnt at temperature reaching max 400 °C, to avoid oxidizing away the nanodiamonds themselves. Then the remaining nanodiamonds are dispersed to an aqueous solution for measurements.

The surface charge relates to the stability of colloidal dispersions. The value indicates the degree of repulsion between adjacent, similarly charged particles in dispersion or suspension. For molecules and particles that are small enough, a high zeta potential will confer stability, i.e., the solution or dispersion will resist aggregation. The higher numeric value the higher is the repulsion between the particles and the better is the stability of a dispersion of particles.

High numeric charge value (zeta potential) indicates high repulsion forces and enhances forming a two- or three dimensional network of nanodiamond particles on the filter material wherein the nanodiamond particle to particle distance may be tailored to 100 nm or less from each other. This forces viruses or any other particles bypassing the filter layer to a close contact with nanodiamonds resulting them to being captured by the oppositely charged nanodiamond particles.

A filter layer may comprise either one surface layer incorporated with nanodiamonds with negative surface charge and another surface layer incorporated with nanodiamonds with positive surface charge. The surfaces of oppositely charged species adsorb both negatively and positively charged species. Alternatively, both surfaces may have similar charge. If filter layer’s both surface layers are incorporated with nanodiamonds, the bulky part of the filter layer acts as a dielectric layer between the filter layer two surface layers.

The filter layer electrostatic performance comprised from adhered nanodiamond particles charge (measured as zeta potential) may be modified by tailoring the nanodiamond concentration and/or by tailoring the thickness of the fibrous layer containing the nanodiamond particles. The nanodiamond particles strong charge warrants also said particles strong adhesion to filter layer body material. This charge is so strong that the particles remain adhered despite subjecting the filter layer to strong airflow.

A filter layer may comprise either one surface layer incorporated with nanodiamonds with negative surface charge and the other surface layer incorporated with nanodiamonds with negative surface charge. The surfaces of same charged species both adsorb positively charged species and thereby further improve the efficiency.

A filter layer may comprise either one surface layer incorporated with nanodiamonds with positive surface charge and the other surface layer incorporated with nanodiamonds with positive surface charge. The surfaces of same charged species both adsorb positively charged species and thereby further improve the efficiency.

A filter layer may comprise the nanodiamonds throughout the filter porous structure. The nanodiamonds can carry either positive or negative charge. The effectivity can be tailored by adjusting the nanodiamond concentration and or the filter layer thickness and pore size.

The net charge of a particle may be determined by the sum of the charges exposed on the surface. This may be calculated from the protein structure(s) at the surface. There are several tools available online to calculate net charges of folded protein structures as well as their surface charge distribution.

If the filter layer is incorporated also by an agent having antimicrobial or an antitoxin properties, such as metal ions, the activity of the microbes and many viral particles will be destroyed or reduced. It is to be noted that also nanodiamonds are believed to have certain independent activity against bacteria and viruses. Examples of suitable agents are metal ions such as Aluminum, Barium, Boron, Calcium, Chromium, Copper, Iron, Magnesium, Manganese, Molybdenum, Nickel, Lead, Potassium, Sodium, Strontium, Zinc and Silver, such as Copper and/or Silver.

In an embodiment where the filtration layer described here is boosted (incorporated) with Silver, or Copper or both of then the contact time for deactivation is believed to be short compared to filters without a metal ion.

The nanodiamonds may have an average primary particle diameter of 4 to 6 nm. Nanoscale size increases the active, highly charged area and provide their good adhesion to the filter layer base material. In addition, nanodiamonds do not substantially lower the gas flow throughout the filter layer. They are not detached as subjected to airflow.

Preferably the nanodiamond is also a single digit nanodiamond. Single digit nanodiamonds provide a large active surface and do not aggregate. In addition, they have a minimal effect to the air flow properties of the filter layer. By term “single digit nanodiamond” is meant a nanodiamond particle substantially in its primary particle form. The average size of a single digit nanodiamond particle is 10 nm or less.

The present invention relates also to a filter construct. An essential feature of such construct is that it comprises at least one filter layer described here. In addition, the construct may comprise one or more selected from the following:

■ one or more pre-filter layer; and

■ one or more filter layer(s); and

■ one or more electrostatic layers; and

■ any combination of those. Prefilter layers may be used for removal most of the larger particles such as dust, hair, and pollen. These are commonly used in e.g. HEPA filtration constructs and may be e.g. coarse mechanical filters. The nanodiamond particles can also be on a prefilter, either on one or both outer surface layers or impregnated throughout the prefilter porous structure.

Protective layers may be used to protect either the active filtration layer from external factors (such as abrasion, touch or oily skin) or to protect skin from the active filtration layer and increase comfort of e.g. a face mask.

Separate electrostatic layers may be included to further improve the filtration efficiency.

In addition, the filter construct may comprise filter layers with charcoal or any other active ingredient.

Filter properties may be discussed based on Under European normalization standards EN 779. As an example, a HEPA filter may remove at least 99.97% of airborne particles 0.3 micrometers (pm) in diameter which complies with the retention capacity of HEPA claim H13 defined by the United States Department of Energy (DOE) standard and averaged retention of > 99.95%.

The filter construct described here may increase the retention may to averaged retention of 99.995% (H14) or even higher.

The advantage of the present invention is that an excellent retention of particles may be achieved by using even a coarse filter material, such as material of G4 class, as a filter material into which nanodiamonds are incorporated. As an example, G4 class filter used as a primary filter removes particles of >5 pm with substantially 100% retention. When a coarse filter material is used, the air flow resistance is low thereby saving energy and/or improving comfort of a face mask. Similarly, as applied as a part of HEPA construct, the pressure drop can be dramatically reduced resulting in great energy savings and the HVAC device lower noise level. In addition, such material is cheaper than finer filter materials. It may also be possible to reach an excellent particle retention even using only a prefilter and one nanodiamond incorporated layer.

The filtration construct may be a face mask, a safety cloth or curtain or a HEPA filter construct. The general structure of a face mask is known. One example of the construction, also used in the experimental section here, is a mask called 4 PLY N95 face comprising total of 4 layers: outside and inside are standard polypropylene (PP) layer, 2nd from outside is the layer comprised of ES Fiber Cotton with incorporated nanodiamonds and 3rd layer is melt brown PP layer (an electrostatic layer).

Another example of the construction, also used in the experimental section here, is a mask called 4 PLY N95 face comprising total of 4 layers: outside and inside are standard polypropylene (PP) layer, 2nd from outside is the layer comprised of ES Fiber Cotton with incorporated nanodiamonds and 3rd layer is F8 level PTFE layer (a filter layer). With F8 level filter layer is meant a filter capable of trapping particles sized 1 to 10 micrometers, according EN 779 standard.

Still another example of the construction, also used in the experimental section here, is a mask called 4 PLY N95 face comprising total of 4 layers: outside and inside are standard polypropylene (PP) layer, 2nd from outside is the layer comprised of G4 glass filter comprised of polyethylene terephthalate with incorporated nanodiamonds and 3rd layer is F8 level PTFE layer (a filter layer).

The filter construct may comprise two or more active filter layers described here. Said layers may be incorporated with nanodiamonds with opposite charges.

The surfaces incorporated with oppositely charged nanodiamonds in one embodiment are not facing directly each other. They may be incorporated on opposite surfaces on one filter material layer. This protects the charge of each surface. Alternatively, they may be separated by a dielectric spacer known within the filed. A spacer may be an air cavity between the layers. Alternatively, said opposite charged surfaces may be applied on the opposite sides of a single filter material layer. Unless the nanodiamonds are applied in excess, the filter material acts as an insulator or a dielectric layer. The advantage of using a dielectric spacer that it prevents neutralization of the charge.

Hence, a dielectric spacer can also be applied between the surface of filter material layer incorporated with nanodiamonds and a filter layer with static charge. Here, the incorporation of such a spacer will prevent the static charged filter layer charge from decaying unwantedly.

The present invention relates also to method for improving a filter’s capacity of capturing particles. The method is characterized by

(a) providing aqueous nanodiamond dispersion; and (b) providing at least one air filter layer; and

(c) applying said dispersion to at least one side of the air filter layer, wherein such formed layer comprises detonation nanodiamonds (ND) at amount of 0.001 to 10 g/m ² calculated based on the outer surface of the filter material.

The nanodiamonds applied may have charge of at least +40 mV or less than -40 mV measured as an aqueous dispersion, optionally before diluted to be applied. The charge may also be at least +50 mV or less than -50 mV or at least +55 mV or less than -55 mV.

The dispersion may be applied by spraying a nanodiamond dispersion on to at least one side of the filter layer and drying. Spray application allows producing filter layers which carry charged nanodiamonds only at one surface. In addition, drying step after spraying may be easier than after immersion.

The dispersion may be applied by immersing the filter to a nanodiamond dispersion followed by drying the filter by evaporating the solvent. An immersion application results in substantially uniform spread of nanodiamonds on the surface of the filter and pores throughout the filter structure (planar directions as well as z-axis direction). It will also allow a higher nanodiamond load and thereby enhanced adsorption capacity.

The drying step (evaporation of the solvent used in dispersion) can be made using known methods, such as high-speed dryer applying heat. Also passive evaporation may be used, especially when the dispersion with high nanodiamond content and thus relatively low solvent content has been used. If desired, the nanodiamond dispersion may be diluted before the application. An active filter layer can be dried to desired moisture content.

The nanodiamonds may be applied as 0.01 to 5 wt.-%, such as about 0.5 wt.-% nanodiamond dispersion. The nanodiamonds may be applied as 0.1 to 3 wt.-%, as or as 0.1 to 1 wt.-%. The higher nanodiamond content, the easier is the drying step. A person skilled in the art is able to adapt the application speed and distance for spray application.

The solvent in said dispersion may be any solvent with a boiling point of 250 °C or less, such as 200 °C or less or 150 °C or less. The boiling point should be reasonable to allow evaporating the solvent after the nanodiamond dispersion is applied to the filter material. Low evaporation temperature requires less energy and is gentle to the filter material. Preferably the solvent is an aqueous solution such as deionized water.

The invention is illustrated below by the following non-limiting examples. It should be understood that the embodiments given in the description above and the examples are for illustrative purposes only, and that various changes and modifications are possible within the scope of the invention.

Examples

Example 1. Preparation of diluted detonation nanodiamond dispersion

280 grams of 2.5 wt.% uDiamond® Hydrogen D aqueous detonation nanodiamond dispersion (containing 7 grams of highly positively charged detonation nanodiamond particles) was diluted with 30000 grams of deionized water. The resulting mixture was mixed by mechanical stirring for 30 minutes time. The resulting mixture nanodiamond concentration was 0.023 wt.%.

Example 2. Preparation of nanodiamond containing ES Fiber Cotton filter medium

The 0.023 wt.% aqueous nanodiamond dispersion received in Example 1 was sprayed evenly onto a moving ES Fiber Cotton surface, followed by drying the resulting filter medium in a high-speed dryer unit. The speed of the ES Fiber Cotton Surface was adjusted to adhere approximately 0.075 g of detonation nanodiamond solids on square meter(m ² ) of ES Fiber Cotton Surface.

The process was repeated on ES Fiber Cotton media pristine side, to arrive into 0.15 g of detonation nanodiamond solids on square meter(m ² ) of ES Fiber Cotton adhered on both sides of the cloth media.

Example 3. Filtering efficiency measurements

(a) Single layer performance

A blank ES Fiber Cotton sample as well as a sample containing 0.075 g/m ² of hydrogen-terminated nanodiamonds on both sides of ES Fiber Cotton received in Example 2 were analyzed for their filtering efficiency applying the filter tester Model SC-FT-1406D, by standard GB2626. The filtering efficiency for 0.3 and 0.5 micrometer particles for the blank ES Fiber Cotton sample was 10%. The filtering efficiency for the nanodiamond containing sample for 24%; i.e. the filtering efficiency was improved by more than 100%.

(b) Performance as part of a protective face mask

Guangzhou Fuzelong Hygiene Material Co., Ltd produced 4-ply N95 mask was chosen as a reference. Said mask is comprised of four different layers, the outer layer being made of polypropylene, the second layer being made of needle punched nylon, the third layer being made of melt-blown, static polypropylene and finally, the fourth layer being in contact with user face is made of polypropylene.

The nanodiamond enhanced sample was made by replacing the reference sample contained needle punched cotton layer with an ES Fiber Cotton based filter material containing 0.075 g/m ² of hydrogen-terminated nanodiamonds on each side of said filter medium.

The filtering efficiency for 0.3 micrometer particles for both samples were analyzed with filter tester Model SC-FT-1802D, by standard GB2626. The reference sample airflow resistance (Pa) and particle filtering efficiency (%) were measured at airflow of 85 L/min.

For the nanodiamond enhanced sample flow resistance (Pa) and particle filtering efficiency (%) were measured at a multitude of different airflows, namely 30, 60, 90 and 99.99 L/min.

The Guangzhou Fuzelong Hygiene Material Co., Ltd produced 4-ply N95 mask (reference) particle filtering efficiency 97.89% and the airflow resistance is 114.6 Pa.

The particle filtering efficiency was measured for fine particles sizing 2.5 microns or less in size (PM2.5).

The nanodiamond enhanced sample airflow resistances and particle filtering efficiencies at different airflow volumes are depicted in Table 1 below.

Table 1. As can be seen from the data, replacing the Fuzelong 4-ply N95 mask contained second filter layer, i.e. the needle punched nylon layer with hydrogen-terminated nanodiamond enhanced ES Fiber Cotton filter enabled improving the particle filtering efficiency from 97.89% at 85 L/min airflow (KN-95; FFP-2 or N95 quality) to over 99%, both with comparable and even higher airflow volumes (KN-99; FFP-3 or N99 quality). In addition, the airflow resistance dropped dramatically from 114.6 Pa (85 L/min) to 65.1 Pa (90 L/min), allowing easier breathing when applying the nanodiamond enhanced filtering material for safety mask use and lower energy consumption if using the nanodiamond enhanced filtering material for other filtering, such as HEPA, ULPA etc.

The nanodiamond enhanced filter material can be made on any available solid or fibrous substrate, synthetic or natural, allowing the airflow through the filter media. The filtering efficiency can be tailored and further increased by adjusting the nanodiamond content per filter surface area. The nanodiamonds can be adhered either on one side of the filter layer, on both sides of the filter layer or throughout the filter material porous structure. The latter is easy to gain by for example immersing the entire filter media (entire layer) into selected nanodiamond dispersion in either water or any other applicable solvent, followed by drying said filter material essentially free from applied nanodiamond dispersion contained liquid media. If applying the nanodiamond particles only on one side of the filter media, the other, non-treated side stays essentially free of any charge. This invention is facilitating making filter structures having several charged layers separated from each other by the non-charged bulky filter material, without increasing the HEPA filter or safety mask airflow resistance. Due to performance improvement it is also possible to carry out high efficiency filtration with filter materials having larger pore sizes and thus, lower airflow resistance.

The filter material charged surfaces can be tuned to carry either non-decaying positive or non-decaying negative charge by selecting the nanodiamond particles from either those with high positive zeta potential or high negative zeta potential (such as Carbodeon uDiamond Vox D in water (5 wt.%), said product being comprised of carboxylated detonation nanodiamond particles exhibiting zeta potential of > - 50 mV).