Field of the invention

The invention relates to a process for the production of a filter membrane for filters intended for the efficient microfiltration of liquids and air, particularly in the field of capture ultra-fine particles.

State-of-the-art

By applying nanofibrous structures - nano-nonwoven textiles for filtration of liquids and air, very high flow rates at filter membranes can be achieved, especially at the beginning of the filtration process. Nanostructures form structures with a mean pore size of around 300 nm and can therefore also be used for microfiltration of liquids, especially water, which is a process that removes all bacteria with high efficiency. The disadvantage of using nanostructures for liquid filtration is their poor mechanical properties. Membranes prepared from them therefore require additional reinforcement by joining (lamination) with suitable support layers.

Methods for joining layers of very fine fibres into laminated structures of filter materials are the subject of, for example, PCT International Patent Application WO 2011/052865, PCT International Patent Application WO 2012/135679, or U.S. Patent Application 2013118973.

From the point of view of the functionality of the filter structure, it appears to be positive if the connection of the individual layers by the connecting intermediate layer is limited in area. In this sense, International PCT Patent Application WO 2008/150548 and U.S. Patent Application Nos. 2004116019 and 2004128732 disclose a method of bonding a layer of very fine fibres and a support layer, in which a bonding adhesive interlayer is applied in limited areas by engraving roller.

The subject of the International Patent Application PCT WO 2013/066022 is again a joining method in which a layer of very fine fibres with a lower melting point (50-170°C) is applied to the suppurt material on one or both sides by electro spinning, and a layer of very fine fibres with higher melting point (80-250°C) is applied over it. During the subsequent thermal lamination, the layer with a lower melting point is partially melted and the layers are joined. Similarly, the connection of filter structures is addressed in Patent EP1985349 of German authors.

A composite comprising at least one layer of nanofibres is known from the prior state- of-the-art; for example, as also represented by document CZ 25797 U1 (NAFIGATE Corporation, a.s.). It is clear from the examples that the polymer nanofibre filter layer can be directly applied to the surface of the carrier layer, which serves as a support material for depositing polymer nanofibres, using calendering and/or a binder suitable for sufficiently durable bonding of the carrier layer and the filter layer. This binder can be applied in the form of powder and/or paste and/or gels and/or liquids, in the form of a regular or irregular grid and/or separate formations, such as fibres and/or particles and/or stripes and/or other formations.

Composite structures that contain nanofibres and are suitable for the removal of microorganisms are also known from the prior state-of-the-art - see International PCT Patent Application WO 2013/013241 (EMD MILLIPORE CORPORATION).

However, when applying the above methods, similarly to ultrasonic, flame, hot gas or air bonding of textiles, the fusible component penetrates into the fine pores of the filter materials and significantly reduces the effective filtration area, i.e., increasing the pressure resistance and decreasing the flow rate.

Principle of the invention

According to the invention, the factor that contributes to eliminating the above- mentioned shortcomings is a method for producing a filter membrane, which in its multilayer structure comprises at least one support layer of polymeric textile with a basis weight of 15 to 200 g/m ² , and at least one associated layer of electrospun nonwoven nanotextile with a basis weight of 0.05 to 8 g/m ² , with a distribution of nanofibre diameters in the range of 40 to 400 nm and mean pore size of 200 to 1,800 nm.

The object of the invention is to compress the said layer of polymeric textile with the said layer of electrospun nonwoven nanotextile at a temperature of 50 to 200°C, preferably at a temperature of 130 to 150°C, and a pressure of 0.5 to 10 bar. A polymeric textile with a wide molecular weight distribution is used as the support layer, the low molecular weight fractions of which have a melting point in the pressing temperature range, and, in the molten state during compression cause bonding to the electrospun nonwoven nanotextile of polymeric material with a melting point above the upper limit of the pressing temperature range.

As a polymeric support textile with a wide molecular weight distribution, a polymeric textile made of partially degraded polyethylene terephthalate is preferably used, the low molecular weight fractions of which have a melting point of 145 to 170°C, and, when compressing the support layer of the polymeric textile with a layer of electrospun nonwoven nanotextile at a temperature of 50 to 200°C and pressure of 0.5 to 10 bar, it additionally causes reinforcing of the support layer when connecting the two layers. A layer of submicron fibres supportd on a polymer from the group consisting of polyvinylidene fluoride, polyurethane, polylactic acid, polyamide, polyacrylonitrile, cellulose acetate, polystyrene, polysulfone, and poly ether- sulfone is preferably used as the nonwoven nanofibre textile layer.

The prepared structure of the support textile, formed of a polymer with a wide molecular weight distribution, connected to the nanofibre layer, can then be provided with a cover layer consisting of polypropylene, polyethylene, polyamide or polyester melt blown, spun bond or spun lace nonwoven textiles or nets, or woven fabrics of synthetic fibres particularly supportd on polypropylene, polyethylene terephthalate and polyamide, or natural fibres such as flax, hemp, cotton or mixtures thereof.

Membranes reinforced with woven or nonwoven polymeric textiles prepared by the process according to the invention already have sufficient mechanical properties necessary for liquid-microfiltration membranes and can also be used for pleating into air filters. The most important variables in the preparation of membranes are pressing pressure and temperature.

Explanation of the drawings

The enclosed drawings serve to further clarify the principle of the invention and are represented as follows:

Figure 1. DSC curves of polyethylene terephthalate polymers with a wide and narrow molar weight distribution.

Figure 2. Image of support nonvowen textile reinforced textile reinforcing by melting of low molecular weight fractions of PET

Figure 3. Image of biofilm formation (right) on the initial nano -structured polyvinylidene fluoride microfiltration membrane

Figure 4. Nanostructured surface of the filter material formed at 130°C Figure 5. Comparison of distilled water flow through a washed and unwashed microfiltration membrane

Figure 6. Collapsed nanostructure of filter material formed at 160°C Figure 7. Graph of pore size reduction and permeability with pressing temperature Figure 8. DSC curves of polypropylene polymer with a wide molecular weight distribution Figure 9. Illustration of pleating and fixing of pleats on filter materials. Examples of invention implementation

Example 1

The water microfiltration membrane was produced by compressing a 70 g/m ² nonwoven support layer made of partially degraded polyethylene terephthalate with a wide molecular weight distribution (see Figure 1), prepared from recycled polymer, and a layer of 2.4 g/m ² electrospun nonwoven nanotextile made of polyvinylidene fluoride nanofibres with a mean nanofibre diameter of around 160 nm. The pressing took place at a pressure of 4 bar and a temperature of 150°C. At this pressing temperature, the low molecular weight fractions in the polyethylene terephthalate support layer were optimally melted, and thus the desired reinforcing of the filtration material took place while the filter nanostructure remained intact (see structure in Figure 2).

Example 2 (comparison)

The water microfiltration membrane was produced by compressing 2.4 g/m ² polyvinylidene fluoride nanofibres with a mean nanofibre diameter of about 160 nm and a nonwoven textile having a basis weight of 70 g/m ² made of partially degraded polyethylene terephthalate with a wide molecular weight distribution like Example 1. However, the pressing took place at a pressure of 4 bar and a temperature of 130°C. The flow of distilled water at a pressure of 1 bar through the membrane dropped from 140000 l/m ² h to 30000 l/m ² h in one hour. The reason of this decrease is the biofilm formation shown in Figure 3.

The filter membrane prepared by pressing with 4 bar at 130°C had a smooth surface, and the structure shown in Figure 4 can be cleaned by repeated washing (with the effect shown in the graph in Figure 5) and by backwashing.

The material prepared by pressing the same materials and at the same pressure as in Example 1, but at a temperature of 160°C, already had a collapsed structure (Figure 6).

The effect of pressing temperature on pore size and air permeability (measured according to ASTM F316-03 from 2011) is shown in Figure 7. Materials with the construction described above can be pressed up to temperatures of 150°C. Example 3

The composition of the two-layer filter material was the same as in Examples 1 and 2, but a fabric with a basis weight of 100 g/m ² , woven from polyethylene terephthalate yarns formed of two fibres plied from 3 yarns with a fineness of 20 tex was used as the support fabric. The polyethylene terephthalate used was formed from macromolecules with a wide molecular weight distribution as in Example 1. The reinforcing of the membrane at a pressing temperature of 130°C was less than in Example 1, but the other microfiltration capabilities of the membrane were maintained.

Example 4

The nanofibre layer used was the same as in Examples 1, 2 and 3, but a nonwoven textile prepared from a mixture of polypropylene and polyethylene terephthalate staple was used as a substrate, where the staple was prepared in a fibre weight ratio of 65:35, with a total basis weight of 30 g/m ² . The polyethylene terephthalate fibres used were made of a polymer with a wide molecular weight distribution. The reinforcing of the filter material was less than in Example 1, however, by pressing at a temperature of 140°C and a pressure of 4 bar, a material with good compatibility with the nanostructure and pleatable with commercial equipment was prepared.

Example 5

The used support nonwoven textile was the same as in Example 1, but the nanofibre layer was alternatively made of polymer- supportd nanofibres from the group consisting of polyurethanes, polylactic acid, polyamide 6, polyacrylonitrile, cellulose acetate, polystyrene, polysulfone and polyether-sulfone.

Example 6

The filtration membrane was prepared by layering polyvinylidene fluoride nanofibres with a basis weight of 2.3 g/m ² on a polypropylene spun bond textile with a basis weight of 50 g/m ² and a molar weight distribution corresponding to the DSC record in Figure 8. The double layer material was pressed down and subsequently smoothed at a temperature of 80°C and a pressure of 1 bar. Example 7

Another example of a filter material has a multilayer structure, which is formed as a sandwich with a layer composition of a polyethylene terephthalate fibre nonwoven textile with a wide molecular weight distribution as in Example 1 and a basis weight of 50 g/m ² - polyvinylidene fluoride nanofibre nonwoven nanotextile - nonwoven textile of polyethylene terephthalate fibres with a wide molar weight distribution as in Example 1 and a basis weight of 50 g/m ² . Such significantly reinforced material can be used for liquid microfiltration at elevated pressure.

Example 8

The eighth example of technical evaluation describes a filter material for air filtration with a multilayer structure, which is formed as a sandwich with a composition of nonwoven textile of polyethylene terephthalate fibres, with a wide molecular weight distribution as in Example 1 and a basis weight of 50 g/m ² - nonwoven nanotextile of polyvinylidene fluoride nanofibres - viscose nonwoven textile with a basis weight of 30 g/m ² . This material can be used for pleating (Figure 9) into air filters with increased efficiency of capturing ultra-fine particles. Depending on the content of nanofibres, from 0.1 to 4 g/m ² , it is possible to prepare materials for filtering air in filtration classes from F9 up to U16.

The starting filter material described in this example with 2.5 g/m ² of polyvinylidene fluoride nanofibres has, in the planar state, a filtration efficiency according to EN143 higher than 99.999% and a pressure drop at a flow rate of 301/min lower than 300 Pa. A similar filter material supportd on borosilicate microfibres, widely used for the industrial production of air filters, shows the same filtration efficiency, but achieves a pressure loss of more than 400 Pa, i.e., higher by more than 30%.

After application of the polymer filter material according to this example with nanofibres for a pleated filter - pleating into pleats 12 mm high, fixing (Figure 9), cutting to the required shape and casting into the housing - the filtration efficiency of the finished filter remains higher than 99.99%, however, the pressure drop decreases to 60 Pa. Example 9

The same composition of sandwich material as in Example 8, but instead of viscose nonwoven textile, polypropylene spun bond and melt blown textiles with basis weights from 15 to 70 g/m ² were used. Compared to brittle borosilicate materials, the polymer filters, due to their elasticity, have better resistance to damage during pleating.

Example 10

The same composition of filter material as in Examples 1 to 3, but the polyethylene terephthalate nonwoven textile or woven textile used as the support material was prepared from a mixture of two polyethylene terephthalate polymers with different polymerisation stages. Polyethylene terephthalate prepared with a lower degree of polymerisation, and thus with a lower molar weight, acts in the fibres as a polymer melting at a lower temperature, which provides a temperature modification of the support textile, which is the subject of this patent.

Example 11

A window net for capturing a high proportion of ultra-fine particles and bacteria from the air was prepared in a sandwich arrangement of two polyester nets with a middle layer of polyvinylidene fluoride nanofibres with a basis weight of 0.06 g/m ² , corresponding to a pressure loss of 8 Pa at an air flow of 30 l/m ² h. The lower polyethylene terephthalate net was characterised by a basis weight of 24 g/m ² , an open area of 69%, a thickness of 110 pm, a square mesh size of 240 pm and a yarn diameter in the warp and weft of 55 pm. The upper polyethylene terephthalate net had a square mesh size of 1,000 pm and a yarn diameter in the warp and weft of 200 pm. Both nets were made of polyethylene terephthalate with a wide molecular weight distribution. The three-layer window net was prepared by pressing between two heated rollers at a temperature of 140°C and a pressure of 4 bar. The capturing efficiency of 70 nm in the area of the maximum penetrating particle size, measured according to EN 1822, was 10% at a frontal air velocity of 5.7 cm/s corresponding to an air flow of 30 l/m ² h.