Improved nanofiber membrane

Abstract

There is herein described an air permeable processed nanofiber membrane, and an associated method of forming an air permeable partially densified membrane. The resultant processed nanofiber membrane has more desirable acoustic qualities over the prior art, due to the partially densifying process.

Description

[0001] IMPROVED NANOFIBER MEMBRANE

[0002] FIELD OF THE INVENTION

[0003] The present invention relates to a processed nanofiber membrane. The processed nanofiber membrane may be used in a vent assembly, such as a vent assembly unit comprising a vent assembly and an electronic housing, and acoustic devices comprising such a vent assembly. The nanofiber membrane may be an electrospun membrane.

[0004] BACKGROUND OF THE INVENTION

[0005] Acoustic membranes in acoustic devices can allow sound to propagate through and past the membrane, and to and from an acoustic device. Acoustic membranes can also prevent ingress of water, dust, and other contaminants.

[0006] Acoustic vents may be used to protect one or more acoustic transducers of an electronic device, such as microphones or speakers, for example, from liquids or particulates, whilst allowing the pressure within an acoustic volume defined between the acoustic transducer and the acoustic vent to be equalised with the atmospheric pressure external of the electronic device within which the acoustic transducer is installed.

[0007] Accordingly, it is necessary for the acoustic vent to be permeable to air to allow the pressure equalisation and to prevent the ingress of particulates and liquids such as water.

[0008] In addition to the above required properties, it is also desirable for the acoustic vent to minimally impact the acoustics of the electronic device such that the performance of the acoustic transducer is maximised whilst protected by the acoustic vent.

[0009] Protective acoustic covers are common in the market, such as porous ePTFE membranes (US6512834B1). These membranes provide good protection, such as from liquids or particulates, and low acoustic insertion loss.

[0010] However, there remains a need for non-PFAS (per- and polyfluoroalkyl substances) materials to be used in such applications. Furthermore, maintaining the protection and improving acoustic characteristics are pushing the limits of materials design. Due to the recent regulatory concerns with the use of ePTFE (PFAS) family of polymers, there is a need for technologies that are versatile over different range of polymers.

[0011] Electrospinning is one of the industry preferred and practiced methods for preparing membranes comprising thin nanofibers, and is a process that can use a wide range of polymers. However, nanofiber membranes are typically of high porosity and membranes of such high porosity are typically low in bubble point and high in thickness and do not have optimal acoustic properties.

[0012] The present disclosure explains how to take the output of an industrial method of making nanofiber membranes (e.g. electrospinning, melt blowing, rotary jet spinning) and improve the membrane to result in property ranges suitable for the application of protecting acoustic transducers from the ingress of water meeting IPX8 standards.

[0013] SUMMARY OF THE INVENTION

[0014] According to a first aspect of the present disclosure there is provided an air permeable processed nanofiber membrane, wherein the air permeable processed nanofiber membrane comprises: a mean thickness of less than 20 microns; and a permeability, as measured by the method described herein, of less than or equal to 5 x10-15m2.

[0015] According to another aspect of the present disclosure, there is provided an air permeable nanofiber membrane, wherein the air permeable nanofiber membrane comprises: a mean thickness of less than 20 microns; and a permeability, as measured by the method described herein, of less than or equal to 5 x10-15m2.

[0016] Throughout this specification, the term ‘processed’ membrane refers to a membrane which has undergone a process in the manufacture of the membrane. The process may be any suitable process which results in a membrane with the resultant parameters, i.e. resultant parameters such as a mean thickness of less than 20 microns; and a permeability, as measured by the method described herein, of less than or equal to 5 x10-15m2.

[0017] The process may for example be a mechanical process. For example, the process may be a compressing, calendering, and / or rolling process. The process may be for example a rolling process, wherein the rollers are heated rollers. The process may be a combination of any of these methods. The compressing process may be done via any form of compressive force, such as plates pressed against one another for example.

[0018] The membrane may be processed chemically, for example through wetting with a liquid (for example a low surface tension fluid or solvent) and drying.

[0019] ‘Processed’ membranes may be defined relative to ‘unprocessed’ membranes, which have not gone through any steps to reduce the thickness and / or permeability.

[0020] The air permeable processed nanofiber membrane may have a thickness of from around 0.5 microns to around 20 microns. The air permeable processed nanofiber membrane may have a thickness of from around 0.5 microns to around 15 microns. The air permeable processed nanofiber membrane may have a thickness of from around 0.5 microns to around 12 microns. The air permeable processed nanofiber membrane may have a thickness of from around 0.5 microns to around 8 microns. The air permeable processed nanofiber membrane may have a thickness of from around 0.5 microns to around 6 microns. The air permeable processed nanofiber membrane may have a thickness of from around 5 microns to around 13 microns.

[0021] The thickness is measured via a thickness measurement, as described herein.

[0022] The air permeable processed nanofiber membrane may have an air permeability of from around 3 x10-19m2to around 5 x10-15m2. The air permeable processed nanofiber membrane may have a permeability of from around 3 x10-18m2to around 5 x10-15m2. The air permeable processed nanofiber membrane may have a permeability of from around 3 x10-17m2to around 5 x10-15m2. The air permeable processed nanofiber membrane may have a permeability of from around 3 x10-16m2to around 5 x10-15m2. The air permeable processed nanofiber membrane may have a permeability of from around 5 x10-16m2to around 5 x10-15m2.

[0023] The air permeability is measured via an Airflow Measurement, as described herein. The air permeable processed nanofiber membrane may have a mean porosity of between 10% and 80%. The air permeable processed nanofiber membrane may have a mean porosity of between 15% and 70%. The air permeable processed nanofiber membrane may have a mean porosity of between 20% and 60%. The air permeable processed nanofiber membrane may have a mean porosity of between 25% and 60%. The air permeable processed nanofiber membrane may have a mean porosity of between 30% and 60%. The air permeable processed nanofiber membrane may have a mean porosity of between 35% and 60%. The air permeable processed nanofiber membrane may have a mean porosity of between 40% and 60%.

[0024] The porosity is measured via a porosity measurement, as described herein.

[0025] The mean bubble point pressure of the air permeable processed nanofiber membrane may range from around 8 PSI to around 150 PSI. The mean bubble point pressure may range from around 8 PSI to around 135 PSI. The mean bubble point pressure may range from around 8 PSI to around 100 PSI. The mean bubble point pressure may range from around 8 PSI to around 70 PSI. The mean bubble point pressure may range from around 8 PSI to around 60 PSI. The mean bubble point pressure may range from around 8 PSI to around 20 PSI. The mean bubble point pressure may range from around 10 PSI to around 18 PSI. The mean bubble point pressure may range from around 14 PSI to around 18 PSI.

[0026] The bubble point pressure is measured via a bubble point pressure measurement, as described herein.

[0027] The transmission loss of the air permeable processed nanofiber membrane at 1 kHz may range from around 5dB to 30dB. The transmission loss at 1 kHz may range from around 5dB to 20dB. The transmission loss at 1 kHz may range from around 5dB to 18dB. The transmission loss at 1 kHz may range from around 7dB to 14dB. The transmission loss at 1 kHz may range from around 7dB to 11dB. The transmission loss at 1 kHz may range from around 7dB to 8dB. The transmission loss is measured via transmission loss measurement, as described herein. The absolute value of the phase angle at 1 kHz may be more than 45 degrees. The absolute value of the phase angle at 1 kHz may range from around 50 degrees to 90 degrees. The absolute value of the phase angle at 1 kHz may range from around 60 degrees to 90 degrees. The absolute value of the phase angle at 1 kHz may range from around 70 degrees to 90 degrees. The absolute value of the phase angle at 1 kHz may range from around 75 degrees to 90 degrees. The absolute value of the phase angle at 1 kHz may range from around 80 degrees to 60 degrees.

[0028] A phase angle which is closer to 0 degrees results in a purely resistive regime and a phase angle closer to + / - 90 degrees results in a purely reactive regime. Any region between +45 and -45 degrees can be considered to be predominantly resistive for a frequency range of 500 - 20 kHz. Beyond + / -45 degrees, for that particular frequency, the membrane can be considered as predominantly reactive.

[0029] The air permeable processed nanofiber membrane may have a mean mass per area of between 0.5 g / m2and 8 g / m2. Throughout this specification, when ranges are given, the endpoints are also considered to be included in the range given.

[0030] The air permeable processed nanofiber membrane may comprise polyurethane or polyimide or polyamide.

[0031] The air permeable processed nanofiber membrane may comprise a mixture of polyurethane or polyimide or polyamide.

[0032] The air permeable processed nanofiber membrane may be non-composite. Noncomposite may here be defined as not comprising layers. Non-composite may be defined as a membrane comprising a unitary layer. Non-composite may also be defined as a nonlayered membrane.

[0033] The air permeable processed nanofiber membrane may be an acoustic cover.

[0034] According to another aspect of the present disclosure, there is provided a vent assembly comprising the air permeable processed nanofiber membrane as previously described. According to another aspect of the present disclosure, there is provided an acoustic device comprising the air permeable processed nanofiber membrane as previously described.

[0035] According to another aspect of the present disclosure, there is provided a method of processing an air permeable nanofiber membrane to partially densify the membrane, the method comprising: compressing the nanofiber membrane through a plurality of rollers, wherein at least one of the rollers is heated.

[0036] One of the rollers may be a heated roller and may be maintained at a temperature of around at least 55 °C to around 170 °C. The roller may be maintained at a temperature of around at least 70 °C to around 170 °C. The roller may be maintained at a temperature of at least 100 °C to around 170 °C. The roller may be maintained at a temperature of at least 100 °C to around 150 °C.

[0037] The heated roller may be made from metal.

[0038] One of the rollers may be a non-heated roller and may be made from a polymer material.

[0039] The nanofiber membrane may pass through the rollers once. This may be referred to as a ‘single nip’.

[0040] The nanofiber membrane may pass through the rollers twice. This may be referred to as a ‘double nip’. The nanofiber membrane may pass through the rollers any number of times, to achieve a suitable compressed product.

[0041] The nanofiber membrane may pass through the rollers at a speed of between 0.2 m / min to 5 m / min.

[0042] The pressure between the plurality of rollers on the nanofiber membrane may be between 80 N / mm to 450 N / mm.

[0043] The method may result in an air permeable processed partially densified nanofiber membrane which has a bubble point pressure which is at least 50% higher than the unprocessed nanofiber membrane. The method may result in an air permeable processed nanofiber membrane as previously described.

[0044] Air permeable, in reference to the processed nanofiber membrane is to clarify that the material must have a measurable value of air permeability.

[0045] Thickness may be measured via non-contact measurement, i.e. no compression of the material by the measurement device.

[0046] BRIEF DESCRIPTION OF THE DRAWINGS

[0047] Embodiments of the present invention will now be described, by way of example only, with reference to the following figures:

[0048] Figure 1 is an example of a processed nanofiber membrane within a captive ring according to the present disclosure;

[0049] Figure 2 shows two scanning electron microscope (SEM) images, one of an unprocessed nanofiber membrane, and one of a processed nanofiber membrane;

[0050] Figure 3 is an illustration of the results of the process to partially densify a processed nanofiber membrane;

[0051] Figure 4 shows an example of a one-stage roller which can be used to process nanofiber membranes according to the disclosure;

[0052] Figure 5 shows an example of a two-stage roller which can also be used to process nanofiber membranes according to the disclosure;

[0053] Figure 6 is a graph showing the relationship between porosity and permeability of the processed nanofiber membranes described herein;

[0054] Figure 7 is a graph showing the relationship between the porosity and the thickness of the processed nanofiber membranes described herein; and Figure 8 is a graph showing the relationship between the transmission loss at 1 kHz and the absolute value of phase angle at 1 kHz of the processed nanofiber membranes described herein.

[0055] DETAILED DESCRIPTION OF THE DRAWINGS

[0056] While the making and using of various embodiments of the present invention are discussed in detail below, it should be appreciated that the present invention provides many applicable inventive concepts that can be embodied in a wide variety of specific contexts. The specific embodiments discussed herein are merely illustrative of specific ways to make and use the invention and do not delimit the scope of the invention.

[0057] To facilitate the understanding of this invention, a number of terms are defined below. Terms defined herein have meanings as commonly understood by a person of ordinary skill in the areas relevant to the present invention. Terms such as "a", "an" and "the" are not intended to refer to only a singular entity, but include the general class of which a specific example may be used for illustration. The terminology herein is used to describe specific embodiments of the invention, but their usage does not delimit the invention, except as outlined in the claims.

[0058] Figure 1 shows an example of an air permeable processed nanofiber membrane 100 as described herein, located within a captive ring 102. The membrane 100 may be housed within any device or apparatus, one example where the membrane may be used is in a vent assembly.

[0059] Vent assemblies can be used in a range of devices for many different purposes. For example, a vent assembly may be provided on a closed container to provide for pressure equalization within the closed container. Closed containers include for example, batteries. In electronic devices that comprise acoustic transducers, vent assemblies can be provided for pressure equilibrium purposes but also to protect the acoustic transducers from contact with contaminants such as particulates or liquids, whilst allowing sound transmission through the vent assembly. The materials used to make up the vent assemblies are typically required to be resistant to the passage of particulates and / or liquids, especially liquid water, whilst also permitting air flow through the vent assembly. In some applications, the impact of the membrane materials on the acoustic properties of the vent assembly also have to be considered.

[0060] Figure 2 shows two scanning electron microscope (SEM) images, one of an unprocessed nanofiber membrane, and one of a processed nanofiber membrane. The specific example is of a polyimide electrospun nanofiber membrane, the unprocessed nanofiber membrane is designated PI-07 (left image) and the processed nanofiber membrane is designated Pl- 07-P (right image) in the description.

[0061] It is clear from the images that the processed membrane, which was modified according to the method of the present disclosure, results in a thinner and less permeable membrane than the unmodified version. This is beneficial for acoustic performance, as outlined in the description.

[0062] Figure 3 is a comparison of a comparative membranes 206, and a partially densified membrane 208 according to the present disclosure. The membrane 206 on the left of the image is a comparative nanofiber membrane which has not been post-processed. According to the methods described herein, the membrane 206 is taken and processed according to methods described herein, the results of which are shown in the processed nanofiber membrane 208 on the right of Figure 3. The thickness and porosity of the processed nanofiber membrane 208 reduces, whilst the bubble point pressure increases.

[0063] Figure 4 shows an example of a one-stage roller 310 which can be used to process membranes 300 according to the disclosure. Figure 4 shows two rollers 302, 304, and a membrane 300 being compressed between them. The membrane 300 may pass through from left to right for example.

[0064] The top roller 302 is a heated roller and is made from steel. The roller 302 may be made from any metals material or any material which may be strong enough to compress the membranes herein described, whilst being able to be heated. The bottom roller 304 is made from Nylon, however the bottom roller may be made from any plastics material, and / or any suitable material for providing enough force to compress the membrane 300.

[0065] The top roller 302 and the bottom roller 304 may be inverted, and the specific arrangement shown in the figure should not be construed to be limiting.

[0066] The top roller 302 is heated and maintained at a chosen temperature. Heating may be done via an electric element. Alternatively, heating may be done by water, oil, or Infrared. The temperature of the heated roller may vary from around 50 degrees Celsius to around 170 degrees Celsius.

[0067] The bottom roller 304 may be actively cooled, to maintain a steady temperature. The bottom roller 304 may be maintained at room temperature.

[0068] The rollers 302, 304 are pressed together to form a compression force, which is exerted onto the membrane when it passes through.

[0069] The compression force may vary from 100 N / mm to 400 N / mm. The units of N / mm describe the force per mm of roll width over which the membrane is subjected to compression between the rollers.

[0070] Figure 5 shows an example of a two-stage roller 410 which can also be used to process membranes 400 according to the disclosure. In this example, there are three rollers; a top roller 402, a middle roller 404, and a lower roller 406. The membrane 400 can be seen to pass between the top roller 402 and the middle roller 404 from right to left. The membrane 400 then passes between the middle roller 404 and the lower roller 406 from left to right. Thus, the membrane 400 is compressed in two-stages. The membrane 400 is heated once, when it passes over the middle roller 404. This example set up should merely be taken as exemplary and should not be construed as limiting.

[0071] In this embodiment the middle roller 404 is heated and is made from steel. The top roller 402 and the lower roller 406 are made from Nylon and are actively cooled using a fan to maintain a steady temperature. The middle roller 404 may be made from any metals material, or any material which is capable of providing a compression force, and which can be heated. The top roller 402 and the lower roller 406 may be made from any plastics or composites material, or any material which can be cooled and can provide an adequate compression force.

[0072] As in the first embodiment, the rollers are pressed together to provide a clamping or compression force which is exerted to the membrane which passes through.

[0073] The compression forces between the rollers may be equal or they may differ. In this particular example, the compression forces are equal to each other, and range from around 100 N / mm to 400 N / mm.

[0074] The temperature of the heated roller may vary from around 50 degrees Celsius to around 170 degrees Celsius.

[0075] In both the one-stage roller 310 and the two-stage roller 410, the line speed at which the membrane travels may be kept constant, or it may vary along the length of the membrane. The line speed may be a constant 1 m / min.

[0076] Figure 6 is a graph showing the relationship between porosity and permeability of the membranes described herein. The comparative example membranes are shown via a circle, and the processed nanofiber membranes which fall within the scope of the claimed disclosure are shown via a dot.

[0077] As shown, there is a clear distinction in the location of the dots and the circles. The inventive examples (dots) are all located within the lower left of the graph (low porosity and low permeability), whereas the comparative examples (circles) are all located within the upper right of the graph (higher porosity and higher permeability).

[0078] The vertical line in Figure 6 shows the upper bound of the air permeability of the inventive example membranes. The two horizontal lines show the lower and upper bounds of porosity, of the inventive example membranes. Figure 7 is a graph showing the relationship between the porosity and the thickness of the membranes described herein. The comparative example membranes are shown via a circle, and the processed nanofiber membranes which fall within the scope of the claimed disclosure are shown via a dot.

[0079] As shown, there is a clear distinction in the location of the dots and the circles. The inventive examples (dots) are all located within the lower left of the graph (low porosity and low thickness), whereas the comparative examples (circles) are all located within the upper right of the graph (higher porosity and higher thickness).

[0080] The vertical line in Figure 7 shows the upper bound of the thickness of the inventive example membranes. The two horizontal lines show the lower and upper bounds of porosity of the inventive example membranes.

[0081] Figure 8 is a graph showing the relationship between the transmission loss at 1 kHz and the absolute value of phase angle at 1 kHz of the membranes described herein. The transmission loss is measured in decibels (dB) and the phase angle is measure in degrees.

[0082] As shown, there is a clear distinction in the location of the dots and the circles. The inventive examples (dots) are all located within the lower right diagonal section of the graph, whereas the comparative examples (circles) are all located within the upper left diagonal section of the graph.

[0083] The line which segments the graph can be described by the equation y = 0.2778x, where the variable y represents the transmission loss at 1 kHz (dB) and the variable x represents the absolute value of the phase angle at 1 kHz (°). The inventive examples all lie below this line on the graph. This line illustrates the equation given in the claims of the application, in reference to transmission loss at 1 kHz. The process described herein generally shifts the properties of a processed nanofiber membrane (compared to its unprocessed precursor) to have lower transmission loss at 1 kHz and higher absolute value of phase angle at 1 kHz, both of which changes are preferable. There is an ongoing need for improved acoustic membranes. Some embodiments described herein can advantageously achieve appreciable acoustic performance, whilst providing a mean thickness of less than 30 microns and a permeability, as measured by the method described herein, of less than or equal to 5 x10-15m2.

[0084] As used herein, the term “membrane” refers to an article, such as a sheet-like article, having three dimensions defined by three mutually perpendicular axes or directions in which two of the dimensions are typically larger than the third. For instance, the two larger dimensions are at least an order of magnitude greater than the smallest, third, dimension. Thus, the smallest, third dimension may represent the thickness of the membrane. This thickness can be measured along the third axis or direction, referred to herein as the thickness direction of the membrane.

[0085] The membrane may have opposing first and second surfaces, in which the surfaces are separated by the thickness of the membrane i.e. the shortest distance between opposing first and second surfaces of the membrane. This distance may be defined by a line orthogonal to the planes of both of the first and second surfaces. Thus, the thickness direction may be a direction parallel to a line defining the thickness of the membrane.

[0086] The first and second axes or directions are both orthogonal to the thickness direction of the membrane. The first and second directions therefore define a plane perpendicular to the thickness direction of the membrane. The plane defined by the first and second directions is therefore parallel to or overlaps with the planar sheet of the membrane.

[0087] As used herein, the term “membrane” refers to the processed nanofiber membrane. The nanofiber membrane may be an electrospun membrane. The nanofiber membrane may also be processed via other solvent spinning or melt spinning processes, like rotary jet spinning or melt blowing.

[0088] Nanofibers are fibers with diameters measured in nanometers, typically less than 1000 nm (1 micrometer). They are characterized by their small size and high surface area to volume ratio, leading to unique properties and diverse applications in various fields. Throughout this specification, the term should also be construed to include any fiber made through an electrospinning process. The membrane has a thickness of from about 1 microns ( m) to about 20 microns (pm), wherein the first and second directions are each orthogonal to the thickness direction. A membrane thickness of from about 1 pm to about 20 pm is beneficial for sound transmission, because generally the thinner the membrane, the better the sound transmission through the membrane. For example, a given membrane having a thickness of > 20 pm would generally have more sound transmission loss relative to an identical membrane differing only in that it has a thickness of less than about < 30 pm.

[0089] The membrane may have a thickness of at least 1 pm. An excessively thin membrane may have poor mechanical and strength properties.

[0090] In some embodiments, the membrane may have a thickness of from about 2 pm to about 20 pm. Such a membrane thickness results in excellent sound transmission properties.

[0091] The membrane may be used in a vent assembly, particularly for membrane applications meeting the ingress protection (IP) standards. The membrane prevents contaminants crossing the membrane and thereby contaminating any acoustic device protected by the membrane, for instance by meeting the IPX8 standard for the prevention of the ingress of dust.

[0092] As used herein, the term “vent assembly” refers to any vent assembly as described below. Vent assemblies can be used in a range of devices for many different purposes. For example, a vent assembly may be provided on a closed container to provide for pressure equalization within the closed container. Closed containers include for example, batteries. In electronic devices that comprise acoustic transducers, vent assemblies can be provided for pressure equilibrium purposes but also to protect the acoustic transducers from contact with contaminants such as particulates or liquids, whilst allowing sound transmission through the vent assembly. The assembly having membrane may further comprise one or more adhesive regions. The one or more adhesive regions may be present on one or both of the opposed external surfaces of the membrane. An adhesive region may be arranged around the perimeter of one or both of the external surfaces of the membrane. The adhesive region may comprise an adhesive, such as a pressure sensitive adhesive, a heat activated adhesive or a UV activated adhesive. The adhesive may be (i) an acrylic pressure sensitive adhesive such as Tesa® 75720 reinforced acrylic adhesive foam tape, Tesa® 4972 acrylic double-sided reinforced tape, an acrylic / silicone adhesive such as Tesa® 61532, a monolithic acrylic adhesive such as Tesa® 63305 or a bio-acrylic such as Tesa® 68875 supplied by Tesa SE or (ii) a phenolic resin / nitrile rubber heat activated film adhesive such as Tesa® 58469 supplied by Tesa SE or (iii) a UV activated acrylic adhesive such as AC-99005DT supplied by Advanced Adhesive Technology.

[0093] The membrane may also resist water entry, for instance meeting the IPX8 standard for the prevention of ingress of water when immersed at a depth of at least 1 meter, as measured by extended Water Entry Pressure (eWEP).

[0094] The membrane also has sufficient air permeability to allow for pressure equilibration between the two sides of the membrane, for instance between the external environment and the inside of an acoustic device through the membrane. This air permeability is represented by an airflow of at least 0.01 L / hr when evaluated based on the normalized airflow test described herein. Throughout this specification, ‘air permeability’ and ‘permeability’ are to be considered interchangeable.

[0095] Some embodiments of the present disclosure are directed to a predominantly reactive acoustic membranes. In a predominantly reactive mode, sound transmits predominantly through vibration of the membrane in the active area, rather than within the gas phase in open porosity of the membrane. This behaviour is associated with lower acoustic insertion loss at higher frequencies, which is beneficial for preserving sound fidelity in communication devices.

[0096] Also significant to sound transmission loss is the mean mass per area of the membrane. The membrane may have a mean mass per area of < 8 g / m2. The membrane may have a mean mass per area of > 0.5 g / m2. The membrane may have a mean mass per area of from about 0.5 g / m2to about 8 g / m2. A membrane mean mass per area of from about 0.5 g / m2to about 8 g / m2is beneficial for sound transmission, because generally the lower the mean mass per area of a membrane, the lower the sound transmission loss. For example, a membrane having a mean mass per area of 10 g / m2would generally have more sound transmission loss relative to an identical membrane differing only in that it has a mean mass per area of less than or equal to 5 g / m2. In some embodiments, the membrane may have a mean mass per area of from about 4.17 g / m2to about 5.62 g / m2. Such a membrane mean mass per area results in excellent sound transmission properties.

[0097] The mean mass per area of a membrane is measured as indicated in the test methods described herein. For a given bulk density, the mean mass per area is related to the thickness of the membrane.

[0098] In some embodiments, the membrane may have a bubble point pressure of at least 3 psi. For example, the bubble point pressure may be in a range of from 2 psi to 150 psi. In some embodiments, the bubble point pressure may be in a range of from 3.8 psi to 135 psi.

[0099] Examples:

[0100] Example processes to prepare polyimide, polyurethane, and polyamide nanofiber membranes are as follows:

[0101] Polyimide:

[0102] In some embodiments, non-composite membranes are initially formed by electrospinning the nanofibers. In certain embodiments, non-composite membranes are manufactured using GCM147(PI-07), GCM139 (PI-04) available from W. L. Gore & Associates, Inc. of Newark, Delaware and using the methods described herein.

[0103] A condensation polymerization reaction between a diamine monomer and a tetracid dianhydride was performed in a kettle with mechanical agitation in a high polar solvent. This process yielded a copolymerized polyimide acid spinning solution. The polymer solution was then spun with electrospinning equipment, in which high voltage was applied to direct the jet towards the other electrode at high speed where a collector is used to collect the fibres which are formed due to the forces on the electrospinning solution. The spun mat was heat treated to obtain an imidized copolymerized polyimide web that contains nanofibers. Two comparative examples, PI-04 and PI-07, were made through this process. Polyurethane: The polyurethane nanofiber membrane with the part number EKO-5 was purchased from Elmarco Ltd (Liberec, Czechia). Different lots were used and are reflected in Examples and data tables as PU-02, PU-08, and PU-12.

[0104] Polyamide: A polyamide 6 membrane was made according to patent US9101860B2, and is noted in data tables at PA-07.

[0105] An example heated compression process can be described as follows. The membrane is post processed to improve the bubble point and reduce the thickness and porosity. The membrane is compressed through rolls that are heated to at least 60 °C and compressed at a pressure of over 100 N / mm. In this process, the airflow of membrane drops, leading to an increase in the bubble point.

[0106] Test methods

[0107] Polymer density determination:

[0108] Polymer density was measured by a helium pycnometer (ACCUPYC® 1340, Micrometrics Instruments Corp., Norcross, GA). Before measuring, the pycnometer was calibrated using a steel metal ball with a known volume. This method determines the solid volume (for example excluding pores) of a material in a sample and dividing the mass of the sample by that volume yields a density in units of g / cm3. The polymer density is the density of the polymer that makes up the porous membrane, which is distinct from the density of the membrane (elsewhere defined as density_sample, the sample density). In later formulas, the polymer density is shown as density_polymer.

[0109] Areal weight determination:

[0110] An Chaus Explorer precision scale was used to determine the weight of a 0.025 m diameter disk (A=0,000213 mm2). The measured areal weight was multiplied by 1 / 0,000213 to yield areal weight in grams per square meter.

[0111] Porosity evaluation: The porosity was calculated by following formula: 100*(1-(density_polymer / density_sample)). The density of the sample, density_sample, was calculated by following formula: (sample areal weight) / (sample thickness), having units of g / cm3.

[0112] Normalized Airflow Measurement:

[0113] ATEQ Airflow is a test method for measuring laminar volumetric flows of air through a membrane sample or vent assembly according to the present disclosure. The sample (i.e. a vent assembly or microporous polymer membrane) is sealed in a fixture that allows for airflow to pass through the active area. In the case of a membrane sample, a circular area of membrane is sealed between an o-ring and a metal plate. In the case of a vent assembly, the part may be secured to a metal coupon or may be otherwise adhered to a solid surface, such that it is fixtured to allow air to pass through the active membrane area of the vent assembly. An ATEQ D520 Flowmeter is used to measure the airflow rate (L / h) through the membrane or vent assembly by challenging it with a pressure differential across the membrane or vent assembly in the range of 4-12 mbar of air pressure. For membrane samples, typical measurements used 2.99cm2area and 12 mbar pressure, but pressure was reduced in cases where the measured airflow exceeded the calibrated range of the equipment.

[0114] Airflow values are normalized by the sample area (e.g. the typical membrane area of 2.99 cm2or the active membrane area in a vent assembly) and test pressure to be presented in units of L / (hr*cm2*mbar). Such pressure and area normalizations allow for measurement of normalized airflow in membrane samples and vent assemblies of arbitrary area and permeability.

[0115] Permeability calculation:

[0116] The Darcy permeability of a sample can be calculated as follows:

[0117] Permeability (m2) = 7.44E-16 * Normalized Airflow(L / (hr*cm2*mbar)) * Thickness (microns)

[0118] Frazier calculation:

[0119] The Frazier permeability reading is the rate of flow of air in cubic cm per square cm of sample area per sec at a differential pressure drop across the test sample of 12.7 mm water column. Air permeability is measured by clamping a test sample into a circular gasketed flanged fixture which provides a circular opening of 17.2 cm diameter. The upstream side of the sample fixture is connected to a flow meter in line with a source of dry compressed air. The downstream side of the sample fixture is open to the atmosphere. The flow rate through the sample is measured and recorded as the Frazier number.

[0120] An estimate of a Frazier air permeability, in units of ccs (cubic cm per square centimeter per sec), can be made using the following equation.

[0121] Frazier = Normalized Airflow(L / (hr*cm2*mbar)) 12.887

[0122] Bubble point pressure:

[0123] The bubble point was measured according to the general teachings of ASTM F316-03 using a Capillary Flow Porometer (Model 3G zh from Quantachrome Instruments). The sample holder comprised a porous metal plate (Part Number: 196450, Anton Paar), 25.4 mm in diameter and a plastic mask (Part Number ABF-300, Professional Plastics), 18mm inner diameter x 24.5 mm outer diameter. The sample was placed in between the metal plate and the plastic mask. The sample was then clamped down and sealed using an O- ring (Part Number: 193798, Anton Paar). The sample was wet with the test fluid (Silicone fluid, 10 cSt, having a surface tension of 19.75 dynes / cm) and the test was run using the built-in procedure that determines the pressure at which fluid is evacuated from the pores and airflow through the membrane is detected.

[0124] Transmission loss and phase testing:

[0125] Transmission loss and phase testing were performed in accordance with ASTM-E2611- 09, which is the standard test method for measuring normal incidence sound transmission loss and phase based on the 4 microphone transfer matrix method. The transfer matrix of the assembly is measured and we use T12 element of the transfer matrix as the acoustic impedance value for all the assemblies described in the examples.

[0126] An impedance tube was used to make measurements across a frequency range of 500 Hz to 20,000 Hz. The inner diameter of the tube was 8 mm. Other dimensions of the tube spacing and size, tube length and wall thickness were all designed in accordance with the requirements called out in the ASTM E1050-12 and ASTM E2611-09 test standards. A JBL 2426H compression driver was mounted at one end of the tube and powered by a Bruel and Kjaer Type 2735 amplifier connected to a 31-band ART 351 graphic equalizer. The measurement system used 4 Bruel and Kjaer Type 4138 microphones connected to a 4 channel Bruel and Kjaer Type 3160-A-042 LAN-XI Frontend with a generator output. Data was acquired and processed using Bruel and Kjaer PULSE Labshop with Type 7758 Acoustic Material Testing Software, version 21.

[0127] The sample assemblies that were tested had an inner diameter of 1 .5 mm, which is smaller than the inner diameter of the impedance tube. A pair of conical adapters was therefore required in order to mount the sample assemblies. The convergent cone had an inlet diameter of 8 mm and an outlet diameter of 1.5 mm. The divergent cone had an inlet diameter of 1.5 mm and an outlet diameter of 8 mm.

[0128] When using conical adapters, additional processing of the data was required to account for the converging geometry of the cones. Theoretical equations were derived to calculate the transfer matrices of the conical adapters and can be found in the literature (Hua, X. and Herrin, D., "Practical Considerations when using the Two-Load Method to Determine the Transmission Loss of Mufflers and Silencers," SAE Int. J. Passeng. Cars - Meeh. Syst. 6(2): 1094-1101 , 2013 & Mechel, F. P. (2008). Formulas of Acoustics. New York, NY: Springer).

[0129] Thickness:

[0130] The thickness was measured using a laser micrometer (Keyence model no. LS-7010). A metal cylinder was aligned between a laser micrometer source and a laser micrometer receiver. The shadow of the top of the cylinder is projected onto a receiver. The position of the shadow was then reset as the “zero” reading of the laser micrometer. A single layer of membrane is draped over the surface of the metal cylinder without overlap and without wrinkles, casting shadow onto the receiver. The laser micrometer then indicated the change in the position of the shadows as the thickness of the sample. Each thickness was measured three times and averaged for each sample.

[0131] Specific Examples: According to the methods described herein to partially densify electrospun membranes, various comparative (unprocessed) membranes were prepared or purchased, and had the method applied to them to result in membranes according to the invention.

[0132] Example 1

[0133] A comparative polyimide electrospun membrane, denoted as PI-04 herein, was used as a precursor to the inventive example. PI-04 had the properties as outlined in Table 1 .

[0134] PI-04 was then compressed at a line speed of 1 m / min between a heated chrome steel roller and a nylon roller at an approximate temperature of 150 °C with a setpoint line force of 400 N / mm. The resultant properties of PI-04-P (P indicates processed version) are given in Table 1 . It should be noted that the processed membrane PI-04-P should not be considered to be an example which falls within the scope of the claims. This is because the PI-04 membrane has a high mass per area of 8.9 g / m2and high thickness of 97.33 pm. The post processing of this membrane results in a porosity of 77%, which results in a material which does not have the performance attributes to fall within the scope of the claimed disclosure.

[0135] Example 2

[0136] A comparative polyimide electrospun membrane, denoted as PI-07 herein, was used as a precursor to the inventive example. PI-07 had the properties as outlined in Table 1 .

[0137] PI-07 was then compressed at a line speed of 1 m / min between a set of double nip compression rollers (double nip means membrane passes through the rollers twice) consisting of a pair of heated chrome steel roller at an approximate temperature of 150 °C and a nylon roller with a setpoint line force of 400 N / mm in each nip. The resultant properties of PI-07-P (P indicates processed version) are given in Table 2.

[0138] Example 3

[0139] A comparative polyurethane electrospun membrane, denoted as PU-02 herein, was used as a precursor to the inventive example. PU-02 had the properties as outlined in Table 1.

[0140] PU-02 was then compressed was then compressed at a line speed of 0.3 m / min between a heated chrome steel roller and a nylon roller at a temperature of 150 °C with a line force of 400 N / mm. The temperature that is stated is the temperature set point for the steel roller. The resultant properties of PU-02-P (P indicates processed version) are given in Table 2.

[0141] Example 4a

[0142] A comparative polyurethane electrospun membrane, denoted as PU-08 herein, was used as a precursor to the inventive example. PU-08 had the properties as outlined in Table 1.

[0143] PU-08 was then compressed at a line speed of 1 m / min between a heated chrome steel roller and a nylon roller at an approximate temperature of 100 °C with a setpoint line force of 100 N / mm. The resultant properties of PU-08a-P (P indicates processed version) are given in Table 2.

[0144] Example 4b

[0145] The same comparative polyurethane electrospun membrane as the previous example, denoted as PU-08 herein, was used as a precursor to the inventive example. PU-08 had the properties as outlined in Table 1.

[0146] PU-08 was then compressed at a line speed of 1 m / min between a heated chrome steel roller and a nylon roller at an approximate temperature of 60 °C with a setpoint line force of 400 N / mm. The resultant properties of PU-08b-P (P indicates processed version) are given in Table 2.

[0147] Example 5

[0148] A comparative polyurethane electrospun membrane, denoted as PU-12 herein, was used as a precursor to the inventive example. PU-12 had the properties as outlined in Table 1. PU-12 was then compressed at a line speed of 1 m / min between a set of double nip compression rollers consisting of a pair of heated chrome steel roller at an approximate temperature of 70 °C and a nylon roller with a setpoint line force of 200 N / mm in each nip. The resultant properties of PU-12-P (P indicates processed version) are given in Table 2.

[0149] Example 6

[0150] A comparative polyamide electrospun membrane, denoted as PA-07 herein, was used as a precursor to the inventive example. PA-07 had the properties as outlined in Table 1. PA-07 was then compressed at a line speed of 1 m / min between a heated chrome steel roller and a nylon roller at an approximate temperature of 70 °C with a setpoint line force of 400 N / mm. The resultant properties of PA-07-P (P indicates processed version) are given in Table 2.

[0151] The present disclosure may also relate to an acoustic device comprising the nanofiber membrane described herein. The acoustic device may be one or both of a speaker and a microphone.

[0152] The application describes how to make improved nanofiber membranes by adding post processing step that partially densifies the membrane, whilst still remaining porous in order to be compatible for venting applications. During this process, the thickness of the membrane decreases resulting in improved acoustic performance. The acoustic mode of the membrane also transforms to more of a predominantly reactive nature which can be analysed through looking at the phase angle (tan delta) of the membrane using the measurement method described herein. The bubble point increases (maximum pore size decreases) resulting in improved protection from the ingress of liquids meeting the standards of IPX8 protection.

[0153] One method to process nanofiber membranes is to use a calendering process (compression of the membrane under a certain temperature).

[0154] An example of how the electrospinning process works is given below.

[0155] A collection of needles are fed with polymer solution, and are connected to a power supply such that an electrostatic potential of tens of thousands of volts is established between the needles and a counter-electrode. The established electric field pulls the polymer solution into the electric field toward the counter-electrode, and nanofibers are formed as the accelerating solution is pulled and solvent is evaporated.

[0156] The created nanofibers are then collected on a “removable processing carrier” e.g. siliconized paper, non-porous Polyethylene Film, or Polypropylene spunbond which is moving continuously through the machine above the spinning electrode and below the counter-electrode, so as after creation of a nanofiber nonwoven membrane the removable processing carrier can be peeled off the nanofiber nonwoven membrane to obtain a standalone nanofiber membrane.

[0157] Whilst specific embodiments of the present invention have been described above, it will be appreciated that departures from the described embodiments may still fall within the scope of the present invention.

[0158] All ranges described herein are exemplary in nature and include any and all values in between. The terms “substantially”, “approximately” and “about” used herein are interchangeable and refer to a measurement that includes the stated measurement and any measurements reasonably close to the stated measurement. Measurements that are reasonably close to the stated measurement deviate from the stated measurement by a reasonably small amount as understood and readily ascertained by individuals having ordinary skill in the relevant arts. Such deviations may be attributable to measurement error, differences in measurement and / or manufacturing equipment calibration, human error in reading and / or setting measurements, minor adjustments made to optimize performance and / or structural parameters in view of differences in measurements associated with other components, particular implementation scenarios, imprecise adjustment and / or manipulation of objects by a person or machine. In the event it is determined that individuals having ordinary skill in the relevant arts would not readily ascertain values for such reasonably small differences, the terms “about” and “approximately” can be understood to mean plus or minus 10% of the stated value.

[0159] Throughout the description and claims, the terms take the meanings explicitly defined herein, unless the context clearly dictates otherwise.

[0160] The phrases “in one embodiment”, “in an embodiment” and “in some embodiments” etc. as used herein do not necessarily refer to the same embodiment(s), though they may. Furthermore, the phrases “in another embodiment” and “in some other embodiments” as used herein do not necessarily refer to a different embodiment, though they may. All embodiments of the disclosure are intended to be combinable.

[0161] The terms “comprises” and “comprising” mean to include but not limited to, such that further features may be present. The terms may also mean to consist of. All references and test methods cited herein are incorporated by reference in their entireties.

[0162] TABLE 1 : comparative examples TABLE 2: Inventive examples

Claims

1. CLAIMS1. An air permeable processed nanofiber membrane, wherein the processed nanofiber membrane comprises: a mean thickness of less than 20 microns; and a permeability, as measured by the method described herein, of less than or equal to 5 x10'15m2.

2. An air permeable processed nanofiber membrane according to claim 1 , wherein the membrane has a mean porosity of between 20% and 70%.

3. An air permeable processed nanofiber membrane according to any preceding claim, wherein the mean bubble point pressure ranges from around 8 PSI to around 150 PSI.

4. An air permeable processed nanofiber membrane according to any preceding claim, wherein the transmission loss at 1 kHz ranges from around 7 dB to 20 dB.

5. An air permeable processed nanofiber membrane according to any preceding claim, wherein the absolute value of the phase angle at 1 kHz is more than 45 degrees.

6. An air permeable processed nanofiber membrane according to any preceding claim, wherein the membrane comprises a polyurethane, polyimide, polyamide or mixtures thereof.

7. An air permeable processed nanofiber membrane according to any preceding claim, wherein the membrane has a transmission loss at 1 kHz equal to or below the line defined by the equation (Transmission loss at 1 kHz, dB) = 0.2778*(Absolute Value of Phase Angle, degrees).

8. An air permeable processed nanofiber membrane according to any preceding claim, wherein the membrane has a mean mass per area of between 0.5 g / m2and 8 g / m2.

9. An air permeable processed nanofiber membrane according to any preceding claim, wherein the nanofiber membrane is made through melt spin processing.

10. An air permeable processed nanofiber membrane according to any preceding claim, wherein the nanofiber membrane is made through solvent spin processing.11 . An air permeable processed nanofiber membrane according to any preceding claim, wherein the nanofiber membrane is an electrospun nanofiber membrane.

12. An air permeable processed nanofiber membrane according to any preceding claim wherein the membrane is non-composite.

13. An air permeable processed nanofiber membrane according to any preceding claim, wherein the membrane is an acoustic cover.

14. An air permeable processed nanofiber membrane according to any preceding claim, wherein the nanofiber membrane is processed via compressing the membrane through a plurality of rollers, wherein at least one of the rollers is heated.

15. A vent assembly comprising the nanofiber membrane according to any of claims 1 to 14.

16. An acoustic device comprising the nanofiber membrane according to any of claims 1 to 14.

17. A method of processing an air permeable nanofiber membrane to partially densify the membrane, the method comprising: compressing the nanofiber membrane through a plurality of rollers, wherein at least one of the rollers is heated.

18. A method according to claim 17, wherein one of the rollers is a heated roller, and is maintained at a temperature of at least 55 °C to around 170 °C.

19. A method according to claim 18, wherein the heated roller is made from metal.

20. A method according to any of claims 17 to 19 wherein one of the rollers is a nonheated roller and is made from a plastics material.

21. A method according to any of claims 17 to 20, wherein the membrane passes through the rollers once.

22. A method according to any of claims 17 to 20, wherein the membrane passes through the rollers twice.

23. A method according to any of claims 17 to 22, wherein the membrane passes through the rollers at a speed of between 0.2 m / min to 5 m / min.

24. A method according to any of claims 17 to 23, wherein the pressure between the plurality of rollers on the membrane is between 80 N / mm to 450 N / mm.

25. A method according to any of claims 17 to 24, wherein the method results in a processed partially densified membrane which has a bubble point pressure which is at least 50% higher than the unprocessed nanofiber membrane.

26. A method according to any of claims 17 to 25, wherein the method results in a nanofiber membrane according to any of claims 1-14.

27. An air permeable processed nanofiber membrane according to any of claims 1-14, wherein the membrane is processed according to the method of any of claims 17- 26.

Images