Patent Description:
The present invention refers in particular to a composite filtering medium in the form of fabric for use as protective medium for electro-acoustic components, in the consumer-electronics field, such as speakers, receivers, microphones, etc..

Different fabrics are known which are used for protecting acoustic devices, with the aim of providing an adequate protection against external agents and at the same time a sufficient acoustic performance.

Conventional composite fabrics used in the sector described above, in which the meshes are coated with a layer of nanofibres, have the drawback of not being able to withstand temperatures of up to <NUM>. In fact, the fibres that form the fabric tend to degrade as a result of heat and, basically, do not possess the thermal profile suited for ensuring the required performance.

A further disadvantage of prior composite materials is represented by the poor adhesion between the nanofibres and the monofilaments that form the meshes of the fabric.

Document <CIT> discloses that when a polymer of the support fibers is compatible with the nanofiber polymer, permits in situ bonding of the nanofiber layer directly to the scrim, without addition of a separate adhesive between the webs.

The aim of the present invention is to provide a composite filtering medium that is improved as compared to the filtering media so far known and presents a higher performance.

In particular, an object of the invention is to provide a new composite fabric with filtering action for electro-acoustic components which, by presenting a filtering performance that is equivalent to or higher than that of the composite fabrics according to the prior art, will be able to withstand temperatures of even up to <NUM>.

A further object of the invention is to provide a composite fabric of the type described above, capable of providing an improved adhesion between the filaments of the fabric meshes and the nanofibres of the coating, even without the use of adhesivizing additives.

Within the above aim, an object of the invention is to provide a filtering medium that will be particularly useful in the production of hearing aids and acoustic devices.

Another object of the invention is to provide a filtering medium capable of ensuring adequate features, at least similar to those of a conventional fabric, but with an improved protective capacity.

A further object of the invention is to provide a filtering medium that may ensure at the same time better acoustic performance and better characteristics of protection from intrusion of particulate.

Yet another object of the invention is to provide a filtering medium that will be able to guarantee values of air permeability and acoustic impedance at least equal to those of conventional fabrics, but with a degree of protection against intrusion of metal dust, textile dust, etc., that is better than that of a conventional fabric.

A further object of the invention is to provide a filtering medium that may advantageously be used for the protection of electro-acoustic components in the consumer-electronics field, such as speakers, receivers, microphones, etc..

Yet a further object of the invention is to provide a filtering medium that will be able to guarantee the widest possible reliability and safety in use.

These and other objects, which will become more apparent hereinafter, are achieved by a composite filtering medium and by a method for the production thereof, as specified in the appended claims.

Further characteristics and advantages of the subject of the present invention will become more apparent from an examination of the description of a preferred, though not exclusive, embodiment of the invention, illustrated by way of indicative and non-limiting example in the attached drawings.

With particular reference to the numerals present in the aforesaid figures, the composite filtering medium <NUM> according to the invention comprises a fabric of the type with weft and warp, formed of PEEK threads or monofilaments <NUM>, on the surface of which polyimide nanofibres <NUM> are deposited by electrospinning.

The monofilaments and nanofibres are characterized by good properties of thermal resistance, expressed in terms of melting temperature and/or glass transition temperature, with a thermal range from room temperature up to <NUM>.

The base fabric used in the preparation of the composite filtering medium according to the invention is formed of threads or monofilaments of polyether ether ketone (PEEK), with in general from <NUM> to <NUM> threads/cm, with a diameter from <NUM> to <NUM>.

Specifically, base fabrics capable of withstanding high process temperatures (up to <NUM>), both in one-step thermal processes and in thermal recycling or reflow processes, i.e. processes in which a number of consecutive cooling/heating cycles are present, are suitable for the invention.

For the finishing operations and further surface treatments it is possible to use metallization, washed and heat-set "white" fabric, coloured fabric, fabric subjected to plasma treatment, hydrophobic treatment, hydrophilic treatment, antibacterial treatment, antistatic treatment, and the like.

Preferred for the invention the monofilament fabric of polyether ether ketone (PEEK), is made with <NUM> threads/cm, a diameter of <NUM>, a mesh opening of the base fabric of <NUM>, a weave with a weight of <NUM>/m<NUM>, and a thickness of <NUM>.

The nanofibres of polyimide (PI) are capable of withstanding high process temperatures of up to <NUM>, both in one-step thermal processes and in reflow cycles, preferably with a diameter of the nanofibres comprised between <NUM> and <NUM>.

Preferred for the invention the polyimide (PI) nanofibres have a diameter of between <NUM> and <NUM>.

The electrospinning process for formation of the nanofibres and subsequent deposition thereof on the base fabric comprises injection of the material for formation of the nanofibres, dissolved in an appropriate solvent or mixture of solvents, through a nozzle in order to spread it on an electrode.

Thanks to the potential difference between the nozzle and the electrode, the nanofibres are formed as a result of evaporation of the mixture of solvents due to the electrical field and stretching of the polymer deposited on the electrode by means of the nozzle.

The nanofibres thus formed are then stretched and subsequently deposited on the base fabric.

Furthermore, the nanofibres <NUM> thus formed, in an innovative way as compared to the prior art, are not only deposited on the base fabric, but also firmly adhere thereto, without the aid of glues and/or adhesive agents applied on the base fabric and/or on the nanofibre layer, thanks to the use of some process parameters that will be clarified hereinafter, which favour a partial and surface swelling <NUM> of the threads <NUM> forming the base fabric.

This phenomenon of swelling <NUM> of the base threads <NUM>, which does not affect the structural properties of the base fabric, favours a firm adhesion of the nanofibres <NUM> arranged on the base threads <NUM>, enabling formation of a product that is more stable from the standpoint of integrity of adhesion between the base fabric and the nanofibres during the various machining processes to which the filtering medium is subjected.

According to the invention, this result is achieved by subjecting to an electrospinning process a solution of the polyimide for formation of the nanofibres <NUM> in a mixture of dimethyl acetamide (DMAc) and N-methyl-<NUM>-pyrrolidone (NMP) solvents, in excess of NMP. During the electrospinning process, polyimide nanofibres are thus obtained, which deposit on the threads <NUM> made of polyether ether ketone (PEEK). During this deposition step, the above-described solution of solvents, still carried by the nanofibres <NUM>, moistens the threads <NUM> over their surfaces of contact with the nanofibres, thus forming on the surfaces themselves softened parts and swellings <NUM>, which, in the electrospinning process, incorporate part of the nanofibres <NUM>, thus contributing to their adhesion or their anchorage to the threads <NUM> (<FIG>).

Consequently, according to the invention, during the electrospinning process not only are the nanofibres <NUM> deposited on the monofilaments <NUM>, but also a phenomenon of adhesion or relative fixing between the threads and the nanofibres takes place, which contributes to the stability of the composite fabric <NUM>. The solvents are mixtures of DMAc and NMP in a ratio of <NUM>:<NUM>, with NMP present in excess, in particular in an amount greater than or equal to <NUM>% by weight in the mixture of the solvents. Hence, in this way, adhesion of the nanofibres is favoured, without impairing the structural properties of the base fabric. <FIG>, <FIG> are SEM images that explain the phenomenon just described.

Table <NUM> provides a comparison of the properties of thermal resistance in terms of percentage variation of air permeability (Δ % of A. ) before and after one or more reflow cycles between the filtering medium according to the invention and the filtering medium of to the prior art at different temperatures and with different times of use, specifically <NUM> for <NUM> hours (temperature and times of use commonly required for the filtering media according to the prior art); <NUM> for times ranging from <NUM> to <NUM> hours (temperature and times of use commonly required for filtering media operating at high temperatures).

Table <NUM> then provides a comparison of the percentage difference of air permeability, measured before and after one or more reflow cycles from <NUM> to <NUM> and from <NUM> to <NUM>, between the filtering medium according to the invention PROTO <NUM> and the filtering medium according to the prior art Aethex <NUM>. Air permeability is measured in l/m<NUM>s-<NUM> at an air pressure of <NUM> Pa.

From this table it emerges that the filtering medium according to the invention has the same filtering capacity as the filtering medium according to the prior art in the thermal range <NUM>-<NUM> and in the typical times of use of the latter and that only the filtering medium according to the invention can be used at high temperatures, for different times, and in a number of reflow cycles. For this reason, the data measured at <NUM> are not available for the composite fabric Aethex <NUM> as they are not measurable. It is known that an indication of damage to a filtering medium during or after its use is an increased or decreased air permeability thereof as compared to its initial state of non-use, in a percentage of tolerance higher than the physiological one of the process, which in general is deemed acceptable within <NUM>%, since this behaviour implies possible damage to the morphology of the filtering medium occurred during its use. Instead, variations of air permeability in excess or in defect with respect to its initial state of non-use comprised in the range of tolerance of the process itself are an indication of an unaltered capacity of protection of the filtering medium during its use. In the specific case, there may be noted a reduction in the air permeability of the filtering medium comprised in the range of tolerance of the process, i.e., lower than <NUM>%, confirming the filtering capacity of the composite medium according to the present invention.

<FIG> illustrates the composite filtering medium according to the present invention, with a view on the surface of the PEEK monofilaments of the base fabric in the electrospinning treatment with the DMAc-NMP solvents in a weight percentage ratio of <NUM>:<NUM> and with polyimide nanofibres, before (A) and after various thermal recycling cycles (B-F), where specifically:.

From the images presented it may be noted that in all the study cases of thermal recycling from B to F, the morphological appearance of the filtering medium according to the invention, understood as homogeneity of coating of the meshes and adhesion of the polyimide nanofibres to the PEEK fabric, thanks to the formation of swellings on the surface of the fabric itself described previously, remains unaltered as compared to the morphological appearance that the filtering medium itself had prior to its use, confirming the thermal resistance of the filtering medium according to the present invention in regard to the thermal-recycling tests performed.

The following Table <NUM> shows the properties of air permeability, pore size, and acoustic impedance of various prototypes of filtering media according to this invention, in comparison with the corresponding properties of the PEEK <NUM> base fabric alone, i.e., without nanofibre coating.

In this table, the pore is that of the fabric, made up of the combination of the pores present on the base fabric and of the pores formed on the nanofibre coating. Air permeability is measured in l/m<NUM>s-<NUM> at an air pressure of <NUM> Pa.

- PEEK <NUM> is the base fabric according to the invention, having <NUM> threads/cm, a diameter of <NUM>, a mesh opening of the base fabric of <NUM>, a weave with a weight of <NUM>/m<NUM>, and a thickness of <NUM>;
- PROTO <NUM> is the composite fabric according to the invention, formed by the PEEK <NUM> base fabric with coating of polyimide nanofibres, obtained by electrospinning of a solution of polyimide in DMAc/NMP in a ratio of <NUM>/<NUM>;
- PROTO <NUM>-<NUM> are the composite fabrics according to the invention, similar to PROTO <NUM> and obtained with different parameters of the electrospinning process.

Table <NUM>. Properties of air permeability and pore size of the base fabric and of some prototypes of the filtering medium according to the present invention.

From Table <NUM> above it may be noted that, in terms of air permeability, in the electrospinning process used in the preparation of PROTO <NUM> to <NUM> according to the invention, the meshes of PEEK fabric are coated in a progressively increasing way with nanofibres, thus obtaining decreasing pore sizes and increasing acoustic impedances.

The following Table <NUM>, instead, shows the properties of air permeability of standard fabrics, where present, having pore sizes comparable with those of the various prototypes according to the present invention, where:.

- PES <NUM>/<NUM>, PES <NUM>/<NUM> is polyester with a mesh opening of <NUM> and <NUM>, respectively, and <NUM>% and <NUM>% of free surface in <NUM><NUM> of fabric, respectively.

Table <NUM> highlights that, with comparable pore size, in the case of PROTO <NUM> and <NUM> there is an air permeability much greater than in the case of the fabric, thus ensuring a better acoustic behaviour. Instead, since no standard reference fabric is available for the comparison with PROTO <NUM> and PROTO <NUM>, it is evident how only the composite fabric according to the present invention can ensure the described properties of air permeability and protection against particulate.

Finally, a further object of the invention is to provide a new composite fabric with filtering action for electro-acoustic components that, in addition to being able to withstand temperatures of even up to <NUM>, has a filtering performance that is equivalent to or higher than that of composite fabrics of the prior art.

In the case under examination, different prototypes have been prepared having the same air permeability as that of composite fabrics according to the prior art, and, given the same properties of air filtering, filtering media have been obtained with pore sizes comparable to or smaller than those of the corresponding composite fabrics according to the prior art.

By adding a plasma treatment to the filtering medium according to the present invention, in combination with the use of polyimide nanofibres <NUM>, in addition to increasing the water-column resistance already known in the case of the filtering media belonging to the prior art, the energy required for removing the oil deposited on the surface of the filtering medium has been drastically reduced as compared to that required by the filtering media according to the known technology, thus obtaining a better protective performance than the latter's.

Appearing in <FIG> are the pressures required for removing completely the oil deposited on the surface of the filtering media according to the prior art and according to the present invention, which have an air permeability comparable with that of the various prototypes according to the present invention.

From <FIG> it emerges in particular that the pressure required for clearing completely the oil from the composite fabric is lower for the composite fabrics according to the invention than for those of the prior art, which renders cleaning of the fabric simpler and faster.

The present invention has a wide range of advantageous applications.

An example of practical application of the present invention regards filtering, even at a high temperature.

As compared to the nanomesh of a conventional type, the filtering medium made of PEEK and polyimide can be used also in several other applications; for example, it can be used in applications where the filtering medium will be subjected to high process temperatures, and it is necessary for its properties to remain unaltered, i.e., for the fabric and the nanofibres not to undergo any modification due to temperature.

An innovative application of the filtering medium according to the present invention regards MEMS (Micro Electro-Mechanical Systems) technology.

Specifically, MEMS devices are characterized by two vent ports, one internal to the MEMS device, which involves the use of a protective filtering medium not necessarily post-treated via die cutting, and one external to the device itself, which involves the use of a protective filtering medium necessarily post-treated using die-cutting means, which allows problems of overheating and equalization of the pressure of the electronic components to be overcome during their use.

This makes it possible to withstand, in the various steps of assembly and use, very high process temperatures, even close to <NUM>, favouring the passage of air between the inside and the outside of the MEMS devices.

The need to protect both of these vents with protective membranes permeable to air and resistant to high temperatures, close to <NUM>, is fundamental for preventing contamination by humidity, particulate, dust, oil, etc., and more in general for preventing reduction in sound transmission performance, for enabling recirculation of air flows, and finally, since they are resistant to high temperatures, for favouring assembly of the MEMS devices previously integrated with protective layers, a technical choice that facilitates and renders more economic the production of a MEMS device, which, if assembled without protection of the vents, can undergo damage during the assembly process.

In the current state of the art, the protection of the internal and external vents of the MEMS devices is implemented in two ways: i) either by not applying any protective layer, with consequent very short service life of the device; ii) or else by applying protective adhesive tapes.

The choice of the application of a protective adhesive tape cannot be considered satisfactory as compared to non-protection of the vent port itself, as these tapes are not permeable to air, and hence, albeit protecting the device from several contaminants, do not enable an adequate recirculation of air, with consequent problems of overheating and non-equalization of the pressure inside the MEMS.

Hence, the use of a filtering medium made with a nanomesh of a known type, with the added advantage of thermal resistance according to the present invention, represents an advance with respect to the current state of the art of MEMS technology, both for internal protection and for external protection of MEMS devices.

In acoustic applications, the invention can be used in those situations where the filtering medium is co-moulded with high-melting polymers, thus ensuring a thermal resistance during the process.

In addition to the use in applications involving high temperatures where the conventional nanomesh could not be used for evident physical limits, the second advantage of the present invention lies in the improved and intimate adhesion between the supporting fabric and the nanofibres.

In this case, in fact, this property enables:.

With firm adhesion of the nanofibres to the substrate the vibrational problem just described disappears, and hence the sound is cleaner.

Typically, it is possible to study the distortion of the sound or the onset of undesired noise caused by microvibrations induced by the nanofibre layer electrospun on the substrate through acoustic analyses referred to as "Rub&Buzz".

<FIG> shows a graph with the Rub&Buzz analyses for a standard nanomesh specimen (Aethex <NUM>), a specimen of PROTO <NUM>, i.e., the filtering medium according to the present invention, and a specimen of fabric Acoustex <NUM> having the same acoustic impedance as Aethex <NUM> and PROTO1.

From the graph appearing in <FIG> it may in general be noted that for all the specimens measured, Aethex <NUM>, PROTO <NUM>, Acoustex <NUM>, the percentage of vibration induced by the filtering medium is below <NUM>% (reference target for commercial acoustic devices).

Moreover, by comparing PROTO <NUM> with the two reference specimens, lower and consequently improved percentage values of Rub&Buzz may be noted for all the specimens PROTO <NUM> as compared to Aethex <NUM> and Acoustex <NUM>, thanks to a better adhesion of the nanofibre layer on the base fabric.

It is to be pointed out that it is possible to add a plasma treatment to the filtering medium according to the present invention, thus achieving benefits on two fronts:.

Moreover, with plasma treatment, the energy required for removing the oil deposited on the surface of the filtering medium is reduced.

It has in practice been found that the invention achieves the intended aim and objects.

A filtering medium has in fact been obtained constituted by a material made up of polyimide nanofibres deposited via electrospinning on a monofilament fabric, of the type weft and warp, made of polyether ether ketone (PEEK) threads or monofilaments.

During deposition the nanofibres are arranged on the threads, on the crossings between them and in the meshes of the fabric; being arranged in the meshes, they reduce the mean aperture and the free surface of the fabric.

In this way, it is possible to obtain values of air permeability and acoustic impedance equal to those of standard fabrics.

At an equal value of permeability/impedance the composite fabric provides a protection against intrusion of dust (metal dust, textile dust, etc.) that is better than that of a standard fabric.

This is due to the random and three-dimensional structure of the nanofibre layer of arranged in the meshes, which makes it possible to reduce the surface of passage, unlike the fabric that employs only its own mesh opening to reduce and/or prevent passage of dust. Thanks to the nanoscale size of the fibres, it is moreover possible to minimize the percentage of closed or non-filtering volume.

In this way, by replacing with the composite fabric a standard fabric with equal permeability in a given application, it is, in fact, possible to guarantee the same pressure drop given the same flow that passes through, thus considerably improving protection against intrusion.

Claim 1:
A method for producing a composite filtering medium comprising a base fabric, of the type with weft and warp of threads or monofilaments (<NUM>) of polyether ether ketone (PEEK), characterized in that said method comprises a step of electrospinning for formation of nanofibres (<NUM>) of polyimide (PI) and a subsequent step of deposition of said nanofibres on said base fabric; said method comprising injection of the material for formation of the nanofibres, dissolved in a mixture of dimethyl acetamide (DMAc) and N-methyl-<NUM>-pyrrolidone (NMP) in excess of NMP solvents, through a nozzle in order to spread it on an electrode, said method comprising application of a difference of potential between said nozzle and an electrode; said nanofibres being formed as a result of evaporation of said solvent or mixture of solvents, due to the electrical field, and stretching of the polymer deposited on the electrode, by means of the nozzle; said nanofibres thus formed being subsequently stretched and deposited on said base fabric, in which, by depositing said nanofibers on said threads (<NUM>) of the base fabric and moistening them with said mixture of solvents, said nanofibres (<NUM>) form, on the surfaces of contact with said threads, swellings (<NUM>) that favour adhesion of the nanofibres (<NUM>) on the threads (<NUM>) themselves, wherein said nanofibres (<NUM>) firmly adhere to said base fabric, without the aid of glues and/or adhesive agents applied on the base fabric and/or on the nanofibre layer, at the said swellings (<NUM>) of the threads (<NUM>) of said base fabric.