Background of the Invention

Field of the Invention

The present invention relates to coatings for use on porous fabrics or foamed structures well as to the finished coated fabrics. Particularly, the invention relates to a process for producing these coatings using plasma-assisted sol-gel techniques.

Description of Related Art

The sol-gel process is a wet-process used in materials processing to produce solid product structures, such as films or coatings, from small molecules, the final products being suitable for use in various application areas. One of the largest application areas is thin films. These films can be produced using the sol-gel technique, for example by spin coating or dip coating the sol-gel material onto a substrate. Other coating techniques include spraying, electrophoresis, inkjet printing or roll coating. Using this sol-gel procedure, novel structures can be formed, as well as novel properties for the obtained products, that cannot be created by any other method. The sol-gel process involves conversion of a starting material into a colloidal solution (sol) that acts as the precursor for an integrated network (or gel) of either discrete particles or network polymers. Typical precursors are metal alkoxides or oxides.

In this chemical procedure, the 'sol' (or solution) gradually evolves towards the formation of a gel-like diphasic system containing both a liquid phase and solid phase whose morphologies range from discrete particles to continuous polymer networks. The sol-gel technique in preparing films is described in, for example WO2013085611A1. In this publication, the film is formed by providing a composition including the sol-gel film precursors and a crystallization aid, and processing the composition to form the film. The sol-gel approach is a cheap and low-temperature technique that allows for the fine control of the product's chemical composition and properties to make it suitable for specific purposes.

For example, DE 10209667 relates to the use of this technique in improving the properties of textile-based filter materials. Likewise, EP 2444549 discloses bioactive coating layers applied on both sides of textile supports in a sol-gel process utilizing a foam-coating technique for spreading the coating onto the substrate.

EP 857229 discloses the application of chemical charge-modifiers to permeable sheets or films by first contacting the sheet with amphiphilic macromolecules, such as proteins, to produce a coating, and then contacting the coated sheet with the chemical charge modifiers, which can be in the form of sol-gels.

WO 2010/029586 discloses a surface treatment method, particularly for materials, fabrics and other laminar elements having similar properties, consisting of the following steps: striking at least one substantially superficial portion of the material with electromagnetic radiation of preset frequency, in order to break up and remove organic and inorganic impurities that are present in the material; subjecting the superficial portion of the material to a stream of plasma, in order to modify its shape; interpenetrating the superficial portion of the material with a layer such as a matrix, having a substantially lattice-like

configuration and a thickness of no more than a few millionths of a meter.

The present sol-gel coating techniques can still be developed further, among others, by improving the adhesion of the coating to the substrates. Summary of the Invention

It is an object of the present invention to provide novel coatings for porous substrate materials that particularly are selected from foams and non-woven materials.

Particularly, it is an object of the invention to provide coatings with surface properties that can be tailored according to the desired end-use of the coated products.

It is a further object of the invention to provide a process for manufacturing such coated substrate materials in a manner that allows tailoring of the surface properties of said substrate materials.

These and other objects, together with the advantages thereof over known coatings and processes, are achieved by the present invention, as hereinafter described and claimed.

The present invention concerns a process for coating porous substrates, including at least one sol-gel coating step.

More specifically, the process of the present invention is characterized by what is stated in the characterizing part of Claim 1.

Further, the coating and the coated substrate of the present invention are characterized by what is stated in Claims 11 and 12, and the uses of the process are characterized by what is stated in Claims 13, 14 and 15.

Considerable advantages are obtained by means of the invention. Among others, the present invention provides a low-temperature technique for coating particularly foamed and non- woven substrates, which further allows for the fine control of the product's chemical composition and properties, for example by making the surface of the final structure hydrophobic or oleophobic, or by giving the surface properties that prevent its fouling.

The sol-gel coating process allows the formation of coating gradients on the substrates, in the depth-direction of the substrate surface. This gradient allows the use of smaller amounts of coating materials compared to the prior coatings. Further, a stronger bonding of the coating to the pre-treated substrate material is achieved, since a partial penetration of the sol-gel coating into the pre-treated surface is achieved, instead of a clear boundary being formed between the surface and the coating layer. In case of open cell foams being coated, shear forces are thus better distributed into the foamed structure, sharp material boundaries are avoided and the resulting bonding and the obtained overall structure is stronger.

This invention enables also gradient of properties in different depth profiles of the web-like material. The obtained structure can have e.g. hydro- or oleophilic inner layer changing gradiently to hydro- or oleophobic (or some other combination of previously mentioned properties or degree of surface energy) top- layer.

The pre-treatment and the sol-gel coating step are both mild and cheap. They also do not harm the fibers of the substrate, and particularly cause no melting or pinhole formation.

Despite being mainly focused on non-woven substrate materials, and foams having corresponding porosities, the process can be used also on woven substrates. Next, the invention will be described more closely with reference to the attached drawings and a detailed description.

Brief Description of the Drawings

Figure 1 is a schematic drawing of the preferred equipment for use in the process of the present invention, for coating non-woven fabrics.

Figure 2 is a schematic drawing of the preferred equipment for use in the process of the present invention, for coating foamed substrates. Detailed Description of Preferred Embodiments of the Invention

The present invention concerns a process for coating porous substrates including at least one sol-gel coating step, which is preceded by at least one pre-treatment step selected from plasma and corona treatments.

The term "porous" is here intended to define the substrates as the types having a high porosity, in untreated form, i.e. a high ratio of the volume of the spaces between the fibers, cell surfaces and particles in the substrate material to the whole volume of the material. The "high porosity" is intended to encompass such ratios, measured and calculated for example as an average open hydraulic flow area, to have a porosity of >20%, particularly >40%, and most suitably >60%.

Suitable porous substrates are non-woven fabrics or foamed substrates, which non-woven fabrics include synthetic sheets or webs formed by mechanical, thermal or chemical treatment of separate fibers, molten plastic or plastic films. Examples of such fabrics include felts and filters.

Preferred substrate materials are polymers, metal or metallized surfaces, foils, fiberglass, or paper- like products, particularly synthetic materials, preferably thermoplastic polymers, such as polyethylene, polypropylene, polylactide (PLA), polystyrene, thermoplastic polyurethanes, nylon, vinyl, polyvinyl chloride (PVC), polyethylene terephthalate (PET), most suitably polypropylene fabrics and polystyrene (PS), polyurethane (PU), as well as foams of such materials.

Particularly preferred thermoplastic foamed substrates are polyethylene foams, polystyrene foams, polylactide foams and thermoplastic polyurethane foams, whereas particularly preferred thermosetting foams are thermosetting polyurethane foams and urea- formaldehyde foams.

If the curing of the coating process includes a thermal treatment, thermosetting polymers are equally suitable for use as substrate materials. These include vinylester fiberglass, thermosetting polyurethanes, vulcanized rubber, urea- formaldehyde and resins, for example epoxy resins and melamine resins, due to their high temperature stability. The surfaces of these substrates can be relatively inert, whereby it is difficult to strongly attach coatings to their surfaces.

According to one preferred embodiment of the invention, the non-woven fabrics are selected from hydrophilic materials, such as the above listed polylactides or polyesters.

According to another preferred embodiment of the invention, the non-woven fabrics are selected from hydrophobic materials, such as the above listed polyolefms, or from fabrics pre-treated with hydrophobizing agents or coated with hydrophobizing or hydrophobic coatings. Suitable hydrophobizing agents (which thus also can be included into the hydrophobizing coatings) include silica, silanes, siloxanes and waxes.

The pre-treatment of the substrate is used, among others, to improve the adhesion of the sol-gel coating to the substrate surface, and particularly to obtain a permanent adhesion as well as coating gradients, for example by increasing the penetration of the coating into the fabric. Corona or plasma treatment incorporates ionized molecules onto the substrate surface. Thus, using a combination of plasma or corona pre-treatment with a sol-gel coating procedure, permanently or irreversibly coated substrates can be obtained, as the sol-gel coating material attaches to these ionized molecules.

Corona treatment is carried out by applying a low-temperature corona plasma discharge to the surface of the substrate, the corona plasma being generated by application of a voltage to electrodes. Preferably, the plasma/corona pre-treatment is carried out in atmospheric conditions, as an "atmospheric-pressure plasma treatment". Carrying out the plasma treatment in atmospheric conditions renders the treatment suitable for industrial scale roll to roll production. Both the atmospheric-pressure plasma treatment and the conventional corona treatment function by charging and ionizing gas molecules (such as oxygen or other gas molecules contained in air), but in the atmospheric system, the density of the generated plasma is larger. This increases the rate and degree of incorporation of ionized molecules onto the substrate surface, thus providing even stronger adhesion to the subsequently applied coating.

As stated above, the pre-treatment is preferably carried out at atmospheric conditions, whereby an ambient temperature is sufficient. In general, high drying/curing temperatures are avoided in order to avoid melting of the substrate. Instead, the curing preferably takes place using UV radiation. The temperature can, however, be temporarily increased to remove the solvent and cure the coating, e.g. up to 100-160 °C, but remaining below the melting, deformation or degradation point of the substrate material.

The corona discharge is generally applied using a frequency at a level of 10-30kHz, preferably 15-25 kHz. Thus, to achieve a final sol-gel coating with a gradient in the depth- direction of the substrate surface, a frequency close to the upper limit of the range can be selected, preferably a frequency of 20-30kHz, more preferably 20-25kHz.

The sol-gel coating procedure involves conversion of a precursor material into a solution (sol). This sol is subsequently, particularly after application of the sol onto the substrate, converted into a gel-like phase by forming an integrated network of either discrete particles or network polymers obtained by reactions of precursor material(s).

Suitable precursor materials include metal oxides or chlorides, such as the oxides or salts of zinc, aluminum, silicon, zirconium and titanium, or the metal oxides can be formed from corresponding metal alkoxides (the starting materials), by e.g. hydrolysis and condensation reactions, during the sol-gel process. Suitable salts include nitrates, sulfates and

carboxylates. Particularly, precursors of a limited molecular mass are preferred, whereby the precursor materials used in the present invention lack biomolecules, such as proteins and enzymes, which are formed of relatively large molecules. A suitable mass limit for the molecules forming the present precursor materials is 500g/mol. According to an embodiment of the invention, the sol-gel coating procedure includes the following steps:

sol formation, where the precursor material(s) are mixed into a solvent to obtain a sol (also called a sol-gel solution),

optionally, addition of further chemicals, such as crystallization aids, e.g.

phosphine oxides, or acid or base catalysts, e.g. hydrochloric acid, hydrosulphuric acid or sodium hydroxide,

preferably, initial agglomerate/network formation, where the precursor material(s) are reacted, e.g. by hydrolysis and polycondensation, into a colloid or an integrated network of particles or polymers, which bind(s) the cured layer to the surfaces of the substrate,

application of the sol onto a substrate,

optionally, a separate step of drying or other separation of the film-forming colloid or network phase from the remaining liquid of the solution, and

curing of the formed network.

Of these steps, the sol formation (as well as the addition of further chemicals into the sol and any other pre-treatments of the sol) is preferably carried out using a batch procedure, whereas the following steps preferably are carried out in-line.

Preferred solvents to be used in the sol formation are aqueous solvents, such as water, or alcoholic solvents, optionally containing one or more of salts, acids or bases. Preferred alcohols are methanol, ethanol and any of the propanols (n-propanol and iso-propanol). The solvent can also be formed from a mixture of any of these.

The concentration of precursor material in the solvent is selected to provide an

advantageous viscosity for the sol, which renders it suitable for spreading out into a desired thickness on a substrate. Such a concentration is preferably 0.05-60w-%, more preferably 0.5-50w-%, and most suitably 5-30w-%. The resulting viscosity is preferably higher than the viscosity of water, i.e. higher than 8.90 x 10 ⁴ Pa s (at 25°C), or it is for example from 9 10 ⁴ to 12 10 ⁴ Pa-s.

The application of the sol onto the substrate (i.e. the coating step) is preferably carried out by spray or dip coating, most suitably by spray coating. The use of spray coating can be carried out on web-like materials in a roll-to-roll process. Further, the use of spray coating allows the formation of gradient coatings having a more solid (thick) coating on the surface of the fabrics or open cell structured foams compared to the inner layers. A relatively thick coating layer can be produced onto the substrate, if so desired, by adjusting the viscosity, the concentration and the wetting characteristics of the sol-gel solution (these characteristics particularly being based on the choice of solvent and precursors). The thickness of the layer depends also on the density of the substrate fabric, as well as the surface characteristics of the fibers, such as their porosity and surface chemistry. When coating fabric substrates a thin and elastic coating which does not break or crack when the fabric and the fibers are bent can be produced by using low- viscosity solutions having a suitable surface tension. As stated above, the coating can also form a gradient in the depth-direction of the substrate surface. This is achieved by adjusting the intensity (e.g. by adjusting the frequency used in the corona discharge) used in the pre-treatment step, whereby the efficiency of the pre- treatment step changes in the depth direction of the formed coating layer, with the highest obtained activity on the surface. This results in a coating that displays higher coating coverage of the substrate on the surface compared to the inner layers. Further, this results in a partial penetration of the sol-gel coating into the pre-treated surface, instead of a clear boundary being formed between the surface and the coating layer. In case of foams coated with the method of the invention, shear forces are thus better distributed into the foamed structure, sharp material boundaries are avoided and the resulting bonding and the obtained overall structure is stronger.

The non-woven fabric substrate or the foamed substrate is preferably used as a web-like structure during at least the sol-gel coating step, and most suitably during the entire process, whereby the process can be carried out as a continuous roll-to-roll process. Thus, the coating process of the present invention can even be used in the same process line as the manufacture of a non-woven or foamed substrate.

In certain conditions, the gel- formation can begin already in connection with the sol- formation, but it is completed in the curing step. Under certain catalysis conditions, a colloid can be formed. Other conditions favour for example the formation of networks.

The curing step is preferably carried out by using fluidized, UV or IR curing, or a combination of two or more of these. The curing can further include a thermal treatment, for example carried out at a temperature of 100-300°C, the use of crystallization aids, or the use of acid or base catalysts, such as hydrochloric acid, hydrosulphuric acid or sodium hydroxide. The reaction taking place can be for example a condensation (or

polycondensation) reaction. The curing generally also results in the drying of the coating, as remaining excess solvent is removed.

Optionally, a separate step of drying the coating, or evaporating or otherwise separating the excess solvent from the cured layer, can be carried out before the curing step, which separate step can be combined with a thermal firing step to induce further

polycondensation.

The term "colloid" is used primarily to describe a broad range of solid-liquid mixtures, which contain distinct solid particles or structures which are dispersed to various degrees in a liquid medium. The term is specific to the size of the individual particles, which are larger than atomic dimensions but small enough to exhibit Brownian motion. This size range (or particle diameter) is typically between 0.1 and ΙΟμιη. When this coating procedure is carried out in a roll-to-roll procedure, the web-like substrate is passed from one roll to another roll, passing various treatment units before being wound up on the second roll. Preferably, the substrate web passes first a pre- treatment unit, where for example plasma or corona treatment of the substrate surface takes place, subsequently a sol-gel coating unit, where for example spray, roll or dip coating of a sol-gel onto the pre-treated substrate takes place, and finally a curing unit, where the sol- gel film that has been spread onto the substrate surface is cured.

The present invention also concerns equipment (Fig. 1) for use in the above described process (preferably arranged in sequence at a roll-to-roll coating line):

1 one or more pre-treatment units,

2 one or more coating units,

3 one or more curing units,

as well as optionally:

4 one or more drying units, preceding the curing unit(s) 3.

This sequence enables production of the above mentioned coatings in-line.

The pre-treatment unit(s) 1 function by plasma or corona treatment. Generally, one such unit is sufficient. The coating unit(s) 2 are arranged downstream from the pre-treatment unit(s) 1. In case of two or more coating units 2, they are typically arranged sequentially, immediately following each other, or with drying unit 4 between. Preferably, the coating units 2 function by spray coating. Generally, one spray coating unit 2 is sufficient, since the thickness of the coating can be adjusted, for example, by adjusting the feed rate, but it is possible also to arrange two or more units in series, whereby the coating can be partially cured between the different coating units. The curing unit(s) 3 are preferably arranged downstream from the coating unit(s) 2. In case of two or more curing units 3, they are preferably arranged sequentially, immediately following each other, but according to another option (when using more than one spraying unit), the coating units 2 and the curing units 3 can be arranged so that they alternate each other. Curing preferably takes place by fluidized curing (such as air curing or fluidized bed curing), or by IR or UV curing, but it is particularly preferred to select at least one curing unit 3 functioning by UV curing. According to another particular option, at least one curing unit 3 functions by air curing, optionally intensified by heating the air or by heating the substrate using IR radiation, whereas at least one subsequent curing unit 3 functions by UV radiation or heat (using IR radiation or heated air).

The drying unit(s) 4 are optional, since the curing as such can cause sufficient drying, particularly when involving heating. Preferably these optional separate drying unit(s) 4 are arranged upstream from the curing unit(s) 3, i.e. before the curing units(s) 3, and are intended to remove excess solvent and thus facilitate the subsequent curing. These drying units 4 can be selected from similar units as the curing units 3.

Thus, according to a preferred embodiment, the equipment includes units that are arranged sequentially, in the following order (see Fig. 1):

a) pre-treatment unit(s) 1 ,

b) coating unit(s) 2,

c) optional drying unit(s) 4, and

d) curing unit(s) 3

preferably at a roll-to-roll coating line. Although the present invention as described above relates to the coating of non- woven fabrics or foamed plastics of similar porosities, the process of the invention is suitable for use also on woven fabrics. In addition to the coating process described above, the present invention also relates to a coating produced as described above, as well as to a coated substrate thus produced.

The coating thickness can vary, but a preferred thickness is between 0.1 and 100 μιη, for example between 0.4 and 6 μιη.

The hydrophobic or oleophobic surface that can be obtained using the coating of the present invention provides the surface protective characteristics, whereby the surface for example repels most fouling products. Said hydrophobic surface is obtained partially by covering the hydrophilic groups of the substrate surface with the formed coating, and partially by reacting the hydrophilic groups of the sol precursors to give a gel coating formed of hydrophobic molecules. Further hydrophobicity and oleophobicity can be obtained by including hydrophobic or oleophobic compounds, such as hydrophobic or oleophobic polymers in the sol used to form the coating or alternatively by using precursors with hydrophobic or olephobic functionalities such as fluorine derivatives.

A further option is to use the coated structure of the present invention in preparing sandwich structures, including further layers, such as a layer containing nanocellulose (or microfibrillated cellulose) on top of the sol-gel coating layer. An even further option is to use conventional long/continuous fibre reinforced skin layers as the top layers in the sandwich structure.

Suitable uses of the coated fabrics of the invention thus include their use as medical packages, surgical clothes or masks, filters and geotextiles. A preferred end-use is as filter components for filtering air, water or process chemicals.