SUBSTRATES AND ARTICLES OBTAINED THEREWITH

This application claims priority to European application No. 17306903.0 filed on December 22, 2017, the whole content of this application being incorporated herein by reference for all purposes.

TECHNICAL FIELD

The present invention pertains to a coating technology for improving surface properties, in particular hydrophobic and/or icephobic surface properties, of substrates such as steel, stainless steel, aluminum, glass or ceramic substrates which are found for example in the automotive, aircraft, naval, building or energy industries.

BACKGROUND ART

It is known that there is a permanent need in the industry to develop technics aiming at modifying the surface properties of materials, particularly for applications where the presence of water and/or ice is not desirable.

For example, in the car industry, it has been proposed to provide water-repellent windows or windshields in order to improve visibility (and therefore safety) when driving under heavy rainfalls or on wet roads.

Another example is represented by ice accretion and ice adhesion deleterious phenomena appearing on different surfaces and which, under cold weather conditions, can result in severe problems on power lines, telecommunications, transportation in general, aircraft or power production by wind turbines. Icing of wind turbines is a problem which not only affects their energy production performance, but also causes mechanical and electrical failures.

As of today, there is no clear or well-established relationship between hydrophobicity (water anti-adhesion) and icephobicity (ice anti-adhesion) of a given material, but some studies show that a highly hydrophobic surface will also present good anti-icing properties.

In order to modify or adapt the surface properties of a substrate material, and limit the potential issues mentioned above, it is now well known to apply on said substrate one or more coating compositions (films, layers) under appropriate coating conditions. In the state of the relevant art, among the most widely used industrial coatings, special mention is to be made of the silicone, the PTFE (polytetrafluoroethylene like ’’Teflon ^® ”) and the polyurethane (PU) based products, which can be easily applied onto many different kind of substrates adapted to numerous applications.

However, a need still exists for proposing coating technics different from the ones disclosed in the prior art.

The purpose of the present invention is therefore to provide a new, useful and efficient coating treatment aiming mainly at enhancing the hydrophobic and/or icephobic surface properties of a substrate.

In this respect, it has now been surprisingly found that the combined use, as coating composition, of a specifically functionalized precipitated silica and of polyurethane polymers makes it possible to achieve advantageously this objective and to prepare articles with improved surface properties, namely hydrophobic and/or icephobic improved properties.

This finding, and others, constitutes the basis for the present invention.

SUMMARY OF THE INVENTION

A first object of the present invention relates to an article comprising a substrate and a coating composition deposited onto the surface of said substrate, said coating composition comprising (i) one or more crosslinked polyurethane polymers and (ii) particulates of precipitated silica functionalized with a linear polyfluorinated compound presenting at least one a,w alcoxysilane end group(s).

According to a first embodiment of the article of the invention, the coating composition is composed of a layer of said functionalized silica particulates deposited onto the surface of a separate layer of said crosslinked polyurethane polymers.

According to a second embodiment of the article of the invention, the coating composition is composed of a single layer containing said functionalized silica particulates embedded inside the crosslinked polyurethane polymers.

The invention also relates to a method for enhancing the hydrophobic and/or icephobic surface properties of a substrate, said method comprising depositing onto the surface of said substrate a coating composition comprising (i) one or more crosslinked polyurethane polymers and (ii) particulates of precipitated silica functionalized with a linear polyfluorinated compound presenting at least one a,w alcoxysilane end groups. The invention also pertains with a method for obtaining an article as defined before or for enhancing the hydrophobic and/or icephobic surface properties of a substrate, said process comprising the steps of:

(a) providing an organic liquid suspension of said functionalized silica particulates

(b) providing an organic liquid solution of crosslinkable polyurethane polymer precursors

(c) optionally mixing said liquid suspension with said liquid solution

(d) depositing subsequently onto said substrate first the liquid solution and second the liquid suspension, or alternatively depositing onto said substrate the mixture obtained at step c).

Another object of the present invention also relates to the particulates of precipitated silica functionalized with a linear polyfluorinated compound presenting at least one a,w alcoxysilane end groups, which are indeed chemical entities new per se, and a process for the preparation thereof

According to an embodiment of the invention, in all its aspects, the polyfluorinated compound grafted on the surface of the precipitated silica particulates is preferably a perfluoro compound, and even more preferably a perfluoro polyether compound.

According to an embodiment of the invention, in all its aspects, the polyfluorinated compound grafted on the surface of the precipitated silica particulates is preferably a perfluoro compound, and even more preferably a perfluoro polyether compound, having two a,w alcoxysilane end groups

According to an embodiment of the invention, in all its aspects, the precipitated silica, before and/or after functionalization as explained herein after, present preferably in its dry state one or more, and even more preferably all, of the following features:

- an average particle size comprised between 0.1 and 5 microns

- a BET specific surface area comprised between 50 and 300 m ² /g

According to an embodiment of the invention, in all its aspects, the deposition of the coating composition(s) is preferably carried out by spraying the same onto the desired substrate, and drying.

The present invention enables to obtain substrates with both super hydrophobic and ice phobic surface properties, with the coatings being durable, robust and abrasion resistant, i.e. convenient for numerous and diversified applications, in particular the applications already known from the prior art such as exposed before.

DETAILED DESCRIPTION OF THE INVENTION

Precipitated silica

Precipitated silicas are well known products, and many processes exist to prepare the same. They are also widely available as commercial products, sold for example by the SOLVAY Company, in particular under the trademark Tixosil ^® . Other precipitated silica providers are the Companies EVONIK, PPG, selling products under the tradenames Ultrasil ^® , Hi-Sil ^® .

The term precipitated silica refers to a silica which may be obtained by precipitation reaction of a silicate, such as alkali metal silicate (sodium silicate for example), with an acid (sulfuric acid for example), to produce a suspension of precipitated silica; any method can be used: namely, addition of acid to a silicate vessel, total or partial simultaneous addition of acid and silicate to a mixture of water and silicate. A drying step (generally with an atomizer) of the cake obtained by filtration of the suspension from the precipitation is carried out. The drying can be preceded by a disintegration operation of the cake. A grinding step may be carried out.

The precipitated silica used in the invention may be prepared for example according to methods of preparation as described in EP 0520862, WO 99/07237 or WO 99/49850.

The precipitated silica used according to the invention can be advantageously a precipitated silica sold under the trademark Tixosil ^® 365 by the SOLVAY Company.

Precipitated silica implemented in the context of the invention preferably has a BET surface area of at least 50 m ² /g, in particular at least 75 m ² /g, especially at least 90 m ² /g, for example between 100 and 400 m ² /g. It may be between 100 and 250 m ² /g, especially between 110 and 250 m ² /g.

The BET surface area can be determined according to the Brunauer - Emmet - Teller method described in "The Journal of the American Chemical Society", Vol. 60, page 309, February 1938 and corresponding to the NF T 45007 (November 1987).

Functionalization of the precipitated silica

According to an essential feature of the invention, the surface of the precipitated silica must be functionalized, which means that a specific moiety will have to be grafted at the surface of the particulates. By grafting, it must be understood that the moiety will be covalently and therefore permanently bound to the particulates thanks to a chemical reaction between reactive functions present at the surface of the silica and reactive functions contained in the moiety.

In the present invention, the moiety corresponds to the linear polyfluorinated compound bearing at least one a,w alcoxysilane end group, said at least one end groups having chemically reacted by direct condensation with the silanol groups present at the surface of the Si0 ₂ particulates to generate a permanent siloxane bond between the same.

The functionalizing compound maybe schematically represented by the following formula:

(RiO)(R ₂ 0)(R ₃ 0)-Si-R-T

in which R ₁ -R ₃ independently from each other designate an alkyl radical, preferably a C ₁ -C ₃ alkyl radical and even more preferably is methyl or ethyl, R designates a linear fluorinated, preferably perfluorinated, bivalent radical, and T is a perfluorinated group, more preferably -CF ₃ or a group of formula - Si-fOF^XORsXOR _f ,) in which R4-R6 independently from each other designate an alkyl radical, preferably a C ₁ -C ₃ alkyl radical ans even more preferably is methyl or ethyl.

Without being bound to any theory, when in contact with the silanol Si-OH groups present at the surface of the particulates, a direct condensation reaction occurs between one siloxy group present at each end of the functionalizing compound and said silanols, thereby creating real covalent bondings of the -Si-O-Si- type between the two species (the functionalizing compound becomes attached like a“loop” at the surface of the particulates), as illustrated by Figure 1.

The process to achieve the desired functionalization may be performed as follows:

- providing a suspension of precipitated silica particulates in an organic solvent

- adding in said suspension a linear polyfluorinated compound presenting a, w alcoxysilane end groups (the functionalizing compound)

- heating the mixture so prepared

- recovering the reaction product, preferably by filtration

- washing said reaction product with an organic solvent

- drying whereby a powder of functionalized precipitated silica is obtained.

The heating is preferably conducted at a temperature comprised between 25 °C and l50°C, most preferably between l00°C and l40°C, ideally at about l20°C. The organic solvent constituting the reaction medium and the washing solvent is preferably non-polar solvents, in particular o-xylene. The time to make the reaction complete may be comprised from 1 hour to 5 hours for example, preferably about 3 hours. At the end of the reaction, the solid phase is separated from the reaction medium, preferably by filtration and under vacuum, and then washed with the organic solvent to eliminate the functionalizing compound not chemically bonded on the silica surface. Finally, the powder is dried before being subsequently processed. At this stage, the weight ratio functionalizing compound / Si0 ₂ in the particulates is generally comprised between 5 and 80 %, preferably between 15 and 70 %.

As per the invention, and as explained before, the functionalizing compound to be used is a linear compound terminated at least on one side, and more preferably on each side, by a trialcoxysilane group (reacting group), and the main chain of which being highly fluorinated, and preferably is perfluorinated (i.e. all the hydrogen atoms borne by carbon atoms have been substituted by fluorine atoms).

Most preferred functionalizing compounds of the invention are perfluoro polyethers bearing a,w alcoxysilane end groups.

Concerning these perfluoro polyethers bearing at least one a,w alcoxysilane end group, they may be chosen among the products having the following formula :

(Ri0)(R ₂ 0)(R ₃ 0)-Si-R’-T

in which :

- Ri, R ₂ andR ₃ , independently from each other, designate an alkyl radical, preferably a C ₁ -C ₃ alkyl radical and more preferably are methyl or ethyl,

- T is either a perfluorinated group, preferably -CF _3, or a group of formula -Si-(OR ₄ XOR5XOR ₆ ) in which R4, R ₅ and R ₆ , independently from each other, designate an alkyl radical, preferably a Ci-C ₃ alkyl radical, and more preferably are methyl or ethyl, and

- R’ designates a linear perfluorinated bivalent radical comprising more than 50 wt. %, preferably more than 90 wt. %, more preferably composed essentially of, and still more preferably composed of statistically distributed repeating units of formula [-CF ₂ -CF ₂ -0-] _m and [-CF2-0-] _n , in which m and n are both integers higher than 0 and the m/n ratio is comprised from ² A to ³ / ₂ , preferably between 0.80 and 1.20, said m/n ratio being further preferably higher than 1.0,

wherein the perfluoro poly ether has generally a number average molecular weight from 35,000 to 45,000.

These compounds are for example described in the document US 7 329 784, the content of which is herewith enclosed by reference.

As it will be apparent to the person skilled in the art of perfluoro polyether polymers, when said group T is a perfluorinated group, the perfluoro polyether is also referred to as a“monoftmctional perfluoro polyether”, while when said group T is a group of formula -Si-(OR4)(OR5)(OR6) as defined above, the perfluoro polyether is also referred as a“bifunctional perfluoro polyether”.

Preferably, the functionality of the perfluoro poly ether, i.e the number of alcoxysilane groups, is at least equal to 1.50, and more preferably at least equal to 1.80. The functionality of the perfluoro poly ether can be calculated for example by using the method such as disclosed in the document EP 1 810 987.

As an interesting example of commercial product which may be used in the present invention, mention may be made of“Fluorolink ^® S10” sold by the SOLVAY Company and which may be represented by the following formula:

As already mentioned, the functionalized precipitated silicas are new and useful products per se, and are therefore also part of the present invention. These products are characterized as being particulates (powder) of precipitated silica functionalized with a linear polyfluorinated compound presenting a,w alcoxysilane end group(s), said polyfluorinated compound being permanently grafted at the surface of the silica owing to a siloxane covalent bound. Alternatively, these products may also be defined as the reaction product, under specific conditions explained before, between precipitated silica particulates and a linear polyfluorinated compound bearing a,w alcoxysilane end group(s). The presence of said grafting owing to a real covalent -Si-O-Si- bound is easily evidenced by classical analytical technics, such as IR spectroscopy or Si NMR. Coating composition, preparation and use thereof

The next steps involved in the present invention will be the preparation of an organic suspension of the functionalized precipitated silica particulates described before, and of an organic solution of crosslinkable polyurethane polymer precursors.

Then, the invention may be further implemented according to two distinct embodiments:

- either said suspension and solution are mixed together, and the resulting mixture is then deposited at the surface of the desired substrate

- or they are deposited separately in two steps, i.e. that the solution of crosslinkable polyurethane polymer precursors is first deposited on the substrate as a bottom coat and then the suspension of the functionalized precipitated silica particulates is deposited as a finishing top coat.

The suspension of the functionalized precipitated silica particulates may be simply prepared by introducing the same in an organic solvent, in an amount which is generally comprised between 1 and 30 % in weight, more preferably between 1 and 10 % in weight.

On the other hand, the organic solution of polyurethane polymer precursors may be prepared by the one skilled in the art by any known methods under usual conditions known to prepare the same, for example by mixing polyols and polyisocyanates in an organic solvent, being understood that the resulting mixture will subsequently become, after a classical polyaddition reaction, a crosslinked polyurethane polymer (see for example WO 2012-170832 and US 2014-0208978).

Moreover, polyurethane polymer precursor compositions are also widely available commercial products, which can be used as such after dilution in an organic solvent, in an amount which will be generally comprised between 50 and 100 % in weight, more preferably between 70 and 95 % in weight.

Examples of “ready to be used“commercial polyurethane polymer precursors compositions are“5441 PU66 4H PU Topcoat Gloss” sold by PPG Aerospace, and “Aviox Finish 77702” sold by Akzo Nobel. The organic solvents used for the preparation of the silica suspension and the polymer precursors solution are advantageously the same, in particular in the case when the two compositions are mixed before deposition on the substrate, and said solvents are preferably chosen among tetrahydrofuran, butyl acetate, ethyl acetate, isopropyl acetate, isobutyl isobutyrate, and mixtures thereof.

When the process of the invention is implemented in one step, the mass percentage of functionalized precipitated silica in polyurethane precursors in the admixture may be conveniently adjusted to obtain different coating properties on the substrate. Preferably, this percentage is comprised between 5 %w and 70 %w.

After deposition and drying of the composition on the substrate, a solid crosslinked polyurethane coating will be obtained.

Concerning the deposition of the coating compositions on the surface of the substrate, suitable methods are known to the person skilled in the art. For example, substrates could be dipped in dip tanks containing the coating compositions in the form of a slurry, and thereafter dried. Other methods worth being mentioned are spin coating and blade coating. However, a preferred process which is particularly efficient in the present invention is a deposition carried out by spraying the liquid coating compositions on the surface of the substrate {spray coating ), providing thin layers in which a spontaneous drying and curing occurs even at ambient temperature of working. The method to be implemented will depend on the thickness desired for the film, which is also dependent from the intended use for the coated substrate. In this respect, the thickness of the coatings of the invention may be comprised between 1 pm and 100 pm, particularly between 1 pm and 70 pm, and even more particularly between 1 pm and 50 pm.

The substrates, the surface property of which needs to be improved, in particular the hydrophobic and/or icephobic surface properties, could be made of any material like steel, stainless steel, aluminum, glass or ceramic, and could be of any shape, such as articles found useful in the automotive, aircraft, naval, building or energy industries, for example windows, wires and wind turbines.

Should the disclosure of any patents, patent applications, and publications which are incorporated herein by reference conflict with the description of the present application to the extent that it may render a term unclear, the present description shall take precedence.

The present invention will now be illustrated by the following examples, which are not intended to be limiting.

EXAMPLES

The raw materials used to prepare the samples tested in the examples were:

a) precipitated silica powder: Tixosil ^® 365 sold by the SOLVAY company and having the following features:

- mean diameter: 3.4 pm

- BET specific surface area: 157 m ² /g

This product will be more shortly identified as T365 thereinafter

b) functionalizing agent: a,w alcoxysilane terminated perfluoro polyether sold under the trademark FLUOROLINK ^® PFPE S10 by the SOFVAY Company.

This product will be more shortly identified as S10 thereinafter.

g) polyurethane precursors:

- polyisocyanate: TOFONATE™ TM HDT-FV sold by the VENCOREX Company

- polyol: SETALUX™ 1907- BA-75 sold by the NUPLEX Company

The polyaddition of these precursors will provide a crosslinked polyurethane (PU). The samples were prepared according to the following steps and strategy:

Functionalization of precipitated silica

lg of T365 is dispersed in 20 ml of o-xylene and 1.4 g of S10 is added to the suspension. The flask is heated at l20°C during 90 min to make the direct condensation of alcoxysilane from functionalizing agent with silanol groups of silica. At the end of the reaction, the slurry is filtrated under vacuum and washed with o-xylene to eliminate the S10 not chemically bonded on the silica surface. Finally, the powder is dried at l20°C overnight. 50 % in weight of S10 is grafted on T365 silica.

Preparation of a dispersion of functionalized precipitated silica

The functionalized precipitated silica (S10/T365) is dispersed in ethyl acetate (from 1 % to 10 %w). The solution is placed on ultrasonic bath during 30 minutes to obtain a homogeneous dispersion. Preparation of a polyurethane precursors formulation

Tolonate TM HDT-LV (3g) is added to butyl acetate (0.92 g) and methyl amyl ketone (0.85 g). Then, the Setalux 1907 BA-75 (9.5g) is added to this mixture and gently mixed with spatula.

Preparation of a polyurethane primer layer on aluminum substrate

At first, the 2024-T3 Alclad aluminum substrate is cleaned with methyl amyl ketone, the polyurethane precursors formulation is sprayed and dried until at least 4 hours. In all cases, this step is needed to enhance interactions of coating with aluminum substrate. The coating is referenced as Sample 1.

Preparation ofS10/T365 top coating on PU primer layer

A polyurethane precursors formulation is sprayed on aluminum substrate (PU primer layer) and dried until at least 4 hours before spraying of the functionalized precipitated silica dispersion at 5 %w in ethyl acetate. The coating is referenced as Sample 2.

Preparation of composite PU-S10/T365 coating on PU primer layer

The functionalized precipitated silica dispersion at 5 %w in ethyl acetate is prepared and added to the PU precursors formulation in order to have different percentage of functionalized precipitated silica particles in PU precursors. Firstly, a polyurethane precursors formulation is sprayed on aluminum substrate (PU primer layer) and dried until at least 4 hours before spraying of the mixture comprising functionalized precipitated silica dispersion at 5 %w in ethyl acetate and PU precursors. 7 coatings prepared with different weight percentages of functionalized precipitated silica particles in PU precursors (5 %, 15 %, 20 %, 25 %, 30 %, 45 % and 60 % in weight). The coatings are referenced as Sample 3, 4, 5, 6, 7, 8 and 9.

The surface properties of the articles (coated substrates) prepared as above are then assessed according to several tests explained below.

A- Contact angle measurement & roll-off angles of the coatings

The water contact angle (WCA) measurement was conducted with a Biolin Optical Tensiometer, using DI water as testing liquid with drop volume of 4pL. For each measurement, 40 images were taken during the first 20 seconds following the droplet deposition, and the average contact angle value was noted. In general, 3 measurements (on different spots) were conducted on each coating in order to have statistically reliable results.

Regarding the roll-off angle measurement, an automated tilting cradle consisting of a frame that supports Biolin Optical Tensiometer on an axle turned by a motor on one end and a pivot on the other is used. The motor is driven by the One Attension software to turn Biolin Optical Tensiometer at precise speeds to defined angles. As the stage tilts, so tilts the camera and thus the image onscreen is similar to what would be seen with a standard contact angle experiment and allows estimating roll-off angles of the coating. Roll-off angles from 0° to 90° can be measured with an angular resolution at 0.1°. The coating is placed on vacuum stage to avoid any movement of the coating.

The results are reported in the Table 1. The super hydrophobic properties (WCA > 150 ° and low roll-off angle) are obtained when functionalized precipitated silica dispersion at 5 %w in ethyl acetate is sprayed on PU primer layer. In the case of composite coating, the super hydrophobic behavior is obtained when at least 30 %w of functionalized precipitated silica in PU precursors formulation is incorporated.

Table 1

A critical amount of functionalized precipitated silica in PU matrix is needed to obtain surface nanoroughness producing superhydrophobic properties.

B - Ice related testing on coatings

• Ice-rain testing

Ice-rain testing of samples 1, 2 & 9 are performed in the IFAM ice-chamber (apparatus available at Fraunhofer Institute for Manufacturing Technology and Advanced Materials IFAM, Bremen, Germany).

This test is used to simulate super-cooled water droplets in contact with our functionalized substrate and is obtained as follows: air temperature = -5°C, substrate temperature = -5°C, rel. humidity = 66 %, wind speed = 9 m/sec, rain duration = 10 sec and visual inspection after 5 min.

From each coating, one sample was subject to testing, the test was performed twice.

A picture of the ice-rain test available at Fraunhofer IF AM (Bremen, Germany) is made available as Figure 2.

After conditioning of test samples, cold water is applied for duration of 5 seconds and ice formation on test samples is being assessed 5 minutes after water spraying. The evaluation is based on visual comparison of the iced surface and is allocated an ice grade level (IG), defined from 0 (ice-free) to 5 (extensive and nearly complete ice coverage) with increasing ice formation on the surface.

For the PU primer layer coating (Sample 1), an IG level of 2 is estimated due to the presence of small to medium sized and isolated ice droplets. Regarding the coating corresponding to a top layer of functionalized precipitated silica particles (Sample 2) on PU, only a very few and relatively small water droplets on the surface are observed (IG level = 0-1). Finally in the case of the composite at 60 %w of functionalized silica particles in PU, no ice is visually detected on the coating (IG level = 0).

These tests demonstrate the interest of functionalized precipitated silica particles to decrease rime ice adhesion. Depending on the targeted application, a top coating or incorporation in PU resin of these functionalized precipitated silica particles are conceivable to limit ice adhesion.

• Clear ice adhesion testing

The adhesion of clear ice is assessed by using an in-house modified notch test device and has been validated by the same experiments available at Fraunhofer IF AM (Bremen, Germany). The pendulum test is illustrated by Error! Reference source not found.3.

During the test, the energy is measured to remove clear ice from the test surface. Ice cubes are being frozen on sample surfaces and removed by a well-defined pendulum. The pendulum amplitude is then correlated to the energy needed to remove the ice. It is expected that a low energy value is correlated with low clear ice adhesion. All experiments are repeated five times and the mean of energy needed to remove ice cube is calculated and expressed in percentage. The samples 1, 2, 4, 7 & 9 are studied. Table 2

The best results in terms of low ice adhesion are obtained for top coating of functionalized precipitated silica particles in PU primer layer and for the composite with 60 %w of functionalized precipitated silica particles incorporated in PU resin.