FR3060597A1 - REVERSIBLE PIEZOCHROME COATING WITH SOL-GEL / POLYMER HYBRID MATRIX FOR DETECTION OF IMPACTS ON COMPOSITE SUBSTRATES - Google Patents

Abstract

The present invention relates to a piezosensitive coating (12) for the detection of impacts (121, 122, 123, 124) on a substrate (10), comprising a matrix in which at least one piezochromic pigment is dispersed, said matrix is a hybrid sol-gel / polymer matrix formed by at least one organic precursor and a polymerized inorganic precursor, the piezochromic pigment is chosen so that said coating changes color from a determined pressure threshold, corresponding to a pressure transition of said piezochromic pigment in said matrix between two polymorphic forms of different colors. The invention also relates to a method of producing such a piezosensitive coating.

Description

Holder (s): AIRBUS GROUP SAS Simplified joint-stock company, UNIVERSITE PARIS 6 PIERRE ET MARIE CURIE Public establishment, UNIVERSITE DE BORDEAUX Public establishment, NATIONAL CENTER FOR SCIENTIFIC RESEARCH Public establishment.

Extension request (s)

Agent (s): IPSIDE.

REVERSIBLE PIEZOCHROME COATING WITH HYBRID SOL-GEL / POLYMER MATRIX FOR DETECTION OF IMPACTS ON COMPOSITE SUBSTRATES.

FR 3 060 597 - A1 (Çy The present invention relates to a piezosensitive coating (12) for detecting impacts (121, 122, 123, 124) on a substrate (10), comprising a matrix in which is dispersed at minus a piezochrome pigment, said matrix is a hybrid sol-gel / polymer matrix formed by at least one organic precursor and an inorganic precursor polymerized, the piezochrome pigment is chosen so that said coating changes color from a threshold determined pressure, corresponding to a transition pressure of said piezochrome pigment in said matrix between two polymorphic forms of different colors.

The invention also relates to a method for producing such a piezosensitive coating.

Reversible piezochromic coating with hybrid sol-gel / polymer matrix for the detection of impacts on composite substrates

FIELD OF THE INVENTION

The present invention belongs to the field of functional coatings used for controlling the integrity of structures, also called structural health monitoring (Structural Health Monitoring in English), it relates more particularly to a reversible piezochrome coating for detecting impacts on substrates mainly composites.

The present invention consists of a pressure-sensitive material comprising a hybrid organic-inorganic matrix consisting of an oxide network and a polymer network, the two networks being interpenetrated at the nanometric scale, and containing one or more types. piezochromic pigments, dispersed in said matrix, which change color from a well defined threshold pressure.

STATE OF THE ART

Composite materials have mechanical properties and lightness that make them widely used for the manufacture of structural parts in various industrial applications and more particularly in the aeronautical industry.

However, these materials are particularly vulnerable to damage caused by shocks, even of low energy, which can occur during the assembly of parts (for example in the final assembly line). The damage generally located at the point of impact (delamination, internal transverse cracks) can be more extensive and propagate under the effect of loading in fatigue, which can thus lead to a reduction in their mechanical resistance significantly. This damage can be underlying or barely visible, requiring costly and time-consuming non-destructive inspections to verify the integrity of the composite. Therefore, a visual impact detection system would allow ultrasonic inspections to be focused strictly on the necessary areas.

The present invention relates more generally to facilitating the detection of impacts on structures whatever their materials of manufacture, and more especially to the detection of impacts on composite supports due to a more delicate visual control on this type of material, the damage being able to have a surface imprint which is difficult to locate while presenting a more deterioration in depth.

Today, composite structures are thoroughly inspected visually (VT), by a global ultrasonic test (US) if necessary, or by any other non-destructive test (CND).

These methods are long and costly and prove to be penalizing for maintenance cycles and can jeopardize the sustainability of industrial processes due to the increasing use of composite materials as structural parts. In the field of safety and the prevention of failure of composite structures, the existing solutions relate abundantly to effective protection against shocks, by anti-shock coatings for example, and only rarely deal with the problem of detecting those -this.

The solutions closest to the invention describe paints or adhesive films containing capsules filled with dyes. Under the effect of a pressure, a shock for example, capsules burst and release the dye which then reveals the impact zone. This type of paint or film allows to locally reveal an impact with an estimate of its intensity but on a low range of pressures and for pressures much lower than the pressures causing damage to composite materials.

This solution, in addition to the fact that it only relates to the detection of low energy impacts, is not reversible. Dye capsules can only be used once, so it may be necessary to reprocess the affected area with the same paint for later use.

In addition, it is difficult to obtain a homogeneous paint on large parts because of the inertia of the control capsules which constitute simple bodies in suspension in a liquid phase and not a miscible phase in the paint.

In the case of the use of pressure sensitive films, it is necessary to introduce an additional step in the industrial process to remove and analyze the film since they are not designed to remain on the substrates.

The challenge is therefore to be able to have a quick and efficient means of locating impacts of determined intensity in order to use the necessary preventive or corrective actions while having a coating adhering to the substrate and capable of being painted by the following.

STATEMENT OF THE INVENTION

Based on this state of affairs, the applicant proposes a solution which overcomes the limitations of the prior art and describes a piezosensitive coating for detecting impacts on a substrate, comprising a matrix in which at least one piezochrome pigment is dispersed.

This coating is remarkable in that the matrix is a hybrid sol-gel / polymer matrix formed from at least one organic precursor and an inorganic precursor polymerized, and in that the piezochromic pigments are chosen so that said coating changes color from a determined pressure threshold, corresponding to a transition pressure of said piezochrome pigment in said matrix between two polymorphic forms of different colors, and this in correlation with the mechanical properties of the substrate (colorimetric transition pressure in the range of damage pressures for composite materials for example).

Indeed, the transition pressure of a piezochrome pigment to the powder state can change as soon as said pigment is mixed in a medium, in this case the coating matrix.

More particularly, the hybrid sol-gel / polymer matrix is formed from at least two precursors from the following three precursors: a metallic or silicon alkoxide, a metallic or silicon organoalkoxide, and an organic monomer.

Advantageously, the hybrid sol-gel / polymer matrix is formed by a silicon alkoxide, a silicon organoalkoxide and an organic monomer.

In a preferred embodiment, the silicon alkoxide is tetraethyl ortosilicate (TEOS), the silicon organoalkoxide is (3methacryloxypropyl) triethoxysilane (MaPTES) and the organic monomer is 2-hydroxyethyl methacrylate (HEMA).

In a preferred embodiment, the silicon organoalkoxide is (3aminopropyl) triethoxysilane (APTES) and the organic monomer is an epoxide.

In an alternative embodiment, the silicon organoalkoxide is (3-glycidoxypropyl) triethoxysilane (GPTES) and the organic monomer is an amine. In this case, it is a silylated amine - diepoxide combination.

Similarly, a silylated epoxy-diamine combination can be used.

In an advantageous embodiment, the piezochrome pigment is a metal oxide of formula AM0O4 with A metal, such as copper, zinc, iron, nickel, ... etc.

In particular, the piezochrome pigment has a formula CuMoi- _x W _x 0 ₄ with x a real number between 0 inclusive and 1 not included, or between 0.01 inclusive and 0.3 inclusive (preferably between 0.07 and 0 , 1), or a formula Coi- _x Mg _x Mo04 with x a real number between 0 inclusive and 1 inclusive.

In the first case, the pigment is said to be doped with tungsten when x is different from 0. In the second case, the pigment is said to be doped with magnesium when x is strictly between 0 and 1.

Advantageously, the piezochrome pigment is coated, before its introduction into said matrix, in a core-shell polymer or in hybrid core-shell nanoparticles.

The present invention also relates to a process for producing a piezosensitive coating on a substrate, said coating comprising a hybrid sol-gel / polymer matrix and at least one piezochrome pigment, and characterized in that it comprises the following steps:

- preparation of a mixture containing said piezochrome pigment, an organic monomer, and at least one alkoxide or a metallic or silicon organoalkoxide;

- stirring of the mixture for at least 30 minutes;

- applying the mixture to said previously prepared substrate;

- total polymerization of the mixture.

In one embodiment, the method comprises the following steps in order:

- preparation of two separate solutions, a solution A containing TEOS in an acidic aqueous solution and a solution B containing a mixture of MaPTES, HEMA, a photochemical initiator and said piezochrome pigment;

- stirring of solution B for 30 minutes;

- mixing solutions A and B;

- stirring of the mixture obtained for 30 minutes;

- spray coating of the mixture obtained in the previous step on the substrate; or by dipping (dip-coating) of the substrate in the mixture obtained in the previous step;

- exposure of the coated substrate to ultraviolet radiation until the coating hardens.

In an alternative embodiment, the method comprises the following steps in order:

- preparation of a solution C containing a mixture of TEOS, APTES and said piezochrome pigment;

- stirring of solution C for 1 hour;

- addition of an epoxy base and water to solution C;

- stirring of the mixture obtained in the previous step for 30 minutes;

- spray coating of the mixture obtained in the previous step on the substrate; or by dipping the substrate in the mixture obtained in the previous step;

- heat treatment of the coated substrate in an oven or in an autoclave at a temperature of 130 ° C.

In an alternative embodiment, the method comprises the following steps in order:

- preparation of a solution D containing a mixture of TEOS, GPTES, water and said piezochrome pigment;

- stirring of solution D for 1 hour;

- addition of an amine base to solution C;

- stirring of the mixture obtained in the previous step for 30 minutes;

- spray coating of the mixture obtained in the previous step on the substrate; or by dipping the substrate in the mixture obtained in the previous step;

- heat treatment of the coated substrate in an oven or in an autoclave at a temperature of 130 ° C.

The fundamental concepts of the invention having just been exposed above in their most elementary form, other details and characteristics will emerge more clearly on reading the description which follows and with reference to the appended drawings, giving by way of 'non-limiting examples of embodiments of a piezochrome coating and its production process in accordance with the principles of the invention.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1a illustrates a coating according to the invention applied to a composite substrate and revealing four impacts of different intensity;

Figure 1b is a section of Figure 1a along the axis A-A, showing schematically two levels of internal damage in the substrate;

FIG. 2 illustrates the evolution of the color of the impacted area on two different piezochromic coatings as a function of the impact energy;

FIG. 3 illustrates compounds of a hybrid sol-gel / polymer matrix according to a first embodiment of the invention;

FIG. 4 represents the steps of a method for producing the coating according to the first embodiment of the invention;

FIG. 5 illustrates compounds of a hybrid sol-gel / polymer matrix according to a second embodiment of the invention;

FIG. 6 represents the steps of a method for producing the coating according to the second embodiment of the invention;

FIG. 7 is a curve illustrating the evolution of the transition pressure of a piezochrome pigment as a function of its concentration of doping metal.

DETAILED DESCRIPTION OF EMBODIMENTS

The present invention meets the need for means capable of rapidly revealing the shocks and impacts undergone by a composite structure, and which can lead to significant internal damage to the structure while being difficult to perceive on the surface of said structure.

Indeed, when a part made of composite material is impacted, by a falling tool for example giving rise to an impact defect, the internal structure of the part can be severely damaged by delamination in the case of a material laminate composite. Damage due to impact defects often requires repair or replacement of the part in question.

In general, a low energy impact does not leave a visible trace on the surface of the impacted part, said part must therefore go through a full ultrasonic test during the quality control and maintenance phases. This ultrasonic control then makes it possible to detect, locate and characterize possible internal damage in the controlled part.

The term "at least" in the context of the present invention means equal to or greater than "the integer following the term. The words "comprising", "comprising" and "containing" as well as their conjugated forms, do not exclude other elements or stages, and the indefinite article "one" or "one" does not exclude a plurality unless indicated opposite.

FIGS. 1a and 1b represent a composite substrate 10 on which a piezochrome coating 12 has been applied, said coating having been subjected to impacts of increasing intensity. The impacts were made by the same tool and therefore have spots 121, 122, 123 and 124 of equal diameters.

Spot 121 corresponds to an impact of 0.1 GPa, the weakest, and therefore to a change in color of the coating at the lowest contrast.

The other spots 122, 123 and 124 correspond respectively to impacts of 0.3 GPa, 0.5 GPa and 0.8 GPa. The color changes of the coating have increasing contrasts with a maximum of contrast for the spot 124 which corresponds to the greatest impact pressure undergone by the coating 12.

The impacts suffered by the substrate 10 cause internal damage illustrated in the longitudinal section of Figure 1b by triangular patterns 112 and 114 more or less dense and deep.

The coating, object of the present invention, allows for this purpose to visually detect and locate the impact zones on a composite substrate for the time of a routine visual inspection. The health control of the part concerned is significantly improved because of a considerable time saving and an ability to detect impacts of low intensity causing no damage in the substrate, the impacts cannot be revealed by the coating only from a well defined pressure threshold.

Ultrasonic testing remains necessary to characterize the damage revealed by the coating, but only concerns areas near the spots that materialize the impacts. Therefore, non-destructive testing can be strictly focused on useful areas.

A more reliable, shorter and therefore less expensive health check for composite structures is thus obtained.

FIG. 2 represents an experimental photograph showing the evolution of the color of the impacted area on two different piezochromic coatings 12a and 12b as a function of the impact energy.

On the coating 12a, the contrast of the color change takes place clearly from 12 joules, while on the coating 12b, the color change is clear from 2 joules of impact energy. The coating 12a therefore has a higher pressure threshold than that of the coating 12b.

The present invention relates to a piezosensitive coating comprising a hybrid sol-gel / polymer matrix comprising piezochromic pigments, said pigments being coated beforehand or not.

The hybrid sol-gel / polymer matrix is produced as its name suggests according to a sol-gel process and an organic polymerization process.

The hybrid sol-gel / polymer matrix is composed of at least two polymerized networks which overlap one another, an organic network and an inorganic network. The organic network is of the polymerizable type, while the inorganic network is made of metal oxides and or of silicon oxides. The two polymerized networks, organic and inorganic, each have a bonding interface extended to the nanometer scale, said networks being preferably linked by a strong bond of covalent or iono-covalent type.

The synthesis of the hybrid sol-gel / polymer matrix is carried out thanks to organic and inorganic polymerization reactions involving molecular precursors in the process used.

The term “hybrid” is used because, on the one hand, of the presence of two organic and inorganic components in the final matrix, and on the other hand, with regard to the interpenetration of the organic and inorganic components resulting in the nanoscale.

The molecular precursors contained in the solution of the process used, undergo polymerization reactions which lead to gelation of the reactive system after deposition.

According to the invention, the solution of the hybrid sol-gel / polymer matrix contains at least two distinct precursors from the following three chemical families: metal or silicon alkoxides, metal or silicon organoalkoxides with at least one polymerizable function, and organic monomers (or pre-polymers). In the case of two precursors from the last two families, the organic monomer preferably has at least one polymerizable function of the same family or complementary to that of the metal or silicon organoalkoxide.

The family of metal or silicon alkoxides groups together alkoxides (or alcoholates) of formula M (OR) _m where M is a metal of oxidation state m or silicon and R is an alkyl organic group of formula C _n H2n + i, OR then being a hydrolyzable and then condensable alkoxide group according to the sol-gel process.

A precursor of the family of metallic or silicon organoalkoxides exhibits at least one polymerizable function and at least two hydrolyzable and condensable OR alkoxide groups according to the sol-gel process. In addition, it also undergoes an organic polymerization reaction of its polymerizable function which will be described later.

A precursor of the family of organic monomers has at least one polymerizable function which leads to the organic network by polymerization.

In the case of a solution containing an organic monomer precursor and a metal or silicon organoalkoxide precursor with at least one polymerizable function, the organic monomer preferably has either the same type of polymerizable function as that of the organoalkoxide is a complementary function that of organoalkoxide. This allows a copolymerization reaction of the two precursors.

According to the invention, the element M of an alkoxide or of an organoalkoxide is a metal (for example titanium, zirconium, aluminum, tin, etc.) or silicon, preferably silicon, titanium or zirconium.

In a preferred embodiment, the solution contains at least one precursor of the family of alkoxides of which M is silicon, said precursor is then a silicon alkoxide of formula Si (OR) 4, or Si (OCnH ₂ n + i) 4 in which n represents an integer.

Advantageously, the tetraethyl orthosilicate of formula Si (OC2H ₅ ) ₄ (or simply Si (0Et) 4) also called tetraethoxysilane, or TEOS, is used, as illustrated in FIG. 3.

In an alternative embodiment, tetramethoxysilane (or TMOS) of formula Si (OCH ₃ ) _{4 is used} , polydimethoxysilane (or PDMOS) of general formula CH ₃ - [O-Si (OCH ₃ ) ₂ ] n-OCH ₃ or the polydiethoxysilane (or PDEOS) of general formula CH ₃ CH ₂ - [O-Si (OCH2CH ₃ ) 2] _n -OCH ₂ CH ₃ .

Other silicon alkoxides can also be used, such as propoxide, isopropoxide and silicon butoxide.

According to the invention, a precursor of the family of organic monomers can be, for example, ethylene dimethacrylate, ΙΉΕΜΑ, benzyl methacrylate, methyl methacrylate, glycidyl methacrylate, 2- (dimethylamino) ethyl methacrylate, methacrylic acid, urethane dimethacrylate, an epoxide, an amine, etc., preferably ΙΉΕΜΑ or an epoxide.

In a preferred embodiment, illustrated in FIG. 3, the hybrid sol-gel / polymer matrix is obtained from a solution containing TEOS as a precursor of silicon alkoxide type, MaPTES as a precursor of silicon organoalkoxide type, and ΙΉΕΜΑ as a precursor of the organic monomer type, MaPTES as well as ΙΉΕΜΑ having the same polymerizable function.

The various precursors are represented in FIG. 3 by their topological formulas.

The ethoxy functions of TEOS and MaPTES hydrolyze in the presence of water in hydroxy (OH) functions, the presence of these hydroxy groups allows the formation of Si - O - Si bridges via condensation reactions thus creating the inorganic network of the hybrid sol-gel / polymer matrix. The methacrylate polymerizable functions of MaPTES and ΙΉΕΜΑ undergo a radical polymerization process leading to the formation of the organic network.

A hybrid sol-gel / polymer based on HEMA is therefore obtained.

In a preferred embodiment, illustrated in FIG. 5, the hybrid sol-gel / polymer matrix is obtained from a solution containing three precursors: TEOS, APTES as a precursor of the silicon organoalkoxide type with a polymerizable function. of amine type, and an epoxide as a precursor of organic monomer type.

The various precursors are represented in FIG. 5 by their topological formulas.

The precursors of the silicon alkoxide and silicon organoalkoxide type, namely TEOS and APTES, undergo the same hydrolysis and condensation reactions as in the embodiment of FIG. 3.

As for the epoxide, it is opened by the amine function of APTES.

A hybrid sol-gel / epoxy-based polymer matrix is obtained.

The piezochromic pigments, sometimes called tribochromatic pigments, which will be introduced and mixed in the hybrid sol-gel / polymer matrices to form the piezosensitive coating according to the invention, are inorganic particles with piezochromic properties, that is to say that 'they change color depending on the pressure, preferably in the visible light spectrum.

The color changes of the piezochromic pigments of the present invention are reversible after application of an external stimulus (for example the temperature). However, there are piezochromic pigments with irreversible color change.

The piezochromic pigments according to the invention are metal oxides of formula AMo0 ₄ , A being a metal, the crystal structures of which are modified under the effect of pressure, thus inducing a change in color by optical contrast under the effect of constraints. mechanical. These pigments are preferably chosen from the following oxides: CuMo0 ₄ , ZnMo0 ₄ , FeMo0 ₄ or NiMo0 ₄ .

Derivatives of these metal oxides doped with tungsten (W) or magnesium (Mg) can also be used as piezochromic pigments.

In a preferred embodiment, the piezochromic pigments used are members of the series below with x a real number between 0 and 1:

CuMoi- _x W _x O ₄

In a preferred embodiment, the piezochromic pigments used are members of the series below with x a real number between 0 and 1:

Co ₁ . _x Mg _x MoO ₄

Advantageously, the members of the first series are used, namely CuMoi- _x W _x 0 ₄ , but with x a real number between 0.07 and 0.1. Such as CuMo ₀ , 925W ₀ , 075O ₄ or CuMoo, ₉ Wo, i0 ₄ for example.

These piezochromic components undergo a first phase transition which can be induced by a pressure applied to said piezochromic components. These components also have thermochromic properties which will not be presented here. The phase transition takes place between two polymorphic forms, each of said polymorphic forms having a distinct and distinguishable color, for example:

- green for the low pressure form (a);

- red-brown for the high pressure form (y).

The change in color between the two phases of a piezochrome compound can occur gradually or suddenly in the vicinity of the transition pressure of said piezochrome compound. The piezochromic pigments used in the present invention are characterized by well-defined transition pressures, the table below references the transition pressures of certain members of the CuMoi- _x W _x 0 _{4 series} :

X compound Transition pressure (in GPa) 0 CuMo0 ₄ 0.25 0.075 CuMOo, 925Wo, 0750 ₄ 0.1 0.1 CuMoo, gWo, i0 ₄ 0.012

The number x represents the rate of tungsten by mass title in the members of the CuMoi- _x W _x 0 _{4 series} , it has been demonstrated that the adjustment of this rate, within the limit of solubility of tungsten, makes it possible to control the transition pressure of the piezochrome compound.

FIG. 7 indeed illustrates the evolution of the transition pressure of the piezochrome compound, on the ordinate, as a function of the molar percentage of tungsten in said piezochrome compound, on the abscissa. It appears in FIG. 7 that the measurement values can be extrapolated by a straight line, the evolution of the transition pressure as a function of the tungsten concentration is therefore linear.

The choice of the doping metal, as well as its proportion, makes it possible to determine the transition pressure as well as the precision of this pressure. The interactions between the carefully chosen piezochromic pigments and the host matrix should be optimized. The determination of these parameters leads to the adjustment of the pressure of the color change of the coating as well as the viscosity of the final mixture of the paint before application to the substrate.

This therefore makes it possible to obtain piezochromic pigments with well-defined transition pressures suitable for detecting impacts of determined intensities, in a wide range of pressures covering different industrial applications.

In the case for example of structural parts frequently subjected to routine impacts, of known intensities, but which can damage said part, the piezochrome pigment chosen for the coating is that having a transition pressure which best corresponds to an order of magnitude of the intensities of these impacts.

The piezochromic pigments according to the invention can be directly introduced into the hybrid sol-gel / polymer matrix of the coating, but also previously coated, on the one hand, to adjust the chemical compatibility between said matrix and said pigments, and on the other hand, modulate the mechanical properties of the coating resulting from the mixture of said matrix and said pigments and thereby the range of pressure sensitivity of said coating.

The coating of piezochromic pigments consists of an encapsulation of said pigments either in a core-shell polymer (polymer core-shell in English), or in hybrid nanoparticles also of core-shell type comprising organosilanes which have at least one polymerizable function.

The core-shell polymer is for example poly (butyl acrylate) (PBuA), poly (methyl methacrylate) (PMMA), etc.

The hybrid core-shell nanoparticles contain organosilanes having a polymerizable function from methacrylate, vinyl, glycidoxy, etc.

In one embodiment, the “shell” part of the polymers or hybrid encapsulation nanoparticles comprises compounds with metal complexing functions, such as organic polymers of the polyvinylpyrrolidone (PVP) type for example, but also organosilanes with the same metal complexing functions, namely amines, hydroxylamines, phosphonic acids, carboxylic acids, ... etc.

These core-shell compounds, polymers and hybrid nanoparticles, thus make it possible to modify the mechanical properties of the coating according to the invention, and therefore to adjust the pressure thresholds from which the coating changes color. They also make it possible, as has already been pointed out, to promote the chemical compatibility between the piezochromic pigments, functionalized, and the host matrix.

In one embodiment, the hybrid sol-gel / polymer matrix of the coating comprises at least two different types of piezochromic pigments.

In one embodiment, the piezochrome coating makes it possible to detect impacts of intensities between 1 and 100 joules, with a change in color of the impacted area of said coating visible to the naked eye.

The present invention also relates to a process for the production of a reversible piezochrome coating with a hybrid sol-gel / polymer matrix, said coating being obtainable from a HEMA-based or epoxy-based matrix.

FIG. 4 illustrates a first embodiment of the method, said method makes it possible to obtain a piezochrome coating containing a matrix based on HEMA and comprises in order:

- A step of preparing two separate solutions, a solution A containing TEOS in an aqueous solution and a solution B containing a mixture of MaPTES, HEMA, a photochemical initiator and at least one piezochrome pigment;

- A step 410 of stirring solution B for 30 minutes;

a step 420 of mixing solutions A and B;

- A step 430 of stirring the mixture obtained for 30 minutes;

a step 440 of spray coating (spray-coating in English), in which the mixture obtained in step 430 is sprayed using a paint spray gun onto the substrate to be coated so as to obtain a film of the piezochrome coating deposited on said substrate previously adequately prepared; or by dipping (dip-coating in English), in which the substrate is immersed in the mixture obtained in step 430 and then removed at constant speed so as to obtain a film of the piezochrome coating deposited on said substrate previously prepared adequately;

a step 450 of treatment by ultraviolet radiation, in which the film deposited on the substrate in step 440 is exposed to ultraviolet radiation.

Advantageously, the precursors are mixed in the following molar proportions: x TEOS, y MaPTES, z HEMA with x + y + z = 1 and 0 <x <0.6; 0 <y <0.9 and 0 <z <0.5 and preferably 0.5 of TEOS; 0.25 from MaPTES and 0.25 from HEMA.

The piezochromic pigments used are for example CuWo, o8Mo ₀ , 90 ₄ and or Coo, 5Mgo, 5Mo04. The coating obtained is then applied in one or more layers, preferably three layers, each layer having a thickness of between 10 and 12 micrometers. An opaque film is thus obtained on the treated substrate.

FIG. 6 illustrates a second embodiment of the method, said method makes it possible to obtain a piezochrome coating containing an epoxy-based matrix and comprises in order:

- A step of preparing a solution C containing a mixture of TEOS, APTES and at least one piezochrome pigment;

- A step 610 of stirring solution C for 1 hour;

- a step of adding an epoxy base and water to solution C;

- A step 620 of stirring the mixture obtained in the previous step for 30 minutes;

a step 630 of spray coating (spray-coating in English), in which the mixture obtained in step 620 is sprayed using a paint gun on the substrate to be coated so as to obtain a film of the piezochrome coating deposited on said substrate previously adequately prepared; or by dipping (dip-coating in English), in which the substrate is immersed in the mixture obtained in step 620 and then removed at constant speed so as to obtain a film of the piezochrome coating deposited on said substrate previously adequately prepared;

a step 640 of conventional heat treatment, in which the substrate coated with step 630 is baked in an oven or in an autoclave at a temperature of 130 ° C.

Advantageously, the reagents are mixed with the following molar quantities: x 'TEOS, y' APTES, z 'epoxy, (4x' + 3y ') H ₂ O with x' + y '+ z' = 1 and 0 <x '<0.6; 0 <y '<0.5 and 0 <z'<0.5 and preferably 0.16 of TEOS; 0.17 APTS; 0.335 epoxy and 1.15 H ₂ O.

The coating can also be applied with a roller, a brush or a brush, after adequate preparation of the substrate.

In one embodiment, the method further comprises a step of dispersing said piezochromic pigments using a three-cylinder mixer.

When applying the coating layer, the target thickness must be respected in order to ensure the mechanical strength and adhesion of the desired layer.

The present invention indeed relates to a piezosensitive coating composed of a hybrid sol-gel / polymer matrix comprising piezochromic pigments, which makes it possible to determine the exceeding of a threshold or a plurality of pressure thresholds in a range determined by a color change of the impacted surface.

In one embodiment, the concentration of said piezochromic pigments is between 2% and 50% by mass.

Concentrating a sufficient quantity of one or more types of pigments in a matrix makes it possible to obtain sufficient contrast to the change in color of the coating. The chemical composition of the pigments and that of the matrix determine the pressure threshold at which the coating changes color.

Detection of the color change after exposure beyond the threshold pressure can be carried out by spectrophotometry, by the eye of a human observer, or by the naked eye of a human observer. A color chart can be used in the case of a three-color system, that is to say comprising two pressure thresholds.

The present invention finds applications in the detection of impacts when checking the integrity of a structure in any industry.

Claims (19)

1. Piezosensitive coating (12) for detecting impacts (121, 122, 123, 124) on a substrate (10), comprising a matrix in which at least one piezochrome pigment is dispersed, characterized in that: - Said matrix is a hybrid sol-gel / polymer matrix formed by at least one organic precursor and an inorganic precursor polymerized; - the piezochrome pigment is chosen so that said coating changes color from a determined pressure threshold, corresponding to a transition pressure of said piezochrome pigment in said matrix between two polymorphic forms of different colors. 2. Piezosensitive coating according to claim 1, in which the hybrid sol-gel / polymer matrix is formed by at least two precursors from the following three precursors: a metal or silicon alkoxide, a metal or silicon organoalkoxide, and an organic monomer . 3. Piezosensitive coating according to claim 2, wherein the hybrid sol-gel / polymer matrix is formed by a silicon alkoxide, a silicon organoalkoxide and an organic monomer. 4. Piezosensitive coating according to claim 3, in which the silicon alkoxide is tetraethyl orthosilicate (TEOS) of formula Si (OC ₂ H ₅ ) 4. 5. Piezosensitive coating according to claim 4, in which the silicon organoalkoxide is (3-methacryloxypropyl) triethoxysilane (MaPTES) and in which the organic monomer is 2hydroxyethyl methacrylate (HEMA). 6. Piezo-sensitive coating according to claim 4, in which the silicon organoalkoxide is (3-aminopropyl) triethoxysilane (APTES) and in which the organic monomer is an epoxide. 7. A piezosensitive coating according to claim 4, in which the silicon organoalkoxide is (3-glycidoxypropyl) triethoxysilane (GPTES) and in which the organic monomer is an amine. 8. Piezosensitive coating according to any one of the preceding claims, in which said piezochrome pigment is a metal oxide of formula AMo0 ₄ with A a metal. 9. A piezosensitive coating according to claim 8, in which A is a metal among copper, zinc, iron, and nickel. 10. Piezosensitive coating according to claim 8 or 9, wherein said piezochrome pigment has a formula CuMoi_ _x W _x 0 ₄ with x a real number between 0 inclusive and 1 not included. 11. Piezosensitive coating according to claim 10, in which x is between 0.01 inclusive and 0.3 inclusive, preferably between 0.07 and 0.1. 12. Piezosensitive coating according to claim 8 or 9, wherein said piezochrome pigment has a formula Coi- _x Mg _x Mo0 ₄ with x a real number between 0 inclusive and 1 inclusive. 13. Piezosensitive coating according to any one of the preceding claims, in which said piezochrome pigment is coated, before its introduction into said matrix, in a core-shell polymer or in hybrid core-shell nanoparticles. 14. Method for producing a piezosensitive coating on a substrate, said coating comprising a hybrid sol-gel / polymer matrix and at least one piezochrome pigment, and characterized in that it comprises the following steps: - preparation of a mixture containing said piezochrome pigment, an organic monomer, and at least one alkoxide or a metallic or silicon organoalkoxide; - Stirring said mixture for at least 30 minutes inducing partial polymerization of said mixture; - applying said mixture to said substrate; - polymerization of said mixture. 15. Method according to claim 14, characterized in that it comprises the following steps in order: - preparation of two separate solutions, a solution A containing TEOS in an acidic aqueous solution and a solution B containing a mixture of MaPTES, HEMA, a photochemical initiator and said piezochrome pigment; - stirring of solution B for 30 minutes; - mixing solutions A and B; - stirring of the mixture obtained for 30 minutes; - spray coating or soaking of said substrate in the mixture obtained in the previous step; - exposure of the coated substrate to ultraviolet radiation until the coating hardens. 16. Method according to claim 14, characterized in that it comprises the following steps in order: - preparation of a solution C containing a mixture of TEOS, APTES and said piezochrome pigment; - stirring of solution C for 1 hour; - addition of an epoxy base and water to solution C; - stirring of the mixture obtained in the previous step for 30 minutes; - spray coating or soaking of said substrate in the mixture obtained in the previous step; - heat treatment of the coated substrate in an oven or in an autoclave at a temperature of 130 ° C. 17. Method according to claim 14, characterized in that it comprises the following steps in order: - preparation of a solution D containing a mixture of TEOS, GPTES, water and said piezochrome pigment; - stirring of solution D for 1 hour; - addition of an amine base to solution C; - stirring of the mixture obtained in the previous step for 30 minutes; - spray coating of the mixture obtained in the previous step on the substrate; or by dipping the substrate in the mixture obtained in the previous step; - heat treatment of the coated substrate in an oven or in an autoclave at a temperature of 130 ° C. 18. Method according to any one of claims 14 to 17, in which said substrate is a substrate made of composite material. 19. Method according to any one of claims 14 to 18, in which the coating obtained conforms to any one of claims 7 to 12. 1/4