The present invention relates to a metallic substrate directly coated with a non-conductive primer, the non-conductive primer being at least partially coated with a paint comprising reduced graphene oxide and a thermosetting polymer, a method for the manufacture of this coated metallic substrate, a method for detecting strain deformation. It is particularly well suited for offshore industry, electronic industry and energy industry.

Very sensitive strain sensing devices are greatly needed for monitoring a state of a structure, such as an expressway, a building, a bridge, an airplane, a ship, or the like, or for early detecting a defect that occurs in an extreme situation such as a natural disaster such as earthquake, typhoon, or the like. Therefore, piezoelectric elements are being researched.

However, most of piezoelectric elements are fragile ceramic materials, and are difficult to be used as a flexible strain sensor. A special manufacturing process is mainly needed for forming a piezoelectric element on a flexible substrate.

The patent application US2014291733 discloses a strain sensing device comprising: a flexible substrate; a gate electrode formed on the flexible substrate; a gate insulating layer configured to cover the gate electrode, and include a part formed of a flexible material; an active layer formed of reduced graphene oxide (R- GO) for sensing a strain, on the gate insulating layer; and a source and drain electrode formed on the active layer.

However, this device is really complex to produce at industrial scale since it comprises a lot of elements. Moreover, this device can be applied to a flexible or elastic electronic device. Nevertheless, it is not adapted to metallic substrate such as steel. Indeed, for example in the energy and offshore industries, there is a need to monitor the strain deformation of wind turbine to early detect defects or critical situations such as cracks (especially important in welded areas), pitting corrosion, very high loads that could be avoided by wind turbine or blades positioning control, etc. and therefore to increase the lifetime of the wind turbine. Finally, in the method of forming the active layer of reduced graphene oxide, graphene oxide is adsorbed onto a gate insulating layer, by using a graphene oxide aqueous solution (0.2 mg/mL) of a graphene oxide nanosheet formed by a graphite striping method of Hummer. An adsorbed and networked graphene oxide layer is exposed to hydrazine hydrate vapor at about 40°C. for 18 hours to thereby be reduced, thereby forming an R-GO layer that is the active layer. However, the formation of reduced graphene oxide is very long and the absorption technique can lead to adherence problem and decrease the quality of the strain detection.

Thus, the purpose of the invention is to provide an easy system to detect and monitor strain deformation of metallic substrates. Additionally, the purpose is to provide a system having a high detection sensitivity and therefore improve the life time of metallic substrates.

This is achieved by providing a coated metallic substrate according to claim 1. The coated steel substrate can also comprise any characteristic of claims 2 to 16.

The invention also covers a method for the manufacture of the coated metallic substrate according to claims 17 to 23.

The invention also covers a method for detecting a strain deformation with the coated metallic substrate according to claims 24 to 25.

Finally, the invention covers the use of the coated metallic substrate according to claim 26.

The following terms is defined:

- Reduced graphene oxide means graphene oxide that has been reduced. The reduced graphene oxide comprises one or a few layer(s) of graphene having some oxygen functional groups including ketone groups, carboxyl groups, epoxy groups and hydroxyl groups.

Other characteristics and advantages of the invention will become apparent from the following detailed description of the invention.

To illustrate the invention, various embodiments and trials of non-limiting examples will be described, particularly with reference to the following Figures:

Figure 1 illustrates an example of one nanoplatelet of reduced graphene oxide according to the present invention.

Figure 2 illustrates an example of a few nanoplatelets of reduced graphene oxide according to the present invention. The invention relates to a metallic substrate directly coated with a non- conductive primer, the non-conductive primer being at least partially coated on at least one side with a paint comprising reduced graphene oxide and a thermosetting polymer.

Without willing to be bound by any theory, it seems that the paint including the reduced graphene oxide and the thermosetting polymer well adheres on the non-conductive primer increasing the lifetime of the coated metallic substrate and very importantly, the loading transfer to the sensor. Indeed, it is believed the reduced graphene oxide is highly dispersed in the paint leading to an improvement of the detection quality. Finally, the paint deposited on the non-conductive primer is an easy and simple system allowing a quick detection of strain deformation.

The reduced graphene oxide can be produced from kish graphite as disclosed in the patent applications PCT/IB2017/000348 or PCT/IB2018/053416. It can also be produced from electrode scraps as disclosed in PCT/IB2018/053643.

Preferably, the non-conductive primer is coated on both sides.

In a preferred embodiment, the coated metallic substrate is covered by a protective layer. The protective layer can be made of thermosetting polymers. In this case, the coated non-conductive substrate is protected against corrosion, etc.

Preferably, the lateral size of the reduced graphene oxide is between 1 and 80pm, more preferably between 40 and 80pm and advantageously between 60 and 80pm.

Preferably, the weight percentage of oxygen in the reduced graphene oxide is between 2 and 20% and preferably between 2 and 10%. Indeed, without willing to be bound by any theory, it is believed that the percentage of oxygen plays a role in the conductivity and electrical resistance of the paint.

Preferably, the reduced graphene oxide is not functionalized by a biopolymer. Indeed, without willing to be bound by any theory, it is believed that the biopolymer can decrease the sensitivity of the strain deformation detection.

Preferably, the reduced graphene oxide is in a form of one or more nanoplatelets. Indeed, without willing to be bound by any theory, it is believed that the form of the reduced graphene oxide can play a role in the detection since it seems that the nanoplatelets can easily form a path in the paint wherein the electricity runs. Figure 1 illustrates an example of one nanoplatelet of reduced graphene oxide. The lateral size means the highest length of the layer through the X axis, the thickness means the height of the layer through the Z axis and the width of the nanoplatelet is illustrated through the Y axis. Figure 2 illustrates an example of a few nanoplatelets of reduced graphene oxide.

Advantageously, the thickness of the paint is below 1 mm and preferably between 25 and 500 microns.

Preferably, the concentration of the reduced graphene oxide in the paint is between 0.05 and 10% by weight, preferably between 0.05 and 7% by weight and advantageously between 0.5 and 4% by weight. Indeed, without willing to be bound by any theory, it seems that having the reduced graphene oxide in the above concentration further improves the detection sensitivity in the case of strain because in that range the conductivity of the network of nanoparticles formed inside the thermosetting resin is more sensitive to deformations allowing to detect smaller strains.

Preferably, the paint does not comprise a thermoplastic polymer. In particular, the paint does not comprise acrylic polymer. Indeed, it is believed that the thermoplastic improves the viscosity of the paint leading to a bad dispersion of reduced graphene oxide and therefore a poor quality of the coated metallic substrate.

Advantageously, the thermosetting polymer is chosen from among: epoxy resin, Polyester resin, Polyurethanes, Polyurea/polyurethane, Vulcanized rubber, Urea-formaldehyde, Melamine resin, Benzoxazines, Polyimides, Bismaleimides, Cyanate esters, polycyanurates, Furan, Silicone resins, Thiolyte and Vinyl ester resins or a mixture thereof.

Preferably, the molar mass distribution of the polymer is below or equal to 1300 and advantageously between 700 and 1200.

Preferably, the non-conductive primer is made of polymers.

In a preferred embodiment, the polymer is chosen from among: Poly(methyl methacrylate), epoxy resin, Polyester resin, Polyurethanes, Polyurea/polyurethane, Vulcanized rubber, Urea-formaldehyde, Melamine resin, Benzoxazines, Polyimides, Bismaleimides, Cyanate esters, polycyanurates, Furan, Silicone resins, Thiolyte and Vinyl ester resins or a mixture thereof.

Preferably, the non-conductive primer does not comprise Poly-4- vinylphenol, polyethersulfone or Polydimethylsiloxane. Indeed, without willing to be bound by any theory, it is believed that the presence of these polymers can reduce the detection sensitivity.

Preferably, the metallic substrate is chosen from: aluminum, steel, stainless steel, copper, iron, copper alloys, titanium, cobalt, metal composite or nickel or a mixture thereof.

Advantageously, the paint does not comprise titanium dioxide or copper.

Preferably, the non-conductive primer is coated with paint strips to form an alternation between painted and non-painted non-conductive primer.

In another embodiment, the non-conductive primer is coated with one entire layer of paint.

The second object of the present invention is a method for the manufacture of the metallic substrate according to the present invention, comprising the successive following steps:

A. The deposition of a non-conductive primer on a metallic substrate,

B. The deposition of a mixture comprising reduced graphene oxide, a thermosetting monomer, a curing agent and optionally a solvent on said non-conductive primer being previously deposited on said metallic substrate and

C. A curing step.

Preferably, in step B), the mixing is performed as follows:

i. The mixing of reduced graphene oxide and a thermosetting base polymer and optionally a solvent,

ii. The addition of a curing agent,

iii. The mixing of the mixture obtained in step B).

Preferably, in step A), the deposition of the non-conductive primer is performed by: spin coating, spray coating, dip coating, film coater, coil coating, brush coating or spatula coating. Preferably, in step B), the solvent is chosen from among others: xylene, n- butanol, ethylbenzene, naphtha solvents, n-butyl acetate, toluene, cyclic hydrocarbons, isopropanol and benzyl alcohol or a mixture thereof.

Preferably, in step B), the thermosetting monomer is chosen from: epoxy resin, ester, urethane, urea/polyurethane, Vulcanized rubber, Urea-formaldehyde, Melamine resin, Benzoxazines, imides, Bismaleimides, Cyanate esters, cyanurates, Furan, Silicone resins, Thiolyte and Vinyl ester resins or a mixture thereof.

Advantageously, in step B), the curing agent is chosen from among: polyamide, polyamide, phenols, amines and polyaddition isocyanate or a mixture thereof.

Preferably, in step B), the deposition of the coating is performed by spin coating, spray coating, dip coating, film coater, coil coating, brush coating or spatula coating.

Preferably, in step C), the curing step is performed by drying at room temperature.

The third object of the present invention is a method for detecting a strain deformation with the coated metallic substrate according to the present invention comprising the following successive steps:

1. the application of electrical voltage to the metallic substrate directly coated with a non-conductive primer, the non-conductive primer being at least partially coated with a paint using an electronic system,

2. the measurement of the electrical resistance variation after deformations of the metallic substrate directly coated with a non- conductive primer, the non-conductive primer being at least partially coated with a paint.

Without willing to be bound by any theory, it is believed that in the paint, the reduced graphene oxide nanoparticles form a conductive network. When the material is subjected to a strain, the internal geometry of the network which is stronger than the thermosetting changes in an important way. The consequence is a change in the electrical resistance of the paint. In this case, preferably, the gauge factor, being the ratio of relative change in electrical resistance to the mechanical strain e, is above 5.

Preferably, in step 1 ), the electronic system comprises a power supply system. Preferably, it is a battery.

Finally, the last object of the present invention is the metallic substrate directly coated with a non-conductive primer, the non-conductive primer being at least partially coated with a paint according to the present invention for detecting strain deformation.

The invention will now be explained in trials carried out for information only. They are not limiting.

Examples:

Example 1 : conductivity test

Steel substrates, having the following chemical composition in weight percent: 0.0670%C, 0.4910%Mn, 0.0220%Cu, 0.0110%Si, 0.0100%S,

0.01 10%P, 0.0180%Ni, 0.0180%Cr, 0.0480%Nb were coated with Epoxy thermosetting resin having a molar mass distribution between 700 and 1200, bisphenol A-(epichlorhydrin) epoxy resin having a molar mass distribution below or equal to 700 and xylene.

Different nanoparticles were mixed with an epoxy resin having a molar mass distribution between 700 and 1200, bisphenol A-(epichlorhydrin) epoxy resin having a molar mass distribution below or equal to 700 and xylene. The mixture was mixed and dispersed using a device called DISPERMAT. Then, a curing agent comprising polyamide was added in the mixture before being mixed. The mixture was deposited on poly(methylmethacrylate) (PMMA).

Then, an electric voltage (10V) was applied on all the trials using an electronic system including a battery. The electrical resistance was determined. The conductivity of all Trials was calculated.

The results are in the following Table 1 :

^* : according to the present invention.

Trials 1 to 4 show a high conductivity and therefore a high sensitivity for detecting leak and strain deformation compared to Trials 5 and 6.

Example 2: Strain deformation test

Steel substrates, having the following chemical composition in weight percent: 0.0670%C, 0.4910%Mn, 0.0220%Cu, 0.0110%Si, 0.0100%S, 0.01 10%P, 0.0180%Ni, 0.0180%Cr, 0.0480%Nb were coated with Epoxy thermosetting resin having a molar mass distribution between 700 and 1200, bisphenol A-(epichlorhydrin) epoxy resin having a molar mass distribution below or equal to 700 and xylene.

Different nanoparticles were mixed with an epoxy resin having a molar mass distribution between 700 and 1200, bisphenol A-(epichlorhydrin) epoxy resin having a molar mass distribution below or equal to 700 and xylene. The mixture was mixed and dispersed using a device called DISPERMAT. Then, a curing agent comprising polyamide was added in the mixture before being mixed. The mixture was deposited on the same non-conductive primer than Example 1.

Then, a tensile loading was applied on all the Trials and the gauge factor, being the ratio of relative change in electrical resistance to the mechanical strain e, was determined. A conventional strain gauge sensitivity being made of constantan® was added in comparison.

The results are in the following Table 2:

: according to the present invention.

Trials 7 to 10 show a high gauge factor and therefore a high sensitivity to detect the strain deformation compared to conventional strain gauge.