Mechanoluminochromic composite material and process for detecting and / or monitoring the structural integrity of an artifact coated with said composite material

Abstract

A mechanoluminochromic composite material and a process for detecting and / or monitoring the structural integrity of an artifact are herein described, said process comprising the steps of providing an artifact superficially and at least partially covered by said mechanoluminochromic composite material, exposing the surface of the artifact to an ultraviolet ray source, irradiating it for a pre-fixed time, and detecting the emissive properties of the composite material while the surface is being irradiated.

Description

[0001] Title: Mechanoluminochromic composite material and process for detecting and / or monitoring the structural integrity of an artifact coated with said composite material

[0002] DESCRIPTION

[0003] Field of application

[0004] In its more general aspect the present invention relates to the field of mechanoluminochromic materials and processes for detecting the structural integrity of materials, particularly of the mechanical type. Prior art

[0005] Nowadays, there are various techniques for monitoring and verifying the structural integrity of materials.

[0006] The structural integrity of materials may be detected by using various non-destructive techniques, such as thermography, ultrasonic testing and acoustic emissions.

[0007] These procedures require time and considerable financial resources, and hidden damage is often difficult or impossible to detect.

[0008] The context is that of monitoring the structural properties (SHM or ‘Structural Health Monitoring’) of polymeric materials and composite materials, in particular of the polymeric matrix fiber composite materials (‘Fiber Reinforced Plastics’ or FRPs), used for structural applications in the automotive, aeronautical and aerospace fields.

[0009] In these fields, the FRPs are gradually replacing metals, as their lightness, at equal mechanical performance, allows for significant fuel savings, thus reducing costs and increasing economic and environmental sustainability. However, their laminar structure, characterised by layers of carbon or glass fibres bonded together by a cross-linked polymeric matrix, makes them prone to the formation of micro-defects that cancompromise their functionality. Particularly dangerous are the damages caused by low / medium speed impacts, which typically leave the outer surface intact but generate micro-cracks and delaminations in the innermost layers of the material (‘Barely Visible Impact Damage’, BVID), that propagate as a result of fatigue loads and, if not detected in time, may lead to catastrophic failure of the structure.

[0010] The most widely used technologies for this type of applications are the fibre Bragg gratings (‘Fiber Bragg Grating’ or FBG) and piezoelectric ceramic-based sensors, such as lead zirconate titanate sheets (PZT). However, all of these technical solutions require the use of several sensors distributed appropriately along the entire component or structure to be monitored, making both the design and production steps of the component more difficult, not to mention the possible harmful effects that the sensors may cause on the structural properties of the material.

[0011] A possible alternative technology is the one identifiable in the mechanoluminochromic materials.

[0012] In the field of mechanoluminochromic materials, organic, inorganic or organic-inorganic compounds, commonly referred to as mechanochromic or piezochromic compounds, are known, which have been proposed for the development of mechanochromic or piezochromic coatings.

[0013] These materials are characterised in that they change their optical properties in response to a mechanical stimulus. The change in optical properties specifically affects the colour of the compound, which is detectable to the naked eye.

[0014] Organic compounds covalently bonded to a polymeric matrix are disclosed in WO2013158379 and in the publication by Yoon et al. entitled ‘Mechanochromic and thermally reprocessable thermosets for autonomic damage reporting and self-healing coatings’, NPG Asia Materials, 2022, 14, art. no. 61.In the compounds disclosed therein, mechanical stimulus causes the breaking of covalent bonds, resulting in a change in the colour of the compound.

[0015] Similar compounds are disclosed in US20140013864 and in the publication by Shree et al. entitled ‘Self-reporting mechanochromic coating: a glassfiber reinforced polymer composite that predicts impact induced damage’, Mater. Horiz., 2020, 7, 598-604.

[0016] Inorganic compounds, such as metal oxides doped with magnesium and tungsten, are disclosed in WO2018115654 and in the publication by Morelle et al. entitled ‘Hybrid piezochromic coatings for impact detection on composite substrates for aeronautic’, Materials Letters, 2019, 253, 140-143.

[0017] In the inorganic compounds disclosed herein, mechanical stimulus causes a crystalline phase transition resulting in a change in colour in the compound. The change in colour occurs when a certain pressure threshold has been exceeded. Furthermore, the change in colour is reversible when thermal stimulus is applied.

[0018] Organic-inorganic compounds, such as coordination compounds having a metal centre with an electronic configuration of 3d4, 3d6or 3d7, a nitrogen-containing ligand and an anion are disclosed in WO2018115654.

[0019] In these compounds, mechanical stimulus causes an electronic spin transition, resulting in a change in the colour of the compound. The change in colour occurs once a certain pressure threshold has been exceeded. Furthermore, the change in colour is reversible when thermal stimulus is applied.

[0020] There are also other technological solutions proposed for the preparation of mechanochromic coatings based on photonic crystals, i.e. ordered nanostructures whose colour depends on the geometry of the nanostructure and the difference in refractive index of the materials thatconstitute it (structural colour): tension or compressive stress gradually changes the geometry of the nanostructure and therefore the colour. The change in colour is reversible when the structure exhibits elastic recovery.

[0021] These solutions are described in CN115584047 and CN111421928 patent applications.

[0022] Therefore, in the aforementioned technological solutions, the additive changes colour as a result of a mechanical stress, modifying the aesthetic properties of the coating in which it is incorporated. Furthermore, in some of the aforementioned technological solutions, the change in colour is reversible by applying a thermal stimulus.

[0023] Therefore, there is a strong need in the field for a material or device that can be used to detect the structural integrity of materials, allowing overcoming the aforementioned limitations.

[0024] Particularly, on the one hand, it is necessary, especially for applications where high stresses are expected and high safety standards are required, such as in the aerospace field, to develop a technology that causes an irreversible change in chromatic properties.

[0025] This requirement is justified by the need of ensuring that any structural alteration undergone by the structures of interest can always be detected and to prevent tampering.

[0026] On the other hand, for some applications it is necessary or even particularly undesirable that, in the event of a structural alteration, the change in chromatic properties can be detected by people not interested in the structural integrity of the concerned structure or device.In this context, there is also a particularly pressing need for providing a reliable, simple and inexpensive technique capable of identifying the early stages of mechanical weakening, in particular, of polymeric matrix fibrous composite materials.

[0027] Therefore, the technical problem underlying the present invention is to provide a mechanoluminochromic material, useful for detecting the structural integrity of materials, that determines an irreversible change in chromatic properties and, at the same time, does not lead to a change in colour when exposed to visible light.

[0028] Summary of the invention

[0029] This problem is solved by a composite material comprising a matrix of a polymeric nature and a mechanoluminochromic additive in the form of a coordination compound or a coordination polymer, said mechanoluminochromic additive being dispersed in the matrix, wherein said coordination compound or coordination polymer has the following general formula (I)

[0030] [(MX)a(L)b]n (I)

[0031] where M is selected from Cu(I), Ag(I) or Au (I), X is selected from I, Br, Cl, CN, SCN or any combination thereof, L is selected from substituted or unsubstituted pyridine, substituted or unsubstituted pyrimidine, substituted or unsubstituted pyrazine, or a phosphine group, a is 1, 2, 3 or 4, b is 1, 2, 3 or 4 and n is a number greater than or equal to 1; or said coordination compound or coordination polymer has the following general formula (II)

[0032] [(M’X’)a L’b’Yc’)]n’ (II)

[0033] where M’ is selected from Cu(I), Ag(I) or Au (I), X’ is selected from I, Br, Cl, CN, SCN or any combination thereof, L’ is selected from substituted or unsubstituted pyridine, substituted or unsubstituted pyrimidine, substituted or unsubstituted pyrazine, Y is a phosphine group, a’ is 1, 2,3 or 4, b’ is 1, 2, 3 or 4, c’ is 1, 2, 3 or 4 and n’ is a number greater than or equal to 1.

[0034] Advantageously, as will be seen herein after in connection with the detailed description, the composite material according to the present invention determines a change in the emission colour when stimulated by ultraviolet (UV) radiation, which is not attributable to a change in the chemical structure (bond breaking) or to a change in the crystalline phase (such as phase transition or amorphisation), but to the formation of surface defects caused by mechanical stress, resulting in a change in the emission colour. The change in the emissive properties does not occur when a certain stress threshold is exceeded (as in the case of chemical bond breaking or phase transition), but is gradual depending on the stress applied and, as a result, may be used to obtain information on the extent of the stress applied.

[0035] With reference to the prior art, in the aforementioned alternative technological solutions, the additive changes colour as a result of a mechanical stress, thus modifying the aesthetic properties of the coating, which are visible to the naked eye.

[0036] The present invention, instead, does not modify the aesthetic aspect of the coating since the colour of the additive does not change when observed in visible light.

[0037] Moreover, very advantageously, the change in the emission colour is permanent, that is to say, it is not reversible. This property prevents the possibility of counterfeiting (intentionally or unintentionally over time, due to temperature, for instance) the damage generated by the stresses and impacts suffered by the material itself.

[0038] According to a first embodiment, X and X’ may be selected from any anion of the group comprising I⁻, Br⁻, Cl⁻, CN⁻ or SCN⁻ respectively.

[0039] For instance, in the case of the general formula (I), the coordination compound or coordination polymer may be [(CuI)3-BrPy]nwhere n is anumber equal to or greater than 1.

[0040] According to a further embodiment, as detailed in the above reported general formula (I) and general formula (II), X and X’ may instead be any combination between two or more anions selected from I⁻, Br⁻, Cl⁻, CN⁻ or SCN⁻, respectively.

[0041] In other words, it is understood that, when X and X’ are any combination between two or more anions selected from P, Br, CP, CN~ or SCNy the unit (MX)aand the unit (M’X’)a> contain a metal cation in combination with at least two any anions selected in the group constituted by P, Br, Cl, CN- and SCNv

[0042] For instance, in the case of the general formula (I), the coordination compound or coordination polymer may be [(CuIo.9Bro.i)3-BrPy]nwhere n is a number equal to or greater than 1.

[0043] By using another graphic representation alternative to the aforementioned general formula (I) and general formula (II), said coordination compound or coordination polymer may be identified by the following general formula (Ibis)

[0044] [(MXzX”z-i)a(L)b]n (Ibis)

[0045] where M is selected from Cu(I), Ag(I) or Au (I), X and X” are selected independently from I, Br, Cl, CN or SCN, L is selected from substituted or unsubstituted pyridine, substituted or unsubstituted pyrimidine, substituted or unsubstituted pyrazine, or a phosphine group, a is 1, 2, 3 or 4, b is 1, 2, 3 or 4, z is a natural or decimal number comprised between 0 and 1, and n is a number greater than or equal to 1;

[0046] or said coordination compound or coordination polymer may be identified by the following general formula (Ilbis)

[0047] [(M’X’zX’”z>-i)a’(L’b’Yc’)]n’ (Ilbis)

[0048] where M’ is selected from Cu(I), Ag(I) or Au (I), X’ and X’” are selectedindependently from I, Br, Cl, CN or SCN, L’ is selected from substituted or unsubstituted pyridine, substituted or unsubstituted pyrimidine, substituted or unsubstituted pyrazine, Y is a phosphine group, a’ is 1, 2, 3 or 4, b’ is 1, 2, 3 or 4, c’ is 1, 2, 3 or 4, z’ is a natural or decimal number comprised between 0 and 1, and n’ is a number greater than or equal to 1.

[0049] Preferably, said coordination compound or coordination polymer may be selected from [(CuI)2Pyz]n, [(CuI)3-BrPy]n’ where n and n’ are a number greater than or equal to 1 or [(Cui)- 4-PhPy]4.

[0050] In other words, the coordination compound or coordination polymer may be selected from [(Cul)2 pyrazine]n, [(CuI)3-Br-pyridine]nwhere n and n’ are a number greater than or equal to 1 or [(CuI)-4- phenylpyridine] 4. Preferably, said matrix of a polymeric nature may be a polymer selected from a thermosetting polymer or a thermoplastic polymer.

[0051] More preferably, said thermosetting polymer is selected from epoxy resins, thermosetting acrylic resins, aspartic resins, polyurethane resins, alkyd resins, amino resins, phenolic resins, silicone resins, resins based on unsaturated polyesters or vinyl ester resins, synthetic rubbers, or natural rubbers.

[0052] More preferably, said thermoplastic polymer is selected from homopolymers and copolymers of vinyl polymers, polyolefins, thermoplastic acrylic polymers, polyvinyl esters, fluorinated polymers, polyamides, polyurethanes, polyesters, polyethers, polycarbonates, cellulose nitrate and other cellulose derivatives, polysaccharides, thermoplastic elastomers, polyketones, polyether ketones, polyimides, polysulfones, polyethersulfones, or polyarylsulfones.

[0053] Still more preferably, said thermoplastic polymer is an acrylic polymer selected from polymethyl methacrylate (PMA) or polybutyl methacrylate (PBMA).In absolute preference, said thermoplastic polymer is different from a fluorinated vinyl polymer, more preferably it is different from polyvinylidene fluoride (PVDF).

[0054] Equally in absolute preference, said vinyl polymer is polyvinyl acetate (PVAc).

[0055] Preferably, said mechanoluminochromic additive is present in the composite material of the present invention in a quantity equal to at least 0.1%, preferably comprised between 0.1% - 98% by weight on the total weight of the composite material, more preferably between 5% and 10% by weight on its total weight.

[0056] Preferably, said mechanoluminochromic additive is in the powder form. More preferably, said mechanoluminochromic additive has an average particle size comprised between 10 nm and 1 mm, still more preferably between 100 nm and 500 microns.

[0057] According to the present invention, the expression ‘average particle size’ refers to a value that indicates the size of the particles (in the form of agglomerations or clusters) constituting the additive of the composite material according to the present invention.

[0058] The particle size is equal to the main size of the ideal solid that exemplifies or comprises the three-dimensional shape of the particle, in particular the diameter of the ideal sphere completely comprising a given particle. More particularly, the mechanoluminochromic additive has an average particle size comprised between 500 nm and 20 microns.

[0059] The particles of the additive according to the present invention have an average particle size as measured according to optical or electronic microscopy techniques, such as scanning electron microscopy, and subsequent image analysis.

[0060] According to a preferred embodiment, said mechanoluminochromicadditive is in the powder form and the particles constituting it are covered at least partially, optionally completely, by a surface protective layer comprising a protective material of a polymeric nature.

[0061] Preferably, said protective material of a polymeric nature is selected from a thermosetting polymer or a thermoplastic polymer.

[0062] More preferably, in said protective material of a polymeric nature said thermosetting polymer is selected from epoxy resins, thermosetting acrylic resins, aspartic resins, polyurethane resins, alkyd resins, amino resins, phenolic resins, silicone resins, resins based on unsaturated polyesters or vinyl ester resins, synthetic rubbers or natural rubbers. Alternatively, in said protective material of a polymeric nature said thermoplastic polymer is selected from homopolymers and copolymers of vinyl polymers, polyolefins, thermoplastic acrylic polymers, polyvinyl esters, fluorinated polymers, polyamides, polyurethanes, polyesters, polyethers, polycarbonates, cellulose nitrate or other cellulose derivatives, polysaccharides, thermoplastic elastomers, polyketones, polyether ketones, polymides, polysulfones, polyethersulfones, or polyarylsulfones.

[0063] Advantageously, when said mechanoluminochromic additive is in the powder form and the particles constituting it are covered at least partially, optionally completely, the composite material according to the present invention is resistant to attack by acids or bases, potentially identifiable among the components and / or additional additives present in the composite material according to the present invention.

[0064] Indeed, certain commercial formulations (e.g. highly acidic or basic) used to produce the composite material according to the present invention may potentially compromise its stability and alter the chemical structure of the additive and its appearance under visible light.

[0065] In this case the mechanoluminochromic additive, before being used for the preparation of the aforementioned formulations, may be subjected toa pre-treatment. This pre-treatment consistently consists in the application of an at least partially covering protective layer onto the surface of the particles constituting the additive, in order to form a layer of material that isolates it from the remaining components of the formulation. This covering protective layer may be made by means of traditional microencapsulation techniques.

[0066] In absolute preference, the composite material according to the present invention may comprise an additional additive selected from organic pigments, inorganic pigments, dispersing agents, additives for controlling rheological properties, stabilisers or any combination thereof.

[0067] Preferably, the composite material according to the present invention is in the film form.

[0068] According to the present invention the expression ‘film’ means that the above composite material, in particular organized in one or more layers superimposed on each other, has an overall thickness comprised between 10 nm and 1 cm, preferably between 50 microns and 500 microns.

[0069] The present invention also relates to a mechanoluminochromic coating, also called coating layer, comprising the composite material as above described.

[0070] Advantageously, by virtue of the properties conferred by the composite material as above described, the mechanoluminochromic coating according to the invention effectively fulfils the independent function of informing the end user about the mechanical stresses that can cause catastrophic failures, enabling simple and reliable identification of the mechanical stresses acting on the material itself.

[0071] Moreover, when used as coating layer, the mechanoluminochromic composite material is not added to the material constituting the structure and the internal layers of the artifact, such as an item, a device or a structural element, that has to be monitored; conversely, it is an integral part of the mechanoluminochromic coating of the invention and thiscircumstance determines a particular advantage from a practical and economic point of view.

[0072] A further simplification arises from the fact that the composite material and the possible coating layer, made from the aforementioned composite material or that contains it, contain a polymeric matrix comprising an easily available and easy-to-use material, thus allowing an easy scalability at an industrial level.

[0073] Advantageously, said coating layer may indeed be applied to laminates without altering the structure and / or composition of the laminate itself. By intervening through a coating layer without modifying the structure and / or the composition of the laminate itself, the structural properties of the laminate itself are not affected, unlike other SHM (‘Structural Health Monitoring’) techniques. Moreover, unlike the sensors commonly used in the SHM techniques, the use of the coating layer does not require the use of wiring, connectors and electronic control units for signal acquisition, amplification and processing, which normally have to be integrated with the component.

[0074] Alternatively and according to a further aspect of the present invention, the aforementioned mechanoluminochromic composite material may be integrated into an artifact of interest, at the time of production of the latter.

[0075] Therefore, the invention relates to an artifact made of the mechanoluminochromic composite material according to the invention. In other words, this means that the mechanoluminochromic composite material of the invention constitutes the components that define the artifact, said artifact being constituted by said components and made integral with them or said artifact comprising said components, which are assembled to each other.

[0076] Therefore, according to this alternative embodiment, themechanoluminochromic composite material is not used to coat the external surface of the artifact, further to making the latter, but it is used for the production thereof.

[0077] As said, the composite material and the mechanoluminochromic coating layer object of the present invention show a change in the emission properties in response to the mechanical forces without compromising the aesthetic qualities of the product. The variation in emission can be easily verified with the naked eye using UV lamps. Furthermore, the change in emission is irreversible, which prevents the possibility of counterfeiting the damage. This technology also allows for a quantitative estimate of the forces that have acted on the material by measuring the variations in the emission spectrum by means of a portable instrument. The composite material and the coating layer of the invention are mechanoluminochromic since they show a change in the emissive properties (luminochromic) further to applying mechanical stresses such as compression, tension, impacts, but also scratching and rubbing, without showing any change in their colour under visible light.

[0078] The change in the emissive properties does not occur when it exceeds a certain stress threshold, but it is gradual depending on the stress applied and, consequently, it may be used to obtain information on the entity of the stress applied. To this end, it is necessary to quantify the change in the emissive properties as emission intensity ratio at two different wavelengths, relating respectively to the emission of the coordination compound or coordination polymer before and after the application of the mechanical stimulus.

[0079] The measurement of luminochromism may be also made through portable, low-cost equipment (only a UV light source and a digital camera, such as a smartphone, are required), enabling rapid, cost-effective, onsite analysis of the integrity of the above described materials. Moreover, acquisition via digital camera and subsequent processing allow the entire strain field to be measured and visualised, and therefore the stressesdeveloped on the surface of the component, unlike what happens with a point measurement of the sensors commonly used in the SHM. This type of analysis may be carried out by untrained personnel (private, domestic use, for instance) or may also be implemented in automated control lines. For instance, a possible use of the composite material or of the covering layer according to the invention is on production or control and maintenance lines of artifacts, specifically items, devices or structural elements ranging in size from microscopic (for instance microchips, mini-prostheses, joints) to large-scale (for instance means of transport, aeroplanes).

[0080] Therefore, the present invention relates to the use of the mechanoluminochromic composite material or of the mechanoluminochromic coating layer according to the present invention for at least partial surface covering of an artifact, such as an item, a device or a structural element.

[0081] Preferably, according to the use provided for in the previous paragraph, the surface covering of the item, device or structural element is total. The aforementioned technical problem is also solved by the use of the composite material or by the coating layer according to the present invention in detecting and / or monitoring the structural integrity of an artifact, such as an item, a device or a structural element, superficially and at least partially covered by said composite material or by said coating layer.

[0082] Preferably, according to the use provided for in the previous paragraph, said composite material or said coating layer superficially covers at least one portion of said artifact, wherein said at least one portion is made of polymeric matrix fibrous composite materials (‘Fiber Reinforced Plastic’ or FRPs).

[0083] This means that the invention also relates to an artifact, such as an item, a device or a structural element superficially and at least partially coveredby the aforementioned mechanoluminochromic composite material or by the mechanoluminochromic coating layer.

[0084] Preferably, the surface covering of the item, device or structural element according to the invention by said mechanoluminochromic composite material or by said coating layer is total.

[0085] More preferably, said item, said device or said structural element is used and / or adapted to the use for structural applications in the automotive, aeronautical and aerospace fields.

[0086] The aforementioned technical problem is also solved by a process for detecting and / or monitoring the structural integrity of an artifact, such as an item, a device or a structural element, wherein said process comprises the following steps:

[0087] a) providing an artifact, such as an item, a device or a structural element, superficially and at least partially covered by the composite material or by the coating layer according to the present invention;

[0088] b) exposing the surface of said artifact at least partially covered by said composite material or by said coating layer to at least one ultraviolet ray source, irradiating said surface for a pre-fixed time;

[0089] c) detecting the emissive properties of said composite material or of said coating layer while the surface is being irradiated.

[0090] Preferably, process detecting step c) according to the present invention is carried out through at least one digital camera, more preferably through a smartphone.

[0091] Preferably, irradiating step b) and / or detecting step c) may be carried out automatically, in particular they may be carried out in automated control lines.

[0092] Preferably, irradiating step b) and detecting step c) may be repeated a plurality of times in succession, more preferably in a rhythmic manner.More preferably, process detecting step c) according to the present invention comprises the following passages:

[0093] cl) detecting the emissive properties of said composite material or of said coating layer while said surface is being irradiated at a time t0;

[0094] c2) detecting the emissive properties of said composite material or of said coating layer while said surface is being irradiated at a time t1, wherein said time t1 is subsequent to said time t0.

[0095] Equally in absolute preference, the process according to the present invention comprises the following additional step:

[0096] d) once detecting step c) has been completed, comparing the emissive properties of said composite material or of said coating layer thus detected and comparing them with the emissive properties of said composite material or of said coating layer previously detected, thus obtaining respective comparison values.

[0097] In absolute preference, during aforementioned comparison step d), the emissive properties of said composite material or of said coating layer detected during passage c2) may be compared with the emissive properties of said composite material or of said coating layer detected during preceding passage cl), thus obtaining a comparison value.

[0098] In equal preference, the process according to the present invention comprises the following additional step:

[0099] e) providing an operator or a control unit or a computer with the emissive properties of said composite material or of said coating layer, thus detected during step c), and / or with said comparison values.

[0100] In absolute preference, comparison step d) and / or step e) may be carried out automatically, in particular they may be carried out in automated control lines.

[0101] Advantageously, by virtue of the use of the composite material or of thecoating layer according to the present invention, the aforementioned process allows detecting and / or monitoring the mechanical stresses acting or having acted on said item, device or structural element superficially and at least partially covered by said composite material or by said coating layer, so as to effectively identify its structural integrity. Once a structural change or damage has been suffered, the emissive properties of the coordination compound or coordination polymer are altered, with a consequent broadening of the emission band and shift of the emission maximum to longer wavelengths.

[0102] While mechanical forces acting on some specific types of items, devices and structural elements may cause catastrophic failures, the process according to the invention thus enables a simple and reliable identification of the mechanical stresses acting or having acted on the material itself, thus being useful from a detection, monitoring and prevention perspective.

[0103] Preferably, the process for detecting and / or monitoring the structural integrity of an artifact, prior to irradiating step b), comprises the following additional step of exposing the artifact to an external mechanical stress. More preferably, the exposure of the artefact to an external mechanical stress occurs actively and in a controlled manner.

[0104] Advantageously, according to the latter embodiment, the process for detecting and / or monitoring the structural integrity of an artifact occurs in the context of performing a testing step or ‘testing’, which associates certain mechanical stresses (impacts, compressions, strains caused by tensile, compressive, shear and bending stresses, scratching) with the change in specific emissive properties of the coordination compound or coordination polymer.

[0105] Particularly, once the correlation between the intensity of the mechanical stress and the emissive one has been determined, it is possible to determine the stresses which the component is subjected to underoperating conditions based on the changes in the detected emissive properties.

[0106] This latter embodiment of the method according to the invention is particularly advantageous in the context of design and prototyping of a component, since it allows detecting the stresses which it is actually subjected to under operating conditions without the need of providing it with sensors and bulky equipment.

[0107] The composite material of the invention, the aforementioned coating layer and their uses find application in the field of aeronautics, automotive industry, sports equipment, motorsport and in the optimisation and control of industrial technological processes, as well as in construction. Particularly, the composite material of the invention, the aforementioned coating layer and their uses find particular application in monitoring the structural properties (SHM, ‘Structural Health Monitoring’) of polymeric materials and composite materials, in particular of polymeric matrix fibrous composite materials (‘Fiber Reinforced Plastic’, FRPs), used for structural applications in the sports, energy, automotive, aeronautical and aerospace fields.

[0108] Indeed, the non-homogeneous structure of said materials, characterised by overlapping layers of carbon and glass fibres or kevlar bound together by a cross-linked polymeric matrix, makes them prone to the formation of micro-defects that may compromise their functionality. Particularly dangerous are the damages caused by low / medium speed impacts in the aeronautical field, which typically leave the surface intact but generate micro -fractures and delamination of the innermost layers of the material (‘Barely Visible Impact Damage’, BVID), which, as a result of fatigue loads, if not detected in time, lead to catastrophic failure of the structure. In this context the invention suggests a technology and a process that are reliable, simple and cost-effective, capable of identifying the early stages of mechanical weakening in these materials.Another context in which the invention may find application is that of prototyping and development of sports items and equipment, such as midsoles in athletics and running shoes, boots, skis, boards, bicycles, motorcycles, cars, helmets and other protective equipment used in contact sports and motorsports.

[0109] In general, the invention finds application in the prototyping of any component that needs to be designed and made of any material and where the stresses which it is subjected to during normal use are unknown.

[0110] In addition to the sports equipment field, said application is particularly useful in components for household appliances, in the automotive and aerospace fields.

[0111] During prototyping, a crucial aspect is the study of any structural criticalities and mechanical strength of the equipment in order to increase its safety and performance. In this context, the proposed invention may be used as a minimally invasive sensor distributed on-site, allowing continuously testing the equipment and detecting the stresses which the component is actually subjected to during the prototyping step. Still in the sports, household appliances, automotive and aerospace fields, the invention may also find application in sports equipment subject to high mechanical loads during use, in order to monitor the structural integrity of components and equipment, prevent fatal damage to equipment and ensure maximum performance, with particular regard to maximum safety for athletes.

[0112] In this context, the proposed invention would allow quickly obtaining extensive information relating to the structural integrity and the stresses which the prototype is subjected to during use, possibly, combining it with structural calculation models in order to make a digital twin of the structure itself.

[0113] In the sports context, examples of this application include helmets andprotective equipment in contact sports and motorsports, components in motorsport vehicles and equipment, sailing, cycling and winter sports, athletics equipment such as poles, running blades and prosthetic components.

[0114] Another context in which the invention can be applied concerns the optimisation and control of industrial technological processes.

[0115] It is possible to produce films capable of mapping pressure on a surface, showing how pressure is distributed over the area of interest. These films could then be used to optimise and control various industrial production and manufacturing processes based on pressure processing, such as calendaring, moulding, welding, and laminating.

[0116] The features and advantages of the present invention will become clearer from the following detailed description, comprising some experimental examples.

[0117] Moreover, further features and advantages of the present invention will become clearer from the detailed description herein after reported of a preferred embodiment thereof, given by way of indicative and non-limiting example with reference to the appended claims.

[0118] Brief description of the drawings

[0119] Figure 1 represents a diagram showing the solid-state emission spectra of a PVAc film loaded with 5% w / w [(CuI)3-BrPy]n(solid line) and virgin powder of [(CuI)3-BrPy]n(dotted line).

[0120] Figure 2 represents, in Figure 1a, a diagram showing the Igreen / Iblueratio based on the strain of the 5% w / w film during a tensile strain test; Figure 2b is a picture taken under ultraviolet light (λexcitation= 365 nm) and Figure 2c is a picture of the same element shown in Figure 2b, but under white light, specifically of a poly(vinyl acetate) film containing 5% w / w [(Cul)3-Br-py]nand tensile-strained until breakage.Figure 3 represents, in Figure 3a, a diagram showing the solid-state emission spectra of the PVAc film containing 5% w / w virgin [(CuI)3-BrPy]n(black line), when subjected to a 1500 N impact (dashed line) and when subjected to a 2800 N impact (dotted line); Figure 3b shows a diagram where the Igreen / Iblueratio depends on the impact force applied to a 5% w / w film; and Figure 3c reports a picture under ultraviolet light (λexcitation= 365 nm) of the 5% w / w virgin film and impacted at 1500 N and at 2800 N.

[0121] Figure 4 represents a picture under ultraviolet light (λexcitation= 365 nm) of the PVAc film containing 5% w / w [(CuI)3-BrPy]nscratched by means of a steel metal tip.

[0122] Figure 5 represents, in Figure 5a, a diagram showing solid-state emission spectra of the epoxy coating containing 5% w / w virgin [(CuI)3-BrPy]n(black line), when subjected to a 1500N impact (dashed line), when subjected to a 2500 N impact (alternating dotted and dashed line) and when subjected to a 3100 N impact (dotted line); Figure 5b shows a diagram where the Igreen / Iblueratio depends on the impact force applied to the 5% w / w coating; Figure 5c reports a picture under ultraviolet light (λexcitation = 365 nm) of the 5% virgin coating and when subjected to a 1500 N, 2500 N and 3100 N impact.

[0123] Detailed description

[0124] As will be seen herein after, the composite material according to the invention and the covering layer comprising the same have proven particularly effective due to the fact that the mechanoluminochromic additive is colourless under visible light, but generates luminescence when irradiated with radiation at an appropriate wavelength in the ultraviolet region, in particular in the wavelength range comprised between 300-405 nm.

[0125] When the additive is subjected to mechanical loads (such as compression, strain or shear stresses) may suffer an alteration of themicro- and macro -structure without changing its chemical composition and crystalline structure. As a result of this alteration, the emissive properties of the compound are altered, with a consequent broadening of the emission band and shift of the emission maximum to longer wavelengths. The mechanical stress does not lead to changes in colour of the compound when exposed to visible light.

[0126] In order to prepare the coating, the mechanoluminochromic additive is included inside a polymeric binder. The polymer may be both thermosetting and thermoplastic. The inclusion of the mechanoluminochromic additive may occur by mixing it with the binder formulation.

[0127] If the polymer is thermosetting, the additive is mixed with a formulation of a not yet cross-linked prepolymer. The prepolymer may be used in the solution or dispersion form, both in organic solvent and water.

[0128] The prepolymer formulation thus obtained will be able to cross-link after its application to a substrate, in the presence of the pre-mixed additive, thus obtaining a thermosetting polymeric composition containing a mechanoluminochromic additive.

[0129] If the polymer is thermoplastic, the additive is mixed with a polymer solution or with a dispersion, both in organic solvent and water, thus obtaining a thermoplastic polymeric composition containing a mechanoluminochromic additive.

[0130] The polymeric composition thus obtained, whether thermosetting or thermoplastic, may be applied to the surface of a substrate, i.e. the surface of the item, the device or the structural element of interest, by the traditional coating application techniques, such as dip coating, flow coating, application with rollers and spray techniques.

[0131] Another possible method for incorporating the mechanoluminochromic additive into a coating consists in applying the non-additive coating to the surface of the substrate and, before the coating hardens, applying themechanoluminochromic additive as a powder onto the surface of the coating by means of spray techniques (powder coating).

[0132] Particularly, the mechanoluminochromic additive used in the following examples appeared as a microcrystalline powder, more particularly consisting of irregularly shaped or needle-like particles, with an average particle size comprised between 500 nm - 20 microns.

[0133] Specifically, as will be seen in the following examples, a composite material comprising [(CuI)3-BrPy]nwas provided, with n greater than 1, as mechanoluminochromic additive at a concentration of 5 and 10% by weight and various thermosetting polymers, alternative to each other, such as epoxy, acrylic, polyurethane and aspartic resins and to thermoplastic polymers such as PMMA, PBMA, PC, and PVAc.

[0134] PVAc was used to produce self-standing films which were tested in tensile, compression and impact modes. For all mechanical stress conditions, it was possible to quantitatively determine the change in the emissive properties and correlate it with mechanical stress.

[0135] The other polymers were used to produce coating on paper, aluminum and FRPs. In these cases, the coatings were subjected to impacts at different mechanical speeds and the change in the emissive properties was quantitatively determined and correlated to the mechanical stress.

[0136] Example 1: preparation of a thin film constituted by the composite material according to the invention (vinyl-based)

[0137] For the preparation of thin poly(vinyl acetate) films containing the mechanoluminochromic additive [(CuI)3-BrPy]n, a polymeric solution in dichloromethane (4% w / v solution) was prepared and, separately, a dispersion of the mechanoluminochromic additive in 2% w / v dichloromethane. The dispersion was stirred at 600 rpm for 5 minutes and then added to the polymeric solution. The resulting formulation was transferred to a PTFE Petri dish. After 48 hours, the films thus formed were removed from the Petri dish and placed into a vacuum oven at 60°Cfor 6 hours to remove any solvent residues.

[0138] The film thus obtained was produced with a nominal chromophore concentration of 5% w / w.

[0139] Example 2: evaluation of the emissive properties of the mechanoluminochromic additive in the polymeric matrix and in the absence of stress

[0140] The film obtained through Example 1 and the virgin chromophores were irradiated with a UV light source to compare the emissive properties thereof.

[0141] Figure 1 reports the solid-state emission spectra (normalised), recorded by using an excitation wavelength of 365 nm.

[0142] By comparing the two spectra, it is possible to observe that the manufacturing process and the incorporation of the mechanoluminochromic additive into the polymeric matrix did not significantly alter the emission properties of the chromophore.

[0143] Example 3: evaluation of the emissive properties of the composite material according to the invention after stress (tensile)

[0144] After verifying that the chromophore properties were preserved in the film on occasion of Example 2, the mechanoluminochromic response of the films was evaluated in several modes, such as tensile, low-speed impact, and scratching.

[0145] Given that the chromophore used shows a variation in the emission colour from blue to green when subjected to mechanical stresses, the ratio between emission intensity in green (recorded at 505 nm) and the emission intensity in blue (recorded at 440 nm) was used to evaluate the mechanoluminochromic response (Igreen / Iblue). The data obtained in this way is in fact independent of parameters such as the light radiation intensity and the analysis geometry used.Starting from tensile, the mechanoluminochromic response was recorded in real time during stress-strain experiments. Figure 2a reports the mechanoluminochromic response depending on the strain. Looking at the graph shown in Figure 2a, it can be seen that the mechanoluminochromic response was immediate upon application of stress, with a response that can be linearly correlated to the applied strain up to strain values of approximately 100%, followed by a plateau at strains greater than 100%.

[0146] By comparing the images of the virgin and strained samples under ultraviolet light, a change in the colour emitted from blue to green was observed (Figure 2b). This difference, instead, was not observed in the pictures of the same samples collected under white light (Figure 2c), thus confirming that the mechanoluminochromic response of the chromophores under analysis only concerns emission and not absorption.

[0147] Example 4: evaluation of the emissive properties of the composite material according to the invention after stress (low-speed impact) Mechanoluminochromic characterisation was also performed following low- speed impacts by collecting emission spectra after applying impacts with forces equal to 1500 and 2800 N. As observed in the tensile experiments, even after the application of impact forces, the emission spectra (reported in Figures 3a) revealed a marked shift toward red of the emission, which was also visible in the images of the impacted composites under UV light (Figure 3c). This result was confirmed by the analysis of the mechanoluminochromic response Igreen / Iblue (Figure 3b), where the presence of a linear relationship was observed between the applied force and the mechanoluminochromic response Igreen / Iblue.

[0148] Example 5: evaluation of the emissive properties of the composite material according to the invention after stress (scratching)

[0149] Surface scratching analyses of the film were also carried out using a steelmetal tip (Figure 4). Analyses of images of scratched films under ultraviolet light showed that even the application of surface shear forces induces a mechanoluminochromic response, leading to a change in the emitted colour from blue to green.

[0150] Example 6: preparation of a thin film constituted by the composite material according to the invention (epoxy-based)

[0151] For the preparation of epoxy-based coatings, a dispersion of chromophore [(CuI)3-BrPy]n(500 mg) was prepared in bisphenol A diglycidyl ether-based epoxy pre-polymer dispersed in an aqueous formulation (7.315 g). The dispersion was left to stir at 600 rpm for 5 minutes, after which the cross-linking agent (2.185 g) was added. The resulting formulation was used to brush coat carbon fibre-based composites. The composites coated with the mechanoluminochromic coating were cross-linked at 60°C for 48 hours.

[0152] The coating thus obtained was produced with a nominal chromophore concentration of 5% w / w.

[0153] Example 7:

[0154] Similar to what was done in Example 4 for the PVAc-based film, the epoxy-based mechanoluminochromic coating as obtained in Example 6 was characterised by collecting the emission spectra following the application of low-speed impacts with forces equal to 1500 N, 2500 N, and 3100 N.

[0155] As observed in the PVAc-based composite, even in the epoxy coating the application of impact forces also leads to considerable red shifts in the emission spectra (shown in Figures 5a), which are also visible in the images of the impacted composites under UV light (Figure 5c). This result was confirmed by the analysis of the mechanoluminochromic response Igreen / Iblue (Figure 5b) where, even in this case, the presence of a linear relationship was observed between the applied force and the mechanoluminochromic response Igreen / Iblue.Instruments and methods:

[0156] Solid-state emission spectra

[0157] The solid-state emission spectra were collected by using a ThorLabs compact spectrometer (CCS200M, equipped with an optical fibre with an apical opening of 200 μm) coupled with an LED source at 365 nm (ThorLabs SOLIS-365c).

[0158] Mechanoluminochromic response under tensile

[0159] Stress-strain tests were conducted using an Instron 4465 dynamometer equipped with the ‘Bluehill Universal’ software package. The dog boneshaped samples were prepared in accordance with the ASTM D1708 standard (effective length = 14 mm, width = 5 mm, thickness = 0.2-0.3 mm) and tested under tensile with a 100 N load cell. The crosshead speed was set at 24 %·s-1. At least three samples of each type were tested for each material.

[0160] The solid-state emission spectra were collected using a ThorLabs compact spectrometer (CCS200M, equipped with an optical fibre having an apical opening of 200 μm) coupled with a 365 nm LED source(ThorLabs SOLIS-365c). The spectra were recorded at a frequency of 2 Hz.

[0161] Mechanoluminochromic response to impact

[0162] Low-speed impact tests were carried out using a drop weight machine. A laser device was installed on the machine to capture the impact speed both before and after the collision, and the laser signal was acquired at 100 kHz. To measure the contact force, a piezoelectric load cell (PCB 208C05) was mounted on the impact tip and the signal was acquired at 100 kHz.

[0163] The samples were impacted with an impactor having a hemispherical steel tip with a diameter of 12.7 mm and a weight of 1.3 kg. For eachforce, three samples of each type were tested. The target was positioned on top of a fibreglass-reinforced plate and a steel structure with a rectangular opening measuring 125 × 75 mm. A lever clamp with four rubber tips was used to hold the sample in place. After centring and securing the sample to the structure, the machine was set to the desired impact height. The solid-state emission spectra were collected using a ThorLabs compact spectrometer (CCS200M, equipped with an optical fibre having an apical opening of 200 μm) coupled with a 365 nm LED source (ThorLabs SOLIS-365c). At least 10 spectra of the impacted area were recorded and subsequently averaged.

[0164] Photographs under white light and UV light

[0165] The pictures were taken using a Canon EOS 1000D camera equipped with an 18-55 mm lens and a 400 nm bandpass filter. The images under ultraviolet light were collected by illuminating the samples with a 365 nm LED source (ThorLabs SOLIS-365c).

Claims

CLAIMS1. Composite material comprising a matrix of a polymeric nature and a mechanoluminochromic additive in the form of a coordination compound or a coordination polymer, said mechanoluminochromic additive being dispersed in the matrix, wherein said coordination compound or coordination polymer has the following general formula (I)[(MX)a(L)b]n (I)where M is selected from Cu(I), Ag(I), or Au (I), X is selected from I, Br, Cl, CN, SCN, or any combination thereof, L is selected from substituted pyridine, unsubstituted pyridine, substituted pyrimidine, unsubstituted pyrimidine, substituted pyrazine, unsubstituted pyrazine, or a phosphine group, a is 1, 2, 3, or 4, b is 1, 2, 3, or 4 and n is a number greater than or equal to 1;or said coordination compound or coordination polymer has the following general formula (II)[(M’X’)a’(L’bYc’)]n’ (II)where M’ is selected from Cu(I), Ag(I), or Au (I), X’ is selected from I, Br, Cl, CN, SCN, or any combination thereof, L’ is selected from substituted pyridine, unsubstituted pyridine, substituted pyrimidine, unsubstituted pyrimidine, substituted pyrazine or unsubstituted pyrazine, Y is a phosphine group, a’ is 1, 2, 3, or 4, b’ is 1, 2, 3, or 4, c’ is 1, 2, 3, or 4 and n’ is a number greater than or equal to 1.

2. Composite material according to claim 1, wherein said coordination compound or coordination polymer is selected from [(CuI)2Pyz]n, [(Cul)3-BrPy]n’ where n and n’ are a number greater than or equal to 1, respectively, or [(CuI)4-PhPy]4.

3. Composite material according to claim 1 or 2, wherein said matrix of a polymeric nature is a polymer selected from a thermosetting polymer ora thermoplastic polymer.

4. Composite material according to claim 3, wherein said thermosetting polymer is selected from epoxy resins, thermosetting acrylic resins, aspartic resins, polyurethane resins, alkyd resins, amino resins, phenolic resins, silicone resins, resins based on unsaturated polyesters, vinyl ester resins, synthetic rubber, or natural rubber.

5. Composite material according to claim 3, wherein said thermoplastic polymer is selected from homopolymers and copolymers of vinyl polymers, polyolefins, thermoplastic acrylic polymers, polyvinyl esters, fluorinated polymers, polyamides, polyurethanes, polyesters, polyethers, polycarbonates, cellulose nitrate and other cellulose derivatives, polysaccharides, thermoplastic elastomers, polyketones, polyether ketones, polyimides, polysulfones, polyethersulfones, or polyarylsulfones.

6. Composite material according to any one of the preceding claims, wherein said mechanoluminochromic additive is present in said composite material in a quantity equal to at least 0.1%, preferably comprised between 0.1% and 98% by weight on the total weight of the composite material, more preferably between 5% and 10% by weight on its total weight.

7. Composite material according to any one of the preceding claims, wherein said mechanoluminochromic additive is in the powder form.

8. Composite material according to any one of the preceding claims, wherein said mechanoluminochromic additive has an average particle size comprised between 10 nm and 1 mm, preferably between 100 nm and 500 microns.

9. Composite material according to claim 7 or 8, wherein the particles of said mechanoluminochromic additive are covered at least partially, optionally completely, by a surface protective layer comprising a protective material of a polymeric nature.

10. Composite material according to claim 9, wherein said protective material of a polymeric nature is selected from a thermosetting polymer or a thermoplastic polymer, preferably said protective material of a polymeric nature being a thermosetting polymer selected from epoxy resins, thermosetting acrylic resins, aspartic resins, polyurethane resins, alkyd resins, amine resins, phenolic resins, silicone resins, resins based on unsaturated polyesters or vinyl ester resins, synthetic rubber or natural rubber, or said protective material of a polymeric nature being a thermoplastic polymer selected from homopolymers and copolymers of vinyl polymers, polyolefins, thermoplastic acrylic polymers, polyvinyl esters, fluorinated polymers, polyamides, polyurethanes, polyesters, polyethers, polycarbonates, cellulose nitrate or other cellulose derivatives, polysaccharides, thermoplastic elastomers, polyketones, polyether ketones, polymides, polysulfones, polyethersulfones, or polyarylsulfones.

11. Composite material according to any one of the preceding claims, in the film form, preferably said film having a thickness comprised between 10 nm and 1 cm, more preferably between 50 microns and 500 microns.

12. Mechanoluminochromic coating comprising the composite material according to any one of claims 1 to 11.

13. Use of the composite material according to any one of claims 1 to 11 or of the mechanoluminochromic coating according to claim 12 for at least partial surface covering of an artifact.

14. Use of the composite material according to any one of claims 1 to 11 or of the mechanoluminochromic coating according to claim 12 in detecting and / or monitoring the structural integrity of an artifact that is superficially and at least partially covered by said composite material or by said coating.

15. Use according to claim 14, wherein said composite material or said coating superficially covers at least one portion of said artifact, said atleast one portion being made of polymer matrix fibrous composite materials.

16. Artifact made of the composite material according to any one of claims 1 to 11.

17. Artifact covered superficially and at least partially by the composite material according to any one of claims 1 to 11 or by the mechanoluminochromic coating according to claim 12.

18. Artifact according to claim 17, wherein said composite material or said coating superficially covers at least one portion of said artifact, said at least one portion being made of polymer matrix fibrous composite materials.

19. Process for detecting and / or monitoring the structural integrity of an artifact, comprising the following steps:a) providing an artifact, such as an item, a device, or a structural element, superficially and at least partially covered by the composite material according to any one of claims 1 to 11 or by the coating according to claim 12;b) exposing the surface of said item, device, or structural element at least partially covered by said composite material or by said coating to at least one ultraviolet ray source, irradiating said surface for a pre-fixed time; c) detecting the emissive properties of said composite material or of said coating while the surface is being irradiated.

20. Process according to claim 19, wherein said detecting step c) is carried out through at least one digital camera, preferably through a smartphone.

21. Process according to claim 19 or 20, wherein said irradiating step b) and / or said detecting step c) are carried out automatically, preferably being carried out in automated control lines.

22. Process according to any one of claims 19-21, comprising the following additional step:d) once detecting step c) has been completed, comparing the emissive properties of said composite material or of said coating thus detected and comparing them with the emissive properties of said composite material or of said coating previously detected, thus obtaining respective comparison values.

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