The present invention relates to artefacts provided with anti-icing means, and intended in particular, but not exclusively, for aeronautical applications. Several factors, during flight, can contribute to ice formation on an aircraft, among which are meteorological and/or aerodynamic factors. Furthermore, in extreme cold and humidity conditions, such as in cold winters of Northern Europe, etc., it is mandatory for the safe take-off of an aircraft to remove and/or prevent a build-up of snow and ice on the wings, tail, etc. The ice on these components may cause changes in their shape, negatively affecting airflow across the surface and hindering the ability to create lift or maintain the aircraft control.

An object of the present invention consists in the development of new ice protection systems of artefacts, such as flat and curved Carbon Fiber Reinforced Composites (CFRCs) and Glass Fiber Reinforced Composites (GFRCs)panels.

According to the invention, this object is attained by an artefact according to claim 1. Preferred features of the artefact of the invention are disclosed by the dependent claims which form an integral part of the description.

E.g., the integration of a “green” flexible film heater in CFRCs and GFRCs panels confers them the function of being able to activate anti-icing or de-icing through electrical power.

The wording “heater film”, “film heater” and “heating element” are interchangeably used throughout the present disclosure.

Further advantages and features of the present invention will be apparent from the following detailed description, provided in a non-limiting way with reference to the following examples and drawings, wherein:

Figure 1 illustrates the chemical formulae of exemplary thermoplastic polymers usable in a heater film of an artefact of the invention;

Figure 2 illustrates on the top optical images of film heaters (the first two films differ from each other in the number of graphene layers in the graphitic layers, the thermoplastic matrix is the same (PVA)), and on the bottom optical images of film heaters showing the film flexibility;

Figure 3 is a schematic representation of a functionalization process in the film heater preparation;

Figure 4 illustrates - a) the electrical conductivity of the composite PVA (50% filler) for different molecular weight: 30-70 kDa (Low Molecular Weight (LMW)), 89-98 kDa (Medium Molecular Weight (MMW)), 146-186 kDa (High Molecular Weight (HWM)); b) the Electrical Conductivity of LMW composite of PVA at different amounts of conductive filler;

Figure 5 illustrates the temperature profiles, as a function of time, for 3, 4 different values of applied constant voltage (Figs. 5a, 5b) and temperature variation (AT) as a function of the applied power (Fig. 5c), for the heating films obtained with two different molecular weight loaded with 60% of filler;

Figure 6 illustrates the temperature profiles, as a function of time, for 4 different values of applied constant voltage (Figs, a, b and c) and Temperature variation (AT), for the heating films obtained with two different molecular weight loaded with 50% of filler;

Figure 7 shows optical images of panel 1 and panel 2;

Figure 8 refers to the temperature detected at the center and at the edges of the sample, at the copper contacts and the evolution of the electrical resistance as a function of the heating time for an applied voltage V = 2.0 volt;

Figure 9 refers to the temperature detected in the center and at the edges of the sample, at the copper contacts and the evolution of the electrical resistance as a function of the heating time for an applied voltage of 3.0 volt;

Figure 10 refers to the temperature detected in the center and at the edges at the copper contacts and the evolution of the electrical resistance as a function of the heating time for an applied voltage of 4.0 volt;

Figure 11 refers to a summary table of the electrical and thermal parameters related to the tests carried out and the AT vs. the applied power;

Figure 12 refers to the temperature detected at the center and at the edges of the sample, at the copper contacts and the evolution of the electrical resistance as a function of the heating time for an applied voltage V = 4.0 volt;

Figure 13 refers to the temperature detected in the center and at the edges of the sample, at the copper contacts and the evolution of the electrical resistance as a function of the heating time for an applied voltage of 5.0 volt;

Figure 14 refers to the temperature detected in the center and at the edges at the copper contacts and the evolution of the electrical resistance as a function of the heating time for an applied voltage of 6.0 volt;

Figure 15 refers to the temperature curves vs applied voltage related to the tests carried out on Panel 2 (on the left side); table of the electrical parameters and thermal parameters (on the right side). The table shows the maximum temperatures reached at 90 s (first region of the linear section) and at 5 minutes (second region - at the origin of the "almost" plateau);

Figure 16 is a summary table of electrical and thermal parameters related to the tests carried out and the AT vs. the applied power;

Figure 17 illustrates a possible configuration of a leading edge divided in different sectors, each having a different thickness of the film heater;

Figure 18 relates to photos showing the consistent damages where repair actions involve the replacement of CFs or GFs fabric (impregnated with resin) on aircraft under stopping conditions;

Figure 19 illustrates the execution of the repairs mentioned with regard to figure 18;

Figure 20 illustrates the bonding procedure of two edges of a heating film in PVA LMW loaded with 60% of filler;

Figure 21 refers to the heating performance of a repaired heating film;

Figure 22 illustrates the film heater repaired and drilled again after the first repair (on the top) and the results of the heating test (on the bottom); and

Figure 23 relates to images showing the adaptability of the film heater to the curvature radius of the leading edge.

Flexible film heater

The flexible film heater is a very thin and light film composed in some embodiments of: a) a polymeric matrix such as a thermoplastic polymeric matrix containing polar groups (compatible with the thermosetting resin used for aeronautical panels), or a thermosetting polymeric matrix optionally functionalized with elastomeric nanodomains. Non-limiting examples of a thermoplastic polymeric matrix containing polar groups are polyvinyl alcohol, vinyl alcohol-based thermoplastic resins, vinyl alcohol copolymers, copolymer of vinyl alcohol and butanediol, alcohol/vinyl- acetate copolymer, polymeric derivatives of cellulose (such as for example carboxymethyl cellulose) and any combinations thereof.

Non-limiting examples of a thermosetting polymeric matrix are epoxy, polyurethane, polyimide, polyester, phenolic, bismaleimides, vinyl esters, and polyamides, optionally functionalized with elastomeric nanodomains.

Non-limiting examples of electrically conductive carbon-based nanoparticles are electrically conductive carbon-based nanoparticles of the heater film are selected from the group consisting of carbon nanotubes, carbon nanofibers, exfoliated graphite nanoparticles, expanded graphite nanoparticles, graphene-based nanoparticles and mixtures thereof.

Some of the preferred embodiments of the invention are described herein below.

Thermoplastic polymeric matrices of the film heater

A preferred embodiment of the polymer matrix of the flexible film heater is a thermoplastic polymer matrix. Several thermoplastic polymers can be used having the characteristic above mentioned. Some examples of chemical structures are shown in Figure 1.

For example, the thermoplastic matrix can be composed of polyvinyl alcohol, here named with the acronym PVA. Different Molecular weight of PVA can be used: (Es. Mw = 30 kDa - 70 kDa / Mw = 89 KDa- 98 KDa/ 146 KDa- 186 KDa, etc.) Polymer matrices, such as the highly biodegradable, hydrophilic, amorphous Butendiol/Vinyl Alcohol/Vinyl-Acetate copolymer (Nichigo G-Polymer - grade OKS 8049), commercially available (provided from Nippon Gohsei Synthetic Chemical Industry, Europe GmbH, Dusseldorf, Germany) and here named with the acronym HAVOH can be also used.

Some examples are shown in Figure 1.

The chemical structures of the two matrix polymers above-mentioned are illustrated in Figure 1.

Useful polymeric matrices can also be characterized by groups such as carbonyl groups or ester groups and/or polar groups such as -NH (see Figure 1c).

Graphene layers and/or thin electrically conductive graphitic layers

As mentioned, the electrically conductive nanofiller is made of carbon-based nanoparticles, preferably graphene-based nanoparticles or very thin graphitic layers of nanometric thickness with an average value preferably not higher than 25 nm, and a large surface area, for which the optimized value of the aspect ratio is preferably higher than 1200. Optical images of some of these films are shown in Figure 2. In particular, it is possible to observe the film flexibility in the images on the bottom of figure 2.

To further increase the film flexibility and resistance to bending (e.g., for applications on curved parts with a reduced curved radius such as leading-edge), it is possible to functionalize the film by using a crosslinking agent and/or a lower amount of graphitic layers in combination with a lower molecular weight of the polymer matrix. The crosslinking agent is among Urea derivatives and is more preferably characterized by at least two carbonyl groups in order to crosslink the polymer chains (es. 1,3 DBA, uric acid, barbituric acid, murexide, etc).

Examples 1-2 refer to the preparation of highly flexible film heaters.

Examples 1-2

Preparation of the Film Heaters (1-2)

For the preparation of the film heater (1) the crosslinking agent "barbituric acid" has been used. To produce the functionalized film heater, the polymer is dissolved in hot water (60- 100 °C) for 1-2 hours under magnetic stirring till the solution is completely clear. The resulting polymeric solution is around 0.4% in weight of polymer to solvent. Thus, a percentage of the graphitic layers (from 40% to 60%) is added to the polymeric solution, which is ultra-sonicated for one hour at room temperature.

After the sonication, the crosslinking agent is added to the solution (generally 20% in weight with respect to polymer amount) and stirred at room temperature for 30 minutes. In this way, after the complete solubilization of this agent, the film is directly obtained by the evaporation of water via solvent casting. The evaporation time can be tuned by increasing, on the exposed surface the temperature value, or removing the humidity of the room. After evaporation, the film is annealed at 1-3 bar around 140 °C for one hour. This step is done to remove voids due to evaporation and to allow the crosslinking reactions of the urea- based agent with the matrix, that is thermally activated during the annealing process.

The production process of the functionalized films is illustrated by Figure 3.

The same procedure used in the previous examples can be used using as polymeric matrix HOVOH with 50 or 60% by weight of graphitic layers. The films obtained by the casting process are flexible and highly homogeneous as the unfunctionalized ones. The electrical conductivity of the functionalized systems, compared to the unfunctionalized one, is similar, proving that the crosslinking does not limit the applicability of the film heater. The values of electrical conductivity are shown in Table 1 with an amount of graphitic layers of 50% by weight. Table 1 - values of electrical conductivity (S/m) of film heaters containing an amount of graphitic layers of 50% by weight. The amount by weight of graphitic layers can be between 20% and 80% and more preferably between 30% and 60%. In order to impart a greater flexibility to the film heater, it is preferred to use a little lower graphitic layer weight (between 30% and 50% by weight) in combination with a lower molecular weight of thermoplastic matrix. Furthermore, the content of graphitic layers can be adapted to the electrical source available on the aircraft.

Concerning this last aspect, it is worth noting that, depending on the specific application, it is relevant to have the possibility to tune the electrical properties of the material depending on the voltage generator and the electrical energy supply available. In this scenario, surely the range of applicability can be improved by appropriately varying the filler content. As expected, by increasing the filler content, the electrical conductivity of the composite film heater generally increases (see Figure 4). Furthermore, the value of electrical conductivity is also affected by the molecular weight of the thermoplastic matrix, as highlight in Figure 4. In Figure 4a, it is possible to observe that for the same percentage by weight of the carbonaceous filler (50%), the electrical conductivity value decreases as the molecular weight of the thermoplastic matrix increases. This is reasonably due to the fact that the conductive network is due to the filler being intercalated between the macromolecule chains. Of course, for the same molecular weight of the matrix, the electrical conductivity of the film heater increases with increasing the filler percentage (as shown in Figure 4b).

However, adding conductive filler in the composite affects also the mechanical properties of the materials, that are crucial for having a flexible system.

Hence, by simply using a different molecular weight of the polymeric matrix, it is possible to produce film heaters with very different electrical conductivity without changing the production process, making the systems applicable for a wide range of voltage generators available.

As reported in Figure 4, the electrical conductivity of the composite is sensitively affected by the molecular weight of the polymeric matrix. For this reason, lower molecular weight polymers guarantee higher electrical conductivities (that are favorable if limited voltages are available), whereas it is possible to choose higher molecular weights if higher voltages are available.

The relevance of the polymeric molecular weight has been investigated in reference to the heating performance of the film heater. This aspect is crucial for the application proposed by the present invention.

Purely by way of example, it is shown hereafter the evaluation of the Joule heating efficiency taking into account the heating kinetic, the voltages needed and the applied power for the two different molecular weights already analyzed for the evaluation of the electrical conductivity.

Figure 5 shows the Temperature Profiles, as a function of time, for 3, 4 different values of the constant applied voltage (Figs, a and b) and the Variation in temperature (AT) as a function of the applied power (Fig. c), for the heating films obtained with the two different molecular weights.

As illustrated by Figure 5, the heating measurements were conducted by applying increasing constant voltages. The voltage increase causes an increase in the temperature of plateau. If we consider, not the temperature reached, but the variation in temperature with respect to the initial one (AT), with the same power, the heat obtained for the different samples are similar. As expected, the dissipative effect is a function of the applied power.

For the same molecular weight an increase in the voltage determines an increase in the plateau temperature.

The two film heaters (based on PVA 30-70 kDa and PVA 89-98 kDa) were also tested, through electrical measurements in direct current (DC), with the aim of evaluating the voltage values and the powers necessary to perform a heating up to about 100 ° C. The heating measurements were conducted by applying constant and increasing voltages. The temperature and current profiles during heating are shown in Figure 6.

Considering also the samples surface, the power densities have been evaluated. The values are reported below in Table 2. Table 2 - Electrical parameters of the film heaters based on two different molecular weight loaded with 50% wt/wt of filler.

Electrical measurements in alternating current (A.C.), at a frequency of 400 Hz, for low and medium molecular weight of the PVA matrix, loaded with 50% of filler have been also performed for application on aircraft of current air fleets. An infrared thermometer pointed at the center of the sample was used for temperature measurements, while the current values, upon applying the voltage, were evaluated using an amperometric clamp. The results obtained are reported in Tables 3 - 4.

Table 3 - Electrical parameters and thermal measurements of the film heater based on the molecular weight between 30-70 kDa loaded with 50% wt/wt of filler.

Table 4 - Electrical parameters and thermal measurements of the film heater based on the molecular weight between 89-98 kDa loaded with 50% wt/wt of filler.

The results shown in Tables 3-4 allow observing that efficient heating is reached for both samples in a very short time (also less than a minute). The film heaters here described have been tested using a two-stage curing cycle (typically of aeronautical structures), for which the second stage can be also carried out up to 180 °C for 3 hours. The film heaters manifest high thermal stability and a melting temperature which can reach the value of 268 °C (depending on the molecular weight of the polymeric matrix and the amount of nanofiller)

Anti/de-icing of CFRCS and GFRCs Panels

The CFRCs and GFRCs panels having integrated the film heater are specifically designed for application in aeronautics (aircraft’s wings - leading edges-, engine cowlings, fuselage, etc). However, from their application, several industries can benefit, among which all transport sectors (automotive, rail, etc.) wind energy (wind turbines), offshore structures, and the civil engineering sector (assembled structures for bridges, footbridges, etc.).

In the specific case of application on aircraft, compared to current pneumatic or thermal metal resistance systems (current wire heating pad systems), the new systems and the adopted methodologies allow very high anti-icing/de-icing efficiencies combined with a remarkable reduction in energy consumption and consequently a reduction of CO2 release into the earth's atmosphere. The adopted technology also reduces air and soil pollution of airports. Concerning this last point, it is well known that when ice, snow, or frost are accumulated in the rest condition of aircraft, usually a deicing fluid, generally a mixture of glycol and water, is sprayed under pressure on the surface of the aircraft components for removing ice and snow before the take-off, with an increase in soil pollution and waste of solvent.

Of course, the ground application of fluids has no effect on the risks which arise from the accretion of frozen deposits on the aircraft at any time after take-off.

Hence, aircraft are designed to have anti/de-icing facilities activable in-flight.

The invention here proposed can be applied both before (aircraft ground de/anti Icing procedures) and after take-off (aircraft in flight).

In particular, the present invention proposes smart high-efficient anti-icing/de-icing composites, which can also be characterized by feature ice-tailored power output performance. It is well known that ice accretion on fast-moving complex surfaces like aircraft wings (with more severe conditions on the shorter radius parts) is generally uneven and varies with the variable flow field. The possibility to apply a new approach adaptive to uneven and variable ice accretion, compared to current pneumatic or thermal metal resistance systems (current wire heating pad systems, etc.), leads to further energy savings. The new anti/de-icing technology allows manufacturing CFRPs or GFRPs capable to combine concomitant and/or consequential benefits, with respect current Anti/de-icing solutions, as described hereafter for different panels configurations.

BENEFITS

1) Remarkable reduction in energy consumption during de-icing or anti-icing operations and slow-down of resource depletion.

The relevant reduction in energy consummation is due to different concomitant factors: a) High efficiency of heating function (and hence of the anti-icing or de-icing function (see examples 1-3) in Section 1 (also without adaptative/modulated solutions);

SECTION 1

In this section are described the tests performed on CFRCs and GFRCs Panels or tests performed on the film heater for the anti/de-icing function in case of permanent damages or in case of maintenance operations.

Different configurations of the panels have been considered during the manufacturing processes.

In particular, panels composed of carbon fiber cloths impregnated with aeronautical resin, fiberglass cloths impregnated with aeronautical resin, and mixed panels composed of both carbon fiber cloths and fiberglass cloths.

In the manufacturing process, where the use of carbon fiber cloths is foreseen, it is mandatory to isolate the electrodes from the carbon fabric.

The insulation can be performed with a suitable conventional insulating spray or through the use of insulated electrodes (on the opposite side to the part that comes into contact with the film heater).

The electrical insulation of the electrodes is useless for the configurations of panels where the film heater is placed between fiberglass fabric.

Just by way of example, hereafter are described some of the analyzed configurations.

Figure 7 shows optical images of Panel 1 and Panel 2.

Panel 1 (belonging to the type of mixed panels composed of both carbon fiber fabrics and glass fiber fabrics) was made by isolating the "heater film" through its interposition between two glass fiber fabrics so as to isolate the entire heating film from the carbon fiber fabrics.

Panel 2 (belonging to the type of panels composed only of fiberglass fabrics) was manufactured by positioning the film heater in the center of the fiberglass cloths. In this case, the insolation of the electrode adhered to the film heater is not necessary.

Panels 1 and 2 have been cured through the typical curing cycle applied by Leonardo to loadbearing components of primary structures.

The heating tests were carried out at room temperature, applying different voltage values for a time of 30 minutes. Through the application of thermocouples at different points on the surface of the panels, a survey was carried out on the temperature values reached in correspondence with the different applied voltages and the positioning of the thermocouples.

Simultaneously with this type of investigation, the time course of the current was detected, from which the resistance values were calculated as a function of time and therefore of the temperature.

The tests performed applying D.C. on Panels 1-2 ensure high efficiency in the joule heating with low power densities.

Furthermore, the heating of the analysed panels, monitored through a thermo-chamber, evidenced a satisfying homogeneity of the temperature over the entire sample surfaces.

Heating Performance of Panel 1. Tests performed in D.C.

Figure 8 relates to the temperature detected in the center and at the edges of the sample, in correspondence with the copper contacts; evolution of the electrical resistance as a function of the heating time (applied voltage V = 2.0 volt)

Figure 9 relates to the temperature detected in the center and at the edges of the sample, in correspondence with the copper contacts; evolution of the electrical resistance as a function of the heating time (applied voltage V = 3.0 volt)

Figure 10 relates to the Temperature detected in the center and at the edges in correspondence with the copper contacts; evolution of the electrical resistance as a function of the heating time (applied voltage V = 4.0 volt)

Figure 11 is a summary table of the electrical and thermal parameters related to the tests carried out; AT vs applied power. The value of AT has been determined considering the difference of temperature detected by the thermocouple at the center of the sample.

Heating Performance of Panel 2. Tests performed in D.C.

Figures 12-15 refer to: the temperature detected at the center and at the edges of the sample, at the copper contacts and the evolution of the electrical resistance as a function of the heating time for an applied voltage V = 4.0 volt (Fig. 12); the temperature detected in the center and at the edges of the sample, at the copper contacts and the evolution of the electrical resistance as a function of the heating time for an applied voltage of 5.0 volt (Fig. 13); the temperature detected in the center and at the edges at the copper contacts and the evolution of the electrical resistance as a function of the heating time for an applied voltage of 6.0 volt (Fig. 14); the temperature curves vs applied voltage related to the tests carried out on Panel 2 (on the left side); table of the electrical parameters and thermal parameters (on the right side). The table shows the maximum temperatures reached at 90 s (first region of the linear section) and at 5 minutes (second region - at the origin of the "almost" plateau) (Fig. 15);

Figure 16 is a summary table of the electrical and thermal parameters related to the tests carried out and the AT vs. the applied power.

Considering the results of Figs. 15-16, panel 2 exhibits a fairly fast response when considering the applied power densities. If we consider the temperature profiles in the linear section, it can be seen that about 70-80% of the maximum temperature value is reached for times less than 3 minutes. For example, for an applied voltage of 8 Volts (2676.1 W/m ² ), a temperature of 90° C is reached in 90 seconds with a heating rate of approximately 84 °C/min.

The tests performed applying D.C. on Panels 1-2 ensure high efficiency in the joule heating with low power densities.

Furthermore, the heating of the analyzed panels, monitored through a thermo-chamber, evidence a satisfying homogeneity of the temperature over the entire sample surfaces.

The high heating efficiency of the panels tested in D.C. was also found for test performed in A.C., as shown in Table 5.

As an example, hereafter, in Table 5 are shown the electrical and thermal parameters detected for Panel 2 applying an alternate current (The AC measurements have been led at 400 Hz)

Table 5 - Electrical parameters and thermal measurements detected for panel 2. Tests performed in A.C. b) Ability to obtain panels with high homogeneity in the temperature of the panels (see examples) without applying wire resistors. Furthermore, unlike wire resistors, the proposed solution preserves the resin from the criticality of the thermal degradation in the areas of the interface between resin and wire. c) Possibility of applying modulated solutions. For example, on the leading edges, it is possible to make adaptative the heating performance by choosing a modular solution with film heaters of different thicknesses suitably placed in the layers of curved panels (or directly applied as treated coatings). Figure 17 highlights that using the same power supply voltage (the same type of current), it is possible to reach different temperatures (for example on different parts of a leading edge) only by choosing the suitable thicknesses and a modular configuration. The film heater can be placed between different layers or directly as a coating.

The layers are among CF woven, GF woven, hybrid materials, metallic or metal alloy materials, etc. A generic example, where it is possible to have layers of different materials that integrate the film heater is shown in figure 17. Fig. 17 is merely exemplificative and does not exclude further possible suitable configuration designs.

Figure 17 highlights the possibility to apply an adaptative, modular solution. It is possible to electrically power the heated profile with the same type of current; hence the heating element are divided into different sectors with different thermal power densities. The thickness of the film should allow managing the different power densities. This allows reducing part of electrical components that increase the aircraft weight and push towards aircraft profiles conditioned by the need for weight balance. d) Weight savings associated with the absence of metal resistors or pneumatic systems (deicing boot systems), different consequential benefits are expected. benefits

As a consequence of the weight reduction, fuel consumption is reduced (preserving the rapid depletion of energetic resources) and consequently the quantity of CO2 released into the earth's atmosphere is reduced (reduction of greenhouse gases). e) There is no need to add power generators different from those already present in aircraft, the new systems can be alimented using direct or alternate current (DC or AC) adapting the anti/de-icing efficiency of the panels to available voltage/current; f) There is no need to add power generators different from those already present in aircraft, the new systems can be alimented using direct or alternate current (DC or AC), leaving unaltered the anti/de-icing efficiency and recovering the not dissipated electrical energy (by joule effect) with suitable electrical wiring.

2) Easiness of manufacturing processes and adaptability to current aeronautical manufacturing processes

According to the invention, the integration of the green flexible film heater in CFRCs and GFRCs panels is advantageous in order to confer them the function of being able to activate anti-deicing or de-icing through electrical power. This film can be placed between the plies of carbon fibers or glass fiber fabrics. For further heating efficiency of the panels, the film heater can be placed under the last CF or GF ply (the ply exposed to the elements). There is no need to use adhesive layers to bind it to the Carbon Fiber (CF) or Glass fiber (GF) fabrics impregnated with thermosetting resin. This last occurrence greatly facilitates the manufacturing process.

In fact, the heating element (film heater) will result perfectly integrated in the panel after the traditional curing cycles used in the aeronautical field. Concerning this aspect, no delamination problems have been detected during the performed tests; after its positioning between the pre-impregnated layers of the reinforcing fabric layers, the active layer (film heater), after the curing process (which can be carried out in autoclave or out of autoclave) will be perfectly adhered to the layers of reinforcing fabric with the solidified resin.

3) In the repair operations of aeronautical panels, it is possible to restore also the functional properties of the CFRCs and GFRCs panels.

The current repair processes can be implemented to add to the restoration of structural properties also the restoration of functional properties (anti/de-icing properties) (see Example 1 in Section 2 - Maintenance operations)

Section 2

Example 1.

Maintenance operations Considering the increasing presence of composite material in aircraft, it has been increasingly necessary to develop repair methods for the accidental damage that these components may receive in service (see Fig.18). Among the developed repair methods, of particular interest is the “hot bond repair”, which involves the removal of damaged material and replacement with layers of carbon fiber and resin to be polymerized directly on the component. This type of repair, which restores the complete structural capability and surface finish prior to damage to the repaired component, is the one most requested by airlines (see Fig. 19). It requires different steps: a) identification of the damaged area and the location of the damage; b) Preformation of the scarfing, exposing 10-12 mm per ply; c) preparation of the mask for cutting the canvases; d) Adhesive film cutting; e) Dehumidification of the area to be repaired; f) Preparation of the thermal profile with thermocouples; g) Vacuum lamination and compaction of plies; h) Sack preparation; i) heating with a thermal blanket and temperature control with thermocouples; 1) Bag removal.

The methods proposed so far take into account the restoration of the structural properties.

In the case of the present invention, the flexible film heater can be applied as a ply, in the current repair techniques, to also restore the functional property (anti/de-icing).

Figure 18 illustrates consistent damages requiring repair actions involving the replacement of CFs or GFs fabric (impregnated with resin) on aircraft under stopping conditions.

Figure 19 illustrates the execution of the repairs: removal of the damaged material layer by layer and replacement thereof. If the panel is the anti/de-icing panel of the present invention, the functional properties can be restored (together with the structural one) through the replacement of the film heater between the plies (as in the original configuration).

In this regard, the example presented hereafter evidences the efficiency of the film heater to self-repair under the effect of the passage of an electric current after the cutting and the overlapping of two corners. Description of the experiment

This test has been carried out using a molecular weight (Molecular Weight (MALLS)) of the thermoplastic matrix between 30,000 and 70,000 Da.

The film contacted with copper electrodes was initially cut into two distinct parts, subsequently, after the superposition of two edges (see Figure 20), the system was subjected to 10 minute of heating at 160 ° C followed by further heating for 10 minutes at 160 ° C at a pressure of about 2 bar.

The outcome of the repair is shown in Figure 20 in which the two overlapping flaps are fully adhered and compacted. The repaired sample has an electrical conductivity higher than 200 S/m. This conductivity made it possible to perform heating tests with low applied voltages.

Figure 21 shows the heating test carried out at a voltage of 6 V, the relative electrical and thermal parameters and the distribution of temperature by means of an image acquired by an infrared camera.

The thermocouple positioned in the center of the sample, in the sheltered area, recorded a rapid increase of the temperature up to 75 °C with an increase of about 53 °C compared to the ambient temperature in equilibrium conditions.

The film heater exhibits high efficiency, in fact, most of the temperature variation occurs in the first 30 seconds, after which the film has a homogeneous or almost homogeneous distribution of the temperature over the entire surface, including the repaired area. The applied power density, even for a repaired system, is more efficient than the requirements established by point 12 of table 1 of Annex A: EWPS 65- EW-0000-N352-200181 Issue 1 dated 24/08/2020. According to such requirements, in order to attain a power density in the range between 8 - 50 kW/m ² for a heating between 7 °C and 21 °C, a DT of 14 °C is required.

An important note in this regard is that if it is planned to apply the heater film as a coating, the repair can be carried out through heating activated directly by the application of electric current (joule effect) on the coating.

4) Damage resistance without loss of heating efficiency of the functional CFRCs and GFRCs panels

Unlike metal wire heaters, the film heaters here proposed manifest greater resistance to damage (also consistent damages) because the damage does not break the metal connections or wires. In fact, the film heater may have a nanometric conductive network on the entire surface.

(see Example N°1 in Section 3).

Section 3

Example N°1 of Section 3 refers to the assessment of the heating performance of the film heater after permanent damage.

Example 1

Evaluation of the Heating Performance after a permanent damage

The film heater based on the lowest molecular weight (Molecular Weight ( MALLS ) ) (30,000- 70,000 Da) PVA, loaded with 60% ABG1045 graphite, was considered for this test. In particular, the test has been performed on the same sample (already repaired- see Es. Y), hence in conditions that would seem very critical. The heating efficiency has been assessed after permanent damage which consisted of drilling the film heater with an 8 mm diameter hole (see Figure 22 on the top). The results of the heating tests are likewise shown in Figure 22.

After the permanent damage, the system has almost the same efficiency both in terms of heating and in terms of applied power density. The thermal images show that the area surrounding the hole is characterized by homogeneous heating, very similar to that detected over the entire surface of the sample.

5) Possibility to adapt the resistance and the film flexibility to the to the radius of curvature of panels, such as (curved panels, e.g. leading edges

Images showing the adaptability of the film heater to the curvature radius of the respective leading edge are illustrated by figure 23.