The invention relates to an in situ thermoset curing process through resistive heating. Furthermore, the invention relates to process for in situ repairing composites and a process for in situ soldering based on the thermoset curing process through resistive heating.

BACKGROUND ART

Curing in polymer chemistry and process engineering refers to the hardening of a polymer material by cross-linking of polymer chains, brought about by chemical additives, ultraviolet radiation, electron beam or heat.

Generally convection oven are used to transmit heat, but these oven technologies are limited to sizes of the pieces to be cured smaller than the dimensions of the ovens or autoclaves and require still long times of curing; therefore other technologies such as ultraviolet, infrared or microwave ovens or autoclaves are also used. For instance, US 2012/01 1 1497 discloses the reduction of the curing time -in comparison to the traditional curing with a convection oven- of a thermoset matrix made of epoxy resin and carbon nanotubes (CNTs). Since CNTs have excellent microwave absorption properties, the microwave energy absorbed by the CNTs can be converted into heat energy and the epoxy resin can be cured by means of a MW oven. However, the temperature of the piece during curing is difficult to control since the energy absorption of the piece depends on several parameters (e.g. volume, filler volume fraction).

Other methods of composite curing are based on Joule effect. Interesting examples are those that intentionally incorporate metal particles in insulating thermosets in order to make them conductive. Curing is performed by a directly current flow through the tailored conducting thermosets [GB2309925]. However, a big amount of metallic particles, about 30-95 wt%, and high powers are required to cure the thermosets by the mentioned Joule effect.

On the other hand, the autoclave method of curing carbon-fiber (CF) / resin composites has remained largely unchanged until the high electrical conductivity of the CFs has been exploited to produce electrodes. For instance, fibres are firstly pre-impregnated in thermosets and further cured by Joule effect, passing an electrical current through the CF-thermoset [Joseph C, Viney C. Electrical resistance curing of carbon-fibre/epoxy composites. Comp. Sci. Tech. 2000; 60: p. 315 - 319]. Electrical resistance curing heats the composite directly, and significantly less energy is used in comparison to oven curing. Nonetheless, this techniques still requires heat transfer across large distances (tens of microns) in the thermoset, which is a drawback since thermosets are very poor thermal conductors. This situation gives rise, for example, to thermal gradients in the sample that result in non-uniform curing.

For the reasons stated above, it is needed to develop a more energy efficient curing process for thermosets that can provide fast heating rates while resulting in uniform curing of the parts, without requiring the addition of large amounts of agents (>1 wt.%) that can compromise other properties.

SUMMARY OF THE INVENTION

Nanocarbons, such as carbon nanotubes and graphene, possess a unique combination of mechanical, electrical and thermal properties parallel to the plane of the graphitic layers, combined with very high specific surface and aspect ratio (area/thickness ratio for graphene). One of the routes to exploit these properties on a macroscopic scale is to disperse them in a polymer matrix. Typically, low volume fractions of nanocarbons result in the improvement of matrix properties and/or the addition of new functionalities to the matrix. Electrical percolation, for example, can be often achieved with less than 0.1 vol% of nanocarbons reaching values in the range of 0.1 - 1 .0 S/m of electrical conductivity.

A first aspect of the present invention relates to a process for in situ thermoset curing which comprises the following steps: a) forming a nanocarbon conducting thermoset by incorporating 0.001 -0.5 wt% of nanocarbons into a thermoset matrix, or by incorporating at least 5 wt% of a thermoset into a nanocarbon matrix, being these weight percents with respect to the final weight of the nanocarbon conducting thermoset, b) optionally incorporating at least an additive to the thermoset obtained in step (a), c) supplying electric power of a value ranging from 1 mW to 100kW to the thermoset of step (a) or (b) to raise its temperature to a temperature value between room temperature and 400 °C using a heating rate between 0.01 °C/min and 1000 °C/min and maintaining the temperature for at least 5 minutes, and d) supplying electric power of a value ranging from 1 mW to 100kW to the thermoset of step (c) to raise its temperature to a temperature value between room temperature and 400°C using a heating rate between 1 and 780 °C/min and maintaining the temperature for at least 5 minutes, wherein steps (c) and (d) are carried out detecting the real temperature of the nanocarbon conducting thermoset and adjusting the current flowing through the nanocarbon conducting thermoset depending upon the detected real temperature.

A comparison between the resistive heating of the present invention and oven processes in terms of power required to cure will depend on the specific characteristics of the oven and mould, amongst others, but resistive heating is by definition more efficient since it converts all the electricity into heat, whereas the oven has thermal losses when transferring heat from its elements to the sample mould. For reference, the present invention requires less than 5 kJ to cure a sample of around 1 cm ³ , in compared to 3 MJ used with a standard laboratory oven (with capacity to cure around 100 pieces of that size).

Curing through resistive heating has the additional advantage of enabling the rapid adjustment of the sample temperature and the power supplied to it in comparison to oven processes. For example, once the exothermic curing reaction starts, the electric power can be reduced accordingly to take advantage of the energy released by the sample, a process that would be more complex in an oven due to its high thermal mass. In addition, the cooling rates after curing are also notably faster when using resistive heating, since it is mainly the sample that needs to reach room temperature, whereas in the oven process after withdrawing the sample from the oven both the sample and the mould need to could down. In a preferred embodiment, the thermoset of step (a) is selected from the list consisting of epoxy or phenolic resins, polyesters, polyurethanes, polyamides, acrylates, elastomeric materials, rubbers, silicones and a combination thereof.

In a more preferred embodiment, the thermoset is an epoxy or a phenolic resin.

In another preferred embodiment, the nanocarbons of step (a) are selected from the list consisting of carbon nanotubes (CNTs), graphene, graphene oxide, carbon whiskers, macroscopic fibres made out CNTs or graphene, films of CNTs, films of graphene and a combination thereof. The term "graphene" as used herein refers also to reduced graphene oxide, monolayer and multilayer graphene and high purity graphene grown by chemical vapour deposition, among others. The term "carbon whiskers" are also known as are vapour-grown carbon fibres, carbon nanofibres and graphite whiskers, refers to graphitic fibres with diameter typically in the range 15nm - 500nm.

In another preferred embodiment, the forming of the nanocarbon conducting thermoset of step (a) is carried out by incorporating 0.001 - 0.1 wt% of nanocarbons into a thermoset matrix.

In another preferred embodiment, the forming of the nanocarbon conducting thermoset of step (a) is carried out by incorporating 5-50 wt% of a thermoset into a nanocarbon matrix.

In a preferred embodiment of step (a), the incorporation of nanocarbons into a thermoset matrix is carried out by means of uniform dispersion techniques, such as milling, calendering, sonication and centrifugation.

The thermosets are electrical insulators. The addition of nanocarbons in low proportions (0.001 -0.5 wt %) provides a certain electrical conductivity which allows resistive heating by passing electric current through them while embedded in the thermoset.

An advantage of incorporating nanocarbons into a thermoset matrix, is the accomplishment of the resistive heat conduction within the material itself. In oven processes instead, heat has to be transferred from the resistive elements to the mould and from the mould to the sample. Even at low CNTs volume fractions of nanocarbons, the separation between the CNTs is below 100 nm, which implies that the distance between heat sources is also below 100 nm. This distance corresponds roughly to the distance that heat flows in the poorly conducting thermoset; thus, it is desirable to make it as small as possible.

The uniform dispersion of the nanocarbons into a thermoset matrix achieves a uniform curing and consequently homogeneous properties of the cured composite. Measuring the glass transition temperature of similar composite samples produced under similar temperatures cycles, it is observed that resistive cured composite samples vary roughly 1 % in their glass transition temperature whereas those cured in the oven vary by 15% across the sample.

In another preferred embodiment of step (a), the incorporation of a thermoset into a nanocarbon matrix is carried out by means of infusion, injection, bath, impregnation, resin transfer moulding, vacuum-assisted resin transfer moulding, impregnation and combinations of these techniques.

Step (b) of the process for in situ thermoset curing relates to the incorporation of at least an additive to the thermoset obtained in step (a). This step (b) is optional. In a preferred embodiment of step (b), additives are selected from the list consisting of reinforcing elements, catalysts, antioxidants, UV stabilizers and fire retard ants.

Catalysts shall be selected from among the list consisting of dimethylamine, monoethylamine, triamines, aminomethyl phenol and a combination thereof.

Reinforcing elements shall be selected from among the list consisting of macroscopic fibres, glass fibres, short fibres, carbon fibres, polymeric fibres, natural fibres and a combination thereof.

Antioxidants shall be selected from among the list consisting of aromatic amines and hindered phenols. UV stabilizers shall be selected from among the list consisting of oxanilides, benzophenones, benzotriazoles, hydroxyphenyltriazines and a combination thereof. Fire retardants shall be selected from among the list containing of phosphates, phosphoric or boric acid, borates, sulphates, aluminium or magnesium hydroxides, magnesium carbonates, hydromagnesite and a combination thereof. In a more preferred embodiment, the phosphates are selected from the list consisting of mono-ammonium phosphate, di-ammonium phosphate, melamine phosphate and a combination thereof.

Step (c) of the process for in situ thermoset curing relates to the supply of electric power of a value ranging from 1 mW to 100kW to the thermoset of step (a) or (b) in order to raise its temperature to a temperature value between room temperature and 400 °C using a heating rate between 0.01 °C/min and 1000 °C/min and maintaining the temperature for at least 5 minutes by means of detecting the real temperature of the nanocarbon conducting thermoset and adjusting the current flowing through the nanocarbon conducting thermoset depending upon the detected real temperature.

In a preferred embodiment of step (c), the electric power supplied to the thermoset ranges a value between 1W and 1 kW.

In another preferred embodiment of step (c), the temperature of the thermoset raised by supplying electric power has a value between 50 and 200 °C.

In another preferred embodiment of step (c), the heating rate ranges values between 1 °C/min and 100 °C/min. Step (d) of the process for in situ thermoset curing relates to the supply of electric power of a value ranging from 1 mW to 100kW to the thermoset of step (c) to raise its temperature to a temperature value between room temperature and 400°C using a heating rate between 1 and 780 °C/min and maintaining the temperature for at least 5 minutes, by means of detecting the real temperature of the nanocarbon conducting thermoset and adjusting the current flowing through the nanocarbon conducting thermoset depending upon the detected real temperature.

In a preferred embodiment of step (d) the electric power supplied to the thermoset ranges a value between 1W and 1 kW. In another preferred embodiment of step (d), the temperature of the thermoset raised by supplying electric power has a value between 50 and 200 °C.

In another preferred embodiment of step (d), the heating rate ranges values between 1 °C/min and 100 °C/min.

Another aspect of the present invention relates to a process for in situ repairing composites according to the thermoset curing process of the invention described above, which further comprises a step b ^" ), between step b) and step c), of placing the thermoset of step b) in contact with the surface to be repaired.

The term "composites" as used herein refers to materials made from two or more constituent materials with significantly different physical or chemical properties, that when combined, produce a material with characteristics different from the individual components. The individual components remain separate and distinct within the finished structure.

A clear advantage of this process is to cure in-situ, without having to take the pieces to be repaired to an oven, particularly for large composite parts such as aerospace panels.

Another aspect of the present invention relates to a process for soldering, according to the thermoset curing process of the invention described above, which further comprises a step b ^' ), between step b) and step c), of placing the thermoset of step b) in contact with the metal surface.

Traditional soldering methods, for example for metal cables, usually involved melting soft metals at temperatures between 200 and 500°C. Such methods might be unsuitable for nanocarbon-based materials, firstly, because they can cause damage to the materials by oxidation and secondly, because molten metals do not normally wet CNT fibres and are therefore unlikely to produce a good soldering contact without prior chemical treatment of the fibre.

Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skilled in the art to which this invention belongs. Methods and materials similar or equivalent to those described herein can be used in the practice of the present invention. Throughout the description and claims the word "comprise" and its variations are not intended to exclude other technical features, additives, components, or steps. Additional objects, advantages and features of the invention will become apparent to those skilled in the art upon examination of the description or may be learned by practice of the invention. The following examples, drawings and sequence listing are provided by way of illustration and are not intended to be limiting of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 Schematic of a setup used to cure a thermoset containing CNTs through resistive heating.

Figure 2 Plot showing that resistive heating can be controlled to follow a predetermined heating protocol. Here, the actual sample temperature is used as an input for a controller that adjusts the electric power delivered to the sample and therefore its temperature. Figure 3 Schematic representation and photographs of a setup used for curing a glass fibre reinforced composite.

Figure 4 Schematic representation and photographs of a setup used for repairing an aerospace composite panel by curing a thermoset through resistive heating.

Figure 5 Process for using resistive heating to cure a thermoset in an array of CNTs to make an electric contact between the array and a metal, lb) shows the porosity of the nanocarbon array that allows infiltration of epoxy into it and therefore makes the epoxy resistive soldering process possible.

EXAMPLES Example 1 :

In this example, CNTs are added to an epoxy resin at a volume fraction of 0.5 wt.% and dispersed using an Exakt80E three roll mill (Exakt Technologies) by progressively reducing the gap between rollers. Dispersion was achieved using a constant output roller speed of 250 rpm (relative speed between rools is 1 :3:9) and decreasing the gap between them 125μηι to 5 μηπ. Once the CNTs are dispersed and the resin is electrically conducting 18 wt.%, Aradure XB 3473 hardener is added to the mixture. The hardener was added to the above CNT/epoxy resin and stirred mechanically later using a Heidolph RZR1 stirrer with a TR 21 radial flow Impeller.

The addition of CNTs to the thermoset makes it electrically conductive, therefore making it possible to pass electric current through it and raise its temperature through Joule heating. As shown in Figure 1 , the mixture is placed in a small container with two pre- placed metal contacts at each end connected to a power supply,. The sample is heated by passing current through the epoxy resin and the temperature is controlled by varying the voltage (power) applied across the electrodes, as shown in Figure 2. Thus, in this method, the heat source is the sample itself. This implies that when the sample temperature has to be adjusted, this can be achieved instantly by changing the power delivered to it. Furthermore, the power can be reduced, for example, to take advantage of the energy released by the exothermic curing reaction. In this example, the power delivered to the 1.5 g sample was approximately 4W, after checking various parameters such as mould shape, size, material, room temperature, use of insulation, etc.

The uniformity of the sample cured by resistive heating is assessed by comparing the glass transition temperature of material removed from the middle of the cured piece to material from the lateral edge of the piece (Tablel ).

The variation of the glass transition temperature is within 1 %. A sample prepared as detailed in this example but cured in the oven under equivalent conditions has a variation in glass transition temperature of 15%.

Table 1 : Glass transition uniformity in samples cured by resistive heating and standard oven processing.

Curing Location from which Glass transition

Sample

procedure extracted temperature (°C)

A Resistive curing Middle of piece 95.0

A Resistive curing Lateral edge 96

B Resistive curing Middle of piece 84.3

B Resistive curing Lateral edge 73 Example 2:

In this example, a thermoset containing CNTs prepared as in the previous example is infused into a standard glass fibre array (the diameter of the fibres ranges 5 to 50 microns) by direct mixing between the two. The structure of the array is typically porous and therefore accessible for thermoset infiltration through capillary forces.

Figure 3 shows a schematic of the set-up used and photographs illustrating the steps followed during the preparation of the composite.

Then, the infused array is placed between two flat electrodes consisting of two aluminium slabs, connected to a power supply. Next, the sample is heated by passing current through the conductive thermoset so that the sample is cured. Using this resistive curing, the heat transfer occurs throughout the whole sample at the same time. In this example, the power delivered to the ~ 10 g sample was in the range 15 - 35 W, It has been chosen taking into account various parameters such as mould shape, size, material, room temperature, use of insulation, etc.

Scanning Electron Microscopy images were obtained by means of EVOMA15 Zeiss SEM. The image lb) of Figure 5 obtained by scanning electron microscopy shows the cross section of the material to be consistently cured through it, with no evidence of curing-induced cracks and any sign of heterogeneity, confirming that the whole volume of sample is cured.

Example 3: Repair of an aerospace composite panel.

In this example, a fibre-reinforced composite panel typically used in the aerospace industry is repaired using the composite prepared as indicated in Example 1 as adhesive, applying it to the panel to be repaired and curing it by resistive heating. The set-up used is schematically shown in Figure 4. The repairing process is shown by photographs also in Figure 4.

The starting material is a laminate that was damaged while carrying out a standard impact test. In order to repair it in-situ, without having to take them to an oven, a small amount of the composite prepared as in Example 1 is spread around the damaged area and then covered with a special Carbon Fibre lamina used for repairs according to aeronautical specifications. The two composite parts, which required no modification, are used as electrodes and joined by curing the epoxy adhesive through resistive heating by passing current through the part for a few minutes. One electric contact is made on each panel, for example using conventional silver paint. In this example, the power delivered to the ~ 3 g adhesive sample was in the range 5 - 30 W,

The bottom image in Figure 4 shows the repaired composite withstanding a weight of 10kg.

Example 4:

In this example resistive curing is done to join a composite prepared as described in Example 2 to a metallic surface. These composites have low density, typically below 1.2 g/cc and can exhibit very high electrical conductivity of 3x10 ⁶ S/m These values are comparable to that of copper on a mass basis and therefore of great interest for several applications, hence the importance of processes such as soldering of the composites to a metallic substrate to achieve an electric contact.

The soldering method is shown in Figure 5. Having placed the composite prepared as described in Example 2 in contact with the metal surface, a small drop of standard epoxy resin is applied to the composite. The thermoset instantly wicks in the composite due to its high internal area, but preserving the electrical contact between composite and metal. Next, a small current is passed through the sample to cure the composite. In this example, the power delivered to the ~ 10 mg sample was approximately 1 .5 W. A mechanically robust contact is obtained between the two conductor materials which exhibit a contact resistance value of 2 Ohm. For instance, a working LED is shown in Figure 5III).