INTEGRATION OF DEVICES AND ELECTRICAL CONNECTIONS IN COMPONENTS OR STRUCTURAL PARTS OF POLYMERIC MATERIAL INSTALLED ON A VEHICLE

The invention relates to a method for the manufacturing of a component or a structural part made of polymeric material, adapted to integrate electrical devices and connections, and a system for carrying out the method. The method includes injection moulding of a composite material including: a non-conductive polymeric matrix; a dispersed phase including at least one carbonization promoter to form carbonaceous conductive structures; and a reinforcing-fibre filler to direct the distribution and orientation of the dispersed phase in the polymeric matrix, wherein injection of the composite material into a mould for forming the component or the structural part includes supplying the material in a spatially more-concentrated way at pre-established regions of the component or of the structural part designed for the incorporation of electrical devices or connections, and supplying the material in a spatially more spread-out way elsewhere.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1shows, in schematic form, the steps of an innovative method for the production of a component or a structural part for a vehicle, made using non-conductive polymeric material and integrating electrical devices or connections. In step100, the composite polymeric material is provided, wherein the material includes a non-conductive polymeric matrix and a dispersed phase of filamentary nano-structures, which are promoters of conductivity. At the same time, in step200, the design of the component or the structural part of the vehicle and the associated mould for injection-moulding forming of the material produced in step100is performed.

The step for forming the component or the structural part is denoted by300and, in the following, at step400, definition of the conductive areas or tracks is carried out on the moulded part using the laser ablation writing technique described in published international patent application WO 2012/055934.

Finally, in step500, the component or part thus produced, which has predetermined conductive areas forming the electrical devices and connections, and is assembled together with other supply components, such as the external connectors.

The provision of the composite polymeric material includes mixing, in a polyolefinic polymeric substrate (for example, commercially available polypropylene (PP) or high-density polyethylene (HDPE)) of a phase of filamentary nano-structures, promoters of conductivity, in particular carbon-based nano-structures such as carbon nanotubes or nanofibres enriched with substances which favour compatibility, namely coupling agents for the reinforcing fibres (for example alkaline hydroxides in aqueous solutions of polymer grafted with maleic anhydride) and a phase of reinforcing fillers, such as glass fibres or fillers of mineral origin. The inventors have noted that the increase in conductivity in the components made using this polymeric material is greater than the conductivity in the components made of composite polymeric material without reinforcing fibres.

The reinforcing fibres, especially those with short glass fibres, have the effect of increasing the degree of dispersion of the carbon nano-structures, which otherwise tend to reaccumulate in the melted mass, creating “islands” which overall prevent the transit of electric charges in the manufactured article, owing to the Van der Waals forces which are generated between the chains of nano-structures. This distribution effect also reduces the so-called skin effect (the formation of a surface film of non-conductive polymeric material), allowing the nanotubes to migrate towards the surface, directed by the reinforcing fibres.

The reinforcing fibres, in particular the glass fibres, are a reinforcing agent which is widely used in the polymer sector. In the tests carried out for optimization of the raw materials to be used in the process described above, it has emerged that in a polyolefin (PP, HDPE) based polymeric matrix, to which carbon nanotubes have been added, the presence of glass fibres, preferably in filaments with a length of the order of 5 mm and diameter of the order of 10 μm, increases the electrical conductivity thereof, even without further carbonization treatment, and facilitates the dispersion of the carbon nanotubes.

The results obtained show that a partially conductive interphase is formed between the glass fibre and the polymeric matrix, owing to the carbon nanotubes which tend to line the glass fibre: the local concentration of the carbon nanotubes inside the glass fibre/polymer matrix interphase provides the material with multi-functional properties, including an increase in the mechanical characteristics due to the presence of the glass fibres, and an increase in the electrical characteristics due to the presence of the carbon nanotubes.

The coupling agents used to improve the adhesion of the glass fibres in the polymeric matrix improve the distribution of the fibres in the matrix itself, making it practically isotropic: the nanotubes which line the fibres are consequently also uniformly distributed in the polymeric matrix, thereby ensuring the homogeneity of the electrical conductivity in the article.

It has been noted in tests that a component made from polyolefinic materials to which carbon nanotubes have been added, owing to the correct dispersion exerted by the glass fibres, has a conductivity at least 100 times greater than an analogous component without a dispersed glass-fibre phase. These results have been obtained with polyolefin-based polymers having carbon nanotube fillers (multiwall CNT) in an amount from 1.5% to 10% by weight, and glass fibre fillers in an amount from 10% to 20% by weight, wherein the values refer to the weight of the composite material.

During the moulding of materials of this type, the flow of the material filling a mould, (the dynamic behaviour of which is comparable to that of a high-viscosity fluid) produces complex interactions which result in: a fragmentation, such that the lengths of the fibres are distributed in a manner typical of a Weibull distribution, as shown inFIG. 2; and a strong influence of the dynamic behaviour of the flow on the arrangement of the fibres, which arrange with preferential orientations.

In particular, the velocity profile of the flow has a high gradient zone in the vicinity of the mould walls, and a zone with a tendentially uniform profile in the central part of the thickness of the mould cavity. Consequently, in the volumes of material which in the centre are subject mainly to transverse deformation, the fibres tend to be arranged in a direction perpendicular to the injection flow, while in the vicinity of the walls the fibres tend to be arranged parallel to the flow, as a result of the shearing stresses. This condition is shown inFIG. 3for a generic mould with a tapered shape, provided with an injection nozzle, where G indicates the injection nozzle, A indicates the advancing front edge of the injected flow, F indicates the arrangement of the fibres, and S indicates the volume of solidified composite material.

The fluid-dynamic conditions created during the process of injection of the composite material mixed therefore define the orientations of the fibres which in turn determine the mechanical and electrical properties of the component. Thus, in view of the foregoing, in order to avoid localized stress of the material during the injection step with an associated loss of the conductive capacities (breakage of nanotubes, deterioration of the matrix, skin effect), during design of the actual mould particular attention must be paid to the following parameters: injection layout, if necessary sequential; mould conditioning layout; and design of the movements of the carriages which are not temperature-regulated.

The component moulding step300is therefore dependent on a suitable design of the mould in step200, which is dependent, in turn, on the design of the component, not only as regards the form and volume dimensions, but also the arrangement of the conductive regions where the integrated electrical connections or devices are to be formed.

The component is obtained by injection-moulding the polymeric compound defined above. Advantageously, the component or the structural part which is to be made should not have small-radius curvatures, and the forming mould should have an optimized spatial distribution of the injection nozzles, which are spatially more concentrated (compact) in the electrically functional areas of the part, and spatially more spread out elsewhere.

The moulding conditions (including, for example, temperature profiles, velocity profiles, temperature-regulation mode of the mould, injection times, pressure profiles) fundamentally determine the electrical conductivity characteristics the manufactured article will have after moulding and following definition of the conductive areas or tracks by writing or laser ablation.

The correct setting of these moulding-related parameters is of fundamental importance in order to produce a component which has sufficient levels of internal conductivity (for example, of the order of 100 ohm/cm) before laser activation of the actual conductive areas. Incorrect moulding parameters may cause a partially isolating skin effect, which would hinder the subsequent laser writing activation step.

The optimum definition of the moulding parameters is generally dependent on the geometrical configuration of the component and the layout of the mould, such that for each new component to be moulded, associated polymeric material, press type, and mould layout, it is necessary to follow a specific procedure of fine-tuning the initial parameters and defining the optimum operational parameters.

For example, in the case of production of a fuel filler nozzle of a HDPE-based composite material including carbon nanotubes and glass fibres in the percentage amounts indicated above, the tests carried out have revealed the following general setting of the parameters necessary for obtaining a good initial conductivity level: mould temperature-regulated to an average temperature of 60° C.; high injection speed; low holding pressure; and high holding time.

In particular, from the tests carried out, it emerges that the distribution of the glass fibres, and therefore the distribution of the carbon nanotubes (namely the nano-structures which promote conductivity), and consequently the homogeneity of the electrical characteristics of the moulded article, are affected by the following transformation parameters: melting temperature, mould temperature, cooling time, injection speed and time, injection pressure, plasticization speed, and holding time and pressure.

The tests carried out for components with a volume of about 300 cm3, such as the fuel filler nozzle described above, show how it is, in any case, advantageous to operate using the following moulding parameters: temperature of the material between 190° C. and 260° C.; temperature-regulation of the mould between 50° C. and 70° C.; injection speed of between about 60 and 150 cm3/s (or, an injection time of between 3 s and 5 s for a volume of 300 cm3); injection pressure of between 60 bar and 80 bar; holding/cooling time in the mould of between 30 s and 60 s; and holding pressure of between 35 bar and 60 bar.

It is noted that, with the polymeric materials to which carbon nanotubes and glass fibre fillers have been added, there is a variable dispersion of the said fillers depending on the radial distance from the injection point. In effect, the carbon nanotubes act as a fluidifying agent for the polymer chains (having a smaller size, the hot molecules of polymers “slide” on the nanotubes). This effect results, with regard to the part, in a high concentration of nanotubes close to the injection point, and a smaller concentration of nanotubes far from the injection point. For this reason, in order to ensure that a functional component is obtained, (namely one where it is possible to form conductive tracks able to form electrical connections or devices), it is preferable to arrange the injection zones close to the electrically functional areas of the part (namely the areas of the component or structural part which are to be used for formation of electrical connections or devices).

By way of example, in the case where it is required to form areas with piezo-resistive characteristics (adapted to the formation of switching devices such as the control buttons for apparatus on-board a vehicle), it has been established that the arrangement of the injection nozzles need to be within a radius of 30 cm from the area assigned so as to undergo a subsequent laser writing treatment for activation of conductive tracks. Greater distances do not ensure an adequate distribution of the glass fibres and the carbon filamentary nano-structures, irrespective of the moulding parameters envisaged.

Once the mould has been prepared, following the design of the component or structural part adapted to integrate electrical devices or connections, and after implementation of the industrial process for moulding this component or structural part, the step for definition and realization of the conductive areas (tracks) is performed. For this purpose the technique of writing by laser ablation and consequent localized pyrolysis known from international patent application WO 2012/055934 is applied.

It has been shown that the localized heating produced by a focused laser beam (which induces selective superficial ablation of the polymer matrix) causes the filamentary nano-structures, which are dispersed within the matrix, to surface and percolate, thus forming a conductive pattern. Moreover, the interaction of the laser beam with the polymer substrate favours the thermal decomposition thereof, and the consequent formation of carbon. The carbon formed in this way acts as a bridge between the nano-structures during the process of ablation of the surface layers, further favouring the formation of the electrically conductive areas (tracks). A laser beam precisely focused on the polymer matrix may be used to obtain deep and stable conductive tracks.

The parameter for the minimum distance between the conductive tracks needs to be controlled, in order to prevent interference (crosstalk) between adjacent tracks. For this purpose, it has been noted that in a polypropylene matrix with a mineral reinforcing filler (for example, talc) in an amount of 10% by weight (referred to the weight of the composite material), in order to improve the dimensional stability and the rigidity (including, if necessary, rubber for improving the impact elasticity), and carbon nanotube filler in an amount of 2.5% by weight (referred to the weight of the composite material), which ensures an electrical conductivity of 1.6 Kohm/cm, such that there be no interference between the tracks, a minimum distance of 10 mm should be maintained between adjacent tracks.

The width and depth parameters of the laser beam for ablation of the composite material need to be controlled so as to obtain a level of specific electrical resistivity of at least 1.6 Kohm/cm on an injection-moulded component with an average thickness of between 2.5 mm and 3.0 mm.

FIG. 4shows a cross-sectional view of a laser-cut track. From test results, in order to obtain a level of electrical conductivity of at least 1.6 Kohm/cm, it is necessary to perform a laser incision with a width which is preferably between 1.10 mm and 1.40 mm, and ideally equal to 1.25 mm, and a depth preferably of between 0.70 mm and 0.90 mm, and ideally equal to 0.80 mm. The focused laser incision forms a groove B in the surface C of the material with an overall triangular and substantially symmetrical cross-section. However, other forms (for example, a trapezoidal form) and different orientations may be obtained by controlling focusing and orientation of the laser beam with respect to the surface of the part.

The inventors have noted that, with a laser beam having a wavelength of 10.6 μm, the optimum speed of laser ablation is 5 mm/min with an effective focal length of 135 mm and an operating power of about 30 W. In order to avoid localized combustion, which results in deterioration of the nanotubes, the laser ablation process is carried out in an inert nitrogen atmosphere. It should be noted that a modified thermal zone Z (with transverse dimensions of about 0.2 mm), where an electrical conductivity effect may still be detected, is created in the region of the groove.

In order to form electrical connections and devices, the geometrical configuration of the conductive tracks (namely the layout of the conductive region, its width, and depth), also needs to be controlled. This is of fundamental importance in the realization of electrical devices, such as switching devices in the form of control buttons integrated in the component, in order to usefully employ the piezo-resistive effect of a conductive track, the variation in resistance caused by a suitable mechanical deformation or the capacitive effect of a conductive track, and the variation in capacity caused by an external body touching a conductive electrode area (such as finger).

FIGS. 5 and 6show two schematic illustrations of a control device (button) integrated in a component using the technology described in this patent application, respectively in a plan view from above, showing the forms and dimensions thereof, and a partial view simulating the movement of the device in an operating condition.

FIG. 5shows a segment10of a conductive track embedded in a matrix of polymeric material12, in which an operating area14, able to be acted on by a user, is defined. The embodiment illustrated shows an operating area with a circular form (the diameter of which is between 20 mm and 30 mm, preferably 25 mm) defined along a winding section10′ of the conductive track10, having a loop-like or meander form (the width of which is between 1.0 mm and 2.0 mm, preferably 1.5 mm). It is to be understood that the depiction of a substantially bell-shaped loop is purely illustrative, and that other winding trajectories may also be provided in the operating area14. Advantageously, the greater the overall length of the conductive track subject to stressing in the operating area (in this case, following a pressure exerted on the area by a user), the greater the effect of variation of the electrical resistivity parameter indicating the action of a user performed on the operating device.

A cut or weakening line20(for example, a thinner zone of the material) is conveniently provided on the polymer substrate around the operating area, in order to facilitate the mechanical displacement (oscillation) of at least one portion of the operating area of the device, with respect to the surrounding surface of the component in which it is integrated, and amplify as far as possible the effect of varying the electrical parameter of the circuit.

FIG. 5shows an open cut line20substantially in the form of cardioid. This cut line defines a base of the operating area14(the area between the end segments E of the cut line and the circumferential arc F without the cut line) having a substantially trapezoidal form, which allows the device to operate in the elastic range without undergoing permanent deformation.

FIG. 6shows a partial, simulated, three-dimensional view of the device, obtained by sectioning the device area along the diametral cross-sectional line VI-VI shown inFIG. 5. It is possible to identify a top surface portion22a,which is subject to a tractional force in the operating condition where pressure is exerted on the area of the device, and a bottom surface portion22b,which is subject to compression in the operating condition where pressure is exerted on the area of the device.

During tests, the piezo-resistive effect occurred when a force of 25 N was applied, with an elastic deformation of about 3 mm. A corresponding variation in the electrical resistivity between the ends of the winding segment10′ of the conductive track was recorded, equivalent to 10%. Where no force is applied, the resistance is equal to R=30 Kohm. Where a force of F=25 N is applied, the resistance is equal to R=33 Kohm.

The minimum variation in electrical resistivity which can be detected over a background noise, resulting from variations in the environmental conditions is in the region of 3% (for example, temperature or humidity, which also produce dimensional variations in the component).

In general, it was shown that, in order for the formed operating device to be able to work in the elastic range, the maximum permissible deformation needs to be equal to about 3 mm. Moreover, in order to obtain the piezo-resistive effect, with the aforementioned characteristics, the operating area14should have a thickness greater than 2 mm and preferably a thickness of 3 mm.

FIG. 7shows an alternative embodiment of the integrated control device (button) in which the operating area14is the same as that shown inFIG. 5, and the segment10of the conductive track has a winding section10″, in the form of a serpentine inscribed in a loop-like or meander envelope curve, with a substantially bell-like form, which has a greater overall length subject to stressing in the operating area than the winding section shown inFIG. 5, increasing the variation effect of the electrical resistivity parameter indicating the action of a user on the control device.

Finally, the component or structural part realized by applying the process steps described above and integrating electrical devices or connections is assembled together with other components forming part of a supply set, for example by successive mechanical processing operations (such as welding, gluing, etc.), able to integrate the selectively conductive component in a supply assembly for subsequent installation on the vehicle. To this end, the component or the structural part has a plurality of metal connection terminals co-moulded on the polymeric matrix, which allow connection of the conductive tracks and the control devices to electrical circuits or systems outside the component, in order to receive or distribute information or power supply current signals.

FIG. 8shows a panel of polymeric material, denoted overall by50, containing conductive tracks52which are parallel to each other and spaced so as not to cause crosstalk or interference between adjacent connections, wherein the tracks have, coupled to their ends, along the edge of the panel, metal (for example, gold) connection terminals54for connection to external electrical (signal or power supply) distribution circuits (for example, wiring of on-board equipment for receiving operating commands from the control devices (buttons) integrated in the polymeric matrix).

It should be noted that the embodiment proposed for the present invention in the above description is intended to be a purely non-limiting example of the present invention. A person skilled in the art may easily implement the present invention in different embodiments which do not depart from the principles illustrated here, and are therefore included within the scope of protection of the present patent, as defined in the appended claims.