CH711917B1 - Covering element in variable colors. - Google Patents

Abstract

The present invention relates to a covering element for a portable object comprising a support on which a first material is arranged, said first material being chosen capable of reflecting different visible wavelengths as a function of the stress applied. The covering element may be a middle part or the back of a case (2), a bracelet (3) or a dial (41) of a timepiece.

Description

Description: The present invention relates to a covering element for a portable object comprising a support on which a first material is arranged, said first material being chosen capable of selectively reflecting different visible wavelengths as a function of the applied stress.

Technological background Portable objects such as watches are known, which are equipped with colored covering elements. In general, the covering elements are colored with a particular color or pattern and this coloring is final.

However, there is a need for a watch provided with a covering element, the color of which may vary. Such a need comes from the desire of consumers to be able to change the color of their watch or portable object according to their desires or mood.

An obvious solution is to provide accessories such as phone cases or spare parts that can be replaced easily by the user.

However, this solution has the disadvantage that it becomes necessary for users to have a quantity of spare parts in stock, these parts can get lost and it becomes necessary to renew the stock with each change of watch or phone .

Another solution is to use pigments whose color can vary, these pigments can be photochromic or thermochromic. These pigments are incorporated into an ink or directly into the material forming the covering element. A change in temperature or brightness is then sufficient to change the color of the covering element.

A disadvantage here is that this configuration is not controllable. Indeed, these pigments react to specific values of temperature or brightness so that the consumer has little control over these elements, he is dependent on them.

There is therefore a need for a covering element whose variation in color is controllable.

Claims (22)

Summary of the invention The invention relates to a covering element which overcomes the aforementioned drawbacks of the prior art by providing a covering element for a portable object which allows a controllable color variation. 1. covering element (10) for portable object (1), comprising a support (12) on which a first material (M1) is arranged, said first material being chosen capable of reflecting different visible wavelengths as a function of a stress applied by a device included in said covering element, said device (14) making it possible to modify the stress applied to said first material. 1) by crystallization, sedimentation, evaporation of a colloidal suspension (the technology is used in particular in P-Ink® and Elast-lnk® inks) 2. covering element according to claim 1, characterized in that the support (12) and the device (14) for modifying the stress applied to said first material are one and the same piece. 2) under the effect of an electric field (as described by Baumberg Advanced Engineering Materials 2013, p 948), 3. covering element according to claim 2, characterized in that the support (12) is a bistable metal strip (32) coated with said first material. 3) under the effect of shear forces by extrusion (as described by Baumberg Stretching the imagination, Textiles, issue 4, 2009, 8-10 and US 2013/0288 035, Manufacture of composite optical materials). The periodic network thus formed is then frozen by UV polymerization. [0040] A ^2nd mode of manufacture of the photonic crystals is to create "pads" of nanometric size on flexible thin film (typically a sheet of PDMS). Researchers from the University of California Berkeley fabricated rows of silicon pixels using lithography techniques, which they then encapsulated in 2 sheets of PDMS (Flexible photonic metrics for tunable coloration, Optical Letters, 2015, p255). lead to a thin and flexible film which can change color at will under the effect of slight deformations. In fact, when a material with a structural color is flexible or deposited on a flexible support, the periodic network can then be modified by varying the geometric dimensions of the material and it is then possible to obtain a material with colors. which change in a controlled manner when the material is mechanically stressed. These geometric dimensions are modified by applying a constraint on said material. These constraints can be a stretch, a compression, a twist, a pinch or any other constraints allowing a modification of the shape and / or dimensions of the material. This first material M1 can then be in the form of a more or less thick film or be directly integrated into the substrate forming the covering element 10. In the case of a film, a first configuration allows to the first material M1 to comprise only a film or a so-called opal strip obtained by the techniques previously described. In a second configuration, said first material M1 comprises the so-called opal film assembled on a flexible film. In a third configuration, said first material M1 comprises the opal film encapsulated in two flexible films. The flexible films are preferably PDMS films or thermoplastic films, preferably polyurethane thermoplastic films. CH 711 917 B1 Consequently, the covering element 10 according to the invention must be provided, in addition to the support 12, with a device 14 making it possible to apply a stress to the first material M1 to make it change color. In a first embodiment, the covering element 10 is configured so that the support 12 and the device 14 for applying a stress on the first material to make it change color are one and the same element . It is then understood that the support is able, by its characteristics, to be manipulated so as to generate a mechanical stress on the first material M1. According to one embodiment, the covering element 10 is a bracelet 30 called "slap strap" or "slap band" in the Anglo-Saxon version. Such a bracelet 30 is composed of a bistable metal band 32 serving as a support 12. The bracelet 30 can be laid flat by tensioning the bistable metal band 32 and then wound again around the wrist by snapping the bracelet. This bistable metal strip 32 can then be coated with a protective layer 34. This protective layer 34 can be a molded plastic material or a film or an ink which hardens in order to improve its aesthetics as visible on the surface. fig. 3a. Advantageously according to this first embodiment, the first material M1 is arranged on this strip of bistable metal. In a first configuration, the first material is arranged to cover the entire surface of the strap 30. The first material M1 can then be in the form of a more or less thick film which can be sewn and / or assembled by bonding, ultrasonic welding, laser welding, infrared welding or extruded directly onto the bistable metal strip 32 as shown in FIG. 3a or also molded onto the support 12. A protective layer 34 (varnish or thermoplastic) can optionally be applied over it. The first material M1 can then be placed either on the metal strip 32 or on the protective layer 34 of the bistable metal strip 32 as shown in FIG. 3b. In a second preferred configuration, for reasons of ease of process, said material M1 is deposited on the bracelet 30 and then molded with a transparent TPU type thermoplastic. In a third preferred configuration, for reasons of ease of process, said first material M1 comprises an opal film encapsulated in 2 transparent thermoplastic films (TPU type) and the material M1 is assembled to the bracelet by thermal welding techniques . In another configuration, the first material M1 is visible in certain specific places on the bracelet. For example, the first material M1 is placed on the bistable metal strip (sewn, glued, overmolded, assembled or extruded) then a second layer 35 (leather, thermoplastic, ...) perforated is placed so as to reveal the first M1 material as shown in fig. 3c. Thus, when the bistable metal strip 32 passes from one state to another, that is to say from the state in which the metal strip is under tension in the state in which the strip of metal 32 is not under tension and is wound, a stress variation occurs on the metal strip. This stress variation is transmitted to the first material M1. However, the first material M1 is capable of reflecting different wavelengths according to the stress applied to it. Consequently, when the slap bracelet 30 goes from one state to another, the variation in stress allows the first material M1 to reflect different wavelengths and therefore to have a bracelet having different designs depending on whether it is wound or not. In a second embodiment, the support 12 and the device for applying a stress 14 on the first material M1 to make it change color are dissociated and the application of the stress to the first material is done by l intermediary of a material M2 capable of seeing its volume vary as a function of a physical quantity, this physical quantity possibly being temperature. More preferably, the material M2 is a liquid with a boiling temperature at atmospheric pressure of between 0 and 60 ° C. Under these conditions, when a temperature higher than the boiling temperature of M2 is applied to the covering element, the material M2 goes into the gas phase and thus sees its volume increase. For example, the material M2 is ethyl chloride, butane, propane or a mixture of these compounds. In a first embodiment, the support 12 comprises at least one recess 40. This at least one recess 40 is used to place a capsule-piston system 43 and the material M2 therein. This capsule-piston system 43 tight to the material M2 consists of a half-shell 44 and a piston 45 as shown in FIGS. 4a and 4b. The recess 40 is closed by a film 42 made of the material M1 or carrying the material M1. Consequently, when the temperature increases, the volume of the material M2 increases and acts on the piston 45. The piston 45 has a rounded external surface which comes into contact with the film of said material M1 when the piston is in the high position. In this way, when the piston 45 is pushed by the material M2, it comes to stretch the material M1 which then changes color. The piston capsule 43 thus formed is then placed in a recess 40 of the covering element. This piston capsule 43 can be glued, expelled, welded, screwed or even crimped in the recess to be held there. In a preferred embodiment, the covering element is incorporated into a thermoplastic component of the portable object (for example bracelet, dial, watch case, etc.) and the thermoplastic component of the portable object carrying the 'covering element 10 is directly molded onto the piston capsule 43 thereby forming the recess 40. In this case, very good adhesion is obtained between the covering element and the portable object in which it is integrated. CH 711 917 B1 In an alternative configuration, said first material M1 comprises an opal film encapsulated in 2 films of transparent thermoplastic (TPU type) and the material M1 is assembled for example by heat welding or by bonding. This configuration advantageously allows for recesses 40 each accommodating a piston capsule 43 as shown in FIG. 4c, these capsules can include specific and different M2 materials. In this embodiment, the variation in coloring of the material M1 capable of reflecting different wavelengths depending on the stress applied to it is possible. Indeed, the material according to the invention is capable of reflecting different wavelengths according to the stress applied so that, for the same material, a difference in stress implies a different variation in color. To do this, a first solution consists in modifying the dimensions: length, volume of the piston capsule 43 or recess 40. These modifications of the dimensions then give rise to a different response to the stress applied by the expansion of the material M2. A second solution consists in using different variants of the material M2, each variant having a different coefficient of expansion. This solution makes it possible to have, for capsules 43 or a recess 40 of the same size, variations in different colors since the material M2 will be different. A third solution consists in using different materials for the embodiments of the membrane 42. Indeed, by using materials each having different deformation properties, it is ensured that the stress applied to the material M1 will be different and therefore the color variation will be different. It can also be provided to have, for each piston capsule 43, a specific M1 material. These possibilities then make it possible to produce a covering element 10 provided with a multitude of zones using this first material M1, the different zones possibly having characteristics allowing different color variations. In a second embodiment, the recess 40 is used to house a capsule 43 'provided with two demicoques 44' made of flexible material as shown in Figs. 5a and 5b. This capsule 43 ′ is then fixed in the recess by gluing or heat welding of one of the half-shells 44 ′. This capsule 43 ′ is also arranged to be filled with an M2 material which expands as a function of a physical quantity such as temperature or pressure. The recess 40 is closed by a film 42 ′ produced with the material M1. Consequently, when the temperature increases for example, the material M2 in the capsule expands and deforms the half-shell 44 '. This half-shell 44 'exerts a stress in turn on the film 42'. This constraint deforms the film 42 ′ and therefore the material M1 reflects another wavelength. Provision may be made for having several capsules 43 ′ by recess 40 as visible in FIG. 5c. In an alternative to this second embodiment, the covering element 10 is itself used as a capsule 43 ′. In a nonlimiting example, the covering element will be a strand 31 or a bracelet link 3. The bracelet strand is then composed of two strips assembled 310 together. This assembly is carried out so that the two strips 310 are fixed by their periphery so that the interior space 312 can serve as a cavity. This cavity 312 cleverly allows a pocket to be housed therein containing M2 material which expands as a function of a physical quantity such as temperature or pressure. Advantageously according to the invention, one or both bands 310 forming the strand 31 of bracelet 3 carry the material M1 capable of reflecting different wavelengths depending on the stress applied to it. This first material M1 is arranged on one and / or the other of these bands in the form of a film or a band or by being directly part of the material forming the said band or bands 310 as shown in FIG. 6. Thus, when for example the temperature varies, the M2 material reacts and expands or contracts, causing a deformation of the band or bands forming the bracelet strand. This deformation is therefore communicated to the first material M1 which, under the effect of this applied stress, sees its structure change and reflect different wavelengths as shown in FIG. 7. In an advantageous variant, the bracelet strand 31 comprises stiffening structures 313 as visible in FIG. 8. These stiffening structures are arranged between the two strips 310 forming the strand and extend longitudinally or transversely relative to said strand. These structures increase the torsional strength of the bracelet strand. These structures 313 are in the form of at least one wall 314 fixed to the two strips. This configuration allows said walls to delimit different zones 315. For example, by having two transverse walls, three distinct zones are created in the bracelet strand. Cleverly according to this variant, these zones can each be filled with a pocket containing a separate M2 material. This configuration makes it possible to have the band or bands forming the strand which are irregularly deformed, the material M2 of each distinct zone reacting differently. Of course, provision may be made for the constituent bands of the bracelet strand to be made of different materials. This advantageously makes it possible to have a material having more advantageous deformation characteristics for the strip carrying the first material. This means that said strip becomes more easily deformable, resulting in better reactivity to the expansion of the second material M2. CH 711 917 B1 In a third embodiment, the stress is applied to the first material M1 by means of a mechanical device 14. This mechanical device is arranged to apply, at the request of the user, a stress on the first material and thus modify its dimensions and / or its shape. For this third embodiment, the device 14 allowing the application of a stress on the first material is a pump device. Such a device is similar to that of US patent 5,113,599 filed by Reebok®. Such a device schematically consists of a bag 141 of plastic material provided with a pumping system 142. Such a pumping system 142 comprises a pump 143 and an exhaust valve 144 for inflating and deflating the bag 141 of plastic material. The pump 143 used consists of a hollow bell 143a provided with a hole 143b allowing the air to be filled with the bell. The pump can also be fitted with a non-return valve 145 using a pair of sheets 145a of plastic material which can be welded via a welding point, to a wall of the pump 143. These sheets are folded substantially in the shape of a U and allow air to pass only in the direction of the pump towards the pocket. To release the air from the pocket, an outlet or exhaust valve 144 is used. The outlet valve as shown in fig. 9 can comprise a piston 144a comprising a spring 144b which biases the piston in the closed position. A flange 144c around the periphery of the piston rod 144a prevents air from leaking between the piston and the outlet fitting because the flange is biased in the closed position and in contact with said fitting. To release air from pocket 141, the piston is depressed by the user so that air escapes around the piston rod. This pump system 142 can then be directly arranged at the pocket 141 or be dissociated and be connected to said pocket via a channel. The direct arrangement at the level of the pocket 141 or at the level of the channel is possible by independently manufacturing the pump system 142 on a plastic base. This plastic base will be welded or glued to the pocket or channel, these being provided with an opening as shown in fig. 9. In a first alternative embodiment visible in FIG. 10, the covering element 10 and the pocket 141 form only one element. In a nonlimiting example, the covering element 10 is a strand 31 of bracelet 3. Such a strand, as described above, comprises two bands 3100 made integral and forming a hollow space 3102 between them. This hollow space 3102 then forms an air pocket. One of the two strips 3100, preferably the strip that the user will see while carrying the portable object, will be provided with the pump system 142 as described. For this, one of the two bands 3100, preferably the band which will be visible to the user, will be equipped with an opening 3103. This opening 3103 is provided for the establishment of the pump system 142. That -this is arranged on a base 142a which will be welded or glued to the band 3100 of the bracelet strand 31 at the opening 3103. Thus, the user can act on the pump system 142 to inflate or deflate the strand bracelet. However, the latter is used to carry the first material M1, that is to say the material capable of reflecting different wavelengths depending on the stress applied to it. This first material M1 is in the form of a more or less thick film deposited on at least one of the two strips 3100 forming the strand 31 of the bracelet, but it can also be provided that the first material M1 forms an integral part of the material. in which the strip or strips 3100 are produced. In a preferred configuration (not shown), for reasons of ease of process, said first material M1 comprises an opal film encapsulated in two films of transparent thermoplastics (TPU type) and the material M1 constitutes at least one of the strands 3100. In this way the production of the bag and its plastic pump system can be easily carried out by a few heat-sealing operations. Consequently, when the user acts on the pump system 142, he can then increase or decrease the air pressure in the bracelet strand 31. If the pressure increases, the stress exerted by this increase in pressure is transmitted to the bands forming the bracelet strand 31 until causing a deformation of one and / or the other of the bands 3100. This deformation is transmitted to the first material M1 which therefore sees its structure change resulting in the reflection of a different wavelength. By acting on the exhaust valve 144 of the pump system 142, the air is evacuated from the bracelet strand 31 which reduces the pressure inside of it. This results in a reduction of the stress exerted on the strips 3100 which then resume their initial shape. The first material M1 then resumes its initial structure and therefore reflects its original wavelength. Of course, provision may be made for the bands 3100 constituting the strand 31 of the bracelet to be made of different materials. This advantageously makes it possible to have a material having more advantageous deformation characteristics for the strip carrying the first material M1. This results in said strip 3100 becoming more easily deformable, resulting in better reactivity to pumping. In a second visible alternative embodiment, the covering element 10 and the pocket 141 are dissociated. In a nonlimiting example, the covering element 10 is a bracelet strand 31. Such a strand, as described above, comprises two bands 3100 made integral and forming a hollow space 3102 between them. Advantageously according to this alternative, the hollow space 3102 between the two strips 3100 is used as housing. This housing is used to accommodate a pocket 141 which can be inflated and deflated and provided with an opening 3103 for housing the pump system 142 as described above. The principle is therefore that the activation of the CH 711 917 B1 pump 143 makes it possible to inflate or deflate the pocket 141 causing its deformation and that of the bands 3100 forming the bracelet strand. According to a first solution visible in FIG. 11, one and / or the other band 3100 forming the bracelet strand carries the first material M1 as described for the first alternative embodiment. Thus, the inflation or deflation of the pocket 141 results in the application or not of a constraint on the bracelet strand which is deformed or not. According to a second solution visible in FIG. 12, it is expected that the pocket 141 is the element which carries the first material M1. This first material M1 is in the form of a film deposited on the pocket but it can also be provided that the first material M1 is an integral part of the material in which the pocket 141 is made. Cleverly, at least one of the bands 3100 of the bracelet strand 31 comprises openings 3103. These openings 3103 allow the user to see the pocket 141 reflecting a certain wavelength through the band or bands 3100 forming the bracelet strand 31. Furthermore, when the pocket 141 inflates under the action of the pump 143, the pocket 141 can be partially inserted into the openings 3103 and therefore deform locally. This local deformation therefore causes a local stress on the first material M1 and therefore a modification of the wavelength that it can reflect. In a third alternative embodiment, visible in FIG. 13, the covering element 1, here a bracelet strand 31, consists of a recess 400 and a flexible film 410 made of the material M1 or carrying the material M1. The hollow / film assembly constitutes the pocket 141. Similarly to the first alternative embodiment, a pump system 142 can then be arranged at the level of the pocket 141 so as to inflate or deflate the pocket 141 and therefore to apply a geometric constraint on the material M1 and therefore to modify its coloring. In a preferred version of this alternative for reasons of ease of process, the recess 400, the pump system 142 and the flexible film 410 sealing the pocket are made of thermoplastic. More preferably, the recess 410 is incorporated into a strand 31 of the bracelet obtained by injection. The M1 material consists of an opal polymer film encapsulated between 2 sheets of thermoplastics. In this way, the covering element 1 can be obtained by simple assembly operations (ultrasound, heat sealing, or the like). In an exemplary embodiment visible in FIGS. 14 and 15, the covering element 1 is a watch dial 41, the portable object being a watch comprising a watch movement 25. In this case, provision is made for having a pocket 141 located under the dial 41, this pocket 141 can be produced from two heat-sealed half-pockets. This possibility of using two half-pockets makes it possible to have a pocket 141 produced with different materials such as to have half-pockets with different rigidities in order to have a half-pocket serving as a support and a half-pocket called active which will easily deform. This pocket 141 can then be placed in the housing formed by the dial 41, the middle part 20 and the plate of the watch movement. This pocket 141 is then supplied with air via a pumping system 142 placed outside, at the level of the watch case 2 so that it can be handled by the watch wearer. Ultrasonic bonding or a joint allows this pumping system to be assembled tightly to the watch case. The pumping system is connected to the bag via a channel 146. This channel 146 is an integral part of the bag 141 and is created by the addition of circular welds to reduce the thickness of the inflated area. The dial 41 has holes 41a so that the pocket 141 is visible to the wearer of the portable object. The part of the pocket visible to the wearer is the part which supports the first material M1. Thus, when the wearer acts on the pumping system, the bag inflates or deflates and the M1 material undergoes a stress which makes it change the wavelength of the light it reflects. In a preferred version of the invention, the pocket carrying the M1 material is located in a window or an openwork dial to leave the pocket visible. When the pump is activated, the pocket carrying the M1 material inflates, the opal pattern changes color and appears in relief in relation to the rest of the dial. For all of the embodiments of the invention, it is possible to modify the base color and the color variation, by playing on the very structure of the opal material constituting M1 during its manufacture. Indeed, as described in the literature, the refractive indices, the size and the spacing of the components of the opal material, make it possible to control its original color and its colors after mechanical stresses. It will be understood that the embodiments described above are not limited to a bracelet or dial but can also be applied to a watch case, a middle part, a bezel, a crown, a push button, a clasp or a buckle, or an ice cream. It will be understood that various modifications and / or improvements and / or combinations obvious to a person skilled in the art can be made to the various embodiments of the invention described above without departing from the scope of the invention defined by the appended claims. It will be conceivable that, in the first embodiment, the covering element is made of a material which absorbs water. Such a configuration allows the covering element to swell with humidity and therefore causes the appearance of a stress on the first material. Also be applied to a watch case, a middle part, a bezel, a crown, a pusher, a clasp or a buckle, or even a crystal. CH 711 917 B1 It will be understood that various modifications and / or improvements and / or combinations obvious to those skilled in the art can be made to the various embodiments of the invention described above without departing from the scope of the invention defined by the appended claims. It will be conceivable that, in the first embodiment, the covering element is made of a material which absorbs water. Such a configuration allows the covering element to swell with humidity and therefore causes the appearance of a stress on the first material. Claims 4. covering element according to claim 1, characterized in that the device (14) making it possible to modify the stress applied to said first material comprises a second material (M2) capable of seeing its volume vary as a function of a physical quantity , said second material comprising a flexible element carrying the first material (M1). 5. covering element according to claim 4, characterized in that the support (12) comprises at least one recess (40) closed by a film (42 ') provided with the first material (M1) and forming a housing, said housing receiving a capsule (43) composed of two integral half-shells (44) filled with said second material (M2). 6. covering element according to claim 4, characterized in that the support (12) comprises at least one recess (40) closed by a film (42) provided with the first material and forming a housing in which a capsulepiston system ( 43) and the second material (M2) are placed, said capsule-piston system (43) consists of a demicoque (44) and a piston (45) so that the variation in the volume of the second material (M2 ) causes the piston to move. 7. covering element according to claim 5, characterized in that each capsule (43) is filled with said second material, the latter being able to be different from one capsule to another. 8. covering element according to claim 6, characterized in that each recess (40) is filled with said second material (M2), the latter possibly being different from one housing to another. 9. covering element according to claim 5, characterized in that the support (12) is formed by at least two parts (310) integral and fixed together by their periphery so as to leave an empty space (312) between these two parts, at least one of the two parts being flexible and carrying said first material (M1), said space forming the recess (40) containing the second material (M2). 10. covering element according to claim 9, characterized in that the support formed by the at least two parts further comprises at least one structure (313) extending between the two parts in order to delimit at least two zones (315 ), each zone can contain a pocket. To this end, the invention aims to provide a covering element for portable object comprising a support on which is arranged a first material, said first material being chosen capable of selectively reflecting different visible wavelengths as a function of the stress applied, said covering element further comprising a device making it possible to modify the stress applied to said first material. 11. covering element according to claim 10, characterized in that each zone (315) contains a second specific material (M2). This invention advantageously makes it possible to have a covering element which can have its color changed without having to apply a layer of paint or without having to add a colored shell. 12. covering element according to claim 1, characterized in that the device (14) for modifying the stress applied to said first material comprises a pocket (141) made of plastic materials connected to a pump system (142) allowing the inflating / deflating the bag, said pump system comprising a hollow bell (143a) provided with a hole (143b) for filling the air with the bell and a non-return valve (145) comprising a pair of sheets ( 145a) of plastic material each welded to a wall of the pump, said sheets being folded substantially in the shape of a U and arranged with respect to each other so as to allow the air to pass only in the direction of the pocket , said device being associated with a flexible element carrying the first material (M1). In a first advantageous embodiment, the support and the device for modifying the stress applied to said first material are one and the same piece. 13. covering element according to claim 12, characterized in that the support (12) is formed by at least two fixed parts (3100) together by their periphery so as to leave a hollow space (3102) between them, said space allowing the pocket (141) to be housed there, one of the parts forming a strand and being provided with an opening for the pump system (142) to be inserted therein and to be accessible. In a second advantageous embodiment, the support is a bistable metal strip coated with said first material. 14. covering element according to claim 13, characterized in that at least one of the two parts (3100) is flexible and carries the first material (M1). CH 711 917 B1 In a third advantageous embodiment, the device for modifying the stress applied to said first material uses a second material capable of seeing its volume vary depending on a physical quantity, said second material being associated with a flexible element carrying the first material. 15. covering element according to claim 13, characterized in that the at least one of the two flexible parts (3100) is perforated, the first material being supported by the pocket forming said flexible element and visible through the perforation. In a fourth advantageous embodiment, the support comprises at least one recess closed by a film provided with the first material and forming a housing, said housing receiving a capsule composed of two integral half-shells filled with said second material. 16. covering element according to claim 13, characterized in that the support (12) is formed by at least two parts (3100) integral and fixed together by their periphery so as to leave a hollow space (3102) between these two parts, at least one of the two parts being flexible and carrying the first material, said space forming the pocket (141) of the pump system, one of the parts forming the strand being provided with an opening so that the system pump is arranged there. In a fifth advantageous embodiment, the support comprises at least one recess closed by a film provided with the first material and forming a housing in which a capsule-piston system and the material are placed, said capsule-piston system consists of a half-shell and a piston so that the variation in the volume of the second material causes a displacement of the piston. 17. covering element according to one of the preceding claims, characterized in that said first material (M1) is in the form of a film deposited on said support (12, 32). In a sixth advantageous embodiment, each capsule is filled with said second material, which may be different from one capsule to another. 18. covering element according to one of claims 4 to 11, characterized in that the flexible element (42, 42 ', 310, 3100) is formed at least in part by the first material (M1). In another advantageous embodiment, each recess is filled with said second material, which may be different from one housing to another. CH 711 917 B1 In another advantageous embodiment, the support is formed by at least two integral parts and fixed together by their periphery so as to leave an empty space between these two parts, at least one of the two parts being flexible and carrying said first material, said space forming the recess containing the second material. 19. covering element according to one of claims 14 to 16, characterized in that the flexible part (42, 42 ', 310, 3100) is formed at least in part by the first material (M1). 20. Portable object comprising a box closed by a bottom and a cover, characterized in that said portable object comprises at least one covering element (1) according to one of claims 1 to 19. In another advantageous embodiment, the support formed by the at least two parts further comprises at least one structure extending between the two parts in order to delimit at least two zones, each zone possibly containing a pocket. 21. Portable object according to claim 20, characterized in that said portable object is a timepiece comprising a case (2) formed by a middle part (20) closed by a bottom (21) and a crystal (22), and a bracelet (3) fixed to the middle via two pairs of horns (24), said bracelet being provided with at least one bracelet strand, and in that the at least one covering element is chosen from the list consisting of in the middle, the back, the bracelet and optionally a dial. In another advantageous embodiment, each zone contains a second specific material. In another advantageous embodiment, the device for modifying the stress applied to said first material comprises a plastic bag connected to a pump system allowing the bag to inflate / deflate, said pump system comprising a hollow bell provided with a hole allowing the air filling of the bell and a non-return valve comprising a pair of sheets of plastic material each welded to a wall of the pump, said sheets being folded substantially in a U shape and arranged one with respect to the other in order to allow the air to pass only in the direction of the pocket, said device being associated with a flexible element carrying the first material. In another advantageous embodiment, the support is formed by at least two parts fixed together by their periphery so as to leave an empty space between them, said space allowing the pocket to be accommodated therein, one of the parts forming the strand being provided with an opening for the pump system to be inserted therein and to be accessible. In another advantageous embodiment, at least one of the two parts is flexible and carries the first material. In another advantageous embodiment, the at least one of the two parts is perforated, the first material being supported by the pocket acting as a flexible element and visible through the perforation. In another advantageous embodiment, the support is formed by at least two integral parts and fixed together by their periphery so as to leave an empty space between these two parts, at least one of the two parts being flexible and carrying the first material, said space forming the pocket of the pump system, one of the parts forming the strand being provided with an opening for the pump system to be arranged therein. In another advantageous embodiment, said first material is in the form of a film deposited on said support. In another advantageous embodiment, the flexible element is made at least in part by the first material. In another advantageous embodiment, the flexible part is formed at least in part by the first material. The invention further relates to a portable object comprising a housing closed by a bottom and a cover, characterized in that said portable object comprises at least one covering element according to the invention. In an advantageous embodiment, said portable object is a timepiece comprising a case formed by a middle closed by a bottom and a crystal, and a bracelet fixed to the middle via two pairs of horns, said bracelet being provided with at least one bracelet strand, and in that the covering element is chosen to be arranged in the list comprising the middle part, the bezel, the crown, a push-piece, the back, the bracelet and the buckle. In another advantageous embodiment, said timepiece further comprising a timepiece movement providing time information to display means comprising a dial, said dial being part of the list in which the element of dressing is chosen. Brief Description of the Figures The advantages of such a covering element will become clear on reading the description which will follow and on examining the drawing which illustrates it by way of example and in which: fig. 1 and 2 schematically represent a portable object according to the present invention; fig. 3a, 3b and 3c schematically represent a first embodiment of the covering element according to the present invention; fig. 4a to 8 schematically represent a first embodiment of a second embodiment of the covering element according to the present invention; fig. 9 to 13 schematically represent a third embodiment of the covering element according to the present invention; CH 711 917 B1 fig. 14 and 15 schematically represent a version of the covering element according to the present invention in which said covering element is a dial; Detailed Description The present invention relates to a covering element 10 for a portable object 1. Said portable object can for example be a timepiece or watch 1. A conventional portable object comprises a case closed by a bottom and a cover and includes a device enclosed in the housing. Of course, such a portable object can be a telephone or a computer or a touch-sensitive electronic tablet or a piece of jewelry such as a bracelet. It can also be a fashion accessory, such as a bag or glasses. In the case where the portable object is a timepiece, the timepiece 1, visible in Figs. 1 and 2, is for example a wristwatch comprising a case 2. This case 2 is formed by a middle part 20 closed by a bottom 21 and a crystal 22. The timepiece also includes a strap 3. The latter is fixed to the middle part, for example via two pairs of horns 24 at the middle part. Bracelet 3 can be made up of two bracelet strands, each strand being attached to a pair of horns and connected to the other strand via a clasp. The timepiece further comprises a mechanical or electronic watch movement providing time information to display means 4. These display means comprise for example a dial 41 and hands 42 or discs or an LCD screen. The covering element comprises a frame 12 also called a support and made of a metallic or plastic material. According to the invention, the covering element 10 uses a first material M1 capable of selectively reflecting different visible wavelengths according to the stress applied to it. Indeed, there are known materials capable of creating so-called structural colors. These materials are composed of at least two elements with different refractive indices and periodically alternate. The size of the elements is of the order of magnitude of the wavelengths of light so that optical interference phenomena (constructive and destructive waves) are created which lead to the selective reflection of certain wavelengths . Interference phenomena and therefore the colors reflected depend on the size of the periodic lattice and the refractive indices of the at least two selected elements. In nature, there are several examples of structural colors. We can cite: butterfly wings, opals. This type of material (called photonic crystals or opal materials) can also be synthesized by techniques known to those skilled in the art. Synthetic photonic crystals are generally produced by orderly assembling mono-dispersed and bi-material spheres (with 2 different refractive indices at the heart and at the periphery). The ordered network of spheres can be produced: 22. Portable object according to claim 21, characterized in that said timepiece further comprising a timepiece movement providing time information to display means comprising a dial (41), said dial being part of the list in which the covering element is chosen. CH 711 917 B1 CH 711 917 B1 CH 711 917 B1 CH 711 917 B1 CH 711 917 B1 CH 711 917 B1 CH 711 917 B1 CH 711 917 B1