Abstract:
The invention concerns a method for making a radiation heating structure comprising a heating film electrically powered to produce Joule heating, a radiating film comprising radiating additives and a thermally insulating film. The insulating film and the radiating film are fixed on either side of the heating film. The structure is obtained by double injection of polymerizable resins in a heating mould, a first resin being filled with radiating additive on the side of the heating film and a second more fluid resin on the side of the insulation.

Description:
FIELD OF INVENTION 
   The invention relates to the field of heating elements, such as radiation heating panels. 
   BACKGROUND OF THE INVENTION 
   Heating structures of this type, which are substantially in the form of a sheet, comprise a heating layer that includes at least one electrical resistor intended to be electrically powered in order to produce Joule heating. This heating layer is advantageously fixed between two reinforcement layers that are preferably electrically insulating. 
   The heating layer is fixed between the two reinforcement layers by injecting a resin that is cured when the temperature is raised, thereby also stiffening the heating structure obtained. 
   To give this structure thermal radiation properties, the injected resin is filled with radiating additives, such as plaster particles. 
   However, the resin thus filled, once cured, does not allow the aforementioned reinforcements and/or electrical resistor to be satisfactorily bonded, and debonding of one element of the heating structure, when in service, has often been observed. 
   The present invention aims to improve the situation. 
   SUMMARY OF THE INVENTION 
   In one embodiment, the present invention provides a radiation heating structure, comprising at least:
         a heating layer comprising at least one electrical resistor intended to be electrically powered in order to produce Joule heating;   a radiating layer, comprising predominantly radiating additives; and   a substantially thermally insulating layer, the insulating layer and the radiating layer being placed on either side of the heating layer.       

   Advantageously, this heating structure is substantially in the form of a sheet, with an insulating face and, opposite it, a radiation heating face. The term “in the form of a sheet” denotes both a plane form and a substantially curved, or even bent, form. 
   The present invention also proposes a process for manufacturing such a heating structure, in which:
         a) a laminate comprising at least the aforementioned electrical resistor and reinforcements is introduced into a mold; and   b) injected into the mold:
           via an opening formed in a first wall of the mold opposite one face of the laminate intended to form the radiating layer, is a first resin that is filled with radiating additives and can be cured in the mold; and   via an opening formed in a second wall of the mold opposite one face of the laminate intended to form the insulating layer, is a second resin that is more fluid than the first resin and can be cured in the mold.   
               

   The insulating character of the thermally insulating layer is advantageously conferred by an insulating sheet that is introduced with the aforementioned laminate into the mold so as to face the second wall via which the more fluid, second resin is injected. Additionally, or as a variant, the second resin may include insulating additives and, despite the presence of such insulating additives, may still be more fluid than the first resin. 
   In one advantageous embodiment, the manufacture of the heating structure is carried out by pultrusion and the aforementioned mold is a pultrusion mold having an entry end and an exit end, between which, in step b), said laminate is made to advance while the first and second resins are being injected. Preferably, this advance is sufficiently rapid to limit any diffusion of the radiating additives into the second wall of the mold. 
   In a preferred embodiment, the respective injection rates of the first and second resins are chosen according to the speed of advance of the aforementioned laminate through the pultrusion mold and so as to limit any diffusion of the radiating additives into the second wall of the mold, while ensuring diffusion of the radiating additives into the heating layer. 
   The object of the present invention is also a mold for implementing the process, which mold comprises:
         a first wall and a second wall opposite said first wall;   first means for injecting a first resin, which can be cured in the mold and is filled with mineral additives, via a first opening in the mold formed in said first wall; and   second means for injecting a second resin, which can be cured in the mold and is more fluid than the first resin, via a second opening formed in said second wall.       

   In a preferred embodiment, this mold is a pultrusion mold and includes, for this purpose, an entry end and an exit end, between which the aforementioned laminate can advance. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other features and advantages of the invention will become apparent on examining the following detailed description and the appended drawings in which: 
       FIG. 1  shows schematically a heating structure S within the meaning of the present invention; 
       FIG. 2  shows schematically a cross-sectional view (along the line of section II-II) of the heating structure S of  FIG. 1 ; 
       FIG. 3  shows schematically a pultrusion installation for the manufacture of heating structures; and 
       FIG. 4  shows schematically the heating structure S advancing through a pultrusion mold  1 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring firstly to  FIG. 1 , the heating structure S has the general form of a substantially curved sheet. The heating structure S is electrically powered via at least one connection module Ml provided on an end edge of the heating structure S. 
   This heating structure S may be intended for heating domestic rooms, as home radiators connected to the electrical mains. In other applications, the heating structure S may be used as a reinforcing structure (such as a reinforcing beam, or else a plinth) in industrial or domestic premises, or even in public places. In such an application, a plurality of heating structures S, in the form of heating panels, may for example be provided, these being joined to one another via electrical connection modules M 1  and M 2 , in order to form the covering of a wall, or else a number of reinforcing beams for a construction, either in industrial premises or in a public place (a bus shelter or the like). 
   The heating structure S may be of not insignificant benefit in other applications, such as heated stadium seats or as home bathtubs (thus allowing water to be kept at the desired temperature). 
   Another particularly advantageous application is in the automobile field. A radiation heating structure of the type shown in  FIG. 1  may be used for demisting a windshield, such a structure forming an integral part of or acting as the dashboard of the passenger compartment of a motor vehicle, or else acting as side reinforcements in the passenger compartment. 
   Referring now to  FIG. 2 , the heating structure within the meaning of the present invention comprises a heating film FC sandwiched between two reinforcement layers C 1  and C 2 . Each reinforcement layer C 1  and C 2  comprises an array FV of glass or carbon fibers, each embedded in respective resins R 1  and R 2 , which cure when the temperature is raised, for example in a pultrusion mold as will be seen later. 
   More particularly, the reinforcement layer C 1  comprises a resin R 1  filled with particles P which act as radiating additives. For example, such radiating additives may be iron, aluminum, wood or vermiculite particles. In an advantageous embodiment, these particles are mineral particles, such as marble particles. In a preferred embodiment, these radiating additives are plaster particles, the plaster having at least the following advantages:
         at high temperature, it releases water, giving the heating structure S a flame-retarding effect;   it is of low cost;   it increases the stiffness of the structure; and   its heat radiation properties give the heating structure S its desired radiating character.       

   Thus, such pulverulent additives, with a high emissivity, give the heating structure S radiation heating properties. The application of such a radiating structure S is advantageous in (but not limited to) open public areas in which a flow of air is regularly circulating and for which convective heating would be prohibitively expensive. In addition, radiation heating provides the feeling of soft heating, with no air mixing, by emission of electromagnetic waves in the infrared range. On receiving these waves, the walls, floors and other elements of a room “convert” them into heat. 
   In the abovementioned applications of the heating structure according to the present invention, it is preferable for the structure S to radiate only via one of its faces F 1  so as to limit the electrical consumption and thus maintain a satisfactory degree of conversion of electrical power into heat. For this purpose, the heating structure S furthermore includes an insulating layer IS on its face F 2 , opposite the radiating face F 1 . For example, the insulator IS may be a sheet of mineral wool, such as glass wool or, preferably, rock wool. 
   The heating film FC contains at least one electrical resistor. For this purpose, said heating film FC may be formed from a plastic film on which one or more resistors have been screen-printed. As a variant, it is also possible to envision using carbon fiber fabric. In yet another variant, there may be an array of conducting wires. In general, it should be pointed out that the heating film FC is formed from one or more types of electrically resistive materials intended to be electrically powered and capable of producing heat by the Joule effect when an electrical current flows through them. 
   Advantageously, the use of a carbon fiber fabric ensures satisfactory impregnation of the resins R 1  and R 2  in which it is embedded, thereby making it possible to achieve good adhesion of the heating film FC in the heating structure S. 
   Thus, in the embodiment in which the heating film is a screen-printed film, it is advantageous to provide openings made in the film. The resins R 1  and R 2  can then interpenetrate during the in-mold injection step. 
   Moreover, it is also possible to provide a heating film FC produced in the form of a fabric of fibers, for example glass fibers, in which fabric an electrically conducting wire is overstitched, or else the fibers of which are impregnated with a conductive polymer. 
   The radiating composite section that the heating structure S thus forms has a first face F 1  with a high radiating power and an insulating second face F 2 , opposite the first one, while the reinforcements FV provide the structure S with satisfactory mechanical strength. The particles P, which are preferably plaster particles and are predominantly in the radiating reinforcement layer C 1 , ensure both a high emissivity and good mechanical strength of the structure S. In  FIG. 2 , it should be noted in particular that the reinforcement layer C 2 , which includes the insulator IS, contains substantially fewer radiating particles P than the reinforcement layer C 1  intended to radiate. In the process for manufacturing the heating structure S within the context of the present invention, the first resin R 1  is initially filled with particles P, in order to form the radiating layer C 1 , whereas the more fluid resin R 2  contains no such radiating additives. 
   A process for manufacturing the structure S, by pultrusion in a preferred embodiment, will now be described with reference to  FIG. 3 . 
   The pultrusion process allows the manufacture of polymer-matrix sections reinforced with continuous reinforcements. The reinforcements, such as carbon or glass fibers or fabrics FV, come from bobbins B placed on supports at the front of the pultrusion machine. Moreover, the heating film FC, in an embodiment in which it is in the form of a carbon fiber fabric, and the insulating sheet IS, in the embodiment in which it is in the form of a rock wool sheet, are placed on supports that give it a freedom of rotation so that all of the bobbins are unwound continuously. Guides and racks  2  orient the fibers, the heating film and the insulating sheet, by placing them under substantially the same tension in order to constitute the backbone of the future composite forming the structure S. Thus presented in an organized form, they are impregnated with resins R 1  and R 2  at the entry of a die  1 , which ensures that the whole assembly is held together and the resins are cured by heating. This die therefore has the shape of a heating mold (hereinafter called the “pultrusion mold”), into which the aforementioned first resin R 1  and second resin R 2  are injected. These first and second resins harden by curing in the pultrusion mold. The various constituents advance along the X axis by means of a traction device  3  located downstream of the pultrusion mold  1 . The station  4  of the pultrusion installation comprises a cutting and ventilation device for thus recovering the heating structure S for which it now remains only to provide one or more connection modules M 1  and M 2  for connecting its heating film FC. 
   Advantageously, the resins injected (arrows R 1  and R 2 ) into the pultrusion mold  1  are thermoplastics. In this embodiment, the station  4  of the pultrusion installation may be preceded by a bending unit for bending the composite section output by the mold  1 , so as to give it a curved or other such chosen shape. For this purpose, polymer matrices intended to form the protection layers C 1  and C 2 , by impregnation of the carbon or glass fibers or fabrics FV, may advantageously be thermoplastic resins of the PBT (polybutylene terephthalate) type or else of the polycaprolactone type, allowing the structure output by the pultrusion mold to undergo a thermoforming operation. 
   Advantageously, the pultrusion makes it possible to obtain shapes of sections that are both plane and curved, or else more complex shapes of solid or hollow cross section. 
   The description now refers to  FIG. 4  in which the laminate LAM of  FIG. 3 , formed by the insulating sheet IS, the heating film FC and the carbon or glass fibers FV, for example in woven form, is fed into the entry end  10  of the pultrusion mold  1 . The laminate, comprising the insulating film IS and the heating film FC, placed among the reinforcement fibers FV, thus penetrates the mold in order to be impregnated with resins R 1  and R 2 . The two resins are then injected (arrows R 1  and R 2 ) via openings  12  and  13  formed in the mold  1  on opposed walls and facing the heating film FC and the insulating sheet IS, respectively. The resin R 2  is a standard resin (of the PBT type or else of the epoxy or other type). It is thus injected without radiating additives, in contact with the thermal insulation IS, into the upper portion of the mold  1 . Satisfactory impregnation is thus guaranteed and better thermal insulation is provided in this region of the heating structure S being formed. The other resin R 1  is injected into a lower portion of the mold  1 . The resin R 1  is more viscous and is filled with radiating additives in order to constitute the radiating matrix of the section. Preferably, the resin R 1  substantially coats the heating film FC, which advantageously has openings in order to promote interpenetration of the two resins R 1  and R 2 . 
   Preferably, the fluid resin R 2  is injected via the opening  13  into an upper wall of the mold  1 , whereas the viscous resin R 1  is injected via the opening  12  placed in a lower wall of the mold  1 , thereby making it possible, through gravity, to limit the contamination of the thermally insulating layer C 2  by the radiating additives. Moreover, the respective flow rates of the resins R 1  and R 2  are controlled according to the speed of advance of the laminate LAM through the pultrusion mold  1 , depending on the radiating additive content of the resin R 1  and depending on the rate of cure of the resins at the temperature of the mold. 
   Typically, for a speed of the laminate through the mold of substantially between 0.5 and 1 m/minute, a flow rate of the fluid resin R 2  of about 0.5 to 1.5 l/minute and a flow rate of the viscous resin R 1  of about 0.5 to 1.5 l/minute for a mass of about 900 kg of radiating additives per m 3  of resin of the thermosetting polyester type are provided. The resins R 1  and R 2 , of the aforementioned type, cure in the pultrusion mold  1  at temperatures of around 100 to 150° C. 
   The preformed heating structure S is withdrawn via the exit end  11  of the pultrusion mold  1  and advances to a bending unit equipped with a press comprising pressing members P 1  and P 2  for giving the structure S a chosen shape by bending, in a preferred embodiment in which the resins R 1  and R 2  are thermoplastics. 
   Finally, the process for manufacturing the heating structure S continues with the fitting of a connection module M 1  in order to electrically power the heating film FC. 
   Of course, the present invention is not limited to the embodiment described above by way of example—it extends to other alternative embodiments. 
   Thus, it will be understood that, in a simplified embodiment of the heating structure S, one of the thicknesses of the reinforcements FV in the layer C 1  or in the layer C 2  may be omitted. However, it is advantageous to keep the electrically insulating reinforcements in the radiating layer C 1 . In this embodiment, a thickness of resin R 2  may be maintained between a thermally insulating sheet IS and the heating film FC without reinforcements FV. 
   In the above embodiment, an insulating sheet IS is introduced into the laminate which is embedded in the resins R 1  and R 2 . In an alternative embodiment, this insulating sheet may be omitted and the insulating character of the face F 2  of the structure is provided by injecting a resin R 2  that is itself filled with insulating additives, such as ceramic particles. The resin R 2 , even filled with such insulating additives, remains more fluid than the resin R 1  filled with radiating additives, such as plaster particles. Of course, it will be understood that the insulating face F 2  of the structure may furthermore comprise both an insulating sheet IS and a resin R 2  filled with insulating additives of the aforementioned type, in applications in which it is advantageous to optimize the insulation of the face F 2  of the heating structure within the context of the invention. These insulating additives have not been shown in the figures for the sake of clarity, but they are predominantly close to the insulating face F 2 . 
   The process described above for manufacturing the heating structure S is advantageously a pultrusion process. As a variant, composite sections for forming the heating structure S may be produced by any other forming technique, such as reaction molding or RIM (Reaction Injection Molding), or compression molding, such as BMC (Bulk Molding Compound) or SMC (Sheet Molding Compound). 
   In particular, within the scope of the present invention, a simple mold for injecting the resins R 1  and R 2  may be provided in which a laminate comprising at least reinforcing fibers FV and a heating film FC are held taut. A viscous resin R 1  filled with radiating additives P and a more fluid resin R 2  are injected into this heating mold via two opposed openings in order to consolidate all of the elements of the structure.