Abstract:
Film of dielectric material, which film contains gas bubbles preferably of a flat shape. To achieve improved electrical properties, the film is given a large internal unipolar charge, which is created by charging the film by means of an electric field intensive enough to produce partial discharges in the gas bubbles and to cause the charges to move into the dielectric material of the film.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to a dielectric cellular electret film containing flat gas bubbles and to a procedure for its manufacture. The invention also relates to a electromechanical transducer elements used for example in pressure, force, acceleration or vibration transducers having one or several dielectric cellular electret film  
         BACKGROUND OF THE INVENTION  
         [0002]    It is known that charges are formed on the surface of dielectric films when the films are rubbed or reeled or when the films are manufactured by previously known continuous manufacturing methods. The electric charges thus produced may generate electrostatic forces and the films may e.g. adhere to different surfaces. However, such electrostatic forces of attraction are very weak and the charges can easily disappear e.g. due to humidity, so the action of these forces may be of a very short duration. When this is the case, the films will not set straight on a surface and they are easily detached e.g. by air current.  
           [0003]    Previously known are also electrets produced from solid material in which charges are generated in the film surface structure by subjecting it to a DC corona treatment. Such electret films can be utilized for example in various transducer and sensor applications, such as e.g. the electret microphone.  
           [0004]    Producing a powerful charge penetrating deep into a solid film material is very difficult because it is not possible to increase the surface potential of such film material to a sufficiently high level during the corona treatment (charging) because of the leakage current flowing through the film. A leakage current flows because in solid material the resistance of the material is reduced as a function of voltage.  
           [0005]    A further known method is to manufacture e.g. from polypropylene a bubble film which contains small, preferably flat gas bubbles placed in an overlapping arrangement inside the film. Due to the whiteness of the film resulting from the bubble structure, such film is an excellent material for printing. This kind of white film is generally used e.g. as packaging material.  
           [0006]    In a film containing flat gas bubbles, resistivity is not reduced in the gas bubbles and the leakage current is not increased with voltage. Therefore, it is possible during charging to achieve a high electric field, e.g. in the range 100-200 MV/m, which in turn produces partial discharges in the bubbles contained in the film.  
         SUMMARY OF THE INVENTION  
         [0007]    It is an object of the present invention to achieve improved electrical properties in dielectric cellular electret films, where the film has a large unipolar charge created inside the film.  
           [0008]    It is a further object of the present invention to achieve a transducer element to convert mechanical stress, such as force, pressure or mechanical vibrations, into electrical signals.  
           [0009]    It is another object of the present invention to achieve a self-adhesive film made self-adhesive by an electrostatic force, a film whose strong adhesion is based on a large unipolar charge created inside the film.  
           [0010]    It is a further object of the present invention to achieve a procedure for manufacturing a dielectric cellular electret film.  
           [0011]    The invention is characterised in that the dielectric cellular electret film having flat gas bubbles contains partial discharges inside the film produced in the gas bubbles to achieve improved electrical properties.  
           [0012]    The invention is further characterised in that the film has partial discharges produced in the gas bubbles and the charges are caused to move into the dielectric material of the film as a result of that the film is provided with a large internal unipolar charge created by charging the film by means of an electric DC field intensive enough to produce partial discharges in the gas blisters and to cause the charges to move into the dielectric material of the film.  
           [0013]    With the film of the invention, very good electric and acoustic properties, such as sensitivity and adhesion are achieved. Moreover, the film of the invention retains its charge for a very long time, typically several years.  
           [0014]    In addition, because of the charge inside the film, an electrostatic force is active on both sides of the film, which means that films will stick together very firmly. This makes it possible to place several films, typically 15-30 pcs and even as many as 50 pcs, on top of each other like the leaves of a so-called flap board. It is also possible to attach other materials, such as paper, onto the film.  
           [0015]    The most significant advantage provided by this type of self-adhesive film is that no separate adhesive substances, such as glue, or fixing tools are needed to make it adhere to a surface.  
           [0016]    The film of the invention sets straight and clean onto a surface. The film can resist e.g. air currents, humidity and heat without being detached from the surface. In addition, it adheres well even on a rough surface. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]    [0017]FIG. 1 presents a cross-section of a swelled dielectric cellular electret film, and  
         [0018]    [0018]FIG. 2 presents an exploded perspective view illustrating the different components that belong to the transducer according to the invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0019]    To produce a cellular dielectric electret film as provided by the invention, a bubble film  101  made of a plastic material, e.g. polypropylene, of a thickness of a few tens of :m, e.g. 50 :m, is charged by means of a very intensive electric field (e.g. in the range of 100-200 MV/m). During charging, partial discharges occur in the flat lens-like, shredded or cavitated gas bubbles  102  which also may be called voids or cells in the film, and the charges are injected into the plastic material of the film. Due to the large resistivity of the bubble structure and the film material, the charges are well retained within the film. As a result, a film is obtained which has a high net charge and whose strong adhesion is based on a large unipolar charge inside the film.  
         [0020]    Because of the powerful net charge, such a film is especially applicable as a self-adhesive film which can be used e.g. as a medium of visual information e.g. by printing the information directly on the film. As the charge inside the film generates an adhesion force on both sides of the film, information printed on paper or other light material can be attached to the film. It is also possible to pile films on top of each other e.g. on a wall to form a so-called flap board.  
         [0021]    One can also cut holes or patterns of arbitrary shape in the film, so it can be used e.g. as an anti-glare barrier in automobiles or for various decorative purposes.  
         [0022]    Furthermore, it is possible to create charges of opposite sign inside the film so that certain areas of the film contain a positive charge and certain other areas a negative charge, e.g. so that positively and negatively charged strips or areas of any shape placed side by side are formed in the film. When the positive and negative charges are equal, the net charge of the film is zero and the film can also be used between metal sheets without generating in the metal sheets voltage differences according to the capacitance law  
               Δ                 U     =     Q     Δ                 WC               (   1   )                               
 
         [0023]    where ΔU is the voltage, Q is the charge of the film and ΔC is the change in capacitance occurring when the sheets are being detached or piled up. The change in capacitance may be of the order of 1 μF/m 2  and, without the use of strips, the voltage differences thus produced may be several kV and therefore even dangerous to life. One of the applications made possible by such a film is its use a self-adhesive protective film on steel and sheet metal plates.  
         [0024]    To improve its adhesion and printability, the bubble film may also be advantageously subjected to an AC corona treatment. The corona treatment is performed before charging. Possible uses of the film may be further increased by providing one side of the film with a coat of slightly sticker-like material, which allows the adhesion of the film to be adjusted both via AC corona treatment and by adjusting the intensity of the charging process described above.  
         [0025]    Further, the amount of net charge formed inside the film can be adjusted by doping the film with charge binding additives, such as ferrochloride, which generally contain impurities which act as charge centres.  
         [0026]    The film can also used as a transducer element to convert mechanical load, such as force, pressure or mechanical vibrations, into electrical signals.  
         [0027]    The film has film layer foamed to be of full-cell type and it has been oriented by stretching it in two directions. It may be coated at least in part on one side or on both sides with an electrically conductive layer.  
         [0028]    The film can be pressure inflated from a prefoamed plastic film, so that the film becomes a strongly foamed film product, involving a high foaming degree and allowing the thickness of the product to be increased without increasing the amount of plastic material. The term “swelled dielectric electret bubble film” is used herein to refer to a foamed film-like plastic product having a permanent electric charge injected into material.  
         [0029]    [0029]FIG. 2 presents a transducer element for converting mechanical stress, such as mechanical vibrations, into electric signals. The element comprises a transducer part  116 , a conductor part  115 , a connector part  114  and an end part  117 .  
         [0030]    In the embodiment of FIG. 2 the transducer of the invention is composed of sheets  107  and  108  of dielectric film, which may be made e.g. of 0.1 mm thick polyester. On the underside of sheet  107 , a signal electrode  109  is printed by the serigraphic technique using e.g. silver or graphite. Printed around the signal electrode  109  is a ground electrode  110 , which reduces electromagnetic inter- ference noice in the signal. It is noted, however, that this ground electrode  110  is not essential to the structure. Printed by the serigraphic technique on the top surface of film sheet  107  is a ground electrode  111 , which may also consist of aluminium foil or other electrically conductive foil suited for the purpose. Seri-printed on the top surface of sheet  108  is a ground electrode  112  and on the bottom surface also a ground electrode  113 . It should be noted that this ground electrode  113  is not essential for the structure in this and other embodiments of the invention, where the transducer is not a differential transducer. Sheet  108  may also consist of e.g. thin aluminium or brass foil or other electrically conductive foil suited for the purpose. It is noted that the ground electrodes  110 ,  111 ,  112 ,  113  are shorter at the end  114  pointing towards the preamplifier than the signal electrode  109 , whereas at the other end  117  the ground electrodes are somewhat longer than the signal electrode. Instead of being serigraphically printed, the electrodes may also be evaporated e.g. from aluminium onto di-electric films using a mask. Between the sheets  107 ,  108  there is an element  118 . This element  118  is composed of three preferably swelled dielectric electret bubble films  119 ,  120 ,  121 . Injected onto the underside of the topmost film  119  is a negative electric charge. Injected onto the top side of the intermediate film  120  is also a negative electric charge, while a positive electric charge is injected onto its underside. Injected onto the top side of the bottommost film  121  is a positive electric charge. After being charged, the films have been glued together. The bottommost films  121  bottom side may also be provided with a metallic electrically conductive surface, e.g. evaporated aluminium, which is to be noted is not necessary. This electrically conductive surface is possible to have also on topside as well as on one or both sides of films  119 (on topside when ground electrode  110  is not printed)and  120  but it is not recommended. With the charging procedure described, a maximal electric charge density is achieved. From the point of view of operation, it is sufficient to have only the surfaces of the intermediate film  120  charged. Such an element responds only to the pressure generated by the vibration of the strings, not to bending at all.  
         [0031]    The element may also consist of two dielectric electret bubble films, in which element  118  unlike charges of the films  119 ,  121  are placed opposite to each other. Such a structure mainly responds to pressure only and very slightly to bending and is thus applicable for converting the vibrations of the strings  103  into electric signals. By placing the films with like charges opposite to each other, an element mainly responsive to bending is achieved.For operation, it is sufficient that element  118  be composed of only one swelled dielectric electret bubble film.  
         [0032]    Between sheets  107  and  108  there is also a dielectric film  122 , which may be made e.g. of polyester, preferably of the same thickness as the film element  118 . This insulation prevents a short circuit between the signal electrode  109  and the ground electrode  112 . Instead of using a dielectric film  122 , it is possible to provide the bottom surface of film  107  at the area  115  or the top surface of film  108  at the area  115  with dielectric insulation serigraphically printed over the electrode(s) on the surface to prevent short circuit. Between the film sheets  107 ,  108  there is also a dielectric film  123  on the other side of the element  118  at the area  117 , preferably of the same thickness as film  122 . Another possibility is to extend the element  118  consisting of dielectric electret bubble films to the end of area  117 , in which case film  123  is not needed. Similarly, it is possible to extend the element  118  to the end of area  114  as well, in which case film  122  is not needed. At one end  117  of the transducer is a metallic connector  106  mechanically pressed through sheets  107 ,  123 ,  108 , shorting the ground electrodes  111 ,  110 ,  112 ,  113 . At the other end  114  is a metallic connector  124  mechanically pressed through sheets  107 ,  122 ,  108  to connect the signal electrode  109  to a signal processing device. The ground electrodes, which are all thus disposed on the outer surfaces of film sheets  107 ,  108 , are grounded e.g. by pressing them between the halves of the casing of the signal processing device. It is recommendable to use a soft, electrically conductive material in this area between the halves of the casing. The grounding can also be implemented by pressing one of the ground electrodes  111 ,  113  against the circuit board of the signal processing device at a point reserved for it, at which point it is also recommendable to use electrically conductive rubber as mentioned above.  
         [0033]    It is obvious to a person skilled in the art that different embodiments of the invention are not restricted to the example described above but that they may instead be varied within the scope of the claims presented below.