Abstract:
A polishing element of the pad type for finishing mineral or organic optical lenses. The polishing element includes a base plate on which fibers are deposited by flocking them onto the surface of the aforesaid base plate in orthogonal fashion, with the aforesaid fibers consisting of viscose and/or polyamide fibers that are affixed onto the base plate by means of an adhesive. The adhesive is based on polyurethane.

Description:
RELATED APPLICATION  
       [0001]     The present application claims priority to French Application No. 03 13541 filed Nov. 19, 2003.  
       TECHNICAL FIELD  
       [0002]     This invention concerns a polishing product element for finishing optical lenses.  
       BACKGROUND OF THE INVENTION  
       [0003]     Optical lenses undergo several transformative steps in order to attain their required geometrical shape and final transparency.  
         [0004]     The surfacing of optical lenses, with a concave or convex face, includes at least three major steps: 
        rough shaping, which makes it possible to attain the general profile,     fine grinding, which makes it possible to smooth the surface, and     polishing, which further refines the surface and imparts transparency to the lens.        
 
         [0008]     Taking into account the diversity of the profiles in optical lenses, specific supports, called “pads,” have been developed that can be mounted in a removable manner on rigid or non-rigid rotating supports, called “tools,” and that are used as components to perform the fine grinding and polishing operations.  
         [0009]     These pads are generally attached directly to the tool via an adhesive interface or else they are held on a material having a very high coefficient of friction, which is in turn attached to the tool via an adhesive interface.  
         [0010]     Lenses are now mostly organic and the difficulties that arise involve not only the technical in the production of the profiles but also the manufacturing lead times for, on the one hand, production on demand within the shortest possible turnaround and, on the other hand, an increase in productivity.  
         [0011]     It is thus necessary to have the possibility to make adjustments in the materials constituting the pads in order to make them appropriate for polishing machines.  
         [0012]     In this regard, the applicant has previously produced pads that are subjects of the French patent FR 2 794 390, comprising a base plate made of a polymeric film, a fabric or non-woven material, consisting of viscose and/or polyamide fibers that are deposited by flocking and are attached by means of an adhesive of the polyvinyl chloride type.  
         [0013]     These pads provide a certain number of advantages, in particular substantial gains in terms of efficiency and polishing time.  
       SUMMARY OF THE INVENTION  
       [0014]     It is the goal of this invention to improve the performance further, especially concerning longevity, by proposing pads that can consecutively polish several optical lens surfaces without impairing the polishing quality or the duration and efficiency of each successive polishing operation performed with the pad before it is finally discarded.  
         [0015]     To this end, the object of this invention is a polishing element of the pad type for finishing mineral or organic optical lenses, which element is intended for mounting on a turning support, comprised of a base plate on which fibers are deposited by flocking them in orthogonal fashion onto the surface of the aforementioned base plate, with the aforementioned fibers consisting of viscose and/or polyamide fibers, which are attached to the base plate by means of an adhesive, characterized in that the aforementioned adhesive is based on polyurethane.  
         [0016]     The base plate preferably consists of a cotton fabric.  
         [0017]     In comparison with known pads, including those according to FR 2 794 390, such pads, when used on tools having a coating made of a plastic foam onto which the pads are affixed by means of an adhesive, have shown a remarkable ability to polish successively a considerably higher number of surfaces with the same pad while using a conventional polishing liquid. Thus, tests have shown that a single pad was capable of polishing about twenty or more surfaces as compared with only a few surfaces at best with a conventional pad, and this in a notably reduced time due to a considerably increased resistance of the fibers to being pulled off, resulting from the association of a base plate made of a cotton fabric with an adhesive based on polyurethane.  
         [0018]     According to another embodiment of this invention, which is even more effective, the base plate consists of a foam, in particular of polyurethane, with the pad then being placed directly on the smooth and hard surface of a tool without its conventional flexible coating and being attached by means of an adhesive. Such a pad makes it possible to polish more than one hundred surfaces with the same efficiency as before, thus confirming the excellent resistance of the fibers to being pulled off.  
         [0019]     The greater longevity of the pad results from the fact that the connection with the tool is much more durable thanks to the polyurethane foam base plate that provides a barrier between the polishing liquid and the adhesive, which barrier is interposed between the pad and the tool and makes it possible to profit, for a much longer time from the excellent anchorage of the fibers obtained via the polyurethane based adhesive, i.e., to polish a much higher number of surfaces with the same pad before the latter detaches from the tool, this detachment being inevitable but the time of which is pushed back thanks to this invention. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]     The present invention will now be described with reference to the drawing in which a single figure shows a diagrammatic cross-section of a pad type element of this invention.  
         [0021]      FIG. 1  depicts a base plate, which is, according to a preferred embodiment, punched out of a cotton fabric. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0022]     The surface mass of the base plate  10  lies, for example, between 200 g/m 2  and 400 g/m 2 , preferably 300 g/m 2 , which corresponds to a thickness of about 0.25 mm.  
         [0023]     The base plate  10  is advantageously punched out in the form of a disc, because the machines that receive it are of the revolving type. The term disc should be taken in its broadest meaning, i.e., a revolving part, but it can of course be of a circular or oval form, with or without petals, with or without indentations.  
         [0024]     This base plate  10  is then flocked by distributing fibers  12  in an adhesive  14 . An excess of the fibers is deposited and aligned in orthogonal fashion on the surface of base plate  10  by processes that are known in this technical field, such as the application of electrostatic fields.  
         [0025]     The fibers are advantageously viscose fibers having a trefoil cross section and a linear density of 5 decitex and a length between 0.3 mm and 1.0 mm, preferably 0.5 mm, which leads to a surface mass between 50 g/m and 120 g/m 2 , preferably 70 g/m 2 . The length is the length of the fibers as they are produced and is thus measured a priori.  
         [0026]     It is also possible to use polyamide fibers, preferably also with a trefoil cross section, with consequently changing parameters but while maintaining the same final parameters.  
         [0027]     The diameter of these fibers lies between 5 and 25 μm, preferably between 15 and 25 μm, for the given lengths.  
         [0028]     In accordance with this invention, the adhesive  14  is a polyurethane based adhesive having a surface mass between 50 g/m 2  and 150 g/m 2 , preferably 130 g/m 2 .  
         [0029]     As an indication of size, the total thickness of the polishing element of the present invention is about 0.50 mm to 5 mm.  
         [0030]     Thus, by combining fibers  12 , a polyurethane based adhesive  14  and a base plate  10  made of cotton fabric, the polishing element of the present invention makes it possible to obtain surprising results, since it is possible to successively polish about twenty surfaces with the same pad while using a conventional polishing liquid, in particular a liquid based on aluminum oxide, as compared with only a few surfaces with a commercial pad of the same diameter and shape. The polishing time is also reduced.  
         [0031]     Finally, a significant gain in productivity has been observed because intervention by an operator in order to change the pad is much less frequent.  
         [0032]     These remarkable results are attributable to the conjunction of the nature of the polyurethane based adhesive  14  and the nature of the cotton fabric base plate  10 , which surprisingly imparts to the fibers a resistance to being pulled off the pad that is significantly higher than that encountered with known pads, including those according to FR 2 794 390, and which makes it possible to perform a much greater number of successive polishing operations with the same pad in less time.  
         [0033]     According to another embodiment of this invention, the base plate  10  consists of plastic foam, preferably of polyurethane foam.  
         [0034]     Whereas the pad with a cotton fabric base plate is used with a tool made of a rigid support coated with an elastic foam on which the pad is affixed using an adhesive in the usual manner, the pad according to this second embodiment is used on a tool without an elastic coating and is affixed directly onto the hard and smooth surface of the aforesaid tool via the adhesive.  
         [0035]     This embodiment makes it possible to substantially improve the longevity of the pad and to thus further increase the ability of the same pad to polish additional surfaces.  
         [0036]     In fact, in the first embodiment described above, the pad is able to polish about twenty surfaces, after which it is discarded. But this discarding is not a consequence of too many fibers having been pulled off, it simply due to the detachment of the pad from the tool because of the degradation of its adhesive connection with the tool because of moisture coming from the polishing liquid.  
         [0037]     In the case of the pad according to the second embodiment, the connection with the tool is much more durable because of the polyurethane foam base plate  10 , which constitutes a barrier between the polishing liquid and the adhesive that is interposed between the pad and the tool and which makes it possible to benefit from the excellent anchorage of the fibers  12  imparted by the polyurethane based adhesive  14  for a much longer time, i.e., to polish a much greater number of surfaces with the same pad before the latter detaches from the tool, this detachment being inevitable, but being retarded.  
         [0038]     As an indication, the performances of pads of this invention were measured under the following test conditions; 
        treated lens: organic lens with an index of 1.5     polishing liquid: aluminum oxide with a particle diameter of 1.3μ;     polishing pressure: 2 bars;     polishing time: 2 min per surface.        
 
         [0043]     To illustrate the resistance of the fibers  12  to being pulled off the pads of this invention, Taber abrasion tests were performed under the following conditions on dry pads according to the first and second embodiments described above: 
        test tool: grinding wheel H 18 (standard grinding wheel);     test pressure: mass of 2×1 kg;     duration: one hundred cycles.        
 
         [0047]     Tests were performed on discs of the same dimensions as conventional pads and pads according to the two embodiments of this invention and showed weight losses of the discs tested due to the pulling off of fibers by the friction of the grinding wheels, which losses were twenty times greater with the conventional pads than with the pads of this invention, the weight losses being identical in the products of this invention on a cotton fabric base plate and on the polyurethane foam base plate.