Abstract:
The invention concerns a triboelectric air filter consisting essentially of a mixture of polypropylene fibers with polymethaphenylene isophatalamide fibers.

Description:
This application is a national stage of International Application No. PCT/CA98/00470, filed on May 13, 1998. 
    
    
     TECHNICAL FIELD OF THE INVENTION 
     The present invention relates to a new triboelectric air filter. 
     TECHNOLOGICAL BACKGROUND 
     It is known that the air filters that are used in the apparatuses where an air filtration is required, are much more efficient if the material forming the filters contain electric charges. 
     Such air filters hereinafter called &lt;&lt;electrostatic filters&gt;&gt; are indeed capable of catching a greater amount of dust than the conventional filters which do not have electric charges. This difference comes from the fact that attractive forces are exerted between the material forming the filters and the dust which, in a natural manner, also contains electric charges. 
     There are presently three methods for obtaining such electrostatic filters. 
     The first one consists in producing a permanent lack of balance at the molecular level, between the electric charges of a given material. The product obtained by this method is usually called a &lt;&lt;electret&gt;&gt;. 
     The second method consists in grafting ions, usually by bombardment, in the material forming the filter. 
     The third method uses the principle of triboelectricity, whereby electric charges are generated when at least two different materials are rubbered together. The product obtained by this third method are usually called &lt;&lt;triboelectric filters&gt;&gt;. 
     By way of example of reduction to practice of this third method, reference can be made to British patent No. 2,190,689 issued in 1989 in the name of National Research Development Corporation, which discloses a triboelectric air filter consisting of a mixture of (1) polyolefin fibers such as polyethylene, polypropylene or ethylene and propylene copolymers, with (2) fibers of another polymer containing hydrocarbon functions substituted by halogen atoms, such as chlorine. In this British patent, the weight ratio of fibers (2) to fibers (1) ranges between 70:30 and 20:80 and preferably between 60:40 and 30:70. 
     By way of other examples of reduction to practice of this third method, reference can also be made to U.S. Pat. No. 5,470,485 issued in 1995 in the name of FIRMA CARL FREUDENBERG, which discloses a triboelectric air filter consisting of a mixture of (1) polyolefin fibers with (2) polyacrylonitrile fibers. Once again, the weight ratio of fibers (2) to fibers (1) ranges between 70:30 and 20:80. 
     In all cases, several important characteristics distinguish the electrostatic filters from each other. These characteristics include the amount of electric charges, the intensity of the generated electrostatic fields, and time duration of the electrical fields. To be efficient, a filter must ideally have a high number of charges which produce intensive electrostatic fields for a period of time at least equal to the life time of the filter. 
     All these characteristics depend on the method used for producing the charges, from the selection and proportions of the constituting materials of the filters and finally from the conditions of use of the filter. The conditions of use are, due to the existence of a standard method for measuring the efficiency of the filters, implicitly considered as constant. Therefore, this distinction is essentially established at the level of the method of manufacture and the selection of the materials forming the filter. 
     SUMMARY OF THE INVENTION 
     It has now been discovered that excellent triboelectric air filters may be obtained when use is made of a mixture of polypropylene fibers with methaphenylene isophtalamide fibers. 
     More particularly, it has been discovered that triboelectric air filters having an efficiency ranging between 90% and 99% can be obtained in such mixtures, even with relatively low amounts of polymethaphenylene isophtalamide fibers. 
     The selection of this triboelectric couple of fibers commercially available and simultaneously having all the requested characteristics, was not evident to make. As a matter of fact, it is only after observation of anomalies in published lists of triboelectric couples that the Applicant has started an exhaustive study which has shown that the combination of the above mentioned fibers, have higher properties. 
     Therefore, the invention relates to a air filter of triboelectric type, characterized in that it consists essentially of a mixture of polypropylene fibers with polymethaphenylene isophtalamide fibers. 
     DETAILED DESCRIPTION OF THE INVENTION 
     As previously indicated, the triboelectric air filter according to the invention consists of a mixture of (1) polypropylene fibers with (2) polymethaphenylene isophtalamide fibers. 
     This mixture is preferably in the form of a non-woven having a weight ratio of fibers (2) to fibers (1) ranging between 5:95 and 50:50, and even more preferably between 10:90 and 30:70. Therefore, as can be noted, the weight ratio of fibers (2) to fibers (1) in the filter according to the invention is much lower than those proposed in the two patents mentioned hereinabove. 
     Any kind of fibers (1) or (2) can be used. As polymethaphenylene isophtalamide fibers (2), use will preferably be made of those sold under the trademarks NOMEX®, APYEIL® and TEIJINCONEX®. 
     The weight of the filter and its surface weight (in g/m 2 ) depends on the requested physical properties, such as the pressure loss and filtration efficiency. Typically, the surface mass of the filters should range between 15 and 500 g/m 2 . 
     As previously indicated, the triboelectric air filter according to the invention has a filtration efficiency of about 90% to 99%. The pressure loss is usually in the range of 0.1 and 2 mm of water. 
     Advantageously, these characteristics are stable over the time. 
    
    
     EXAMPLES 
     Tests were carried out on a filter according to the invention identified hereinafter by the symbol F 1 . By way of comparison, similar tests were also carried out on a filter of the same composition and the same surface weight but modified by addition of an antistatic agent. This filter is identified by the symbol F 2 . Further tests were finally carried out on a third filter of the same surface weight available in a market. This known filter is identified hereinafter by the symbol F 3 . 
     
       
         
               
             
               
               
             
           
               
                 TABLE I 
               
               
                   
               
               
                 IDENTIFICATION OF THE FILTERS 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 F1 
                 Filter according to the invention in a form of non-woven having a 
               
               
                   
                 surface weight of 135 g/m 2  and a weight ratio of fibers (2) to 
               
               
                   
                 fibers (1) equal to 10:90. The fibers (2) had a 1.5 denier titled. 
               
               
                   
                 The fibers (1) had a size of 3 deniers. 
               
               
                 F2 
                 Filter identical to F1 with, in addition, an antistatic agent for 
               
               
                   
                 inhibiting electric charges. 
               
               
                 F3 
                 Filter with a surface weight equal to 135 g/m 2 , available on a 
               
               
                   
                 market and produced by a triboelectric process as disclosed in U.S. 
               
               
                   
                 Pat. No. 5,470,485 
               
               
                   
               
             
          
         
       
     
     The results obtained with these different triboelectric filters are as follows: 
     
       
         
               
               
               
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                   
                 Surface 
                   
                 Pressure 
                   
               
               
                 Identification 
                 weight 
                 Efficiency 
                 loss 
                 Quality 
               
               
                 Selection 
                 (g/m 2 ) 
                 (%) 
                 (mm of water) 
                 Factor 
               
               
                   
               
             
             
               
                 F1 
                 135 
                 98 
                 0.5 
                 7.8 
               
               
                 F2 
                 135 
                 46 
                 0.5 
                 0.8 
               
               
                 F3 
                 135 
                 94 
                 0.4 
                 7.0 
               
               
                   
               
             
          
         
       
     
     In the above Table, the quality factor is a numerical value which has been calculated with the following equation        Q   =       -   ln            100   -   Efficiency       100   ×     Lost of  pressure                                  
     The quality factor permits classifying the efficiency of the filters. The higher the value of &lt;&lt;Q&gt;&gt;, the higher is the quality of the filter. 
     As can be seen, the filter F 1  according to the invention has proved to be more efficient that the known filter F 3 . The poor results obtained with filter F 2  confirm that the electric charges have an important influence of the efficiency of the filter. 
     The efficiency of the filter F 1  according to the invention has shown good stability over the time. This filter was used over two years and no significant difference was noted between the initial value of the efficiency and the subsequent measurements made 4 months and 24 months after its manufacture.