Abstract:
A breathable moisture barrier suitable for use in enclosing flexible electrodes, foam substrate and other electrical components within a substantially liquid impervious environment without significantly impairing the ability of electrical sensors to accurately measure temperature and humidity variations within the surrounding environment such as the seating compartment of an automobile. It is further disclosed that a breathable polymeric material having a sufficiently high moisture vapor transmission rate (MVTR) will permit water vapor to be transported across the barrier and quickly reach equilibrium within the enclosed seat sensor mat assembly and to allow the humidity compensation of the occupant sensing system to function correctly. The breathable moisture barrier of the present invention provides a sufficiently high MVTR to allow rapid humidity equilibration within the sensor mat so as to better track rapid changes in the passenger compartment environmental conditions upon starting the car and applying either air conditioning or heating.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
   This application claims priority to U.S. Provisional Patent Application No. 60/332,077, filed Nov. 20, 2001. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates generally to a breathable, moisture-seal fabric and, more particularly, to a water impervious, breathable moisture barrier enclosing a flexible electrode antenna assembly suitable for use in an occupant sensing system. 
   2. Background of the Invention 
   U.S. Pat. Nos. 5,914,610 and 5,936,412 disclose that the ability to determine the position, orientation or presence of a person within a defined space is important in applications ranging from medical treatments to safety and security. For applications where determining the position, orientation or presence of a person within a defined space is important, electrode sensor arrays have been developed to allow automatic monitoring of the defined space. Such sensor arrays and methods for resolving a presence in a defined space are taught in the above-referenced patents. 
   Although the method for resolving a presence or activity in a defined space using a sensor array is known, the ability to adapt those sensor arrays to a particular environment is not addressed in the prior art. Specifically, in uses where the sensor array is used to monitor or detect the presence or activity of a person, additional factors come into play which may greatly impact the ability of the sensor array to provide accurate electrical field readings. For example, an expected use of these sensor arrays and methods, as described in U.S. Pat. Nos. 5,914,610 and 5,936,412, is in an automobile seat for regulating the deployment of airbags. Additionally, commonly owned Patent Convention Treaty Application No. PCT/US01/04057 filed on Feb. 8, 2001, incorporated in its entirety herein, sets forth and describes a flexible electrode antenna having sufficient flexibility, comfort and durability for use in close proximity to an individual such as an automobile seat. However, it must be noted that none of these references adequately address the issue of preventing the absorption of liquids into the sensing system which significantly impairs the ability of the system to make occupant classification determinations. 
   Therefore, there is a need for a breathable moisture barrier enclosing the sensing system (consisting of electrode antennas, foam substrate and other electrical components) to provide a substantially liquid impervious seal while having a high enough level of breathability so as not to interfere with the readings of a temperature/humidity sensor located inside the seat sensor mat assembly. 
   SUMMARY OF THE INVENTION 
   The present invention provides a breathable moisture barrier suitable for use in enclosing flexible electrodes, foam substrate and other electrical components within a substantially liquid impervious environment without significantly impairing the ability of electrical sensors to accurately measure temperature and humidity variations within the surrounding environment such as the seating compartment of an automobile. The invention comprises a breathable polymeric material having a sufficiently high moisture vapor transmission rate (MVTR) to permit water vapor to be transported across the barrier and quickly reach equilibrium within the enclosed seat sensor mat assembly and to allow the temperature and humidity compensation of the occupant sensing system to function correctly. The breathable moisture barrier of the present invention provides a sufficiently high MVTR to allow rapid temperature and humidity equilibration within the sensor mat so as to better track rapid changes in the passenger compartment environmental conditions upon starting the car and applying either air conditioning or heating. 
   In one preferred embodiment, the breathable moisture barrier is a microporous, polypropylene film bonded to a polypropylene nonwoven. In alternative embodiments, other breathable polymeric fabrics or films having a sufficiently high MVTR, a suitable level of durability and other performance attributes required for car seat applications, including comfort, noise, flame retardancy, and thermal stability, may also be used. 
   As used herein, these terms have the following meanings: 
   1. The term “breathable” describes a material that is vapor permeable but liquid and particulate impermeable such as films used in garment and diaper applications. 
   2. The term “microporous” describes a material with a structure that enable fluids to flow through them and has an effective pore size that is at least several times the mean free path of the flowing molecules (from several microns down to 100 Angstroms). 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A more complete understanding of the present invention may be obtained by reference to the following Detailed Description when taken in conjunction with the accompanying drawings in which like reference numerals refer to like parts and wherein: 
       FIG. 1  is a perspective view of a seat sensor mat assembly featuring a foam mat, a flexible electrode antenna and a humidity/temperature sensing element all enclosed within a breathable moisture barrier constructed in accordance with one embodiment of the present invention; 
       FIGS. 2   a-c  illustrate a top plan view, a side elevational view and a bottom view, respectively, of the seat sensor mat assembly in accordance with the embodiment of  FIG. 1 ; 
       FIG. 3   a  is a detailed side elevational view illustrating construction features of the seat sensor mat assembly near a corner of the embodiment shown in  FIG. 2   b;    
       FIG. 3   b  is a detailed side elevational illustrating construction features of the seat sensor mat assembly along a bottom portion of the embodiment shown in  FIG. 2   b;    
       FIG. 4   a  is a cutaway side view of the seat sensor mat assembly taken along section line A—A; 
       FIG. 4   b  is a detailed cutaway side view clearly showing the positioning of the temperature/humidity sensor within the seat sensor mat assembly shown in  FIG. 4   a;    
       FIG. 5  is a series of offset graphs illustrating the effect of varying the breathable area on humidity sensor equilibrium time; 
       FIG. 6  is a series of offset graphs illustrating the effect of using different commercially available polymeric materials having varying levels of MVTR on humidity sensor equilibrium time; and 
       FIG. 7  is a logarithmic chart illustrating the theoretically calculated time required to reach equilibrium at 25° C. and 50% relative humidity as a function of the MVTR. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Although those skilled in the art will readily recognize that multiple unique constructions may be created for use as a breathable moisture barrier in an occupant sensing system, the present invention is described herein primarily in relation to one preferred construction. In particular, the present invention is described herein as a foam mat having flexible sensor electrodes and a humidity/temperature sensor attached thereto, all enclosed within a breathable polymeric fabric laminate to render the apparatus substantially liquid impervious and having sufficiently high MVTR as to not interfere with the functioning of the sensor apparatus. Alternate constructions in addition to those described herein are considered within the scope and the spirit of the invention. 
   In one example of a seat sensor mat assembly for use in automotive applications, the assembly will consist of a number of electrically conductive sensor strips adhesively attached to the top and bottom surfaces of an open celled, flexible polyurethane foam core. Each sensor strip is connected to an electronic control unit which operates the occupant sensing system by measuring impedance changes in an electric field around each of the sensor strips based on an occupant&#39;s size and presence. The seat sensor mat assembly is designed to be incorporated into a hollowed out portion of the bottom cushion of an automobile seat. 
   Since the occupant sensing system uses electric field sensing technology, water or other liquids spilled on the seat could impair the ability of the sensing system to function properly. Water and other conductive liquids prevent the system from sensing occupant changes accurately because the conductive liquid creates coupling and shorts between adjacent conductive strips on the seat sensor mat assembly. 
   One solution to this problem would be to enclose the seat sensor mat assembly entirely within a liquid impervious polymeric film. This is much like wrapping or coating the seat sensor mat assembly with a non-porous, non-breathable polymer film to keep water out. However, this solution presents additional problems by creating very different environmental conditions, namely temperature and humidity, inside the seat sensor mat assembly and in the surrounding passenger compartment of the automobile. The dielectric properties of the materials and gases of the environment surrounding the conductive strips are temperature and humidity dependent. Sealing the entire assembly in a manner which is impervious to both liquids and gases make temperature and humidity compensation of the electric field data significantly more difficult. This solution may ultimately require a temperature and humidity sensor both inside the seat sensor mat assembly and externally within the passenger compartment. Comfortability issues may also arise as air that is trapped inside the sealing material will be unable to escape and will be compressed as the seat is compressed. It is also notable that should any moisture become trapped inside the seat sensor mat assembly, it is quite possible that it will create mildew and other nondesirable conditions inside the assembly. An additional comfort problem may arise from perspiration trapped in the seat trim that is in contact with the occupant. 
   Accordingly, a breathable moisture barrier would provide a number of advantages in an occupant sensing system. A breathable moisture resistant fabric can be used to create a positive seal of the open celled foam preventing water uptake. Additionally, breathability permits a uniform temperature and humidity environment between the seat sensor mat assembly and the passenger compartment. This uniform environmental condition allows a single temperature/humidity sensor to be incorporated into the seat sensor mat assembly to characterize the environment of both the assembly itself and the passenger compartment. Moreover, temperature and humidity compensation of the electric field data is made easier due to the consistent dielectric properties of the materials and gases within the seat sensor mat assembly, seat trim materials and the passenger compartment. A breathable moisture barrier should also prevent any moisture from collecting or mildewing in the seat sensor mat assembly, seat trim materials or seat bun. Although a breathable membrane will usually not allow a sufficient amount of air to escape from the seat sensor mat assembly as a passenger sits down, ventilation holes may be easily incorporated on the bottom surface of the assembly and a transfer adhesive, used to adhere the sensor mat to the seat bun may be further utilized at the edges of the ventilation holes to prevent moisture from entering the assembly. 
   Although any number of breathable moisture barrier films or fabrics are available, one preferred embodiment of the present invention would utilize a microporous polypropylene film laminated to a polypropylene nonwoven and available commercially as PROPORE brand fabric from the Minnesota Mining and Manufacturing Company (3M). See U.S. Pat. Nos. 4,539,256, 4,726,989, 4,902,553 and 5,238,623, incorporated herein by reference, for a more complete discussion on the physical properties and the manufacture of microporous polypropylene films. This particular film/nonwoven laminate offers a number of advantageous properties including relatively high moisture vapor transmission rates (MVTR of about 8,000 g/m 2  24 hours), 345 kPa (50 psi)) water holdout, abrasion resistance due to the presence of the nonwoven layer, good bondability using acrylic transfer adhesives, and relatively low cost due to the use of polypropylenes as the base material in comparison to other moisture resistant breathable fabrics such as GORETEX available from W. L. Gore and Associates and based on polytretrafluroethylene polymers. 
   The film may contain adjuvants such as antioxidants, flame retardants and the like, to the extent that such adjuvants do not critically impair the moisture vapor transmission or mechanical properties of the film. Useful antioxidants include phenolic compounds such as pentaerythritol tetrakis(3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate), available as IRGANOX 1010, from CIBA Specialty Chemicals, and ANOX 20, available commercially from Great Lakes Chemical. Useful flame retardants include halogenated organic compounds, organic phosphorous-containing compounds, and inherently flame retardant compositions. Halogenated organic flame retardants include biphenyls such as 2,2′-dichlorobiphenyls, octabromodiphenyls, and halogenated diphenyl ethers containing from 2 to 10 halogen atoms. Useful organic phosphorus additives include phosphorous compounds such as phosphoric acids, phosphorous-nitrogen compounds, halogenated organic phosphorous compounds and the like. These can be used alone or mixed with e.g., chlorinated biphenyls or antimony oxide. Inherently flame retardant polymers are those which do not support combustion or are self-extinguishing. Examples include poly(vinyl choride), poly(vinylidine chloride), polyimides, polyether ketones, and the like. See U.S. Pat. No. 6,171,689, incorporated herein by reference, for further discussion of flame retardants. The adjuvants may be added individually or mixed together, e.g., flame retardant may be mixed with antioxidant and added together for the sake of simplifying process steps during manufacture. The nonwoven layer may also contain adjuvants such as antioxidants, and the like. 
   In one embodiment of the invention, the composition for the breathable moisture barrier film includes octabromodiphenyl oxide, available commercially from Great Lakes Chemical as DE-79. 
   Referring now to  FIG. 1 , a perspective view of a seat sensor assembly constructed in accordance with the present invention is shown. The seat sensor assembly  10  comprises a polyurethane foam core  20  a number of electrically conductive sensors  30  and a temperature/humidity sensing element  40  disposed within an opening  25  formed in the foam block  20  for measuring temperature and humidity changes both within the assembly and the interior of the automobile itself. The entire assembly  10  is then covered in a substantially liquid impervious, breathable fabric  50 . Optionally, air ventilation holes  60  may be provided in the bottom of the assembly  10  to allow air to escape from the assembly  10  as an occupant sits on the seat cushion of the automobile. The breathable moisture barrier  50  may be adhered to the foam core  20  of the assembly  10  by an acrylic transfer adhesive, not shown. 
   Referring now to  FIGS. 2   a - 2   c , a top plan view, a side elevational view and a bottom view, respectively, of the seat sensor assembly of  FIG. 1  is shown. Note that  FIGS. 2   a - 2   c  further illustrate the positioning of the electrically conductive sensor strips  30 , the temperature/humidity sensor  40  and the air ventilation holes  60  in the seat sensor assembly. 
   Referring now to  FIGS. 3   a - 3   b , detailed drawings are provided to better illustrate features of the side elevational view shown in  FIG. 2   b.  In  FIG. 3   a,  a detailed drawing of a corner portion of the seat sensor assembly constructed in accordance with the present invention is shown. Again, as with  FIGS. 1-2   c,  there is a foam core  20  with electrically conductive sensors  30  and a barrier material  50  attached thereto by a layer of acrylic transfer adhesive  70 . Similarly,  FIG. 3   b  shows the foam core  20 , the conductive electrodes  30 , the barrier material  50  and a layer of transfer adhesive  75  on the underside of the barrier material to adhere it to a seat cushion, not shown. 
   Referring now to  FIG. 4   a,  a cutaway side elevational view taken along section line A—A is shown. This figure illustrates the positioning of the foam core  20 , the opening in the foam core  25 , the temperature/humidity sensor  40  and the barrier material  50 . Similarly,  FIG. 4   b  shows a detailed drawing of a portion of  FIG. 4   a  and better illustrating the positioning of the foam core  20 , the opening in the foam core  25 , the temperature/humidity sensor  40 , the barrier material  50  and the bottom layer of transfer adhesive  75 , again used to attach the seat sensor mat assembly to a seat cushion, not shown. 
   Referring now to  FIG. 5 , a series of offset graphs are presented to show test cell conditions at 70% relative humidity and then changing to 90% relative humidity at a steady temperature of 40° C. For this figure, the test cells were created by cutting an open celled polyurethane foam mat into a 10.2 cm×10.2 cm×1.9 cm (4 in×4 in×0.75 in) block having a central opening extending therethrough and fitted with a temperature/humidity sensor. The control test cell did not have any additional coverings. The upper surface of the moisture resistant test cells were covered with a Hytrel 4056 thermoplastic elastomer available form DuPont and having an MVTR value of about 400 g/m 2  24 hr. The moisture resistant test cells were then sealed on the sides and bottom with a non-breathable polymer film. Additionally, to better investigate the influence of breathable surface area, it was possible to cover a portion of the top surface of the test cell to reduce the breathable area from a 10.2 cm×10.2 cm (4 in×4 in) square to a 5.1 cm×5.1 cm (2 in×2 in) square. 
   As shown in  FIG. 5 , the uncovered foam cell, noted here as environmental chamber conditions, came to equilibrium almost immediately and had a distribution of humidity readings about the set points of 70% and 90% relative humidity. The Hytrel covered foam test cells were then tested in both 5.1 cm×5.1 cm (2 in×2 in) and 10.2 cm×10.2 cm (4 in×4 in) configurations. The 5.1 cm×5.1 cm (2 in×2 in) test cell exhibited large differences from the environmental chamber conditions after the initial change in the relative humidity set points and a long equilibration time to reach the actual environmental conditions. By comparison, the 10.2 cm×10.2 cm (4 in×4 in) test cell exhibited much smaller differences from the environmental chamber conditions after changing the set point and reached equilibrium approximately 4 times faster than the 5.1 cm×5.1 cm (2 in×2 in) test cell. This demonstrates the influence of breathable area on the ability of the humidity sensor to track changes in environmental conditions. 
   Referring now to  FIG. 6 , another series of offset graphs showing the effects of MVTR on humidity sensor equilibrium time is shown. The test cells were produced much like the Hytrel 4056 cells above with 5.1 cm×5.1 cm (2 in×2 in) breathable surfaces of Macromelt 6239, Tegaderm, and Propore KN-2311 having the properties noted in Table 1. 
   
     
       
             
             
             
             
             
           
             
             
             
             
             
           
         
             
               TABLE 1 
             
             
                 
             
             
                 
                 
                 
               Thickness 
               MVTR 
             
             
               Material 
               Purpose 
               Construction 
               μm (mil) 
               (g/m 2  24 hr) 
             
             
                 
             
           
           
             
                 
             
           
        
         
             
               Hytrel 
               Breathable 
               Thermoplastic 
               25.4 (1) 
               400 
             
             
               4056 
               Film 
               polyester 
             
             
               (DuPont) 
                 
               elastomer 
             
             
               Propore 
               Breathable 
               Microporous 
                356 (14) 
               8,000 
             
             
               KN-2311 
               Fabric 
               polypropylene 
               (1.5 mil 
             
             
               (3M) 
                 
               film/ 
               film + 
             
             
                 
                 
               polypropylene 
               12.5 mil 
             
             
                 
                 
               nonwoven 
               nonwoven) 
             
             
               Tegaderm 
               Breathable 
               Polyethylene 
                 38 (1.5) 
               90 
             
             
               (3M) 
               Film 
             
             
               Macromelt 
               Non-breathable 
               Polyamide 
                152 (6) 
               less than 10 
             
             
               6239 
               Film 
             
             
               (Henkel) 
             
             
                 
             
           
        
       
     
   
   Note that at 70% relative humidity, the Macromelt covered sample does not appear to reach equilibrium, the Tegaderm covered sample takes about 500 minutes to reach equilibrium whereas the Propore fabric covered material reaches equilibrium in a matter of several minutes and closely tracks the data of the test chamber itself. These results are again mirrored as the relative humidity is increased to 90% at a time of about 800 minutes. Again, it is clear that the Propore fabric having the greatest MVTR most closely tracks the change in environmental chamber conditions. 
   Referring now to  FIG. 7 , a graph illustrating the theoretically calculated equilibrium time to reach temperature of 25° C. and 50% relative humidity inside a 40.6 cm×40.6 cm×1.9 cm (16 in×16 in×0.75 in) seat sensor mat assembly from various initial environmental conditions as a function of the MVTR is shown. For automotive seat applications, it would be desirable to have equilibration time of less than about five minutes, and more preferably less than about 1 minute, so as to better track rapid changes in the passenger compartment environmental conditions upon starting the car and applying either air conditioning or heating. As shown here graphically, at initial conditions of 80% relative humidity and temperatures of 30° C., 40° C. and 50° C., there is a straightforward logarithmic relationship between the MVTR value and the time required for the seat sensor mat assembly to reach equilibrium conditions of 25° C. and 50% relative humidity. To reach an equilibrium time goal of less than five minutes, the minimum MVTR value falls somewhere between 100 and 1000 g/m 2  24 hours and, more specifically, in a range of about 400 to 600 g/m 2  24 hours. Obviously, a material having a particularly high MVTR value of about 8,000 g/m 2  24 hours can reach equilibrium in significantly less than five minutes or as shown here, about 0.1 minutes. This theoretical value is based on a calculation using only the 40.6 cm×40.6 cm (16 in×16 in) top surface of the 1.9 cm (0.75 in) thick assembly as the breathable area. However, increasing the breathable surface area by, for example, making the bottom surface breathable or increasing the overall volume by, for example, increasing thickness will have a significant impact on the equilibration time. Note that, for ease of comparison, the data shown in  FIG. 7  have been further summarized in Table 2, as shown below. 
   
     
       
             
           
             
             
             
             
           
             
             
             
             
           
         
             
               TABLE 2 
             
           
           
             
                 
             
             
               Equilibration Time to 25 C, 50% RH 
             
           
        
         
             
               Initial Condition: 
                 
                 
                 
             
             
               MVTR 
               30 C, 80% RH 
               40 C, 80% RH 
               50 C, 80% RH 
             
             
               (g/m 2 /24 hours) 
               Time (min) 
               Time (min) 
               Time (min) 
             
             
                 
             
           
        
         
             
               1 
               399.5 
               972.0 
               1877.8 
             
             
               10 
               39.9 
               97.2 
               187.8 
             
             
               100 
               4.0 
               9.7 
               18.8 
             
             
               500 
               0.8 
               1.9 
               3.8 
             
             
               1000 
               0.4 
               1.0 
               1.9 
             
             
               5000 
               0.1 
               0.2 
               0.4 
             
             
               10000 
               0.0 
               0.1 
               0.2 
             
             
                 
             
           
        
       
     
   
   Although preferred embodiments of the invention have been described in the examples and foregoing description, it will be understood that the invention is not limited to the embodiments disclosed, but is capable of numerous rearrangements and modifications of the parts and elements without departing from the spirit of the invention, as defined in the following claims. Therefore, the spirit and the scope of the appended claims should not be limited to the description of the preferred embodiments contained herein.