Patent Publication Number: US-2021178932-A1

Title: Electrically Conductive Urethane Foam

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to U.S. Provisional Application No. 62/581,189, filed on Nov. 3, 2017. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a polyurethane foam having electrically conductive characteristics, More particularly, the invention relates to an electrically conductive urethane foam with improved durability and integrated with an automotive seat permittivity sensor. 
     2. Description of Related Art 
     Various sensor systems integrated into automotive seats are known in the art, One known system for recognizing the occupancy of a seat contains measurement strips of electrically conductive polyurethane passing through a seat cushion and connected to a computer for evaluation. 
     Other sensors using a layer of an electrically conductive cellular foam material or similar materials are also known in the art. An example known tactile sensor comprises, in part, a first layer including an electrode, a second layer of electrically conductive cellular foam material having an electrical resistance which varies in dependence upon deformation of the foam, and an electrically conductive third layer having elastomeric properties. Another known tactile sensor comprises, in part, an upper layer assembled with a lower layer of conductive foam elastomer, a first electrode in contact with a top surface of the upper layer, and a second electrode in contact with a bottom surface of the lower layer. 
     However, all of these examples may have limited durability and performance in an automotive seating application due to the properties of available conductive foams. Current conductive foams may have a poor polyurethane base structure. The current foams may be subject to quick mechanical breakdown such that these foams may be unsuitable for use in automotive seating. These foams may have poor hysteresis and may have thickness distortion. Further, these properties generally may result in poor durability when used for automotive seating and similar applications. 
     It is desirable, therefore, to improve the properties of conductive foam. New novel applications are possible if sufficient conductivity and durability is achieved. It is also desirable to have foam that can be constructed to act as a variable resistor such that as the foam deflects, conductive particles get closer together, causing electrical resistance to decrease. Further, it is desirable to improve the durability of the foam so that variable resistance foam may be employed in high-loading applications such as automotive seating. Finally, it is desirable to integrate an electrically conductive foam as part of an automotive seat sensor system. 
     SUMMARY OF THE INVENTION 
     A high durability electrically conductive urethane foam acts as a variable resistor such that as the foam deflects, conductive particles get closer together, causing electrical resistance to decrease. The electrically conductive foam is further integrated into an automotive seat sensor system. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Advantages of the present invention will be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings wherein: 
         FIG. 1  is side perspective view of a permittivity sensor according to an embodiment of the invention; 
         FIG. 2  is a chart showing electrical performance of a conductive foam according to an embodiment of the invention; 
         FIG. 3  is a side view of an automotive seat according to an embodiment of the invention; and 
         FIG. 4  is a schematic view of first and second embodiments of a seat cushion having a permittivity sensor according to embodiments of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       FIGS. 1 to 4  illustrate an electrically conductive foam integrated into an automotive seat sensor system according to embodiments described herein. Directional references employed or shown in the description, figures or claims, such as top, bottom, upper, lower, upward, downward, lengthwise, widthwise, left, right, and the like, are relative terms employed for ease of description and are not intended to limit the scope of the invention in any respect. Further, cross section views of automotive seat cushion assemblies, flexible printed circuit assemblies, and foam are shown to illustrate their layers and components, but such views are not necessarily to scale. Referring to the Figures, like numerals indicate like or corresponding parts throughout the several views. 
       FIG. 1  illustrates a side perspective view of an electrically conductive foam  10  integrated into an automotive seat permittivity sensor  20  according to one embodiment of the present disclosure. The permittivity sensor  20  provides a way to measure the amount of compression of an automotive seat  30  (shown in  FIG. 3 ) for use in an automotive vehicle. As shown in  FIG. 1 , the permittivity sensor  20  comprises, in part, the electrically conductive foam  10  assembled with a flexible printed circuit  40 . The flexible printed circuit  40  may comprise an electrical circuit  44  typically printed on a suitable base film  48  such as a polyethylene terephthalate film (PET) or a polyimide film (PI). 
     Electrically conductive urethane foam is a polyurethane foam that can exhibit electrically conductive characteristics and is very desirable in a number of applications. Foams can be classified as one of insulative, mild electrostatic conductivity, and conductive. The preparation and processing of the foam determines the electrical conductivity characteristics. No special preparation is required for insulative foam. Mild electrostatic conductivity can be obtained with a foam by embedding and/or coating with electrically conductive particles. These particles are often ionic salts. In order to obtain a conductive foam, very high conductivity is obtained through special particles coated and bonded on the surface of the foam. With this method resistance levels of 3 ohms to 3,000 ohms can be achieved. 
     Some example conductive foams are electrostatic conductors used in the electronics industry. These known conductive foams may be used to protect integrated chips from shorting out from an errant static charge and may be used as a manufacturing aid in the chip manufacturing plants. These foams may be capable of very high resistance levels in excess of 1.0×10 6  ohms and may be surface coated foams and/or may have ionic salts impregnated into the foam. 
     However, current conductive foams may have a poor polyurethane base structure. The current foams may be subject to quick mechanical breakdown such that these foams may be unsuitable for use in automotive seating. These foams may have poor hysteresis and may have thickness distortion. Further, these properties generally may result in poor durability when used for automotive seating and similar applications. 
     If sufficient conductivity and durability are achieved, then new novel applications are possible. As discussed above, these conductive foams can be constructed to act as a variable resistor. As the conductive foam deflects, conductive particles get closer together, causing electrical resistance to decrease. With sufficient durability, variable resistance foam can be employed in high-loading applications (i.e. automotive seating). Since deflection of the foam is directly related to the load that the foam is experiencing, conductive foam of the present disclosure can now be used as a key element in a pressure sensing system for automotive seating. The conductive foam according to the present disclosure has high resiliency, high durability, can be very responsive to pressure changes, and can rebound/respond in the millisecond timeframe. 
     A novel electrically conductive foam according to one embodiment of the present invention is a high durability foam having very low hysteresis, high resiliency, very low compression set, and maintains these properties over the life of a vehicle. Further, this novel conductive foam has high electrical conductivity that varies with deflection. Special conductive particles are securely bonded to the base foam. A unique binder loading keeps the conductive particles attached but does not interfere with foam softness and conductive performance. 
     A preferred embodiment of this novel conductive foam exhibits very high durability with a hysteresis load loss of about 6.4% under constant force pounding and with height loss under about 1% though life testing, when tested per ASTM D3574-11-13. Further, this conductive foam has a high resiliency when tested with 305 mm ball rebound testing per ASTM D3574-11-H. 
     Referring to Table 1 shown below, a preferred base foam formula comprises, in parts per hundred polyol (PPHP), about 95 PPHP high reactivity 6000 molecular weight capped triol polyether polyol such as Voranol® CP-6001 manufactured by DOW®, about 3 PPHP glycerine/sucrose initiated polyether polyol such as Carpol® GSP-355 manufactured by Carpenter®, about 2 PPHP glycerin-initiated polyether polyol such as Carpol® GP-5171 manufactured by Carpenter®, about 0.425 PPHP silicone surfactant such as Tegostab® B-8734 manufactured by Evonik®, and about 0.33 PPHP amine catalyst 1,4-Diazabicyclo[2.2.2]octane solution such as Dabco® 33LX manufactured by Evonik®, about 0.175 PPHP amine catalyst bis-(2-dimethylaminoethyl)ether in dipropylene glycol such as Jeffcat® ZF-22 manufactured by Huntsman®, about 0.055 PPHP delayed-action catalyst made up of bis-(2-dimethylaminoethyl) ether in dipropylene glycol which has been partially neutralized with formic acid such as Jeffcat® ZF-54 manufactured by Huntsman®, and about 1.85 PPHP water. This preferred formula has an (A+B) Ratio @ 100 Index of about 38.6. The preferred base foam formula further comprises diphenylmethane diisocyante having a free NCO content of about 29.4% and a functionality of about 2.15 such as SUPRASEC® 7007 manufactured by Huntsman®. The preferred base foam is manufactured using generally known polyurethane foam manufacturing methods. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Preferred Base Foam Formula 
               
            
           
           
               
               
               
               
            
               
                   
                 Manufacturer 
                 Chemical 
                 PPHP 
               
               
                   
                   
               
            
           
           
               
               
               
               
            
               
                   
                 DOW ® 
                 Voranol ® CP-6001 
                 95 
               
               
                   
                 CARPENTER ® 
                 CARPOL ® GSP-355 
                 3 
               
               
                   
                 CARPENTER ® 
                 CARPOL ® GP-5171 
                 2 
               
               
                   
                 EVONIK ® 
                 TEGOSTAB ® B-8734 
                 0.425 
               
               
                   
                 HUNTSMAN ® 
                 JEFFCAT ® ZF-22 
                 0.175 
               
               
                   
                 HUNTSMAN ® 
                 JEFFCAT ® ZF-54 
                 0.055 
               
               
                   
                 EVONIK ® 
                 DABCO ® 33LX 
                 0.33 
               
               
                   
                   
                 WATER 
                 1.85 
               
               
                   
                   
                 (A + B) Ratio @ 100 INDEX 
                 38.6 
               
               
                   
                 HUNTSMAN ® 
                 SUPRASEC ® 7007 
               
               
                   
                   
                 NCO % 29.4/Functionality 2.15 
               
               
                   
                   
               
            
           
         
       
     
     A preferred embodiment of the conductive foam is prepared by bonding high electrically conductive particles to the base foam using a latex binder according to the present disclosure. The preferred electrically conductive particles have a small sub-micron size with high electrical conductivity such that low loading levels are needed to achieve desired conductivity. An example of preferred conductive particles is very pure carbon black such as Ketjenblack® EC-600JD manufactured by AkzoNobel®. A preferred latex binder is a carboxylated styrene-butadiene emulsion to securely bind the conductive particles to the base foam and which stays flexible through the life of the product. A preferred source of a suitable latex binder is Rovene® 4180 manufactured by Mallard Creek Polymers®. A preferred dispersing agent comprises, in part, 1-methoxy-2-propylacetate and n-butanol. A preferred source of a suitable dispersing agent is Efka® PX 4310 manufactured by BASF®. 
     An electrical bonding agent formula for a preferred embodiment of an enhanced conductive coating comprises: about 100 parts water, about 1.75 parts latex binder such as Rovene® 4180, about 1 part conductive particles such as Ketjenblack® EC-600JD, and about 0.5 parts dispersing agent such as Efka® PX4310. The preferred conductive coating is prepared using generally known manufacturing methods for mixing coatings. 
     The electrically conductive foam according to one embodiment of the present invention is formed by a wet add-on application of the enhanced conductive coating to the base foam until the base foam is thoroughly saturated. The saturated base foam may be pinch-rolled to remove excess coating material. The remaining wet add-on for about a 6 mm thick base foam part is about 259 g/ft 2  (2788 g/m 2 ). The saturated base foam may be oven cured at about 180° F. until dry. 
     As shown in  FIG. 2 , the electrically conductive foam, prepared according to the above disclosure, has improved electrical performance when prepared according to the above-preferred embodiment.  FIG. 2  shows the change in resistance (ohms) as the thickness (mm) of the conductive foam is reduced by compression. This results in a conductive foam suitable for use in automotive applications. Further, electrically conductive foam according to the present disclosure may be integrated into a seat sensor, as described in the following embodiments. 
     One embodiment of an automotive seat has a cushion in which a portion of a foam pad is filled with conductive particles. An example automotive seat  30  shown in  FIG. 3  generally comprises a seat back  50  and a seat cushion  60  and may be configured as a front and/or rear seat of an automotive vehicle. The seat cushion  60  may be constructed with a foam pad  70  in which a portion of the foam pad  70  is filled with conductive particles. As shown schematically in  FIG. 4 , a permittivity sensor  20  may be assembled with the automotive seat  30  and may interface with the foam pad  70 . The permittivity sensor  20  may comprise a layer of conductive foam  80  attached to a PET/PI flexible printed circuit  40  as shown in  FIG. 4 . Preferably, the layer of conductive foam  80  comprises electrically conductive foam having a base foam formula as shown in Table 1 coated with the enhanced conductive coating of the present disclosure. The PET/PI flexible printed circuit  40  may comprise any flexible printed circuit configuration suitable for an intended application. Exemplary flexible printed circuit alternatives  40 A and  40 B are illustrated in  FIG. 4 . PET/PI flexible printed circuits  40 ,  40 A,  40 B generally comprise an electrical circuit  90  printed onto a polyethylene terephthalate film (PET) or a polyimide film (PI)  100 . 
     A first embodiment of a seat cushion  60  according to the present disclosure comprises a first polyurethane (PU) foam slab  110  mounted on an upper surface  120  of the conductive foam pad  80  which forms an upper layer  130  of the permittivity sensor  20 , as shown in  FIG. 4 . The seat cushion  60  further comprises a second polyurethane (PU) foam pad  140  assembled with a lower surface  150  of a lower layer  160  of the permittivity sensor  20 , i.e., assembled adjacent to the lower surface  150  of the PET/PI flexible printed circuit  40 ,  40 A,  40 B. 
     Also shown in  FIG. 4  is a second embodiment of a seat cushion  60 A according to the present disclosure. The seat cushion  60 A comprises a conductive polyurethane (PU) foam slab  170  mounted on an upper surface  180  of a permittivity sensor  20 A. The permittivity sensor  20 A may comprise the layer of conductive polyurethane foam slab  170  attached to a PET/PI flexible printed circuit  40  as similarly shown for permittivity sensor  20  in  FIG. 4 . The seat cushion  60 A further comprises a polyurethane (PU) foam pad  190  assembled with a lower surface  200  of the permittivity sensor  20 A, i.e., assembled adjacent to a lower surface  200  of the PET/PI flexible printed circuit  40 . As in the first embodiment of the seat cushion  60 , the PET/PI flexible printed circuit  40  of the second embodiment of the seat cushion  60 A may comprise any flexible printed circuit configuration suitable for an intended application such as exemplary flexible printed circuit alternatives  40 A and  40 B illustrated in  FIG. 4 . 
     One benefit of the conductive foam, prepared according to the above disclosure, is improved durability combined with high resiliency. A second benefit of the disclosed conductive foam is improved electrical conductivity. The improved electrical conductivity in combination with the high resiliency results in a foam that may be very responsive to pressure changes and can rebound/respond in the millisecond timeframe. These benefits result in a conductive foam which is suitable for automotive seating applications since these applications require high durability as well as high resiliency. 
     An additional benefit is integrating this improved conductive foam, prepared according to the above disclosure, into a seat sensor, such as a permittivity sensor, for an automobile seat. Further, seat cushion assemblies comprising, in part, a permittivity sensor and the conductive foam, are disclosed which benefit from the performance characteristics of the improved conductive foam. 
     The invention has been described in an illustrative manner, and it is to be understood that the terminology, which has been used, is intended to be in the nature of words of description rather than of limitation. Many modifications and variations of the present invention are possible in light of the above teachings. It is, therefore, to be understood that within the scope of the appended claims, the invention may be practiced other than as specifically described.