Static dissipative mat
RELATED CASE 
     This application is a continuation-in-part of our original application, Ser. No. 243,776, filed Mar. 16, 1981, now U.S. Pat. No. 4,363,071. 
    
    
     BACKGROUND OF THE INVENTION 
     The invention relates to a static-dissipative web construction, suitable for example as a floor mat to enable personnel-accumulated static electricity to safely discharge from a person standing on the mat. 
     Various static-discharge mat constructions have been proposed, ranging from such highly conductive configurations as to permit the hazard of substantially instantaneous discharge, to slow-leaking constructions which exhibit undesirable dependence upon ambient humidity. Between these extremes, U.S. Pat. No. 4,208,696 to Lindsay, et al. describes a multi-layer static-dissipative web wherein an open-weave fabric in the form of cotton scrim is rendered electrically conductive (using carbon in a latex binder) and is interposed between upper and lower layers of relatively low conductivity, to produce mat constructions having an overall volume resistivity between 10 10  and 10 11  ohm-cm and surface resistance in the order of 10 8  ohms per square; Lindsay, et al. predicate their results on the foraminous nature of their conductive open-weave fabric. While the Lindsay, et al. product is in many respects satisfactory, it is prone to delamination, and for many applications an order of magnitude reduction in surface resistance is desirable, i.e., to the order of 10 7  ohms per square. 
     BRIEF STATEMENT OF THE INVENTION 
     It is an object of the invention to provide an improved static-dissipative web or mat of the character indicated. 
     It is a specific object to produce such a web or mat that is inherently not prone to delamination and which exhibits a surface resistance in the order of 10 7  ohms per square, as measured pursuant to ASTM Standard D257-76. 
     Another specific object is to provide such a web or mat construction having the further property of dust collection in the presence of pedestrian traffic thereon, as well as the property of repeated washability with conventional detergent solutions. 
     A general object is to provide such a web or mat construction which is of relative simplicity, which uses readily controllable and available component materials, and which is inherently relatively insensitive to environmental humidity. 
     The invention achieves the foregoing objects by employing a thin continuous film of carbon-compounded polymeric material, such as a bonded conductive flexible plate on the underside of an upper polymeric layer of low conductivity, the thin film having a surface resistance in the order of 10 2  ohms per square, as measured pursuant to said ASTM Standard D257-76. By employing polymeric material of the same nature, e.g., polyvinyl chloride in both these layers, as well as in an expanded conductive cushioning bottom layer, laminar bonding is optimized, and sensitivity to varying humidity is minimized. Also, the continuous nature of all layers, and the continuous nature of their interface bonding, provides a continuous volume within which electrostatic charge may dissipate and distribute over the thin film of the conductive intermediate layer, as distinguished from the discrete paths in which charge dissipation must be channeled in the foraminous-scrim network configuration of Lindsay, et al. A washable dust-collecting property is imparted by compounding with the upper polymeric layer a tackifying resin selected for its ability to remain thoroughly dispersed with the vinyl and plasticizer of the upper layer. 
    
    
     DETAILED DESCRIPTION 
     The invention will be described in detail for a preferred embodiment, in conjunction with the accompanying drawings, in which: 
     FIG. 1 is an enlarged sectional view through a mat construction of the invention; 
     FIG. 2 is an exploded view in perspective, to permit better identification of components of the construction of FIG. 1; 
     FIGS. 3a to 3f are views similar to FIG. 1, to illustrate successive stages in fabrication of the construction of FIG. 1; and 
     FIG. 4 is a schematic diagram to illustrate steps in a continuous process for making the construction of FIG. 1. 
    
    
     In FIGS. 1 and 2, the invention is shown in the form of a static-dissipating web or mat comprising three bonded layers 10-11-12 of differently conductive polymeric material, such as polyvinyl chloride. The top or upper layer 10 and the bottom or lower layer 12 may each be of the same solid-cast construction, but as shown, the lower layer 12 is an expanded version of the same polymeric material; and both layers 10-12 incorporate one or more conductive ingredients to enable each of layers 10-12 to have a volume resistivity in the range 10 7  to 10 12  ohm-cm. The inner or intermediate layer 11 is a thin film of preferably the same polymeric material containing an electrically conductive ingredient such as carbon black and exhibiting a surface resistance in the order of 10 2  ohms per square, as measured pursuant to ASTM Standard D257-76. A highly satisfactory conductive vinyl film for use at conductive layer 11 is known as Condulon, a trademark and product of Pervel Industries, Inc., Plainfield, Conn. 
     In a specific illustrative example, the solid upper and expanded lower layers 10-12 are both of polyvinyl chloride, with added conductive plasticizer, which may be commercially available products known as Markstat AL-15 or di-octyl-adipate (DOA)*, or a combination of the two. If mixed, it is preferred that the proportion by weight of the AL-15 to the DOA be 2:1, their combination accounting for 12 percent of the total dry-ingredient mix. In the illustrative example, the solid top layer 10 was 35 mils thick, the inner layer 11 was 2 mils thick, and the expanded lower layer was 88 mils thick, for an overall thickness of 125 mils. The upper and lower layers each exhibited a volume resistivity of about 10 9  ohm-cm and a surface resistance of 10 8  ohms per square, and the surface resistivity of the intermediate layer was 300 ohms per square. Overall surface resistance of the consolidated mat was measured at the exposed surface of top layer 10 to be 10 7  ohms per square, and the time for discharge of a 5 kV potential was 0.05 second. 
    
     A washable dust-collecting &#34;sticky&#34; or &#34;tacky&#34; property is imparted to the top layer 10 by adding a tackifying resin to the vinyl and plasticizer of the mix from which the upper level is formed. In general, any of the generally available tackifiers may be used, provided that it is selected for ability to remain thoroughly dispersed (i.e., virtually in solution) with the vinyl and plasticizer. We have thus far found it best to employ a pentaerythrital ester of a tall-oil rosin as the tackifying ingredient; an illustrative specific such product is commercially known as Zonester &#34;100&#34;, being a product of tall-oil rosin manufacture, by Arizona Chemical Company, Fairlawn, N.J. In such compounding of Zonester &#34;100&#34; with the other top layer ingredients, the proportion of the tackifier should be in the range 25 to 45 percent of the total weight of ingredients, the preferred mix involving substantially one-third tackifier by weight. For the indicated materials below the lower end of the stated range, the exposed surface of top layer 10 is not sufficiently tacky; and above the upper end of the stated range, the tackifier is excessively present, being susceptible to take-up in shoe material of pedestrian traffic. 
     Continuous manufacture of the described web will be described in connection with FIGS. 3 and 4, commencing with a supply reel 15 of suitable casting web 16 continuously advancing from left to right, in the sense of FIG. 4. The casting web 16 may be a release-coated fabric or paper, and in the latter event the casting surface thereof is preferably embossed (as in FIGS. 3a), for ultimate aesthetic purposes at the exposed upper surface of layer 10. 
     A first casting of liquid-mixed polymeric-coat ingredients including the tackifying component is made at 17 and the same is cured or fused at 18, thereby establishing the solid layer 10 atop the casting paper 16, as shown in FIG. 3b. A separate supply of conductive film for inner layer 11 is available from a reel 19 and is guided for bonded lamination to layer 10, under heat and pressure, at heated squeeze rolls 20, to produce intermediate product shown in FIG. 3c. A second casting of liquid-mixed polymeric-coat ingredients (this time with an expanding component) is then made at 21, so that the developing product appears as in FIG. 3d, wherein the numeral 12&#39; will be understood to designate the as-yet unexpanded liquid coat applied at 17. Passage through an expanding oven 22 enables controlled uniform expansion of the coat 12&#39; to its ultimate thickness, as bottom layer 12, the same being consolidated in a fusing oven 23, with the appearance shown in FIG. 3e. The product is now completed by stripped removal of the casting paper and its separate accumulation at 24, leaving finished product (FIG. 3f) available for reel accumulation at 25. The now-exposed upper surface of the top layer 10 is sticky (tacky) to the touch, with great ability to retain dust and, on contact, to extract dust from shoes, wheel treads, or the like. 
     Static-dissipating vinyl mat material, produced as described is found to meet all stated objects. Static-dissipating conductivity is an order of magnitude (i.e., 10 times) better, that is, a surface resistance of 10 7  ohms per square, as compared with 10 8  ohms per square of a vinyl product of the Lindsay, et al. patent, and delamination is virtually impossible. Ambient humidity and/or water immersion are found to have no significant effect on electrical properties. The inherent capability of Condulon to discharge a 5 kV charge in 0.02 second enables design modification to increase the speed of electrostatic discharge, from the 0.05 second time observed for the described mat, either by creating a less-thick top layer 10 or by increasing the proportion of conductive plasticizer in the low-conductivity layers 10-12. Generally, the thickness range of layer 10 may be between 25 and 50 mils, the thickness range of layer 11 may be between 1 and 5 mils, and the thickness range of layer 12 may be between 25 and 125 mils, expansion being optional and dependent upon ultimate use. 
     The tacky exposed upper surface of top layer 10 is not only effective in removing dust or dirt particles from pedestrian footwear, wheel treads, and the like, but it also lends itself to repeated washing with commonly used detergent solutions, such as Spic &amp; Span and Pinesol, for total regeneration of the tack. And of course, the sticky surface is static dissipative. Use is therefore particularly attractive at entrances to hospitals, electronic &#34;clean rooms&#34;, and the like. 
     While the invention has been described in detail for the preferred embodiment, it will be understood that modifications may be made without departing from the scope of the invention.