Cellular retroreflective sheeting

Abstract

Cellular retroreflective sheeting comprises a first film of thermoplastic material which has been formed to define arrays of minute cube-corner reflective elements on one face thereof. The reflective elements are covered with a second thin thermoplastic film and the two films together pass over a patterned roller where a series of sonic welding heads rapidly bond the films together forming small cells of retroreflective cube-corner elements in the pattern of the roller. Because the heat generated in the welding process is localized to the pattern of the roller, cell walls may be formed having narrow thickness yet good bonding of the films is achieved. Further, residual heat does not appreciably transfer to adjacent retroreflective elements, therefore, distortion of adjacent elements is minimized and a sheeting construction is achieved having superior brilliance and durability as well as maximum retroreflective capability over a wide angular range of incident light. Also disclosed is a method for forming such sheeting.

What is claimed is:  
     
         1 . A method of forming flexible retroreflective hermetically sealed cellular sheeting comprising the steps of: 
 providing a first thin continuous thermoplastic film having a plurality of cube-corner retroreflective elements formed on one face thereof;    providing a second thin continuous backing film of thermoplastic material;    juxtaposing said first and second films with said second film covering said retroreflective elements;    passing said juxtaposed films over a patterned roller with said second backing film in contact with said roller, said roller having a continuous array of upstanding closed walls defining cells; and    applying sonic vibration to said juxtaposed films at said roller as said juxtaposed films are passed thereover to thereby form continuous welds between said films in the pattern of the roller, whereby said welds form discrete hermetically-sealed cells of multiple retroreflective elements thereon with air gaps between said elements and said second film and thereby providing a cellular retroreflective sheeting.    
     
     
         2 . The method of  claim 2  wherein said continuous cell walls are approximately 0.010 inch in thickness.  
     
     
         3 . The method of  claim 1  wherein said vibration is applied by a plurality of ultrasonic welding horns disposed lengthwise of the longitudinal axis of said roller.  
     
     
         4 . The method of  claim 4  wherein said welding horns are arranged in staggered relation along the length of said roller.  
     
     
         5 . The method of  claim 4  wherein said welding horns overlap to assure continuous weld.  
     
     
         6 . The method of  claim 1  including the step of directing air against said welding sheeting to thereby cool said sheeting.  
     
     
         7 . The method of  claim 1  including the step of conducting liquid through said roller to thereby control the temperature of said roller.  
     
     
         8 . Cellular retroreflective sheeting comprising: 
 a first film of thin transparent thermoplastic material having a plurality of cube-corner reflective elements formed on one face thereof;    a second film of thin thermoplastic material disposed adjacent to said one face and covering said reflective elements; and    a plurality of continuous welds formed between said films by ultrasonic vibration and bonding said films together, said welds defining a continuous array of closed hermetically-sealed cells each multiple retroreflective elements with air gaps between said reflective elements and said second film.    
     
     
         9 . The sheeting of  claim 8  wherein said second film is an acrylic copolymer.  
     
     
         10 . The sheeting of  claim 8  wherein said second film is white in color.  
     
     
         11 . The sheeting of  claim 8  wherein said welds have a thickness of 0.010 inch or less.  
     
     
         12 . The sheeting of  claim 8  wherein said cells have an area of approximately 0.015 square inch.  
     
     
         13 . The sheeting of  claim 8  wherein said cells are hexagonal in shape.  
     
     
         14 . The sheeting of  claim 8  wherein said welds have a thickness of approximately 0.010 inch or less and said cells have an area of approximately 0.015 square inch.  
     
     
         15 . The sheeting of  claim 14  wherein each cell has approximately 300 cube corner retroreflective elements therein.  
     
     
         16 . The sheeting of  claim 14  wherein said cells are hexagonal in shape.