Abstract:
A thermal indicator using indicia formed by opaque ink applied to a thermal imaging material element is described. The ink is substantially the same color as the imaging element background and remains substantially hidden until the imaging element background changes color when heated. Time-temperature exposure indicating labels can be prepared using a printer with direct thermal type and ink jet type print heads.

Description:
RELATED APPLICATIONS 
     This application is a Divisional Application of, and claims priority to, U.S. application Ser. No. 11/365,190, filed Mar. 1, 2006, now U.S. Pat. No. 8,083,423 entitled “THERMAL INDICATORS,” which is hereby incorporated by reference herein in its entirety for all purposes. This application is also related to the following other Divisional Applications of U.S. application Ser. No. 11/365,190, U.S. application Ser. Nos. 13/314,567 and 13/314,714. 
    
    
     BACKGROUND 
     Direct thermal printing is a recognized means of printing quietly without toners or inks. It is a relatively mature technology that has been around for over forty years. Its use by retailers for printing of cash register receipts, mailing labels, etc. is now commonplace. 
     In direct thermal printing, a print head selectively applies heat to paper or other sheet media comprising a substrate with a thermally sensitive coating. The coating changes color when heat is applied, by which “printing” is provided on the coated substrate. For dual-sided direct thermal printing, the sheet media substrate may be coated on both sides. 
     Time-temperature indicators using thermally sensitive color change materials are well known. For example, indicator devices that relay on diffusion of a dye through a polymer are described in U.S. Pat. Nos. 6,214,623; 5,746,792; 5,057,434; and 4,212,153. 
     SUMMARY 
     A thermal indicator using indicia formed by opaque ink applied to a thermal imaging material element is described. The ink is substantially the same color as the imaging element background and remains substantially hidden until the imaging element background changes color when heated. Time-temperature exposure indicating labels can be prepared using a printer with direct thermal type and ink jet type print heads. 
     Other features, advantages and variations of the invention will be apparent from the following description and the appended drawings and claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a thermally sensitive label with hidden opaque printing before exposure to excessive heat. 
         FIG. 2  illustrates the thermally sensitive label of  FIG. 1  following exposure to excessive heat, where the opaque printing has been made visible. 
         FIG. 3  shows schematically a direct thermal printer with an ink jet print head. 
     
    
    
     DESCRIPTION 
     By way of example, various embodiments of the invention are described in the material to follow with reference to the included drawings. Variations may be adopted. 
       FIG. 1  illustrates a thermally sensitive label  10  with hidden opaque printing  20  before exposure to excessive heat. The label  10  comprises a substrate with one or more thermally sensitive coatings, e.g., permitting direct thermal printing  30  on the label  10  in a manner well known in the art. The label  10  may also include printed material such as a logo  40  added by direct thermal printing, inkjet printing, lithographic process, flexographic printing or the like. Conventional direct thermal printing paper can be used for the exemplary labels  10 . 
     In the example shown in  FIG. 1 , the initial background color of the label  10  is white. The hidden opaque printing  20  would match this background color so as to be essentially invisible before the label  10  is exposed to excessive heat. In this example the color of the opaque printing  20  would be white. The visibility of the hidden opaque printing  20  in  FIG. 1  is for illustration only, where again the opaque printing  20  would be substantially invisible in actual practice against the background of the label  10 . The hidden printing  20  could be added to the label  10  by inkjet printing, for example. 
     When the label  10  is exposed to excessive heat, the background color of the label  10  turns dark, as shown in  FIG. 2 , exposing the opaque printing  20  and rendering it visible, e.g., to reveal a message. The illustrated label  10  in  FIGS. 1 and 2  could be used, for example, for safe guarding medication in pill bottles from excessive thermal exposure. The safe guard would be an integral part of the label on the bottle. In this embodiment the opaque printing  20  is a warning message preferably pre-printed on a white direct thermal label  10  using opaque white ink. The white on white printing is initially invisible. When the label  10  is exposed to excessive temperature the entire label images, or turns dark. The initially invisible white printing  20  becomes visible. This is shown in  FIGS. 1 and 2 . The activation temperature when the label  10  turns dark can be selected based on requirements for safe guarding particular medication in containers to which the label  10  is applied. 
     The white warning message or printing  20  may be optimally placed on a portion of the label not thermally imaged by thermal printing  30  or printed with logo  40 . This is depicted in  FIG. 1 . However by adjusting the opacity of the white ink it is possible to place the invisible print  20  on areas  30  of the label  10  that are thermally printed if desired. This can be accomplished by adjusting the opacity of the white to allow the thermally imaged areas  30  to appear gray through the white pre-printing. As long as the thermal printing is sparse an observer will not detect the hidden message  20  before heat activation. 
     The activation temperature for revealing the hidden message  20  can be adjusted by changing the thermal sensitivity of the label  10 . The substantially opaque white message  20  may be applied over a protective layer as desired. Alternatively, or in addition, a substantially transparent protective layer may be applied to the label  10  on top of the printing  20 . The label  10  is not limited to white thermal paper and the hidden message  20  is not limited to white opaque ink. Other colors of paper and inks may be used. Similarly, the label  10  is not limited to black thermal imaging, other imaging colors being possible. 
     In another application of a thermal label or thermal paper  10 , the hidden message  20  could be used as a security feature. When the paper  10  is thermally printed an area can be intentionally thermal printed to expose the hidden print  20 , authenticating the media. 
     The foregoing description above presents a number of specific embodiments or examples of a broader invention. The invention is also carried out in a wide variety of other alternative ways which have not been described here. Many other embodiments or variations of the invention may also be carried out within the scope of the following claims. 
     As shown in  FIG. 3 , a printer  50  for printing of time-temperature indicator labels  10  can be constructed using a direct thermal printing print head  60  to print first indicia  30  on the labels  10 , and an ink jet print head  70  to print second indicia  20  on the labels  10 , using well known print head technologies. The printer  50  includes a platen  80  opposing the thermal print head  60  and the labels  10  are presented for printing on a web moving through the printer  50  along a feed path  90 . Such a printer  50  would comprise a supply of ink (not shown) for the ink jet print head  70 , where the ink is of a color that substantially matches that of the thermal media elements  10  prior to activation due to excessive heat.