Thermochromic Indicators

Abstract

A thermochromic indicator includes a coating on the substrate. The coating comprising a solid crystalline particles having a melting point of 0-130° C. At temperatures below the melting point, the coating is opaque. At temperatures above the melting point, the coating becomes translucent. The change in opacity can be reversible or irreversible.

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         13 . An assembly which comprises
 (1) a substrate which comprises a temperature sensitive product or a packaging material used to contain a temperature sensitive product or a label to be applied to a temperature sensitive product or package, and   (2) a temperature indicating film, layer or coating which   (a) comprises a first component consisting of a side chain crystalline (SCC) polymer which (i) has a crystalline melting point, Tp, of 0-135° C., (ii) is in the form of solid particles having a median particle size greater than 0.03□ and less than 10□, and (iii) is at least partially cross-linked, and   (b) comprises a second component, consisting of air or gas voids, the primary source of light scattering for the temperature indicator, which are mixed with, between and surrounded by the first component, and optionally   (c) a third component, consisting of small amount of binder polymer to (i) improve adherence to a substrate, and (ii) maintain mechanical integrity and void structure of the indicating film or coating, which   (d) undergoes a change in degree of light scattering, manifested by a visible change in opacity or translucency, wherein the change occurs on first heating to a temperature that is less than 5 degrees C. above Tp and wherein the change in the indicator substantially remains when the temperature is reduced below Tp.   
     
     
         14 . An assembly according to  claim 13  wherein the SCC polymer is a copolymer of two or more co-monomers, and which primarily comprise repeating units in which side chains comprise linear polymethylene radicals containing 12-30 carbon atoms. 
     
     
         15 . An assembly according to  claim 13  wherein the SCC polymer is a copolymer comprising repeating units containing n-alkyl groups which contain 12, 14, 16, 18, 20 or 22 carbon atoms. 
     
     
         16 . An assembly according to  claim 13  wherein the SCC polymer is a copolymer which consists primarily of units derived from one or more n-alkyl acrylates with n-alkyl groups containing 12, 14, 16, 18, 20 or 22 carbon atoms. 
     
     
         17 . An assembly according to  claim 13  wherein the temperature indicating film, layer or coating is prepared by drying a liquid coating or ink formulation containing the SCC polymer and binder components suspended or emulsified in water or a mixture of water and/or other volatile liquids. 
     
     
         18 . An assembly according to  claim 17  wherein the binder polymer component is an emulsion polymer with a median particle size less than 0.15 micron and a minimum film formation temperature, MFFT, that is below the Tp of the SCC polymer and, preferably, at least 10° C. below the Tp of the SCC polymer, and ideally below 20° C. so as to promote formation of a continuous and adherent dry film following evaporation of volatile liquids from the liquid coating or ink formulation. 
     
     
         19 . An assembly according to  claim 13  wherein the binder polymer component is a non-particulate water-reducible or water-soluble polymer with a minimum film formation temperature, MFFT, that is below the Tp of the SCC polymer and, preferably, at least 10° C. below the Tp of the SCC polymer, and ideally below 20° C. so as to promote ambient temperature formation of a continuous and adherent dry film following evaporation of volatile matter from the liquid coating or ink. 
     
     
         20 . An assembly according to  claim 13  wherein the SCC polymer is prepared by an emulsion or suspension polymerization process to yield a median particle size at least 0.2μ and at most 10μ, especially from 0.3μ to 5μ, and most especially from 0.5μ to 2μ. 
     
     
         21 . An assembly according to  claim 13  wherein the substrate, in order to add support and durability to the assembly, is a label made from plastic film, paper, or fabric onto which may be applied a thin and otherwise easily damaged temperature indicator coating. This SCC polymer coated substrate, with the help of an adhesive, is subsequently affixed to temperature sensitive materials or the packaging used to contain them and provide better adhesion and durability than applying the temperature indicator coating directly to the material and/or packaging. 
     
     
         22 . An assembly according to  claim 13  wherein the temperature-sensitive product is a vaccine, a pharmaceutical, blood or other sensitive biological or tissue products and wherein the change in the indicator is in the translucency by heating the assembly above Tp. 
     
     
         23 . An assembly which comprises
 (1) a substrate which comprises a temperature sensitive product or a packaging material used to contain a temperature sensitive product or a label to be applied to a temperature sensitive product or package, and   (2) a temperature indicator which   (a) comprises a crystalline polymer or wax which (i) has a crystalline melting point, Tp, of 0-135° C., (ii) is in the form of solid particles having a median particle size greater than 0.03μ and less than 10μ, and   (b) comprises a matrix polymer, wherein the matrix polymer is a different polymer than the crystalline polymer particles and wherein the difference between the refractive index of the crystalline polymer and the matrix polymer is at least 0.03 greater at a temperature below Tp than at a temperature above Tp and the difference between the refractive index of the crystalline polymer and the matrix polymer is not more than 0.03 at a temperature below Tp, and   (d) undergoes a change in degree of visible light scattering, manifested by a visible change in opacity or translucency, wherein the change occurs on heating to a temperature that is less than 5 degrees C. above Tp and wherein the change in the indicator substantially returns to its original appearnce when the temperature is reduced below Tp.   
     
     
         24 . The assembly of  claim 13 , wherein indicator is a crystalline wax emulsion combined with binder polymer. 
     
     
         25 . The assembly of  claim 24 , wherein the indicator is irreversible. 
     
     
         26 . The assembly of  claim 23 , wherein the indicator consists of crosslinked crystalline SCC particles suspended in a fluid matrix. 
     
     
         27 . The assembly of  claim 26 , wherein the indicator is reversible.