Thermochromic jet ink

The thermochromic jet ink compositions of the present invention undergo color changes in response to changes in thermal environment. Additionally, the inks comprise a combination of solvent, dyes and other essential ingredients which make them suitable for ink jet printing, or printing which is effected without contact between the printing device and the substrate on which the printed characters are deposited. The present invention provides thermochromic ink compositions with enhanced thermal specificity. Moreover, these inks are capable of indicating that the substrate has been exposed to such a specific thermal environment for at least a specified minimal period of time. The color change so effected is irreversible and does not require steam or moisture to leach out selected components of the composition thereby providing a more rapid and distinct color change. The most notable use of the present invention is to indicate completion of pasteurization or sterilization of the substrate or its contents.

BACKGROUND OF THE INVENTION 
The invention of this application relates to ink jet printing compositions 
and their use as color change indicators. The compositions are 
particularly useful as sterilization and/or thermal exposure indicators. 
Ink jet printing is a well known technique by which printing is effected 
without contact between the printing device and the substrate on which the 
printed characters are deposited. Briefly described, ink jet printing 
involves the technique of projecting a stream of droplets of ink to a 
surface and controlling the direction of the stream electronically so that 
the droplets are caused to form the desired printed image on the 
substrate. That technique of non-contact printing is particularly well 
suited for application of characters onto irregularly shaped surfaces, 
including, for example, the bottom of beverage containers. 
In general, ink jet printing techniques impose rigid requirements on the 
ink compositions. To be suitable for use as a jet ink, the compositions 
must meet rigid requirements of viscosity and resistivity, solubility, 
compatibility of components and wettability of substrate; the ink must be 
quick-drying and smear resistant, must be capable of passing through the 
ink jet nozzle without clogging and must permit rapid clean-up of the 
machine components with minimum effort. 
Apart from the jet ink applications of the ink, another area of interest in 
the art is the provision of printable pasteurization or sterilization 
indicators. Such compositions have heretofore usually utilized pigments 
and pigment combinations and have been applied primarily in determining 
whether proper sterilization heat has been applied to objects used in 
medical and surgical procedures. In the food packaging industry, for 
example, where metal cans, plastic pouches or coated metal cans are 
printed, filled with product, sealed and the package subjected to 
conditions of high temperature and moisture during pasteurization or 
sterilization techniques, there is a need for such compositions that are 
printable by either contact or jet ink techniques, that exhibit sufficient 
adhesion to the substrate to withstand the moisture and high temperature 
conditions of sterilization and which, at the same time, undergo a visible 
and permanent color change. 
The provision of such an ink composition which shows a distinct difference 
in color between an unsterilized and a completely sterilized package 
provides for ready visible inspection and permits tracing a particular 
package or packages after they have been processed. The invention thus 
provides a positive and visible indication that sterilization or 
pasteurization has in fact been carried out on the product to which the 
ink is applied. 
Thermochromic inks and paints have been known for some time. 
U.S. Pat. No. 3,471,422 describes a temperature responsive printing ink 
which is responsive to autoclaving. This ink is based on a pigment which 
initially is of one given color and which upon exposure to the elevated 
temperature condition reacts with a sulfur generated by a second component 
of the ink to generate a second color. 
U.S. Pat. No. 4,166,044 describes a temperature responsive jet printing ink 
useful as a sterilization or pasteurization indicator. This ink comprises 
either (1) a colorant which reacts under the elevated temperature 
conditions to change its color, or (2) a combination of colorants which 
together form an initial given color and under steam sterilization 
conditions changes color due to leeching. 
Similarly, U.S. Pat. No. 4,179,397 discloses an extractable/nonextractable 
dye combination for thermotropic ink compositions which meet the rigid 
requirements for ink jet printing techniques. Finally, U.S. Pat. No. 
4,155,895 performs similarly to the thermotropic ink described in U.S. 
Pat. No. 4,179,397 but that rather than relying on the leeching out of a 
more soluble dye at elevated temperature in the presence of steam or water 
the '895 patent utilizes a thermochromic dye that reacts at elevated 
temperature in the presence of steam to undergo a visible color change. 
These prior art inks show a color change upon exposure to a temperature 
level/time period either because (1) there is a change in the molecular 
structure of the color component and the new molecule displays a different 
color, or (2) a color component is selectively extracted to enable the ink 
to show as a new color. 
It is, therefore, a principal object of the present invention to develop an 
ink which indicates by a color change that it was exposed to an 
environment having a temperature in excess of a predetermined level for at 
least a specified minimal period of time. It is an additional object of 
the present invention to exploit the inherently more temperature specific 
process of chemical degradation in this case, of heat-sensitive dyes, to 
show more precisely and dramatically the exposure of an article printed 
with an indicator ink to a predetermined temperature level/time period. 
Another object of the present invention is to make a jet printable ink 
that could display a desired message having a predetermined color on a 
container or package in response to a specific temperature level/time 
period. 
These and other objects of the invention will be apparent from the 
description which follows. 
SUMMARY OF THE INVENTION 
The foregoing objects are accomplished by providing an improved 
thermochromic ink composition particularly adapted for use as a jet ink 
composition which utilizes a unique combination of dyes. More 
specifically, a dye with a desired heat sensitivity is combined with a 
heat resistant dye and a suitable organic carrier to form an ink. Upon 
exposure to the prescribed temperature for a predetermined length of time 
the heat sensitive dye will degrade and the color of the ink will change 
to that of the heat resistant dye. Preferably the color of the message 
printed with the ink composition of this invention should change color 
when exposed to an elevated temperature such as a temperature at or above 
about 116.degree. C. particularly a temperature from about 116.degree. C. 
to about 127.degree. C. for a period of time, such as for a time of about 
15 minutes to about 30 minutes. The advantage of the described ink lies in 
the fact that there is more precise control of the temperature of 
conversion and resulting dramatic color change which occurs upon exposure 
of the ink of the invention to the predetermined temperature/time 
conditions. The temperature control is possible by taking advantage of the 
greater temperature dependence of chemical decomposition over that of 
aqueous extraction, and the color change is accomplished by use of a 
stable dye which exhibits a vividly different color from the combination 
of heat sensitive/heat stable dyes. Thus, a definitive color change occurs 
after exposure to predetermined temperature conditions. Further, because 
chemical decomposition takes place the color change is irreversible. 
DETAILED DESCRIPTION OF THE INVENTION 
As described hereinabove, the ink compositions of this invention comprise a 
thermally degradable/nondegradable combination of dyes, a suitable 
carrier, a resin, and other optional components each of which should be in 
carefully balanced proportion to achieve successful operation of the ink 
in a jet printing apparatus and suitable properties as color-change 
indicators. 
In general, the inks of the present invention exhibit the following 
characteristics for use in ink jet printing systems: (1) a viscosity from 
about 1.6 to about 7.0 centipoises at 25.degree. C., (2) an electrical 
resistivity from about 50 to about 2000 ohm/cm., and (3) a sonic velocity 
from about 1200 to about 1300 meters/sec. Electrolytes can be added to 
adjust the specific resistivity of the ink. Usable electrolytes include 
dimethylamine hydrochloride and hydroxylamine hydrochloride. 
The inks should dry quickly, within about 4 seconds, with good adhesion 
when printed on bare tinplate or aluminum or on polymeric coated tin or 
aluminum cans. It will be understood by those familiar with the art that 
these inks will exhibit a color change in a temperature/time dependant 
manner. That is, as the temperature is increased, the time period required 
to bring about a color change will decrease. Similarly, a longer period of 
exposure of the ink to sterilization conditions will facilitate a color 
change at a relatively lower temperature. The inks should exhibit a 
visible color change when exposed to a temperature at or above 116.degree. 
C. for a length of time of 15 minutes or more. Most preferably the color 
change will occur upon exposure to a temperature of about 116.degree. C. 
for about 15 minutes. 
Suitable heat-sensitive dyes for use in the inks of this invention are 
those which undergo decomposition and hence a visible and permanent color 
change when exposed to the desired temperature, such as temperatures above 
116.degree. C. In addition to such thermochromic characteristics the dye 
must also be soluble in the ink solvent system and compatible with 
components thereof. 
Preferred heat-sensitive dyes are typically selected from such dyes as 
solvent based dyes and basic dyes. One skilled in the art will be able to 
ascertain other such operable dyes through reference to information as 
contained in the Color Index Guide. 
Exemplary of such dyes are those available and known in the art as Basic 
Green 4, commercially available as Victoria Green from Keystone Aniline 
Corp., and Solvent Yellow 56, commercially available as Sudan Yellow 150, 
from BASF Wyandotte Corp. 
It is contemplated that other dyes not specifically enumerated, but of 
sufficient compatibility and solubility, and reactive under the described 
conditions to undergo decomposition and loss of colorant effect, may also 
be employed. 
In general these heat-sensitive dyes are present in the composition in 
amounts varying from about 0.5% to about 2.5% by weight of the 
composition. 
Suitable heat stable dyes for use in this invention are those which are 
unresponsive to wide variations in temperature, are selected so as to be 
visually distinct from the heat-sensitive dye, are soluble in the chosen 
liquid carrier, and are compatible with other components of the system. 
A preferred class of heat-resistant dyes are those which may be classified 
as solvent based dyes having a high temperature of degradation, such as at 
least about 150.degree. C. 
Exemplary of such dyes are those available and known in the art as Solvent 
Red 33, Basic Violet 4, Solvent Blue 98, and Solvent Red 164. These dyes 
are commercially available by their trade names; Luxol Fast Red B (Morton 
Thiokol, [M.T.]), Ethyl Violet (Keystone Aniline Corp.), Automate Blue #8 
(M.T.), and Automate Red 164 (M.T.), respectively. 
It is contemplated that other heat-stable dyes not enumerated but of 
sufficient compatibility, solubility and colorant effect may also be 
employed. 
In general these heat-stable dyes are present in the composition in amounts 
varying from about 1.0% to about 4.0% based on the weight of the ink 
composition. 
Although a variety of solvents may be utilized as the liquid carrier 
system, the principal combination is typically that of a lower aliphatic 
alcohol and a lower aliphatic branched ketone. Preferred is the 
combination of methanol and methyl ethyl ketone. 
Suitable resins for use in this invention include those which are of the 
class of phenolic type resins. One such commercially available resin is 
Varcum 8357 from BTL Specialty Resins Corp. 
Surfactants may also be added to impart desirable characteristics to the 
liquid ink composition. Suitable surfactants include nonionic surfactants 
such as fluorinated alkyl esters (i.e. commercially available as FC-170C 
from 3M Co.). 
Other optional components may be included in the inks of the present 
invention, such as plasticizers. One such suitable plasticizer is that 
commercially available from Monsanto as Saniticizer 8. It also may be 
desirable to add humectants, to prevent the jet tip from drying, such as 
propylene glycol methyl ether. 
The following examples are illustrative of ink compositions according to 
this invention which are effective thermochromic jet inks.

EXAMPLE 1 
The following composition was formulated: 
______________________________________ 
Methyl Ethyl Ketone 29.9% 
Methanol 39.0% 
Varcum 8357 13.0% 
Dimethylamine hydrochloride 
0.6% 
FC-170C (surfactant) 1.5% 
Saniticizer 8 (plasticizer) 
2.0% 
Basic Green 4 (unstable) 
2.4% 
Luxol Fast Red B (stable) 
3.6% 
Propylene glycol methyl ether 
8.0% 
100.0% 
______________________________________ 
The resulting ink had a viscosity of 1.86 cps, a resistivity of 460 ohm/cm, 
and a sonic velocity of 1260 meters/second. 
The ink was jet printed on bare tinplate and aluminum, and coated tin and 
aluminum cans. The ink quickly dried to a black image within four seconds 
and had good adhesion. The labelled articles were subjected to conditions 
of retorting, sterilizing, pasteurizing, or autoclaving at 116.degree. C. 
for 20 minutes. In response to these conditions the printed indicia 
visibly changed color from black to red. 
EXAMPLE 2 
Replacing Luxol Fast Red B in Example 1 with Solvent Red 33, all other 
factors and ingredients being identical the same results were observed 
with the minor exception being that the initial ink composition exhibited 
a blackish-green color which then became red under the above described 
conditions. 
EXAMPLE 3 
Replacing the Basic Green 4 and Luxol Fast Red B of Example 1 with Solvent 
Yellow 56 and Basic Violet 4, respectively, all other factors and 
indredients being identical resulted in a color change of black to 
blue/violet under the prescribed conditions. 
EXAMPLE 4 
Replacing the Basic Violet 4 of Example 3 with solvent Blue 98, all other 
factors and ingredients identical, gave a color change of green to blue 
under the prescribed conditions. 
EXAMPLE 5 
Replacing the Solvent Red 33 of Example 2 with Solvent Red 164, all other 
factors and ingredients being equal, the same adhesion and quick drying 
characteristics were observed and the black ink changed to red under the 
prescribed temperature level/time period exposure.