Freeze indicator

A method is disclosed for substantially eliminating undercooling in a liquid cooled below its freezing point which comprises utilizing a nucleating agent system comprising: PA1 (1) a nucleating agent which is a metal compound, insoluble in the liquid, the metal compound and liquid having similar molecular space groupings; and PA1 (2) at least 0.075 wt. % based on the metal compound of a soluble salt of a metal which is the same metal as that of the metal compound. In a preferred embodiment the valence state of the metal of the salt is the same as that of the metal of the metal compound. In another embodiment a metal compound of limited solubility, e.g., less than 1% by weight in the liquid, more preferably about 0.15 to about 0.25 wt. %, is utilized to serve the function of both the insoluble nucleating agent and the soluble metal salt. Where the liquid is water, deuterium oxide or mixtures thereof the preferred metal compound is cupric sulfide and the preferred salt is cupric sulfate.

BACKGROUND OF THE INVENTION 
This invention relates to freeze indicators. More particularly, it relates 
to freeze indicators which may be adjusted to provide precise information 
to the user that a product has been exposed to a certain low temperature, 
usually near the freezing point of water. 
Freeze indicators which utilize the expansion characteristic of water to 
break a frangible ampule are well known in the art; see for example, Smith 
U.S. Pat No. 3,545,400. Once for device shown in the Smith '400 patent is 
exposed to temperatures below the freezing point of water, the water 
freezes into ice and expands causing the ampule to break. After the ice 
was formed and after the surrounding temperature returned to a point above 
the melt point of the ice, the water was absorbed on a dye loaded pad, 
thus giving an indication that the device has gone through a freeze stage 
and back through a thaw stage. 
Problems have arisen in giving an accurate indication of the passage of the 
device through the normal freezing point of water, i.e. 0.degree. C., due 
to the undercooling effect of water which will permit water to remain in 
its liquid state substantially below its normal freezing point as for 
example, as low as -16.degree. C. This problem has been partially overcome 
by the addition of certain nucleating agents to the water. An example of 
this is shown in British Patent No. 1,245,135, issued to Scheller. 
Scheller discloses the technique of adding powdered glass to an ammonium 
chloride solution to avoid undercooling. 
U.S. Pat. Nos. 3,956,153 and 3,980,581, issued respectively to Chadha and 
Godsey, disclose the use of nucleating agents having substantially similar 
space groups to thermal responsive materials used in disposable 
thermometers. Chadha '153, discloses the use of anthraquinone as a 
regenative nucleating agent. 
It is known that over a period of time a nucleating agent will become 
"poisoned". Not wishing to be bound by theory this poisoning effect is 
believed to result either from contamination by the medium into which it 
is incorporated or by some as yet unexplained change with time in the 
crystal structure of the surface of the nucleating agent. The solution to 
this "poisoning" problem which Chadha proposed was to incorporate into the 
thermally responsive material a nucleating agent which is slightly soluble 
in the thermally responsive material at a concentration in excess of the 
solubility. The result is that with each remelt and nucleation cycle a 
fresh surface of nucleating agent is presented which effectively nucleates 
the thermally responsive medium. Of course where the preferred nucleating 
agent for a system is insoluble. The approach of Chada cannot be utilized. 
An improved freeze indicator has been disclosed in U.S. Pat. No. 4,191,125 
to Johnson. That '125 patent discloses a device comprising a water filled 
frangible ampule, a nucleating agent and a surfactant. Suitable nucleating 
agents which are disclosed include cupric sulfide, ferrous sulfide, zinc 
metal, molybdenum sulfide, tungsten sulfide, beryllium aluminum silicate 
and silver iodide, all of which are sunbstantially insoluble in water. 
These insoluble nucleating agents are susceptible to the poisoning effect 
discussed above. 
SUMMARY OF THE INVENTION 
In a freeze indicator which comprises a frangible ampule containing water 
and a nucleating agent which is an insoluble inorganic salt of a metal, it 
has been surprisingly found that the effectiveness of the nucleating agent 
can be maintained over an extended period of time by including in the 
water-nucleating agent mixture a poison inhibitor which is a water soluble 
salt whose cation is the same as that of the nucleating agent. For example 
where the nucleating agent is cupric sulfide, the poison inhibitor can be 
cupric sulfate. Where the nucleating agent is silver iodide the poison 
inhibitor can be silver fluoride. Where the nucleating agent is zinc metal 
the poison inhibitor can be any water soluble zinc compound, e.g., zinc 
chloride.

DETAILED DESCRIPTION OF THE INVENTION 
This invention relates to a method of inhibiting the poisoning of a 
nucleating agent used in a water based freeze indicator, and the 
composition of the nucleating agent and poison inhibitor. The term "poison 
inhibitor" as used in the specification and claims is used as a matter of 
convenience and not intended to describe a mechanism by which the useful 
life of a nucleating agent is extended. In the practice of the invention 
disclosed in U.S. Pat. No. 4,191,125 to Johnson, it was found that while 
any nucleating agent within the scope of the invention would be effective 
immediately after the preparation of the ampules, with time, its 
effectiveness deteriorated as shown by breakage tests. 
To test the effectiveness of a particular nucleating agent a batch of 
several hundred ampules designed to have a particular freezing point would 
be prepared, and divided into lots of fifty. A new lot would be frozen 
each day by holding at the particular temperature for which it was 
designed to break for about one hour. The number of ampules broken would 
be recorded. The tests would be repeated for at least seven days. An 
effective nucleating agent should result in the breakage of fifty out of 
ampules fifty in each test. 
To aid in the understanding of the instant invention the disclosure of 
Johnson, U.S. Pat. No. 4,191,125 will be repeated herein in detail. The 
invention may be more fully appreciated by reference to the drawings. 
Referring now more particularly to FIG. 1 there is provided freeze 
indicator 1 which includes frangible housing 3 which may be made of glass, 
polystyrene or any frangible material, inert to the liquid, which can be 
filed with liquid and sealed. 
Referring now to FIG. 2, frangible container 3 houses a liquid, such as 
water, which undergoes expansion upon freezing, thereby fracturing the 
frangible container when the environment around the indicator passes below 
the freezing point of water. In order to avoid the undercooling effects 
which depress the freezing point of water significantly, a nucleating 
agent is added to the water. The nucleating agent most preferred is one 
which has substantially the same molecular space grouping as the frozen 
water. This provides for faster and more complete crystal growth when the 
environment passes below the freezing point of water. 
Examples of acceptable nucleating agents are cupric sulfide and beryllium 
aluminum silicate. Other acceptable materials are ferrous sulfide, zinc 
metal, molybdenum sulfide, and tungsten sulfide. Also, silver iodine has 
been shown to provide adequate results. 
In order to increase the surface area of contact between nucleating agents 
and the water, a surfactant of wetting agent has also been added to the 
mixture. Suitable surfactants include Atlas G-2127, Tween 80, Ultrawet 601 
and Triton X-100, all of which are commercially available. Tween 80, 
represented by the chemical expression polyoxyethylene 20 sorbitan 
monoleate. Triton X-100 is a para-isooctyl polyethylene glycol phenyl 
ether containing an average of ten ETO moieties per molecule. 
In order to fine-tune the device so that an indication is given for a 
predetermined temperature, an amount of deuterium oxide may be added to 
the water. Deuterium oxide (D.sub.2 O) has a normal freeze point around 
4.degree. C. By adding the proper amount of D.sub.2 O to H.sub.2 O, the 
freeze point of the mixture may be raised accordingly to accommodate 
particular needs. Even by using the above mentioned nucleating agents, it 
has been found that the device, without D.sub.2 O added, freezes at about 
-4.degree. C. By formulating a mixture of 98% D.sub.2 O and 2% H.sub.2 O, 
the freeze point is raised to approximately 0.degree. C. Various freeze 
points between 4.degree. C. and 0.degree. C. may be provided by adding 
lesser and lesser amount of D.sub.2 O below 98%. Since frozen deuterium 
oxide has the same molecular space groupings as frozen water, the same 
nucleating agents as mentioned above may also be used to overcome the 
undercooling affect. 
As can be seen in FIG. 2, the frangible ampule is protected from damage 
before freezing by a semi-rigid plastic blister 4. This blister 4 has 
various ridges 5 which provide mechanical strength to the device so that 
the ampule will not break if handled roughly. The blister can be vacuum 
formed of polyvinyl chloride, or any other suitable deformable plastic 
material which is inert to the liquid system of the indicator. 
Immediately below the frangible ampule is indicator pad 6 which is a layer 
of absorbent material such as Whatman 1 MM paper, available from Whatman 
Company. A water soluble dye 7 is printed on the backside of indicator pad 
6. When the ampule 3 is broken, an amount of unfrozen water is released 
from the ampule and poured onto pad 6, and is absorbed down to dye layer 
7. The water will dissolve the dye, causing the dye to migrate to the top 
of the pad nearest the ampule. Since blister 4 is an optically clear 
material, a visible indication of freeze is then provided. 
In most prior art freeze indicators, a thaw must occur in order to 
determine that the environment had ever undergone freeze due to the fact 
that upon freezing, the liquid, such as water, becomes solid and cannot 
possibly wet an indicator means. 
It is uncertain as to why in Johnson's device, this wetting occurs 
immediately upon freezing and breakage of the ampule. However, it is 
possible that there is only sufficient solidification of a portion of the 
water to break the ampule, but enough liquid remaining present to give an 
immediate color change on the indicator paper. Also, as the water freezes 
and expands into ice, the pressure in the remaining part of the ampule 
increases, causing a depression of the freezing point of the remaining 
water. When the ampule breaks, there is a sudden decrease in pressure 
inside the ampule, causing the water to be quickly propelled onto the 
indicator pad before it can freeze. Also, the surfactant assists in the 
removal of the water from the cracked ampule by lowering the surface 
tension between the water and the fractured ampule, thus providing a dual 
function for the surfactant, the other function being to increase the 
surface area of contact between the liquid and the nucleating agent. 
Referring again to FIG. 2, the blister cover 4 is sealed to backing 8 
around edges 9 of the device by heat scaling. An adhesive 10 is provided 
on the bottom of backing 8 so that the freeze indicator may be readily 
attached to packages which need such an indicator. A paper cover 11, which 
is prelable from the adhesive, is applied over the adhesive 10 to protect 
the adhesive prior to use. 
As can be seen from FIGS. 1 and 2, the ampule 3, which in the embodiment is 
glass, includes constructed neck 12. The constructed neck is at liquid 
fill height of the ampule when the ampule is upright. Air space is 
therefore provided above the restricted neck in region 13. The ampule is 
sealed with either an epoxy or a glass melt seal as indicated at 14. The 
air space of 13 provides for volumetric expansion of the liquid due to 
heating, such expansion being smaller than the volumetric expansion due to 
freezing. In this embodiment, approximately two percent (2%) air space is 
provided. The fill level of the ampule is indicated by line 15 shown in 
FIG. 1. The air space which is provided in region 13 should be within the 
limits of one to six percent (1%-6%) of the total volume of the ampule. 
Another embodiment of the ampule is shown in FIG. 3 in which a bulb-type 
container with capillary extension 16 is provided. The bulb is filled to a 
level 17 with the mixture of water, surfactant, and nucleating agent. A 
part of the nucleating agent, which in this embodiment can be cupric 
sulfide, is indicated at 18. The volume of the air space in the capillary 
portion of the ampule 16 is again within the range listed above. Thus the 
air space is small enough to permit breakage of the ampule due to the 
expansion of the freezing liquid, but large enough to allow thermal 
expansion of the liquid without breaking the ampule. 
In the manufacture of the prior art freeze indicator device as disclosed by 
Johnson, a specification of 97% purity was set for the cupric sulfide. 
From time to time difficulty was experienced in the operation of the 
device in the operation of the freeze indicator due to excessive amounts 
of soluble contaminants in the cupric sulfide, resulting in depression of 
the freezing point of the indicator solution. As a consequence they fail 
to function at the design temperature. In order resolve this problem the 
specification for cupric sulfide was changed to 99+%. The product received 
had a purity of 99.7%, In using the high purity CuS a new problem 
developed. While initially all indicators tested broke at the test 
temperature, after aging two days, 20% of the indicators did not break. 
After four days only half the indicators tested broke, and after one week 
only ten percent broke. 
If the manufacturer of the freeze indicator is the user and fresh 
indicators are used within one day of manufacture no problem is 
encountered. However, if the indicators are shipped to the end user, and 
are allowed to stand for a period of time faulty results may be 
experienced in that less than all of the indicators will break on use. 
By a process of elimination it was concluded that the offending component 
of the indicator composition was the cupric sulfide. Washing of the cupric 
sulfide did not resolve the problem. The cupric sulfide was tested for 
cupric ion both in retains samples of CuS which were satisfactory and of 
samples which did not give acceptable ampule breakage. The test consisted 
of adding potassium iodide to a slurry of cupric sulfide. Surprisingly, 
the expected precipitate was found in the retain samples but not in the 
suspect lot of CuS. 
Tests for cuprous ions on sample lots from other suppliers showed that CuS 
which did not properly nucleate the water to result in breakage of the 
ampules was free of cuprous ions. These samples were about 99.0 to 99.5% 
CuS. It was postulated that the cause of the problem was "poisoning" of 
the nucleating agent over time. This "poisoning" effect is a common 
problem in nucleating agents; see for example U.S. Pat. No. 3,956,153 
(Chada) discussed above. From the quantity of precipitate resulting from 
the iodide test it was suggested that cuprous ion was not the only source 
of copper ion. 
Surprisingly, the addition of cupric sulfate to the water system of the 
indicator was found to resolve the nucleation problem and resulted in 
successful vial breakage at test conditions. It has been found that at 
least 0.075 wt % cupric sulfate, based on the cupric sulfide, when added 
to the water system of the indicator results in protection of the cupric 
sulfide nucleating agent from poisoning. Preferably at least 0.09% cupric 
sulfate can be used; more preferably at least 0.10% cupric sulfate is 
used. Use of larger amounts of cupric sulfate, e.g., 0.2%, while effective 
to insure proper nucleation after aging, begins to have an effect on the 
freezing point of the indicator solution and must be taken into account 
when designing the freezing point of the device. The specification for 
cupric sulfide is 99+% pure CuS. 
Not wishing to be bound by theory, it is believed that the mechanism which 
operates to protect the CuS from poisoning is the continual transfer of 
copper ions in and out of solution maintaining a dynamic equilibrium 
between the insoluble cupric sulfide and the soluble cupric sulfate. The 
result is a continual replenishing of the surface of the cupric sulfide 
with fresh copper ions. 
To demonstrate the effectiveness of cupric sulfate as a poison inhibitor 
the following tests were preformed. A batch of identical ampules were 
prepared with the of cupric sulfate as the only variable. The ampules were 
tested in lots of 50 for specific time periods. 
EXAMPLE I 
A freeze solution having a freezing point at 0.degree. C. was prepared 
having the following formulation: 
______________________________________ 
Component Quantity 
______________________________________ 
Deuterium Oxide 1 liter 
Synotol 119* 2.6 grams 
methylene blue dye 0.38 grams 
Cupric sulfide 250 grams 
______________________________________ 
*An alkylolamide surfactant manufactured by PVO International, Inc. 
Freeze indicators were prepared by placing the ampules in a well stirred 
tank containing the freeze solution, drawing a vacuum on the system to 
remove air from the ampules and releasing the vacuum, thereby filling the 
ampules. The filled ampules were then sealed. The ampules had a volume of 
1 cc. The ampules were designed to break at 0.degree. C. were tested at 
-3.degree. C. An amount of cupric sulfate was added to the composition as 
shown in Table I. 
The amount of cupric sulfate added to test ampules in groups of fifty vials 
each, was varied over a range of 0.0 to 0.2% by wt. based on the CuS. The 
results are shown in Table I. 
TABLE I 
______________________________________ 
BREAKAGE TEST -3.degree. C. 
Cupric Sulfate* Two 
(wt. %) One Day One Week Two Weeks 
Months 
______________________________________ 
0% 50/50 41/50 5/50 1/50 
0.05 48/50 38/50 10/50 3/50 
0.1 50/50 50/50 48/50 50/50 
0.2 15/50 -- -- -- 
______________________________________ 
*Based on the weight of cupric sulfide 
The ampules containing 0.2% cupric sulfate did not pass the break test 
because this concentration of cupric sulfate depressed the melting point 
below 0.degree. C. 
EXAMPLE II 
The experiments of Example I were repeated for ampules designed to break at 
-4.degree. C. The following formulation was used in preparing the ampules: 
______________________________________ 
Component Quantity 
______________________________________ 
Distilled water 1 liter 
Synotol 119* 12.6 grams 
Safranin O dye 1.88 grams 
Cupric sulfide 250 grams 
______________________________________ 
*An alkylolamide surfactant manufactured by manufactured by PVO 
International, Inc. 
The test temperature was -7.degree. C. the results are shown in Table II. 
TABLE II 
______________________________________ 
BREAKAGE TEST -7.degree. C. 
Cupric Sulfate* Two 
(wt. %) One Day One Week Two Weeks 
Months 
______________________________________ 
0% 50/50 46/50 12/50 2/50 
0.05 50/50 49/50 21/50 5/50 
0.1 50/50 49/50 50/50 50/50 
0.2 10/50 -- -- -- 
______________________________________ 
*Based on the weight of Cupric Sulfate 
Not wishing to be bound by theory it is believed that at the 0.2% level of 
copper sulfate the freezing point of the solution has been depressed 
sufficiently to prevent breakage at the test temperature. While the 
ampules are not satisfactory for a -4.degree. C. monitoring device they 
will work at a lower temperature. If desired excess cupric sulfate can be 
used to control the freezing point of the ampule. However, for the purpose 
of this invention at least 0.075 wt % cupric sulfate must be utilized, 
preferably at least 0.09%, most preferably at least 0.1 wt % based on the 
cupric sulfide. 
The concept of this invention may be described generically as a nucleating 
agent system comprising a substantially insoluble nucleating agent in 
combination with a poison inhibitor. The nucleating agent system is 
defined by selecting a nucleating agent which is a metal compound 
insoluble in a liquid which expands upon freezing, the metal compound and 
liquid having substantially similar molecular space groupings, a metal 
salt, soluble in the liquid as the poison inhibitor, the metal salt being 
a salt of the same metal as the metal compound, thereby providing a source 
of soluble metal ion of the same metal as the cation of the inhibitor. The 
poison inhibitor must have a solubility in excess of the concentration at 
which it will be used, e.g., at least 0.15 grams per 100 ml of water at 
room temperature. Preferably, the solubility of the poison inhibitor is at 
least 0.3 grams per 100 ml; more preferably the solubility is at least 1 
gram per 100 ml; most preferably at least 10 grams per 100 ml. 
It is well known that cuprous iodide acts as a nucleating agent for water 
in ice formation. The phenomenon is used in artificial rain making and in 
producing "snow" for ski slopes where natural precipitation has not 
occurred. Based on this knowledge freeze indicators were prepared using 
cuprous iodide as the nucleating agent. Tests on freshly prepared 
indicators resulted in 100% breakage after 10 minutes at the test 
temperature. After one day of aging, however, only 60% of the test 
specimens broke after one hour, suggesting a poisoning effect was at work. 
The addition of 0.1% of cupric sulfate, based on the iodide, to indicator 
system resulted in improved nucleation. 
Illustrative, non-limiting examples of poison inhibitors are shown in Table 
III. As used in the specification and claims, the term metal compound 
includes zinc metal. 
TABLE III 
______________________________________ 
Nucleating Agent Poison Inhibitor 
______________________________________ 
Cupric Sulfide Cupric sulfate 
ferrous sulfide ferrous sulfate 
molybdenum sulfide 
molybdenum tetrabromide 
silver iodide silver fluoride 
zinc metal zinc chloride 
cupric iodide cupric sulfate, 
cuprous chloride 
______________________________________ 
The level of poison inhibitor, based on the nucleating agent, is the same 
as that disclosed for the cupric sulfide/cupric sulfate system. Expressed 
in weight percent based of the weight of nucleating agent the amount of 
poison inhibitor used is at least 0.075%, more preferably at least 0.09%, 
most preferably at least 0.1%. The poison inhibitor must have a solubility 
in excess of the concentration at which it will be used, e.g., at least 
0.15 grams per 100 ml of water at room temperature. Preferably, the 
solubility of the poison inhibitor is at least 0.3 grams per 100 ml; more 
preferably the solubility is at least 1 gram per 100 ml; most preferably 
at least 10 grams per 100 ml. 
Those skilled in the art having access to this disclosure will appreciate 
that by selecting a metal compound which has finite solubility in the 
order of about 0.15 wt. % to about 1.0 wt % based on the weight of the 
liquid, the inhibitor and poison inhibitor will be the identical compound. 
Preferably, the solubility of the compound will be about 0.15 to about 0.5 
wt. %, more preferably about 0.15 to 0.3 wt. %, e.g., 0.25 wt. %. The 
upper limit of solubility is based on the practicality of controlling the 
freezing point of the solution. Large solubility levels will make it 
impossible to have both insoluble inhibitor present and a minor amount of 
soluble ion, while still being able to control the freezing point of the 
solution. 
In the preferred embodiment of the invention the nucleating agent system 
comprises an in soluble metal compound in combination with a soluble salt 
of the metal wherein the valence of the metal in the metal salt is the 
same as the valence of the metal of metal compound. By way of 
illustration, where the metal compound is cupric sulfide the preferred 
salt is a soluble cupric salt such as cupric sulfate; where the metal 
compound is cuprous iodide the preferred metal salt is a soluble cuprous 
salt such as cuprous chloride. 
In one embodiment of the invention as shown in FIG. 4, the indicator pad 6 
of FIG. 2 is replaced by a swatch cotton cloth 19 which obscures the 
ampule from view, and a dye is incorporated into the liquid. When the 
liquid freezes, thereby breaking the ampule, the dye containing liquid is 
absorbed by the cotton swatch indicating that freezing has occurred. The 
swatch of cotton cloth incidentally serves to protect the ampule from 
shock breakage. The invention is described in terms of a swatch of cotton 
cloth. However, any material wetted by the liquid may be used, e.g., paper 
or other cellulosic fiberous material and liquid wettable synthetic 
materials. Illustrative, non-limiting examples of dyes useful in the 
practice of this invention are methylee blue and Safranin O. 
While Johnson '125 discloses the use of a surfactant it has been found that 
the use of the surfactant is optional.