JP2557362B2 - Photoactivatable leuco base time-temperature indicator - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to time-temperature indicators, and more particularly to indicators that are inactive until activated by actinic radiation.

2. Description of the Prior Art The use of color change indicators to monitor the time-temperature history of perishable articles has been shown in several patent specifications. These include U.S. Pat. No. 4,189,399 (February 19, 1980).
Issued daily, Peter) and US Pat. No. 4,212,153 (198
(Kidnius et al., Issued July 15, 0) specification is included.

If the perishable article to be monitored also has a short useful life and / or requires refrigeration, it is desirable, if not essential, to use an indicator that is inactive until activated. Peter U.S. Pat. No. 4,208,186 (June 17, 1980)
No. 4,276,190 (June 30, 1981) and a diacetylenic composition having an inert form which is activated by contact with activating steam. . Activation by converting the salt form of the diacetylenic monomer into the acid form is described in US Pat. No. 4,373,032 (198
Preziogi et al., Published Feb. 9, 2013).

U.S. Pat. No. 3,768,976 (issued October 30, 1973, Hu et al.) Discloses a temperature-time integral indicator based on temperature-dependent oxygen diffusion into a package containing an aqueous solution of a redox dye. The dye is dark in the reduced state and colorless when oxidized. Similar indicators with free radical sensitive dyes and peroxides on the support are shown in US Pat. No. 3,966,414 (June 29, 1976, Catab et al.).

Photoactivation of various chemical processes has been reported. For example, it is known that certain onium salts are photoinitiators for cationic polymerization (eg, J.V.
Klibero, polymer, engine. and. Rhino. (Polyme
r Eng.and Sci) 23, 953 ( 1983);. and Jay Bui Crivello et al., Jay. polymer. Rhino (J.Polymer Sc
i.), Symposium No. 56 , 383 (1976).

The photogeneration of hydrohalic acid is Maslovsky, Apra. Optics (Appl.Optics) 13, 857 (1974 ) and U.S. Pat. No. 4,247,611 (Jan. 27, 1981 issue, Thunder et al) is shown in.

SUMMARY OF THE INVENTION According to the present invention, photoactivatable
Time-temperature indicator
Is (i) a thermally inactive compound comprising a leuco base, and (ii) an acid that converts the leuco base into a thermally active product when irradiated with actinic radiation. It comprises a mixture of photosensitive compounds. Preferably the mixture is dispersed in a medium and coated on a support.

In operation, the present invention comprises: (a) irradiating the photoactivatable indicator with actinic radiation to render it thermally active; (b) measuring the reflectance of the indicator at a particular wavelength; (c) Time-temperature exposur
e) measuring the reflectance of the indicator at the specified wavelength after the increase, and (d) the increment of the time-temperature exposure using the pre-established relationship between the change in the reflectance of the indicator and the time-temperature exposure. A method of measuring the increase in time-temperature exposure, comprising the step of: The method is particularly useful for measuring the exposure of perishable articles by first subjecting the article to a photoactive variable time-temperature indicator and then following the steps described above.

The term "time-temperature indicator" as used herein and in the claims means a composition that responds to the integral action of time and temperature in a measurable and predictable manner. Activation of the time-temperature indicators of the present invention is by photogeneration of "acids", which term is also understood to include Lewis acids, Bronsted acids and the like.

BRIEF DESCRIPTION OF THE FIGURES FIG. 1 represents the time dependence of the reflectance at 632 nm of the activated indicator level of the present invention kept at room temperature and 6 ° C.

FIG. 2 shows the time dependence of the reflectance change rate with respect to the indicator coating thickness and photoactivation.

DETAILED DESCRIPTION OF THE INVENTION Many food products (food and non-food) are perishable. When the perishable articles are packaged, it is not readily known when they have exceeded their useful life. It is more difficult to determine exactly where an article is on a fictitious graph where its degradation is plotted as a function of time. Since the rate at which a perishable article degrades is generally a function of its integral time-temperature exposure (at least within a limited time-temperature range), a time-temperature indicator is one that is relevant to the freshness of a perishable product. Is a useful tool for The indicator must consist of a composition with readily measurable physical properties that change in a reproducible manner with time-temperature exposure. For convenience, color is used, but other characteristics are suitable. For immediate indicators, the window of time for the color to change in the temperature range of interest must correspond to the window beyond which the perishable product deteriorates.

For products that change significantly in a relatively short period of time (eg, several days) or at relatively low temperatures (eg, 0 ° C. or below), some form of control activity that ensures that the color change does not begin until the desired point. Needs to be converted. One possible means of activation is by light, or "photoactivation". Potential benefits of photoactivation include:

(I) the color change is activated at a specific time point, (ii) the activation process is completely non-invasive, and (iii) the degree of activation can be controlled by dose, which Therefore, a range of time-temperature characteristics can be displayed by a single indicator.

Some of the disadvantages or problems include:

(I) may be activated by ambient light exposure, (ii) potentially difficult to reproduce the activating dose,
And (iii) activating radiation may accelerate the color change.

Suitable time-temperature indicators are based on the color development of leuco bases. Leuco bases are colorless forms of dyes such as diphenylmethane and triarylmethane dyes and are therefore considered to be precursors to these.
(Detailed information about these dyes-manufacturing method, special manufacturing, etc.
Can be found in Kay Ben Catalaman, Chemistry of Synthetic Dyes , Volume II, Academic Press, New York, 1952, p. 705 et seq.

The following triarylmeta-based leuco bases are generally preferred for the present invention.

R is independently H, (C ₁ -C ₂ ) alkyl, hydroxyalkyl group, sulfonated alkyl group, or substituted phenyl group. R'is independently H, C ₁ -alkyl group,
Or a sulfite residue. R ″ is independently H or a C ₁ -alkyl group.

Ar is And R ₁ -R ₅ is irrespective of H, (C ₁ -C ₄ ) alkyl group, halogen group, amine residue, N (C ₁ -C ₄ ) alkyl group, carboxylic acid residue, sulfite It is a residue, a hydroxyl group or a substituted phenyl group. Ar may be substituted with naphthalene or a substituted naphthalene, in which case the leuco base is diphenylnaphthylmethane leuco.

A He-Ne laser emitting at 632 nm is a convenient light source for monitoring the reflectance of the indicator of the present invention. When using this laser, the preferred leuco base is ultimately green,
It gives a blue or purple color. Specifically, Malachite Green Leuco (II), Brilliant Green Leuco (III) and Crystal Violet Leuco (IV)
Is particularly preferable.

By selecting from these and other suitable leuco bases and mixtures of two or more thereof, a wide variety of desirable colors are obtained.

It is known that leuco bases are converted into dyes in the following two steps.

In the formula, HY is an acid. Surprisingly, when a mixture of leuco base and photoacid is exposed to actinic radiation, its color development rate is temperature dependent. That is, the color development is time-
Direct temperature exposure. The mechanism underlying this action is not known, but it is likely that a carbinol or carbinol homologue is formed in the photoactivation step, which then reacts with photoacid to initiate color development. For simplicity, the description of this method assumes that the dye is formed by the (time-temperature dependent) action of photoacid on the leuco base.

It is known that many compounds generate an acid when excited by actinic radiation. These photoacids include o-nitrobenzaldehyde and substituted o-nitrobenzaldehydes; trihaloalcohols (X ₃ ROH, where X is halogen and R is alkyl having at least 2 carbon atoms); and formula φ _n (I, S) (P, Sb, As) F
_m (where n is 2 or 3, m is 4 for P, S
6 for b and As).
When choosing a particular compound, the following points must be considered.

Since the response to time-temperature exposure generally depends on the degree of photoactivation, the photoacid generator does not respond to ambient light exposure, or the indicator is protected from this type of exposure (e.g. the use of opaque coversheets). Or by keeping the indicator in the dark). In addition, the photoactivation process must be reproducible. This can be conveniently and easily accomplished, for example, by using a reproducible light source, or by monitoring photoactivated irradiation. An advantage of the indicators of the present invention is that the leuco base system does not substantially develop color when exposed to radiation well above the amount required for photoactivation.

It is convenient to use wavelengths in the visible range for activation,
These can also cause problems. When time-temperature exposure is monitored by changes in reflection density, it is important that the light whose reflectance is measured is one that is not capable of producing additional activation. Since it is easy to adopt visible light reflection, it is preferable that no activity occurs at a wavelength of 400 nm or more. For example, a substance that can be activated at a wavelength of about 200 to 400 nm is preferable.

A photoreaction that can be easily completed would be desirable. This is because the activation obtained is relatively independent of the exact dose. However, in a preferred form of the indicator of the present invention, higher levels of photoactivation result in an indicator that responds more rapidly (ie, darkens) at constant temperature. Thus, in this case, a single substance may act as an indicator over a wide range of times and temperatures depending on the degree of photoactivation.

Photoacids must be thermally stable in the environment of use.
That is, it must not be heat-activatable.

A preferred photoacid generator is o-nitrobenzaldehyde (ONB), which provides effective photochemical conversion to the corresponding nitrosobenzoic acid as described below.

Benzoic acid is a strong acid (pKa ~ 4.2) as compared with normal aliphatic carboxylic acids (pKa ~ 4.8) and has a group (elec
The benzoic acid ortho-substituted with the tron withdrawing group) is still strong (o-nitrobenzoic acid, pKa ~ 2.2; o-chlorobenzoic acid, pKa ~ 2.9). Therefore, o-nitrosobenzoic acid (VI) is a stronger acid than the aliphatic carboxylic acid.

Is an alternative to ONB as tourist compound is 2,2,2-bromo ethanol (CBr ₃ CH ₂ OH). This compound produces HBr when irradiated with ultraviolet rays. However, the similarly activated 2,2,2-tribromoethanol system has a slower response at constant temperature than the system using ONB. Therefore, ONB is preferred.

The practical time-temperature indicator of the present invention consists of a mixture of leuco base and photoacid dispersed in a medium tube and coated on a support. Preferably the medium is a polymer such as polyvinyl alcohol (PVA), polyvinyl acetate (PVAc),
Or it consists of a mixture of both. PVAc is preferred. This is because it has greater solubility in organic solvents, such as alcohol / water mixtures, than PVA. Since leuco base and ONB are also soluble in organic solvents, good quality PVAc coatings can be produced. Generally, the higher the alcohol concentration in the alcohol / water solution, the faster the time-temperature induced discoloration in the coatings prepared therefrom. About 95% alcohol is the best in practice. PVAc is
This is because it does not dissolve in 100% alcohol. An indicator consisting of malachite green leuco base and ONB dispersed in PVAc is preferred.

Any suitable method can be used to apply the indicator to the support, including, for example, spraying (using an airbrush, for example) or application using a doctor knife or Mayer rod. Depending on the coating method used, it is necessary to control the viscosity of the solution by, for example, the molecular weight of the polymer medium. Any suitable support can be used for coating, including paper, thermoplastics, metals and the like. Paper is preferable because it is inexpensive and easy to handle. A faster color change can be achieved by using a thicker coating, which can be prepared, for example, by forming multiple coatings in a single area. The combined effect of coating thickness and degree of light activation on the rate of discoloration of the coating allows the freshness of products with shelf-life differences of 30 or more from each other to be monitored using a single indicator composition.

Any coversheet that prevents unwanted photoactivation can also provide an oxygen barrier, which is useful for indicators that develop color when exposed to oxygen. Finally, the cover sheet can protect the coating, thereby preventing damage to the coating during handling. Polyester is one of a wide variety of materials suitable for cover sheets.

The following examples are presented so that the invention might be more fully understood. The specific techniques, conditions, materials and data reported are set forth to illustrate the principles and practice of the invention and are not to be construed as limiting the scope of the invention.

Example 1 60 mg of malachite green leuco [p, p'-benzylidene bis (N, N-dimethylaniline] (purchased from Fisher Scientific Company) was added to 1 ml of an organic solvent.
I put it inside. (Alcohol, acetone and chloroform were used in separate tests with similar results). This solution was then mixed with 30 mg of o-nitrobenzaldehyde (ONB) crystals. A piece of paper was dipped in the resulting solution. The coated pieces had a faint gray color and remained in that state for several days when stored in a dark place. However, irradiation with UV for a few seconds “activates” the strip,
It became a very faint green color. The photoactivated samples became more green at different rates when left at different temperatures (room temperature, refrigeration temperature, etc.).

Example 2 A mixture of 4 g of polyvinyl alcohol (PVA) in 100 ml of water was added.
Aqueous medium (gel) by heating to 80 ° C for a few minutes
Was prepared. PVA (from Aldrich) had a molecular weight of 115,000 and was 99-100% hydrolyzed. PVA gel 1 ml
And 60 mg of Malachite Green Leuco were mixed well by milling each other. The mixture was applied to a 12.5 mm x 100 mm paper strip by means of a Mayer rod or by silk screen and then dried at room temperature in a dark hood. 15 mg of ONB (photoacid generator) was dissolved in 1 ml of reagent grade ethanol. Dried coated strips (12.5 x 6.3 mm) were dipped in ONB solution for a short period of time and laminated as described below. The coated strip was heat-sealed between two transparent heat-sealing polyester films (thickness 100 μm, available from Capac) to create a “label”. The initial reflectance was 95-99%. For time-temperature monitoring of non-activated labels, some of these labels were stored at room temperature in the dark and some at refrigerated temperature. Light activation of other labels 100W for individual labels
It was performed by irradiating with a mercury arc lamp for a certain period of time (5 to 20 seconds). The polyester film transmitted light having a wavelength of 300 nm or less. 85-90% reflectance after photoactivation
(Still looked substantially colorless).

The decrease in reflectance over time at constant temperature was monitored by measuring the reflectance of the indicator label against a standard (reference) barcode using an optical scanning wand with a 632 nm beam. In practice, a square hole was cut out in the bar code label and the indicator label was placed below the hole. Since the two labels were held together, the wand scanned both the barcode label and the indicator label. Over a period of time, the reflectance of the indicator label decreased at a temperature-dependent rate. A general plot for room temperature (22 ° C) and refrigeration temperature (6 ° C) is shown in Figure 1. The reflectance value is the average of at least 10 scans. Labels that had not been activated were also monitored for 1 month or longer, and no color was observed at 6 ° C. or room temperature. When Crystal Violet Leuco was used instead in the same system, the final color produced was purple. Similarly, when a 5: 1 mixture of malachite green and crystal violet leuco base was used, a blue color was obtained.

Example 3 10% polyvinyl acetate (PVAc, molecular weight 120,00 in various mixtures of alcohol and water with an alcohol content of 60-90%)
0, Aldrich) to prepare a polymer gel. A mixture of 300 mg of leuco base (malachite green) and 25 mg of ONB in 10 ml of each of these polymer gels was applied on Whatman # 41 paper with a Perche airbrush. The coated sample was dried in a dark hood for 4 hours, then Example 2
Laminated and photoactivated by the method of. The color development of the activated label at room temperature and 6 ° C. was measured by the method of Example 2. The color development rate was proportional to the alcohol content, the fastest for the sample prepared in 90% alcohol and the slowest for the sample prepared in 60% alcohol.

Example 4 A coating mixture was prepared using the method of Example 3 by blending leuco base, ONB, and PVAc gel prepared in an alcohol / water mixture with each other. This coating mixture was applied to a piece of paper by spraying with a Perche airbrush. Several samples were prepared with two or more layers of coating of the same material. The coated strip was dried and laminated. The reflectance at 632 nm was monitored after photoactivation while the samples were stored at refrigeration temperature (6 ° C). The results are shown in FIG. Samples A and B each had one coat of coating mix, but they differed in the duration of photoactivation (350 nm). The more the activation was, the higher the rate of change of reflectance was. Samples C and D each had a two-layer coating, but again at different rates depending on the photoactivation period. Samples E and F were made by applying a third coating over the two-layer coating. In this case as well, the speed changed depending on the photoactivation period. Immediately after photoactivation, the reflectance (R) of each sample ranged from 85 to 90%, but for simplification, all starting points were 87% in FIG.
As shown.

Example 5 Mixture of alcohol and water (75% alcohol) 100 m
A polymer gel was prepared by adding 10 g of polyvinyl chloride (molecular weight: 120,000) to 1 l and heating the mixture at 80 ° C. for 2 hours. A coating mixture was prepared by blending 300 g of leuco base (malachite green) and 25 mg of ONB with each other in 10 ml of this polymer gel. The mixture was airbrushed through the slit to the blank portion of the barcode label. The thickness of the coating film was adjusted by controlling the number of times the airbrush passed over the slit. The applied barcode label was dried in the hood and stored in the dark. The coated area was activated by UV irradiation for 10 seconds.
To prevent further photoactivation by ambient light,
The application area was covered with a UV-absorbing plastic strip. Label development was monitored by scanning directly on the activated barcode label.

Direct application onto the blank portion of the bar code label has the obvious advantage over systems requiring encapsulation in that direct printing methods can be employed to apply the indicator material.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (56) Reference JP-A-59-163543 (JP, A) JP-A-59-188528 (JP, A) British patent 1161058 (GB, A)

Claims (10)

(57) [Claims] 1. A thermally inactive compound comprising (i) a leuco base, and (ii) an acid which converts the leuco base into a thermally active product when irradiated with actinic radiation. A photoactivatable time-temperature indicator comprising a mixture of photosensitive compounds. 2. An indicator consisting of the mixture according to claim 1 dispersed in a medium and coated on a support. 3. The indicator according to claim 2, wherein the thermally inactive compound comprises one or more leuco bases of triarylmethane dyes. 4. The indicator according to claim 3, wherein the leuco base comprises malachite green leuco. 5. The indicator according to claim 3, wherein the photosensitive compound is o-nitrobenzaldehyde or 2,2,2-tribromoethanol. 6. The method of claim 2 wherein the medium comprises polyvinyl alcohol, polyvinyl acetate, or a mixture of both.
Indicator as described in the section. 7. A polyester cover sheet is further included.
The indicator according to claim 2. 8. (a) (i) a thermally inactive compound comprising a leuco base, and (ii) conversion of said leuco base into a thermally active product upon irradiation with actinic radiation. Irradiation with a actinic ray of a photoactivatable time-temperature indicator consisting of a mixture of acid-forming photosensitive compounds is considered to be thermally active, and (b) the reflectance of the indicator at a specific wavelength is measured. And (c) measuring the reflectance of the indicator at the above specified wavelength after increasing the time-temperature exposure, and (d) and using the pre-established relationship between the change in the reflectance of the indicator and the time-temperature exposure. A method for measuring the increase in time-temperature exposure, which comprises the step of calculating the increase in time-temperature exposure. 9. The method according to claim 8, wherein the photoactivatable indicator is a mixture of o-nitrobenzaldehyde, malachite green leuco and polyvinyl acetate. 10. A thermally inactive compound comprising (a) (i) a leuco base, and (ii) a leuco base which is thermally activated when irradiated to actinic rays in a perishable article. A photoactivatable time-temperature indicator consisting of a mixture of photo-sensitive compounds that produce an acid to be converted into a substance, and (b) irradiating the indicator with actinic radiation to render it thermally active, (c) ) Measuring the reflectance of the indicator at a particular wavelength, (d) measuring the reflectance of the indicator at the particular wavelength after increasing time-temperature exposure, and (e) the change in reflectance of the indicator and time-temperature exposure. A method of measuring the increase in time-temperature exposure of a perishable article by calculating the increase in time-temperature exposure using a pre-established relationship between.