GB2338554A - Thermal dosimeter - Google Patents

Abstract

The thermal dosimeter comprises an indicator dye which exhibits a detectable change in its absorption spectrum on exposure to oxides of nitrogen (NO X ). The dye may be a nitrodiphenylamine or anthraquinone-based dye. The change is a change in colour and/or fading or intensity increase in the original colour. The dosimeter may include an oxide of nitrogen-evolving substance, e.g a discrete nitrate ester (butanetriol trinitrate) or polymeric nitrate ester (nitrocellulase). The dye and optional evolving substance can be contained in a liquid or solid reation medium, the latter in the form of a film. For use in temperature history determination for rocket propellant, medicines and munitions.

Description

2338554 THERMAL DOSIMETER The present invention relates to a thermal

dosimeter device, to methods for making the thermal dosimeter device, and to methods using the thermal dosimeter device.

A thermal dosimeter is a device which can show an easily measurable, time and temperature dependent change that reflects a full or partial temperature history of a product to which it is attached or otherwise associated. Such devices are of interest to the food industry, in particular for monitoring frozen foods and dairy produce. To this end,. time temperature indicators (TTIs) have been developed and are commercially available.

Time temperature indicators have three main operating principles, they may be diffusion-based, enzyme-based or polymerisation based (Taoukis et al. Food Technology: October 1991, 70-82). These TTIs integrate, in a single measurement, the full timetemperature history from the time of activation and can be used to indicate an,effective average" temperature (Teff) during distribution which theoretically can be correlated to continuous temperature-dependant qualityloss reactions. Diffusion Based TTI The indicator consists of a pad saturated with the migrating chemical mixture, serving as a reservoir. Superimposed on the pad is the end of a long porous wick (the track) along which the chemical can diffuse. Before use, the pad is separated from the wick by a barrier film so that no diffusion occurs. Upon activation by removal of the barrier, diffusion starts if the temperature is above the melting point of the chemical mixture. The response of the indicator is the distance of the advancing dye from the origin. It has been shown that a plot of distance moved squared vs time gave a straight line which followed the Arrhenius relationship as a function of temperature. Because of the diffusion based design, the activation energy of this badge is limited to 33-50 kJ mol-'. However the application of a simultaneous colour reaction along the track can be used to create new tags with activation energies up to 125 kJ mol-'. Enzymatic Based TTI The indicator is based on a colour change caused by pH decrease resulting from a controlled enzymatic hydrolysis of a lipid substrate. Before activation, the indicator consists of two separate.compartments, in the form of plastic minipouches. one compartment contains an aqueous solution of a lipolytic enzyme, such as pancreatic lipase. The other contains the lipid substrate absorbed in a pulverised polyvinyl chloride carrier which is suspended in an aqueous phase with a pH indicator. Different combinations of enzyme substrate types and concentrations can be used to give a variety of response life and temperature dependence. Hydrolysis of the substrate (eg tricaporin) causes release of an acid (capronic acid) and a drop in pH, which translates into a colour change of the pH indicator. Because the substrate and enzyme can be varied, activation energies of 63-2145 kJ mol-1 can be obtained. Polymerisation based TTIs These are based on the ability of disubstituted diacetylene crystals to polymerise through a lattice controlled solid state reaction. The reaction proceeds via 1,4-addition polymerisation and the resulting polymer is highly coloured due to the resulting unsaturated highly conjugated backbone. Side groups have little effect on the colour but affect the reaction properties of the monomer. The change in colour measured as a decrease in reflectance is the is basis of the TTI, and the response follows typical first order kinetics. The dyes are printed as bar codes containing data on the product and the reflectance is measured by scanning with a laser optic wand. The tags have activation energies of 84-100 kJ mol-', but the catalyst can be varied to change this.

The time period over which prior art TTIs have been expected to function is appropriate for the storage of foodstuffs, commonly weeks to months. However, this is too short for many applications. For example, a thermal dosimeter for use in monitoring the service life of propellants for rocket-motors may need to function for up to 20 years.

Moreover, the time temperature indicators of the prior art function over a temperature range which is too narrow or too low for many applications.

One substance which is problematic to monitor at present is nitrocellulose (NC) based propellant, which comprises nitrocellulose and is commonly used in rocket motors and munitions such as gun ammunition. Nitrocellulose slowly decomposes exothermically to form oxides of nitrogen. The presence of these gases and the increase in temperature and pressure then promotes a further gas evolution. In this way the decomposition of nitrocellulose based propellants can be termed llautocatalyticll and may eventually lead to autoignition. Stabilisers are added to nitrocellulose propellants to remove nitrogen oxides, thus interrupting this catalytic cycle.

At present, the methods available to assess a propellant service life include destructive testing whereby the remaining stabiliser content of the propellant is analyzed. However this method is expensive. Alternatively the service life of a propellant can be predicted by models which are based on accelerated aging trials.

1 is An effective thermal dosimeter for monitoring the service life of the propellant of a rocket motor would be desirable, but would need to function over a wide temperature range. For example, the dosimeter may need to function at temperatures up to about 500C. In some circumstances, such dosimeters would need to function at temperatures below OOC, although normally the lowest temperature encountered would be around OOC.

The present inventors have found that a phenomenon known as "gas fume fading" can be used as the basis of a thermal dosimeter which can be used, for example, for long term and/or high temperature applications. "Gas fume fading" is the phenomenon by which certain dyes, on exposure to an NOx gas, manifest a change in colour of the dye and/or a change in intensity of the original colour.

The invention has relevance to the monitoring of the state of the propellant used in some rocket motors, to predict the remaining service life of the propellant. The invention also has relevance for general thermal monitoring, particularly for those applications in which a TTI capable of operating over a long time span and/or over a wide temperature range is required. Examples of such applications include monitoring of medicines, especially in hot climates, and monitoring munitions, which may need monitoring for long time periods. Summary of the Invention

In accordance with a first aspect of the present invention, there is provided a thermal dosimeter device comprising an indicator dye which exhibits a detectable change in its absorption spectrum on exposure to oxides of nitrogen (NO,,).

It has been found that the gas fume fading reaction whose use underlies the present invention may be used in a thermal dosimeter which may function over is a longer time-period, and a higher temperature-range than previously described time-temperature indicators.

Preferably the change in absorbtion spectrum is detectable as a visible colour change, i.e. in the range of about 390-750 nm, although where the change lies in the ultra-violet or intra-red, this is also contemplated as being within the invention.

The dye selected for use in the invention may be one which manifests at least one of the following effects upon exposure to NO,:

(i) a change in colour; (ii) a loss in intensity of the original colour (fading); iii) an increase in intensity of the original colour. Normally, the dye chosen will manifest a change in colour and a loss in intensity of the original colour, that is classic "gas fume fading".

The dosimeter may also comprise an NO. evolving substance, for example a nitrate ester, such as nitrocellulose. The NO. evolving substance evelves NO, over time at a rate dependent on the temperature.

When the thermal dosimeter device of the invention is exposed to NO,, for example as evolved by the NO. evolving substance, the indicator dye is caused to change its colour and/or alter the intensity of its existing colour, which may then be detected (either intermittently or continuously) by a suitable detection means, such as a colorimeter (spectrophotometer). These changes are directly related to the temperature history of the dosimeter. The detection means can be calibrated, so that when a particular colour and/or intensity is detected, this may be correlated with a known ',thermal dose,,. This way, a product with which the thermal dosimeter device is associated can be monitored to determine the stage reached in its shelf is life or when the end of its shelf life has been reached.

The indicator dye and optionally the Nox evolving substance are preferably contained in a reaction medium in which the gas fume fading reaction can take place. The concentration of the NOx-evolving substance in the reaction medium can be adjusted to vary the rate of dye depletion.

In one embodiment of the invention, the reaction medium is a liquid in which the indicator dye is dissolved or dispersed. The precise nature of such a liquid is not critical to the invention, although it is important that it should be inert to the extent that it does not interfere with the gas fume fading reaction. Preferably the liquid is chosen such that the indicator dye and optionally also the No. evolving substance is soluble in it. A preferred liquid medium is dioctylphthalate (DOP). Where the reaction medium is in a liquid form, it will be contained in a suitable container or receptacle, which may be sealed.

In another embodiment the reaction medium is a solid, in which the indicator dye and optionally also the NOx-evolving substance are dispersed. Such solid embodiments are advantageous in that they have an inherrent strength and are relatively easy to handle.

Typically, the solid medium will be in the form of a film in which the indicator dye is present. One or more agents may be added to act as a solid medium and give the film the desired chemical and physical properties. For example, the film may include a plasticizer such as dioctylphthalate (DOP), which increases the workability or flexibility of the film, and helps to bind the film together. The film may include an inert binder such as cellulose acetate butyrate (CAB), which acts as a bulking agent.

The solid medium may be formed in a manner to incorporate therein the dye. Thus, a film forming composition containing the dye may be prepared and then cast to form a suitable film which is then caused or allowed to cure, harden or dry.

The solid medium, incorporating the dye and optionally the NOx-evolving substance may be mounted on a structure, such as a glass or plastic carrier to facilitate storage, transport and use of the dosimeter, and protect the film from physical damage. The use of such a carrier would also protect the active ingredient(s) in the film from agents in the environment which may affect the activity.

Techniques for forming films are well known in the art. one presently preferred method involves combining the various ingredients by dissolving them in acetone, pouring out the mixture into a suitable receptacle and then evaporating off the acetone. However there are a number of other methods which are known in the art which would be applicable for forming films such as solventless casting, or using hot rollers. The thermal dosimeter may be in the form of a badge, for example in which the film is sealed between glass plates.

In one embodiment of the invention, the thermal dosimeter device is itself formed or associated with the NO, evolving substance which evolves, for example by a chemical decomposition reaction, NO. gas in a time and temperature dependent manner. Typically, said No. evolving substance is a nitrate ester, such as nitrocellulose.

In this embodiment of the invention, the NOx evolving substance may be integrated in the thermal dosimeter device, for example by incorporation in or application to a liquid or solid medium. In such an arrangement, the shelf life of the thermal dosimeter device prior to use may be relatively short, although this can be increased by refrigeration of the device.

is m An alternative arrangement is one in which the No. evolving substance is not in gaseous communication with the indicator dye, for example by the provision of a barrier means between the NOX evolving substance and the indicator dye, whereby any NOx evolved prior to the intended use of the thermal dosimeter device does not come into contact with the dye. When the thermal dosimeter device is to be used, the barrier is removed so allowing gaseous communication between the NO, evolving substance and the indicator dye.

In another embodiment of the invention, the thermal dosimeter device is not itself.formed or associated with any Nox evolving substance, but is intended to be used in relation to a product which contains such a substance. For example, the propellant used in a rocket-motor may contain nitrocellulose whose decomposition can be monitored using a thermal dosimeter device in accordance with the invention to determine the stage of its service life that the rocket-motor has reached. The Indicator Dye The indicator dye for use in the TDD of the invention is one which exhibits a change in its absorbtion spectrum on exposure to gaseous oxides of nitrogen (NOj. It is thought that No. hydrolyses to form an active acid species which interacts with the dye. Preferably the change in absorbtion spectrum is detectable as a change in visible colour and/or an alteration in intensity of its original colour.

The indicator dye may be, for example, a nitrodiphenylamine (NDPA) dye or an anthraquinone-based dye having an amine functionality at the 1position. Such anthraquinone dyes normally absorb light in the blue end of the spectrum, and exhibit a blue to red colour change on exposure to NO, are preferred.

Particularly preferred are the anthraquinone dyes is in which the amine functionality at the 1-position is a secondary amine. Preferably the amine radical has an electronegative substituent in the para position relative to the amine radical (i.e. at the 4-position). The presence of the electronegative substituent para to the amine radical increases the electron density at the amine nitrogen and it is theorized that this increases the ease of electrophilic attack at the amine nitrogen, and hence the susceptibility to "gas fume fading". Advantageously the electronegative substituent is another secondary amine radical. Preferably the two secondary amine radicals are identicali The antraquininone dyes may contain other substituents as is well known in the art. For example, groups such as -S03 may be added to increase the solubility of the dye in water.

The dye may be of the general formula N 1,1RL 0 H 2 wherein R, and R2 are each independently substituted or unsubstituted alkyl or substituted or unsubstituted aryl. Preferably R, and R2 are each a lower (Cl-C6) alkyl radical optionally substituted by a hydroxy group. The most preferred dyes are listed in the Table below. In these dyes R1 and R2 are the same straight chain alkyl groups and are as follows:

Indicator Dye R,/R2 Disperse Blue 14 (DB-14) CH3 Solvent Blue 59 (SB-59) C2H5 Solvent Blue 35 (SB-35) C4H9 Solvent Blue 14 (SB-14) C_,H7.., is Preferably R, is the same as R2 -without wishing to be bound by theory, the present inventors believe that dyes in which R, and R2 are identidal groups have improved performance because the total number of different reaction products produced is reduced. Thus the likelihood of producing reaction products which absorb light at the relevant wavelength and interfere with the colour change is reduced. The NO- evolving substance The NO, evolving substance is preferably a nitrate ester. The preferred nitrate ester is a polymeric nitrate ester such as nitrocellulose. Another suitable nitrate ester is a discrete (i.e. non polymeric) nitrate ester such as butanetriol trinitrate.

According to a second aspect of the present invention there is provided a method for monitoring the temperature history of an article which method comprises the use of a thermal dosimeter in accordance with the first aspect of the present invention. In a preferred embodiment of this second aspect of the invention, the method is used for monitoring the remaining service life of a propellant.

According to a third aspect of the present invention there is provided the use of an indicator dye which produces a detectable change in its absorbtion spectrum on exposure to oxides of nitrogen (NO,,) to monitor the temperature history of an article.

is For a better understanding of the present invention, and to show how the same may be put into effect, reference will now be made to the following examples. In the examples, reference is made to Figure 1 to 9 of the drawings, in which:

Figure 1 shows a thermal dosimeter in accordance with the present invention, in which the indicator dye and NOX-evolving substance are incorporated into a solid substrate in the form of a film, which film is sandwiched between two glass plates; Figure 2 shows the absorbtion characteristics over time of DB-14/NC/DOP films aged at 30OC; Figure 3 shows the absorbtion characteristics over time of DB-14/NC/DOP films aged at 40OC; Figure 4 shows the absorbtion characteristics over time of DB-14/NC/DOP films aged at SOOC; Figure 5 shows the relationship between the coefficient of a linear fit and wavelength for DB 14/NC/DOP films aged at 30, 40 or SOOC; Figure 6 shows the aging trials for DB-14/NC/DOP films aged at 30, 40 or SOOC over time; Figure 7 shows the relationship between the coefficient of a linear fit and wavelength for DB3/NC/DOP films aged at 30, 40 or SOOC; Figure 8 shows the relationship between the coefficient of a linear fit and wavelength for DAAQ/NC/DOP films aged at 30, 40 or SOOC; and Figure 9 shows the depletion in dye absorbtion against time for DAAQ/=N/CAB films.

EXAMPLE 1

Aging of DB-14/13.1%N NC/DOP Badge Films A film containing a small amount of DB-14 was cast from acetone using propellant-grade nitrocellulose (13.1%) and a plasticizer (DOP). The seals were mounted between two glass slides and sealed using a is li sealant, as shown in Figure 1. This method was used to produce 12 badges for aging trials at 30, 40 and 500C. The badges were aged in a fan- assisted hot air oven between 30 and 500C and periodically removed for visible absorbtion measurements using a Perkin Elmer L9 spectrophotometer.

Figures 2, 3 and 4 show averaged results for badges aged at 30, 40 and 500C respectively (absorption data was "normalised"). It can be seen that a well defined change in the absorption profile is observed during the aging of these films.

This data can be used to calculate the temperature dependent rates and the activation energy of the NC decomposition process (two parameters used to define the characteristics of a thermal dosimeter). In doing this certain assumptions needed to be made.

1) The decline in dye concentrations is first order (the decomposition of the nitrocellulose in the rate determining step). The films absorption can be used as a measure of the dye concentration. However to do this a wavelength needs to be found where the products of the dye reaction do not absorb.

3) At such wavelength a plot of time against [(initial absorption)/(absorption at time t)l gives a linear plot whose slope is the rate constant (k) at that temperature.

4) Once the rate constants at each temperature have been calculated, a plot of ln k against temperature gives a straight line whose gradient equals E,,,,JR(E.,= activation energy and R = gas constant).

To find the wavelength where a plot of time against lnHinitial absorption)/absorption at time tH gives a linear fit, the correlation coefficient of a linear fit on the absorption data at all wavelengths was calculated. This was then plotted against the wavelength. Data from badges aged at 30, 40 and SOOC are shown in Figure 5.

It can be seen that no linear region exists at every temperature investigated (a correlation coefficient of 1 indicating a perfect linear fit). Around 600-650 nm a linear region exists at 30 and 400C but not at 500C. It is very unlikely that this nonlinearity at SOOC would be due to a change in the reaction order and so it is proposed that it must be due to a change in the nature of the dye products at 500C with these products absorbing in the 600-650 nm region. If this is true then the nonlinearity will result in a higher than expected absorption and since the absorption at time t would be higher than expected then 1nHinitial adsorption)/(adsorption at time tH would be lower than expected. A plot of time against ln[Unitial absorption)/(absorption at time tH for absorption data collected at 600 nm is shown in Figure 6. It can be seen that the ln[Unitial absorption)/absorption at time t)l values for badges aged at SOOC are indeed lower than would be expected for a linear fit.

The activation energy of the badge was calculated, and the results are shown in Table 1. The activation energy of 109 kJ mol-' corresponds well with reported values for the hydrolysis of nitrate esters (the expected NC degradation process at the low temperatures used for aging). EXAMPLE 2 Aging of DB-3/13.1W NC/DOP and DAAQ/13.1M NC/DOP Badge Films In parallel to the work detailed in section 5.0, films incorporating DB-3 and DAAQ were also aged at 30, 40, and 500C. Figure 7 and 8 depict a plot of the correlation coefficient against the wavelength for these badge films run at the three temperatures. Unlike the DB-14 badge films, no region of good linear fit was found at any of the three temperatures when trying to calculate rate constants. EXAMPLE 3 Aging of DAAQI=N/CAB Badge Films Films were produced by a similar method as for the previous examples, incorporating the discreet nitrate ester butanetriol trinitrate (BTTN) with cellulose acetate butyrate (CAB) as an inert support and DOP as plasticisier. They were then aged at SOOC and their absorbtion profiles monitored. FigureS shows the depletion in dye absorbtion at 60Onm against time. It can be seen that there is a general fall in depletion though the low levels of BTTN present lead to a slow depletion rate. In general though it can be said that it should be possible to use a discrete nitrate ester with an inert substrate and plasticiser to produce a thermal dosimeter.

is -is- is Table 1: Absorption data for DB-14 badge films.

Badge Temp Absorption at 650 nm Number (OC) 0 312 528 816 1152 1512 1872 11 30 2.66 2.24 2.13 2 1.9 1.77 1.68 30 2.39 2.02 1.95 1.85 1.7 1.67 1.59 9 30 1.98 1.62 1.58 1.44 1.37 1.36 1.25 8 40 2.95 2.06 1.73 1.45 1.18 0.93 0.68 7 40 2.52 1.77 1.51 1.28 1.08 0.9 0.73 6 40 2.12 1.45 1.24 1.03 0.82 0.66 0.51 40 1.97 1.36 1.13 0.94 0.79 0.6 0.48 The above raw data was collected using a PerkinElmer Lambda 9 spectrophotometer. The data has not been "normalised" though it has been presented to 2 decimal places. As can be seen, results are presented for samples aged at 0, 312, 528, 816, 1152, 1512 and 1872 hours.

TABLE 2: Calculation of rate constants for DB-14 badge films at 30 and 400C.

is Badge Temp in (Abs initial/Abs at time t) Gradient r2 Number (OC) 312 528 816 1152 1512 1872 11 30 0.17 0.22 0.28 0.34 0.41 0.46 0.000183 1 30 0.17 0.2 0.26 0.34 0.36 0.41 0.000156 0.97 9 30 0.2 0.22 0.32 0.36 0.38 0.46 0.000161 0.95 8 40 0.36 0.53 0.71 0.92 1.16 1.47 0.000689 1 6 40 0.38 0.54 0.73 0.94 1.17 1.43 0.999664 1 40 0.37 0.56 0.74 0.91 1.19 1.41 0.000657 1 7 40 0.35 0.51 0.68 0.85 1.03 1.24 0.000553 1 A plot of In(abs initial/abs at time t) was needed to calculate the rate constant k). It was noted that badge 7 produced a significantly different k value when compared to the other badges aged at the same temperature. Table 3: Calculation of activation energy for DB-14 badge films.

Badge I/T (K) In k Number 11 0.003299 -8.61 0.003299 -8.76 9 0,003299 -8.73 8 0.003193 -7.28 6 0.003193 -7.32 5 0.003193 -7.33 Regression Output: Constant Std Err of Y Est R Squared No. Of observations Degrees of Freedom 4 X Coefficient(s) -13195.5 Std Err of Coef. 458.5623 34.82785 0.059161 0.995193 6 i - 17- The gradient of a plot of In k against I/temperature (K) should give a straight line whose gradient equals E',,,/R (where R is the gas constant 8.3143 JKlmol-1) Therefore in this case -13195.5 Eact Eact/8. 3143 -13195.5 8.3143 109711.35 J M01-1 109kJ mol-1 is I

Claims (17)

1. A thermal dosimeter device comprising an indicator dye which exhibits a detectable change in its absorption spectrum on exposure to oxides of nitrogen (NO.).

2. A thermal dosimeter device according to claim 1, wherein the absorbtion spectrum change is detectable as a visible colour change.

3. A thermal dosimeter device according to any previous claim, wherein the indicator dye manifests at least one of the following effects upon exposure to NO (i) a change in colour; (ii) a loss in intensity of the original colour (fading); iii) an increase in intensity of the original colour. A thermal dosimeter device according to any claim, further comprising an NO,-evolving

4. previous substance.

5. A thermal dosimeter device according to claim 4, wherein the NO,evolving substance is a discrete or polymeric nitrate ester.

6. A thermal dosimeter device according to claim 5, wherein the NO,evolving substance is nitrocellulose.

7. A thermal dosimeter device according to claim 5, wherein the NOXevolving substance is butanetriol trinitrate.

8. A thermal dosimeter device according to any previous claim, wherein the indicator dye is a nitrodiphenylamine (NDPA) dye or an anthraquinonebased dye.

9. A thermal dosimeter device according to claim 8, wherein the indicator dye is an anthraquinone-based dye having an amine functionality at the 1position.

X

10. A thermal dosimeter device according to claim 9, wherein the amine functionality at the 1-position is a secondary amine functionality, and wherein a secondary amine functionality is also present at the 4position.

11. A thermal dosimeter device according to claim 10, wherein the secondary amine functionalities are the same.

12. A thermal dosimeter according to any previous claim, wherein the indicator dye is of the general formula 0 N,Rl a> 0 RAI-1 R wherein R, and R2 are each independently substituted or unsubstituted alkyl, or substituted or unsubstituted aryl.

13. A thermal dosimeter according to any previous claim, which comprises dioctylphthalate (DOP).

14. A thermal dosimeter according to any previous claim, which comprises cellulose acetate butyrate (CAB).

15. A method for monitoring the temperature history of an article which method comprises the use of a thermal dosimeter in accordance with any previous claim.

16. A method according to claim 15, which method is used for monitoring the remaining service life of a propellant.

17. The use of an indicator dye which produces a detectable change in its absorbtion spectrum on exposure to oxides of nitrogen (NOj to monitor the temperature history of an article.