WO2007082984A1

WO2007082984A1 - Device for detecting changes in temperature, method of manufacturing the device as well as use of the device

Info

Publication number: WO2007082984A1
Application number: PCT/FI2006/050545
Authority: WO
Inventors: Teemu Ruotsalainen
Original assignee: Teemu Ruotsalainen
Priority date: 2005-12-08
Filing date: 2006-12-08
Publication date: 2007-07-26
Also published as: FI118163B; FI20051268A; FI20051268A0

Abstract

A device suited for detecting changes in temperature and a method for manufacturing the same, as well as its uses. The device comprises nanoscale fragments, which consist of structures having a high light scattering efficiency and a high opacity, which structures, as a consequence of a change in the temperature, are capable of collapsing and forming larger-size structures having an essentially weakened light scattering efficiency and opacity. According to the invention, the nanoscale fragments are arranged on a layer of base material so that they at least partly cover this material optically and prevent the observation of the base material layer and, correspondingly, when breaking down, allow the observation of the base material. The device can be used as a temperature indicator and in the manufacture of thermographic paper.

Description

DEVICE FOR DETECTING CHANGES IN TEMPERATURE, METHOD OF MANUFACTURING THE DEVICE AS WELL AS USE OF THE DEVICE

The present invention relates to a device according to the preamble of Claim 1 for detect- ing changes in the temperature.

Such a device comprises nanoscale fragments consisting of structures having a high light scattering efficiency and a high opacity, which, as a result of a change in the temperature, are capable of collapsing and forming larger-size structures that have an essentially impaired light scattering efficiency and opacity.

The invention also relates to a method according to the preamble of Claim 13 for manufacturing a device suited for detecting changes in the temperature and the use of the present devices as temperature or heating indicators in packages or in connection with the same to detect exceeding of a predefined limiting temperature.

Recently, efforts have been made to find ways of manufacturing advantageous devices that are suited for temperature monitoring particularly for applications of the food industry. Utilizing the properties characteristic of nanoscale or nearly nanoscale fibres, drop-like particles and the combinations thereof constitutes a solution to the said situation. The diameters or the smallest dimensions of these fragments are a few micrometers or less. At the moment, there are commercial disposable sensors that detect the limiting temperatures, the most well-known of them being MonitorMark™ made by 3M, FreshCheck™ by Lifelines, and Checkpoint® by Vitsab. However, the products mentioned above have their weaknesses:

The operation of MonitorMark™ is based on a diffusion, that of FreshCheck™ on a polymerization reaction and that of Checkpoint® on the enzymatic hydrolysis of lipids. The above reactions are slow, which is why the sensors react slowly on external stimuli and, therefore, are unreliable as temperature meters.

The manufacture and the colour change of fibres of nanoscale or nearly nanoscale radii are described in publication Pedicini, A. et al. Thermally Induced Color Change in Electrospun Fiber Mats, Polymeric Materials: Science and Engineering, 2002, 87, 373. The colour change dealt with in the publication is based on polymeric (PC, PEO and PMMA) fibres that are blended with carbon black, which, below their glass transition temperatures and melting points, are white or off-white, but, when combining in a molten state, form larger structural entities, exhibiting the colour characteristic of the carbon black.

Various structures made of nanofibre or nanofibre mixtures and manufacturing methods are described in US patent publication 6,110,590, among others. The publication describes a method of manufacturing nanofibre from silk. Published application WO 2004/016839, in turn, describes equipment for manufacturing nanofibre by electrospinning, and a nozzle construction used in the process.

Literature or publications that describe the known technology do not present a method of using nanoscale or nearly nanoscale fibres, drop-like particles or structures consisting of the combinations thereof in temperature control. Neither is the manufacture of nanoscale fibres for temperature monitoring and observation dealt with.

The purpose of the present invention is to eliminate at least some of the problems related to the known technology. Particularly, the purpose of the invention is to provide temperature detectors or temperature indicators, which are capable of even detecting small changes in the temperature.

The present invention is based on the idea that a device suited to monitoring temperatures uses nanoscale fragments consisting of polymers or polymer mixtures, which are brought on top of a bottom layer as a layer of their own so that, at a basic temperature, they cover at least part of the surface of the bottom layer, preventing an observer from seeing this part of the surface of the bottom layer.

The solution is based on a phenomenon, according to which the nanoscale or nearly nanoscale fibres, drop-like particles or their combinations, which consist of small molecular compounds, polymers or their mixtures (including mixtures blended with fillers), are white, almost white or off-white and they have a high opacity, which is a result of the high light scattering efficiency of the structures. However, as a result of a change in the temperature or, in practice, a raise in the temperature, the nanoscale fragments loose at least part of their light scattering efficiency and, thus, their opacity, whereby the bottom surface can be seen through the layer formed by the nanoscale fragments.

On the basis of the above, the invention relates to a device that is suited for temperature monitoring and comprises: - an base material layer and

- a surface consisting of nanoscale or nearly nanoscale fibres, drop-like particles or their combinations, which is fitted on top of the base material layer, the surface's fibres, particles or combinations thereof forming a structure, which is capable of collapsing when the device is brought to a temperature that is above the fragments' melting point, glass transition temperature or both.

The device according to the invention is used, for example, in a temperature monitoring method, wherein the melting point, the glass transition temperature or both of these for the nanoscale fragments, which typically consist of a polymer or a polymer mixture, are selected so that the structure of the material collapses under heating, causing a considerable weakening of the light scattering efficiency and the opacity in the nanoscale structures formed by the polymer, the polymer mixture or the polymer mixture blended with fillers. If, on the surface of the base material, there is an easily distinguishable specific colour, or a pattern or other marking has been formed thereon, the change in the temperature is easy to notice when the layer covering the surface looses its opacity, whereby the specific colour or a corresponding covered pattern or marking of the base material layer become visible.

More specifically, the device according to the present invention is characterized by what is stated in the characterizing part of Claim 1.

The method according to the invention, in turn, is characterized by what is stated in Claim 13, and the uses according to the invention are characterized by what is stated in Claims 15, 20 and 21.

Compared with the known technology, the invention provides considerable advantages. By using various processing methods, such as electrospinning, it is possible, according to the present invention, to manufacture from simple raw materials, such as ordinary commercial polymers or the combinations thereof and from mixtures blended with fillers, durable de- vices of uniform quality for temperature monitoring, which, however, react easily and are also inexpensive to manufacture and use.

The colour change of the temperature indicators made of nanoscale fibres and particles is based on the structural changes caused by the changes in the temperature of the structures formed by the fibres or the particles or the combinations thereof, the initial temperature, in other words the limiting temperature under observation of the structural changes being very accurately adjustable.

The devices according to the invention can be used as temperature indicators, in electronic components and in the manufacture of thermographic paper.

Other details and advantages of the invention appear in the following detailed description.

Figures Ia to Id show a series of four photographs, which show how the colour of a temperature indicator made of polyethylene oxide changes when the limiting temperature is exceeded.

Figures 2a and 2b show the principles of the changes in the glass transition temperatures of the polymer mixtures according to the mutual volume fractions of the polymer components. Figure 2a presents the situation for homogeneous polymer mixtures and Figure 2b, correspondingly, the situation for heterogeneous polymer mixtures.

Figures 3 a and 3b show TEM images of the bicontinuous structure of a nanofibre formed from a PS-P4 VP(PDP) _{1 0} complex, the image in Figure 3a is taken in the direction of the fibre and in Figure 3b, correspondingly, perpendicular to the fibre.

As disclosed above, the device according to the invention for detecting temperature changes comprises nanoscale fragments, which consist of structures having a high light scattering efficiency and a high opacity, which structures, as a result of a temperature change, are capable of forming larger-size structures that have an essentially weakened light scattering efficiency and opacity. These fragments include, for example, extremely small (i.e., "nanoscale") fibres, particles and the combinations thereof. Generally, nanoscale means that the smallest dimension of the fibres or particles or corresponding structures or their combinations is on average smaller than about 1 micrometer; preferably e.g. the diameters of the fibres are about 10 to 500nm. Generally, the sizes of the fragments are in the range of or smaller than the wave lengths of visible light.

The term "fragments" refers to small fibres and fibrils, particles (including spherical and drop-like particles) and structural units with indefinite forms, which consist of molecular compounds, polymers or their mixtures, the latter possibly containing fillers as blend components.

The particles, particularly the fibres and the fibrils, can be manufactured by e.g. electro- spinning, as described in our previous patent application FI 20040493, which is incorporated herein by reference. The electrospinning is a processing method, wherein the con- sumption of the material to be defibrated is generally 10 - 0.001 g/m², preferably 0.1 - 0.001 g/m² depending on the thickness of the fibre layer.

According to this technology, the device for temperature monitoring according to the present invention is made of at least one polymer by a method, wherein:

- a defibration solution or melt containing at least one polymer or polymer mixture is produced, its viscosity being over 0.0001 N/m²s, preferably over 0.1 N/m²s, and

- the polymer or the polymer mixture is defibrated by electrospinning so that the defibration solution or melt is fed through a capillary tubing that is charged to a first potential to a receiving area that is charged to a second potential.

A typical potential difference and distance between the electrodes mentioned above are preferably 0.1 - 10 kV/cm, especially about 0.5 - 5 kV/cm, and 10 - 1000 mm, especially about 50 - 500 mm, correspondingly.

The simple electrospinning described above is quite a cost-effective way to produce a device suited for temperature monitoring. However, it should be emphasized that the manufacturing method of the present invention is not limited to the electrospinning, but other known manufacturing techniques suited for this purpose can be used as the manufacturing method of the nanoscale or nearly nanoscale fibres, drop-like particles or the combination thereof. The fibres and the particles can be manufactured from polymers, especially from thermoplastic materials, such as polyethylene oxide (PEO), polycarbonate (PC), polystyrene (PS) as well as acrylate and methacrylate polymers, such as polymethylmethacrylate (PMMA). The polymer is selected so that the glass transition temperature and/or the melting point of the nanoscale fibre or particle made from it is (are) in a suitable proportion to the desired limiting temperature. The limiting temperature in that case refers to a temperature, the exceeding of which is to be indicated. Depending on the materials, changes in the structures of the nanoscale fragments can either take place before the temperature reaches the glass transition temperature and/or the melting point, correspondingly, or not until above the same. Accordingly, for example, a change in the structures of the polyethylene oxide fibres begins from the melting point of the powdery PEO (59.5°C), as shown by the appended Figures Ia - Id. A change in the structures of the polymethyl-methacrylate fibres/particles does not take place until distinctly above the glass transition temperature. The transition temperature of the PC fibres is about 135°C and that of the PMMA fibres about 160°C.

The invention can also employ mixtures of polymers, whereby the polymers have different glass transition temperatures/melting points. This application is examined in detail below. Furthermore, blend components can be used to influence the temperature, where the structural alterations begin.

The various properties of the polymer components are combined in the materials made of the polymer mixtures. For example, homogeneous (and also heterogeneous) polymer mixtures can be used to attain better mechanical properties than those of the polymer components that form the mixture. However, for the invention, a more important property of the homogeneous polymer mixtures is adjusting the glass transition temperature (T_g). For homogeneous polymer mixtures, which are typically formed from chemically similar poly- mers (A, B), only one glass transition temperature is observed, which is obtained by the average value scaled by the volume fractions (q>A, φβ) of the glass transition temperatures (T_gA, T_gB) of the mixed polymers. In other words, the glass transition temperature of the homogeneous polymer mixtures can be adjusted "gradually" between the glass transition temperatures of the polymer components that form the polymer mixture by changing the mutual volume fractions of the polymer components (Strobl, G., Polymer mixtures, in the book: The physics of polymers, Springer, Berlin 1997, pp. 83 - 128) (see Figure 2a). For the heterogeneous polymer mixture, in turn, two glass transition temperatures are found, which are identical with the glass transition temperatures of the polymer components, i.e., the degree of the glass transition increases with the volume fraction increasing (see Figure 2b).

In the case of polymers, the eutectic phase behaviour applies in normal air pressure, which is of the same kind as the behaviour of water and alcohol, for example, whereby the melting point of the mixture is lower than that of the initial materials. Also the melting point of the polymer solvent systems can thus be adjusted by adjusting the volume fractions of the polymer and the solvent. For example, in the case of the mixture of polyethylene oxide and resorcinol, two so-called eutectic points are observed in the phase diagram, with the mole fraction of the resorcinol being 0.10, the melting point is 43 °C and with the mole fraction being 0.49, the melting point is 86°C.

The glass transition temperatures and the melting points of block copolymers, graft copolymers and random block copolymers can be adjusted by the selection of the polymer blocks, by chemical treatment or by adjusting the mutual volume fractions or mole fractions of the polymer blocks, as Papageorgiou G. Z. et al. have proven in their article of the Polymer International magazine called Melting point depression and cocrystallization behavior of poly(ethylene-cobutylene 2,6-naphthalate) copolymers (Polym. Int., 2000, 53, 1360 - 1367), the content of which is incorporated herein by reference. As an example, the block polymers of polyethylene-2,6-naphthalene (PEN; T_g 122°C, T_m 268°C) and polybu- tylene-2,6-naphthalene (PBN; T_g 71°C, T_m 242°C) can be mentioned. By systematically adjusting the volume fractions or, in this case, the mole fractions of the blocks, the glass transition temperature (T_g) and the melting point (T_m) of the block polymers (EB) can be systematically adjusted (see Table 1). Table 1

Regarding the polymer mixtures, the fractions of the polymer components can freely vary, provided that the desired properties are achieved for the glass transition temperature or the melting point, correspondingly. Generally, in the case of the two polymers, the molar ratio of the A and B components is within a range of 0.1:99.9...99.9:0.1, typically about 10:90...90:10.

In the embodiments described above, the polymers may also be replaced by components that are smaller than polymers, e.g. crystallizing compounds. In this case the structure of the temperature indicator will collapse at the melting point of the mentioned crystals.

As stated earlier, the nanoscale or nearly nanoscale fractions obtained from most of the thermoplastic polymers using the above methods are white, almost white or off-white and they have a high opacity, which is due to the high light scattering efficiency of the above- mentioned structures. When the nanoscale or nearly nanoscale components similar to those above are processed to form surfaces on the surfaces of different colour bases, the reflection of the colours characteristic of the bases is totally or almost totally prevented.

The shapes of the larger-size structures, which are formed as a result of the change in temperature, vary between a thin film, netlike or drop-like according to the surface energies. Whichever the shape of the collapsed structures, their light scattering efficiencies and opacities are essentially weakened as a result of the collapsing, whereby the characteristic colour of the base or the patterning on the surface is allowed to reflect and it comes into view.

As the change in temperature causes the structures to collapse, even a momentary exceeding of the limiting temperature is enough to cause the specific colour or the pattering on the surface to irreversibly come into view. The temperature indicator according to the invention is thus disposable, and not even the structural alterations caused by the momentary temperature rises can be changed afterwards, i.e., the results of the temperature changes cannot be tampered with.

The phenomenon described above can be utilized in temperature monitoring and observation. The term "temperature indicator" that is used in connection with the invention thus refers to a device suited for temperature monitoring.

In a preferred application of the invention, the nanoscale fragments form an unbroken layer on the surface of the base, covering a selected part of the surface of the base material. In the temperature indicator, at least 5%, preferably at least 10%, more preferably about 15 - 100% of the surface of the base material is covered with such a layer, whereby this portion of the base forms the said temperature indicator. It is appropriate that such a large part of the surface of the base is covered, that the covered surface becomes visually observable at least from a distance when the layer covering it becomes transparent or translucent as a result of a temperature rise.

The thickness of the layer always varies according to the colour or the pattern of the nanoscale fragments and the base material that is to be covered; however, but generally it is about 0.5 - 1000 micrometers, particularly about 1 - 500 micrometers, preferably about 10 - 100 micrometers.

As the base material of the temperature indicator according to the present invention, any suitable, preferably at least essentially planar substrate can be used, such as paper, board, metal foil, glass, polymer film or metal or polymer plate, or a combination thereof, such as plastic-coated paper, board, glass or metal plate. The base material can be self-supporting, whereby its basis weight with respect to the papers and boards is typically about 80 - 500 g/m². However, the base material is not limited to planar substrates only, but the temperature indicator function according to the invention can also be provided by laminating, on an uneven surface, a sufficiently thick layer of the fragments presented above.

The entire base material can be of a uniform colour, which was also called a specific colour above, or it can be patterned with a continuous or discrete patterning at least on the covered area. Colour signals or written or patterned messages can be formed under the covering layer.

The base material can also be translucent or transparent, whereby the temperature indicator can be provided with a light source + sensor -combination, e.g., a source of low-energy light (e.g. IR light) + a sensor, of which one is arranged behind the base material and the other one in front of the covering layer. Thereby, the collapsing of the covering layer can be verified as an increase of the transmission.

The colour or the patterning of the base material can also be varied in different places of the material and, when so desired, two or more temperature indicators can be used for the same base, preferably consisting of different polymer mixtures, their structural alterations taking place at different temperatures. Thus, a temperature monitoring device is provided, which can be used to observe a temperature that applies to a certain interval. When the temperature even momentarily exceeds the limiting temperature of a polymer mixture that changes at a lower temperature, a certain base material colour of patterning becomes visible from under this mixture, but, since the limiting temperature of the other temperature indicator has not yet been exceeded, the colour or patterning of the base material in that place has not yet become visible, whereby it is obvious that the temperature at least momentarily has been in the interval between the limiting temperatures of the temperature indicators in question.

In a general solution, the colour change of the temperature indicator is based on the structural alterations caused by the temperature changes and the resulting physical changes, their starting temperature and monitored limiting temperature being accurately adjustable. In this respect, the temperature limit can be set at an accuracy of about half a degree, ac- cording to the polymer material, as disclosed by the example below and the appended figures.

According to another preferred embodiment, the colour changes of the temperature indicators are based on UV activation or activation by light of another wavelength or another level of energy, such as light formed by a laser or a LED, whereby a partial embrittlement of the fibre structure is caused by radiating UV light or other light. For this embodiment, polyacrylate used in the electronics industry, such as polymethyl methacrylate (PMMA) or some other polymer that decomposes in UV light, as well as, optionally, also a UV activator are added to a polymer solution that is preferably treated by electrospinning. The defi- brated polymer mixture is thus manufactured from at least two polymers. A polymer, such as polyacrylate, which is UV-decomposed in fibre that is defibrated at correct volumetric ratios, preferably 0.1 :0.9 - 0.9:0.1 , and another polymer component form a bicontinuous structure similar to that in Figure 3. The polymer phase that melts at a higher temperature forms the supporting structure. The structure of the bicontinuous fibre will not collapse before this polymer, which melts at a higher temperature, has reached its melting point or glass transition temperature (e.g., the melting point of PMMA is about 150 - 170°C), even if the other of the polymer components of the fibre were already in liquid form. The temperature indicator manufactured from the fibre material is activated by radiating the fibre with UV light, whereby the molecular mass of the polymer, which melts at a higher tem- perature, preferably polyacrylate(s), is reduced and is no longer capable of functioning as the supporting structure of the fibre. After the treatment mentioned above, the structure of the fibre collapses as early as at the melting point or glass transition temperature of the polymer that melts at a lower temperature. Thus, the polymer that melts at the lower temperature works as the temperature indicator after the polymer that melts at the higher tem- perature has been degraded by the UV light.

If the remaining phase is not in liquid form, the structure according to the above-mentioned second embodiment will not collapse even if the other polymer phase was removed or degraded. A temperature indicator of this type thus facilitates particularly the treatment of temperature indicators that function at low temperatures before they are attached to the product that is monitored. In addition to bicontinuous structures, also a so called core-shell technique can be used to manufacture the temperature indicator with activation. In this technique a double nozzle is used to form a protective shell on the component melting or crystallizing at a lower temperature using the component melting at a higher temperature. After the protective shell has been removed by activation, the structure of the temperature indicator may collapse as early as at the melting point or glass transition temperature of the component melting at a lower temperature.

According to another preferred embodiment, the activation is performed by selective dissolving, whereby a solvent dissolving the polymer functioning as the supporting structure is chosen as the solvent. The structure remaining after the solvent treatment mainly consists of the polymer melting at a lower temperature, whereby the structure of the temperature indicator collapses as early as at the melting point or the glass transition temperature of the polymer melting at a lower temperature.

By correctly selecting the melting point, the glass transition temperature or a combination thereof for the polymer or the polymer mixture, the method or the device according to the present invention can be used in various applications.

For example, the temperature indicators according to the present invention are suited to monitoring the temperatures of food casings, medicines and many other heat-sensitive products at the various stages of the cold chain, among others. Furthermore, the tempera- ture indicators can be used to monitor the high temperatures of foodstuffs, such as the degree of ripeness of ready meals or semi-finished meals. The temperature indicators can also be utilized in the electronics industry, e.g., by quickly and easily finding any failures caused by overheating of components, which are provided with temperature indicators, by means of the colour change of the temperature indicator. The temperature indicators can also be combined with the RFID technology, for example by using the temperature indicator as the memory of an RFID sensor, whereby, by monitoring the memory, it is easy to observe, whether the temperature of the product that is monitored by the sensor combination has remained above the maximum temperature set for it.

According to the first preferable practical application, the method according to the inven- tion is used to produce temperature indicators that are suitable for temperature monitoring, enabling a new, inexpensive and effective solution for controlling the fulfilment of regulations, which relate to the temperatures set for separately packed foodstuffs and medicines in the cold chain, among others, or to discover a faulty manipulation of the packaged products in the cold chain.

When placing the temperature indicator, for example, in a meat package packaged for consumers so that warming up of the package and, at the same time, of also the temperature indicator product-specifically correlate with the warming of the contents of the package, and by selecting the melting point, the glass transition temperature or both of the polymer or the polymer mixture so that the temperature of the package and also of the temperature indicator reach the said limiting temperature when the contents of the package have warmed up to or above the critical limiting temperature set product-specifically, the temperature indicator can be used to monitor the limiting temperatures set for foodstuffs, among others, in the various stages of the cold chain.

According to another preferred practical application, the devices that are suitable for tem- perature monitoring according to the invention are used to monitor the heating and cooking of convenience foods and uncooked and semi-finished products, which are prepared in ovens and microwave ovens. In this connection, the term "heating indicator" is used for the temperature monitoring devices suitable for the above-mentioned task. The colour change of the heating indicator indicates that the meal is warm enough and done to be enjoyed, making it easier to observe the required degree of ripeness or degree of warmth and, at the same time, reduces the spoilage of meals due to overheating.

When placing the heating indicator in the package to be heated so that warming up of the casing of the package correlates with warming up of the product in the package, and by selecting the melting point, the glass transition temperature or both of the polymer or the polymer mixture so that the temperature of the product-specific package and, at the same time, of also the heating indicator reach the said limiting temperature when the contents of the package have warmed up to a product-specifically intended temperature, the invention can extremely well be applied to monitoring the warming and cooking of convenience foods and uncooked and semi-finished products. According to a third preferred practical application, the heating indicators are used to monitor various products to indicate whether they have been heated adequately and to high enough temperatures. The heating indicator is suitable for example as indicators in the process of sterilizing the instruments used in hospitals. These instruments are typically packed and sterilized in overpressurized steam, while the temperature typically is between 120 °C and 135 °C. By attaching the heating indicator to the inside or the outside of the sterilizing package before the sterilization process, it becomes possible to easily detect whether the packed instruments have gone through the sterilization process.

According to a fourth preferred practical application, the temperature indicators are utilized in the electronics industry, for example, in tracing a component that has been damaged because of overheating, by attaching to or enclosing the electronic component in the temperature indicator. Generally, electronic components are damaged at high temperatures, whereby also the temperature indicator should be customized to have a high limiting temperature. For the polymers of such applications, polymers melting at high temperatures, such as polystyrene, polycarbonate, polymethacrylates, polyolefϊns are well-suited.

According to a fifth preferred practical application, the temperature indicators are combined with the RFID technology, whereby the temperature indicator can function, for example, as the memory of an RFID sensor. By monitoring the refraction coefficient, the light scattering or the opacity of the surface of the temperature indicator, the RFID sensor is capable of identifying, whether the temperature of the product being monitored by the sensor combination has, at some time, exceeded the maximum temperature set for it. The combination of the temperature indicator and the RFID can also be designed to activate and send a message/an alert on a change in the refraction coefficient, the light scattering or the opacity of the surface of the temperature indicator.

According to a sixth preferred practical application, the method according to the present invention is used in the manufacture of thermographic paper. Generally, thermographic paper is used in receipts, tickets, stickers, etc. An advantage of this application over the methods previously used is its resistance to UV light, i.e., the printout of the paper will not fade with time, which is a problem with the thermographic papers commonly used. According to a seventh preferred practical application, temperature indicators are used, the polymer or the polymer mixture of which being selected so that its melting point or glass transition temperature is close to the room temperature, whereby, for example, the collapsing of the polymer or the polymer mixture is provided by means of heaters found in house- holds, such as hair dryers, bringing the colour or the patterning of the base material into view. By using this application, for example, Advent calendars can be made, the shutters of which can be opened by means of a hair dryer, among others, whereby the picture or the text, which is under the shutter in the calendar, comes into view.

The following non-limiting example describes the invention

Example 1

The limiting temperature of a temperature indicator made of polyethylene oxide was customized to 59 °C. The functioning of this temperature indicator on exceeding the limiting temperature is shown in Figures Ia - Id.

The said temperature indicator was manufactured from polyethylene oxide (Aldrich) with a molecular weight of 100,000 g/mol. The aqueous solution of 15% by mass that was made of polyethylene oxide was defibrated by electrospinning onto a metal-coated sticker-based target that was in zero potential and was dyed red. The polymer solution was fed through a capillary tube that had an inner diameter of 1.8mm and a length of 51mm and was charged to a potential of 15kV. The distance between the tip of the capillary and the target perpen- dicular to it was 15cm, whereby the electric field strength was lkV/cm.

The operation of the device according to the invention is shown in the appended drawings:

Figure 1 shows a technologic prototype, which functions at a high temperature (the critical temperature is 59°C) and which has been heated by a Linkam TP 93 heating device. In Figure Ia, the temperature is stabilized to a temperature of 58°C. The temperature in Fig- ures Ib, Ic and Id has been raised by 2°C a minute and the heating has lasted for 15s, 30s and 45s, whereby the temperature achieved by the prototype, in the situations according to these figures, has been 58.5°C, 59°C and 59.5°C, respectively. An arrow has been added to the figures in the series to make it easier to observe the temperature indicator. Figure 3 shows two TEM images of the bicontinuous structure of a nanofibre formed from a PS-P4 VP(PDP) _{1 0} complex in the direction of the fibre (Figure 3a) and perpendicular to the fibre (Figure 3b). The diameter of the nanofibre in Figure 3b is about 150nm, whereas in Figure 3a it is about 250nm.

Claims

CLAIMS:

1. A device suited for temperature monitoring, comprising: nanoscale fragments consisting of structures with a high light scattering efficiency and a high opacity, which structures, as a result of a change in the temperature, are capable of collapsing and, with the fragments fusing together, forming larger-size structures that have an essentially weakened light scattering efficiency and opacity, characterized in that

- the nanoscale fragments are arranged on a layer of base material so that they at least partially cover this material optically, preventing the observation of the surface of the base material layer and, when collapsing, allow the observation of the surface of the base material layer, correspondingly.

2. The device according to Claim 1, characterized in that the nanoscale fragments consti- tute an unbroken layer that covers at least 5%, preferably at least 10%, particularly preferably about 15 - 100% of the surface of the base material.

3. The device according to Claim 1 or 2, characterized in that the nanoscale fragments consist of at least one type of a small molecule or polymer or a mixture of several poly- mers.

4. The device according to any of Claims 1 to 3, characterized in that the nanoscale fragments consist of at least two polymers, the first of which forms a supporting structure for the fragments and melts at a higher temperature than the other, and, optionally, of a UV activator, which together constitute a bicontinuous structure.

5. The device according to Claim 4, characterized in that the first polymer is a polymer that degrades in UV light, such as polymethyl methacrylate.

6. The device according to any of Claims 1 to 4, characterized in that the device comprising nanoscale fragments is activated by selective dissolving, whereby a solvent dissolving the polymer functioning as the supporting structure is chosen as the solvent.

7. The device according to any of Claims 1 to 6, characterized in that the structure of the bicontinuous fibre will not collapse until the polymer that melts at a higher temperature has reached its melting point or its glass transition temperature, even if the other of the polymer components of the fibre were already in liquid form.

8. The device according to any of Claims 1 to 5, characterized in that the temperature indicator made of at least two polymers can be activated by radiating the fibre with UV light, whereby the molecular mass of the polymer that melts at a higher temperature is reduced, and it is no longer capable of functioning as the supporting structure of the fibre, and whereby the structure of the fibre collapses as early as at the melting point or the glass transition temperature of the polymer that melts at a lower temperature.

9. The device according to any of Claims 1 to 5, characterized in that the temperature indicator made of at least two polymers can be activated by radiating the fibre with light from a laser or a LED, whereby the molecular mass of the polymer that melts at a higher temperature is reduced, and it is no longer capable of functioning as the supporting structure of the fibre, and whereby the structure of the fibre collapses as early as at the melting point or the glass transition temperature of the polymer that melts at a lower temperature.

10. The device according to any preceding claim, characterized in that the surface of the base material has a specific colour or an identifiable pattern has been formed thereon, and the surface for this part is covered by a layer of nanoscale fragments having a white, almost white or off-white colour that prevents a total or almost total reflection of the specific colour or the pattern of the base material, as a result of which the coated base material re- fleets outwards a colour that is exclusively or almost exclusively specific to the nanoscale fragments.

11. The device according to any preceding claim, characterized by comprising nanofibres or nanoparticles, which are manufactured from at least one polymer by a method, compris- ing: preparing a defibration solution or melt containing at least one polymer or polymer mixture, having a viscosity of over 0.0001N/m²s, preferably over 0,01N/m²s, and bringing the polymer or the polymer mixture into fibrous form by electro spinning so that the defibration solution or melt is fed through a capillary tubing charged to a first potential to a receiving area charged to a second potential, whereby the potential difference and the distance between the electrodes mentioned above are 0.5 - 10kV/cm and 10 - 1000mm, respectively.

12. The device according to any preceding claim, characterized in that the melting point, the glass transition temperature or both of the material or the mixture of materials to be defibrated is selected so that the structure of the material that is defibrated, formed into drops or formed by a combination thereof collapses under heating.

13. A method of manufacturing a device suited for detecting changes in temperature, characterized in forming, on the surface of a base material layer, a layer comprising nanoscale fragments consisting of structures having a high light scattering efficiency and a high opacity, the structures being capable of collapsing as a consequence of a change in the temperature and forming larger-size structures that have an essentially weakened light scattering efficiency and opacity, whereby the nanoscale fragments are arranged on the base material layer so that they at least partially cover this material optically and prevent the observation of the base material layer and, correspondingly, when breaking down, allow the observation of the material.

14. The method according to Claim 13, characterized in that the surface of the base material is provided with a specific colour or an identifiable pattern is formed on the surface, and

- with respect to the colour or the patterning, the surface is covered by a layer of nanoscale fragments having a white, almost white or off-white colour, which totally or almost totally prevents the reflection of the specific colour or the pattern of the base material, whereby the coated base material reflects outwards exclusively or almost exclusively the colour specific to the nanoscale fragments.

15. Use of the device according to any of Claims 1 to 12 as a temperature or heating indicator.

16. The use according to Claim 15, characterized in that the device is used in a package or in connection thereof to indicate an exceeding of a predefined limiting temperature in the package.

17. The use according to Claim 15, characterized in that the temperature indicator is used to control the handling of packaged products in a cold chain.

18. The use according to Claim 15, characterized in that the device is used to monitor the warming or cooking of convenience foods as well as raw and semi-finished products that are prepared in an oven or a microwave oven.

19. The use according to Claim 15, characterized in that the device is used to monitor the temperature of components in the electronics industry.

20. Use of the device according to any of Claims 1 to 12 as the memory of an RFID sensor.

21. Use of the device according to any of Claims 1 to 12 in the manufacture of thermographic paper.