WO2022261367A1

WO2022261367A1 - Improved temperature indicator element

Info

Publication number: WO2022261367A1
Application number: PCT/US2022/032901
Authority: WO
Inventors: Hiroki Kigami; Koichi Nakamura; Kurato AKIBA; Kimihiko Yoshimatsu; Nahid Mohajeri
Original assignee: Nitto, Inc.
Priority date: 2021-06-10
Filing date: 2022-06-09
Publication date: 2022-12-15

Abstract

Described herein are thermochromic indicator elements comprising a porous indicator layer which provide improved shelf life and durability for extended temperature monitoring, and methods for making and using the same.

Description

IMPROVED TEMPERATURE INDICATOR ELEMENT

Inventors: Hiroki Kigami, Koichi Nakamura, Kurato Akiba, Kimihiko Yoshimatsu, and Nahid Mohajeri

CROSS-REFERENCE TO RELATED APPLICATIONS

[001] This application claims priority to United States Provisional Application No. 63/209,338, filed June 10, 2021, which is incorporated by reference herein in its entirety.

FIELD

[002] The present embodiments are related to a temperature indicator element for indicating the attaining of a predetermined temperature.

BACKGROUND

[003] In many types of manufacturing processes, ovens or heating units have temperature indicators that show the temperature inside the oven or heating unit. However, this temperature can often be different from the actual temperature of the object being heated because it takes time for an object in an oven or a heating unit to warm. Temperature indication labels may be used to more accurately determine the actual temperature of an object being heated. However, they are generally for short term use, and there is a need for additional temperature indicators that are suitable for long term use.

SUMMARY

[004] The disclosure relates to a thermochromic indicator for environmental temperature having increased durability and/or activity lifetime.

[005] Some embodiments include a temperature indicator for indicating the attaining of a threshold temperature, the indicator comprising: a polymer matrix; and a visual indicator element which changes appearance as a result of reaching the threshold temperature.

[006] Some embodiments include a method for making a temperature indicating element comprising: mixing polymer particles, the polymer particles having a melting temperature at a threshold temperature, in a liquid hydrocarbon lubricant to form a polymer mixture; forming a porous film of the polymer mixture; and heating the porous film of the polymer mixture to remove the hydrocarbon lubricant. [007] Some embodiments include a method for indicating temperature comprising: thermally coupling a temperature indicator described herein to an object to be heated to a threshold temperature; and wherein the polymer matrix of the temperature indicator comprises a plurality of open pores; wherein a polymer in the polymer matrix has a melting point that is about the same as the threshold temperature; wherein the temperature indicator and the object are heated to a temperature that is at least about as high as the threshold temperature; and wherein, as a result of being heated to the temperature that is at least about as high as the threshold temperature, a sufficient number of the open pores are closed to increase the transparency of the polymer matrix to at least 50%.

BRIEF DESCRIPTION OF THE DRAWINGS

[008] FIG. 1 is a depiction of a possible embodiment of an indicator element described herein at a first temperature.

[009] FIG. 2 is a depiction of a possible embodiment of an indicator element described herein at a second temperature.

[010] FIG. 3 is a depiction of a possible embodiment of a thermochromic indicator element described herein.

[011] FIG. 4 is a depiction of a possible embodiment of a thermochromic indicator element described herein.

[012] FIG. 5 is a depiction of a possible embodiment of a thermochromic indicator element described herein.

[013] FIG. 6 is a depiction of a possible embodiment of a thermochromic indicator element described herein.

[014] FIG. 7 is a depiction of a possible embodiment of a thermochromic indicator element described herein.

DETAILED DESCRIPTION

General

[015] This disclosure relates to temperature indicator that indicates, e.g. by visual appearance, when a desired temperature, also referred to as a threshold temperature, is reached or exceeded during a heating process. Generally, the temperature indicator may include a polymer matrix and an indicator element, such as a visual indicator element, which changes appearance as a result of reaching the threshold temperature.

[016] The indicator element may operate by a physical or chemical change that occurs as a result of reaching the threshold temperature. For example, the indicator element may operate as a result of a melting process, such as melting of a polymer in the polymer matrix. For example, melting may change the appearance of the indicator element. The indicator may also operate as a result of other types changes in property, such as a change in electrical conductivity.

[017] For indicators where the indicator element changes appearance as a result of a melting process, the change in appearance caused by the melting may be a result of a change in polymer matrix pore size, porosity, light transmittance, air permeability, or some other property that is affected by melting. In some embodiments, the change in appearance of the visual indicator element is a result of partial or complete melting of a material having a porous structure. In some embodiments, the change of appearance of the visual indicator element appearance can be a color change that can be determined by light transmittance and/or air permeability change.

[018] An indicator element, such as a visual indicator element, may have a variety of suitable structure or forms. In some embodiments, the visual indicator element may have a structure that combines the polymer matrix with a plurality of pores within the polymer material. For example, this polymer matrix with a plurality of pores may be in the form of a porous indicator layer. In some embodiments, the porous indicator layer can comprise polymer having plural defined open pores within the polymer material.

[019] The visual indicator element may operate by melting of a polymer of the polymer matrix. For example, a polymer in the polymer matrix may have a melting temperature at or near the threshold temperature. Flowever, the melting temperature may be different than the threshold temperature. For example, other components in the polymer matrix may cause the polymer to melt at a lower temperature than the melting temperature of the pure polymer. Additionally, the structure of the indicator may be such that the temperature of the polymer is different than the temperature of other parts of the indicator, such as a part of the indicator that is closest in temperature to the object being heated. Thus, for example, the threshold temperature may be within about 1 °C, about 3 °C, about 5°C, about 10 °C, or about 30 °C (for example, if the melting temperature is 330 °C, the threshold temperature might be in a range of 300-360 °C). Melting of the polymer may cause some or all of the pores within the polymer material to close, fill, or change shape. This change in the number or shape of the pores may then affect the visual appearance of the polymer matrix. For example, it may make the polymer matrix more transparent. [020] For embodiments where the polymer matrix becomes more transparent as a result of reaching or exceeding the threshold temperature, the transparency of the polymer matrix may be increased by at least 10%, at least 20%, at least 30%, at least 50%, or at least 70%. In some embodiments, the transparency of the polymer matrix is increased by at least 50%. As a result of reaching the threshold temperature, the transparency of the polymer matrix may approach 100%, e.g. 95%, 99%, 99.9%, etc.

[021] The temperature indicator may include additional structural features, such as an adhesive or adhesive layer, a backing layer, a release liner, etc. In some embodiments, the temperature indicator can comprise an adhesive layer. In some embodiments the temperature indicator can comprise a backing layer. In some embodiments, the temperature indicator can comprise a release liner.

[022] FIG. 1 and FIG. 2 illustrate how some embodiments of temperature indicators may be structured and how they may operate. For example, prior to heating to the threshold temperature, some temperature indicators may have a structure such as that shown in FIG. 1. In FIG. 1, temperature indicator 10 comprises indicator layer 12. Indicator layer 12 may comprise a polymer matrix 14 and plural pores, such as pores 16 within polymer matrix 14. When temperature indicator remains below the melting temperature of the polymer matrix, temperature indicator 10 retains the structure shown in FIG. 1.

[023] In some embodiments, the plural pores, e.g., pores 16, can contain a material within the pore. The material within the pores may have an index of refraction that is different from the polymer matrix material, e.g., air. These pores can be defined by air bubbles formed in the polymer matrix during mixing and or formulation. Alternatively or additionally, these pores can be defined by interstitial spaces defined between discrete particulate polymer materials, e.g., particles, powder, and / or microspheres. These pores and/or air voids can affect a haziness, opacity or lack of transparency in the polymer matrix.

[024] FIG. 2 illustrates what may happen to the structure of indicator layer 10 when the temperature reaches threshold temperature, or the melting temperature of a polymer of the polymer matrix. As shown in FIG. 2, melting of the polymer may cause the plural pores to be fused, dissolved, and/or dissipated. This may remove the plural pores, and/or the material contained within the pores that has a different refractive index than the polymer of the matrix, due to collapse, fusion, or other change, etc. Removal of these pores, etc. produces a film layer 12 having a relatively uniform refractive index. Thus, the transparency of the polymer matrix may increase as a result of the change in or removal of the pores within the polymer. As a result, reaching the threshold temperature can be visually detected. For example, the unmelted polymer matrix can be opaque and/or white, and the melted polymer matrix can be transparent and/or colorless. [025] In some embodiments, as shown in FIG. 3, the polymer matrix 12 can comprise plural particles 14A defining pores 16 therebetween.

[026] While depicted herein as spherical shapes, the pores and particles of FIGS. 1, 3, 4-7, or any other relevant Figure, as well as pores and particles of any other embodiments, may have irregular shapes. For example, pores may have a shape defined by the space between adjacent polymer matrix particles. The particles may have shapes such as flakes, irregularly shaped particles, irregular powder granules or comminuted materials.

[027] In some embodiments, as shown in FIGS. 4, 5, 6 and/or 7, the thermochromic indicator 10 can further comprise an adhesive layer 18. In some embodiments, the thermochromic indicator can further comprise a release liner 20. In some embodiments, the thermochromic indicator can further comprise a backing layer 22. In some embodiments, the thermochromic indicator can further comprise a color contrast layer 24. In some embodiments, the color contrast layer can be disposed behind, beneath the polymer matrix 12 so that when the polymer matrix 12 becomes more transparent, gas permeable, and/or less porous, the color contrasting layer, differing in color from the indicator layer/polymer matrix layer, is visually perceivable. To adhere backing layer 22 or color contrast layer 24 to indicator layer 12 directly, these layers can be made from thermoplastics like PET, PP, PE ,etc. When these layers are laminated, adding heat and pressure makes thermoplastic resins flow into the porous structure of the indicator layer. As result, a mass anchorage effect is expected. In some embodiments, backing layer 22 or contrast layer 24 and indicator layer 12 can be laminated with adhesive layer 18A (FIG. 7).

[028] It is believed that this construction can enable immediate indication of the presence of the targeted or desired local environmental temperature despite a time interval from the initial disposition of the indicator element to the time of the perceived indication of desired environmental temperature.

Indicator / Polymer matrix layer

[029] In some embodiments, the indicator can comprise a polymer matrix having a melting temperature at a desired indicated temperature, or a threshold temperature. In some embodiments, polymer can be selected for having a melting temperature and or melting temperature that corresponds to the threshold temperature. In some embodiments, for example, if the selected polymer is PTFE, then the melting temperature can be, for example, greater than 330 °C, greater than 340 °C and/or greater than 350 °C. The melting point of PTFE can be about 345 degrees C as unbaked (unsintered). It may be varied by grade, molecular weight, size of pellet (granulated) or powder grain. Flowever, typically, it is about 345 +/- 10 °C. Also, whether the PTFE is crystalized or not (by baking, sintering) may influence its melting point. For example, when PTFE is sintered (crystalized), the melting point may be lowered to around 327 °C.

[030] In some embodiments, where the PTFE is crystalline, either by the film processing (recrystallization occurring during cooling down) or as incorporated into the construction, the melting temperature and thus the threshold temperature for changing to transparent from opaque, can be about 330 °C. In some embodiments, where the PTFE is not sintered/non-sintered (amorphous), either by the film processing or as incorporated into the construction, the melting temperature (T_m), can be about 350 °C, and thus the threshold temperature may be around 350 °C, e.g., in a range of about 300-400 °C or about 340-360 °C. Other polymers can be selected to provide the desired melting temperature, target desired indicator transparency change. See Table 1 below.

TABLE 1

[031] In some embodiments, the polymer matrix comprises polytetrafluoroethylene (PTFE). In some embodiments, the PTFE can be crystalline.

[032] In some embodiments, the polymer matrix comprises poly(acrylonitrile), syndiotactic.

[033] In some embodiments, the polymer matrix comprises poly(6-aminocaproic acid).

[034] In some embodiments, the polymer matrix comprises poly(caprolactam).

[035] In some embodiments, the polymer matrix comprises poly(decamethylene adipamide).

[036] In some embodiments, the polymer matrix comprises poly(decamethylene sebacamide).

[037] In some embodiments, the polymer matrix comprises poly(etheretherketone).

[038] In some embodiments, the polymer matrix comprises poly(ethylene terephthalate).

[039] In some embodiments, the polymer matrix comprises poly(ethylene).

[040] In some embodiments, the polymer matrix comprises poly(hexamethylene adipamide).

[041] In some embodiments, the polymer matrix comprises poly(hexamethylene sebacamide).

[042] In some embodiments, the polymer matrix comprises poly(hexamethylene suberamide).

[043] In some embodiments, the polymer matrix comprises poly(methyl methacrylate).

[044] In some embodiments, the polymer matrix comprises poly(methyl methacrylate).

[045] In some embodiments, the polymer matrix comprises poly(methylene oxide).

[046] In some embodiments, the polymer matrix comprises poly(4-methylpentene).

[047] In some embodiments, the polymer matrix comprises poly(propylene).

[048] In some embodiments, the polymer matrix comprises poly(styrene).

[049] In some embodiments, the polymer matrix comprises poly(trans-l, 4-butadiene).

[050] In some embodiments, the polymer matrix comprises poly(vinyl alcohol).

[051] In some embodiments, the polymer matrix comprises poly(vinyl chloride).

[052] In some embodiments, the polymer matrix comprises poly(vinyl fluoride).

[053] In some embodiments, the polymer matrix comprises poly(vinylidene chloride).

[054] In some embodiments, the polymer matrix comprises poly(vinylidene fluoride).

[055] In some embodiments, the polymer matrix comprises poly(io-undecanamide).

[056] In temperature indicators having a polymer matrix with a plurality of pores in the form of a porous indicator layer, such as a porous film, the layer or film may have any suitable thickness. Thickness may influence the time that is required for the indicator to detect that the threshold temperature has been reached. For example, a thinner film has a smaller heat capacity, and thus its temperature increases more quickly. Thickness may also influence material handling. If a film is too thin, it may be difficult to handle.

[057] In some embodiments, the porous film or layer may have a thickness that is at least about 0.001 mm, about 0.05 mm, about 0.01 mm, or about 0.1 mm. In some embodiments, the porous film or layer may have a thickness that is up to about 0.5 mm, about 0.8 mm, about 1.0 mm, about 2.5 mm, about 10 mm, about 25 mm, about 50 mm, about 75 mm and/or about 100 mm. In some embodiments, the porous film can have a thickness that is within about 10%, within about 20%, within about 50%, or within about 80% of one of the following thicknesses: about 0.1 mm, about 0.2 mm, about 0.35 mm, about 0.4 mm, about 0.6 mm, and about 1 mm. For example, a thickness that is within about 50% of 0.4 mm would be a range of about 0.2-0.6 mm. In some embodiments, a porous layer or film comprises a porous PTFE film having thickness between 0.01-5 mm, for example, 0.05-2 mm. [058] In some embodiments, the indicator layer can comprise a porous polymer film. In some embodiments, the indicator layer can comprise an organic polymer. In some embodiments, the organic polymer can be a fluorinated organic polymer. In some embodiments, the organic polymer can be polytetrafluoroethylene (PTFE). As shown in FIG. 3, the organic polymer can comprise plural particles, e.g., plural pellets and / or powder particles. In some embodiments, the plural particles can have an average particle diameter of between 0.001 mm, 0.01 mm, 0.05 mm, 0.1 mm, 0.25 mm to 0.50, 0.75, 1.0 mm, 2.0 mm and/or any permutation of the aforementioned values. In some embodiments, the plural particles have an average particle diameter between 0.1-1 mm.

A porous polymer film or layer may potentially be used as temperature indication label without any additional components. Flowever, single layer of adhesive with liner or double coated tape can be applied on one side of this PTFE film. These structures may make it easier to position the label and/or hold the label in the desired position (see FIGS. 6 and 7).

Adhesive layer

[059] In some embodiments, the temperature indicator can comprise an adhesive layer. The adhesive layer may comprise an adhesive polymer. In some embodiments, the adhesive polymer can be a pressure sensitive adhesive, hot melt, and/or heat curable adhesive.

[060] In some embodiments, the adhesive polymer can comprise acrylic base adhesive, polyurethane, and/or synthetic rubber base adhesive. Silicone adhesive leaves very little, or no residue and its residue is easy to clean. On the other hand, other types of adhesive can leave behind a significant amount of residue and they are very hard to clean. Thus, in general, an adhesive residue of acrylic adhesive, polyurethane adhesive, or synthetic rubber base adhesive may be more difficult to remove than silicone adhesive.

[061] In some embodiments, the adhesive polymer can be curable. After adhesive is dried and cured, it can be laminated with the indicator layer (also known as transfer coat). To apply silicone base adhesive as transfer coat, Pt cure system silicone adhesive can be better than a BPO (benzoyl peroxide) curing system. A BPO curing system tends to be reacted with fluoro-silicone release agent on a release liner. That can make the liner release from the adhesive heavier and difficult to remove.

[062] In some embodiments the adhesive element can comprise a backing layer. In some embodiments, the backing layer can be transparent. In some embodiments, the backing layer can be opaque. In some embodiments, the backing layer can comprise polyethylene, polypropylene, polyethylene terephthalate, polyurethane, or a similar material. In some embodiments, the backing layer can comprise paper, nonwoven material.

Release liner

[063] In some embodiments, the thermochromic indicator element can comprise a release liner. In some embodiments, the release liner can be disposed upon a bottom layer of the indicator, e.g., applied on the temperature indicator layer or adhesive layer and/or adhesive layer. The release liner may be removed from the adhesive layer to allow the adhesive layer (and the thermochromic layer comprising the adhesive layer) to attach at desired monitored location. In some embodiments, the release liner can comprise paper, polyethylene, polypropylene and a release agent. The release agent may help the paper to be cleanly removed from the adhesive layer. In some embodiments, the release agent can be silicone, fluoro silicone, carbamate, or a similar material.

[064] In some embodiments, the release liner may comprise a carbamate compound release agent or silicone release agent. In some embodiments, if the indicating layer comprises a silicone adhesive, a release agent can comprise a fluoro-silicone release agent, e.g., Loparex, S2 CL PET 5100/000. In some embodiments, the release liner may comprise oriented polypropylene, polyethylene terephthalate, polyethylene, or paper.

Color contrasting layer

[065] In some embodiments, the temperature indicator can comprise a color contrasting layer. In some embodiments, the color contrasting layer is disposed beneath the porous indicator layer. In some embodiments, the porous indicator layer is interposed between the color contrasting layer and the external environment, e.g., the viewer. In some embodiments, the color contrasting layer may be a color that is not white and/or translucent. The color contrasting layer is not visible when the temperature indicator is at the first temperature, for example below the melting temperature. When the porous indicator layer reaches its melting point, it becomes transparent or at least less opaque. As a result, the differently colored color contrasting layer becomes perceivable from behind the more transparent, less porous indicator layer.

[066] In some embodiments, the backing layer can be the color contrasting layer if the backing layer is colored different from the opaque porous indicator layer, e.g., the backing layer can comprise a second color different from the opaque porous indicator layer.

[067] By using a colorimeter, the chromaticity/transparency can be measured. Note that the chromaticity is expressed by the lightness index of the L*a*b* color system (CIELAB1976). The absolute value of the difference is the measured chromaticity between the standard whiteboard and the thermochromic indicator-element. This value is reported as "DE" for before and after exposure to high temperature.

From the above results the appearance change and/or transparency can be evaluated by the following equations:

DE difference = | DE after - DE before |

[068] In some embodiments, the appearance change and / or transparency can be evaluated by transparency measurement. A suitable methodology for evaluating transparency can by ASTM D:1003-00 (ASTM D1003-00, Standard Test Method for Flaze and Luminous Transmittance of Transparent Plastics, ASTM International, West Conshohocken, PA, USA).

Methods

[069] A temperature indicator as described herein can be made by any suitable method. For example, a temperature may be prepared by a method comprising mixing the polymer particles described herein in a liquid hydrocarbon lubricant to form a polymer mixture. A porous film may be formed from the polymer mixture, and/or heated to remove the hydrocarbon lubricant. In some embodiments, a porous polymer film may be formed by a method comprising heating and compressing the polymer mixture. In some embodiments, the method can further comprise stretching the heated porous polymer film. In some embodiments, the polymer particles have, or may be selected for having, a melting temperature around the threshold temperature.

[070] In some embodiments, the polymer matrix can comprise plural polymer particles and the plurality of open pores comprise the interstitial spacing defined between the plural polymer particles. In some embodiments, the polymer matrix may be prepared by a method comprising stretching the polymer matrix when the polymer matrix is heated near the melting temperature of the polymer. [071] For methods comprising mixing polymer particles in a liquid hydrocarbon lubricant to form a polymer mixture, the polymer precursor can be mixed with a hydrocarbon lubricant. In some embodiments, the liquid carrier lubricant can comprise a C to C hydrocarbon. In some embodiments, the liquid carrier lubricant can be dodecane. In some embodiments, the weight ratio of hydrocarbon lubricant to polymer material can be between about 5-50, about 10-40, about 10-20, about 20-30, about 15-20, about 20-25, about 25-30, about 30-40, or about 20 parts hydrocarbon lubricant to 100 parts by weight fine powder polymer, e.g., 20 parts dodecane to 100 parts fine PTFE powder. In some embodiments, the method can comprise forming a porous film of the polymer mixture. In some embodiments, forming the porous polymer film can comprise molding the polymer mixture. In some embodiments, the porous polymer film can be obtained by extruding a mixture of fine polymer powders or pellets and liquid lubricant into sheet form or milled by roller and making it into a sheet form.

[072] In some embodiments, molding the polymer mixture can comprise paste extruding the polymer mixture to form a rod-shaped product. In some embodiments, the paste extruding can occur at 15 °C to about 60 °C, about 15-30 °C, about 30-40 °C, about 40-50 °C, about 50-60 °C, about 30-35 °C, about 35-40 °C, about 40-45 °C, about 45-50 °C, or at about 40 °C. In some embodiments, the polymer mixture can be baked (sintered, crystallized) at a temperature between 320 °C and 370 °C. In some embodiments, when the polymer mixture is sintered, the polymer resulting polymer sheet can be stretched into two times to thirty times in machine direction and cross-web direction. In some embodiments, pellets or powders can be partially fused but still retain pores or spaces between pellet and pellet; or powder grain and powder grain. When it is elongated (or expanded, or stretched), pores can become larger. PTFE film as temperature indicator can be used with or without stretching. In some embodiments, this extruded film can be expanded in machine direction and/or cross web direction. This expanded film can have high air permeability and/or high light transmittance.

[073] It is believed that the stretching can facilitate maintaining the porous morphology within the stretched polymer sheet. In some embodiments, the film can be subjected to additional expansion processes. In some embodiments, the method can comprise heating the porous film polymer mixture to remove the hydrocarbon lubricant, but retaining the spacing defined between polymer particles and/or spacing defined by the hydrocarbon lubricant prior to evaporative removal. In some embodiments, the polymer film polymer mixture can be heated at 100 °C to about 200 °C, about 150- 200 °C, about 140-180 °C, or about 150-170 °C, e.g., 160 °C for dodecane.

[074] The temperature indicators described herein may be used in a method for indicating temperature. This method may comprise thermally coupling a temperature indicator described herein to an object to be heated to a threshold temperature. In this instance, "thermally coupling" means that the temperature indicator and the object to be heated are coupled, e.g., fastened or otherwise kept in proximity to one another, so that the temperature indicator remains at about the same temperature as the object to be heated. In some embodiments, the polymer matrix of the temperature indicator comprises a plurality of open pores, and the polymer in the polymer matrix has a melting point that is about the same as the threshold temperature. Because of the thermal coupling, the temperature indicator and the object are heated, e.g., together, to a temperature that is at least about as high as the threshold temperature. In this method, as a result of being heated to the temperature that is at least about as high as the threshold temperature, a sufficient number of the open pores are closed to increase the transparency of the polymer matrix. For example, the transparency, or the total transmittance (e.g., of visible light), may be at least 25%, 50%, 75%, 90% after the threshold temperature has been reached.

EMBODIMENTS

Embodiment 1. A temperature indicator for indicating the attaining of a desired temperature, the indicator comprising: a. a polymer matrix; and b. a visual indicator element, the visual indicator element changing appearance as a result of pore size, porosity, light transmittance or air permeability.

Embodiment 2. The temperature indicator of embodiment 1, wherein the visual indicator element comprises a plurality of defined open pores within the polymer matrix herein , and a melting temperature at a desired indicated temperature, the polymer matrix having plural defined open pores providing at least a first level of transparency, wherein the application of temperature above the desire indicated temperature melts the polymer matrix and closes a sufficient amount of open pores to provide a second higher level of transparency.

Embodiment s. The temperature indicator of embodiment 2, wherein the second level of transparency is greater than 50 %.

Embodiment 4. The temperature indicator of embodiment 1, wherein the polymer matrix comprises plural particles.

Embodiment s. The temperature indicator of embodiment 1, wherein the polymer matrix comprises polytetrafluoroethylene (PTFE).

Embodiment 6. The temperature indicator of embodiment 5, wherein the PTFE is unsintered polytetrafluoroethylene. Embodiment 7. The temperature indicator of embodiment 6, wherein the melting temperature is greater than 330 °C.

Embodiment 8. The temperature indicator of embodiment 5, wherein the PTFE is non-sintered polytetrafluoroethylene.

Embodiment 9. The temperature indicator of embodiment 8, wherein the melting temperature is equal to or greater than 345°C .

Embodiment 10. The temperature indicator of embodiment 1, comprising a porous film, the porous film comprising the polymer matrix of embodiments 1-9.

Embodiment 11. The temperature indicator of embodiment 10, wherein the porous film has a thickness between 0.01 mm and 100 mm.

Embodiment 12. The temperature indicator of embodiment 10, wherein the polymer matrix comprises plural particles sized having an average particle diameter between 0.01 mm and 1.0 mm. Embodiment 13. The temperature indicator of embodiment 1, further comprising an adhesive layer.

Embodiment 14. The temperature indicator of embodiment 1, further comprising a release liner. Embodiment 15. The temperature indicator of embodiment 1, further comprising a backing layer.

Embodiment 16. A method for making an indicating temperature element comprising: a. mixing polymer particles, the polymer particles having a melting temperature at the desired indicated temperature, in a liquid hydrocarbon lubricant to form a polymer mixture; b. forming a porous film of the polymer mixture; and c. heating the porous film polymer mixture to remove the hydrocarbon lubricant. Embodiment 17. The method of embodiment 16, wherein forming a porous film comprises heating and compressing the polymer mixture to create a porous film polymer.

Embodiment 18. The method of embodiment 16, further comprising selecting a polymer particle having a melting temperature at the desired indicated temperature.

Embodiment 19. The method of embodiment 16, further comprising stretching the heated porous polymer film.

Embodiment 20. A method for indicating temperature comprising:

Providing a polymer matrix defining a plurality of open pores therein, the polymer matrix having a melting temperature, the polymer matrix disposed at a first temperature below said melting temperature and having a first transparency at said first temperature; Applying a second temperature to the polymer matrix, the second temperature above the melting point of the polymer matrix and sufficient to melt and close at least some of the plurality of defined open pores, wherein sufficient pores are closed to increase the transparency of the polymer matrix to at least 50%.

Embodiment 21. The method for indicating temperature of embodiment 20, wherein the polymer matrix comprises plural polymer particles and the plurality of open pores comprise the interstitial spacing defined between the plural polymer particles.

EXAMPLES

[075] It has been discovered that embodiments of the thermochromic temperature indicator element described herein have improved durability as compared to other thermochromic temperature indicator elements. These benefits are further demonstrated by the following examples, which are intended to be illustrative of the disclosure only, and are not intended to limit the scope or underlying principles in any way.

An example was created and tested as described herein.

[076] Polytetraflouroethylene (PTFE) porous film was produced by the following method. Specifically, 20 parts by weight of a liquid lubricant, dodecane,( Isoper M Tokyo yuso, Tokyo Japan ) is uniformly mixed with 100 parts by weight of a PTFE fine powder ("Polyflon [registered trademark] PTFE F-104" manufactured by Daikin Co., Ltd., standard specific gravity: 2.162). A mixture was obtained. Next, the mixture was molded by paste extrusion at 40 °C to obtain a rod-shaped molded product. The obtained rod-shaped molded product was passed between a pair of metal rolling rolls to obtain a long sheet. Then, the obtained long sheet was heated to 160 °Cto remove lubricant. Thickness of film was 0.35 mm as completed. This film was not expanded with using additional expansion process.

Test method & results

[077] PTFE film made as described above was placed in an oven at various temperature for 5 min. to see if its appearance is changed.

Result

[078] Table 2 shows the results.

[079] Original color of PTFE temperature indicator was white (opaque).

[080] When PTFE film as temperature indicator was exposed to 350 °C, which is higher than the melting point of this film, the appearance of film was changed. The visually perceived white color gradually changed to translucent and or transparent. It can be expected that air permeability and/or light transmittance of specimen before and after heated at 350 degree C was different.

[081] When the film was exposed to 370 degree C, the originally white color of film had disappeared and turned to transparent.

[082] More significant differences in air permeability and/or light transmittance can be expected between before and after specimen was heated up.

[083] Samples that were exposed to 350 degree C and 370 degree C, did not return back original color after those samples were cooled down.

[084] Thus, PTFE temperature indicator label can show if temperature reaches higher than its melting point that is about 350 °C.

Table 2

[085] Unless otherwise indicated, all numbers expressing quantities of ingredients, properties such as molecular weight, reaction conditions, and etc. used in herein are to be understood as being modified in all instances by the term "about." Each numerical parameter should at least be construed in light of the number of reported significant digits and by applying ordinary rounding techniques. Accordingly, unless indicated to the contrary, the numerical parameters may be modified according to the desired properties sought to be achieved, and should, therefore, be considered as part of the disclosure. At the very least, the examples shown herein are for illustration only, not as an attempt to limit the scope of the disclosure. [086] The terms "a," "an," "the" and similar referents used in the context of describing embodiments of the present disclosure (especially in the context of the following embodiments) are to be construed to cover both the singular and the plural, unless otherwise indicated herein or clearly contradicted by context. All methods described herein may be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. The use of any and all examples, or representative language (e.g., "such as") provided herein is intended merely to better illustrate embodiments of the present disclosure and does not pose a limitation on the scope of any embodiment. No language in the specification should be construed as indicating any non-embodied element essential to the practice of the embodiments of the present disclosure.

[087] Groupings of alternative elements or embodiments disclosed herein are not to be construed as limitations. Each group member may be referred to and embodied individually or in any combination with other members of the group or other elements found herein. It is anticipated that one or more members of a group may be included in, or deleted from, a group for reasons of convenience and/or patentability.

[088] Certain embodiments are described herein, including the best mode known to the inventors for carrying out the embodiments. Of course, variations on these described embodiments will become apparent to those of ordinary skill in the art upon reading the foregoing description. The inventor expects skilled artisans to employ such variations as appropriate, and the inventors intend for the embodiments of the present disclosure to be practiced otherwise than specifically described herein. Accordingly, the embodiments include all modifications and equivalents of the subject matter recited in the embodiments as permitted by applicable law. Moreover, any combination of the above- described elements in all possible variations thereof is contemplated unless otherwise indicated herein or otherwise clearly contradicted by context.

[089] In closing, it is to be understood that the embodiments disclosed herein are illustrative of the principles of the embodiments. Other modifications that may be employed are within the scope of the embodiments. Thus, by way of example, but not of limitation, alternative embodiments may be utilized in accordance with the teachings herein. Accordingly, the embodiments are not limited to embodiments precisely as shown and described.

Claims

1. A temperature indicator for indicating the attaining of a threshold temperature, the indicator comprising: a polymer matrix; and a visual indicator element which changes appearance as a result of reaching the threshold temperature.

2. The temperature indicator of claim 1, wherein the change in appearance of the visual indicator element is a result of a change in pore size, porosity, light transmittance, or air permeability.

3. The temperature indicator of claim 1 or 2, wherein the change in appearance of the visual indicator element is a result of partial or complete melting of a polymer in the polymer matrix.

4. The temperature indicator of claim 3, wherein the polymer in the polymer matrix has a melting point that is about the same as the threshold temperature.

5. The temperature indicator of claim 3, wherein the visual indicator element comprises a plurality of pores within the polymer matrix, wherein partial or complete melting of the polymer in the polymer matrix changes the number, size, or shape of at least some of the plurality of pores, resulting in a change in transparency of the polymer matrix.

6. The temperature indicator of claim 5, wherein the change in transparency of the polymer matrix comprises an increase in transparency of the polymer matrix.

7. The temperature indicator of claim 6, wherein the increase in transparency of the polymer matrix is greater than 50%.

8. The temperature indicator of claim 1, wherein the polymer matrix comprises plural particles.

9. The temperature indicator of claim 1, wherein the polymer matrix comprises polytetrafluoroethylene (PTFE).

10. The temperature indicator of claim 9, wherein the PTFE is unsintered polytetrafluoroethylene.

11. The temperature indicator of claim 10, wherein the PTFE has a melting temperature that is about equal to or greater than 330 °C.

12. The temperature indicator of claim 11, wherein the PTFE has a melting temperature that is about equal to or greater than 345°C .

13. The temperature indicator of claim 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, or 12, comprising a porous film, wherein the porous film comprises the polymer matrix.

14. The temperature indicator of claim 13, wherein the porous film has a thickness between about 0.01 mm and about 100 mm.

15. The temperature indicator of claim 13, wherein the polymer matrix comprises a plurality of particles having an average particle diameter between about 0.01 mm and about 1.0 mm.

16. The temperature indicator of claim 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, or 15, further comprising an adhesive layer.

17. The temperature indicator of claim 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, or 16, further comprising a release liner.

18. The temperature indicator of claim 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, or 17, further comprising a backing layer.

19. A method for making a temperature indicating element comprising: mixing polymer particles, the polymer particles having a melting temperature at a threshold temperature, in a liquid hydrocarbon lubricant to form a polymer mixture; forming a porous film of the polymer mixture; and heating the porous film of the polymer mixture to remove the hydrocarbon lubricant.

20. The method of claim 19, wherein forming a porous film comprises heating and compressing the polymer mixture to create a porous film polymer.

21. The method of claim 19, further comprising selecting a polymer particle having a melting temperature at the threshold temperature.

22. The method of claim 19, further comprising stretching the heated porous polymer film.

23. A method for indicating temperature comprising: thermally coupling a temperature indicator of claim 1 to an object to be heated to a threshold temperature; and wherein the polymer matrix of the temperature indicator comprises a plurality of open pores; wherein a polymer in the polymer matrix has a melting point that is about the same as the threshold temperature; wherein the temperature indicator and the object are heated to a temperature that is at least about as high as the threshold temperature; and wherein, as a result of being heated to the temperature that is at least about as high as the threshold temperature, a sufficient number of the open pores are closed to increase the transparency of the polymer matrix to at least 50%.

24. The method for indicating temperature of claim 23, wherein the polymer matrix comprises plural polymer particles and the plurality of open pores comprise the interstitial spacing defined between the plural polymer particles.