Do You Know What Temperature Sensitive Labels Are?

Temperature sensitive labels are tools that utilize special chemicals to change color at specific temperatures.

They are widely used in numerous fields, ranging from vaccines, medicines, and electronic components to perishable foods, and from manufacturing to logistics.

Through intuitive color changes, they allow people to quickly determine whether the temperature of a product is within the safe range, safeguarding product quality.

(I) Working Principle Revealed

• Capillary Principle: Take the simplest temperature - sensitive label as an example. It contains two fluids, one clear and one colored, in a tiny capillary tube. When the temperature reaches a preset threshold, the clear fluid (usually a volatile organic compound such as isopropanol or hexane) expands and pushes the colored fluid, triggering a permanent color change. 
• Phase - Change Indicators: These labels use the transformation of materials between solid and liquid states to indicate temperature changes. Materials like paraffin waxes and fatty acids have precise melting or solidifying points. For example, during endothermic phase change (melting), the material absorbs heat and changes from solid to liquid; during exothermic phase change (solidification), it changes from liquid to solid, which is used to monitor temperature drops. 
• Leuco Dyes: Leuco dyes can transform between a colored form and a colorless (leuco) form through a reversible redox reaction at a specific temperature. They are mixed with substances like weak acids and solvents to precisely control the color - change temperature. For instance, Crystal Violet Lactone (CVL) turns from colorless to blue when heated. 

• Crystal Violet Lactone (CVL)	One of the most common leuco dyes	Changes from colourless to blue when heated
  Fluoran dyes	A family of leuco dyes with various colour changes	Examples include: a) 2-Anilino-6-dibutyl amino-3-methyl-fluoran (changes to orange) b) 3-Diethylamino-6-methyl-7-anilinofluoran (changes to red)
  Spiropyrans	Often used in combination with other dyes	Can switch between colourless and coloured forms with temperature changes
  Rhodamine B Lactam		Changes from colourless to pink/red when heated
  Bisphenol A	Often used as a developer in combination with other leuco dyes	Helps stabilise the colour change

• Liquid Crystals: Liquid crystals have the characteristics of both liquids and crystals. When the temperature changes, their molecular structure adjusts, altering the wavelength of reflected light and thus presenting different colors. Cholesteryl esters and cyanobiphenyls are commonly used liquid crystals in labels. By mixing different liquid crystals, specific temperature ranges and color changes can be achieved. 

(II) Types of Labels Analyzed

• Irreversible Temperature - Indicator Labels
  • Multi - Level Labels: These labels permanently change color when different preset temperature thresholds are exceeded. They can monitor the gradual increase of temperature, with each level corresponding to a specific temperature point. 
  • Time - Temperature Indicators (TTIs): TTIs consider both time and temperature factors. Through irreversible chemical reactions (such as oxidation, polymerization, or enzymatic reactions), they record the exposure of products to unsuitable temperatures. A color change  reflects the potential risk of product quality. 
  • Cold - Mark Indicators: These labels are activated when the temperature drops below a specific threshold, ensuring the protection of products sensitive to low temperatures. 
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  • Warm - Mark Indicators: They change color when the product's temperature exceeds a certain limit, providing an early warning of overheating.
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• Reversible Temperature - Indicator Labels
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  • Dual - Temperature Indicators: These labels can monitor both high and low temperature limits (sometimes a temperature range). They can be reset and reused, making them suitable for products that are sensitive to temperature fluctuations and need to be protected from both overcooling and overheating. 
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  • Reversible Temperature Labels (Based on Cholesteric Liquid Crystals): Cholesteric liquid crystals in these labels reflect different colors of light at different temperatures. For example, after specific manufacturing adjustments, it shows blue below 2°C, green between 2 - 8°C (the ideal storage temperature for many pharmaceuticals), and red above 8°C. When the temperature drops, the label's color can return, facilitating continuous monitoring. However, long - term exposure above 122°C can damage the liquid crystals, affecting the label's performance, and its shelf life is about 18 - 24 months.
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II. Temperature - Sensitive Labels vs. Electronic Devices: What are the Advantages?

• Direct Surface Temperature Reading: Temperature - sensitive labels can be directly attached to the product surface, accurately obtaining the surface temperature and avoiding errors caused by indirect measurement of electronic devices. 
• Instant Feedback: The small mass of the materials in the labels enables rapid response, promptly capturing temperature changes and reducing delays and errors caused by heat conduction.
• Simple and Cost - Effective: They require no complex operation training or high - end equipment and are inexpensive.
• No Heat Loss: Unlike electronic probes, temperature - sensitive labels do not conduct heat away from the surface, ensuring the accuracy of measurement results.

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