* What is thermochromic pigment?
1. Reversible Thermochromic Pigment
The phenomenon of color changing based on temperature is called thermochromism, and the representative product that utilizes this phenomenon is thermochromic pigment. The principle of color changing based on temperature is that when the temperature of a solid solvent inside a micro capsule increases, it melts into liquid. This is when the two substances that account for the color separate and become colorless (clear); when the temperature drops again, they bind together to become colored. The image below describes the principle of thermochromic pigment.
Thermochromic pigment can set the color changing temperature between 0℃ and 70℃. Thermochromic pigment has a characteristic of becoming colorless (clear) when heated above the set temperature, returning to its color when it is cooled below the set temperature. Various applications can be developed using this.
2. Hysteresis Thermochromic Pigment
The existing thermochromic pigment is considered reversible thermochromic pigment. Such thermochromic pigment becomes colorless when heated above the set temperature and returns to the color immediately when the temperature drops below the set temperature. Recently, a thermochromic pigment that utilizes the hysteresis phenomenon wherein the temperature at which the color disappears and reappears is lowered further has been applied. Hysteresis is a physics term that refers to the phenomenon of irreversible deformation when a certain amount of force is applied. The coloration from this phenomenon is indicated as follows:
When we look at the image above, the color disappears at temperature of 60℃ or higher, and it has to drop below 0℃ for the color to reappear. This explains that hysteresis color behavior occurs in the daily temperature range (0℃ - 60℃).
3. Application of reversible thermochromic pigment and hysteresis thermochromic pigment
1) Application of reversible thermochromic pigment
① Sports industry: Applied to an ice hockey puck to indicate the timing of puck replacement; the puck color changes from purple à to white when the temperature is above freezing point.
In 2019, PPG selected LCR Hallcrest as technical partner to develop thermochromic coating for the hockey puck to be used during the 2019 Bridgestone NHL (National Hockey League) Winter Classic. LCR Hallcrest is a representative manufacturer of thermochromic pigment, and PPG is a global company that manufactures automobile and industrial pigment. It is a good example of mutual cooperation and joint development of pigment and coating applied to the massive sports industry of the US represented by NHL.
② Interior decoration: Thermochromic mug cups are widely applied with novelty designs by coating the surface.
③ Medical body temperature checking band and sticker: As the global COVID-19 pandemic began at the end of 2019, people all around the world became particular about 37.5℃. By questioning “How can we check temperature more easily?”, thermometric devices such as thermometers and devices with thermochromic pigment were released. Wearable temperature sensors also apply reversible thermochromic pigment that indicates the temperature to check the human body temperature. Such research and application scope are expected to expand.
Wearable temperature sensors use the thermochromic micro material (TC-M) or nano capsulized phase change material (NPCM) with PVA (polyvinyl alcohol/water-soluble polyurethane) composite high-molecular matrix as membrane. Body temperature band is a silicon band around the wrist to check the body temperature immediately.
④ Application to the textile industry: Thermochromic garment that dyed leather and cotton yarn with micro-capsulized thermochromic material in core fabric are sewn into clothes. A recent application includes camouflage military uniforms.
The application of thermochromic pigment to textiles would need more research. In order to mix capsulized pigment and PET and reel out thread, the durability of the yarn is especially important. When dying a fabric, fastness to wash, friction, and sunlight after dying is an important element, but there are still doubts as to whether the problem of fastness has been resolved. They are said to have marketed this as a unique and rare item.
2) Application of hysteresis thermochromic pigment
① Erasable pen: Erasable pens have become famous worldwide with the brand Frixion ball (PILOT). This stationery product was designed by applying the principle of color fading by friction heat.
For stationery products, it is applied in various colors, writing tools, and inks such as gel ink, marker ink, fluorescent marker ink, and colored pencils. At ActivON, we apply hysteresis thermochromic pigment for our stationery products.
② Overheat detection cap terminal: At temperatures between 60℃ and 70℃, hysteresis discoloration and abnormality due to overheat can be confirmed.
Hysteresis discoloration and degradation of wire can be easily confirmed below the temperature of degradation (80℃). This item can effectively prevent fire from overheating wires in industrial fields.
SpotSee, an IoT (Internet of Things) and platform company in the US with a global network of over 300 sales and technical service partners in 62 countries, has recently acquired TMC and LCR Hallcrest in January 2021, thereby merging the two global leaders in the temperature monitoring product and indicator fields. TMC and LCR Hallcrest develop and manufacture thermochromic products, thermo-labels, and thermometer strips. With focus on monitoring and reporting changes in temperature, the purpose of TMC and LCR Hallcrest is in line with the strategy of SpotSee® to grow the temperature portfolio and expand to new markets. Like SpotSee®, TMC and LCR Hallcrest's products are widely applied from medical device monitoring to food logistics and shipping.
Likewise, many attempts are being made in diverse industries in the global market for the application of thermochromic pigment. Although there are still tasks to be resolved, research and development should continue to enable the sustainable application of materials that are safe for the human body.
2. https://www.hallcrest.com/news, PPG and National Hockey League Extend Exclusive Paint Partnership
3. https://doi.org/10.1016/j.apenergy.2019.04.077 “Facile flexible reversible thermochromic membranes based on micro/nanoencapsulated phase change materials for wearable temperature sensor, Volume 247, 1 August 2019, Pages 615-629