Thermochromic Ink | Color Changing Ink |

Apr 03, 2017

All About Thermochromic Ink

black ring

Thermochromic inks are compounds that are sensitive to temperature and, as a result, change colors. First developed in the 1970s for different household uses, thermochromic materials are named for the Greek words for heat and color: “thermos” and “chroma”. The original thermochromic inks and dyes only changed color after prolonged exposure to heat.

There are reversible and irreversible thermochromic inks. Those that are irreversible leave a permanent mark once a peak temperature has been reached, while reversible inks can vary between normal and altered states several times. If you’re curious about temperature sensitive ink and its uses, read on for interesting facts from!

Types of Color Changing Ink

Thermochromic effects are produced by one of two primary materials: liquid crystals, which are the same materials that are used in computer and cell phone displays, and carbon-based dyes called leuco dyes. While both are used in thermochromic inks, leuco dyes have a greater range of applications and are easier to work with.

How Liquid Crystals Work

Liquid crystals have traits of both liquids and solids. The type that produce thermochromic effects are found in a form called nematic—meaning that their molecules are arranged in layers and typically point in the same direction. If light is shined on these nematic liquid crystals, some of the light will reflect back with what is known as iridescence, which is the phenomenon that produces a multicolor effect on the surface of a soap bubble. It is caused by incoming light waves that reflect off nearby objects.

Depending on how close the crystals are, the color of the reflected light will change. Heating and cooling nematic crystals will increase or decrease the space between them, altering the color reflected—from black to red—through each color in the spectrum, up to violet. The result is a colorless form at room temperature, with a spectral change occurring within a range of 5 degrees Celsius with a bandwidth of 24 - 29 degrees Celsius.

What Liquid Crystals Are Used For

Since liquid crystals give a precise measurement of temperature, they are predominantly used in items like baby thermometers and the temperature gauges inside of refrigerators. These types of bands are typically manufactured in the form of microscopic capsules, which are embedded in a polymer.

Of all types of nematic liquid crystal ink, chiral has the most vibrant color change. This thermochromic ink is used in liquid crystal displays, or LCDs, and for printing, design, and crafts, as well as for educational purposes.

Liquid crystal ink can be brushed or sprayed onto many surfaces. It has been used in innovative ways, including jewelry, clothing, CD covers, footwear, and business cards. It has also been used in the process of thermal mapping, and as a utilitarian solution to warn consumers that their food may be too hot. It has even made its way into the realm of interior design and automobile paint.

It is best to apply liquid crystal thermochromic inks to a black background, although they do work over other strong colors like reds and blues.

How Leuco Dyes Work

cold and hot mug

Leuco dye, from the Greek word for white, “leukos,” is a dye that has the ability to switch between two distinct chemical forms. The switch in color is usually reversible and occurs as the result of applied heat, light, or changes in pH. Irreversible changes only take place as a result of oxidation or chemical reduction.

Leuco dyes are chemicals based on carbon that change colors when heat causes their molecules to change back and forth between two different structures. These structures are the non-leuco (colored form) and the leuco (colorless form).

Non-leuco and leuco molecular forms absorb and reflect incoming light in different ways, so when they are printed on cotton or paper, they appear to be distinctly different colors.

While liquid crystals shift up and down the color spectrum as their temperature changes, leuco dyes are mixed in a variety of ways to produce several color-changing feats. They can also do this at a wide range of temperatures, unlike liquid crystals. Leuco thermochromic inks are less detailed than liquid crystals when it comes to temperature indication. A color change states that the object is either hot or cold—with no variance in-between.

What Leuco Dyes Are Used For

Leuco dyes are mainly used in battery testers, flat thermometers, indicators on bottles that change color when a beverage reaches a certain temperature (e.g. cold soda or warm maple syrup), or novelty items such as clothing and jewelry. They also form the basis of most pH indicators for swimming pools and thermal printer paper.

When promotional items or products employ temperature sensitive ink for entertainment purposes, liquid crystal is usually not used since the material is too expensive or sophisticated for such uses. It is leuco dyes that are responsible for coffee cups that have hidden messages when filled with steaming beverages. They are also the compounds behind color-changing “Hypercolor” shirts that morph colors when warm hands touch them.

Leuco thermochromic inks are used for these purposes because their microscopic capsules are produced easily with usual screen printing methods and adhere well to both paper and cotton. This enables these products to be mass-produced at fair prices, while liquid crystals require specialized, costly printing.

The Popularity of Color Changing Ink

In the past, temperature sensitive inks were used more as a gimmick than anything else. Color-changing straws, spoons, and t-shirts, along with mood rings and “heat and reveal” or “chill and reveal” promotions were extremely common in the 1970s and 1980s. Since then, the novelty has worn off and thermochromic ink has been employed for more practical purposes.

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