HOW DO MOOD PENCILS AND PENS ACTUALLY WORK?

Resource Center » How Does a Mood Changing Pencil, Pen, or Mug Actually Work?

During the 1970s, mood rings became very popular when thermochromic inks, crystals, and dyes were introduced to industry. Mood-changing materials use temperature-sensitive liquid crystals that change color according to skin temperature. Today, this technology is used in inks, paints, and papers for a variety of purposes. There are two general types of thermochromics: thermochromic liquid crystals and leucodyes. For instance, adding temperature-sensitive ink or paint to plastics can give an ordinary cup or pen the ability to change color. Many popular promotional items, like stadium cups, pencils, pens, water bottles, and beer steins are now available as mood-reflecting items.

So, how do mood-changing materials work? It's a fairly complicated process. Thermochromic liquid crystals are primarily used when accurate temperatures are needed; because these crystals change color when temperatures change, they can be used for room, refrigerator, aquarium, and medical thermometers. The crystals’ molecules exist in layers that regularly change their orientation according to temperature. Some of the molecules change color depending on light reflection from some of the wavelengths of the crystal structures. In this changing orientation, light passing through the crystal is reflected back as a color. A change in a crystal's temperature results in a molecular change in the crystal and in the color reflected from light passing through it. A high temperature will reflect a blue-violet color and red-orange is reflected back in a low temperature state. Because blue occurs in the light spectrum at a shorter wavelength than red, increasing the temperature results in a longer and redder wavelength.

Liquid crystals require special equipment when used in the printing process. When used in inks and dyes, the crystals are frequently microencapsulated in suspension form. Liquid crystals are usually more expensive than other printing technologies and they are more sensitive to high temperatures, ultraviolet light, and some solvents and chemicals.

Leucodyes, paints, papers, or inks temporarily change color depending upon temperature. They are easier to work with and are used in situations where absolutely accurate temperature readings are not critical. The leucodyes, like liquid crystals, are microencapsulated by a protective wall to help them maintain their color-changing properties and to help protect them from the properties of the inks with which they are mixed. The leucodyes or paints work in the same manner as the liquid crystals, with molecules structured in layers that change their orientation according to temperature.

These color-changing inks can be used in screen-printing and offset lithography, along with other printing processes. They are combined with standard inks and are used for an ever-widening range of applications, from textiles to product stickers. Using basic color theory, when a leucodye color like blue is added to a red ink, it produces purple ink. When cool, the product will be purple, and when the temperature rises, the color will change to red. Any number of colors can be formulated in this way. The temperature range for such changes can also be varied.

Some common uses for these paints and dyes are making cups or mugs that change color according to the temperature of the liquids they hold, promotional pens that change color when held, bath toys to predict general hotness or coolness of a baby’s bath water, and clothing with colors that change according to the amount of body heat that's generated.

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