How to use Thermochromic Pigments Correctly?

Table of Contents

Thermochromic pigments are state-of-the-art products but structurally weak compared with many other pigments and dyes. To take full advantage of this product while keeping it out of harm’s way, the following precautions are to be observed in the application of thermochromic pigments.

I. Effect of thermochromic pigments Temperature

Thermochromic pigments usually change colors within a certain temperature range, while in a setting beyond that range thermochromic pigments might not achieve color change as expected or unsatisfactory color change results. Therefore, it’s necessary to understand and confirm the proper temperature range that fits the thermochromic pigments being used. In addition, ambient temperature also affects the color change results of thermochromic pigments for excessively high or low ambient temperature might result in improper color change. So the application of thermochromic pigments in suitable ambient temperature is strongly advised.

II. Thermochromic pigments Dosage

We recommend using 2% to 5% thermochromic powder in resin or plastic castings. When using thermochromic powder, if the material is already colored, we recommend adding no more than 2.5%. Also note that improper use of plastic fillers may cover the color of thermochromic pigments. Thermochromic powder is used in printing inks and coatings in an amount of 8%-12%.

III. Thermochromic pigments Mixing and Friction

Thermochromic pigment microcapsules are susceptible to mechanical damage when mixed with materials. Hence friction must be reduced in the preparation of such pigment before it’s employed. We recommend using a slow speed mixer, hand mixing, a three-roll mill or other “gentle” mixing method. Using a high-speed agitator such as a ball mill will quickly destroy the microcapsule structure of the calorific value color-changing pigment.. Besides, In addition, we should consider adding dispersants during mixing to prevent repeated extrusion and shorten the mixing time.

IV. Thermochromic pigment Mediums Requirements

Due to the thin wall of thermochromic pigment microcapsules, do not use solvents containing three or fewer carbon atoms, such as methanol, ethanol or acetone, because these small particles can easily penetrate the microcapsule wall and destroy the pigment structure, causing color deformation or pigment fading, resulting in colour deformation or pigment discoloration. We use solvents adapted to select solvents containing particles with six or more carbon atoms.

If we must use small particle solvents, we should control the amount used. The solvent will quickly evaporate after being sprayed and printed, and won’t leave damage to pigments.

Let’s take a look at the effects of different solvents on thermochromic powders. The data shows time needed to form visible deformations in pigment performance (at 20°C). It’s to be noted that only pure solvents were used for this experiment, and usually in mediums are mixed with solvents, binders, emulsifiers and other compounds which may react with thermochromic pigments, therefore this table is for reference only.

MethanolDMFEthanolAcetoneEthanol 40%Isopropyl alcoholEthyl acetateEthylene glycol/diethyl etherButyl acetateButanone
Up to 5 hoursUp to 48 daysUp to 48 days1/3-2 months2/3-3 months30-180 days60-150 days90-300 days90-300 days90-180 days
Water (pH 2-8)TolueneCyclohexanoneBenzyl alcoholGasolineMineral oilTurpentinePlasticizer DOPXyleneCyclohexane
Over 3 YearsOver 1 YearsOver 1 YearsOver 1 YearsOver 1 YearsOver 1 YearsOver 1 YearsOver 1 YearsOver 1 YearsOver 1 Years
the effects of different solvents on thermochromic powders

Before using thermochromic powder, make sure there are no small particles of solvent in the ring. Even a small amount can damage the pigment. So tests are suggested before a new medium is put into use.

V. UV resistance

Thermochromic pigments have low UV resistance. When we use thermochromic pigments for coloring, the surface of the product needs to be coated with a layer of UV protection paint. For outdoor products, it is mandatory. Pigments may be added to UV hardened resins (duration of hardening is short), so UV light won’t damage pigments within the range of visible light.

VI. Thermochromic Pigments Water-based Mediums

Thermochromic pigments can be added to water-based paints and inks. As it is mentioned above mediums shall not contain solvents with particles having less than 4 atoms of carbon. The Medium pH should remain at 2-8 (optimally 2.5-5). In alkaline media with a pH value above 8, thermochromic pigments will eventually be damaged.. Addition of a few nonionic surfactant will help thermochromic pigments disperse in mediums more easily.

VII. Thermochromic Pigments for Inks and Paints

When the temperature is above a certain level, they become transparent. The index of refraction of thermochromic pigment microcapsules is 1.5-1.54, and if binders used in paint/ink showcase significant difference in the index of refraction, then we will see “fog” effect instead of translucent layers after the color changing temperature is reached. Generally speaking, thermochromic pigments go well with alkyd and acrylic resins while don’t make good partners with polyamide resins and cellulose nitrate.

Differences in the index of refraction have also some impact on the choice of solvents. Solvents like toluene, xylene and benzene methanol (the indexes of refraction are 1.496, 1.493 and 1.54 respectively) have similar index of refraction as thermochromic pigments and work well with them. But if we use isobutyl alcohol with significantly different index of refraction (1.377), then we will observe smaller colour concentration than the actual one. But this effect is reversible, and the color returns to normal after the solvent evaporates..

Pigment microcapsules act like clear glass. Regular thin glass are transparent, but glass as thick as 100mm has green colour. If we want a completely transparent surface, we can apply a layer of thermochromic material on it. When pigments are already colored, it’s enough to have patterns on the surface covered.

Phosphates, bromides and chlorides will damage the calorific value color pigments and should not be present in the rings.

VIII. Possible Effects of Thermochromic Pigments

After reaching desired temperature, thermochromic pigments will become transparent. Pigments below this temperature will witness color mixing that might generate four effects:

Thin-layer pigments (such as paints or inks) become transparent after reaching the specified temperature. Patterns beneath the pigment layer will be revealed.

When used on thicker objects (such as resin castings), the pigment will turn white after reaching the specified temperature, and the refraction of thousands of microcapsules creates an effect similar to dense fog.

Pigments can also be colored with other dyes or pigments; it’s crucial to use them correctly to prevent the second coloring agent from obscuring the effect of thermochromic pigments. For instance, if we dye resin with green transparent dye and add red thermochromic pigments that will change color at 30°C. According to color blending rules, the resin, when below 26.5°C (at which the red pigment begins to fade), will appear yellow. As the temperature rises from 26.5°C to 30°C, the resin will shift from yellow to green, returning to its original green color at 30°C.

Using several different thermochromic pigments (different colors and color changing temperatures) makes the availability of products possible with several different colors as the temperature changes.

IX. Storage of Pigment and Thermochromic Products

Thermochromic pigments are sensitive to UV light, so it’s important to keep it from direct sunlight and stored in shades. Finished thermochromic products should be coated with UV-resistant paint, and this is particularly important for those often exposed to direct sunlight.

Thermochromic pigments can alter colors beyond the limit of attempt without significantly altering their performance. So pigments and products containing them should be stored in an environment several Celsius degrees below the color changing temperature (for example, pigments that change color at 30°C begin to fade at 26.5°C, so storing them at 24°C or lower is safe). It’s to be noted that freezing will also damage thermochromic pigments.

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Written by ——

Jeff graduated from Hubei University of Technology with a major in materials science and engineering. He has rich knowledge in materials. After graduation, he worked on color shifting powder pigments. He has rich experience in toner research and development and manufacturing, and is an excellent writer.

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