When Temperature Talks: How Thermochromic Pigments Transform Materials

5 Minute Read

By Danielle Williams

Thermochromic pigments, such as Chromazone® pigments and slurries, enable formulators to create products with dynamic and heat-sensitive, colour changes. These smart materials have found their way into various applications, from functional safety indicators to eye-catching promotional products (Figure 1).

For example, reversible thermochromic pigments are used in novelty mugs that reveal hidden designs when filled with hot beverages, packaging that signals when food or drink is at the optimal temperature for consumption, security labels for brand protection and anti-counterfeiting, and temperature-sensitive warning indicators on surfaces or medical devices.

Thermochromic heat changing spoon, mugs and cups

Figure 1. Thermochromic pigments such as Chromazone® can be used in a variety of applications from novelty promotional products, through to safety indicators or brand protection applications.

Principles of Thermochromism

Thermochromism in Chromazone® pigments is based on microencapsulated leuco dyes that undergo a reversible structural change when exposed to specific temperatures. Below their activation temperature, these dyes are in a coloured state; upon heating, the solid matrix they are suspended in melts, and the molecular structure shifts, rendering the pigment colourless or lighter. This transition is fully reversible: cooling restores the original colour. The activation temperature can be tailored between -10°C and 69°C to suit specific application needs, with the colour change typically occurring within a 3–5°C window around the rated temperature.

Principles of reversible thermochromism

Figure 2: Principle of reversible thermochromism. When heated about their activation temperature, Chromazone® thermochromic pigments will undergo a colour change and change from coloured to colourless.

Types of Materials Available

Chromazone® thermochromic pigments are supplied in two primary forms to facilitate a broad spectrum of industrial uses:

Aqueous Slurries: These are dispersions in water, ideal for water-based coatings, paints, and inks. They are available in standard activation temperatures (15°C, 31°C, 47°C) and a range of base and custom colours. The slurries are designed for gravure, flexo, and screen printing, as well as for paint and coating systems.
Free-Flowing (FF) Powder: This form is recommended for plastic and non-aqueous systems, such as polymer masterbatch production. The powders are easily incorporated into plastics during extrusion or moulding, allowing for temperature-sensitive films, moulded parts, or speciality effects in consumer goods.

Formulation Considerations for Chemists

When formulating with Chromazone® thermochromic pigments, several critical factors must be managed to ensure optimal performance and longevity:

1. Environmental Sensitivity

UV Light: Prolonged exposure to UV can degrade the thermochromic effect. Incorporating UV absorbers and using protective coatings can improve durability, but lightfastness remains a limitation with this type of pigment (Blue Wool Scale 1–2).
Heat: Extended exposure to temperatures above 50°C can cause permanent loss of functionality. However, the pigments can withstand short bursts up to 200°C, as seen in polymer processing.

2. Chemical Compatibility

pH: Formulations should remain below pH 8, as higher alkalinity significantly reduces pigment stability. Ammonia is incompatible; substituted ethanolamines are preferred for pH adjustment.
Solvents: Chromazone® slurries are sensitive to many solvents. Ethanol, ketones, and NMP should be avoided, while isopropanol and isobutanol are acceptable up to 3.5% of the formulation.

3. Mechanical Handling

Mixing: Low-shear mixing is essential to prevent rupture of the microcapsules. High-shear mixing, ball, or bead milling must be avoided.

4. Substrate and Colour Strength

Opacity: Chromazone® pigments are translucent and best displayed on white or light backgrounds. Chromazone® Black offers the highest opacity, especially with heavy deposits.
Loading: Colour strength depends on pigment concentration and film thickness. Formulators should optimise these parameters for the intended visual effect.

Applications in Coatings, Inks, and Polymer Masterbatches

Reversible thermochromic pigments can be used in a wide range of applications in coatings, inks and polymer masterbatches:

Coatings/Paints: These are used for interactive wall paints, safety indicators, or decorative finishes that respond to touch or ambient temperature.
Inks: Employed in security printing, interactive packaging, promotional materials, and novelty items, using screen, gravure, or flexographic processes (Figure 3)
Polymer Masterbatches: Incorporated into plastics for temperature-indicating utensils, toys, or packaging that visually signals temperature changes during use or transport.

Thermochromic food packaging

Figure 3: Thermochromic inks used on food packaging can indicate when food or drink is at an optimal temperature for consumption and result in eye-catching packaging

Summary

Thermochromic pigments like Chromazone® offer chemists a versatile toolkit for creating responsive, engaging, and functional materials. With careful attention to formulation parameters and environmental sensitivities, these pigments unlock a world of possibilities in coatings, inks, and plastics, transforming ordinary products into interactive experiences.

For more information and to discuss your application, contact your Lawrence Industries account manager or call us to discuss your requirements.

Coatings and Construction

Danielle studied Chemistry at the University of York, where she earnt her MChem with a focus on Green Chemistry. She started with us in 2013, specialising in Coatings and has since progressed to a Business Development Manager looking after both Construction and Coatings accounts. Since 2020, Danielle has also been in charge of overseeing our marketing activities.

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