In life, nothing stays the same. Unless it is the colour of the paint on your car! You now dislike that blue, but there is no way you would want to go to the time and vast expense of painting it. But, sometimes the things we love, our possessions and paint can really change colour, with the help of thermochromic ink technology. Thermochromic inks use the process of thermochromism, which means materials that change their shade or hue in response to temperature. So why not paint your car and watch it morph. With the right paint on a warm day, it could change from the blue you hate to a cheery and dynamic yellow.
Thermochromic inks have been around a while and first hit the masses in the 70s, as the mood ring, that used the body heat as an indication of an emotional state.
More recently, a 'cook-it-stick' using a thermochromic coating on the tip indicated when the meat on the BBQ was cooked to perfection and safe to eat, by changing from black to red. Some beer cans had a graphic that appeared when the contents were cool enough to provide the best cold and refreshing taste.
Since the early days of the mood rings, thermochromic inks have come on leaps and bounds and evolved year by year. They can be found still in all sorts of novelty items, but have found many useful serious applications like thermometers, drink containers, shoes, clothing, baby accessories e.g. baby feeding spoons and how warning label to name just a few.
Many companies utilise these eye catching graphics to sell their products and create exciting brand awareness.
Is it magic?
Thermochromic inks use their magic in many ways. There are two important categories of these inks - thermochromatic liquid crystals or TLC's and leuco dyes.
Is it a liquid or a solid?
Liquid crystals are in fact what their name indicates; a material that has several properties of a liquid but then crossed with structural elements characteristic to crystals. If you look through the microscope at a liquid crystal, you will see a fluid that displays textures.
Liquid crystals are vibrant and dynamic. Their property changes depending on the conditions in their environment; TLC's exhibit varying colours in response to the change in temperature. At lower temperatures, liquid crystals are mainly in a solid, crystalline form. In this lower temperature condition, TLC's don't reflect much light at all, therefore, appearing black.
Apply some warmth, increase it a little at a time, and you'll see the TLC's start to shift from black to almost every spectral colour. This is because, as temperature rises, the space between the molecules of the crystals change, so they reflect light differently.
The process to integrate TLC's onto a product to make it change colours requires a little work. This is because the liquid crystals must first be microencapsulated into millions of tiny capsules that are only a few microns in size. This process or encapsulation offers some protection for the TLC's and importantly it maintains their thermochromic properties.
These capsules are then blended with other substances and used in products, like baby room thermometers. Place the thermometer in a baby's bedroom and you'll see a rapid change of colour that is an indicatation of an accurate temperature.
This precise measurement of Temperature accuracy is a strong argument for the use of TLC's. Their colour consistency can indicate cold or heat levels to within +/- 0.5 degrees or less. It is worth noting though that TLC's performance can suffer with repeated exposure to outside influences like UV light, moisture or chemicals. In their manufacture, they do often require dedicated equipment for manufacture into various products, which can add significant expense to a manufacturer's production costs. Saying that, these costs are coming down all the time, with many organisations having ability to carry out this production.
Leuco dyes and inks, though, are a different story.
Thermochromic Liquid crystals can be rather difficult to incorporate into products like clothes, labels or other goods. But this is where Leuco dye inks, come into their own, as they feature more robust chemical makeup that allows product creators to employ these inks for all types of applications.
A well known applications of leuco dyes was used on cans of Coors beer. They feature a graphic of a mountain and at room temperature, the mountains appear white. But when cooled to the perfect drinking temperature (around 7 Celsius), and the mountains change colour to a bright blue. When the beer becomes warmer, the mountain again reverts back to its original colour white. This change can happen many, many times.
Like TLC's, the leuco dyes are also microencapsulated into tiny droplets. These are only about 3 to 5 microns in size, and that prevents them from reacting or being damaged by other chemicals.
Leuco dyes are coloured when they're at a cool temperature but as the heat increases, they become nearly clear or translucent. This then allows them to reveal any patterns, words or colours that may be printed on the underlying layer of the ink. In most cases, Leuco dyes can also be mixed with another colour. This means that as the temperatures changes, a two-tone colour effect occurs. For example, if you mix blue and yellow together, you have an ink that appears green at a low temperatures but yellow when the temperature rises.
I would love to say it is magic but there is a basic science behind the way the inks work. The tiny capsules contain a colourant, plus an organic acid and also a solvent. When kept at lower temperatures the solvent remains in a dense state as the colorant and acid are kept in close proximity to one another. As a result of this, they create colour by reflecting light. But as the solvent warms up, the colourant and the acid start to separate so there's no visible colour, so this in turn exposes underlying inks.
Take the cook-it-stick as an example. At room temperature, the stick shows a black tip; when warmed, the black area fades, revealing the red tip that indicates the correct cooking temperature.
With regards temperature accuracy, leuco dyes can be less accurate than TLCs. This means you cannot depend on them for applications where you really need a very precise temperature reading, like a thermometer. Nevertheless, leuco dyes can be incorporated into all sorts of fascinating and amusing products. You could almost say 'the sky's the limit!'