Definitions of Mixing Principles for Thermochromics


The post this week talks about definitions of mixing principles for thermochromic textiles. Thermochromic leuco dyes changes from coloured to colourless on heating. It is also possible to provide a change from one colour to another, by mixing the thermochomic dye with permanent pigments. I have worked with chromic dyes for over 10 years, during which I have also taught others of how to work with them. I have found a lack in written and graphical explanations of the practicalities of mixing leuco dyes. Experience has demonstrated that each new designer, interested in the use of thermochromic dyes, has had to develop an understanding without relevant guidance.

The in this post suggested approach, which was presented in Chapter 4 in my PhD thesis, was developed based on my conclusions from years of practice using thermochromic dyes as well as on theoretical considerations. These conclusions resulted in the construction of a number of illustrations to explain the mixing principles for thermochromic leuco dyes, both purely for leuco dyes as well as combined with permanent pigments. These illustrations is meant to offer practical guidelines for designers when working practically with the chromic dyes as well as a communicative tool, for example when the designer needs to convey the spectrum of colour change of the dye to another person.

One defines several colours within one colour, when one works with thermochromic dyes. The displayed colour of the textile correlates to the active or inactive chromic dye. I have therefor, defined two stages: ‘Colour state 1’ – the ‘inactive colour state’ represents the visible colour when the thermochromic dye is cold. ‘Colour state 2’, defined as the ‘active colour state’ represents the visible colour when the thermochromic dye is warm/heated. Colour state 1 equals the visual colour of the mixed thermochromic dyes if chromic dyes are used without permanent pigments and the visual colour of all mixed dyes and pigments if both thermochromic dyes and permanent pigments are used. Colour state 2 is represented by the colour of the fabric substrate if only thermochromic dyes are used (since the dye is more or less colourless) and the colour of the permanent pigment if permanent pigments are used together with the chromic dyes.

Mixing more than one leuco dye with different activation temperatures can provide more than two colour states. Depending on the number of different activation temperatures used, several colour states can be obtained.

Mixing thermochromic dyes with permanent pigments requires that the designer needs not only to establish the balance between the relative percentages of dyes and permanent pigments to provide the desired inactive colour (state 1), but also the desired colour of state 2, the colour of only the permanent pigment, when the leuco dye is active. Changes in the ratio between the components in the dye recipe of leuco dyes mixed with permanent pigments can provide a range of hues, as with traditional subtractive mixing. However, there are two ways to approach the variation in colour outcome in designing, since there are two visible colours (states 1 and 2). The designer can decide either to affect colour state 1 only or both of the colour states.

The graphics below illustrates (left) mixing principles for mixing several thermochromic leuco dyes with different activation temperatures and (right) mixing several thermochromic leuco dyes with different activation temperatures as well as using a permanent pigment.

To read more about the mixing principles see section 4.1.1 in my PhD thesis Thermochromic textiles and sunlight activating systems: an alternative means to induce colour change. Click here to download

To read an overview of the whole thesis project, click here.