Thermochromic microcapsules

Figure 2.18 Lightness (L ) hysteresis of cotton printed with thermochromic microcapsules.

In terms of the substrate for thermochromic composites, it has been found that characteristic temperatures are strongly dependent on the surface applied to. In that respect, significantly narrower temperature ranges were obtained for thermochromic microcapsules printed on the surface of the 6- rm thick aluminium foil. 

PCMs are either in a solid or liquid state during their application. To prevent dissolution while in the liquid state, PCMs are enclosed in small plastic spheres with diameters of only a few micrometers. These microscopic spheres containing PCM are called PCM-microcapsules. Microscopic spheres that contain colour-changing polymers are called photochromic microcapsules or thermochromic microcapsules. 

The process of colour loss and the reverse process of colour gain represented over hghtness, hue, and chromacity values in dependency on temperature bear significant similarity to the concentration vs the volume of the reagent titration curves Skoog et al., 1999. The decrease in the ions within the thermochromic composite is conditioned by the temperature increase. As a result, the number of colourless molecules of the pH-sensitive dye within the microcapsule at Ti vs T2 increases (in the case T2 > T ) (Table 2.7). 

Very much like the photochromic systems described previously in this chapter, organic thermochromic composites are prone to degradation caused by various external stimuli. To evaluate the extent of the damage and to determine the conditions under which the microcapsules may be used without the fear of lost functionality or complete loss of the colour-changing phenomenon, the following investigation was done. 

Microcapsules, i.e. organic thermochromic composites, were found to be completely destroyed after only one washing cycle according to the ISO 105C01 standard (Figure 2.20). 

Most of the tests done to determine the fastness properties caused severe deterioration of the microcapsules, except for the mbbing, in which the microcapsules resisted major damage from the Crock-meter and retained most of their thermochromic capabilities. 

To promote better adhesion and shielding of the microcapsules it may be necessary to increase the amount of binder in thermochromic ink formulation. However, this has been found to significantly influence the coloration of the thermochromic substrate. The author has researched the difference in the reflectance curves obtained... 

Based on the high number of preserved, intact microcapsules in the case of the washed thermochromic textile, the colouristic parameters alone could not be used to evaluate the functionality of the thermochromic textile, while the SEM micrographs suggest the presence of a sufficient number of preserved microcapsules to retain the thermochromic property (Figure 2.23). 

Figure 2.21 Surface of the thermochromic textile printed with red microcapsules before the testing (5000 X original magnihcation).

Masterbatches of polyethylene and polypropylene containing microencapsulated thermochromic materials are already commercially available from various manufacturers. The thermochromic materials used in these microcapsules are... 

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