Colour-changing textile coatings containing multi-layered mica pigments : an experimental study

Sammanfattning: This study investigates the possibility of producing angle-dependent colour-changing textiles with thickened polyurethane (PU)-based aqueous dispersion containing multi-layered mica pigments using a direct coating method. Colour changing textile coatings with the latest generation of layered mica pigment (EP) have a promising future for functional purposes such as authentication and aesthetic eye-catching applications. The surface structure and colour of the substrate as well as the coating formulation combined with different coating parameters will strongly influence the colour-changing effects of textile coatings containing EPs. Scanning electron microscopy (SEM) showed that the substrate surface roughness increased when constructed of multifilament yarns with high thread count, spun staple-fibre yarns with higher and spun staple-fibre yarns with lower thread counts. Multi-angle spectrophotometer measurements of EP-coated samples showed that the colour differences in form of CIE L'a'b'-coordinates varied to a great extent, depending on detection angles, surface roughness and colour of the substrates. Increased surface roughness generated less plane-parallel orientated EP-platelets and consequently a less pronounced colour-changing effect on coated textiles. The lightness, L'-values, for white substrates were much higher than the corresponding values obtained with black substrates. Thus white substrates greatly affect the colour-changing effects, which become less intense and much more difficult for the human eye to distinguish. Different types of rheology modifiers (RMs), dispersion in oil of sodium polyacrylates (LDPSAPs), hydrophobically modified alkali-swellable acrylic emulsions (HASEs) and hydrophobically modified ethoxylated polyurethanes (HEURs) provided EP-formulations with similar viscosity profiles. However, it was found that the choice of RM as well as variations in gap heights and coating speed could increase the solids deposit, corresponding to more and better dispersed EPs within the coated layer as well as more plane-parallel oriented platelets as confirmed by SEM. Multi-angle spectrophotometer measurements with six detection angles showed that the variations of the absolute values of L' and a' were more pronounced between detection angles of -15° and 15°, corresponding to the angles where the strongest colour changes can be visually observed. Slow-coated samples with a higher deposit were measured to be lighter and of higher chroma compared to samples coated at a higher speed. The colour-changing effects were found to be governed by the choice of RM and coating parameters in terms of variations of the amount of coating deposited onto the samples.