A modern approach to gloss control1 July 2001
Summary This new aqueous polyurethane chemistry enables very low gloss levels to be created that perform well, look good and have an attractive tactile feel while being compatible with all the usual leather finishing components in producing high flexibility and abrasion resistance. The film created with this new approach to gloss control has a unique and interesting surface morphology that is inherently low-gloss yet produces coatings that are very lush, soft and velvety. Gloss levels as low as a few tenths of a percent, even at high angles of incidence, have been achieved with some of the early prototypes without adding any dulling agents. Because they were almost dead-matt, the appearance tended not to look like leather and to be rather unattractive. The ideal is not necessarily the film with the lowest gloss. But add this film to a gloss finish to reduce the level of gloss and the results are quite different. There is no refractive index mismatch as in finishes dulled with high reflective index mineral dullers or very hard organic dullers. Finishes, therefore, exhibit low haze, favouring the appearance of a jet black and rendering other colours more intense. What is more, because the new polymer is both highly durable and a film former in its own right, not only is it well suited to achieving dulling, it can survive for 2,000 hours in Weather-Ometer tests without loss of gloss or appearance. The resulting film is continuous and flexible and investigations using a scanning electron microscope have shown that the sub-surface morphology does indeed correspond to a truly continuous film. The accompanying micrograph at 3,000x magnification (Figure 1) showing a film cast onto a Leneta C5 chart and freeze-fractured, provides evidence of a few small voids in the film even though continuous film formation has proceeded extremely well beneath the surface. At the surface, adjacent particles have welded together and the film formation process has not resulted in the elimination of surface detail. Regardless of drying temperature from ambient up to 120°C and higher, dried-film morphology at the surface and beneath follow different regimes. The surface appearance compares quite dramatically with that obtained with conventional dulling agents, as seen in Figure 2. This very clearly shows how the film morphology retains the appearance of the surface particles. This is the feature that contributes to the mechanism by which the film works. Because a sphere is rotationally symmetrical, tilting the sphere does not change the paths of the refractive rays. So the film resulting from Stahl's new polymer does not affect the paths of the light falling on it, unlike a typical almost flat film resulting from the application of conventional dullers. Although based on well-known laws of optics, the mathematics are highly complex but show conclusively that rays of light reflected off the surface particles will be directed into adjacent spheres at low to moderate angles of incidence. Almost all of the reflected rays enter with the original or an adjacent sphere with minimal primary or secondary reflection. This is the secret to the success of the new film. Effect on gloss and haze The characteristics of low gloss and low haze are both dependent on the optical effects resulting from the fact that the surface morphology of this film does not destroy the particulate appearance of the surface of the finish. The particulate appearance remains; it is not destroyed in a film that envelopes all around it, filling the spaces and creating an almost flat surface. As far as gloss is concerned, nearly all the light striking the surface of the spherical particles on the surface is refracted into the coating. Inevitably some is reflected, but very little. Most of this will enter the adjacent sphere and will be refracted in some direction other than the specular angle. So almost no light is reflected from the surface. The appearance is low gloss. Whatever light enters the coating goes straight through. There is no scattering from the dulling agent within the coating. Since haze is primarily caused by scattering of light at the surface and within it, no scattering of light means effectively no haze and thus the colours of the finish are apparently intensified. How does this affect finishing? The film morphology is regular and solid, as would be expected with any film used in leather finishing. However, because high refractive index duller need not be used in the finish, refractive index mismatch is absent, haze is almost eliminated and colour rendition becomes more intense. Low gloss finishes are given a liveliness and appearance that has hitherto simply not been possible, a factor that will undoubtedly be of interest to a fashion world always looking for something new. From a performance point of view, this new gloss chemistry can be used in any ratio with other binders and is not subject to Critical Pigment Volume Concentration limitations that are normally the case where dullers are used at high loadings to achieve very low gloss. The effect on appearance, performance and feel of low gloss leathers is unique and certainly make it possible to make a much wider use of low gloss effects wherever leather is used.