Appearance of textured and pigmented polymeric surfaces
Licentiate thesis, 2007
The appearance is of increasing importance in the production of premium products since it is strongly connected to the quality impression. The surface appearance of any object may be described in terms of colour and surface characteristics such as gloss and texture. The aim of the present work is to gain an improved understanding of the relations between these attributes and also to their relation to the human visual perception of appearance.
The relation between colour and gloss of injection-moulded plastic specimens was evaluated by means of a spectrophotometer and a glossmeter. An increase of the lightness L* of the specimens gave a higher measured gloss of the textured field, with a low gloss appearance. This can most likely be attributed to a contribution from bulk scattering, which is linked to the reflectance from within the sample. Furthermore, the influence of texture on the colour was assessed by evaluating the colour difference between a smooth field and a more textured area on the specimens. When the surface was textured, the colour changed; it mainly became lighter and less saturated. The magnitude of the change was affected by the colour of the material and this change caused by increasing the surface roughness could be predicted in a satisfactory manner using a model developed for xerographic printing paper. The effect of surface texture on the measured gloss was evaluated for polymeric surfaces with very small differences in texture and gloss using a modification of the general scalar Kirchhoff approximation. The variations in surface topography responsible for the gloss differences were however for several of the studied specimens too small to be reflected in the topography measurements; thus the model could only partially be applied.
In order to study the visual perception of appearance, sensory evaluation was applied i. e. a psychometric approach in which an observer panel is employed. Here, an influence of the colour on the visually perceived gloss of textured specimens was noticed. The gloss was considered higher for the darker specimens and hence a negative correlation between the measured gloss and the perceived gloss was at hand for these textured surfaces, which differed in colour. The visual perception of gloss and texture in the case of specimens of the same colour was also evaluated using a human test panel. The observers were in fair agreement with the measured physical values, although the observers were in some cases able to discern differences between specimens not revealed by the measurements. On specimens differing significantly in both texture and gloss, the agreement between the measured surface topography parameters and the perceived roughness was poorer. It is suggested that higher gloss of a textured surface may enhance the perception of a higher roughness, and that this could possibly account for the discrepancy.
Key words: appearance, colour, gloss, surface texture, polymer, perception