Dry hiding effect
The dry hiding effect ( English: dry deck effect ) is an effect from the paint formulation. It describes the increase in the hiding power of a coating during drying due to the fillers it contains .
description
Typical paint formulations usually work below the critical pigment volume concentration ( CPVK ), i.e. the pigment and filler surfaces are completely covered by the surrounding paint system. Above the CPVK, the pigment surfaces are no longer completely covered. In addition to the interface between pigment and binder, there are also interfaces between pigment and air above the CPVK.
The difference in the refractive indices ( n ) of titanium dioxide (n = 3.1 (rutile); n = 2.52 (anatase)) and the binder matrix (typically n = 1.5) is high enough to produce the scattering responsible for the white appearance is. The refractive index of typical fillers such as barium sulfate (n = 1.64) is lower, so that these substances appear as transparent when they are completely enveloped. The difference between the refractive indices of typical fillers and that of air (n = 1) is significantly greater, so that the fillers act as a white pigment.
Since in the freshly applied system there is no interface to air, but to water (n = 1.33) or solvent (n = 1.3–1.5), these systems are often still transparent when wet, but become during the process increasingly covering the drying process. This fact also explains the term dry hiding effect .
Examples
So that interior emulsion paints dry as porous as possible and vapor diffusion open , a formulation above the CPVK is chosen. In the case of simple qualities, attempts are usually made to minimize the addition of titanium dioxide for reasons of cost . In order to still obtain an opaque coating, more cheap fillers are added, the opacity of which only becomes effective after the paint has dried (dry hiding effect). Usually, the binder content is also reduced in order to obtain more interfaces between filler and air and in this way to strengthen the effect.
High-quality interior wall paints use a higher proportion of titanium dioxide and therefore appear to be opaque when wet.
Despite many excellent properties, lime paint is used less often today, since even mixed lime paint has to be applied in several thin layers and initially appears almost completely transparent until it dries. Since pure lime paints should be applied to a well-soaked substrate, it takes a correspondingly long time before the hiding power and appearance of the paint can be assessed. In the trade, however, lime and lime casein paints are offered to which additional binders and pigments as well as fillers are added, which facilitate processing and reduce the dry-hiding effect .