Lifting and wrinkling defects form similar final appearance through similar physicochemical properties. These defects stem from the simultaneous decrease in paint volume due to the evaporation of the solvent/crosslinking at the bottom layers of the film; and increase in the volume formed at a thin upper layer that partially hardens and loses the elasticity due to the migration of the unreacted oligomers from bottom layers to the upper layers.
Fig.1 wrinkling problem is schematized due to early polymerization of the polymers located at the paint/air interface of the paint film.
Clearly, the main reason resulting in wrinkling defect are a) volumetric mass as the quantity of the decrease in the volume, b) the change in the viscosity of the wet film during drying and c) evaporation rate.
Fig 2: Schematic representation of wrinkling defect observed on the air-dry finishing paint
Lifting shows some similarities with wrinkling from some aspects. However, the reasons of the defect morphology and the steps of the formation are different.
For example, a wet paint which has been characterized by fast dust drying attribute, dries and cures without the formation of wrinkles. In case of the application of a second layer including strong solvents (i.e. toluene, butyl acetate) on top of that previous cured paint layer, solvents will engage with the first layer resulting in a volumetric swelling unless the compactness of the crosslinking of the first layer is enough to prevent the penetration of the solvents. As the continuation, the leaving of the solvents causes a volumetric decrease. By being not completely dry, the bottom parts of the first layer show a homogeneous swelling while the top parts will be exposed to a morphological defect as in wrinkling. As the defect occurs due to the second layer application, it is called as lifting.
Lifting issue may also occur when a second application is performed (repainting, repair painting) to the systems comprising a thermoplastic bottom (as in wet-on-wet applied base-layer varnish systems) and a crosslinked top.
Fig. 4: The lifting of a dry film having insufficient crosslinking due to the solvents of overcoating layer
Fig 5.Lifting defect occurred on the secod layer of paint system (overcoating layer)
Wrinkling defect could be prevented with the solutions that ensures complete chemical drying throughout the paint film. For instance, by using secondary driers providing a faster drying (Zirconium, Pb soap) in an alkyd based paint formulation.
Similar precautions should be taken in the formulation of the first layer to prevent the lifting defects. In addition, avoiding application of over coating layer with strong solvents could be an alternative solution.