Overflatetørke

Overflatetørke is a drying phenomenon in which the outermost layer of a material loses moisture much faster than the interior. This creates a surface layer that feels dry or hard while the underlying material remains wet or plastic, a condition often referred to as skin formation.

Mechanisms and factors: Surface drying is driven by heat, airflow, and low surface humidity, combined with the material’s porosity and moisture content. When moisture migrates from the interior to the surface, a dry skin can form and impede further diffusion of moisture, potentially leading to trapped moisture, stresses, and defects. The risk is higher in coatings, paints, mortars and plaster, wood products, textiles, and composites.

Implications in various fields: In painting and coatings, overflatetørke can cause poor adhesion of subsequent coats, cracking, wrinkling, or blistering. In concrete or plaster, rapid surface drying can lead to plastic shrinkage, surface cracking, or dusting. In wood and composites, it can cause surface checks and warping when internal moisture migration lags behind surface drying.

Control and mitigation: Managing drying conditions—slower heating, moderated airflow, higher ambient humidity—or using retarders and appropriate curing schedules can reduce overflatetørke. Applying multiple thin coats instead of a single thick coat, and ensuring adequate curing time before further processing, also helps. In some cases, pre-conditioning the material to achieve balanced moisture distribution is important.

Measurement and monitoring: Moisture meters (pin and non-destructive), relative humidity measurements, and in-situ monitoring of surface appearance (gloss, tackiness) help detect excessive surface drying. Infrared thermography can reveal temperature-driven drying patterns.

Etymology: The term is used in Danish and Norwegian contexts to describe this phenomenon; in English prose it is often translated as surface drying or surface skin formation.