FLOW AND LEVELLING
In most cases, superior surface properties can not be achieved without the addition of surface control additives that alter the surface properties of the coating. Depending on the kind of additive used.
● Orange peel : Surface bumpiness or waviness that looks like the skin of an orange. It is often caused by poor levelling and is a common defect in both spray- and roll applied coatings.
● Benard cells : Hexagonal cells with marked centers produced by circulation in the film caused by gradients of concentration, density and/or temperature.
● Floating : Benard cell circulation influences color shade and appearance; observed as mottled，blotchy, or streaked appearance of a paint film.
● Craters：Small bowl-shaped depressions often having drops or bands of material at their centers and raised circular edges in a coating film. Caused by contaminants of lower surface tension.
● Fish eyes ： Crater-like holes whose centers which consist of a uniform flat painted region, surrounded by a depression, followed by a ridge of paint. They are caused by un-dispersed fluid or airborne droplets (silicones, water, dried soap, dust, wax, and oil). Large fish eyes can be found individually and small ones are often found in small densely packed clusters.
● Picture framing (edge crawling) ： De-wetting of the applied coating and the appearance of fat edges or picture framing around the edges of a panel or metal part. It is generally caused by increase of surface tension on the edge during drying. Low viscosity increases this phenomenon.
● Flooding : Surface color is uniform but different to the original one caused by uneven distribution of pigments in the film during drying.
● Crawling : De-wetting of the applied film from the substrate due to surface tension differentials.
● Telegraphing : Flow of paint induced by temperature gradients or contaminant of the substrate surface.
CHOOSING A SURFACE CONTROL ADDITIVE
●Polysiloxanes (silicones) have a very high surface activity and therefore are often used as surface control additives. Commercial silicone-based surface control additives are modified by polyethers, polyesters or alkyl side groups to improve recoatability and inter-coat adhesion. Modification parameters are silicone content, molecular weight, and modification degree.
● Polyacrylate surface control additives based on special designed (meth)acrylic monomers with low surface
tension. These additives can improve flow and leveling. Some additives are also effective as air-release agents.
● Perfluoro-modified polyacrylate can improve flow and leveling, not stabilize the foam,not affect the recoatabilityand can effective reduce surface tension, improve substrate wetting and anti crater.