Silicon containing or acrylic based surface additives are used frequently and successfully to reduce the surface tension of the coating for the prevention of many coating defects. At this point, it is suggested to control the product with differentiated surface chemistry and surface tension through the use of additives in order to eliminate recoating issues especially in stove dried coatings.
The fact that silicon additives are dissoluble in wet coating enables them to homogeneously reduce the surface tension of wet coatings. Otherwise, silicon compounds with low surface tension that cannot mix in with the coating may result in point surface tension defects such as cratering.
A large portion of the silicon compound in stove dried primer rapidly migrates to the air-coating interface. If the heat energy transferred to the film during the stoving of wet coating is not sufficient to break the organic modification bonds of the silicon compound, the composition of the silicon compounds will not change at the stove outlet. When the topcoat is applied on the stoved primer, the silicones on the surface of the stoved primer film dissolve in the solvents of the topcoat this time migrating to the topcoat-air interface. It may be considered that a problem-free adhesion quality will be attained between the primer and the topcoat due to the decrease in the silicon compound at the interface. (Figure.1, 120ºC/20’migration mechanism during stoving)
On the other hand, the amount of heat transferred to the primer in the stove may result in the breaking of certain chemical bonds on the silicon compound. This breakage may lead to the formation of molecules on the primer surface which are not compatible with the coating and which have low solubility. Since these molecules are not compatible with the topcoat applied on the primer, they may not move towards the surface of the topcoat and result in adverse impacts on intermediate adhesion by remaining at the primer topcoat interface through reacting with reactive agents in the topcoat. (Figure.1, 150ºC/40’ layer rich in incompatible silicon compounds in sample stoving)
The severity of the aforementioned problem for stove dried coatings is expected to increase for cases where the primer is stoved for long periods of time at high temperatures and the topcoat is cured less.
nsufficient surface cleaning for the base coat, presence of oil, dirt and material remains on the surface and high pigment/binder ratio for the formulation of the base or topcoat can be considered among reasons that may have an adverse impact on the adhesion of a coating batch in addition to the probable mechanisms subject to silicon containing surface additives.