Slip and Levelling Additives
The surface of coating, although it might be for different purposes protective or decorative or both, it is the eye-catcher for it users and consumers. It is exposed to “the world” and has not only to withstand some severe circumstances like wheatering, but is responsible for the appearance of the coating, colour, brightness, gloss and the “feel”. It all starts at the surface. 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.
Polysiloxanes (silicones) have a very high surface activity and therefore are often used as surface control additives. 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.
Dimethylpolysiloxanes (PDMS) are used for different purposes depending on their degree of polymerization. Low molecular weight products are used as leveling agents. Increasing the molecular weight creates a higher degree of incompatibility with the coating medium and can generate a defoaming action. However, in most cases, recoatability is a problem with this product group and methylalkyl polysiloxanes are superior in this respect.
Polyether-modified siloxanes can be tailored to certain coating types and are superior to the dimethylpolysiloxanes. However, in certain applications, hydrolytic stability may be a problem. Self-condensation of hydroxyl functional polysiloxane can give rise to the formation of incompatible products with a strong tendency for cratering.
Polyester-modified siloxanes exhibit a higher stability against thermal degradation and improvements in compatibility. These products provide long-term slip and water repellency.
Homo- and copolymers based on (meth)acrylic monomers are well known polyacrylate surface control additives. In some cases, they are incompatible in the paint system, which leads to the development of haze in clear-coats. This problem can be solved by choosing an acrylic leveling agent with lower molecular weight and improved compatibility. Gloss levels in solid colors are normally not affected by the incompatibility. In addition to their positive impact on flow and leveling, acrylic homo- and co-polymers are effective as air-release agents. Since they are not reducing the surface tension of the coating to the extent of silicone-based products, the wetting of substrate surfaces is improved (substrate wetting).
Perfluoro-modified surface control additives are the most effective compounds to decrease surface tension, however, recoatability and foam stabilization and cratering may occur. These undesired side-effects depend very much on the system parameters that have to be optimized and adjusted to gain optimum results. Controlling the parameters of molecular weight, polarity, degree of fluorine modification, curing conditions, and additive concentration in the formulation must be evaluated carefully.