Paints, varnishes and printing inks are applied as thin layers on a variety of substrates. The applied films have relatively large surface areas compared to their volume, and a correspondingly large contact area with respect to the substrate. Therefore, during film formation, numerous problems can occur, such as inadequate wetting of the substrate, poor flow, crater formation and pigment floating.
Phenomena that develop at the surface of a film play an important role in the formation of defects, and can also influence film properties. The addition of small spreading substances, such as silicones, can significantly affect these phenomena.
Substances are identified as spreading substances if they reduce the surface tension of a liquid, for example, by concentrating at the air interface, and forming a new, lower energy interface with the air.
Numerous substances have spreading properties in water, which has a very high surface tension of 73 mN/m. The molecules of these so-called surfactants possess a non-polar segment oriented towards the air and a polar segment oriented towards the liquid. Silicone oils and modified siloxanes demonstrate spreading properties in solvent-based systems because of their inherently low surface tension.
Silicone oil spreading substances influence several surface properties simultaneously. They: