Introduction of Plasticizers in Coating Formulations
Plasticizers are used to control the film formation process of coatings based on physically drying film forming materials. Proper film formation is essential in order to meet demands on specific coating properties such as dry film appearance, substrate adhesion, elasticity, in combination with high level of hardness at the same time.
Plasticizers function by reducing film formation temperature and elasticize the coating; plasticizers work by embedding themselves between the chains of polymers, spacing them apart (increasing the “free volume”), and thus significantly lowering the glass transition temperature for the polymer and making it softer.
The molecules in polymeric film forming materials, such as nitrocellulose (NC), typically show low chain mobility, explained by strong molecular interaction (explained by van der Waals forces) of the polymer chains. The role of the plasticizer is to reduce or entirely prevent the formation of such bridging bonds. In case of synthetic polymers this can be achieved by incorporating elasticizing segments or monomers which sterically hinder molecular interaction; this chemical modification process is known as “inner plasticization”. For natural products or hard polymers of poor processing, the option is the external use plasticizers in the coating formulation.
Plasticizers interact physically with the polymer binder molecule, without a chemical reaction and form a homogeneous system. The interaction is based on the specific structure of the plasticizer, typically containing polar and non-polar moieties, and results in lowering the glass temperature (Tg). In order to ensure high efficiency, the plasticizer should be able in penetrating the resin at film forming conditions.
Classic plastizers are low molecular weight materials, such as phthalate esters. However, more recently phthalate free products are preferred as the use of phthalate esters is restricted due to product safety concerns.