Development of low VOC additives to extend the wet edge and open time of aqueous coatings

Abstract

One of the challenges of formulating waterborne coatings is achieving an acceptable balance of properties both during the film application and drying process as well as in the final film. This difficulty is compounded by regulations relating to the reduction of volatile organic compounds (VOCs) in architectural coatings. There is a competition between the requirements for adequate workability time of the coating with appropriate film formation and recoat behavior. The period in which irregularities in a freshly applied coating can be repaired without resulting in brush marks is referred to as the open time, while the period in which a coating can be applied over an existing paint film without leaving lap marks is deemed the wet edge time. Aqueous coatings generally employ dispersed high molecular weight polymers as binders. These binders often provide short open times when the coating is dried since the dispersed polymer particles tend to be immobilized quickly in the edge region of an applied coating. As a result, the viscosity of the coating increases rapidly, which leads to a limited window of workability. Small molecule alkylene glycols such as ethylene and propylene glycol are routinely incorporated in aqueous coatings as humectants, but are limited in utility since they are considered to be VOCs. This paper describes the process by which new, low VOC additives were developed to improve open time and wet edge in aqueous coatings. Mechanisms for the enhanced performance will be outlined through use of rheological techniques. Paint performance properties will be exemplified in a typical architectural formulation.