Thermodynamic Affinity versus Final Properties in a Latex Film Coalescence: A Combined Hansen Solubility Parameter, DLVO Theory, and Water Contact Angle Approach

Coalescing agents have been widely used in latexes for better film formation. However, there is no widely accepted quantitative criterion for selecting an appropriate coalescing agent, and the origin of mechanical properties variation of the films is not well understood. In this work, using the Hansen solubility parameters (HSP) approach, the thermodynamic affinity of a styrene-acrylic latex with four coalescing agents, including ethylene glycol, propylene glycol, dipropylene glycol, and dipropylene glycol mono <i>n</i>-butyl ether, has been studied. The film formation was evaluated with pendulum hardness together with the DLVO interaction analysis and time-dependent water contact angle measurement. The results showed that HSP and the relative energy difference (RED) values can be used as a quantitative criterion for predicting thermodynamic similarity. However, in most cases, they were insufficient to describe the final film properties, and other factors such as evaporation rate were seen to be influential. DLVO interaction analysis showed that the lower hardness of samples with RED lower than and close to 1 was not due to insufficient film formation, and water contact angle showed that coalescing agent retention is responsible. However, the RED greater than unity produced harder films, associated with lower retention. The samples with the highest RED, which had the highest evaporation rate, were left from the film more easily, leading to the highest film hardness. The results revealed that lower RED can result in a more coalescing agent retention, which competes with the insufficient film formation, both of which lead to reduced film hardness.