Modelling cracking during film formation of soft core/hard shell latexes

Waterborne dispersions of hybrid polymer particles consisting of a soft core covered by patches of hard polymer (soft core-hard "shell") are very promising to overcome the film formation paradox (formation of mechanically strong films at low temperature). However, the presence of hard phase at the exterior of the particles creates stresses during film formation that often results in cracking of the films. Due to the technological importance, mathematical modelling of cracking during drying of waterborne dispersions is an active area of research. These models developed analytical expressions that predict cracking for homogeneous and hard core-soft shell dispersions. However, they cannot justify the experimental results obtained with soft core-hard "shell" dispersions. As the development of analytical equations for film formation from soft core-hard "shell" dispersions looks an insurmountable challenge, in this work, a different strategy was used and particle deformation and stress generation were calculated by numerically solving a modified Navier-Stokes equation. A good agreement between experimental results and model predictions was achieved.