Void formation and opacity in latex films with aluminum phosphate particles

The opacity of dry films cast from mixed dispersions of styrene-acrylic latex and amorphous aluminum phosphate particles changes widely, depending on particle concentration and on particle previous drying history. SEM and TEM micrographs from film trimmed surfaces or thin cuts show that particle-latex adhesion is very good, even though the film contains closed voids. Measured values of scattering coefficient calculated from films reflectance using Kubelka–Munk's model were compared to the void numbers and sizes, showing that the observed voids account for film opacity. The most opaque films show contrast ratio equal to 88% and a broad distribution of void sizes, peaking at ca. 650 nm diameter. Film scattering coefficient of the observed voids was calculated using Mie's theory to evaluate their contribution to film opacity showing that the measured and estimated values are in fair agreement. Calculations show that scattering efficiency for 650 nm—diameter air voids trapped within the phosphate particles or resin, that have the same refractive index, is only 20% lower than the calculated values for optimum size rutile particles dispersed in the same resin. Mechanisms for void formation in this system are discussed.