Impact of resin molecular weight on drying kinetics and sag of coatings

• Published in: Progress in organic coatings 2024-09, Vol.194, p.108618, Article 108618
• Main Authors: Issa, Marola W., Barancyk, Steven V., Rock, Reza M., Gilchrist, James F., Wirth, Christopher L.
• Format: Article
• Language: English
• Subjects: Coating defects; Drying; Non-invasive imaging
• ISSN: 0300-9440; 1873-331X
• DOI: 10.1016/j.porgcoat.2024.108618

Coating performance is influenced by factors inherent to formulation design and processing conditions. Understanding the complex interplay of these attributes enables for mitigating defects, such as sag, which is a gravity-driven phenomenon that impacts the aesthetic and functionality of a coating. The work herein investigates the impact of resin molecular weight and solvent choice on the drying kinetics and sag velocity in polymer films. These films, ranging in thickness from ~60 μm to ~120 μm were formulated with 45 % by weight polymer resin in one of two solvent packages with different relative evaporation rates (RER). Gravimetry was initially used to track drying rate and a one-dimensional diffusion model was utilized to compute the apparent solvent diffusivity. In addition, the film thickness was tracked with optical profilometry. Results from these measurements showed that for fixed molecular weight the drying rate increased by approximately two-fold for the high RER solvent, whereas the apparent diffusivity tended to increase with increasing polymer molecular weight. Films formulated from higher molecular weight resins had greater initial viscosities and thicknesses for identical draw down blade clearance. By extension, the higher apparent diffusivities at greater molecular weights were attributed to effects of prolonged evaporation times for the thicker films. The sag velocity was measured through the thickness of the film for these systems at a 5° incline using the Variable Angle Inspection Microscope (VAIM). Measurements showed an increase in sag velocity for thinner and less viscous films, which was somewhat surprising both because a thinner film will experience lower gravitational stress and quicker drying times as compared to a thicker film. From these data we conclude that formulating a coating with higher molecular weight resin, although likely to increase drying time, will tend to deter sag because of the large impact of viscosity on these phenomena. •Films containing higher molecular weight resins were thicker for identical draw down blade clearance.•Films containing higher molecular weight resin dried more slowly than those of lower molecular weight resin.•Data suggests near surface drying was dominant during times consistent with flash.•Thicker films made from more viscous material experienced milder sag.