Using an added liquid to suppress drying defects in hard particle coatings

TL;DR

This study demonstrates that using capillary suspensions with added liquid can produce uniform, defect-free hard particle coatings by inhibiting lateral drying and controlling stress, eliminating the need for polymers.

Contribution

The paper introduces a polymer-free method using capillary suspensions to improve coating uniformity and reduce defects during drying.

Findings

Capillary suspensions dry uniformly without defects.

High yield stress inhibits lateral drying and causes even shrinkage.

Bridges between particles prevent air invasion and extend drying period.

Abstract

Hypothesis: Lateral accumulation and film defects during drying of hard particle coatings is a common problem, typically solved using polymeric additives and surface active ingredients, which require further processing of the dried film. Capillary suspensions with their tunable physical properties, devoid of polymers, offer new pathways in producing uniform and defect free particulate coatings. Experiments: We investigated the effect of small amounts of secondary liquid on the coating's drying behavior. Stress build-up and weight loss in a temperature and humidity controlled drying chamber were simultaneously measured. Changes in the coating's reflectance and height profile over time were related with the weight loss and stress curve. Findings: Capillary suspensions dry uniformly without defects. Lateral drying is inhibited by the high yield stress, causing the coating to shrink to an even height. The bridges between particles prevent air invasion and extend the constant drying period. The liquid in the lower layers is transported to the interface via corner flow within surface pores, leading to a partially dry layer near the substrate while the pores above are still saturated. Using capillary suspensions for hard particle coatings results in more uniform, defect free films with better printing characteristics, rendering high additive content obsolete.