From Wet to Protective: Film Formation in Waterborne Coatings

Abstract

The importance of anticorrosive coatings cannot be overstated, as they play a vital role in safeguarding assets and infrastructure across various industries. Within this context, the emergence of waterborne (WB) coatings stands out for their paramount significance, offering a sustainable alternative to traditional solvent-based (SB) coatings and addressing pressing environmental concerns. Despite their eco-friendliness, the complexity of their film formation mechanism and the lack of understanding present challenges in enhancing the performance of waterborne coatings for corrosion protection. These coatings are created by dispersing polymers in water, which then form a protective film on a substrate. The process involves stages like water evaporation, particle packing, and polymer interdiffusion, often leading to films with defects and inferior protection against extremely corrosive environments compared to SB coatings. This Review scrutinizes the interplay of factors affecting film formation in application, including coalescing agents, environmental factors, and application conditions. A comparative analysis between SB and WB coatings is also featured to shed light on the performance gap under harsh conditions. This Review discusses analytical techniques for studying film formation, aiming to guide future research toward improving WB coatings' durability and effectiveness. In compiling this collective wisdom, this Review emphasizes the translation of theoretical understanding into practical knowledge, equipping formulators with actionable insights to optimize WB coatings for real-world application and performance.

Keywords: anticorrosive; coalescing agent; film formation; latex; waterborne coating.

Figure 1

Most common applications of WB coatings in different industries.

Figure 2

Optical images evidencing the nonuniform horizontal drying of an applied WB coating (200 nm polystyrene particles in water) with 10 wt % solid concentration. Reproduced with permission from ref (36). Copyright 2013 IOP Publishing Ltd.

Figure 3

Schematic representation of the film formation stages for different coating systems at a simplified condition: (a) for SB coatings (dissolved polymer) and (b) for WB coatings (dispersion of latex in water).

Figure 4

Cryogenic SEM (cryo-SEM) images of latex after the evaporation of water for (a) 3 min, (b) 6 min, (c) 8 min, and (d) 15 min, going from a close-packed (ordered) structure to a deformed structure with distinguishable boundaries. The small features between particles shown by arrows are trapped frozen water. Reproduced with permission from ref (48). Copyright 2005 Elsevier.

Figure 5

Visual evidence of the flat contact area between adjacent deformed particles, where particle identity is preserved (TEM image), and different perspectives of a rhombic dodecahedron, showcasing six square faces and six rhomboidal faces on the vertical sides, with three on the upper portion and three on the lower section. Reproduced with permission from ref (52). Copyright 2015 Royal Society of Chemistry.

Figure 6

All of the proposed mechanisms contributing to particle deformation in WB coatings.

Figure 7

Schematically represented and experimentally observed meniscus at the surface of a wet monodisperse latex film coated on a silicon substrate. Reproduced with permission from ref (48). Copyright 2005 Elsevier.

Figure 8

(a) Schematic of receding waterfront mechanism, (b) schematic phase diagram, (c) surface diagram of dimensionless groups, and (d) dimensionless numbers and the initial model. Reproduced with permission from ref (59). Copyright 1999 American Chemical Society.

Figure 9

A mind map of different experimental techniques to explore film formation in WB coatings categorized by each stage.

Figure 10

(a) Schematic representation of the MS-DWS setup to study the film formation through a nondestructive, in situ procedure. (b) Kinetics of A.S.I.I. observed during the formation of a WB sample on a glass plate, illustrating distinct phases. Reproduced with permission from ref (74). Copyright 2008 Elsevier.

Figure 11

Bode plots of WB film formation with data fitted to models featuring the following circuit elements: (a) tacky film (30 min): R_coat = (6.88 × 10⁴) ± (1.29 × 10³) Ω, CPE_coat = (106.4 × 10^–9) ± (7.14 × 10^–9) S s^α, α = 0.874 ± 0.00529; (b) semidry film (60 min): CPE_coat = (5.87 × 10^–9) ± (7.24 × 10^–11) S s^α, α = 0.874 ± 0.00529, W_diffusion = (21.1 × 10^–9) ± (1.70 × 10^–10); (c) completely dry film (120 min): R_coat = (1.69 × 10⁸) ± (1.49 × 10⁶) Ω, CPE_coat = (7.55 × 10^–9) ± (4.74 × 10^–11) S s^α, α = 0.948 ± 0.00670, and water absorption profile of (d) SB and (e) WB films with varying thickness during a 9-day immersion period. Reproduced with permission from ref (19). Copyright 2012 Elsevier.

Figure 12

A schematic representation of the mechanism through which coalescing agents facilitate the coalescence of latex particles: (a) how coalescing agents leave behind the latex particles after evaporation and (b) the corresponding phenomenon without coalescing agents.

Figure 13

(a) Flow curves of various dispersions and (b) median particle size versus the loading concentration of the coalescent. Reproduced with permission from ref (15). Copyright 2017 John Wiley and Sons.

Figure 14

Evaporation rate based on the relative weight of the sample compared to the initial state resulted from the TGA analysis: (a,b) various temperatures (constant relative humidity) and (c) relative humidities (constant temperature). Reproduced with permission from ref (109). Copyright 2011 American Chemical Society.

Figure 15

(a) The interparticle distance and volume fraction during film formation upon various environmental conditions, and (b) different stages of the proposed film formation mechanism. Reproduced with permission from ref (109). Copyright 2011 American Chemical Society.

Figure 16

AFM height images of freshly applied (a) and cured films at 40 °C for 24 h (b). Reproduced with permission from ref (110). Copyright 2018 Shenyang Pharmaceutical University. Published by Elsevier.