Influencing Factors of Thickening of Waterborne Coatings During Storage

This article focuses exclusively on waterborne coating systems and summarizes six core, practical causes of post-storage thickening, which are highly consistent with on-site production conditions.

01. pH Value Fluctuation (Most Common Inducement)

1. Low system pH and insufficient neutralization
Alkali-swellable thickeners (ASE/HASE) cannot be fully unfolded and activated. They undergo gradual association during storage, resulting in continuous thickening.

2. Slow pH drop during storage
Resin hydrolysis, additive decomposition, and neutralization of ammonia or amine neutralizers by carbon dioxide in the air cause continuous pH decline, leading to post-thickening and swelling of the coating system.

3. Excessively high pH value
Long-term strong alkaline conditions accelerate emulsion hydrolysis and degradation & entanglement of resin molecular chains, which also causes an increase in coating viscosity.

02. Thickener System Problems

1. Alkali-swellable thickeners
Extremely sensitive to pH and electrolytes. Slow cross-linking and association occur during storage, causing sustained post-thickening under static conditions.

2. Polyurethane associative thickeners
Affected by the adsorption of surfactants, pigments and fillers, the hydrophobic groups undergo secondary gradual association during storage, leading to continuous rise in static viscosity.

3. Cellulose thickeners
Poor hydrolysis resistance. Long-term storage causes molecular chain degradation and structural entanglement, easily resulting in abnormal viscosity increase.

4. Excessive dosage or poor compatibility of thickeners
Unreasonable collocation or over-addition of thickeners continuously strengthens the static network structure of the coating system and raises viscosity.

03. Emulsion Stability Defects

1. Unstable emulsion glass transition, particle size and coating structure
Slow swelling and micro-flocculation of emulsion particles occur during long-term storage.

2. Secondary reaction of residual substances
Residual monomers, initiators and functional groups in the emulsion trigger slow secondary reactions, resulting in tight accumulation of particles and increased overall viscosity.

3. Poor water and alkali resistance stability of emulsion
Long-term static placement causes slight demulsification and flocculation, which thickens the entire system.

04. Influence of Pigments, Fillers and Powders

1. Poor dispersion of pigments and fillers & post-desorption of dispersants
Pigments re-flocculate and agglomerate during storage to form a network structure, causing structural thickening.

2. Self-contained electrolytes and soluble salts in fillers
Excessive salt content in kaolin, talc, calcium carbonate, titanium dioxide and other powders destroys the hydration structure of thickeners and induces coating thickening and swelling.

3. Inorganic anti-settling thickening fillers
Bentonite, magnesium aluminum silicate and other fillers fully hydrate and unfold during long-term static storage, forming a strong thixotropic network and causing significant static thickening.

05. Interference of Additives and Electrolytes

1. Hydrolysis and failure of dispersants and wetting agents
The breaking of surface charge balance causes system instability and viscosity increase.

2. Excessive electrolyte content in the system
Metal ions and salts compress the electric double layer of emulsion and destroy the hydration layer of thickeners, which is a high-frequency cause of post-thickening of waterborne coatings.

3. Inappropriate types of biocides and neutralizers
Slow chemical reactions occur between unsuitable additives and resins/thickeners during storage, destabilizing the system and causing thickening.

06. Storage Environment and External Conditions

1. Temperature
High-temperature storage accelerates hydrolysis, oxidation and association reactions, greatly aggravating post-thickening; low-temperature storage causes extrusion of emulsion particles and poor hydration, resulting in temporary thickening.

2. Poor sealing performance
Slow water volatilization and concentration, together with continuous carbon dioxide ingress that reduces pH value, jointly exacerbate coating thickening.

3. Storage time
Longer storage time leads to accumulated side reactions, flocculation and molecular association, making post-thickening more obvious.

Quick Distinction of Thickening Types

• Obvious viscosity reduction after stirring and re-thickening after standing: Physical structural thickening (caused by flocculation, anti-settling agents and associative thickeners).

• Still high viscosity, gelling or caking after stirring: Chemical instability or emulsion micro-flocculation, belonging to coating deterioration.