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Thickening of water-based coatings after storage – meeting factory standards, viscosity increases after several days to weeks of storage, and in severe cases, it may become jelly like, flocculent, and precipitate. Its essence is the slow shift of the binding network, dispersion state, and chemical equilibrium within the system over time. Starting from the rheological mechanism, this article summarizes and classifies the phenomenon of post thickening, identifies its core causes, and proposes a comprehensive prevention and control plan for the entire process.

1、First of all, what type of thickening does your post thickening belong to?

Without the need for complex instruments, one can quickly locate the attribution direction by opening the barrel and observing the features

Performance characteristics, possible attribution direction

Within 24 hours after production, the viscosity significantly increases

2. Insufficient hydration or bridging flocculation of thickeners

3. Slow and continuous thickening in the later stages of storage

4. Pigment base chemical reaction or dispersant failure

5. Dilute during mixing, quickly rebound after standing still

6. Abnormal thixotropy, excessive thickening agent binding network

7. The upper clear liquid is thin, and the bottom has hard sediment

8. Pigment flocculation and sedimentation, dispersion system collapse

9. Accompanied by color fading and decreased luster

10. Chemical conflict between pigments and resins

2、4 Core Causes and Solutions

Cause 1: Improper combination or feeding of thickeners (highest risk)

Phenomenon characteristics: Within 24 hours after the production of the coating, the viscosity increases, the mixing becomes thinner, the static rebound occurs, and in severe cases, irreversible bridging flocculation is formed, and the rigid network continues to densify.

Bottom level logic:

HEC hydration is insufficient, and it continues to absorb water and expand during storage;

HEUR is not pre diluted, and hydrophobic groups adsorb at multiple points to form bridges.

Solution:

Adopting HEUR as the main component and HEC compound system, with HEC accounting for no more than 30%;

HEUR must be pre diluted according to the recommended ratio in the product specification and slowly added. Direct flushing of the original solution is strictly prohibited;

• The viscosity recovery curves of different lotion thickener combinations were tested by rheometer to screen the optimal combination.

Cause 2: Implicit chemical reaction between pigment and base material

Phenomenon characteristics: Slow thickening in the later stage of storage, accompanied by color fading, thickening or decreased luster.

Bottom level logic:

Acidic pigments (carbon black) neutralize salt with alkaline resins to increase viscosity;

Zinc based anti rust pigments are saponified with resin carboxyl groups to produce granular zinc soap;

Insufficient surface treatment of titanium dioxide results in excessive surface active sites, which compete for the adsorption of dispersants and disrupt the dispersion balance of the system, leading to flocculation.

Solution:

During the formulation design phase, conduct compatibility screening between pigments and base materials;

Prioritize the use of surface treated inert pigments;

Highly active pigment systems are stabilized by adding chelating agents or metal passivators.

Cause 3: Dispersant failure or insufficient dosage

Phenomenon characteristics: thin upper clear liquid, hard sedimentation at the bottom; After strong mixing, the viscosity temporarily returned to normal, but after construction, the covering force decreased and the color became blurred.

Bottom level logic:

The dosage of dispersant was not calculated based on the specific surface area of the pigment, and the remaining amount was insufficient;

Dispersants compete with thickeners for adsorption and are replaced and desorbed;

The aggregation and sedimentation of pigment particles adsorb free resin, causing a local viscosity surge.

Solution:

Accurately calculate the minimum amount of dispersant based on the BET value of the pigment;

Select polymer block copolymer dispersants that match the system;

Adjust the ratio of thickener to dispersant and choose a synergistic formula.

Cause 4: Enlarged defects in storage or construction environment

1. The environment itself will not directly cause thickening, but it will significantly accelerate internal reactions and amplify formula defects:

2. Environmental variables, impact mechanisms, threshold control, high temperature

3. The rate of resin oxidation polymerization increases by 2-3 times per liter at 10 ℃

4. Storage temperature 10-30 ℃

5. Poor sealing and solvent evaporation disrupt the balance of solubility, leading to resin precipitation and thickening

6. Strictly seal the packaging, store in full barrels, and dilute excessively during construction

7. Adding more than 10% water at a time can disrupt the equilibrium of film formation, causing sagging and loss of gloss

8. The dilution ratio should be controlled within the recommended range

3、 Three step closed-loop prevention and control system

Step 1: Formula pre validation

• Test the viscosity recovery curve of different lotion thickener combinations through rheometer to select the best combination;

• Acrylic acid lotion is recommended to be matched with non-ionic thickener to avoid anionic initiation of late flocculation.

Step 2: Factory thermal storage inspection

Perform 50 ℃ x 15 day thermal storage acceleration experiment for each batch;

Simulate extreme storage and transportation environments, identify coagulation or excessive cross-linking risks in advance;

Batch with abnormally high viscosity or precipitation shall not be shipped out of the factory.

Step 3: Standardized Construction Control

• Dilution: Add ≤ 5% water at a time, stir at 600-800rpm for 10 minutes, and use deionized water;

• Tool: Match the construction roller according to viscosity (choose 8mm fluff for medium viscosity coating);

• Temperature: When<10 ℃, preheat to 25 ℃ 24 hours in advance; Add 0.1-0.3% anti settling agent at>35 ℃;

Dual component: Use strictly within the activation period (2-4 hours) and mix as needed.

Quick self inspection checklist for post thickening (recommended to save)

Has HEUR been pre diluted and fed according to the recommended ratio;

Is the proportion of HEC ≤ 30%;

Whether the dispersant is used according to the BET value calculation;

Has the active pigment undergone compatibility screening;

Has the finished product undergone 50 ℃ x 15 day thermal storage verification;

Whether the storage temperature is 10-30 ℃ and the packaging is sealed throughout the process;

Is the on-site single water addition ≤ 5%.