Guide to the use of dispersants for shoe coatings: a comprehensive analysis from principle to practice
In the production of sports shoes, leather shoes and other footwear products, the uniformity of the color of the upper,
the glossiness and the durability of the coating often depend on a key additive in the coating - the dispersant. It is like a
"micro conductor" that directly affects the final performance of the coating by regulating the distribution state of pigments
and fillers. This article will unveil the mystery of dispersants for shoe coatings from the principle of action, usage methods
to precautions.
1. The core role of dispersants: breaking agglomeration and building a stable system
Pigments and fillers are the source of paint colors, but these tiny particles (usually nanometer to micron level) are very easy to
attract each other in liquids due to surface energy to form agglomerates. This agglomeration will lead to increased viscosity of
the coating, poor leveling, and even problems such as color spots and sedimentation. The core function of the dispersant is to
evenly disperse these particles and maintain a stable state through physical or chemical effects.
Mechanism of action:
Dispersant molecules usually consist of two parts:
Anchoring groups: such as carboxylic acids and phosphates, which can be tightly adsorbed on the surface of the pigment to form
a "protective layer";
Solvated chains: such as long-chain structures such as polyethers and polyesters, which extend into the coating matrix and prevent
particles from colliding and aggregating through steric hindrance effects.
For example, in water-based shoe coatings, dispersants wrap pigment particles through an "anchor-stretch" structure, reducing the
particle size distribution to 1/3 of that when not dispersed, thereby significantly improving the uniformity of the coating. Experimental
data show that the gloss of coatings using polymer dispersants can be increased by more than 20%, and the color difference ΔE value
is reduced to less than 0.5 (the industry standard is ≤1.5).
2. How to use dispersants: a practical guide from the laboratory to the production line
1). Pre-dispersion test: determine the optimal dosage
The dosage of dispersants needs to be adjusted according to the type of pigment, coating system (water-based/solvent-based) and
process requirements. Excessive use may cause free dispersant residues to affect the performance of the coating film; insufficient dosage
will not effectively disperse the particles.
Operation steps:
Take 2g of coating base material and add 100ml of acetone (or deionized water for water-based system);
Add 2g of color paste and 2g of pigment in turn, and stir well;
Add dispersant drop by drop (the recommended initial concentration is 3%-5% of the masterbatch mass), and stir for 5 minutes after each
addition;
Stand for 20 minutes and observe the precipitation. If there is no precipitation and the system is uniform, it is the best dosage.
Case reference:
When a shoe material company produces black water-based leather coating, the above method is used to determine the dispersant dosage
to be 4.2% of the pigment mass, which reduces the coating viscosity from 120KU to 85KU, and at the same time increases the pigment
loading from 30% to 40%, reducing the production cost by 18%.
2). Grinding and dispersion: efficient crushing and stabilization
In the preparation stage of color paste, dispersant needs to be used together with pigment and grinding media (such as zirconium beads)
to crush the pigment particles to the target particle size (usually 0.1-0.3 microns) through high shear force.
Key parameters:
Grinding time: adjusted according to the hardness of the pigment, generally titanium dioxide requires 2-4 hours, carbon black requires 4-6
hours;
When to add dispersant: It is recommended to add it all at once at the beginning of grinding to fully wet the pigment surface;
Temperature control: The grinding temperature of the water-based system must be lower than 50°C to avoid dispersant decomposition.
Effect verification:
Use a laser particle size analyzer to detect the D50 (median particle size) and D90 (90% particle size) of the color paste after grinding.
High-quality dispersants can make D50≤0.2 microns, D90≤0.5 microns, and the particle size distribution curve is narrow peak-shaped.
3). Paint mixing and coating: maintain dispersion stability
During the paint mixing stage, it is necessary to ensure that the dispersant is compatible with components such as resins and solvents
to avoid re-flocculation of the pigment due to system changes.
Notes:
pH adjustment: The pH of the water-based system needs to be controlled between 8-9 to enhance the adsorption capacity of the
dispersant anchoring group;
Dilution ratio: When the dispersant concentration is too high, it can be diluted to 5%-10% with a solvent before use;
Stirring speed: The stirring speed is recommended to be 500-800rpm during the paint mixing stage to avoid introducing too many
bubbles.
Coating effect:
After spraying, the coating surface of the coating using the dispersant can form a uniform "mirror" effect, with a gloss of more than
80° (measured at a 60° angle), and no defects such as brush marks and orange peel.
3. Common misunderstandings and solutions for the use of dispersants
Misunderstanding 1: The more dispersant, the better
Consequences: Excessive dispersant will cause the coating film to become brittle, reduce water resistance, and even cause
"floating color" (stratification of different pigments).
Solution: Determine the optimal dosage through pre-dispersion testing, generally not exceeding 8% of the masterbatch mass.
Misunderstanding 2: All dispersants can be used universally
Consequences: Different pigments (such as organic pigments and inorganic pigments) need to be matched with special
dispersants. For example, carbon black requires an amine-containing polymer dispersant, while titanium dioxide is suitable
for carboxylate dispersants.
Solution: Select dispersants according to the type of pigment, or consult suppliers for matching solutions.
Misunderstanding 3: Ignoring the compatibility of dispersants and resins
Consequences: Poor compatibility will cause dispersant precipitation, forming "oil spots" or "lumps".
Solution: Give priority to dispersants with similar polarity to the resin, or improve compatibility by adding compatibilizers
(such as coupling agents).
4. Future trends: smart dispersants and green manufacturing
With the improvement of environmental protection and performance requirements in the footwear industry, dispersant
technology is developing in the following directions:
Low VOC dispersants: Use bio-based solvation chains to reduce volatile organic compound emissions;
Self-healing dispersants: Automatically release dispersants and repair pigment distribution when the coating is damaged
through light response or thermal response mechanisms;
Nanocomposite dispersants: Compound inorganic nanoparticles (such as silica) with organic dispersants to improve
dispersion efficiency and coating hardness.
For example, a starch-based dispersant developed by a certain company has a biodegradability rate of over 90%, has
passed the EU REACH certification, and has become the preferred additive for water-based sports shoe coatings.
Conclusion
From pre-dispersion testing in the laboratory to grinding and mixing paint on the production line, the use of dispersants
runs through the entire process of shoe coating preparation. By scientifically selecting dispersant types, accurately
controlling dosage, and avoiding common misunderstandings, companies can significantly improve coating performance,
reduce production costs, and meet environmental protection and sustainable development needs. In the future, with the
popularization of intelligent dispersion technology, shoe coatings will usher in a new era of higher performance and wider
application.