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Expert Work | pH Control and Adjustment in Waterborne Metallic Paints
The water-based metallic paint here refers to water-based paint with aluminum silver paste metallic pigment added.
Compared to water-based paints that incorporate general pigments and toners, water-based metallic paints have stricter requirements and control ranges for the pH value of the system. This is because aluminum paste is generally stable only when the pH value is greater than 7 and less than 9, so the pH value of the system must always be maintained within this range.
When the pH value of the coating system is less than 7 or greater than 9, the aluminum pigment surface will react, turning black and bloating. When the pH value is stable at 8-8.5, the storage stability of aluminum powder and coating system is the best.
Commonly used neutralizers for adjusting pH value include: ammonia, triethylamine (TEA), dimethylethanolamine (DMAE), and 2-amino-2-methyl-1-propanol (AMP-95). Their basic properties are as follows:
| Relative molecular weight | Boiling point (℃) | |
| Ammonia (26%) | 17（NH3） | -33（NH3） |
| TEA | 101 | 89 |
| DMEA | 89 | 134 |
| AMP-95 | 89 | 165 |
Ammonia has a strong odor, evaporates quickly, has poor storage stability for the paint, and has poor wettability for pigments, so it is not suitable for use in metallic paints.
Suitable neutralizers include TEA, DMEA, and AMP-95. Choosing the right neutralizer based on its boiling point, volatilization rate, alkalinity, and paint film drying conditions will help achieve the best overall performance.
When forced drying at room temperature or low temperature, DMEA is generally used as a neutralizer, while AMP-95 can be used for high-temperature baking drying. When adjusting the pH of the coating system, DMEA or AMP-95 should be diluted with water to approximately 10-20% before adding, stirring while adding to avoid resin agglomeration and deterioration caused by direct addition of high amine concentrations and strong alkalinity.
For water-based two-component polyurethane system metal paint, the effect of neutralizer amine substances on the activation period must also be considered. As we know, amine substances are accelerators for polyurethane curing, so during the construction process, they will inevitably have a certain impact on the usable time of the coating and may shorten the activation period of the coating.
Therefore, during the production process of water-based metallic paint, it is necessary to select a suitable neutralizer and strictly control the dosage and process to obtain the best coating performance.