Aluminum-plastic panel surface treatment technology: a complete analysis of magnesium hydroxide dispersant selection
As the core material of modern building curtain walls and interior decoration, the surface treatment process of aluminum-plastic panels directly affects the weather resistance, flame retardancy and decorative effects of the products. Among many functional additives, magnesium hydroxide is widely used due to its excellent flame retardant properties, but the selection of dispersants often becomes a key bottleneck affecting the final performance. This article will explore in depth the technical selection points of magnesium hydroxide dispersants and provide manufacturers with feasible solutions.
1. The dual mission of magnesium hydroxide in aluminum-plastic panel coatings
In the three-layer composite structure of aluminum-plastic panels, the pretreatment coating undertakes a vital functional mission. Magnesium hydroxide (Mg(OH)₂) is an inorganic flame retardant, and its addition amount usually accounts for 30-45% of the solid content of the coating. The dispersion state of this white crystalline powder directly determines two core properties:
1. Optimization of flame retardant efficiency**: Uniformly dispersed magnesium hydroxide particles can decompose and absorb heat at 380°C, while releasing water vapor to dilute combustible gases to form a dense magnesium oxide protective layer
2. Surface quality assurance**: Improper particle size control will cause defects such as "fish eyes" and orange peels in the coating, affecting the flatness of the subsequent coating process
Laboratory data show that when the particle size D50 is controlled at 1.2-2.5μm, the LOI value (limiting oxygen index) of the coating can be increased to more than 32%, while the 60° glossiness remains in the high-quality range of 85-90GU.
2. Five golden rules for dispersant selection
2.1 Chemical compatibility matching principle
Different resin systems have significant differences in their adaptability to dispersants. For PVDF (polyvinylidene fluoride) systems, it is recommended to use fluorine-modified polymer dispersants, whose fluorine atoms on the molecular chain can form a strong interaction with the resin. Experimental comparison found that when using FM-12 dispersant, the storage stability of the coating increased from 72 hours to 180 hours, and the thixotropic index decreased by 27%.
2.2 Quantitative evaluation of dispersion efficiency
Using dynamic light scattering (DLS) to monitor the dispersion process, an excellent dispersant should be able to make the particle size distribution span ((D90 - D10)/D50) ≤1.2 within 30 minutes. A certain company introduced ultrasonic dispersion + phosphate dispersant with an HLB value of 12.5 to stabilize the Zeta potential of magnesium hydroxide above -45mV and reduce the sedimentation rate to 0.8%/month.
2.3 Thermal stability guarantee mechanism
In the baking process of 180-200℃, the thermal decomposition temperature of the dispersant must be higher than 230℃. It is recommended to use acrylic copolymers with a molecular weight of 8000-12000. Its thermogravimetric analysis (TGA) curve shows that the 5% thermal weight loss temperature reaches 245℃, which is much higher than the conventional process requirements.
2.4 Environmental compliance requirements
With the implementation of GB 30981-2020, nonylphenol polyoxyethylene ether (NPEO) dispersants have been banned. The current mainstream solution is to use APEO-free polyether-modified siloxanes, such as TEGO Dispers 755W, which has a VOC content of <2% and is REACH certified.
2.5 Cost-effectiveness balance model
Through LCC (life cycle cost) analysis, although high-quality dispersants have a higher unit price, they can reduce the amount of magnesium hydroxide by 15-20%. After a curtain wall company switched to D-325 dispersant, the cost per ton of paint decreased by 380 yuan, and the flame retardant grade was increased from B1 to A2.
III. Diagnosis of common problems on the production site
3.1 Conflict between dispersant and defoamer
When continuous shrinkage occurs, it is necessary to check the compatibility of the dispersant and the silicone defoamer. It is recommended to use a dispersant with a branched molecular structure (such as BYK-2155). Its synergistic effect with BYK-088 defoamer can stabilize the surface tension at about 28mN/m.
3.2 High-temperature coarsening phenomenon
When particle size aggregation occurs at the drying tunnel outlet, the anchor group density of the dispersant should be tested. The use of a polymer dispersant containing multiple carboxylic acid groups (such as EFKA-4585) can form a more solid steric hindrance effect. After aging at 200℃/30min, the D50 growth rate is controlled within 8%.
3.3 Batch stability difference
Establish a mathematical model for the amount of dispersant added: W = 0.02×S×ρ/(1 + 0.15C), where S is the specific surface area of magnesium hydroxide (m²/g), ρ is the slurry density, and C is the solid content. After a factory applied this formula, the viscosity deviation of different batches dropped from ±15% to ±3%.
IV. Industry cutting-edge technology trends
Microencapsulation dispersion technology is causing changes. By encapsulating the dispersant in a silica shell, pH-responsive release can be achieved. Tests show that in a coating system with a pH of 8-9, this technology increases the dispersion efficiency by 40% and reduces the amount of additives by 15%.
Graphene-modified dispersants show unique advantages. The G-Dispersant series of products developed by a research institute uses the two-dimensional structure of graphene to enhance the steric effect, increasing the critical bulk concentration (CPVC) of magnesium hydroxide from 58% to 63% while maintaining excellent leveling properties.