I. What is Moisture?
In industrial production and materials science, moisture generally refers to the total amount of water contained within a substance. However, based on its form and state, water can be further subdivided into the following levels:
1. Surface-Adsorbed Water (Free Water)
Definition: Moisture physically adsorbed onto the surface and pores of a material, usually originating from ambient humidity.
Characteristics: Weakest binding force, easiest to remove by room temperature drying (e.g., oven drying).
Example: Moisture adhering to the surface of powder particles after being placed in humid air.
2. Internally Bound Water
Water of Crystallization: Exists in the form of water molecules within the mineral crystal lattice (e.g., gypsum CaSO₄·2H₂O, copper sulfate pentahydrate CuSO₄·5H₂O). Stronger binding force, requires higher temperatures (e.g., several hundred degrees Celsius) to remove, and can lead to crystal structure destruction.
Structural water/hydroxyl water: Exists in the form of hydroxyl groups (-OH) in the mineral lattice (e.g., Al₂Si₂O₅(OH)₄ in kaolinite, Al(OH)₃ in aluminum hydroxide). It has extremely strong binding forces and requires extremely high temperatures (typically > 500°C) to be removed as water molecules, leading to fundamental changes in the material's structure.
3. Chemically bound water: Water participates in chemical reactions, becoming a component of new compounds. This concept sometimes overlaps with water of crystallization, but emphasizes its irreversible chemical change.
In the daily production, trade, and application of inorganic powder materials, "moisture" usually refers to the most easily changed "surface-adsorbed water (free water)."
II. What does moisture mean for inorganic powder materials?
For inorganic powder materials, moisture content is far more than a simple "content" indicator; it is a critical parameter affecting quality, processing, and economics, with multifaceted effects:
1. Impact on Material Properties
Flowability: Moisture is one of the most important factors affecting powder flowability. Even trace amounts of moisture can form "liquid bridges" between particles, generating capillary forces, leading to powder agglomeration and a sharp decrease in flowability (such as the hygroscopic agglomeration of sugar and flour). This is disastrous for conveying, packaging, and automated metering.
Bulk Density and Filling Properties: Moisture affects the friction and agglomeration state between particles, thus altering the loose packing density and tapped density.
Electrical Properties: For powders used as insulating materials (such as ceramics and mica), moisture significantly reduces their volume resistivity.
Optical Properties: For pigments (such as titanium dioxide) and fillers, moisture affects their whiteness, hiding power, and dispersibility.
Chemical Reactivity: Moisture can sometimes trigger or promote undesirable chemical reactions, such as the oxidation of certain metal powders and the prehydration of cement.
