Aluminum hydroxide: Why can’t it be used directly?
Inorganic amphoteric hydroxides—aluminum hydroxide (Al(OH)3, ATH)—possess highly efficient flame retardant, smoke suppressant, and filling properties. Upon thermal decomposition, it does not produce toxic or corrosive gases and can be used as a flame retardant filler in polymeric organic materials. Currently, the use of ATH as a flame retardant is increasing year by year, and ATH has become the most important inorganic flame retardant globally.
Modification First, Then Flame Retardancy
Generally, manufacturers typically fill flammable materials with powdered aluminum hydroxide (ATH) or coat the surface of flammable materials with a flame-retardant coating containing ATH to improve the flame-retardant properties of polymeric organic materials.
Furthermore, because ATH contains three hydroxyl groups (-OH), its surface is asymmetrical and highly polar. The surface hydroxyl groups exhibit hydrophilic and oleophobic properties, making it prone to agglomeration when added to polymeric organic materials, directly affecting the material’s mechanical properties.
Therefore, aluminum hydroxide needs to be surface modified before use.
Surface Modification of Aluminum Hydroxide
Surface modification is one of the key technologies for optimizing the properties of inorganic powder materials, playing a crucial role in improving the application performance and value of inorganic powders. Surface modification of inorganic particles refers to the adsorption or encapsulation of one or more substances on the surface of inorganic particles, forming a core-shell composite structure. This process is essentially a composite process of different substances.
Types and Characteristics of Modifiers
There are many types of powder surface modifiers, but there is no standard classification method. Modifiers for inorganic powder modification are mainly divided into two categories: surfactants and coupling agents.
(1) Coupling Agents
Coupling agents are suitable for various composite material systems of organic polymers and inorganic fillers. After surface modification with coupling agents, the inorganic material’s compatibility and dispersibility with the polymer are increased. The surface of the inorganic material changes from hydrophilic and oleophobic to oleophilic and hydrophobic, increasing its affinity with the organic polymer.
Coupling agents are diverse and can be classified into four main categories based on their chemical structure and composition: organic complexes, silanes, titanates, and aluminates.
(2) Surfactants
Surfactants are substances that can significantly alter the surface or interfacial properties of a material when used in very small amounts. They include anionic, cationic, and nonionic surfactants, such as higher fatty acids and their salts, alcohols, amines, and esters. Their molecular structure is characterized by a long-chain alkyl group at one end, similar to polymer molecules, and polar groups such as carboxyl, ether, and amino groups at the other end.
How can the modification effect be determined?
Is modified aluminum hydroxide reliable? How reliable is it? This requires evaluating and characterizing the modification effect.
Currently, the flame-retardant effect of aluminum hydroxide flame retardants can be evaluated through direct methods such as testing the material’s oxygen index, vertical and horizontal flammability index, smoke production, thermogravimetric analysis, and mechanical properties during combustion; or indirectly by measuring powder absorbance, activation index, and oil absorption value to indirectly test its modification effect.
(1) Absorbance
Unmodified ATH has hydrophilic and oleophobic hydroxyl groups on its surface, allowing it to dissolve in water or settle freely to the bottom. After modification, the surface of ATH becomes hydrophilic and oleophobic, with surface properties completely opposite to the unmodified form. It cannot dissolve or settle to the bottom and can only float on the surface. However, modified ATH can dissolve or precipitate well in oils (such as liquid paraffin).
(2) Activation Index
Unmodified ATH has very strong polarity due to the nature of its surface hydroxyl groups (-OH), allowing it to dissolve or settle freely in water with similar properties. After modification, ATH has a layer of lipophilic groups attached to its surface, with surface hydroxyl groups (-OH) encapsulated within. The better the modification effect, the higher the lipophilic group coverage rate of the ATH surface, and the more modified ATH floats on the water surface.
(3) Oil Absorption Value
Measuring the oil absorption value requires adding castor oil to ATH and stirring. Before modification, ATH, due to its hydrophilic and oleophobic properties, requires more castor oil to form spheres. After surface modification, it becomes hydrophilic and oleophobic, improving the dispersibility of ATH in the polymer and reducing voids formed by powder agglomeration.