Acid aluminum phosphate for the binding and coating of materials

This is a review of acid aluminum phosphate for the binding and coating of materials. The acid aluminum phosphate in the form of a solution obtained by dissolving aluminum hydroxide in phosphoric acid, with a P/Al molar ratio of 23, is effective for the binding of fibrous or particulate materials. Porous ceramic articles are made using this binder by wet forming, followed by heat treatment. The binder resides at the junction of the adjacent fibers in the porous fibrous ceramic article. No free binder was observed in fibrous or particulate articles. The porosity is 45%–90% in fibrous articles, and is 25%–82% in particulate articles. Porosity above 61% in particulate articles is attained by adding carbon particles and subsequent removal of them by combustion. The binder proportion is 3–10 wt% in fibrous articles, and is ∼0.1 wt% in particulate articles. Porous ceramic articles made using a silica binder instead of the acid aluminum phosphate binder are inferior in permeability, mechanical properties and creep resistance, and exhibit a wider pore size distribution. Free silica binder was observed in articles made with the silica binder. The acid aluminum phosphate with a P/Al ratio of 12 or 23 is also effective for coating materials, particularly graphite for the purpose of oxidation protection.