Alumina Hydrate

Alumina Hydrate, also referred to as Aluminum Trihydroxide or ATH for short, is a white powder used as a flame retardant and smoke suppressant when added to polymers such as rubber and carpeting. It’s commonly seen used as an ingredient.

Thermal decomposition of boehmite and gibbsite leads to lamellar or fibrous forms of alumina hydrate, depending on preparation method, which can then be converted by hydrothermal treatment into bayerite or g-Al2O3 materials.

1. Flame Retardant

Corundum (a-Al2O3), the natural polymorph of alumina, is hard and chemically inert, having an extremely high specific surface area of 5m2/g-1 and being the primary raw material used for creating abrasives. G-type aluminas, on the other hand, are more reactive than corundum and used as catalyst supports; they can be produced through heat treating gibbsite or boehmite and come with various BET (N2) surface areas.

Commercial aluminas are produced from gibbsite using the Bayer process, which involves leaching followed by seeded precipitation of purified gibbsite. This results in an array of small spherical crystals as well as larger particles composed of tabular and prismatic crystals forming aggregates.

These aluminas can be identified by their presence of four Al(OH)3 polymorphs with planar pseudohexagonal tabular structures, all sharing one similar crystal structure with two rows of edge-sharing octahedra linked together by one row of five bridging hydroxyl groups that connect them (Figure 3.1). Their distinguishing features are their stacking sequences and geometry of interlayer and intralayer hydrogen bonds as well as varying occupancy of nonspinel sites relative to those shared between tetrahedral and octahedra (Figures 3.1).

Heating under controlled water vapour pressure can convert g-Al(OH)3 into the more stable a-Al(OH)3, making for easier adsorption and restructuring of surface structures. Lower temperature and higher water vapour pressure help facilitate this transformation by freeing bound water molecules on the alumina surface, leaving free hydroxyls exposed on its surface for gas absorption as well as structural rearrangement.

Alumina hydrates are well known for their ability to adsorb gases while simultaneously serving as flame retardants, with their behavior in air depending on both the nature of the gas being adsorbed and its shape.

In general, aluminas with higher surface areas tend to exhibit better flame retardant properties than those with lower surface areas. These differences are likely related to their increased surface areas allowing more oxygen into their pores; their hydration being determined by temperature and humidity conditions during production which in turn influences their structure and properties.

2. Anti-Smoke Suppressant

Anti-smoke properties of alumina are closely tied to its ability to adsorb volatile gases, making alumina an excellent desiccant and recovery agent for various gases such as hydrogen sulphide (H2S). Furthermore, prior heat treatment of this material has proven highly effective for these applications and allows it to uptake and retain large quantities of H2S at high temperatures; with absorption increasing with rising temperature.

Adsorption of H2S by alumina depends on its structure; disordered structures tend to exhibit lower activity than more ordered structures. Many processes have been developed which produce highly ordered forms of hydrated alumina for anti-smoke applications; usually via heat treatment of some form of hydroxide, oxide-hydroxide or hydrous alumina gel material. Furthermore, its adsorption properties depend heavily on how and at what temperature it was heated – optimal results being attained at temperatures lower than 600 degC.

Notably, exposed alumina to water vapour will quickly and irrevocably lose its BET surface area (Sing, 1973). Soaking microporous hydrous alumina gels in liquid water results in non-porous bayerite formation as well as rapid degradation in specific surface area (Sing, 1973).

Gibbsite, a highly porous form of Al2O3, can be produced commercially through the Bayer process by leaching hot caustic aluminate solution with added seeding. Gibbsite generally exhibits plate and prism crystal structures; however, obtaining pure gibbsite from this industrial process is difficult due to small concentrations of alkali metal cations which cannot be washed off using dilute HCl solutions.

There have been various methods developed for producing high surface area aluminas without using significant amounts of alkali metals, typically “sol-gel” techniques which use hydrolysis of aluminum alkoxide to form a gel which is aged before thermal drying before being hydrothermally transformed to yield powder a-Al2O3 with diverse particle size distributions.

3. Opacity Enhancer

As a filler, alumina hydrate enhances the opacity of glazes through gas bubbles that diffuse into glass surfaces to absorb light. Furthermore, this filler helps enhance fining of glazes through encouraging coalescence of finely dispersed gas bubbles that penetrate glaze melt.

Alumina hydrate can also be used in polymer composites as a barrier enhancer. When mixed together with non-polar polymers, its particles can be uniformly dispersed throughout without forming aggregates – helping prevent any erosion when subjected to impact.

Alumina Hydrate (Al2O3) is a form of alumina with high specific surface area and high pore volume content, producing amphoteric properties such as basic and acidic properties. Produced through reacting alumina with hydrochloric acid and water, Alumina Hydrate powder is often supplied as fine white powder which is suitable for applications including ceramics, refractories and industrial materials.

Alumina hydrate has a complex surface structure due to the presence of both hydroxyl groups and coordinated water molecules, with their combined presence creating a complex surface structure. When exposed to air, however, hydroxyls are removed by exposure leaving high-energy Al3 + sites exposed on its surface; in subsequent hydration processes however these sites become replaced with high-energy cations at an extremely rapid rate, leading to shorter casting times when castables made with this material are being castable.

Clay bodies and glazes often rely on this material as its melting temperature is much higher than calcined alumina, though only small amounts should be added since too much can reduce fluidity of slurry, increasing viscosity. Furthermore, this additive promotes opacity as well as increasing color intensity when used to color certain pink glazes.

Surfactants can help enhance the opacity of alumina hydrate production, by decreasing water presence on its surface and taking advantage of its high specific surface area.

4. Filler

Gibbsite (g-Al2O3) stands in contrast to nonporous boehmite, which has an inflexible structure composed of hard, stiff crystals. Instead, gibbsite’s platy structure features small crystals which form pseudohexagonal tabular structures on which plates and prisms assemble to form pseudohexagonal tabular plates and prisms with pseudohexagonal tabular structures resembling pseudohexagonal plating structures on which it sits as raw materials in industrial production of alumina.

At temperatures between 220 and 600degC, ionic bonding in alumina hydrate begins to break down, liberating water molecules that contribute to its inherent flame retardant properties.

Hydration and calcination of alumina hydrate produce amphoteric alumina trihydroxide which dissolves readily in both acids and alkalis, making alumina hydrate an extremely versatile filler material.

Alumina hydrate’s powdered or granular texture makes it easier for potters to work with than vermiculite or other coarser fillers, and some potters prefer it over vermiculite or other fillers such as vermiculite. Alumina hydrate is an essential ingredient in many silver clay kiln wadding recipes as it helps prevent the clay from sticking to its shelves during firing – simply dusting the shelf lightly with some alumina hydrate before firing will eliminate surface tension and allow free movement as it shrinks as surface tension subsides – in some kilns it also acts as an insulator against heat transfer from its surroundings reducing heat transference during firing reducing heat transference by controlling its use as an insulator from its heat transfer capabilities as an insulator between materials used.