By: Dr. Ed Weil
Research Professor, Polytechnic Institute of NYU (off-campus)
IP Fellow, University of Akron Research Foundation
Adjunct Professor., SUNY Stony Brook (off-campus)
Consultant in flame retardancy; U.S. patent agent
ATH is, by weight and for all applications, the largest flame retardant at this time, although it has been exceeded on a monetary basis by tetrabromobisphenol A.
There are two main categories, ground and precipitated. Both varieties have rather similar thermal properties, but differ in their particle size and shape. Both have their origin in the ore, bauxite, which is almost always processed by the Bayer Process. Bauxite is dissolved in strong caustic, filtered to remove iron and silica minerals and other contaminants, then the solution is cooled and the aluminum trihydroxide allowed to crystallize out. The process is controlled by seeding or selective nucleation, precipitation temperature and cooling rate. The size of the crude product is generally above 50 µm. It is commercially wet- or dry-ground to produce median size ranges of “ground Bayer hydrate” from about 1.5 µm to about 35 µm, and varying in color from almost white to beige. The main impurities, which depend on the ore source, are iron compounds, silicates, soda, and some organic materials.
To make purer, whiter, ATH, the Bayer hydrate may be redissolved in caustic and reprecipitated. The reprecipitated ATH usually has a median size range between 0.25 and 3.0 µm, lower iron, silica, soluble soda, and organic content, and lower surface areas than comparable sized ground ATH.
Enhancement of the flame retardant action of ATH with zinc borates has been established. In EVA, improved char (and inorganic barrier) formation is noted. Testing show lowered peak rate of heat release in the cone calorimeter, and causes a major depression of the second (burn-off) peak of heat release.
Studies have confirmed the zinc borate enhancement of the action of ATH in EVA and polyolefins. Evidence has also been published for the beneficial (perhaps synergistic) action of alumina monohydrate (boehmite) added in the range of 5% to ATH in the range of 65%. The zinc borates are also recommended for use in EPDM, particularly in combination with ATH, MDH, or a clay. In formulations of elastomers with high loadings of carbon black, especially those with high surface area, afterglow may be noted and can be controlled by adding zinc (or barium) borates.