ATH

Product=20 Knowledge=20
ATH =EF=BF=BD Filler=20 With An Attitude

Aluminum trihydroxide, = variously referred=20 to as ATH, alumina trihydrate, hydrated alumina, or simply = hydrate,=20 is a primary ingredient in most solid surface materials, = accounting=20 for as much as 70% of the total product. The powdery = substance is=20 odorless and non-carcinogenic, and provides translucency and = whiteness to solid surface materials. Because it contains = thermal=20 characteristics similar to the acrylic and polyester = polymers with=20 which it is mixed, the final cast product exhibits excellent = physical properties.

Of great importance to = commercial solid=20 surface applications in particular, ATH is a non-smoking,=20 low-toxicity, halogen-free flame retardant. When exposed to = fire,=20 ATH begins to decompose into water and anhydrous = (water-free)=20 alumina. The water acts as a heat sink, cooling the plastic = and=20 significantly slowing its degradation into combustible fuel. = In=20 other words, when exposed to a fire 400 degrees or hotter, = the water=20 released by ATH acts as a type of built-in fire=20 extinguisher.

ATH is derived from bauxite, = which is one=20 of the most plentiful substances in the earth=EF=BF=BDs = crust. The natural=20 ore is refined from red dirt into a fine, white powder = through one=20 of two processes, known as the Bayer and Sinter processes. = Further=20 refining of the ATH powder through heating results in = anhydrous=20 alumina, which is used in the production of aluminum=20 metal.

In the sinter process, the = bauxite ore is=20 heated along with reactive components, such as sodium = carbonate and=20 sodium hydroxide to about 850 degrees Celsius. At this = temperature=20 sodium aluminate is formed, which is leached from the = mixture with=20 water. The solution is then clarified and cooled, and the = ATH is=20 precipitated using seed crystals, as in the Bayer process = described=20 below, or by means of gasification with carbon dioxide=20 gas.

Because of the extremely high = temperatures used in the sintering process, all organic = matter=20 present in the original bauxite ore is destroyed, which = results in=20 an ATH that is white in color. The sinter process predates = the Bayer=20 process, which came into popular use because it is more = economically=20 viable for most western producers to use. Sintering, today, = is most=20 often seen in less industrialized nations.

In the Bayer process, finely = ground=20 bauxite is fed into a steam-heated digester unit, = where it is=20 mixed under pressure with a hot solution of caustic soda. = There, a=20 chemical reaction takes place, which results in a solution = of sodium=20 aluminate, commonly referred to as green liquor. The green = liquor is=20 then separated from the waste during clarification, = and=20 filtered to remove any remaining impurities.

The liquor solution is next = pumped into=20 six-story tall precipitator vessels where it is mixed with = millions=20 of tiny seed crystals of alumina hydrate. This is known as=20 precipitation. These seed crystals are the foundation = upon=20 which further alumina hydrate crystals deposit. The mixture = is=20 continuously agitated so that the particles remain in = suspension,=20 enabling the crystals to grow. Finally, the entire mixture = is pumped=20 out of the precipitator for further processing.

After cleaning and drying, a = portion of=20 the hydrated alumina is removed for processing for use in = the=20 chemical industries. This is the raw material we know as = ATH. The=20 rest of the precipitated alumina hydrate =EF=BF=BD about 95% = of the total --=20 continues through the Bayer process into calcination, = where=20 the material is heated to release its chemically bonded = water, then=20 dried and sent off to smelters for the production of = aluminum=20 metal.

Certain large, fully = integrated=20 producers, such as Alcoa or Alcan, reprocess the ATH a = second time,=20 using fresh caustic soda to dissolve the alumina trihydrate=20 particles. "That forms a new sodium aluminate liquor which = is quite=20 pure," explains Ray Shaw, Market Manager =EF=BF=BD Hydrates = for Alcan=20 Chemicals. "We remove impurities from the solution, then we=20 crystalize the ATH from this new solution, and it comes out = as a=20 very white crystal."

After its second refining, = the ATH is=20 washed to remove the sodium aluminate liquor from the = surface, and=20 then dried. "It is a fairly rounded particle at that point," = says=20 Shaw. "In some cases, the unground powder is small enough to = use in=20 solid surface applications."

Shaw explains that the ATH = particle can=20 be engineered to possess certain properties and = configurations=20 during precipitation. "It is all part of the process = control," he=20 says. "The unground particle can be engineered to whatever = size you=20 want. Temperature, concentration of the solution, agitation, = and=20 things like seed crystals affect the type of crystals that = are=20 precipitated from the solution. The science is well=20 understood."

For those applications = requiring smaller,=20 or tightly controlled, particle sizes, the ATH is sent to = various=20 kinds of mills for grinding. "The roller mill is very = popular in=20 North America," he explains. "You can grind to whatever = particle=20 size you desire. In solid surface applications some people = go all=20 the way down to an average size of 5 or 6 = microns."

Companies, such as J.M. Huber = or R.J.=20 Marshall, which specialize in providing a full range of = mineral=20 fillers to the cast polymer industry, purchase single-stage = Bayer=20 ATH from any number of alumina refineries located around the = world.=20 Then they further process the material to suit the specific = needs of=20 their customers.

According to Don Mills of = Huber=20 Engineered Materials, preparing these crystals for use as = fillers=20 involves cleaning, grinding, and sizing of the material. = "When we=20 get the ATH from the refinery it is still in a crude form," = he=20 explains. "It is an agglomeration of hexagonal shaped = primary=20 crystals. Think of it as growing rock candy, but you have a = bunch of=20 crystals that grow together. The reason it is done that way = is to=20 make particles big enough to be handled."

During processing, impurities = like iron=20 and sodium ion embedded within the ATH crystal are removed, = and the=20 particle is ground to size. "We also modify the particle = size=20 distributions in order to come up with surface chemistries = and=20 particle morphologies (the shape of the particle surface) = which=20 create good physical properties in the applications and = market areas=20 we serve," Mills says.

Both Shaw and Mills are = careful to point=20 out that the proper size and type of ATH used in a solid = surface=20 sheet is highly dependent upon the total "system" of product = components, which includes the type of resin, catalyst and = pigments=20 used. "As a major ingredient of solid surface, it is = critical that=20 the correct ATH be blended with the resin," Shaw says. "It = has a=20 profound effect upon visual performance and physical = strength,=20 weather-ability, stain resistance and long life of the=20 product."

So what is the proper = combination of=20 filler, resin, catalyst and pigment? "There is no one right = ATH=20 product for everybody," Shaw continues. "It is up to each=20 manufacturer to select the right one for him. You have to = tie all=20 the elements together into a formula which = works."

Which means that the way = solid surface=20 products perform depends more on the inter-relationship of = the=20 combined ingredients than on which brand or type of resin is = being=20 used. In fact, according to Mills, the amount, size and = shape of ATH=20 used in an acrylic solid surface sheet, for example, will = even have=20 a significant impact on its thermoforming=20 characteristics.

"When you say something is an = acrylic,=20 that tells you what kinds of linkages you have between = blocks of=20 monomers," he says. "It doesn=EF=BF=BDt tell you the average = molecular=20 weight of the polymer, what the cross-link density is, or if = there=20 are any other impact modifiers which probably are introduced = at some=20 point. ATH, because it is a primary component, plays a = primary role=20 in that. There is a lot of engineering that goes into the = design,=20 and when you change one thing, you also change something=20 else."

This inter-relationship which = exists=20 between the primary ingredients in solid surface can be a = limiting=20 factor in new product development. Yet, because the = percentages and=20 types of filler, resin, catalyst or pigment can be = manipulated into=20 so many different permutations, the possible range of new = solid=20 surface products seems practically unending.

"There is no exact science to = this," Shaw=20 says, "which is why there are so many different flavors of = solid=20 surface out there. There is a basic kind of recipe and many=20 variations and different kinds of resin, many different = kinds of=20 ATH. You just have to get the right combination. That is = where the=20 skill of the plant chemist comes in =EF=BF=BD getting the = proper blend of=20 properties to get the right product."

Editor=EF=BF=BDs Note: This = article is Part One=20 in a series dealing with the products and processes involved = in=20 making solid=20 surface.