As an Extender & Filler Pigment in:
MAIN END USE APPLICATION(S):
As an Extender & Filler Pigment in:
Ceramics and Refractory
MANUFACTURE AND PROPERTIES:
Alumina Hydrate is generally prepared from Aluminium Sulfate (iron free) and Sodium Carbonate. After washing, the resultant product can be dried by the aid of hot air currents. It is then pulverized. The white powder consists of anglomerates of a pigment made up of very small particles. The chemical make-up of this hydrate can be represented by:
(Al2 [OH]6) x (Al2 [SO4]3) y
The values for x and y depend upon the conditions during the manufacture. However, the value for x is considerably greater than the value for y. The specific gravity of the dried hydrate is 2.43 and the refractive index is 1.535. The pH value is 6.3 to 6.7 and that signfies that the product is acidic in chemical behavior and has livering tendency. To overcome this problem, we developed a special quality which is less prone to liver than the preceding grade. The pH value of this quality is 6.8 to 7.4 and has completely replaced the preceding grade among printing ink manufacturers.
Specific Gravity : 2.43
Bulk Density : 0.32
Oil Absorption : 65 ?5 per 100 gms.
pH : 6.8 to 9
Moisture : 22 to 26 %
Residue on Sieve of 240 Mesh : Max. 1 %
Livering : As per standard sample
Transparency : As per standard sample
Whitishness : As per standard sample
Refractive Index : 1.535
4 Microns : 93.8 %
2 Microns : 84.4 %
1 Micron : 53.1 %
0.5 Micron : 39.0 %
0.2 Micron : 35.9 %
0.1 Micron : 32.8 %
FINISH AND GLOSS:
It has been observed that Alumina Hydrate improves the finish of dried ink films. If gloss varnish can be used, the addition of same increases the finish still further. However, the use of an excess of gloss varnish makes the print appear "varnishy" and yields relatively poor trapping in the case of multicolour printing. "Hydrate" is superior to Magnesium Carbonate, Calcium Carbonate, Blanc Fixe and Clays when it comes to a moderate finish or a high gloss. A combination of hydrate and gloss varnish is frequently used.
LITHOGRAPHIC INK EXTENDER:
Because the Alumina Hydrate has acidic properties, it does not change appreciably the acidity of the lithographic fountain solution during the printing operation. This is not the case with alkaline extenders, such as Magnesia (Basic Magnesium Carbonate) and Calcium Carbonate. These tend to neutralize the acid in the fountain solution, for they can shift it to weakly acidic or even to the alkaline state. "Greasing" is the general failure when this happens. The hydrate is therefore a useful ingredient in lithographic inks.
TRANSPARENCY & PRINTING QUALITIES:
In multicolour printing where inks trap over each other, it is obviously an advantage to have transparent inks after the first one. Alumina Hydrate qualifies very well as a transparent extender when an ink needs to be reduced in colour strength for multicolour printing.
Because the particles of Alumina Hydrate are very small, it possesses suitable printing qualities. It is frequently used to improve the printing qualities of high specific gravity pigments, such as Chrome Yellows, Orange Mineral and Molybdated Chrome Orange.
USE IN MOISTURE - SET INKS:
The majority of the moisture-set inks (often called steam-set inks) contain a resin that is very acidic in nature. This resin is a fumarated resin and has an acid value that is over 300. When carbonate extenders are present in moisture set inks, extreme foaming occurs due to the formation of copious bubbles of carbon-dioxide gas as the result of a chemical reaction involving the acidic resin.
The selection of clays or of blanc fixe as extenders in these inks is not entirely satisfactory because of their relatively poor printing qualities. In a number of inks, a blend of clay and Alumina Hydrate can be used as the extender to arrive at a low cost and acceptable printing qualities. However, if the printing run be a long one, it is safer to select a good printing extender to prevent piling on the rollers and on the printing plates.
Strangely enough, Alumina Hydrate does not seem to body up more than a trace when the fumarated rosin vehicle is used. Zinc Oxide and Orange Mineral liver readily with this vehicle.
PREVENTION OF FADING BY ALKALIES:
More present day pigments are faded or changed in hue by alkaline substances than by acidic ones. The majority of the Triphenylmethane and Diphenylmethane pigments, such as Methyl Violet and Alkali Blue, shift their chemical structures to the colourless state when alkali is present. The popular Iron Ferrocyanide Blues readily fade with alkalies.
If Calcium Carbonate or Magnesium Carbonate (Magnesia) are selected in the ink as the extender, together with moderate or low concentrations of the above pigments, fading can occur in the presence of considerable moisture.
As an example of this, an alkali blue tint faded inside a humidified cigar box, but not on the outside label. In this case the 100 % relative humidity activated the Magnesium Carbonate-Alkali Blue reaction, yielding a fading. However, at 40 - 50 % relative humidity the reaction did not occur.
The use of Alumina Hydrate as the extender solved the problem by preserving the blue colour at both relative humidities.
Another example of fading occured when a parchment iron blue ink film was boiled during fifteen minutes with hot water, the iron blue faded to a weak tan colour of iron oxide. However, the change of the extender to Alumina Hydrate, which is mildly acidic, preserved the blue colour.
Thus, the use of Alumina Hydrate, is a safeguard in cases where alkaline materials can be activated by the presence of sufficient moisture that can yield fading of sensitive pigments.
There is a possibility of livering when a pigment is able to react with a vehicle or its component, yielding a metallic soap.
If the product has approximately the same viscosity as the component, then only a trace or no bodying action is obtained. If the quantity of the reactive vehicle such as a resin or a heat polymerized drying oil be small, then the livered state will not be reached. And if the reactivity of either the pigment or the vehicle be low, the amount of body change will be small. Only when the pigment and the vehicle are both in high concentrations and very active will livering occur, provided, of course, that the product is highly viscous.