Brushstrokes - October 2011 - Surface Coatings Association of New ...
Brushstrokes - October 2011 - Surface Coatings Association of New ...
Brushstrokes - October 2011 - Surface Coatings Association of New ...
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Fig. 2 Some white pigment sales.<br />
Fig. 3<br />
As mentioned above White lead is the oldest white pigment<br />
in use today. It was known at least 400 B.C. The first plant<br />
in America was built in 1804 by Samuel Wetherill & Sons<br />
in Philadelphia. There have been recent advances in the<br />
improvement <strong>of</strong> manufacture which is enabling the industry<br />
to make better white lead pigment. It is manufactured by<br />
five different processes. These vary primarily in the type <strong>of</strong><br />
raw material used.<br />
The oldest process is probably the Dutch process, which<br />
uses refined metallic lead in the form <strong>of</strong> perforated discs. It<br />
takes about three months to make the white lead. The Carter<br />
process dates from 1885. It uses powdered lead in revolving<br />
wooden cylinders and takes only 12 days. The Euston process<br />
puts the refined lead in solution and precipitates the white<br />
lead. Feathered lead is used as the raw material, which is<br />
made by running molten lead into water. The Sperry process<br />
is the electric process using a lead anode and iron cathode.<br />
The electrolytes, sodium acetate and sodium carbonate, are<br />
separated by a membrane. The Thompson-Stewart process is<br />
a recent development similar to the Carter process in that lead<br />
oxide is formed. However, all the lead oxide is first formed;<br />
then carbon-dioxide is added and controlled to form a definite<br />
chemical compound: 4PbC03.2Pb(OH)2.PbO.<br />
Basic carbonate white lead has the ability to impart adhesion,<br />
toughness, elasticity, and durability to a paint. It is used in<br />
various types <strong>of</strong> paint, principally in exterior paints. Basic<br />
carbonate white lead is the only white pigment which will<br />
produce a durable exterior paint if used alone without other<br />
pigments. A large proportion <strong>of</strong> the total white-lead production<br />
is used in white-lead pastes, which are thinned to paint form<br />
by the painter or other consumer. The two types <strong>of</strong> commercial<br />
white-lead paste are heavy paste and s<strong>of</strong>t paste. The first is<br />
composed <strong>of</strong> about 91 per cent white lead and 9 per cent<br />
linseed oil, while the latter contains about 89 per cent white<br />
lead, 9 per cent linseed oil, and 2 per cent turpentine. The<br />
linseed oil used is refined oil with an acid number <strong>of</strong> 6 to 12.<br />
About 95 per cent <strong>of</strong> the total production <strong>of</strong> white lead is<br />
consumed by the paint industry, with a small amount used in<br />
putty and by the ceramic and other industries.<br />
Basic lead sulfate is quite widely used as a paint pigment.<br />
It is called white basic lead sulfate, basic sulfate white lead<br />
or “sublimed white lead.” White basic lead sulfate is a quite<br />
recent pigment in comparison to basic carbonate white lead.<br />
It was originated in 1855 by E. O. Bartlett. He was at that<br />
time making zinc oxide directly from zinc ores by the American<br />
process. Applying the same principles to the production <strong>of</strong> a<br />
lead pigment from lead ore, he found that it was possible to<br />
produce a white powder having pigment properties.<br />
The first plant was built in 1876 in Joplin, Missouri, where lead<br />
ore deposits were sufficiently free from other metals for the<br />
production <strong>of</strong> white basic lead sulfate. It is manufactured by two<br />
processes, called the fume process and the chemical process;<br />
these are self-explanatory as far as manufacturing is concerned.<br />
Almost the entire production <strong>of</strong> white basic lead sulfate goes into<br />
mixed-pigment exterior paints, either directly or as the basic lead<br />
sulfate portion <strong>of</strong> blended leaded zinc oxides.<br />
White basic lead sulfate, like basic carbonate white lead, has<br />
the ability to impart to paints adhesion, toughness, elasticity,<br />
and durability, but is not as effective in this respect as is the<br />
basic carbonate. In exterior mixed-pigment paints, it is used<br />
principally as a substitute for basic carbonate white lead.<br />
White basic lead sulfate may be used for the “white lead”<br />
content <strong>of</strong> many specification paints, such as Federal<br />
Specification paints TT-P-40 and TT-P-81. White basic lead<br />
sulfate is a less expensive pigment than basic carbonate<br />
white lead, and this has been an influencing factor in its<br />
use. White basic lead sulfate is not generally used for singlepigment<br />
paints or pastes but is used in conjunction with<br />
other pigments in ready-mixed exterior paints. Ninety-seven<br />
per cent <strong>of</strong> the total production <strong>of</strong> this material is used in<br />
the paint industry, the remaining 3 per cent being used in<br />
the rubber and other industries.<br />
Zinc oxide pigments are made primarily by either the American<br />
or French process. In both these processes the characteristics<br />
<strong>of</strong> the pigment are determined by:<br />
1. The temperature <strong>of</strong> the oxidation <strong>of</strong> the fume;<br />
2. The time the zinc oxide is held at a high temperature;<br />
3. The composition <strong>of</strong> the gases;<br />
4. The rate <strong>of</strong> cooling.<br />
SURFACE COATINGS ASSOCIATION OF NEW ZEALAND 3