May- White Hiding Pigments

May- White Hiding Pigments

A selection of historical paint references

by Peter Walters

Titanium Dioxide, the theme of this edition of Brushstrokes, is, in spite of its now almost universal use as the white opacifying pigment in most industries, a very recent addition to our formulating armoury. Titanium Dioxide has only been produced commercially since 1919, initially by two companies, sharing technology and development efforts, in Fredrikstad, Norway & Niagara Falls, USA.

The first Titanium Dioxide products consisted of 25% anatase TiO2 co-precipitated with 75% barium or calcium sulphate. These pigments gave between three and four times as much hiding as the then commonly used white lead. This quantum leap in performance sounded the death knell of white lead as an effective opacifying pigment. However initial cost of these ‘high performance’ pigments relegated their early use to quality and specialised paints, providing the white lead industry with breathing space.

The crucial dates in the adoption of TiO2 as our preferred white opacifying pigment are 1920, the first commercial TiO2 composite products became available for purchase, 1928, ‘Pure’ anatase pigments, 1940, ‘Pure’ rutile pigments and 1951 the first surface treated TiO2 products became commercially available. During the 1950’s TiO2 replaced most previous white opacifiers as the pigment of choice.

This timeline is reflected in the historic texts in my personal library. The Certain-teed Paints Varnishes handbook of 1928 mentions Titanox, a 30% titanium oxide co-precipitated with 70% barium sulphate, and Titanolith, a very new co-precipitated three component pigment consisting of 15% titanium oxide, 25% zinc sulphide and 60% barium sulphate. The handbook contains the comment "Titanium oxide itself is a very excellent pigment but it is so expensive that it could not be put to general use." Even the article on Titanox states that "Its relatively high cost causes it to be replaced whenever possible by combinations of other pigments which possess equally good properties."

By 1935 The Modern Painter and Decorator stated, under an entry for Titanium Oxide, that it was a "comparatively new addition to the range of white pigments" that had the chief disadvantage of an "expense which has prevented it from being so widely employed as it deserves to be."
My extract for our theme is from von Fischer's comprehensive textbook "Paint and Varnish Technology" published in 1948 by the Reinhold Publishing Company. I have chosen to reproduce the first part of Chapter 5 as it comprehensively contrasts and compares all white opacifying pigments used in the coatings industry at a point in time when Titanium Dioxide was just coming to dominate this field. A coatings text, therefore, had to compare TiO2 with the existing technologies that the coatings technologists at the time would have been familiar with. This textbook was also published prior to the introduction of surface treated grades of TiO2 and at a time when anatase TiO2 was considered the appropriate choice for exterior white house paints because of the "self-cleaning" properties brought about by its free-chalking characteristics.

White Hiding Pigments

Pigments Department, E.I. duPont deNemours and Co., Inc., Wilmington, Delaware

In this chapter there will be a general consideration of the various white hiding pigments in common use. This subject is a very large one; therefore it will not be covered completely but only the more important points will be considered. There isn't any good short definition of what we mean by a white hiding pigment. However, it may be defined as a material which "tends to color something white."

In order to clarify the topic under consideration it is best to list the white hiding pigments which will be taken up. The following are included:
White lead (basic carbonate) Titanium dioxide (Rutile)
White lead (basic sulfate) Titanated Lithopone
Zinc oxide Zinc sulfide-barium
Leaded zinc oxide Zinc sulfide-calcium
Lithopone Zinc sulfide-magnesium
Zinc sulfide Antimony Oxide
Titanium-barium pigment Lead titanate
Titanium-calcium pigment Basic lead silicate
Titanium-magnesium pigment Zirconium oxide
Titanium dioxide (Anatase) Lead tetra-phosphate

Of these 20 white pigments, white lead is the oldest, while titanium dioxide (pure and extended) is the most important today. The last three are relatively new in the pigment field. Probably more will be learned about these in future years.

Figures 2 and 3 summarize the production figures of three important pigments. A study of these graphs reveals interesting information on the production of these materials. Attention is called to the effect of the depression years and how the introduction of a new pigment tends to affect one already in production.

May- White Hiding Pigments

As mentioned above White lead is the oldest white pigment in use to-day. It was known at least 400 B.C. The first plant in America was built in 1804 by Samuel Wetherill & Sons in Philadelphia.

There have been recent advances in the improvement of manufacture which is enabling the industry to make better white lead pigment. It is manufactured by five different processes. These vary primarily in the type of raw material used.

The oldest process is probably the Dutch process, which uses refined metallic lead in the form of perforated discs. It takes about three months to make the white lead. The Carter process dates from 1885. It uses powdered lead in revolving wooden cylinders and takes only 12 days.

The Euston process puts the refined lead in solution and precipitates the white lead. Feathered lead is used as the raw material, which is made by running molten lead into water. The Sperry process is the electric process using a lead anode and iron cathode.

The electrolytes, sodium acetate and sodium carbonate, are separated by a membrane. The Thompson-Stewart process is a recent development similar to the Carter process in that lead oxide is formed. However, all the lead oxide is first formed; then carbon-dioxide is added and controlled to form a definite chemical compound: 4PbC03.2Pb(OH)2.PbO.
Basic carbonate white lead has the ability to impart adhesion, toughness, elasticity, and durability to a paint.

It is used in various types of paint, principally in exterior paints. Basic carbonate white lead is the only white pigment which will produce a durable exterior paint if used alone without other pigments.A large proportion of the total white-lead production is used in white-lead pastes, which are thinned to paint form by the painter or other consumer. The two types of commercial white-lead paste are heavy paste and soft paste.

The first is composed of about 91 per cent white lead and 9 per cent linseed oil, while the latter contains about 89 per cent white lead, 9 per cent linseed oil, and 2 per cent turpentine. The linseed oil used is refined oil with an acid number of 6 to 12.

About 95 per cent of the total production of white lead is consumed by the paint industry, with a small amount used in putty and by the ceramic and other industries.
Basic lead sulfate is quite widely used as a paint pigment. It is called white basic lead sulfate, basic sulfate white lead or "sublimed white lead." White basic lead sulfate is a quite recent pigment in comparison to basic carbonate white lead.

It was originated in 1855 by E. O. Bartlett. He was at that time making zinc oxide directly from zinc ores by the American process. Applying the same principles to the production of a lead pigment from lead ore, he found that it was possible to produce a white powder having pigment properties.

The first plant was built in 1876 in Joplin, Missouri, where lead ore deposits were sufficiently free from other metals for the production of white basic lead sulfate.
It is manufactured by two processes, called the fume process and the chemical process; these are self-explanatory as far as manufacturing is concerned. Almost the entire production of white basic lead sulfate goes into mixed-pigment exterior paints, either directly or as the basic lead sulfate portion of blended leaded zinc oxides.
White basic lead sulfate, like basic carbonate white lead, has the ability to impart to paints adhesion, toughness, elasticity, and durability, but is not as effective in this respect as is the basic carbonate.

In exterior mixed-pigment paints, it is used principally as a substitute for basic carbonate white lead.

White basic lead sulfate may be used for the "white lead" content of many specification paints, such as Federal Specification paints TT-P-40 and TT-P-81. White basic lead sulfate is a less expensive pigment than basic carbonate white lead, and this has been an influencing factor in its use.

White basic lead sulfate is not generally used for single-pigment paints or pastes but is used in conjunction with other pigments in ready-mixed exterior paints.
Ninety-seven per cent of the total production of this material is used in the paint industry, the remaining 3 per cent being used in the rubber and other industries.

Zinc oxide pigments are made primarily by either the American or French process. In both these processes the characteristics of the pigment are determined by:
(1) The temperature of the oxidation of the fume;
(2) The time the zinc oxide is held at a high temperature;
(3) The composition of the gases;
(4) The rate of cooling.

The French process uses purified metal which is melted and vaporized at 1650 to 1850F under controlled conditions. The main difference in the American process is that this process uses ore instead of refined metal.However, the effects for which it is used in paints can be summarized, together with the known or suggested explanations for the results:
"Use of Zinc Oxide in Paints
To aid mixing and grinding
Consistency control
Penetration control and sealing
Hardening or solidifying the film
Gloss and gloss retention
To minimize discoloration and yellowing
Reduced chalking
Color and tint retention
Dirt shedding (self-cleansing), washability, and ease of polishing
Mildew control
Resistance to moisture and to blistering under moist conditions
Neutralizing and inhibiting"

Leaded zinc oxide was first introduced into the paint industry about 1896 as a result of developments on smelting Colorado complex zinc and lead ores. With improvements in the manufacturing process the product was standardized. Several grades were developed containing 5, 20 and 35 per cent lead sulfate and basic lead sulfate, the remainder being zinc oxide. The more recent developments have been the introduction of a 50 per cent grade of leaded zinc and the manufacture of leaded zinc oxides by blending white basic lead sulfate and lead-free zinc oxide.
A considerable portion of the leaded zinc oxide on the market today is entirely or in part a mechanical mixture. Practically the entire production of leaded zinc oxide is used by the paint industry.

It is used in conjunction with other white pigments in exterior house paints. When leaded zinc oxide is used in a paint it usually furnishes the entire amount of zinc oxide required in the particular paint.

Co-fumed leaded zinc is made in the same general manner as Amer-ican process lead-free zinc oxide. Dry-mixing zinc oxide and basic lead sulfate in the proper proportions yields blended leaded zinc oxide.
Lithopone was not very important for some time after its introduction.
This is often characteristic, and some attribute this condition to the fact that people do not like to make changes. However, the fact that lithopone was not light-stable prior to about 1920 no doubt had a definite effect on its acceptance.

The improvement in lithopone at this time was very marked and made it possible for the material to be used much more widely than previously.

ithopone is primarily used in paints, rubber, textiles, paper and printing inks. It is receiving very stiff competition from titanium dioxide and extended titanium dioxide.

High strength lithopones are made by blending lithopone with zinc sulfide or titanium dioxide. The former are made in several "strengths," the 50 to 55 per cent zinc sulfide grade being the most popular. This is also known as zinc sulfide-barium pigment.

Blends with titanium dioxide are called "titanated lithopones," and usually contain about 15 per cent of titanium dioxide. Also in this same class are the zinc sulfide-calcium and zinc sulfide-magnesium pigments; these are blends of zinc sulfide with calcium sulfate or magnesium silicate.
Zinc sulfide is a relatively high-strength pigment, but it has not been used as widely as the blends with lithopone or extenders.

Extended titanium pigments were introduced shortly after World War I. First to be produced was titanium-barium pigment, consisting of 25 per cent titanium dioxide and 75 per cent barium sulfate. Later came titanium-calcium pigment, with 30 per cent titanium dioxide, and a 30 per cent titanium-barium pigment.

Relatively recently, titanium-magnesium pigment (30 per cent titanium dioxide and 70 per cent magnesium silicate) was made for use in house paints.
These pigments, particularly straight titanium dioxide, met great resistance because of their relatively high price per pound, and a long educational program was required to convince the pigment users that these prices were really low when considered on the basis of cost per unit of hiding power developed in the finished product.

Titanium dioxide has the highest hiding power of any of the white pigments, but its manufacture is more complicated and more expensive than any of the other pigments which have been considered. Figure 4 shows the manufacture of titanium dioxide pigment.
Examination of this flow sheet shows how complicated the manufacture of this pigment really is. In order to manufacture pigment of a high quality today, extreme care must be taken in all steps, and impurities controlled very accurately.
Previously the subject of hiding power was mentioned. It is this quality of titanium dioxide that makes it very important in the paint field.
The comparison of the hiding power of various pigments is given in Figure 5. There may be some disagreement in the actual values of hiding power. This disagreement is due primarily to the fact that different conditions are used in measuring it. However, in general, the order given in Figure 5 will be found under usual conditions.