Document OEYvQr4rYEVJVnRDggMqYQR2M

White Hiding Lead Pigments I-A-c E. J. DUNN, JR. Consultant NL Industries, Inc. Hightstown, New Jersey 08S20 I-A-c-1 BASIC LEAD CARBONATE Pigment White 1 (77597) Basic Carbonate White Lead (BCWL) White Lead, Basic Carbonate White Lead Dutch White Lead Berlin White Flake White Krems White Silver White Slate White Chemical Formula Pb3OgC2H2 (MW 775.67) 2PbC03-Pb(0H)2 Chemical Composition (wt %) Pb 80.26 O 16.51 C 3.10 H 0.13 100.00 In the early part of the twentieth century it was a common practice for the manufac turer of white lead to add his name to the product so that white lead carried many brand names other than those listed above. As consolidations in the industry took place, brand names were replaced by manu facturing process names. Dutch, Carter, Electrolytic, Chamber, and Precipitation Process typified the designations for white lead in the era of roughly 1915 to 1940. In today's market this pigment is called basic lead carbonate, basic carbonate white lead, and white lead. Basic lead carbonate is most representative. Government specifications often allow partial substitution of some other lead pig ments for white lead in certain paint formulas. This introduced names such as basic sulphate white lead and basic silicate white lead. Such practice has added con fusion since these products are not con stituted of basic lead carbonate. This du bious practice appears to be phasing out of the picture as newer lead pigments and lead coated silica pigments have been brought to market. Chemically, basic lead carbonate has the formula 2PbC03 Pb(0H)2 (equivalent to a theoretical lead carbonate content of 68.9%). Actually the chemical composi tions of basic lead carbonate pigments in the trade vary with the process of manu facture. The largest tonnages of white lead were originally produced by the old Dutch stack process which produced a lead car- 65 66 E. J. Dunn, Jr. bonate content ranging up to 75% by weight. This meant it contained up to 15% of free or uncombined normal lead carbonate (PbC03). Some properties of the pigment suffer as the lead carbonate content is increased. Contemporary white leads have a lead carbonate content in the range of 62 to 66%. These lower lead carbonate contents provide a finer particle size and impart higher tinting strength, hiding power, brightness, oil absorption, and paint con sistency. These low carbonate white leads presumably consist of two different basic lead carbonates: 2PbCO3-Pb(0H)a (most prevalent) and 4PbCCV2Pb(OH)aPbO. X-Ray diffraction patterns verify the presence of these two lead carbonates as well as the free normal carbonate. Each lead carbonate has a distinctive X-ray pattern.1 Some typical chemical compositions of the basic lead carbonate formed by various manufacturing processes2 are given in Table 1. TYPICAL PROPERTIES Basic I,ead Carbonate Carter-Tank Finished Precipitated Appearance Fine white powder Density (g/cm3) 6.75-6.85 (Ib/gal) 55.6 Optical Brightness, glycerine rub-up (%>) 92 Tinting strength 160-190 Hiding power (fl2/lb) 20 Refractive index 1.94-2.09 Specific surface area. BET (m7g) 1.6 Crystal nature System Hexagonal Habit Lamellar to flaky Oil absorption (lb/100 lb) 11 Fine white powder 6.85-6.95 57.5 93 220-250 25 -- -- Hexagonal Flaky 15 White leads are chemically active pigments. They react with both the free acidic Fig. 1. While lead soaps (700x). portions of vehicles and with the breakdown acids that develop from paint vehicles as paint films age. These reaction products shown in Fig. 1, which are called lead soaps,8 reinforce the paint film. Fortunately these white lead reaction products are formed at a favorable (slow) rate that im parts the right type of plasticity to the paint film for good stabilization. Active white lead pigments also prevent syneresis or softening of the paint film. The beneficial rate of reaction with white lead helps to retain the film flexibility necessary for good adhesion and durability. Legislation prohibiting the use of lead ingredients in amounts greater than 1% w of lead in interior paints (or exterior paints that arc accessible to children) has been passed by several states. The Food and Drug Administration (1971) is considering i%>w as the top limit for total lead content. Such legal restrictions are discouraging the use of lead pigments in trade sales paints and to some extent in industrial paints. ECONOMICS The production and consumption of white lead pigments are quite dependent on the price of metallic lead. From 1926 to 1945 the New York price of metallic lead varied l-A-c While Hiding Lea< Table I factured b; PbO COa PbC03 Combined Basic PbC from 3it/lb to 8(C/lb New York prices of sequent period (195 of metallic lead so to more than 16) period, the use of t dined correspondin creasingly uneconc density, high price-j had to be considers of pigment necessai However, the benefi lead pigments still w; for specific purpos Fig. 2 shows the re! lead and basic let prices.4,7 At presen 2 lie/lb (it has reach* Flfi. 2. United States basic lead pigments (19 lead metal also given for Carbonate was priced at : in 1946. GLD33819 I-A-c White Hiding Lead Pigments Table 1 Chemical Composition of Lead Carbonate Pigments as Manu factured by Various Processes (%w) Dutch Carter Corrosion Normal Tank Lead Finished Precipitated Carbonate PbO CO, PbCO, Combined H20 Basic PbO 86.3 11.4 69-74 2.3 28.0 86.6 11.0 66-68 2.34 28.8 87.2 10.7 64-67 2.1 34.5 88.0 9.9 62-64 2.03 37.3 83.5 16.5 100 -- -- 67 from 3^0/lb to 80/lb. Figure 2 presents the New York prices of metallic lead for a sub sequent period (1950-1968).4 As the price of metallic lead soared from about 40/lb to more than 160/lb during this overall period, the use of white lead pigment de clined correspondingly since it became in creasingly uneconomical to use a high density, high pricc-per-pound material that had to be considered in terms of the volume of pigment necessary for a gallon of paint. However, the beneficial properties of white lead pigments still warrant their use in paints for specific purposes. An inspection of Fig. 2 shows the relation between metallic lead and basic lead carbonate pigment prices.4-7 At present (1971) white lead is 2 l$0/lb (it has reached a high of 23.10/lb). Figure 3 illustrates the estimated United States consumption pattern for basic lead carbonate pigment for 1968 in terms of white lead shipments.* The production of all lead pigments now approximates only 160 million lb whereas in the early part of the twentieth century well over 400 million lb of lead pigment were produced annually in the United States.4 As shown in Fig. 4, the consumption of basic lead carbonate has also fallen from about 90 million lb in 1950 to a low of 24 million lb in 1968. Imports of white lead approximate 4,000,000 lb/yr, but ordinarily exports pretty well balance out imports. White lead in oil was the backbone of the exterior house paint business up until about 1940. By that time both titanium dioxide and zinc pigments had established their merits and two-coat systems were initiated. However, the use of basic lead carbonates on a reduced scale has still persisted. The use of basic lead carbonate is still showing a decline and, in view of the present con cern with toxic materials,8-9 this trend is not likely to be reversed. HISTORICAL BACKGROUND Fig. 2. United States pricing pattern for selected basic lead pigments (1950-1969). Pricing trend for lead metal also given for the same period. Basic lead carbonate was priced at 8|e/lb and lead metal at 8e/lb in 1946. One of the earliest descriptions of white lead is given by Theophrastes10 (originating in the fourth century B.c.). Similar data relative to the production of white lead, then known to the Greeks as "Psimithium" and to the Romans as "Cerusa," are found in Pliny" and Vitruvius." The white lead industry began to expand GLD338P0 68 E. J. Dunn, Jr. |.A-c White Hiding L< 50 fTT(f method of manufc small amount of v t precipitation proc '% starting material (t V, white lead pigmenl Fig. 3. Estimated United States consumption pattern for basic lead carbonate pigment (million lb) for 1968. Total: 25.6 million lb. tremendously in the early part of the seven teenth century due to the introduction of the Dutch stack process. Subsequently, various processes to manufacture white Fig. 4. Annual United States shipments of basic lead carbonate forthe period 1950-1968. lead were developed such as the German chamber process, the French precipitation process, and the Carter, Electrolytic, Euston, and Thompson-Stewart processes. Many variations of these processes were also adopted. In the early part of the twentieth century, white lead production was at an all time high of approximately 340,000,000 lb/year.6 Approximately 40 plants manufactured white lead in the United States by the Dutch stack process. When the price of lead doubled in the late 1940s, the Dutch stack process became obsolete since it was uneconomical to operate. The Carter process for making white lead from atomized lead was pursued as the Dutch process was phased out of produc tion. The manufacture of white lead from a leady litharge was introduced about 1940 and finally a combination process of cor rosion-tank finishing became the main MAJOR REASONS Basic lead carboi ; foolproof pigment; paints.13 Three of i ; ties are its slow acidic components parting good stat impart adhesion, ; brushing. Oil-bas normally have a i lead present. Whiti f power, possesses | oil, is readily wet (: lead-in-oil compos degree of absorp: | below 310 nm, and i properties. It lowe ? mission, imparts if keeps water solub | white lead is a filr ; many times too c year of weatherini cleaning by slow cl a portion of a cha dioxide is often us bonate in linseed i come this deficienc; jj Since lead sulfid ' lead carbonate p y stained black whe; environments with present. PIGMENT MANUFA ^ today's economj processes used fc white lead: the c 1 process and the pi , the corrosion-tank f t oxide is charged cylinders.14 Moisti surface of the ch; GL033821 1-A-c White Hiding I.rad Pigments 69 method of manufacture. Along with this, a small amount of white lead is made by the precipitation process using litharge as a starting material (used primarily for the dry white lead pigment business). MAJOR REASONS FOR USE Basic lead carbonate is one of the most foolproof pigments to use in linseed oil base paints.1* Three of its more beneficial proper ties are its slow rate of reactivity with acidic components of vehicles (thereby im parting good stabilization), its ability to impart adhesion, and its unusual ease of brushing. Oil-based primers for wood normally have a goodly portion of white lead present. White lead contributes hiding power, possesses a high affinity for linseed oil. is readily wet (and easily flushed to give lead-in-oil compositions), provides a high degree of absorption for near-UV light below 310 nm, and imparts some fungicidal properties. It lowers moisture vapor trans mission, imparts blister resistance, and keeps water solubles at low levels. Since white lead is a film building pigment, it is many times too durable during the first year of weathering to provide good self cleaning by slow chalking. For this reason, a portion of a chalking grade of titanium dioxide is often used with basic lead car bonate in linseed oil base paints to over come this deficiency. Since lead sulfide (PbS) is black, basic lead carbonate paints tend to become stained black when exposed to industrial environments with hydrogen sulfide (H,S) Present. material) as the cylinder slowly rotates. Catalytic quantities of acetic acid are also automatically sprayed on the charge as new surfaces are exposed. Carbon dioxide from flue gas is metered into the center of the rear end of the cylinder. The moisture activates the metallic lead particles, helping to oxidize them to litharge, and the carbon dioxide from the flue gas reacts to form the basic lead carbonate. These reactions heat up the charge to about 185"F. After about 2i days in this corrosive steaming atmos phere, the charge is balled up into small lumps and at this point it is emptied, dis integrated, and water classified. The clas sified product is pumped to large circular reaction tanks where the final stage of carbonation is achieved. Flue gas is pumped into the bottom of these reaction tanks while the slurry is agitated with a strong turbine mixer. The final carbonation takes about i day, so that the batch is completed in a total of about 3 days. The second process for the manufacture of white lead in the present market is the precipitation process.15 A litharge slurry is fed to a large turbine agitated mixing tank and oxidized by blowing air into the slurry to eliminate any metallic lead (if present). When the metallic lead is eliminated, flue gas is pumped into the slurry to carbonate the product, usually to a carbonate content of 64 to 66%. This is a short-time reaction, taking less than 1 day for completion. Time of completion is dependent on the size of the batch being carbonated. Instrument control is very important for this process. PIGMENT GRADES PIGMENT MANUFACTURE In today's economy there are primarily two Processes used for the manufacture of white lead: the corrosion-tank finishing process and the precipitation process. In Ihe corrosion-tank finishing process a leady oxide is charged into slowly revolving cylinders.14 Moisture is sprayed on the surface of the charge (about 3 tons of There are primarily two grades of basic lead carbonates marketed. One is based on the corrosion-tank finishing process and the other on the precipitation process. The greater volume of white lead is made by the corrosion-tank finishing process. In both processes particles are generally formed with a hexagonal outline. If the particles are allowed to grow during'processing, the hexagonal outline is generally well es tablished. The controls in processing nor- 6* rf* malty keep the pai better optical anc hexagonal outlim veloped with finer Micrographs* The third or depth carbonate particl other two dimensi the microscope, decidedly flaky. T be a little thicker t particle imparts t particle size than dimensions obsei scope. Photomicr of 1000 diameters lead and norma lustrated in Fig. t (high tinting strei pronounced hexa tide size is a littl cipitated white le. the contour band i edge of the parti' particles are incli than the high bt lead. By comparing t it is evident that tl white lead are les irregular in shape possess a thinner Because of these cipitated white le tinting strength, higher oil absoi consistency. Normal lead c: grow to a larger f lead carbonate pa in shape, as she crystal habit wot Their crystal oul defined. White lead p; linseed oil and tl particles to aggr< textured film f 70 glomerates need t GLD33823 I-A-c While Hiding Lead Pigments 71 ntally keep the particles fine so as to develop better optical and physical properties. The hexagonal outline is not always well de veloped with finer particle size. 15/i in size for the eye to detect their presence in a film. Cumulative Particle Size Distribution Curve- Micrographs-' The third or depth dimension of a basic lead carbonate particle is much less than the other two dimensions that are visible under the microscope. White lead particles are decidedly flaky. The coarser particles may be a little thicker than the finer ones. A flaky particle imparts the effect of a much finer particle size than is indicated from the two dimensions observable under the micro scope. Photomicrographs at a magnification of 1000 diameters of the two types of white lead and normal lead carbonate are il lustrated in Fig. 5. The particles of H.T.S. (high tinting strength) White Lead have a pronounced hexagonal outline. Their par ticle size is a little coarser than in the pre cipitated white lead. From the thickness of the contour band (the dark line outlining the edge of the particle) it is evident that the particles are inclined to be a little thicker than the high basicity precipitated white lead. By comparing the micrographs of Fig. 5 it is evident that the particles in precipitated white lead are less distinct, more indefinite, irregular in shape, of finer particle size, and possess a thinner third or depth dimension. Because of these characteristics, the pre cipitated white lead normally has a higher tinting strength, higher hiding power, higher oil absorption, and higher paint consistency. Normal lead carbonate particles usually grow to a larger particle size than the basic lead carbonate particles. They are prismatic n shape, as shown in Fig. 5, and their crystal habit would be classed as chunky. Their crystal outline is usually quite well defined. White lead particles wet easily with linseed oil and there is little tendency for Particles to aggregate or produce a rough textured film from agglomerates. Ag glomerates need to be above approximately Figure 6 gives the cumulative particle size distribution curve for H.T.S. White Lead particles as measured from two dimensions of the particles. The average diameter by surface mean is 1.1 /i. This is an ideal average diameter to use for comparison of many types of materials. White lead might be made of a little finer particle size for higher optical properties. However, it is a little difficult to achieve because of the general tendency of basic lead carbonate 100 80 CumuUtivt overlife pertidee (wt %) 80 70 60 60 40 30 20 10 0 Fig. 6. Cumulative particle size distribution curves for selected basic lead pigments (white hiding types). GLD33824 72 E. J. Dunn, Jr, particles to grow during precipitation and carbonation. The range of particle size is fairly broad, which allows tight packing of the particles for a structurally strong paint film. The flaky character of the particles impart body and paint consistency. The number of par ticles in 1 cm* (solid) approximates 3.6 trillion. In general, the higher the number of particles per cubic centimeter, the higher the hiding power, tinting strength, and oil absorption. A particle size distribution curve for precipitated white lead would be of slightly finer particle size than for the H.T.S. White Lead. The reflectance curves for the two types of white lead are quite similar, with rela tively high reflection of UV light (over 80%) through the 310 to 400 nm range. Below 310nm the UV reflection falls off rapidly (becoming less than 10% below 270 nm). From 210 to 270 nm, white lead is a strong absorber of U V light. In the visible range of the spectrum, from 400 to 700 nm, white lead provides very high reflectance, making it a good white pigment (most reflectance values are above 97%). Over the near-lR range, white leads have relatively high reflection values compared to magnesium oxide (set atl00% reflection). Spectral Reflectance Curves3 The spectral reflectance curves for two commercial grades of basic lead carbonate are given in Fig. 7 for both the visible and the near-UV range. These curves were obtained from the dry packed powders. PIGMENT SPECIFICATIONS AND MANUFACTURERS Table 2 lists the basic lead carbonate pig ments that are currently manufactured by Fig. 7. Spectral reflectance curves for selected basic lead pigments in the near UV and visible spectrums. Data courtesy Titanium Pigments Division. NL. I-A-c White Hiding L Table 2 Commerc Standard Specificath Company Coding BNK EP HMD MGL NL the five mqjor Ui meet two industry Regulations on Let Prior to the midd , rior surfaces cont; V During the 193C titanium dioxide/i to replace white 1 application. Subs | problem arose i i poisoning in chil f chewing and eatir | the walls of im l buildings in slum i ^ Discussion of t tablishment in 19 dard of the Arne Institute (Z66.1). I-A-c-2 BAS] SULFATE Pigment White 2 ( Basic Sulfate, Wh Sublimed White L Super Sublimed V Bartlet White Lea Lewis White Lea< Chemical Formul Normal lead su PbS04 (MW Monobasic lea< PbSCVPbO I-A-c While Hiding l.ead Pigments 73 Table 2 Commercial Pigments Meeting Two Standard Specification for Basic Lead Carbonate Company Coding ASTM D8i-43(55)or Federal TT-W-00251g BNK EP HMD MGL NL AAA E-PA E-PAAA HLP-A HLP-AA HLP-AAA Scotch Laddie Dutch Boy HTS Dutch Boy i 11 the five major United States producers to meet two industrial specifications.18 I Regulations on Lead Content ' Prior to the middle 1930s, paints for interior surfaces contained a high lead content. During the 1930s the more economical titanium dioxide/extender pigments began to replace white lead pigments for interior application. Subsequently a health-hazard problem arose in that occasional lead poisoning in children was traced to the chewing and eating of paint film chips from the walls of improperly maintained old buildings in slum areas. Discussion of this situation led to the es tablishment in 1955 of the voluntary stan dard of the American National Standards Institute (Z66.1). This standard limited the lead content in paints intended for chil drens' toys, furniture, and interior surfaces to a maximum of 1.0% lead based on the total paint solids. More recently the Food & Drug Admini stration (FDA), U.S. Department of Health, Education, and Welfare, issued a proposed regulation that would declare certain heavy-metal-containing paints as hazardous substances that require special labeling or warning for child protection under the Federal Hazardous Substance Act. This proposal reduces the acceptable level of lead (as metal) in certain paints to 0.5% of the nonvolatile content of the paint. It also limits the levels of antimony, arsenic, cadmium, mercury, and selenium in the nonvolatile coating portion to 0.05%. individually or in total (reduction from the earlier 0.06% limit specified by the ANSI Standard Z66.1 of 1964). The acceptable level of water-soluble barium at 1.0% of the barium present is reaffirmed. It is ex pected that this FDA regulation will be come operational in late 1972. These rulings have caused changes in certain paint formulations and will result in further changes in the future. However, the present major uses of lead pigments in primers and finishes for exterior house surfaces and for various metal protective systems for outside use have not been essentially affected by this legislation except for the requirement of a warning label. These regulations will supersede any state requirements. I-A-c-2 BASIC LEAD SULFATE Pigment White 2 (77633) Basic Sulfate, White Lead Sublimed White Lead Super Sublimed White Lead Bartlet White Lead Lewis White Lead Chemical Formula (Mixture) Normal lead sulfate PbS04 (MW 303.27) Monobasic lead sulfate PbSCL-PbO (MW 526.48) Chemically, basic lead sulfate is a broad term that embraces many compounds with varying degrees of basicity above the nor mal (nonbasic) lead sulfate. The lowest basicity lead sulfate (stoichiometric pro portions) is monobasic lead sulfate with 42.4% combined or basic PbO present. The lead sulfate pigment marketed to the paint industry has a basic or combined PbO con tent usually in the range of 15 to 28%.2 Therefore, the usual basic lead sulfate that GLD338P6 74 E. J. Dunn, Jr. is sold is a mixture of normal lead sulfate and monobasic lead sulfate. Government specifications allow the sub stitution of basic lead sulfate for basic lead carbonate in certain paint formulas. This has introduced pigment names such as basic sulfate white lead and basic silicate white lead even though these products do not contain white lead (basic lead carbonate). It is expected that such names may be phased out of paint terminology as new and more specific lead pigments are brought to market. Blue Basic Lead Sulfate A variant of the white basic lead sulfate pigment is the older product called blue basic lead sulfate that was and still is mar keted as a metal protective pigment. Blue basic lead sulfate is a mixture of compounds consisting of approximately 78% of mono basic lead sulfate, 10% lead sulfide (PbS), 4% lead sulfite (PbS03), 4% zinc oxide (ZnO). plus 4% carbon and undetermined material. The use of blue basic lead sulfate as a metal protective pigment seems to be rapidly declining. A concise survey of this slate-gray pigment is given by Rose.17 TYPICAL PROPERTIES1" Appearance Density (g/cm') (Ib/gal) Bulking value (gal/lb) Brightness in oil (%) Oil absorption (lb/100 lb) Screen analysis through 325 mesh (%) Basic Lead Sulfate White powder 6.4 53.3 0.0188 83.0 8.0 99.9 ECONOMICS The price of basic lead sulfate pigment tends to vary with the price of metallic lead. During the 1950s it varied by about 3tf/lb from an average price of I6k/lb. During the 1960s it averaged 18i0/lb and in 1971 (May) was quoted at 21 Je/lb.7 Annual United States consumption of basic lead sulfate pigment by the paint industry has declined from about 10 million lb in 1965 to less than half this amount in 1970. In view of the concern relative to lead toxicity (as contributed by paints), further erosion in sales can be antici pated.8,9 HISTORICAL BACKGROUND White basic lead sulfate was first manutured about 100 years ago. It was obtained as a portion of zinc oxide pigment because in the early days both zinc oxide and lead sulfate pigments were made directly from the ores by sublimation using controlled atmospheres. At that time a by-product was obtained during the lead ore smelting that was called blue lead and basic sulfate blue lead. Sub sequently it was made under better con trolled furnace conditions to produce a white powder having pigment properties. In recent years the demand for the basic lead sulfate product has declined. In 1935 a new basic lead sulfate was made by a precipitation process19 that gave the pigment improved optical and physical properties and higher basicity. However, it was not particularly economical and was taken off the market about 1960. MAJOR REASONS FOR USE White basic lead sulfate is used only in multiple or mixed pigment formulations. Its low basicity, and therefore low reac tivity. makes it a good pigment to mix with zinc oxide. Leaded zinc oxides may con tain up to 50% basic lead sulfate and a popular level is 35% (35% leaded zinc oxide). Because of its lower density com pared with basic lead carbonate, basic lead sulfate is often substituted for the carbonate pigment. Basic lead sulfate is used in both prime and finish coats. The high basicity lead sulfate compounds are used primarily to stabilize plastic com- f l-A-c White Hiding positions. Convi ments are used t the highly basic introduce probl stability. 1 yji.. f jfAjf `|v' | >$ j '( `fv *'v | PIGMENT MANl White basic lea< duced by a fum atomized into a j excess air and s lead sulfate. The that is drawn o system by suctio is collected (th shaken to relieve into bins for pack j PIGMENT GRADI { Only one grade < l is marketed (Si t 41) by EP. Its \ given in Table 3. 1 An electron phi basic lead sulfate and reveals the marily roundish o parent, and cha roughened surfact The cumulative curve for the com pigment is given diameter is 1 p,m sonably uniform ir When mixed wi Ts Sc N. Su M CLD338?7 I-A-c While Hiding Lead Pigments 75 positions. Conversely, the low basicity pig ments are used to stabilize paint films since the highly basic grades could conceivably introduce problems with paint package stability. V \ .-*1. * PIGMENT MANUFACTURE White basic lead sulfate is currently pro duced by a fume process. Molten lead is atomized into a jet flame in the presence of excess air and sulfur dioxide to form the lead sulfate. The lead sulfate forms a fume that is drawn off through a long cooling system by suction to a bag house where it is collected (the bags are periodically shaken to relieve the pigment so that it falls into bins for packaging). PIGMENT GRADES Only one grade of white basic lead sulfate is marketed (Super Sublimed E-P No. 41) by EP. Its chemical composition is given in Table 3. An electron photomicrograph (1000 X) of basic lead sulfate pigment is given in Fig. 8 and reveals the pigment particles as pri marily roundish or irregular in shape, trans parent, and characterized by a slightly roughened surface.2 The cumulative particle size distribution curve for the commercial basic lead sulfate pigment is given in Fig. 6.18 The median diameter is 1 gm and the particles are rea sonably uniform in size. When mixed with linseed oil, basic lead Fig. 8. Basic lead sulfate (lOOOx). sulfate exhibits an average brightness of 83% across the visible spectrum. This makes it a good white pigment for house paints. Below 290 nm. basic lead sulfate has a low reflectance and acts primarily to absorb UV light. PIGMENT SPECIFICATIONS AND MANUFACTURERS There are three main specifications for the procurement of basic lead sulfate: Military ASTM Federal Ml 23-46 D82-44 (1955) TT-W-261c All are met by Super Sublimed E-P No. 41 produced by the one United States manufacturer of this pigment (EP).18 Table 3 Chemical Composition of a Commercial Basic Lead Sulfate Pigment (EP) Combined PbS04 (%) or Basic PbO (%) Normal lead sulfate (theoretical) Super Sublimed E-P No. 41 (commercial) Monobasic lead sulfate (theoretical) 100.0 83.5 57.6 0.0 16.5 42.4 *v. g u d 3382E 76 E. J. Dunn. Jr. I-A-c-3 BASIC LEAD SILICATE Pigment White 16(77625) Basic Silicate White Lead Chemical Formula, Representative PbaSi2H2Os(MW 807.8) 3PbO 2Si02 H20 Chemical Composition, Representative (wt %) Pb 77.00 Si 6.95 H 0.25 O 15.80 100.00 PbO 83.0% Si02 14.8% Basic lead silicates that are used for pig mentary purposes are generally variations of monobasic lead silicate. A wide range of compositions may be produced, but com monly they have a molecular composition close to that given above. The commercial mixed lead silicate pigment, however, usually has a much higher silica content as given in Table 4. Table 4 Chemical Composition of Commercial Basic Lead Silicates PbO (%) Si02 (%) High Lead (E-P 202) High Silica (E-P 303) 83-85 14-16 49 51 X-Ray diffraction analyses show these two products to be quite different. The hy drated lead silicate pigment (E-P 202) has a characteristic pattern that distinguishes it from the mixed pigment (E-P 303) contain ing the free silica. The National Bureau of Standards has published an article describ ing the various lead silicates that have been prepared as definite compounds.20 TYPICAL PROPERTIES Basic Lead Silicates E-P 202 E-P 303 (Low Silica) (High Silica) Appearance Density (g/cm*) (lb/gal) Bulking value (gal/lb) Brightness in oil (%) Oil absorption (lb/100 lb) Screen analysis . through 325 mesh (%) White powder 6.4 53.3 0.0188 86.0 16 99.5 White powder 4.0 33.3 0.030 13 99.9 ECONOMICS Over a period of many years the basic lead silicate pigments, in general, have been priced at slightly lower levels than the basic lead sulfate pigments. The price of basic lead silicate has remained relatively stable for a 20-year period (1950-1969) and as with the other lead pigments has paralleled the pricing pattern for metallic lead as shown in Fig. 2. As of early 1971 the high lead (85% PbO content) silicate pigment was quoted at 24c/ lb whereas the low lead (48% PbO content) silicate pigment was priced lower at slightly less than 200/lb. These prices compare with a price of 2 He/lb quoted for basic lead sul fate at that time. HISTORICAL BACKGROUND In 1942 a United States patent21 was issued disclosing the manufacture of lead silicate pigments. Hence these pigments represent relatively new products that were first marketed in the United States in the early 1940s. The mixed basic lead silicate variant was introduced in the early 1950s. The first basic lead silica pigment was promoted as a stabilizer for vinyl chloride l-A-c White Hiding I.e; plastics since it wa use in insulated ] coating formulatio to high electrical i it was adapted for i ing pigment. Rest perience has showi act to provide dun the mixed lead silic about 51% silica an was brought on th< however, are refei white leads. MAJOR REASONS !j.;: Basic lead silicate that react with i T I' breakdown acids ar f industrial composi 1 vinyl chloride plasi 1 They are low tin i have low hiding po tributions in these importance. Basic lead silica rable paint films sin to chalking (althoi occasionally a pro! is helpful in water-t reducing and con s K P. Fig. 9. Photc 303 (right). GLD33829 E. J. Dunn, Jr. ES Basic Lead Silicates E-P 202 E-P 303 Low Silica) (High Silica) White powder 6.4 53.3 White powder 4.0 33.3 0.0188 86.0 0.030 16 13 99.5 99.9 any years the basic lead in general, have been wer levels than the basic nts. The price of basic :mained relatively stable od (1950-1969) and as l pigments has paralleled for metallic lead as shown l the high lead (85% PbO gment was quoted at 24tl / lead (48% PbO content) as priced lower at slightly 'hese prices compare with quoted for basic lead sul- V i k I-A-c White Hiding I.ead Pigments 77 plastics since it was particularly suitable for use in insulated polyvinyl chloride wire coating formulations where it contributed to high electrical resistivity. Subsequently it was adapted for utilization as a white hid ing pigment. Research and practical ex perience has shown that basic lead silicates act to provide durable films. Around 1950 the mixed lead silicate pigment18 comprising about 51% silica and 49% basic lead silicate was brought on the market. Both products, however, are referred to as basic silicate white leads. MAJOR REASONS FOR USE Basic lead silicates are reactive pigments that react with acidic components and breakdown acids and thereby act to stabilize industrial compositions (paint films, poly vinyl chloride plastics). They are low tinting strength whites and have low hiding power and hence their con tributions in these areas are of secondary importance. Basic lead silicates tend to produce du rable paint films since they impart resistance to chalking (although photo-sensitivity is occasionally a problem). Basic lead silicate is helpful in water-base primers for wood by reducing and controlling cedar and red- wood staining. This high lead content pig ment is also claimed to confer a degree of corrosion resistance to ferrous substrates.22 The mild reactivity of basic lead silicate helps to stabilize linseed oil films. This pig ment is also an effective stabilizer for poly vinyl chloride plastic compositions. PIGMENT MANUFACTURE Lead silicate pigments are prepared by fus ing silica and litharge at a temperature of approximately 1800F. When the fusion is complete, the fluid melt is allowed to run from the furnace into a large volume of water which granulates the product. The granulated compound is wet ground in a ball mill for 24 hr, reducing it to pigmentary par ticle size. The ground slurry is removed from the ball mill, flocculated, dried, and pulverized. The pigment can be dried at high temperatures since it is not heat sen sitive (although it is important to retain some water of hydration). PIGMENT GRADES There are essentially only two grades of basic lead silicate for the paint trade as listed under the section on Typical Proper- ;k g r o u n d States patent*1 was issued inufacture of lead silicate these pigments represent products that were first United States in the early J basic lead silicate variant i the early 1950s. c lead silica pigment was tabilizer for vinyl chloride Fig.Photomicrographs (lOOOx) of Basic Lead Silicate 202 (left) and Mixed Lead Silicate 303 (right). GLD33830 ties. A third grade (E-P 201) is made pri marily for the stabilization of plastics. The photomicrographs (1000X) shown in Fig. 9 illustrate the appearance of two types of basic lead silicate pigment. In general these pigments have a coarser par ticle size than either the basic lead carbonate or the basic lead sulfate pigments. This is shown by making a comparison of the per cent comulative oversize particles above 10 ^im among the three basic lead pigments in Fig. 6. PIGMENT SPECIFICATION AND MANUFACTURERS There are no industry specifications for the procurement of basic lead silicate pigments. The only United States manufacturer of lead silicate pigment is Eagle-Picher In dustries, Inc. I-A-c-4 BASIC LEAD SILICO SULFATE Oncor 45X Coated Silica Pigment Basic Silicate White Lead Basic lead silico sulfate is a coated silica pigment comprising three chemical com pounds; a core of silica (Si02) and a coating (mixture) of gamma tribasic lead silicate (PbSiOa-3PbO) and monobasic lead sulfate (PbSCL-PbO). Chemical Composition, Overall (wt %) The composition of basic lead silico sul fate is roughly equivalent to i gamma tri basic lead silicate. I monobasic lead sulfate, and i silica (core). X-Ray diffraction pat terns demonstrate the presence of silica and verify the formation of both the gamma tri basic lead silicate and the monobasic lead sulfate.1 Microscopical analyses confirm the presence of the lead compounds as a strongly adherent composite coating on the silica core. PbO SiO* S03` 47.9 47.9 4.2 100.0 Basic lead silico sulfate pigment is unique in that it has a core of silica and a coating of mixed basic lead salts." It was the first in a series of such pigments developed for the paint industry. TYPICAL PROPERTIES Appearance Density (g/cm3) (lb/gal) Bulking value (gal/lb) Hiding power (ft2/lb) Tinting strength Oil absorption (lb/100 lb) Basic Lead Silico Sulfate White powder 4.0 33.3 0.030 10 70 15 ECONOMICS Basic lead silico sulfate was designed as an economical lead-base white pigment." Pig ments are formulated in paint compositions primarily on a volume basis and the usual substitution of one pigment for another is also on a volume basis. Thus a formulation requiring 170 lb of basic lead carbonate (6.8 g/cm3) requires only 1001b of basic lead silico sulfate (4.0g/cms) on a volume re placement basis. This substitution repre sents a tremendous saving in raw material cost since not only are fewer pounds used but the price per pound is also lower. Clearly basic lead silico sulfate'is more economical than the more dense (higher specific gravity) basic lead sulfate or basic lead carbonate pigments. From 1950 to 1963 the price of basic lead silico sulfate was substantially the same as the basic lead silicate. From 1964 to 1969 I-A-c White Hiding Lead I the basic lead silico priced somewhat high (17tf/lb to 1840/lb).*-7 HISTORICAL BACKG1 Basic lead silico sulfa lead pigment that w 1948. Research had e; that the stabilizing re* paint films were prima as evidenced by mien and chemical analyse quence to these findii ment of solid phase : pigments consisting of as silica) and a reacti' shell (such as an inorg Basic lead silico s pioneer pigment wher lead silicate was put i (over the silica core) ti a paint film and monoh added to stabilize the silicate." Basic lead silico sull was manufactured con 1940s so that experier in the field is now ove exposure has confirme ico sulfate is a good dui greatest utilization d pigment house paints ( solvent-base types). MAJOR REASONS FOI ;V.i. Aside from cost savin sulfate is used for the i fers to paint systems. It is unique in that i through of redwood ; latex base paints.26 TI paints penetrates into the stain, and then imrr through the paint film it. The basic lead silic some discoloration of t prevents the stain from GLD338 31 F~J. Dunn, Jr. ITION AND y specifications for the lead silicate pigments. States manufacturer of it is Eagle-Picher In- i ; - jf basic lead silico sulvalent to i gamma trimonobasic lead sulfate. X-Ray diffraction pate presence of silica and of both the gamma trind the monobasic lead ical analyses confirm : lead compounds as a )mposite coating on the fate was designed as an se white pigment.-4 Pigd in paint compositions me basis and the usual pigment for another is isis. Thus a formulation iasic lead carbonate (6.8 ly 1001b of basic lead ;/cm3) on a volume re'his substitution repre; saving in raw material are fewer pounds used jnd is also lower. Clearly 'fate' is more economical i (higher specific gravity) or basic lead carbonate 63 the price of basic lead ubstantially the same as ate. From 1964 to 1969 l-A-c While Hiding Lead Pigments 79 the basic lead silico sulfate pigment was priced somewhat higher as shown in Fig. 2 (I7e/lb to 18k/ib).2-7 h is t o r ic al bac k g r o u n d Basic lead silico sulfate is a relatively new lead pigment that was first marketed in 1948. Research had established about 1930 that the stabilizing reactions of pigments in paint films were primarily surface reactions as evidenced by microscopical techniques3 and chemical analyses.25 The natural se quence to these findings was the develop ment of solid phase reactions to produce pigments consisting ofan inactive core (such as silica) and a reactive outside coating or shell (such as an inorganic active lead salt). Basic lead silico sulfate represented a pioneer pigment where the gamma tribasic lead silicate was put in the surface coating (over the silica core) to impart durability to a paint film and monobasic lead sulfate was added to stabilize the whiteness of the lead silicate.13 Basic lead silico sulfate (Oncor 45X/NL) was manufactured commercially in the late 1940s so that experience with this pigment in the field is now over 20 years. Practical exposure has confirmed that basic lead sil ico sulfate is a good durable pigment with its greatest utilization developing in multi pigment house paints (both water-base and solvent-base types). ma j o r r e a s o n s f o r u s e Aside from cost savings, basic lead silico sulfate is used for the many benefits it con fers to paint systems. It is unique in that it reduces the bleedthrough of redwood and cedar stains in latex base paints.2* The water from latex Paints penetrates into redwoods, dissolves the stain, and then immediately bleeds back through the paint film and badly discolors it. The basic lead silico sulfate may allow some discoloration of the primer coat but it Prevents the stain from progressing into the finish or subsequent coats. If severe bleed ing is evident in the prime coat, a safe prac tice is to apply a second coat of primer before applying the finishing coat(s). The commercial basic lead silico sulfate (Oncor 45X) mixes readily with most colors and provides good stabilizing properties. It can replace basic lead carbonate in oil-base primers and basic lead sulfate in multipig ment paints (whether the basic lead car bonate is present as such or as a part of leaded zinc oxide). Basic lead silico sulfate pigment is the most versatile and economical pigment for formulating white or light tint exterior coat ings (including primer and finish coats for both oil- and water-base coatings). Since it has the lowest density (specific gravity) of all the active white (hiding) lead pigments, its utilization results in substantial cost savings. The chemical activity of basic lead silico sulfate is similar to that of basic lead car bonate. Thus it stabilizes by forming lead soaps with acidic components and degrada tion products that normally would soften a paint film. The low tinting strength of this pigment materially aids the formulation of a wide range of colored paints. PIGMENT MANUFACTURE Basic lead silico sulfate is manufactured from a mixture of silica, litharge, and sul furic acid. The silica is wet ground overnight in a ball mill. The ground slurry is then pumped to a reaction tank containing a slurry of litharge. Acetic acid in catalytic amounts (0.1% wt) is added to the slurry. Dilute sulfuric acid is prepared and slowly added to the litharge/silica slurry mixture to form the required amount of monobasic lead sulfate. Mixing is continued for an ad ditional hour after all the sulfuric acid has been added to insure homogeneity. The pre cipitate is filtered, dried, and calcined in a rotary kiln to produce the lead-coated silica particle. The calcined product is disinte grated and packed in 501b bags for the trade. g LD33832 PIGMENT GRADES There are two grades of basic lead silico sul fate pigment for the paint industry (Oncor 45X Green Code and Red Code). The Green Code product is the more broadly used pigment. The Red Code, de signed for blister-resistant wood primers, has 4.0% P205 substituted for the 4.2% SOj in the Green Code grade. Approximate av erage compositions for the two grades are given in Table 5. The particle size characteristics of the two Oncor 45X pigments are similar. The particle size is very dependent on the par ticle size of the ground silica used for the particle core. The average surface mean diameter is 5 to 6 fi. 1 IT T [ x B*iic 1**3 silico suHste - 10 (Oncor 45X) - T i S I4 T) 3 3 i* / / 1 1 1 1= - / __ Dibasic lead phosphite (Dyphos/NL) Table 5 Average Chemical Composition of Rasic Lead Silico Sulfate Pigments (wt %) Oncor 45X(NL) Green Code Red Code PbO SiO, SO;) pso 5 47.9 47.9 4.2 -- 48.0 48.0 -- 4.0 05 0.4 0.3 0.2 0.1 --1 J___1___!________1_____1_____1_____ l_____ 20 30 40 SO SO 70 to 90 100 Cumulative underage particle* (wt %) The cumulative percent distribution curve for the basic lead silico sulfate pigment par ticles as made by the microscopical pro jection method is given in Fig. 10. The specific surface area of this Oncor 45X sample is 1.2 nT'/cm3. Figure 11 is a photomicrograph (1000X) illustrating the appearance of the uncoated and coated silica pigments. The reaction of the uncoated silica to form a coated product is quite evident.*7 The crystal habit of both pigments would be classed as irregular or chunky. The spectral reflectance characteristics of basic lead silico sulfate (Oncor 45X) are graphed in Fig. 7 and show that this pigment absorbs U V light up to 285 nm. Above that, the reflectance rapidly increases to 76% at 360 nm and on up to 87% at 400 nm. In the Fig. 10. Cumulative particle size distribution curves for basic lead silico sulfate (Oncor 45X/NL) and di basic lead phosphite (Dyphos/NL). visible range it provides an average reflec tance of 94% but falls a little lower in the blue wavelength region. In the near-IR it is as highly reflective as magnesium oxide. PIGMENT SPECIFICATIONS AND MANUFACTURERS There are no standard industrial specifica tions for the procurement of basic lead silico sulfate. It is manufactured and sold only by NL Industries, Inc. as Oncor 45X. I-A-c White Hiding Ij Fig. 11. Phc original uncos I-A-c-5 DIB/ Dibasic Lead Pho: Dyphos Chemical Formuh 2Pb0PbHP03Chemical Compos Pb 8.P O1 H I0( Dibasic lead phosj is the dibasic lead Its commercial ( quite close to its 90.2-91.7%; P 3. 105C, 0.3% maxii Hr: 't.* m ift I-A-c White Hiding Lead Pigments 81 Fig. It. Photomicrographs (lOOOx) of basic lead silico sulfate (Oncor 45X) (left) and original uncoated silica (right). I-A-c-5 DIBASIC LEAD PHOSPHITE Dibasic Lead Phosphite PG Dyphos Chemical Formula Pb3POS5H2 (MW 742.7) 2Pb0PbHP03-iH20 Chemical Composition (wt %) Pb 83.7 P 4.2 PbO 90.2% O 11.8 H 0.3 100.0 Dibasic lead phosphite PG (pigment grade) is the dibasic lead salt of phosphorous acid. Its commercial chemical composition is quite close to its theoretical formula (PbO 90.2-91.7%; P 3.7-4.5%; moisture loss at 105C, 0.3% maximum). TYPICAL PROPERTIES Appearance Density (g/cm3) (lb/gal) Tinting strength, minimum Refractive index Hiding power (fp/lb) Oil absorption (lb/100 lb) Average diameter surface mean (gm) Dibasic Lead Phosphite White powder 6.67 55.5 250 2.25 38 14 2.5 ECONOMICS Dibasic lead phosphite has remained very stable in price over the past 20 years (with in a price range from 56p/lb to 59g/lb from 1950 to 1970). It was first used for the stabilization of plastics but its effective anticorrosive properties and its ability to eliminate the bleed-through of cedar and redwood stains in emulsion primers have since broadened its use. It is presently produced by three manufacturers. GLD33B34 HISTORICAL BACKGROUND Dibasic lead phosphite was patented in 1949-' and marketed shortly thereafter. Its early use was primarily as a stabilizer for vinyl-type plastics with a chlorine con tent. This use has expanded with the years. Since basic lead phosphite is basic in nature, it is an active pigment that stabi lizes films in a manner similar to other basic PIGMENT MANUFACTURE lead pigments. Its anticorrosive properties are good and this white pigment is being Dibasic lead phosphite is made by pre utilized in increasing amounts in light or paring a litharge slurry to which acetic acid pastel colored anticorrosive primers and in catalytic amounts (0.1%) is added. The finishes (especially useful on highway guard slurry is heated to 100F. A clear solution rails). of phosphorous acid is then prepared using phosphorous acid crystals. This is heated to MAJOR REASONS FOR USE 100F and slowly added to the well-mixed heated slurry until a pH in the range of 7.0 to 7.1 is reached. Agitation is continued for Fig. 12. Electronmicroi grade of Dibasic Lead Phi Dibasic lead phosphite pigment is a versa an added half-hour to insure equilibrium tile product and is consumed by both the conditions. The batch is flocculated, the Both the particle s plastics and paint industry. In the plastics excess water is siphoned off, and the resi portant to the proper! industry it acts as an effective stabilizer due is filtered and dried. The product is film. Acicular particl for polyvinyl chloride resins or other vinyl cooled to room temperature and carefully long direction of the resins having a chlorine content. disintegrated. Since the compound is sen relieve stress withoi In the paint industry it acts as an anti sitive to the heat generated by the disin corrosive agent in metal protective coatings tegration process, there is the possibility f fi Acicular particles a and as an antistaining agent in latex coat of it taking fire. If this happens, there is ings (over cedar and redwoods). a reddish-yellow glow in the product and When dibasic lead phosphite is intro that portion of the batch must be isolated r ef er enc es duced in modest amounts into white or and removed or the whole batch will burn pastel colored metal protective coatings, it improves the blister and corrosionresistant qualities of the paint. Usually the incorporation of 0.6 to 1.0 lb of the phosphite pigment per gallon of paint will provide improved resistance to corrosion for exposures to saline environments. It is effective in alkyd, linseed oil, and epoxy ester vehicles or their combinations. Di basic iead phosphite will also impart anti corrosive properties to emulsion primers designed for the protection of steel. In latex (and spoil) by changing composition. PIGMENT GRADES There is only one grade of dibasic lead phosphite for pigment use (Dyphos/NL). The crystal habit of this pigment grade is acicular as shown in the electron photo micrograph (1,400 >5) of Fig. 12. As better resolution is applied to the ultra-fine par ticles, they also are shown to be acicular 1. From the X-Ray Files tories of NL Industrie* 2. From the Files of th< of NL Industries. Inc. 3. Dunn, E. J.. Jr., " Industry." Offic. Dig. No. 247,288-300. Jun 4. BM.p. 633. 5. Rausch. G. A.. Miner< New York, 1921. 6. BM.p.649. 7. Published Price Lists i St. Louis, Mo. primers for wood, it prevents the nail head (only a small percentage by weight of the * Gage. J. C. and Litchfi rusting that often makes a paint job un sightly. The effectiveness of dibasic lead phos phite in reducing the wood staining of latex paints is in proportion to the level of pigment present. Usually 0.4 to l.OIb/gal particles appear roundish or irregular). A cumulative percent particle size dis tribution curve for the commercial dibasic lead phosphite (Dyphos) is given in Fig. 10. The average diameter is about 3.0 jx and the specific surface area is 2.0 m3/cms. of Lead from Paint Fih tinal Tract," J. Oil C< 236-243 (March 1969) 9- Kirby. Sheila M.. "L Prod.. 61(4). 25-30 (Af 10. Theophrastes. History Sir John Hill, London, [-A-c White Hiding I.tud Pigments 83 consistency) due to their natural interlock ing tendency that produces resistance to flow as compared to roundish particles. The spectral reflectance curve for di basic lead phosphite is given in Fig. 7 and reveals that this pigment strongly absorbs the UV wavelengths up to 310nm. From this point the reflectance increases rapidly until it reaches a value of 70% at 350 nm and 94% at 400 nm. Dibasic lead phosphite is a high bright ness white pigment possessing a dry powder reflection of approximately 98% across the full visible spectrum. The near-IR reflec tion for this pigment is quite similar to magnesium oxide. Fig. 12. Elecironmicrograph (1400x) of pigmem grade of Dibasic Lead Phosphite PG/N1.. PIGMENT SPECIFICATIONS AND PIGMENT MANUFACTURERS Both the particle shape and size are im portant to the properties imparted to a paint film. Acicular particles add strength in the long direction of the particle and tend to relieve stress without rupture of the film. Acicular particles also add body (paint There are no standard specifications for the procurement of dibasic lead phosphite. Dibasic lead phosphite is produced by three United States manufacturers (NL, MGL, FER) but only NL supplies a pig ment grade. f .REFERENCES 1. From the X-Ray Files of the Hightstown Labora tories of NL Industries, Inc. 2. From the Files of the Hightstown Laboratories of NL Industries. Inc. 3. Dunn. E. J., Jr.. "Microscopy in the Paint Industry." Offic. Dig. Paint Varn. Prod. Clubs. No. 247,288-300. June (1945). 4. BM, p.633. 5. Rausch, G. A., Minerals Industry, McGraw-Hill, New York, 1921. 6- BM, p.649. 7. Published Price Lists of American Paint Journal, St. Louis, Mo. 8. Gage. J. C. and Litchfield, M. H., "The Migration of Lead from Paint Films in the Rat Gastro-Intestinal Tract," J. Oil Colour Chem. Assoc.. 52(3), 236-243 (March 1969). 9- Kirby, Sheila M., "Lead in Paint," Paint yarn. Prod., 61(4), 25-30 (April 1971). 0. Theophrastes. History of Stones, Translated by Sir John Hill. London. 1774, p. 223. 11. Konig. R., "History of Pigments," Pliny Historia Naluralis, Fette, Siefer Anstrichmellel, 62, 130 (i960). 12. Vitruvius, The Ten Books of Architecture VII, 12th English Translation. Harvard Univ. Press, Cambridge. 1926. 13. Stewart, A., "Lead Pigments In Paints," Offic. Dig. Paint Varn. Prod. Clubs. 22, 1100-1113 (1950). 14. Dunn, E. J., Jr., U.S. Patent 2,323,982, "Manu facture Of White Lead," (July 13.1943). 15. Thompson, G. W. and Stewart, A., U.S. Patent 2,218,940, "White Lead," (October 22.1940). 16. Guide to U.S. Government Paint Specifications, National Paint and Coatings Association, Washington, D.C., 19f/i Ed. (1972). 17. Rose, C. H., "The Lead Pigments," in Protective and Decorative Coatings, Vol. 2 (Matiello, J. J., Ed.), Wiley. New York, 1942. 18. From the Files of Eagle Picher Industries (Data Sheet), Cincinnati. Ohio. / 84 E. J. Dunn, Jr, 19. Stewart, A., U.S. Patent 2.249,330, "Process for Making Basic Lead Sulfate." (July 15. 1941). 20. Gcllncr, R. F.. Creamer. A. S. and Bunting. E. N., "The System PbO-SiOj." A'nr. Bur. Stand. J. Res., 13.237 (1934). 21. Turlett, F. L. and Vahrenkamp, G. J.. U.S. Patent 2.268.913. "Lead Pigments," (January 6, 1942). 22. RM. 23. Williams, F. J. and Pitrot. A. R., U.S. Patent 2,477,277, "Composite Lead Sulfate/Lead Silicate Pigment and Process of Manufacture," (July 26.1949). 24. Peterson, E. P., "Basic Silicate White Lead 45X: A New Pigment Concept," Offic. Dig. Paint yarn. Prod. Clubs. 23.208 (1948). 25. Dunn, E. J., Jr., "Chemical Reaction in Metal Protective Paints," Paint, Oil A Chemical Review, J09,5-8,July 25(1946). 26. Brochure on Basic Lead Silico Sulfate-(Oncor 45 X), NL Industries, Inc. 27. Dunn, E. J., Jr., "A Pigment Particle Surface," Paint yarn. Prod., 42,9 (1952). 28. Kebrick. L. M,, U.S. Patent 2.483,469, "Basic Lead Phosphite Pigments," (October 4,1949). Antii W. A. GU Pigments An NLIndustrii Hightstown, Pigment White 11 Antimony Trioxid Antimony White Antimony Pigmen Antimony Bloom Antimony Sesquk Antimonious Oxit Flowers of Antimi Chemical Formuli Sbi03 (MW 291 Chemical Compos Sb 80 10( GLD33837