Document O1OXqMOzV70k50bZQz8d7RjKM

Patented July 26,1949 2,477,277 UNITED STATES PATENT OFFICE 2,477,277 COMPOSITE LEAD SULFATE-LEAD SILICATE PIGMENT AND PROCESS OF MANUFAC TURE Fnutdi J. Williams, Port Washington, and Adrian R. Pttrot, Hempstead, N. T, assignors to Na tional Lead Company, New Fork, N. I, a cor poration of New Jersey Application March 27,1946, Serial No. 657,534 4 Claims. (CL 106--297) 12 This invention relates to pigments and more other objects will become apparent from the fol particularly to a novel composite lead sulphate- lowing description. lead silicate pigment and a process for its man This invention in its broadest aspects contem ufacture. plates first the formation of an Intimate mixture Basic lead sulphate has long been known as a S containing PbO, SOj, and 810* which may pref paint pigment. It is prepared commercially gen erably be in the form of an intimate mixture of erally by either volatilising lead sulphide ore or lead sulphate or basic lead sulphate, or free lith pig lead and sulphur and collecting the product arge and lead sulphate or basic lead sulphate, and as a finely divided sublimed basic lead sulphate; silica; and the subsequent calcination of this or adding sulphuric acid to a slurry of lead mon 10 mixture at a temperature of from about 400* C. oxide which may contain a small amount of lead to about 706* C. and preferably from about 600* acetate as a catalyst. While basic lead sulphate C. to 650* C. for a period of from 1 to 3 hours. is somewhat widely used as a pigment, its prop The calcination results in a chemical and/or erties are such that it is generally used in con physical combination of the ingredients of the junction with other pigments in the formulation 15 mixture to form the novel composite lead sul of so-called mixed paints. This is necessary phate-lead silicate pigments. because lead sulphate is generally deficient in The proportions of ingredients used in the weathering properties and, in order to obtain a practice of this invention may be varied over a long weathering paint film, this deficiency must considerable range and still be considered to fall be counteracted by the substantial presence of 20 within its scope. The drawing. Pig. 1, represents other pigments. a diagram wherein the percentages of PbO, SO* Silica, SiO*, is used as a pigment when finely and SiO, are plotted on a triline&r diagram (tri divided; however, its low refractive index gives angular graph). The point I represents 100 per it little hiding strength, and since it has low re cent PbO, 3 represents 100% 80s, and 4 repre- activity with common paint vehicles, it contrib 2fi sents 100% 8iO,. The shaded area on the graph utes little to the quality of the paint. Silica is is bounded by straight lines Joining the points most often used as a diluent or extender pigment S, I, 7.1, which points correspond respectively to in low cost paints. Lead silicate pigments have compositions listed below: also been known, although their commercial use has not heretofore been widespread. Lead sili 30 Points: Parts by weight 33PbOISOs . 66810, cate pigments are made by fumadng a mixture of PbO and silica at high temperature under ? minim ... SIPbO 6SOs 63SiOs __72PbO ISSOs 15810s which conditions various lead silicate compounds 3-------------- ... 77PbO 380, 30810, may be formed. These pigments have generally shown good weathering properties; however, they 35 The shaded area within these lines covers com have the very serious disadvantage of being un positions having ingredients In the proportions stable when exposed to light. This instability contemplated in this invention. On a molar results in objectionable darkening of the paint basis, it will be seen that the proportions of the film containing lead silicate when exposed to the ingredients may be varied such that for each mol weather and makes it substantially impossible to 40 of SO, present, there are from 2 to 10 mols of formulate white or light tinted coatings using PbO, and for each mol of PbO, there are from these pigments. 0.75 to 7.5 mols of 810* It will be found that pig This invention has for its principal object the preparation of a composite lead sulphate-lead sil icate pigment of superior weathering quality, of good tinting strength, and which is stable in that it win not darken on exposure to light. Another 45 ments made according to this invention, having a composition within the shaded area an the graph will have tinting strengths ranging from 40 to 120 when determined according to Gardner in his "Examination of Paints, Varnishes, and Lacquers," 8th edition, 1939, page 35, and will object is to provide a pigment containing a sub show good weathering properties and stability to stantial proportion of silica in chemical and/or 60 light. physical combination without the normal dele If it is desired to produce pigments according terious effect of such silica content on the pig to this invention having tinting strengths in the ment properties. Another object is to provide a higher ranges, that is between 100 and 120 or long weathering pigment of low lead content and above as determined by Gardner, the Ingredients 3,477,377 34 tion will fall within the heavier shaded area on is gritty In character. The calcination time the graph, Fig. 1, bounded by straight lines drawn joining points 7, 8, 8, II, II.' These points should generally be between 1 and 3 hours and preferably about 2 hours. Batches calcined under indicate respectively the composition tabulated these preferred conditions are white in appear- below: S ance and completely soft and not gritty. They Points: Parts by weight 7 72PbO 1380] 16810* I77PbO 380* 20810* 851PbO 280* 47810* 10 43FbO 280* 55810* 11 41PbO 480* 55810* 10 are thus ideally suited for incorporation into paints with a minimum of grinding. In order that this invention may be more clear ly understood, the following example will illustrate the preparation of composite lead sul phate-lead silicate pigment. The ingredients PbO, SO*, and 810* may be in EXAMPLE I corporated into the mixture In any convenient 18 liters of water, 2223 grams of litharge (lead form. Also raw materials may be used which 15 oxide), 2000 grams of finely divided silica and 40 do not contain only these radicals, but which grams of 99% acetic acid were placed in a 25 may give these products when subsequently liter capacity glass vessel equipped with a wooden heated In the calcination stage, for instance, in turbine type stirring device. While the slurry place of lead oxide, we may use metallic lead, lead was maintained in vigorous agitation, 231 grams carbonate, or even a previously compounded 20 of sulphuric add (95%) previously diluted to 500 lead sulphate. As a source of silica, we may use milliliters with water, was added slowly over a precipitate silicic acid or fine native silica. We period of 1 hour. The system then consisted of have found that sand stone or quartzite which Is a slurry of hydrous tribaslc lead sulphate and readily obtainable In pure form Is quite satis finely divided silica in a dilute lead acetate solu- factory when ground to extreme fineness In a 25 tlon as determined by chemical and microscopic suitable mill such as a pebble mill. The SO* means. The solid portion was separated on a Ingredient is preferably used In combination with Buchner funnel, washed free from lead acetate lead oxide as a lead sulphate or basic lead sul and dried. The dry product was calcined in a phate. It is essential that an intimate mixture muffle furnace at 600* C. for 2 hours. The cal- of the Ingredients be obtained. 30 dned product was a soft powder with whiteness Good results may be obtained by mixing dry equivalent to the best grade of basic carbonate basic lead sulphate and silica or PbO, Pb80 and white lead. On analysis, the following constants silica. In the proper proportions. Better results were found. are obtained when these are mixed In an aqueous slurry. It is much to be preferred, however, to precipitate lead sulphate or basic lead sulphate in the presence of finely divided silica in a slurry while agitating continuously. This may be ac complished by placing the finely divided silica and litharge and sufficient water to form a slurry 35 40 %PbO 47.85 %SO* ---------------------------------------------- 4.29 %SiO* ---------------------------------------------- 47.86 Tinting strength ____________________ 120 Oil absorption------------17.9 g. oil/100 g. pigment Specific gravity_____________________ 4.07 Into a vessel equipped with a suitable agitating To test the paint making properties of the or stirring device. At this point. It is preferable, composite lead sulphate-lead silicate prepared although not necessary, to add a small amount above, a paint was made using this material mixed of acetic acid or some other add catalyst for with linseed oil, drier, and volatile thinner at a the purpose of dissolving a small portion of the 45 pigment volume ratio of 29. Three coats of litharge, thus supplying lead ions to the solution. this paint were applied to cedar panels which To this slurry, sulphuric acid is slowly added with were exposed in Florida in comparison to cimnar vigorous agitation during which addition lead panels using a 29 pigment volume ratio white sulphate or basic lead sulphate is formed. The lead paint. The white lead panels started to amount and character of this lead sulphate is SO chalk in 16 weeks while the panels painted with dependent on the ratio of litharge to sulphuric the paint pigmented with the product of this acid as will be evident to those skilled In the art. It is possible to produce monobasic lead sulphate Invention had not started to chalk in 34 weeks which indicates excellent durability. Further 2PbOSO* or hydrous tribaslc lead sulphate more, the paint films containing the composite 4PbOSChH*0 or a mixture. Further, it is pos 65 lead sulphate-lead silicate pigment remained in sible to have the tribaslc sulphate plus excess lead tact and white throughout the exposure and oxide when the ratio of PbO to SO* is greater showed no tendency toward darkening or dis than 4 to 1. Such mixtures up to a ratio of coloration. lOPbO to ISO* are within the scope of this in A unique feature of this invention is the com- vention. 80 paratlvely high 810* content which may be pres When the addition of sulphuric acid to the ent in the products iff this invention without slurry is complete, the precipitate is dewatered proportionate impairment of the physical prop by filtering and drying. This may be done by erties of the pigment. The amount of 810* may any of the standard methods generally employed, be substantially greater than that necessary to cr the slurry may be transferred directly to a 05 form lead silicate with the amount of PbO pres continuous rotary kiln in which the first portion ent, considering the PbO combined with the 80* will act as a drier and the subsequent portion as lead sulphate. This phenomenon suggests that as a calclner. The dried slurry is calcined at a a physical as well as a chemical combination of temperature of from about 400* C. to about 700 the Ingredients occurs during calcination, and C. and preferably from about 600* C. to 650* C. 70 that the chemically uncombined silica becomes The temperature of calcination is important, coated with lead silicate and/or lead sulphate. since temperatures below 400* C. are generally It will be found that if the proportion of silica is ineffective. Temperatures substantially exceed increased over that given in the limits shown, ing 700* C, complete the reaction speedily, but apparently uncoated silica will be evident in the often produce a partially sintered product which 75 product, and the amount over and above that 0000-NLI-000018992 a,477,377 56 which could have been used according to this ment consisting essentially of PbO, SOa, SiOa in Invention will be present as a diluent or extender physico-chemical combination, the amounts of with proportionate lowering of the pigment prop PbO, SO3, and SiOa analytically present being in erties. such proportions by weight as may be represented Generally speaking, the proportions of the in B by a point lying within that area of a trilinear gredients will Influence to some degree the prop diagram of composition PbO-SOa-SiOa defined as erties of the calcined pigment. The higher PbO hounded by straight lines joining in series points content compositions will show somewhat greater represented by the compositions: tinting strengths. Higher SiOa content will give lower specific gravity. A higher SOj content will 10 provide somewhat greater stability toward light, although all compositions within the general shaded area shown on the graph will be shown 33PbO: ISOa: 66SIO2 31PbO:6SOa:63SiOa 72PbO: 13SOj: 15SiOj 77PbO:3SOa:20SiOa to have satisfactory stability. As an example of 2. A composite lead sulphate-lead silicate pig the general trend shown in the pigment proper IS ment of tinting strength at least 100 consisting ties when the composition of the pigment Is essentially of PbO, SOa, SiOa in physico-chemical varied, the following table gives principal prop combination, the amounts of PbO, SO3 and SiOa erties of several different finished pigment com analytically present being in such proportions by positions. weight as may be represented by a point lying Ta b l e I Properties of Lead Sulphate-Silica Pigments 20 within that area of a trilinear diagram of com position PbO-SOj-SiOa defined as bounded by straight lines joining in series points represented Exampie Per Cent by Weight PbO so. SiOi Tinting Oil Ab Strength sorption Weather ing Quality by the compositions: 72PbO:13SOa:16SiOa 25 77PbO:3SOa:20SIOa 51Pb0:2S0s:47S10i 2............... 3............ 4.............. 6.............. e.............. 7.............. 40.12 48.75 48.65 47.18 3237 76.00 1.76 2.60 2.90 6.64 2.90 3.00 49.12 48.75 48.55 47.18 64.73 21.00 85 16.9 Excellent. 43PbO:2SOa:55SiOa 105 19.7 Do. 41PbO:480]: SSSiOa 100 16.1 Do. 100 60 100 15.8 21.6 10.8 Do. Good. 30 3. A composite lead sulphate-lead silicate pig Excellent. ment consisting essentially of PbO, SO3, SiOa In physico-chemical combination, said PbO, SOa, The novel lead sulphate silica pigment products and SiOa being analytically present in proportions of this Invention have numerous advantages over by weight of about 47.8% PbO, 4.3% SOa and other white oigments and are definitely superior 35 47.4% SiOa. to mixtures of pigments having the same ana 4. A process for manufacture of composite lead lytical ingredients taken in the same proportions. sulphate-lead silicate pigments which comprises They combine to give properties of both basic forming an intimate mixture consisting essen lead sulphate and lead silicate without the poor tially of lead monoxide, PbO, at least partially weathering qualities associated with (me and the 40 converted to lead sulphate, PbSOx, and silica SiOi light instability associated with the other, and the amounts of PbO, SO3, and SiOi analytically also many contain a substantial amount of silica present in said mixture being In such proportions without proportionate deleterious effect from the by weight as may be represented by a point lying presence of this generally low grade pigment. within that area of a trilinear diagram of com The weathering qualities of paint made with the 45 position PbO-SOa-SiOa defined as bounded by new pigments have been shown to be excellent straight lines joining in series points represented and even better than basic carbonate white lead by the compositions: which has long been accepted as the standard 33PbO:lSOa:60SlOi white pigment Another advantage of the new pigments are their light weight. Since a large 60 31PbO: 6SO): 63S10* 72PbO: 13SOi: ISSlOa proportion of the composition may consist of 77PbO: 3SOj: 20SiOi silica which has a low specific gravity, the spe cific gravity of the combined pigment may be and heating said mixture at a temperature of considerably lower than for either lead carbonate from about 400* C. to about 700s C. for a period or basic lead sulphate white lead. This low spe 65 of from l to 3 hours. cific gravity Is obviously a useful factor In certain FRANCIS J. WILLIAMS. applications where light weight pigments are de ADRIAN R. PITROT. sirable and may be further advantageous when considering pigments on a volume basts as, for REFERENCES CITED Instance, when formulating by the gallon. 00 The following references are of record In the It will be understood that the examples shown file of this patent: are given merely for the purpose of illustration and that other variations and embodiments may UNITED STATES PATENTS be considered to fall within the scope of this Number Name Date invention as limited by the appended claims. 06 2,391,118 BartonDec. 18,1946 We claim: 2,383,284 Barton------------------- Aug. 21,1945 1. A composite lead sulphate-lead silicate pig- 0000-NLI-000018993