Document LKGoy9Z2yYjrXa5g8Bq1KQb8b

Patented Feb. 2, 1954 BAIiSitot [tMj u>. wri*rii^Tjj)lj tlLt // 2,668,122 UNITED STATES PATENT OFFICE ,2 668,122 COMPOSITE LEAD CHROMATE-LEAD SILICATE PIGMENT -/- ? S ^ Adrian R. Pitrot, Uniondale, N. Y., assignor to National Lead Company, New York, N. Y., a corporation of New Jersey No Drawing. Application October 10, 1951, Serial No. 250.813 2 Claims. (CL 106--298) 12 This invention relates to a novel corrosion in- molar proportions, therefore, must insure that hibitive lead chromate pigment. sufficient PbO is present to combine with the Basic lead chromate, PbOPbCrO*, has long chromate radical and also that sufficient silica been known as one of the best corrosion inhibi- is present to combine with the remaining PbO tive pigments being generally in a superior class 6 and also to provide some proportion of excess along with such well known pigments as red lead free silica. As noted, both the C1O3 and SiOa and zinc chromate, commonly known as zinc yel proportions are dependent on the PbO content. low. Extensive commercial utilization of basic While a proportion of free silica within the range lead chromate is limited, however, by its rela noted is desirable in the product of the instant tively high material and manufacturing costs. 10 invention, the more active anti-corrosion com The principal object of this invention is to pounds cannot be diluted to an indefinite ex provide a pigment which will have the excellent tent by free silica. Therefore, the molar propor corrosion inhibitive properties of basic lead chro tion of silica present is restricted to the upper mate and which can be produced economically. limit of the range within which satisfactory pig- The novel pigment of this invention may be 15 ment properties are obtained. prepared by forming an intimate mixture con While the pigment product of this invention taining PbO, CrCb and SiC>2. This may con as hereinbefore described consists essentially of veniently be in the form of an intimate mixture PbO, Cr03 and SiOa, incidental impurities which of basic lead chromate, litharge and silica. This may occur in the raw material or which may be mixture is then calcined at an elevated tempera 20 picked up in the manufacturing may be present ture. The calcination results in chemical and in minor amounts. These will generally not ex physical combination of the ingredients there ceed a total of about 1% and will ordinarily be of to form a composite lead chromate lead sili of a type not significantly effecting the desired cate pigment containing free silica in which the properties of the pigment or the process by which silica is present without impairment of the cor 25 it is produced. rosion inhibitive properties of the combined and While the methods of mixing the ingredients modified lead chromate pigment, which exhibits may vary broadly as will be appreciated by those the characteristic surface properties of basic lead skilled in the art, litharge, normal or basic lead chromate. chromate, and silica may be dry mixed in the As a source of the chromate ingredient, it is 30 proper proportions although better results are preferred to use chromic acid which is readily generally obtained when these ingredients are available, but other chromate materials such as mixed in the form of an aqueous slurry. An ad lead chromate, basic lead chromate, ammonium vantageous method is to precipitate basic lead dichromate, or potassium dichromate for in chromate in the presence of finely divided silica stance, may be employed. As a source of silica 35 in an aqueous medium with constant agitation. precipitated silicic acid or fine native silica may When a water slurry containing litharge and be used. It has been found, for instance, that a finely divided silica is agitated, for instance, lead quartzite which is readily obtainable in powder chromates are readily precipitated by the slow form is quite satisfactory when ground to suffi constant addition of chromic acid. It is general- cient fineness in a suitable mill such as a pebble 40 ly preferable to add a small amount of a so-called mill for example. The proportions of the PbO, Cr03 and SiOa analytically present in the pigment of this inven tion may be varied over a considerable range. On a molar basis, a satisfactory product is ob tained when there are from 2 to 10 mols of PbO for each mol of C1O3 and from 0.75 to 7.5 mols of SiOa for each mol of PbO. The preferred range to produce the best combination of pigment prop solution catalyst such as acetic acid, for instance, for the purpose of accelerating the formation of lead chromates by supplying a greater quantity of lead ions to the solution. The quantity and " the basicity of the lead chromate formed will de pend on the relative amounts of chromic acid and litharge employed. It is best to produce hy drous tribasic lead chromate, erties is a combination of from 4 to 7 mols of 60 3Pb0.PbCr04.y2Ha0 PbO for each mol of Cr03 and from 2 to 5 mols of SiOa for each mol of PbO. The molar proportions of the constituents as noted above are important and critical. It is assumed that the Cr03 combines with the PbO during the fumacing operation and in addition, the excess PbO over that combining with the CrOa will combine with part of the SiOa present. or monobasic lead chromate, PbO-PbCrO*, or a mixture of these two with or without the pres ence of free unreacted litharge. Employing this 55 method of the addition of chromic acid to an agitated slurry of litharge even in the presence of silica, the most basic lead chromate in the system will form first. It is generally prefer Therefore, the finished product will comprise lead able that the proportion required to form the chromate, lead silicate and free silica. The SO most basic lead chromate be employed, in which 0000-NLI-000018994 N 2737 case at least 4 mols of litharge will be present for each mol Of OcOz. At the completion of the reaction, the slurry The above panels were then exposure tested in comparison to panels- similarly painted with coatings of red lead and with zinc chromate in of lead chromate and silica is dewatered and alternate immersion testing machine which con dried. The dewatering operation may be any one 5 sisted of a mechanical device which repeatedly of several standard methods such as filtering: and and alternately lifted the panel from complete drying or feeding the slurry directly ter a con immersion in synthetic sea water and the air. tinuous inclined rotary kiln. The dried slurry Each cycle consisted of 4 minutes' immersion and is calcined at a temperature of. between about 4 minutes in air, After 63 days, it was found that 600 to 650 C. for a period cf from about 1 to 3 id the painted panels prepared with the lead chro hours. The calcination temperature is quite im mate silica pigment of this invention were either portant in that temperatures below about 400- slightly better or equal to those painted with red 450 C. are generally ineffective to produce the lead and were definitely superior to those painted desired physico chemical combination while tem with zinc chromate. Numerical ratings for blis peratures above 700 C. result in a sintered prod 15 tering and corrosion based on physical appear uct with an undesirable gritty character. Batches ance in accordance with the standard system of of product calcined at the proper temperature the American Society for Testing Materials in are completely reacted, are desirably soft, and which 10 is perfect and 0 is complete failure, readily ground to pigment particle size require gave at rating of 9.5 to the lead chromate silicate ments. 20 paint and to the red lead paint while the zinc In order that this invention may be clearly chromate paint had a rating of 7 after 63 days. understood, the following example illustrating the The combination of the constituents in the pig preparation of the lead chromate silica pigment ment product of this invention is physico-chem of this invention is offered. ical, that is, in part physical and in part chem Example 25 ical. It is evident that chemical compounds are formed by dry reaction between the CrCb, PbO 14 Vi liters of water, 2223 grams of litharge and Si02 and also that the free silica is physi and 2070 grams of finely divided silica were placed cally combined with these reaction products. in a 25 liter glass vessel equipped with a proper The free silica particles appear to be coated with stirring device and reacted with the addition of 30 lead silicate and lead Ghromate to produce a sur 2.68 grams of glacial acetic acid, all at room tem face anti-corrosion pigment effect. perature. 252 grams of chromic acid anhydride It will be generally appreciated from the above were dissolved in 2 liters of water and while the results that the novel composite lead chromate slurry of litharge and silica was maintained with lead silicate pigment product of this invention constant agitation, the chromic acid solution was 35 combines the excellent corrosion resistant metal slowly and constantly added thereto over a pe riod of lVi hours. At the end of this time, the protective pigment properties of lead chromate pigments with economy in the presence of silica, slurry was filtered, dried and calcined in an elec thus offering an economical superior corrosion in- tric muffle furnace in suitable dishes at a tem nibitive pigment product. perature of 650 C. for 2 hours. The product was 40 While this invention has been described and found to possess a fine soft texture, was free of illustrated by the example shown, it is not in grittiness- and was found on analysis to contain: tended to be strictly limited thereto and other Per cent Lead monoxide------------ -------- ------------ 49.9 Chromic acid anhydride---------- ------------- 5.55 Silica ----.-------- ---------- --------- 43.9 modifications and variations may be employed within the scope of the following claims. 1 . I claim: 1. A composite lead chromate lead silicate pig ment consisting essentially of PbO, CrOj, and This product was disintegrated of its soft agglom SiOa in physico-chemical combination, said PbO, erates ina swing hammer type mill. CrOi and SiOa being analytically present so that The-pigment product of the example was found there are from 2 to 10 mols of PbO for each mol to possess excellent rust ihhibitive-properties when of Cr03 and from 0.75 to 7.5 mols of SiOs for formulated into'a paint and applied to metal sur each mol of PbO. faces. Por example, the Wad chromate lead sili 2. A composite lead chromate lead silicate pig cate pigment of the example was ground into a ment consisting essentially of PbO, Cr03, and paint together with vehicle and thinner in the SiOa in. physico-chemical combination, said PbO, following amounts: Cr03 and SiOa being analytically present so that Per cent there are from 3 to 7 mols of PbO for each mol Pigment, the example-------------- 50.5 of GrG3 and from 2 to 5 mols of SiOa for each Vehicle.------------------------ -- 49.6 mol of PbO. 60 100:0 ADRIAN R. PITROT. Vehicle: References- Cited in the file of this patent Alkyd resins------ ------------------~ 75.9 Thinner and drier-.-------- 24.1 UNITED STATES PATENTS 100.0 65 Number 2,944,244 Name Date HarshawJune 16, 1936 The* above* paint was applied by spraying to cleaned' strips Of cold rolled steel. Three coats were:applied for a total film thickness of 0.0025 inch. The panels* were allowed' to* dry in air for 70 2,172,997 2,477,277 2,588,367 EdwardsSept. 12,' 1939 Williams- et al'.____July 26 1949 Read--------------------- Aug. 7, 1951 l0*days>. 0000-NLI-000018995