December, 1926
I N D U S T R I A L A N D ENGINEERING C H E M I S T R Y
presence of material which has no so-called “hook” for soluble nitrocellulose, such as some oils and some resins, may be noted from such actions as sweating under heat. The presence of a volatile plasticizer may be noted by the marked increase in brittleness from heat exposure and in such case many of the steps in the analysis must be made with care. Some resins and certain pigments cause extreme discoloration on heating. Analysis
From these preliminary experiments the analybt has gained more or less definite impressions as to the type and amount of nitrocellulose present, the type and composition of the solvent, of the plasticizers, softeners, resins, and pigment. He is then ready to start the actual analysis. A wide variation in point of attack is possible and the following is offered as one of many. A 100-gram sample of the lacquer to be analyzed is stirred into a liter beaker containing about 800 cc. of toluene. The swelling which occurs will increase the volume to fill the beaker. The pigment stays with the jelled nitropellulose and practically all the other ingredients go with the liquid phase, which is easily separate$ by filtering. Continued extraction of the nitrocellulose residue or precipitate with benzene gives
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a fairly complete separation, though some of the materials other than pigment are left as contaminants of the nitrocellulose and some of the latter is carried with the liquid phase. The combined filtrates are next evaporated to practically constant weight a t as low a temperature as possible, the nitrocellulose residue being also dried to constant weight. The nonvolatile material of the lacquer has now been divided into two parts whose weights represent per cent of the total lacquer and the rest of the analysis is governed by this resinplasticizer ratio. Acetone is added to the nitrocellulose residue and by repeated centrifuging the pigment is separated from the nitrocellulose. The analysis for stabilizers is a very uncertain process, as they are present in relatively small amounts and if physical tests are not included which show their presence it is almost impossible t o identify them. The analysis of the solvent requires starting with a new sample and me shall not touch on that analysis. From this point on the analytical procedure is guided largely by information given by the preliminary tests and such other tests as may be mentioned in the other papers. At the conclusion of the analysis, the correctness of the findings is tested by “building” a lacquer from the analytical results and comparing it with the original material.
Separation and Analysis of Pigments in Lacquer’ By F. W. Hopkins MURPHYVARNISHCo., NEWARK,N
N E of the most difficult problems which confronts the analyst is the separation of pigments from lacquer, especially where the pigments used are p-nitranilines, toluidine reds, other toners of like nature, or c:ubon blacks. These pigments being so finely dispersed that they are in a colloidal suspension, it is practically impossible t o separate them either by extraction or by the use of a solvent or a mixture of solvents and centrifuging. When a white pigment is used in tinting these pigments, it is possible to separate the white. A &gram sample of the lacquer is mixed with 20 cc. of methylene chloride, which throws the nitrocellulose out of solution and carries with it the pigment. The nitrocellulose is extracted by shaking with a mixture of equal parts of benzene and ethyl acetate and centrifuging from 6000 or 7000 r. p. m. White lacquers in which the pigments used are zinc oxide, lithopone, zinc sulfide, or titanium oxide may be ignited directly or after the solvents are evaporated, a 20-gram sample being used. After a qualitative analysis in which zinc is found, that which is soluble in acetic acid is determined and reported as zinc oxide. That which is insoluble is determined and reported as zinc sulfide. When zinc and barium are found qualitatively, it would indicate lithopone. Separating the zinc and barium, the zinc is insoluble in acetic acid and should be determined and reported as zinc sulfide and the barium determined and reported as bwium sulfate. If titanium were present it would be determined by the reduction of the titanium and titration with a ferric salt.
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Method
The best general method with pigments such as Prussian blues, chrome yellows, and chrome greens is as follows: 1 Presented a s a p a r t of the Symposium on “Analysis of Lacquers” before the Mldwest Regional Meeting a n d the Meeting o f the Section of P a i n t a n d Varnish Chemistry of t h e American Chemical Society, Madison, Wis., M a y 27 to 29, 1926.
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T o a 5-gram sample of the lacquer methylene chloride is added and the nitrocellulose is thrown out of solution carrying the pigment with it. The solvents are either evaporated over an oil bath or upon centrifuging, the nitrocellulose and pigments are thrown down, and the solvents decanted. The mixture of nitrocellulose and pigments is placed in a dense alundum crucible 13/’s inches (3 cm.) in diameter. The crucible is placed in a Bailey-Walker extraction apparatus with 25 cc. of a mixture of equal parts of benzene and ethyl acetate and heated over a water bath for 3 hours to extract the nitrocellulose, plasticizers, and resins. The pigment is then heated in the oven a t 105” C . to a constant weight. In blue lacquers, the pigments are usually Prussian or ultramarine blues, and nitrogen and iron determinations are made from which the percentage of Prussian blue is reported. To differentiate between the Prussian and ultramarine blues, the lacquer is ignited and a qualitative test for iron is made. Yellow lacquers are usually made from chrome yellows and the lead is determined as lead sulfide and chromium as chromic oxide. Green pigments in general are a mixture of chrome yellow and Prussian blue. Sitrogen and iron determinations are made from which the Prussian blue is calculated, and the lead and chromium separated. The lead is determined as lead sulfide and the chromium as chromic oxide. Alternate Method
A 10-gram sample of lacquer is taken and the nitrocellulose, which carries the pigment with it, is thrown out of solution by agitation with water. The solvents and water are removed by distillation over an oil bath. The pigment is extracted from the residue by shaking with a mixture of equal parts of benzene and ethyl acetate and centrifuging a t a high speed.
Because of the colloidal nature of pigments in lacquers, t’hese pigments may be determined qualitatively, which gives approximately the necessary information as to the nature and amount of pigments. The presence of hydrogen sulfide in petroleum vapors in certain oil fields in Texas constitutes a serious health and safety hazard which necessitates the exercise of constant precaution, according to the Bureau of Mines.
