Pigment Determination in Carbon Black and Iron Blue Paints FRANCIS E. ROBINSON, The Glidden Company, Reading, Pa. A simple, rapid, and accurate method for quantitative separation of very fine pigments from oil and varnish vehicles is based upon the fact that certain materials yield light, flocculent precipitates which settle rapidly, carrying along the pigment that is present.
A
RELIABLE method for accurately separating very small particle size pigments in oil ( 3 , 5 )and varnish ( I , 2) vehicles has never been satisfactorily worked out. Carbon black, iron blue, chrome greens containing iron blue, and finely ground whites are the most common causes of trouble. It is somet,imes possible to filter the fine pigment through a very fine filtering medium but further trouble is experienced because the filter is clogged by the fine particles. AJthough in some cases it may not, be neces, sary to make a perfect separation and some of the pigment may be passed through the filter, in other cases an exact determination and a clear filtrate are required. The basic principle of the method described in this article can be applied to any pigmentvchicle combination; it is only necessary to find the proper settling agent to collect the pigment and the proper solvent to dissolve the vehicle and to precipitate the settling agent on the pigment. Since the pigment particles are too small to be retained on the filtering surface, it is necessary to collect them by adding an agrnt (nitrocellulose, for example) that when precipitated will carry along with it the finely divided pigment. Two methods are outlined in this article; the principle.of each is the same; the methods differ only in application and procedure. The first method, which depends upon nitrocellulose to collrct the pigment, can be applied to all pigments; it has the disadvantage that the extracted pigment will contain nitrocellulose, although the vehicle will be free from irnpuritics. This method involves no chemical reaction upon any pigment and can be used on d!:irIy any vehiclc except emulsion vehicles or poorly soluble vehicles. Certain vehicles, such as run Congo, short oil alkyds, ester gum, and maleic resins sometimes require special treatment, which consists of redissolving and then reprecipi tating the nitrocellulose in order t,o obtain complete extraction of the vrhicle. The second method makcs use of a glyceryl phthalate varnish (the phthalate is added if not already present) as the settling agent, prccipitated on the pigment in the form of potassium phthalate. Since alcoholic potassium hydroxide is used in this reaction, no alkali-soluble pigments such as lead chromate pigments can be present. I t s principal application is to carbon black and other insoluble pigments, to emulsion vehiclep, and to poorly soluble vehicles that, cannot be determined by other methods. It, is based upon the ordnanre method for alkyd resin determinat'ion (4) and is very accurate. REAGENT
XITROCELLULOSE Sor.clms. Add 20 parts hy weight of the dry R.S. 0.25-second nitrocellulose to 80 parts by weight
of ethyl acet,ate. -4fter solution is complete, determine the exact nonvolatile content of the solution. PROCEDURE
.\LETHOD 1 (Sitrocellulose Method). Weigh nccurately a 2- or 3-gram sample of the paint or enamel and pour into a tared 50-cc. centrifuge tube. ildd 10 cc. of ethyl acetate (85% ester), and stir until sample and ester are completely mixed. Pour into this mixture a weighed sample (ahout 3 grams) of the nitrocellulose solution, and stir until a smooth mixture is obtained. Precipitate this mixture by adding slowly, drop by drop, about 30 cc. of high-solvency aromatic Hi-Flash naphtha, stirring rapidly during the addition. Then place the tube in a water bath and raise to a temperature of 180' F.; hold at, this temperature overnight 01' until the ester has complrtely evaporated. 364
.4t the end of this time, remove from the water bath, cool to room temperature, refill to top with more high-flash naphtha, and centrifuge until the top liquid is clear. Pour off the clear liquid and refill with benzene, place in a !vat,er bath a t 140" F., and allow to stand for about 1 hour. Cool and centrifuge as before. Repeat the centrifuging with the benzene as before and finally wash with petroleum ether, omitting the water bath. Dry a t 150" F. to constant weight. Calculate pigment by subtracting t,heknown amount of nitrocellulose from the total weight. METHOD2 (Phthalate Method). Weigh accurately 7 or 8 grams of the sample into a 250-cc. stoppered Erlenmeyer flask and add 5 cc. of butyl Cellosolve (ethylene glycol monobutyl ether) to aid compatibility of the vehicle. Shake t'he flask carefully, so that the sample and Cellosolve are thoroughly mixed. Add 3 or 4 cc. of a long oil glyceryl phthalate varnish (unless already present) and shake until a ,smooth mixture is obtained. I t is not necessary to w i g h the glyceryl phthalate varnish, as it does not enter into the calculation. I n case a smooth mixture does not result at this point, add more Cellosolve slowly until a smooth mixture is oht,ained. S e x t add 125 cc. of 0.6 N potassium hydroxide in alcohol ( 5 grams of potassium hydroxide) and shake thoroughly. Stopper and allow to stand about 3 hours a t 130" F. to allow for precipitation. At t,he end of this time remove from oven, cool to room tempemture, add !50 cc. of ethyl ether, and restopper; allow to stand a t least 1 hour. Filter through a tared dry Nter paper (Whatman S o . 32, for example), washing with 50 cc. of alcohol-ether (1 to 1) mixture, using five 10-cc. portions. Dry in an oven a t 200" F. for 10 or 15 minutes, remove from oven, and replace in filtering funnel. Wash with warm water (160' F.) until all the soluble potassium phthalate is dissolved and the pigment is washed free of the phthalate. Make the final washing with ethyl alcohol to shorten the drying period. Ilry in :in oven a t 220" I?. t o constant, weight.
Table
I.
Pigment Determinations in Different Vehicles
Pigment Carbon black White pigments Iron oxide Iron blue Carbon black Carbon black
Vehicle Congo varnish Emulsion Alkyd Rosin varnish Linseed oil .4lkyd varnish
Method Phthalate Phthalate Phthalate Sitrocellulose Sitrocellulose Sitrocellulose
Pigment Theoretipal, % 6.60 50.0 43.7 51.3 3.20 3.0
Pigment Determined.
%
6.60 46.3 43.7 51. 7 3.90
3 4.
Because of the great variety of enamel mid paint, vehicles, many different types of solubilities aye encountered. Table I ,shows some applications of t'he two mct,hods and the results obtained. Since the samples were all commercial grades and subject to slight variat,ions, t,he results are reasonably close. The use of nitrocellulose limits the vehicle solubility t o esters and aromat'ic hydrocarbons. However, the number of cellulose derivatives is large enough t o cover practically all vehicles. Ethyl cellulose, benzyl cellulow, cellulose acetat,e, cellulose acetate butyrate, and other agents may be used for vehicles in nhieli nitrocellulose is not satisfactory. The choice of liquids is very extensive. For solvents there are esters, ketones, and ether;.; for precipit,ating liquids there are aromatic and aliphatic hydrocarbons, terpenes, and alcohols. The choice is determined by the ability of the cellulose product to precipitate on the pigment while the vehicle remains in solution. The precipitation method of pigment separation results in considerable time-saving in moat cases. The material filters very ra.pidly on a vacuum filter and doe.I* terials, Synthetic Type", HOMB-ES-No. 680% Par. G-10. p. 20 (November 15, 1943). (5) U. %Government, "FederalSpeo. for Ready-Mixed BlackPaint". TT-P-61, Par. F-2g, p. 4, Washinaton, D. C., Government Printing Offioe, 1931.
Printing Office. 1941.
(3) Gardner, H. A,, "Physiod and Chemiod Examination of Paints,
Renewing the Pit ter-Calke Surface in Small-scale Vacuum mati1ons R. S. BURNETT A
ern
Regionsl Research Laboratory, N e w Orleans, La.
THISa
note describes a device whicu iiit~ ~)eit.~i useu su~ct;ss~mly on laboratory and semipilofplant scale to expedite the vacuum filtration of alkaline aqueous dispersions of vegetable protein extracted from peanut and cottonseed meals. I t s effectiveness suggests t h a t i t might prove useful in clarifying various gummy or coUoidsl solutions. It can be easily and inexpensively constructed. The device consists of a hand-operated rotary scraper which can be used with either a tahle-top Biichner funnel or a stoneware suction filter. By means of the scraper h e colloidal and gummy materids which accumulate on and clog the surface of the filter cake can he periodically removed during liltration donlong with B thin layer of t.he filter aid used. By means of this periodic scraping ahe filter cake surface can he renewed as frequently as is neces-
the sermer to travel thrauzh a h u t 3 inches of cake. As the
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A similar scraper designed far"use k i t h a stoneware suctiun filter is shown a t the right. The lower hearing is threaded and the scraper assembly is fastened t o the top of the f l t e r by s c m v clamps which me welded to a support resting on the filtcr. Better results are ohtained when no more vacuum is employed than is necessary to maintain a steady flaw through the filter. hlore rapid filtration of alkaline solutions can he obtained when the E l e r aid is supported by a glass lilter cloth instead of a cotton cloth or filter paper. This i s probably due to t,he fact t h a t glass
