Paint Technology and Tests Part 8

=Carbon Black.= This form of very pure carbon results from the combustion of gas. Its gravity, 1.09, is lower than that of lampblack, which shows a gravity of 1.8. It is used in much the same way and for the same purposes as lampblack. In physical appearance it shows great similarity to the particles of lampblack.

=Lampblack.= This pigment, made from the combustion of oils, consists very often of more than 99% carbon. It has wonderful tinting value. The particles show a fine, fibrous structure with a tendency toward agglomeration. They differ greatly in physical appearance from those of either graphite or bone black, being exceedingly more uniform than the latter.

[Ill.u.s.tration: Zinc Chromate]

[Ill.u.s.tration: Prussian Blue]

[Ill.u.s.tration: Ultramarine Blue]

[Ill.u.s.tration: Chrome Green]

[Ill.u.s.tration: Bone Black]

[Ill.u.s.tration: Carbon Black]

=Graphite.= Graphite, both in the natural and artificial form, contains impurities such as silica, iron oxide and alumina, but the natural form has a much greater percentage of these foreign materials, in some cases as high as 40%. Graphite is usually mixed with other pigments, such as red lead and sublimed blue lead, thus serving better as a paint coating.

The difference in physical appearance of the various carbon pigments is interesting, as each pigment has characteristics of its own. In graphite we find a great tendency toward agglomeration or ma.s.sing of particles.

=Mineral Black.= Mineral black is a pigment made by grinding a black form of slate. It contains a comparatively low percentage of carbon and consequently has low tinting value. It finds use as an inert pigment in compounded paints, especially for machine fillers. The pigment has a flocculent appearance, the particles showing a strong tendency to ma.s.s.

Photomicrographs of two combination paint pigments are here given, to show the various pigments as they appear under the microscope, when in combination.

PERCENTAGES OF OIL REQUIRED FOR GRINDING VARIOUS DRY PIGMENTS INTO AVERAGE PASTE FORM

White lead (corroded) 9% White lead (sublimed) 10% Zinc lead (American) 12% French process zinc oxide 17% American process zinc oxide 16% Blanc fixe 30% Barytes (natural) 9% Paris white (whiting) 20% Terra alba (gypsum) 22% Floated silica or Silex 26% Kaolin (China clay) 28% Asbestine 32% Blue, ultramarine 27% Blue, Chinese or Prussian 50% Black, gas carbon 82% Black, lamp 72% Black, drop 60% Black, bone 50% Brown, mineral 24% Brown, vand.y.k.e 50% Chrome yellow, lemon 23% Chrome yellow, medium 30% Chrome yellow, orange 20% Chrome yellow, dark orange 15% Chrome green, Chem. pure light 21% Chrome green, Chem. pure extra dark 25% Chrome green, 25%, color light 13% Chrome green, 25%, color extra dark 17% Graphite (pure) 40% Indian red, (98%) 20% Ochre, yellow, American 26% Ochre, yellow, French 28% Ochre, golden 28% Red, Venetian 23% Red, Oxide 25% Red, Tuscan 27% Red, Turkey 28% Red, lead 12% Red, lake 55% Sienna, Italian, raw 52% Sienna, Italian, burnt 45% Sienna, American, burnt 38% Sienna, American, raw 40% Ultramarine green 28% Umber, Turkey, raw 48% Umber, Turkey, burnt 47% Umber, American, burnt 36% Umber, American, raw 38% Verona green (terra verte or green earth) 32% Vermilion, English (quicksilver) 14% Vermilion, American (chrome red) 16% Paris green, American 23% Zinc chromate (permanent yellow) 15%

[Ill.u.s.tration: Lampblack]

[Ill.u.s.tration: Graphite]

[Ill.u.s.tration: Mineral Black]

[Ill.u.s.tration: Asbestine and Whiting]

[Ill.u.s.tration: Silica and Asbestine]

CHAPTER IV

PHYSICAL LABORATORY PAINT TESTS

For the paint chemist who desires to familiarize himself with the more recent a.n.a.lytical methods worked out in American laboratories, reference may be had to treatises on the a.n.a.lysis of paints, by Gardner and Schaeffer,[16] and Holley and Ladd.[17] a.n.a.lytical methods are not included in this chapter, the writer's desire being to treat the subject from the standpoint of the physical properties of painting materials.

The work outlined herein is of a nature that affords a wide field of research, and a brief study will doubtless suggest similar work to the student of paint.

[16] The a.n.a.lysis of Paints and Painting Materials. McGraw-Hill Book Co., New York, 1910.

[17] Mixed Paints, Color Pigments and Varnishes. John Wiley & Sons, New York, 1908.

=Preparation of Paint Films.= The study of paint films is one that has become of vital importance, and is receiving at the present time great attention. Among the many methods which have been suggested and attempted for securing paint films, a few already well known may be mentioned.

By painting upon zinc and eating away the zinc with acid: The objection to this method is very evident, namely, the action of the acid upon the paint coating, which is likely to be very severe. Another method has been to spread paraffin on a gla.s.s plate, and painting upon this surface. When the paint is dried, the paraffin is melted off and thus the film is obtained. This method is open to objections, in that the paraffin surface is not a comparable one upon which to paint, and also that the complete removal of the paraffin is not a.s.sured.

Another method consists in covering a piece of gla.s.s with tin foil, painting out the film upon the foil, and after drying properly, to remove the sheet of foil with its coating of paint and immerse in a bath of mercury which, by amalgamation of the tin, leaves the paint film.

We now come to a method worked out in our laboratories, which can be recommended as being not only simple but efficient and practical. It has been found that a size from noodle glue, when painted upon ordinary fair-quality paper, makes a surface from which the paint may be subsequently stripped. The paint is applied in the ordinary way to the paper, which is held during the operation by thumb tacks, and allowed to dry. The paint may be separated by immersion in water kept at about 50 degrees Centigrade. By this method large films may be obtained, but it has been found very unhandy to work with films exceeding an area of eight inches square. When the film of paint has been detached from the sized paper through the dissolving of the noodle glue, the paint film is then immersed in a fresh solution of water, in order to remove whatever excess of noodle glue there may be remaining. A gla.s.s rod is then introduced into the bath, in which the paint film is floated upon the gla.s.s rod, which is then hung up to dry in a suitable container to prevent the acc.u.mulation of dust, etc.

[Ill.u.s.tration: Bottles Showing Relative Permeability of Films by Amount of Whiting Formed Within]

=The Permeability of Paint Films.= A series of tests were made to determine the water-excluding values of various combinations of painting pigments ground in pure linseed oil. White pine boards, six inches long, four inches wide, and one inch thick, were carefully prepared and numbered and given three coats of a white paint formula of the corresponding number. After drying, the boards were carefully weighed and immersed in a tub of water for three weeks. After removal, the surfaces of the boards were dried with blotting paper and the boards weighed. The gain in weight, corresponding to the amount of water penetrating through the pores of the wood, was observed. The boards were again immersed and at the end of two months the following results were obtained:

Grammes of water Formula absorbed No. through paint

1. Soya bean oil 120 2. Linseed oil 102 3. Calcium sulphate 93 4. Barytes 88 5. Asbestine 74 6. Corroded white lead 59 { Basic carb.--White lead 25% } { Basic sulph.--White lead 20% } 7. { Zinc oxide 25% } 58 { Calcium sulphate 25% } { Calcium carbonate 5% } 8. Sublimed white lead 56 9. Zinc oxide 56 { Zinc lead white 30% } 10. { Zinc oxide 40% } 42 { Basic carb.--White lead 20% } { Calcium carbonate 10% } 11. { Basic carb.--White lead 50% } 42 { Zinc oxide 50% } { Basic carb.--White lead 38% } 12. { Zinc oxide 48% } 38 { Silica 14% }

The test boards were then exposed, with their content of water, to the action of the sun's rays. Blistering of the painted surfaces took place in many cases, caused by the rapid withdrawal of the water and its consequent action on the paint film. The tests seem to indicate that a mixture of white lead and zinc oxide, with or without a small percentage of the inert pigments, is not as subject to the action of the water as the single pigment paints. In order, however, to corroborate these tests, it occurred to the writer to develop a more visible means of demonstrating the pa.s.sage of moisture through paint films.

[Ill.u.s.tration: Bell Jar Apparatus for Testing Permeability of Paint Films

Paint films sealed over mouths of Bottles containing Lime Water.

Carbonic Acid Gas generated under Bell Jar pa.s.ses through Plate Films and precipitates Calcium Carbonate]

Another series of white pine boards were therefore soaked in a solution of iron sulphate for several hours. After removal, the surface of each board was dried and coated with one coat of the paints previously tested. After thorough drying for forty-eight hours, there was placed on the surface of each board a few drops of a solution of pota.s.sium ferrocyanide. This solution has the effect of producing a blue coloration with iron sulphate, and in every case when it was placed on a paint of considerable porosity, the solution penetrated through and formed a blue coloration beneath the paint. The results corroborated the original tests referred to above.

A series of sheets or films of paints were then prepared according to the method referred to on page 71. These films were placed over gla.s.s dialyzing cups, allowing the inner surfaces to sag so as to hold a small amount of dilute ammonium chloride solution. Distilled water was placed on the reverse side of the dialyzing apparatus and the tests started. At the end of six days the distilled water in each test was examined and the following results obtained:

Test No. 1 (corroded white lead and asbestine film) allowed the pa.s.sage of 0.002 gm. ammonium chloride. Test No. 2 (corroded white lead and zinc oxide film) allowed the pa.s.sage of 0.0003 gm.

ammonium chloride.

Tests were also made with dilute solutions of other salts such as ferric chloride, having a dilute solution of pota.s.sium sulpho-cyanide on the reverse side of the apparatus. In the latter case the formation of a pink color, characteristic upon the mingling of these solutions, was obtained in a few hours.

=Film-Testing Machine.= A film-testing apparatus, termed a "filmometer"

by its originator, Mr. R. S. Perry, was constructed, with the following features: A graduated upright tube is fixed by means of sealing wax to two metallic plates which carry an evenly bored hole, exactly under the hole in the upright tube. This hole measures exactly one square centimeter in area, and is circular. The upright tube is graduated into lineal centimeters and is called the pressure tube.

[Ill.u.s.tration: Gardner Accelerated Test Box]

[Ill.u.s.tration: Perry Film Testing Machine]

Attached to the lower end of this pressure tube, close to the metallic plates which serve as carriers for the paint film to be tested, is a side-neck, which is inclined at an angle of 45 degrees to the pressure tube, and serves the purpose of introducing the mercury, as will be described later. Immediately under the openings in the metallic plates which carry the film are arranged two pieces of iron inclined at a 90-degree angle, so arranged that when the pressure of mercury is applied and causes rupture of the film, the falling mercury shall be caught between these two insulated plates and cause contact. These two plates are connected up by wire with a pair of magnets, thence to an electric bell, and from there to storage batteries which supply the current.

A film of paint is tested in the following manner: A piece of film one inch square is cut out and placed between the two metallic plates which hold the film immediately under the pressure tube. Mercury is run in from a burette through the side-neck and applies pressure upon the film by gravity. As the mercury is run in it rises of course in the tubes until this pressure becomes so great as to finally break the film. At this point the mercury will run out, and, falling upon the two insulated iron plates immediately below, will cause contact and close the circuit which rings an electric bell, which is a signal for the operator to shut off the inflow of mercury through the side-neck from the burette.

The pressure tube is also supplied with a piston which is made of a piece of thin iron wire having a disc attached to its lower end. As the mercury rises in the pressure tube this iron wire is pushed up, being very delicately counterpoised over a wheel. Upon the breaking of the film the mercury runs out, but upon falling upon the two iron plates underneath causes contact to be made, which causes the current to run through the pair of magnets before mentioned, which, becoming electrified, attract the piston in the pressure tube, giving a reading for the maximum height of the column of mercury.

[Ill.u.s.tration: Diagram of Perry Filmometer]