Paint Technology and Tests Part 15

=Chalking Test.= Small strips of black felt, about one inch square, were firmly attached to a block of wood, and by a clamp having the same pressure in each case, the wood with its surface of black felt was fixed to the panel. This apparatus, which resembles a blackboard eraser, is firmly drawn across the panel in one direction for a certain definite distance, during which time it gathers all the chalked surface presented by the painted wood. Upon detaching the apparatus from the panel it is observed that the black cloth becomes whitened to an extent proportionate to the chalking that has taken place on the given area.

After each one of the panels had been treated in the same manner by the same operator, the black cloths were a.s.sembled on one large board and photographed. A definite standard of chalking was made up, and the operator was enabled to put down opposite the report on each panel the degree of chalking which had taken place, No. 1 representing the least amount and No. 10 the greatest amount of chalking.

=Degree of Whiteness Shown by Panels.= It was a very simple matter to gauge the whiteness of the various panels, by comparing them with a series of standard boards painted with three coats of white paint.

Florence Brand, New Jersey zinc oxide, was used as the standard for whiteness and termed "No. 1." In making "No. 2" standard, to the zinc oxide was added .01% of lampblack. By adding .02% of lampblack to the zinc, standard "No. 3" was obtained, and so on, increasing the amount of lampblack in each case by .01%. These standards were run up to "No. 30,"

and the various panels on the different fences compared with them. The degrees of whiteness are recorded in progressive numbers, No. 1 being the standard for whiteness and No. 30 the darkest. The Atlantic City panels ranged from 3 to 8 in the scale of whiteness, while the Pittsburg panels required the use of the entire range of standards.

=Resistance to Abrasion.= The apparatus used for determining the abrasion resistance of a paint was made of a gla.s.s tube about six feet long, having an internal bore of 7/8 inch. This was supported in an upright position over a dish which held the panel under test at an angle of 45 degrees. The abrasive material consisted of No. 00 emery, which was dropped into the tube through a funnel having a bore of 5 mm. When the emery reached the bottom of the long tube it scattered itself so as to strike a surface on the panel about an inch in diameter. The emery was constantly poured in until the paint coating had worn away, showing the bare wood. The weight in pounds of emery powder required to show the disruption of the coating is recorded and reported as the measure of the "abrasion resist." The panel requiring the greatest weight of emery to cause abrasion is evidently the most resistant to abrasion. Paint is often subjected to serious abrasion, through the blowing of sand, especially at the seash.o.r.e, and to withstand such action should contain a proportion of pigments especially resistant to abrasion, such as silica, zinc oxide, asbestine, and barytes.

[Ill.u.s.tration: Apparatus for Determining the Abrasion Resistance of Paints]

[Ill.u.s.tration: Formula No. 1, A. C.]

[Ill.u.s.tration: Formula No. 2, A. C.]

[Ill.u.s.tration: Formula No. 3, A. C.]

[Ill.u.s.tration: Formula No. 4, A. C.]

[Ill.u.s.tration: Formula No. 5, A. C.]

[Ill.u.s.tration: Formula No. 6, A. C.]

NOTE: The author wishes to acknowledge the a.s.sistance of Dr. J.

A. Schaeffer in the preparation of the photomicrographs herewith shown.

[Ill.u.s.tration: Formula No. 7, A. C.]

[Ill.u.s.tration: Formula No. 8, A. C.]

[Ill.u.s.tration: Formula No. 9, A. C.]

[Ill.u.s.tration: Formula No. 10, A. C.]

[Ill.u.s.tration: Formula No. 11, A. C.]

[Ill.u.s.tration: Formula No. 12, A. C.]

[Ill.u.s.tration: Formula No. 13, A. C.]

[Ill.u.s.tration: Formula No. 14, A. C.]

[Ill.u.s.tration: Formula No. 15, A. C.]

[Ill.u.s.tration: Formula No. 16, A. C.]

[Ill.u.s.tration: Formula No. 17, A. C.]

[Ill.u.s.tration: Formula No. 18, A. C.]

[Ill.u.s.tration: Formula No. 19, A. C.]

[Ill.u.s.tration: Formula No. 20, A. C.]

[Ill.u.s.tration: Formula No. 33, A. C.]

[Ill.u.s.tration: Formula No. 34, A. C.]

[Ill.u.s.tration: Formula No. 35, A. C.]

[Ill.u.s.tration: Formula No. 36, A. C.]

[Ill.u.s.tration: Formula No. 37, A. C.]

[Ill.u.s.tration: Formula No. 38, A. C.]

[Ill.u.s.tration: Formula No. 39, A. C.]

[Ill.u.s.tration: Formula No. 40, A. C.]

[Ill.u.s.tration: Formula No. 45, A. C.]

[Ill.u.s.tration: Formula No. 46, A. C.]

[Ill.u.s.tration: Formula No. 47, A. C.]

=Making Photomicrographs.= The photomicrographs which are herewith shown were made in the following manner: A part of a panel was placed upon the stage of the microscope and held firmly in place with clips. By varying the adjustment and carefully running over the field the condition of the surface was readily given, using the same eye-piece and objective throughout the tests, and obtaining a magnification of thirty-three.

Great care was exercised to secure an average field showing the general and typical appearance of every panel. Little difficulty was experienced in so doing, as the laboratory panels gave very representative surfaces of the large panels on the fence. The instrument was then inclined horizontally and the eye-piece was fitted into the camera nose. In the back of the bellows of the camera was placed the ground gla.s.s for focusing. To secure illumination the light from an electric arc lamp was reflected from a mirror directly upon the painted surface of the panel, which in turn was reflected through the camera on to the ground gla.s.s. The plate-holder was then put in position and six-second exposures were made, afterward developing and printing.

=Checking and Cracking.= What was termed "fine matt checking" at the First Annual Inspection was not visible at the time to certain members of the Inspection Committee, but it is an established fact that the checking was an existing condition, as the photomicrographs have shown.

This checking has a very peculiar characteristic in that the lines are very narrow and hair-like, being somewhat interlaced and peculiarly forked. That this hair matt checking is a preliminary condition which afterwards develops into matt checking and into marked or heavy checking seems to be indicated.

It appears from an examination of the photomicrographs of the paint films that a paint coating closely resembles the surface of the earth, and is subject to the same basic laws that have caused the various geodetic changes in the earth's crust. Observation of a dried pond or lake bed will disclose types of fissuring and cracking similar to those shown by dried paint coatings in which the oil has been fully oxidized, and especially in the case of paints containing pigments which act upon the oil to produce compounds brittle in nature.

At Atlantic City the panels were all clean and free from dirt, presenting continuous exposure of the films, and thus maintaining conditions for active checking. At Pittsburg, soon after the panels began to chalk, the large amount of dust and black soot in the atmosphere completely covered the panels with a very thick, resistant coating of carbon, which acted as a seal or protector, preventing disintegration to a great extent. This coating was extremely hard to remove, and photomicrographs, before and after removal of this coating by rubbing with a damp cloth, failed to reveal marked checking on any of the formulas except those made of strictly pure basic carbonate-white lead. The checking, even on these, was not as marked as at Atlantic City. It is presumed that after the chalking had taken place and the chalked pigment had been washed from the panels, the gradually increasing coat of carbon and lead sulphide had protected the panels from checking, or possibly the atmosphere of Pittsburg, which in other respects had deteriorated the panels to a greater extent than at Atlantic City, did not have the extreme action in causing checking that the Atlantic City atmosphere seemed to have effected.

[Ill.u.s.tration: Combination Formula No. 1, Pittsburg

BEFORE WASHING

Mottled surface due to external coating of impurities.]

[Ill.u.s.tration: AFTER WASHING]

[Ill.u.s.tration: Formula No. 4, Pittsburg

BEFORE WASHING]