Applicator for the Preparation of Uniform Paint Films E. J. D U " , JR., AND C. H. BAIER National Lead Co. Research Laboratories, Brooklyn, N. Y.
DOCTOR BLADE
A machine for spreading uniform paint films has been devised which is extremely simple i n design and operation. The principle is a doctor blade having enough mass to flow automatically down an inclined plane by the pull of gravity. This device gives films which are free from the unevenness produced with hand-drawn doctor blades. Films 4 inches wide and approximately 30 inches long are prepared for exposure work, but any reasonable size desired can be made. Clear oils, varnishes, lacquers, enamels, outside house paints, thixotropic, or free-flowing flats all give uniform films by this method. With modified doctor blades, a second coat may be applied over a properly dried first coat.
GUIDE,.-. PINS - ...
CLEAUNCE
FIQURE1. DOCTOR BLADETROUGH FOR SPREADINQ PAINT
decided upon i t became a question of the type of doctor blade t o select. The authors chose a square trough type, illustrated in Figure 1, which holds the paint and gives uniform flow with a good clean edge. The four sides of the square trough are made of steel 4 inches long, 1.5 inches wide, and 0.25 inch thick. The four sides are pinned together and held in place by screws, and are brought to a common plane by lapping upon a flat surface. One side is cut to a definite number of thousandths of an inch below the other three sides and then reassembled. When completed, the doctor blade must be tested by preparing films on plate glass. Looking through these films with transmitted light the degree of uniformity of the paint film is readily observable, and if necessary the doctor blade is touched up with a fine stone until it gives a uniform film.
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HE preparation of uniform films for various types of finishes has been a rather vexing problem.
I n 1922, Walker and Thompson (8) published the s inningdisk method for the preparation of uniform films. Modiications of this principle were the offset spinning disk and Scofield's (6) spinning machine, with a sloping surface. Arlt ( I ) published the spra -gun method of producing films of controlled thickness. Fgwed films have been illustrated by Stoppel (@, and dipped films by Bruins (3). Films made with a roller-ty e machine have been described by Strieter (7). Doctor blaze machines, the type most commonly used, are well illustrated by the Parks ( 4 ) Film-0-Gra h, Brier and Wagner ( 2 ) basket blade, and Bird or Bradly brade. All the above machines and their many modifications have served a very useful purpose, but the doctor blade principle seems to be the most generally useful for handling all kinds of formulations.
Rings could be used, as well as squares, for single-coat application work. Their overhang at the bottom would facilitate emptying and cleaning, but the rings are more troublesome tomachine, produce a somewhat different flow effect, anddo not give as satisfactory an edge. Furthermore, in two-coat work there would not be enough bearing surface on the rings for uniform results.
With the foregoing information on the subject, a n investigation was made to develop a quick and simple means of producing large enough paint films, u-ith a sufficiently high degree of uniformity to be suitable for exposure and general physical test purposes.
For two-cost work the doctor blade is made about 0.5 inch wider than for the first coat. Doctor blades for two-coat work are machined both front and rear to a definite number of thousandths of an inch clearance. The front side has to be machined high enough to avoid touching the first coat that was applied. A good clearance for much of the authors' work seems to be about 0.003 or 0.004 inch. The rear edge or doctor blade may then be machined to approximately 0.005-inch clearance for applying the second coat; thus it in no way touches the first coat that was applied, but holds the paint in and gives a very uniform second coat.
Description of Machine During the early stages of development, i t was found t h a t drawing doctor blades by hand gave poor results, especially when rather large-sized films were desired. A milling machine carriage and a hydrostatic flow arrangement were tried for producing a uniform movement of either the doctor blade or the base upon which paint was applied. Every turn of the milling machine was recorded in the film in the trials and therefore i t could not be used. Hydrostatic control of t h e movement gave satisfactory results b u t was too cumbersome for general use. Finally, the pull of gravity down a n inclined plane on a fair-sized doctor blade was tried, and proved t o be very effective. Once the inclined flow arrangement ,was
FIGURE 2. DEVICE FOR SUPPORTING PANELS 427
428
Vol. 13, No. 6
INDUSTRIAL AND ENGINEERING CHEMISTRY
TABLE11. DRYFILMTHICKNESS AND RATEor FALL OF DOCTOR BLADE (Various tmee of mints for different elevations of inclined .d a m e.> Elevation of Inolined Plane from Baas 9 Inches 11 Inches 15 Inohes Rate of Film Rate of Film Rate of Film T Y I )of~ Paint fall thickness fail thiokness fall thioknesr Seconds Mila Seconds Mila Seoonds Mils
.5
1.2
FIGURE 3. APPLICATORFOR SPREADING UNIFORM FILM
The standard (Figure 2 ) for holding the stock on which the Elms are t o he applied is made of regular 1 X 1 inch angle iron welded together at the ends. A swivel bar at one end and a hinge
i& at &y
point while riding daw& the incline. Raiising the inclined plane to a height of 9 inches gives a very satisfactory
1.4 1.1
...
... ,..
... ... ...
...
... ...
... ... ...
for some typical surface coating materials as spread on plate glass with a doctor blade of 0,003-inch clearance are shown in Table 11,which a180 includes the time in seconds required by the doctor blade to slide down for various angles of the incline. It is apparent that the higher the incline, the faster the doctor blade flows down and the thicker the paint film produced. The viscosity or plasticity of the paint formulation materially affects the speed a t which the doctor blade falls down the incline (Table 11). To indicate the high reproducibility of preparing films on plate glass, the data of Table I11 are given. These figures show that on good-grade plate glass there is a high degree of reproducibility. If panels are selected the reproducibility will be within 0.1 or 0.2 mil. as noted in Table 111.
Figure 3, thewhole machine is set UP an a stindard, so that a p a . may he used to catch the paint ai the bottom of the incline. This &pparatushas been made available through R. P. Cargille, New York, N. Y. TABLE I. Panel 1 2
3 4 5
6 7 8
PILI4
THICKNESS IN MILS
4 Inches
1.2 1.7
1.8
1.2 1.8 1.7
1.4 1.4
AT ALONG STEEL PANEL
8 Inches 1.3 1.6 2.2 1.4 1.8 2.4 1.4
1.6
12 Inches
1.4 1.7 2.2 1.4 2.1 1.6 1.3 1.5
POINTS MEASURED 16 Inohes 1.6 2.0 2.0 1.3
20 Inohes
1.6 1.6
1.8 1.6
2.1 1.4 1.8 1.2
1.5
1.2
1.6
1.4
FIGURE 4. TYPICAL FILMS TAKENWITH TRANSMITTED LIGHT Left, Rat paint.
Right, outaide house paint
Reproducibility of Apparatus An important feeture in the preparation of uniform films is the base or stock upon which paint is applied. The more uniform t h e surface of the stock, the more uniform the finished films. Good-grade plate glass usually has a variation of about 0.0005 inch, while cold-rolled steel in long pieces is inclined to give a little more variation, some of which is due to springing or bending the steel while handling. When the steel is properly handled better results are obtained, To indicate the reproducibility obtained with 30-inch cold-rolled steel panels eight films were spread on eight different panels, and film-thickness measurements were made every 4 inches along the center of each panel. Results given in Table I are typical values and give the expected reproducibility, considering the large area covered and the cold-rolled steel base. On plate glass dootor blades have given excellent results, as indicated by Figure 4. These photographs were taken with transmitted light and are very uniform. Film-thickness data
FIGURE
5.
U E V I C E FOR MEASURINO 'I'HICKNESS
ANALYTICAL EDITION
June 15, 1941
TABLE111. FILM THICKNESS IN MILS O F FILMSSPREAD PLATE-GLASS PANELS Paint Exterior Interior Exterior Exterior Interior Interior
(Points measured along panels) 4 Inches 8 Inches 12 Inches 16 Inches 2.1 2.0 1 8 2 4 2.1 2.4
2.1 2.0
1.s
2.4 2 1 2.4
2.1 1.9 1.9 2.4 2.2 2.4
2.2 2 0 1 s 2.2 2.2 2.4
ON
20 Inches 2.2 2.0 1 7 2.2 2.2 2.5
429
film. For met film thickness the spindle is screwed through the film to the metal, but for dry film thickness measurements the film is chipped or dissolved away. This makes an inexpensive but fairly accurate means of measuring film thickness. A flashlight bulb, attached for measurement of film thickness on metal, flashes on when the spindle makes contact with the metal panel. Some film thickness measurements were checked with a microscope, shims, and a regular micrometer on stripped films, so they may be considered reasonably accurate.
Acknowledgment The authors have doctor blades that will produce films of many different thicknesses. However, the dried film is always a bit thinner than the clearance of the doctor blade used: A film put on with a 0.003-inch blade may be anywhere from 0.5 to 2.5 mils thick, depending on the formulation of the paint and the perfection of the stock upon which the film is applied. Measurement of the wet and dry film thickness of clear linseed oil films as well as pigmented paints containing no volatile shows t h a t linseed oil shrinks appreciably on drying, which is a factor of importance in formulating. For example, when a paint film containing approximately 30 per cent pigment by volume shrinks 25 per cent on drying, the resultant dried film would contain approximately 40 per cent pigment by volume. The method used for measuring the film thickness is indicated in Figure 5. The micrometer head is brazed to the block, so that the spindle may be readily brought to the surface of the paint
Many thanks are due the Xational Lead Company Research Laboratories for allowing the authors to publish this paper and to A. Stewirt, director of research, under whose supervision the work developed.
Literature Cited Arlt, H. G.. Bell Labs. Record, 14, 216 (1936). Brier, J. C., and TTagner, A. M., IND. ENG.CHEW,20, 759 (1928). Bruins, P. F., Ibid., Anal. Ed., 9 , 376 (1937). Parks, A. M.,“Physical and Chemical Examination of Paints”, H. 8.Gardner, 9th ed., p. 99, Washington, Institute of Paint and Varnish Research, 1939. Scofield, F., National Paint, Tarnish and Lac.\xuer .issoc., Circ. 593
(Jan., 1940).
Stoppel, E. A., Proc. A m . SOC. Testing Materials, 23, Part I , 285 (1923). Strieter, 0. G., Bur. Standards J. Research, 5, 248 (1930). Walker, P. H., and Thompson, J. G., Proc. Am. SOC.Testing Materials, 22, l l , 465 (1922). PRESENTED before the Division of Paint and Varnlsh Chemistry a t t h e 100th Meeting of the American Chemical Society, Detroit, Mich.
Graphite Heating Baths WM. I. HARBER’, The Edwal Laboratories, Chicago, Ill.
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HE heating of glass flasks used in distillations, extractions, and reactions has always presented a problem.
For this purpose, open flames, hot plates, or liquid baths are used at present in the laboratory, b u t these methods either present serious fire hazards or are inefficient. Some year3 ago, the author undertook to run a series of ammonolysis reactions at high temperatures (330’ C.) for extended periods ( I ) . The use of oil baths with their disadvantages of decomposition and fire hazard was precluded. Metal baths were found to form mossy clumps after a while with attendant loss of material. A glance at some of the more authoritative laboratory manuals showed no discussion of graphite as a heating medium, but the possibility of its use was mentioned by LassarCohn ( 2 ) . A powdered graphite was tried as a heating bath material, and was found suitable for both reactions and distillations, especially where high temperatures were required-. g., the fractional distillation of high-molecular-weight fatty compounds. Heat conductance of a properly proportioned bath was excellent; once a desired temperature was reached there was no difficulty in maintaining it. The utility of this material in ordinary laboratory operations was amply confirmed by its increased use among the author’s colleagues. The technique in the use of graphite baths is the same for ordinary laboratory distillations as for large-scale operations, where graphite has been found to be an excellent heating bath 1
Present address. Swift and Co., Union Stock Yards, Chicago, I11
material. .4n iron container is selected of such a size as to be only slightly larger than the flask t u be heated, and enough graphite is placed in the container to form a floor for the flask, a 0.25-inch layer being ample. The flask is placed on the layer of graphite and held in position by the conventional system of clamps. Enough graphite is added to surround the flask, after which the material is tamped to form a tight packing. When it is desired to note the bath temperature, a thermometer is suspended near the wall of the flask, with the bulb at the he’ ht of the lower third of the flask. The bath is heated with e a e r a Bunsen or M6ker burner, depending upon the temperature desired. Variation in size of the graphite particles produced no significant difference in efficiency of heating. The flake size (Dixon No. 1, large flake) seems best suited, for i t does not soot and is cleaner to handle. The cost of graphite and its permanent quality encourage its use for both ordinary and large-scale laboratory operations.
Acknowledgment The author wishes to thank E. S. Glauch of The Joseph Dixon Crucible Co., Jersey City, X. J., for supplying liberal samples of various grades of graphite for experimental purposes.
Literature Cited (1) Harber, doctoral dissertation, “High-Molecular-Weight Aliphatic Amines and Their Derivatives”, pp. 75-9, Library, Iowa State College, 1940. (2) Lassar-Cohn, “Organic Laboratory Methods”, English tr., p. 24, Baltimore, Williams and Wilkins Co., 1928.
