Section Barytes FilmSection Barytes Film
Section Barytes Film
A view of these sections of paint films under the microscope gave to the operator a better idea of the structure of a paint than had ever been afforded heretofore. It was easy to perceive the relative position of the pigment particles and the three coats. The penetration of one coat into another was easily discernible, and measurements were made upon the sections in order to determine the average thickness of coat and its general appearance.
Sections were also made of Inside and Outside White upon wood. These sections revealed under the microscope the thickness of the coats and also the penetration of the priming coat into the wood. Appended is atablegiving microscopic measurements.
Polar Micro-Examinations and Photomicrographs.By Polar Micro-Examination is meant the examination of pigments under polarized light. A polarizing apparatus, though not an essential in the hands of the paint chemist, is nevertheless much to be desired, for by its help deductions may be drawn as to the contents of a paint, which by other means might not be possible. The polarizing apparatus as marketed by most manufacturers of the microscope is attached in the following manner:
The diaphragm immediately under the sub-stage container is swung out and opened to its widest limit, allowing the insertion of the polarizer. This polarizer carries one of the pair of Nicols prisms and is countersunk to allow of the introduction of gypsum or selenite plates. The analyzer fits over the eyepiece on the tube.
The use of polarized light upon paint is valuable on account of its action upon crystalline substances. The re-enforcing pigments, such as Asbestine, China Clay, Gypsum, Silex, Barytes, etc., are crystalline and consequently act upon the polarized light. In most cases these pigments are used in ready-mixed paints in small amounts, varying between 5 and 25%. When a slide containing a small amount—for example, less than3%—of these crystalline pigments is examined under the microscope by ordinary transmitted light, they will often escape observation, owing to the small amount in which they are present. However, in the case of polarized light, this could hardly happen.
Barytes under Polarized LightMicroscopic View of Barytes under Polarized Light
Microscopic View of Barytes under Polarized Light
A slide of paint containing these re-enforcing pigments is prepared in the usual manner. On examining this under the microscope and using the polarizing apparatus, the crystalline pigments are at once detected by revolving the analyzer. At one position of the analyzer, one sees an ordinary field, as with transmitted light, but if one revolves the analyzer, the field gradually becomes darker until total darkness is obtained throughout, except in such places where crystalline substances are present, when the crystal is shown up with beautiful distinctness. Photomicrographs of various single pigments and pigment combinations are shown underChapter III.
Effect of Pigments on Oil.Certain pigments have the property of acting upon the linseed oil in which they are ground, forming metallic linoleates which accelerate the drying of oil. This is especially true of lead and zinc pigments. The inert crystalline pigments, when ground in linseed oil and painted out, distribute the oil so as to allow a great surface to be exposed to the air. Thus by physical action, and possibly catalytic or contact action, these inert pigments stimulate the drying of oilpaints in which they are ground. Lead and zinc paints, of course, have the greatest drying values on account of the added effect of the linoleates formed, as outlined above. The writer has made a series of tests in which the action of various pigments upon linseed oil is shown. The tests were made in the following manner:
Five grams of each of a series of commonly used paint pigments, including those of inert crystalline nature as well as the more valuable amorphous pigments which are considered more or less chemically active, were ground separately in an agate mortar, with 5 grams of raw linseed oil. The ground paste in each case was placed in a marked glass beaker, and allowed to stand in a dustless section of the laboratory for one month. The oil-pigment paste from each beaker was then separately extracted with benzine to remove the linseed oil from the pigment. The benzine solutions of oil were then heated to remove the benzine and the residue of oil burned to ash in crucibles. The ash from each test was weighed, and if it ran above the percentage of ash determined on a blank sample of linseed oil (namely, .003%), the ash was analyzed qualitatively for metallic constituents. The following table of results shows the percentage increase in ash, as well as the constituents of ash on the various samples tested:
Observation of these results shows that pigments such as Barytes, Blanc Fixe, and Silica have no chemical action on the linseed oil. The results on Asbestine and China Clay also are negative, the extremely slight increase in amount of ash from these samples probably being due to traces carried over mechanically into the oil mixture; the last named pigments being more fluffy and difficult to separate from oil. Slight action seemed to be apparent in the case of whiting, a pigment somewhat alkaline in nature. A longer test might have shown this pigment to have possessed still greater action. Corroded white lead showed considerable action, resulting in the formation of lead linoleate or some other organic compound. Zinc oxide and lithopone, the latter pigment containing 30% of zinc sulphide, both indicated action on the oil. Chrome yellow (chromate of lead) showed some action, as did also Prussian blue, the ash from the last named pigment showing a heavy percentage of iron oxide.
Red Lead showed a most astounding gain in these tests, chemical action of the pigment on the oil being apparent soon after the tests were started, as shown by the formation of a hard cake with the linseed oil.
The Raw Linseed Oil which was used in these tests had an acid value of 1.84%, which is very low. The neutralization of this free fatty acid by some of the alkaline pigments used, may account for part of the increased percentage of ash, but in most cases the pigments, and more especially the basic pigments, had a direct saponifying action upon the glycerides of the oil.
Laws of Paint Making.To secure a proper comprehension of the composition of paints, and to be able to interpret the functions of their various constituents, requires an understanding of the general physical principles involved. The modern grinder has accepted the Law of Minimum Voids, and upon this law he bases the design of paint formulæ, aiming toward the production of what have been properly termed Scientifically Prepared Paints. Perry’s formulation of the Law of Minimum Voids in a paint coating, and the analogy which he has drawn between a scientifically prepared paint and a well-proportioned concrete, was the result of genuine scientific thought following observation and experimentation. It must be admitted that analogies are not always safe to draw conclusions from, but it surely is no fallacy in reasoning to draw analogies between these two materials, when they resemble each other in so many ways. To carry out processes of reasoning, and to formulate laws from such close analogies, is certainly a step in the right direction.
A graphic summary of the analogies between a properly proportioned concrete and a paint, are shown on next page.
Although this table graphically summarizes the principles involved, the matter is presented with greater clearness in the following:
Law No. 1—The law of minimum voids to be observed in constructing a paint formula—this law having already been accepted as mathematically correct and technically proved in the technology of concrete and cement.
Corollary—The requisite thickness of a paint film together with the utmost attainable strength and impermeability can best be obtained by a properly proportioned blend of pigments of three or more determinate sizes.
Law No. 2—The law of the flat arch in paint coatings—i.e., the fact that in studying the fundamental physical principles governing the strength and durability of a paint coating it is necessary to regard the coating as consisting of a series of flat arches, in which the pigment particles of largest characteristic size serve as the piers or supports for the flat arches of which the continuous film is composed.
Corollary A—The strength and durability of a paint coating is determined by the strength and durability of the piers or supports (which consist of the characteristic pigment particles of the largest size).
Corollary B—Owing to their inherent strength and durability the pigment particles of largest characteristic size which serve as supports for the paint coating should consist, in part at least, of chemically inert pigments, such as natural crystalline barium sulphate, calcium carbonate, magnesium silicate, etc.
Corollary C—It follows directly that the thickness of a paint coating is determined by the particles of pigments having the largest characteristic size, even if that pigment be present only in moderate percentage. Upon this principle depends the comparatively great thickness of film and moderate spreading rate of paints composed of such pigments as basic carbonate—white lead, red lead, barytes, etc., and the strongly contrasted thinness of film and high spreading rate of paints composed of the sublimated pigments such as lamp black, zinc oxide, basic sulphate—white lead, zinc-lead white, leaded zinc, etc.
In commenting upon the announced laws set forth above, Heckel says: “The recognition of these laws was an exercise of pure deduction. Paint manufacturers before Mr. Perry’s announcement were producing paints containing three or more pigments with particles of varying characteristic sizes; but their procedure was based largely on empirical knowledge, the result of accumulated experience, due to a conscientious endeavor to produce the highest type of paints for economic service. In the absence of any law to govern or to limit the use of the reinforcing pigments, inexperienced manufacturers had brought upon the market paints which were badly proportioned as to the several pigments, or burdened beyond the limits of effectiveness with reinforcing pigments. To all paint manufacturers Perry rendered a substantial service in deducing for them the laws set forth in his address. In the results following a recognition of these laws there was nothing new or startling, but Perrywas the first to give the principles from which it can be determined in advance whether a paint formula will prove to be physically good or bad in practice.
Paint Chasers, Mixers and GrindersSeries of Paint Chasers, Mixers, and Grinders
Series of Paint Chasers, Mixers, and Grinders
Overhead Churn MixerOverhead Churn Mixer
Overhead Churn Mixer
Battery of Modern Underdriven Paint Mixers and GrindersBattery of Paint Mixers and Grinders of Modern Underdriven Type
Battery of Paint Mixers and Grinders of Modern Underdriven Type
Shrinkage of Paint Pigment after Grinding in OilPhotographs courtesy of Ernest HeathView showing Shrinkage in Bulk of Paint Pigment after being ground in Oil. Filled Barrel on Right with the Oil forms one-third Barrel Paste as shown in Barrel on Left
Photographs courtesy of Ernest Heath
View showing Shrinkage in Bulk of Paint Pigment after being ground in Oil. Filled Barrel on Right with the Oil forms one-third Barrel Paste as shown in Barrel on Left
Careful Dressing of Bull Stone Mill from GrinderView showing careful Dressing of Bull Stone Mill from Grinder
View showing careful Dressing of Bull Stone Mill from Grinder
“As has been before stated, he was not the first to recognize the law governing minimum voids, but by that scientific use of the imagination which Tyndall so highly commends, he recognized, as by inspiration, the fundamental similarity existing between a film composed of solid particles cemented together by a semi-solid homogeneous menstruum and a layer of concrete composed of solid particles cemented together by a solid homogeneous medium. His application of the law permits the paint manufacturers to design a paint formula with full knowledge of the controlling conditions, so that it shall produce a coating neither too thick, and therefore uneconomical and subject to excessive internal strains, nor too thin, and thus weak and inefficient for protection. That Mr. Perry’s contention was well-founded, other paint technologists have since demonstrated; notably Mr. Wirt Tassin, in his microscopic studies of paint films in situ, and Prof. G. W. Thompson who, in his address to the Penna. Association of Master Painters at Reading, said:—“I want to agree with Mr. Perry * * * where he says that a pigment should be made up of particles of different sizes. Mr. Perry also draws a further parallel between paint and concrete where he refers to the form of the reinforcing pigment particles and suggests that in paint coatings as in concrete a field can befound for the chemically inert pigments with rod-like or hair-like structure, to strengthen the film, just as the steel rods and iron mesh are used to reinforce concrete in structural work—a suggestion which, since the first publication of the address, has been widely accepted as a practical aid in the manufacture of good paints.”
Use of Inert Pigments.There seems to be no reasonable doubt as to the efficiency of a small amount of inert pigments in paint, and the writer has often compared the manufacture of paint of the above type to the making of various alloys wherein zinc, copper, and other metals are added to gold in order to make a product possessed of greater durability, etc.
Color Grinding MillsBatteries of Color Grinding Mills
Batteries of Color Grinding Mills
There has been considerable inquiry as to just what is meant by the statement that “a moderate percentage of inert pigments, combined with properly adjusted mixtures of white lead and zinc oxide, have given wonderful service in all the tests.” The writer has been asked to define what “moderate” means. A “moderate percentage of inert pigments” should be defined as that amount of natural crystalline pigments that will, when mixed with white lead and zinc oxide, not materially detract from the hiding power of white lead and zinc oxide. It is possible to mix a certain percentage of these crystalline pigments with white lead and zinc oxide, and, by thorough grinding, incorporate them in such a manner that the mixture will show nearly as good a hiding power as the straight white lead and zinc oxide.When certain limits have been reached, however, and these limits must be determined by the manufacturer and painter in making practical tests, the further addition of inert pigments lowers the hiding power of the paint and therefore lowers the value of the paint. These remarks do not apply to artificial crystalline pigments, such as precipitated whiting, which possess greater hiding values than the natural pigments.
Perry’s Principles of Paint Making.Parts of the original paper[15]in which Perry so clearly set forth the principles from which the preceding laws were formed, follow:
[15]Physical Characteristics of a Paint Coating. R. S. Perry. Michigan Chapter, Amer. Institute of Architects, 1907.
[15]Physical Characteristics of a Paint Coating. R. S. Perry. Michigan Chapter, Amer. Institute of Architects, 1907.
Sealing Quality or Imperviousness of the Coating.“It has been emphasized that for durability and protection, the strength and imperviousness of a paint coating are vital factors. The protective value of the paint coating of course ceases with its chalking or disintegration, but, while it is true that the protecting or final life of the coating ceases with this disintegration, it is also true that a paint coating has always during its true life more or less porosity from the nature of the linoxin or oxidized linseed oil. Therefore during its protecting life the degree of its imperviousness influences its resistance to attack upon its own life and its protection of the underlying materials. The more impervious the paint coating without loss of strength, the slower the oxidation or disintegration of the paint coating itself and the greater protection to the underlying material.
“A coating of linseed oil alone is not only weak, but the simplest and crudest experiments will show its porosity and this porosity increases rapidly with progressive oxidation, the porosity of course definitely hastening the over-oxidation or chalking. In proportion, therefore, to our success in filling the voids in the linseed oil film with proper pigment materials, we will in that degree succeed in excluding agencies of decay, not only from the mass of the paint coating itself, but also from the surface to be protected. These conditions are exactly parallel in the requirements and performance of the best-made concrete, and Taylor & Thompson in their work on concrete have clearly stated that to obtain imperviousness there must be freedom from voids, and that to obtain these conditions, the materials used must have at least three determining sizes.
Voids between Larger and Smaller ShotEqual Volume (One Cubic Centimetre) of Each Size of Shot Taken. Note that the Smaller Shot Cover more than Half as much again as the Larger Shot and the Voids are Smaller.
Equal Volume (One Cubic Centimetre) of Each Size of Shot Taken. Note that the Smaller Shot Cover more than Half as much again as the Larger Shot and the Voids are Smaller.
Two Determining Sizes of Solid Particles in ConcreteDiagram Illustrating Two Determining Sizes of Solid Particles in Concrete
Diagram Illustrating Two Determining Sizes of Solid Particles in Concrete
Three Determining Sizes of Solid Particles in ConcreteDiagram Illustrating Three Determining Sizes of Solid Particles in Concrete
Diagram Illustrating Three Determining Sizes of Solid Particles in Concrete
“‘It is a fact that with particles of different sizes as against uniform size the densest mixture can be obtained. This is so evident as to require no proof.’ It follows that the least density and hence the largest percentage of voids occur when the grains are all of the same size, and it is shown that the most voids occur in a mass of large particles. The least voids occur when the voids between the large particles are filled with smaller particles and when these smaller voids between the smaller particles are in turn filled with still finer particles. In other words—particles with three determining sizes will fill up a given space more completely than particles of two determining sizes and very much more completely than particles of one size.
Elasticity and Strength.“The paint coating here again is governed by many of the laws which govern the similar material, i.e., concrete. We find, by again referring to Taylor & Thompson, on Concrete, page 275, that tests at the Watertown Arsenal on concrete convinced the investigators that the ultimate strength of a concrete is identical with the shearing strength of particles of stone making up the aggregate.
“This means that in its ultimate form the good concrete will crack or shear through the broken rock contained therein, and resistance to shearing is directly proportionate to the strength of the broken rock chosen for the mixture. The film of semi-liquid linseed oil when fresh is extremely weak, but as it hardens, its characteristics and physical properties will obviously be those qualities which are a composite of the qualities of the solid particles and of the semi-solid linolein incorporated together in the paint coating. These physical properties of the suspended and incorporated pigments profoundly modify the film in this respect.
“The dried vehicle, linoxin, is notable for its elasticity, and it is weak in crushing and tensile strength, and in hardness or resistance to surface wear. The fact that it is a semi-solid furnishes an opportunity to modify and improve those characteristics of a solid in which it is deficient. The semi-solid, rubber-like linoxin between the coarser particles of the pigment obviously uses these coarser particles as supporting points. The medium sized particles of the second group of alteration products serve the same purpose as the broken rock in concrete. The coarser particles absolutely do not, and can not, serve the purpose of stiffening or of reinforcing or modifying the consistency andqualities of the semi-solid linoxin, for a number of reasons, one of which may be mentioned, namely, that particles of the first, or coarse, class have a determining size which is a large fraction—a heavy percentage—of the total thickness of coating, and are in some instances thicker in diameter than the thickness of an oil coating not reinforced with the fine or fire group.
“We must think of the coarser particles as piers. The mixture of linoxin with the other two groups of particles in the spaces between these coarser particles, or piers, is the true paint body and consists of flat reinforced arches which have the extra support of falsework, in the shape of the structural material on which the coating rests. Asbestine pulp, a natural product and one of our most important natural reinforcing pigments, serves not only in the coarse group as supporting particles for the linoxin arch, but also because of its peculiar properties serves the more important purposes of reinforcement. It retains, no matter how finely ground, its peculiar needle-like, or rod-like, form of particles, and obviously serves the purpose of reinforcing the flat arch of linoxin, exactly as iron bars or iron netting serve in reinforced concrete arches. The medium sized particles of the second group of pigments produced by chemical alteration or precipitation, serve the purpose of the broken rock in concrete, and together with the coarser supporting particles and the finest reinforcing particles, give minimum voids and a maximum imperviousness to agencies of internal decay.
“It goes without saying that the pigments of any one group contain particles of dimensions which fall into the other two groups, but no one pigment supplies the correct proportion of each of the three required dimensions, and each pigment has so large a percentage of approximate dimensions as to bar it from exclusive use in the other two groups. Given similar homogeneous coatings under identical conditions, we recognize the law that elasticity will vary directly with thickness. Direct deduction from this law teaches us that of two paint coatings equal in wear, in strength, opaqueness, and in all other qualities except thickness, we should choose the thinner coating. Therefore if we have two paint coatings fulfilling every requirement, the first compounded with pigments giving a thicker coating and the second with pigments yielding a thinner coating, we must choose the second formula and obtain the thinner coating.
Adhesive Power.“The adhesion of the linoxin to the coarse group of particles and to the underlying material is vital to the life of the paint coating. If the coating parts from the surface beneath, we have scaling or peeling. It is universally admitted that this will result from use of zinc oxide as the sole pigment. We have only to conceive of our flat arch of reinforced linoxin and leave out our points of support, to realize that this is the inevitable result if the coating be subject to extreme exposure, although good results may be obtained from zinc oxide used alone, as, for instance, in interior house painting where extreme changes of temperature and exposure are avoided.
“Three major lines of force hold our linoxin in place—adhesion toward the underneath surface, adhesion to the coarse particles, and cohesion within the linoxin itself. These lines must be represented by a flat arch of linoxin with a downward pointing magnet therefrom, to represent adhesion to the surface. Magnets on each side of the arch pointing toward the supporting coarse particles, and two magnets within the arch and pointing toward each other, or to the centre of the arch, these latter to represent the force of cohesion.”
The Pigment Contention.During the year 1906 officials of the North Dakota Agricultural Experiment Station examined a number of paints on sale in the northwestern States. The presence of large quantities of inert pigments as well as water, in some of these paints, prompted agitation for State laws requiring the formula-labeling of paints. Certain paints made of white opaque pigments such as white lead and zinc oxide were exempted from the statute. The white opaque pigments used in these paints were believed by certain manufacturers as well as by many prominent paint authorities of high standing to be benefited in their wearing value by the addition of small percentages of inert crystalline pigments, such as barytes, silica, China clay, etc. Laboratory experiments had already determined that these inert crystalline pigments had a certain definite action in increasing the life of paints, but it had become evident that they should be used with discretion, in moderation, and with a proper understanding of their limitations, if the best results were to be obtained. The addition of very large quantities of such pigments was not indulged in by discriminating manufacturers, but the exact percentage to use was a matter of great doubt, even to the most experienced. In order to determine just what percentage of crystalline pigments, admixed with white opaque paint pigments, would give the best service and results, it seemed imperative that practical paint tests should be made. A series of paint tests on commercial brands of paint had already been started at the Fargo Agricultural College, and, at the suggestion of the Paint Manufacturers’ Association of the United States, another series of practical paint tests were instituted, and carried out under the supervision of Dr. E. F. Ladd, Director of the North Dakota Experiment Station.
Test Fences to Solve the Problem.It was apparent that the pigment question could be solved only through field testsmade on a comprehensive basis and placed under the control of scientific and technical societies of renown, so that they might be fair and unbiased from every standpoint. In order to secure a comparison of the wearing of different paint formulas in various sections of the country and under differing climatic conditions, another series of tests was started in the East soon after the North Dakota tests had been started. Simultaneously fences were erected at Atlantic City, N. J., and Pittsburg, Pa. The site of the Atlantic City fence is a strip of land running due north from Atlantic and Savannah Avenues and within a short distance from the Atlantic Ocean, the exposure being a severe one. The site of the Pittsburg fence is back of the athletic field of the Carnegie Technical Schools, the fence running east and west and being exposed to the heavily charged sooty atmosphere coming from the many industrial plants near by.
Construction of Framework of Fences.At these two locations framework fences were built, upon which were placed a series of painted panels. Heavy yellow pine posts six inches square were set in the ground about six feet apart and to the depth of about four feet, upon a concrete base. The posts were solidly tamped and then braced at the top with supplementary studding braces two inches thick. Connecting the posts was a line of studding six inches by two inches, forming a solid framework, the bottom of which was approximately fifteen inches from the ground. The bottoms and tops of the fences were protected by heavy boards two inches thick, so that the moisture and rain might be prevented from working itself up into the wood. The whole fence was sheathed with twelve-inch planed white pine, thus forming a solid background for the test panels.
Lumber for Panels.The lumber for the test panels was most carefully selected, being of three grades—white pine, yellow pine, and cypress. A large amount of each grade of lumber was secured, and after the best portion had been made up into panels, the panels were inspected by an expert lumber classer; nearly 40% being rejected on account of the presence of knots or sappy places which appeared upon the surface. Each of the panels finally passed upon as suitable for the test was branded with a hot iron with consecutive numbers running from 1 to 186. The grade of wood used for each panel was indicated by an abbreviated mark—W for white pine, C forcypress, and Y for yellow pine. In order that a record of each panel might be kept on file, previous to the application of paint to the panels, a complete series of photographs was taken of the panels in sets of four. This work seemed advisable so that the future failure of paint on any one panel, which might be thought due to faulty wood, could be either verified or refuted by a reference to the series of photographs made of the bare panels.