Two-Coat System of House Painting - Industrial & Engineering

Two-Coat System of House Painting J

F. L. BROWNE Forest Products Laboratory, Madison, Wis. The history of two-coat painting for new exterior woodwork and of two-coat systems of painting is reviewed. Distinction is made between (a) twocoat initial painting with self-priming (one paint used for both coats), ( b ) two-coat painting with special primer and finish paint made far application in thick coats, and (c)two-coat painting with special primer but with conventional prepared paint for finish paint. Experimental data are presented to show that, with a given paint, durability depends on the thickness of coating whether the thickness is provided by two thick coats or by three thinner ones. When modern paints of high opacity are used, the thickness of coating needed for good durability is much

greater than that required merely to hide the wood. Two-coat painting of types a and c often gives rise to short-lived paint jobs because the necessity of building satisfactory film thickness has not been fully appreciated by paint manufacturers, painters, and paint users. Tw-o-coat systems of type b are less likely to be applied too sparingly. The experimental data demonstrate a superiority of well-designed special primers over self-priming for painting such woods as Douglas fir and southern yellow pine. But they show also that primer and finish paint should be designed carefully for use together because a primer may give good results with some finish paints and prove incompatible with others.

PRIMING COAT. The first coat of paint applied in initial painting. PRIMER.The paint used for the priming coat. SELF-PRIMINQ.Use of the same paint for primer and for subsequent coats exce t that the aint may be thinned differently with oil and volatig for the digrent coats. SPECIALPRIMER.A paint used for the priming coat over which some other paint is used for finish coat. UNDERCOAT. A coat of paint applied over previous paint and to be itself covered by at least one further coat in the same paint job. The previous paint may be a priming coat in the same paint job or, in repainting] the coating left from the previous paint job. The undercoat is never, in the writer’s definition, applied to bare wood or other supporting material and is never the last coat of a paint job. The terms “body coat” and “intermediate coat” have been used by others in the significance of undercoat, and some call the first coat in repainting a “priming coat”. UNDERCOATER.The paint used for the undercoat. FINISH COAT. The last coat applied in any one paint job. FINISH PAINT. The paint used for finish coat. TWO-COAT SYSTEM.Essentially synonymous with “two-coat work” but used articularly when the two-coat work departs in some way from oyder practice. The principal applications are to two-coat initial painting with either self-priming or special riming and to two-coat repainting with special priming. gome writers do not consider two-coat initial painting with self-priming to be a two-coat system. The present confusion in use of the term might have been avoided if painting with two kinds of paint had been popularized under the name “two-paint system”.

T

HE two-coat system of house painting during the past decade has passed from a practice ignored or condemned by the industry to a recommendation made by a majority of paint manufacturers; some of them advocate i t enthusiastically whereas others accept it in public but regard it in private with misgiving or disapproval. Much painting with two-coat systems has been thoroughly satisfactory and much of i t has proved seriously lacking in durability ( I S ) . The successful work demonstrates that a substantial economy is realizable from the practice. The unsuccessful work demonstrates that the conditions necessary for satisfactory two-coat painting are not yet understood thoroughly by all manufacturers and users of paint. Definitions of Terms Some terms that commonly appear in discussions of painting are ambiguous because they have not been defined authoritatively and are used variously by different writers. The sense i n which this writer uses them is therefore set forth in the following paragraphs. The definitions, however, represent merely the writer’s own preferences which may not be shared by other workers in the field. PAINTJOB. All of the operations of applying one or more coats of paint within a comparatively short time and covered in the typical case by a single contract with a painter. The term is also applied to the layer or layers of paint in position on the surface after the job is completed. INITIAL PAINT JOB OR INITIAL PAINTINQ. The first paint job on a bare surface of wood or other supporting material. The bare surface may be a newly erected one or an old one from which all previous paint had been completely removed. REPAINT JOB OR REPAINTINQ.A paint job on a previously painted surface from which the previous paint has not been removed completely. PAINTCOATINQ.The entire amount of paint in position on a surface. On a surface that has been painted several times, the paint coatin consists of several paint jobs in successive layers. COATOF ~ A I N T . A single layer of paint spread in the liquid condition at one time and allowed to harden. A paint job is usually made u of two or more coats of’paint. TWO-COAT $OR= OR TWO-COAT PAINTING. A paint job in which two coats are applied. There is, of course, one-coat work, three-coat work, etc.

Meticulous adherence to the distinctions between priming coat and undercoat and between undercoat and finish coat sometimes becomes cumbersome. For example, it is occasionally convenient to speak of applying two coats of finish paint over a special primer or of repainting with a special primer and a finish paint. I n commercial practice special primers are called variously “primer”, “primer-sealer”, “undercoater”, “primer and undercoater”, “foundation coat”, or are given trade names that strongly suggest the use for which they are intended.

History of Two-Coat Initial Painting For a t least half a century prior to the open recognition of the two-coat system, two-coat initial painting was regarded as cheap work and false economy, though much of i t was un900

July, 1941

INDUSTRIAL AND ENGINEERING CHEMISTRY

doubtedly done. Accepted practice required a minimum of three coats for initial painting and two coats for repainting. House paints were designed and recommendations for using them were based on the assumption that these standards would be observed. I n the course of long experience and competition, paints and painting procedures were adjusted so that three coats would supply sufficient paint to ensure satisfactory results but would avoid any wasteful excess of paint. Serious rebellion against these standards began about 1920 when a boom in house construction following a long stagnant period found labor costs far higher than they had ever been before. Speculative builders resorted to two-coat work to reduce cost of application, often with the frank recognition that temporary paint jobs which would require repainting within a year or two were all that was expected. Good initial appearance with uniform gloss and reasonable opacity were essential, but painters soon.found that they could not provide a uniform gloss and reasonable opacity by merely eliminating the undercoat of three-coat work. The thin priming paints characteristic of the old three-coat system do not seal wood effectively against absorption of oil from the next coat of paint. I n addition, the level of opacity of the paints of 1920 was such that any material diminution in thickness of coating resulted in failure to hide the wood acceptably. To meet the demand for two-coat initial painting, therefore, painters had to invent a new technique of application in which the full amount of paint previously applied in three coats was put on in two by greatly reducing the spreading rate of both coats. The new technique was one of painting in thick coats. The economy was effected entirely in labor, not in material. The greatly diminished spreading rates, however, involved the further requirement that the paint for both coats be unusually high in content of total pigment so that the consistency would be favorable for application in thick coats without running and sagging and without wrinkling during drying. The painters could make the necessary paint mixtures with paste paints, such as lead-in-oil, but the prepared paints of 1920 were generally too low in pigment content to be suitable for two-coat initial painting. We may call this technique of initial painting the two-coat system with self-priming. When done honestly by painters who had mastered the technique, it proved far more than a temporary expedient and compared favorably in durability with good three-coat work. When done incompetently, the shortcomings were usually apparent before the painter could escape responsibility for them. As a result the practice spread beyond the field of speculative building and gained wide acceptance. Paint manufacturers slowly began to recognize it by printing directions for two-coat initial painting on their labels, first on paste paints and later on prepared paints as well. Usually these directions indicated that twocoat work was considered less satisfactory than three-coat work, and the directions called merely for elimination of the undercoat of three-coat work without changing the thinning for either primer or finish paint and without pointing out the need for lower spreading rates in two-coat work.

History of Special Primers The three-coat system from long tradition was generally considered a one-paint system; that is, self-priming was the accepted practice. The industry was long opposed to special primers for house painting in spite of their general use in painting metal, in enameling woodwork, and in much industrial finishing. Objections to special primers for house painting arose from the fact that the industry had waged a

901

long fight against the use of inferior materials, such as yellow ocher paint, cheap paint, and odds and ends accumulated about the paint shop, as priming coats for house painting. Favorable consideration of special primers for house painting began with the purpose of improving the durability of paint on the heavier softwoods in which the wide bands of summerwood lead to relatively early crumbling or flaking of coatings (4,7,21,29,Sd, 33, 34) or of improving the resistance of coatings to moisture blistering (33). Interest in adapting them to two-coat initial painting arose somewhat later. For holding coatings intact over summerwood, aluminum priming paint made with a suitable vehicle has proved outstandingly satisfactory (4, 6 , 7, 18, 21) but has been regarded with disfavor by most paint manufacturers and is unsuitable for twocoat systems because of its color. The studies made of special primers overthrew the traditional theory that priming paint for wood should be low in content of pigment and high in content of volatile thinner in order to promote deep penetration into the wood. Deep penetration, in fact, proved to be wasteful and even harmful because it consists of vehicle alone, separated from the paint film, and isolated in the cavities of wood cells beneath the painted surface (14, 24, 32, $3). Useful penetration is limited t o the filling with paint-i. e., with pigment and vehicle together-of those wood cavities that open directly into the surface on which the paint is applied. Beyond that point pigments cannot penetrate because the openings between cavities are too small to permit their passage. When used according to the self-priming technique, ordinary house paints penetrate wastefully and can be improved by restraining undue penetration. The principle of designing primers to restrain wasteful penetration of vehicle has become known as controlled penetration, a term now firmly established because of its use in advertising (38). One means of controlling penetration is the incorporation of bodied oil in the vehicle (17, 28, SS), a fact that was disclosed some years before its application in special primers was realized (16). Another means that seems to be less widely appreciated is formulation with a high level of pigmentation favoring application at low spreading rate-that is, in thick coatings (6, 29). A thick layer of pigment, especially if made up of pigments that adhere strongly to the vehicle, exercises a capillary competition with the wood and holds more of the vehicle in the paint. Most of the modern special primers make use of both of these methods, though some of them rely more on bodied vehicle than on high pigmentation. Complete elimination of zinc oxide from special primers is recommended by one school of thought on the subject (29,31) and is opposed by another (32, 34). Most of the primers now on the market either contain no zinc oxide or contain much less than is used in the finish paint sold with them, but one of the early primers on the market contains much more zinc oxide than the finish paints sold with it. Conclusions about the use of zinc oxide in special primers must be subordinated to the more fundamental problem of compatibility between primer and finish paint. The finish paints on the market differ so widely in properties (19, 96, bo, 31) that no one primer can be expected to prove compatible with all of them. When the finish paint contains no zinc oxide, the primer should be zincless (8); on the other hand, some zincless primers produce alligatoring and intercoat flaking of finish paints containing zinc oxide, which is corrected when zinc oxide is incorporated in the primer also (29, 32). Leaders of both schools of thought concerning the place of zinc oxide in primers agree that compatibility is a dominant consideration and that the offering of any one primer for general use under all finish paints cannot be expected to give the best results in all cases (29, 34). For that

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Vol. 33, No. 7


reason a special primer and a finish paint, as a rule, should be manufactured for use together as a painting system. Since the special primers with controlled penetration lend themselves readily to a high level of pigmentation favoring application in thick coats, may be made with a high content of opaque pigment, and offer great protection against spotted chalking and fading, they were proposed as a particularly effective means of meeting the demand for two-coat initial painting (29, S I ) . The proposal contemplated special formulation of the finish paint for application in thick coats and the offering of primer and finish paint together as fully equal or superior to the older three-coat painting. The term “twocoat system” as first advertised t o the public (17) meant a primer and finish paint of this kind. The directions for initial painting giveh on the label described two-coat work only and limited thinning of either product to very moderate additions of oil or volatile, if any. Most manufacturers were unwilling t o abandon the threecoat system t o commit themselves fully to two-coat initial painting. Some tried offering a two-coat system with special primer and finish paint, in addition to their older line of prepared paint, but this overburdened dealers’ paint stocks and confused sales programs. The compromise generally adopted is to add only one new product, a special primer with controlled penetration, to the regular line of prepared paint. The primer and the prepared paint together are then offered as a two-coat system. I n such twocoat systems, however, the primer alone takes the entire burden of making up for the elimination of an undercoat. That it cannot reasonably be expected to do so is indicated by the fact that so many of the manufacturers’ directions express a preference for three-coat work. The special primer supplementing a conventional prepared paint leads to exceedingly complicated directions for application. Some paint labels now attempt t o describe as many as four methods of initial painting, (a) three-coat work with selfpriming, (b) three-coat work with special primer or with the special primer used for both primer and undercoat, (c) twocoat painting with special primer, and ( d ) two-coat painting with self-primer. As a rule, no difference in thinning is directed for two-coat and for three-coat work, and nothing is said about changing the spreading rates. Careful attention to spreading rate in its relation to thickness of coating is now more necessary than it was in 1920 because of the much greater opacity of modern paints. I n the two-coat painting, with self-priming of 1920, failure to apply enough paint usually betrayed itself by incomplete hiding of the wood. With many special primers and finish paints of the present time, however, satisfactory hiding can be obtained with only half the thickness of coating necessary for good durability, Hiding, therefore, can no longer be relied upon to gage adequacy of film thickness. Moreover, with the two-coat painting of 1920 the painter dared not stretch his paint too far because of the danger of uneven gloss or “suction spots”. With modern special primers having controlled penetration, scanty paint jobs run little danger of uneven gloss or of spotted chalking and fading. Their shortcomings do not show up until the coating begins to break up prematurely. The logical development of two-coat initial painting is onecoat repainting. So far there seems to be little commercial exploitation of the possibility on the part of paint manufacturers although one maker of a two-coat system recommends it to some of his customers but does not yet print it in his label directions. I n practice much one-coat repainting has been done for many years. When repainting is done a t reasonable intervals, before the old paint starts to crumble or flake seriously, one generous coat of highly pigmented paint such as is suitable for the finish paint of a two-coat

system is sufficient. The writer’s home, for example, has been satisfactorily maintained since its erection in 1923 with one coat of paint every four years. When repainting has been too long deferred, however, and crumbling or flaking has exposed areas of bare wood, two-coat repainting is necessary. For such conditions the special primers with controlled penetration have obvious advantages which are emphasized in the advertising of the special primers. The Forest Products Laboratory has been concerned with the problems of two-coat initial painting since its painting studies began in 1924. The first series of exposure tests dealt with the painting characteristics of the native softwoods, but the second series, started in the fall of 1924, was the first of many dealing with two-coat initial painting. I n all of them careful attention was paid to the thickness of coating.

Relation between Spreading Rate and Film Thickness The series of tests on the native softwoods ( I ) revealed an extremely wide variation in the spreading rate a t which different painters apply paint, even when the paints and their thinning proportions are identical and the surfaces and conditions of application are as similar as it is possible to make them. The data on spreading rate, which have been published in detail (S), show that the personal habit of the painter governed the spreading rate to a much greater degree than the nature of the wood or the kind of paint. Among eleven painters, each painting Bty-six panels, the range in average spreading rate was 561 to 869 square feet per gallon for priming coat, 732 t o 1286 for undercoat, and 705 to 1335 for finish coat. Observations of practical work on painting houses show that these data are by no means extreme; on the contrary, they underestimate the variations encountered in commercial painting practice where painters are accustomed to thinning paint as they see fit in addition to brushing it out according to individual habit. Unfortunately, spreading rate has been considered almost entirely from the point of view of economy in consumption of paint, and its bearing on the thickness of coating necessary for good durability has been largely overlooked (11, 20). If paint is spread uniformly on a smooth, nonabsorptive surface, the thickness of the wet film in inches, T,, is related to the spreading rate in square feet per gallon, 8, by the equation, T , = 231

+ 1448

The wet film thickness can be measured directly, immediately after application, by means of a Pfund film gage (20, 22). The wet film shrinks in thickness as the volatile ingredients evaporate. A small increase in volume of the drying oil during drying (87) may be neglected. The thickness of the dry film, therefore, is Td

=

231 nu

+ 144 8

where nu is the fraction of the nonvolatile ingredients (pigment plus nonvolatile vehicle) in the paint by volume. Bare wood, however, is an absorptive surface; part of the priming paint sinks into the lumina of wood cells that open into the surface, and part of the oil from the paint penetrates still deeper into the wood (14, 22A, 24). There is no convenient technique for measuring these losses, and they undoubtedly vary with the nature of the wood, the composition of the paint, and the spreading rate; but many observations a t the Forest Products Laboratory indicate that reasonable agreement between estimated and directly measured thickness of dry a m is obtained by assuming that half of

July, 1941

INDUSTRIAL A N D ENGINEERING CHEMISTRY

903

TABLEI. 1924 TESTSOF TWO-COAT INITIAL PAINTING WITH SELF-PRIMINQ BY ELIMINATINQ THE UNDERCOAT OF THREE-COAT WORK

Panel No.’

81 82 83 84

Two-Coat Work (Left-Hand Halve6 of Panels) Three-Coat Work (Right-Hand Halves of Panels) Estd. Estd. Paint thiokness Initial Total Paint thickness Initial nonvolatile of d w appearDurapaint nonvolatile of dry appearapplied coating anoe bility applied applied ooating anae Gal./lOOO sq. fi. Inch Months UaZ./1000 sq. ft. Inch Paint L on White Pine 1.01 0.85 0.0011 Badb 32 1.20 0.94 0.0012 Badb 1.41 1.20 0.0016 Badb 34 2.11 1.71 0.0024 Poor 1.86 1.60 0,0022 Poor 2.87 2.35 0.0034 Fair 3.00 2.61 0.0035 Fair 0 4.44 3.64 0.0062 Good

Total paint applied

&.

85 86 87 88

1.05 1.68 2.26 3.54

0.87 1.39 1.90 3.00

0.0011 0.0018 0.0026 0.0041

89 90 91 92

1.03 1.47 1.97 3.68

0.87 1.26 1.70 3.22

0.0011 0.0017 0.0023 0.0035

93 94 95 96

1.07 1.88 2.29 3.68

0.88

0.0011 0.0022 0.0026 0.0043

Paint (LZso)m on White Pine Badb 32 1.36 Badb 32 2.73 Poor 45 3.81 Fair 45+0 5.25 Paint L on Southern Yellow 27 Badb Badb 27 Poor, 27 38 Fair

Durability

Months 32 38 45+0 45+c

1.10 2.26 3.18 4.38

0.0015 0.0032 0.0047 0.0063

Bad Poor Fair Good

32 45 45+: 45+

1.06 1.75 2.49 4.22

0.0014 0.0024 0.0035 0.0056

Badb Bad Fair Good

27 27 32 45+6

1.01 1.99 3.00 4.31

0.0015 0.0028 0.0044 0.0063

Bada Bad Fair Good

a2

Pine

1.33 2.14 3.01 5.17

Paint (LZso)io on Southern Yellow Pine

a

1.59 1.94 3.14

Badb Badb Poor Fair

27 27 27 32

1.35 2.40 3.59 5.17

a?

32

45+*

The panels were exposed vertically facing south at Madison Wis. in December 1924. 01 the riming :oat. Coating still considered servioeable when the tests were discontinued a8er 45 months.

b Coating showed flat spots (uneven gloss) beCQusCof inadeqdaey C

the nonvolatile vehicle and all of the volatile thinner are lost during drying so that Td

= 231

[l

- +(nu - p ) - (1 - nu)] +. 144s

where p is the fraction of total pigment in the paint by volume. This equation reduces to

Td

= 231 (nu

+ p ) t 2 X 144 8

The assumptions that lead to this equation represent an oversimplification of the processes involved. Other workers may disagree with some of the assumptions. In the writer’s experience, however, they achieve empirically a rough a p proximation of film thickness sufficient for the present purpose. Dry film thickness can be measured directly by carefully cutting sections through the painted surface for observation under a microscope with micrometer eyepiece

@@.

To illustrate the calculation of dry film thickness, assume that a priming coat containing p = 0.25 and nu = 0.70 is applied to bare wood at 450 square feet per gallon, and a finish coat containing nu = 0.90, a t 550 square feet per gallon. For the priming coat, Td = 231 (0.70 4-0.25) + 2 X 144 X 450 = 0.00169 inch, and for the finish coat, T d = 231 X 0.90 + 144 X 550 = 0.00262 inch. The estimated thickness of the resulting coating is, then, 0.00169 0.00262 = 0.0043 inch. During exposure to the weather the film thickness shrinks, at first as a result of contraction of the drying oil (16)and later from chalking and erosion. No data are available on the rate a t which these changes in film thickness take place, but the prevailing opinion seems to be that there is comparatively little change in thickness during the first two years.

+

Two-Coat Painting with Self-priming I n a series of tests of two-coat initial painting with selfpriming started in 1924 at Madison, Wis., the two-coat work was done by merely omitting the undercoat of the three-coat work, just as the label directions of many paint manufacturers still recommend. Left-hand halves of test panels received two-coat jobs and rightihand halves, three-coat jobs. Panels were painted a t four levels of spreading rate

which were roughly for priming and finish coats, respectively: (a) 1600 and 3000, (b) 1200 and 1300, (e) 950 and 900, and (d) 650 and 600 square feet per gallon. The first level represented the scantiest and the fourth level the fullest application practicable on vertical surfaces with these typical paints of that period. The tests were made on panels of white pine and of southern yellow pine. Details of the technique of testing of the Forest Products Laboratory (IO) and of the methods of evaluating results (IS)have been published. Paints are described in the concise terminology and symbols of the laboratory’s system of classification (9). Two paints were used in the 1924 tests. One was pure white lead paint, L, mixed for finish coat with p = 0.246 and nu = 0.962, for undercoat with p = 0.272 and nu = 0.724, and for priming coat with p = 0.181 and nu = 0.761. The second was paint (LZ&,, mixed for finish coat with p = 0.226 and nu = 0.922, for undercoat with p = 0.201 and nu = 0.820, and for priming coat with p = 0.174 and nu =

0.807. Table I reports the amount of paint and of paint nonvolatile applied to each test area in gallons per 1000 square feet, the estimated thickness of the coating in inches, the initial appearance of the coating, and its durability. The judgments of initial appearance are based on adequacy of hiding of the grain of the wood and on uniformity of gloss. To hide the wood completely, a coating not less than 0.005 inch thick proved necessary. Most house owners, however, were satisfied a t that time by a degree of hiding corresponding to the rating fair which was obtained with coatings approximately 0.0035 inch thick. All coatings that attained ratings of fair or good in initial appearance dried with uniform gloss and weathered without spotted chalking, indicating that the priming coat in two-coat work or the priming and undercoat in three-coat work succeeded in sealing the wood against absorption of oil from the finish coat. With the paints used, hiding power was a fairly reliable guide to the amount of paint required both to escape uneven gloss and spotted chaking and to attain reasonably good durability. Both of the paints used develop characteristic checking patterns comparatively early, usually during their second year of exposure. With paint L the checking pattern is reticulate

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1936 TESTSO F

SPECIAL PRIMERS I N

Vol. 33, No. 7

REPAIXTIXC AFTER 40-MONTH EXPOSURE ( X 3)

Previous paint L in good condition for repainting repainted with paint L for finish coat and t h e following for the undercoat: area 1162, paint L itself; 1153, primer (TLidsg p / n v 0.41; 1157, primer S p/nv 0.32; 1158, primer SZe p / n o 0 . 3 2 .

practically from the beginning, but with paint (LZ&O it begins with a parallel pattern and becomes reticulate later. With both paints, checking began a little earlier and became somewhat more conspicuous in three-coat than in two-coat work; i t eventually became more conspicuous in the thicker coatings than in the thinner ones. The results indicate that durability is a function of the thickness of the coating, not of the number of coats. At any given film thickness the durability was about the same whether the thickness was built up with two coats or with three. The paints used, however, were designed primarily for use in threecoat work and were too low in pigment content to be convenient for application in the thick coats necessary for good two-coat work. Under many conditions of weather there would undoubtedly have been trouble with wrinkling during drying of the finish coats applied a t the lowest level of spreading rate in these experiments. The results therefore indicate that two-coat initial painting by mere omission of the undercoat of three-coat work is impracticable even when

priming and finish paints are applied in the thickest coats possible. To make two-coat work practicable, the paints must be properly designed for application in thick coats. Accordingly, further exposure tests were made in 1926 in which paint (LZ30)8ov a s redesigned with higher content of pigment and was then thinned differently for two-coat and for three-coat work. As made up, the paint contained p = 0.242 and nv = 0.923. This was considered a high level of pigmentation in 1926 but not a t the present time. The paint was used unthinned for finish coat in two-coat work, but for finish coat in three-coat painting it was thinned with 0.5 pint of linseed oil per gallon. The thinning for all coats together with the spreading rates, amount of paint applied, estimated thickness of coating, and durability are recorded in Table 11. The paint was tested in white, ivory, and gray colors on panels of southern yellow pine. Coatings of practically equal thickness were obtained in both two-coat and three-coat work, and the resulting durabilities were likewise about equal. All of the coatings dried

July, 1941


905

1936 TESTS OF SPECIAL PRIMERS I N REPAINTINQ AFTlR @MONTH EXPOSURE ( X 3) Previous paint (LZsdw in ood condition for repaintin , repainted with paint (LZsdsa for finish coat and the following for the undercoat: area 1100, paint (fZso)so itself; 1107, primer $TLd@p / n v 0.41; 1171, primer 8 p / n v 0 . 3 2 ; 1172 primer 82s p / n n 0 . 3 2 .

with uniform gloss and color, and during exposure gloss was lost and chalking and fading developed uniformly on all panels. The durabilities observed agreed closely with those found for coatings of similar thickness of paint (LZ30)W on southern yellow pine in the 1924 tests. As in the 1924 tests, checking began a little earlier and became more conspicuous in three-coat than in two-coat work. The results, therefore, disagree with the ancient dictum that “three thin coats are better than two heavy ones” (26). Between 1926 and 1931 six other series of exposure tests were started by the Forest Products Laboratory in which two-coat and three-coat initial painting with self-priming were compared. Besides Madison, exposures were made a t Tucson, Ariz., Fresno, Calif., and St. Paul, Minn. I n addition to paints L and (LZ30)80, the following paints were tested: LZ~S,LZZ,, (LZ37)7z, (L&o)so, (TZza)sa, and (sz30)69. Tests were made on western red cedar, redwood, ponderosa pine, northern white pine, red pine, and southern yellow pine. I n one of the series four-coat initial painting was com-

pared with three-coat and two-coat painting. All of these series indicated that the durability depends upon the thickness of the coating, regardless of the number of coats. The following conclusions were published in 1934 (2) as a result of the tests made a t St. Paul in cooperation with the Northwestern Paint and Varnish Production Club and the Minnesota chapter of the Painting and Decorating Contractors of America : With the white lead paint and the titanium and zinc paint the best two-coat job proved equal or superior in durability on the whole t o the best three-coat job on the same boards. With the lead and zinc phi& the best two-coat job was slightly inferior to the best three-coat job except on red cedar, where it was better. By reason of the higher pigment concentrations and lower spreading rates followed in the two-coat painting, roughly similar total quantities of paint were applied in the two-coat and threecoat jobs. It is evident that two-coat painting when done in the manner followed in these experiments is thoroughly practicable and ives coatings that closely approach good three-coat work in duraiility and prove distinctly better than poor three-coat work.


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Vol. 33, No. 7

TABLE11. 1926 TESTSOF TWO-COAT INITIAL PAINTINQ WITH SELF-PRINING BUT WITB THINXINO PROPORTIONS READJVSTED FOR TWO-COAT WORK No.

of

Thinning Proportions, Pints/Gal. Finish Undercoat coat TurTur-‘ penpenOil tine Oil tine

Primer TurpenOil tine

Panel N0.a

Coats Applied

273 274 277 278

3 2 2 2

1.25 1.00 0.00 0.00

2.0 1.5 0.0 1.5

275 276 279 280

3 2 2 2

1.25 1.00

0.00 0.00

2.0 1.5

291 292

3 2

1.25 0.00

Spreading Rate, 9s. Ft./Gal. UnderFinish Primer coat coat

Gal. of Paint Applied/ 1000 84.F t . NonTotal volapaint tile

Estd, Thickness of D r y Coating, Inch

Dvability hlontds

.. ..

Paint (LZao)so,Ivory T i n t , on Southern Yellow Pine 2.5 0.5 0 781 794 775 0.0 0 574 490 0.0 0 544 535 0.0 0 535 474

3.83 3.78 3.71 3.98

3.06 3.27 3.42 3.40

0.0043 0.0044 0.0045 0.0046

30 31 30 33

0.0

0 .. .. ..

Paint (LZ3d80, Gray Tint, on Southern Yellow Pine. 2.5 0.5 0 719 934 SO6 0.0 0 690 524 . 0.0 0 610 562 0.0 0 645 588

3.70 3.36 3.42 3.25

2.97 2.93 3.16 2.78

0.0041 0.0040 0.0042 0.0037

30 30 30 30

1.0 1.0

0

2.5

Paint (LZao)so, White, on Southern Yellow Pine 0.5 0 694 847 800 0.0 0 492 500

3.87 4.03

3.21 3.55

0.0044 0,0047

31 33

0

..

1.5

..

.. .. .. ...

... ... ...

... .. ...

... ... ...

...

a The panels were exposed vertically facing south a t Madison,

...

Wis., in January, 1926.

As indicated by Table I and confirmed by the later tests, the desirable thickness of coating for good durability with most house paints seems to be of the order of 0.0045 t o 0.0055 inch. Thicker coatings may be still more durable, but the checking or cracking patterns developed as the paint ages tend to become too conspicuous. To attain a film thickness of 0.0045 inch in two-coat work, the primer should be applied a t a spreading rate of approximately 450 square feet per gallon and the finish coat a t 500 square feet per gallon. To do so under practical working conditions, paints are required of higher consistency and higher content of pigment than the industry was accustomed to offering until the last few years. The Wood Handbook of the Forest Products Laboratory (19) stated: “Much painting of new wood is now done with only two coats, but this practice frequently leads to unsatisfactory results. When properly done, two-coat painting is practicable, but it requires more skillful workmanship than three-coat painting.” Two-Coat Painting with Special Primer I n 1936 the Forest Products Laboratory started a series of exposure tests on two-coat and three-coat initial painting with special primers. The purpose was to study the case in which a special primer is offered to supplement a conventional line of prepared paint, or simply for use “under any good house paint”. It was not intended to represent the case in which the finish paint as well as the primer is designed for application in thick coats, and the two are offered together as a twocoat system meeting the requirements for the best standards of painting. The special primers used in the tests were either commercial products submitted by their manufacturers with the request that they be tested, or were made on formulas proposed by manufacturers of raw materials and known t o be in commercial use. Eight finish paints were

used over each primer; seven were made a t this laboratory to represent the range in types of prepared paints on the market a t that time, and the eighth was white lead paint mixed from commercial soft-paste white lead. The tests were made on panels of Douglas fir and southern yellow pine exposed vertically facing south a t Madison, Wis. These woods were chosen because one object of the test was t o learn whether the special primers give better service than self-priming on such woods, and if so, how they compare for that purpose with aluminum primer. The series occupied five units (numbers 19 to 23, inclusive) of test fence 3. Each unit was 6 feet long and 7.5 feet high, and was covered with sixteen boards of 6-inch drop siding, alternately Douglas fir and southern yellow pine. Each unit was marked off into four vertical strips, each 1.5 feet wide, for priming with three special primers and self-priming, respectively. For subsequent coats of finish paint the units were subdivided horizontally, a pair of boards for each of the eight finish paints. I n Table IV the number of the fence unit on which each primer was tested is recorded so that the reader can tell which of them were tested on the same boards. The primers and priming procedures tested in three-coat work were as follows: SELF-PRIMING.The finish paint thinned as indicated in Table 111. 1. ALUMINUM PRIMER. 1.75 pounds of commercial aluminum aste (65 per cent by weight aluminum powder) in 1 gallon of odied linseed oil vehicle containing 63 per cent by weight nonvolatile (viscosity, 1.4 poises). It was applied at the average spreading rate S = 780 square feet per gallon. 2. FLAKE LEADPRIMER.6.1 pounds of commercial flake lead paste (90 per cent by weight flake lead) in 1 gallon of the above bodied linseed oil vehicle, applied at S = 850 square feet per gallon. 3. MICAPRIMER.2 pounds of commercial, surface-coated

E

USEDIN 1936 PRIMER TESTSAND SPREADING RATESAT WHICHTHEYWEREAPPLIED TABLE 111. FINISHPAINTS Two-Coat Work Finish coat Finish Paint

p 0.256 0.276 0.186 0.243 0.250 0.347 0.268 0.270

7&W

0.934 0.920 0.930 0,920 0,893 0.962 0.924 0.931

S 530 670 605 670 620 480 530 590

Self-primer P nv 0,228 0.830 0.245 0.818 0.165 0.826 0.216 0.818 0.222 0.794 0.309 0.855 0.821 0.238 0.240 0.827

S 430 570 520 595 560 495 595 580

P 0.256 0.276 0.186 0.243 0.250 0.347 0.268 0.270

Finish coat nv 0.934 0,920 0.930 0.920 0.893 0.962 0.924 0.931

S 610 585 690 660 690 540 600 570

Three-Coat Work Undercoat P nv S 0.270 0.730 570 0.757 0.280 800 0.231 0.846 600 0.275 0.758 635 0.275 0.754 580 425 0.360 0.898 0,280 0.757 700 0.270 0.730 680

Self-primer nu 0.781 0.884 0.938 0.906 0 . i9o 0.884 0.253 0 .s s 5 0,190 0.884 0.190 0.884 P 0.180 0.190 0.148 0.185

S 670 705 580 720 600 525 640 625


July, 1941

mica in 1 gallon of the above bodied linseed oil vehicle, applied at S = 685 square feet per gallon.. - 4. PRIMING OIL A. A commercial seal for wood offered as a “clear mill primer” for lumber and said to “function as an undercoater or first coat of paint”. It is described as “a blend of nitrocellulose with light treated oil, reduced with nitrocellulose solvents”. It contained 16.7 per cent b weight nonvolatile and waa applied at S = &5 s uare feet per gallon. 5. PRIMIP;Q OIL A commercial priming oil sold by a paint manufacturer for use in priming new wood to be painted with his house paint. It apparently contained bodied tung and linseed oils. The content of nonvolatile was 86.5 per cent by weight and it was applied a t S = 740 square feet per gallon. 6. PRIMINQ OILC. A mixture of 2 pounds of bodied tung oil, 2 pounds of bodied linseed oil, 4 pounds of raw linseed oil, 1.2 pounds of turpentine, 0.3 pound of lead-manganese naphthenate drier, and 1 pound of Carbitol. The mixture had a viscosity of 3.2 poises and was applied at S = 735 square feet per gallon.

907

%.

A t left, two-ooat work with primer (TL24)08 p / n v 0.41; a t right, two-coat work self-primed. Finish on two upper boards, paint (TLzEZ*~)IW, on two lower boards, paint (SL11Zza)rs.

Primers and priming procedures tested in two-coat work were as follows: SELF-PRIMING.The finish paint thinned as indicated in Table 111. 7. (.TL&s .p/nv 0.41. A commercial zincless pruner with controlled penetration sold rn part of a two-coat system. It contained p = 0.276, n v = 0.670, and was applied a t S = 475 square feet per gallon. 8. (TLa7)76 p / n u 0.29. A commercial zincless primer with controlled penetration sold to supplement a line of prepared paint. The label said: “Recommended as the best primer under all exterior house paints.” It contained p = 0.254, nv = 0.869, and was applied at S = 500 square feet per gallon. 9. (TL6)ra p / n v 0.18. A commercial zincless primer with controlled penetration sold t o su plement a line of prepared paint. The A t left, two-coat work with primer (TL27)76 p / n a 0.29; at right, two-coat work with primer d e l said that it is ‘‘a perfect first coater for (TLe)e p / n v 0.18. Finish on two upper boards, paint (TLzEZZE)IO~, on two lower boards, priming painted or un ainted exterior surfaces paint (SLIIZZI)i s . in preparation for finisKing with any oil paint”. It contained p = 0.104, n v = 0.585, and was applied a t S = 720 square feet per gallon. 10. (TL+ p / n v 0.24. A commercial zincless rimer with controlled penetration sold to su pyement a line of prepared paint. The late1 said: “Any good qualit of prepared paint or white lead and linseeioil paint may be applied over this primer with excellent results.” It contained p = 0.178, nu = 0.72, and was applied at S = 490 square feet per gallon. 11. TLtlZltp/nv0.17. A commercial rimer high in zinc oxide and with controllec?penetration, sold to supplement a line of prepared paint. The label said that this primer “will improve the appearance and durability of any suitable finish coat paint”. It contained p = 0.164, nu = 0.954, and was applied at S = 505 square feet per gallon. 12. PRIMER X. A commercial primer containing zinc oxide made with “portland cement and processed oils” to be thinned with a special thinner of undisclosed composition and A t left, three-coat work with aluminum primer: a t right, three-coat work with self-priming. used under house paint, porch paint, wall Finish on two upper boards, paint ( T L ~ Z ~ ) Lon O Itwo . lower boards, paint (SLuZd7a. paint, masonry paint, etc. The composition was not revealed beyond these broad state1936 TESTSO F SPECIAL P R I M E R S AFTER &MONTH EXPOSURE ments. It was amlied at S = 555 sauare feet per gallon. 13. (sz9)69p / n v 0.34. A commercial primer with controlled penetration and a low content of zinc oxide. It according to a formula recommended by a manufacturer of was not offered for priming new wood but for undercoating in paint igments. It contained p = 0.277, n o = 0.863, and was initial painting and in repainting. For initial painting the manuappliex at S = 480 square feet per gallon. facturer recommended priming with priming oil B. The product 15. SZ6 p / n v 0.32. Made at the Forest Products Laboratory contained p = 0.209, nu = 0.625, and was applied at = 475 according to a second formula recommended by the above pigsquare feet per gallon. ment manufacturer. It contained p = 0.275, nu = 0.862, and was applied at S = 625 square feet per gallon. Primers 14 and 14. S p / n v 0.32. Made at the Forest Products Laboratory

..


908

15 both had controlled penetration, but one was zincless whereas the other contained a small proportion of zinc oxide. Both have been offered commercially.

The composition of the finish paints and the spreading rates a t which they were applied are reported in Table 111. Composition and consistency of the paints were adjusted to represent typical use of prepared paints for finish coat in threecoat initial painting-that is, application a t an approximate spreading rate of 700 square feet per gallon. Paint (LZ&, however, was made for application a t 550 square feet per gallon because it was a second-grade paint of low content of opaque pigment necessitating thicker coats to obtain sufficient hiding power. The painter actually applied most of the paints more generously than was intended in the threecoat painting. As already pointed out, the purpose of the study required that the same finish paints be used for the twocoat work and that they be applied a t spreading rates not much lower than were followed in three-coat painting. For self-priming in two-coat work the finish paints were thinned with 1 pint of turpentine per gallon and applied as generously as was found practicable, which was roughly 500 to 600 square feet per gallon. The resulting two-coat jobs with selfpriming represent fairly the maximum thickness of coating that can reasonably be expected when conventional prepared paints, designed primarily for three-coat painting, are used in two-coat painting, but they do not represent the thickness of coating obtainable with properly designed two-coat systems. The three-coat jobs with self-priming, on the other hand, represent more generous application than is commonly obtained in commercial painting but not more so than should readily be obtainable.

TABLEIV.

1936 TESTSOF TWO-COAT AXD THREE-COAT PAINTING WITH

Test Fence Unit No.

Primer

1. 2. 3. 4. 5. 6.

Self-primed Self-primed Aluminum primer Flake lead primer Mica primer Priming oil A Priming oil B Priming oil C

13. (8Z9)69P/n'D0.34 14. S p/nv 0 . 3 2 15. SZsp/nvO.32

'

SPECIAL PRIMERS= Paint Total NonPaint volatile Applied Applied GaE./lOOO a q . ft.

Estd. Thickness of D f y DuraCoating bility Inch Months

Three-Coat Painting 4.22 4.88 19 4.22 4.88 20 3.53 4.56 19 3.47 4.46 19 3.63 4.74 19 3.02 4.68 20 4.63 3.98 20 3.85 4.64 20

0.0059 0.0059 0.0051 0.0051 0.0052 0.0045 0.0045 0.0045

35 35 50 34 40 36 31 34

Two-Coat Painting 21 3.59 3.14 3.14 22 3.59 3.14 23 3.59 3.01 3.83 3.73, 3.34 2.42 3.11 3.09 3.77 3.50 3.71

0.0042 0,0042 0,0042 0.0042 0.0044 0.0034 0.0041 0.0044

28 30 28 35 37 30 41 31 26

2:

i:::

g:::

g:g8:2

23

3.33

2.99

0.0041

i:

35

a The data reported are averages for the eight finish paints tested oyer

each priming procedure. A copy of the complete tabulation for each finish paint separately can be obtained from th,e Forest Products Laboratory within 12 months from the date of this publication. b Data lacking.

The undercoats for the three-coat work are obviously mixtures obtainable from paste paints but not from prepared paints. Since these paints were being used in other series of exposure tests in 1936, it was convenient to make use of them in this series on special primers. The results are summarized in Table IV. For brevity, the averages for all eight finish paints over each primer are reported.

Vol. 33, No. I

With self-priming, the three-coat jobs were materially more durable than the two-coat jobs, BS was expected from the difference in the estimated thickness of the coatings. It should be pointed out, however, that the two-coat jobs, even without the benefit of special primers with controlled penetration, held gloss uniformly just as long as the threecoat jobs or the two-coat jobs with special primers. Uneven gloss and spotted chalking are caused by inexcusably scanty application of paint and do not require special primers for their correction. The most durable coatings by far were those done in threecoat work with aluminum primer. This proved true with each of the eight finish paints. The average durability of 50 months is 9 months greater than the next best average durability recorded and 15 months greater than the durability with self-priming. Mica primer improved the durability of three-coat work by 5 months on the average, but flake lead impaired the durability slightly. Substitution of a suitable unpigmented priming oil for the priming coat of three-coat work may, from these results, leave the durability practically unimpaired even though the film thickness is somewhat reduced. A slight saving in cost of material for the paint job might be effected in that way, but there is serious risk of early development of conspicuous alligatoring ( 7 ) of the finish paint when an unpigmented primer is applied generously enough to form a continuous coating over the wood. Priming oil B in three-coat work gave an average durability of 31 months whereas the special primer made by the same manufacturer (primer 13) gave an average durability of 36 months in two-coat work. The economy effected through saving the labor required for the third coat of paint is so much greater than any possible saving in cost between a clear priming oil and a pigmented primer that the priming oils do not commend themselves for practical house painting. Among the two-coat jobs, the one with primer X proved less durable than the two-coat jobs with self-priming. Since this is the one primer tested whose composition the manufacturer shrouds in mystery, no further comment is necessary. All other special primers with controlled penetration made more durable coatings than the two-coat work with self-priming although in most cases the estimated film thickness was of the same order of magnitude. Five of the nine special primers with controlled penetration made two-coat jobs equal or superior in durability to the three-coat jobs with self-priming, even though the estimated film thickness of the three-coat jobs was materially greater. Undoubtedly still better durability would have been obtained if these primers had been used for three-coat work or if the finish paints had been designed for and applied in thicker coats to build up the film thickness to that of the three-coat jobs with self-priming. The durability of the two-coat jobs with the different special primers varied greatly, from 26 to 41 months. The relatively poor performance of primer 9 is clearly due largely to its low content of total pigment and of total nonvolatile, the high spreading rate a t which it necessarily was applied, and the smaller film thickness resulting. Among the primers of group T L (primers 7 to 10, inclusive), the durability increased as the content of white lead increased; among the primers of groups S and SZ (primers 13 to 15), those containing zinc oxide performed better on the whole than the one containing neither zinc oxide nor white lead. The one primer containing a high proportion of zinc oxide (primer 11) proved relatively inferior. The two best primers in these tests (primers 8 and 11) were relatively high in content of total nonvolatile, nv, and had a ratio of pigment to nonvolatile, p / n v , of 0.24 and 0.29. This level of p / n v is approximately that of finish paints and is much higher than that

July, 1941


of old-fashioned priming mixtures, but it is far short of the ratio recommended by some investigators (69, SI). The extremely high ratios of p / n v , such as that of primer 7, are obtainable only by reducing nu materially, a procedure that seems to be disadvantageous in primers with controlled penetration just as it has been shown to be in self-priming

($1.

In these tests primers 7, 8, and 9 proved compatible with all of the finish paints; that is, the defects characteristic of the finish paint, such as chalking, checking, cracking, crumbling, and flaking, developed in the same way over selfpriming and over the special primers and remained a t all times similar in pattern and visibility, differing only in the rate at which crumbling or flaking over the bands of summerwood progressed toward the end of the period of durability. Primer 10 slightly hastened checking of finish paints (LZ16)aa and ( T L Z ~ Z Zbut B ) ~retarded ~~ checking of paints (TLae)llo(r) and (LZao)so. I n an exposure test made in 1935 a t 45’ facing south, primer 10 caused conspicuous alligatoring of a commercial finish paint ( T L I ~ Z I ~p )/ n~v~0.29. , Primers 11 and 12, which contained substantial proportions of zinc oxide, caused earlier and more conspicuous checking of finish paints L and (TL89)llo(r)and caused alligatoring of paint L a t age 24 months but did not alter the characteristics of any of the other finish paints. Primer 14, on the other hand, caused alligatoring of all finish paints containing any zinc oxide but left the characteristic behavior of paints L and (TL3~)ll~(r) unaltered. The alligatoring over primer 14 began a t age 18 months and led to intercoat 5aking (flaking of the finish coat from the priming coat rather than of the whole coating from the wood), which was not related to the distribution of summerwood in the boards. The alligatoring and intercoat flaking were more pronounced, the higher the content of zinc oxide in the finish paint. Primers 13 and 15, which contained small proportions of zinc oxide, did not alter the normal behavior of finish paints containing zinc oxide and made the checking of paints L and (TLa~)ll~(r) only slightly coarser in pattern than normal. Finish paint (TL&o(r) was characterized by greater discoloration with accumulated dirt than any of the other finish paints. The discoloration was normally uniform (for example, over self-priming) for the first two years, after which the dirt was sloughed off irregularly and gave a badly spotted appearance. Over all primers containing zinc oxide (primers 11, 12, 13, and 15) the sloughing of dirt began within one year and was practically complete over primers 11 and 12, so that for the remainder of the exposure period these test areas were among the cleanest and whitest on the fence. Flake lead primer likewise caused early sloughing of dirt from paint (TL&0(r), a fact that presumably is related to the reaction between flake lead and linseed oil already mentioned.

Repainting with Special Primers for Undercoating Repainting tests to compare self-undercoating with the use of special primers for undercoating were made in 1936 with primers 1, 7, 8, 9, 11, 13, 14, and 15. There were available the “control” areas of the test panels used in the 1930 primer tests (7), which had been exposed te the point of marked crumbling or flaking, and some panels of western red cedar which had been exposed to the point of chalking and checking but not of crumbling or flaking. The previous paints were paints L and (Lz3O)SO in three-coat work, self-primed. The panels with old paint L were repainted with paint L for finish paint and those with old paint (LZm)g with the same lead and zinc paint for finish paint. I n each case half of the panel received a special primer for the undercoat and half was selfundercoated. All of the repainting was done in two-coat

909

work except that two coats of white paint were applied over aluminum primer to obtain the required hiding. The repainted coatings proved materially more durable than the initial paint jobs. At the last inspection, after 45 months, the repainted areas had not gone long enough to permit tabulation of durabilities. It was clearly indicated, however, that with paint L for finish paint, the following special undercoaters are making more durable repaint jobs than are being obtained with self-undercoating: aluminum primer, ( T L z ~ p) /~n v 0.41, (TL27)76 p / n v 0.29, (sz9)69 p / n u 0.34, S p / n v 0.32, and sz8 p / n v 0.32. Primers (TLe), p / n v 0.18 and TLIIZsz p / n v 0.17 are making less durable jobs than self-undercoating. With paint (LZao)w for finish paint, primer S p / n v 0.32 has caused severe alligatoring and intercoat flaking, just as it did in the tests of initial painting when the finish paint contained zinc oxide. The repainting tests have brought out the compatibility relations between the special undercoaters and the finish paints particularly clearly. With aluminum undercoat the normal checking pattern of both finish paints was made coarser and more conspicuous. There was no such effect over aluminum primer in the tests of initial painting although it is observed in initial painting a t times (6). Undercoaters (TL& p / n v 0.41 and (Sz9)69 p / n v 0.34 made the checking of both finish paints somewhat less conspicuous, whereas primers (TL27)76 p / n v 0.29 and (TL& p / n v 0.18 made the checking of both paints slightly more conspicuous. Undercoater S p / n v 0.32 did not alter the checking of white lead paint materially, but caused alligatoring and intercoat flaking of paint (LZS0)SO as already pointed out. Undercoaters SZa p / n v 0.32 and TLl1Z62 p / n v 0.17 made the checking of paint L coarser but did not materially alter that of paint (Lz&0.

Literature Cited (1) Browne, F. L., Am. Paint Varnish Mfrs.’ Assoc., Sci. Sect., Circ. 219, 125 (1924), 290,202 (1926); Oflcial Digest Federation Paint & Varnish Production Clubs, 95, 106 (1930). ( 2 ) Browne, F. L., Am. Paint Varnish Mfrs.’ Assoc., Sci. Sect., Circ. 404,596 (1931), 445,454 (1933); Oflcial Digest Federation Paint & Varnish Production Clubs, 121, 1068 (1932); A m . Paint J., 19, 7 (Dec. 10, 1934). (3) Browne, F. L., Drugs, Oils & Paints, 42, 230 (Dec., 1926), 268 (Jan., 1927) ; Painters’ Magazine, 54, 10 (Jan., 1927). (4) Browne, F. L., IND.ENG.CHEM.,22,847 (1930). (5) Ibid., 25, 835 (1933). (6) Ibid., 26, 369 (1934). (7) Ibid., 27, 292 (1935). (8)Ibid., 28, 798 (1936). (9) Ibid., 29, 1018 (1937). (10) Browne, F. L., J. Chem. Education, 10,529 (1933). (11) Browne, F. L., Paint, Oil Chem. Rev., 97, 11 (May 16, 1935); R y . Eng. Maintenance, 31, 241 (1935). (12) Browne, F. L., Paint, Oil Chem. Rev., 97, 10 (Aug. 8, 1935); 100, 9 (April 14, 1938). (13) Browne, F. L., Proc. Am. SOC. Testing Materials, 30, 11, 852 (1930). (14) Browne, F. L., Proc. Wood Painting Conf., Madison, Wis., Sept. 13 and 14, 1929; IND. ENG.CHEM.,23, 290 (1931). (15) Cabot, S., U. 8. Patents 1,662,999 (March 20, 1928), 1,791,119 (Feb. 3, 1931); Drugs, Oils & Paints, 43, 46 (July, 1927). ENG.CHEM.,21, 621 (16) Clark, G. L., and Tschentke, H. L., IND. (1929). (17) Devoe and Raynolds Co., Saturday Eve. Post, advertisement, p. 113 (April 18, 1936). (18) Edwards, J. D., and Wray, R. I., IND.ENO.CHEW.,17, 639 (1925), 19, 975 (1927); Oficial Digest Federation Paint & Varnish Production Clubs, 122, 15 (1933). (19) Forest Products Lab., Wood Handbook, pub. of U. S. Dept. Agr., 1935. (20) Gardner, H. A., Natl. Paint, Varnish Lacquer Assoc., Sci. Sect., Circ. 586,91 (1939). (21) Gardner, H. A., Paint Mfrs.’ Assoc. U. S., Circ.281, 111 (1926); Am. Paint Varnish Mfrs.’ Assoc., Sci. Sect., Circ. 314, 432 (1927). (22) Gardner, H. A., “Physical & Chemical Examination of Paints, Varnishes, Lacquers and Colors”, 9th ed., p. 103 (1939).


910

(22A) Ibid., p. 134, Fig. 215. (23) Hart, L. P., Ball, G. L., Jr., and Johnson, E. E., Natl. Paint, Varnish Lacquer Assoc., Sei. Sect., Circ. 567, 175 (1938). (24) Haslam, J. H., and Werthan, S., IND.ENG. CHEM.,23, 226 (1931). (25) Jacobsen, A. E., Oficial Digest Federation Paint & Varnish Production Clubs, 146, 215 (1935). (26) Mills, B. S., Painter’s Hand-Book, p. 42, Eckstein White Lead Co., 1887. (27) Morrell, R. S., “Varnishes and Their Components”, p. 21 (1923). (28) Pittsburgh Plate Glass Co., “Wdlhide Exterior Primer”, April, 1933. (29) Robertson, D. W., “House Paint Primers and Undercoaters”, Titanium Pigments Go., May, 1936.

Vol. 33, No. 1

(30) Robertson, D. W., Oficial Digest Federation Paint & V a r n i s h Production Clubs, 146, 228 (1935). (31) Robertson, D. W., and Jacobsen, A. E., IND.ENQ.CHEM.,28, 403 (1936). (32) Schmutz, F. C., Palmer, F. C., and Kittelberger, W. W., “Improving Paint Service on Wood with Special Priming”, New Jersey Zinc Co., March, 1936. (33) Schmutz, F. C., Palmer, F. C., and Kittelberger, TV. W., IND. ENG.CHEM., 22, 856 (1930). (34) Schmutz, F. C., Palmer, F. C., and Kittelberger, W. W., Oflcial Digest Federation Paint & Varnish Production Clubs, 141, 355 (1934).

PRESENTED at a joint meeting of the Twin-Cities Paint, Varnish, and Lacquer Association and the Northwestern Paint and Varnish Production Club, a t St. Paul, Minn.

Drying Air with Phosphoric Acid in a Packed Tower M. M. STRIPLIN, JR. Tennessee Valley Authority, Wilson Dam, Ala.

a drying agent for gases. From their data it is apparent that dustrial use is to pass it phosphoric acid can be employed to dry air sufficiently for through apacked tower counteruse in processes where atmoscurrent to a stream of a liquid pheric air would not be suitdesiccant. There are installaable. tions of this type in operation using sulfuric acid. Greenewalt I n addition to vapor pressure ( 2 ) published some data on data, i t is essential in the design drying air with sulfuric acid in of large-scale drying towers to a 2-inch-diameter wetted-wall have data from which to calculate the required volume of a column. given tower packing. I n the The present work was underabsence of such data the design taken to supply data for the may result in a tower either too design of drying towers using small to remove the desired phosphoric acid as the desicK c a = 0.01 G lb. moles/(hr.) (cu. ft.) (atm.) quantity of moisture from the cant. The development of this air or one larger than necessary. p a r t i c u l a r m e t h o d w a s of New vapor pressure measurements for Absorption tower performance interest in connection with proc95.0 and 99.0 per cent H3P04 are correlated data suitable for large-scale esses requiring dry air which in convenient form with previous data of plant design must be obtained yield phosphoric acid in such from similar large-scale equipquantities and concentrations other concentrations to permit the calcument or from moderately large that it may be employed in the lation of ICGU. pilot plants. Sherwood (9) drying tower. The use of a stated that data obtained in drying - _ agent _ .produced continutowers less than 6 inches in diameter are of relatively little ously in the process is an advantage because such a procedure value for design purposes, but that data obtained in larger eliminates the cost of regenerating a drying agent. towers serve as a reasonably adequate basis if used properly. The importance of phosphoric acid as a drying agent for The purposes of this paper are to present experimental gases has been overlooked to a great extent until the last few data on the drying of air with phosphoric acid obtained by years, possibly because of its cost and the lack of equilibrium operation of a 10-inch packed tower, and to illustrate the use partial pressure data for this acid. Such data are necessary of the data in the design of a large-scale drying tower. before the degree of dryness that will be obtained can be predicted and before a rational design of large-scale drying equipDrying Equipment and Procedure ment can be undertaken. Perry and Duus (6, 7) gave some data for the partial presFigure 1 is a diagram of the apparatus used for drying air and sure of water over aqueous solutions of phosphoric acid, and Figure 2 shows the design of the acked tower. The latter was constructed from 10-inch standarrf steel pipe, coated inside with called attention to the technical importance of this acid as

A

N ESTABLISHEDmethod of drying air for in-

~

Data are given for the absorption of water vapor from air by phosphoric acid in a 10inch-diameter tower packed with 1-inch Raschig rings. The influence of gas and liquor rates on the over-all capacity coefficient, is given. Variations in the air rate are shown to have a much greater effect on KGu than variations in the acid rate. With an acid rate, L , of about 460 pounds/ (hour)(square foot), the effect of variations in the air rate, G, between 128 and 556 pounds/(hour)(square foot) is represented by the equation,

Two-Coat System of House Painting - Industrial & Engineering

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