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226
sures,” Mech. Eng., 49, 124 (1927); J . Am. Welding SOC.,6, No. 3, 84-7 (1927). (6) Hodge, J. C., “Application of Fusion Welding to Pressure Vessels,’’ I b i d . , 9, No. 10, 93-116 (1930). (7) Jasper, T. M., “Pressure Vessel Welding,” I b i d . , 8 , S o . 6, 12-27 (1929).
(8) Joys, “Pressure Vessels-Destruction Test on Gasoline Cracking Still,” I b i d . , 4, No. 10, 65-70 (1925). (9) Kinsel, A. B., “Design of Dished and Flanged Heads,” I b i d . , 6 , S o . 5 , 33-4 (1927). (10) Miller, S. W., “Examination of Ruptured Head of Ethylene T a n k , ” Mech. E n g . , 49, 117-23 (1927); J. A m . W e l d i n g SOL.,6, P o . 3, 73-84 (1927). (11) Miller, S. W:, “Fusion-Welded Pressure Vessels,” I b i d . , 6, N o . 2, 23-40 (1926).
Vol. 23, KO.2
(12) Miller, S. W., “Inspection of Welds in Pressure Vessels,” I b i d . , 4, NO,6 , 41-5 (1925). (13) Miller, S. W., “Maximum Allowable Working Fiber Stresses in Pressure Vessels,’’ I b i d . . 7, No. 9 , 21-7 (1928). (14) Miller, S. W., “Rationale of Safe Welding in Pressure Vessels,” I b i d . , 4, No. 7 , 34-45 (1925). (15) Miller, W. B . , “Welding in Enameled Lined T a n k Industry,” I b i d . , 4, No. 9, 23-7 (1925). (16) Moore, “Fatigue Tests of Pressure Vessels,” I b i d . , 8 , No. 10, 42-52 (1929). (17) Rockefeller, H. E., “Oxy-Acetylene Welded Construction of a Large High Pressure Storage T a n k , ” Ibid., 6, No. 5, 16-32 (1927). (18) Rockefeller, H. E . , “Procedure Control in Pressure Vessel Welding,” I b i d . , 7, No. 3, 23-38 (1928). (19) Schuster, L. E., I b i d . , 9, No. 5, 18-64 (1930).
Studies in the Painting of Wood I-Influence of Wood Structure on Paint Behavior1 J. H. Haslam and S. Werthan T ~ NEW B
JERSEY
ZINC
COMPANY.
P A L M K R T O X , PA
A new method for studying the structure of wood several objections to this prowith relation to paint films is presented. The method cedure. The dye is often adits effort to i m p r o v e involves a microscopic examination of sections stained sorbed by the dense wood p r o t e c t i v e coatings, by selective dyes. structure or upon the surface has made considerable progA brief description of the wood structure is followed of the pigment, rendering it ress in paint technology. At by a discussion of the effect of wood upon the protective useless in the study of oil the s a m e t i m e institutions coating, including data on the changes occurring in penetration. Also, the pressuch as the Forest Products spring and summer wood under the influence of moisence of the dye may cause a Laboratory and universities ture. retardation of the oxidation of have carried out many inves* Photographs and information are given, showing the oil, altering the period of tigations of wood structure. that the structure of the wood surface and nature of penetration. Observation of The proper c o r r e l a t i o n of the vehicle control the degree and uniformity of penethe penetration of a dyed oil these data and correct intration. The study indicates that a slight uniform under low magnification has terpretation of the findings penetration of the vehicle of the paint into the wood shown a layer of clear liquid should lead to a better underis more desirable than deep irregular penetration. just in advance of the colored standing of the relationships In conclusion, the authors discuss the several relaoil, suggesting that a meaBure between paint and wood. tionships existing between the factors studied and of the p e n e t r a t i o n of the Astudy of the literature inpaint durability. color is not a measure of the dicates that, although some total D e n e t r a t i o n . These i n v e s t i g a t i o n s have been carried out abroad, there is little published information dealing objections have been overcome in the ievelopment of a new with the relationships that exist between applied coatings and technic in which the specimens are stained after being cut. wood surfaces (8,4). I n this article an attempt has been made Apparatus and Technic to show the importance of a correlated study of wood structure and the applied film. This work involved microscopical exFor the preparation of the mounts, a Minot automatic amination of the wood structure; study of the penetration of precision microtome with modification in the form of a this structure by the vehicle of the protective coating; deter- freezing attachment is used. Figure 1 illustrates the apparamination of the changes that take place in the paint filmand tus, showing the adaptable freezing unit and carbon dioxide wood surface during aging and exposure; and investigation of gas tank. The dyes used for staining the sections have been the effect of these changes on adherence, porosity, and paint chosen for their ability selectively to stain wood and oil. durability. It has been found that brilliant green,* a water- or alcoholIn general it has been assumed that the life of a protective soluble stain, is entirely satisfactory for staining wood and coating depends on the paint composition, and it is only that Soudan I11 or IV3 and scarlet R4are quite effective in recently that the microstructure of wood has been considered the oil. This gives a green and red contrast which is more an important factor in paint durability. Unquestionably, satisfactory for microscopic examination than if other preflong life depends on proper formulation, but proper formula- erential stains for wood, such as hematoxylin or safranin, tion should provide a well-adhering film of the best pigment- are used. binder combination after the paint has been applied to the 1 A 1 per cent aqueous solution of brilliant green, a sulfate or double wood and the vehicle adsorption by the wood satisfied. zinc chloride of tetraethyldiaminotriphenyl carbinol, also known as emerald One method suggested in the literature for the study of green, malachite green, or ethyl green paint films and their behavior on wood is the use of dyed 3 Soudan 111 is benzeneazobenzene-azo-&naphthol, an oil soluble stain vehicles in the paints to be examined. There are, however, t h a t is made up in 1 per cent strength in alcohol and diluted with 50 per
HE paint industry, in
T
Received September 19, 1930. Presented before the Division of Paint and Varnish Chemistry a t t h e 80th Meeting of t h e American Chemical Society. Cincinnati, Ohio, September 8 to 12, 1930. 1
cent water. 4 Scar!et R is the sodium salt of xylen-azo-8-naphthol disulfonic acid, an oil-soluble stain, a deeper red than Soudan 111. This stain is also prepared by dissolving 1 per cent in alcohol and diluting 50 per cent with water.
The sections are cut fnmi a specimen about
l/z
inch
(1.3 nil.) square of the p n e l uiider examinat.ion. I t is not
necessary to freeze the siircimens if the paint is on wood and is to be exnriiiried for pcnetration of the oil. A drop of water on tlie end of the block or edge of the knife of the microtome facilitates tlie cutting, giving a more unifonn section. For most purposes R-ctions 10 to 15 microns in thickness are satisfactory. It is, Iiurever, necessary to use sect,ions 1 to 2
successfully cut with a culd kiiife. Cold toluene is used to remove the becswax and rosin and has so far shown no indication of affecting the film. The sectioned films if not stained may be mounted in balsam. Figure 2 illustrates the type of sections that may be secured by this method and that arc used in the study of pigment concentrations in the film. Figure 3 is a highly magnified (1500x1 seetion of the upper xurface of a paint film. It is possible to observe the distribution of the pigment throughout the film. In investigations of this sort great care must be exercised in choosing representative sections, as the area actually photographed is indeed quit8 sniall in comparison with the total paint surface. These sections nevertheless offer a means of studying changes that take place in the film during aging and failure, Preliminary investigation shows tliat there is a vast amount of information that can he derived from this source. Structure of Wood
Automatic Micmmime with COP Yreezine A l f e c h m e n f tised to Obfnln Micrcmecfionu
Figwe I - M i n s f
Ily selectively staining sections of painted blocks of wood, it has been possible to get an insight into the complex structure of wood and obtain some explanations for certain paint failures. Altlioiigli two pieces of different woods having received the samc treatment may appear uniform and equally satisfaator), for painting, the same paint applied on the two pieces often behaves quite differently. Figure 4 shows actualsize photographs of flat-grain surfaces of white pine, redwood. fir, and cypress. These surfaces were all prcpared in the same manner and appear quite uniform for painting. Figure 5 is n portion of the same white pine, redwood, fir, and cypress srirfaces magiiified 5OX. Each surface is quite different structurally and there fundamental differences cxcrt an effect upon tlie life of the paint film applied to them and often cause variations in failure of the same paint. For convenience, wood is grouped into two general classifications known as hardwoods and softwoods. These two main classes are again subdivided into many species and
niicrons thick for the study of filnr structure, aud approximately 5 mirrons for iiLservations of pigment concentrations. l'lie sections are daced on slides which have nreviouslv been cleaiied and cove& with a film of cgg albom&i, xliieh; when dry, servcs to attach the specimens to the slide. If the srctionx are of freshly applied paint, they are allowed t o oxidize for 8 to 10 hours. The slide bearing a section is then placed in . . the solution of brilliant green for I t.o 2 minutes, which stains oiily the wood fibers. If the wood is too deeply stained, it may be detdorized with dilute a!cohol and then washed in water. The slide is then placed for 2 niinutes in the solut,ion of Soudan 111 or I V or scarlet R, which stairis tile oil red. Passing thespecimen through liydrochloric acid fumes and then washing clear with water tends to intensifv the red color.6 For Immanent preservat.ion the specimen may be mounted in Allen's niediiim6 and protected with a cover glass. JIaIsnm w a s found to be unsntisfactory. The Euscope as described by Exton (91, has proved t o be a valualrlc in- FiBure 2 4 r o s a Section of 8 Two-Cwnt FiBure 3---Cr088 Section of Yilm Shew. Pains llilm Shewin* Pl4msnf Concen- in$ Pigment Dismhufion st Upper surstrument iii observing nnd pliotographing the frafion8. 40x face. 15aox sections. 'Shen it is desirable to prepare sections of paint films varieties. l'lie two large classes may readily be identified by for a study of the structure of the film, a soniewhat different microscopic. examination, but i t is sometimes difficult to technic must be used. These films may bc soft and contain differentiate between species. unoxidized oil arid when sectioned smear or become greatly Oak, a typical representative of tlie hardwoods, as seen distorted. To avoid this and keep t.he film intact it is im- in Figure 6, exhibits a very complicated structure of the bedded in a combination of becswax and rosin (2:1) and the wood e1einent.s and a complex system of tissue which serves entire block frozen to the point at which the wax can no longer very often as a means of identification. Hardwoods have not be dented with tlie finger nail. The specimens can then be been considered in this work because they are not generally 6 In the presentatioo of the paper autochrome places wwe qW which used in outside construction. cannot be reproduced in the written paper. The woods used in the exterior of buildings are generally 6 A!len's medium consists of a strained ~ o l u l i o nof g u m arabic of the the softwoods, and it has been necessary to limit this inconsistency of glycerol. to which '/I volnme of &cero1 and ~olume vestigation to the more common of these. The cells of the af formaldehyde has beeo ~radiially ineorporeted. Thin medium sets hard. softwoods bear a more or less simple relationship to one
two rrouos of fibers. It will be rioted that the Y
ure 10-the change from spring wood to suirimer wood is a eradual one. This characteristic va-
pointed out later. Among the outstanding features of these fibers are the pits, and they are so important in paint teclmology that it is desirable to give a detailed account of their structure and function. The work done by Scarth (9) shows clearly tlie striicture and the general arrangeinent of these pits (Figure 12). At intervals along the cell wall there will be found bulges toward the inside of the fiber, and directly opposite in the adjoining fiber there is a corresponding bulge so t.hat these structures resemble saucers, one inverted upon the other. At the apex of these convex surfaces there is a small orifice. Thus far the structure is quite simple and would remain so were it not for a membrane which is stretched laterally across the center. TlLis ineinbrane is a continuation of the lignin matrix, which serves to cement the fibers toFigure 4-Actual Siac. Flat-Grain Surfaces Showing Appmrent Unifurmlfy for gether. IIowever, according to Bailey (I) this Palntlng membrane is so construct,ed that it allows tlie A--while pine; R-redwoad: C-iir; D - - c ~ p r e s massaze of liouids and zases. The center Dartion. ~,~ another and lack entirely the comp1exity arid specialized or torus, of this mernbraric is solid, but around it, radiating character exhibited by the hardwoods. The stnicture of the as the spokes of a wheel, thwe are narrow slits through which softwoods has been thoroughly covered by botanists. There very fine colloidal niaterial may pass. The diffusion of are three types of cells that are of major interest to the gases may take place quite readily provided the torus of these Daiiit industry-the tracheids, the medullary rays, and the res& ducts. The resin ducts aie n o t found in all the softwoods, and therefore canriot be classed as a characteristic property of these woods. However, their presence cannot be ignorcd because they play an important part in the woods in which they occur. TRAcHEIDcAccording to Jeffrey (5) and Penliallow (81, the tissue of softwoods consists chiefly of tracheids or fibers. This primary tissue may be divided into t,wo groups on a basis of their formation in growth. The spring wood, formed during the rapidly growing period in the life of the tree, is made up of large, elongated, tapering elements with bordered pits. These elements are dovetailed together a t their tapering ends and their arrangement may be seen in Figure 7. These spring-wood fibers have thin walls, the radial aalls being covered with bordered pits. Theother groupof fibersma,kesupthesumiuerwood tissue, which is formed during tlie more or less quiet period in the life of the tree. These fibers are also tapered and are dovetailed together, but their walls are considerably thicker and the hordered pits are confined to the tangential wall. Figure 8 pictures the summer-wood fibers, showing the absence of pits on the radial surface of the fiber. Figure 9 is a cross section of white pine a t the point of transition from the summer wood to sorinz wood. There it is emv to see the Figure 5-Magn1Bt.d Surface (59 x) of Flat-Graln Wood Showing Irregularities locatioH ift h i pits, the differencein i&knes and Variation in Surface to Be Painted the cell wall, and the relative void space in the A-white pine: R-redwmd; C--Elr; D-ypresn I
Y
230
--x
Figure iI-Cross Sections Showine Change8 in Size of Cells of Spring and Summer Wood. SO A -fir, 13-redwood, C-ypresb
appro~.
Figure 14-Crosa Section of W h i t e Pine S h o w i n g Size of B Longitudinal Resin Canal wifh Respect fo Surrounding Cells. 270 x
Fieuro IZ--Fibers and Pit S f r ~ c f u 88 r ~Found in Spruce (Scarth)
Figure l3--Cross Section of e Log Showing Position of Flat-Grained a n d Edge-Grained Boards with Respect to Medullary Rays
Figure 15--Longitudinsi Section of White Pine s h o w ing s Radial Resin Canal. 60 X
medullasy ray cells are in direct communication with a great many tracheids These medullary rays are important in still another way, serving as the easiest and quickest means of transferring gas through a board. Any pressure set up in a board exerts itself along the medullary rays and these, in
flabgrained lumber, terminate at the pinewood interface. Studies now in progress indicate that these structures are directly related to failure by blistering. RESINDum-The last group of cells of importance in softwoods are the resin duck. Very often, in some species
Pihure 16-Actual
of wood, tlie resin is responsible for unuighbly paitit failures, siieli as small globules of resin exuding through tlie film, and
dark brown streaks fornied as the resin deconiposes or as pcding of the filni from exposed resin canals in which t,he resin has hec,oiue hard and brittle by oxidation. 'These resin cansls are considerably larger tliari tlie surroiiiiding cells, as may he seen in Figure 14, which shows the longitudinal resin canals of white pine. The lateral resin ducts (Figure 15) are smallcr and often are in communication with resin pockets 'These canals are in coininnidcation with adjacent, tihers and tissue. Solvents or heat. iiiakc the resinous material o f such a consistency that it will flow froin tlie resin canals to the surrounding tissue and to the surrace of the hoard. The radial ducts tenninate a t the surface of flat-grained luinber, while the longitudinal tubes may be exposed for many inches, as shown in Figure 16, which is a tangential (fiat grained) surface of vellow Dine. Both the radial and loneitudinal canals are responsible for some of the irregular penetration of oils, as the resin is soluble in the oils and thus aids in penetration of the oils from the paint down into the wood. Quite contrary to the general opinion, resinous woods arc readily penetrated by oils. For example, yellow pine, a very resinous wood, showed the greatest ease of penetration. The penetration was greatest radially, and passed through the resin0"s ha& of the summer wood more rapidly than in any of the other woods used. ~~~
of water is greatest in a tangential direction, averaging approsiiiiately twice that of the radial expansion and p e r h a p s fifty times that of the longitiidinal expansion. This condition is explained by the wood structure. From the general assernhlage of t,he cells in wood the sina,ll e!ongation in a longitudinal direction is perhaps due to the fact that the fibers may slip past each other and their increase in length not result in a direct elongation of the \rood. The greatest increase is in a tangentid direction because in this direction t i e fibers have tlie greatest degree of freedoln. I n the radial (Tiection the fibers are firmly attached to the medullary rays so that the resultant change is more restricted than the tangential expansion. The swelling of an individual fiber free from restraint will be the same radially and tangentially, and it is the arrangement of the cells that accounts for the variations in the degree of swelling in the different dimensions of a piece of wood.
S i r e Surface of Yellow Pine Showfile B Lonflltudinal R c d n Canal That Has Exuded Real" w o n t h e Surface
Table 1--0ver-.411 E.srm)snslonof Yellow P h e /1/
~
Pipure 17-Failure
76.0 4.0
3.0
4.07 4.69 SXPINSION
o ~ ~ dry ~ . ~ I I Average erpailSLoo
