January, 1935
I N D U S T R I A L A N D E N G I X E I3 R I N G C H E M I S T R Y WOODSCHOSENFOR TESTS
The hardwoods were selected for study partly for their commercial importance in the hardwood lumber industry and partly to obtain representation of the different types of wood structure. Table I lists the woods used together with their significant structural and physical characteristics. The data for specific gravity and rings per inch are for the boards used in these experiments. For the purpose of comparison four softwoods representing the range in softwood painting characteristics were included in the study.
gallons (11.4 liters) of raw linseed oil, 2 gallons (7.57 liters) of turpentine, and 0.125 gallon (0.475 liter) of liquid paint drier for the priming coat, with 0.5 gallon (1.9 liters) of raw linseed
TESTPANELS The test panels were made by mounting 0.5 X 5.5 inch (1.27 X 14 cm.) bevel siding on supporting panels of hemlock sheathing of the kind commonly used on houses. The supporting panels were 18.5 inches X 6 feet (0.47 X I .83 meters). In general, four boards 5 feet 10 inches (1.78 meters) lorig and of one species were mounted on a supporting panel; of these boards two were plainsawed or flat-grain and two were quarter-sawed or edge-grain' unless only plain-sawed boards of the species were available. The hemlock supports projected 1 inch (2.54 cm.) beyond the siding to offer nailing surface for attaching the panels to the framework of test fences. These nailing surfaces were then covered with vertical strips of wood much like corner boards on a house. The following hardwoods were obtained only in plain-sawed boards: black ash, sugar maple, and aspen. The quartersawed boards of yellow birch were of 3.5-inch (8.9-cm.) size; consequently, three of them mere mounted in place of' two 5.5-inch boards. The surface of each panel was marked off into three equal test areas, 18.5 X 23.5 inches (0.47 X 0.60 meter) in size, one area for each of the three paints or painting systems studied. In addition to the panels for ainting, test panels were made for exposure of the wood to weat%erwithout any protective coating. These panels were of the same general construction but the boards of siding were only 34.5 inches (88 cm.) long, and t w o sets, representing two woods, were mounted on a single supporting panel.
A
REDWD
1
35
R W 2 AP
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os
PAINTS USED
"
~
'
as
"
'
'
" " 040
'
'
~
a45
"
'
~
'
ax,
"
'
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'
05s
"
~
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'
060
'
"
'
'
065
'
070
SPEOfK GRAV/TY OF WOOL7
Two of the coatings studied were linseed oil paints of the kinds commonly used on houses. For the third, however, a more complicated painting system consisting of filler, primer, surfacer, and enamel was chosen because it has been common practice to use such systems in many places where hardwoods are or were formerly used out of doors-for example, in railway coach construction. The two house paints were a pure white lead paint and a lead and zinc paint. The white lead paint was made of softpaste basic-carbonate white lead (85 per cent pigment by weight) by thinning 108 pounds (49 kg.) of the paste with 3
FIGURE2. RELATION BETWEEN DURABILITY OF PAINTCOATISGS -4ND SPECIFIC GRAVITY OF W O O D
oil, 1.5 gallons (5.68 liters) of turpentine, and 0.125 gallon of liquid paint drier for the second coat, and with 2.5 gallons (9.46 liters) of raw linseed oil, 0.125 gallon of turpentine, and 0.125 gallon of liquid paint drier for third coat. The lead and zinc paint had the following composition:
I n the hardwood lumber industries it is customary to speak of "plainsawed" and "quarter-saased" boards while in the softwood industries the terms "flat-grain" and "edge-grain" or "slash-grain" and ' vertical-grain" are preferred I
TABLEI. PROPERTIES OF KlSD OF \$rOOD Ring-poroue hardwoods with large pores: Chestnut (Castanea dentata) Black ash (Frazinus nigra) Red oak (Quercu6 sp.) Red oak (Quercus sp.) White oak (Quercus sp.) White oak (Quercus sp.) Shagbark hickory (Hicoria ouata) Diffuse-porous hardwood with large pores: Black walnut (Juglans nigra) Diffuse-porous hardwoods with small pores: Yellow poplar (Liriodendron tulipifera) Aspen (Populus tremuloides) Red gum (Lipuidambar styracipua) Yellow birch (Betula lutea) Sugar maple ( A c e saccharum) Softwoods: Northern white pine (Pinus strobus) Redwood (Sequoia sempervirens) Western larch (Lariz occidentalis) Southern yellow pine (Pinus sp.)
43
REQIONOF GROWTH OF LUMBER Appalachian Lake States Lake States Appalachian Lake States A4ppalachian Ohio
Pigment, 64% by weight of the paint, c,omposed of: Basic-carbonate white lead, % , b y welght Lead-free zinc ,oxide, % by weight Magnesium silicate. % .,_ bv - weight Liquid, 36% by weight of the paint, composed of: Raw linseed oil, % by weight Turpentine, % by weight Liquid paint drier, % b y weight THE
60 30 10
89 5.5 5.5
WOODSTESTED
SP. GR. OF BOARDSOVEN-DRYWEIGHT No. OF ANNUALGROWTH AND .&-DRY VOL. RINQSPER INCH(2.54CM.) AV. Max. Min. AV. Max. Min.
0.409 0.487 0.591 0.562 0.664 0.548 0.698
0.457 0.600 0.628 0.574 0.770 0.595 0.783
0.349 0.382 0.549 0.471 0.584 0.510 0.634
1s 30 14 22 1s 28 17
32 40 16 24 22'
44 20
10 16 12 16 11 18 12
Iowa
0.660
0.593
0.531
10
12
8
Appalachian Lake States Southern Lake States Lake States
0.395 0.488 0.515 0.638 0.688
0.472 0.542 0.574 0.776 0.811
0.334 0.434 0.472 0.588 0.618
35 16 10 35 39
68 22 12 56 56
12 10 8 20 14
Lake States California Inland Empire Southern
0.344 0.481 0.577 0.539
0.395 0.609 0.686 0.678
0.288 0.370 0.506 0.416
19 36 28 12
22 52 47 15
14 24 12 10
41
1 S 1 ) I ! S 'C I( I A I ,
A 1, D
k; N G I N Eli, K 1 N G C 1.1 E hl 1 S 'I It Y
Vol. 27, No. 1
and sandiog sirrPacer. The sanding surfacer had the following composition : 75.0
11.3 8.7 6.0
39.0
60 4
The enaniel had the following coinposition T'iwnent, 57.4% by rejeht, ~ o u i ~ c u eoi: d l'itnnux li. % by reig1it linir.-carboiiste ivliite lend, 7' by reislit 1.erd-freo zinc: oxide. y, by weight I.W"id. 42.5% by reiplit. cornpored ui: Ilodied linseed oil, % by wcieht l',,ryentins, "/o by weight
45.0 35.0
20.0 20.0 58.0 41.0
The l,urlied liiiseed oil for tlie sanding siirfacer and tlie enarnel was ninde by heating vaniisii makers' Iirisecd oil at 300" C. for 2.75 honrs, adding one thirty-second volume oE liquid paint drier, and holding a t 300" C. for 15 minutes longer. It had a viscosity of npproximately "X" in the Gardner viscosity tube standards (12.9 poises). The liquid paint drier for all of the products used was a linoleate drier containing a small proportion of rosin and Iiaving 3.56 per cent metallic lead and 0.36 per cent metallic manganese by weight. I'AINTlXG AND EXPOSiJllE O F PANELS
AI1 painting was done indoors at Madison, \Vis. Ahout oiie week was allowed bctrveen coats for drying; during a part of this period the panels were taken out of doors for esposore to sunshine. Priming-coat paints were applied at spreading rates approximating 500 square feet per gallon, other paints a t about GOO square feet per gallon. Exposures were made in the vertical position facing south a t Madison, Wis., Seat,tle, \Vash., Tucson, A r k , Palmerton, Pa., and Gainesville, Fla. Inspections were made yearly at oiitlying stations, three t.iines a year at Madison, by the writer or by Don arouse of the Forest Products Laboratory. At t.lie last inspections before this report was witten, the exposures liad Ia3ted for 42 months at Gainesville and Palnierton, 47 months at Tucson and Seattle, and 48 mouths at Madison.
The compositioii OS t l i c wood filler (in per cent by weight) \vas as follow.:
A composite p i ~ m e n t(.onairtia~y ul 2,5% tiianiuiii dii,ride and 75% barium sultnte, by veight.
For npplication this paste filler w11s tliirmed w-ill) twpentirle, brushed mi the wood, allowxd to trim dn11 by evaporation of tlie turpentine, and then worked into the pores OS the wood with clean rags. The white lead prinier was the same as the printing coat described for the white lead house paint. "Half and IralE" was a niixture of equal volumes of whiLe lead primer
RESULTSAND Discussio~ Iinrnediately after paiiiting, it was apparent t.ttat coniirioii liouse paints do not make entirely satisfactory coatings on hardwoods Tvitli large pores because paint applied by brushing does not succeed in filling the pores properly. As a result the port? often remain visible as slight depressions in the coating, and the grain of the wood is not entirely concealed (Figure 1). Examination of the depressions ovcr the pores by nieans of the microscope Srequently revealed them as actual cavities in the coating where tlie paint, while still fluid, was drawn down along the walls of the pores or else was broken open by a bubble of air expanding from the pore. Appxrzntly paint that is fluid enough to apply hy brush is too fluid to plug or bridge large pores. Iiardwouds with large pores, therefore, are poorly adapted to ordinary house painting niet.liods. To pliig the pores suflicioiitly to allow paint to be applied sinootlily, stiff pigrnent paste (wood filler) must be worked into them mechanically as is done in the conventional operation of wood filling. Tests have not yet been made to determine whether any of the hardwoods retard the drying of paints lacking white lead or zinc sulfide when applied in the presence of rnucli moisture and absence of sunlight. Redwood and cypress among the softwoods are known to exert such an effect (0). Unverified reports from industrial sources point to difficulty on that score on some of the liardwoods that are relatively rich in tannins, such as the oaks, chestnut, and walnut,.
tkcept as pointed out in the two preceding paragraplis, tiie nature of t,he wood paint,ed has little or no effect on the behavior of paint coatirigx during the early part of pailit life either oii liardn.oods or on .softwoods. Suioiling, lo= of gloss, chalking, fading, atid checking proceed in the mariner charnct,eristic of tlie kind of paint and conditions of exposure but vitliiiut regard for the kind of wood beneatli. Aster tlie iiaint has become sufficiently embrittled witli age for disintegration to begin, lioircver, the nature of the mood largely determines the rate a t ivliieh disintegration progresses and the coos~iiciaiii.~nc-s of the areas of n.niirl laid bare. In softwoods tlie priiilvrriinant factor is tlie width of the lmrd.; of summerwood. In the Ilard~wodstlic specific grayit? oS tlie wood ~ n d the size i d the pores seem to be the predominant Factors. Tlie beliarior of the enibrittled rxmtings with respcct to riiailitenaim of tlieir integrity is suiriniarized in 'fable TI. The methods of inspccting and rating coatiiigs in u,?e by the Forest, I'rodricts Laboratory have lieen pu1)Iislied elsewhere (2, 4). Tal,le I1 records the dural.iility of tlie coat.ings and the rating in integrity at, the last inspection ai. each station and a composite rating for average durability at all five stations. The durability is defined as the time elapsed before the integrity of t,Iie coatirig rcaches the rating of jtoor, which is considered the limit to wliich repainting slioiild be deferred if satisfactory rriainteiiaiice is to bc ured. Coatings alloived tlJ go unpainted beyond that time are in the period of paint neglect. Tlie composite ratings for all siations, given iii tlie last coluiiiii of Table 11, are tlie average durabilities dxilated on tire assuniption that coatings remaining poor or better in integrity at t l i e last inspection would have acldiiional useful lives as follows: 12 niontlis if last rated fair or better, S WETH SMkL.1. I'OrrSS. i . ' L A K I < ; OF TAR-PED M A Y 1%kWnZ,Y h U 13 and specific gravity within each group except in the case of wooo, HUT EI*N I N €fE*"Y S o ~ W o O n . eneniel finish on hardwoods with large pores. The reason OB lIsr?,~r,:.~ IAWITI:I) POI$ A LONG Tine 'TO TEE H.vos OF S U I ~ I L H for this discqxmcy is pointed out. in a subsequent paragraph. ,Yoon K i t h any oire group of moods thc durability OS paint is gen- .iftei47-inontii expumre st Tucaon lead and sin,: paint (ohore) on soatbein erally greater tile lighter the wood (Figure 3). In previous yellow pine ahd (below) on biroii. publications tlie writ.er has attributed this teiidcncy to better meclranieal adlicsioii of eniitrittled paint on lighter woods, ently tends t
